Forensic Polymer Engineering

Forensic polymer engineering

Related titles: Advances in polymer processing: from macro to nano scales (ISBN 978-1-84569-396-1) This comprehensive book reviews the latest advances in polymer processing techniques. Part I reviews fundamentals such as materials and their properties. Part II discusses advances in moulding. Part III covers alternative processing technologies such as calendaring and coating, foam and radiation processing. Part IV addresses nanotechnology, whilst the final part of the book considers post-processing technologies such as joining, machining, finishing and decorating polymers. Failure analysis and fractography of polymer composites (ISBN 978-1-84569-217-9) Fractography is the study of fracture surface morphology. This important book discusses the main types of failure mechanism in polymer matrix composites. It discusses the causes and characteristics of failure mechanisms such as delamination. It also includes case studies on failures due to material, design and manufacturing defects as well as those caused by in-service conditions. Properties and performance of natural-fibre composites (ISBN 978-1-84569-267-4) This important book reviews the wealth of recent research into improving the mechanical properties of natural-fibre thermoplastic composites. The first part of the book discusses the main types of natural fibres used in composites and the way the fibre–matrix interface can be engineered to improve performance. Other chapters cover the increasing use of natural-fibre composites in such areas as automotive and structural engineering, packaging and the energy sector. Details of these and other Woodhead Publishing materials books can be obtained by: • •

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Forensic polymer engineering Why polymer products fail in service

Peter Rhys Lewis and Colin Gagg

Oxford

Cambridge

New Delhi

Published by Woodhead Publishing Limited, Abington Hall, Granta Park, Great Abington, Cambridge CB21 6AH, UK www.woodheadpublishing.com Woodhead Publishing India Private Limited, G-2, Vardaan House, 7/28 Ansari Road, Daryaganj, New Delhi – 110002, India www.woodheadpublishingindia.com Published in North America by CRC Press LLC, 6000 Broken Sound Parkway, NW, Suite 300, Boca Raton, FL 33487, USA First published 2010, Woodhead Publishing Limited and CRC Press LLC © 2010, Woodhead Publishing Limited The authors have asserted their moral rights. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the authors and the publishers cannot assume responsibility for the validity of all materials. Neither the authors nor the publishers, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from Woodhead Publishing Limited. The consent of Woodhead Publishing Limited does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing Limited for such copying. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Library of Congress Cataloging in Publication Data A catalog record for this book is available from the Library of Congress. Woodhead Publishing ISBN 978-1-84569-185-1 (book) Woodhead Publishing ISBN 978-1-84569-780-8 (e-book) CRC Press ISBN 978-1-4398-3114-4 CRC Press order number: N10190 The publishers’ policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp which is processed using acid-free and elemental chlorine-free practices. Furthermore, the publishers ensure that the text paper and cover board used have met acceptable environmental accreditation standards. Typeset by Toppan Best-set Premedia Limited Printed by TJ International Limited, Padstow, Cornwall, UK

Contents

1 1.1

1.2

1.3

1.4

Preface Acknowledgements

xv xxi

Introduction Product failure 1.1.1 Non-metallic elements 1.1.2 Bonding Covalent bonds Electrostatic bonds Hydrogen bonds van der Waals bonds Properties of polymers 1.2.1 Polymers High performance polymer fibres Other uses 1.2.2 Natural materials 1.2.3 Properties of elastomers Failure modes 1.3.1 Mechanical failure Loading patterns Load path Stress concentrations 1.3.2 Chemical attack Oxidation Hydrolysis Ultraviolet radiation 1.3.3 Stress corrosion cracking 1.3.4 Environmental stress cracking (ESC) Methods of investigating product failure 1.4.1 Sifting the evidence

1 1 2 2 3 6 7 9 11 11 15 15 17 18 19 20 21 22 23 26 27 27 28 29 30 32 32 v

vi

1.5

1.6 2 2.1 2.2

2.3

2.4

2.5

2.6

2.7

Contents Witness evidence Records Surviving remains Public information sources 1.5.1 Textbooks 1.5.2 Event reporting 1.5.3 Public domain 1.5.4 Materials and product standards 1.5.5 Disasters References

33 33 34 35 36 36 37 37 38 40

Examination and analysis of failed components Introduction Processing methods and defects 2.2.1 Injection moulding 2.2.2 Extrusion 2.2.3 Other moulding methods 2.2.4 Other shaping routes Mechanical testing 2.3.1 Tensile testing 2.3.2 Creep and stress relaxation 2.3.3 Composite materials 2.3.4 Photoelastic strain analysis Techniques for recording product failures 2.4.1 Visual observation 2.4.2 Forensic macroscopy 2.4.3 Radiography Forensic microscopy 2.5.1 Optical microscopy 2.5.2 Scanning electron microscopy 2.5.3 ESEM Types of product defect 2.6.1 Mechanical defects 2.6.2 Fatigue 2.6.3 Friction and wear 2.6.4 Environmental failure Molecular analysis of polymer properties 2.7.1 Sampling 2.7.2 Chromatography 2.7.3 Infra-red spectroscopy 2.7.4 Fingerprint spectroscopy 2.7.5 Beer-Lambert law 2.7.6 UV spectroscopy

42 42 42 43 47 48 48 50 50 51 52 53 55 55 56 56 58 59 61 62 63 64 66 67 68 69 69 70 72 76 76 77

Contents

2.8 2.9 2.10 3 3.1 3.2

3.3

3.4

3.5

3.6

3.7 3.8

2.7.7 NMR spectroscopy 2.7.8 Other methods 2.7.9 Thermal analysis Integrity of results Conclusions References Polymeric medical devices Introduction Failed catheter 3.2.1 Thermoplastic elastomers 3.2.2 Accident at childbirth 3.2.3 ESEM of the failed end 3.2.4 Material and mechanical testing 3.2.5 Degradation theory 3.2.6 Conclusions Failure of connectors 3.3.1 Connector failures 3.3.2 Premature cracking of connectors 3.3.3 Disclosure 3.3.4 Literature 3.3.5 Joint expert examination 3.3.6 Injection moulding 3.3.7 ESC/SCC hypothesis 3.3.8 Discussion 3.3.9 Balloon catheters and angioplasty Failure of a breast tissue expander 3.4.1 Failure of tissue expander 3.4.2 Loading pattern 3.4.3 Conclusion 3.4.4 Other cases Failure of sutures 3.5.1 Wound opening 3.5.2 Analysis of new suture 3.5.3 Possible causes of failure 3.5.4 Outcome Failure of breathing tubes 3.6.1 Development of sight tube 3.6.2 Faulty tubes 3.6.3 Trial 3.6.4 Lessons Conclusions References

vii 79 80 81 85 86 88 89 89 90 91 92 94 96 99 101 102 102 103 106 106 107 110 110 111 112 114 114 118 119 120 121 121 122 123 124 124 124 126 129 130 130 132

viii

Contents

4 4.1 4.2

Polymer storage tanks Introduction The Boston molasses disaster 4.2.1 Causes of failure Failure of polypropylene storage tanks 4.3.1 Catastrophic failure 4.3.2 Investigation 4.3.3 Stress concentration 4.3.4 Cause of failure 4.3.5 Other problems 4.3.6 Paint accident 4.3.7 Rotational moulded tanks 4.3.8 UV degradation 4.3.9 Failed battery cases Failure of fibreglass storage tanks 4.4.1 Chopped strand mat 4.4.2 Catastrophic failure on Teesside 4.4.3 Plant damage 4.4.4 Wall and base sections 4.4.5 Reassembly of failed parts 4.4.6 Fracture locus 4.4.7 History of usage 4.4.8 Thermal properties of composite Reconstructing the events leading to failure 4.5.1 Thermal expansion 4.5.2 Hoop stress 4.5.3 Strength of material 4.5.4 Reactions in tank 4.5.5 Failure sequence 4.5.6 The bund Dealing with the aftermath 4.6.1 Standards 4.6.2 Acid storage tanks 4.6.3 Other failures 4.6.4 Glass fibre attack Setting new standards References

134 134 134 136 137 138 138 140 141 143 143 144 148 152 153 154 154 155 157 160 162 163 164 166 166 167 167 168 168 169 170 170 171 172 173 173 175

Small polymeric containers Introduction Failure of battery containers 5.2.1 Military batteries 5.2.2 Failures

176 176 177 177 178

4.3

4.4

4.5

4.6

4.7 4.8 5 5.1 5.2

Contents

5.3 5.4

5.5 5.6

5.7

5.8 5.9 6 6.1 6.2

6.3

ix

5.2.3 Investigation 5.2.4 Material analysis 5.2.5 Conclusions 5.2.6 Aircraft batteries 5.2.7 Patent action Failure of buckets 5.3.1 Weld line formation Exploding batteries 5.4.1 Fire brigade incident 5.4.2 Material quality 5.4.3 Hydrogen explosions 5.4.4 Personal injury 5.4.5 Hindenburg disaster, 1937 Failed truck battery cases 5.5.1 First failures Failures in miner lamp battery casings 5.6.1 New design in polycarbonate 5.6.2 First failures 5.6.3 Solvent cracking 5.6.4 Strain birefringence 5.6.5 Property checks 5.6.6 Polishing 5.6.7 Moulding conditions 5.6.8 Stress concentrations 5.6.9 Practical applications 5.6.10 Colliery experience Improving design to prevent failure 5.7.1 Alleged hydrogen explosion 5.7.2 South African lamps 5.7.3 Further developments Conclusions References

179 180 181 182 182 184 186 187 188 190 190 191 193 195 196 198 200 200 201 203 206 207 211 213 215 217 219 220 221 223 224 225

Polymeric pipes and fittings Introduction Fracture of PVC water piping 6.2.1 Factory crisis 6.2.2 Analysis of broken pipe 6.2.3 Reconstruction 6.2.4 Stresses on pipe 6.2.5 Cause of failure Failure of PVC water pumps 6.3.1 Rising mains

226 226 227 227 228 232 233 235 235 236

x

6.4 6.5

6.6

6.7

6.8 6.9 7 7.1 7.2

7.3 7.4

7.5

Contents 6.3.2 Fatigue tests 6.3.3 Machined PVC problem 6.3.4 Mediation Failures in gas pipelines 6.4.1 Fracture surface Failures in ABS pipes and fittings 6.5.1 Immingham docks 6.5.2 Conclusions Compressed gas explosion 6.6.1 Cracked pipe 6.6.2 Mechanics 6.6.3 Controversy Failures in polybutylene pipes and acetal resin fittings 6.7.1 Acetal fitting fracture 6.7.2 Literature review 6.7.3 Acetal albatross 6.7.4 Degradation mechanism 6.7.5 Pipe failures 6.7.6 Recent developments Conclusions References

238 240 242 243 248 250 250 253 253 254 257 258 259 259 260 263 264 266 267 268 270

Polymeric seals Introduction Failure of elastomeric seals in brakes 7.2.1 Scanning electron microscope (SEM) of fracture surface 7.2.2 Elastomer analysis 7.2.3 Explanation of accident The Challenger disaster Failed elastomeric seals in a semi-conductor factory 7.4.1 Failed diaphragm seal 7.4.2 More failed seals 7.4.3 Ozonolysis 7.4.4 Independent analyses 7.4.5 Sources of ozone 7.4.6 Chasing the problem 7.4.7 Conclusions Failures in TPE radiator washers 7.5.1 New washers 7.5.2 Leaks in CHS systems 7.5.3 Simulation experiments

272 272 273 274 276 277 278 281 282 283 287 287 289 290 292 293 294 294 296

Contents

7.6

7.7 7.8

8 8.1 8.2 8.3 8.4

8.5

8.6 8.7

8.8 8.9

8.10 8.11

xi

7.5.4 Direct examination 7.5.5 Hydrolysis Failures in silicone mastics 7.6.1 Fire station training building 7.6.2 Sealant analysis 7.6.3 Calorimetry 7.6.4 Conclusions Conclusions References

298 299 301 301 302 303 304 306 308

Tools and ladders Introduction Failure of polypropylene hobby knives 8.2.1 Accident reconstruction Failure of polystyrene components in hobby knives 8.3.1 Knife inspection Failure of handles in angle grinders 8.4.1 Fracture surface 8.4.2 British Standard for tools 8.4.3 Another handle failure 8.4.4 The fracture 8.4.5 Conclusions Failure of security caps for gas cylinders 8.5.1 Storage failures 8.5.2 Development of torque test Failure of an ABS handle 8.6.1 Scanning microscopy Failure of chairs manufactured from polypropylene 8.7.1 Material analysis 8.7.2 Another failure 8.7.3 Litigation Failure of swimming pool steps 8.8.1 Fatigue crack Failed polyamide fittings in ladders 8.9.1 Failed stepladders 8.9.2 Scanning microscopy 8.9.3 Another accident 8.9.4 Scanning microscopy 8.9.5 Product design, quality and testing Conclusions References

310 310 311 311 313 314 316 317 320 320 320 323 324 325 327 328 331 331 333 334 335 336 336 338 339 342 343 344 345 346 348

xii

Contents

9 9.1 9.2

Components in transport applications Introduction Failure of tailpack in a motorbike accident 9.2.1 Seized wheel 9.2.2 Deeper analysis 9.2.3 Bungee cords 9.2.4 Alternative theory 9.2.5 Critical speed 9.2.6 Skid mark analysis 9.2.7 Tailpack design Failure of drive belts 9.3.1 Belt remains 9.3.2 Brittle fracture surface 9.3.3 Sequence of events 9.3.4 Other composite belts Failure of tyres 9.4.1 Truck tyre failure 9.4.2 Oxygen and ozone cracking 9.4.3 Sequence of events 9.4.4 Modern tyre technology Failed Rilsan nylon fuel pipes 9.5.1 First encounters 9.5.2 Spider lines 9.5.3 Sir John Gielgud 9.5.4 Fiat fuel lines 9.5.5 A car fire in Ireland 9.5.6 Murphy infants-v-Fiat spa 9.5.7 Other Mirafiori fires 9.5.8 Global markets 9.5.9 Fires in tunnels Stress corrosion cracking of nylon connectors Conclusions References

349 349 350 350 352 354 355 357 358 358 359 360 362 363 365 365 366 369 370 371 374 374 376 377 379 383 385 387 388 389 390 393 394

Consumer products Introduction Failure of Noryl plugs 10.2.1 Microscopy 10.2.2 Material analysis 10.2.3 Injection moulding conditions 10.2.4 Conclusions Failure of Noryl busbar plugs 10.3.1 Quality control

396 396 397 398 402 405 407 408 410

9.3

9.4

9.5

9.6 9.7 9.8 10 10.1 10.2

10.3

Contents 10.4 10.5 10.6

10.7

10.8

10.9 10.10 11 11.1 11.2

11.3 11.4 11.5

11.6

11.7

xiii

Residual current devices (RCDs) 10.4.1 Patent action Failure of kettle switches Failure of fittings on luggage carriers 10.6.1 First accident 10.6.2 Fracture and other surfaces 10.6.3 Failure sequence 10.6.4 Conclusion 10.6.5 Second accident 10.6.6 Fracture surface 10.6.7 Contamination Failure of ABS joints on bike carriers 10.7.1 Damaged shells 10.7.2 Stress analysis 10.7.3 Conclusions Failure of HDPE baby cot latches 10.8.1 Broken latch 10.8.2 Analysis Conclusions References

411 413 414 416 417 417 419 420 420 421 424 425 427 429 429 430 431 433 434 437

Conclusions Introduction: causes of product failure Poor manufacturing methods 11.2.1 Faulty moulding 11.2.2 Assembly problem 11.2.3 Medical devices Poor design 11.3.1 Stress concentrations Poor choice of materials Environmental stresses 11.5.1 Stress corrosion cracking 11.5.2 Oxidation and ozonolysis 11.5.3 Environmental stress cracking 11.5.4 Data compilations Access to information 11.6.1 Published literature 11.6.2 Old problems 11.6.3 The internet 11.6.4 Wikipedia References

438 438 438 439 439 440 441 442 443 444 445 446 448 449 449 450 451 452 453 454

Index

455

Preface

Forensic methods have improved dramatically in recent times, increasing the chances of catching criminals, resolving disputes and enhancing product quality. It is common knowledge that forensic science has enabled many old, cold cases to be solved, especially unsolved murders committed years ago, provided the evidence was preserved at the time for modern analysis. But there has been similar, although less well known, progress in forensic engineering, the subject that deals with accidents, disasters and product failure of all kinds. Modern techniques have shed much light on the Tay and Dee bridge disasters, for example; disasters from a different era of technology (1, 2). Re-examination of the remaining evidence from old railway accidents such as that at Shipton-on-Cherwell in 1874, have revealed the nature of the fracture which derailed an entire train, causing 34 deaths among the passengers (3). Metal fatigue was an important failure mode in these Victorian disasters, but was for long unrecognized and failures continued without respite. Despite the advance in understanding in the 20th century, the problem continues down to the present in all engineering fields. While case studies of metal product failure are well published today, those of other materials remain neglected, especially of non-metals such as glass, ceramics and polymers. Failures of plastic and elastomeric products are poorly published, perhaps a not unexpected problem given the reluctance by companies to advertise their failures, academic disdain of practical subjects, and their relatively recent introduction as engineering materials. However, some recent compilations have added much new and useful information of direct use to product designers. They include the pioneering books by Meyer Ezrin (4), David Wright (5) and John Scheirs (6) as well as our own previous work which presented a wider view of both metal and polymer product failures (7). In that book, we presented our cases as a narrative from failure to cause of the problem, with details that are often ignored, such as: xv

xvi • • •

Preface the parallels between failures in different materials previous examples of similar failures alternative interpretations by other investigators.

Many failure modes, for example, are common to many different types of material, especially fatigue from repeated loading below the nominal failure loads, corrosion or changes in a material as a result of interaction with its environment, creep rupture, wear and other mechanisms. So knowledge in one discipline can provide clues as to how failure occurred in other areas. Knowledge of several subjects is, indeed, often essential when products made of several different materials fail, such as the bridge bearing discussed in Chapter 2. The track record of parallel product failures is another topic of often vital interest because much will already have been determined and causes established, providing a context for a current investigation. Thus the information from the USA about an ongoing court case involving thermoplastic pipes was crucial in resolving a case involving a pipe junction failure in the UK (Chapter 6). While such information was frequently obscure in the past, the world wide web is exposing many such cases to public view and easy access. It can not only help resolve disputes, but also aid designers in selecting materials knowing the environment in which products have to perform reliably. A third area we have emphasised both here and in our previous book is the role of alternative failure explanations. More often than not, complete information is rarely available to the investigator, so assumptions about the loads and environments must be made in order to pursue the failure causes. Litigation cases frequently restrict important information from one side or another, at least until the disclosure phase, requiring the investigator to keep an open mind about the failure or failures. But some investigators jump to conclusions which are often not justified by the evidence, and that opinion often coincides with the views of the client who is funding the action. Client bias is in fact very common, but must be resisted when performing an investigation. It is in that client’s own long-term interests to know just how a product failed, which is the function of an independent investigation. If bias creeps into a report, then costs mount as litigation proceeds to an inevitable and unfavourable conclusion. It is far better to know the bad news early rather than later, a seemingly obvious comment, but one frequently missed during litigation. So we have included clear evidence of misleading or mistaken reports from other investigators, such as that from a study of radiator washer cracking (Chapter 7), where a report made incorrect deductions from the failed washers, and reached the wrong conclusions. Much extra work was then needed to find the real cause of the problem. Another example is given in

Preface

xvii

Chapter 10. It involved cracked transformer plugs which could electrocute the user, a problem that was raised by the supplier in the UK. They imported the plugs from Japan, and the cases were in turn made in China. A Japanese group suggested a cause which we could not confirm, and they used a single method rather than relying on several independent methods. We suggested a quite different source of the problem, faulty moulding in China. The Chinese produced moulding records which confirmed our diagnosis, and the problem was solved for the affected batch of plugs. Missing evidence is another problem often faced by the investigator. It is prevalent of course in fires, the key initiation point frequently destroyed by the fire itself. However, traces which do survive can hold the key to the solution of the problem, as discussed in more detail in Chapter 9 dealing with vehicle accidents. The material evidence in medical failures is also sometimes lost, especially if the product is disposable, such as with sutures used to stitch wounds (Chapter 3). Other agencies may lose samples, and failed samples may be discarded after inspection by the manufacturer, as in other cases discussed in Chapter 3. The extra uncertainties introduced make investigation yet more difficult, and explains why many legal cases take so long to resolve. Poor reporting on failures is not endemic to litigation but extends into the domain of the designer and manufacturer, where failed products should be studied in depth so as to prevent future failures. It is one hope that this and other failure compilations will help reverse that problem, by making failure case studies much more widely available to the specialist engineer. One way it can be achieved is by publication in learned and technical journals, and one such journal that has established a firm foundation is Engineering Failure Analysis edited by Dr DRH Jones. An increasing number of specialist papers dealing with non-metals are to be found there, helping to widen access to the study of product failure and ways to circumvent the many problems that ensue. Some of the cases published in this book are also published in that journal. The theoretical basis for the study of polymeric product failures is established and laid down in Chapter 1, along with the special terminology needed with long chain materials. Polymer science is a relatively new subject, dating back to the 1920s, although materials like gutta-percha (insulation in electrical and communication cables), natural rubber, celluloid and Bakelite were well exploited in the Victorian period. New polymers are still being synthesized, and an understanding of the basics is normally needed, even when examining well-known polymers such as polyethylene, which displaced gutta-percha for cable insulation in the 1930s. The analytical tools used for examining failed products are discussed in Chapter 2 with some background to their utility, publication and limitations. A compilation of

xviii

Preface

both common and specialist terms of use throughout this book is also available elsewhere (8). The case studies proper begin at Chapter 3 with an examination of failed medical products, both transitory and permanent implants with a large polymer component. It is one of the most active areas of interest, and unlike many other areas, reasonably well published in the specialist medical literature. Chapters 4 and 5 encompass large and small containers, where polymers are well established as materials of construction. Both small and much larger failures can lead to extensive collateral damage when the fluid contents are released by cracking of the container walls. Pipes are discussed in Chapter 6, where polymers have revolutionized practice, especially for utility transportation. But mistakes in using polymers have occurred, and one such problem was so widespread in North America that it resulted in one of the largest, most expensive and long running class actions ever. Polymers have long been used for sealing pipe systems, and they are the subject of case studies in Chapter 7, including an example of a very expensive problem in a pneumatic system controlling a semi-conductor fabrication factory in Japan. Rubber seals failed and shut down several machines, not just once but several times, leading to loss of production. Tools and related products follow in Chapter 8, and include products such as knife handles, power tools and ladders as well as all-plastic furniture. When such products suddenly fail, the safety of the user is immediately at risk. Modern cars contain many hidden safety-critical components such as fuel lines, as well as visible products such as tyres. Failure can have devastating consequences in driven vehicles, including both motorbikes and trucks. Road traffic accidents are the subject of Chapter 9, which on investigation proved to be traceable to the failure of polymer components. Polymers are ubiquitous in consumer products such as electrical insulation in plugs and other electrical equipment, where failure can result in electrocution, so great care is needed to prevent failure. They are also used for key anchors in luggage and baby cots, for example, where failure can result in serious personal injury (Chapter 10). We believe that it is only by publicizing case studies of product failure that designers and producers will change their practices and procedures to eliminate risks to users, improving not just product safety but also their own reputations as manufacturers. And it is not as if many of the design changes needed are costly or difficult to make. A single example will suffice among the many discussed in the main text. The strength of many products could be increased easily by ameliorating stress concentrations, especially sharp corners on the inner sides of enclosures. It can be achieved by rounding out sharp corners on tool edges and corners, an operation taking only a few minutes depending on tool complexity. Many other examples are described in the text.

Preface

xix

References (1) Lewis, Peter R. Beautiful Railway Bridge of the Silvery Tay: Reinvestigating the Tay Bridge Disaster of 1879, Tempus, 2004. (2) Lewis, Peter R. Disaster on the Dee: Robert Stephenson’s Nemesis of 1847, Tempus Publishing (2007). (3) Lewis, Peter R. and Nisbet, Alistair, ‘Wheels to Disaster!: The Oxford train wreck of Christmas Eve, 1874’, Tempus (2008). (4) Ezrin, M. Plastics Failure Guide: Cause and Prevention, Hanser (1996). (5) Wright, D. Failure of Plastics and Rubber products: Causes, Effects and Case Studies involving Degradation, RAPRA (2001). (6) Scheirs, J. Compositional and Failure Analysis of Polymers: A Practical Approach, Wiley (2000). (7) Lewis, Peter Rhys, Reynolds, Ken and Gagg, Colin. Forensic Materials Engineering: Case Studies, CRC Press (2003). (8) Walker, PW (Ed), Lewis, Peter R, Braithwaite, N, Reynolds, K and Weidmann, G. Chambers Materials Science and Technology Dictionary, Chambers (1993).

Dr Peter Rhys Lewis [email protected] Colin Gagg [email protected]

Acknowledgements

First and foremost, we would like to thank the numerous insurance companies, loss adjusters, manufacturers, other experts and lawyers for providing all of the cases in this book. Oldham Batteries Ltd deserve special mention for their support of our work, including Dr Richard Acton, Technical Director at the time, and Bob Booth of the Technical Department. We would also like to extend our appreciation to all those fellow experts with whom we have collaborated and who have proved open to argument and discussion. PRL would like to acknowledge work done by former research students, especially Drs Geoff Attenborough, Dave Anderton, Phil Hargreaves, Paul Hawkins, Kamal Weeraperuma, and Bob Ward, and the support of Sir Geoffrey Allen FRS in encouraging research with industry. The Consumer Research Labs and World Bank helped support the work on PVC pipes. We also thank EPSRC for supporting our post-graduate course in Forensic Engineering (T839) which aims to provide students with a basic foundation in the subject. The earlier course Design and Manufacture with Polymers (T838) received similar support, both being run in collaboration with the Polymer School at London Metropolitan University as part of an integrated graduate development scheme or IGDS. Students on the courses have participated actively in day schools, showing great enthusiasm for the subject. Our colleagues at London Met helped produce T838, including, Drs John Brydson, Mike O’Brien, Bob Dyson and Mark Alger. Professor Rod A Smith, FRAEng, Dr Colin Goodchild, Professor Roy Crawford FRAEng and Dr DRH Jones gave encouragement to the project. The Royal Academy of Engineering and the Open University supported numerous visits by PRL to the USA to read papers at the FAPSIG group of the Society of Plastic Engineers (SPE) based on case studies of polymer failure. He thanks Drs Meyer Ezrin and Donald Duvall, and Professors Jan Spoormaker and Alex Chudnovsky for their interest in his work. Thanks also go to Rebecca Dolbey, and Drs David Wright and Roger Brown of RAPRA Technology Ltd for interactive discussions. PRL would also like xxi

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Acknowledgements

to thank staff at the Shrivenham campus of Cranfield University during his tenure on the Forensic Engineering and Science Masters course, especially Drs Mike Edwards, John Bellerby, Donald Peach and David Lane. Much of the detailed research discussed would not have been possible without the help of our many colleagues, especially Drs Rod Barrett, G Weidmann, Sarah Hainsworth (Leicester University) and Jim Moffatt with technical assistance from Gordon Imlach (FTIR, DSC, SEM), Stan Hiller (optical microscopy), Richard Black (microscopy) and Naomi Williams (SEM), Charles Snelling and Peter Ledgard (machining), Richard Hearne and Ian Norman (lab superintendent). We have had stimulating discussions of the subject with Nancy Ashburn, Salih Gungor, Martin Rist Professors Jeff Johnson, Lyndon Edwards and Mike Fitzpatrick. We would both like to thank our families for their support, especially Sue Gagg for proofreading and David Lewis for providing information on current coal mine practice. Dr Patrick Lewis has helped in several literature searches involving medical device failures, and Fiona Lewis in providing administrative support.

This book is dedicated to students of forensic engineering and science

1 Introduction

1.1

Product failure

It comes as no surprise that products have a limited life in service. But what many might find surprising is the very great range of possible causes of failure, from a large and now very diverse range of materials. The failure modes of most metals are well established, simply because most have been used in service for many years. They have been well studied both in the laboratory and in practical applications, so there is a voluminous literature on the way they fracture, or fail in other ways. That, of course, does not stop further failures, but it does make failures from known causes less likely. Engineers and designers have a large property database available to them to check whether or not a particular metal or alloy is fit to be used under a specified set of circumstances. Such is not the case with most nonmetallic materials, especially those that have been discovered or invented within the recent past, especially polymers. Their failure mechanisms are the subject of this book, but we have not taken the usual academic approach of separating the failure mechanism from the product which fails, but rather discuss each incident as a case study in its own right. Case studies are important for several reasons. Product failures must be discussed in context, when the cause or causes of failure can be related to the way in which the product has been used (or abused). Secondly, if further failures of a particular type are to be prevented in future, then the causes must be identified so as to take remedial measures. It necessarily implies that all the product features which are relevant to its failure have to be examined for establishing the causal chain of events leading to its final demise. The first step in establishing the causal chain is simply to provide a chronology of events, so that each step is isolated and sequenced. Only then can the causes be tackled, using appropriate analytical tools. The details of each incident have to be described so that the critical facts can be sorted from the mass of irrelevant detail. This enables a fuller picture of the accident to be achieved, and it is also much more interesting to the reader if he or she wishes to draw parallels with related incidents within their own experience. There is no better way of illustrating the basic principles of polymer technology than by way of a detailed case study. It focuses attention on a specific aspect of the polymer structure, or the way it has been made, or the design of the product which has failed. 1

2

Forensic polymer engineering

So by way of prelude to the case studies described in this book, it is essential to provide the technical backdrop to the diversity of polymers used in products today. They provide properties unavailable in metals, such as transparency, low weight, high strength and insulation, for example. Low weight is at a premium for transport of goods and people, and one area where polymers have expanded in use. That success forms the backdrop to this book. After all, to understand the way materials succeed helps to understand why they fail. The starting point is the range of nonmetals available in the natural spectrum of elements. General definitions of terms and some explanatory text is provided in two dictionaries (1, 2).

1.1.1 Non-metallic elements The breadth of materials is not only very wide, but growing at an unprecedented rate today. To the existing fixed number of naturally occurring metals (about 72 of the 92 elements) have been added many different alloys, and compositions for particular kinds of properties. The much smaller number of non-metallic elements (about 20) exert an influence way beyond their number, which is actually only about 14, when the unreactive noble gases are excluded. That select group of elements includes most important of all, the elements: carbon (C), silicon (Si), oxygen (O), hydrogen (H), nitrogen (N), sulphur (S) and phosphorus (P). They are abundant in the Earth’s crust, and are all reactive both with one another, and with the metals. With the large class of metals and alloys, they include polymers, ceramics and glasses, all of which are important classes of useful solid materials. But in order to understand their physical and chemical properties, a brief discussion of the way they are held together at a molecular and atomic level is useful. The bonding then gives rise to various kinds of structure, depending on whether or not the bonds are directional or otherwise.

1.1.2 Bonding All solid materials are held together by bonds between the atoms of which they are made. The major classes of bond include: • • • • •

metallic covalent or chemical electrostatic hydrogen bonds van der Waals bonds.

The first three bond types are the strongest, the last two the weakest, and it is natural that the first group dominate the major classes of material. The

Introduction

3

covalent and hydrogen bonds are highly directional in space compared with the non-directional metallic, electrostatic and van der Waals bonds. They can therefore be symbolized by lines in flat representations, as in the following pages, and covalent bonds give rise to many important engineering materials, especially polymers and composites. Covalent bonds Excluding the metallic bond, covalent bonds occur when elements combine together and form a stable compound. The simplest example is the hydrogen molecule, written symbolically as H2, because it is a compound of two hydrogen atoms linked by a single covalent bond: H—H The bond is symbolised by the line between the two atoms, and hydrogen is said to be a diatomic molecule. It exists as a gas under normal conditions, is the lightest gas known and so has been used for lifting airships, for example. It is highly explosive in mixtures of air or oxygen, a problem encountered in a range of failing products. It occurs in a similar covalently bonded form in many compounds with carbon, such as in the thermoplastic polyethylene: —[CH2—CH2]n— This representation is known as a repeat unit, because when repeated endlessly, it creates a very long chain molecule. The real material is thus made from a mixture of such long chains. Structural complexity occurs when further groups are added to the simple PE repeat unit, so polypropylene has a methyl group added: —[CH2—CH(CH3)]n— But hydrogen also occurs in more complex repeat units, not just with carbon but also with other elements such as nitrogen and oxygen, as in the thermoplastic material nylon 6, with repeat unit: —[CO—CH2CH2CH2CH2CH2—NH]n— All polymers can be described by a repeat unit, or combination of different repeat units (copolymers), as shown for a few simple polymers in Table 1.1. The monomers from which they are made are also shown, together with the molecular weight of the repeat unit (MR). The latter can be calculated using standard atomic weights and knowing the repeat unit formula. Thus since the atomic weight (relative atomic mass) of carbon and hydrogen are 14 and 1 respectively, M is (2 × 12) + (4 × 1) = 28. Copolymer structure gives an added level of complexity, as shown in Fig. 1.1 for the

4

Forensic polymer engineering

Table 1.1 Repeat units and size in some common polymers Monomer

Repeat unit

MR

PE PP PVC PS BR NR CR PA6

CH2=CH2 CH2=CHCH3 CH2=CHCl CH2=CHC6H5 CH2=CH—CH=CH2 CH2=CH—C(CH3)=CH2 CH2=CCl—CH=CH2 CO — (CH2)5 — NH HO2C—(CH2)4—CO2H H2N—(CH2)6—NH2

—[CH2—CH2]— —[CH2—CHCH3]— —[CH2—CHCl]— —[CH2—CHC6H5]— —[CH2—CH=CH—CH2]— —[CH2—CH=C(CH)3—CH2]— —[CH2—CCl=CH—CH2]— —[NH—(CH2)5—CO]—

28 42 62.5 104 54 68 88.5 113

—[NH—(CH2)6—NH—CO—(CH2)4—CO]—

226

{

{

PA 6,6

—

Polymer

Copolymer repeat units

Homopolymer (polystyrene)

Alternating copolymer (SAN)

Graft copolymer (HIPS)

Graft terpolymer (ABS)

Random copolymer

Repeat units Styrene

Acrylonitrile

1.1 Co-polymer repeat units.

Butadiene

Introduction CH3

CH3 C

CH

CH3 C

CH2

CH

CH

C CH2

CH2

S

S

S CH2

CH

C

CH2 CH

CH3

CH2 C

CH CH2

CH

S

CH

5

CH2

CH CH

C

CH3

CH3

1.2 Cross-linking of natural rubber.

various structures formed from styrene, butadiene and acrylonitrile monomers. Polymers can also be classified as thermoplastic and thermoset, terms which describe their behaviour on heating. Thermoplastics can be heated repeatedly with little change in properties, while thermosets cross-link on heating. Cross-linking binds all the chain molecules together by covalent bonds, so that the shape of the material is permanent when the reaction has occurred (Fig. 1.2). Thermoplastic polymers comprise the majority of synthetic polymers, although thermosets are a small but important class of polymers for adhesives (such as epoxies) and composite materials, where they are used as the matrix to bind reinforcing fibres together (epoxies and polyesters for example). Although all polymers can be formed into fibres, a small class of thermoplastics have traditionally been used in fibre form. They include nylon 6, nylon 66 and PET (polyethylene terephthalate). Natural fibres such as silk and cotton are also important for textile manufacture. Yet another way of classifying polymers is by the way they are made. The broad division is between chain-growth and step-growth polymers, the former made by initiating chains using special catalysts so that long chains form very quickly from monomer (M): nM → —[M]n— Examples include PE, PP and polystyrene, and they usually possess a double covalent bond, from which reaction occurs. Step-growth polymers are made by each monomer unit reacting one at a time with another monomer: M + M → M—M + M → M—M—M . . . . . Examples are common, with all nylons, PET, polycarbonate among those formed stepwise. High molecular weight polymer is achieved only slowly, and molecular weights of commercial grades tend to be relatively low compared with chain growth polymers. The molecular weight is simply the

6

Forensic polymer engineering

molecular weight of the repeat unit (MR) multiplied by the number of units in each chain (n): M = nMR

1.1

In most polymers, there are chains of different length, so two ways of defining the average are the number average and weight average molecular weights, Mn and Mw respectively:

∑ (N M ) ∑ (N ) i

Mn =

i

i

1.2

i

i

and

∑ (W M ) ∑ ( N M ) = = ∑ (W ) ∑ ( N M ) i

Mw

i

2

i

i

i

i

i

i

i

i

1.3

i

where Ni and Wi are the number of chain molecules of molecular weight Mi respectively. The weight average molecular weight is always greater than the number average except for monodisperse polymers. An important single variable which defines the breadth of chain distribution is the dispersion, D: D = M w Mn

1.4

When all the chains are of equal length, D must be unity and Mw and Mn are identical. Such so-called monodisperse polymers can be made, but commercial polymers are usually polydisperse. For step-growth polymers, D = 2, and chain growth systems produce much greater dispersities (typically about 10). In three dimensions, covalent carbon with single bonds is tetrahedral (Fig. 1.3), that is, the four single bonds point to the corners of a tetrahedron if the carbon atom is at its centre. If generated regularly in space, it generates the diamond structure, but by contrast, graphite is the more common form of carbon found in nature, where the carbon atoms are arrayed in flat sheets. This is due to the trigonal bonding present in double-bonded carbon. The three bonds point to the corners of an equilateral triangle with carbon at the centre. Polyethylene forms a linear chain, but still preserving the tetrahedral shape of the carbon bonding with the hydrogen atoms. It forms a linear zig-zag conformation when crystalline (Fig. 1.4). Electrostatic bonds The next class of bond type occurs universally in combinations of metals with non-metals. The electrostatic bond forms between charged elements

Introduction

7

H

C H

H

H

1.3 Tetrahedral carbon atom.

(ions), and one of the simplest examples is magnesium oxide used in crucible constructions, and has the formula MgO or Mg++O− −. Oxides are widely used in high-temperature resistant materials owing to the high energy of the electrostatic bond, which needs a correspondingly high temperature to split the bond apart. They therefore find wide use in molten metal containment, turbine blades, and similar exceptional applications. All such ceramics are highly crystalline because the ions pack closely with one another in a highly regular way. Another major class of material is the inorganic glasses, normally based on a mixture of oxides fused together so that long-range order is lost and the material is non-crystalline. Depending on the oxides used, they also tend to exhibit high transition temperatures. One common component of glasses is silica, SiO2. When melted and then cooled slowly, silica is non-crystalline, and the silicate tetrahedral are randomly linked together. However, it can be mixed with metal oxides to form common glasses such as soda-lime-silica glass: Na2O/CaO/SiO2. The material is non-crystalline and thus transparent. It can be spun into glass fibre, a very common reinforcement for polymer composites, either in chopped fibre form so that products can be injection moulded, or in continuous fibre for use in more substantial products such as storage tanks, boat hulls and building products. Hydrogen bonds A much weaker bond occurs in many natural organic materials, as well as water itself, of formula: H2O or H—O—H

8

Forensic polymer engineering

a)

b)

The diamond structure

Graphite structure based on trigonal carbon atoms H HH HH H C C C C C C C

c)

d)

H HH HH HH H Portion of linear polyethylene chain in zig-zag conformation

Crystalline polyethylene

1.4 Variety of carbon structures.

The main bonds in the water molecule are covalent, but water is almost unique in the weak intermolecular bonds it forms between adjacent molecules: H—O—H--H—O—H--H—O—H They make water much more viscous than would be the case if hydrogen bonds didn’t exist, but are also significant in the way they occur in many natural materials such as wood and fibres (as cellulose), and in DNA/RNA, the building blocks of life. They also occur in synthetic polymers such as

Introduction

9

CH2 CH2 CH2

C

C O

H

O

H

CH2 CH2

CH2

CH2

CH2

CH2 CH2

CH2

CH2

CH2

CH2

N

CH2

CH2

N

CH2 N

CH2

CH2

CH2

H

C

O

H

C

O

H

C CH2

CH2 CH2

O

N

N

N CH2

O

C

CH2

CH2

H

CH2

CH2

CH2

CH2

1.5 Sheet structure of nylon 6,6.

nylon, where the bonding occurs between NH and OH groups of the repeat unit in the crystalline form of the polymer (Fig. 1.5). That their density within a structure is important for their properties is shown by the variation of melting point (Tm) with chain length for different polyamides (nylons), the melting point falling with increase of chain length between the amide group active in bonding (Fig. 1.6). Polyurethanes show a similar fall, but polyesters show no correlation since they are not hydrogen bonded. van der Waals bonds The weakest bonds of all occur between covalent molecules, as in gases, in liquids and solid materials, such as polymers. This is why hydrogen gas, for example, can only be liquefied at very low temperatures, where the thermal vibrations of molecules is low enough for the molecules to be held loosely together by the weak van der Waals bonds. One form of the bond occurs in polymers along the chain, and gives rise to rotational isomers. All chains can rotate about their carbon–carbon chain bonds (Fig. 1.7), and the resistance to rotation is determined by van der Waals interactions and steric hindrance (that is, the interference from the physical size of the atoms on

10

Forensic polymer engineering 260 240

(b) Polyamides

220

Temperature (°C)

200 180 160

(c) Polyurethanes

140

HDPE

120 100 80

(a) Polyesters

60 1

2

3

4

5 6 7 8 9 10 11 12 13 14 15 Number of carbon atoms

1.6 Structure and melting points of polymers.

Rotation about carbon-to-carbon bond

Hydrogen Carbon

1.7 Rotational isomerism.

the side parts of the chain). Thus polypropylene (PP) will have greater steric hindrance than polyethylene because the methyl extra side group (—CH3) is larger and thus interferes more than a simple hydrogen atom (—H). The resistance to rotation is therefore greater in PP than polyethylene or PE, giving it a much higher Tg, and also inhibiting crystallization. The different shapes of a single chain created by rotation about the chain links gives rise to different conformations, while the structure determined by the covalent bonds is termed the configuration. The configuration is necessarily a more permanent feature of a polymer since it is locked in at polymerization, while the conformation varies with temperature and environment. But it is a truism to say that it is the weakest part of a structure that determines its stiffness and ultimate strength, so such bonds represent a

Introduction

11

force which cannot be ignored, and indeed, exert an influence in failure studies out of proportion to their significance in the spectrum of forces holding solids together.

1.2

Properties of polymers

As one might expect, the properties of materials are dominated by the type of bonding between the atoms and molecules. The strongest covalent bond occurs between carbon atoms, with diamond and graphite as exemplary solids for their extremely high melting temperatures, and mechanical properties such as stiffness and strength. Both exist in highly crystalline forms (Fig. 1.3), and are exploited commercially for those almost unique properties. Diamond crystals are used as a powerful abrasive on oil drilling tools, for example, and graphite for crucibles, and in another form, carbon fibre, as a very strong reinforcing agent in composite materials. But both are relatively expensive, so find limited application at present compared with the majority of materials. They can be thought of as representing the apex of the property pyramid, every other material falling below their maxima.

1.2.1 Polymers The largest class of non-metallic material is represented by synthetic polymers, long chain molecules constructed from different repeat units (3–5). Their properties are determined not by their strongest bonds (the covalent bonds in the chains), but by their weakest, the van der Waals bonds between the chains. This is basically why they generally exhibit low melting and other thermal transitions compared with ceramics and glasses. On the other hand, relatively low transitions mean that they can be shaped easily, and indeed, shaped into very complex products with alacrity. Polymers can be partly crystalline, or amorphous, depending first on the regularity of the repeat unit. If it is symmetrical and regular then crystallization is possible, but may not always be achieved. Polyethylene is always partly crystalline because the repeat unit is very simple and symmetrical, and the melting point, Tm is an important characteristic of the polymer. There is another important distinction between two groups of polymers: the elastomers and plastics. The thermal behaviour of all polymers can be described in the form of a so-called viscoelastic master curve (Fig. 1.8), where stiffness (tensile modulus, E) is plotted against temperature. The example chosen here is that for several different forms of polystyrene, the normal amorphous type, a cross-linked version and a crystalline type. At low temperatures, thermal motion of the chains is low, and the influence of van der Waals bonds high, so the polymer is stiff. As temperature rises, the chains become more

12

Forensic polymer engineering Tg Glass

Tm Rubber

Viscous flow

10 1 GN m–2

9

Log [ER(10)/N m–2]

8

Crystalline

7 1 MN m–2

6 Cross-linked 5

4

Atactic

3 A 60

80

100

120

140

160

Tm

B 180

200

220

1 kN m–2

240

Temperature (°C)

1.8 Viscoelastic master curve (A = low molecular weight; B = high molecular weight).

mobile, so the chains can rotate and overcome the weak intramolecular bonds. This temperature is the glass transition temperature (Tg) and is another characteristic for a specific polymer. The polymer reaches a plateau, where it behaves like a rubber, showing reversible and long-range elasticity. Further increase in temperature causes viscous flow, although this region is inhibited by crystallinity or cross-linking. An amusing material which possesses all these attributes over a foreshortened time scale is ‘pottyputty’: it can easily be rolled into a ball, which when dropped bounces like a rubber. But when hit with a hammer, it shatters like a brittle glass. And if left unattended, it will flow like a liquid. It encapsulates all the viscoelastic states over a much smaller time frame than conventional polymers. Since crystallization and the transition to a rubbery state are controlled by the same inter- and intra-molecular bonds, one would expect a correlation between the melting and glass transition temperatures (Fig. 1.9). The relation is linear and Tg is roughly two-thirds of the melting point when the temperature is expressed in degrees absolute (Kelvin): Tg = 2/3 Tm

1.5

Introduction

13

700 PTFE

Melting point, Tm (K)

600 Nylon 6,6 PET

500 PVDC PP

400

Cellulose triacetate PC

Nylon 6

HDPE

300

NR cis-PB

200 100 100

150

200

250

300

350

400

450

Glass transition temperature, Tg (K)

1.9 Relation between melting point and glass transition point.

A list of thermal data for various polymers is shown in Table 1.2. At yet higher temperatures (towards the right in Table 1.2), the notional polymer will degrade, usually involving chain break-up to smaller chains or fragments. Such degradation affects both viscosity and strength in just the same way, because the chain assembly is held together by chain entanglements, and when chain length falls below a critical value, the strength (and viscosity) fall catastrophically. The problem is directly related to the entanglement of the long chains with one another: above the threshold, they will tangle and so the chain assembly is held together by knots and loops between the chains. But below the threshold, there is no entanglement and the strength is very much lower and drops fast with falling molecular weight. Typical critical entanglement molecular weights vary in the following way: Polyethylene Polycarbonate Nylon 6 Polypropylene Natural rubber, PMMA, PEO PTFE Polysobutylene Silicone rubber Polystyrene

4000 4600 5200 6700 10 000 13 200 17 000 29 000 356 000

Above these values, the strength is much greater and rises with molecular weight. At the other end of the scale of chain length, the strength is extremely high, but the polymer may be difficult to process into shaped

14

Forensic polymer engineering

Table 1.2 Thermal properties of polymers Polymer

Tg/°C

Tm/°C

ΔHt J mol−1

kJ kg−1

−90 −100

131 115

8 110

289

Polypropylene (isotactic) Polyisobutylene

−10 −65

176

10 970

261

1,4-cis-Polybutadiene 1,4-cis-Polyisoprene 1,4-trans-Polyisoprene

−85 −70 −65

1 28 74

9 200 4 390 12 700

171 64 187

Polystyrene (isotactic) Polystyrene (atactic) Styrene/acrylonitrile copolymer

239

8 360

80

97 107 −75 −40 −125

175 66

6 650 8 280

222 188

Polyvinyl chloride Polyvinylidene chloride Polyvinyl fluoride Polychlorotrifluoroethylene Polytetrafluoroethylene

80 −15

(210) 190 197 210 327

7 520 5 020 6 100

163 43 61

Polymethyl methacrylate (atactic) Bisphenol A polycarbonate

105 142

Polyethylene (high density) (low density)

Polymethylene oxide Polyethylene oxide Polydimethyl siloxane

Polyacrylonitrile Aromatic polyesters

Aliphatic polyamides

2,T 4,T 6,T 10,T 4,1 6 6,6 6,10 10,9 10,10

267

125

317

5 020

95

65

276 234 161 138 152 225 267 226 216 214

23 400 31 770 35 530 45 980 42 220 21 320 43 050 50 160 36 780 34 690

122 144 143 151 192 189 191 178 113 103

components owing to the very high viscosity. However, the high strength is such an attractive property for many engineering products, that machining is used to shape components. Examples include ultra-high molecular weight polyethylene, PTFE and nylon, both of which are used as highperformance bearings, for example. The importance of cross-linking and crystallization in delaying the onset of creep and flow has led to new polymers such as thermoplastic elastomers (TPEs), which have most of the

Introduction

15

advantages of thermoplastics, but have networks stabilized against chain movement by physical cross-links in the form of domains or crystallites. Control by the weakest intermolecular bonds means that stiffness and strength are also low relative to other materials, bearing in mind the large variation to which strength values are subject. Stiffness can also vary over wide limits, depending on fillers usually present in commercial materials. The values given for various mechanical properties in Table 1.3 are for unfilled, reasonably pure samples measured at a slow rate at ambient temperatures (ca 25°C). Increase in test rate almost always lowers strength because chain movement cannot respond quickly enough at high rates, a direct consequence of viscoelasticity. High performance polymer fibres However, there are some important exceptions to relatively poor mechanical properties, especially where they can be made in a form where only the covalent bonds are strained (6, 7). Such are the high performance fibres, such as aramid (Kevlar and Twaron) and UHMPE (Dyneema and Spectra). The structure of aramid is created essentially by binding the short linear chains in nylon into a so-called aromatic or benzene ring. Chain rotation is stopped and the hydrogen bonds between the chains hold the assembly into a very rigid and inflexible network (Fig. 1.10). Their mechanical properties are exceptional, and compare with carbon fibre as well as conventional materials (Table 1.4). They are used both alone in ballistic fabrics and for heavy duty applications such as ships hawsers and tendons, and also in composites with thermosets. They compare very well with high-strength metals such as steel, with high merit indices for stiffness (E/ρ and strength (σ/ρ), where E is the tensile modulus, σ the tensile strength and ρ the density. They are a measure of the stiffness and strength per unit weight, so are widely used for aerospace (body components and rotor blades, etc.) and other advanced transport vehicles, such as Formula I racing cars. Other uses Polymers are also essential constituents of paints and many coatings, adhesives and foams. Paints are applied not just for aesthetic reasons but also to protect the underlying wood or metal from microbial attack and corrosion. They are normally filled with pigment, which lowers their strength, but may also form part of the protection of the underlying surface (especially of metals). But they also fail with time, being subjected to the same failure modes as bulk polymers, both physical and chemical (such as UV attack). Adhesives are a special class of polymer developed for binding two surfaces together, and similar comments apply. Foams are essentially

Polymer

High-density polyethylene (HDPE) Low-density polyethylene (LDPE) Polypropylene (PP)

Unplasticized poly(vinyl chloride) (UPVC) Polystyrene (PS) Acrylonitrile-butadiene styrene (ABS) Poly(methyl methacrylate) (PMMA)

Polycarbonate (PC) Poly(ethylene terephthalatc) (PET)

Nylon (PA6.6)

Natural rubber (NR)

Fabric-filled phenol formaldehyde (PF)

Class

Thermoplastic polyolefins

Thermoplastic vinyl polymers

Thermoplastic polyesters

Thermoplastic polyamide

Elastomer

Rigid thermoset

Table 1.3 Mechanical properties of polymers

1400

930

1140

50

50

∼0.003 8.0

80

60 54

65

50 42

50

33

10

30

σTS/MN m−2

2.0

2.5 3.0

3.2

1180 1210 1390

3.4 2.5

1060 1050

1.5

910 3.0

0.2

920

1450

1.0

E/GN m−2

960

ρ/kg m−3

0.7

∼700

200

125 275

2.0

2.5 80

30

400

800

600

Breaking strain/%

7.5

–

10.0

60.0 3.5

2.0

1.7 15.0

3.2

6

>50

10

Izod impact strength/J m−2

Introduction O C

H N

O C

N

O C

N

O C

H

H

Fibre axis H

C O

N

17

N H

C O

N H

1.10 Molecular structure of aramid fibre.

Table 1.4 Mechanical properties of fibres Fibre

Aramid

Kevlar 29 Kevlar 49 Kevlar 149 (Spectra 900)

Polyethylene Polypropylene E-glass Piano wire (steel) Carbon fibre Ultra high modulus Very high strength

Density ρ kg m−3

Tensile modulus E GN m−2

Tensile strength σTS GN m−2

E/ρ ×10−3

σTS/ρ ×10−3

1400 1440 1440 970 910 2550 7860 1960 1750

60 124 179 117 36 72 210 520 235

2.8 3.1 3.1 2.6 1.0 2.4 3.0 1.9 3.0

42.9 86.1 124.3 121.0 39.6 28.2 26.7 265.3 134.3

2.0 2.15 2.15 2.68 1.10 0.94 0.38 0.97 1.71

composites of polymers with gas-filled voids, and are used as insulators, packaging and fillers for composite structures.

1.2.2 Natural materials Synthetic polymers have many parallels in the natural world, and indeed, first exploitation of long chain materials started with natural substances such as amber (fossil tree resin), shellac (extract of beetle carapice), woods of all kinds, natural fibres such as flax, jute, cotton and ramie, and many other such derivatives. The first elastomers to be used by man comprised the dried latex from certain tree species (especially Hevea Brasiliensis), a material rapidly supplied by a bulk industry and known as natural rubber. It is still widely exploited, but now supplemented by a range of synthetic elastomers.

18

Forensic polymer engineering

Exceptionally, natural rubber has a simple repeat unit (Fig. 1.1), in contrast to most natural materials. The ultimate in structural complexity occurs in proteins, where the sequence of units in the chains is unique and highly variable, so the concept of a repeat unit is redundant. It mirrors the complexity of its progenitors, DNA and RNA, where the sequence of base pairs is unique. There are numerous structural proteins, such as collagen and elastin, rigid and elastic proteins which help support our own bodies. However, they are rarely used as commercial materials nowadays. Silk is different, but an expensive protein for everyday use. A similar comment may be made about gossamer, the protein of spiders’ webs, which may find industrial application in the distant future if biotechnologists succeed in making it on a large scale. It will compete with the high-performance fibres, if it is ever commercialized. Cellulose and derivatives have long been of structural use in many different forms. Wood derives its stiffness from cellulose fibres, and its strength as a composite from a lignin matrix. The fibres occur freely in cotton, flax, ramie and jute, among others. Rayon is a synthetic fibre made from native cellulose (5).

1.2.3 Properties of elastomers The single most important development in rubber exploitation in a range of products came with Goodyear’s invention of vulcanization in 1848, where sulphur bonds are formed between chains to form cross-links (Fig. 1.2). They stabilize the ensemble of chains by preventing flow and creep, as shown by the viscoelastic master curve (Fig. 1.8), so rubber products will not deform permanently when loaded, a severe problem before Goodyear’s invention. Rubber boots could deform so badly in hot weather as to become totally useless, for example. A blizzard of uses followed his intervention, including apparel (such as the waterproof Mackintosh), galoshes, bushes, bearings and anti-vibration mounts of all kinds, and ultimately, the rubber tyre. It came at a crucial time in the industrial revolution, providing protection for the many machines used in transport (especially steam locomotives) and with the internal combustion engine, the automotive. The tyre remains the single most intensive use of the natural material, and a host of synthetic elastomers such as neoprene, butyl and polybutadiene have also been developed, partly as a result of the shortage of natural supplies during the Second World War. Lightly cross-linked elastomers are unique materials in that they experience large strains when loaded, sometimes up to seven or eight times their original length, as is common knowledge with the behaviour of elastic bands (8). The behaviour can be described by the equation:

Introduction

σ = G ( λ − λ −2 ) =

E ( λ − λ −2 ) 3

19

1.6

where σ is the stress, G the shear modulus, E the tensile modulus and λ the elongation ratio (defined as the ratio of the new length to the original length). It is a non-linear stress-strain curve, quite unlike that shown by the initial linear behavior of Hookean solids (such as most metals). In addition, the modulus can be described in terms of the number of network chains, N, Boltzmann’s constant, k and the absolute temperature, T: G = NkT

1.7

The equation predicts that the shear modulus increases with increased temperature, a surprising inference since the opposite is true for other materials. The effect is relatively small, however. But the most important way of increasing the stiffness of rubber is by reinforcement with carbon black, a nano-particle material widely used as both a pigment and filler for many other polymers. It is essential for tyres where carbon black also toughens the elastomers used in their construction and provides a degree of protection against UV rays in sunlight. Natural rubber (NR) is just one of a wide range of elastomers used in products today, and a typical car tyre comprises several different materials blended together. The tread has to grip the road, so energy absorption is important in maintaining a high coefficient of friction. It is usually a blend of NR and SBR (styrene-butadiene rubber), the latter being added to increase the hysteresis of the mixture. The tread is reinforced either by a steel or an aramid breaker in the conventional radial ply tyre. By contrast, the side wall must flex easily without heat build-up, so is a blend of NR and polybutadiene, a low hysteresis elastomer. The reinforcing plies are usually of rayon, a cellulosic fibre. The lining is of butyl rubber, a material resistant to diffusion of air through the wall. Many other rubbers are used in cars, with NBR, or nitrile-butadiene rubber, being used widely. The nitrile units are added in the copolymer to resist swelling when in contact with fuel, so it is used in fuel pipes as well as in uses where contact with fuel is likely (9–11).

1.3

Failure modes

The ways in which polymers fail include a wide variety of behaviour. However, it is convenient to follow what might be termed ‘conventional’ failure modes as observed in metals, since many will have encountered such problems during routine use of metal products. Many car owners, for example, will have been shown metal components by mechanics at their local garage which have fractured by fatigue, perhaps. Crankshafts and

20

Forensic polymer engineering

piston rods come to mind. They will also have seen rusted body parts, although the problem has diminished in recent years with the use of more corrosion-resistant metals and polymer composites.

1.3.1 Mechanical failure Classical failure modes from application of load include the following distinct generic terms, normally easy to recognize if the loads and dimensions of a product are known at specific times in its life. Fracture is one of the most common failure modes, but can occur under different circumstances, such as: • • • • • •

Overload: applied stress is greater than the strength of the product. Creep: distortion of a product under a constant load. Creep rupture: the end-point of creep with separation of the product. Stress relaxation: decay of stress at a constant strain. Fatigue: failure from repeated loading. Wear: removal of material at a surface.

Fracture is common in failed polymer products, and one which is usually obvious to the observer. There are some subtleties, however. For example, hairline cracks are very difficult to spot visually even on external product surfaces, and impossible if they occur on hidden parts of a product. They represent serious flaws from which complete growth to product separation can occur when low loads are applied to the product. Special methods of detecting such cracks have therefore been developed over the years, especially with metal products. An even more insidious problem lies with internal cracks which do not impinge at all on outer surfaces, especially in composite materials. Cracks are usually formed by brittle fracture, but may also be found with traces of ductility. A sure test for remnant ductility is to fit broken parts together, and see if an exact match is possible. If not, then some ductile deformation must be present. Thus broken ceramics are easy to fit together (at least, when the number of separate parts is small), but many broken plastic products are difficult to re-assemble due to remnant distortion. There are many subdivisions of these processes known to metallurgists, which perhaps rings a note of confusion in the layperson. Thus ‘fretting’ is simply a type of wear caused by repeated movement of two surfaces against one another, so there are elements of a fatigue-like process occurring in this failure mode. Such is typical of all real failure modes, when different modes combine with one another to produce a failed product. So although each distinct mode may be easy to recognize when occurring alone, it becomes more difficult when in combination with others, which is really where the fun starts for investigators. Polymers are especially susceptible

Introduction

21

to creep, distortion and stress relaxation with time, a direct consequence of viscoelasticity (Fig. 1.8). However, such effects are inhibited by crosslinking and crystallization. One symptom of product overload is crazing, when many small cracklike features form within a plastic. Such crazes are often visible within transparent plastic products such as drinking beakers (Fig. 1.11), growing slowly with time, and changes in temperature and exposure to aggressive chemicals (such as during washing-up). They form during yielding of the material, and the internal space within a craze is foam-like (Fig. 1.12), with a lower density than the surrounding bulk solid. They are unique to polymers, and usually the precursors to true cracks (12, 13). Loading patterns In addition, there may be several different ways in which the component is loaded, with the generic types including: • • • •

Tension: pulling a product apart. Compression: crushing the product (opposite of tension). Bending: levering a body about a fulcrum. Shear: straining the part sideways.

1.11 Axial crazes in PMMA beaker.

22

Forensic polymer engineering Solid polymer

Open crack

Craze

1.12 Schematic craze profile.

• •

Torsion: twisting a product about an axis. Impact: two bodies colliding with one another.

The concepts are generic, and real loading patterns are usually much more complex, combining one or more of the elements together. Simple loading patterns are, however, easy to reproduce in a testing rig, so that much data is usually (but not always) available somewhere in the literature. And there are many examples of simple loading situations, such as a rope holding a heavy object (tension), a bridge column bearing the weight of a span (compression), a tree branch bearing its leaves (bending), bearing surfaces acting against one another (shear), and a rotating shaft from an engine (torsion). Bending is interesting because it produces compression on one side of the bent object (away from the point of load), and tension on the reverse side. Since most materials are weaker in tension than compression, failure usually occurs on the tension side of a bent object. However, there are always exceptions, such as the failure of a bent live branch, which fails by shear of the composite fibres of the wood, the cracks growing at right angles to the applied load. Dead wood usually fails by simple fracture on the tension side, owing to degradation of the composite structure. Any of the loading elements can occur with most of the failure modes already mentioned, so that the rope might fail by tension overload or shear overload. Fatigue in compression is rare, however. In real cases, the loading situation is often indeterminate, especially for consumer products, which may be (and usually are) subjected to all of the above loading regimes at some time in their lives. If unknown, then that regime must then be inferred from the way the product has failed. Load path It is helpful in analyzing loading patterns to realize that forces must be connected in a chain through a product, so as to form a path. When there

Introduction

Ring crack

23

Radial crack

A

1.13 Hairline cracks formed by point impact in pipe.

are several or many different components in that path, the load takes different forms along the path. Take a very simple example, one component pressing against a flat surface of another. The local load in the flat surface will be compressive, but will fall moving away from the point of contact until it reaches zero. Then there will be a compensating tension force in the surface to balance the compressive force. This is why tension cracks occur some distance away from the point of contact in say, a glass sheet impacted by a projectile. They form concentric rings of cracks. Damage to the material at the tip will also create radial cracks, so the final result of impact will be the characteristic cobweb pattern of brittle cracks found when a PVC pipe is broken by point loading (Fig. 1.13). The simple beam in bending is another simple example of how loading creates a combination of tension and compression across the thickness of the sample, together with a shear component for thick samples. The idea of load path through more complex configurations of many different components is helpful in elucidating how specific parts came to break rather than others, because it is the weakest component which governs the strength of the whole assembly. Stress concentrations One way in which the applied stress in a body is much greater than expected is the presence of stress concentrations. They are local variations in shape where the stress lines through the product are forced together (Fig. 1.14), and so magnified. Simple examples include: • •

cracks in or at the edges of bodies holes in flat sheets

24

Forensic polymer engineering

a

b

1.14 Stress concentration in flat bar.

1.15 Internal corners in accelerator pedal.

• voids within products • corners and fillets • changes in profile of shafts • screw threads. Such design features are often inevitable in a product shape, but the magnitude of the stress concentration (Kt) can be minimized by forethought. Compilations of stress concentration factors allow designers to do just this (14, 15), but, life being what it is, they are often left unchanged in a new product, until a sudden application of load in service initiates a crack at the stress raiser. Corners are a common flaw in polymer products produced by the tool maker in the steel mould. They are reproduced exactly in the polymer shape, and remain to wreak havoc when the time is ripe (Fig. 1.15). A simple formula for the stress at a notch tip is shown below. It relates the stress concentration, Kt to the length of the notch, D and the radius of curvature at the tip, r (Fig. 1.16): Kt = σ/σ0 = 1 + 2(D/r)½

1.8

So when r = D, Kt = 3, and Kt = 1 when the feature disappears. The formula shows that a round circular hole, and a semi-circular notch in the edge of a sheet triples the applied load. Other simple examples include a

Introduction

r 2D

r D

25

r D

1.16 Stress concentrations in flat bars.

spherical void, which doubles the stress. Sharp cracks are much more serious, with the stress rising many times at the tip, and sharp inner corners have a similar deleterious effect on the strength of the product. Fatigue always starts at a design stress raiser, so their presence and position is a crucial factor in examining failed products. They usually determine the weakest part of a product from which cracks grow. A surprising way in which stress concentrations can be exploited is socalled rubber toughened polymers. By introducing minute rubber particles into a thermoplastic, the strength can be raised greatly because each tiny particle acts as a spherical stress raiser. A crack is started at the edge of the rubber sphere, and since there are so many, the energy needed to break the material is very much greater than if they were absent. Typical examples include ABS, rubber toughened PVC and nylon. It is a fallacy to say that stress raisers are always bad. For example, packaging should be strong enough to protect the contents, but weak enough to be broken when the product is needed. A small cut or notch is often provided for the user to initiate a crack or tear, suitably large enough to be visible, yet not obtrusive. Polypropylene is widely used for packaging as thermally sealed film, as bubble packs or large containers for meat and fish in supermarkets (gas filled to prolong product life). But designers frequently fail to provide a simple stress raiser to allow easy access to the contents, allowing the consumer to injure him or herself when opening it with a sharp knife (readily accessible in a kitchen where the product is to be cooked). On the other hand, in products where non-metals are essential to protect the user from scalds or cuts, such as pan or knife handles, it is vital to eliminate the deleterious effects of stress raisers. A large carving knife for cutting a wedding cake suddenly failed, injuring the bride and ruining the special event (Fig. 1.17). The ceramic handle was poorly designed, with the tang of the blade exerting excessive leverage on the handle, the stress concentrated at the tang end. It is a classic example of an accident waiting to

26

Forensic polymer engineering

End of tang (just visible)

Glue line Sliver

1.17 Fractured wedding knife.

1.18 Failed hobby knife.

happen. Plastic-bodied knives should also be capable of brusque use without collapsing when first used (Fig. 1.18).

1.3.2 Chemical attack If simple mechanical failure can become complex rather quickly, chemical attack of a product starts off being complex (16–19). That there are many ways in which materials are attacked is a reflection of the complex mixtures of chemicals to which they can be exposed in service, starting with the atmosphere around us. The problem becomes even more serious when the product is loaded, or possesses in-built forces which can be relieved by crack formation. Frozen-in strain is unique to long chain molecules, and is often produced during manufacture, when the chains are extended by flow of the melt in the steel tools. On cooling, the chains remain in an extended, but unstable state, ready to resume a coiled up conformation when triggered by external influences such as heat, load or certain chemicals. Perhaps the most common form of attack occurs through oxidation in its many different manifestations, although hydrolysis is also a major degradation mechanism for one class of polymers.

Introduction

27

Oxidation Oxygen (O2) in the air (ca 21% by volume) is the most active ingredient, but there are several other compounds which also degrade polymers (and metals as well). The composition of trace gases is variable, such as water vapour (measured by humidity), which changes depending on the prevailing weather and air temperature. Sulphur dioxide (SO2) is an aggressive pollutant (volcanoes, fuel burning) which oxidizes and combines with water to form sulphuric acid. Ozone (O3) is a more powerful oxidant than its parent, oxygen, and another product of pollution. It is produced by the action of sunlight on air contaminated with hydrocarbons (such as unburnt petrol), and photochemical smog is a serious problem in many cities in the world. The gas may only be present in parts per billion (ppb) but degrades many rubbers very quickly, and is extremely toxic to life. Chlorine (Cl2) is widely used as a cleaning agent or disinfectant for the same reason, and is also very aggressive to polymers. Oxidation in general must always be an expected agent of attack, simply because oxidizing agents are universally present around us. Another common household cleaning agent is bleach, which releases chlorine from a dilute solution of sodium hypochlorite. While useful in attacking and destroying germs, it will also degrade many thermoplastics.

Hydrolysis Acid and alkali work against step-growth polymers by hydrolysis, that is, the chain is broken down by cleavage at the functional group linking the chains together. The functional group in nylon is the amide or peptide group (—CO—NH—), so the reaction in hydrolysis would be . . . MMM—CO—NH—MMM . . . → . . . MMM—CO2H + NH2—MMM . . . Because the chains are halved in length at each step, the molecular weight drops rapidly, and once the entanglement threshold is reached, the material falls to pieces. As with ozone attack, the extent of reaction depends on the strength and concentration of acid or alkali, strong acids such as nitric, sulphuric and hydrochloric being more effective than weak acids. But there are some anomalies in comparing acid and alkali hydrolysis. Thus polycarbonate is unaffected by strong acids, but severely attacked by alkali. This is why it can be used for acid containment quite safely. On the other hand PET is not hydrolyzed by alkali, but is attacked by acids. Acids and alkalis are present in many common liquids, such as carbonated soft drinks (dilute phosphoric acid), cooking ingredients like vinegar (dilute acetic acid), baking soda and cleaners (sodium bicarbonate or carbonate).

28

Forensic polymer engineering

Owing to its ubiquity, water can affect most polymers deleteriously. It can either act directly by hydrolyzing step polymers or in other ways. The problem shows up during processing or shaping, when high temperatures increase the chances of hydrolysis. Since most processing temperatures are greater than 100°C, any traces of liquid water in the feedstock will vaporise to form unwanted bubbles in the product. Most polymer feeds must therefore be dried thoroughly before moulding. Ultraviolet radiation A common cause of failure in many polymers occurs by exposure to UV radiation, commonly encountered in sunlight. The radiation occurs at wavelengths shorter than the blue end of the visible spectrum (hence the term, ultraviolet) and are of course invisible to the naked eye. The interactions with long chain molecules are complex, and beyond the scope of this book, but some general comments are possible. In the first case, UV encompasses a wide range of wavelengths, the shorter being most damaging (essentially because the energy of the radiation increases with shorter wavelength). Much of the most damaging radiation is, however, absorbed by the ozone layer in Earth’s upper atmosphere. But that still leaves sunlight with a substantial UV content. And it varies with climate, altitude and weather, so levels are rather unpredictable. Theoretically, those polymers with double bonds or other absorbing functional groups are most at risk of UV degradation, but theory falls well behind practice because many commercial polymers, which have no absorbing groups, contain defects which do absorb. They may also have small amounts of co-monomers which are absorbing. Thus PE apparently has just simple CH2 repeat unit, but in practice, contains a small number of C=O or carbonyl groups produced by oxidation during high-temperature processing, so will eventually be attacked by UV. The effects of UV attack include cracking and the formation of a degraded layer on the surface, and pigments can also be bleached, in an effect known as ‘whitening’. On the other hand, some polymers may contain groups which are nonabsorbing but which are susceptible to UV attack. The best common example is polypropylene (PP), which has a tertiary hydrogen atom (H) present in every repeat unit: —[CH2—CH(CH3)]— It is less stable than the surrounding hydrogen atoms, and can be stripped off with less energy, and so represents a weak point in every unit. Because it is universally present, PP is very susceptible to UV attack. Figure 1.19, for example, shows the tops of two traction batteries degraded by UV, the white areas in the battery at left showing a particular problem of thermally

Introduction

29

1.19 Fractured and whitened battery top.

welded polymer susceptible to radiative attack. Products which are exposed to sunlight should be protected from UV, and several fillers or additives are available for this purpose. They are absorbing compounds not dissimilar in function to sunscreens for skin, which is equally susceptible to UV damage.

1.3.3 Stress corrosion cracking An important failure mode produced by chemical attack of polymers is stress corrosion cracking (SCC) by analogy with a similar problem encountered with metals (16–19). Trace amounts of powerful reagents can induce microcracks, which then grow slowly under applied loads or through another problem known as frozen-in strain. The classic example comes from the 1920s in India, when cartridges exploded in the rifles rather than firing a bullet. The cause was traced to hairline cracks in the brass cases, which in turn were caused by small amounts of ammonia gas emitted by dung heaps. It attacked the copper, forming a complex, so cracks were initiated where internal stresses (or residual stresses) were greatest. The solution was to anneal the brass after manufacture. Similar problems occur in polymers, but the nature of chemical attack is different. As will be described in a case study later, a vehicle sprang a slow leak of diesel fuel, which went critical while on the road, causing multiple accidents to following cars. The leak had been caused by sulphuric acid attacking a nylon connector in the fuel pipe. Analysis of the remains showed that a small drop of acid had leaked from the battery above, initiated an SCC crack which grew slowly until sudden and total failure. The damage

30

Forensic polymer engineering

done to nylon fabrics such as tights and stockings by traces of acid is well known to users, and acid or alkali spills on most clothing of any composition will quickly cause irreparable damage. Many cleaning agents contain quite concentrated acid or alkali, and wise users will wear protective garments, such as polyethylene gloves or aprons. This polymer is immune to such attack since it is a chain-growth polymer. And it is not only polymers which are attacked. Attack of many polymers by oxygen, ozone and chlorine are other forms of SCC in polymers, requiring a low stress or strain threshold for crack growth. Figure 1.20 shows the acetal junction on a water supply system under a laboratory sink, which failed suddenly and caused much damage to the computers in the department below. It had cracked at an early stage in its life, and grew slowly with time until it could no longer withstand the water pressure, and failed. The cause was traced to the low level of chlorine in the potable water supply, an unlikely possibility (it was thought at the time). However, other evidence was discovered from a trial in the USA which confirmed the problem.

1.3.4 Environmental stress cracking (ESC) A further problem (not found in metals and alloys) encountered with polymers involves attack by organic fluids (16–19). There is no permanent

1.20 Stained fracture in acetal fitting.

Introduction

31

chemical change, but the effects are the same as with SCC. Cracks are initiated and grow when product surfaces are exposed, and are only, and unfortunately, detected when the product falls in half, or leaks its contents. Just this happened when a pub landlord experienced a series of fractures of blow-moulded beer containers. As is usual with these examples, there is no obvious cause, and the problem remained a mystery until careful investigation exposed the truth. He had been cleaning the containers with a powerful detergent, and the detergent initiated microcracks in the walls, which then grew uncontrollable when loaded with beer. The manufacturer increased the molecular weight of the PE, and users were advised to change detergents, and the problem ceased. Detergents such as Igepal are in fact used in standard tests to check for ESC (environmental stress cracking) in products likely to be so exposed in service. Step-growth polymers are also susceptible to ESC, especially noncrystalline or amorphous materials like polycarbonate. It is cracked by relatively low levels of active chemicals like methylene and ethylene chloride (which are also solvents for the polymer) and alcohols like methanol (CH3OH). Figure 1.21 shows a battery case used in a miner’s lamp which cracked and leaked when in use in the colliery. The cracks were caused by ESC induced by the solvent welding process used to assemble the viewing

Leak

1.21 Leaking polycarbonate battery case.

32

Forensic polymer engineering

windows and lid. The solvent released chain orientation produced by injection moulding the parts. Transparent chain-growth polymers such as PMMA, polystyrene and SAN are also very sensitive to a wide range of organic fluids, threatening the integrity of structural uses, and visually unacceptable for simple products such as drinking beakers and decorative articles. So what causes ESC under such apparently unusual circumstances? Organic polymers and organic fluids share common structures, carbon backbone chains and small carbon molecules respectively. They are frequently compatible, and the small molecules can usually diffuse into the long chain assemblies of polymeric materials. If the reagent is powerful enough, dissolution will occur, but if not, a thin layer of liquid will be present in exposed surfaces. The fluid swells the polymer and the surface layer expands. It is also weaker mechanically, so any applied stresses may initiate microcracks at vulnerable zones (typically at stress raisers such as corners, thread roots, and holes). Frozen-in strain (chain orientation) provides a driving force for crack growth, and must exceed a threshold value for growth to proceed (just like SCC). The growth rate will increase with stress and orientation levels, and may be intermittent for products used only occasionally. Thus fracture surfaces examined after product failure will frequently show lines where the crack has stopped, and then been reawakened by reapplication of stress.

1.4

Methods of investigating product failure

The way in which many different factors can affect the strength of a product clearly makes investigation of a specific failure difficult, unless the list of variables can be reduced or eliminated. That is just what is involved in systematic examination of failure: it is a process of elimination by careful collection of all the facts surrounding a particular incident.

1.4.1 Sifting the evidence But what counts as evidence? The answer to that question depends on several, if not many, different kinds of facts associated with accidents, whether of metal or non-metal products (or composite products constructed of different materials). The following can be regarded as a minimum list. First is the testimony of any individuals who saw what happened before, during and after the accident. Then there are the circumstances surrounding the incident, such as the time and date, the environment, weather and so on. The technical records are an invaluable source of information and often available routinely for equipment failure (but not always). But the material evidence itself is probably the most important focus of enquiry, followed by the details of its history and provenance.

Introduction

33

Witness evidence If product failure has resulted in death, injury, or damage to property, statements from those in the vicinity will often be available. The earlier that statements have been gathered, the better. Memory fades, and the later a witness is asked to recall events, the greater the chances of error, especially if litigation has started. Bias creeps into statements and there is usually a lack of technical detail, because the interrogator is normally a lawyer with no technical expertise. If there are one or more victims, their memory may be affected by the accident. For example, falls from ladders are among the most common to occur to consumers, but falls from a height often cause mental shock, and amnesia about the events just prior to the fall. Often accidents occur so quickly that the witnesses or victims have great difficulty recalling the sequence of events, making the material remains the only mute evidence to the incident. Circumstantial evidence is frequently the only reliable evidence available, and the material evidence must be checked against any witness statements available.

Records Documentary evidence is a vital source of the facts of a case, especially if equipment is monitored regularly and automatically. In industrial cases, documentation is often copious as a result of health and safety legislation, and includes: • • • • •

maintenance sheets design data manufacturing records quality control information applicable standards.

With such a wealth of information available, the problem is one of sifting the records for the gems that will reveal how and why the incident occurred. For example, industrial processes are now usually automatically monitored by various sensors in the equipment. Variables, such as time, temperature, pressure and volume of contents, are measured and recorded remotely in computer databases. However, the data is usually indigestible until analyzed, and visualized in the form of graphs or diagrams. Trends can then be seen to help interpretation of the facts of the matter in hand. A great advantage of such data compilations is their objectivity, and bias is easily detected. Thus a faulty detector, such as a thermocouple, will show up when compared with other thermocouples in the system. A cross-check is available from calibration records. Systematic analysis will also expose whether or not the records are a fair representation of events. Sensors may be missing

34

Forensic polymer engineering

from critical parts of the system, although information can be inferred from the data supplied. Computer records are not infallible: computers crash, data can be lost or mangled, as any PC user knows to their cost, but they are generally an invaluable source of unbiased measurements. Quality standards such as ISO 9000 require systematic record keeping of processes, materials and designs, and can give an insight into the past history of a particular product. Design-specific standards are a way of assessing compliance, but since they are drafted by committees composed of the manufacturers, they must be regarded cautiously. And in most cases, they are historic documents, and may not have been modified for the latest developments. Most contracts will specify compliance with one or more standards, so standards have an important status in the eyes of the courts. Surviving remains The material evidence which survives from a failure provides mute but revealing evidence of product history, often the key to unlocking the way it failed (16–19). It could be: • • • •

the broken battery which exploded in the face of a mechanic the damaged bearing from a swing bridge the remains of a ruptured storage tank a fractured crutch, and so on . . .

Such remains are normally preserved as the material evidence for further investigation, the proof positive of the cause of an accident and the justification of a case for compensation. But there are occasions when the broken product was formed as a result of the incident, rather than causing it. So the material evidence has to be seen in its context, and not in isolation. For example, a broken plastic ladder tip started an investigation to determine the cause of a ladder slip, when the user was severely injured by falling to the ground. But inspection of the accident scene showed that the tip broke after the ladder had started slipping down the wall, and could not therefore be taken as the cause of the accident. The trace evidence of its journey down the wall, scrape marks on the wall, was visible evidence of how the tip was broken. Single items always prompt the question: was this broken part the only one to have suffered fracture? Sometimes, there are many similar broken products, suggesting faulty design, a rather more serious position for the product manufacturer, because rapid action must be taken to withdraw existing products in service and so prevent further incidents. The product must be redesigned to withstand service conditions, or alternatives provided which are capable of resisting the working environment. Thus many fractured or leaking miners lamps indicated one or more serious design

Introduction

35

flaws, and immediate action to provide alternative light sources to enable the colliery to keep working. Each broken product needs examination to provide a picture of the pattern of failure, and if a common failure mode is found, details of each individual failure are unnecessary. Statistical analysis of many failures can provide further clues about the design flaws causing those failures, helping the designer to improve the product. Design defects represent a serious challenge to the credibility of a manufacturer, and remedial costs can escalate rapidly. This is why it is so important to investigate early and ameliorate, or better, eliminate the escalation. Every product failure demands individual treatment, and usually starts with simple visual examination, careful measurement of its dimensions and determination of its condition compared with an equivalent intact component. Comparison is a simple way of checking if the parts really are identical, and if not, the reason for divergence. Many products are now identifiable from logos, date stamps and manufacturing codes either printed or embossed on the product in a concealed position of the product outer surface. If the material is unknown, or degradation is suspected, it must be analyzed for the constituent parts: the matrix polymer, filler and any minor additives (such as UV absorbent). The analysis should aim to be non-destructive, but if necessary, sampling needs to be away from critical features such as fracture surfaces. Although direct comparison with unaffected product or component is ideal, it is not always possible. Visual inspection aims to identify the following product features as a minimum: • • • • • •

overall dimensions distortion in dimensions fit with matching parts surface quality traces of wear identity marks.

The search for key details does not stop at the fracture surface, however. Cracks which have not grown to completion are one objective of the search. Such sub-critical cracks provide evidence of the way the component has been loaded in service, and might show why failure has occurred in the first place. Thus discovery of sub-critical brittle cracks on a PP storage tank showed the tank to be under-designed for its function, and sub-critical cracks on the acetal plumbing fitting indicated that SCC was a failure mode to be brought into the picture (Fig. 1.20).

1.5

Public information sources

One way in which product failure can be studied is through the literature, and standard texts that are available, albeit of limited extent. There is a

36

Forensic polymer engineering

long tradition of publicizing the causes of failure, but usually only when that failure has been so severe in loss of life or property as to be classed as a disaster or catastrophe. Lesser failures have either become so common as to attract little attention (car accidents), or receive no publicity at all by being deliberately suppressed. That raises interesting questions about freedom of information, and prevention of further accidents of the same kind. The amount of information available on the world wide web is prodigious, but indigestible unless the searcher has a clear idea of the information needed.

1.5.1 Textbooks There are several standard texts which are useful background for analyzing product failure, as discussed already. There is a shortage of case study compilations on polymers but some substantial works have been published in the last decade or so. Among them are the books by Ezrin (16), Wright (17) and Scheirs (18). A recent work discusses numerous case studies of both metal and polymer failures from a forensic viewpoint, including intellectual property (19). And several reviews also exist in the literature concerning design in polymers (20) and product failure (21) published in the series about polymer technology from RAPRA, an excellent and rewarding source of often obscure but vital source of further and detailed information.

1.5.2 Event reporting It is likely that the vast majority of equipment failures not causing death, injury or great loss of property are not reported publicly. The company suffering such minor incidents will probably circulate employees details of the problems, and remedial measures taken, but the matter will end there. On the other hand, in some activities, both accidents and near-misses must be reported and publicized. The outstanding example is the aircraft industry, where legislation forces all incidents to be reported to inspection authorities, and remedial measures taken, all under public scrutiny. Nearmisses of flying aircraft, for example, are widely publicized in the press. The railways, too, are obliged to report SPADs (signal passed at red), incidents which do not result in any accident but are an indicator of a potential problem. The problem became reality in the Ladbrook Grove crash of October 1999, when a local train passed a red signal and collided with a fast express, with great loss of life. However, there is a growing body of literature publishing investigations into a wide range of product failures. The academic journal Engineering Failure Analysis (22) is devoted to publishing case studies of failed products, but it remains relatively isolated compared with strictly academic

Introduction

37

journals. Several volumes of papers taken from the journal are available (23). Although non-metallic failures are reported there, the majority involve metal products. Loss Prevention Bulletin (24) specializes in failures occurring in chemical plant, focusing on a wide variety of failure modes and their effects on the companies concerned. There is a long line of major disasters within living memory, which have created enormous damage to workers at the affected sites and further afield. Flixborough (1974), Bhopal (1984) and Buncefield (2005) are just a few of the disasters which will be remembered by the wider public. This and other accidents of chemical plant are discussed by Kletz (25). Civil engineers have a long and distinguished record of publicizing failures, reflecting the safety-critical nature of large structures such as bridges, dams, buildings and tunnels, for example. When failure occurs, it is likely to be dramatic and life-threatening.

1.5.3 Public domain Among the foremost worldwide databases are those from Espace (26) and the US Patent Office (27). They itemize patents from the principle patenting countries, primarily the USA, Europe and Japan, and complete patents can be downloaded free. But why should patents be a source of information on failures? Inventors claim new products, which solve particular problems, and product failure represents an important part of those problems. So because glass is brittle and fails at rather low loads, laminated and annealed glass addresses the problem by toughening the material. Similarly, toughened plastics like ABS were developed to address the problem of brittleness in polystyrene. In addition, failure of new products can be studied simply by turning to the patent which establishes that product. Registered design databases such as that run by the UK Patent Office can also be a valuable source of pictures of a design at the date of registration. The UK trademark database is useful for determining ownership and identity of many commercial products. All such databases are readily accessible from the world wide web. There are specialized databases for specific areas of failure, which are of great use in monitoring specific designs. Thus the FDA has a very large database of failures of medical devices at MEDWATCH, allowing an investigator to follow the failure history of specific hip joints, heart valves, stents and similar implants (28). Since there is usually a plethora of different designs, identification by tradename or trademark on the compilation gives the required information.

1.5.4 Materials and product standards Unless products comply with specific standards, the ability to sell can be compromised in many markets. Compliance with product standards and

38

Forensic polymer engineering

regulations is now a major issue for many manufacturers. British Standards Institution (BSI) and the American Society for Testing and Materials (ASTM) have produced standards covering safety, performance and reliability of most products, including the influence of mechanical and environmental factors. In addition, each standard is reviewed and updated periodically, thus ensuring continued relevance. Equivalent standards exist in Europe (ISO), Germany (DIN), Japan (JIS), etc., and all are good reference sources (2, 3). Within any standard, much of the required ‘background work’ is often included and could be of particular value in product liability disputes. A comparative analysis between a relevant standard, and the product or component in dispute may be needed. It should quickly become apparent whether the subject component did not conform to a standard. However, standards may be of limited use. In many new products there may be no standard at all, or an old standard which has limited applicability. Standards are produced by a committee of testing experts and industry representatives, and the final draft is a compromise between conflicting interests. Thus there is no UK standard covering thermoplastic tanks, and the German standard DVS 2205 must be used as the only alternative. Many medical devices lack coherent standards because their development is growing fast, and the committees have not caught up with the latest products. So caution is required when looking for an appropriate standard. And who standardizes the standards? There is a range of styles even within an organization, some providing only minimal information, others more lengthy and even obscure. There are ranges of standards covering test methods for polymers, and they are much quoted in data for specific polymers supplied by manufacturers. Unfortunately, the values quoted are usually ideal and the reality seldom meets that ideal, even for basic data like moduli. Part of the problem lies in the test samples chosen because they are pristine and clean, and never exhibit the problems of contamination found in real components. With tensile tests, moulded dumbbells are even more unrealistic. All technical data sheets should thus be regarded sceptically. The closer tests are to real conditions of exposure, the greater their credibility.

1.5.5 Disasters Certain disasters from the recent past achieved a certain notoriety at the time, and such was their scale that public inquiries were established to investigate their causes. The numerous disasters on the railways come to mind, such as the Tay Bridge disaster of 1879 (29) and the lesser known but infamous fall of the Dee bridge at Chester in 1847 (30). Railway accidents have also recently been examined with specific reference to metal fatigue (31). Marine disasters were so common in the Victorian era as to

Introduction

39

be largely forgotten now, unless through an imaginative journalist’s pen, such as the mysterious abandonment of the Mary Celeste. But the tragedy of the sinking of the Titanic in 1912 was so enormous that it is remembered by every generation through a new film or book. Similarly, the destruction of the Hindenburg airship in 1937 seemed to presage or echo the widespread human misery suffered at the hands of the Nazis. Such disasters continue to fascinate the public, not least because of a degree of uncertainty about the precise causes. While the Dee and Tay bridge disasters involved only metals (the cast iron girders and columns respectively), the Hindenburg (and the R101 airship of 1930) failed from the low strength and flammability of their gas containers and outer envelopes. They were composed of cotton fabrics reinforced with cellulose nitrate or acetate, and doped with rubber latex and other highly inflammable substances. Lightweight organic composites have been widely used in air and spacecraft for a number of years, and have been involved in a number of disasters, not least the Challenger and Columbia accidents. In the first disaster, in January 1986, the O-rings on one of the booster rockets failed, and the propellant exploded though the gap, engulfing the entire structure shortly after take off. The Viton rubber (a fluorinated copolymer) was very inflexible at the near freezing temperatures at lift-off, and could not seal the booster sections correctly. The problem was well known, but managers ignored the warnings. The second disaster, in October 2003, also involved damage during take off, and once again was recorded on cameras, just like the first disaster. The critical damage was caused by a large piece of foam insulation falling away as the rocket rose, and striking carbon-fibre composite on the wing. The damage fatally weakened the shuttle, but was missed at the time. When the craft attempted re-entry, the entire structure disintegrated. The accident exposed the testing regime at NASA to be flawed, because the problem was appreciated, but tests only conducted with very small pieces of foam, much smaller than that which actually flew off at launch. The final demise of Concorde, the supersonic airliner, was signalled by an horrific accident at Paris in July, 2000. It occurred when a tyre blew out during take-off, and a large fragment penetrated the fuel tank above. The jet fuel was ignited by the engine nearby and the Air France plane eventually crashed, killing all on board. The small fleet was grounded, and after a short period when flights were resumed on the British versions, was scrapped entirely in 2003. Tyre blow-outs were not uncommon on other Concordes well before the Paris accident, although after the fatal crash, the cross ply tyres were replaced by a radial ply design, and the fuel tanks were reinforced by a rubber/aramid lining. Non-metallic failure as a cause of major accidents and disasters is an unfortunate reflection of the lack of awareness of their limitations, and it

40

Forensic polymer engineering

is also unfortunate that the benefits of new materials are often overplayed and exaggerated when first introduced into the market. Engineers are then forced into the embarrassing position of explaining minor unanticipated failures, and being brought to account when lives are lost. The case studies discussed in subsequent chapters have been assembled largely from our own notebooks, supplemented by examples in the public domain. If failures of a similar type are to be prevented in the future, then publishing cases is a powerful way of educating designers and engineers of the shortfalls in the design and behaviour of polymers products.

1.6

References

(1) Walker, PW (Ed), Lewis, PR, Braithwaite, N, Reynolds, K and Weidmann, G, Chambers Materials Science and Technology Dictionary, Chambers (1993). (2) Walker, PW (Ed), Chambers Dictionary of Science and Technology, Chambers (2000). (3) Billmeyer, FW, Textbook of Polymer Science, 3rd edn, Wiley (1984). (4) Mills, N, Plastics: Microstructure and Engineering Applications, 2nd edn Butterworths (2005). (5) Brydson, J, Plastics Materials, 7th edn, Butterworth (1999). (6) Lewis, PR, High Performance Polymer Fibres, RAPRA Reviews, 9(11), (1999). (7) Lewis, PR, Highly oriented polymers in de Wit, Demaid and Onillon (Eds), Case studies in manufacturing with advanced materials, North Holland (1992), 97–122. (8) Treloar, LRG, Physics of Rubber Elasticity, 3rd edn, Oxford reprint (2005). (9) Naunton, WJS, The Applied Science of Rubber, Edward Arnold (1961). (10) Blow, CM (Ed), Rubber Technology and Manufacture, Newnes-Butterworths (1971). (11) Morton, M (Ed), Rubber Technology, Van Nostrand-Rheinhold (1973). (12) Andrews, EH, Fracture in Polymers, Oliver & Boyd (1968). (13) Hull, D, Fractography, Cambridge (1999). (14) Pilkey, WD, Peterson’s Stress Concentration Factors, 2nd edn, Wiley, New York (1997). (15) Young, WC, Roark’s Formulas for Stress and Strain, 6th edn, McGraw-Hill (1989). (16) Ezrin, M, Plastics Failure Guide: Cause and Prevention, Hanser (1996). (17) Wright, D, Failure of Plastics and Rubber products: Causes Effects and Case Studies involving Degradation, RAPRA (2001). (18) Scheirs, J, Compositional and Failure Analysis of Polymers: A Practical Approach, Wiley (2000). (19) Lewis, Peter Rhys, Reynolds, Ken and Gagg, Colin, Forensic Materials Engineering: Case Studies, CRC Press (2003). (20) Lewis, PR, Designing with Plastics, RAPRA Reviews, 6(4), (1993). (21) Lewis, PR, Polymer Product Failure, RAPRA Reviews, 10 (3), (2000). (22) Jones, DRH (Ed), Engineering Failure Analysis, Elsevier (1995). (23) Jones, DRH (Ed), Failure Case Studies, Volumes 1, 2 and 3, Elsevier (2000). (24) Donaldson, T, Loss Prevention Bulletin, IChemE, Rugby, England.

Introduction

41

(25) Kletz, T, Learning from Accidents, 3rd edn, Gulf Professional Publishing (2001). (26) http://www.patent.gov.uk (27) http://www.uspto.gov (28) http://www.fda.gov/medwatch (29) Lewis, PR and Reynolds, K, Forensic Engineering: a reappraisal of the Tay bridge disaster, Interdisciplinary Science Reviews, 27(4), (2002), 287–298. Peter R. Lewis, Beautiful Railway Bridge of the Silvery Tay: Reinvestigating the Tay Bridge Disaster of 1879, Tempus, 2004. (30) Lewis, PR and Gagg, C, Aesthetics – v – Function: the fall of the Dee bridge 1847, Interdisciplinary Science Reviews, 29(2), (2004), 171–191. PR Lewis, Disaster on the Dee: Robert Stephenson’s Nemesis of 1847, Tempus Publishing (2007). (31) Peter R Lewis and Alistair Nisbet, Wheels to Disaster!: The Oxford train wreck of Christmas Eve, 1874, Tempus (2008).

2 Examination and analysis of failed components

2.1

Introduction

Various experimental methods are needed to examine the physical remains when polymer products fail. It may include many samples, or just a unique example, so care is needed in the choice of methods. It is also valuable to examine intact samples of a failed product to check their integrity using similar methods. A central aim of this chapter is to review the range of methods for analyzing polymers and it includes the: • • •

methods for characterization of polymers theoretical background to particular methods most appropriate methods for a given failure.

However, it is worth reiterating that the cause of many failures can be detected by careful examination using low-power magnification coupled with some basic chemical or physical testing and analysis. Inspection of the failure will show the forces involved, whether the load was applied cyclically or was a single overload, the direction of the critical load, and the influence of outside forces such as residual stresses or strains. Knowing the roots of the failure, the investigator can pursue both the causes of failure and possible human errors. The way products are made is important for the features that can arise from the particular way a polymer is shaped, and when those features turn into defects.

2.2

Processing methods and defects

Shaping of polymers occurs via several routes, especially: • • • •

injection moulding extrusion rotational moulding compression moulding.

Each produces characteristic faults, most of which are detected by the machine operator. Many, however, are difficult to spot without access to microscopes or other methods, so defective products can enter the chain. 42

Examination and analysis of failed components

43

Injection moulding machine Plastic granules

Hopper

Reciprocating screw

Heater

Barrel

Injection

Mould cavity

Nozzle

Mould

Moveable platen

Clamping

2.1 Injection moulding (Wikipedia Commons).

2.2.1 Injection moulding The method involves injection of molten polymer into a shaped tool which can be separated at the end of the moulding cycle (Fig. 2.1). The tool has at least one gate where the polymer enters, and in some cases where the shape to be created is complex, several gates. Since the metal parts of the tool must be able to separate, there are several important design rules. Polymer products frequently need supporting ribs, so they must be aligned with the direction of withdrawal of the tool, for example. The cycle time is dominated by the cooling period (Fig. 2.2) caused by their low thermal conductivity, and many polymers must be cooled slowly so as to relieve internal strains and stresses which can result from quenching. Otherwise, a seriously defective product can be created. Where holes are needed in a product, the flow of the melt has to part, creating the problem of weld lines where they rejoin. Voids and sinks can act as stress concentrators if in the load path when the product is in service. Moulding features can include: • • • •

frozen-in stress and strain poor fusion at weld lines voids in the centre of thick sections sink marks at the surface.

Other features may be created where the polymer granules are not dried correctly. Since thermoplastic polymers have to be processed well above the Tg or melting point, Tm, it is usually well above the boiling point of water, so any traces of moisture will create voids and surface splay marks, for example. It may also degrade the molecular weight by hydrolysis.

44

Forensic polymer engineering Start

Close mould Inject Remove moulding

Open mould Cool

tio ara p pre Melt

n

2.2 The moulding cycle.

Whether a moulding feature becomes a defect depends on its location on the product, its further treatment and its final environment. Thus a weld line in an unstressed part of the product may never cause failure. However, if it falls along a load path, it can act as a nucleus for a brittle crack. The polymer melt viscosity is an important variable in the process because it effectively controls not just the way the process works, but the strength of the final product. The root variable is molecular weight, which determines both properties: the greater molecular weight of the polymer, the greater both the melt viscosity and the product mechanical strength. The shear stress, τ of a Newtonian fluid like water is related to the shear rate, γ and the viscosity η by the simple equation τ = ηγ

2.1

However, polymer melts are governed by a so-called rate law where the shear stress is more sensitive to shear rate, or the rate at which the melt moves when sheared: τ = η(γ)n or log τ = log η + n log γ

2.2

Examination and analysis of failed components 104

PV C( rig id)

Acrylic at 240 °C

happ/N s m–2

103

at 19 0° C

45

Polypropylene copolymer (MFI 4) at 230 °C

LD polyet hylene (MFI 20) at 170 °C PBT at 240 °C

Po ly (sta ether nda sulp rd g hon e rad e) a t 35 0 °C

Nylon 66 at 285 °C 2

10

PV C

(fl ex ibl e) at 17 0° C

101

Injection moulding range

100

101

102

103 · Shear rate (g )/s–1

104

105

2.3 Variation of apparent melt viscosity with shear rate.

and log η = log τ − n log γ with the exponent n of negative value. In other words, the melt viscosity decreases with increasing shear rate, and such fluids are generally known as ‘pseudoplastic’ in nature, a specific example of a non-Newtonian liquid (Fig. 2.3). So as the shear rate increases, the shear stress falls. The diagram shows how several different polymers react as shear rate rises, and there are considerable variations between them. Thus acrylics such as PMMA and PVC fall fastest compared with more rigid chains like those of PBT (polybutylene terephthalate) and polyethersulphone. The MFI is the melt flow index, an empirical measure of melt viscosity used by moulders, and inversely related to molecular weight. The shear rates encountered in the pipes of moulding machines are typically above 103 sec−1, so the melt viscosities used in moulding are those to the right of the diagram. There is a subtle implication that some polymers are more difficult to mould than others, those polymers with inflexible chains generally being more difficult than simple chains like LDPE. Greater care is needed for such polymers, which includes polysuphones, polycarbonates as well as PBT. Similar care is needed with composites such as short glass reinforced materials such as GF nylon owing to the thickening effect of the short fibres present in the melt.

46

Forensic polymer engineering

The melt viscosity below entanglement is proportional to molecular weight: η = k Mw

2.3

But when the chains start to entangle with one another, then the melt viscosity rises very steeply according to a power law: η = k Mw

3.4

2.4

The influence of the two equations is illustrated for some polymers by Fig. 2.4, where the molecular weight is now plotted in terms of the number of atoms in the backbone chain, NB. This is why injection moulding grades of polymers tend to be chosen near to the entanglement molecular weight so as to minimize melt viscosity. However, the tensile strength will lie at the lower end of expectations, and will be sensitive to any mechanism which cuts or degrades chains. It might only need a few single chain scissions to lower the molecular weight locally to below entanglement, the tensile strength drops dramatically and a brittle crack is initiated. Injection moulding is the most sophisticated moulding technique, and the tools are an expensive part of the process, their cost being determined by

4

(Log h) – K (N s m–2)

3

2

Polystyrene Polyvinyl acetate

1

Polyisobutylene

0

Polydimethyl siloxane

–1

–2

2

3 Log NB

2.4 Critical entanglement molecular weight.

4

Examination and analysis of failed components

47

their complexity. Production runs must be long to justify their cost, and there are various way of increasing the rate, by using multi-cavity tools, for example (1, 2).

2.2.2 Extrusion The process of injecting a stream of molten polymer through a die of constant section is known as extrusion, and it is generally simpler than injection moulding. Pipe, sheet and profiles are made using the method, but there are several kinds of feature in the final product which can be deleterious. Perhaps the most important is the equivalent of weld lines: so-called ‘spider lines’ which are aligned along the axis of a pipe, for example. They are formed by the internal metal supports for the die head, where the melt divides before reforming. If the melt is too cold, then reformation is poor, so leaving lines along the extrudate. They can be seen clearly in the section of a pipe in Fig. 2.5. The section also shows another problem encountered in all processes: poor mixing of ingredients, in this case carbon black in polyethylene. To achieve the best product strength, such fillers must be mixed to make a

2.5 Section of poorly mixed pipe showing spider lines.

48

Forensic polymer engineering

uniform material, combining both high dispersion and an even distribution of particles. Owing to the simplicity of extrusion, much greater molecular weight material can be used, so extruded products tend to be stronger than moulded products. The problem of residual strain is also usually much reduced in extrudates. This is why pipe is generally very strong, failures tending to occur at joints.

2.2.3 Other moulding methods Rotational moulding is a way of shaping products without two matching tools. Only an outer tool is used, and a weighed quantity of powder is added to the tool, which is then rotated in an oven. The particles gradually melt and fuse to create a uniform wall, which can then be removed at the end of the cycle. Since high pressure is not used, higher tool temperatures are needed, so raising the chance of thermal degradation. The inner surface can also be rough owing to incomplete particle fusion. Both oxidation and geometric irregularity can weaken this surface, making products weak to external impact loads (3). Compression moulding is a primitive form of injection moulding, but yet widely used for elastomers and some thermosets. It suffers from the poor control of melt flow, although tools are simple and production rates high. The features, which may turn into defects, follow those of injection moulding. Perhaps the most sophisticated method is used in tyre building, where temperature control is crucial to achieving the best properties of the many different parts of the product. With materials of extremely high molecular weight, which are impossible to mould or extrude, sintering is a possible forming route. The process involves use of very high pressures to compress powder particles together into very simple shapes in a closed mould. It is used for ultra-high molecular weight PE or UHMPE, and PTFE, for example. Shaping by machining can then be achieved, although costs are high, since each product must be shaped individually.

2.2.4 Other shaping routes Welding is an important secondary process used to bond components together to make a composite product. Thermal or fusion welding presents the problem of temperature control. Since high temperatures are needed, the problem of oxidation is ever present, so good control of the process is needed to achieve a reliable bond. Those problems can go undetected until too late, as when a pipe exploded at a chemical works on Teesside (Fig.

Examination and analysis of failed components

49

Butt weld

Gasket

2.6 Pipe failure at chemical plant on Teesside.

Discolouration in brittle crack, occurring over several months 350 mm pipe

Pipe wall 20 mm thick

2.7 Section of butt weld showing discolouration of brittle crack.

2.6). The polypropylene pipe was in a scrubbing line designed to remove strong acids from effluent gases, and was operated at 50°C and a pressure of 0.5 bar up to 1.7 bar when it failed suddenly. The fracture surface (Fig. 2.7) showed that a brittle crack had been growing for some time in the butt fusion weld, judging by discolouration in the crack. An exhaustive investigation showed there to be a faulty heater element in the welding machine, causing it to operate 50°C below the specified temperature of 220°C for the polymer. This operation temperature of about 170°C compares with the normal melting point of about 176°C, so it was likely that the opposing sides

50

Forensic polymer engineering

of the weld had not fused correctly, and formed a weld line. The feature opened up slowly when the pipe was working, until 8 months after installation, it failed catastrophically. Three other faulty welds in other pipes were also found when the relevant parts were dissembled. Precautions were introduced to prevent future problems by calibrating the heater elements in the welding machine on a regular basis.

2.3

Mechanical testing

Since many product failures involve fracture, mechanical testing can be important for establishing the state of the material (3). However, it is of limited value in most investigations because ample material is needed for testing, material which is usually unavailable. Large samples of flat sheet are needed to cut dumbbell specimens, normally an impossible requirement. It is destructive, so cannot be used for unique samples. Polymer products are often highly anisotropic, so any results are of limited diagnostic value.

2.3.1 Tensile testing Component failures can occur by traumatic overloading, as a result of poor design, incorrect material selection, manufacturing defects or environmental factors. Mechanical properties of a failed component are, therefore, of prime interest in any failure investigation, as they will provide an insight as to how the component would perform under ‘service’ loading conditions. Some of the most widely quoted mechanical properties are those determined by a tensile test. A common measure given by a test is the tensile modulus, E, which is simply the initial slope of a stress-strain curve defined by: E = stress/strain = σ/α

2.5

where the stress, σ is just the applied force, F per unit cross-section area of the test piece, A: stress = σ = F/A

2.6

and the strain, α is the relative extension from an original sample length l0 to length l: strain = α = (l − l0)/l0

2.7

As well as elastic modulus, a wide range of mechanical data can be generated from tensile tests, such as tensile strength, elastic limit, yield point, cold drawing and so on. The tensile test rarely tells the whole story, and further mechanical information may be required to conclude a failure inquiry.

Examination and analysis of failed components

51

2.3.2 Creep and stress relaxation But polymers are essentially viscoelastic materials, so their mechanical properties are subject to the time scale of examination as well as the local temperature. The tensile stress relaxation and creep moduli are often quoted as ER(t) or EC(t) where the time in brackets is stated, 10 seconds being a common standard. These are the values quoted in the table in Chapter 1, and are often used in design calculations (3, 4). With all polymers, their temperature sensitivity is high, particularly so for polymers with relatively low melting (Tm) and glass-transition (Tg) temperatures – such as polyolefins. As part of their structure is in the amorphous elastomeric state at room temperature, their creep rates will in general be higher than materials such as PVC, PMMA and PC, which have glass-transition temperatures at or well above 100°C. Permanent distortion of polymer products is a symptom of poor design or exposure to higher loads or temperatures than expected. Figure 2.8, for example, shows a glass reinforced radiator reservoir distorted by contact with hot water in the cooling system of a brand new car on which the tank was being tested. The product had been injection moulded into a cold tool, so producing high levels of residual strain. The near boiling conditions of exposure allowed those strains to relax and distort the product, despite the high Tm of the nylon 6,6 matrix of 267°C. In a test of thermoplastic polypropylene from a failed storage tank, sample dumbbells were cut from the surrounding sheet as well as across a thermal weld. As will be discussed in detail in Chapter 4, the tank failed from just such a weld, and a test of the weld was thought useful. The bulk extruded sheet proved very strong, cold drawing above its yield point, but all the welds failed by brittle cracking across the centre of the weld. The fractures appear to have started small pinholes in the outer surfaces of the

2.8 Failed car radiator tank showing gross distortion (top).

52

Forensic polymer engineering

welds, and their weakness probably reflects the recrystallization of the material inevitably involved where the polymer was melted and then reformed. The weld strength of about 21 MNm−2 compared well with the data sheet value of about 20 MNm−2. As will be seen later, the failure was in fact caused by another problem unrelated to the polymer used in its construction.

2.3.3 Composite materials The spread of moduli is extremely wide, from a few kNm−2 for elastomers through common thermoplastics of about 2 MNm−2, up to about 180 GNm−2 for aramid fibres (5). It makes them ideal candidates for composite materials with a thermoset or thermoplastic matrix reinforced by a variety of fibres or particles. Their moduli can be modelled in several ways, a simple model of parallel loads giving the result: Ec = E1V1 + E2 (1 − V1)

2.8

where Ec is the composite modulus, E1 the fibre modulus and E2 the matrix modulus. The volume fraction of fibres is V1. The parallel model in reality fails to recognize the great anisotropy of all composite materials, a factor which has to be taken into account when designing composite products and the relation between the load path and the fibre orientation. Composites are attractive for highly demanding components because of the higher moduli and strengths available, so are widely used in aerospace and automotive applications such as the radiator tank already mentioned. It was moulded from short glass fibre (30% by weight) nylon 6,6, but failed during testing by a brittle crack developing at one point on its axis (Fig. 2.9). Water leaked through the crack when it had grown through the wall,

2.9 Brittle crack on tank near external buttress.

Examination and analysis of failed components

53

and the engine of the new car seized up without warning. The crack had formed as a result of the pressure of the cooling system on the wall, and acting on a weld line which had formed along the axis of the component (6). Tensile testing of the material revealed the anisotropy of the moulding, and only proved possible because the sample was not involved in litigation (it had been submitted by the manufacturer who wanted a detailed analysis). A new sample was also tested for comparison with the failed specimen. Two samples were tested to break from each tank, with the following strength recorded from machined dumbells: New sample, lateral to axis New sample, parallel to axis Failed sample, lateral Failed sample, parallel

σf σf σf σf

= = = =

84 81 55 80

MNm−2 MNm−2 MNm−2 MNm−2

All samples showed virtually identical strain to break of about 10%. The sample tested across the axis showed the lowest strength, probably caused by a small stress concentration in the outer surface. Flow lines could be seen in all the samples where the fibres were oriented during moulding from a central gate (Fig. 2.8). It was interesting to note that all values were well below the strength given in the material supplier’s data sheet of 140 MNm−2 at a strain of 6%. So although the test did show a small difference between good and failed material, it did not point to the reasons for failure.

2.3.4 Photoelastic strain analysis Indirect stress or strain analysis is a versatile method for investigating possible or actual failure of a product or part. Failure can be from externally applied stress or from residual (moulded-in) stresses. Both external stress and moulded-in strain (or a combination of both) can cause a part to fail prematurely. It is more straightforward to detect failure due to poor design, or excessive service forces. However, residual stresses and strains are altogether different. Here, poor moulding practice can generate residual strain just about anywhere, anytime. Photoelastic inspection will allow detection of frozen-in strains, allowing identification of failure, with the method revealing the actual levels of orientation in the part. Some transparent plastics such as polycarbonate are highly birefringent and lend themselves to photoelastic stress analysis. The part is placed between two polarizing media and viewed, in the crossed polar position, from the opposite side of the light source. Fringe patterns are observed – without applying external stress, thus allowing observation of moulded-in or residual strains in the part. Figure 2.10 shows a set square containing

54

Forensic polymer engineering W

W

P

2.10 Birefringence in polycarbonate set square showing gate at P and weld line at WW.

2.11 Birefringence in girder section showing stress raiser at upper corner.

residual moulding stresses that are clearly visible under the photoelastic viewing method. A high fringe order indicates areas of high chain orientation whereas low fringe order represents an unstressed area. Close spacing of fringes represents a high strain gradient, whereas uniform colour will be an indicator of uniform strain in the part. The injection point of polymer at P shows high residual strain, and the corners to the central hole an exceptional level. A weld line formed beyond the hole is also clearly visible (WW). Plastic models can be used to simulate ‘in-service’ conditions. Both applied and residual stress fields can be exposed using models of structures in photosensitive material placed between polarizing filters in the crossed polar position. Figure 2.11 shows the stress fields present in a section of a

Examination and analysis of failed components

55

bridge beam used in the first rail crossing of the Dee at Chester. It failed in May 1847 by brittle fracture. One possible initiation point lay in the corners present in the cavetto moulding shown in section on either side of the lower part of the cast iron structure. Straining the lower flange showed that the upper corner was the most seriously strained and thus the likely cause of the failure. The method is used widely for examining how structures respond to various load conditions (7).

2.4

Techniques for recording product failures

The simplest investigation tools are often the best, and simple observation is such a basic method that it is frequently forgotten by text books. However, it is perhaps the single most useful method for both recording and analyzing failures. Photography of the failed product and the context of failure at the site of an accident provides a permanent record, frequently useful at a later stage of an investigation.

2.4.1 Visual observation The power of observation is a basic asset for the forensic investigator. However, there is a tendency to neglect the fundamental importance of our eyes – the simple power of visual observation, so that others may see what we have seen and the importance attached to it. Sharp observation will allow more than scrutiny of artefacts under examination – it may also provide an insight into the situation that led to failure in the first instance. Observations of this nature allow assessment of the circumstances from which physical evidence was gathered, and provide insight as to a possible train of events leading to failure. It takes the form of: • • • •

a visit to the scene of the accident or failure selection of evidence for laboratory examination examination of ‘scene of incident’ photographs (and/or pertinent documentation) taken by bodies such as the police or factory inspectorate. inspection of witness statements from the accident.

Visual observation is critical for crack detection. A human eye with 20/20 vision is able to resolve features as small as 75 μm in size at a distance of 25 cm. It is possible under perfect conditions (on a mirror-polished surface) to detect a crack with a crack opening dimension (COD) as small as 10 μm. However, the minimum detectable COD becomes much larger if the surface is rough or not perfectly clean. Surface features such as scratches and machining marks present visual ‘noise’ that will effectively mask any cracking. This is why other methods to aid crack detection are normally needed.

56

Forensic polymer engineering

2.4.2 Forensic macroscopy Forensic photography demands more than a ‘point-and-shoot’ approach to recording information. The general area in or around the vicinity of the incident is always important but it is of little use photographing the surrounding area when the clue to failure lies in one small area or a surface feature that would pass unnoticed by a non-specialist onlooker. Careful examination and photography of the fractured artefact should follow, concentrating on any relevant details on external surfaces that are visible to the eye without any magnification. The photographic record should provide information on size and condition of all pieces, and should show the relationship of any fracture to its component parts. As many pictures should be taken as is thought necessary to define and isolate key features on samples. At a later date such pictures, as an aide-memoire, may well become invaluable. Rapid advances in digital imaging technologies have greatly improved many aspects of forensic photography. Digital imaging makes it possible to capture, edit, and output images faster than processing conventional film. It is also possible to import individual frames of video for enhancement. Techniques that used to be applied in the darkroom through trial and error can now be used on a computer, and the results are immediately visible on screen (8). On the other hand, conventional film still preserves more data than many high resolution digital images, so is still useful for record purposes. And analysis of old photographs in cold cases is an important area of research. They can, of course, be scanned to produce digital images, and enlarged to show details of interest. In an investigation of degraded rope used for stabilizing polytunnels used in agriculture, there were visible colour differences between a new coil and the failed polypropylene rope (Fig. 2.12). Tensile tests showed that the old rope failed at between 25% and 50% of the new samples. Closer inspection showed that fraying occurred in the outer strands, possibly by degradation induced by exposure to strong sunshine (Fig. 2.13), a conclusion that could only be tested (and confirmed) by infra-red spectroscopy. The rope had been used in South Africa where sunshine levels are generally much higher than in the UK.

2.4.3 Radiography Use of X-rays to reveal a hidden interior to a solid object is of course well known, and can be vital during investigations as a non-destructive way of analyzing unique samples (9). Soft X-rays as used in hospitals (ca 40keV) are the ideal source for polymer products, since they share similar densities

Examination and analysis of failed components

57

2.12 Failed rope coil compared with new rope.

2.13 Damage to outer strands in failed rope compared with unaffected cut rope.

to human flesh and bone. Since the absorption of the rays is primarily related to the atomic weight of the material, the method should distinguish the position of heavier objects within a soft matrix, or lighter objects in a heavy matrix (such as cracks). A source of radiation is directed toward a sample, with a sheet of radiographic film having been previously placed behind the object (Fig. 2.14). The density of the image formed on the film is a function of the quantity of radiation transmitted through the object, which in turn is inversely proportional to the atomic weight, density and thickness of the object. A contact radiograph of an injection moulded vehicle panel containing 22% by volume of 13 mm diameter glass fibres is shown in Fig. 2.15 (a), along with a photographically enlarged radiograph of the same panel at a different point of flow (Fig. 2.15 (b)). The same method can be used for tracing cracks, although care is needed in examining in several different sample orientations, since they can be

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Film pack or X-ray imaging system

X or γ source

Forensic sample, package, system or artefact

2.14 Contact radiography.

(a)

(b)

2.15 Contact radiographs of fibre orientation in composite polymer.

easily missed if the cracks and beam are not exactly parallel. The method was crucial in a previous case involving gas moulded chair arms, arms with hollow interiors but of irregular shape (6).

2.5

Forensic microscopy

Simple visual examination of a failed specimen is fast, cheap and the eye has good ability to perceive both depth and colour, but resolution is limited and subjective. Higher magnification inspection is needed using microscopy.

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Methods include stereo microscopy, reflected light microscopy and scanning electron microscopy (SEM). For non-metallic materials an environmental scanning electron microscope (ESEM) is essential. Samples can be examined without the need for a conductive coating to stop charging of the surfaces. One big advantage of scanning electron microscopy is that it can be combined with energy dispersive X-ray analysis (EDX) to obtain information on elemental distribution. On the other hand, sample manipulation is more difficult and all colours are lost using SEM/ESEM.

2.5.1 Optical microscopy Reflected light microscopy is used to study the microstructure of opaque materials. Contrast in the image results from differences in reflectivity of the microstructure. The maximum magnification achievable is limited to about ×1000. For polymeric materials, thin sections can also be examined in transmitted mode with polarizing filters to give information on the forming process, provided of course that the material is not filled with an opaque reinforcement like carbon black. It is often useful, when possible, to polish and etch a section to reveal hidden details. But the method is partially destructive, so permission must be granted if the sample is unique. By far the most useful tool, however, is stereomicroscopy. Stereo microscopes take advantage of the brain’s ability to superimpose two images from different angles and perceive spatially accurate 3D objects. In the stereomicroscope this is achieved by transmitting two images from the sample inclined by a small angle (10–12°) to yield a stereoscopic image when the sample is viewed through the eyepieces. Stereomicroscopes allow images to be obtained with excellent depth perception but limited resolution. The images can be recorded through a digital camera onto a PC (where the image is taken through a single camera and thus the 3D effect is lost). However, stereomicroscopes are essential for examining fracture surfaces in detail and have a similar advantage to visual examination in that they carry good colour information. That capability is vital in inspecting samples for trace evidence, such as smears of paint or contaminant. But an even less costly way of examining samples is the digital microscope. It has become common, owing to its ease of use compared with conventional microscopes. It works without conventional optics such as eye-pieces, and plugs into a computer via the USB port. The image is seen immediately on the computer screen, where it can be saved and filed. It consists of a small CCD camera with an in-built light source and works at up to about ×200, filling a gap between macroscopy using a conventional 35 mm camera, and stereomicroscopy. Figure 2.16 shows, for example,

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2.16 Ozone cracks in diesel fuel pipe for CHS boiler.

2.17 Ozone cracks from corners in numerals.

ozone cracks in a high-pressure diesel fuel pipe from a central heating boiler. The pipe sprang a leak when the deepest crack penetrated through to the bore of the textile-reinforced nitrile rubber (NBR) tube. The pipe had lasted for about 10 years before failure, and according to the maintenance engineer who replaced it in March 2009, is a very common problem. The failure was also interesting for showing how such cracks often start from stress raising features on pipes, such as logos impressed into the pipe just after manufacture (Fig. 2.17). The ozone came from switches on the controls, where sparks create the gas at very low levels. Although this failure was detected in time, when it occurs elsewhere there may be much more serious effects, such as fire if the fuel vaporises and ignites, as cases will show in later chapters. Ozone cracks can also bring production lines to a halt if seals in pneumatic systems are attacked.

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2.5.2 Scanning electron microscopy Scanning electron microscopy is widely used for examining materials because there are a wide range of signals that result from the interaction of the electron beam with the solid and the technique gives both high resolution and good depth of focus. A considerable amount of microstructural and microchemical information can be obtained; the spatial resolution is usually less than a micron and can approach 5–10 nm. When an electron beam interacts with a solid material, a range of signals can be produced from the electron beam–specimen interaction. The signals that are most useful are secondary electrons, which give information on the topography of the material, and backscattered electrons, which give information on the composition of the material. X-rays ejected from the sample surface are characteristic of the atomic species from which they are produced. They give valuable information on the microchemistry of the surface and X-rays are often used in a forensic investigation to identity contaminants or inclusions, or simply to check whether the material has the correct composition. One of the drawbacks of scanning electron microscopy is that generally the sample needs to be electrically conducting in order to prevent imaging artefacts from charge build-up on the specimen surface. This is a problem with non-metallic materials which are generally insulating. There are several strategies that can be used to avoid this problem. The traditional method has been to use a conductive coating of either carbon or a metal such as gold. Gold coating can sometimes provide benefits in a surprising way. The fracture surface of the failed radiator (Fig. 2.9) was examined by breaking open the tank and was inspected using stereomicroscopy (Fig. 2.18). The tide marks produced by the leakage of cooling water can be seen very clearly on the upper free surface, but little detail in the fracture itself. After gold coating, the same surface was examined in the SEM (Fig. 2.19). The

2.18 Optical micrograph of brittle crack in radiator tank of GF nylon 6,6.

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2.19 SEM micrograph of gold coated fracture surface.

2.20 Optical micrograph of gold coated fracture showing weld line and cold slugs.

surface seemed to show a weld line at the lower edge of the fracture. When re-examined in the stereomicroscope, however, the problem became much clearer (Fig. 2.20). The highly reflective area in right centre is a ‘cold slug’ or the remnants of a granule which has not fully melted and fused, and the weld line at the base of the fracture is very clear. There are smaller remnants of such granules, and there can be no doubt that the tank was made very early in the production run. The first samples are usually flawed because the melt in the barrel of the moulding machine (Fig. 2.1) has not reached the equilibrium temperature. They should always be rejected by the operator, but for some unknown reason, one such sample entered the assembly line.

2.5.3 ESEM An alternative approach to coating samples in conventional SEM is to use an environmental scanning electron microscope or ESEM. In this microscope differential pumping is used between the detector (which is placed

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63

Primary eletron beam A

Detector Cascade electrons

+30–300V

Sample

Gaseous atom Positive ion Nagative ion

2.21 Environmental scanning microscopy.

under the pole piece of the final lens) and the column to prevent the gas affecting the beam in the electron column. Gas is bled into the sample chamber and is ionized by the electron beam. The gas ionization process additionally gives signal enhancement from a cascade amplification process (Fig. 2.21). The imaging gas is usually chosen to maximize the imaging signal and therefore is commonly either air/nitrogen in low vacuum systems or water in environmental scanning electron microscopes. Care has to be taken in the use of these imaging gases with energy dispersive X-ray analysis that the gas doesn’t mask elements that are important for analysis purposes. Additionally, care must be taken to avoid introducing imaging gases that may cause degradation of the sample being examined (10). An example where ESEM becomes vital to an investigation is shown in Fig. 2.22. The fracture shown occurred on a diesel fuel pipe connection, and caused a series of accidents in Scotland, in one of which a driver was seriously injured. The fracture surface showed numerous striations typical of incremental crack growth, and they occurred over a period of days, demonstrating that the leak should have been detected before the accident. The case is discussed in a later chapter.

2.6

Types of product defect

Both design and manufacturing engineers generally operate rigorous ‘quality control’ procedures at every stage of design and manufacture. It is therefore unusual for faulty products to enter service. However, on occasion faulty goods do manage to enter service with an inherent defect.

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9

56 4

7

12 10 11

13

8

3? 2 1

2.22 ESEM micrograph of fatigue crack in nylon 6,6 fuel line connector.

A defect is an imperfection that renders a product unsafe for its intended use and, as suggested above, is introduced either at the design stage, or at the point of manufacture. A design defect exists when a whole class of products are inadequately planned, and often poses unreasonable risks to consumers. A car manufacturer’s design of a vehicle with the fuel tank positioned so that it explodes in low-speed collisions is defective, for example. When the design is defective, even products perfectly manufactured are defective. On the other hand, a production or manufacturing defect arises when a sound design plan is not followed and the product is improperly manufactured. Such ‘manufactured-in’ defects can be patently obvious, or of a latent nature: •

•

‘Patent defects’ are defects which are plainly visible or which can be discovered by reasonable inspection or customary tests – hence the saying ‘patently obvious’. Visible surface cracking or blow holes are two examples of patent defects. ‘Latent (or hidden) defects’ are defects which are not plainly visible and which cannot be determined by reasonable inspection or customary tests, and which are unknown when the item is accepted. Internal voids or sub-surface cracking are examples of latent defects.

2.6.1 Mechanical defects Mechanical failure arises from application of external forces that cause a product or component to deform, crack, or break when the yield strength

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of the material is exceeded. The applied force may be tensile in nature, compressive, torsional, or impact – with the force being applied over short or long time spans, and at varying temperatures and/or humidity conditions. Any engineered system or individual component can fail from application of a single overload force. Single (or traumatic) overload can produce either a ductile or a brittle fracture mode. Ductile and brittle failures are terms that simply describe the amount of macroscopic plastic deformation that preceded fracture. A ductile failure is one where there is substantial distortion or plastic deformation of the failed part. Normally, a component will fail in a ductile manner when it plastically deforms, and the steadily reducing cross-section can no longer carry the applied service load. Ductile failure can be identified from: • •

the high degree of deformation and distortion that will be present around the fracture zone tearing of material accompanied by appreciable gross plastic deformation and exhibit necking.

The term ‘brittle fracture’ is used when a part is overloaded and breaks with no visible distortion and little or no plastic deformation. Here, a crack will form: • • •

with little or no necking without gross plastic deformation the fracture surface sometimes appears smooth and polished.

In a brittle overload failure, the crack will begin at a point of maximum stress, and then grow across the section. Separation of the two halves isn’t quite instantaneous, but crack speeds are very high, approaching the speed of sound in the material. One of the results of this is that the direction of the fracture path is sometimes indicated by chevron marks that point toward the origin of the failure. The type of fracture, ductile or brittle, should be compared with the nature of the material. Brittle fractures often appear in normally ductile materials. This indicates that either the load was applied very rapidly or some change has occurred in the material, such as low temperature embrittlement or degradation, where the material will no longer act in a ductile manner (11, 12). At ambient and elevated temperatures, most materials can fail at a stress which is much lower than its ultimate strength. This group of failure modes are time-dependent, and termed creep deformation and creep rupture. More generally, materials or components undergoing continuous deformation over time under a constant load or stress are said to be creeping. Elastic, plastic, and viscoelastic deformation can all be included in the creep

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process, depending on the material, service temperature and time of deformation.

2.6.2 Fatigue There is a particularly insidious mechanical failure mode that is responsible for a high proportion of in-service failure. Fatigue is the progressive, localized, and permanent structural damage that occurs when a material is subjected to cyclic or fluctuating strains under nominal stresses that have maximum values lower than the yield strength of the material. These stresses are always below the normal strength of the material, and yet will still cause catastrophic failure. Cracks are initiated at stress concentrations in the product and grow progressively until the affected part can no longer support the applied load (13). Any tight radius can act as a stress raiser (Chapter 1), severely curtailing service lifetime of a product or device. This can be illustrated by considering a failure that overtook an upmarket, therefore expensive, vacuum cleaner. It had been in use for some eighteen months when a spring catch unexpectedly failed in a brittle way. The catch held an extension tube. A large number of this particular model of vacuum cleaner were failing prematurely, and at an identical position. The catch in question had been injection moulded in ABS, normally a tough and ductile polymer, and flexed about a tight radius each time the pipe was removed and replaced, inflicting two cycles per use. Observation of the fracture surface revealed a multi-start low-cycle fatigue failure (Fig. 2.23) that had initiated from a tight radius moulded as part of the catch profile. The intensified stress at the initiation

2.23 Fatigue striations in ABS vacuum cleaner part.

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sites induced brittle cracks which then grew slowly at each load application. The obvious solution to this failure was a simple design change – an increase in the bend radius was all that was required to alleviate the problem.

2.6.3 Friction and wear There is a further mechanical mode that can result in premature service failure – wear. Friction is the resistance to motion between two surfaces that are forced to slide relative to each other. Frictional properties of materials in intimate contact will result in wear of surfaces when such contacts slide, impinge or oscillate relative to each other. Friction and wear are of considerable importance when considering the efficiency and/or operating lifetime of a product or component – friction will result in wasted power and generate heat, whereas any ensuing wear will lead to poor working tolerances, loss of efficiency and may ultimately lead to premature failure (14). The wear process has been defined as ‘the progressive loss of substance from the operating surface of a body occurring as a result of relative motion at that surface’. Wear is relatively gradual, with the exception of galling. Here, excessive friction between high spots will results in localized welding. Subsequent splitting creates further roughening of rubbing surfaces, accelerating the breakdown process. In contrast to outright breakage, product or machine performance may degrade slowly rather than cease suddenly, so defining a point of failure may not always be obvious. Products or components that have ostensibly failed by ‘wear’ are often encountered, and it is often necessary to establish if the rate of wear was acceptable and reflected good engineering practice or not. Fretting is a problem caused by hard particles trapped at a bearing surface and attacking the weaker of the bearing surfaces. It can produce very rapid rates of wear, sometimes with unexpected results. A swing bridge at a marina on the south coast ceased operation when the bearings wore so severely as to stop deck movement. When extracted, the steel pins showed abnormal and localized wear, while the ultra-high molecular weight nylon 6 sleeves showed little damage. The bearings were not sealed against the environment, and salt, sand and other debris had entered, becoming embedded in nylon surfaces. The sleeves then acted as a very effective abrasion agent and wore the pins excessively (Fig. 2.24). The sleeve size was too great and correctly dimensioned parts were used to solve the problem. Thermal failure of products can and will occur from exposure to extremely hot or extremely cold environments. At abnormally high temperatures the product may warp, twist, melt, or even burn. In addition, polymers (like most materials) tend to become brittle at low temperatures, when even the slightest load excursion may cause the product to crack or shatter. Thermal

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2.24 Failed bridge bearing caused by abrasion of steel pin by debris on nylon sleeves.

fatigue can arise when products are subjected to cyclical temperature fluctuations.

2.6.4 Environmental failure Plastics exposed to aggressive environments are susceptible to many types of attack. Ultraviolet rays, humidity, ozone and heat are major environmental factors that seriously affect many polymers. The effect can be anywhere from simple loss of colour, slight crazing and cracking, to a complete breakdown of the polymer structure (15, 16). When ultraviolet attack occurs, the material may have a colour shift, become chalky on its surface, and/or crack. There are a number of methods to reduce this problem, such as the addition of carbon black to the polymer will usually absorb most UV radiation. Chemical inhibitors are available for most polymers, which improve the UV resistance. Figure 2.25 shows ABS mill bobbins which were made by injection moulding flanged ends, and then solvent welding to extruded tube. They were designed to replace the original wooden bobbins. The problem lay in the solvent, methylene chloride, a very powerful organic fluid which created environmental stress cracks in the flanges. The brittle cracks were not noticed at the time and grew slowly, then when the bobbins were fully loaded on the spinning line, they suddenly broke, spewing nylon fibre across the factory floor. Multiple cracking occurred radially, and the fracture sur-

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2.25 ABS mill bobbins replacing wooden version at left.

faces showed no signs of ductility whatsoever, a characteristic of environmental stress cracking or ESC. The polymer involved, ABS, is amorphous and especially sensitive to ESC if injection moulding was poor. There was indeed evidence of poor moulding practice, such as numerous weld lines visible in the product surfaces. It was thus likely that tools were cold, and so frozen-in strain higher than normal. High barrel temperatures could also oxidize the reinforcing butadiene particles which make ABS a tough material. Once those particles are degraded and destroyed, the polymer becomes brittle. Nowadays, such bobbins are moulded in polypropylene or HDPE to a different design, usually as a one- or two-piece product rather than three separate mouldings welded together. Snap-fit parts eliminate the need for welding.

2.7

Molecular analysis of polymer properties

Since polymers are chain molecules, measurement of molecular weight is an important aspect of characterization. However, the groups of atoms within the chain are just as important in determining polymer properties, so identifying those groups is also often critical. Spectroscopy is the main method, and there are several different and independent spectroscopic techniques (17). The extra and unusual groups introduced by oxidation can be identified in the same way, although their levels are often initially low and so care is needed in sampling and recording spectra.

2.7.1 Sampling Taking specimens is a vital aspect of spectroscopy, and techniques like chromatography, and many different forms can be used. They include:

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• thin slices cut from a sample • thin films cast from solvent • surfaces of flexible polymer • polymer solutions. Thin slices can be examined directly in IR spectroscopy but material must be thin enough for the incoming IR beam to penetrate, so are normally in the range 10–100 microns. Slices can be made using a microtome, effectively a device which uses a steel blade to cut the mounted sample in a controlled and accurate way. An ultramicrotome uses a glass or diamond blade to cut even thinner slices for electron microscopy. Thin films can be cast using a suitable strong solvent such as methylene chloride (CH2Cl2) to create the polymer solution. Casting onto a ceramic tile makes a good planar film which can be removed easily, although vacuum treatment is usually essential to remove the last traces of solvent. Attenuated total reflection or ATR is a way of examining surface spectra in the infra-red, where the surface of a flexible polymer can be pressed against a single selenium crystal. Liquid samples are routinely and easily examined by all techniques, but may not be representative of the solid material, and key information lost. The spectra must also be adjusted for that of the solvent, and makes interpretation more difficult. On the other hand, it is effectively the only route for several methods such as GPC, UV and NMR spectroscopy. Liquids are usually examined as a thin film on sodium chloride plates inserted into the IR beam, and finely divided solids can be mixed with a paraffin liquid such as Nujol to give a paste which can be smeared over the discs. Examining solid samples is preferable for most failure studies, since variations from place to place can be more easily monitored, and thin films or sections are easy to prepare. However, cast thin films allow filler to be filtered or it settles, and the polymer can be examined alone. Fillers are usually present in commercial samples, and add to the complexity of IR spectra by obscuring large areas of absorption. Great care is needed in sampling because it is necessarily destructive, so analysis of unique specimens may be impossible unless ATR can be used. Where large numbers of failed products are available, then sampling is usually not a problem. Varying the position of sampling can locate external exposed areas of a product where UV radiation may have affected the polymer, and then compared with internal parts which have always been in darkness. Heat affected zones in welds can be studied by careful sampling, using bulk material as the standard for comparison.

2.7.2 Chromatography GPC, or gel permeation chromatography is unique to polymers. It is the best method for characterizing the complete molecular weight distribution

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of a polymer. GPC can determine several important weight parameters, including the most fundamental characteristic of a polymer – its molecular weight distribution. Weight and distribution values are of importance, as mechanical strength is directly related to molecular weight, and will influence many of the characteristic physical properties of a polymer. Subtle batch-to-batch differences in these measurable values can cause significant differences in the end-use properties of a polymer. The technique uses a polymer solution in suitable organic solvent that is introduced into a column containing a cross-linked gel. As the solution passes down the column, the smaller chains are absorbed by the gel, leaving the longer chains to be eluted first. The smaller chains are then released, so giving a distribution curve for the sample. This is why the method is sometimes known as ‘size exclusion chromatography’ (Fig. 2.26). Since different polymers behave in different ways, each must be calibrated in the solvent chosen for analysis (often THF, tetrahydrofuran), usually using a set of monodisperse polymers. The molecular mass distribution will typically show a single peak and a tail either side. Degradation of chain length

Sample mixture

Separation begins

Partial separation

Separation complete

Separated samples

2.26 GPC column.

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Forensic polymer engineering 1.25 1.00

Brittle

Wn (log M)

Virgin 0.75

Good batch

0.50 0.25 0 2.0

3.0

4.0 Log M

5.0

6.0

2.27 GPC for molecular weight analysis.

that occurs in oxidation will show as a lateral shift of the whole curve to lower molecular mass. The method was used where a polypropylene tank failed in service (18). GPC analysis showed material embrittlement when compared to virgin material and a sample taken from a tank that had not failed (Fig. 2.27). GPC is not the only method available, however. Viscometry, osmometry and light scattering are other routes to measuring molecular weight, but are time-consuming compared with the rapid results from GPC (Table 2.1). Other forms of analysis include pyrolysis gas chromatography or PGC, where the products of thermal decomposition can be separated and examined. Since polymers have widely different ways in which they degrade when heated, the spectrum of products can be distinctive for a particular material.

2.7.3 Infra-red spectroscopy Infrared spectroscopy is a key method for qualitative analysis of organic and many inorganic compounds. It relies on the absorption of infra-red light by molecules, while other spectroscopic methods relate to other parts of the spectrum of radiation (Fig. 2.28). The energy, E of different forms of radiation is given by the formula: E = hν = hc/λ

2.9

where h is Planck’s constant (= 6.626 × 10−34 Js ), ν the frequency of radiation, λ the wavelength and c the velocity of light (= 299,792,458 ms−1). So the higher the frequency, the more energetic the radiation, with X-rays more energetic than visible light for example. Infra-red radiation is commonly present in sunshine and firelight, and is relatively benign compared with the more energetic UV light. The visible spectrum occurs between

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Table 2.1 Methods for analysis of polymer structure Level of structure

Analytical technique

Information gained

(1) Chemical composition and chemical structure

Elemental analysis C,H,O,N; Group analysis, e.g. OH, CO2H, etc.; Infra-red (IR) spectroscopy; UV spectroscopy; NMR spectroscopy; Pyrolysis gas chromatography (PGC), etc.

Type (and proportion) of repeating unit(s)

(2) Molecular dimensions, degrees of polymerization and molecular mass

Viscometry

An average molecular mass (viscosity average) An average molecular mass (number average) Molecular dimensions and an average molecular mass (weight average) Molecular mass distribution

Osmometry Light scattering measurements Gel permeation chromatography (GPC)

(3) Type of molecular aggregation

Thermal analytical techniques: differential thermal analysis (DTA); differential scanning calorimetry (DSC); differential mechanical thermal analysis (DMTA) Electron microscopy

Crystalline and amorphous phases; glass transition temperatures, crystalline melting points; levels of crystallinity: viscoelastic behaviour Fracture surfaces, orientation phase composition, crystalline state

Optical microscopy Scanning electron microscopy (SEM) Birefringence

about 380 nm (violet) and 780 nm (red) or between 0.38 micron and 0.78 micron in wavelength (19). FTIR (Fourier Transform Infra-red) is simply a sophisticated form of spectroscopy where the thin sample can be scanned repeatedly so as to improve resolution. IR spectroscopy is used to characterize all polymers as well as solvents, pigments, fillers and additives, which is why it is so useful in polymer investigations. The method is based on the principle that interatomic vibrations absorb at specific frequencies of infra-red radiation (20). The frequency of radiation absorbed is governed by the types of bonding present, so the C–H bond stretching absorbs at a quite different frequency (ca 3000 cm−1) to that of the bond bending at about 1500 cm−1 (Figure 2.29).

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1 Alternating current 103 106

Radio frequencies

109

Micro waves

Nuclear quadrupole resonance Nuclear magnetic resonance Electron spin resonance Rotation

1012 Infra-red radiation

Vibration

Visible light

Outer elecrton transition

1015 Ultraviolet light 1018 X-rays

Inner electron transition

1021 g -rays

Nuclear transition

2.28 Electro-magnetic spectrum and interactions with matter.

Wavenumbers in cm–1 5000 4000

O

3000 2500

H and N C

C

2000

H stretching

C

O stretching C

H stretching

C and C

1500 1400 1300 1200

N stretching

N stretching

C

3

4

5

6

O stretching

C

N stretching C

1000

900

800

N

H bending

C

C

H bending

7

700

C stretching

H bending H bending

O 2

C

C stretching N

1100

H bending 8

9

10

11

12

13

14

Wavelength in microns

2.29 Infra-red correlation table.

The simplicity of the absorption spectrum of a medium density polyethylene (MDPE) cast film is shown in Figure 2.30. The spectrum is limited to the region 600 to 1700 cm−1, so the C–H absorption peak is omitted, and the main peak near the centre at 1480 cm−1 represents the C–C stretching mode. The peak to the right-hand side represents branch points in the linear chain where a secondary carbon bond exists, that is one where a carbon is linked to three carbon atoms rather than two. It changes the way the joint

15

Examination and analysis of failed components Wavelength/μm 7 8 9 10

C C

15

C stretch

O stretch C

1800

1600

H bend

1400 1200 1000 Wavenumber/cm–1

800

100 90 80 70 60 50 40 30 20 10 0 600

% Transmission

6

75

2.30 IR spectrum of MDPE showing oxidation.

vibrates, so absorption occurs at a slightly different wavenumber. The other absorption peak of interest is that at just above 1700 cm−1. The correlation table (Figure 2.29) shows it to be a carbonyl absorption peak, and it should not occur at all in MDPE. It has been formed by oxidation, which may occur by several possible mechanisms, but especially by processing at too high a temperature and/or exposure to UV radiation such as sunlight: —CH2—CH2—CH2— + O2 → —CH2—CO—CH2— Such groups now represent points of weakness which can initiate chain breakage by further oxidation. Carbonyl groups can absorb UV, so scission is possible: —CH2—CO—CH2— + UV → —CH2—CHO + CHO— Each chain end has an aldehyde group which is again susceptible to further oxidation to carboxylic acid. The conventional way of protecting sensitive polymers is to use additives, especially anti-oxidants and UV absorbers, often small aromatic molecules added at a low level (1–3%) to the compound. Another, less costly, additive used to protect outdoor applications is carbon black. In most applications, carbon black in concentrations of 1% is known to protect PE from the effects of UV radiation. Figure 2.31 shows the almost identical spectrum of a sample of LDPE, a type of polyethylene made by an entirely different process. It shows a slightly lower degree of branching and oxidation, judging by the slightly lower absorption peaks at the 1400 cm−1 position and the carbonyl wavenumber. But like the MDPE spectrum, it shows a high level of carbonyl groups, more than enough to cause serious loss of strength and cracking in the bulk materials, as will be discussed in a later chapter. It needs only a very small degree

Forensic polymer engineering

6

1800

1600

Wavelength/μm 7 8 9 10

1400 1200 1000 Wavenumber/cm–1

15

800

100 90 80 70 60 50 40 30 20 10 0 600

% Transmission

76

2.31 IR spectrum of LDPE showing oxidation.

of oxidation to initiate cracking because chain cleavage affects overall molecular weight so quickly and deleteriously. The challenge for IR spectroscopy is to detect those very low levels of carbonyl groups, often masked by background noise, in a suspect polymer.

2.7.4 Fingerprint spectroscopy But more complex polymers increase the number of absorption peaks very quickly, and it is not always possible to identify all the individual peaks. Introduction of aromatic groups, as for example in polystyrene or polycarbonate, increases the number of peaks below about 1500 cm−1 greatly. The region below 1500 cm−1 is often known as the fingerprint region owing to the multiplicity of peaks which can have a characteristic pattern of use in pinpointing a specific polymer or additive in a set of samples. The pattern of absorbencies can identify the basic form of polymer, using spectral libraries to pin-point a material more accurately (21). With commercial polymers, the fingerprint region is frequently obscured by fillers, but they can be removed by solvent extraction of the polymer, since most inorganic fillers like calcium carbonate, talc and carbon black are totally insoluble in organic fluids.

2.7.5 Beer-Lambert law As the amount of light absorbed is proportional to concentration of a chemical species, this method can also be used quantitatively (19). It is done

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by using the Beer-Lambert law, an empirical law which relates absorption to variables like percent transmission, T, film thickness, t, concentration, c and extinction coefficient, ε: log10(1/T) = εct

2.10

For a single film sample, the only two variables are the transmission and concentration, so for two different groups, 1 and 2, then: log10(T1/T2) = εc2t/εc1t = c2/c1

2.11

In other words, the relative peak height is directly proportional to the relative concentration. A peak which is constant throughout (such as C–H) can be chosen as the arbitrary standard, so a measure of the unknown group is possible. This provides a quick and easy way of estimating relative amounts of specific groups in a given sample. The main problem is one of drawing a common base line, because the background is often very variable, as shown by the polyethylene spectra just illustrated. So some care is needed in recording and then analysing spectra to record quantitative data. In a recent study of oxidation in a plasticizer, polybutene, the extent of oxidation was tracked by measuring the carbonyl peak height as a function of the concentration of an added anti-oxidant when the samples were aged at a constant rate. The plasticizer is used in a new type of flexible sealant for window glazing, but needs protection against oxidation during processing.

2.7.6 UV spectroscopy At the opposite end of the visible spectrum is the ultraviolet or UV region (Fig. 2.28). It is a much more energetic part of the spectrum of light, as may be judged by the effect of UV on skin and many polymers. Entities which absorb UV radiation include the carbonyl group, and virtually all aromatic groups (containing benzene rings and related structures). So any polymers containing such groups can be analysed using UV spectroscopy. The method is used mainly with solvent extracts, such as when checking a polymer for the presence of protective additives. For example, a sample of polyethylene taken from a cracked PE product was finely divided and extracted with a non-solvent for the polymer, methanol or MeOH. It involved warming the solvent and polymer mixture, and then decanting the solvent. A cracked MDPE mancab was treated in the same way as a standard sample known to contain a stabilizer. The two UV spectra are compared in Fig. 2.32, the standard showing the clear trace of an aromatic additive, while the mancab exhibited very little absorption. The

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% Transmission

0

B

50

A

100 300

400 Wavelength/nm

500

600

2.32 UV spectrum of extracts showing effect of added stabilizer (B and A).

O CH2CH2CH2CH2CH2CH2CH2CH3

OH C O

2.33 Structure of phenolic UV stabilizer with long side chain.

known additive is 2-hydroxy-4-octoxy-benzophenone, a UV absorber and protective chemical for PE. Its structure is shown in Fig. 2.33. With two benzene rings linked by a carbonyl group, the compound absorbs UV light very strongly, which is what it is meant to do for its protective action. The long side chain inhibits migration from the polymer. In general UV spectra are very simple compared with IR spectra, yielding relatively few

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79

absorption peaks (19). There are compilations of standard spectra to check attributions.

2.7.7 NMR spectroscopy NMR is sometimes a useful analytical tool for determining the content and purity of a sample, as well as its molecular structure. It is achieved by detecting atomic nuclei with spin in molecules, by absorption at resonance (19, 22). The nuclear magnetic field felt by a molecule is modified by the chemical environment (a chemical shift) so protons can be differentiated in different parts of molecules. The method is particularly valuable for polymeric investigation, enabling different stereoisomeric forms to be identified for example. The sensitivity of the technique allows ‘fingerprinting’ of unknown materials. Here, the unknown compound can either be used to match against spectral libraries or to infer the basic structure directly. Once the basic structure is known, NMR can be used to determine molecular conformation in solution as well as studying physical properties at the molecular level such as conformational exchange, phase changes, solubility, and diffusion. However, resolution is dependent on applied field strength; the stronger the magnetic field, the greater the resolution. The method proved valuable in solving an identity problem with building plasters. A company had developed a secret ingredient for improving the viscosity response of its plaster. It was polyacrylic acid (PAA) from a particular manufacturer and was added at a level of 0.1%. It extended the period when a high viscosity is needed of the wet plaster during final application to a wall. Its repeat unit is: —[CH2—CH(CO2H)]— and the sodium salt has the structure: —[CH2—CH(CO2−Na+)]— The salt form of the polyelectrolyte was normally used because of its solubility in water, and this polymer is used widely where high water absorption is needed, such as in nappies and plant mulches. A manager at the factory left suddenly and started up making a similar product which then competed with the original plaster. The director of the first factory suspected that the new product used the same additive, and had used proprietory information when he was employed there. He sued the new company, but had to prove that the added ingredient was identical with his own. FTIR of solvent extracts from each plaster proved unequal to the task of matching the additives. The problem lay in the complexity of additives used, especially another ingredient, bone glue, present to the extent of 4%. It is a complex mixture of natural biopolymers, such as

80

Forensic polymer engineering CH CH2

)DMF solvent ) TMS

) 8

7

6

5

4

DMF solvent

3

)

2

1

0

2.34 NMR spectrum of polyacrylic acid (PAA).

degraded protein, and effectively masked the PAA additive. But if a selective solvent to extract only the PAA could be found then the problem might be resolved. Dimethyl formamide or DMF of formula H—CO—N(CH3)2 was found to be such a selective solvent for PAA, but the IR spectra still proved of low resolution. Deuterated DMF was needed to lower spin levels, with the result shown in Fig. 2.34. The lower spectrum (at 600 MHz) shows pure PAA used by the first company, and the upper shows the DMF extract of the plaster from the second company. The major resonance peaks are identical, albeit of different intensity, but the deciding factor was the nearly identical background. The minor peaks showed that even the impurities in the PAA were identical, proving that the same source of the additive was used in each plaster (Fig. 2.35). The case did not proceed to trial but was settled out of court.

2.7.8 Other methods A key indicator of acid or alkaline conditions is the pH, defined as: pH = −log10[H+]

2.12

Examination and analysis of failed components

81

CH2

)DMF solvent ) CH

)DMF solvent )

8

7

TMS

6

5

4

3

2

1

0

2.35 NMR spectrum of DMF extract of suspect plaster.

where [H+] is the concentration of hydrogen ion, or rather the concentration of the hydroxonium ion since free protons are hydrated to form [H3O+] in aqueous solutions. Pure water has a pH of 7.0, while acids have lower values, and alkaline solutions greater values, up to pH = 14. Measurement is straightforward using an electronic pH meter with a glass electrode. Estimating pH can be useful in degradation studies where oxidation of a polymer or plasticizer occurs and produces small molecule carboxylic acids, so pH in trial oxidation studies can be used to follow degradation. It may also be essential when judging the potential for polymer hydrolysis, because the reaction normally increases with either strong acids (very low pH) or strong alkalis (high pH). Step-growth polymers are most sensitive to hydrolysis, and include polyesters such as PET and polycarbonate, polyamides such as nylon 6,6 and polyoxides such as acetal. The latter are degraded in acid conditions (low pH), while PET and polycarbonate are hydrolyzed under alkaline conditions (high pH).

2.7.9 Thermal analysis Thermal analysis is the application of a precision controlled temperature program that allows quantification of a change in a material’s properties

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Table 2.2 Thermal methods of analysis Weight changes

Energy changes

Dimensional and mechanical changes

Isobaric Isothermal

Differential thermal analysis (DTA)

Thermo-gravimetric analysis Derivative thermo gravimetric

Calorimetry Differential calorimetry (DSC)

Penetration Thermal mechanical analysis (TMA) Modulus Dynamic mechanical thermal analysis (DMTA) Dilatometry Expansion Contraction

Table 2.3 Thermal transitions (a) Physical 1st order transition

2nd order transition

Melting Crystallization Crystal-crystal transition Liquid-crystal transition Glass transition Glass transformation

(b) Chemical Curing Cross-linking Vulcanization Polymerization Oxidation Degradation

with change in temperature (23). The range of thermal analysis procedures available is shown in Table 2.2. Of these methods, differential scanning calorimetry (DSC) is probably the most important. The technique measures the enthalpy of a substance while it is being heated up at a controlled and constant rate. Valuable information concerning a material’s composition and its response to temperature may be obtained. Any temperature excursion can be attributed to physical or chemical events within the sample substance, as listed in Table 2.3. Thermal analysis is one of the key routes available for characterization of polymeric materials. Differential scanning calorimetry is a technique used to examine the thermal properties of materials under carefully controlled conditions. Only milligram amounts of material are needed, so although the method is destructive, sampling is minimal. The sample is

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83

Fusion peak Second-order (glass) transition

Exotherm

lsothermal

Deflection directly proportional to specific heat No-sample baseline

lsothermal

Temperature (K)

2.36 Schematic DSC thermogram.

heated at a constant rate (typically 10°C per minute), and the heat flow into or from the sample automatically recorded by the instrument. Heat is absorbed when a sample melts (an endothermic change), and the melting point (Tm) is often characteristic of the chemical composition of the material under study. The method is exceptionally useful for polymer investigation, where additional information includes the glass transition temperature (Tg, the temperature when the plastic becomes elastomeric) and the decomposition temperature, when the material decomposes. While metals and alloys melt at a sharply defined temperature, polymers melt over a range of temperatures, and that range is also sensitive to heating rate, molecular weight of the polymer, as well as changes in chemical composition (24). Figure 2.36 shows schematically the various types of information provided by DSC. Figure 2.37, for example, shows the DSC curve of polyethylene terephthalate, a polymer used widely in packaging. The polymer is crystalline, with a melting point at about 248°C and a Tg centred at about 84°C. In most packaging, the polymer is transparent because the crystallites are smaller than the wavelength of light. The variable degree of crystallinity in polymers is illustrated by the behaviour of various grades of polyethylene (Fig. 2.38). The highest melting points and degrees of crystallinity are shown by HDPE, as one might expect from the greater densities shown by the material. There follow MDPEs and finally low density polyethylenes, with correspondingly lower melting points. The degree of crystallinity can be calculated from the area under the curves using an appropriate base line, with the results shown in the table (Table 2.4). Polypropylene shows a high Tm of about 176°C, but it is commonly copolymerized with ethylene, the extent of the lowering described by the equation: 1 1 R XE − o = Tm T m ΔH f

2.13

84

Forensic polymer engineering Method Sample every 1.00 sec. Gas [1] B [off] Ramp from 30.0 °C to 280.0 °C at 10.0 °C/min

2 1

Heat flow (mWatts)

0 80.63 °C Tg = 83.54 °C

–1

26.69 mJ/mg 235.62 °C

86.95 °C

–2 –3 –4 –5

247.88 °C

–6 –7 0

50

100

150 Temperature (°C)

200

250

300

2.37 Thermogram of PET from soft drink bottle.

Weight: All samples 7.1 mg Range: 10 mcal s–1 Scan rate: 10 °C min–1

HDPE

Heat flow rate

MDPE

20

LDPE

40

60

80 100 T (°C)

120

140

160

2.38 Melting behaviour of various polyethylenes.

where T0m is the homopolymer melting point, Tm the melting point of the copolymer (both expressed in degrees Kelvin) with XE the mole fraction of ethylene units. ΔHf is the heat of fusion of the homopolymer (taken as 10.97 kJ mol−1) and R the gas constant (= 8.314 J K−1 mol−1). Only a small amount of added ethylene units will lower the melting point. Thus 5 mole

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85

Table 2.4 Crystallization properties of polyethylenes Sample type

Tm °C

Crystallinity/%

LD LD MD MD HD HD

106 113 115 118 132 133

32 39 44 50 76 79

per cent of added ethylene lowers the melting point by about 8° to 168°C. A similar effect occurs with the glass point, which with this copolymer is important for its resistance to freezing temperatures.

2.8

Integrity of results

Care is needed to ensure that all analytical data retain their integrity. One test for integrity is reproducibility: can another expert perform the same test and achieve the same result? It means that all experimental conditions must be recorded and published, or passed on to other experts. It is also a test worth performing on one’s own results, to check that the spectrum really is showing what it is meant to show and not an artefact. Use of automatic recording instruments demands that calibration tests are performed at regular intervals to check machine integrity. For DSC, a low melting metal, indium of known melting point of 156.6°C is analyzed during routine calibration. A standard spectrum of polystyrene is used for the same purpose in FTIR spectroscopy, as it possesses numerous absorption peaks at known wavelengths. Internal standards such as tetramethyl silane (TMS) are routinely used in NMR to measure chemical shifts of an unknown compound. Several different techniques that measure the same quantity can also used to corroborate results independently. Many of the more recent methods can also be checked by an inter-laboratory test, where a compound of known composition is sent to each lab and analyzed blind. This has been done for GPC, and revealed frighteningly large discrepancies, showing that confidence can be put on a set of results obtained on one instrument when recorded at the same time, but comparison of results from different instruments may be dangerous. The importance of the reproducibility test was highlighted recently during the investigation of failed electric plugs. The Noryl casings were cracking and exposing live wires to the fingers of the user, making the manufacturer directly liable for any injuries. The plugs in question had been moulded at a factory in China, and it was our opinion that the process itself

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had caused widespread cracking of the transformer plugs. However, a forensic group in Japan disputed our conclusion. They claimed that too much fire retardant had been added to one batch of the material in Europe, so weakening the moulded plug casing. They based their conclusions on alcoholic extracts of the plugs analyzed using FTIR. When we analyzed the same plugs using SEM-EDX (a method quite independent of FTIR), we could not corroborate their findings, and requested checks on their method. We never received their experimental details, and the original conclusion of faulty moulding was finally accepted. Quality checks in the factory have been improved to prevent a re-occurrence of the problem. When unique polymer samples are collected, the need for preservation is paramount. All samples should be protected using polyethylene bags, and for fragile specimens, stored in the dark at a low temperature, such as in a refrigerator. Handling should be minimal, preferably using plastic gloves to prevent contamination from sweat, especially if EDX is used for elemental analysis. The same general principles apply to all the techniques of analysis and inspection described in this chapter, and if not applied rigorously, can lead to the kind of ambiguity and contradiction which led to the dispute in the first place. Although lawyers will often want the bare minimum of testing to be undertaken in a dispute, they do not have to appear in a witness box to defend their results.

2.9

Conclusions

There are several points to make concerning failures of engineered products. First, it is surprising how much can be gleaned from the material evidence of failure, especially when linked to witness or other documentary evidence. In effect, the investigator will try to explain the failure by sifting the evidence carefully so as to reveal key or critical parts that show how the incident occurred. On many occasions, this may be limited to a bundle of documentary evidence, where the expert will be required to ‘tease-out’ relevant information on the matter in hand. Fracture surfaces are very revealing if preserved intact. Every so often, the microstructure of a small particle will reveal its origin, and hence allow a sequence of events to be constructed. Often the sifting process will be long and painstaking (particularly when a large and complex device, such as an aircraft, fails), but at other times it may be the speed of reaction to unfolding events which solves the problem in hand. Thus a photograph taken ‘just in time’ can shortcircuit an investigation, particularly when the evidence is deliberately removed or tampered with at a later time. Secondly, there is no doubt that experience and a working knowledge of typical failure modes of products can help elucidate the problem. To rec-

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87

ognize how a component or system failed, an understanding of how it works (and was manufactured) in the first place is essential. Reverse engineering of a product can help failure investigation. By stepping back through the transformation stages, the investigator will then be in a better position to determine most probable or expected point of failure within a component or system. This is ‘the weakest link’ principle in action (as described in Chapter 1). Then when it shows abnormal features, another cause of the failure must be sought. Although there are many possible experimental techniques which may be used with polymers, most investigations quickly reduce to a handful of methods. Pre-eminent among them is microscopy of failed surfaces, optical followed by ESEM, together with DSC and FTIR spectroscopy. Although the cost of any failure investigation increases as the complexity and depth of analysis become more sophisticated, the resultant benefit will be a far more complete recognition of the true origins of demise. However the concept of ‘cost’ should not be limited to the ‘monetary’ value of the investigation. A wider view should be taken that will encompass any business aspects associated with the failure in question. Consumers expect higher cost products to translate into higher quality goods that have a lower failure rate and longer lifespan than that of lower cost equivalents. This will apply to components and materials alike. When components and materials fail to meet stated performance specifications or service requirements, the consequences can be loss of manufacturing time and customer business. In general, lost business revenue will far outweigh costs of investigation. Moreover, any failure occurring in-service may have legal ramifications (possible litigation) in addition to business implications (loss of customer goodwill), along with potential loss of company reputation. The choice of methods controls investigative cost, with some far exceeding others. NMR, for example, is always more expensive than FTIR, which in turn is greater than optical microscopy or simple macroscopy. Many reports referred to us for a second opinion have often used the right methods but ignored failure modes to which polymers are particularly susceptible, especially ESC and SCC. Others are biased to the client, never a way of solving problems. The costs of those reports are wasted, which is often why second opinions are sought. Society is always looking for someone to ‘blame’ for any misadventure, and is far more eager to undertake litigation as a process to redress any perceived loss, damage or injury associated with failure of a product. Rapid and effective use of appropriate failure analysis at the outset of investigation can short circuit potential market place pitfalls, thus facilitating positive customer retention along with continued company growth. However, it is vital that appropriate expertise is tapped, especially in specialist areas like polymers. Although many forensic engineers will claim

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expertise in metal failures, it may not qualify them for examining polymer failures, where the basic failure modes vary significantly from those of metals.

2.10

References

(1) Cracknell, P and Dyson, R, Handbook of thermoplastic injection mould design, Chapman and Hall (1993). (2) Maier, C, Advances in Injection Moulding Technology, RAPRA Review reports No 72 (1994). (3) Crawford, RJ, Plastics Engineering, Butterworth-Heinemann, 3rd edn (1998). (4) Mills, N, Plastics: Microstructure and Engineering Applications, Edward Arnold, 2nd edn (1993). (5) Lee, SM (Ed), International Encyclopedia of Composites, VCH Publishers (1991). (6) Lewis, Peter Rhys, Reynolds, Ken and Gagg, Colin, Forensic Materials Engineering: Case Studies, CRC Press (2003). (7) Holister, G, Experimental Stress Analysis, Cambridge University Press (1967). (8) Blitzer, HL and Jacobia, J, Forensic digital imaging and photography, Academic Press (2002). (9) Halmshaw, R, Non-Destructive Testing, Edward Arnold, 2nd edn (1991). (10) Kitching, S and Donald, AM, Beam damage of polypropylene in the environmental scanning microscope: an FTIR study, Journal of Microscopy, 190, 1998, pp. 357–365. (11) Andrews, EH, Fracture in Polymers, Oliver and Boyd (1968). (12) Williams, JG, Stress Analysis of Polymers, Longman (1973). (13) Hertzberg, RW, Fatigue in Polymers, John Wiley (1988). (14) Bowden, FP and Tabor, D, Friction and Lubrication of Solids, CUP (1995). (15) Wright, David, Failure of Rubber and Plastic Products, RAPRA (2001). (16) Ezrin, M, Plastics Failure Guide, Hanser-SPE (1996). (17) Crompton, TR, Characterisation of Polymers, Smithers RAPRA (2008). (18) Erzin M, Lavigne G, Dudley M and Pinatti, L, Case studies of plastics failure related to molecular weight or chemical composition, ANTEC 2005, pp 3469– 3474, 2005. (19) Dodd, RE, Chemical Spectroscopy, Elsevier (1962); Pavia, DL, Lampman and Kriz, GS, Introduction to Spectroscopy, 3rd edn, Thomson (2000). (20) Conley, RT, Infra-red Spectroscopy, 2nd edn, Allyn and Bacon (1975); Günzler H and Gremlich HU, IR Spectroscopy: An Introduction, Wiley (2002). (21) Hummel, DO, Polymer and Plastics Analysis, 2nd edn, Hanser Verlag (1978). (22) Günther H, NMR Spectroscopy: Basic Principles, Concepts, and Applications in Chemistry, 2nd edn, WileyBlackwell (1995). (23) Wunderlich, B, Thermal Analysis of Polymeric Materials, Springer (2005). (24) Brydson, J, Plastics Materials, 7th edn, Butterworth (1997).

3 Polymeric medical devices

3.1

Introduction

If one were to examine areas of great advance in the use of new materials, medical devices would surely be among the first to be noticed. One reason why polymers are now so widely used is their similarity to the natural materials from which our bodies are built. They have similar mechanical properties, and so are flexible in response to body stresses. Some polymers are inert and unreactive to body fluids, and can all be designed into products of some complexity with great ease. The body environment is highly reactive since it is in a continual state of producing energy for body functions (such as muscle movement), with many complex chemical pathways in both the fluids (such as blood) and tissues (such as muscle and bone). Enzymes, or biochemical catalysts, target specific molecules in changing their structure, whether degrading them to simpler units, or changing their make-up. But there are some relatively simple environments where the body breaks up the molecular constituents of food into much simpler units, and uses a strongly acidic environment to achieve that end. Thus starch is effectively degraded to glucose monomer units by acidic hydrolysis in the stomach, the glucose then becoming a vital energy source for muscles. Implants must be able to resist such attack in other aggressive environments in the body, temporary implants like catheters for short periods, and permanent implants like hip joints for many years. On the other hand, degradation can be exploited in the case of sutures for stitching wounds, where the stitches disappear over a timescale matching the healing process. In general, many common polymers show good biocompatability, but care is needed to ensure their high purity owing to the problem of leaching of possibly toxic additives which are usually added to commercial plastics to lengthen their lifetime. Additives like anti-oxidants cannot be used for fear that they will contaminate the body. That then raises problems of enhanced sensitivity to degradation, especially thermal degradation during moulding, for example. UV absorbing additives present the same problem of leaching, toxicity and the chance of degradation before use. As if those problems were not difficult enough in themselves, there is another problem: sterilization. All devices to be used within the body must be totally sterile, so that no bacterial or viral contamination of the patient is possible. Equipment feed lines to patients must likewise be sterile, especially in the 89

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inner surfaces which make contact with fluids such as serum, blood, infusions of drugs or liquid nutrition. So how is it achieved? There are several processes currently in use: heat, ethylene oxide gas and gamma radiation. Each represents a different way of killing bacteria or viruses lurking on products, but exposure times and dose rates must be judged carefully to eliminate any possibility of affecting the polymer or polymers involved. Heat sterilization, for example, must be matched to the thermal behaviour of the polymer, not exceeding the Tg, and never the melting point of the material. Ethylene oxide is less aggressive, but cannot be used with polymers where there is any possibility of chemical reaction with the repeat unit. Gamma radiation is a highly energetic form of radiation, which can initiate degradation in sensitive chain molecules. Experiments prior to supplying new devices will normally show what doses are effective only against extraneous bacterial contamination. Whatever form of sterilization is used, must be prevented or eliminated with devices for medical use. It implies ‘clean room’ conditions of manufacture, with well-sealed moulding shops, positive pressure of the internal (filtered) atmosphere to prevent ingress of dust, and a very high level of cleanliness. The feedstock polymer is usually a specific grade developed for a particular product, with traces of metal catalysts (or any other remnants of polymerization which might be harmful) removed for the potential leaching risk. The moulding conditions must be chosen so as not to expose the hot melt to excessive temperatures when degradation starts to occur. And such traces might be difficult if not impossible to see by eye alone, remaining hidden unless special checks are made of product quality. Most regulatory bodies, such as the FDA (Federal Drugs Administration) in the USA and the MHRA (Medicines and Healthcare Products Regulatory Agency) in Britain, will insist on a programme of tests to ensure that a new product or device will not prove damaging to patients. The testing will usually include toxicity tests, integrity tests (such as for mechanical strength under expected loading conditions in the body) and in vivo tests as a final check on compatibility with the body. This might include tests using animals, the first balloon catheters being tested in this way, for example. For testing must be rigorous and demanding so as to assure the integrity of the final product. In reality, it does not always happen, as some of the following cases show very clearly. And there is always the chance of unexpected damage, not caught by the rigorous quality testing demanded of medical products.

3.2

Failed catheter

Catheters are such a common item in hospital practice that they are usually taken for granted by all who use them. They are the plumbing tubes for infusing patients with drugs in intensive care, but if they break, damage to

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91

the patient can follow, or worse. They are easy to manufacture by extrusion, where hot molten polymer is pushed by a screw through a narrow circular die. The bore is created by an internal cylinder (or torpedo) within the die so that the difference between them forms the wall of the tube. Catheters of varying stiffness can be formed by varying wall thickness, but also by varying the material of construction.

3.2.1 Thermoplastic elastomers Thermoplastic elastomers (TPEs) are a relatively new class of polymer that offer a wide range of modulus because their microstructure can be controlled during polymerization. They are often block copolymers, made by reacting two or more different monomers together in such a way as to provide two or more different types of chain within the same molecule. Because different polymers are usually incompatible with one another, socalled domains are formed of one of the polymers. In the first of its type to be made in the 1960s, SBS (Styrene-botadiene) copolymers form regular arrays of domain, which can be globular if styrene is the minor constituent (Fig. 3.1). Such domains act as a kind of physical cross-link because they anchor the flexible and elastomeric polybutadiene chains, the physical properties being much superior to polybutadiene alone. Creep is much reduced, so that shaped products retain their integrity. The stiffness of the

Polystyrene

Polystyrene

Polystyrene

Polystyrene

1000 Å

3.1 Microstructure of styrene-butadiene block copolymer.

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Forensic polymer engineering

3.2 Microstructure of block copolyester.

material is similar to that of PB alone, but if the styrene content is increased, the modulus increases in step (1). A different type of TPE is made from polyester and polyglycol (polyether) chains (2). The stiffness is in general greater than the elastomeric part alone at high polyester content, thanks to the presence of crystalline domains (Fig. 3.2) rather then amorphous globules, as is the case with SBS materials. Other TPEs, which have all the advantages of thermoplastics, but a greater range of stiffness, include many varieties of polyolefin, such as ethylene propylene copolymers, where physical cross-linking is achieved by crystallization of small stereoregular block of one or the other component (1). And there are also block copolymers of nylon and rubbery segments. Such a commercial material is Pebax (made by Elf Atochem), trade name for a range of nylon TPEs. Like the polyester TPEs, they offer advantages for catheters owing to the great range of wall stiffness, giving doctors greater manipulative control of IV (intravenous) and catheters designed for insertion into the body.

3.2.2 Accident at childbirth One application found for Pebax was in catheters for infusing an epidural anaesthetic into pregnant women during labour. The anaesthetic may be requested to ease the lower abdomen pain when giving birth. So how is it administered? The thin (1 mm outer diameter) catheter of length one metre, is sealed at one end, and three tiny holes created by a hot wire in the adjacent side for transmitting the drug directly into the spinal fluid of

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93

the patient (Fig. 3.3). The so-called distal end of the catheter is threaded through a hollow steel needle (a so-called Tuohy needle) which punctures the spine (Fig. 3.4). It emerges into the fluid core of the backbone, and the drug can then be drip-fed safely into the patient. After birth, the Tuohy needle is withdrawn, carrying the catheter with it.

3.3 Tip of thermoplastic nylon catheter showing bleed holes.

3.4 Tuohy needle used for epidural anaesthetic.

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But a problem occurred in 1990 when a Mrs K was giving birth to her first child (6). Following safe delivery of the baby boy, the needle was withdrawn but the catheter tip found to be absent. Inspection of the remaining catheter showed that the tip had broken away across the proximal infusion hole, and remained in the patient’s spinal fluid. Any operation to extract the small piece of plastic was out of the question, because surgical intervention might cause greater damage than justified. The tip was sterile and apparently presented no further risk to Mrs K. However, she thought otherwise, and brought an action against the hospital, and the makers of the catheter. In preparing expert reports, the failed catheter was clearly key evidence for the case, one way or the other. When first examined, the long length of remaining catheter proved to have been held in storage pinned to black card, and the proximal end through which the epidural fed showed signs of brittleness. Nurses remembered having problems attaching the proximal end to the drip, and had to tape the parts together. There were brittle cracks present here, and at several other places along its length. In addition, there appeared to be a slight yellowish tinge to the failed catheter. The expert acting for Mrs K examined the failed end of this length in a solicitor’s office with a hand lens, not an ideal way of assessing the evidence. He thought he could see traces of score marks running across the failed end, perhaps created by the catheter being withdrawn over the sharp end of the Tuohy needle. This should not have happened, because there are strict guidelines given to hospital staff that the Tuohy needle should be taken out first, and then the catheter withdrawn through the needle. In his opinion, the staff had been negligent by withdrawing the catheter first, and so damaging the end.

3.2.3 ESEM of the failed end The dispute entered a new phase when experts were appointed to act for the hospital and manufacturer. A joint meeting agreed that high resolution microscopy of the fractured catheter could help resolve the main issues, whether to confirm or negate the score marks claimed by the claimant’s expert. In one of the first uses of ESEM, the distal catheter end is shown in Fig. 3.5. Although covered with dust from the solicitor’s office, there appeared to be no trace of the score marks claimed by the claimant’s expert. Another new catheter had been damaged by withdrawal through the Tuohy needle, and its surface (Fig. 3.6) was quite different to the failed sample. It does show score marks from tiny defects in the sharp edge of the needle blade, and cut debris at the edge. So how had the catheter actually failed? Another part of the proximal end was also examined, and its failure surface examined using conventional SEM (made conducting by a thin gold film). It exhibited a brittle glassy fracture over most of its end surface, and a

Polymeric medical devices

3.5 ESEM image of the fractured distal end of a catheter showing bleed hole in section.

3.6 ESEM image of the distal catheter end of another catheter.

95

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Forensic polymer engineering

3.7 Brittle fracture in proximal end of failed catheter.

longitudinal crack, confirming what the nurses had said. Most interesting however, was the presence of a ductile tear of very similar type to that shown in the failed tip (Fig. 3.7). Our interpretation of the features shown by the distal end (Fig. 3.8) indicated that the part had fractured in a mainly brittle, but also partly ductile way. There were two large flat zones next to the infusion hole with a thinned and torn part at the furthest extremity of the surface. There was no trace of the cut marks at all. Far from indicting the hospital staff, the evidence at both the distal and proximal ends showed a brittle catheter. Such a device should remain tough and ductile in response to loads, so how could it have become brittle?

3.2.4 Material and mechanical testing There was now some basis for tests to check the material quality of the pure polymer. It would be a multifold attack, first tensile testing of the remaining length of catheter, and second by infra-red spectroscopy near the brittle part of the tubing. Density tests showed little difference between the new and failed catheters, while DSC showed a single melting point at about 170°C in both samples, consistent with a separate and intact polyamide phase. However, there was a large difference in the heat of fusion (ΔHf) a measure of the degree of crystallinity, as measured by the area under each peak of the new and failed catheters (Fig. 3.9). The upper curve of a new catheter showed a much smaller melting peak than that of the failed catheter, and the melting points were slightly different: Standard new catheter Failed catheter

Tm = 184°C, ΔHf = 33 Jg−1 Tm = 171°C, ΔHf = 45 Jg−1

The results were confirmed independently by the other experts, but what do they indicate? One possibility is that polyether chains degraded in

Polymeric medical devices

Ductile shear Lips

Brittle fracture zone

Inner lip

Tear zone

Longitudinal scratches

97

Hole

Ductile fibre

Tear zone Ductile fibre

Main ductile fracture tip Brittle fracture zone

Shear lips Possible origin (mirror zone)

3.8 Fracture surface map of broken distal catheter end.

2 33.38 mJ/mg 170.34 °C

Heat flow (mWatts)

1 0

1.10 mJ/mg 35.08 °C

–1

40.62 °C

166.73 °C 44.89 mJ/mg

–2

184.11 °C

1 2

–3 –4 –5 –6 171.10 °C

–7 –8 0

50

100

150 200 250 Temperature (°C)

3.9 DSC curves: new at top, failed below.

300

350

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length, so fewer nylon blocks were held by polyether chains, and so free to crystallize, increasing the heat of fusion. The small drop might indicate a loss of nylon block chain length too, because polymer melting points decrease with decreasing chain length. The length of catheter provided several samples for straining to failure in simple tension, the results showing the catheter to be much weaker than a new catheter tube. Samples from the proximal end were too brittle to withstand bending around the grips of the tensometer, but the distal end samples were tested successfully. The results on two such lengths of catheter were as follows: Mean tensile strength Mean extension to break

= 8 Newtons = 15%

These results could be compared with five results obtained on new catheter: Mean tensile strength Mean extension to break

= 28.2 Newtons = 650%

So the new polymer exhibited a tensile strength well over three times the strength of the failed catheter, with no evidence of yield at all in any of the samples tested. Conventional infra-red spectroscopy yielded very little, with the spectra from both new and failed samples being effectively identical, a common result for polymers which may be in the early stages of degradation. However, the expert for the manufacturer chose a new method to analyze a very small piece from the failed catheter. It was FTIR microscopy, a technique which had just been developed by an instrument maker. It involves passing an infra-red microbeam on a chosen area of the sample in an optical microscope. The spectrum shown in Fig. 3.10 compared with a new sample of catheter. Although the spectra look similar, there are in fact subtle but significant differences, as noted by the arrows. Slight shoulders on bigger peaks indicate traces of compounds not present in new polymer, and their position pointed towards low molecular weight esters produced by photo-oxidation (possibly UV attack), as suggested by an independent survey carried out by French workers in the 1980s (3). Although the FTIR experiment was carried out by the expert acting for the manufacturer, the tensile tests were carried out in the presence of all the experts, so could not be disputed later when the case went to trial. Some NMR spectra were obtained on catheter material, indicating that the polymer comprised polyglycol and polyamide 12 chains of structure: PTMEG, polytetramethylene glycol or —[CH2CH2CH2CH2O]n— and Polyamide 12 (nylon 12) or —[(CH2)11—NH—CO]n—

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0.90

FLA 234-tip of wedge 2 0.85 600-wide end of wedge 0.80 0.75 0.70 0.65 0.60

Absorbance

0.55 0.50 0.45 0.40 0.35 0.30

1725 cm–1

1175 cm–1

0.25

1740 0.20 cm–1 0.15

1290 cm–1

0.10

FLA 600 B

0.05 0.00

FLA 234

1515 cm–1

1325 cm–1

–0.05 1800 1750 1700 1650 1600 1550 1500 1450 1400 1350 1300 1250 1200 1150 1100 1050 Wavenumbers (cm–1)

3.10 FTIR microscopy traces for good (top curve) and failed catheter with anomalies arrowed.

The melting point of about 170°C was consistent with the commercial brochure technical data supplied by Atochem (4). However, it was surprising that a wholly aliphatic polymer should be sensitive to UV degradation, simply because the only chromophore in the repeat unit is the carbonyl group in the peptide bond (—CO—NH—). On the other hand, the French workers found that photo-oxidation did occur, mainly within the polyether parts of the molecule. Alternatively, or in addition, the polymer may have been exposed to excessively high temperatures during moulding. Hydrolysis was less likely, since the nylon blocks appeared unaffected in the failed catheter. Although GPC would have revealed the extent of chain breakdown, results were not obtained in time before the dispute was resolved.

3.2.5 Degradation theory The sum total of all the tests (DSC, tensile and FTIR) now pointed to chain degradation of the catheter, but at what stage? There was no evidence that it had been carelessly exposed at the hospital to direct sunlight, and in any case, ordinary window glass screens out the most harmful part of the UV spectrum in sunlight. It was more likely that degradation had started earlier in its history. More information emerged from the manufacturer, enabling a flow diagram of the probable sequence of events in its life to be constructed (Fig. 3.11). There was no evidence that any other catheters made from the two batches, FLA 234 and 235, had degraded in a similar way. Moreover, quality tests at several stages had not detected any problem, so

100

Forensic polymer engineering Dates Not known

Unknowns Polymerization

QC methods

Transport of granules

Not known

Extrusion in Ireland two batches FLA 234 and FLA 235

Coil storage extrusion QC, temperature control, light exposure

Coil transport

Not known

Cut to length, washed in freon (hot?)

Incoming QC tests, light exposure, auto or semi-auto

Internal transport

Not known

Heat sealing of one end (T > 170 °C?); three side holes made by hot needle (+ultrasonics?); printing of depth marks

Temperature control, light exposure, auto or semi-auto, QC checks

Internal transport

Not known

Hand coiled, packed by hand, packs packaged and boxed, then sealed

QC sampling, final checks, light exposure, human error

Transport

Not known

Gamma radiation of whole boxes to sterilise, 2.5 MRad, continuous exposure

Radiation control, checks on radiation levels and dose

Not known

Storage at warehouse

Seals intact?

Not known

Storage at hospital

Seals intact?

Use and failure

Human error?

3.11 Flow sequence of catheter manufacture.

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how could it have happened? The fact that the brittle areas were isolated even on the one metre length of the failed tubing produced one possible explanation. It was this: extruded tube would have been stored in coils, and it is possible that the failed catheter was made from a length of tubing on the outside of the coil, where it might have been exposed to direct sunlight. Perhaps it occurred just after extrusion or at some stage in transport. A brief exposure to sunlight might have been enough to start the degradation in a small way, but then accelerated by gamma sterilization at a further stage in its manufacture (Fig. 3.11). No anti-oxidant or UV absorber would have been used owing to the problem of leaching, so leaving the polymer unprotected. High moulding temperatures could also have enhanced the onset of UV degradation. An alternative possibility could involve ESC or SCC, but no data was available on the fluids which are known to crack the material. It is assumed that the manufacturer would have tested the polymer against known medical fluids to check its resistance, but there was always a chance that a new fluid cracking agent contacted the catheter after its removal from its protective packaging just before use.

3.2.6 Conclusions The action proceeded towards a trial in the High Court, although all the evidence showed that a manufacturing defect was the most likely cause of the accident: • • • •

The nurses discovered the proximal end to be brittle after it had been inserted. DSC showed a big increase in heat of fusion, and a lowered Tm compared with a new catheter. FTIR microscopy showed traces of degradation products. Failed catheter showed low tensile strength in tests.

There was still substantial disagreement between the experts, despite a meeting held between them. However, some new evidence emerged after the meeting, and just before the trial was due to commence. There was an unexplained feature of the damage to the catheter tip. What load could have caused the fracture to have occurred? After all, there should be no load at all if the catheter is enclosed by a hollow steel needle, and the end is simply resting in the spinal fluid. The answer came from a sample of a used catheter from a recent successful birth by epidural. The tip was intact, but was distorted at the tip: the consultant reported that it had been compressed by the adjacent vertebrae, and been deformed by the compressive load (Fig. 3.3). There was no sign of brittle cracking. This fact helped resolve the dispute, and the case was settled just before the trial was scheduled to start. The claimant received damages from the manufacturer, and

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the hospital was exonerated. The brittleness at the proximal end had only been discovered after the tip had been inserted into the spinal fluid, and the birth could not be suspended until a new catheter had been fitted. A literature search failed to find any other reported examples of Pebax catheters failing by brittle cracking, but informal evidence for cracking of other catheters came during the 2001 ANTEC conference in the USA, when a short paper of this case study was read in the Failures Analysis and Prevention section (5). One delegate mentioned that he had had a similar intermittent problem with HDPE catheter tubing, also probably caused by UV exposure. Great care in manufacture of medical grade polymers for catheters is clearly needed.

3.3

Failure of connectors

Intravenous (IV) catheter lines find extensive use in intensive care, and for drip feeds to many other groups of patients (the elderly, chronically sick and premature babies, for example). It is natural, then, that systems have been developed for allowing different drugs to be fed through the same tube, for other fluids to be supplied, such as serum and TPN (total parental nutrition, a synthetic equivalent of milk). Multiple supply implies use of junctions (Y-junctions, for example), connections and ways of supplying drugs via hypodermic needle. There are many such medical plumbing systems available to medical staff in hospitals, and indeed for self-medication to chronically ill, but stable patients, who have been transferred home. Many systems were developed in the 1980s and 1990s, and are still being actively developed further. Different materials have been used for the catheters of such systems, including silicone rubber, for example. It is a very stable polymer, is stable to relatively high temperatures and inert to most body and medical fluids. It is normally supplied as cross-linked tubing for extra dimensional stability. The catheter ends are supplied with connectors to enable infusion of drugs via hypodermic needle, which typically comprise a rubber seal embedded in a plastic connector. The needle can be pushed through the seal and retracted, the rubber relaxing back and so apparently providing a secure way of delivering a measured dose with no extraneous contamination.

3.3.1 Connector failures However, there have been problems with the quality of the thermoplastic fittings. The connectors at the ends of the catheter are often injection moulded from polycarbonate, and in some cases have shown brittle cracks. Such cracks are difficult for medical staff to spot in time, and can lead to bacterial contamination of the fluid supply to the patient. The problem was

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highlighted by investigators in the early 1990s when large numbers of such devices first started to appear on the market (7). Splits in hubs were often encountered, especially in connections known as luers (from the first user of such devices), where a smooth conical end is pushed into a female socket. The problem is the hoop stress applied by the pushing action, tending to initiate cracks from defects on the edge of the joint or elsewhere (such as poorly formed gates, the point where plastic has been injected in the tool of the moulding machine). In addition, any variation in fitment dimensions will put extra hoop stresses on the female socket. Another device uses a screw fitting, where the male part is twisted into the female luer. However, there are several problems with this fitting, too. Such fittings are covered by an ISO standard (8), but there still appear to be problems of fitment. There are two problems. One is that screw fittings are insecure unless some means of locking the fitting can be made. The problem is well known to motor engineers, and various devices have been developed to stop the joint unscrewing. The second problem is the fit between connectors from different suppliers. Owing to small dimensional differences, the joint can unscrew quite readily, so making the composite joint unsafe.

3.3.2 Premature cracking of connectors The problem of premature cracking of connectors became critical when one design was introduced into the British market in the mid-1990s (9). The connectors are designed to join lengths of catheter, and are typically used in Hickman lines, which are usually made in silicone rubber (Fig. 3.12). Each device is made by welding two parts of the outer casing together to form the final shape (Fig. 3.13). The device is 25 mm long and 10 mm at its widest point. The case conceals an inner working mechanism, which

3.12 Hickman IV line fitted with polycarbonate connectors.

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Forensic polymer engineering

3.13 Cracked joint in polycarbonate connector.

4 mm

3.14 Section through connector to show internal structure.

consists of a stainless steel helical spring behind a latex rubber seal. The stainless steel tube within the spring runs along the centre line of the design and provides the pathway for the liquids used in the IV line (Fig. 3.14). The rubber seal has a re-sealable cut at its centre to allow luers to be inserted easily for a new line. Such a female luer also has an external screw thread

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for secure attachment of the connection. The other end of the device consists of a male luer which can be connected to the main line, and is also fitted with a screw thread. The device was tested to standards current in the early 1990s (8), and appeared to fulfil all requirements. The device offered a sealed unit so as to prevent contamination of the central feed line by pathogenic bacteria. The problem of infection in hospitals is well known, and hospital authorities have been tackling it by a variety of routes including cleaning and disinfection of working surfaces, improved staff hygiene and so on. IV lines need extra special care since the lines offer direct entry to the body to pathogens, bypassing the normal defences of the body. Lines carrying nutritional fluids such as TPN (total parental nutrition), a synthetic liquid equivalent to milk, are especially at risk since they offer nutrition to bacteria as well as the patient. That the design was faulty emerged later (9), during a court action brought by the mother of a premature baby born in a hospital in the southwest of England in 1995. The premature baby was fed intravenously via a Hickman line, but suffered infections in the early part of 1996 when the new connectors started to be used by that particular hospital. According to nurses, doctors and the mother herself, the connectors kept cracking, and would last no longer than a day. Sometimes brittle cracking was so bad that they had to be replaced even more frequently. On one occasion, when the baby was being transferred between hospitals by ambulance, the Hickman line snapped and was retained by the mother after surgical removal (Fig. 3.12). It was to play a central role in the subsequent proceedings. Both of the green polycarbonate shrouds on the connectors exhibited brittle cracks (Fig. 3.13). In June 1996, the baby contracted meningitis while cracked connectors were still in use, and almost died. It later was found that the little boy had suffered brain damage which medical experts assigned to the near fatal episode in June 1996. The mother then started proceedings in 2002, accusing the hospital of negligence in using such devices, and the manufacturer for supplying faulty products. The surviving samples retained by the mother were examined using macrophotography but were not subjected at that stage to the more revealing methods of optical or electron optical microscopy. The solicitor in charge of the case was very protective of the samples, and it later transpired that they were the only surviving examples of many similar failed connectors. The low magnification survey showed that brittle cracks were present at the gate of the green shroud, where molten polymer was injected during manufacture. The cracks were entirely brittle and extended over large parts of the outer shroud. No examination was made of the inner parts of the devices, since it would involve dissembly of the joint, involving extra stress on the samples.

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Forensic polymer engineering

Brittle cracks tend to occur at the gate because this is where frozen-in strain or chain orientation is greatest, a common problem with many polymers and where polycarbonate is especially susceptible if moulded incorrectly. Early work with mining products had shown that polycarbonate battery cases cracked in a similar way when exposed to organic solvents (methylene and ethylene chloride) used during solvent welding the product (see Fig. 1.2 and Chapter 4). It was concluded at an early stage in the investigation that the connectors had been poorly moulded, although their birefringence could not be measured directly since they had been filled with pigment.

3.3.3 Disclosure Several years later, in 2006, the case advanced with the claimant asking for disclosure of design, manufacture, testing and failure documents from the defendants. The failure reports from UK hospitals made interesting reading. There were records from first introduction of the device in 1994, of numerous and sometimes distressing complaints from many different hospitals of the problem of cracked connectors, and the MDA (Medical Devices Agency, precursor to the MHRA) was asked to investigate. However, the problem was not tackled until many months later by the company. The first failure reports made by internal workers at the manufacturer were unimpressive: • • •

reports lacked a systematic approach with only two photographs given in evidence users were blamed for ‘forcing’ joints open with forceps no samples were preserved.

The extensive literature on polycarbonate seems to have been ignored. So as a direct result, no serious action was taken to either withdraw the faulty products from supply to hospitals, or to re-examine the design and manufacture of the connectors. Even when a new design using different polymer was introduced in early 1996, the older designs continued to be used by hospitals, including that where the premature baby was being treated.

3.3.4 Literature In fact, there had been many warnings published in the technical press about the problems of using polycarbonate in luers and connectors of the type made in France and the subject of the investigation, and they were published before the French design was launched. Particular warnings were

Polymeric medical devices

107

expressed in papers presented to the Failure Analysis Group of the SPE (Society of Plastics Engineers) at their annual ANTEC conferences. Stubstad was one of the first researchers to warn of the problems of premature cracking of polycarbonate luers in a paper at ANTEC 1992 (10). He reported that female luers were susceptible to lengthwise brittle cracking owing to the hoop stress imposed by the incoming male luer, especially if there were any dimensional differences between the two. However, the underlying problem could be cold moulding, where high chain orientation exists in the luer, and encourages brittle cracking (11). Failures in clinical situations were also being reported in the medical literature, and were detailed in a review in Neonatal Network in 1992 (12). The method examined was ECMO (extra corporeal membrane oxygenation), where, of 445 accidents, 45% caused loss of blood, with 55% involving cracked circuit components like connectors. A specific accident involving polycarbonate is detailed in another paper (13). ‘Bathing alcohol’ was accidentally spilled onto a polycarbonate casing to an oxygenator, causing it to crack. The fluid was composed of 70% ethyl alcohol, 1.6% acetone and other organics. The oxygenator was being used during a heart operation on an elderly man, and was life-threatening. Many other papers of the 1990s reported cracked connectors, but failed to identify the polymer used in the devices. For example, a hub on an epidural catheter connector cracked and emergency methods adopted (14).

3.3.5 Joint expert examination As trial approached (set for May 2008), the experts engaged by each of the three parties to the action, organized examination of the connector remains. Optical microscopic inspection of the connectors was the first to be used; both the external shroud and the inner recesses of the two devices were examined. The results were dramatic because they revealed two key features which were not obvious in the original inspection: 1. many surfaces were contaminated by stains and particles, and 2. brittle cracks extended to the inner recess. The traces of yellow stains over much of the exterior and some of the inner parts of both connectors implied that they were original to their period of use (Fig. 3.15). They were probably urine stains from the baby, since the connectors would have been in close proximity to his body, probably lying on his skin. The particles trapped in the sharp facets at the remnant of the gate were probably traces of faeces and coagulated blood. The fracture surfaces of the cracks were also contaminated both with the yellow stain and by particulate matter, so that contamination and crack formation were probably contemporaneous.

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Forensic polymer engineering

3.15 Optical micrograph of gate showing contamination.

2 mm

3.16 Connector end showing extensive cracking.

Contamination extended to the inner recess of the connector attached to the red end of the Hickman line when the joint had been separated. But more important, the tip of the male luer exhibited a brittle crack very close to the part of the device where the inner steel pipe ended (Fig. 3.14). This observation was unexpected since it had not been seen before (since the joint had only been dissembled once, and the tip not examined in detail) but showed that brittle cracking was much more extensive than had been appreciated. That conclusion was reinforced by the observation of brittle cracks in the base of the recess (Fig. 3.16). Clearly, the external cracks had

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109

penetrated the body of the device and allowed contamination to enter the recess. The second male luer showed little contamination but a brittle crack near the tip. The quality of the seal was tested using air pressure and a test in water appeared to show no leaks of air (a test familiar to cyclists for a leaking inner tube). However, it was not at all clear whether such a test would have shown a hairline crack which had penetrated through to the bore of the tube at the tip owing to the slow rate of air movement through such a small lesion (also familiar to cyclists when a tiny hole exists in the tube: a wellknown cause of slow punctures). If the cracks had extended into the side of the tip then there would have been a path for pathogens to enter the bore of the tube where contamination of the TPN feed or drug line was possible. The three experts followed up by re-examining the same samples using ESEM at the University of Surrey shortly afterwards. Although constrained by the limits of the specimen stage, the exam confirmed the existence of large cracks at the base of the recess of the male luer (Fig. 3.17). The crack in the base was about 20 micron wide, and thus easy for 0.2 micron diameter bacteria to penetrate. Although the experts did not find a crack at the tip of the male luer, the top of the steel inner pipe appeared misplaced in the polycarbonate moulding. No X-ray analysis was carried out at the time which might have revealed the nature of the contamination clearly seen in both optical and electron microscopy. Similar results were obtained with the other sample.

600 μm

3.17 ESEM of inner crack in connector.

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Forensic polymer engineering

3.3.6 Injection moulding Consideration of the bundles of documents disclosed by the manufacturer produced some moulding records, but they were not contemporary with the first design of the connector, and in fact dated to several years later when the company returned to use polycarbonate after using polyester. Those records showed that a tool temperature of 80°C was being used currently, a temperature within allowable limits (according to technical brochures from the manufacturers, and our experience of the problem). No further moulding details were forthcoming despite repeated requests, and it was said by the defendants that ‘such moulding details would be recorded on “a scrap of paper” ’. Even in the early 1990s, most injection moulding machines were fully computerized, and all setting conditions (melt pressures and temperatures, tool temperatures, etc.) recorded automatically. The reason why it is important to record such details is very simple: when it comes to repeating a batch run, it is essential to use the setting conditions already established for that product. It was likely that the original design had been cold moulded, producing high levels of chain orientation in the polycarbonate parts of the device.

3.3.7 ESC/SCC hypothesis Environmental stress cracking (ESC) or SCC seemed the most likely cause of the brittle cracks seen in the retained samples, but no records had been kept by the hospital of what fluids the connectors had made contact with during service. Polycarbonate is sensitive not only to solvent cracking such as those seen on battery cases, but a range of other common liquids that are likely to be found in hospitals, as the literature search had shown. It was very unlikely that the connectors came into contact with methylene or ethylene chloride, but much more likely that cleaning fluids used on wards may have made contact. Such liquids as bleach (sodium hypochlorite) and strong detergents are used for disinfection, and contact with them could initiate brittle cracks. They do so by stress corrosion cracking (SCC) rather than ESC, since the alkaline content will hydrolyze polycarbonate by attacking the carbonate group in the repeat unit. Indeed, methanolic KOH or NaOH degrade the material extremely rapidly back to monomer, a method for destructive removal of the polymer (15). Other organic fluids might also include acetone, organic alcohols and ethers, which can act as ESC agents (16, 17). However, it was known that TPN itself can attack polycarbonate, knowledge which was publicized before 1994/5 especially in the US literature (10), and so the company should have been very wary of introducing the connectors without extensive testing in TPN. Stubstad, for example, in his 1992

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paper (11) had referred to the problem of cracking in ‘fat solutions’, which includes TPN. Other liquids containing lipids and used for carrying drugs were also a problem, judging by the failure reports from UK hospitals in 1994/5. However, it was difficult to explain how a fluid in the bore could have caused such extensive external cracking in the retained connectors. Unfortunately, the manufacturer had disposed of all failed connectors which had been sent back for inspection by many hospitals (and the MDA), so that there was no prospect of a more complete analysis of the problem. It was also of some interest to observe from the numerous hospital failure reports that detection of cracking usually occurred by nurses seeing fluid leaking from the devices, so in many cases of reported failures, cracking must have linked the bore with the external world. Even the external cracks visible on the retained connectors were not spotted either by the baby’s mother or the solicitor dealing with the case when the action was first started, such is the small size of the device, and the difficulty of observing small hairline or even partly open cracks (Figs 3.12 and 3.13). Given the literature failure reports, a survey of the FDA website (18) showed a number of reported failures of luer connectors on the ‘Maude’ compilation. The records (made anonymously by medical staff) showed failures of IV sets in 1991, both involving fracture or detachment of luers. Several reports mentioned failure of the French design, although the records did not provide the detail needed to pinpoint the exact cause of fracture. FDA enforcement notices also showed that a number of recalls were made in the same period. The French company issued a recall of 60 000 infant feeding systems in France in 1994 because the ‘end cap may become loose’. A larger recall of 60 million was made by another company in the same year owing to cracks produced by certain solutions in the female luer. Two further recalls were made in 1996 and 1998, the first being a recall of the subsidiary of the French company of 3068 neonatal catheters due to cracking in the female luer lock, and a smaller similar recall made in 1998. So recalls could be adopted by the manufacturer in question, although none appeared to have taken place in the UK.

3.3.8 Discussion It was most likely that the brittle cracks seen on many connectors, and in some examples quoted by hospitals, leading to total disintegration, were caused by ESC or SCC or by a combination of both failure modes. Although the cracks on the retained connectors had not apparently reached critical state, they were very close to penetrating the inner bore of the feed tube (Fig. 3.16). They supported the mother’s contention that connectors were cracking on a daily basis, needing regular replacement before they in turn had to be replaced.

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The experts for the defendants resisted these conclusions, however, despite the advice given by the court to all experts to act independently of their clients’ wishes. The net result of several expert meetings produced a much more reasonable document just before trial, allowing the lawyers to proceed to a fair settlement without the need for what would have been a very expensive trial. There were up to 30 experts on all sides in the action, most of whom were medical experts rather than scientists or engineers. Substantial damages were paid to the mother of the disabled child.

3.3.9 Balloon catheters and angioplasty A life-saving operation which emerged during the 1980s is angioplasty, where a folded thermoplastic balloon is inserted into the artery of a heart patient (Fig. 3.18). Typically, the patient is suffering from blocked arteries, where fatty deposits accumulate on the walls of a blood vessel (artherosclerosis). They not only restrict the blood supply, but fragments can break away and cause strokes or heart attacks. The folded balloon is carried on a flexible probe, a hollow tube carrying a guidewire for manipulating the device when being threaded through the artery to a blockage or restriction on the artery wall. The passage of the probe is followed using an X-ray body scanner or similar device, and when the balloon-carrying tip reaches the affected part, it is slowly inflated to 6–8 bar so as to crush the restriction and so improve blood supply. A further development of the device involves threading a hollow stent over the balloon. It is normally a perforated metal cage and is designed to expand when the balloon is inflated. At its maximum extent it is in close contact with the artery wall, and should remain there

3.18 Balloon catheter and guide wire for angioplasty operation.

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when the balloon is deflated and withdrawn at the end of the procedure. The stent remains in place because the metal has been deformed plastically. It supports the artery wall, where weakness may have developed over time because of the build-up in fatty deposits (plaque). The technique was first developed by Gruntzig working in Switzerland (19), and further developed by Drs Palmaz and Schatz, surgeons working in Texas (20, 21) (Fig. 3.19). The stent technique is widespread, with many different designs available, and it is estimated that over one million such operations are carried out every year. It can eliminate the need for open-heart surgery, with all the risks involved. It is minimally invasive, involving insertion of the catheter through a small incision in the groin of the patient (with just the use of a local anaesthetic) to the spot where it is needed (Fig. 3.20). It has a high success rate, and is literally a life saver.

3.19 Compact and expanded stents.

3.20 Expanded stent in artery acting against fatty deposits.

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There were two major medical problems encountered with the method when first used. The deposits could grow back again (restenosis), requiring yet more intervention. And secondly, if the fatty deposits are old, they are frequently hard and calcified. It is difficult to compress the deposit, and more drastic methods must be used. Several other mechanical problems have also been encountered. Balloon catheters can be made from a variety of polymers, including PVC (first used by Gruntzig), amorphous PET, PE and copolymers, but although tough and reliable balloon materials, they can fail under internal pressure from intrinsic defects or defects formed during the procedure. Failure is more likely when a stent covers the balloon, because of a hard metal structure in close proximity to a softer material. Although sharp parts are obviously avoided, they can arise if failure of the stent occurs (21). An elaborate kit is needed to fish the broken parts from the artery. Use of balloon catheters and stents has been extended to the many other passages within the body, following the widespread development of endoscopy to explore the body. It seems clear that such operations will grow in use as the technology develops, reducing the need for major invasive surgery.

3.4

Failure of a breast tissue expander

There is a large range of implants available to surgeons for replacing diseased tissue which needs to be removed. Many employ silicone polymer for its inertness in the body, and low modulus compatible with those of body tissues. One such device is the breast tissue expander. The balloon device is designed to be implanted after mastectomy under the chest muscles, and gradually filled with saline solution via a bulb connected to the balloon. When complete, the device will be removed and replaced with a permanent breast implant. Silicone elastomer is reinforced by PET fibre (at the rear of the balloon), with a silicone catheter connecting the balloon to the bulb implanted just under the skin above. The major problem with silicone rubber, however, is its very poor mechanical properties, especially in tension.

3.4.1 Failure of tissue expander The consequences of failure of implants are always serious for the patient, involving trauma and loss of saline into her body. Just this happened to one woman one night after several weeks fitment of the device following mastectomy. The device had been filled at regular intervals and was apparently at or near capacity. The patient had already experienced the psychological shock of discovery of cancer, and loss of her breast, so the sudden loss of

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3.21 Fractured breast tissue expander.

her shape was severe. On visiting her consultant, the device was extracted under anaesthetic and found to have fractured where the catheter joined the bag (Fig. 3.21). The bag was then made available for independent examination. A check was made using FTIR, showing there to be no apparent problem with the polymer, all absorption peaks observed corresponding with the known spectrum for polysiloxane. However, the bag was supplied in a contaminated state with sodium chloride crystals visible on the inner surface in addition to congealed blood. There were relatively few absorption peaks owing to the thickness of the sample (ca 100 micron) However, in order to preserve the device intact, it was necessary to fold the membrane for insertion into the sample chamber of the spectrometer. The bending stress at the fold created a tear, showing the poor strength of the material when subjected to relatively low loads. Optical microscopy showed that the critical fracture extended across the catheter where it joined the bag, and showed how the fracture extended between two shoulders from the bag extension, one above and the other below the crack surface (Fig. 3.22). The survey confirmed the lack of clear features on the fracture surface itself, although a cusp was found at one edge representing the junction of two brittle cracks emanating from a common single origin on one side of the surface (the cusp is at right in Fig. 3.22, with possible origin at left near the shoulder). ESEM was needed to search for possible defects not detected in the optical microscope. An oblique shot confirmed the fracture to be relatively featureless (Fig. 3.23). There was no evidence for fatigue striations on the surface, so slow intermittent failure across the catheter could be excluded. The zone near the join with the bag showed many defects, and indeed can be just seen at a deep cleft on the right (G) where the bag joins the tube.

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3.22 Oblique view of fracture surface showing cusp.

G

3.23 Oblique view of fracture in ESEM.

There appeared to be microcracks present here, which might explain how failure occurred. A more extensive survey revealed more cracks wherever the catheter met the bag, suggesting that either severe stress had created the cracks, or that these zones were inherently weak (Fig. 3.24). A set of three cracks was seen in the neck near the origin (Figs 3.25 and 3.26) but they were not oriented to initiate a critical crack across the catheter. Just below was the remnant of a larger crack oriented at right angles to the first set, and which was very close to the main critical crack, suggesting that it initiated the final failure (Fig. 3.26). The many sub-critical cracks present in the sample suggested that the load of the whole bag was concentrated at the interface between the bag and catheter, and probably further raised at the sharp corner (Fig. 3.27). The interface might be regarded as

Polymeric medical devices

3.24 Close-up showing cracks at interface.

O

3.25 View of origin of main fracture.

C2 C3

3.26 Close-up of origin.

C1

117

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Forensic polymer engineering Upper shoulder

Main fracture surface

O Microcracks

Crack in adhesive layer

Lower shoulder

Tube Microcracks

1000 μm

3.27 Fracture surface map.

Table 3.1 Record of fills of breast expander Date of fill

Volume of saline added, ml

Comments

Aug. 8

250 cc

Operation with 550-ml capacity bag inserted by surgeon

Aug. 22 Sept. 5 Sept. 19 Oct. 3 Oct. 10

50 cc 100 cc 170 cc 100 cc 150 cc

post-Oct. 17

–

Total 400 cc added Total 570 ml added Total 650 cc [sic] added (expander is probably leaking) Patient experiences total loss of fluid from bag

the weakest zone for other reasons: it is where the catheter is adhesively bonded to the bag, so if the adhesive, itself a silicone polymer, had been poorly cured, then problems could follow.

3.4.2 Loading pattern So microscopy had provided good evidence that the device had failed through poor manufacture. But the history of the device was rather more complicated than at first thought. The sequence of infusions of saline solution would be important in explaining why the device failed. The notes of the patient’s consultant showed a steady increase in volume of solution added to the nominal capacity of the bag of 550 ml (Table 3.1). The bag had been fitted by the consultant with 250 ml already present in the bag, and was followed by a further increment of 50 ml on 22 August. A 100 ml

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portion was added on 5 September, giving a total of 400 ml added to that point. Then on 19 September, a volume of 170 ml was added, exceeding the nominal capacity by 20 ml. It was after this third addition that the patient, Mrs H became disturbed because her chest did not appear to have grown. However, she did not inform her consultant. Further additions were made in October of 100 and 150 ml, and the consultant (presumably observing no increase in bag size) commented that the ‘expander is probably leaking’. Some time after 17 October, Mrs H experienced total loss of volume, and on visiting the surgery, the device was found on exploration to have fractured (Fig. 3.21). A check on the various additions was made by examining the elastomeric seal in the bulb. There were six puncture marks in total, confirming the testimony of the consultant (giving due allowance for a single mis-hit by the hypodermic needle). The rubber seal is designed to retract after puncture so as to retain the contents, although it is unlikely that any leakage occurred here simply because the saline drips down into a bag at a lower level. No defects could be found in the dome, in any case. The final total of 650 ml shown in the consultant’s notes was wrong. Her own records showed that a total of no less than 820 ml of saline had been added by then, well beyond the capacity of the bag.

3.4.3 Conclusion If the action had come to trial, there is no question that the uncertainty over the total solution added to the expander would have led defendant lawyers to attack the credibility of the consultant. A close relationship had grown up between the claimant and the consultant, and the former decided not to pursue the action for fear of indictment of the consultant. Such circumstances are not uncommon in medical litigation, where existing trauma can be deepened by open discussion of the case in open court. The case against the manufacturers, Mentor of California, was thus never tested openly, and no discovery made of previous problems with the device, or evidence about tissue expander design and manufacture. It is possible to suggest that the device was defective for the following reasons: • • •

the catheter possessed too small a diameter for the expected load of the full bag the critical zone where it met the bag was poorly made, with stress raisers present at the deep corners the adhesive silicone used to bond bag and catheter was probably overcured, causing embrittlement.

It is not known what quality checks were in place with the device, or what tests had been performed before introduction in the market. The facts pointed to a badly designed product, probably combined with poor manu-

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facture (22), a conclusion supported by evidence from the largest market for medical devices, the USA.

3.4.4 Other cases

Tensile strength (kgf/cm2)

Litigation in the USA has been very extensive for a long period following the introduction of breast implants there in the 1980s. The situation was the subject of a class action and Dow Corning declared for bankruptcy, with many millions of dollars being awarded in compensation, mainly for failures and leaking permanent implants. There were also claims for damage caused by leakage of silicone used to fill those implants. Whatever the merit of those claims, there is no doubt that many of the implants fractured in the body, and the devices (like the one featured here) had been under-designed for their role. In the final event (May 2000), the Federal authorities only authorized two manufacturers (Mentor Corporation and Inamed Corporation) to make these devices, and under strict government control (23). They use saline liquid as the infill rather than silicone gel. The design and manufacture of these devices has apparently improved, and other countries which also use them (such as the UK) will benefit from the tough attitude of the US authorities (24, 25). It is not difficult to see why design of silicone implants is so important. Figure 3.28 shows the nature of the problem (26). The tensile strength of silicone elastomer is very low at 25°C and certainly much lower than EPM (ethylene propylene rubber) or natural rubber (NR). The strength drops very slowly with rise in temperature (unlike the much steeper drops for other rubbers), but this is no consolation for users of the material at body temperatures (ca 40°C). The material is also very weak in repeated loading,

300

200 FPM 100 Si

0

50

NR

EPM

100 150 Temperature (°C)

200

3.28 Tensile strength of various elastomers.

250

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and will fatigue easily at low applied loads, so where body movement is normal and expected, product design must be conservative. It means eliminating or ameliorating stress concentrations (such as the deep corners where the bag meets the catheter, Fig. 3.21), using thick sections of material and ensuring adhesives are cured correctly. Many attempts have been made to strengthen silicone rubber using a wide variety of different fillers, but a solution is still awaited. In other implants, such as IOLs (intra ocular lenses), silicone is an excellent choice for the replacement lens. They are optically clear and replace a diseased lens, where cataracts have reduced if not eliminated vision in the eye affected. The silicone lens is rolled up and injected through a small slit in the outer eye covering; it then unrolls and fills the cavity. The device is only lightly stressed throughout injection, and almost unstressed when in the eye, so the chance of mechanical failure is very low.

3.5

Failure of sutures

Catgut is a traditional monofilament used to stitch wounds together, but there are now many alternative fibrous materials available. Another natural product, braided silk fibre can also be used, and individual doctors will make a choice appropriate for particular kinds of wound. Stitches which dissolve in body fluids to produce harmless products have been known for many years, the polymers of interest producing non-toxic monomers or starting units. One of the most widely used absorbable suture materials is polyglactin 910, a polysaccharide (commonly sold under the tradename Vicryl). The material is a copolyester of lactic acid and glycolic acid, both of which are harmless products easily excreted by the body: —[OCH2CO]— —[OCH(CH3)CO]—

glycolic acid repeat unit lactic acid repeat unit

The lactic acid content is about 10% in Vicryl, the repeat units being randomly distributed among blocks of glycolic acid. The crystallinity is lower than pure glycolic acid, but amorphous zones are needed to enhance breakdown by absorbing body fluids. It is hydrolyzed by body fluids at a rate comparable with wound healing, so has disappeared when it has finished holding tissues together and the tissues are self-supporting.

3.5.1 Wound opening Absorbable sutures are ideal for internal wounds, such as those made after childbirth, but things may not always go to plan. A Mrs P was recovering after successful delivery of a baby boy, using an emergency Caesarean section. The following morning, she began to bleed heavily and

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was transferred to the labour suite and underwent corrective surgery. However, she suffered continuing problems with the outer wound, which was oozing a pink sticky fluid. The conventional stitches were removed, but about 10 minutes after, she stood up, and the wound opened. She was horrified to see her intestines spilling out, an incident witnessed by her shocked partner. She subsequently brought an action against the hospital and makers of the Vicryl sutures used to stitch her uterus. The medical records show that following delivery of the baby, her uterus was stitched in two layers with Vicryl, and externally with Prolene (polypropylene). She lost a considerable amount of blood during recovery, about 1.44 litres (including that lost at delivery). An ultrasound scan showed her uterus distended with blood clots, about 1 litre being removed under general anaesthetic via her cervix. A clinical note made the next day stated that the ‘uterus had well contracted’. Her outer dressing was changed owing to oozing of fluid, but appeared to have diminished by the next day. Later that same day, the external Prolene sutures were found to be ‘digging into her skin’, so were cut and taped with paper sutures. The following day the wound was clean with only slight oozing, and the paper sutures removed. The attending midwife said that the wound was clean and dry, and the outer Prolene stitches removed ‘easily’. The notes then record wound dehiscence (opening), and she was transferred quickly to theatre where the wound was restitched by another doctor not involved in the original stitching. He made the observation that ‘..all the layers of the wound closure were still present, but the sheath suture had snapped in the middle..’ The statement made it clear that the Vicryl suture had not broken at the knots, and the knots had not slipped. But the failed suture had been discarded after the operation, making investigation of the evidence impossible, a not uncommon problem in medical negligence cases.

3.5.2 Analysis of new suture Although the failed suture had been unfortunately lost, equivalent new sutures were made available for inspection. One of the lengths was strained to break on a tensometer, by tying a granny knot to form a loop and then stringing the loop over round supports on the machine. Two tests gave the results: Tensile breaking load = 70 N, failure strain = 74% (in free fibre) Tensile breaking load = 58 N, failure strain = 96% (at knot) So there is clearly substantial variation in strength, depending on the knot, slippage being a problem and perhaps knot orientation as well. Knots are well-known stress concentrators in ropes and cords, and behave similarly in braided fibre. At the high rate of test, the broken ends showed

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3.29 Fractured test end of Vicryl suture.

melting of individual fibres to form blobs of solidified molten polymer (Fig. 3.29). On the other hand, the failure loads of about 7 and 5.8 kg are high compared with expected loads in soft tissue. The thermal properties of the polymer showed a main melting point of about 200°C, with subsidiary peaks at 180°C, 126°C and 73°C. It is known that the suture was coated with another polyglactide of different composition, probably exhibited by the large peak at 180°C while the smaller peaks represent lubricants such as calcium stearate. Finally the fibre proved rapidly soluble (several hours immersion) in strong caustic soda of pH 14, showing that alkaline conditions were needed for hydrolysis. Some body fluids are very slightly alkaline.

3.5.3 Possible causes of failure So how could the suture have failed at a critical moment? The company Ethicon publishes guidelines on their Vicryl suture (27), stating that the sutures retain 75% of their original tensile strength after 2 weeks implantation in rats, and 50% at three weeks. Since the sutures failed the day after they were emplaced, it was thus unlikely that they failed by dissolution in her body. However, the doctor who restitched the wound said that the Vicryl suture had fractured in the middle. Unfortunately the suture was discarded, so no forensic examination could be made of the remains, and his assertion could not be tested. It is unusual for cords to break centrally, fractures tending to occur at knots or other attachment points, as experiment had shown. Alternatively, the stitch could simply have not been tied correctly in the first place using approved knots and placements in the soft tissues to be joined together.

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Evidence from the staff involved in the original stitching was contradictory, but it was known that very junior staff had been involved when the patient was stitched.

3.5.4 Outcome The action did not proceed in the absence of clear evidence of either medical negligence or a defective suture, and the patient could not be compensated for her distress. It is unfortunate that the physical evidence of failure in many medical cases is frequently lost or discarded, perhaps because the items are disposable anyway. However, it may leave patients uncompensated, and product manufacturers uncertain of the state of their product. If the design and manufacture of products is to be improved, then analysis of failures is vital to determining the cause or causes of the specific problem in question. That task is impossible if the failed products are no longer available for examination. Even a photograph of a failed product is better than no product at all, for much can be learnt from good photographs of failures.

3.6

Failure of breathing tubes

The medical appliances market has developed greatly in recent years with the demand for ever better patient care both in hospital for acute cases, and at home for patients with chronic ailments. Respiratory illnesses are among the most common such ailments, and often require breathing apparatus for supplying the patient with humidified air or oxygen in a controllable way. The breathing equipment for such applications must be made to a very high standard, so that bacterial contamination of the bore is impossible. The case reported here concerns the quality of a large transparent sight tube developed for use in breathing apparatus, the material being injection moulded polysulphone. The alleged defects related to the quality of finish of the tubes, rather than any structural or functional problems. The manufacturer of the breathing apparatus brought an action against the toolmaker, alleging that the tool for making the tubes was insufficient for moulding sight tubes in polysulphone.

3.6.1 Development of sight tube The breathing equipment was already in existence when the decision was made to develop a moulded sight tube. The tube sat at the top of a longer metal pipe, and enclosed a float giving visual indication of flow rate in the tube (Fig. 3.30). The float must not fall below the lower marker so as to

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Topcap 181901 Filter 181902 Flow tube assy. 181924 Float rod 181907 Circlip 181908

Screws×2 M6×8

Float 181906

Patient inlet 30 mm 181909 Patient inlet bdy, assy. 181923

‘V’plate 181935

3.30 Section of breathing tube assembly showing float.

3.31 Original PMMA sight tube.

ensure air or oxygen is being sent to the patient. The transparent tube was machined from acrylic resin (high molecular weight PMMA) to a high quality, but at correspondingly high cost (Fig. 3.31). Not only was the process very labour intensive (and thus expensive), but it also required two parts attached together. Injection moulding would offer economies of scale and part consolidation. A tool was commissioned and the first prototypes in acrylic proved encouraging (Fig. 3.32), although many moulding defects were present, such as severe weld lines (the vertical lines either side of the tube). The decision was made to use UDEL polysulphone (made by Union Carbide), a strange decision in hindsight, given the high cost, and difficult moulding problems presented by this polymer. Apparently, it was felt that

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3.32 Prototype moulded sight tube in polysulphone showing severe weld lines.

alternative transparent plastics like polycarbonate (and acrylic itself) were too susceptible to ESC by attack from common fluids used regularly in hospitals, such as ether and alcohol. The manufacturer clearly envisaged high sales to justify the considerable capital investment needed for an injection moulding tool. Several moulders were then engaged to manufacture the tubes.

3.6.2 Faulty tubes Since the critical proof of the quality of a tool is in the moulding, the set of about 40 faulty sight tubes held by the plaintiff was central to the dispute. The tubes had been moulded by three different moulders. The tubes were examined for the defects alleged by the plaintiff, with very mixed results. It was clear that the moulders used by the plaintiff had experienced severe problems in moulding polysulphone, largely because of its high melt viscosity and Newtonian behaviour with increasing shear rate in the tool (28). Most common thermoplastics exhibit shear thinning, which means that the melt viscosity drops substantially in the runners of the tool, and thus makes moulding to shape much easier (Fig. 3.33). Thus LDPE and polystyrene are usually much easier to mould than polycarbonate or polysulphone because their melt viscosity drops fast as the shear rate increases in the narrow runners into the tool cavity. In addition, the tools must be held at high temperature to minimise frozen-in strain in the final product. Accurate temperature control is needed because the melt viscosity is more sensitive to change in temperature than other polymers. The raw granules must be

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(poise) 105

Viscosity

UDEL Polysulfone (P 3500) 350°C UDEL 104 Polysulfone P 1700 350°C Polycarbonate 315°C

103

10

Low Density Polyethylene 210°C Polystyrene 200°C 102

103 (sec–1)

Shear rate

3.33 Melt viscosity of polysulphone as a function of shear rate.

3.34 Flow lines in moulding shown by shadow (arrow).

dried before moulding to eliminate surface defects such as splay or splash marks. Such manufacturing constraints had not been considered by at least one of the moulders, who had needed to buy an oil circulation system to heat the tool so as to mould the polysulphone. Of the many defects seen in the forty or so sample tubes, most if not all were moulding defects caused by poor material preparation, cold tools, and inexperience in moulding this material. Figure 3.34, for example, shows flow lines in the barrel of the tube

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3.35 Flash marks caused by wet polymer granules (arrow).

3.36 Inclusions in moulding (arrow).

caused by poor temperature control, the flow lines being visualized by their effect on the shadow cast on graph paper. The tube shown in Fig. 3.35 displays splash marks caused by inadequate drying of the granules, an effect usually appearing on exposed surfaces, and clearly unacceptable for a sight tube where optical clarity was essential. Sink marks in the barrel were relatively common due to low pressures in the tool. Inclusions were seen in some samples (Fig. 3.36). Colouration of the samples was also highly variable (Fig. 3.37), showing a range of temperatures at which the melt and tool had been held. So what did the study show? Detailed inspection of the available tubes showed that all defects were attributable to moulding problems, and could

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3.37 Colour variation in set of mouldings, darkest arrowed.

not be blamed on the tool. To add insult to injury, several samples showed score marks caused by the operator levering the moulding from the cavity. Yet despite the overwhelming evidence of poor moulding practice, the plaintiff decided to proceed with the case.

3.6.3 Trial The dispute came to trial in London in the mid-1990s. But the trial itself was to yield yet further revelations. In the first place, the lawyer for the plaintiff had been briefed only at the last minute and had a poor grasp of the case essentials, so the opening speech had to be made by the defendant’s barrister! Then it was the plaintiff’s role to enter the witness stand and be questioned. The defendant’s barrister was very well equipped to cross-examine, and as is usual in such trials, much depended on the documentary evidence which had accumulated over the years during which the dispute had festered. There were reports from the moulders describing the problems of using the tool with polysulphone, but the judge noticed that the last sentence of the last page of one report appeared to be ungrammatical. One possible explanation (offered by the judge) could be that the report had been doctored by photocopying with part obscured by plain paper (perhaps to remove embarssing text), so would not be copied. The judge ordered that the original be produced in open court. It was never produced because the case took another dramatic turn. It was clear from cross-examination on the stand that the plaintiff could not remember key features of the details of the dispute, and when he returned for further cross-examination on the third day of the trial, he broke down and accepted an offer from the defendant to settle the case. The defendant withdrew his counterclaim in return

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for the plaintiff withdrawing his claim. But there was a sting in the tail of the case: the plaintiff had to pay most of the costs of litigation, including the large costs accrued and bringing the case to trial. The latter costs were greater than the original sum in dispute. So none of the voluminous technical evidence was ever heard in open court, although the judge had clearly read the salient reports in the action.

3.6.4 Lessons It was never clear why the manufacturer specified polysulphone for the sight tube. Although injection moulding would have lowered unit costs in the long term, it was still a risky venture and depended on achieving high sales of the apparatus. To specify polysulphone was even riskier because the injection moulders chosen were inexperienced in moulding the material. The tool was sufficient to mould the material, but the quality of the end product was mainly in the hands of the moulders and not the toolmaker. It was not a complicated product, having a simple shape. The only element of complexity lay in the screw threads at either end of the tube (requiring a rotating core), but this was not an insuperable problem in moulding. The dispute should not have continued to a full trial. It should have been resolved much earlier, and saved all the substantial costs of litigation. Indeed, when inspecting the tubes at the plaintiffs’ premises, it was suggested by one of us (PRL) that some form of mediation should be possible. It was rejected. Several attempts were made by the two experts to agree matters before trial, but none succeeded, the plaintiff’s expert supporting his client’s case on every issue. The role of experts should be totally unbiased, seeking to help the court on technical issues only.

3.7

Conclusions

There are many problems in tackling medical device failures. They include • • •

a reluctance to retain and store failed samples returned by hospitals the poor quality of reports analyzing samples (poor photography, or no photography at all) the tendency to blame users without investigating the problem in depth.

They are not normally expected in other areas of engineering failure (although of course not unknown), but tend to recur in case after case of failed medical products. That included the case of the cracked connectors, although two samples were preserved by the mother of the affected baby and supported her evidence. The failed suture disappeared completely, leaving the victim unable to prove negligence. In short, there seems to be

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little general awareness among product designers of the importance of failure analysis in improving product quality, especially for safety-critical products like catheters and ancillary equipment. It is also certainly true that medical staff are aware of their importance, do often retain samples and report about the circumstances of failure. But so frequently those samples are sent away to the manufacturer and then discarded. That is exactly what happened in the connector case, and indeed their existence could only be inferred from the failure reports and an occasional photocopy made of the devices on letterheads. It is clear that in this particular case, the manufacturer was aware of the problem, and changed the material to polyester but allowed the weak initial designs to continue in service. The device is still supplied, but in a more robust version, and the rate of brittle cracking is apparently much lower. The literature of medical device failure is voluminous because responsible staff are aware of the safety-critical nature of many of the products used to assist patients. Doctors and consultants do report and describe the failures encountered in their practice, both in technical and professional journals, and often about individual pieces of equipment. They are an invaluable aid for investigators researching specific designs. However, not all relevant details are published as might be hoped, such as the materials of construction or the loads to which they have been exposed. But the stresses experienced by implants such as breast expanders or external devices like catheters are often difficult to estimate with any degree of accuracy. The traditional engineering approach involves providing products with a high safety margin, using very conservative estimates of load so that the device should be capable of surviving normal loads without failure. Standards also provide a useful way of assuring medical staff that a medical product will resist normal handling and body stresses. But standards authorities usually lag by several years, so that standards appear after failures have been widespread. The standard for mammary implants, for example, only appeared in 2000, several years after the first failures were experienced by patients (29). The standard describes numerous tests to evaluate the mechanical strength of implants, and if applied rigorously, should help to improve product performance. Fatigue behaviour remains uncertain, however, especially when body loads can vary so greatly from person to person, tending to be higher the younger the patient. The artificial hip joint is perhaps the most obvious implant that will experience millions of cycles during its lifetime. To the mechanical problems must be added the effects of exposure to different fluids in many different environments. They include liquids used for disinfection of hospital surfaces (strong detergents, bleach and alcohols) as well as drug carrier fluids, and anaesthetics such as ether. The implant is exposed to many different body fluids and active enzymes, and must be tested against all expected environments before use in humans. Polymers

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are susceptible to many fluids, especially if products are made with significant degrees of residual strain, such as those produced by cold moulding. Brittle cracking may follow contact with active agents, often initially as hairline cracks, but which then grow along the path of greatest residual strain or chain orientation. Such cracks are very difficult to spot by medical staff, who naturally usually have more important jobs. When those cracks grow to completion, a product may leak internal fluids or simply fall apart, and it is usually then that staff notice the problem. If the device failure is a serious threat to the patient, then emergency action is needed, and the failure reported. Manufacturers must take those reports extremely seriously to avert further failures, and not simply blame staff for product abuse. All such failures need investigation by qualified engineers to locate the fault or defects, and recommendations accepted, if medical device design is to improve in the future. The alternative is further accidents, and patient injury, or worse.

3.8

References

(1) Holden, G, Legge NR, Quirk, R and Schroeder, HE, Thermoplastic Elastomers, 2nd edn, Hanser Publishers (1996). (2) Adams, RK, Hoeschle, GK and Witsiepe, WK, Thermoplastic Polyether Ester Elastomers, in Holden et al., op cit. (3) Gauvin, P, Philippart, J-L and Lemaire, J, Photo-oxydation de polyetherblock-polyamides, Makromol. Chem, 186, 1167–1180 (1985). (4) Elf Atochem tech brochures, available on the web at http://www.pebax.com (5) ANTEC 2001, Dallas, USA. (6) Lewis, PR and Gagg, C, Failure of an Epidural Catheter, Engineering Failure Analysis, 16(6), 1805–1815 (2009). (7) Scheirs, J, Compositional and Failure Analysis of Polymers: a Practical Approach, John Wiley & Sons Ltd, p 352 (2000). (8) ISO 594/1 (1986) Conical fittings with a 6% (Luer) taper for syringes, needles and certain other medical equipment; EN 20594-1 (1993). (9) Lewis, PR, Environmental stress cracking of polycarbonate catheter connectors, Engineering Failure Analysis, 16(6), 1816–1824 (2009). (10) Stubstad, J, Female Luers – The Frequent Failers, ANTEC Proceedings, 291–293 (1992). (11) Stubstad, J, Troubleshooting Plastics, Medical Device and Diagnostic Industry, 100–103 April (1992). (12) Vilardi, J, Franck, LS and Powers, R, ECMO Accidents: a survey of the incidence of mechanical failure and user error, Neonatal Network, 11, 25–32 (1992). (13) Niles, SC, Ploessl, J, Sutton, RGT and Steinberg, JB, Oxygenator Failure due to contact with Bathing Alcohol, J Extra-corporeal Technology, 24, 69–71 (1992). (14) Kwan, ESK, Stich, RAH and Shrem, LA, Salvage of a Flow-directed Microcatheter after Hub Failure, Am J Neuroradiology, 17, 868–869 (1996).

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(15) Lewis, PR and Ward RJ, Polishing, Thinning and Etching of Polycarbonate, J Coll and Interface Sci, 47, 661 (1974). (16) Kambour, RP, A review of crazing and fracture in thermoplastics, Macromol Revs, 7, 1 (1973). Moskala, EJ and Jones, M, Evaluating ESC of Medical Plastics, Medical Plastics and Biomaterials (May 1996). (17) McElwee, D and Snyder, EJ, The use of tapered plastic luer connectors in neonatal extracorporeal membrane oxygenation, Heart and Lung, 25, 324–329 (1996). (18) Federal Food and Drugs Administration website at www.fda.gov for several databases dealing with product failure. (19) Mueller, R and Sanborn, T, The History of Interventional Cardiology, Am Heart J 129, 146–172 (1995). (20) Myler, R and Stertzer, S, Coronary and Peripheral Angioplasty: Historic Perspective, Textbook of Interventional Cardiology (2nd edn) Vol. 1., Topol, E. (Ed.), WB Saunders Co., Philadelphia, 1993. (21) King, SB, Angioplasty From Bench to Bedside to Bench, Circulation, 93, 1621–1629 (1996). (22) Lynch, W, Handbook of Silicone Rubber Fabrication, Van Nostrand Reinhold (1978). (23) FDA, Study of rupture of silicone gel filled breast implants, available at http:// www.fda.gov. (24) Young, VL and Watson, ME, Breast implant research: where we have been, where we are, where we need to go.: Clinic in plastic surgery, 28(3), 451–483 (2001). (25) Brandon-H-J et al., Variability in the properties of silicone gel breast implants. Plastic and Reconstructive Surgery 108 (3), 647–655 (2001). (26) Blow, CM (Ed), Rubber Technology and Manufacture, Newnes Butterworth, 135 (1971) (27) Ethicon website at http://www.ethicon.com. (28) Union Carbide technical brochure, Moulding of Udel polysulphones (1995). (29) British Standards, Non-active surgical implants – Body contouring implants – Specific requirements for mammary implants, BS EN 12180:2000.

4 Polymer storage tanks

4.1

Introduction

Thermoplastic and thermoset polymers have successfully displaced many traditional materials for two reasons: they are substantially lighter than competing materials like metals and glasses, but yet possess comparable if not greater strengths. In addition, they can be shaped easily at relatively low temperatures. It is well illustrated by containers, such as tanks and reservoirs. Storage of very large quantities of fluids has traditionally been dominated by steel tanks, which are composed of steel sheets bent to shape and then welded together to form the final structure. The strength of a steel tank is determined by the strength of the weakest parts, usually the welds and holes in the structure needed for access pipes. Catastrophic failures have occurred of such tanks, and usually by fracture initiated at these locations, as the case of the Boston disaster illustrates. Other problems can arise from oily residues in tanks, which can form an explosive atmosphere. But polymer storage tanks can also fail catastrophically, as the case studies in this chapter demonstrate. Such tanks are widely used industrially for holding liquids as various as fruit juice, pickling acids for steel makers and caustic soda for use in making cleaning agents for farmers. Some of these fluids are extremely corrosive and toxic, so special care is needed in designing tanks for storing them prior to use.

4.2

The Boston molasses disaster

Perhaps the most notorious tank failure occurred in Boston at midday on 15 January 1919, when a storage tank suddenly broke after being filled to capacity with 2.3 million US gallons (8 700 000 litres) of liquid molasses. The wave of viscous fluid (consisting mainly of sugar in concentrated solution), killed 21 firemen and dockworkers in the vicinity. It demolished buildings nearby as well as part of the nearby overhead railway (Fig. 4.1). Little was left of the tank because numerous brittle cracks had grown across the structure, and it collapsed in pieces on the ground. In its original state it was 50 feet (15 metres) high and had a diameter of about 90 feet (27 metres) and was made from steel plates riveted together. The tank had been filled to capacity eight times in its two-year life since construction. The pattern 134

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4.1 Scene of devastation after tank collapse in Boston (Wikipedia Commons).

0

1

1 = 75 ft.

2 Boston

es ship

harbour

Molass Shed

City wharf Tank Union freig ht tr ack

Com me rcia l

stre et

Completely demolished Tro lley sur face trac k be low

4.2 Plan of Boston north harbour showing damage to buildings.

of damage (Fig. 4.2) showed that the initial failure occurred on the landward side of the structure facing the buildings next to the overhead railway, which itself was also severely damaged. A train on the overhead railway had just passed the spot and was lucky to survive; the driver climbed down from his cab and had the presence of mind to stop an approaching train before it arrived at the collapsed viaduct (1). Rescue attempts to save the

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many individuals who had been engulfed by the sticky liquid went on for many hours, and as time went by, hopes of saving trapped victims inevitably faded. Firemen and police continued to work for many hours to retrieve the bodies. Many of the lucky but injured survivors numbering more than 50 recovered at the local hospital. An additional problem faced rescuers because the molasses was starting to harden, and workers found it increasingly difficult to wash away the viscous fluid to start the clear-up operation. Many people concluded from the state of devastation that the tank had exploded, such was the extent of destruction near the base of the tank where the tank seemed to have failed first. The accident left the city in a state of shock, and the rescue attempts lasted several days owing to the sheer amount of debris and devastation.

4.2.1 Causes of failure The failure created much speculation as to the causes: sabotage using explosives and poor structural design being among the most frequently discussed possibilities. The tank owner, US Industrial Alcohol, was a proponent of the sabotage theory, but many independent commentators opted for structural defects. The evidence of the tank debris would be crucial to the investigation, and as much as possible was preserved for examination by the experts. There were many bomb attacks by Italian anarchists at the time, so sabotage could not be entirely dismissed. Although US Industrial Alcohol was not indicted on any criminal charge, a long civil action was started by the injured and relatives of the dead. The trial lasted three years and there appeared initially to be some support for the sabotage theory from explosives experts. However, their testimony was largely discredited. No one had been seen tampering with the tank beforehand, so an external bomb was unlikely. Dropping a bomb inside the tank was also very difficult, since there was a closed roof, and nobody had been seen on the roof at all preceding the collapse at about midday. Finally, if a bomb had been used, the shock wave would have smashed glass windows well away from the tank in surrounding buildings. No shattered windows were found away from the severely damaged structures close to the tank. Further testimony from the Plaintiffs revealed many details of how the tank had been built two years before it failed. The wall thickness had been specified as 0.312 inches (8 mm) at the top increasing to 0.687 inches (17.5 mm) at the base to allow for the increasing hydrostatic pressure towards the bottom of the structure, just like a dam has to be much thicker at the base to resist the impounded water. But the steel delivered was thinner, varying from 0.667 inches (16.9 mm) at the base to 0.284 inches (7.2 mm) at the top. So the steel was about 3% thinner at the base and 10%

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thinner at the top, increasing the hoop stress in the walls over what would be expected. Moreover, the tank had never been tested before use by filling with water to capacity – a normal test used to find any leaks in the riveted joints and assess any distortion on the side walls. It later turned out that the tank had indeed leaked the heavier molasses fluid copiously during its short life, and locals had collected it for their own use. At that time, molasses was used as a sugar substitute, and was also the main source of alcohol when fermented. Indeed, fermentation could occur during transport and storage and released carbon dioxide, which in an enclosed space resulted in high pressures building up within the space. The weather at the time of the failure was unusually warm, and it was suggested that extra pressure within the tank had occurred owing to premature fermentation. Examination of the many steel fragments found on site showed that fracture first occurred at a circular manhole near the base of the tank and spread out from there upwards along the joints. One expert witness from MIT said that insufficient rivets had been used along the joints between the steel plates. He also observed that the hoop stress exerted on the walls at the base was 31 000 psi giving a safety factor of only 1.8. He would have expected a safety factor of 3 to 4. The court found against the company US Industrial Alcohol and they paid out $300 000 compensation, equivalent to about $30 million today in one of the first ‘class actions’ brought in the state of Massachusetts. The tank was built below spec, was never tested correctly, and showed continual leakage from poor joints between the plates when used. Fatigue from the joints is a possible failure mode caused by the repeated emptying and filling, and is a possible explanation of the Boston disaster because the tank failed after just eight complete fills from an inlet pipe near the base of the structure. It is likely that the crack, which grew uncontrollably, started where a joint met the edge of the inlet hole, so magnifying the inevitable stress concentration there. However, no such conclusion was reached by the court, so the final cause remains something of a mystery.

4.3

Failure of polypropylene storage tanks

The use of thermoplastic materials in large storage tanks has become common owing to their resistance to many chemicals which attack steel, while stainless steel is a very expensive alternative choice of material. They are simple to build, being composed of sheets of polypropylene thermally welded to make the final structure. Failures have occurred from a variety of causes, especially poor design and lack of adherence to the standards

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available (DVS 2205 in particular). The case study we published previously (2) covered the failure of a 30-tonne tank at Warrington on 23 August 1994, where it had been used to store a very corrosive fluid, 40% caustic soda. The fluid was destined to be diluted and added to a detergent solution used for cleaning dairy equipment on farms.

4.3.1 Catastrophic failure The failure fortunately occurred on the night shift, so no one was injured, but there was substantial damage to equipment both in the factory and in adjacent premises from the caustic liquid. The manager was still there in an office overlooking the factory floor when he heard a ‘popping’ sound followed by the sound of ‘rain’, and when he turned round he saw a jet of fluid emerging from the tank above the so-called ‘bund’, a brick wall surrounding the tank to prevent such accidents. The tank had been filled that morning, so was full at the time, and this was only the fourth complete fill of the structure. Attempts were made to stop the flow by tapping off fluid but by the time it was achieved, a large volume had leaked away to cause property damage. The fire brigade were called to seal the area off and prevent the fluid entering local streams, where environmental damage would have been severe.

4.3.2 Investigation The tank comprised large panels of 12 mm thick polypropylene panels welded as shown in Fig. 4.3, with three horizontal buttresses encircling the inner cylinder. The failure had occurred in the centre part which lacked support from a buttress, and closer inspection showed that the panel was curved, the crack from which the jet had emerged lying at the centre of a weld (Fig. 4.4). The curvature can be seen on the plate if viewed side-on to magnify the effect. The fracture surface was seen directly, when the tank was cut up for analysis, and it showed a very simple sequence of events, with just four crack fronts. Since there had only been four fills from new, it was easy to see that each crack front represented the sequence of fills. The origin lay at a small pin-hole about 1.2 mm deep in the outer surface at the centre of the panel. The crack grew laterally rather than into the wall until the third fill, when it changed direction and grew into the thickness, finally reaching the inner surface directly beneath the origin (Fig. 4.5). The curvature of the centre panel showed that the plastic had crept over time under load, suggesting that it had been under-designed. If that were so then other vertical seams along the same horizontal line should be

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Top buttress

Upper single panel Centre buttress Lower single panel Base buttress

W1 W3

W2

Jet Bund wall

Factory floor

4.3 Schematic section of failed tank showing jet of caustic soda.

4.4 Close-up of centre panel of tank with large arrow pointing to initial crack.

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O1 1

2 3

4

4.5 Micrograph of crack origin showing four phases of growth.

similarly affected. Another set of similar but sub-critical cracks was found on an adjacent seam, confirming that the tank wall was probably too thin to resist the hydrostatic pressure of the contents. However, since a weld had shown a deep pin hole, the material was tested to see if it was within the strength claimed by the manufacturers. Tensile testing of several samples showed it to be within specification and reasonably strong for the intended purpose, with a tensile strength at yield of about 33 MNm−2. The welds when tested proved to be slightly weaker than the bulk material, as one would expect, and there was some small variation, depending on the tiny defects present in particular samples. Their strength averaged about 21 MNm−2, a value about 30% lower than the bulk material. The polymer was also examined using DSC and FTIR, nothing anomalous being found. The failure appeared to be a mystery then, because there was no material cause to explain the fracture. The design avenue of analysis proved more interesting.

4.3.3 Stress concentration Since the depth of fluid was known, the hydrostatic pressure, P could be calculated, and from that a value for the hoop stress, σH in the wall at the origin of the failure. The pressure is given by the equation P = hρg

4.1

where h is the height of the fluid above the origin, ρ is the density of the fluid (1500 kgm−3) and g the acceleration due to gravity (9.81 gs−2). So P is given by P = 1500 × 2.0 × 9.81 = 29.4 kPa

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The hoop stress is then σH = PD/2t

4.2

where D is the diameter of the tank and t the wall thickness so

σH = (29 400 × 1.35)/0.0117 = 3.4 MNm−2.

So why had the crack grown if the hoop stress was only a small fraction of the strength of the weld of about 21 MNm−2? The first effect must be the stress concentration effect of the pin hole, and should be about 21/3.4 or about 6. In other words the stress will have been concentrated at the base of the pin hole by about six times the nominal tensile hoop stress acting across the weld. And using a simple formula for stress concentration (3), it was possible to calculate the value knowing the dimensions of the defect (as measured from Fig. 4.5). σ = σ0 (1 + 2[D/r]½)

4.3

where (σ /σ0) is the stress concentration, D the depth of the defect from the free surface and r the radius of curvature at the inner tip of the defect. Figure 4.5 gives D = 1.5 mm and R ∼ 0.1 mm. So (σ /σ0) = 1 + 2 × [15]½ ∼ 9. The value for the stress concentration of about 9 is greater than that found experimentally, although another estimate using Peterson (4) gave a value of about 5. But there was another factor, and one only appreciated well after the event, when we inspected the welding process. In order to make the final weld to form a cylindrical hoop, the welded flat panels were bent round and welded using a hot plate against which the two edges to be joined were held. Since the stress remained in the final structure, and was not relieved in any way, it lowered the strength of the tank considerably. Knowing the curvature of the tank, the extra stress imposed amounted to 1.8 MNm−2. So the net result was a stress at the weld where the crack started of about 5.2 MNm−2, much larger than was originally estimated. The net stress was now only about a quarter of the strength of the material, so a stress raiser of only 4 is needed for crack initiation, well below the independent estimates calculated above.

4.3.4 Cause of failure It was thus clear that the tank had not been built correctly. It could not resist the steady increasing hydrostatic pressure from the heavy contents because there was an unprotected or weakened part in the centre of the

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structure. The single thickness of wall could not resist the pressure and started to creep under the steady load. A tiny pin hole in the vertical weld at the centre of the panel concentrated the stress locally to beyond the strength of the material and a crack started to grow from the bottom of the hole. And as the crack grew, the stress magnification grew too, so the crack grew faster at each fill of the tank. There came a point when the crack turned from growing laterally to growing inwards, and when it reached the inner surface, fluid started to jet out from the open crack. An inspection of the relevant standard from the German Welding Institute or DVS (5) showed that the design deviated from the norms specified. The tank should have been built with a wall of steadily increasing thickness down the side, rather than by a series of hoop buttresses which left the centre of the tank unsupported. In short, it should have been built like a dam wall rather than like a barrel (Fig. 4.6). Comparison of the structure as built shows how far the design fell outside the DVS 2205 standard (Fig. 4.7). Failure by creep rupture was inevitable. And detecting the problem before the final event would not have been easy since the crack as it grew would have been almost invisible in the dark corner of the factory where it was found. The sub-critical cracks were only found after the event by rubbing the weld surface with powdered chalk to give contrast. However, careful testing beforehand of such tanks should have shown there to be a problem. Although the density of water is lower than caustic soda, the centre section would have crept over time and been detected by simply measuring the circumference with a tape measure. As with the Boston tank, such an hydraulic test would have prevented the tank from failing and enabled the manufacturer to redesign the structure.

Dam design

4.6 Schematic of tank design with different walls.

Barrel design

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Failed tank Height above base (m)

3 Location of crack 2

DVS 2205 1

0 0

10

20

30

40

50

Wall thickness (mm)

4.7 Discrepancy between DVS 2205 and tank structure.

4.3.5 Other problems The story might have ended there, with the insurers of the tank maker paying out the very substantial costs of the clean-up. But the same manufacturer, a small business, had built numerous other tanks to the same faulty design. All had to be inspected for sub-critical crack along central welds, and a number of small cracks were found. The tanks had to be reinforced or replaced given the danger they posed. For example, one location, a wire works in Staffordshire had two tanks next to one another, one holding an acidic liquid (ferric chloride solution), the other an alkaline fluid (caustic soda). If either had leaked, there was the chance of reaction producing heat and hence a fire could have broken out. However, neither had been filled to capacity since construction owing to restricted access to the site, so cracks were unlikely owing to the lower hoop stresses on the walls. Other tanks had been used to store relatively benign liquids such as fruit juice and soap solutions, but some of the tanks did show cracks and had to be reinforced given that had the tanks failed, even fruit juice can cause a great deal of damage if suddenly released. Fortunately, many were under-utilized and therefore unaffected.

4.3.6 Paint accident A different kind of problem arose when a thermoplastic paint storage tank suddenly failed just after installation at a paint factory. The bottom of the tank was raised from the base and inclined to aid release of the viscous

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Weld

Weld

4.8 Faulty paint tank.

contents (Fig. 4.8). The base was supported by four polypropylene panels, but they were clearly insufficient to support the load imposed by the contents, and the weld holding the base to the side wall gave way suddenly in an accident in 1998. It was the first time the tank had been used, and the resulting deluge of paint created a considerable mess on the factory floor. When the four panels collapsed, all the load of the contents was only supported by the weld with the wall. Although the strength of the weld was not examined directly, it was clear from its appearance that it was not a well-formed weld, judging by the extrusions of polymer surrounding the joint. Welds are in any case weaker than bulk material, and once a crack had started at any point, a single crack would grow very easily through the remainder. The design had not built to the rigorous specifications of DVS 2205, which recommended a steel frame to support the inner bottom of the tank. This would have provided full support for the inclined base and paint without the fear of failure. Fortunately, only one tank was involved and the insurer met the costs of the clean-up and restitution of the storage tank to another design. As in the case of the caustic accident, the paint tank had been made by a very small company who were clearly unaware of the relevant German standard, or even basic design principles with polymeric materials.

4.3.7 Rotational moulded tanks Another type of failure can occur in poorly designed underground storage tanks. They are widely used for sewage storage, water and fuel storage and so on. In the 1970s, one entrepreneurial company decided it would pioneer thermoplastic tanks for sewage storage using rotational moulding to create the structure in just one single manufacturing operation. The method

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involves placing powder polymer such as polyethylene into a steel tool, and rotating the tool within a large gas oven. As the powder was distributed evenly over the wall, it melted and formed the shape of the product. The movement of the oven needs care to ensure that an even wall thickness is produced, but this is not always achieved in practice. One positive advantage of the method is the virtual absence of residual stress or strain in the walls, a problem encountered in the Warrington failure. However, another problem may occur: premature oxidation. Because the temperatures are higher than used in normal injection moulding, the inner surface of the product exposed to the air in the free space may oxidize the inner surfaces. In the present case, numerous very large tanks of capacity about 30 tonnes or 30 000 litres were designed (Figs 4.9, 4.10, 4.11) and installed at several different water treatment plants around the country by a pump manufacturer. After only a few months with their bases buried up to 10 metres below ground, several installations reported failure of the pumping equipment. Inspection of the tanks showed that the walls had deformed inwards under hydrostatic pressure from groundwater, despite being protected by an external concrete shell (Fig. 4.12). The tank possessed a thick wall of polyethylene (HDPE) and was designed with buttresses, but still failed to

4.9 Section of submerged sewage tank.

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4.10 Plan of submerged thermoplastic storage tank.

4.11 A tank ready for testing.

4.12 Inward deformation of a tank wall.

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resist modest hydrostatic pressures (given by equation 4.1 above). As might be expected, deformation was most serious with those tanks having a high external water table and no internal contents to balance the external loads. Tests of the design in a water tank showed that deformation of the walls was rapid (Fig. 4.13) and ended with the complete collapse of the tank (Fig. 4.14). Although the tank had thick 8 mm walls with horizontal and vertical buttresses, it could not resist the hydrostatic pressure, and it was unlikely that redesign could have improved the creep resistance. The manufacturer had to admit that the design was fundamentally flawed, and the pump

4.13 Start of wall deformation in test tank.

4.14 Complete collapse of walls in test tank.

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maker resorted to a GRP design, which did have walls which were much more resistant to catastrophic creep.

4.3.8 UV degradation The same rotational moulder also experienced a series of problems with failure of some of his other products, the largest of which were ‘mancabs’ for the shelter of workmen at the roadside (Fig. 4.15). Large brittle cracks were visible in the 8 mm thick walls (Fig. 4.16), but the cracks were most extensive on the roof of one of the cabs (Fig. 4.17), eliminating its function

4.15 Cracked mancab.

4.16 Close-up of brittle crack in mancab.

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4.17 Severe cracking on roof of mancab.

as a shelter. The seriousness of the roof cracks hinted at the cause, because roofs are inevitably exposed to greater exposure to sunshine, a potent source of ultraviolet radiation or UV. The inference was confirmed by sampling polymer on the roof and comparing the infra-red spectrum (Fig. 4.18a) with that of a bulk sample (Fig. 4.18b). Both exhibited carbonyl absorption just above about 6 microns (or about 1720 wavenumbers), the peak intensity being greater for the sample from the outer skin than that from the bulk polymer. The outer layers of polymer will be affected by UV degradation first, attack occurring at branch points in the polymer chains or carbonyl groups produced by contact with oxygen in the air when being shaped. Although relatively infrequent in HDPE, there can be enough to make the material sensitive to attack. The branch points where a side group or chain is attached possess a secondary carbon atom, which forms a free radical more easily than other carbon atoms in the polymer chain. At the high temperatures used in rotational moulding these are attacked first: —CH2—CH(C)—CH2— → —CH2—CH(.)—CH2— + O2 → —CH2—CH=O + O=CH—CH2— The chain is broken when the free radical reacts with oxygen in the air to form two aldehyde chain ends, which can then react further to form carboxylic acids: —CH2—CH=O + O2 → —CH2—C(OH)=O The carbonyl group also absorbs UV radiation, so degradation starts at those points and rapidly progresses. The carbonyl groups absorb infra-red radiation over a small range of wavelengths, and so can be detected in degraded polymer. Only a small number of chains are broken initially, but their effect on the tensile strength of the material appears very quickly in the form of brittle cracks. The

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5.0

6.0

7.0

8.0

9.0

10

15

100 90 80

6.5%

70 60

40 30 20 10 0 100

a)

% Transmission

50

90 80

4%

70 60 50

30 20 10 b) 2000

0 1800

1600

1400

1200

1000

800

% Transmission

40

600

4.18 IR spectra of HDPE mancab materials showing oxidation.

strength of all polymers is exponentially dependent on molecular weight, so very few chain breaks can have a disproportionate effect on strength, a feature of other degradation mechanisms such as stress corrosion cracking (SCC). The inference was confirmed using GPC to compare the inner and outer surfaces of the hut (Fig. 4.19). The distribution has shifted to lower molecular weights as a result of UV attack. The same manufacturer also made road cones, those devices used to guide motorists during the neverending works that afflict our roads, especially motorways. After only a few months’ exposure outside, rotationally moulded cones were found to be severely cracked (Fig. 4.20). The cone shown had probably been damaged by impact with a car, judging by the ductile deformation visible on the upper part of the device, but many brittle cracks were visible elsewhere. FTIR analysis confirmed the same diagnosis

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Weight fraction

37 Inner surface

48

Outer surface

26

Elution count

4.19 GPC spectra of inner and outer surfaces of mancab showing chain degradation caused by UV attack.

4.20 Failed traffic cone showing extensive brittle cracking.

as before, with UV degradation the culprit. Another product which can suffer the same problem is plastic garden furniture, and some plastics are more sensitive than others to UV attack, especially polypropylene, which has secondary carbon atoms in every repeat unit, so the chances of reaction are very much higher: polypropylene repeat unit: —[CH2—CH(CH3)]n— The solution for polymers exposed externally to sunlight is to add a small amount of a UV absorbing additive, usually a small aromatic compound which absorbs the UV preferentially, dissipating the energy as low grade heat. Garden furniture and roadside furniture are both usually well protected nowadays, although other products can suffer the same problem caused by even occasional or intermittent exposure, such as the catheter

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examined in the previous chapter. But a cheaper alternative is to add say 1% carbon black, which has a similar protective effect by absorbing all radiation in the surface layers of the material, but the result is not always acceptable for consumer products.

4.3.9 Failed battery cases Just the same problem occurred to a large battery manufacturer when their large traction batteries were fitted to fork lift tractors used by the Israeli Army during one of their many wars. The batteries were made by thermally welding blue dyed polypropylene lids to black PP cases. The tops were exposed to direct and bright sunlight in the Middle East, when the batteries were being recharged. They would normally be recharged at night but for some reason they decided on daytime charging, with the covers removed to expose the batteries. UV attack took place in the lids, being shown by fading of the lids at the welds (Fig. 4.21). The problem was confirmed by GPC (Fig. 4.22), where the molecular weight is plotted against the weight fraction of chains. As in the previous case, the whole distribution is shifted to lower molecular weights for the upper exposed part of the lid when compared with the underside. The critical entanglement molecular weight is also shown in the figure because this value represents the point where tensile strength drops steeply to a much lower value. But why should the welded parts of the cases have been attacked preferentially? The answer came by a detailed examination of the welding procedure used at the factory. There was a faulty heating element on one of the welding machines, resulting in over-heating so that the weld material had already started to degrade, but this time by a thermal mechanism. When exposed to sunlight, the welds were preferentially attacked, resulting

4.21 Battery case at left degraded by UV attack along weld.

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Lid (Upper surface)

Lid

Weight fraction

(Lower surface) Mc

Mn Mn Mn Mn

(Upper)(Lower)(Upper)(Lower)

102

103

104 105 Molecular mass

106

107

4.22 GPC spectra comparing inner and outer surfaces of PP cases.

in brittle cracks developing around the rim. The fact that the blue dye was also attacked is another facet of UV degradation because many dyes and pigments are also affected by UV, fading with time of exposure. The problem is well known to art conservationists, who need to protect old works of art (especially watercolours) from the pernicious effects of sunlight. Textiles may be affected similarly, in a process known as ‘photo-tendering’. The carbon black-filled cases remained unaffected because they were shielded form direct sunlight, although they have an intrinsic resistance to sunlight from their carbon black component, a well known UV absorber. The company not only calibrated their heating apparatus more regularly, but also added a UV additive to the blue lids to provide a more reliable product, even for use in unusual circumstances.

4.4

Failure of fibreglass storage tanks

The limitations of thermoplastic tanks alone can be circumvented by reinforcing the tank by an outer shell of composite material. In fact, composite tanks have been used for many years for a variety of applications, including septic tanks and storage facilities of all kinds. Several different types of composites have been used for the shell, the most common being glass fibre/ polyester and glass fibre/epoxy. However, there are also different ways in which the glass fibre itself is used. The cheapest option is to apply glass in the form of chopped strand mat (CSM), where each fibre is only about 2.5 cm long and exists in a random configuration in a so-called prepreg mat where the fibres are loosely held together by a binder. The strength of the material is not as high as filament wound composite where all the fibre

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present is continuous. On the other hand, filament winding requires special equipment and precise control, so is inevitably much more expensive. Other materials include woven cloth, intermediate in properties between CSM and filament wound products.

4.4.1 Chopped strand mat CSM is commonly used to make boat and canoe hulls, for example, and in that form is applied by hand, using rollers to add the polyester resin which provides the matrix for the fibre mats. Layers are built up as each applied layer cures, and at a relatively slow rate because heat is given out during cure. If a succeeding layer is applied too quickly defects can occur within the material due to the heat build-up. Bubbles are a typical defect caused by the excessive temperatures produced. In the case of canoes, only two layers are needed, but boat hulls require many more, the precise number depending on the size of the boat. Up to 40 layers are used on minesweeper hulls, for example. Since the fibres are randomly oriented in each layer, the material is equally stiff and strong in any direction in the plane of the layers, so it is not the ideal kind of reinforcement needed for tank shells. It is the hoop stress, the stress acting around the circumference, which is the greatest stress, being twice the longitudinal stress, which acts along the axis of the tank. This is why cracks tend to grow vertically in the sidewalls of gravity tanks, like the example of the Warrington tank examined earlier. In making storage tanks, the liner is often made of polypropylene sheets welded together like the Warrington tank, and it then provides an ideal structure onto which the mats can be added one at a time. Allowances have to be made for the various inlet and outlet pipes, and often extra reinforcing layers are added here because they usually represent the weakest parts of the structure. Polypropylene is highly resistant to many chemicals, so makes an ideal lining. Other lining materials include uPVC, PVDF (polyvinylidene fluoride) and ECTFE, or ethylene and chlortrifluoroethylene colpolymer. They tend to have better high temperature and oxidative resistance than PP, but are much more expensive. However, failures have occurred in such tanks, despite the existence of a long standing British Standard covering their design and construction (6) and considerable experience of composite tank usage, as the following case study describes.

4.4.2 Catastrophic failure on Teesside The chemical industry in Britain has changed rapidly in the past decade as companies have changed hands, old plant is demolished and new plant is

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erected to satisfy new needs and products. One of the largest centres is Teesside, where, largely for historical reasons, many refineries and ancillary process plants have been built. The area is also a centre for the steel industry, stimulating companies like BOC (British Oxygen Company) to have production facilities to supply gases for iron and steel making. Such plant is designed to run automatically with little human intervention, mainly because key variables like temperature and pressure of specific processes are monitored continuously and controlled by computer. That means fewer staff employed on site, and fewer human checks that the equipment is running correctly. But total chaos can result when a major piece of equipment suddenly fails. Just that happened in 2003, when a fibreglass tank used to store hot water from a ‘polishing’ unit collapsed, releasing 100 tonnes of near boiling water. Fortunately, there was no one around at the time to be injured, but the damage to other equipment next to the tank was severe. The cost of the damage was high, and since the process stopped, there were production losses to add to the total damages claimed from the insurers. The unit used a catalyst to make the product, and the hot water was produced by regenerating (or polishing) the catalyst at regular intervals. The water was alternately acidic and basic with neutralization occurring in the tank. The first investigation was conducted by the The Welding Institute (TWI), a highly respected professional organization well skilled in such matters. Their report had been commissioned by the company who had built the plant, and the insurers wanted an independent report given the high cost of the damages claimed (in excess of £300 000). The builders were a large firm of contractors to the chemical industry, and the insurers of the tank constructors needed an independent report to ensure that all matters were investigated thoroughly and without prejudice. The initial report had pointed to a defective tank, but key information was missing, such as the conditions of operation and whether the builder of the tank had described those conditions of use fully to the men who built the tank, for example.

4.4.3 Plant damage The original state of the storage facility is shown in Fig. 4.23, with access to the top via a steel ladder. The immediate aftermath of the failure was recorded as shown by Fig. 4.24, where the outer wall remains supported by a steel frame, and Fig. 4.25 which shows in detail the collapsed inner wall which originally supported the hot contents. The steel ladder at left in Fig. 4.24 shows the original height of the tank, now much lower owing to the removal of the centre of the structure. When examined some time later, the remains had been removed from site and dumped unceremoniously well away from the plant (Fig. 4.26). The picture shows the massive base plate

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4.23 Composite storage tank before accident.

4.24 Collapsed storage tank on Teesside.

4.25 Close-up of side of tank.

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4.26 Remnants of the tank when inspected.

upon which the structure had originally been erected, plus parts of the original side walls. The structure of the tank is shown by the section in Fig. 4.27, a simple structure which has an additional part, a bund wall built integral with the tank itself. Figure 4.28 shows the surrounding equipment used for polishing and regenerating the catalyst. The outer wall is intended to prevent any leakage of the contents from reaching the outer environment. From the previous figures, it is clear that the bund wall had completely failed to achieve that particular objective. So had the walls of the tank itself been sufficient to retain the full contents of the tank under the conditions of use for many months since installation? The answer could only be found by sampling the base and side wall near the base joint and sectioning them to count the number of layers of CSM used by the builders. The walls are critical in resisting the internal pressure, especially at the base where the hydrostatic pressure is greatest.

4.4.4 Wall and base sections The samples as collected are shown in Fig. 4.29, with Figs 4.30 and 4.31 showing the sections cut precisely at right angles to the base joint in the base and wall respectively. They show a uniform 6 mm thick inner lining plus a varying outer composite wall, within which several layers of CSM are visible. There is a problem with these sections, however. The base section is considerably thicker than the side wall, a situation which contradicts the need for a side wall to resist the hydrostatic pressure from the contents. The base of course will be supported by the concrete foundations on which the structure has been built, but the side walls have no external support, and must resist the hoop stress without failure.

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6.55 m

6.50 m

4.70 m

ø 4.20 m ø 5.20 m

4.27 Tank section.

The sections show that the base was built with six CSM layers (each 1.5 mm thick), but they decreased to five at the corner; the diminishing thickness is especially visible in Fig. 4.30. But the side wall was only between 4 and 5 layers thick, or about 7 mm, thinner than specified (all layers had to be 6 layers thick according to the specification of this tank). The reason for the uncertainty lies in the so-called gel coat, which is the outermost layer made of a thin tissue of fibre mat. Taking that away gives an outer wall of only four layers, much thinner than specified. Both sections also showed many voids within the GRP layers (especially visible in Fig. 4.31), typical of a structure made by hand manufacture, where such defects are inevitable owing to incomplete penetration of the mat by viscous polyester applied during lay-up. The final picture is an oblique photograph of the corner showing the way a brittle crack grew along it during the critical failure (Fig. 4.32). The detail is of interest because it shows traces of aluminium foil left over from testing of the thermoplastic liner for pin holes. The test is used with a spark gun

Polymer storage tanks

Ammonia Hydrochloric acid Other chemicals Effluent tank Polishing unit

4.28 Plan of tank at right with polishing tanks at top.

4.29 Fragments of base showing junction of base (left) and wall (right).

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4.30 Section of tank base showing inner lining and composite bottom with diminishing wall thickness towards corner.

4.31 Section of side wall with inner liner at top, CSM layers below.

and any through-thickness holes are revealed by the trace of the spark through to the conducting foil behind the sample. It should have been removed at the end of testing because its presence could weaken the joint.

4.4.5 Reassembly of failed parts The way the tank failed was explored by placing parts of the base, side wall and bund wall next to one another in a reconstruction of this crucial part of the tank where the failure started (Fig. 4.33). It was immediately

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161

4.32 Close-up of base corner showing metal foil still left in joint.

4.33 Reconstruction of failed section with visible bulging in tank wall.

apparent that the tank wall was severely distorted outwards, especially when the inner tank wall was compared with the flat and unaffected outer or bund wall. The distortion can only have been produced from the hot contents when the tank was full, and so exerting maximum hydrostatic pressure against the side walls. The curvature was measured directly simply by using a straight edge laid against the inner side of the wall (Fig. 4.34). It showed a maximum bulging of 2.3 mm on the part of the wall shown in the picture. However, the pipe inlet was less affected, presumably because the junction resisted creep of the wall locally. The stress levels here will have been higher than elsewhere, and might help to explain how the failure occurred.

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4.34 Side wall at pipe junction showing delamination around hole.

The creep of the walls showed that the tank wall was clearly underdesigned for holding large quantities of water, the lowest part being most at risk from the hydrostatic pressure. The thin walls used here could only encourage creep since the hoop stress is higher than if a thicker wall had been used. However, the situation was rather more complex than appeared at first sight, especially when the history of tank usage was made available.

4.4.6 Fracture locus One problem in investigating failures in composite materials is trying to map the fracture surfaces. By their very nature, crack growth in composites follows many different paths within the product, so there is no unambiguous fracture surface to examine and relate any features to the way the product cracked. But there are some details that allow reasonable inferences about the origin of a particular failure, and the way failure progressed. For example, the fact that a welded seam in the polypropylene liner runs into the hole and across, and failed along its length suggest that the critical crack started at or near the hole made for the pipe (Fig. 4.35). This makes sense because holes are stress concentrations by their very nature, increasing the local stress at the top and bottom edges by roughly a factor of 3. The idea seemed to be confirmed by the local damage around the hole, because it is clear that delamination occurred between the liner and the outer GRP shell well before the final demise of the tank (Fig. 4.35). The separation extended some way into the side walls, up to about 10 cm in places. So this feature has added some complexity to the problem of explaining the failure, and one which needed further information for a detailed interpretation.

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4.35 Pipe hole in side wall with gap from delamination.

4.36 Parts of side wall reassembled with crack growth along seams.

Nevertheless, reassembling the two separate parts shows the likely origin and crack paths from the pipe hole which caused the catastrophic rupture of the tank (Fig. 4.36). The smaller pipe outlet hole above the larger hole will have been exposed to smaller stress from the contents since it is higher from the base, and no seam passes though the hole.

4.4.7 History of usage The tank involved in the Warrington failure had a simple usage history: four fills followed by creep rupture. But the GRP tank had been operating

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fail

90 80 100 fail

90

60 80 50 70 40

T/°C

% fill

70

60 30 50 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct

20

2002 2003

4.37 History of tank conditions (fill at left, temperature at right).

for much longer, and its history was rather more complicated. Since the plant was run automatically with computer control, there were voluminous records available which allowed the history of the tank to be reconstructed, with two sets of data being of greatest interest: the fill levels and the temperature (T). The data are plotted on Fig. 4.37, with the fill denoted by the axis at left and the temperature at right (measured on the outlet stream from the base of the tank). The tank was built in early 2002, and was used to hold waste solutions from the polishing process in the first months of use. However, the temperature of the contents fell to ambient on five separate occasions during the first year of operation. Presumably the plant was being developed so was not working at full capacity. However, the tank never fell below 70% fill, so the wall was under high hydrostatic pressure from the very beginning. And it was at complete fill on four occasions, the last time just before the final failure in August 2003. After September 2002, the temperature lay between about 85 and 95°C, and the contents were at 90°C with a near full tank on the day of the failure. The question that arises is whether or not the polymers were capable of resisting such temperatures while under high hydrostatic pressure. And the best way of answering that question was by using DSC to determine the thermal behaviour of the materials used in the walls.

4.4.8 Thermal properties of composite The sections were sampled for liner and GRP from the base and side wall, and additional standards used for comparison. The liner samples were quite

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normal, showing a high melting point of about 165°C, consistent with the standard sample of polypropylene (Fig. 4.38). Since the melting point lies well above the temperature of the contents, the lining will be able to resist the contents adequately with a good margin of safety. The GRP samples were not crystalline at all, but exhibited a glass transition temperature or Tg lying at about 65°C, somewhat lower than a standard GRP sample (Fig. 4.39). It effectively means that at about 70°C, the material changes from behaving like a rigid plastic to a much more flexible elastomer. The tensile modulus of a rubber is about 3 MNm−2, while that of a rigid plastic is about 3 GNm−2, a difference of a factor of 1000. Although

Standard polypropylene Tank inner wall 5 mW

Tank inner base

20

40

60

80 100 120 140 160 180 200 220 °C

4.38 DSC curves of polypropylene liner.

4.39 DSC traces of GRP materials.

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the GRP shell was reinforced with glass fibre (so the effect was ameliorated), it would have become considerably more flexible on exposure to the hot contents of the tank. In other words it will have deformed much more easily when stressed. Here then lay a possible answer to the problem of creep deformation visible in the wall of the tank. The creep rate would have been much higher for an elastomeric solid than a rigid plastic shell, and it was likely that the poor temperature resistance of the shell led directly to the creep of the main wall. The problem will have been ameliorated to some degree by the glass reinforcement and by the temperature resistant lining.

4.5

Reconstructing the events leading to failure

The data thus showed that not only was the main wall too thin, but was also far too flexible at 95°C to resist the hydrostatic pressures developed near the base when the tank was full or nearly full. The onset of failure probably starts with the early thermal cycling from ambient to temperatures above 95°C, the last major excursion taking place in September 2002 (Fig. 4.37).

4.5.1 Thermal expansion The effect can be estimated knowing the coefficients of expansion of the two materials, (αL) which are quite different (7): Polypropylene: αL ∼ 90 × 10−6 K−1 GRP (CSM): αL ∼ 20 × 10−6 K−1 The expansion in the tank circumference can be calculated for a rise in temperature (ΔT) of 70°C assuming an estimated circumference of the tank of 13.2 m: Since

αL = ΔL/L0ΔT

4.4

where ΔL is the change in length of a sample of original length L0: Polypropylene ΔL ∼ 8.2 cm GRP (CSM) ΔT ∼ 1.8 cm So the polypropylene expands to a much greater extent than the GRP on heating the tank up. This will increase the effective hoop stress acting in the tank wall. On cooling down, the reverse process, the polypropylene will contract to a greater extent than the GRP shell, and it is here that delamination is the most serious danger because part of the stress produced by unequal contraction is at right angles to the wall thickness, so tending to pull the two materials apart.

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The problem of differential thermal expansion is well known in other materials, and is exploited in bimetallic strips, which are used as simple switches in electric circuits. Thus an electric kettle uses such a switch to turn the kettle off when it boils, and so acts as a safety device which is responsive to rise in temperature.

4.5.2 Hoop stress However, it is directly relevant to determine the hoop stress acting in the wall of the tank at the pipe junction (without the added thermal effect). Using equation (4.2) then the hoop stress is σH = PD/2t where D is the diameter of the tank (4.2 m) and t the wall thickness. Assuming that the wall is intact, then the thickness near the base is about (6 + 7) or 13 mm thick by direct measurement (Fig. 4.31). The diameter of the tank is 4.2 metres (Fig. 4.27) and the pressure is given by equation (4.1), so P = hρg where h is the height of liquid above the pipe when the tank is full. The density is assumed to be 1000 kgm−3, although will probably be slightly greater given that salts are formed from reactions between acids and alkalis from the polishing unit. Since the centre of the pipe is 15.5 cm above the base, then the height of liquid above is (6.5 − 0.155) or 6.345 m, so P = 6.345 × 1000 × 9.8 = 62.18 kNm−2 Hence σH = (62.18 × 4.2 × 1000)/(2 × 0.013) = 10.0 MNm−2 The figure may be compared with the earlier estimate made on the Warrington tank of about 3.4 MNm−2, the higher value being due to the greater height and head of liquid in the composite tank on Teesside. The hoop stress acting on the two sides of the pipe hole will now be magnified by the effect of the hole by a factor of about 3, so the effective net stress acting at the hole will be about: (σH) eff ∼ 30 MNm−2.

4.5.3 Strength of material So how strong are the polymers used in the construction of the wall? In the Warrington failure, the strength of sheet polymer was about 33 MNm−2, a

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value which fell to about 21 MNm−2 across the weld. Although the strength of the chopped strand mat was not measured directly, there was some guidance from the literature, with Brydson (8) giving values in the range: σ (CSM

GRP)

∼ 55–117 MNm−2

compared with a much higher value for woven cloth of σ

(woven GRP cloth)

∼ 200 MNm−2

The variation presumably reflects the variability of quality achievable by hand application, such as the presence of more or less voids. But even at the lower value, the outer shell will have resisted the hoop stress of about 30 MNm−2, unless other factors were at work.

4.5.4 Reactions in tank Detailed records were available for the pH of the tank contents, and they showed many large differences in pH over the 2 year life of the tank, with differences of up to about 11. This implied that the contents had been highly acidic (lows of about pH1), or highly alkaline (highs of about pH12), with neutralization occurring in the tank itself and so producing yet more heat. The effluent tank had, in other words, been used as a chemical reactor and was not simply a storage container. The heat will have helped raise the temperature of the contents, as reflected by the temperature recordings from the effluent outfall.

4.5.5 Failure sequence It was now possible to pull the various threads of this particular failure investigation into focus. The structure was under-designed for the task it was intended to perform, especially in the thinner wall, than either specified or needed to support the hot contents. Furthermore, the tank was defective in another way: the polyester matrix forming the outer composite shell was not resistant to high temperatures actually used in the tank up to 95°C, not far off boiling. The tank was used for about 19 months before finally failing, and it is likely that delamination started at its most vulnerable point at an outlet pipe near the base. Separation of the liner and composite shell occurred because the thermal expansion behaviour of the two polymers are quite different, and probably started at one of the last major cooling cycles in September 2002. A delamination crack formed around the pipe outlet and grew with time as smaller excursions in temperature and fill conditions occurred. When the void around the pipe was large enough, the polypropylene weld was exposed to very high loads, which can be

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estimated using the hoop stress formula, readjusted for a single thickness of 6 mm liner: σH = (62.18 × 4.2 × 1000)/(2 × 0.006) ∼ 22 MNm−2 With a stress concentration factor at the edge of the hole, this effectively becomes an effective hoop stress of: (σH ) eff ∼ 66 MNm−2. Such a value is in excess of the strength of about 21 MNm−2, and as soon as the hoop stress reached this level, weld failure was inevitable. It may, for example, have been triggered by the small cooling phase in mid-August 2003 (Fig. 4.37). The crack once started will have grown vertically in the weld, so splitting the tank into two parts (Fig. 4.36). However, the lower crack hit the lower joint between the base and the sides, and the crack ran circumferentially as well, reducing the structure to a total wreck. It is worth noting that the PP liner was probably welded in the same way as the Warrington tank, judging by the way the failed liner was seen to be flat rather than curved after the accident (Fig. 4.33). That implies that there was an inherent residual stress present in the formed liner, increasing the likelihood of failure, as already discussed above concerning the failure of the Warrington tank. A wave of hot water hit the thin bund wall, which immediately failed, so releasing the contents of the tank into the local environment (Fig. 4.25). The tank had been inspected only 3 hours before the final event by an operator, who luckily was absent at the time of failure. He reported that the bund space was dry at that time of his last inspection. The original investigator suggested that the tank had experienced about 30–40 cycles of filling, but few were complete fill-empty cycles where the change in loading conditions is most severe. He thought that fatigue might be occurring at the pipe joint, but even detailed examination of the failed plastic weld failed to show any evidence for fatigue of the kind clearly shown by the Warrington tank. He also performed some analyses on the GRP wall material, calculating the glass content by burning away the polyester. The GRP showed a lower glass content than expected of about 25%, rather than the 30% recommended by the British Standard 4994. So the composite strength probably lay at the lower end of the expected strength range mentioned above.

4.5.6 The bund That the overall design of the tank was also flawed is shown by the behaviour of the bund. It was a very thin composite wall of only 6 mm total thickness and there was no way it could withstand an impact from the nearly

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full contents of the tank. It was doubly flawed because it concealed the state of the inner tank wall. It completely enclosed the lower part of the inner wall (Fig. 4.27), so any distortion would not have been visible for inspection (access was by a small door at the top). If an independent bund well separated from the tank had been used, the distortion might have been seen, and so the accident averted. The use of an integral bund violates the basic concept of a bund, that is, to prevent the tank contents reaching the environment should a leak of any kind occur. It might be feasible to contain a small leak, but a massive failure would be impossible to contain safely. It would have been far better to have incorporated the bund wall into the main structure to add the extra reinforcement needed for supporting the contents. At the same time, an independent separate bund wall of the conventional kind should have been constructed.

4.6

Dealing with the aftermath

During the investigation a number of aspects of the case were exposed. They included statements by the management of the company that installed the tank and equipment (they had sub-contracted the tank itself to another company) that suggested that they thought the tank had been specified to resist water at 105°C. The drawing on which the tank was based, actually stated 90°C as the working temperature. Unless the tank had been pressurized, it would have been an impossibility since water boils at 100°C. Needless to say, the material would have been even less likely to have resisted the higher temperatures, and failure would probably have occurred even faster than it did. The same contractors produced a marketing brochure before the accident (and picked up in their offices after the incident), which stated that ‘The new hydrogen plant . . . [on north Teesside] . . . broke several records. It’s the largest in England, the fastest ever built (in an amazing 19 months) and was completed ahead of schedule, on budget with no lost time incidents.’ Tank failure after completion of the plant seems to illustrate just why safety and structural integrity may sometimes be sacrificed for speed and cost. But fibreglass storage facilities can be built safely, provided that close consideration is given to the appropriate standards and codes of practice.

4.6.1 Standards BS 4994 is a relatively old standard dating from 1989, and is very comprehensive in its recommendations (6). One important aspect of the standard is the classification of storage tanks into three categories, classes I, II and III. The highest category class I tank needs independent verification and

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extensive tests on the materials of use, as well as calculations of the strength, weld and bond points, ash tests to determine glass content; with all rigorously documented with records of the tests including hydrostatic tests with water. The classification depends on the: • • • • • • •

nature of the contents (toxic, corrosive or flammable) chemical compatability of the liner with the contents design temperature design pressure capacity its geometry and support safety-critical nature.

The highest category (I) is automatic for toxic, corrosive or flammable contents, so on that criterion alone, the tank should have been treated as class I. The liner is known to be compatible with the contents, but the design temperature of 95°C is well above the recommended limit of a heat distortion temperature of 60°C for the tank walls. The heat distortion temperature is closely related to Tg, and in this case a value of 62°C was quoted by literature used by the tank builders. In fact the standard recommends a value of 42°C as the maximum temperature which could be used safely for this polymer grade. The design pressure was static head only, but the standard recommends that any tank sizes above 50 m3 must be a class I tank (it had in fact a capacity of 100 m3). The only other criterion of note is the safety of the tank. Since failure could have killed or injured any bystander, then it had to be a class I storage vessel. The specification of the tank stated quite wrongly that it was a category III tank, but on several independent criteria it should clearly have been a class I vessel, with all the checks needed by the standard. It was not even known whether a simple hydrostatic test had been performed before the bund was added, or even after it had been added. Like the Warrington and Boston tanks, such a simple test of filling it with water might have prevented catastrophic failure. It would certainly have caused visible creep of the sidewall if left for a short time.

4.6.2 Acid storage tanks GRP tanks are widely used for storage of other hazardous fluids, such as hydrochloric acid. A small tank farm at Immingham dock is shown in Fig. 4.40, where the acid is stored prior to shipment. The acid is used for secondary oil recovery in North Sea oil and gas fields where it is pumped down oil and gas wells. The tanks are all GRP and the largest stand about 20 metres high. The entire set of tanks is protected by a solid bund. They are made from either woven mat or filament wound GRP, and well reinforced at the

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4.40 Acid storage facility at Immingham Docks.

4.41 Reinforced lower walls of composite storage tanks.

base (Fig. 4.41). The extra layers of reinforcement can be seen clearly in the lower parts of the structures; the pile of chalk present has been used to absorb small leaks of acid caused by seal breakdown. The tank farm is shown because a problem arose of alleged degradation of ABS plastic pipework in the pumping arrangements used in the complex, and is discussed in a later chapter. So composite structures are safe to use when built with highstrength materials to the best standard. However, other problems can arise in tanks that have been built to standard and with sound materials.

4.6.3 Other failures A number of other failures of composite tanks have been reported in the literature, and show that other failure modes can occur in storage tanks. A

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173

very large GRP tank used to store dairy products collapsed suddenly 10 years after installation, when a crack near the base grew from an internal defect. The tidal wave of liquid demolished an electricity sub-station and two men standing nearby just managed to avoid serious injury. The structure was made from dual layer laminate, with a polypropylene liner and CSM shell, although the shell comprised many more layers than the failed Teesside tank. The tank was filled and emptied daily with about 20 000 gallons of liquid, and was also cleaned daily with a hot caustic solution at 77°C. It was also occasionally cleaned with nitric acid. Inspection showed that small cracks grew in the welds of the liner, and they were mended, presumably using a hot torch. The origin was traced to a region in the wall at the base, where milk product had seeped through a small hole or crack in the weld into the shell. The weld had been repaired at some time in the past, but was obviously unsuccessful. The investigation (9) ascribed the failure to stress corrosion cracking (SCC) of the polyester matrix of the shell possibly by acid attack with crack growth encouraged by the daily stressing of the base from the product fill. However, the author failed to publish good images of the fracture surfaces and analysis of the loads on the structure, especially photographs of the fracture. Hydrolysis of the polyester might be one route to SCC, but the contents were also exposed daily to caustic solution at a high temperature, so there is a possibility that hydrolysis may have occurred by that mechanism. In addition, there was the chance of attack of the glass fibres of the shell.

4.6.4 Glass fibre attack The degradation of the glass fibre reinforcement is cited by several authors as another mechanism which can cause tank failure (10). A necessary corollary of those theories is that the liner must have leaked acid contents before such attack, so implying a faulty liner. Ezrin describes the fracture of a 3.7 m diameter and 6.2 m high GRP tank where a single brittle crack started at a circular manhole near the base, not dissimilar to that described here. The tank was under-designed and it failed three years after installation. The E-glass fibre used was also sensitive to acid attack from the dilute sulphuric acid contents, and SCC was suspected.

4.7

Setting new standards

A survey commissioned by the Health and Safety Executive (HSE) in 2003 reported several other failures of dual laminate and other polymer storage tanks (11). Although they concentrated initially on helix-wound HDPE tanks used to store HF, they extended the review to include many other

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materials, and the very wide range of hazardous chemicals and fluids stored in those structures. After discussing the various design procedures recommended by existing standards, they pointed out the importance of allowing for cyclical loading (not allowed for by the standards), a point already raised by the two case studies of the Warrington and Teesside tanks. Cyclical loading must be expected for most storage tanks and fatigue cracks will always start at the weakest points in the structure, especially at welds and joints. They also emphasized the potential role of radiative heating by the sun for external tanks. The authors pointed out the importance of hydrostatic testing before use, as required by most standards, and added some interesting details, such as adding detergent so as to lower the surface tension and allow the contents to penetrate cracks or other defects more easily. It is also important to use water at a temperature applicable in use, a test which would probably have prevented the Teesside accident if conducted over a reasonable time scale of several days to allow for equilibration of the wall. Inspection for hairline cracks is important for all loaded structures, but another test they recommend is measurement of the circumference for any signs of creep: the test is simple and easy to perform, with a reliable guide to creep of the walls. The different kinds of defect produced by faulty welding is clearly a widespread problem, and they emphasize the importance of automatic methods. However, the inherent stress produced by bending sheets to form the final joint is not mentioned by the authors of the review. They go on to describe the types of problem experienced in storage ranks and discuss the collapse of a GRP horizontal tank which had been underdesigned by a manufacturer who had no previous experience of tank building, a problem which ranks with those discussed in this chapter. Failures of HDPE tanks used for storing very strong acids such as nitric acid and hydrofluoric acid (HF) are described, as well as others where cracking has been induced by the acid itself. Nitric acid is especially pernicious because it also a strong oxidizing agent. Examples of brittle cracking are shown in detail. Foundations are important for providing a stable base, concrete being the best option. Sand foundations may be unsuitable and can shift with time, placing the tank under severe stress. Bund design can be a problem, one example being of a GRP tank which floated after waste water was accidentally allowed to flow into the bund, putting the pipe joints at risk. But they do not mention the problem of integral bunds, a strange omission given the problems which can follow, such as hiding the main wall, as occurred at Teesside. But if open to the atmosphere, then the accumulation of rainwater must be prevented. They discuss the use of DVS 2205 and BS 4994 in design, some of the problems of which would be addressed by a forthcoming standard (12). The

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review is an excellent summary of the problems of thermoplastic tanks, and ways to prevent future failures.

4.8

References

(1) Puleo, Stephen, Dark Tide, The Great Boston Molasses Flood of 1919, Beacon Press, Boston (2003). (2) Lewis, PR, Gagg, C and Reynolds, K, Forensic Materials Engineering: Case studies, CRC Press (2004). (3) Walker P (Ed), Lewis, PR, Reynolds, K, Weidmann, G and Braithwaite, N, Chambers Dictionary of Materials Technology, Chambers (1995). (4) Peterson, RE, Stress Concentration Factors, Figure 128, page 195 (1974); also in Pilkey (op cit), chart 4.50 (1997). (5) German Welding Institute (DVS) 2205, Standard for thermoplastic tanks (1992). (6) British Standards Institute, Specification for vessels and tanks in reinforced plastics, BS 4994 (1987). (7) Open University, Design and Manufacture with Polymers T838 (1999), Data Book, Table of thermal properties of polymers. (8) Brydson, J, Plastics Materials, Butterworth, 7th edn (1999). (9) Hull, D, Fractography, Cambridge University Press (2002). (10) Ezrin, M, Plastics Failure Guide: Cause and Prevention, Hanser (1996). (11) Stonehill, J, Bainbridge, H and Heyes, PF, Specification and Inspection of Thermoplastic Storage Tanks, Health and Safety Labs (HSL), HSL/2006/21 (2002). Available on the web at http://www.hse.gov. (12) BS-EN BS EN 13121-3:2008 GRP tanks and vessels for use above ground. Design and workmanship; British-Adopted European Standard / 29-Aug-2008 / 236 pages.

5 Small polymeric containers

5.1

Introduction

When large tanks and reservoirs fail, the consequences are usually disastrous because they tend to fail when full, so that the contents flood the vicinity and cause serious damage. Although small containers do not present the same problem, they do present problems of their own when they fail. Battery cases are probably the most dangerous because they usually contain very strongly acidic or basic electrolytes, such as sulphuric acid or caustic soda. If they leak, then serious personal injury can occur to the user, or physical damage to nearby equipment. In addition, the equipment powered by the battery starts to fail, or, when the electrolyte leaks away, fails altogether. So the function of the equipment is lost. Thus a leak from a battery which powers a lamp underground means that the miner loses his light, a potentially very serious incident which could result in an accident. A leak from a car battery can corrode other components and cause an accident. Other failures of battery containers can be caused by an internal spark, which can ignite gas inside the battery, causing an explosion, which can injure anyone unlucky enough to be nearby. Both thermoplastics and thermosets are almost universally used now for enclosing electrical equipment of all kinds, where their excellent insulating properties are exploited. Thermoplastics have displaced thermosets, especially materials like ‘hard rubber’, which were formerly used widely. But case failure in either material can allow live parts of the inner conductors to be contacted by consumers, with the possibility of electrocution. The integrity of cases is critical to safety, even for such small and apparently innocuous items like plugs on consumer products. The growth in a wide variety of such products imposes constraints on product design as well as the way they are made. More often than not, those products are made in the tiger economies of the East, such as India, China and Indonesia, where quality control and design experience is still growing. There may be a long chain of production, assembly and supply to the consumer, and an early mistake can have far-reaching consequences. Injection moulding can be problematical, with products seemingly correct but yet basically flawed internally. When those mouldings are assembled, small cracks can be created which are almost impossible to detect visually until the final product is stressed by the consumer, when the product fails. 176

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177

But so many small storage products are made from thermoplastics that failures of such mundane items as plastic buckets can cause serious accidents. Their design needs careful thought not just for the integrity of the device but, recognizing that fracture of key parts is possible, then mitigating the consequences by providing some redundancy in the product. Such ideas are widely used in many safety-critical components on aircraft, for example, where redundancy is a key part of the design process, and the principle should be applied to all safety-critical products.

5.2

Failure of battery containers

As portable sources of energy, batteries have multiplied in their applications ever since their invention and development in the Victorian period. They may be simple primary cells where the case itself is a metal like zinc, which corrodes in a caustic electrolyte to provide the current. The case corrodes from the inside and by the time the corrosion reaches the outside of the can, the cell is exhausted and the product disposed of by the user. Damage from the battery is thus rare but can occur if the corrosion is faster at one point, so allowing leakage faster than expected. Such cells are much more commonly now replaced by improved cells where the chemical mechanism is different. Secondary cells are also common, ranging from very large stand-by batteries available in emergencies, when the public power supply fails, to submarine batteries used for electricity storage, through truck and car batteries to smaller versions for motorcycles. Owing to the low cost of the raw materials, lead-acid batteries are the most common systems used, although many other types, using different reactions and materials, are available for powering electronic devices such as mobile phones and laptops, for example.

5.2.1 Military batteries Although glass cells were commonly used for many batteries in the early days, the possibility of breakage was so high and the results so severe, that alternative materials such as hard rubber were used. It was originally developed by Charles Goodyear when he discovered the vulcanization of rubber by sulphur in 1842, and is one of the first of many new materials developed in the Victorian period. It is very highly cross-linked natural rubber, with up to about 30% added sulphur plus some fillers such as carbon black or even powdered anthracite coal. The material known as ‘ebonite’ is a superior grade of hard rubber and widely used for musical instrument stems such as flutes and oboes (1). Hard rubber became a common material for use in battery cases since it could be moulded into a variety of fairly complex shapes relatively easily.

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However, the material as used in batteries is very brittle and easily damaged by abrasion. That is shown by the problems that can occur in tank battery cases, where large containers are made for heavy duty use in power storage. The design problem with battery cases basically comes down to achieving a compromise between making a container tough enough to withstand handling during manufacture and then placement (and replacement) into engine compartments. Most of the time, the container sits in that compartment without any extra stresses on the external case, but has to withstand those initial handling stresses without damage or cracking. So the material specification can be lowered (so saving costs) but to what extent? If lowered too far, the container may then fail, defeating the primary objective.

5.2.2 Failures The first failures of a 12-volt tank battery were discovered on the production line, when the steel handles fitted came away from the casing (Fig. 5.1). The handle fitting was held to the case by a single screw on the underside of the case, and the hard rubber had cracked, so releasing the screw (Fig. 5.2). The internal enquiry showed that up to 10 600 batteries in service could have been affected, plus 1409 batteries impounded in the factory. A further 683 items were found to have faulty handles. It was a serious situation, not just for the loss of production, but also for the batteries in service because of the possibility of personal injury to anyone moving them. They

5.1 Hard rubber tank storage battery.

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179

5.2 Steel handle held by single screw to case.

requested an independent investigation, which we conducted on several failed cases supplied by the company. Attempts made to repair cracked cases with resin put into the cracks were unsuccessful since adhesion was very poor. Tests at the factory indicated that the failed batteries had been made from poor material, the poor ebonite showing a distinctly lower impact strength 0.24 J/12.7 mm notch compared with 0.39 J/12.7 mm notch.

5.2.3 Investigation The first task was to examine the cases for the nature of the defects, which were quickly found at the base and sides of the single screw hole drilled into the material after removal of the handle itself. The samples all showed star cracks from the sides of the hole, some penetrating the base of the hole to the free surface (Figs 5.3, 5.4). In other examples, the base had spalled away completely. The material itself was very weak, as confirmed by checks on tensile test bars machined elsewhere from the cases. They were tested to destruction and the fracture toughness (KIc) calculated from the measured strengths (σ): KIc = Y σ (π c)½

5.1

where Y is a factor related to sample geometry for a single edge notched sample, and c the crack depth. The tests produced a mean result of: KIc (hard rubber) = 0.36 MNm−3/2 at 1 mm/min crosshead speed falling to a value of about 0.1 at a much higher test speed of 1000 mm/min. The value is substantially lower than glass, polypropylene or steel (2): KIc (glass) = 0.8 MNm−3/2 KIc (PP) = 3.0 MNm−3/2 and KIc (steel) = 140 MNm−3/2

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Forensic polymer engineering Star crack

Plan (above, outer)

Side elevation

5.3 Plan and section of handle.

Tests on screw holes and the effect of different diameters were also made, showing that shallow holes produced cracks very readily. Operators had used the correct sized drill bit but had not always drilled them as deep as needed. When the screw is inserted to connect the handle, extra stress will be put on the adjacent material.

5.2.4 Material analysis But what really surprised us was the very poor composition of the material, whether ‘poor’ or ‘good’. Analysis was difficult because rigid thermosets cannot be examined using FTIR, and generally produce poor DSC thermograms. However, a direct approach was used to examine samples, with some interesting results. Sections of the material were easy to polish to examine the composition, however, which turned out to be (according to the supplier) natural rubber heavily cross-linked with sulphur and filled with powdered anthracite (a type of coal of very high carbon content). The sections showed the anthracite particles shining by their specular reflections in the optical metallurgical microscope. Inspection of the image showed that very high levels of filler had been used, weakening the matrix polymer. Sections showing a brittle crack (cc) next to a thread (t) showed just how brittle the material had become (Fig. 5.4). The particles were highly angular with sharp edges and points, and showed a very wide distribution of sizes, none of which had strengthened the material. Other sections showed up flow lines and probable weld lines within the bulk, yet another weakening effect in what was

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c

C t

5.4 Sectioned case showing screw thread and brittle crack.

a seriously weak material for the job of supporting 30 kg weight of the contents. The volume fraction of anthracite estimated from the sections indicated a loading of about 75%. Carbon black itself is a very good reinforcing agent when used in car tyres at much lower loadings, but the particles are much smaller and much better distributed to achieve the desired increase in strength and modulus.

5.2.5 Conclusions The investigation concluded that failure had been exacerbated by poor drilling during manufacture, but the material itself was mainly to blame. Why and how such a grade came to be used in the first place remained a mystery. The drawings were so old as to be partly illegible as to when it had been introduced, and the company itself could not tell us either. The supplier blamed the failures on lower quality anthracite ‘. . . on account of the recent miners strike . . .’ but that could only have been a partial explanation. Variations were also found in the shape of the metal handle which could increase the stress on the screw and hence encourage cracking. The thickness of the steel plate used had recently been reduced from 1.84 mm to 1.71, reducing its stiffness, and so making distortion under load easier. In other words, more of the load was transferred to the case and made failure more likely. To put it bluntly, this was a seriously flawed design: to use just a single screw to hold such a heavy product in what was a very weak and brittle material defied belief. The casing needed very thick walls (½ inch/12 mm) to resist drops and minimize any stress concentrators such as corners. But screw holes themselves will magnify the stress by at least three, so extra special care is needed when adding essential devices like handles. While the immediate cause of the failures might have been under-drilled screw holes and thin steel handles, the long-term problem of using very poor materials would have to be addressed before serious claims might be made against

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the company. Far better materials were available and it is understood that this battery and others have since been redesigned using polypropylene.

5.2.6 Aircraft batteries It also later turned out that the same company were having related problems with 12 V aircraft batteries, and the root cause was the same. Problems appeared in 1985 on the now defunct aircraft, the Canberra. Leaks of sulphuric acid electrolyte had occurred from the vent holes, so endangering the aluminium airframe. The RAF had quarantined a large number of similar batteries owing to the risk of leakage. Each of six vent holes in the top of the case was fitted with a transparent plastic non-leak valve, which screwed into the hard rubber casing of the battery. Inspection of the threads in the cases showed severe damage such as cross-threading, degrading the capability of the joint to seal the contents (Fig. 5.5). The threads were made during moulding by using a retractable core which is unwound from the solid product at the end of the moulding cycle to allow the product to be withdrawn from the tool. The operator then deflashes any excess material extruded through gaps in the tool mating surfaces. The independent investigation concluded that although deflashing could help initiate loss of thread, the cause of failure lay in the material being incapable of resisting abrasion from the plastic thread of the valve. It was clear that such designs should be scrapped, and hard rubber replaced by a much tougher and reliable material like polypropylene. Once again, the root problem was the poor material of construction.

5.2.7 Patent action In fact several large battery companies in the UK had already started a programme of introducing polypropylene into car battery cases, by far the

C

5.5 Stripped thread on aircraft storage battery.

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largest product of the lead-acid battery market. That fact emerged in a patent battle which came to trial in the Patents Courts in 1977 (3). Thin wall containers were patented in the USA (extended to the UK) by Polycase (Fig. 5.6) and it was clear that the material had many advantages over hard rubber for battery casings. The walls were only 2–3 mm thick, so saving a substantial amount of polymer compared with the 6–8 mm of hard rubber cases. The material was much tougher so that the cases could withstand rougher treatment when handled. In addition, details such as threads and ribs could be designed into the product with greater confidence so that they would be safe during use. Numerous ribs were needed to stiffen the casings owing to the rather low tensile modulus of the polymer. The lids could be welded thermally with ease owing to the sharp melting point of about 165°C of the material. A number of UK manufacturers developed very similar cases in PP, some of which were also patented, but the Polycase patent won the infringement action since it had established priority. All cases that used the same principle had to pay royalties to the patent owner. It was clear that the use of the thermoplastic would replace hard rubber as the standard casing material, although other thermoplastics came to be used as their often superior properties came to be recognized.

5.6 The thin walled battery container Polycase UK 999,584.

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The replacement of hard rubber did not come easily. The companies had to invest large sums of money in the complex tools needed to make the cases, which were then manufactured using external trade moulders. The assembly lines also had to be modified to suit the new materials. The independents certainly had the expensive injection machines to supply the products on a fixed-price contract. Experience in using some of the new polymers, however, was in short supply and would create unforeseen problems.

5.3

Failure of buckets

Some tough materials like LDPE came into question when they also failed to resist imposed loads. The common domestic bucket moulded in thermoplastic materials was a big improvement over the traditional galvanized steel bucket, being much lighter, and thus easier to use. It was fitted with a steel wire handle. However, an employee was using such a bucket when it suddenly failed at one of the projecting lugs (Fig. 5.7), and she was scalded by the spill of hot water she was carrying in the device at the time (Fig. 5.8). Since she was working at a factory, she could be compensated for her injuries if the insurers were convinced that it was not her fault that caused the accident. They approached us for an independent assessment of the incident. The lug had broken in a brittle fashion across its diameter, the fracture starting at one of the inner corners (Fig. 5.9). The exposed surfaces showed wear from abrasion, and both lugs showed creep so that the handle holes had become elongated. So what had caused such a sudden and instantaneous failure in a tough material? A full bucket of water imposed a load of 45 Newtons on each of the two lugs. The round lugs of a steel bucket have a cross-section of about 15 mm2, giving a stress of about 3 MNm−2 in each lug. Since the tensile modulus of steel is 210 GNm−2, the elastic strain is very small at about 1.4 × 10−5. With

5.7 LDPE bucket with fractured lug.

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5.8 Close-up of broken lug.

O

5.9 Fracture surface of lug, with origin O at corner.

a much smaller modulus of only about 0.2 MNm−2, LDPE would show substantial deformation for the same load (2). The lugs had to be thickened to limit the strain to a more acceptable figure. In the event, the designer chose to increase the section area to about 57 mm2, so reducing the stress to about 0.8 MNm−2, with a strain under load of about 0.4%, well within the tensile strength of the polymer of about 10 MNm−2. This was considered a reasonable and acceptable degree of deformation at the time. However, the real stress on the lug would be greater owing to the stress concentration of the hole itself, which was estimated at about six times the nominal stress, so giving a stress of about 4 MNm−2 on the device (still well within the material strength). There was a sharp corner in the design (Fig. 5.8) which may have increased the stress further, but the crack did not start here but on the opposite side of the lug. There had to be another explanation for the failure.

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There was no obvious evidence of fatigue, such as striations concentric to the origin where the crack had grown intermittently, and there was no evidence from the bucket itself of abuse or mishandling by the worker. For example, there were no abrasions or cuts at or near the lug which could have caused the fracture. Tensile test bars cut from the sides of the material proved it to be of normal strength, and it had not been affected by UV degradation either, as FTIR spectra showed. Although brittle cracks could be induced by exposing the material to carbon tetrachloride, as far as was known the bucket had not been in contact with either this or any other organic fluid.

5.3.1 Weld line formation The final possibility considered was perhaps a problem with the injection moulding process used to make the product in a single operation. The object had been made from a single gate in the base of the bucket, so that molten plastic entered the tool here, and progressed steadily up the sides to the top and the lug recesses. A problem frequently arises where the molten polymer front has to move around an obstacle in its path, thus splitting the melt into two separate streams. When the melts rejoin at the other side of the obstacle, a weld line can often be formed if the polymer has cooled too much in its travel. It is effectively an area within the solid where material has failed to unite to form a coherent bond. Weld lines are not unusual if the tool is unheated, because the polymer melt will cool as it touches the steel tool surfaces, especially at the end of its run. Several weld lines were indeed found in the region of the lug on the outer surfaces, confirming that one such weld line was the cause of the failure. Needless to say, the culprit weld line was destroyed by the fracture. Although one might have expected a weld line to have formed at the apex of the lug, it is not unusual for them to occur at other points than expected. If, for any reason, one of the melt fronts travels slightly more slowly than the other, then they will meet asymmetrically, as in fact happened here. The weld line represented a permanent defect which could have failed at any time the bucket was loaded to the brim. It was effectively a nascent crack waiting to be parted. The worker concerned was compensated for her injuries, and the wellknown company who had made and sold the item contacted us for further help on the design. Upstanding parts will always be susceptible to weld lines, however hard the moulder tries to eliminate them, so an alternative solution was needed. Fortunately other manufacturers had faced the same problem and had found the solution: lugs set below the upper rim, and so reinforced by the wall as well as an outer part. In the event that the lug failed, the end of the handle would still be held by the recess, giving the

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5.10 Recessed lug for failsafe.

product failsafe performance. Such designs are now universally adopted, and provide the user with much greater protection against sudden and unexpected failure (Fig. 5.10). By eliminating all possible failure modes, the investigation arrived at the only feasible mechanism by which the product failed, despite the lack of direct proof. But then most failure analysis usually relies on circumstantial evidence and experience of previous failures.

5.4

Exploding batteries

An unexpected problem occurs with lead-acid batteries when they suddenly explode. It can occur when one of several failure mechanisms occurs independently of the quality of the plastic case itself. In the era before socalled ‘maintenance-free’ batteries, users needed to top up each cell of their car batteries with distilled water. It was needed because water is lost by evaporation and, more likely, by electrolysis. The latter is an undesired by-process of the function of a lead-acid battery, which normally stores electricity on charging by converting lead sulphate (PbSO4) to the high energy compound lead dioxide (PbO2) in the presence of sulphuric acid. The concentration rises to 40% in a fully charged battery, a highly aggressive liquid. However, the much simpler reaction also occurs on the free metal surfaces dipped into the acid, especially when all the available lead sulphate has been converted to the lead oxide: 2H2O (l) → 2H2 (g) + O2 (g)

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So liquid water is converted into two gases in exactly the right composition to recombine to form water again by the reverse reaction: 2H2 (g) + O2 (g) → 2H2O (l) However, the reaction is usually slow, and pressure builds up above the electrolyte unless vented to the atmosphere (often through leaky screwed stoppers). On older batteries the gas mixture could explode when ignited by the electric sparks ever present in car engine compartments, although the damage was usually not serious since the air space is large. On the other hand, a serious fire could occur if petrol fumes were ignited. On maintenance-free batteries, the cells are designed so that the two gases recombine smoothly, usually at a higher pressure than atmospheric. They are fitted with valves that can open at higher inner pressures (at about 6 psi, for example) if recombination is inhibited. Unfortunately, valves can be blocked by the grease and dirt in engine compartments, so that pressures can build up to unacceptable levels. This in itself could cause the case to burst, but a more likely result is an internal short circuit which causes the mixture of gases to explode, as the following examples illustrate.

5.4.1 Fire brigade incident In July 1987, a stand-by 100 amp hour 12 V battery exploded at a station of the Humberside Fire Brigade. It was one of a set of about 100 used by the brigade and was under regular and constant charge, ready for use at any time. The battery had been delivered new in December 1986, so the failure could not be attributed to damage from old parts, for example. The battery was dissected so that the damaged parts could be examined in detail. The top exhibited cracking of cell 4, and cell 3 was dead (Fig. 5.11), but careful inspection showed that there was some sooting on the underside of cell 2. All the valves were intact although that from cell 2 showed signs of burning, so accounted for the soot remains found on the underside of the lid. The valve was made form polystyrene, a highly flammable polymer. Each individual cell was then tested and all proved in a state of charge apart from cell 3, which was dead. The casing was cut open and the plates from each cell examined one by one. The plates from cell 4 showed showed a ‘sulphation ghost’ on one of the positive plates indicating a possible short circuit. The plates were badly corroded. The plates from cell 3 were also badly corroded, and there had been dendritic growth between the plates through the microfine glassfibre separators. Such growth is unusual in such a young battery and had shorted the plates so that the whole cell was dead. The mechanical strength of the lead connectors within each cell were also tested and proved satisfactory, with no fractures. Cell 2, however, showed

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5.11 Internal explosion in large lead-acid battery case with sooting arrowed.

5.12 Critical crack in battery top.

signs of shorting at the top corner of one of the grids, and the other intact cells also showed signs of positive plate corrosion. The lid fractures showed that no less than five simultaneous brittle cracks had been formed on the outer lid surface, which had grown to completion to form the final flap (Figs 5.12 and 5.13). There could be no doubt from the outward inclination of the flap and the large number of origins that failure had occurred from an internal explosion. The valves were checked and all proved to be in working order, releasing gas pressure when it had risen to 6 psi above atmospheric.

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O

O

5.13 Crack origins from inner corners.

Bomb B.

60

Spark

2m 14

32 mm

217 mm.

Spark

53 mm

33 mm.

153 mm

6 mm8 . 32 mm.

Bomb C.

m.

Bomb A.

Spark

63 mm.

m

m

5.14 Hydrogen explosion experiments (Berthelot).

5.4.2 Material quality The tensile behaviour of the wall material was also examined by cutting tensile dumbbell bars from sheet taken from the battery and testing at two strain rates. The tests of the polypropylene proved that the material was strong and ductile, although the strength dropped with increasing strain rate, a perfectly normal response. The breaking strength was about 32 MNm−2 at a strain rate of 100 cm/min crosshead speed, the material showing a clear yield point, then cold drawing before fracture. The densities were also normal, at about 0.94 g/cc. FTIR spectroscopy showed no carbonyl groups present, so the material had not been oxidized. The method gave an ethylene content of 3% by weight, so the material was actually an ethylene-propylene copolymer.

5.4.3 Hydrogen explosions There is an extensive literature on hydrogen explosions, both as a subject of scientific study and the damage they can cause. The subject had been investigated in the Victorian era first by Berthelot and co-workers and discussed in Bone and Townend (4), who used small bombs fitted with a piston (Fig. 5.14), the smallest being not dissimilar in volume (300 cc) to

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that above each cell (about 340 cc, depending on the electrolyte level). They measured the pressure developed in various gas mixtures, the greatest pressure of 7.6 atmos being developed for a stoichiometric mixture of hydrogen and oxygen. The flammability limits of hydrogen in air are very wide, from 4% to 75%, and the detonation limits narrower, from 18.3% to 59% at atmospheric pressure. The limits are proportionately wider for a pure oxygen atmosphere. The ignition temperature in air is very low, at 585°C, and the flame can reach a temperature of just over 2000°C. The damage evidence suggested that the gas mixture in cell 2 had burnt rather than exploded, judging by the soot marks on the underside of the lid, so the combustion mixture fell outside the detonation limits. However, it was possible to determine the hydrogen concentration knowing that the oxygen index of polystyrene is 18.5% (5). This means that it will only burn freely in air with more than 18.5% oxygen. It works out at about 12%. There would have been some arcing in this cell to ignite the gas mixture, but arcing must have been less in cell 4, allowing the pressure to rise before a detonating explosion blew the lid out. So the explosion had been caused by a failure of the cells, which occurred rapidly in the few months since it had been installed. One cell went dead and the remaining cells were then overcharged. Sparking within the two cells next to the dead cell led first to a fire in the one, and then an explosion in the other. But more serious explosions can occur in older cells where the gas-release valves fail and allow higher pressures of gas to accumulate.

5.4.4 Personal injury Reporting on industrial incidents is often much easier than accidents where litigation is contemplated because of the loss of evidence and time lapse between an incident and its investigation. It makes the job more difficult but more challenging. We were approached in 2004 to investigate an accident where a garage mechanic had lost an eye when a 12 V car battery exploded. He said that he was renovating a vintage car and went to remove the battery, which was situated in an awkward position under one of the seats in the car interior. The battery seemed to be flat, so it would need recharging, and needed to be removed for that to take place. As he came to move the battery, it suddenly exploded. Since he was close to it when it disintegrated, he was severely injured by fragments of plastic from the top as well as an acid spray raised by the explosion (Fig. 5.15). He wanted compensation for his injuries, having lost his job at the garage and without good prospects. The top of the battery in a picture he took shortly after the accident showed extensive fracturing of the top, with four of the six cells exposed to view. Electrolyte levels were high, showing that the battery was not old.

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5.15 Top of exploded car battery.

5.16 Side view showing bulging ends of battery case.

There appeared to be no distortion of the plates. Unfortunately, none of the pressure-relief valves were collected after the accident, so could not be pressure tested. When examined, the battery ends showed extensive deformation (Fig. 5.16), despite the fact that the accident had happened a year before in 2004, and the battery had been stored as it was, after the incident. Plastics creep under load, and it was clear that the permanent deformation or bulging of the ends could only have been produced by long-term high internal pressure. One end had also cracked at the centre of the panel, suggesting that the final event in over-pressurization was a violent internal explosion, as might be expected with the top showing extensive fragmentation (Fig. 5.15). When examined, the plates had sulphated and become distorted, but were originally in a good condition after the accident.

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A photograph taken at the time of the accident showed the distortion in the casing was present then, so the bulges in the ends could not have been produced by plate expansion, for example. The lead metalwork inside the battery appeared intact and undamaged. The brittle cracking of the case was very similar to that found in the previous investigation, showing that the polypropylene could not withstand the high pressure generated in hydrogen explosions. But what caused it to explode? There were several possible theories. In the first place, the bulging of the ends suggests that over-charging had occurred, perhaps after one cell had failed. It is also likely that the pressure relief valves had failed to release the pressure, allowing a dangerous mixture of hydrogen and oxygen to accumulate above the electrolyte. When the battery was jarred by the mechanic trying to move it, a spark inside the battery ignited the gases, which exploded with great violence and shattered the case. It is interesting to note that the damage to the top was considerably greater than that in the last example studied, where only one cell had been broken. That implies that the internal pressure was much greater, so increasing the power of the explosion. One cell igniting would have triggered similar explosions in the other cells. Battery explosions are more common than might be supposed, several examples being described by their users on the web (6), and more serious incidents have occurred in battery charging rooms, where much greater volumes of hydrogen can accumulate (7). The subject is of great topical interest owing to the possible use of hydrogen as a fuel for cars powered by fuel cells, which generate electricity by allowing the gas to react with oxygen on a catalyst substrate. The technology is well developed and organizations like NASA have long experience in dealing with liquid hydrogen fuel for rockets (8).

5.4.5 Hindenburg disaster, 1937 However, hydrogen in the past has caused some disasters, most notably the fire that engulfed the Hindenburg airship in 1937 (Fig. 5.17). The precise cause is still a matter of debate, but the facts are undisputed (9). On the night of 3 May 1937, the Hindenburg left Frankfurt for Lakehurst, New Jersey. It was the pride of Germany, the country having pioneered the use of giant airships supported by hydrogen held in cells within a rigid aluminium alloy frame. The idea had been developed by Zeppelin before the First World War (when they were used to bomb Britain). They had been the most successful in exploiting the commercial possibilities by developing commercial services, and had been supported by the Nazi government. Hydrogen gas was used as the main lifting medium because the safer helium gas had been embargoed by the US government. When it arrived at

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5.17 The Hindenburg disaster, 1937 (Wikipedia Commons).

Lakehurst on the evening of 6 May, the weather was threatening and thunderclouds had only just cleared the area. As it came into land, it dropped water ballast to trim its attitude because the rear end was falling compared to the rest of the fuselage. A mooring rope was dropped to the ground just afterwards, when the ship was 90 metres from the ground and 244 metres from the mooring mast. A small fire was then spotted at 7.25 pm by witnesses on the ground, and the fire spread rapidly from near the rear fin as the fire took hold (Fig. 5.17). The sequence of events was recorded live on air by the reporter Herb Morrison as the craft descended to the ground in flames. Of the 36 passengers and 61 crew, 13 passengers and 22 crew died. In addition a member of the ground crew was killed. Most deaths were not caused directly by the fire but by jumping from the burning airship. Those passengers who rode the airship on its descent to the ground survived. The subsequent enquiry (10) investigated several possible theories, including sabotage. They concluded that a static electricity spark (from the thunderstorm) had ignited a leak of hydrogen, probably when the mooring rope was dropped and earthed the structure. However, the disaster remains an active area of investigation owing to the uncertainty in the evidence. Recent work by Addison Bain (11) has shown that the fabric was a highly inflammable mixture of cotton fibre, cellulose dope, iron oxide and aluminium powder (Fig. 5.18). The composition is similar to that of a thermite mixture (widely used for incendiary bombs in the Second World War) and Bain suggests that the fire started with sparking of the fabric, and then spread to the stored hydrogen. Whatever the exact cause, the disaster ended the reign of the airship and the remaining ships were scrapped; the aluminium was recycled into military aircraft of the Luftwaffe. Britain’s

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5.18 The outer fabric of the airship (Wikipedia commons).

own airship programme had been halted in 1930 after the R101 disaster, when the heavily laden airship crashed at Beauvais on its way to India and caught fire, killing 48 passengers and crew. The tragedy was blamed on poor quality outer fabric which tore open during the storm the ship was negotiating at the time. However, the advent of new high-strength polymer fibres like aramids and modern elastomers has rejuvenated the industry, at least for small airships. They use helium rather than the much more hazardous hydrogen for lifting purposes. Industrial accidents also occur, such as that at Laporte Chemicals in 1975, when an electrolytic cell used for hydrogen production exploded. One man died from severe burns when the caustic soda electrolyte was expelled (12). The accident was caused by corrosion within the cell, of which some warning had been given just prior to the incident, but went unheeded. The explosion caused severe damage to the building housing the process. The investigation showed that oxygen and hydrogen was caused by unexpected corrosion within the cell, and the gas mixture ignited by an internal short, not unlike the battery explosions already discussed above.

5.5

Failed truck battery cases

With the widespread adoption of polypropylene for car battery casings prompted by the Polycase patent (Fig. 5.16), the next target was to be truck and traction batteries. They are usually heavy duty, needing more stored energy for the greater demands of lorries and trucks when compared with car usage. Traction cases are usually larger again, being used on locomotives, for example. Because they inevitably contain more lead in the form of plates and grids, they are much heavier, so putting extra demands on the case. It implies that detail design and choice of the best grade of polymer

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must be to higher standards than accepted for car batteries. We were involved in the design stage when the company asked for evaluation of the strength of the cases first moulded.

5.5.1 First failures The first design was rather fragile, as several returns from users showed (Fig. 5.19). The particular complaint came from Ford, who returned smashed cases for evaluation. Like other major users, they had specifications for proving batteries, most important of which was a pendulum impact test involving a 1 kg sphere moving through 1 metre to hit the case. Since the energy, E is simply: E = mgh

5.2

where m is the mass of the striker, g the acceleration due to gravity (9.81 ms−2), and h the height through which it moves, the test has an energy of 10 joules, a rather modest value. Various parts of the product can be tested since the ball is small compared with the size of the container, and each part must exceed 10 joules. However, the cases were failing in many different places, so we were asked to examine the design and report back. The examination took several forms: the grade of polypropylene used, the way it had been moulded and the geometry of the design. A check using FTIR spectroscopy showed the material had not degraded or oxidized, and

5.19 Cracked prototype truck battery lids.

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that regrind polymer had not been used. However, GPC analysis showed that the grade of polymer was the lowest strength material supplied by ICI. It also had the highest MFI (melt flow index), a rough guide of its molecular weight. The moulding process appeared correct, but the design included numerous sharp inner corners which acted to weaken the product drastically (Fig. 5.20). Although the specification drawing gave recommended radii, the product had much higher radii when examined directly. Thus the lower inner corners had a radius of 0.5 mm compared with the specification of between 1 and 1.6 mm. It was clear that the toolmaker had not conformed to the drawing in these key design details, and so unintentionally weakened the product. At buttresses, the corners were 0.05 mm compared with a recommendation of 1 mm. In addition, the extruded bead of molten polymer at the thermally welded lid of the case had radii of about 0.02 mm, and extremely sharp corners which initiated brittle cracks when impacted. While such radii are beyond easy control, the other stress concentrators could be easily rectified by simply rounding out the sharp external corners of the steel tool core. It was clear that the lowest grade of polymer had been used to make the cases, and should be changed immediately to an improved grade. The higher impact grades of polypropylene are copolymers with ethylene, and also have lower melt flow indices (higher molecular weight). The two

0.5 (1.0)

0.5 (1.6)

5.20 Sharp inner corners on truck cases: actual radii in mm with specification in parentheses.

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changes (increased radii and higher grade) resulted in an acceptable product which passed approval tests. However, it seemed that trade moulders always used the lowest grades of polymer in the absence of specific instructions, probably because it made moulding much easier and gave higher production volumes since the cycle time is generally lower. Such behaviour guarantees product failure, and is costly to track down and change because by the time the mistakes are discovered by testing, a large number of products have been made, and must be scrapped. In the worst case scenario, where testing is absent or ineffective, such defective products enter the market place and cause accidents. Toolmakers should have no excuse for leaving sharp radii in products, but many seem quite unaware of the damage they can create by not meeting a clear specification on the drawing with which they are supplied. The presence of sharp corners is of course not limited to heavy batteries, but has been used in many other products, where they continue to provide a convenient route to premature failure, often at the user’s expense. This may be one reason why many plastic products have such a poor reputation with consumers. Designers really should know better, although few have any detailed knowledge of the mechanical behaviour of the materials which they specify. A final point was made in the report. The carrying handles were a particular hazard given the problem of a similar design in the failed bucket. The stress concentration factor in the battery was estimated to be about 5.2, which together with the distinct chance of a weld line, made it an unacceptable feature. It was recommended to be removed and alternative means of carrying the case adopted. Carrying handles are now usually pivoted under the rim, while car batteries generally either have lips on the main case or a strap to aid lifting. However, some designs still retain an exposed lug on the lid, and should be treated with respect when lifting. The much larger traction batteries and standby batteries for computer protection (in the case of a power cut) are installed only once with specialist handling equipment. But even here, there can be problems as is described in the case of a fire on the Hong Kong transit system in the next chapter.

5.6

Failures in miner lamp battery casings

Coal mining underground has long been a hazardous occupation owing to the ever-present flammable gas methane, with explosive limits 5.3–17%, and detonation limits 6.3–13.5% at normal atmospheric pressure, much narrower than hydrogen. Nevertheless, methane explosions have caused some of the worst pit disasters in the long history of coal mining, and still continue today in countries such as China and India with their rapidly developing energy base. The first important safety lamps were invented independently by Davy and Stephenson in 1815, and were based on the

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need to restrict access of the outer atmosphere to the burning flame of the lamp. Davy used an iron gauze, while George Stephenson used a system of narrow pipes. Neither lamp was a good source of light, and actually gave a false sense of safety, since they were easily damaged and rendered unsafe. It only needed a single break in the gauze to allow explosions, a situation easily caused by rusting of the gauze, and deaths from methane explosions continued to rise. The worst mining disasters were yet to come, because methane explosions raised coal dust from the galleries, and it exploded in turn. Such dust explosions were much more serious because they could engulf the entire pit. Michael Faraday and Charles Lyell investigated just such a devastating explosion at Haswell colliery, Co. Durham in September, 1844 which killed 95 miners, and concluded that coal dust was to blame. Such disasters culminated in the disaster at Senghenydd colliery on 14 October 1913, killing 439 miners (13). Many attempts were made to improve the design of the safety lamp, including use of a glass screen around the flame and multiple rather than a single gauze, but the light was always very poor when cutting a black mineral in the dark. Matters improved when electric lights (14) came into use as late as 1911, powered by a portable battery (iron-nickel or lead-acid). At first they were carried by hand, but a head lamp was invented in the 1920s for attachment to a hard hat, and so allowing the miner free use of his hands. The safety of the lamp relied initially on metal cases, but nonmetals such as hard rubber soon came into use. But the introduction of polypropylene for car and truck batteries prompted manufacturers in the USA to design a new kind of case, in thermoplastic polycarbonate. The material was tough (at least in extruded sheet) and was advertised for bullet-proof glazing. The material was stiff enough to be used in 2–3 mm thick walls, and details such as belt loops and ribs could be designed into the case and made in one step by injection moulding. The existing design used in British collieries consisted of hard rubber with a central screw vent for topping up the electrolyte in the single cell. There had been problems with leaks from this vent – not unsurprising given the problem of leaks from larger battery cases. The miners’ lamp battery was different in design from traction truck or car batteries: they are designed for providing an initial deep discharge of power when cranking the engine during start-up, but thereafter, they are fully charged and provide a smaller amount of power for lights, heating and sparking (if a petrol engine) but are constantly recharged by the generator. They should always be at a state of near complete charge. By contrast, miners’ lamps are deep discharged during each 8-hour shift, and must be recharged on a frame in the lamp room of the colliery when they are not being used. They require much more robust separators (often a glass fibre mat) between the positive and

200

Forensic polymer engineering

negative plates to prevent dendritic growth of material between the plates, which if it succeeds, short circuits the cell and loss of power results.

5.6.1 New design in polycarbonate However, polycarbonate is not easy to injection mould because the molten polymer has a high viscosity compared with conventional polymers, and is also non-Newtonian in behaviour (Fig. 3.3). This means that the viscosity does not fall very fast as the shear rate increases in the narrow pipes of the moulding machine and tool, so it can present difficulties, especially to trade moulders who were experienced in conventional polymers like PMMA, polystyrene and ABS. The lower molecular weight grades needed for injection moulding possessed lower strength than the higher grades used in extrusion, so there might be problems in designing polycarbonate products. There are other problems, too, especially in terms of the problem of unwanted chain orientation, a problem of frozen-in strain. If the tools are held at ambient temperatures, or even cooled (as is common with many other polymers), then the material is quenched so that the polymer chains are locked into non-equilibrium shapes. If the temperature is raised, the product can distort, and the strength also falls in areas of high chain orientation. Cracks may be initiated during manufacture from those areas in the moulding. Following the introduction of polycarbonate battery cases in the USA in the 1960s, other companies worldwide tried to imitate the product. Unfortunately, designs were introduced without sufficient testing or evaluation under the severe conditions used in collieries, resulting in large numbers of failures. The design was different from the US case, probably to avoid any intellectual property problems (such as design copyright for example). But the changes in design were to produce problems of many other kinds, and quite unexpected to the engineers concerned.

5.6.2 First failures We were approached by a large Manchester battery company, Oldhams of Denton, in the mid-1970s when failures were reported from several collieries across the north of England, especially in Lancashire and Yorkshire. The National Coal Board (NCB) asked for an independent investigation of the problem, which was clearly affecting working conditions in the pits. Indeed, the National Union of Mineworkers had threatened legal action over damage to clothing from the acid spillages from brittle cracks in the cases. The failures had been occurring since July 1974 when the first new design in polycarbonate had been introduced. With a total of nearly 60 000 new

Small polymeric containers 40 000

100 000 Number of boxes 10 000

Number of boxes

201

Failed boxes

30 000 1000

Failed boxes 20 000

100 10 000

0

10

1 1972

1973

1974 year

1972 1973

1974 year

5.21 Histograms of failed mining lamp cases, normal at left, logarithmic at right.

lamps sent to pits over the two-year period to 1974, the failure rate varied from 1.7 to 6.8%, and the average over the whole period was 3.3%. The majority of returns from the lamp rooms (where most failures were found) were battery cases which leaked acid (60%) followed by damaged cases (25%), the rest comprising broken belt loops (7%), lid-case leaks (0.7%) and other unspecified fractures. Although the overall figure of 3.3% might seem small, there were 1964 returned batteries to be examined, and in fact it was a very serious situation because it indicated that the basic design could be faulty. To impress on management the urgent need for attention, two graphs of failures were presented (Fig. 5.21), one being a normal linear histogram, the other one plotted on a logarithmic base to emphasize the failure rates. The main symptom of failure was leakage of sulphuric acid from the windows fitted to the front of the 4 V cases, devices intended to allow the electrolyte to be topped up by lamp room attendants, where the lamps were stored when not being used (Figs 5.22, 5.23). The brittle cracks occurred around the edges of the windows, and grew with time following manufacture. Failures were occurring in new lamps and so it was necessary to inspect the way they were made at the factory.

5.6.3 Solvent cracking The critical step in assembly was the joining of the various parts together. It was done by solvent welding using a powerful organic solvent (a mixture

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Forensic polymer engineering

5.22 Various designs of miners’ lamp, most recent in front.

Leak

5.23 Leaks in polycarbonate case from ESC.

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203

of methylene chloride and ethylene chloride, CH2Cl2 and C2H4Cl2). Such solvents are normally used for dry cleaning clothes, where they are very effective. The solvent penetrated and softened the polymer before the two parts were pressed together, and the joint allowed to dry. It included the two windows and the top to the main moulding. Using such a volatile liquid gave an immediate problem in the welds, because bubbles formed very quickly as the solvent evaporated. However, such defects could not explain why brittle cracks occurred away from the weld itself. Another mechanism was operating. But one recommendation was to use a solution of polymer in the volatile mixture so as to produce a more viscous adhesive, and less prone to splashing or volatilization. We also recommended using smaller volumes of adhesive, and testing every lamp by applying a pressure test to each window to check that good seal had been formed. A later test using a torque wrench on every battery loop was introduced to ensure strong belt loops. The cracks were ESC (environmental stress cracks) where solvent interacted with the polymer and the cracks grew slowly. It is well known that swollen polymer is much weaker mechanically than the solid, but an applied stress is needed to encourage crack growth. Although the problem of residual stress is found (such as in storage tank liners), there is another effect in polymers known as ‘residual strain’ where unwanted chain orientation from moulding can relax, and so form cracks when exposed to certain organic liquids. The orientation can be detected using polarized light, so it was necessary to have transparent sample boxes made to check moulding conditions. At the same time, the moulders should supply data on their moulding conditions to check that they were complying with manufacturer’s recommendations.

5.6.4 Strain birefringence The transparent cases were examined by simply placing them between crossed polars using Polaroid sheet, the same sheet plastic used in sunglasses. Any chain orientation shows up as coloured fringes when seen in white light, or black and white fringes when viewed in monochromatic light. The effects of welding could then be compared directly. The photographs showed that there were high levels of orientation near the window, and that it was modified by solvent welding (Fig. 5.24). The relation between birefringence Δn and the principal stress difference Δσ is simple: Δn = Q.Δσ

5.3

where Q is the strain optical coefficient, which for polycarbonate is 80 × 10−12 m2N−1. The sequence of colours produced is of a series of red fringes of decreasing intensity as the birefringence increases. The first order red is

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Forensic polymer engineering

5.24 Brittle cracks around windows.

equivalent to a stress of 2.6 MNm−2, the second order red to 4 MNm−2 and so on. In the colour pictures a first order red can be seen close to the edge of the window, while higher order reds lie further away. The red fringes are represented by dark lines in the black and white figures. Solvent welding changes the pattern, but the first order red fringe remains. So a strain existed in the edge next to the solvent weld, and solvent-induced cracks grew from the moment the batteries were assembled. Photographs of the front of the case showed the problem of orientation in the face of the box, with high-order fringes located at abrupt changes in the melt flow (Fig. 5.25). The injection points at the gates are visible at the bottom of the picture, while high-order fringes can be seen next to the edges of the box where the flow has been forced around the corners of the product. The walls also show high orientation near corners and edges near the top of the case (Fig. 5.26). Such zones were studied later when other defects in the design became apparent. The method is quite general to many other transparent polymers, including polystyrene, HIPS and SAN as well as polyurethanes, all of which have high stress optical coefficients. Simply placing a sample between crossed polars reveals not just the flow patterns made by the polymer during moulding, but also such features as weld lines and the point of injection. High chain orientation occurs at corners, where the molten polymer is forced around the obstacle (the corner of the tool). This is one reason why sharp corners are especially pernicious in product design owing to the high stress concentration combined with high orientation: a deadly mixture.

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205

5.25 Strain birefringence patterns around windows before and after welding.

5.26 Overall pattern of birefringence in miners’ case.

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Forensic polymer engineering

5.6.5 Property checks It was prudent, however, to check that the polymer had not been degraded during manufacture, a problem which can arise if the polymer has not been dried correctly. Polycarbonate like many other step-growth polymers is susceptible to hydrolysis at high temperatures if even traces of moisture are present. So samples before and after moulding were checked using GPC, a method which gives an idea of the molecular weight distribution of the polymer. If hydrolysis had occurred, then the distribution would be expected to fall. However, the samples all showed identical or very similar distributions. The mechanical properties of tensile dumbbell-bars machined from cases were checked, and found to correspond in showing a yield point before failing after yield, and the start of cold drawing (Fig. 5.27). However, the red filled polymer showed lower tensile strength compared with transparent samples. Used red boxes showed yet lower strength, a reflection of the surface damage and abrasion, and an indicator of the notch sensitivity of the polymer. The lower strength of red filled polymer is a symptom of the effect of fillers on properties: very few increase the strength and most in fact lower the strength by providing particulate stress concentrations within the material. A major recommendation of the initial report would therefore be to switch grades to a transparent polymer. The suggestion was adopted, although a black dye was added to obscure the battery innards. The fracture surfaces of the window cracks showed completely brittle characteristics, and details showed how the cracks had started and grown. The macrograph of the top of the cracked window showed the presence of numerous holes in the weld caused by premature evaporation of solvent (centre) and cracks appear to have started here and grown intermittently through the sides of the window. Closer inspection of the crack surface showed as series of lines roughly at right angles to the crack growth direction, suggestive of a fatigue process, perhaps induced when the batteries A B C D

100 Nominal stress MNm–2

(Rectangular dumbbell) Yield point Cold drawing Normal fracture Fracture after polishing D

A

C

50 B Polished

As received, machined 0

0.5

1.0

1.5

Nominal strain

2.0

5.27 Tensile stress–strain curve for sheet polycarbonate.

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207

were used by miners. Information from the NCB (and our own experience) suggested that miners used the battery hung from the back belt, as a wedge when passing through narrow workings. Such use would put extra stresses on the window area of the case. Further research revealed an innovative new way of examining the properties of the material.

5.6.6 Polishing The effect of surface quality on strength led us to develop methods for polishing the surface, and it turned out that this polymer could be polished by chemical milling, a method long used by metallurgists in examining thin foils for electron microscopy, for example. After screening many chemicals, a solution of potassium hydroxide (KOH) in methanol (MeOH) was found to produce the fastest rate of attack (15). The effect on the strength of polycarbonate was rather dramatic, with the cold drawing region extended well beyond the zone just after the yield point where it usually fractured (Fig. 5.27). The method also produced numerous etch pits when examined under the optical microscope (Fig. 5.28), the shape and distribution depending on the mechanical history of the part examined. Thus polymer strained near the yield point (a) showed circular etch pits, while beyond yield the number increased markedly (b). At fracture the shape of the etch pits changed so that they became elongated at right angles to the strain axis (c). There could be little doubt that they represented craze formation within the bulk material. The method of etching polycarbonate is actually widely used for counting damage from radioactive materials in the badges used by personnel working with such hazardous materials (16). The new phenomenon was investigated by following the change of profile of scratches of known width and depth inscribed on the surface of PC samples using a diamond point mounted in a rig (and known affectionately

50 μm

a

b

c

5.28 Etch pits in strained PC: a) unstrained, b) after yield, c) at fracture.

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Forensic polymer engineering

as ‘little scratcher’). The method had been used by others to measure the thermal relaxation of grooves in silicon-iron (17). The rate of polishing could be followed using double-beam interference microscopy with yellow sodium light, a convenient monochromatic light source (Fig. 5.29). The composite picture shows two sets on interferograms, the one at left for a weak solution of 2.6 N, that at right for a maximum strength solution of 6.29 N. The original surface is shown at the top of each figure, with successive polishing phases below. There are two ruled grooves of 40 microns and 24 microns width on the original surface, plus many minor or adventitious scratches. As might be expected, smaller shallower scratches are removed first, while the largest require removal of more of the overall surface (groove depth was measured by counting the number of fringes from the surface to the deepest part of the groove). The phenomena were analyzed in terms of the relevant diffusion equation (18), which after a number of simplifying assumptions shows that the relative crack depth (b0/b) was given by the simple equation: ln (b0/b) = 2πu/a

5.4

where u is the depth of polymer removed and a is the wavelength of the groove. The equation appears to be confirmed after the initial etching phase by the linear parts of the curves shown in Figs 5.29 and 5.30.

a a

b

b

c

c d

e

50 μm

d

5.29 Interferometer traces of scratch polishing.

50 μm

Small polymeric containers

18 μm

0.8

45 μm

209

68 μm

log bo/b

0.4 110 μm

0

–0.4 0

u

80

40

160 μm

120

5.30 Polycarbonate polishing as a function of crack size.

6.92N

0.4

5.24N

log bo/b 4.24N

0

3.56N 2.60N –0.4 0

Etching

Polishing 100

200 μm

u

5.31 Polishing as a function of caustic concentration.

The most concentrated solution, a vicious brew which also dissolved skin very quickly (so needed ample personal protection), could thus be followed and the depth, b of the cracks measured as a function of depth of removal of polymer (Fig. 5.30). Each experiment showed that polishing was preceded by an etching phase, easily explained because it takes some time for a viscous layer to build up over the polymer surface (Fig. 5.31). It is this viscous layer of degradant in solution which is the mechanism of polishing because active agent has to diffuse through the layer to reach the surface. Naturally those scratches which are deeper within the layer will be less rapidly attacked than the shallower ones. The method allows ultrathin films

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Forensic polymer engineering b

E

c E

2 μm

a

200 nm

5.32 Etch pit E in TEM, details at b and c.

O

CH3 O

C CH3

O

C

O

O

MeO

+

CH3 C CH3

O

CH3 O

C CH3

CO3CH3

5.33 Polishing mechanism of polycarbonate.

of polymer to be made and an etch pit in one sample allowed us to calculate the surface roughness produced by the method (Fig. 5.32). The film shown has been polished to 5.9 microns and the insets show best and worst surface quality, which varies between about 10 up to 50 microns. It is the hydroxyl ions (OH−) of the alkaline fluid which attack the carbonate group, the functional group in the polymer, as shown by the reaction mechanism of Fig. 5.33. It is well known that polycarbonate is sensitive to alkali degradation and stress corrosion cracking in alkali, and our experiments really took the phenomenon to an extreme. The research programme highlighted the sensitivity of polycarbonate to alkaline degradation with very rapid attack by many reagents. The same research was extended to other engineering polymers and showed how others were similarly susceptible to hydrolytic degradation, such as aramid fibre and PET in concentrated sulphuric acid, and polyimide in hydrazine, for example. The polishing method proved valuable in being able to polish a battery case away and so study the variation of birefringence with depth through the thickness of the box, and it confirmed that much of the unwanted orientation occurred in the surface layers where attack from the solvent welding treatment could initiate (Figs 5.34 and 5.35). But the key issue of reducing lamp failures was to tackle the problem at source, at the moulders.

Optical retardation mm–1 (m × 10–7)

Optical retardation mm–1 (m × 10–7)

0

211

18 16 14 12 10 8 6 4 2

18 16 14 12 10 8 6 4 2 1.0 2.0 3.0 (mm) Sample 3 chemically etched

18 16 14 12 10 8 6 4 2 0

Optical retardation mm–1 (m × 10–7)

Optical retardation mm–1 (m × 10–7)

Small polymeric containers

0 1.0 2.0 3.0 (mm) Outside Centre Inside of case of case Sample 4 sectioned mechanically

18 16 14 12 10 8 6 4 2

1.0 2.0 3.0 (mm) Outside Inside Sample 5 sectioned mechanically

0

1.0 2.0 3.0 (mm) Sample 1 chemically etched

5.34 Destructive examination of miners’ lamp cases.

Equivalent 30 tensile residual 24 stress MNm–2 18

Section surface

12 6

Centre of section

0

2

4 6 8 10 12 14 Distance from injection point (cm)

5.35 Residual strain in miners’ case from gate to end.

5.6.7 Moulding conditions The quality of the boxes was highly dependent on the moulding conditions used by the trade moulder. The moulder was asked to produce boxes under different tool temperature conditions so that we could study the birefringence of the polymer, especially in relation to the solvent welded zones.

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Forensic polymer engineering

The trade moulder said that he used a tool temperature of 60–70°C, which was at the very lower end of the manufacturer’s (General Electric Plastics, USA) recommended a lower limit of about 80°C. We asked that he mould transparent cases at up to 110°C so that we could track any changes in properties. The results showed a dramatic decrease in chain orientation and also gave an insight to the distribution of orientation across the case thickness (Fig. 5.36). Samples from the base near to the gate of the box gave the highest strains, while those from the top the lowest, an expected result because orientation is always greatest near the gate, where molten polymer is forced into the steel tool. The equivalent frozen-in stress, σ, can be calculated with the equation: r = Qσd

5.5

20

20

18

18

16 14

70/60/290°C Cavity/core/melt T’s

12 10 8 6

100/90/290°C Cavity/core/melt T’s

4 2 0

6 2 4 8 10 12 14 Distance from injection point (cm)

Optical retardation mm–1 (m × 10–7)

Optical retardation mm–1 (m × 10–7)

where r is the retardation (metres), Q the stress optical coefficient and d the sample thickness. The values shown along the length of the box (Fig. 5.35) show how the stress is greatest at the surface everywhere when compared with the centre of the moulding, just the conditions which promote environmental stress (or strain) cracking. In a similar experiment, the tool temperatures were compared (Fig. 5.36). They showed how increasing the temperature lowered the degree of orientation quite dramatically, especially at the base. Essentially, increasing the tool temperatures allows the chain molecules to relax more easily to an equilibrium state, rather than being quenched into an unstable state.

16

70/60/290°C Cavity/core/melt T’s

14 12 10 8 6 4

100/90/290 Cavity/core/melt T’s

2 0

2 4 6 8 10 12 14 Distance from injection point (cm)

5.36 Moulding conditions and residual strain in cases.

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213

The solution to the problem thus took several forms, including change to the material (red to cloudy black), improving the adhesive and increasing the temperature of the tools. But there was another recommendation, which was perhaps the easiest to implement of all. It concerned the notch sensitivity of the polymer.

5.6.8 Stress concentrations While surface scratches could lower the product strength, much more important was the fixed geometry of the design itself. Because polycarbonate is notch sensitive, any sharp corners will act as serious stress concentrators, and those points will be the weakest parts of the structure. And the design exhibited numerous sharp corners when examined closely. One important criterion for testing the design lay in a drop impact test, which mimicked the use a battery might experience in use, when dropped accidentally in the lamp room, for example. The interior corners at the base are critical because they lie behind impact zones and could initiate brittle cracks if the outer corners impacted the ground. The stress concentration Kt, is simply the ratio between the real stress at a point, σmax and the nominal applied stress, σnom: Kt = σmax/σnom

5.6

The standard compilations like Peterson did not have the relevant diagram for internal corners, but a paper had been published which did just this (19) and so could be used for estimating the stress concentration of an internal corner (Fig. 5.37). For the geometry of the box, then the key variables are d (wall thickness), h, the floor thickness and R, the radius of curvature at the corner: d = 3.5 mm h = 4.0 mm so h/d = 4.0/3.5 = 1.14. The set of stress concentration curves could used to evaluate the factor Kt for various radii of curvature at the corner given this value of h/d, the horizontal axis in Fig. 5.37. In the first design of battery box, R was 0.1 mm, so R/d = 0.1/3.5 = 0.028 Taking the extreme value of h/d = 1 at right in Fig. 5.37, then the relevant curve lies above the highest curve of R/d = 0.1, and gave Kt approaching a value of 2. The boxes were then modified to a radius of 0.4 mm, which gave R/d = 0.4/ 3.5 = 0.15

214

Forensic polymer engineering σ

d R W

W bd σmax

b = thickness of model

h

σ=

1.6 d 30

20 R d 0.1 0.2 0.3 0.4 Position of maximum stress independent of load position for h/d > 0.5

σ max σ

10

0 0.4

0.6

0.8

1.0

h d

5.37 Stress concentration at inner corner in box.

This yielded an approximate value of Kt ∼ 1.3. To obtain an estimate of the radius for no stress concentration, then Kt = 1 when R/d = 0.25, so R/d = 0.25 and so R = 0.25 × d = 0.25 × 3.5 = 0.6 mm. So analysis suggested that the minimum radius of the lower corner should be about 0.6 mm. This value could easily be achieved by simply smoothing the sharp corners of the core of the tool (that part which creates the shape of the interior of the box) using emery paper, not a costly operation given the importance in increasing the strength of the product. It proved difficult in convincing moulders that sharp corners lowered the strength of the boxes until one of us challenged one of them to test a new case for himself by smashing the base with a large hammer. When it cracked,

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215

5.38 Cracks at inner corners in leaking miners’ lamp.

he appreciated the problem, and the radii were changed. Brittle cracks from the base inner corners were common at that time (Fig. 5.38).

5.6.9 Practical applications Another example of the importance of minimizing radii came with a failed lamp which we were asked to analyse in 1979. The battery was relatively old, which was encouraging because it suggested that most batteries were by then starting to achieve their specification life of 3 years. This time, the battery had failed from one end by splitting along a vertical seam produced by mating of the tool parts (Fig. 5.39). The positive or active plates had expanded and put the ends of each cell under internal pressure, so that the ends distorted and eventually cracked along the weakest part, the small sharp corner at the seam. The inner surface was pitted where the holes in the protective sleeve around the plates had met the surface. The chemical attack of the material had been made by the lead dioxide (a powerful oxidant) extruded from the holes and making contact with the polymer surface. The pendant methyl groups in the bis-phenol part of the repeat unit were probably oxidized, and the chains split here rather than at the carbonate group. They formed the deep etch holes seen in the sections (Fig. 5.39). The boxes cracked along the sharp corner in the tool mating line, where tool wear had created the mismatch. The tool had made up to about 700 000 polycarbonate boxes and was showing its age. It was replaced with a new tool incorporating the latest design modifications we had recommended, and the lives of batteries increased from the low initial lifetimes. Belt loops were another weak zone in the design where brittle cracks developed, and were encouraged by local high orientation (Figs 5.40 and 5.41). External cracks such as that shown grew on the outer surface but could grow through the thickness and so produce a leak of the electrolyte. So the final stage in making a safe casing was to ask the toolmaker to

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Forensic polymer engineering

5.39 Brittle crack induced by positive plate expansion (outer side at left).

5.40 Crack at belt loop corner.

remove sharp corners by radiusing the tool. Since the steel tool forms the cavity within which the product is made, sharp corners in the product can be ameliorated by rounding sharp corners on the core of the steel tool, a simple and inexpensive procedure. The modifications produced a significant increase in impact strength.

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217

5.41 Residual strain at belt loop corners.

Indeed, we built test rigs designed to drop full batteries onto a concrete floor, and the drop height increased considerably as the new modifications were introduced. We also introduced a new method, a drop ball test, where a cast iron ball was dropped from various heights onto cases or batteries. The cast iron balls were actually cannon balls kindly lent by the Woolwich Arsenal, and they also increased in weight from 7 kg initially via 10 kg to 20 kg. The impact energy, E is just mgh, where m is the mass of the ball, g the acceleration due to gravity (9.81 ms−2) and h the drop height. So a case which survived a drop impact of a 10 kg ball from 2 metres possessed an impact energy of 196 Joules. With the improvements in design and moulding, the later cases were well capable of withstanding such high impacts, showing that the intrinsic strength of the material could be achieved when well made with a robust design.

5.6.10 Colliery experience The change in fortune of the first new design of the T type batteries was shown by the records we obtained from two Lancashire collieries, Parsonage near Wigan and Sutton Manor, near St Helens (Fig. 5.42). Although the design life was 3 years, most batteries failed in the first months of use, and there followed a steady number of failures, with none surviving beyond about 2 years. However, as the bonding improved and testing of key components was introduced, the lifetime began to improve steadily (Fig. 5.43). However, the record of the red cases was still rather poor, with most failing to reach the scheduled full life. By 1976, new black cases were starting to replace the red cells, and the life of the batteries increased yet again (Fig. 5.44). It was shown by records of the South Yorkshire region of the NCB, and which were part of a memo sent to the Senior Inspector of Mines (Table 5.1).

Forensic polymer engineering 80 Total no. of failures per qtr

Sutton Manor Colliery – battery case failures (E.Entwistle) 60 T type (polycarbonate)

40

R type (rubber)

20

0

73/74

74/75

76

Total no. of failures per qtr

80

Faulty cells

77

78

79 Year

Parsonage Colliery – battery case failures (D.Power)

60

40

T type cases

20 Faulty cells 0

73/74

75

76

77

78

79 Year

5.42 Failure records from two Lancashire collieries.

100

Number of failed batteries

218

80

60

40

20

Normal life of a battery

0 1 2 Lifetime (years)

3

5.43 Polycarbonate lamp lifetime before design changes.

Mean length in service (months)

Small polymeric containers

219

16

12

8

4

1974–75

1975–76

1976–77

1977–78

5.44 Failure records after design changes.

Table 5.1 Increase in battery life (numbers in parentheses)

Nov 1977–Jan 1978 Feb 1978–April 1978 May 1978–July 1978 August 1978–Oct 1978 Nov 1978–Jan 1979

Black cells

Red cells

1.04% 1.93% 2.99% 3.78% 2.66%

9.3% (3782) 8.42% (2612) 12.56% (1879) 16.5% (966) 7.77% (479)

(7 877) (9 079) (10 664) (11 369) (11 578)

So the black cells were replacing the red cells during this period and showing a much lower failure rate, although there was still substantial room for improvement. At this stage we were recommending ameliorating sharp corners both externally and within the product, and they increased the reliability of the lamps yet further. Moreover, we met several electrical engineers and lamp room managers, who all reported favourably on the new design in smoky black polymer compared with the red cells. There were several new design modifications needed, however.

5.7

Improving design to prevent failure

So the various modifications in material, design geometry and manufacture produced a safer product, although further changes were made in the light of other problems. The belt loops required strengthening in the light of loop fractures in the pits and, in a redesign, were increased in width as well as being buttressed and all corners being well rounded. The several trade moulders involved in making the cases and lids were visited to impress on them the importance of using hot tools, as well as using simple quality

220

Forensic polymer engineering

control tests to ensure that chain orientation was minimal. An obvious test was to use crossed Polaroid sheets to examine the birefringence of box parts. Then, when everyone thought that the problems were over, new failures occurred separately in Australia and in South Africa.

5.7.1 Alleged hydrogen explosion An incident had occurred in the Pasminco mine in New South Wales in 1989. The battery was maintenance-free, so there were no windows in the case. It was said that the battery had exploded, a very dangerous occurrence in a mine, so a thorough investigation was needed. The damaged battery showed loss of a chunk from the front face (Fig. 5.45), and it was possible to show how the failure had occurred by careful examination of the pattern of cracks. The failure started at the lid-box joint with two cracks travelling down into the box, although a third crack had been initiated much further down the face. It had originated on the interior of the box so must have been caused by a blow to the front. There were two crack intersections (I1 and I2) where the two sets of cracks had met (Fig. 5.46). The box had also been painted using a spray can, and tests on sprayed polymer showed it to be weakened by the treatment, although it did not initiate the cracks in the Pasminco battery. Paints have a carrier fluid, a light organic solvent,

5.45 Damaged case from Pasminco mine.

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Lid

O2 S

Case O1

I1 I2 O3

Paint zone

5.46 Possible crack path map.

which can initiate ESC cracks, and there had been a series of road accidents where polycarbonate crash helmets had failed in a brittle way, having been spray painted (20). An independent test with a new battery used a spark plug inserted into the case to explode the gas mixture. It produced similar brittle cracks, although the plates of the failed product showed no short circuits. In addition, a spray painted box was tested and failed through paint-induced cracking. It was concluded that the failure was not caused by an internal explosion, but rather by an external impact, which must have been of some force. The exercise was valuable in pointing out the hazards of spray painting the lamps, and the company in Australia advised to cease the practice.

5.7.2 South African lamps A subsidiary of Oldhams in South Africa produced lamps for their extensive mining industry. Working conditions in some of the deep gold mines were severe, and the base of their battery was protected by a rubber boot fitted to the base of the box. We were asked to compare the design of the case with those made by two trade moulders in Britain. The South African battery was maintenance-free while Oldhams were still using top-up cells, although the window had been reduced to a small hole in the front of the

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f

5.47 Strain birefringence patterns in cases of South African batteries, poor at right.

5.48 Strain birefringence patterns in cases of British batteries, poor at right.

case, with the non-spill device welded from the inner rear of the box. The semi-transparent faces of the battery cases were compared using strain birefringence. The comparison showed the high levels of frozen-in strain in the African battery and the lower level of the well-moulded British case (Figs 5.47 and 5.48). In order to forestall failures of the former, it was decided to provide trade moulders with a standard set of birefringence patterns against which they could judge box sections. When visiting new moulders needed to make extra cases, such a test was shown as a way of judging moulding conditions to achieve the best properties from the product. What proved most difficult was to convince toolmakers that removing or ameliorating sharp corners in the product would also be beneficial. It seemed to run counter to their perceived wisdom, where sharpness prob-

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ably equates with crispness and even aesthetic delight. They clearly do not see the product their tool makes. The investigator had to demonstrate the poor state of the boxes by smashing the base with a sledgehammer, and challenging the toolmaker to do the same. And it is by no means an attitude restricted to toolmakers of lamp products, as other cases elsewhere in the book describe. Common examples where sharp corners weaken products include screw thread roots, shoulders, holes, recesses, and numerous other design geometries. Although they may be needed for product function, all need careful design to minimize the inevitable stress concentration.

5.7.3 Further developments For example, the use of a semi-transparent case allowed users to monitor the condition of the cell plates. Owing to the impact and wear the case received in use, shedding of the plate materials occurred, leading to an accumulation of powder lead sulphate at the base. It gave rise to concerns among lamp room supervisors, who complained to the manufacturer. While there was loss of capacity, the batteries still had reserve power and so were still serviceable. However, the company developed PVC bags to surround the plates, a device which simply collected the powder shed by the plates and didn’t tackle the basic problem. Out of sight is out of mind. A much more important development has been the incorporation of methanometers in the headlamp, so giving every user advance warning of dangerous amounts of methane gas in the immediate environment. Methanometers have long been standard equipment for pit deputies, although most still carry flame safety lamps for back-up. The height of the ‘blue cone’ when methane burned within the lamp gives a direct measure of the methane content in the general air. There was some reserve in the new battery which could be used for powering such a device. It would improve the capability of the lamp in protecting the individual miner. But like the development of the new battery, there would be a long development phase needed to ensure that it, too, would be capable of resisting the severe working environment of pits. The existing Bakelite casing of the headlamp needed redesigning for insertion of the sensors, and extra inserts were needed for the battery lid for the associated electronic circuitry. The headlamp itself was redesigned in thermoplastic materials, with several problems of compatibility and moulding defects encountered. Thus a screw thread rim for holding the cap glass in place suffered failures due to the stress concentration at the thread root, and the tool was modified accordingly to prevent further failures. The process of changing the design and manufacture of the miners’ lamp lasted several years, and was ultimately successful in reducing failures so as to make it a safe and reliable product. In hindsight, most of the problems

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could have been anticipated by research and development before the new lamp was introduced into the pit environment, and so prevented the unacceptably high failure rates. Although polycarbonate was not a completely new material in the 1970s, experience with both manufacture and use was very limited and designers were not aware of the limitations of the polymer in moulded products. The most serious limitations included the notch sensitivity of the product and its sensitivity to ESC, environmental stress cracking, especially with high levels of chain orientation. By the time a reliable product had been developed, the coal industry itself was in serious trouble following a divisive miners’ strike, and demand dropped. However, the product was licensed to manufacturers in India and South Africa, for example, where underground mined coal has grown as the British industry has declined. The lamp is widely used in deep metal mines such as the gold mines of South Africa, and the coal mines of India. But the company itself was absorbed by Hawker-Siddeley, and then closed entirely in 2002, partly because of the poor factory conditions, the problems of working with lead and its highly toxic compounds, and the competition from battery makers in the Far East. The Chinese, in particular, have developed impressive mining products such as miners’ lamps for their rapidly expanding coal mining industry, and the latest models incorporate methanometers as well as small radio location devices which enable managers to pinpoint individual workers. Lithium batteries are also now being used for their greater power to weight ratio.

5.8

Conclusions

The development of new polymers and their application to safety-critical products has not been easy. Many mistakes were made in design and manufacture, resulting in numerous product failures. However, traditional materials such as hard rubber also presented severe quality problems, and the new materials were far superior when made correctly within appropriate design limits. Battery cases are effectively containers for chemical reactors, and must resist all that happens within, as well as the often highly corrosive electrolytes. The demands on the product have increased greatly as personal computers and mobile phones have become commonplace, and internal explosions have raised further problems of product integrity. The strength of the container presents the designer with a dilemma: most of its time, the container will be protected within a compartment and suffer very little stress. However, moving the battery raises the possibility of impact blows from falling, for example. Moreover, if the contents explode or expand, then the case will suffer, and expose the user to possible injury. Great care is thus needed in the choice of material and the way it is shaped, as well as the geometrical details of the product.

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225

References

(1) Brydson, J, Plastics Materials, 7th edn, Butterworth (1999). (2) Open University, Design and Manufacture with Polymers (T838), Data Book (1999). (3) Joseph Lucas (Batteries) Ltd. and Another-v-Gaedor Ltd et al. Reports of Patent cases (RPC), 10, 297–396 (1978). (4) Bone, WA and Townend, DTA, Flame and Combustion in Gases, Longmans Green and Co, Chapter XIX, p 217 ff (1927). (5) Van Krevelen, DW, Properties of Polymers, 3rd edn, Elsevier, p 732 ff (1990). (6) Examples are shown at http://www.tractorshed.com/fordnclub/npic6856.htm and http://www.rayvaughan.com/battery_safety.htm with descriptions of the incidents. (7) The site at http://www.indpwrbattery.com/hydrogen.aspx describes such an explosion. (8) http://www.nasa.gov/vision/earth/technologies/hydrogen.html discusses the use of hydrogen in space exploration. (9) Open University, Forensic Engineering (T839), Block 3 Catastrophes (2000). (10) Trimble, S, Report of Airship ‘Hindenburg’ Accident Investigation, Air Commerce Bulletin, 9(2), 21–37 (1937). (11) Bain, Addison, The Freedom Element: Living with Hydrogen, Blue Note Books (2004). (12) HM Factory Inspectorate, The explosion at Laporte Industries Ltd., Ilford, 5 April 1975, HMSO (1976). (13) Duckham, Baron and Helen, Great Pit Disasters: Great Britain 1700 to the Present Day, David & Charles (1973). (14) Oldham, CE, Vintage Centenary Issue, Oldham Batteries Ltd (1965). (15) Lewis, PR and Ward, RJ, Polishing, thinning and etching of polycarbonate, J Colloid and Interface Science, 47, 661 (1974). (16) Doi, M, Fujimoto, K and Kobayashi, S, Etch Pit Formation Model During Chemical and Electrochemical Etching in Polycarbonate Foil, Radiation Protection Dosimetry 37, 5–12 (1991). (17) Mills, B, Jones, H and Leak, GM, Thermal grooving in 3% silicon-iron, Metal Sci J, 1, 9 (1967). (18) Wagner, C, Contribution to the Theory of Electropolishing, J Electrochemical Soc, 101, 225 (1954). (19) Allinson, IM and Bacchus, KM, Design of internal load bearing flanges, Paper 13 in Experimental Stress Analysis. Proc 4th Int Conf on Experimental Stress Analysis, IMechE, Cambridge (1970). (20) Mills, NJ, Plastics: Microstructure and Engineering Applications, 3rd edn, Butterworth (2005).

6 Polymeric pipes and fittings

6.1

Introduction

The pipes that transport fluids between reservoirs are not dissimilar in the problems they present when failure occurs. Fluid leaks can cause substantial damage to property, and are often more insidious than the sudden flood that comes when a container fractures. They can thus go unnoticed until the leak triggers an accident or a fire, for example. Like containers, polymers have been widely adopted in many product applications, especially in the distribution of utilities like water and gas, the disposal of waste water and sewage, and in communications (such as carrying fibre-optic lines). They are much lighter to manhandle and are usually tough enough to withstand rough treatment during installation (1). They do not suffer the problems of corrosion that afflict steel pipes, and are generally of a low enough cost to be feasible for replacement lines when conventional distribution systems are renovated, for example. They may also be used to line existing pipes. Polyethylene and PVC are two of the most common thermoplastics used for piping, but others such as ABS are also used in special applications like pressurized air systems. Growth in their use has been very high in all nations of the world over the past two decades, either for replacement of older systems in conventional materials like cast iron, steel and earthenware, or for entirely new networks. When fractures occur in buried pipelines caused by internal pressure, of whatever material, the cracks tend to run along the axis of the pipe, and must be replaced at great cost since long lengths of pipe may have been destroyed. However, plastic pipes are sensitive to certain organic fluids and if they contact a stressed part, environmental stress cracking (ESC) can cause sudden and quite unexpected fracture, sometimes with disastrous results (2). When polybutene pipe was adopted for domestic hot water systems in the USA, for example, there were numerous failures from stress corrosion cracking (SCC) such as by oxidation (3). Plastic pipe can also suffer creep when imposed external loads are excessive for one reason or another, especially when the foundations move or when loads above buried lines increase. The fittings that accompany pipe systems are usually, but not always, constructed of the same polymer. Failure of or at joints is often more likely because joints are frequently injection moulded and thus inevitably of lower 226

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molecular weight and therefore of lower strength. They exhibit inevitable stress concentrations which magnify the overall stress at such features, so fittings such as bends, collars and two-way joints represent the weakest points in most load paths. Fittings are attached either by thermal welding (e.g., polyethylene) or by using a solvent cement, as in ABS and PVC. Welding of either type needs special care to ensure a tight, reliable and leak-proof joint.

6.2

Fracture of PVC water piping

When large water mains fracture, substantial damage can occur as the water floods out, as many road users will testify by the inconvenience when they break under, or by the sides of roads. The original water network was largely created during the Victorian period with the growth of industry and its great thirst for water supplies, closely followed by the demands from domestic users for clean and potable sources. Most of the original network utilized cast iron pipes of substantial construction, some of which is still in use today, although being replaced by thermoplastics as investment in the infrastructure improves. Cast iron is a very brittle material, and failures are frequent when the overburden on buried pipes changes for whatever reason, or when broken by careless workmen installing other systems nearby, or when water inside a pipe freezes. The expansion when the temperature rises splits the pipe (4). Utility companies frequently follow similar paths when supplying towns and cities, and they may not always coordinate their knowledge of their own network with those of other companies. As thermoplastic pipes replace older systems, there is a problem of compatibility of the new with the old: the properties of the former are quite different from cast iron or steel, especially in the loads that can be used safely on such pipes. Where two different systems meet then failure can occur at the junction, as the following case illustrates.

6.2.1 Factory crisis We were approached by the insurers, General Accident, when a factory in Flitwick, Bedfordshire was flooded by a pipe fracture in 1985. Because the factory made and upholstered furniture, the damage was very extensive, the water ruining much of the stock. The claim on the insurers was therefore large, and the insurers needed to know the cause of the failure. The accident happened at 7.00 am on 16 October 1984 when a rising main suddenly fractured and released a large volume of water into the factory. Thirty workers were there at the time, so discovery was immediate, and one operative nearby was knocked off his feet by the force of the jet of

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The fracture

6.1 Original photograph taken by loss adjuster.

water emerging from the broken PVC pipe. The pressure was sufficient to flood the premises with 4 inches of water in a few seconds, and the jet destroyed work in progress, sewing and cutting machines. Although they were able to turn off the sprinkler main within a few minutes, much damage had by then already occurred. The loss adjuster reported that the 6 inch diameter plastic main had broken about an inch above the concrete floor where the main emerged from the buried pipe line, in a Polaroid photograph taken by the adjuster after the accident (Fig. 6.1). The fracture occurred in a joint with a steel pipe above via a flanged junction. The system had been in place for about 7 years, but about 3 months prior to the accident, a tail end air valve was fitted to the steel pipe above the flange junction (Figs 6.2, 6.3). The joint was formed by bolting the two flanges together. An identical rising main without an air valve in an adjacent room remained intact, suggesting that the new fitment was associated with the failure. The loss adjuster thought that the failure had been caused by fatigue, but he failed to provide any supporting evidence for his theory. The water was at an internal pressure of 7 bar, but was completely still, ready for use only when the sprinklers were activated by fire. Each rising main was connected to a pipeline buried about 1.2 metres below the concrete floor (Fig. 6.4).

6.2.2 Analysis of broken pipe The examination of the fractured PVC pipe (Fig. 6.5) was clearly essential to explaining the failure. When the end had been removed from the steel

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6.2 Air valve fitted 3 months before accident.

6.3 Diagram of new air valve fitted to sprinkler system.

pipe, and the buried part removed by breaking up the concrete bed, it was essential to match the parts to see if anything could be gleaned about the stresses on the pipe. There were several points of interest in the fracture itself. In the first place, the fracture was entirely brittle and secondly, the cracks were circumferential, which meant that it was not the high internal water pressure which caused the failure, since pressure failures always occur along the length of the pipe rather than circumferentially. The fracture was rather

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6.4 Plan of sprinkler system.

6.5 Fractured collar of rising main on sprinkler system showing cusps.

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complex, partly because the pipe end had been damaged by picks used to extract it from the concrete in which it had been embedded. However, those parts which had been damaged were relatively easy to distinguish from the original cracks. But what did the fracture surfaces show? It was possible to follow the cracks as they ran around the pipe from such details as crack intersection and chevrons (or hackles) and to infer that the fracture probably started at the stress concentration of the flange shoulder where the pipe had been solvent welded to the injection moulded flange (Fig. 6.6). The cracks grew along the shoulder at two zones (Figs 6.7, 6.8), deviating into the pipe under a shear stress. Defects were present in or close to the corner of the solvent welded joint between the collar and the pipe, and the latter appears from crack junctions to have been the first to have propagated. The growth of two cracks had formed two cusps where they met (Fig. 6.5).

6.6 Fracture close-up showing solvent welded joint.

6.7 Close-up of fracture at solvent welded joint.

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6.8 Close-up of fracture at joint showing void between pipe and collar.

Fracture

6.9 Enlargement of original photo of fracture.

The stress concentration diagram provided by Pilkey (5) provided an indication of the magnitude of the effect at the shoulder radius. Since t = h and t/h = 1, the centre line applies, but what was the radius at the external corner? The wall thickness was 6.9 mm, and the radius of the solvent weld varied from less than 1 mm up to perhaps 2 mm. The maximum value of t/r was 4 so the minimum radius on the graph is 6.9/4 = 1.725, with a Kt factor of 2, so the minimum stress concentration factor was about two.

6.2.3 Reconstruction The two major parts could be matched together so as to reconstruct the pipe before fracture, using a blow-up of the loss adjuster’s photograph on which to base the reconstruction (Fig. 6.9). The original parts mated very roughly but there was some distortion of the pipe as well as parts missing from the fracture. Using the elliptical shape of the parts in the original photograph, the broken ends were reconstructed as shown in Fig. 6.10. The

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6.10 Reconstruction of fracture to show twist and separation.

picture showed that the broken ends had separated and twisted with respect to one another. The white marks were used as a datum line, showing the degree of twist between the two parts. The displacements were: angular twist = 12.5 degrees or 0.218 radians, and vertical separation of parts = 39 mm So the rising main had been under significant tension and torsion just before the final failure. But what were the stresses and how did they relate to the known strength of the material?

6.2.4 Stresses on pipe To consider all of the stresses acting on the pipe, it was necessary to consider the internal pressure as well as the tension/torsion from an unknown source. In all pipes, the hoop stress (σH) is twice the longitudinal stress (σl) when pressurized (Fig. 6.11): σH = P(D − t)/2t

(6.1)

σl = P(D − t)/4t

(6.2)

and

where P is the internal hydrostatic pressure, D the mean pipe diameter and t the wall thickness. Since the internal pressure was 7 bar or 0.7 MPa, the mean diameter of the pipe was 161.1 mm with a wall of 6.9 mm, the wall stresses are thus σH = P(D − t)/2t = (7 × 105 × 154.2)/ 2 × 6.9 = 7.8 MNm−2 and

σl = 3.9 MNm−2.

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σL σq

σr

6.11 Pipe stresses from hydrostatic pressure.

The hoop stress was well within the strength of uPVC, but the imposed strains were quite different in magnitude. Assuming that the pipe was buried by 1.2 m, then the tensile strain was simply α = 0.039/1.2 = 3.25% Using book values for the creep modulus with time (6), then after 3 months, σ = 31 MNm−2, after 7 years, σ = 27 MNm−2. The short-term strength of the PVC (using samples cut from the pipe) was measured directly in a tensometer and the strength at yield was σY = 58 MNm−2 The material was tough and ductile, showing that it had not degraded in any way (a result confirmed by FTIR spectroscopic analysis). The toughness was also measured directly on pipe material from single edge notched specimens, giving a value of K1c = 2.6 MNm−2, a value in line with literature estimates and comparing well with those mentioned in the previous chapter. The total tensile stress on the pipe before failure was thus at least σT = (27 + 3.9) ∼ 31 MNm−2. With a stress concentration at the shoulder of the joint, that gives a total stress of at least 60 MNm−2, a value in excess of the short-term strength of the material, and explains why the overloaded pipe suddenly fractured.

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6.2.5 Cause of failure So our conclusions were reasonably clear: that the sudden fracture and ensuing flood was caused by excessive tension and torsion in the PVC pipe prior to the final event. But what created the overload? It is reasonable to suppose that the fitment of the air valve created the torsion, especially if the end of the steel pipe fitted to the side of the valve box had not coincided with the entry point. It would have then been possible to twist the valve box slightly to ensure connection of the joint. The geometry shows (Fig. 6.2) that a leverage action could have been used, and the twist taken up by the most compliant part of the system, the PVC pipe. However, that still left the tension in the pipe unexplained. One possibility could have arisen from the original fitment of the rising main. It would have been laid first with the concrete floor, and the plans show that it was laid on a bed of sand in a trench within the concrete. The trench was about 4 feet below ground level. Although the exact details of the way the system was installed are unknown, it is likely that the plastic pipe system was laid first, followed by the steel sprinkler system in steel above, and then the two bolted together. Water would then have been allowed to enter the system, and it is at this stage that major stresses were imposed on the pipes. First of course there would have been the hoop and longitudinal stresses from the water pressure, but there would also have been a substantial load from the column of water acting on the vertical pipe and the water present in the horizontal main below the floor. If the sand foundation settled over time, then the column would have tensioned the PVC pipe to criticality. Failure of foundations is common with pipe systems, where shifts in sand footings can leave pipes unsupported and so put weak joints under severe strain. Mixing pipes with greatly differing properties is a problem because the less stiff or weaker pipes will take all the resulting stress, increasing the likelihood of failure by overload.

6.3

Failure of PVC water pumps

PVC is used very extensively for water supply in the developing world where there have been problems in producing high quality pipe. One critical use occurs in rising mains for handpumps. They are a primary source of drinking water for villages, especially in Africa, India and other parts of Asia with large rural populations. Much research has been undertaken by charities such as Oxfam to improve the design of hand pumps, and several standard products are used widely across the world, such as the Afridev and India Mark II. Their importance in attempts to bring clean water to rural populations cannot be over-stated (7). Further attempts have been

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made to improve designs for VLOM (village level operation and maintenance) so that worn parts such as bearings can be replaced easily and quickly, for example. PVC pipe is used both for networks and rising mains in wells since it is reasonably tough, light for ease of installation and of relatively low cost. It is also a relatively stiff polymer compared with HDPE, for example, so less effort is wasted when drawing water. But problems have been encountered where local stresses are high, especially in rising mains where all of the load is concentrated at the top where it enters the pump. One of the most common types of hand pump involves a reciprocating lever which moves a pump rod up and down within the main. There is a non-return valve at the bottom of the well immersed in the water, so that at each stroke when the pump is primed, water is pulled into the empty main and rises at each stroke. The water rises steadily until it reaches the outlet at the pump and is collected. So when not in use, the rising main is supporting the load of a full column of water ready for next use (Fig. 6.12). Although steel has been used extensively, rust is a big problem and can clog the valve at the base, as well as cause stress corrosion cracking at threads in the joints between pipe lengths, and so lead to loss of the main down the well. PVC mains have also failed, and causes loss of the water supply until the equipment is repaired. It is not at all easy to fish a fallen pipe from the borehole (not much greater in diameter than the main itself), so failure has serious consequences for the villagers who rely on the well for their water supply. If they return to use polluted rivers or streams, disease can follow rapidly. The danger of cholera, for example, is seen by the 2008 epidemic in Zimbabwe, where water supplies broke down across large areas of the country.

6.3.1 Rising mains We conducted research designed to explore the failure mechanisms of rising mains, research funded by the Consumer Laboratories and the World Bank in the 1990s. It produced results which shed some light on some unexpected failure modes. As the previous case study showed, PVC is welded using a solvent, or rather a solution of PVC in a suitable powerful organic solvent. Extrusion of the pipe itself can cause problems, and some skill is needed in producing the best quality, especially for the demanding role in rising mains. The process involves extruding PVC powder into pipe, and care in temperature control is vital to ensure the strength of the final product. The particles must fuse together to form a uniform material without voids and other defects. A simple test for determining the quality of fusion is to immerse a tapered section of pipe in methylene chloride for

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Rod hanger Chain connecting link

Rising main Pump rod Piston Cylinder Valve

6.12 Hand pump (India Mark II design).

about 20 minutes. If fusion is poor then the pipe surface becomes granular and whitening occurs. Most samples showed granulation and whitening with very few showing correct behaviour like the central sample in the figure. Samples from a selection of different developing countries showed a wide range of behaviour, those at left and right showing whitening and granulation in Fig. 6.13. GPC analysis showed a range of molecular weights indicating different sources, as one might expect. PVC also degrades at high temperatures to form hydrochloric acid gas by the reaction: —[CH2—CHCl]— → —[CH=CH]— + HCl and the double bond will oxidize to form carbonyl groups within the long chains. They are points of weakness and can be detected using FTIR spectroscopy. In fact, carbonyl levels were found to be low in all the pipes examined.

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6.13 Methylene chloride test for PVC water pipes.

So the most serious defect found in the samples was insufficient fusion of particles. While it might not affect pipe used for low pressure water supplies, it can affect the fatigue lives of rising mains. The effect can be estimated from the Paris equation (8): Nf = K/(Δσm). 1/ (a0)

(m-2)/2

(6.3)

where Nf is the fatigue life of pipe cycled at 1 Hz and a0 is the maximum flaw size (m and K are constants). The stress range (Δσ ) is 5.4 MNm−2 for a pipe lifting water from 45 m. For a flaw size of 1 micron, then Nf is 3.8 × 107, but if a flaw size of 100 microns occurs, then the fatigue life drops to 6.7 × 106. So the occurrence of much larger voids between particles lowers the fatigue life by a factor of about six. It was thus recommended that pipe manufacturers took much greater care in making pipe dedicated to rising mains, although further tests showed that the joints needed even greater care.

6.3.2 Fatigue tests As part of the intensive tests to which rising mains were subjected, we tested a jointed pipe under realistic fatigue conditions. We used a frequency of 80 cycles per second and a stress range of 10.8 MNm−2 with a mean of 5.4 MNm−2 under sine wave cycling. The pipe failed at only 1.3 × 105 cycles, much lower than might have been expected, and it failed not in the pipe itself but rather at the joint. It had been solvent welded by others for our test, and rather than fracturing, the joint was pulled apart at the weld (Fig. 6.14). The joint had been very poorly bonded and a simple measure of the unbounded area was made by tracing the bond out and then weighing the mass of paper cut-outs. It turned out that only 23% of the total area

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6.14 Failure by joint pull-out in a fatigue test.

6.15 Tracing of bonded area (black) and unbonded joint (grey).

6.16 Fatigue crack at corner of solvent welded joint.

available had been bonded, so making the joint much weaker than expected (Fig. 6.15). Fatigue cracks had started not in the numerous stress concentrations of the bond itself, but rather at the outer corner where the pipe joined the socket (Fig. 6.16). Microscopic inspection showed that the pipe itself was also poorly fused at this point, so external defects were aided by internal voids between the particles. Another surprising feature of the failure also emerged: the pipe and socket did not form a regular but rather an

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eccentric joint, so solvent welding was inhibited from the outset of forming the bond between them. It was the pipe itself which was eccentric to the extent of about 1% on its diameter, giving a joint gap of between zero and 0.55 mm, the socket having an eccentricity of only 0.1%. So the strength of rising mains was (as expected) dependent on the quality of the topmost joints. There is a standard for making such joints (9), which recommends in great detail the procedures to be used. It is vital that fresh cement be used, and the surfaces to be joined carefully abraded before applying the cement to ensure total wetting. The joint must be allowed to dry, necessarily a long process since all solvent must diffuse away from the deepest parts before the joint is fully dry. A further survey of solvent cements from around the developing world also showed great variation in the solvent, added polymer and composition. One of the problems in performing such research was its worldwide nature. It was clearly possible to obtain samples of new pipe and cement, but obtaining failed or fractured pipe and fittings was much more difficult owing to the natural tendency to discard failures when installing replacement rising mains. However, great improvements have occurred in the design and standardization of hand pumps in the developing world, especially where good practice spread from countries such as Sri Lanka to India and Africa. PVC pipe manufacture reached a high standard in the 1990s and many villages benefited from the high quality of installations in the countryside.

6.3.3 Machined PVC problem A rather unusual application of rigid PVC involves building apparatus from machined parts, largely because the polymer is very easy to machine into complex and intricate components. A company in Coventry had the idea of building a wrapping machine using such PVC components. Their concept included incorporating water cooling channels within the PVC parts to control the process. They built a working prototype using a combination of light alloy components, slab PVC and acetal bearings (Fig. 6.17), and relied on a toolmaker to machine the various PVC parts, solvent welding the parts together to make the inner water cooling channels. However, when the wrapping machine was first switched on in 1998, numerous leaks occurred from the PVC components and rendered the process inoperable. The plastic packaging company sued the toolmaker for their losses. We were asked to examine various parts and report on the root cause of the problem. One particular part was chosen for close scrutiny. It was a bar 640 mm long with a section of 39 by 20 mm and when examined, proved to be slightly curved, being about 2 mm out of true. The bar had been solvent welded to form the water channel which ran along its length, and was drilled

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6.17 Bag wrapping machine using grey PVC components.

6.18 Polished section of long arm showing large gap in joint.

at either end (ID about 8.5 mm) to accept the water supply. Macroscopic inspection showed that the joint between the two halves exhibited a small gap of about 0.6 mm (Fig. 6.18). The same picture shows cross-threading on the upper part of the screw thread, no doubt caused by numerous attempts to discover the source of the leak or disconnect the supply to stop the leakage. Since the water pressure was about 2.6 bar or 38 psi, the inner source of the leak was established. External examination showed a similar problem, with similar sized gaps in the joint, and since the bar had leaked here in service, a path between the two gaps was present (Fig. 6.19). No doubt the many other leaks in the system were caused in a similar way by poor joints. So how had the faulty joints occurred? Several possibilities can be suggested: poor application of the solvent cement so that not all the joint area was covered, poor cement or insufficient clamping of the parts as they were joined. The correct cement had been used so poor application and low clamping pressures seemed the likely explanation. On the other hand, some joints showed extruded cement, so clamping was probably sufficient. That

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6.19 Polished section of outer joint showing gaps.

left poor application without complete wetting of the surfaces as more likely, possibly made more difficult by distortion of the underlying material. The distortion may have been produced by asymmetric heating during machining, so one side of a part expanded to a greater extent than another. Alternatively, residual strains (chain orientation) in the original material could have been affected asymmetrically. We concluded that the parts should have been pressure tested before supply, and greater care should have been used during machining the PVC.

6.3.4 Mediation The case was due for trial but, as is increasingly common nowadays, the judge asked for the experts on each side to try to agree a joint statement. While some points could be agreed, there was an impasse over the quality of machining, whether testing had been attempted and over the competence of cement welding. The defendant toolmakers maintained that they had performed their work to a high standard, despite the overwhelming evidence to the contrary. The next step involved mediation between the parties, another increasingly common way of settling disputes before trial. It normally involves a barrister establishing the facts by meeting each party separately, and sitting in different rooms. The lawyer attempts to come to a view of the basic problems by asking increasingly sharp questions of each party. By progressively forcing the issues, the lawyer is effectively cross-examining each party on the credibility of their allegations, not in an open court but rather in complete privacy (and in the absence of the other party). The costs are high but much lower than a full trial, and confidentiality is maintained. The defendants were advised by an expert with little experience of polymers, and the damage to the PVC components could not be denied or

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explained away very easily. The case settled with compensation awarded to the packaging company against the toolmakers. It illustrated the importance of allowing for the properties of polymers during machining operations and achieving good welds by following good practice during adhesive bonding.

6.4

Failures in gas pipelines

Polyethylene pipes are now the norm for gas pipelines across the world, but their increasing use has not been without problems. Flooding from broken water pipes can clearly cause great physical damage and lives are rarely lost, but escapes of gas are much more serious owing to the risk of explosion. Traditional materials like cast iron and steel have been the cause of many gas explosions, whether through brittle fracture of the cast iron, or by rusting of steel mains. Thus a gas explosion which killed four people from one family in Larkhall, South Lanarkshire, in December 1999. The owner of the buried pipeline, Transco, was fined £15 million for the accident in 2005. The failure was caused by deep corrosion of the ductile cast iron main (10 inch (25 cm) diameter), which the company apparently thought had been replaced by an MDPE gas main, such was the state of their records. While many such pipelines have indeed been replaced, there are still many in situ which represent a real hazard from leaks to the environment. Another explosion occurred on 22 October 2000 on Linfield Street, Dundee and it was traced to a fractured joint on the 4 inch (10 cm) cast iron main (10). Two people died and the investigation showed several previous gas leaks had occurred in the vicinity. At the time of the report, Transco estimated that only about 50% of the old mains had been replaced by MDPE, and their replacement would be speeded up. The cause was probably subsidence due to ground movement from other excavations (e.g., drainage pipes) near the affected joint. However, there have been several serious explosions from MDPE gas lines in which lives have been lost and large-scale physical damage has occurred in the USA, for example (11). The main problem is brittle behaviour in a polymer which is nominally tough and ductile. In its 1998 review, the NTSB described three such disasters, the first of which occurred at Waterloo, Iowa on 17 October 1994. The explosion and fire which followed the escape of gas killed six people and injured seven more, destroying a building and damaging several others in the vicinity of the gas escape. In another accident in San Juan, Puerto Rico in November 1996, 33 people died while one person died after a gas explosion in Texas in 1997. The Waterloo explosion occurred after gas leakage from a junction between a ½ inch (12 mm) diameter MDPE plastic service pipe and the steel main (Fig. 6.20). Brittle longitudinal cracks had grown from the

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Steel tapping tee coupling nut Steel tapping tee

Plastic service pipe

Steel main

6.20 Junction between steel main and plastic service pipe at Waterloo.

6.21 Close-up of fractured MDPE pipe from Waterloo, Iowa.

junction, and eventually fractured finally, releasing gas (Fig. 6.21). The pipe had been made more than 20 years before, so crack growth had occurred over a long period of time (12). During the first introduction of the MDPE, testing simply involved over-pressurization until rupture, and ignored the problems of short-term brittle fracture by ESC, for example. Long-term life was predicted from short-term rupture experiments, and neglected to allow for the downturn in hoop strength at longer times (Fig. 6.22). It is a perpetual problem in many safety-critical products with testing regimes which seek to predict long-term behaviour from often very short-term tests. Such tests are even more critical in buried pipes where leak detection is inherently difficult, and replacement expensive and time consuming (as well as disruptive to road users, for example).

Polymeric pipes and fittings

Extrapolated line

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Calculated long term hydrostatic strength

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6.22 Creep rupture curves for MDPE gas lines. Initial assumption (top) and actual (lower).

The huge explosion in Puerto Rico in 1996 was caused by brittle cracking of an MDPE pipe made in 1982 (Fig. 6.23). Propane gas escaped and pooled in an adjacent cellar, since, unlike natural gas or methane, it is heavier than air. Eventually a spark from an air conditioning unit ignited the gas with devastating effects (13). Brittle cracks were seen at thermally welded joints,

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6.23 Humberto Vidal store in Puerto Rico after propane gas explosion caused by brittle cracking of gas pipe.

6.24 Brittle crack in pipe from the San Juan disaster of 1996.

where there was a stress concentration formed by the external corner (Fig. 6.24). The immediate cause of the cracking was ground subsidence beneath the pipe, so transferring greater loads from above onto the pipe junctions. A water pipe had been installed under the gas pipe a few years before the explosion and failure to backfill correctly allowed the gas pipe to bend, the loads at the joints being much greater than expected. The holding company,

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Enron Inc, settled compensation claims without a trial and without admitting liability. The final case discussed in the NTSB review report of 1998 was a gas explosion at Lake Dallas in Texas in 1997 (11). The cause was a metal pipe pressing on the plastic MDPE pipeline, which created a longitudinal brittle crack (Fig. 6.25). Although the NTSB made several recommendations to improve testing of pipes, better control of weld formation and care where other pipelines existed or were later installed (including foundations), further explosions have continued to occur. The NTSB were, after all, recommending improvement of new installations, and there were numerous lines already in existence where problems could still occur. One such devastating explosion was at Dubois in Pennsylvania in 2004 (14). It destroyed a house and killed both occupants at 8.54 am on 21 August (Fig. 6.26). The cause was a gas leak from the butt fusion weld of a 2 inch (5 cm) diameter gas line

6.25 Brittle crack (circled) caused by steel pipe resting above it.

6.26 Dubois gas explosion debris of house in August 2004.

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Forensic polymer engineering

External bead at butt-fusion joint

Leak at butt-fusion joint

2-inch plastic main line

6.27 Rupture of butt fusion weld from Dubois explosion.

to the house (Fig. 6.27). The joint had been formed six years before by the hot plate method, where the two ends are pressed flat against a hot plate, the plate removed and the two ends then pressed together. The NTSB found that the joint was ‘mitred’, or in other words, the ends were angled at about 2 degrees to one another, rather than being completely aligned with one another. The beads of molten polymer which form each side of a butt fusion joint should be uniform, but were asymmetric in this case.

6.4.1 Fracture surface The fracture was examined in some detail using both optical microscopy and ESEM. The single origin was identified at one side of the weld, and had grown around the pipe along the line of the joint (Fig. 6.28). Voids were found at several parts of the surface, perhaps indicating that excessive temperatures were used, and so degrading the polymer. Alternatively, the two parts may not have fused sufficiently to have formed a good joint. The bond was formed from coiled pipe, more difficult to form a straight un-mitred joint correctly, and it was found that when 40 more similar joints were removed and examined in the vicinity, a high proportion were defective.

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Intermal bead

Fracture origin area

Plane of fractures Voids

6.28 Fracture surface of thermal weld from Dubois explosion.

The NTSB concluded that the faulty joint was the root cause of the escape of gas and the deaths of the two residents of the house demolished in the accident. Unfortunately, the investigators were not able to ascertain the original position of the pipe, since the gas company had smashed part of the pipe near the failed joint. It seemed clear, however, that ground movement had placed the butt weld under severe stress for the fracture to have occurred. The continued failure of gas pipes reflects all the problems of faulty welding often made many years before the pipe was made and laid, and suggests that all possible attempts should be made to detect any gas leaks very early so as to prevent catastrophic failures. The explosive power of even a small gas cloud released by a leaking pipe is so high as to demolish houses and other buildings completely, and casualties are likely in this scenario. While each accident may improve testing, construction methods and inspection procedures, there is a high likelihood that such unfortunate accidents will continue owing to the backlog of faulty pipes and joints waiting to fail. In the UK such explosions also continue to occur with distressing regularity, many it has to be said from corroding steel or iron lines which should have been replaced years ago. One hopes that the US experience will ensure that laying and welding methods used currently will inhibit further failures of plastic lines.

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6.5

Failures in ABS pipes and fittings

ABS thermoplastic is also widely used for pipes in various applications. In one case, it was alleged that ABS pipework at a hydrochloric acid storage depot at Immingham docks could cause serious leaks by degrading in contact with acid. In another case, a large diameter ABS pipe used at a glass works exploded, causing substantial physical damage in the vicinity. In both instances, there was fundamental disagreement about the causes of the failures between the experts asked to investigate.

6.5.1 Immingham docks The depot concerned stored concentrated (35%) hydrochloric acid ready for shipment to North Sea oil and gas fields for secondary and tertiary recovery of hydrocarbons (Fig. 6.29). The acid is pumped into old wells to attack the bedrock, and so improve its porosity, thereby allowing more hydrocarbons to be collected. The case was brought in 1999 following installation of the pipework in mid 1997 for conveying acid between the various storage tanks and the dockside. The case against the installer was based on a leak of acid at the plant, although no failed or cracked samples had been preserved. Various stained samples from the system were taken when concern arose over the polymer used. Several, but not all the pipes had been replaced by polypropylene, which was said to be more resistant to the acid than ABS. The few samples which were available for examination included a blanking plate (200 mm in diameter and 20 mm thick) used to seal a dead end (Fig. 6.30). It was stained to a depth of about 0.75 mm, presumably by contact with the acid. DSC analysis showed that the effect of the contami-

6.29 Hydrochloric acid storage facility at Immingham docks.

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nation was to lower the Tg of the material from about 108°C to about 100°C. FTIR spectroscopy showed very little difference between intact and stained samples, although there was a small carbonyl peak detected in the contaminated sample. There was no sign of cracking or deterioration of its mechanical properties. A polypropylene pipe which had been used at a similar storage plant in Great Yarmouth was also available for comparison (Fig. 6.31). It showed a similar zone of contamination (without cracks) and was analysed using the same techniques. DSC showed a small drop of only 2°C in the melting point of 165°C, but no difference could be detected in the IR spectrum, possibly because the method used hot decalin as solvent, so any volatile compounds present could have evaporated away. The next step involved spectral analysis of the raw hydrochloric acid stored at the site. Although none was made available immediately, the fluid

6.30 ABS blanking plate showing contamination at centre.

6.31 Contamination of polypropylene pipe from another HCl facility.

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Forensic polymer engineering

is supplied commercially for cleaning concrete surfaces. One of the samples purchased showed a strong yellow colouration and so a sample of the acid was requested from Immingham. It proved very similar, with a deep yellow colour, not dissimilar to that shown by the contaminated zones. The colour suggested that UV spectroscopy could indicate the nature of the contaminant, and it produced a spectrum showing two peaks (Fig. 6.32). Ferric chloride is also coloured deep yellow, and such a compound could easily have been produced by reaction with iron pipes, for example. The UV spectrum showed peaks in quite different positions, so the contaminant of the acid must have been different in structure. The most similar compounds

abs 4.000

2.000

0.000 190

250

300

350

400 nm

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300

350

400 nm

abs 4.000

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6.32 UV spectra from commercial acid (upper) and ferric chloride solution (lower).

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were phenolics, but could not be confirmed. The data sheet on the acid admitted the presence of various contaminants, including traces of free chlorine, but there was no visible cracking so SCC did not appear to be occurring in the ABS polymer. So we concluded that the colouration was probably caused by phenolics or other organic contaminants diffusing into the surfaces of exposed plastics from the acid, including both ABS and PP, with the effect of slightly plasticizing the polymer but with no other deleterious effects.

6.5.2 Conclusions Another expert report concluded that ABS was unsuitable for exposure to the acid, using no direct evidence but rather data sheets of exposure. These are lists compiled and supplied both by raw materials suppliers and independent labs like RAPRA (who authored the report condemning ABS). They are simply lists of many different chemicals and the assessment of the effect on the polymers considered. But they can be based on old data, and neglect the development of better grades of polymers. RAPRA relied on one of their own lists, which was contradicted by other lists from manufacturers but of more recent date. However, support from some such lists could not displace forensic analysis of fractured samples, and it was likely that the failure could have occurred by another mechanism such as a poor joint or external impact and damage, for example. Since no records of the incident appeared to have survived, the case could not proceed very far without more direct evidence. The conflict was eventually settled by mutual agreement, but it was interesting to observe that the pipework at the plant included both ABS and polypropylene, even after the dispute arose (Fig. 6.33). If ABS was so badly affected, then surely all ABS pipes should have been replaced. However, the interaction of different chemicals with thermoplastic materials must inevitably remain a topic of continuing research, since the downside can be catastrophic failure leading to loss of property, disruption to businesses and personal injury, as the following case shows.

6.6

Compressed gas explosion

We usually associate explosions with escapes of flammable gases (hydrogen, methane, propane and so on), and while they are unfortunately not uncommon, it is also true that explosions can result from the sudden escape of any highly compressed gas, whether flammable or not. Compressed air is widely used in industry as a power source for pneumatic tools, for example, and compressed air is also used for quenching glass in the manufacture of car windscreens. The air supply is produced by a

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6.33 Dark grey ABS pipe at top left, light grey polypropylene at lower centre linked to steel main at Immingham storage facility.

compressor connected to a set of accumulators, or tanks, from which the air can be tapped as and when needed to quench the glass (Fig. 6.34). But on 2 October 1998, such a system suddenly exploded on an industrial estate in Winchester, the air release being sufficient to demolish an adjacent wall, and several BMW cars in the showroom on the other side of the wall were wrecked. The site of the explosion lay in a large diameter ABS pipe leading from the accumulators (Figs 6.35 and 6.36), one side of the pipe having ripped out during the event. There were also other longitudinal cracks in the same pipe, showing that growth of internal cracks must have caused the accident.

6.6.1 Cracked pipe On inspection, the interior of the pipe (1.22 m long, 16.8325 mm in diameter and with a wall of 13.2 mm) exhibited numerous sub-critical cracks aligned along the axis (Fig. 6.37). The cracks appeared in swarms rather than being

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s

ter

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ss

pre om

Mezzanine floor

r

ie Dr

C 1 4˝

Pneu butterfly

1 6˝

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ch en Qu nk ta

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Fracture throughout this section

ers

iv ce Re

6.34 Schematic sketch of pneumatic system at glass works.

2300 mm

382 mm

6.35 Section showing storage tank at left and main pipe with bends.

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Forensic polymer engineering

6.36 Fractured ABS pipe from compressed air explosion.

6.37 Inner bore of ABS pipe showing sub-critical cracks and main fracture.

distributed evenly through the inner surface, and it was clear from the major fracture surfaces that one such crack had exceeded a critical size and grown suddenly to completion. The inner side of the cracks appeared entirely brittle while the outer side was white, indicating craze formation and a normal phenomenon in ductile polymers like ABS. The boundary between the two regions was very sharp and well defined (Fig. 6.38). In other parts of the pipe, the boundary disappeared as all the fracture surface was white in colour. The interior of the pipe showed traces of blue paint indicative of the blue painted exterior having impacted and scraped along the inside, presumably as a direct result of the explosion. Optical microscopy showed how crazes could form at the ends of brittle cracks to form a shear band which could interact with adjacent shear bands. But other areas showed faint traces of superficial contamination with cracks initiated from the path of contamination (Fig. 6.39). Such evidence pointed

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6.38 Close-up of main fracture surface.

6.39 Optical micrograph showing craze nucleation from diagonal contamination.

to ESC caused by some unknown organic fluid which probably made contact with the lining of the tube just before the final event. Fatigue could be excluded as a cause of failure since there was no trace of striations within the crack surfaces. And any problem with the blue paint causing cracking could also be excluded since all the cracks and crazes were entirely on the inner bore.

6.6.2 Mechanics No problems with the structure of the polymer could be detected by either DSC or FTIR spectroscopy, so oxidized or degraded polymer was excluded as a failure mode. The hoop stress of the thick pipe was calculated using the equation (15) σH = q[(a2 + b2)/(a2 − b2)]

(6.4)

where q = the maximum pressure = 10 bar = 106 Pa = 106 MNm−2, a = outside radius = 84.25 mm, b = inside radius = 71.05 mm.

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So the hoop stress, σH = 5.92 MNm−2. The makers of the pipe stated that the tensile strength of the ABS was 45 MNm−2, so the material was well within its capabilities. We concluded that the pipe had failed by entry of a fluid contaminant which had nucleated brittle cracks by environmental stress cracking (ESC) of the ABS surface. In this failure mode, brittle cracks grow very quickly when a stressed surface comes into contact with the organic fluid. Amorphous thermoplastics like ABS are generally more sensitive to a wider range of such fluids than crystalline polymers like PE or PP. But what could be the source of the organic fluid? The most obvious source was the compressor motor because they generally work using oil, although oil should be prevented from entering the air system by filters (Fig. 6.34). However, if the oil contained volatile organics, they could easily escape the filters if gaseous, and travel through the pipes to be condensed in the accumulators (or receivers). They would naturally condense to form a small pool at their bases, and when enough had collected, be driven out by the force of the blast of high pressure air when needed for quenching the glass surfaces. The oil used in the system was stated to be a type of mineral oil which was not approved by the pipe manufacturer for use with ABS, but no further analysis was obtained to test the hypothesis further.

6.6.3 Controversy But another investigator from Burgoynes (a well-known set of consulting engineers) came to a quite different conclusion. He preferred the idea of fatigue, despite the lack of evidence of striations in the fracture surfaces. Fatigue in all materials tends to occur at known stress raisers in a design such as corners and holes. But in this case, the cracks were spread inside a smooth polymer surface, and were not associated with stress concentrations of any kind. Fatigue cracks also tend to be highly localized and not widely distributed, although can be multiple if enough stress risers are present. A meeting to discuss these problems was arranged, and there were many more samples to examine from other pipes in the same system. They also exhibited many internal cracks on the smooth bore, all of which were sub-critical. Some liquid was found later at the base of the receiver next to the broken pipe, but it had not been analyzed or preserved. There was no meeting of minds on the issue, and a rancorous session between the experts ended without conclusion. It transpired that the Burgoynes expert was an expert in ceramic failures, and had little experience of polymer failures. There were a number of missed opportunities which could have resolved the issue more positively, especially by the first investigator from the insurers. Samples of the liquid in the receiver and the compressor oil should have been collected, and analyzed to see if there was

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any connection, for example. It may not have occurred to the investigator that ESC was a possible failure mode, and he thus neglected to gather all the available evidence when on the scene of the accident. It is a constant concern for all investigators who are first to view the remains of an accident, that he or she misses nothing which turns out to be important at a later stage. Abundant photographs of the site are also always important even if most turn out to be valueless. One photograph just might show a crucial detail overlooked at the time, but which is lost when the damage is removed. It is a recurrent problem with many insurance claims because loss adjusters and assessors are not normally trained in forensic methods, and subsequent investigators have to rely on poor quality photographs because the real evidence is long gone. The case could not proceed to trial owing to legal problems, and the issue was settled by mutual agreement. The manufacturer did, however, develop a grade of ABS pipe with a chemically resistant inner lining so as to prevent this kind of problem recurring, and as far as is known, this unusual type of failure has not been repeated.

6.7

Failures in polybutylene pipes and acetal resin fittings

While many different types of thermoplastic have been used successfully for cold or potable water supplies, including PVC, polyethylene and polypropylene, some relatively recent introductions have not fared so well, especially when used for hot water supply in domestic situations. The problem was first encountered in the USA in the 1990s, when large companies started supplying a relatively new material for plumbing, polybut-1ene. The material is a hydrocarbon polymer analogous to polypropylene, but with a larger side group. The isobutyl group is –CH(CH3)2 compared with the methyl group –CH3 present in PP. The extruded polybutylene pipe was installed in numerous home hot water systems, but the material degraded internally by cracking, and failed catastrophically in many homes. The fittings used to connect the pipes together also failed, but at least they were in a well-known polymer, acetal resin, so perhaps the installers should have known better.

6.7.1 Acetal fitting fracture The problems were encountered with acetal resin joints, a problem we also investigated in the UK. A flood had been experienced at Loughborough University in 1988, when an acetal fitting under a sink had suddenly fractured and flooded the computer department below, causing considerable physical damage (16). The failure occurred over the weekend on a Sunday,

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when no-one was around to detect the leak and stop the flow, so the flood continued unabated until discovered the following morning. The failed acetal fitting was situated in a hot water supply system used to feed hot water from a wall-mounted heater direct to the tap above (Fig. 6.40). The system had been installed about three years before the failure. However, it was situated on the cold water side of the system, so was not exposed to high temperatures at all. The component (marked in black in the figure) was not loaded by any external force such as the weight of the heater on the wall since all such loads were supported by a stiff steel bracket. However, another expert acting for the claimants, maintained that the heater had been badly installed and the claim was due for a High Court hearing. That the fracture started soon after fitment to the system and then grew slowly can be judged by the brown deposits covering a large part of the surface (Fig. 6.41). Inspection of the inside of a kettle in a common room below showed that the deposits came from the local water supply. It was a mixture of brown iron oxides and calcium carbonate produced by slow evaporation of water, and with the failed fitting, was produced by leakage through a very narrow crack. The investigation we carried out indicated multiple crack initiation in the screw threads over a long period of time (16). The fitting had been injection moulded, and exhibited a number of severe flow lines indicative of cold moulding. We could not match any new mouldings from the supplier with that fitted in the system, so it looked like a maverick faulty product was the root cause of the failure. Normal tightening loads when the joint was formed started a brittle crack from a weld line and slow crack growth led to failure. The expert from Burgoynes insisted that the heater coming off the wall had overloaded the fitting, or alternatively, the screw fitting had either been over- or under-tightened. We could not agree and it seemed as though the case (L’boro University-v-Wm Moss et al.) would proceed to a full trial.

6.7.2 Literature review Events then took an interesting turn. A literature search by RAPRA conducted when one of us was writing a review on designing with plastics (17) turned up a news item from a US journal in 1991, which mentioned a court case concerning failure of plastic plumbing systems in Texas (18). It stated that a court case in the state had been settled (Babb et al.-v-Shell, US Brass, Hoechst, and DuPont) with damages awarded to the claimant. It involved failure of polybutylene pipes and acetal fittings causing flooding and consequential damage to the Babb house, the latter being of direct interest to the present case. In fact it transpired that it was a class action with many distressed householders suing the installers and manufacturers. The transcript and expert reports were sent to us by the claimant’s lawyers

Polymeric pipes and fittings Tap Hot

Cold

Bench

Upper hot valve

Spindle Spacer support bracket (3mm thick sheet steel) Lower cold valve To cold tap

Partial fracture of plastic junction

Cold supply to heater Cold feed Cupboard

Single screw attachment to wall

Hot feed from heater

Cold water mains

Water heater

Floor

6.40 Section of hot water supply system which failed and caused flood.

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Forensic polymer engineering

6.41 Fractured acetal fitting showing contamination from water supply.

and they were very revealing. They showed that low levels of chlorine in the water (0.3 ppm) could initiate brittle cracks in the acetal fittings. Those results had been known from internal tests by the companies before the materials had been introduced in households, but the management of the companies involved had suppressed the results or ignored them (19). Some of the details revealed during the trial were disturbing. Counsel for the plaintiffs talked about the problems his clients had faced (19): When I say the word ‘leak’, I normally think of that little drip . . . That’s not what I’m talking about. . . .I’m talking about turning on a water faucet loose in your attic. I’m talking about turning it on behind the wall. I’m talking about people who go to see their folks at Thanksgiving and come back with two inches of water in their house. I’m talking about people who go to turn on their lights and have water coming out of the fixture.

Such dramatic evidence focused attention on the many domestic catastrophes which occur when supply pipes fracture as a result of SSC from chlorine in the water. The evidence also showed that the problem had been known by Shell from the early 1980s, yet they continued to market and sell the product into new homes. The large variation in failure types was to be expected since plumbing systems varied enormously in Texas as well as in many other states. Fracture occurred at those points in an individual system where pressure was greatest, so where the hoop stress was largest. But it also tended to occur at joints where metal hoops had been used to compress the PB pipe over fittings. It also varied with chlorine content of the water supply, as one would expect for an SCC mechanism. With so many variables, the role of the expert witness was crucial, and Alexander Chudnovsky was the key expert who reported to the court.

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6.7.3 Acetal albatross He described numerous failed pipes and fittings that he had examined showing severe internal degradation such as whitening of the polybutene bore from its original grey colour caused by exposure to chlorinated water. He talked about the deep cracks which developed into the pipe wall, most of the bore being covered by such brittle cracks. He had also examined documents provided by Shell in disclosure, which from the early 1980s had described the problems of the acetal fittings in graphic terms as the ‘acetal albatross’. Since the fittings generally failed first, Shell blamed them for the complaints they were receiving rather than the polybutene pipe they were supplying. Chudnovsky also thought that the fittings failed in about half the time of the pipe. Although stabilizing compounds were added to both acetal and polybutene, they could be leached from the pipe and fittings, and in general had a limited life in protecting the polymers from attack by chlorine. As with all anti-oxidants, they have a limited life in absorbing free radicals generated by oxidation from whatever source. When they react, they are effectively neutralized, so the overall concentration is reduced. At complete exhaustion, the chlorine attack resumes. The chlorine content of the water may vary, but is continuously replenished by the cold water intake (19). Various estimates were made of the life of plastic systems based on failures from the first installations made in the 1970s, from about 10 years for caravans and outdoor systems to 13 years for houses. They compared poorly with a design life of 50 years estimated by the manufacturers. But such estimates showed great variations depending on local usage temperatures and the quality of the water supply. The US Army reported on the problem at some length and deprecated the use of acetal and polybutylene systems (20). The expert reports on the acetal fittings which failed in Texas included direct analysis of failed parts using FTIR and DSC. The heavily degraded inner bores of acetal fittings showed a decreased melting point owing to the lower molecular weights caused by chain cleavage. X ray analysis using scanning microscopy also showed substantial levels of chlorine in the chains, demonstrating directly that chlorine SCC was the root cause of the internal cracking. The fittings were more sensitive owing to the low molecular weight grades used for moulding. A separate review by Donald Duvall for the court summarized the results of several court cases across the country, especially the extent to which the problem was appreciated by the material suppliers, and then passed that knowledge onto its customers. There were two types of test used by Celanese Corporation (the producers of acetal copolymer in the mid 1970s, and owned by Hoechst of Germany): shortand long-term tests.

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In the first test, acetal tensile bars were exposed to hot water with 5 ppm of added chlorine. The level of chlorine fell to 0.2 ppm and remained there even though fresh water with 5 ppm chlorine was added continuously to the test bath. It had clearly been absorbed by the polymer bars, the surfaces of which whitened with surface degradation. The bars were strained after immersion and showed brittleness very quickly (19). Hoechst also performed numerous longer term immersion tests in 1975 which showed chlorine attack and deterioration of the polymer in 0.5 ppm chlorinated water. The life of products was predicted to be 5 years at 40°C falling to one year at 60°C under an applied stress of 300–600 psi. But the data was not shared by Celanese with their customers, and they ignored their own conclusions before launching acetal fittings into an unsuspecting American market. Celanese also conducted field data on likely chlorine levels in the USA and discovered that levels could lie between 0.2 and 2.5 ppm in drinking water supplies. Direct experience of acetal product failures were also reported from Germany. Acetal components were made for water meters, but had to switch production to another polymer after failures. Similar problems occurred in Spain with impellor blades that were in contact with potable water. Tests of acetal in contact with toilet bowl disinfectant showed rapid degradation of the polymer, presumably owing to the high levels of chlorine in common bleach solutions (19). It was clearly a major failure by Celanese in the USA either to publish or disseminate the results of their own tests made in Europe, tests backed by practical experience of the polymer in contact with chlorinated water supplies as early as 1975. The other major supplier, DuPont, should also have been aware of the potential problem with their version of the polymer, and should have refused to allow the material to be used in continuous contact with potable chlorinated water supplies.

6.7.4 Degradation mechanism Acetal resin comes in two forms, a homopolymer of repeat unit —[CH2— O]— capped with stable end groups which inhibit unzipping of the chains, and a copolymer with acetaldehyde where the larger repeat units also block unzipping. After the polymer had been discovered, it proved too unstable to market, so these two strategies were adopted by manufacturers to stabilize the material (21). However, neither form can resist strong acids or chlorine, the latter being a very powerful oxidizing agent (which is why it is used widely for cleaning and killing bacteria). In both polymer forms, chlorine attacks by abstracting hydrogen, probably in a free radical mechanism: —[CH2-O]— + Cl2 → —[CHCl-O]— + HCl

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and the product hydrolyzes at the carbon-oxygen bond: —[CHCl-O]— + HCl → —[CH2-O]-CHCl2 + HO-[CH2-O]— So chains are broken quickly and the very weak polymer starts to crack open, especially at stress concentrations where the local stress is greater than elsewhere. The rate of attack is increased with rise in water temperature, as the Arrenhius equation predicts for an exothermic reaction. Indeed, domestic cleaners such as bleach (a solution of sodium hypochlorite) release chlorine by the mechanism: NaClO + H2O → NaOH + HClO HClO + HCl→ H2O + Cl2 It is believed that the hypochlorous acid (HClO) is the potent oxidizing species. It is well-known that hot bleach is more effective in removing stains than cold hypochlorite solution. The free chlorine is easily detected by its characteristic acrid smell, as it is in chlorinated water supplies as well as when bleach is used, either as a solution or as bleaching powder. Here was the explanation of a chlorine stress corrosion cracking (SCC) mechanism for the fitting at Loughborough. Checks with the local water company established that chlorine levels in the cold water could rise to as high as 0.9 ppm owing to the practice of sending a ‘slug’ of chlorine down the pipes to prevent bacterial contamination after work had been carried out on the pipes (mending leaks in the road, for example). Attack would occur at weak areas such as weld and flow lines in the threads of the defective fitting, with cracks growing slowly with time to form a small gap in the thread, which leaked water slowly. Local evaporation produced a selfsealing deposit there of brown calcite. The final failure probably came with a sudden pressure surge in the cold water supply, such as water hammer, when a valve closure can initiate a powerful shock wave which travels through the pipes. Plumbers had been working elsewhere on the site that weekend and could have inadvertently triggered the final fracture. Chlorine was later detected on the thread surfaces using ESEM, so confirming SCC as the failure mechanism. The action settled with all parties bearing their own costs, but could have been settled much earlier if the information from the USA had been known more widely. It would be unlikely today owing to the availability of information on the world wide web. It has effectively globalised information so that failures in one country are, or should be, readily accessed by users in another, often thousands of miles away. The polymer department at Loughborough University, one of the largest in the country, agreed with our diagnosis of the problem. They could not have participated in the action, however, being implicitly biased to their own institution.

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6.7.5 Pipe failures The Babb case in Texas also highlighted widespread failures of the polybutylene pipe used in the hot water systems, where attack occurred not only from chlorine in the water but also from dissolved oxygen. The interior of pipes showed an extensive network of deep cracks, both radial and longitudinal, on the inner bores. Whether or not the acetal fittings or pipe failed first depended very much on local conditions in each household, such as exact temperatures used, as well as the state of individual components and the elapsed time since installation. The size of the settlement of several million dollars reflected not only the damage caused by sudden flooding, but also the need to replace intact systems before flooding occurred. But at least one positive outcome of the case was that house owners were warned of a possible if not probable problem. Clearly, there would be a spread of failures, the first being experienced on the earliest installations, which went back to the late 1970s. A schematic way of representing the problem is shown in Fig. 6.42, where stress is plotted against the time for product failures (on a logarithmic scale to cover the very wide range of stresses and times). The figure is directly comparable to Fig. 6.22 for pipes where the hoop stress is the critical stress in the pipe wall. There are three generic modes:

• •

I Mechanical failure in the early life of a product when faulty parts cause premature cracking, for example, or when the parts are badly fitted. II Mixed mechanical-chemical when aggressive chemicals attack sensitive product parts. III Chemical when widespread attack by chemicals on many product parts occurs.

I Mechanical Log (stress)

•

Mechanochemical

II

Chemical

III

Log (life time)

6.42 Schematic diagram of failure modes of polymer products.

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Chemical attack tends to occur in the middle age of a product, especially when the concentration of the attacking reagent (such as chlorine in potable water) is at very low concentrations. Since attack is also increased at higher temperatures (as in hot water pipes) the curve shown in the figure will be foreshortened, so that failure occurs earlier than shown, as shown by hot water systems in the USA. Stresses are high, so there is a strong impetus for cracks to grow by ESC or SCC mechanisms. Old age of perhaps just a few years after installation is dominated by widespread attack by chemicals even at low stresses, but it must be borne in mind that frozen-in orientation also enhances attack in all phases of the life of a product (so real stresses experienced by the product may be larger than planned). As with all the failure modes, local stresses will always be greater at stress concentrations, and brittle cracks will start there first. As in the failures in gas pipes, tests at short times (hours or days in length) cannot be simply extrapolated to predict lifetime but must take other failure modes into account. A routine and established way of performing such tests is by means of exposing stressed samples to the chemicals suspected or known to occur in the environment to which that material will be exposed in service, and there are several standard tests, such as the ‘Bell telephone test’ where bent strips of polymer are immersed in the relevant liquid until failure occurs (22).

6.7.6 Recent developments As might be expected, numerous studies have been undertaken by research groups in the USA to study the problem systematically after the widespread problems started to emerge, often by experts such as Chudnovsky, who had appeared in the court cases. Much of the research has focused on the way SCC cracking develops and progresses in polybutene using FTIR spectroscopy to follow the reactions (23, 24). But other research was directed to advising on better alternative materials. The most important candidate was cross-linked polyethylene, or PEX, and exhaustive tests were performed by Chudnovsky et al. on PEX pipe in contact with chlorinated water (25). They used high temperature pressurized water and passed it through the pipe while monitoring pH and chlorine level in the water. Automatic sensors were triggered at the first sign of leakage. They also examined failed bores using FTIR to detect the carbonyl peaks produced by oxidation. Owing to the much greater resistance of the polymer to oxidation, high temperatures were needed to degrade the material, typically 115°C (when the normal boiling point is raised by the pressure of the supply) down to 95°C. They fitted the raw data to a rate process equation and were able to extrapolate to the expected temperatures of domestic hot water supplies of 80 psi internal pressure and 60°C. They predict a life of PEX pipe of 93 years with a

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95% lower confidence limit of 52 years under these conditions with highly chlorinated water of 4.3 ppm chlorine. They regarded their estimate as conservative because chlorine levels are generally lower, and continuous high temperatures are generally not used in domestic systems. A final point they made in their paper concerned chlorine levels and their effect on lifetime. They compared the effect on life of two levels of chlorine (0.1 and 2.3 ppm) with a neutral unchlorinated water supply. Taking the latter as unity, then 0.1 ppm chlorine lowered the life by a factor of 1.4, while the higher level lowered the life by 2.3. In other words, even the very low level of 0.1 ppm of chlorine had a significant effect on pipe life. The reason for the greater resistance of PEX lies in the very simple repeat unit without side groups as in polypropylene or polybutylene: —[CH2-CH2]n—

—[CH2-CH(CH3)]n—

—[CH2-CH(C3H7)]n—

PEX

polypropylene

polybutylene

Those side groups sensitise the single hydrogen atom on the substituted carbon atom, because the free radical formed by its removal is more stable than that present in PEX or HDPE for that matter. So when oxidized by chlorine or any other oxidative process, the more complex polymers will degrade much faster and at lower temperatures than PE. The low level of crosslinking will also help to stabilize the material as well as lowering creep under load. The plastic pipe fiasco in the USA continues up to the present day, with the announcement in 2008 of a class settlement in Tennessee. It must make the collection of cases, state-by-state, one of the longest running liability suits in history. And over a billion dollars has been pledged by Shell and Celanese to settling the final claims, which have also spread to Canada. The legal results of many cases have been summarized, especially in the case of Cox-v-Shell et al. (26).

6.8

Conclusions

The history of use of plastic pipe shows the importance of prior testing under realistic conditions in order to achieve good lifetimes, provided of course that the knowledge so gained is actually used widely and correctly. Plastic pipes are used increasingly in demanding applications where failure can lead not just to domestic floods as in the case of acetal joint and PB pipe failures, but to explosions where they carry pressurized gas. The failures of such pipes is inevitably increasing, partly because the standards applied to the first installations were not as stringent as they are now, a common facet of all standards as new knowledge of failures and new test methods result in improved specifications. So some buried pipes remain at

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risk, that risk increasing with age. One obvious problem arises from disturbance to the foundations in which the pipes are laid, and ground movement such as subsidence putting extra and unanticipated stresses on the weakest parts such as joints in the system. That risk is greatest on old networks, especially of brittle cast iron, although ductile iron is also liable to corrosion in a wet and oxidizing environment, as the several recent gas explosions in the UK demonstrate. Stress and environmental cracking are two important mechanisms in pipe failure which can lead to premature fracture and catastrophic escape of the contents of the pipes. Both can occur under the right circumstances in all materials, inducing brittleness in seemingly tough and ductile materials. It demands a rigorous approach to investigating failures using the best available methods (27). Then if systematic design or installation issues emerge, such as poor or even non-existent records from the original installation, those issues must be addressed as quickly as possible so as to prevent further problems of the same kind. That problem emerged in the gas explosions in Scotland, Transco not being aware of the existence of old cast iron pipes in several areas. Rigorous investigation also presumes that it will be conducted without preconceived ideas, and soundly based on the evidence that survives, as the ABS acid problem, as well as the ABS pipe explosion showed. Pipes above ground are less susceptible to this kind of infrastructure problem, but may still be in areas difficult to access, such as buried within buildings and so out-of-sight. The small leaks which might otherwise be observed can thus be missed, until crack growth reaches catastrophic levels, by which time it is too late to prevent large-scale damage. Internal crack initiation is the main danger, especially when ESC or SCC occurs on the bore of the wall, so cracking is impossible to detect without destructive intervention. This was the case with the ABS compressed air line, and, on a much larger scale, in the acetal and polybutylene fiasco in so many domestic water systems in North America. The acetal fittings were more susceptible to failure since they were injection moulded and thus exhibited a high degree of frozen-in strain, although the polymer itself is intrinsically more liable to chemical degradation, a fact known from the inception of the polymer in the 1950s. If poorly moulded, such fittings can also fail in cold water supplies, as the example from Loughborough showed. Polybutylene is inherently less sensitive to degradation, and, being extruded, also exhibits a much lower degree of chain orientation. But when those acetal fittings failed, even cursory examination of the pipes would have shown traces of inner degradation, and the first failures should have alerted the manufacturers to a deep-seated problem. Their investigations of fitting failures were clearly flawed for not having detected the parallel problem with the pipework, and so prevented a long drawn-out problem.

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Other polymers, such as PVC, have also been used widely, especially for potable and waste water containment across both the developed and developing world. The polymer is less strong than MDPE, so design must allow for its greater susceptibility to premature failure by brittle cracking. That implies large radii at fillets and corners of fittings, care during moulding, and vigilance when parts are being solvent welded together. Codes of practice and standards are widely available to help users, and many countries (such as Sri Lanka) have used the material wisely and with great success in their water supply programme, a critical area of development in third world countries for prevention of water-borne diseases such as cholera. Other countries have been less successful, many in Africa, by having restricted access to capable manufacturing industries and engineering expertise. As with all product failures, progress will only occur when those failures are published and publicized as widely as possible, and action taken by those responsible for product design and manufacture. Other applications of PVC, such as the welded wrapping machine case, demand high levels of competence to prevent leakage and failure.

6.9

References

(1) Stafford, T, Plastics in Pressure Pipes, RAPRA Review Reports, 9(6), (1998). (2) Wright, David C, Failure of Plastics and Rubber Products, RAPRA Technology Ltd (2001). (3) Lewis, Peter Rhys, Reynolds Ken, and Gagg, Colin, Forensic Materials Engineering: Case Studies, CRC Press (2004), p 184ff. (4) Maker, John, Failure Analysis for Grey Cast Iron Water Pipes, AWWA Distribution System Symposium, Reno, Nevada (Sept 1999). (5) Pilkey, W D, Peterson’s Stress Concentration Factors, Wiley Interscience, 2nd edn (1997), Chart 3.5, p 157. (6) Ogorkiewicz, R M (Ed), Thermoplastics: properties and design. A collective work produced by Imperial Chemical Industries Limited, Wiley (1974). (7) Saul Arlosoroff et al., Community Water Supply: The Handpump Option, UNDP/The World Bank, Washington USA (1987). (8) Hertzberg, R W, Deformation and Fracture Mechanics of Engineering Materials, John Wiley (1976); Fatigue of Engineering Plastics, Academic Press (1980). (9) BS 4346-3:1982 Specification for PVC-U joints and fittings for use with PVC-U pressure pipes and specification for solvent cement (1982). (10) HSE (Hazardous Installations Directorate), Investigation of the Explosion at Linfield Street, Dundee, 22 October 2000, report (2003); available as download at http://www.hse.gov.uk. (11) NTSB, Special Investigation Report, Brittle-like behavior in Plastic Pipe for Gas Service (1998); available for download at: http://www.ntsb.gov/ Publictn/1998/SIR9801.pdf (12) NTSB Pipeline Accident Brief, Waterloo Iowa Explosion (April, 1998); available for download at: http://www.ntsb.gov/publictn/1998/PAB9802.pdf

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(13) NTSB Pipeline Accident Report, San Juan Gas Company, Inc./ENRON Corp. Propane Gas Explosion in San Juan, PUERTO RICO, on Nov 21, 1996 (1997); available for download at: http://www.ntsb.gov/Publictn/1997/PAR9701.pdf (14) NTSB Pipeline Accident Brief, Dubois Penn, August 2004; available for download at: http://www.ntsb.gov/publictn/2006/PAB0601.pdf (15) Young, W C, Roark’s Formulas for Stress and Strain, McGraw-Hill, 6th edn (1989), Table 32, p 638. (16) Open University, Forensic Engineering T839, Block 1 case study; Lewis, P R, Degradation of an Acetal Plumbing Fitting by Chlorine, Fapsig-SPE session, Orlando (2000). (17) Lewis, P R, Designing with Plastics, RAPRA Review-reports, No 64 (1993). (18) Anon, Plastic pipe is expensive for industry, Chemical Reporter, 18 March (1991). (19) Armstrong, James and Duvall, Plaintiffs Exhibits in Chris and Diane Babb-vShell Chemical Co et al., Matagorda County Court, Texas (1992). (20) US Army Center for Public Works, The Use of Plastic Plumbing Materials (1996); available as download from: http://www.wbdg.org/ccb/ARMYCOE/ PWTB/pwtb_420_49_6.pdf. (21) Barker, S J and Price, M B, The chemistry of degradation and stabilization of poloxymethylenes, Section 2.3, p 22, in Polyacetals, Iliffe Books – The Plastics Institute (1970). (22) Brown, R P, Handbook of polymer testing, CRC Press (1999), p 362. (23) Bigg, D M et al., Analysis of the degradation of poly(1-butene) pipe through oxidation induction time tests, Advances in Polymer Technology, 24 (3), 215–225 (2005). (24) Chudnovsky, A et al., Experimental and theoretical investigation of stress corrosion crack (SCC) growth of polyethylene pipes, Polymer Degradation and Stability, 94 (5), 859–867 (2009). (25) Chudnovsky, A et al., Chlorine resistance testing of cross-linked polyethylene piping materials, Fapisg-SPE Dallas (2001); available for download at: http:// www.janalab.com/pdf/ANTEC%202001%20Paper%202.pdf. (26) Hensler, D et al., Class Action Dilemmas:Pursuing Public Goals for Private Gain, Rand Corp (2000), Chapter 13 Polybutene plumbing pipes litigation: Cox-v-Shell et al. (27) Farshad, M, Plastic Pipe Systems: Failure Investigation and Diagnosis, Elsevier (2006).

7 Polymeric seals

7.1

Introduction

Small bore polymer tubing easily absorbs movement, but many conventional piping systems in steel or other rigid materials are still widely used for fluid transport. Yet they too need a way of sealing the system reliably in the face of vibration, especially where they join dissimilar materials. The answer is to close the system with a flexible polymer, and seals are of great importance in all engines and machines with moving parts (1). Sealants serve a similar purpose where gaps in a building, for example, must be closed against ingress of rainwater. Seals have been critical parts of engines ever since the invention of the steam engine by Savery, Newcomen and Watt in the early part of the Industrial Revolution, with natural materials such as hemp fibre, leather and bitumen, for example, being used. The failure of such seals in Brunel’s famous atmospheric railway highlighted the importance of reliable materials which could not be consumed by rats or degraded by the environment (two of the problems with Brunel’s seals). It was discovery of vulcanization by Goodyear in 1844 which gave to the world a path to a better material capable of absorbing movement and vibration (2), a step heralding the invention of the humble O-ring and all manner of seals of different shapes and dimensions. Many new elastomers were developed in the 20th century such as polychloroprene (one variant being known as Neoprene), fluorinated rubber (such as Viton) as well as general purpose rubbers such as NBR (so-called nitrile rubber) and SBR (styrene butadiene rubber). Polyurethanes have also been developed both as cross-linked rubbers and thermoplastic varieties for sealing purposes (3). We rely on such small components as seals of all shapes and sizes to keep cooling systems, heating networks, pneumatic lines and all kinds of engines operational so it is important to be aware of the failure modes of seals. A brake seal is a good example of a safety-critical seal, which, should it fail when driving, can cause a serious accident through total loss of braking power. In manufacturing industry, air lines are an important way of transmitting energy, as the ABS pipeline in the previous chapter showed. But they are also used for controlling the manufacture of another important device, the semi-conductor, universally used in electronics in an application not usually 272

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appreciated. Failure of seals in such lines can curtail manufacture with large consequential losses unless simple precautions are adopted. Similarly, seals in hot water central heating systems are crucial in preventing leakages from the pipes, and if a new material is introduced, it must be capable of resisting both water and high temperatures. Testing such materials before use is essential, and only under conditions which are realistic in their simulation of reality. Sealants, mastics and grouts are also used extensively in buildings to prevent entry of water from the environment, or in specialist applications, one of which includes special non-toxic smokes used to simulate fires in training buildings for the fire brigade. Such sealants must be able to resist the organic components of the smoke but if otherwise, real fires can result. Again, realistic testing is essential if those grouts or mastics are to provide a reliable sealing action.

7.2

Failure of elastomeric seals in brakes

Seals are needed in braking systems to isolate the hydraulic fluid from the mechanical load activated by the driver when attempting to slow the vehicle while driving. Loss of braking power is very serious while driving and can cause accidents, as the case presented in this section shows. The case was referred from insurers in 1982 in relation to a serious accident with a van which had been involved in a crash. The rear brakes had failed, leakage of brake fluid having been found when the vehicle was examined after the accident. The van had travelled about 10 000 miles between fitment of the brake cylinder and failure. When the brake cylinder was removed and stripped, it was found that a small piece of rubber had broken away from the lip seal which isolated the hydraulic circuit (Fig. 7.1). The small chip was roughly ellipsoidal in shape

7.1 Brake piston with fractured rubber seal in original position, removed at right.

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7.2 The fracture surface showing fatigue striations.

and had broken away through the lip of the seal where hydraulic pressure from the brake fluid pushed the edge of the lip into contact with the sides of the brake cylinder. The seal appeared to be under-size, judging by the gaps at both top and bottom in the recess within which it lay. It was also interesting to observe that there were also gaps between the edges of the chip and the fracture surface in the bulk of the seal (left-hand picture of the figure). Inspection of the opposing fracture surfaces showed numerous striations characteristic of a fatigue failure (Fig. 7.2). The piston had a diameter of 19 mm and sat within the cylinder of bore 19.06 mm, so the seal had a gap of 0.06 mm to fill when in position.

7.2.1 Scanning electron microscope (SEM) of fracture surface The sample needed more detailed examination in a scanning microscope. It was cut vertically along the side opposite to the fracture so as to separate it from the aluminium die-cast piston of outer diameter 19 mm and length 48 mm. The seal sat in a recess of diameter 12.15 mm in the end of the piston. The diameter of the seal shrank by about 1 mm when removed, showing that the seal was under tensile strain when in situ, in addition to that already noted on the mismatch between the chip and the seal (Fig. 7.1). So the original strain in the unbroken intact seal was likely to have been greater owing to relaxation of the seal by its fracture and by removal from the piston. It was coated with a thin layer of gold to enhance contrast and reduce the chances of a build-up in charge on the non-conducting surface of the rubber. The nearly complete fracture surface from one side of the fracture in the seal is shown in Fig. 7.3. It shows progressive growth of a brittle crack from the outer corner of the seal up through the body until it met the channel to the lip and grew further to the free surfaces so as to form a sepa-

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O

7.3 SEM micrograph of one side of fracture surface of chip, origin lower left.

7.4 Cuts in outer lower edge of seal (arrow).

rate chip. The direction of crack growth can be judged by the flap formed at the top of the sample by the crack branching just before it met the outer free edge. The other half of the fracture surface showed a similar pattern of the striations from the corresponding outer corner of the seal. So what had initiated the two separate cracks? Deep cuts were found at the lower outer corner of the seal (Fig. 7.4) and there were matching scratches found on the outside of the piston on the land adjacent to the seal

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7.5 Scratches in outer land of piston next to seal recess (top).

recess (Fig. 7.5). Inspection of the interior of the cylinder in which the piston worked also showed traces of similar scratches (as well as a tide mark from the resting position of the rubber seal). However, there was no trace of any particles left in, or on any of the samples. It seemed clear that contamination by sharp abrasive particles had caused significant damage to both the piston and the seal it held.

7.2.2 Elastomer analysis The material of the seal itself was analyzed by exposing the rubber to several different organic solvents, and the swelling measured and compared with known standards (4). Since different rubbers swell to different extents in organic fluids, the particular swelling properties are diagnostic of a specific elastomer. One way of identifying different cross-linked rubbers is by comparing the swelling in three solvents: petroleum ether, benzene and aniline. A small sample of regular section (such as strip) is exposed to the solvent until swelling ceases (usually several hours), and the swelling calculated from the change in dimensions. The closest match to the brake seal values was found to be SBR rubber, a general purpose elastomer widely used in car tyres, for example. Although such a swelling test would now be difficult, since benzene is barred from laboratories as a known carcinogen, swelling tests with other more benign reagents are still useful as diagnostic analytical tool. However, such identification tests have been displaced by IR spectroscopy using ATR (attenuated total reflection) where a sample is pressed against a selenium crystal and the spectrum obtained by multiple reflection of the beam. Using SBR in brake seals is unusual because it has poor resistance to oils and might explain why the seal appears to have shrunk in its recess (Fig. 7.1). For example, SBR is usually plasticized with extender oils, and these can be leached from the material by brake fluid. If that had happened, then

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the volume would shrink, as seems to have occurred. With shrinkage comes an increase in tension or hoop stress in the circumference of the seal, also seen both in the fracture and when removing the seal from the piston. Finally, SBR has poor resistance to fatigue, and much lower than natural rubber, for example (4).

7.2.3 Explanation of accident The direct cause of the failure of the brake seal was abrasive cuts in the outer corner, probably caused by sharp particles which contaminated the fluid. They initiated two fatigue cracks, which grew steadily with use of the brakes on the van. Each application of the brakes by the driver will have imposed an extra hydrostatic stress on the seal. When the brake pedal is pushed down, a lever pushes against the piston, which in turn pushes against the closed hydraulic system. The pressure developed in the fluid acts against the seal lip, pushing it against the walls of the cylinder, but also putting the entire seal under pressure. The hoop stress thus increases throughout the seal, and will be concentrated further at stress concentrations, especially deep cuts (effectively proto-cracks). It is possible that the cuts were formed when the piston and seal were first installed, so growth occurred from first use of the brakes. The net stress at the roots of the two cracks involved was a product of fitment and the start of shrinkage produced by extraction of the extender oil. Growth will have been slow at first, and dependent on the degree of braking used by the driver. A critical point was reached when the cracks reached the base of the lip channel. Slow leakage of brake fluid will have started at this point, the rate of leakage being very slow, since the crack opening will have been very small (and the viscosity of the brake fluid being very high). However, that rate will have increased as the cracks grew into the lip of the seal. The van mileage of 10 000 clearly represents high usage but the nature of that driving was unknown: motorway driving will have involved much smaller brake use than urban driving, for example. When the van was serviced, the brake reservoir probably needed topping up, but clearly the state of the seal was not examined. At the same time as fluid leaked out, air probably entered the system, making the brakes feel ‘spongy’. Air is highly compressible, so pedal pressure will compress the air first and then the brakes, reducing the efficiency of the system. The next critical point was reached when the two cracks met and formed the chip, and brake fluid will have leaked copiously at this point, with total loss of braking to the vehicle from the open cracks (Fig. 7.1). It could well have occurred when maximum pedal force was applied by the driver in an emergency.

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The root cause of the failure was a poorly designed brake seal, which was probably contaminated at fitment by sharp particles; the particles cut the edge of the seal and started two fatigue cracks which grew to completion. Great care is needed during all vehicle maintenance to ensure the correct size and type of seal is chosen by the mechanic. In any case, the driver should have been alerted by sponginess in his brakes, and asked for the brakes to be bled and thoroughly examined for any defects. The replacement seal was quite different in design, with no lip at all, suggesting that the original may have been a simple mistake by the mechanic. Whatever the cause, there was no epidemic of brake failures, so the failure must be put down to human error and poor maintenance.

7.3

The Challenger disaster

That seals are frequently critical parts of many devices and machines was brought home to one of us (PRL) during routine domestic maintenance. He had been puzzled by the failure of a shower head attached to the hot and cold water supply in his bathroom. Demands from other users, especially his daughter, prompted an investigation. When the joint between the water supply and the head was disassembled (with great difficulty owing to build-up of calcium carbonate from the hard water), he found that an O-ring had jammed inside the valve, and was blocking the pipe. The previous owner had fitted an oversize ring, which then extruded under pressure and blocked the valve. The solution was simple: to remove and replace with the correct size ring. But there may be other problems with O-rings, especially their behaviour at low temperatures when fitted to rocket casings exposed to external temperatures. And that is just what happened when the Challenger shuttle exploded shortly after take-off from Cape Canaveral on 28 January 1986 killing all seven crew aboard. It was not so much the shuttle itself, but rather one of rockets used to boost it into space that exploded, and NASA had been warned that there might be a problem. Film of the disaster showed that just before the final explosion, a jet of flame issued from one of these joints on one side of one of the two booster rockets. It had failed to contain the burning contents, and the gap grew as the joint eroded away, ending with an explosion which threw the Challenger into the sea below (Fig. 7.6). The rocket concerned was made from several steel cylinders which were united at their ends by special joints. A key part of the seal between the two cylinders was a set of two Viton fluoroelastomer O-rings of diameter 0.28 inches (5 mm) set into the inner side of the joint (Fig. 7.7). The joint was further protected from the effects of the hot propellant by insulation and a seal of zinc chromate putty. The elastomer of the O-rings is resistant to high temperatures, but that resistance drops rapidly as the temperature

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7.6 Leak of flame from booster rocket (NASA).

Propellant Segment tang

Leak test port plug and packing Grease bead Pin retainer clip Pin retainer band

Insulation Primary O-ring Secondary O-ring Propellant relief flap AFT facing inhibitor Zinc chromate putty Insulation

Clevis pin

Pin retainer band cork insulation Segment clevis

Forward facing inhibitor Insulation Propellant

7.7 Section of the field joint on booster rocket.

falls below ambient. And there had been failures of the rings in previous tests, which gave warning of this, the weakest part of the structure. The problem can be appreciated by considering the effect of internal pressurization as the propellant ignites and starts to burn during lift-off (5). The pressure forces the steel cylinders to expand well away from the joints, so bending the joint in the opposite direction (Fig. 7.8). It opens a momentary gap on the inner side, and the rubber of the O-rings must fill that gap in the fraction of a second to maintain the integrity of the seal. As was pointed out by Richard Feynman at the subsequent official enquiry, just immersing a sample of the rubber in ice water was sufficient to reduce its resilience so that an O-ring could not react quickly enough

Pressurized joint

Exterior

Interior

Exterior

Forensic polymer engineering

Interior

280

Unpressurized joint

7.8 Effect of internal pressure on joint.

when stressed (6). This is the crucial property needed in a rocket casing O-ring, because it must maintain the seal when the joint is vibrated by the forces exerted on the outer casing by combustion processes within the rocket. The response time of the elastomer decreased very rapidly as the temperature fell (Fig. 7.9), and freezing conditions had occurred overnight at the launch site. Indeed, there was some evidence that water had penetrated the field joint, further compromising its integrity. Needless to say the entire joint has been redesigned to incorporate extra O-rings, and rigorous tests have ensured that joints have behaved correctly during launches of the shuttle. The disaster also revealed shocking lapses in management of the project. Engineers at Thiokol Corporation (manufacturers of the rockets) had warned NASA of the possible problem before the launch, and advised against a launch after the cold weather had been reported. However, NASA over-rode their advice and proceeded. Their negligence had a severe effect on the entire US space programme at the time and invigorated development of unmanned missions. The development of the shuttle, however, itself introduced further problems with the crash of the Columbia on 1 February 2003, when it disintegrated over Texas during re-entry into the Earth’s atmosphere, with the loss of all seven crew members (7). That disaster involved failure of polymer components, too, because it was afterwards found that a lump of polyurethane insulation on the exterior of one

Polymeric seals

281

O-ring recovery vs. time

O-ring recovery (inches)

0.060 0.050 Initial compression

0.040

Temperature

0.030

75°F 60°F

0.020

50°F 40°F 30°F

0.010

25°F 10°F

0.000 0.0

0.2

0.4 0.6 Time (sec)

0.8

1.0

7.9 O-ring recovery with time and temperature.

of the rockets came free during launch and impacted the wing of the shuttle. The impact created severe damage to the carbon fibre composite wing, which went unnoticed by the crew during their successful research in space. However, the damage was critical and the wing disintegrated during reentry. It was a failure of testing, because impact tests had been conducted using small lumps of insulation, but nowhere near the size of the large lump which actually caused the accident. As with the Challenger disaster, the events could be seen, albeit at very low resolution, on the videotape of the launch, but only after the event.

7.4

Failed elastomeric seals in a semi-conductor factory

The first notice we had of a failure in a pneumatic line came when a company representative visited us with a failed diaphragm seal. The rubber diaphragm seal in an air line had cracked and the company who had supplied it wanted diagnosis and advice on prevention of a recurrence of the problem. The seal worked in the air line at a semi-conductor factory in Japan, and closed a chamber supporting the air bearing of the main stage holding the chip (Fig. 7.10). A new rubber seal is shown from above and below in Fig. 7.11, and it is a critical component because the air bearing is dependent on the integrity of the seal. The air bearing ensures that the stage is absolutely motionless to enable accurate etching of the chip surface when making the chip circuitry. Since they typically are of the order of fractions of a micron in resolution, any vibrations or movements can wreck the quality of the chip. The seal was small, measuring only 10 mm in diameter,

282

Forensic polymer engineering

7.10 Air bearing with critical diaphragm seal at centre (arrow).

7.11 New seal showing recess at left and plain surface above at right.

but sitting at the centre of a steel disc of diameter nearly 40 mm. The seal was 2 mm thick and the membrane 0.5 mm thick.

7.4.1 Failed diaphragm seal The failed seal is shown in Fig. 7.12. The damage included erosion of the edge and fine cracking of no apparent preferred orientation next to the hole that accepts a steel axle when in position on the etching machine. It was made of NBR, or nitrile butadiene rubber, which is a copolymer of acrylonitrile and butadiene monomers: acrylonitrile repeat unit:

—[CH2—CH(CN)]—

butadiene repeat unit:

—[CH2—CH=CH—CH2]—

Polymeric seals

283

7.12 Damage to surface of NBR diaphragm seal.

Its composition varies between 15 and 30% acrylonitrile, the latter groups providing oil resistance to the butadiene elastomer. The homopolymer polyacrylonitrile is a fibre forming polymer known as ‘acrylic’ as well as forming a thermoplastic material in SAN or styrene-acrylonitrile. The butadiene content gives the material elasticity but is also most vulnerable to oxidation via the double bonds in the chain. Carbon black filler provides some limited protection but the material is vulnerable to degradation. The damage looked like oxidative attack and erosion, perhaps during manufacture, and the company were advised to check the conditions under which the seals were made. The state of the new seals supplied for purposes of comparison also showed manufacturing defects such as weld lines, probably produced by poor temperature control of the steel tools used during compression moulding the devices (Fig. 7.11). The same picture also shows excessive flash rubber near the central hole, as seen on the lower surface of the seal at left in the figure.

7.4.2 More failed seals But it was not the end of the story. A few weeks later, the same company came back with another problem. The seals were cracking in a different way, and this time, chip production had been lost since the air bearings had lost pressurization (Fig. 7.13). Several seals were affected, so production on etching machines was lost. The attack was highly localized with a single circumferential crack with a very rough fracture surface (Fig. 7.14). It turned out that it was not the only rubber part involved. O-rings used to seal other parts of the machines were also failing (Fig. 7.15). Cursory inspection with a magnifier showed characteristic brittle cracks running across the diameter, one of which had run to completion. The ring shown

284

Forensic polymer engineering

7.13 Brittle crack in diaphragm seal.

7.14 Fracture surface of diaphragm seal.

7.15 Fractured O-ring seal.

Polymeric seals

285

had a diameter of 2.5 mm and a circumference of 70 cm, and fitted a large seal between chambers in the same pneumatic system. It was clear that a detailed investigation would be needed to pinpoint exactly the nature of the cracking in the diaphragm seal in view of the loss of chip production. The first step was to examine the seals using ESEM, a method of high enough resolution to reveal the characteristic crack features which might point to the cause of the problem. It was also a method ideally suited to examining such a small component, the size of which would challenge the skills of an optical microscopist. The very first low magnification pictures did indeed reveal that the crack had grown along a sharp inner corner of the seal (Fig. 7.16), and a sub-critical crack was also present at the corresponding inner corner close to the axis of the product (Fig. 7.17). Both corners represented stress concentrators, and were the most likely to be attacked if the diaphragm was

Acc.V Spot Magn Det WD 15.0 kV 5.0 26× GSE 9.2 0.4 Torr

1 mm

7.16 Fractures in diaphragm seal.

Acc.V Spot Magn Det WD 15.0 kV 5.0 75× GSE 11.3 0.4 Torr

200 μm

7.17 Sub-critical crack next to steel post.

286

Forensic polymer engineering

subjected to only small pressures. The membrane between the two corners was only 0.5 mm in thickness, so only a very small degree of cracking could cause total loss of function. The main fracture surface was rough and showed no inner structure (such as striations) and was very similar to that previously seen in the optical microscope (Fig. 7.14). The inner sub-critical crack appeared to show a tendency to branch into the membrane, perhaps caused by breakage of the membrane at the outer corner, putting a small bending moment on the membrane. One advantage of ESEM is the facility to perform X-ray analysis, and comparison of the elemental composition of the crack and an intact surface showed that the crack had a higher oxygen content than unaffected rubber (Fig. 7.18). Such evidence pointed to ozone rather than oxygen attack, a cps C 30

O

20 S

Zn 10 F

Si Al Mg

Ca

Zn

0 0

2

4

6

8 Energy (keV)

cps C

50

O

40 30 20

Zn

10 F

S

Al Mg Si

Ca

Zn

0 0

2

4

6

8

Energy (keV)

7.18 X-ray emission spectra from normal surface (top) and ozonized fracture surface (bottom) showing increase in oxygen content.

Polymeric seals

287

well-known failure mode of many elastomers (all those containing double bonds).

7.4.3 Ozonolysis Ozone gas is an allotrope of oxygen, and one of its most active forms because the gas attacks organic materials by way of their double bonds, which it cleaves very quickly: —CH=CH— + O3 → —CHO + CO2H— In this generic example, ozonolysis produces an aldehyde and a carboxylic acid group at each of the new chain ends, so accounting for the increase in oxygen content of the crack surfaces. But the most important effect is to break the chain: —CH2—CH2—CH=CH—CH2—CH2—CH=CH—CH2— CH2— + 2O3 → —CH2—CH2—CHO+CO2H—CH2—CH2—CHO+CO2H—CH2—CH2— In this case, the chain is broken twice, and since the strength of polymers is critically dependent on chain molecular weight between cross-links, the strength drops and cracks develop (8–11). In surfaces attacked by ozone, one would therefore expect to see higher levels of atomic oxygen from the aldehyde and carboxylic acid chain ends left after ozone attack (12, 13). In fact, in an elastomer like nitrile rubber, there is no atomic oxygen present at all in the pure polymer, but various process aids such as stearic acid and its salts are used in commercial materials. Since such acids or esters do contain oxygen, it will be found in the ESEM spectrum, so the extra oxygen produced by ozonolysis will enhance the relative amount found, as observed (Fig. 7.18). That salts were present can be judged by the trace amounts of several metals found in the seals, such as calcium, magnesium, zinc and aluminium.

7.4.4 Independent analyses The semi-conductor factory was situated near the east coast of Japan, and gas analyses were conducted by the company on the air supply system itself. It was achieved by conventional chemical means, and provided a clear picture of the state of the air in the pneumatic system. The semi-conductor line comprised, at least in principle, four branches, each of which had four machines (Fig. 7.19). But only eight machines were being used, the other positions being empty pedestals. The diagram shows that seven of the eight machines had been affected, the largest number of seal failures having occurred on the machines present on the first two branches, while the

288

Forensic polymer engineering

CDA I-LINE

KrF-LINE

3

M3251

5 M3526

5

3

M5770

M9059

PAS machine with E-chuck replacements PAS machine with number of 3 broken ABC W/F GAS Pedestal only

M6260

M8047

1

1

Ion Implanter

3 M5728

M9189

AGV line

AGV line

AGV line

Ion Implanter

2

2

0

Ion measurement

Ozone measurement

Organic measurement

7.19 Plan of fabrication lines showing failed seals observed.

apparently unaffected machine was at the end of its branch, at the furthest end of the air supply. It suggested that whatever was causing the damage was being exhausted by attack. This is just what happens during ozone attack, as shown by the equations above, that ozone is absorbed by the rubber, and will be depleted further along in the system. Failures tend to decrease along each branch as the gas is absorbed. The two points where analyses were taken are shown in Fig. 7.19, with samples from the first branch. The sequence of failures is shown in Fig. 7.20, with the first being obtained in January 2001 and building up progressively in many different rubber components as the months passed. Thus although by the middle of the year, the failures had appeared to cease, they suddenly started again in earnest by early 2002. The failures had occurred over a wide range of different seals (as shown by the symbols) and were also occurring at a lower level in the apparently unaffected machines, which are identified by their numeric code in the table. The results of analysis are shown in the table (Table 7.1). It shows the concentrations of ozone and nitrogen dioxide (NO2) at the level of parts per billion (PPB) and also expressed in nanogrammes per cubic metre of the air in the system (ng/m3). The table compares the concentrations in two quite separate lines in late 2003 when the crisis was at its height, the lower part of the table showing that the gases were effectively either absent or at minimal levels in the Canon line compared with the ASML line. The levels of ozone were at a maximum of 2.7 PPB down to 0.9 PPB and varied between these two values over the period in which measurements were

Polymeric seals

289

Amount of hardware replacements: 3526 6260 9189 9059 5770 8047 5728

SAT

ABC WF GAS

ABC WF VAC

ABC R-CHUCK GAS

AIRPOOT

28-Jun-04

29-Mar-04

29-Dec-03

29-Sep-03

30-Jun-03

31-Mar-03

30-Dec-02

30-Sep-02

1-Jul-02

1-Apr-02

31-Dec-01

1-Oct-01

2-Jul-01

2-Apr-01

1-Jan-01

3251

E-CHUCK ASSY

7.20 Development of a crisis.

Table 7.1 Chemical analyses of air in two separate pneumatic systems day

Ozone (O3)

NO2

O3 meas.

R-CR (ASML)

T-CR (Nikon/Canon)

27 28 29 30 01 02 19 20 21

Nov 2003 Nov 2003 Nov 2003 Nov 2003 Dec 2003 Dec 2003 Dec 2003 Dec 2003 Dec 2003

PPB

ng/m3

PPB

ng/m3

2.7 1.7 0.9 2.1 1.9 1.5 <0.1 <0.1 <0.1

5790 3640 1930 4500 4070 3210 210 210 210

1.56 0.06 0.07 0.16 0.17 0.07 0.02 0.03 0.03

3200 120 150 328 350 150 41 65 33

taken. The levels of NO2 appeared to parallel those of ozone, although the association was not strong. So these results when taken together confirmed that ozone was the cause of the problem, but what could be the source of the gas in such an enclosed system? And secondly, why had the problem only started recently?

7.4.5 Sources of ozone The investigation now turned to a more detailed examination of the pneumatic system itself and possible sources of the gas. In general, ozone is produced in several different ways, including (14):

290 • • •

Forensic polymer engineering

during electrical discharge such as sparking during silent electrical discharge by the action of sunlight on a polluted atmosphere.

The association between electrical discharge and ozone is strong, since it is known that ozone is produced during thunder storms when lightning occurs (15). The passage of large high-energy sparks through the air causes a cascade of reactions between oxygen and nitrogen, including the creation of nitrogen dioxide and eventually nitric acid when it reacts with water. But the same extremely high temperatures also create ozone, and are present even in small sparks often present near electrical machinery. And visible or audible sparks are not the only source of the gas because silent discharge of static electricity, for example, can also create the gas. Thus high concentrations can be found near xerographic photocopying machines, and indeed wherever discharges can occur either as a part of a process, or accidentally by leakage of electricity. The final source of the gas is by photochemical interaction between the energetic UV component of sunlight and the atmosphere, especially when volatile organic components (VOCs) are present (16). The most common VOCs are those produced by vehicle engines, both diesel and petrol powered, including traces of those fuels and their combustion products in exhaust fumes. It leads to local pollution and, when bright sunlight shines on the mixture, to the formation of ozone and nitrogen oxides (NOx). The problem thus occurs most frequently in or near city centres, and many cities around the world suffer such gaseous pollution. Examples include The Los Angeles basin in the USA and Mexico City in the Americas, but it is also a problem faced by many other developed cities like London and Paris as well as cities in the developing world such as New Delhi, Shanghai and Beijing (17). And such polluted air can travel some way before it is removed either by interaction with unsaturated organics or washed from the atmosphere by rain, for example.

7.4.6 Chasing the problem So one line of attack on the problem examined the local atmospheric conditions near the factory in Japan, while another lay in examining equipment at or near the air intake: the compressor and associated filters (Fig. 7.21). The compressor itself was apparently a conventional design of rotary pump using oil, so the filters fitted to the output gas were designed to trap any oil droplets that accidentally entered the compressed air supply. This was also why the elastomeric seals were made from nitrile rubber, a material well known for its resistance to oil. None of them would react with ozone, however, so the compressor could be the source of the problem, once

Polymeric seals

M

Hepa filter M

291

Compressors: Atlas Copco, type: ZR-155

Filter: Atlas Copco, type: PDJ 375F (0.001 [um])

Filter: PALL, type: MCY1001FRH13 (0.001 [um])

1

Hepa filter M

Absorption dryer: Atlas Copco, type: MD-300W (silica gel)

Hepa filter

7.21 The compressor and air filters on the pneumatic line.

external pollution had been eliminated. Despite an extensive review of the atmospheric pollution in the mainly rural environment, there was no convincing evidence to show that the gases were present in air sucked into the system from the outside. Indeed, such was confirmed by analysis of the air intake using the chemical methods used for the line itself. The composition of the gases present in the system at various times in late 2003 (Table 7.1) suggested an electrical source, since high levels of NO2 and O3 seemed to occur together. A local and nearby source was the most likely explanation, either close to the air intake or within the compressor itself. The electrical motor driving the compressor could not be the source, since the workings were totally independent of the gas line, so another explanation was sought. On detailed examination, it was found that the compressor had itself been changed on the line recently, from a conventional rotary type to a new design which used counter-rotating helical rotors to create the air pressure. The space between the rotors gradually decreased so that the ‘nip’ between the two rotors slowly diminished, and so the air pressure rose (Fig. 7.22). The rotors themselves were plastic-covered so as to ensure a smooth air flow, as well as protecting the polished metal surfaces. Now when nonconducting polymers are used in such a way, the chances of static electrification arises. Many plastics when rubbed physically can produce high local charges, which cannot leak away easily through the bulk, since the materials

292

Forensic polymer engineering

7.22 New design of compressor.

are insulating by their very nature (18). Such triboelectrification (as it is called) is commonly encountered in dry atmospheres, where leakage through the air is also hindered. The phenomenon can, for example, cause sparks to be generated when a driver steps from a car owing to triboelectrification by tyre contact with the road with the charge building up on the metal car frame acting as a Faraday cage. Even removing a nylon shirt can generate small but very powerful sparks. It was therefore the likely source of the problem in producing tiny amounts of both ozone and nitrogen dioxide in the air supply, the gases attacking and slowly cracking the crucial diaphragm seals. The solutions to the problem were several. They included changing the elastomer of the seals to an ozone-resistant material like Viton or EDM, or installing ozone absorbing filters after the compressor (especially those containing activated carbon, a very effective absorber). A recommendation to remove the stress concentrating corners by rounding off the tool was also made as a sensible precautionary measure to reduce the possibility of cracking (Fig. 7.10). Such filters were introduced and the seal corners radiused, so eliminating the problem at source. We were sent samples from another lithography line shortly afterwards, where similar cracks were found. Pro-active maintenance had given the manufacturers fair warning of the problem. However, the cracks had not grown to completion and another series of failures was prevented before they could cause production problems.

7.4.7 Conclusions So what were the lessons of the incident? In the first place, the cracking had a severe impact on the normal operations of the semi-conductor factory. Each time a seal cracked, chips were destroyed during their production, and the line had to be halted while new seals were installed, so causing consequential losses in production, which were estimated to be costing the company $1.5 million per week, such is the level of investment in the lithography machines. The costs of prevention were trivial by comparison. The problem might have been circumvented by using a more resistant rubber, but the problems of new compressors were clearly totally unexpected. And it is surprising to learn that tiny concentrations of ozone can have such far-reaching effects on items that nobody assumes can fail. The reason why ozonolysis can be so devastating is the almost quantitative

Polymeric seals

293

reaction with double bonds, and indeed, has been used widely in analysis of organic compounds for structure determination because if the products are analyzed and identified, it is then possible to predict the starting structure. The reaction can also be turned around to another useful function: detecting small levels of ozone in a gas by simply suspending rubber bands and counting the number of cracks and their shape as they are formed. So despite the fact that most seriously stressed elastomeric products are nowadays well protected against ozone attack (and oxygen, too), there may always be safety- or function-critical components waiting to fail because no one considered ozone attack at all likely. Ironically enough, another Japanese company had alerted the industry to the potential problem of ozone attack in 2001 (19), but few were apparently following or reading the relevant literature. The report was made by SMC of Japan, one of the largest manufacturers of pneumatic equipment in the world, who posted it on their website and so made it available to everyone at the time. The document provides examples of cracked seals of all kinds, and describes the problem of the presence of only trace amounts in a pneumatic air supply system. Sources of the gas are discussed thoroughly, and the specific problem of scroll compressors mentioned as a particular problem. They manufacture a range of seals where the problem is suspected, using ozone-resistant elastomers. The importance of reviewing available literature is one of the themes in many of the case studies discussed in this book: a new problem may just be a recurrence of one that has happened before, then buried and forgotten until the next outbreak. But the availability of relevant information has changed out of all recognition with the world wide web. Vast amounts of data can be accessed both on commercial websites as well as in the academic literature with the use of appropriate key words. Much information is also of course available in conventional text books, although specific failure cases are often difficult to find, perhaps owing to a natural reluctance to reveal embarrassing results. Text books are also frequently out-of-date, and often do not cover as many areas of technology as would be desired. The SMC decision to post details of their experiences (clearly based on their own failures) is heartening, and one hopes that other companies working in other areas of technology will follow suit.

7.5

Failures in TPE radiator washers

Washers are another type of seal where function and performance are critical to the integrity of the system in which they operate. Central heating systems (CHS), for example, are almost universally used today in most domestic and public buildings, replacing less efficient open fires for providing reasonable warmth throughout the winter months as well as a

294

Forensic polymer engineering

convenient source of hot water for baths and kitchens. The hot water heated in a boiler is distributed to metal radiators located in every room, to provide space heating, such water being circulated in a closed system topped up when necessary by a cistern. The same supply is also sent through a copper reservoir to heat water for direct use by the consumer. Such systems must be sealed effectively against leakage through the many joints needed on the extensive copper pipework, including seals on the radiators, for example. If such seals leak, then consequential damage to property can be extensive if not detected in time. Various materials have been used by plumbers, including winding fibre around the pipe end, which jams into any gap in the joint. PTFE tape has also been used in a similar way. Rubber washers made from conventional cross-linked materials are also widely used on taps. But such seals do suffer significant problems, fibre and tape often leaking slowly due to small gaps present on formation by crushing the material into the joint. Conventional rubber washers deteriorate with time, especially when exposed for long periods to high water temperatures, by oxidation and cracking, leading to leakage. In an attempt to address these problems, a plumbing supplies company in the West Midlands experimented with thermoplastic elastomers or TPEs, a relatively new class of rubbers that have become available over the last two decades. Their use in catheters was described in a previous chapter, for example. As that case study showed, however, their physical and chemical properties must be thoroughly explored when being introduced into the market. The present case reinforces that very basic message.

7.5.1 New washers One of the big advantages of TPEs lies in their ease of manufacture using injection moulding rather than transfer or compression moulding for conventional washers. Very large quantities could be made quickly using multiple cavity tools, cutting costs. Waste polymer can also be recycled, while conventional rubber waste must be scrapped. So the company proceeded to trial washers made using polyester TPEs, a block copolymer of polyester and polyether (Fig. 3.2). The former blocks crystallize to reinforce the elastomeric network of flexible polyether chains, so providing stability to any products. The trials showed that such washers worked well in both cold and hot water taps, so the company proceeded to adopt them for use in central heating systems.

7.5.2 Leaks in CHS systems However, leaks were detected by several users after a few months, especially local authorities who had new CHS systems recently installed in

Polymeric seals

295

7.23 Radiator with cracked washer.

schools and old people’s homes. A private school, Benenden, in Kent, also reported failures of washers and leaks, damaging nearby furniture and fittings such as carpets. Figure 7.23 shows the upper vent plug on one such radiator. The radiator came from Stratford old people’s home. Failures were being reported across such a wide range of installations that a thorough investigation was needed, and the loss adjusters instructed forensic engineers. Their report concluded that the washers had been damaged by plumbers over-tightening the joints, or alternatively, the joints were not tightened enough. Although plumbers can make mistakes, the failures suggested widespread incompetence by many different plumbers, an unusual and noteworthy situation. The insurers (via solicitors) approached us for an independent assessment of the problem as the matter had proceeded to a court action. Our first task involved direct examination of failed joints, and it quickly became obvious that the first investigators were mistaken. Failed washers taken from the leaking radiator from Stratford were clearly cracked across their sections (Fig. 7.24). Indeed, when another washer from the first investigators was examined, it, too, showed severe cracking, cracks which were present when originally examined, as a reprint of the photograph from their report showed. The arrows in Fig. 7.25 mark the cracks in the washer as received and the reprint with an identical set of defects at right. The first investigators had completely ignored direct evidence of cracking, presumably in an attempt to reject the large claims for damages from the aggrieved consumers. Such biased reports are actually quite common, but increase costs dramatically by demanding re-investigation especially when litigation has commenced.

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Forensic polymer engineering

7.24 Cracked seals fitted to steel plugs.

7.25 Failed washer as received (left) compared to photograph supplied by first investigation (right).

7.5.3 Simulation experiments In fact, other failed samples showed a range of other damage, including extrusion of washer polymer around the joint (Fig. 7.26), for example. Perhaps it was this evidence that allowed the first investigators to accuse plumbers of over-tightening the plugs. Other failed plugs showed traces of white paint on joints, leading them to allegations of under-tightening because paint could only have penetrated the joint if a gap was already present (Fig. 7.27). But they had ignored alternative explanations, and specifically that the material had itself deteriorated. Another investigation by independent consultants at RAPRA used experiments on brand new washers to show that the polymer had indeed changed with time and temperature of exposure. Table 7.2 shows the results of several experiments designed to test the material for withstanding compression, just as it would when used in a radiator joint. Exposure of washers in plugs for 3 weeks at 85°C (the temperature of the hot water in some CHS systems) showed a large reduction in the torque needed to unscrew the joint from 27 to 7 Nm.

Polymeric seals

297

7.26 Failed washer on blanking plug showing extrusion of polymer (arrow).

7.27 Cracked washer showing extrusion (arrow) and paint on seal surfaces (air vent plug).

Table 7.2 Exposure experiments for Hytrel washers (RAPRA) Experiment

Torque test Nm

Hardness Vh

New washers Plug and vent fitment

– 27 Nm to fit Mean of 7 Nm to undo after 3 week exposure at 85°C Compressed by 1.79 mm in total, original thickness of each washer 2.28 mm 2 Nm to release after 3-week exposure at 85°C

3.9 4.3 4.1 5.8 from vent 6.8 from plug

Washer stacks (5 thick)

6.2 5.1 5.7

298

Forensic polymer engineering

A stack of new washers showed a similar effect and, in both experiments, the hardness of the polymer had increased sharply (20).

7.5.4 Direct examination These results were confirmed by quite independent estimates of the thermal properties of the polymer. Comparison of failed and new washers showed a substantial increase in the heat of fusion from 30.7 to 43.3 J/g of polymer, an increase of over 40% (Fig. 7.28). The result could only be explained by crystallization of the main hard blocks, the polyester segments in the chain network. That would explain the marked increase in Vickers hardness observed independently. There was virtually no change in melting point and little change could be seen by FTIR spectroscopy. Optical microscopy of at least one fracture surface showed traces of striations (Fig. 7.29), a possible result of thermal stresses on the washers as the CHS system cycled between hot and cold conditions. But what could have caused the brittle cracking? It is true that crystallization will lead to stress in the washers as the material contracts, but cracking seemed an unusual response when, ordinarily, the polymer is tough and ductile. The cracks could not have been caused by a weld line because several cracks were seen on single washers (Fig. 7.25). Further explanation was needed.

2 0

210.56 °C

–2

30.72 mJ/mg

Heat flow (mWatts)

–4 205.95 °C

–6

218.89 °C

43.32 mJ/mg

–8 –10

220.03 °C –12 –14 100

120

140

160

180

200

220

Temperature (°C)

7.28 DSC thermograms of new and failed washers.

240

260

280

Polymeric seals

299

7.29 Fatigue striations on fracture surface.

7.5.5 Hydrolysis When polyester TPEs were originally developed by DuPont in the USA (21), they were aware of the problem of hydrolysis of the material at high temperatures. They advised product designers to match conditions of use very carefully to the degradation of specific grades. They issued a technical advisory leaflet showing how the tensile strength half-life (defined as the time needed for the tensile strength of the product to reach half the value found in new products) varied with temperature of exposure (22). The graph they supplied for one grade of Hytrel showed the relation between temperature and tensile half-life, being a linear relation when plotted using log-reciprocal ordinates (Fig. 7.30). One of us (PRL) was involved in the original development of the material in 1968, when he showed that thermal degradation had inhibited formation of high molecular weight polymer. By redesigning the reactor, the problem was eventually overcome. The specific grade shown (lower line) was similar to that used for the washers, and shows that at a continuous exposure temperature of 50°C, for example, the tensile half-life is about 120 days, but drops to only 40 days when the temperature rises to 70°C. The life time drops yet further as radiator temperatures rise further. But it might be objected that radiators are used intermittently and not continuously, so the washer life will be longer than the graph shows. There was, however, evidence from the sheltered accommodation that owing to the greater needs of senior citizens, radiators were kept at high temperatures for longer than is normal in most domestic situations. It would certainly explain why failures were first reported there. Damage was of course accumulative. Cracks would have been started early in the lives of washers by chain cleavage through scission, a familiar mechanism studied in earlier chapters.

300

Forensic polymer engineering Temperature, °F [°C] 158 212 250 122 [70] [100] [120] [50]

77 [25] 100 000

Tensile product half-lifeb (days)

72D Hytrel YEARS 20

10 000 72D Hytrel (90 parts) + Hytrel 10 MS (10 parts)

10 5

1 000

2 1

~120 100 ~40

10 3.4

72D Hytrel alone

3.2

3.0

2.8

2.6

2.4

Temperature reciprocal (103 K–1) bAfter water immersion at indicated temperature

7.30 Thermal stability of polyester TPE to hydrolysis.

Both polyesters and polyethers are known to be sensitive to hydrolysis, which is why they must be dried thoroughly before being subjected to the high melt temperatures involved in moulding. Hydrolysis is also enhanced by strong acids, and although radiator water itself is non-acidic, the reaction tends to be self-propagating, since acid is produced by hydrolysis. Once chains started to break, a fissure was created and hot water penetrated further provided hoop stresses were present to encourage crack growth. Only small stresses were needed, and they may have been present as a result of crystallization. Any compressive stresses present originally were reduced greatly by the process (Table 7.2), so it might be expected that crack growth occurred late in the failure sequence, maybe even as the washers were removed. Whatever the detailed failure mechanism, the widespread installation of the polyester or Hytrel washers without testing was an unmitigated disaster, and all had to be replaced, with a large damages bill on top for those radiators which had allowed leakage to occur. The case did not proceed to trial since all the experts agreed on the basic causes, so court hearing costs were saved. The washers were eventually redesigned in EPDM elastomer, a polymer with intrinsic resistance to hydrolysis.

Polymeric seals

7.6

301

Failures in silicone mastics

Builder’s putty is a common everyday product, and has been supplemented by a range of other sealants for specific applications. Thus silicone mastics are used where water resistance is needed, and many other types have been developed using different polymers. The products usually consist of a polymer plus plasticizer and a filler such as calcium carbonate, or chalk. Builder’s putty itself consists of a blend of chalk and linseed oil, an unsaturated vegetable oil which slowly cross-links with time. The liquid reacts with oxygen in the air in a slow and predictable way to form a hard skin as the gas diffuses through the surface until the entire mass is hard and brittle. Brittle cracks become evident at a later stage in the process, so the glazing must be replaced because water can enter and initiate rot or rust of the frame. The glazing loses its ‘tack’ or stickiness owing to the depletion of unsaturated double bonds in the linseed oil chains, so is usually easy to remove and replace. Indeed, various grades of linseed oil are available where the reaction is speeded up, as in boiled linseed oil, and there are many other natural oils which have even higher rates of oxidation, such as tung and teak oils. Mastics are used not just to provide a seal for windows, but in many applications where a seal against the environment is needed. And where that environment contains gases or fluids that can interact with the seal, there may be problems of seal deterioration and failure. So a mastic material must be chosen very carefully, and tests conducted to ensure that it will resist the environment it is intended to seal against.

7.6.1 Fire station training building Fire brigades require staff to train for the many different problems which can arise during accidents, conflagrations, floods and other hazardous situations. Fires of course present some of the worst hazards, among which entering the smoke filled environment of a burning building can be considered one of the most dangerous. The London Fire Brigade erected a building at their Southwark headquarters for just such training, effectively a multi-storey shell which could be filled with a synthetic smoke, and which was equipped with a ducting system to spread the smoke into those parts being used during training. The ducting was made from pressed steel sheet sections joined conventionally by bolts, the joints being sealed with mastic to prevent leakage of smoke into those parts of the building where it was not needed. The smoke was generated by heating a paraffin oil (23) to high temperature, and jetting a stream of nitrogen gas through the fluid to create an aerosol with oil droplets of 0.5 to 2 microns in diameter. The generator sat

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outside the building and smoke was ducted to various parts of the building when needed. However, the building suffered a real fire shortly after the building was completed. Smoke escaped and condensed on insulation nearby, where it caught fire, although there were no casualties. Such an embarrassing incident led to an enquiry, where the condition of some of the sealants was found to have deteriorated and allowed smoke to escape at an unexpected location. Proceedings were issued against the installers of the sealants, and we were asked to investigate in 2006.

7.6.2 Sealant analysis The sealants were examined in situ at the building, and showed a wide variety of states. In one room, there were signs of the sealant running down the front of the ducting (Fig. 7.31), while an adjacent port in the ducting proved to be quite adequately sealed by a conventional cross-linked elastomer. Another seal elsewhere had, however, liquefied prematurely, leaving traces far from the original joint (Fig. 7.32). Samples were taken for further examination using infra-red spectroscopy and DSC, both methods that should yield useful data on the composition and chemical state of the failed samples. Smears of the oil found on most exposed surfaces were also taken for analysis, as well as a sample of the oil used for smoke generation. Several commercial mastics were purchased for comparison with the failed samples. The smears of oil proved identical with the oil used for smoke production, a not unexpected result since aerosol droplets would have been deposited everywhere during training sessions. It possessed a simple FTIR spectrum of a hydrocarbon mixture, and was easy to identify. The polymer spectra proved more complex, again as expected, but indicated that several mastics had been used, especially those based on silicone, polyethers and

7.31 Sealant drip from a joint.

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7.32 Extensive deterioration of sealant.

acrylic polymers plus added fillers. It was interesting to note that the region below about 1800 cm−1 was especially prominent with intense absorption (the so-called ‘fingerprint’ region). Most polymers absorb strongly here, but so do many fillers such as calcium carbonate as well as oils.

7.6.3 Calorimetry But the critical evidence came with DSC analysis. The first analysis showed no distinctive endotherms, especially given that data sheets showed that the oil boiled ‘above 280°C’, so the temperature range was chosen to end a little above this temperature. In fact, it boiled at a much higher temperature, as a check thermogram confirmed (Fig. 7.33). The sample showed a boiling range of 390–470°C, peaking at about 420°C. Paraffin oils will generally boil over a range of temperatures since they are made by distillation of complex precursors. It reflects their composition of a range of oligomers of varying molecular weight. The second sample was of a beige-coloured sealant from the fire brigade building, and it, too, showed a single peak at high temperatures (Fig. 7.34). The peak was broad as in the oil, centred at about 435°C, somewhat higher than for the oil itself. On the other hand, oils can lose their lower boiling components, and it suggested that the failed sealant contained a significant amount of oil. The clinch came with analysis of another failed sealant of different colour and origin (Fig. 7.35). It showed a more complex thermogram, with three major endotherms, one of which occurred at a peak temperature of 438°C, very similar to that of the other failed sealant. It was thus reasonably clear that the sealants had absorbed considerable amounts of the oil which had formed on their exposed surfaces by coalescence of smoke droplets, both within and outside the ducting.

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7.33 Paraffin oil thermogram showing single boiling point at 420°C.

7.34 Beige sealant showing single boiling point at 435°C.

7.6.4 Conclusions The analyses showed clearly that the failed sealants had liquefied as a result of absorbing oil from the smoke over a period of time. This was shown both by DSC and indirectly by absorption in the fingerprint region of the FTIR spectra. Paraffin oil has a plasticizing effect on many polymers, and the viscosity drops as more fluid is absorbed, so explaining the dripping of the sealants from the joints. The effect was confirmed by mixing the oil with the new sealants, most showing plasticization (24).

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7.35 Brown sealant showing multiple endotherms.

It was also clear that some joints had been well sealed with conventional elastomers, and why mastic materials were needed at all remains mysterious. Perhaps there were some joints which could not be easily filled by strip rubber, or maybe the contractors ran out of supplies. In any case, if such materials were used, they should have been tested using the fluid to which they would inevitably be exposed in use. The High Court case settled soon afterwards. New mastics and sealants continue to be developed, especially with synthetic oligomeric oils like polybutene. It is a low molecular weight polymer with a polyisobutylene repeat unit, the high molecular weight polymer being used in rubber products which need such properties as impermeability to gaseous diffusion, such as tyre inner tubes or linings. A typical polybutene oil will have a molecular weight of 600 for example. Owing to the way it is made, each chain has a double bond at one end, which improves ‘tack’ but also makes the fluid liable to oxidation. Such fluids are used in engine oils (25) and have also been used in mastic or putty, where it is mixed with butyl rubber and calcium carbonate filler. It is used for glazing purposes in buildings. However, some degradation problems have been encountered owing to reaction of a single double bond present at one end of the chain in the grades commonly available commercially. It is susceptible to oxidation and other reactions. Typically, the mastic loses tack and connection with surfaces to which it has been applied. The mastic is eroded by water falling onto the glazing joints, allowing water into buildings and causing consequential damage. It is thought that high temperatures used during manufacture causes premature oxidation of the double bonds, producing carboxylic acid groups which are water soluble.

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The solution to the problem includes preventing oxidation by using a nitrogen gas blanket during manufacture and adding anti-oxidants to inhibit the degradation process occurring.

7.7

Conclusions

Sealants play a crucial role in many different machines and structures by protecting them from their environments. Cross-linked rubbers have traditionally played a central role in sealing such devices, and usually have considerable resistance to many different chemicals, especially when special additives are used in their composition. Examples include engine seals, where the main crankshaft must be sealed with a lip to prevent oil seeping away. Viton lip seals are widely used in this critical role for their resistance to oil, heat and high pressures. Seals also provide a critical role in hydraulic systems such as braking circuits, and if they fail for whatever reason, the driver can lose control of his vehicle if the seal fails and braking power is lost. Fatigue of rubber products can occur for many reasons, but is a symptom of under-design, poor choice of material, dimensions, resistance to hydraulic fluids or poor maintenance. It is deadly in its effects because the driver will be unaware of any problem until the final stages of crack growth when leakage of fluid occurs, quickly followed by total loss of fluid as the crack becomes critical and grows to completion. Rubbers display quite different physical properties, a simple example being rebound resilience (Chapter 1). A solid polybutadiene ball will bounce high (about 75%) since the rubber is highly resilient at 20°C, while a natural rubber ball will bounce to about 60% of the original height and a butyl ball only 10%. But the resilience varies strongly with temperature, and always drops with decreasing temperature. In the case of the Viton O-rings, the drop in resilience was critical in the Challenger disaster in January 1986, and allowed propellant gases to escape from a booster rocket during lift-off. Lower than expected air temperatures cooled the rubber to near freezing temperatures, and the rings could not react quickly enough to vibrations at the field joint. The hot gases escaped through a tiny gap, which then grew fast as the rubber burnt away. The resulting explosion threw the space shuttle into the sea and all the astronauts died. The problem was understood from previous incidents, but NASA and Thiokol management over-rode objections and catastrophe followed. The design has now been modified with more O-rings, but why wasn’t it done before the event, rather than after? Rubber seals in pneumatic systems are vital to normal working and when failures started occurring in a semi-conductor factory in Japan in 2001, many chip-making units were affected, since they were all controlled via the same line, and fed by the same air supply. At first, attention was

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diverted by an oxidized NBR diaphragm seal, but it soon became apparent that ozonolysis was the root cause of the problems. ESEM examination of a cracked seal showed cracks growing from two sharp inner corners, and fracture surfaces enriched in atomic oxygen. Independent analysis of the air stream showed traces of ozone and nitrogen oxides indicative of generation by electric discharge. It is likely that a new design of compressor was the source of the gases, and filters in the system were not capable of absorbing such contamination. New filters cured the problem. Another large pneumatic maker had warned of the problem before the problem arose, but the report was not seen or acted upon. Changes in equipment can sometimes create unexpected consequences and should be thoroughly researched before introduction. Replacing conventional seals by thermoplastic rubbers can also produce unexpected problems. New washers were made for central heating systems in Hytrel polyester elastomer, and were used successfully in hot water taps. However, when used on radiators, where they were exposed continuously to high temperatures, they hardened by crystallization and cracking followed. The seals shrank and allowed leaks to develop. The first reported leaks came from public establishments that maintained high temperatures in their properties. Technical literature available before the failures warned about the problem of hydrolysis, but was not seen by the manufacturer. Tests should have been conducted before introduction to ensure that the material could withstand such exposure. The washers are now moulded in heat- and hydrolysis-resistant elastomer. The use of mastics to seal building components such as ducting and glazing is widespread, but those sealants must be able to resist their environment. Many new materials have been developed using thermoplastic polymers, plasticizers and fillers. Some were used to seal ducting at a fire brigade training building that was used to conduct synthetic smoke to chosen parts of the building. The smoke consisted of an aerosol of paraffin oil, and some of the sealants were plasticized by the oil, so they liquefied and the seals failed. It allowed oil to condense on insulation, causing a real fire. The sealants should have been tested before use. New sealants have been developed using polybutene, a low molecular weight oligomer, but problems have been experienced when it is used as a sealant for glazed roofs in buildings. The problem may be caused by premature oxidation. New ways of using both conventional and new materials should always be tested by exposing those materials to the conditions expected in service. And those conditions should be the worst expected, an objective which is not always easy to achieve in practice, as the ozone problem in the semiconductor factory demonstrated. But there is a wealth of information available in the literature, increasingly provided by sources on the world wide web. Similar problems may have been found before in another application

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of the same material, and can give guidance to prospective future uses. Some general principles can also give clues as to possible potential pitfalls, such as the high reactivity of double bonds in chain molecules, especially to oxidative processes. Product failures, however, are not widely disseminated unless already in the public domain through recalls, court cases or alerts published in the technical press. In the absence of any such warnings, there is no better way of researching product integrity than by carefully designed direct testing. But even when cases reach court, bias frequently occurs among some experts, who provide clients with the opinions they would like to hear rather than the truth of the matter. It certainly happened with the Hytrel washers, and lengthened the case as a result, and the costs of consulting other experts. Investigations should be independent because no one gains from a poor and misleading report, least of all those who instruct such experts. If there is a fundamental problem, it should be exposed, analyzed and publicized so that yet more problems of the same kind do not recur.

7.8

References

(1) Hickman, J A, Polymeric Seals and Sealing Technology, RAPRA Review Reports, 8 (12) (1997). (2) Slack, Charles, Noble Obsession: Charles Goodyear, Thomas Hancock and the race to unlock the greatest industrial secret of the nineteenth century, Texere, New York (2002). (3) Morton Maurice, (Ed), Rubber Technology, Van Nostrand-Rheinhold, 2nd edn (1973). (4) Naunton, W J S, The Applied Science of Rubber, Edward Arnold (1961). (5) Rogers Commission report, Report of the Presidential Commission on the Space Shuttle Challenger Accident, (1986). (6) Richard P Feynman, with Ralph Leighton, What Do You Care What Other People Think? Further Adventures of a Curious Character, W W Norton & Co Ltd (1988). (7) Columbia Accident Investigation Board, Report of Columbia Accident Investigation Board (2003). (8) Rugg, J S, Ozone Crack Depth Analysis for Rubber, Anal. Chem., 1952, 24 (5), 818–821. (9) Andrews, E H, Fracture in polymers, Oliver and Boyd (1968). (10) Andrews, E H et al., Ozone Attack on Rubbers, Chapter 12 in Bateman, L, The Chemistry and Physics of Rubber-like Substances, Maclaren (1963). (11) Lake, G J, Ozone Protection of Rubber, Rubber Chemistry and Technology, 43, 1230, (1970). (12) Anachkov, A et al., Kinetics and Mechanism of the Ozone Degradation of Nitrile Rubbers in Solution, Polymer Degradation and Stability, 19, 293–305 (1987). (13) Solansky, S S and Singh, R P, Ozonolysis of Natural Rubber: A Critical Review, Progress in Rubber and Plastics Technology, 17, 13–57 (2001).

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(14) Wayne, R P, Chemistry of Atmospheres, Oxford University Press, 2nd edn (1991). (15) Griffing, G, Ozone and Oxides of Nitrogen Production During Thunderstorms, J Geophysical Research, 82, 943–950 (1971). (16) Gaffney, J S and Marley, N A, Atmospheric Chemistry of Organic Oxidants and Their Precursors (1999), available for download at: http://www.atmos.anl. gov/ACP/Gaffney.pdf (17) Kent, A, Air Quality Reports available at: http://www.airquality.co.uk/reports/ reports (18) Van Krevelen, D W, Properties of Polymers, Elsevier (1994), Chapter 11, p 330. (19) SMC Corporation, Ozone Resistant Pneumatic Equipment, Document P-E00-6A (2001), available to download free at: http://www.coastpneumatics. com/pdfs/smc/70AIOZONE.pdf (20) Lewis, P R and Brown, R P, Failure of TPE washers in central heating systems, Fapsig session, ANTEC-SPE, San Francisco (2002). (21) Witsiepe, W T, Thermoplastic polyether-polyester elastomers, in Holden, G, Handbook of Thermoplastic Elastomers, Hanser-Verlag (1996), Chapter8. (22) DuPont technical brochure HYT-114, Hytrel 10 MS A Hydrolytic Stabilizer Concentrate (1996). (23) Shell Ondina Oil, materials handling safety data sheet, available for download at: http://www.dopsolutions.com/Literature/910-0027-001-1%20Material%20 Safety%20Data%20Sheet%20-Shell%20Ondina%20Oil%20EL.pdf (24) Brydson, J, Plastics Materials, Butterworth, 7th edn (1999). (25) A typical range of polybutene liquid oligomers is offered by Ineos, and their properties described briefly at: http://www.ineosoligomers.com/28Characteristics_of_Indopol_polybutenes.htm

8 Tools and ladders

8.1

Introduction

Manufacturing in Western countries has changed dramatically in the last two decades, as product designers have switched to developing nations, especially in the Far East such as China, Malaysia, Tawain and South Korea. Labour costs are much lower there, and so it is possible to manufacture components and even complete products at much lower cost than in the West. Much of the process equipment is new, so exploits the latest advances in technology, such as computer control of operation and so on. However, experience with some polymers seemed to be poor as new products emerged, and there were a number of failures on imported goods, such as cheap knives and electric plugs. If the handle on a knife fails, there is a chance that the user will be injured since their hand will be close to a very sharp blade. Power tools offer extra risks because they may use high speed parts to achieve their function: angle grinders, for example, use a very high speed abrasive disc and if the handle fails suddenly, the user is put at serious risk of severe injury. Both the design and material used must take account of the high stresses to which small and power tools can be subjected when in use. Security guards for gas cylinders is an innovative way in which the toughness of thermoplastics like polypropylene can be exploited, but if manufacture produces weaknesses such as weld lines, then the product can fracture prematurely and have the opposite effect to that intended. The expansion of the injection moulding industry has led to many new designs in thermoplastic of traditional products like chairs, tables and other furniture. But wood is a composite and normally tough material, and many apparently tough polymers can be embrittled through poor choice of grade, designs which incorporate stress concentrators, or simply by poor manufacture. Chairs in particular can be subjected to high loads from users, and sudden failure can cause serious personal injury. But polymers are also widely used in metal products where their failure can also cause sudden collapse. Ladders are especially safety-critical products, where failure of rubber feet or tips can propel the user onto the ground. Similar comments apply to stepladders, loft ladders and steps used for swimming pools. 310

Tools and ladders

8.2

311

Failure of polypropylene hobby knives

The Stanley knife has become so widely used as to become something of a design icon, yet large numbers of plastic handled knives which accept the same sharp blade have been designed and are widely used. Some of the first designs to be imported, however, suffered rapid failure owing to the poor way in which the handles were designed, and the users suffered cuts, sometimes of depth and severity (knife on left in Fig. 8.1). A workman was cutting the backing from wooden floor tiles, and while using a plastic hobby knife, cut into his right index finger, severing a nerve and an artery. The depth of the cut was great, and despite emergency surgery, the individual continued to have problems with using his right hand. The accident happened in 1981, and a claim was made against the insurers of the sellers of the knife. At the time we reported to the insurers in 1983, the case had advanced to court action.

8.2.1 Accident reconstruction Although the failed knife had not been sent for inspection, an exactly similar design was sent and proved distinctly dangerous. Pushing the blunt side of the blade against a desktop caused the blade to rotate into the plastic casing, but fortunately without cutting the user’s fingers or hands (Fig. 8.2). The problem was caused by using a relatively soft plastic to hold the blade in position, as the innards of the product showed (Fig. 8.3). The metal blade was held in place against rotation by just two pieces of soft plastic, a small

8.1 Various designs of small hobby knives, suspect knife at left.

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8.2 Blade rotation produced by pressure against blunt edge of blade.

8.3 Dissembled hobby knife showing design of component parts.

knob which mated with a U-shaped depression in the side of the blade, and a strip of plastic of the body acting against the sharp edge of the blade. They were clearly insufficient to prevent the blade turning into the fingers of users, especially if the tool was held in a power grip (1, 2) (Fig. 8.4). The material used was a polypropylene copolymer with ethylene, the content being 4 to 10 weight %, making the plastic more flexible than a homopolymer by suppressing crystallinity. The parts were all injection moulded, and the product was of Italian origin. The amount of force needed to rotate the blade in its plastic handle was measured using a hydraulic tensometer, a task needing some care in positioning the knife so that an accurate reading of the compressive load could be found. A load of about 3 kg was found to cause rapid rotation of the blade into a position where it could do serious damage. Such a load is easily achieved by the hand pressing down, and so such an accident was entirely feasible. A Stanley knife tested under the same conditions showed that failure occurred at about 35 kgf, well over ten times the failure force used in the failed knife. A survey of similar knives available for sale showed their designs to be much superior, either using stiffer plastics or having a small

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8.4 Power grip (top) and precision grip of knife.

metal insert to prevent rotation of the blade into the hand of the user. The medical evidence showed that the injuries to his right hand were consistent with the power grip of Fig. 8.4. Although the original failed knife was not available for inspection, photographs showed that the damage was very similar to that found in the tests on new knives. The final question was how he came to put the blade into compression, a not unreasonable point, since the product is designed to be used so that load acts in the opposite direction to that in which it failed. The answer comes from seeing how such knives are used in service, the sharp point often being used to make a deep hole, when, for example, first making a cut into a tough material like a floor tile. This is when the tip can rotate in the direction in which it failed, as pressure is applied to the tip, sometimes acting against the blunt side of the blade. The workman was compensated for his injuries.

8.3

Failure of polystyrene components in hobby knives

One might have expected that such knives would be removed from the market so as to prevent further injuries. But not so, because yet another hand injury was caused in the early 1990s by a not dissimilar design, but this time it was stamped very clearly with the logo ‘Made in England’. The product had been used to cut plastic trim at the time, and the user claimed that the blade suddenly rotated and cut deep into his index finger. Since

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the enquiry came from his solicitor, the user wanted an objective explanation of the failure so as to sue the supplier. So what had happened in the accident? In his witness statement, the user, a fisherman by trade, said that he was cutting the trim when the blade flew out, cutting him slightly. He put the blade back in and tightened the central screw hard, but it flew out again, rotating upwards, and this time it made a deep gash in his right index finger, cutting down to the bone. His account was supported by two policemen who happened to be present. The medical report showed that he had severed a nerve, and he suffered considerable distress since his hands were a vital part of his work. He had been off work for many weeks as a result of the damage to his hand, much like the previous case.

8.3.1 Knife inspection This time, the failed knife was available for direct examination (Fig. 8.5). The two matched mouldings which formed the case were attached by a single screw, as the user had described. Unlike the Stanley knife, however, there was no means to withdraw the blade into the handle. Closer inspection of the dissembled knife showed that the upper part of the casing had been damaged consistent with rotation of the blade up and into his finger (Fig. 8.6). Small pieces of plastic must have fractured here to allow the steel blade to turn on its mounting within the casing. The same picture showed that the two parts of the casing did not fit together at all well, and there was a gap between them even when the parts were tightened via the central screw. When the knife was opened (Fig. 8.7), the extent of the damage could be seen to the plastic as well as blood stains from the user’s finger (looking like rust stains on the metal blade, but present on the casing as well).

8.5 Failed knife (top) compared with Stanley knife below.

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8.6 Close-up of damaged plastic top.

8.7 Knife opened to show blade fixing, damage and blood stains on the blade.

The damaged part is shown in more detail in Fig. 8.8. Various discrete fractures could be seen at A, an old pit covered in dried blood, main damage from top of blade B, secondary damage at C and further damage at D. The two cracks B and C intersected at a cusp K in the top surface. The cracks suggested that a brittle material had been used for the casing, and was confirmed by FTIR spectroscopy. It showed that the material was polystyrene, well known as a brittle material and therefore not commonly used for stressed products. However, traces of ductility could be seen in the form of whitening of the black polymer (at B and C in Fig. 8.8, for example), suggesting that high-impact polystyrene may have been used. No trace of the polybutadiene could be found in the spectrum, and if it had been used, processing probably oxidized the rubber particles. Traces were left, so some remnant ductility was present, but not enough to toughen the material. A mechanical test showed that the blade rotated upwards at an applied force of 8–10 kgf, somewhat greater than in the previous example, but still

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K B

C

D

A

8.8 Close-up of damage near tip of knife.

substantially less than a Stanley knife. Other failure modes were seen on further tests, such as fracture from the bolt hole, an obvious stress concentrator in the product in a brittle material. The product was not just poorly designed but also poorly made, showing unacceptable distortion at a key point where the blade was held. The accident was caused by such a defect and the poor mechanical properties of the plastic casing, and when the user applied pressure while cutting the trim, the blade rotated and made contact with his index finger. He was holding the knife in the precision grip (1, 2) (Fig. 8.4), and the top of the blade cut deep into his index finger. Although the sharp edge of the blade itself did not make contact, the so-called blunt side is also very sharp, and capable of cutting deep into tissue. The case seemed clear, but the defendants (a large store that sold the knife) were obdurate. A second expert was asked to examine and report on the accident. He concluded that the user had not used the knife correctly, although he produced no evidence to support his opinion. It seemed to be an example of an expert producing an opinion biased towards his client and not directed to the truth of the matter, an all too frequent problem in many court cases. However, the defendants had paid a sum of money into court and the claimant decided to accept the damages to compensate him for his serious injuries.

8.4

Failure of handles in angle grinders

So serious injuries can result from faulty knives, but handles are safetycritical in many other hand-held products. Power tools are a case in point. They are often equipped with a separate handle which screws onto the side of the casing which holds the electric motor, providing the user with extra control over the precise position of the tool when cutting, grinding or drilling. If the handle fails during use, then very serious injury can be caused if the moving part contacts the body of the user.

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8.9 Angle grinder showing handle socket.

In the first case, the polycarbonate handle of an angle grinder fractured suddenly in 1998, and the revolving wheel struck the user in the groin, injuring him severely (Fig. 8.9). He had bought the machine about two years before the accident, and was well used to operating power tools, having been a metal fabricator for many years (according to his witness statement). He was using it to sand down some woodwork, so was supporting the main body with his right hand and holding the handle with his left when it fractured. He dropped the tool and managed to reach the house, leaving a trail of blood behind him. His life was saved by the arrival of the ambulance, but he spent some time in hospital where he received transfusions owing to the great loss of blood. He sued the suppliers of the tool. The grinder was not a well-known brand, and was of a type that has been imported in large numbers from the Far East, and sold at discount prices in supermarkets. It was one of the smallest such machines on the market.

8.4.1 Fracture surface The fracture occurred where the guard joined the stem, above the screw attachment to the grinder (Fig. 8.10). The underside of the guard itself showed severe contamination leading from the gate in the form of discoloured polymer with sharp boundaries (Fig. 8.11). The material of construction was found to be polycarbonate, a polymer which needs careful control of process conditions to achieve its greatest strength, as was seen with mining products. The fracture surface on the machine side showed impact damage along one edge, probably caused when the user dropped the device and it hit the floor (Fig. 8.12). The rest of the fracture showed growth of a single crack which met at one point to form a cusp. However, the handle side of the fracture revealed numerous striations (width about 30 microns in diameter) next to the sharp outer corner, caused by fatigue (Fig. 8.13). It was estimated that the striations covered about a quarter of the section

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8.10 Grinder handles compared with failed handle at right.

G

8.11 Fracture side of handle guard showing contaminated polymer from gate (G).

area before the section could no longer support the applied load, and it suddenly failed. So what had initiated the fatigue striations? There appeared to be several points on the outer corner where the process had started (Fig. 8.13) and their position was not coincidental. They appeared to occur where there were tiny defects in the corner such as weld lines and voids, the latter being associated with the visible contamination of the guard (Fig. 8.11). The bending load imposed on such a handle will have been low, and will have been concentrated at the sharp outer corner where defects were present.

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5 mm

8.12 Fracture surface from screw side of handle showing impact damage.

2 mm

8.13 Fatigue striations from sharp outer corner of handle guard.

An estimate from the measured radius of curvature of the corner of 0.05 mm gave a stress concentration factor, Kt, in excess of about 5 (3). Using the same calculation gave a Kt of about 3 and 1.45 for the handles shown in Fig. 8.10, newer handles supplied with other angle grinders of similar design. If there was a void present at the sharp corner, then Kt will have been at least 10. So a small load of say 1 kgf applied to the handle will have been effectively 10 kgf at the corner. The tensile strength of the polymer was

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unknown but would probably have been low owing to the contamination and/or degradation seen in the guard.

8.4.2 British Standard for tools In response to the action, the tool suppliers said that they had not seen any other failures of this type, and that they tested a selection of the tools to a British Standard (4), a test involving a drop impact test of some severity. However, they did not specify what numbers they had actually tested. The standard does not specify any detailed test of handles. It was very clear from the investigation that a faulty handle had indeed been supplied with the tool when purchased, and the injured user was compensated before trial. His occasional use of the tool had created small hairline cracks to develop at the very weak outer corner to the guard, cracks which grew at every use of the tool. The cracks will have been difficult if not impossible to spot given that they would be black against a black background. When they reached a critical size, the handle suddenly broke and the user was severely injured. One might have thought that such tools would be recalled for replacement of the faulty handles, but one of us (PRL) saw similar defective handles on new grinders in a local supermarket after the case settled. One hopes that the faults were found before further accidents occurred. The poor design was corrected later by increasing the radius of the outer corner, but the quality of some imported new tools leave a great deal to be desired.

8.4.3 Another handle failure A not dissimilar accident occurred on 20 March 2003, when the plastic handle on an angle grinder suddenly broke as the user was cutting metal using an abrasive disc (Fig. 8.14). He had bought the machine about a year before and had hardly used the device before the accident occurred. The discs on grinders are rotated at a very high speed of several thousand rpm, so when the handle failed, the grinder dropped onto his hand and he was very seriously injured as the rotating wheel cut deeply into his flesh. He then sued the manufacturer, and we became involved in 2006 after the manufacturer claimed that their tests showed that the handle needed a force of 75 kg to break, a value well above the load any user could reasonably apply to the handle. The problem needed resolution.

8.4.4 The fracture The broken handle showed a completely brittle failure (Fig. 8.15) near the point where it joined the body of the grinder by means of a screw thread.

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8.14 Fractured handle from angle grinder.

8.15 Matching parts of fracture surfaces of angle grinder handle.

Exposing the matching fracture surfaces showed that the break had occurred across the top of the metal bolt embedded in the plastic, and it showed several serious defects (Fig. 8.16). Comparison of the two surfaces showed that the plastic had shrunk away from the top surface of the bolt, and a large weld line was present in the shrunken surface (at right in the figure). The same surface also showed traces of a number (48) transferred from the head of the bolt to the molten plastic during injection moulding. It showed

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8.16 The upper fracture surface showing shrunken interior and large weld line plus faint traces of a number ‘48’ from the bolt head. Impact damage at lower left.

8.17 Oblique close-up showing origin at edge of voids in plastic.

that the plastic made contact and then shrank back as the polymer cooled, a common problem when the melt is too hot and the cooling too short. The depth of maximum shrinkage was 1.7 mm at the centre. The origin of the fracture was clearly identifiable by the convergence of hackles in the surface (Fig. 8.17). So what was present at the origin? There was clear evidence of voids between the metal edge of the bolt and the surrounding plastic, formed in the same way as the shrunken centre of the upper part of the handle (Fig. 8.18). They would have acted as stress concentrators, by magnifying an innocuous local stress applied by the user to a level where it challenged the tensile strength of the material. The material of the handle was said to have been a glass-filled grade of nylon, or polyamide 6 (a report from the manufacturer said that the material was ‘PA6 + GF30’). However, FTIR spectroscopy indicated a mixture of nylon 6 and 66, an inference supported by DSC, which showed two sepa-

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8.18 Close-up of fracture origin with void at interface with bolt, and flat part at base caused by handle guard hitting the floor.

Glass Transition 68.88 °C Onset Midpoint 70.29 °C

2 mW

Integral normalized Onset Peak 60 1

80

–85.80 mJ –10.73 Jg^ –1 196.12 °C 216.53 °C

Integral normalized Onset Peak 220 240 260

100 120 140 160 180 200 2

3

4

5

6

7

–28.70 mJ –3.59 Jg^ –1 244.06 °C 255.62 °C 280 300°C 8

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8.19 DSC thermogram of material used in handle.

rate melting peaks, one at 216°C, and the other at 256°C (Fig. 8.19). The polymer was a mixture of two different polyamides, nylon 6 and nylon 66. Nor could any trace of glass fibres be found in the samples taken for analysis, contrary to the information from the manufacturer that the material contained no less than 30% short glass fibres. If that were the case, fibre ends should be seen everywhere on the fractures. None in fact could be seen at all. It seemed that the specification may have been changed, or that a non-recommended material had been used.

8.4.5 Conclusions The fracture surfaces showed that premature failure had occurred from pre-existing defects formed during manufacture of the handle. But the

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manufacturer insisted that their tests of new handles showed them to be quite strong enough to resist normal forces applied by users. It is interesting to note that their intact handles also broke in a brittle way at a similar position near the bolt head, confirming that this was the weakest area of the handle. This is a consequence of the very big difference in stiffness between steel and nylon, so that any loads on the handle are borne solely by the polymer. The most likely loads are bending, when the hand of the user pulls or pushes the handle to control the position of the grinding wheel. When the handle is bent, the maximum loads are transferred to the polymer, and a small part to the interface between the steel bolt and the adjacent plastic. The failure load on new handles tested by the manufacturer showed that a load of 75 kgf was needed to cause fracture, although fracture could also be induced by impact loads. They therefore suggested that the user had dropped the grinder to cause the failure, although they could not explain why or how the user had been injured so seriously. They also said that of 30 000 such grinders sold, none had showed a similar failure. Unfortunately, the failed test samples were not made available for comparison with the accident handle, and the tester had not recorded the state of the fracture surfaces or inner defects they exposed. However, the severity of the voids and prominent weld line suggested that the accident handle was a maverick sample made in the early stages of a moulding run when injection conditions had not been fully established. If the molten nylon had been too hot, for example, shrinkage would have been severe (as found) and numerous voids formed in the polymer. Such samples should not have passed quality control, but sometimes they can creep through, perhaps because they seem of good quality judging simply by external appearance. A maverick handle was enough to explain the fracture at very low loads from the user, and there also remained the problem of the specification. The defendant’s theory that the grinder had been dropped did not stand scrutiny: the external surfaces of the handle showed very little wear or damage caused by impact. The only impact damage found was on the fracture surface itself (Figs 8.16 and 8.18), showing that it had dropped after the accident and not before. The case was settled with damages for the injured user.

8.5

Failure of security caps for gas cylinders

There have been many innovative ways in which polymers have been used to create new designs fulfilling unexpected functions, such as the development of security guards for gas cylinders used in hospitals. The product was designed to be fitted around the tops of gas cylinders to protect the valve as well as show the user what gas was present in the cylinder by colour

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8.20 Safety guard for oxygen cylinder.

coding the plastic (Fig. 8.20). Although not strictly a consumer product, the case study is included here because the way the problem was solved involved the same principles discussed in the cases of tool failure. A problem was discovered in 1988 by the manufacturer, an injection moulder in Leicester, who found that caps were falling apart before ever being used on gas cylinders. The devices were made in talc-filled polypropylene in four colours and made with two flaps which are brought together in a secure joint around the top valve of the cylinder. The flaps are hinged at two points to allow rotation of the flaps (Fig. 8.21). The non-return parts of the joint can be seen at extreme left and right. The cap is removed at the hospital by applying a hexagonal spanner to the front of the fitted device. Although disposed of after removal from a cylinder, the unattached caps must be able to resist dropping and handling loads before fitment.

8.5.1 Storage failures However, whole batches were found to be failing at the critical hinges when held in storage, and had to be returned to the moulders for scrapping. Indeed the warehouse at the factory was full of rejected products, a sight which horrified the owners and led to a panic phone call to help resolve the crisis. There were several possible causes, poor material being the one favoured by the moulders, perhaps not unsurprsingly. Samples of material were thus cut from a selection of good and failed mouldings to test the theory using FTIR spectroscopy and DSC. However, no anomalies could be found at all using these methods, so poor material by degradation, for example, could

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H H

8.21 Security cap before fitment showing hinges (HH) and gates (arrows).

be excluded. The spectroscopy showed a high ethylene content of about 10%, making the material both tough and very flexible. Attention turned to the way they were moulded, since this was now the likely source of the problem. The device was moulded via no less than four gates: one at the centre of the hub of the cap (Fig. 8.21 shows the sprue remnant of this gate), two on each of the side flaps and one at the centre of the device diametrically opposite the first gate, as shown by the arrows in the photograph. There were a number of irregularities in the moulding parameters, especially the barrel temperatures of the moulding machine. The setting sheet showed the sequence: 211°C/212°C/219°C while the thermostat set temperatures were: 236°C/243°C/263°C These large differences could change the product in unexpected ways. A large number of new devices were then examined for any defects, and were also tested by fitting on a cylinder head. A key observation came very quickly when considering the integrity of the device. The weakest parts were the hinges about which the flaps rotated to lock the device in place. They consisted of plastic only 0.5 mm thick and running the length of 20 mm along each side of the flaps at the base of the hub (HH in Fig. 8.21). The two lengths of polymer had to be strong enough to resist handling and bending to form the hinge, but yet weak enough to give way when the cylinder was needed. It was noticed that many samples that failed prematurely exhibited weld lines at or on the hinge, the weld lines having formed by impingement of the three streams of molten polymer entering opposite

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8.22 Weld lines in thin polymer hinges of cap guard (arrows).

the hub of the device. Failed samples invariably possessed such weld lines in the hinge rather than in the bulk polymer (Fig. 8.22). And those weld lines varied erratically from sample to sample. Of sixteen samples tested, only five survived fitment to the oxygen cylinder (Fig. 8.20). The remainder showed splits or cracks in the hinges. A quantitative test was needed, as well as close attention to moulding conditions.

8.5.2 Development of torque test What was needed was a simple objective test that could meet the needs of the end user and yet be easily applied in the factory. To develop the test, a mole wrench and spring balance were used under standard conditions to measure the torque strength of the fitted caps. The caps that showed hinge cracks gave very low failure torques, as might be expected, with values ranging from 0 to a maximum of 8.3 Nm, but the average result was only about 4 Nm. By contrast the five intact caps gave an average of 8 Nm with a maximum value of 19 Nm (but failed in the case rather than the hinge). The strength also varied depending on the colour. There was a linear relation between the length of weld line in the hinges and their torque strength, confirming that weld lines were indeed the cause of the problem of premature failure. Such a spread of results was quite unacceptable for the customer, who needed a reliable guard cap that would fail only when a spanner was used on the device. Moreover, the failure torque needed to be relatively high to necessitate use of a spanner. The ergonomic literature gave some interesting results (Fig. 8.23) which showed that when the hand gripped cylinders of various surface textures and diameters, there was a maximum torque that the hand could exert. The maximum was lower for women, and the

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Forensic polymer engineering 1.2

Ta kgf. m

Ta kgf. m

1.0 0.8 0.6 0.4 0.2

Men

Women

0 0

40

80

120 D (mm)

0

40

80

120 D (mm)

8.23 Maximum torque in hand grip of cylinder of various diameters, D.

obvious choice was that for males, since it was greater (2, 5). Using the data shown in the figure, and the diameter of the hub as 35 mm, then the maximum torque that can be exerted is about 0.8 kgf-m or 8 Nm for a knurled steel cylinder. So moulding conditions should be varied, first to move the weld lines away from the hinge, and second to give the hinges a torque strength of at least 8 Nm. This would mean that the hub could not be removed by hand action, requiring a spanner to twist the guard free. Returning to the factory with the recommendation that moulding be stopped to reset the moulding machines, did indeed save the situation. The change in conditions moved the weld lines away from the hinges, and samples were again taken for testing. The strength of the new samples averaged 12 Nm and was much more consistent. Far from being a material problem, the moulders should have taken greater care in monitoring their moulding conditions and inspected their products to eliminate the damaging weld lines.

8.6

Failure of an ABS handle

Wherever the consumer frequently uses a product then handles are normally designed into the product. The low-level water closet is a case in point, and failures can be both dangerous and inconvenient. In one such case, the consumer cut her hand when using such a handle, and claimed damages from the manufacturer. She supported her claim by instructing two experts, both of whom concurred that the design was poor and the material (ABS plastic coated with chrome metal for a polished finish) suspect. They thought that it had failed by simple fatigue. But the case was not as straightforward as the reports indicated. The investigation started with close inspection of the failed handle itself, showing

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that it had cracked across its major axis in a brittle way and so leaving sharp edges to the fracture (on which the user had been injured). Figure 8.24 compares an intact and the cracked handle, showing that it failed roughly halfway along its length, starting on the upper visible side. The fracture surface revealed numerous striations in the upper part, followed by a fast crack region (Fig. 8.25 with interpretation in the fracture map of Fig. 8.26). The handle must have failed progressively from a surface defect on the upper wing, the brittle crack growing slowly at every use until it reached a critical size and the handle suddenly broke into two parts. This must have been when the claimant was injured by the sharp edge of the cracked handle.

8.24 Intact handle at left, failed handle right with brittle fracture (arrow).

Origin

8.25 Fracture surface of handle with striations from origin in top wing and showing crack growth direction.

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Forensic polymer engineering Load

Fracture surface map (WC handle) Origin Tidemarks of variable pitch

Upper rib

B

Centre rib A Scratches

Lower rib

Shear lip

Shear lip

8.26 Fracture surface map of handle break showing crack growth direction.

8.27 Top surface of handle showing scratches and peeling of chrome coating.

The key question now was why had the crack started and then grown slowly to completion. Further inspection of the failed device gave several clues as to the causes of failure. In the first case, there were several scratches near to the fracture surface, one of which was severe (Fig. 8.27), and from which the chrome had peeled back to reveal the white ABS plastic beneath. The second clue came when the underside was examined (Fig. 8.28). It showed numerous etch pits where the ABS polymer had been attacked by an aggressive chemical. The pits were globular in shape, indicating a chemical cleaner which had formed droplets at the bottom of the handle, from which attack of the underlying plastic had occurred. It was now possible to formulate an idea of how the crack had started. Constant and daily use of the handle had caused scratches to occur in the chrome protective layer,

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8.28 Underside of handle showing etch pits in lower wing.

perhaps from a ring on the hand of the user. Regular cleaning with a powerful reagent such as bleach had then attacked the polymer below and caused a stress corrosion crack to develop and grow with every application of load by the hand of the user.

8.6.1 Scanning microscopy The theory was confirmed by SEM and EDX analysis, large quantities of elemental chlorine being found on the fracture surface. Bleach is usually a strong aqueous solution of sodium hypochlorite, which liberates free chlorine when used, and indeed is the basis of the powerful cleaning action. However, it also attacks many plastic materials, and will promote brittle or SCC cracks on an unprotected polymer such as ABS. The case was withdrawn just before trial. It is interesting to observe that the initial investigations jumped to conclusions that were quite wrong, largely because the investigators failed to look further than the fracture surface itself. Damage elsewhere on the failed handle showed the collateral damage of the reagent which had actually caused the critical crack. There was one final point: the user will have seen the crack opening every time she used the handle, especially towards the end of its life, and it should have been a warning of a broken product in need of replacement. Unfortunately, she took no action until the handle broke and she was injured. But there could be no case against the manufacturer for making a faulty product.

8.7

Failure of chairs manufactured from polypropylene

Increasing confidence among designers has led to a veritable explosion in plastic products, among which furniture is notable, especially chairs. Their

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use has expanded greatly in recent times, both in public venues like cafes and restaurants as well as private homes. With their resistance to the weather, they have become a popular choice for garden furniture after initial problems with UV resistance. Manufacture of a complete product in one operation ensures costs are low, so they have been widely adopted for public venues involving large numbers of people. But there have been other problems with the choice of polymer and their intrinsic strength. When a plastic chair fractures suddenly under the weight of the user, he or she can be injured seriously. Problems can occur with choice of the material, the way it is moulded and the design of the product, especially at critical parts such as inner corners where the stress imposed by the user is concentrated. Failures of chairs of any kind are not uncommon, owing to deterioration during continuing use. Wear at joints is an obvious failure mode, especially on old chairs where the screws have worked loose in the wooden frames. Glues will deteriorate and dowels can fail if rotten. But the cases described here involved new chairs, and specifically a plastic chair moulded in Italy. A woman in Lichfield in the Midlands was injured in 1996 when the back leg suddenly fractured, and she fell to the ground, in a case referred by the insurers. The chair had been bought new only a few days before the accident, so failure could not have been caused by ‘wear and tear’. The back legs had failed suddenly by brittle cracking (Fig. 8.29), one of the legs being completely detached from the rest of the chair. The other rear leg showed sub-critical brittle cracks from the rear upper corner. Closer inspection showed that there were two critical cracks which had intersected to form a cusp at the corner itself (Fig. 8.30). In the optical microscope, the material (chalk filled polypropylene) showed large internal voids from which the cracks had initiated (Fig. 8.31). The stress concentration at a void is about

8.29 Failed new chair caused by brittle fracture with leg reassembled.

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8.30 Close-up of fracture showing crack growth directions.

8.31 Optical micrograph of fracture origin showing voids from which cracks grew.

2 for a spherical void, increasing as it becomes elongated. And since the corner itself also represented a serious stress raiser, there could be little doubt that the local stress had exceeded the tensile strength of the polymer.

8.7.1 Material analysis Spectroscopic and calorimetric analysis confirmed the material as polypropylene, with a small carbonyl peak in the FTIR spectrum, showing that some degradation had occurred during processing. The existence of voids within thick moulded sections is caused either by degradation producing gas, or by an intrinsically weak polymer unable to resist depressurization at the end of the injection cycle. It is likely that a combination of both

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processes occurred in this instance, so the material was too weak to resist normal loads. The insurers suggested that the user was ‘a heavy lady’, a proposition which turned out to be false. Although she was pregnant at the time, her weight was only 11 st 13 lbs (or 74.4 kg) rather than 16 stones (or 102 kg) suggested by the insurers. In any case, the British Standard for chairs provides for a minimum supported weight of 1100 N or 112 kg for chairs of ‘careful domestic’ type. The same Standard (6) also suggests that such chairs be tested to such static loads, as well as resistance to fatigue and impact loads. It is possible that the failed chair was a maverick, although a later case involving the same design indicated a deeper problem.

8.7.2 Another failure Following compensation to the injured woman from Lichfield, a not dissimilar failure occurred in August 1998 at a holiday camp. The female user was supervising her children in the swimming pool, and was sitting in the white plastic chair when the front leg suddenly fractured, and she was injured when she hit the concrete floor edging the pool. She sued the company running the camp and the manufacturer of the chair. It was very similar to that which had failed in 1996 (Fig. 8.32), differing only in the design of the drainage holes in the base. Inspection of the fragments showed that the fracture had started near the front corner (Fig. 8.33), while microscopy showed the origin had occurred from a void near the corner (Fig. 8.34). As in the previous case, there was no evidence of fatigue, surface damage or UV degradation.

8.32 Fractured plastic chairs: at left chair which caused injury to user stacked on intact chair.

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8.33 Close-up of accident chair showing crack growth directions from origin near corner.

2 mm

8.34 Micrograph of voids near centre of chair leg corner.

A new chair of identical design was provided and failed in a similar way when the front leg was pulled by hand. It was estimated that the leg cracked with a pull of only about 12 kgf. Although the weight of the injured woman was unknown, such a low strength suggested that the chair was fundamentally flawed. This type of load would be generated by the woman leaning forward so putting all her weight at the front of the chair, with a heavy downward load tending to splay the front legs outwards.

8.7.3 Litigation Because the injured woman had initiated a court action, the defendants asked for an independent report from another expert. He examined the same samples and re-measured the force needed to shear the front leg away at about 20 kgf. He analyzed the material and found a chalk content of 27–28%, a rather high level for a polymer. But he came to different conclusions because the design had been tested by official testing agencies in Italy and Germany, but of course, such testing will be misleading if moulding

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conditions varied from batch to batch (as seen in the security guard samples). Although he confirmed the presence of voids, he thought that they had not contributed to the failure. But lightning rarely strikes twice in the same spot, and the failure of a second chair in the same material to a very similar design suggested that the design was faulty. Despite requests to obtain moulding data (such as setting sheets and QC tests details), they were never produced, and the case settled with compensation for the injured woman. It is surprising what little attention manufacturers give to the detail design of products, details which can weaken a product so much that it endangers users. Polymers are especially liable to degradation and it is often seen by the occurrence of voids in what should be solid material. They remain hidden within the bulk and so are truly patent rather than latent defects, waiting for a critical stress that will cause sudden fracture. Other sudden fractures were encountered during the same period in the author’s own garden, although happily without injury, and it is to be wondered how many failures go unreported. As with many other common low-cost products, failures are rarely reported if no injury has occurred, which is perhaps why many manufacturers claim no knowledge of premature failures of their products, despite circumstantial or hearsay evidence to the contrary.

8.8

Failure of swimming pool steps

But other safety-critical products can fail, too, such as steps beside swimming pools to allow access to the water (Fig. 8.35). A 12-year-old girl suffered leg cuts when a plastic step suddenly broke while she was ascending the ladder in June 1995. The step had only been installed for four weeks as new, so the failure was unusual, especially since the rest of the steps were made from the same material (ABS) and had been there several years. The step appeared to have broken from two points near the supporting tubes to which it was fixed, so as to give a large free fragment and two parts still attached (Fig. 8.36). Close inspection of the fracture surfaces showed that the larger attached part had broken first, the critical crack growing from near the support in the upper part of the step (Fig. 8.37). The step then cracked again to produce the final form. The exposed surfaces were covered with fine ‘crazy-paving’ cracks indicative of oxidative cracking, and there were also numerous weld lines in the moulding.

8.8.1 Fatigue crack SEM showed the origin of the critical fracture with numerous striations below (Fig. 8.38). There appeared to be a series of pits or cracks on the upper surface from one of which the crack grew progressively every time the step was used. The user’s father weighed substantially more (16 stone,

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8.35 Swimming pool ladder with top step replaced after failure; lower steps of same design as failed step.

8.36 The broken step seen from behind showing brittle cracks revealing ribs on underside.

8.37 Close-up of end which failed first showing part of fracture origin.

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0042

10KV

×17

1 mm

WD36

8.38 ESEM micrograph showing faint fatigue striations at upper right near origin.

100 kg) compared with the girl’s weight of only about 7 stone (45 kg), but when the cracks reached a critical state, only the smallest weight would have been needed for sudden fracture. The step showed signs of extensive weld lines, and was probably moulded cold rather than when the machine had reached equilibrium temperatures. ABS is normally a tough and ductile polymer, and should not behave in a brittle way under moderate loading. Calorimetry showed a slightly lower glass transition temperature (Tg) than an intact step, and extra peaks detected in the FTIR spectrum pointed to degradation during moulding. There was a noticeable yellowing of the failed step when compared with an intact step. The crack had started at a weld line, a line of inherent weakness where the polymer has not fused correctly. Even though the load near the top of the step was lower than below, the stress was enough to cause an inherent defect to grow slowly to failure.

8.9

Failed polyamide fittings in ladders

Using polymers in safety-critical parts has become widespread in a range of consumer products, not just in special ladders such as that considered above, but in most extension ladders and stepladders of all kinds. Although most are made from aluminium alloy, key polymeric parts are often supplied for the feet and tips, as well as joints. But care is always needed in interpreting failed or broken components. In a previous case study, an extension ladder dropped suddenly and the user suffered severe injury (7). He blamed the plastic tips because they were found fractured after the

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8.39 Trace of skid mark where loss of rubber fitting caused ladder slip.

event, but the incident was more complex. Analysis of the mechanics showed that the ladder fell because it had been leant at too shallow an angle against the wall of his house. The tips fractured as a result of the slip, and were not the cause of the accident, so his case could not proceed. On the other hand, a painter fell from a ladder when it had been leant at the correct angle (about 75 degrees to the horizontal). Inspection of the skid marks left by the feet showed that one of the rubber feet was defective, and made the ladder unstable (Fig. 8.39). When the user put his weight on the ladder, it was pushed back into the hollow stile, so the aluminium stile end was resting directly on the tiled surface. The coefficient of friction was much lower, and when the user was near the top of the ladder, the bottom suddenly slipped away, and he fell about 20 feet (6 metres) to the ground and was severely injured. His employer was responsible for maintaining the ladder in good condition, and so he received compensation. Stepladders are also widely used both in the workplace and in the home, and although of limited height compared with extension ladders, users who fall can either suffer injury themselves or injure others standing nearby.

8.9.1 Failed stepladders In another case, the joints suddenly failed and although the user himself was not hurt, his wife standing nearby was hit in the face and suffered injury. The investigation was made in 2007 following the earlier accident, not on behalf of the injured party, but on behalf of the ladder manufacturer, who wanted to determine the cause or causes. The device was a three-way combination ladder with two short aluminium ladders held together by two moulded nylon connectors (Fig. 8.40). The blue connectors were designed to allow movement of the ladders into different combinations, both as a

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8.40 Combination stepladder with blue feet, tips and connectors.

8.41 Close-up of connectors showing locking mechanism.

conventional stepladder and as a short extension ladder. The two parts could then be locked together by means of two fittings in slotted composite plates (Figs 8.41 and 8.42). The upper fitting and the plate are separate mouldings held together by a rivet. Failure had occurred in the upper fixing, with two projecting knobs (or stubs) which abut above the aluminium steps, and so hold the ladders in position. One theory put forward by the manu-

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A

1 2

4

(10.0)

3

(2.0) Lo

ck Unlock

A

Section A – A

8.42 Moulded composite connector riveted together (3) and sharp corner (arrow).

8.43 Fracture surfaces of stubs with poor mixing at left and void at right.

facturers was that the user had incorrectly placed either one, or both stubs below the steps, so the ladders were insecure. When they slipped, the stubs impacted the rung below and they snapped. The failed ladder showed that both stubs had broken in the accident, so allowing the ladders to slip apart (Fig. 8.43). It was reasonable to suppose that the weakest one had broken first, and then the other failed by overload. But which stub had failed first and what design factors weakened either or both of them? Inspection of the fracture surfaces showed some defects, such as a void within one of them, and a band of dark blue within the other. The former might indicate degradation, the other poor mixing of the pigment with the matrix polymer, but neither were causes of premature failure since

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8.44 Intact stubs on ladder showing abrasion marks on bearing surfaces.

the fractures had actually started at the very sharp external corner where each stub met the parent fixing (Fig. 8.42 shows the sharp corner in section). Examination of intact stubs showed the abrasion damage where the stubs had rested against the metal steps of the ladder (Fig. 8.44). Considerable pressure must have been produced against the stubs as users ascended the ladders, and put them into bending, with stress concentrated at the sharp corners. An estimate of the stress raising effect of the corner was made using Peterson (8), and gave a value of Kt ∼ 5 with a corner radius of 0.05 mm. The material was said to be a glass fibre filled nylon with about 30% filler content. The material is used widely where strength and some resistance to temperature is needed such as under-the-bonnet applications. A previous case study concerned a failed radiator tank, for example, where poor moulding produced weld lines which caused a leak, and eventual seizure of the engine (9, 10) (Figs 2.8 and 2.9). The corresponding corners of the metal steps also showed considerable distortion due to the local applied loads transferred by the weight of the user standing on the ladder. Such an obvious interaction should have been considered during the design phase of the product and eliminated by reinforcement of those steps.

8.9.2 Scanning microscopy The solution to the problem of tracing the weakness came by examining the fractures in ESEM. The remains of a serious weld line penetrating the corner was found at one point near the left-hand outer corner (at right in Fig. 8.43, ESEM image in Fig. 8.45). It can be recognized by the smooth areas leading from the surface from the corner into the fracture surface. The weld line represents polymer that has not fused, so a line of weakness occurs, and is in effect a proto-crack in the material. When such a feature

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EHT = 3.00 kV Signal A = SE1 Date: 25 May 2007 WD = 9 mm Photo No. = 2

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ZEISS

8.45 ESEM micrograph of fracture near corner showing smooth areas of weld line.

occurs at a sharp corner, the stress raising effect is very high indeed, considerably greater than the estimate of 5 from the geometry of the corner itself. The weld line represents poor moulding practice, since polymer should be hot enough during injection into the tool to fuse correctly. However, the gate of the part lies at the base of the part so polymer melt has a long path to the stub, and can cool down substantially before meeting other melt fronts. Such defects tend to occur at the start of a batch run, and must always be rejected, even though the part seems correct dimensionally and visually. The occurrence of poorly mixed polymer visible in one of the fracture surfaces tended to confirm the diagnosis (Fig. 8.43). The theory that the user erected the steps incorrectly was not supported by the evidence, because the top step showed distortion downwards at the two corners to the stiles, and the damage could not have been produced by impact. Only lower steps could have been damaged like this, so the distortion to the top step must have been made by steady pressure during use.

8.9.3 Another accident As the failure was being researched, another failure of the same design was reported, and although the plastic parts were of black polymer, the failure mode was very similar. The failure of another new ladder had been experienced by a man trimming his 1.8 m high hedge, and he was injured when the stepladder collapsed. The stiles slid back and he fell through the rungs with the trimmer still working. Fortunately, he was not injured seriously,

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8.46 Broken stubs on stepladder with trace evidence of use from white paint marks.

but his legs were bruised by the fall. Judging by the traces of white paint on the stubs and elsewhere, he had been using the ladder for other jobs before the failure (Fig. 8.46). The white paint traces also show contact abrasion when the ladder was used after painting. Both stubs had failed where they joined the major part of the fixing, breaking from the sharp corner as before. Comparison of the failed with a tested ladder showed that the material appeared rather less intensely black in the failed material, so scanning microscopy was used to examine the fracture directly.

8.9.4 Scanning microscopy The inspection proved interesting in showing that areas of the composite appeared depleted in polymer with gaps and voids between the exposed glass fibres (Fig. 8.47). Such a condition could be caused by degradation, a problem possible in nylons by inadequately dried polymer prior to moulding. The hot nylon in the barrel of the moulding machine degrades by hydrolysis. Such voids present at a sharp corner increase the stress there since stress raisers are multiplicative rather than additive. So if a spherical void (Kt = 2) is present at a corner, the net effect is to double the stress concentration at the corner (11), from Kt ∼ 5 to ∼ 10. Once again, the evidence pointed to faulty mouldings rather than user abuse, since the same distortion of the top step corners had occurred. If the stubs had been placed below rather than above the top step, the slip occurs very quickly and should be obvious to the user. Placing just one stub above and the other below was not only difficult but also easy to spot and rectify.

Tools and ladders

100 μm

EHT = 3.99 kV Signal A = SE1 Date: 28 Jun 2007 WD = 14 mm Photo No. = 7

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8.47 Micrograph of fracture corner showing matrix depletion (arrow).

8.9.5 Product design, quality and testing The two failures thus had a common cause: faulty mouldings that were insufficient to bear expected loads. They raised serious questions about the way the product had been introduced into the consumer market, especially in terms of the original design, the extent of testing of prototypes, the monitoring of moulding conditions, and the quality control of the new composite connections. And there had been other failures, like the brittle fracture along one edge holding the connector to the ladder stile (Fig. 8.48). In this case, brittle fracture had occurred along an edge of the large plate moulding which was attached to the stile of the ladder. The entire length of plastic had fractured, so releasing the fixing entirely. Another rib has also fractured (at lower left in the picture), and a large and deep weld line could be seen running from the top into the side of the moulding. It is likely that another weld line also caused the long fracture at right. The product was withdrawn from the market following these failures. Even though the majority of the products may have been strong enough for regular use, the risk of only a small number failing suddenly was too difficult for the company to face. Much more serious injuries could have occurred and a subsequent court case would have been expensive to fight. The negative publicity would also have damaged the reputation of the company. Traditional designs of stepladder continue to be made and sold, but they normally rely on a folding table at the top of the two ladders which gives a very firm anchor to the device. Alternatively, a set of sliding supports links the two ladders together near the base.

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8.48 Broken composite plate probably caused by poor weld line control.

Accidents still occur since ladders are so widely used in both domestic and work environments, and can give rise to serious injury. Users can be quick to blame the ladder, but often they have over-reached sideways during painting, for example, and the stepladders have toppled. Sometimes, the user lands heavily on one leg, which becomes distorted. They then say that the leg has buckled due to faulty metal or joints, but the damage in many such cases has occurred after the event rather than during the accident and so cannot be blamed for the accident (12).

8.10

Conclusions

The growth of the Far East as a manufacturing centre has been remarkable over the last decade, with the price of tools falling dramatically from imports into Europe and the USA. It has been a positive benefit to business and domestic users, but should not mask the often poor quality of some tools. In the worst cases they led to severe personal injury, and in less serious cases to loss of function or loss of product. In the cases of the plastic handled knives, the design was so poor as to beggar the question how such products could have been made in the first place. Then there are products

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like the handles to angle grinders which are badly moulded, but when made correctly, perform well in service. That the designer realized the error in the sharp outer corner of one of the devices became obvious when other handles of the same model were compared. The consequences of failure when in use are horrendous because a grinding wheel can cut deep into soft tissue of the body, and is almost impossible to stop before damage has been done. The safety guard for oxygen cylinders was an important advance for easy recognition by hospital staff of the correct grade of gas to be delivered to patients, but failed through poor monitoring of moulding conditions, allowing weld lines to enter the critical hinge on which the design was based. The moulders quickly jumped to the wrong conclusion by blaming the material, when they should have been keeping a closer eye on their own machines. They were faced with a total loss of credibility, but the solution lay in their hands. They should also have been testing their products, but a simple test was devised so they could monitor production very easily. A handle fitted to a low level WC failed and cut the user, but the failure was more complex than initially met the eye. Inspection showed that the chromium layer protecting the surface had been deeply scratched, possibly by a ring on the hand of the user, and allowed stress corrosion cracking of the underlying plastic, ABS resin. A brittle crack grew over a period of time, by attack from a chlorine-based chemical such as bleach, and would have been visible to the user every time she used the device. She should have replaced the handle before it finally failed. The case against the maker of the handle could not be sustained. Consumers should be aware that cracked products are more likely to fail suddenly than those without visible cracks, and they should be discarded and replaced before any final fracture. Chairs are commonly available in thermoplastics, especially for external use, and can also fail suddenly, but often without any prior warning of imminent failure. Even a fall from 30 inches (0.76 m) or so can cause serious injury to the back, and a particular design of polypropylene chair made in Italy failed suddenly shortly after purchase. The material of construction was poorly processed to shape, and numerous voids were evident inside the material at and near to the fracture. The strength of the chair was lowered further by geometric stress raisers, and would have failed the standard tests available (if it had been tested). Then another chair of similar design and material failed, again injuring the user, but this time became a court action. The chair design had been tested and approved by independent test houses, but investigation showed similar defects as before led to premature brittle fracture. The case was settled before trial. Like many hand tools, ladders of all kinds are potentially lethal instruments if they fail when the consumer is using the device. Falls from ladders

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are one of the most common causes of death and serious injury in the home, especially among older people. Extension ladders cause the most injuries owing to their intrinsic instability, especially if the user is near or at the top. They should be tied at the top, or footed and certainly leant at 75 degrees to provide some security against slippage or toppling. Stepladders seem more stable but can still fail by toppling, although few expect the device itself to fail. But that is just what happened in several recent accidents when key connectors holding the ladder parts together suddenly fractured, all on one specific design which had recently been introduced onto the market. The parts had been injection moulded in GF nylon, and early unexpected failure was caused by moulding defects, either weld lines or porosity, within the polymer at a very sharp corner in the design. It should be a routine exercise to eliminate all sharp corners in polymer mouldings by polishing the tools before, rather than after processing to shape. But quality control also failed the company and allowed defective products to enter the market and endanger users. The hard lessons learnt in other products should be circulated more widely, which is one of the objects of the present book.

8.11

References

(1) Pheasant, S and O’Neill, D, Performance in gripping and turning: a study in hand/handle effectiveness, Applied Ergonomics, 6.4, 205 (1975) (2) Drury, CG, Handles for manual materials handling, Applied Ergonomics, 11.1, 35 (1980). (3) Pilkey, W, Peterson’s Stress concentration factors, Wiley-Interscience, 2nd edn (1997), Chart 3.10. (4) BS EN 50144–1:1996, Safety of hand-held electric motor operated tools; Part 1 General Requirements. (5) Pheasant, S, Body space-anthropometry, ergonomics and design, Taylor and Francis, Chapter 16, page 231 (1986). (6) BS4875 Part I: 1985, The strength and stability of furniture. (7) Lewis, Peter Rhys, Reynolds, Ken and Gagg, Colin, Forensic Material Engineering: Case Studies, CRC Press (2004), Chapter 8, p 273 ff. (8) Peterson, RE, Stress Concentration Factors, Wiley-Interscience (1974), Figure 73, p 98. (9) Lewis, Reynolds and Gagg, op cit, Chapter 7 p 218 ff. (10) Lewis, PR, Premature Fracture of a composite nylon radiator, Engineering Failure Analysis, 6, 181–195 (1999). (11) Walker, P, Lewis, Reynolds et al., Chambers Materials Science and Technology Dictionary (1993), p 303. (12) Lewis, Reynolds and Gagg, op cit, Chapter 3, p 61 ff.

9 Components in transport applications

9.1

Introduction

Car use is so ubiquitous that it is taken for granted, but yet the number of accidents remains high despite the many measures taken to reduce them over the years. They are still one of the most common causes of death and serious injury, with 53 485 in 1992 dropping to 39 407 in 2002, of which there were 4229 deaths in 1992 and 3431 in 2002 (1). Although the UK has the lowest death rate of European countries (6.1 per 100 000 population in 2001), it is small consolation to the relatives of those victims, or the large number of injured motorists. The main causes are adverse weather, alcohol consumption and excessive speed, causes which have been tackled by tougher legislation and changes in social attitudes. There is no doubt that car design has also improved dramatically over the past 20 years, with improvements in passenger safety including seatbelts, airbags protecting against frontal impacts, more efficient brakes and protection zones designed into the structures of cars. Critical car components have improved lives and reliability, the car tyre being an outstanding example, where the radial has now all but displaced the cross-ply tyre. Sudden blow-outs are now much rarer than formerly. Laminated glass has replaced toughened glass windscreens, so driver vision is retained despite severe damage, since the fractured parts are held together by a viscoelastic interlayer, a polymer polyvinyl butyral (PVB). Composite materials have displaced many metal parts, and promise to displace yet more, as their lower weight and better impact resistance offer greater fuel efficiency and safety: nowhere more dramatically shown than by the crash resistance of Formula I racing cars. But there are still many vulnerable components, such as fuel lines and brake seals, for example. Other users are prone to accidents, especially motorcyclists: two wheels are inherently less safe than four when traction on the road suddenly drops, for example. Diesel fuel spills on the road are an especial hazard for motorcyclists. When a bike is involved in an accident, the machine inevitably falls over and the user is usually thrown from the machine, and can sustain horrific injuries since he or she will be travelling at the same speed as the machine. There are no seatbelts for motorcyclists and they are directly exposed to the weather, while improved engine design has led to the potential for much higher speeds than is normally achievable in four wheel 349

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vehicles. Thus motorcycles have 120 deaths or serious injuries per 100 million vehicle kilometres travelled, compared to a figure of just 4 for motorists (using UK DOT statistics for 2005). Both rates have been decreasing, but the disparity between motorcyclists and motorists reveals the far greater hazard of driving on two wheels rather than four.

9.2

Failure of tailpack in a motorbike accident

The hazards of motorbikes are nowhere more revealing than in the following case of a biker who lost part of his leg during an accident which occurred in France in 2002. The accident involved a troupe of bikers who had been travelling together, travelling to Tours to watch the Le Mans Grand Prix race on 17 May in clear, dry weather. As they were travelling along the A28 motorway in northern France, the tailpack of the leading biker (on a Yamaha 600 cc bike) suddenly became detached from the rear pannier of his machine and fell from the back of the bike. It became entangled with the rear wheel, causing it to seize suddenly. Since the machine was moving at speed, it skidded and the biker was thrown clear. However, the biker behind him swerved to avoid the fallen bike, and he himself skidded and was thrown into the carriageway. Unfortunately, he slid into the central reservation, where a steel post holding the barrier severed his leg below the knee. His life was saved by the ambulance team which arrived, and by subsequent treatment in the local hospital. After recovery, he sued the lead biker of the group and the maker of the bag which had fallen from the rear of the motorbike. The tailpack had been in use for about a year after purchase.

9.2.1 Seized wheel The local gendarmerie examined the accident scene very soon afterwards and recorded the state of the vehicles where they had finally stopped, as well as the various skid marks left by the lead motorbike (Fig. 9.1). The seized wheel was the focus of attention because it might be possible to reconstruct the sequence of events from the position of the bag and its contents wrapped around it (Fig. 9.2). The close-up of the rear wheel seemed to show that a heavy duty steel cable (from a cycle lock stored in the tailpack) was probably the cause of seizure when it became jammed between the tyre and the rear suspension. The damage to the base of the tyre clearly showed the point at which the wheel had locked, with a hole in the tyre surrounded by heat affected rubber tread (Fig. 9.3). The first investigator (acting on behalf of the lead biker) was the first to examine the tailpack following its retrieval from the wheel. He attempted a reconstruction of the fall of the tailpack using the original bike (restored to working order). It had been attached by rubber bungee cords to the rear

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9.1 Skid mark from rear wheel of lead motorbike after wheel seizure.

9.2 The lead motorbike after the accident showing rear wheel and tailpack with bungee cords wrapped around number plate.

9.3 Puncture in rear wheel after wheel seizure.

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9.4 Reconstruction of tailpack perched on rear seat of bike.

seat (Fig. 9.4), and the investigator thought that it had slipped sideways so as to jam the rear wheel, as shown by the conjectural reconstruction of Fig. 9.5. There was a problem with the theory, however. When the reconstruction is compared with the actual remains (Fig. 9.2), the position of the black bungee cords is quite different. In the accident, the cords have become wrapped around the rear number plate, while the reconstruction shows no such situation. The way the bag fell during the accident must have been different.

9.2.2 Deeper analysis To address the problem, we felt that a detailed examination of the actual remains was needed. The tailpack itself was severely damaged, as one would expect from its final position resting on the wheel (Fig. 9.2). There were several tears and abrasion marks on the bag showing where the rapidly revolving tyre had made contact with the outer cover (Fig. 9.6). The lock comprised a thick steel cable about 10 mm in diameter, with a loop at one end and a lock at the other (Fig. 9.7). The two ends had been originally given further protection by PVC sheaths covering the outer parts, but they

Components in transport applications

9.5 Sideways slippage of tailpack.

9.6 The damaged tailpack.

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9.7 The damaged bike lock with transfer of PVC.

had become severely damaged when in direct contact with the moving wheel. The damage pattern showed that the lock and loop were near one another in a tighter loop than shown in the picture, judging by the black PVC transferred from them to the adjacent cable. The polymer had been melted by the frictional loads from the tyre, and its transfer showed the original configuration of the coiled cable. Since neither lock nor loop is visible in Fig. 9.3, they must have been under the tailpack and facing forwards. So what can be said about the original position of the tailpack? One reference point is the white flash tape fixed to the large panel of the tailpack seen in Fig. 9.6, and visible in the accident (Fig. 9.2) and the reconstruction (Fig. 9.4). The position after the accident was consistent with the reconstruction, but another point arose about the contents of the bag and what their position told us. According to the statement of the biker who packed the tailpack, it contained mainly clothing (jeans, T shirts, socks, underpants, trainers, a sweat shirt and wash bag, helmet visor, documents, nylon waterproof jacket and the bike lock). We estimated the total weight of clothing as about 5.6 kg, and the lock weighed 1.2 kg, giving a total of 6.8 kg. The only items which were clearly identifiable were the lock and the blue jeans (Fig. 9.2 and 9.3), so the rest must have been buried out of site within the bag. The fact that the lock came into contact soon after the bag fell suggests that it was packed outside (a cargo net was used on the bag), perhaps on the top. Since the lock was the heaviest single item, its position could have made the tailpack unstable when in its original position on the rear seat (Fig. 9.4).

9.2.3 Bungee cords The final point of interest was the role of the straps and bungee cords: were they sufficient to hold the tailpack securely? There were two bungee cords

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attached to the base of the bag and they were apparently the only way of securing the bag to the bike. They were meant to be looped under the seat and then re-attached to the bag by D-rings sewn into the outer fabric of the tailpack, as suggested by the reconstruction. That meant some sacrifice of lateral stability, as already shown above. Bungee cords are composed of a core of continuous vulcanized rubber strips enclosed by a woven fibre sheath. The sheath is easily deformed, so the physical properties are mainly determined, at least initially, by the elastomeric core. However, the sheath limits the maximum strain to about 200%, the inner core being capable of much greater extension. In the first place, the bungees from the tailpack were of relatively small diameter (about 6 mm) compared with the bungees on the new bag bought by the first investigator of nearly 8 mm diameter. He stretched the bungees from the accident out under the seat (Fig. 9.4) and found that the front cord was stretched 1350 mm and the rear bungee to 900 mm, and by performing simple tests estimated that the front cord exerted a tension of 2.45 kgf, and the rear cord only 0.9 kgf. Moreover, the stress exerted by the cords relaxed with time, falling to about 2.2 kgf and 0.7 kgf after 30 minutes. We confirmed the results by conducting creep experiments where cords are subjected to a constant load and the change in length determined. After 30 minutes, the length increased by about 10%, so one would expect a drop in stress of about 10% for a highly stretched bungee. The experiments showed that the force holding the bag in place dropped significantly after 30 minutes, so making the tailpack less secure than the user might have expected. So not only were the bungee cords weaker than on a new tailpack of the same make, but they were also less reliable than, say, a simple rope, owing to stress relaxation with time. The relaxation is probably caused by slow movement of the textile sheath as the fibres pack together more efficiently under load. Stress relaxation in the rubber cords is much smaller by comparison.

9.2.4 Alternative theory It was more likely that the tailpack had fallen backwards from the rear seat, and then become entangled with the wheel. The final position of the bungee cords supported the idea because they were both draped over the number plate (Fig. 9.2), and not under the rear seat, as in the reconstruction (Fig. 9.5). Moreover, there was little evidence of any side winds at the time of the accident, and indeed, the only air movement will have been by virtue of the speed of the motorbike. It will of course have been acting along the axis of the bike tending to overturn the bag over the rear of the machine (Fig. 9.6). Thus if the bike was moving at 50 mph then the front face of the tailpack would be pressed rearwards by the wind force.

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M1

P Wg (M3)

M2

9.8 Schematic diagram of bag stability showing forces acting on the bag (shaded).

It remained to estimate the turning moment from known loads and forces acting on the tailpack (Fig. 9.8). The force exerted by moving air can be calculated using the following formula (2): P = 0.003 V2

9.1

where V is the air velocity in miles per hour (mph) and P is the air pressure in pounds per square inch (psi). So a vehicle speed of 80 mph produces a wind pressure of 0.13 psi (or 91 kgfm−2 in metric notation). The area of the tailpack facing the air flow is about 0.3 metres square, so the net force, F acting on the bag can be calculated from the formula: F = PA

9.2

where A is the area facing the air flow. Hence F = 91 × (0.3)2 = 91 × 0.09 = 8.2 kgf when the motorbike speed is 80 mph. This force will exert a moment, M given by the formula M = Fd

9.3

Components in transport applications

357

where d is the distance from the centre of gravity of the tailpack. Taking that point at dead centre of the bag, then the moment, M1 is M1 = 8.2 × 0.15 = 1.23 kgf-m That moment produced by the air acting on the tailpack will be opposed by the force of the bungee cord of a maximum of about 2.45 kgf. If the rear edge of the seat (the rear corner of the bag) is the pivot, then the moment from the front cord, M2 would be M2 = 2.45 × 0.3 = 0.735 kgf-m But there is another moment which must be taken into account, the inertia of the bag itself (of weight 6.8 kg): M3 = 6.8 × 0.15 = 1.02 kgf-m So the total moment, MT acting against the air pressure is simply: MT = M2 + M3 = 1.755 kgf-m The figure is less than the moment produced by air flow of 1.23 kgf-m, so the bag will not overturn when the motorbike is moving at a speed of 80 mph.

9.2.5 Critical speed The question then arises of what speed will cause the bag to overturn. The critical pressure Pc must equilibrate with the greatest moment resisting overturning of the bag, so 0.15 × 0.09 × Pc = 1.755 kgf-m So Pc = 1.755/0.01305 = 134.5 kgfm-2 or 27.65 psi But by equation 9.1, Pc = 0.003(Vc)2 So (Vc)2 = Pc/0.003 = 27.65/0.003 = 9167 Hence Vc = (9167)1/2 = 95.74 ∼ 96 mph The speed of the motorbike must exceed about 96 mph before overturning of the tailpack can occur. There are many assumptions built into this simple argument, such as the effect of the driver’s body on airflow onto the bag and uncertainties in the exact way the bag was attached by the bungee cords, for example. It is also questionable that the centre of gravity was at the mid-point, especially if the heavy bike lock was placed on the top, as seems likely from its resting position on the rear tyre (Fig. 9.2). It would, however, make toppling of the bag more likely at a lower speed.

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9.2.6 Skid mark analysis An independent report was made on the likely speed of the motorbike from the length of the skid mark made by the lead motorbike (Fig. 9.1). Using kinematic analysis applied to the length and orientation of the main skid mark, the report concluded that the bike must have been travelling at 98–102 mph when wheel seizure occurred, a result in agreement with the stability analysis shown above. The kinematic result was also in agreement with known times of departure from the last halt, the time of the accident and the distance travelled to the accident site. As in many witness statements produced in vehicle accidents, the speeds were under-estimated when compared with the inferences made from the physical remains. This is what the driver of the lead motorbike stated: ‘we had been travelling at around 80–85 mph . . . I remember being on the outside lane and the bike snaking. I remember nothing until I awoke in the recovery room at the hospital.’ The speed limit on the motorway at the time was about 80 mph. The biker added that the group had been travelling for about half an hour before the accident, although the bag itself had been fitted well before, so stress relaxation will have weakened the tension in the bungee cords.

9.2.7 Tailpack design The design of the bag, especially the first version bought by the biker, was deficient on several grounds. The two bungee cords provided were much too small in diameter to provide enough tension to give a secure restraint when fitted to the rear seat, and such cords are in any case poor attachments owing to stress relaxation. The design did not allow for the longitudinal securing of the bag to prevent air pressure acting against the exposed surface of the bag, a severe deficiency since air pressure would act against the bag tending to topple it from the bike, and then, almost inevitably, into the rear wheel. Given that most motorbikes can travel at speeds in excess of 100 mph (sometimes greatly in excess), any accessories designed and sold to be fitted onto the exterior of such machines should be capable of resisting high air pressures. It implies that they should be streamlined and present a minimal area of exposure. In fact rigid panniers are available, firmly attached to the frame of the bike, and are preferable for carrying travel goods (albeit at greater cost). It was unclear whether the manufacturer had carried out tests to ensure the security of the tailpack, such as wind tunnel tests, for example. When the case had progressed past exchange of the several expert reports, disclosure from the defendants proved revealing. There had been many similar accidents from this design of bag, some involving fatalities.

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The accident in France in 2003 was in fact the twelfth of a series of accidents where failure of the tailpack had occurred. The first had occurred as long ago as 1998, when a bungee cord had snapped and the rear wheel locked as the bag fell away, followed by a similar accident a year later, which proved fatal to the rider. A bungee hook had straightened in another, separate accident at about the same time in September 1999. It was the front bungee which failed in these accidents, the critical bungee for opposing air pressure acting on the bag. Further accidents in the new millennium also involved bungee cord failures, probably caused by leverage from the air flow acting against the bag. Another fatal accident occurred in May 2001, the bag having been seen to wobble just before toppling and locking the rear wheel. Further accidents of the same nature occurred through to 2004. The bag supplier, Oxford Products Ltd, was prosecuted successfully by Oxford Trading Standards in April 2003, following the series of similar accidents, and a warning notice published in Motorcycle News the following month. The same company now supplies several different designs of storage bags, which appear to be of much better design, with textile strap attachments preferred over bungee cords. The case illustrates the importance of designing products which are fit for their purpose under all conditions, especially when travelling at the high speed capability of many motorbikes. While all other parts of motorbikes are very effectively streamlined (including the riders who wear close-fitting leather costumes), it is essential that riders are warned of the importance of secure fitment of accessories, and that those accessories are fit for purpose. The simple analysis of the static equilibrium of the tailpack should have been, in hindsight, applied to the design in question before launch in the market. Interestingly enough, similar analysis can also be applied to much larger structures, such as bridges and buildings, where stability in the high winds generated in storms and hurricanes is of crucial importance. The Tay rail bridge failed in high winds in 1879, largely because the structure was supported by fatally flawed cast iron towers.

9.3

Failure of drive belts

A rubber composite drive belt is a safety-critical component which transmits power to the wheels of a motorbike or scooter. If it seizes when the machine is travelling at speed, then the sudden stoppage of the drive mechanism will cause a serious accident. Just this happened in September 2001, when a driver travelling at about 45 mph was suddenly thrown from his scooter when the machine seized up, and the scooter skidded out of control. The driver sustained injuries to his arm and wrist. Examination of the scooter quickly established that all the teeth on the rubber drive belt had

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9.9 Remains of stripped belt drive compared with new belt.

been stripped from the backing (Fig. 9.9), and clogged up the belt case, so preventing further movement. The scooter had been well maintained, and had only travelled 3012 miles before the accident, after being bought new in October 1999. The servicing manual stated that the drive belt should be inspected every 10 000 km or 2 years, and replaced every 15 000 km or 3 years. The question arose of the structural integrity of the belt, and whether the belt had failed as a result of an intrinsic defect or by another cause. Since the belt was totally enclosed in a case, it was protected from external damage. A new belt is shown in Fig. 9.9 for comparison.

9.3.1 Belt remains The first task was to examine the damaged remains, the stripped belt and the many fragments, especially the single and double teeth. The belt consisted of an outer fabric reinforced layer enclosing the glass fibre filament core. There were 51 single teeth in the collection extracted from the belt casing after the accident, seven double teeth and bundles of entangled fibres from the rubber teeth reinforcement. Since the new belt had 77 teeth, there seemed to be a discrepancy in numbers, although it was resolved when enough large teeth fragments were found in the bundle of fibres to make up the numbers. The single teeth were very similar to one another, showing irregular breaks at the junctions between successive teeth (Fig. 9.10). The double teeth were also very similar, most showing incipient or sub-critical cracks between the teeth, where failure had started to occur but been halted, presumably by the sudden seizure of the wheel. But one of the fracture faces appeared quite different from the rest (Fig. 9.11): it is the

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9.10 Stripped single teeth from failed belt.

9.11 Stripped double teeth from failed belt with unique sample on upper left.

one at top left, facing right. It deserved further examination after all the samples had been inspected. Analysis using FTIR and DSC showed the organic fibres to be polyester with a melting point of 257°C, and the rubber matrix probably NBR, or nitrile butadiene rubber filled with fine particles of carbon black. NBR has considerable resistance to swelling by oil, so is preferred for use in engines and drive belts where exposure to oil could occur. Careful examination of all the fragments in the optical microscope failed to reveal any evidence for possible failure mechanisms such as oxygen or ozone cracking, the former usually being seen as fine ‘crazy-paving’ network of brittle cracks, the latter as deep cracks at right angles to the tension. Both types occur only on outer, exposed surfaces, as does the kind of deterioration seen in vulcanized rubber exposed to active organic fluids such as some oils and some light organic liquids that can swell the rubber. There was no evidence that the material had hardened through exposure to the

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heat of the engine or friction, for example. Rubber can also indeed soften with time, but the product stiffness was very similar to the new belt, so this possibility could be rejected too. So another mechanism must have caused the failure. The most obvious mechanism in a cyclically strained product such as a drive belt is fatigue, a mechanism common in hard rigid materials such as steel and other alloys. It is also a failure mechanism seen in rubber products subjected to high cyclical strains. It may seem strange that an inherently flexible material like rubber is susceptible to fatigue, but it is a widespread phenomenon which can occur in any material or product subjected to a varying load. Because it is so commonly associated with metals, we naturally think that the problem of intermittent crack growth (fatigue) is unique to those materials, but the reality is otherwise. Since fatigue failure often occurs as a single crack, it could explain why just one of the samples showed a different fracture surface than the rest of the teeth.

9.3.2 Brittle fracture surface The edge of the double-toothed fragment was worthy of much more detailed examination, especially in the optical microscope, Fig. 9.12 showing the fracture surface in profile. The double-tooth fragment was interesting because it was the only example where the fracture had grown into the crown of the tooth, as shown by the section of Fig. 9.13, where it is compared with the common failure mode across the roots of the teeth. So the sample now exhibited two distinctive properties not shared with any other teeth. The fracture itself showed one main zone of interest, a large curved shape and intersecting a smaller zone at one side, at sharp cusps (Fig. 9.14). The larger zone ran from the base of the belt just above the continuous glass fibre reinforcement to the tip of the tooth, and across the fibre reinforcement within the tooth itself. Numerous ledges running across the width of

9.12 Close-up of brittle fracture on double tooth.

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9.13 Comparison of critical tooth pair (top) with another to show crack path through head of tooth.

9.14 Brittle fracture surface across belt tooth showing horizontal fatigue striations.

the fracture surface were visible, suggesting intermittent crack growth over a period of time. The unique nature of the fracture pointed to a fatigue crack growing slowly across the belt until the crack had reached the glass reinforced layer below (Fig. 9.14).

9.3.3 Sequence of events It was now possible to reconstruct the way failure had occurred. There was probably an initial defect (such as a void or poorly cross-linked rubber) in the zone above the glass filament layer from which a brittle crack grew slowly from first use of the engine. The defect was probably near the lower

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reinforcing layer, and several severe cracks were seen just above the glass filaments. The worst such defect grew slowly under tension as the belt was pulled by the cogged wheel driven from the engine. The polished appearance of the fibres in the tooth was probably caused by shear and compression as the crack closed when bent around the cogged wheels when driving the road wheel. The loads also developed high frictional forces and considerable heat because when the surface was examined in the SEM, some of the fibres had melted, so local temperatures must have been greater than 257°C. The crack became critical when it reached the glass reinforcing cord because the crack could be seen turning into this layer (lower part of Fig. 9.14). The critical crack then grew suddenly around the belt just above the glass filaments in a single event, so leaving a remnant of rubber remaining where the two crack fronts intersected (Fig. 9.15). It was also likely that other cracks had started at most of the other teeth in a similar position, which in turn suggest that the rubber compound used was defective along most of the circumference of the belt, especially at the junction of the teeth with the glass layer. When the critical tooth crack reached the glass layer, it grew uncontrollably along the interface, teeth being sheared away successively as the belt met the one or other of the cogged wheels until the whole belt was denuded of teeth and there was no drive transmitted at all to the rear wheel. The debris clogged up the free space within the casing, and the wheel seized. It was clear that the engine drive wheel was resting against one tooth, where the rubber had jammed and formed a wall. The rear wheel of the scooter seized solid and the vehicle skidded, causing the accident. The drive belt was thus defective from the outset when the bike was new, and the injured user entitled to compensation from the supplier. Such a defective belt should not have been supplied to the bike manufacturer, and other belts from the same batch could have caused similar incidents elsewhere, although none were known at the time of writing. The defective belt could have been spotted during maintenance but was not due for renewal, and thus was missed by mechanics.

9.15 Fragment of rubber left after crack growth along glass filaments.

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Other teeth failures of drive belts include partial loss of teeth, and damaged belts are shown on several websites, with the proliferation of practical blogs and other sites related to car or auto maintenance. Such non-critical defects as loss of teeth can often be detected by changes in the sound emitted by the drive, and are caused by single defects at the base of the teeth rather than the more serious problem discussed above.

9.3.4 Other composite belts Many different designs of belt are used in critical areas of vehicles, such as cam drive belts, and radiator V-belts, the failure of which can cause major damage to those vehicles when failure occurs. The cam belt is especially important because if it fractures, the timing sequence of the engine is lost and pistons can impact valve heads, so ruining the engine (which must often be replaced). This is why they have to be replaced at very regular intervals (usually at set mileages) irrespective of whether they show external damage or not. The internal defects of the present example demonstrate the wisdom of the policy, because such defects will not be visible externally. Failures also include radiator fan belts, formerly very common, but with improvements in design and manufacture fortunately much rarer now. As with drive and cam belts, the loads are supported by the spirally wound glass fibre filaments, supported by NBR loaded with carbon black filler. NBR is the preferred elastomer since it is oil-resistant.

9.4

Failure of tyres

The design of car tyres has improved greatly, especially with the advent of the radial type of construction, where a stiff breaker layer beneath the tread effectively improves road-holding and transfers vibrations to the sidewall (3). The inner tube is usually now part of the inside wall, and in compression so that punctures by sharp objects will not have an immediate and devastating effect on the air pressure. The elastomers used have also been carefully selected to provide the best combination of properties for the device. The tread has to have a high coefficient of friction for traction and road-holding, and is usually a blend of NR and SBR, the latter providing high hysteresis, and hence a high frictional coefficient. By contrast, the sidewall must flex easily at high frequencies, so a low hysteresis elastomer such as polybutadiene forms a high proportion of the blend with NR. This means that less heat is produced in the sidewalls when driving than say, SBR. Solid polybutadiene rubber balls are novelty items because they bounce so high, with high rebound resilience, yet who realises that their use in tyre sidewalls helps improve the drive of a car? The inner lining is made from a modified butyl elastomer, such as bromobutyl or chlorobutyl

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rubber, which has high resistance to the diffusion of air through the layer. The breaker is made either of steel filaments or aramid fibre to provide the needed stiffness, while the beads which hold the tyre to the wheel rims are usually high tensile steel wire. The rubber tread also has other key functions, such as its role in removing surface water on the road while driving: it is provided by the tread pattern, and if wear reduces the channels of the tread, the tyres may not be able to grip the road effectively. There is strict legislation for tread depth and heavily worn tyres must be scrapped during routine maintenance. Rubber cannot resist sharp objects on the road, such as nails or other metal objects, but punctures are now less frequent owing to the improvement in reinforcing materials such as aramid (which is also widely used in ballistic-resistant materials). Owing to their complex composite design, all tyres are hand-built, although automated as much as possible, and with strict quality control procedures to check the materials used in their construction. The tyre building machine is a rig designed to accept sheet rubber and other parts to make the final shape before vulcanization, when the rubber parts are cross-linked to give the tyre its stability and integrity. Car tyres are easier to build on such a rig (essentially a rotating drum to which the parts are added in strict sequence) than the larger tyres used on trucks, tractors and other large vehicles. Very large tyres are correspondingly more expensive and so tend to be kept in service for longer, often longer than justified on grounds of safety, as the following case illustrates.

9.4.1 Truck tyre failure The case was referred after a fatal motorway accident near Doncaster in the winter of 1978/9 following a tyre blow-out on the front wheel of a truck (Fig. 9.16). The tyre (1.22 m in diameter, 36 cm wide and 46 cm rim to rim) showed substantial damage at the crown of the tread, where brittle cracks in the carcass and tread converged (Fig. 9.17). It had been made in 1953 and was 26 years old at the time of the accident, having been on the truck during all of its life. It had been made by Michelin using natural rubber and rayon as the reinforcing fibre for the tyre plies. The shape of the tread indicated that it was designed to work as a tractor rather than a truck tyre, so would be more suitable for moving over mud and earth rather than the smooth tarmac of a high-speed motorway. The condition of the tyre (and the cause of the accident) was central to a civil case before the courts (K James-v-Charles Clarke Ltd). The damage showed a complex series of large cracks in the tread and reinforcing plies (Fig. 9.18). There appeared to be two main cracks running along the axes AB and CD as shown in the figure, which intersected at the

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9.16 Remains of a truck from a fatal motorway accident.

9.17 Failed truck tyre from fatal accident.

C B O

ch

A

D

9.18 Oblique view of damaged tyre (a section has been cut out to the right of D).

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f4 C

f3 f2 f1

A

O

D

9.19 Close-up of damage to crown of tyre with origin at O.

probable origin of the damage, shown as O in the picture. But there were many subsidiary cracks, some of which were fresh and part of the final blow-out, others which were filled with dirt and must have preceded the final event. Closer examination showed that the top of the reinforcing plies had been damaged in several areas at the crown, and are denoted by f1, f2 f3 and f4 in Fig. 9.19. It is likely that such damage (which looked old rather than fresh) had been caused by impact blows to the tyre during its life. Such damage can occur when a vehicle is moving at speed and encounters a hard object such as a kerb or rock in its path. Such impacts have the form of a so-called Hertzian stress profile, where a sharp object presses into the surface and creates compression immediately below the point or zone of impact, but bending further out around the zone. It is the latter which can cause serious damage since the outer parts of the exposed surface will thus be in tension. One immediate effect can be delamination of the tread and carcass since the two layers differ greatly in stiffness, and it is likely that the two layers became separated after the most severe of such impacts. Damage at and near the origin was severe, and extended well into the substantial ply layers, which comprised 14 layers of unidirectional ply set at angles to one another. Further away on the tread, further evidence of prior damage was visible as ‘chunking’ of the outer parts of the tread (such as shown by ‘ch’ in Fig. 9.18). Shallow abrasion wear was present over most of the tread. There was no evidence of tyre over- or under-inflation or wheel misalignment, judging by the tread wear. Neither was there any evidence of a manufacturing defect in the structure of the tyre, malicious damage to the tyre exterior, or damage at the rim. There was no indication of heat damage to the rubber, a symptom of heavy use at high vehicle speed (4).

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9.4.2 Oxygen and ozone cracking Superficial brittle cracking was present over much of the outer exposed surfaces of the tyre, especially oxygen and ozone cracking, where the rubber had been attacked and degraded by these gases. They were especially evident in the tyre sidewall, the characteristic ‘crazy paving’ of oxygen attack being very clear. At the left can be seen the oriented deep cracks typical of ozone attack on the rubber (Fig. 9.20). Ozone cracks always grow at right angles to the strain in the rubber, and were also seen in the tread grooves at the bases of the prominent bulges in the tread pattern. The oxidation cracks were about 2.5 mm deep across the sidewall compared with a maximum depth of about 3 mm for the ozone cracks, both being a substantial proportion of the total depth of about 7 mm of the wall (Fig. 9.21). On the other hand, no critical cracks were seen to have grown into the ply layers. The extensive cracking would have been visible to anyone checking the condition of the tyre and its suitability for high speed driving on a motorway.

9.20 Oxygen and ozone cracking in outer section of sidewall.

9.21 Depth of cracking in sidewall.

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The tyre was probably composed of natural rubber, which is known to be very susceptible to oxygen and ozone cracking. The finely-divided carbon black filler used in the compound has little effect on cracking.

9.4.3 Sequence of events From the observed damage, it was possible to distinguish between the recent and earlier damage. Ozone and oxygen cracking was widespread and long standing, but not in itself critical. On the other hand, the traces of old impact damage under the tread showed the kind of deterioration which probably caused the final blowout, especially at the origin (Fig. 9.18). When the tyre was examined for ply damage, it became clear that the fibres of the plies showed frayed ends, while those further away from the origin had been cut cleanly by the rapid growth of the brittle cracks during the blowout (Fig. 9.22). Following an earlier impact, critical damage had occurred to the plies at the crown of the tyre, and the tread was probably moving freely as a result of delamination between the tread and reinforcement. The tyre pressure

O

C

9.22 Fraying of tyre cords at origin, sharp ends further away (arrow).

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was 105 psi, and the plies will have been carrying most of the biaxial stress in the tyre. The individual rayon filaments will have been in tension and subjected to repeated cyclical stresses when the vehicle was driving, causing fatigue cracks to grow steadily through the fibres, which became frayed and fractured at and near the critical damage. Rayon is a synthetic polymer fibre of cellulosic origin and was formerly widely used for reinforcement of rubber. As the critical crack grew steadily into the carcass, there came a point when the remaining plies could no longer support the inner pressure, and the tyre exploded, The truck was completely destabilized and turned over, crushing the cab and the driver within (Fig. 9.16). Another individual was severely injured. The prior impact damage was hidden under the tread, but the chunking of the tread, which was probably present as a result of impact, should have given warning to the driver of a problem before the accident. The tyre should in any case have been replaced simply from the state of the ozone and oxygen cracking. Such a tyre should never have been fitted to a truck for motorway driving at high speed. The diamond shape of the fracture (Fig. 9.18) was caused by the alternate orientation of successive plies in the carcass. The hoop stress is resisted by the ply orientation, and so slow fatigue cracks grow along a ply between the fibres, but must cross the fibres of the next ply, producing the frayed edges seen in the picture. When the critical fatigue crack grew explosively, the remaining plies fractured without fraying, as seen in Fig. 9.22.

9.4.4 Modern tyre technology Cross-ply tyres for cars have now been superseded by radial tyres, with most driving stresses absorbed by the sidewall, so the tread gives a more stable footprint on the road. The materials used are much improved, with speciality elastomer blends for different parts of the structure. Rayon has been replaced by high-tensile nylon fibre or polyester, with aramids frequently used in the tread breaker belt to replace steel cord. The problem of tyre blowout is in any case much reduced nowadays owing to tighter regulation of tyre condition, especially when garages perform routine maintenance. Tread wear past a legal limit ensures that tyres are replaced earlier in their lives, improving car safety (but incidentally increasing the worn tyre mountain, a considerable recycling problem). Many large tyres are still made to a similar design as that which failed, but there are severe restrictions on their use on roads, as legislation has advanced (a fact which is visible on the logos and messages embossed on every tyre sidewall today). Aircraft tyres are exceptional in that they have to resist great loads when planes take off and land. Many were cross-ply until quite recently when a tyre burst occurred on the Concorde aircraft (Fig. 9.23) taking off from

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9.23 Concorde seen from below, with fuel tanks above landing gear and wheels.

9.24 Piece of titanium strapping which initiated the Paris disaster.

Paris on 25 July 2000. One of the tyres hit a piece of titanium lying on the runway (it had dropped from a previous plane: Fig. 9.24). The tyre burst (Fig. 9.25) and a large chunk of rubber was thrown up. It penetrated the fuel tank in the wing, and the resultant spill and fire caused the plane to crash with everyone aboard killed (5). A new radial tyre was designed as a result, as well as aramid-reinforced fuel tank liners, but they failed to rescue the fleet, which were all scrapped (6). There had been numerous previous instances involving burst tyres on Concorde, and why development of a better tyre was not tasked sooner remains unknown. Rubber components are of course widely used for other components in vehicles simply because they can mitigate the effects of both high and low frequency vibrations inevitably produced by moving vehicles. One key item in all vehicles is the solid bearings used in the vehicle suspension. In order to attach the bearing to the chassis, they are made by bonding steel plates

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9.25 Concorde tyre fragment showing transverse cut.

9.26 Failed bearing from Renault Espace at left (intact bearing at right).

to the rubber, and often also incorporate steel plates within the structure to modify the acceptable compressive strain without affecting the shear behaviour. But they too can fail, and sometimes with distressing results. Fig. 9.26 shows such an engine bearing which failed suddenly when one of us (PRL) was cornering the car, and resulted in the front wheel assembly falling away. Although the vehicle was halted without an accident, the damage was severe and expensive to rectify. The bearing in the picture failed by shear fatigue at the steel-rubber bond from engine movement, especially during sudden starts or stops. The samples also show some superficial ozone cracks, which are not generally a problem with such bearings simply because the rubber blocks are mainly in compression, and the cracks cannot penetrate to the inner parts of the bearing. Small tension loads are expected on outer surfaces, where the cracks form, but are soon halted when they reach the tension zone. Similar, more complex bearings are widely used in many high performance machines, such as helicopter rotor shaft bearings, where they buffer the high loads during start-up and wind-down of the rotors, and so limit the

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9.27 Helicopter rotor bearing with multiple rubber layers.

loads on sensitive metal components, and so prevent fatigue cracks forming and growing (Fig. 9.27).

9.5

Failed Rilsan nylon fuel pipes

The fuel pipes of vehicles are also safety-critical components of all vehicles, and since they are exposed to high engine temperatures, engine oils and fuel, have to be made to a high specification (4). They are frequently made from vulcanized rubber, or sometimes from thermoplastics. The consequences of fracture can be horrendous, because the spray of fuel emitted when the crack finally penetrates the bore is usually quickly ignited in the engine compartment, where electrical sparks are omnipresent. The danger is greater with highly volatile petrol, but diesel fires are not unknown. Diesel spills into the road can cause a separate hazard for the slick quickly forms a slippery layer for other road users. Problems with car fires were encountered on several different car models in the UK in the late 1970s and 80s, and returned with a vengeance in Eire in the 1990s as the result of a failed recall on some models (7).

9.5.1 First encounters The problem came to our attention in the later 1970s, when car fires were reported on new Ford Cortinas (Mark IV) by insurers, and we were asked to investigate one such incident (although many had been reported. The car involved (Fig. 9.28) only had 10 miles on the clock, and was being delivered to a car dealer. As with many fires, the remains to examine were decidedly daunting owing to the fire damage. The fuel hoses were the

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9.28 Fire-damaged Ford Cortina under examination.

9.29 Remains of fuel pump inlet hose with large hole shown by arrow. The sample at right (carburettor inlet hose) shows regular cracks along its length.

subject of immediate attention since the fire appeared to have started somewhere near the fuel pump and the fuel lines running to the device, judging by burning of nearby plastic components such as the air filter cover, battery and fan heater housing. The top of the 12V polypropylene battery case had partly collapsed as a result of the intense heat produced by the burning petrol. There were traces of molten plastic on one of the hoses. The remains of the burnt petrol hose near the fuel pump showed substantial charring (Fig. 9.29). The original hose carried a textile sheath (at extreme left in the figure) and the main hose from the tank to the pump was severely damaged (centre), showing that it had caught fire quickly, and may have been close to the source of the fire. This sample of pump inlet hose was about 28 cm long, with an ID (internal diameter) of about 8 mm and an OD of 15 mm. By measuring the intact length on another model of the same type, the fragment shown represented all of the hose involved. There were at least three points where penetration of the tube into the bore had occurred, two of which were caused by the fire. However, the third hole was different in shape. It consisted of a longitudinal gash near the pump

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9.30 Close-up of fuel hose showing split at extreme right (lower arrow) and mark in bore at top (upper arrow).

inlet end, and is also visible in the close-up of Fig. 9.30. The split appears to have run spirally along the tube, and the tube also appears to have possessed a twist when in situ. The rubber used was NBR, which tends to stiffen and harden with intense heat, as several flame tests on new tube showed.

9.5.2 Spider lines The end of the tube shown in Fig. 9.30 showed a small mark running longitudinally (shown by the upper arrow), which might indicate a problem encountered during extrusion. So-called ‘spider lines’ can be produced where the metal bars which support the die core create a division in the molten polymer. If the rubber was not at the correct process temperature then the lines would be preserved as an incipient split running along the bore of the tube. When pressurized, the split could grow under the inner hoop stress, and soon reach the outer surface, especially if the rubber had been over-cured so that it was stiff and brittle. It was concluded that this is what had most likely happened and since a large length of tubing might have been made originally, there would be a problem on many new models, as indeed, a press item showed shortly after our report had been produced. The Daily Telegraph of 10 June 1981 reported that: An urgent investigation is being carried out by Ford engineers after 12 cases of fire breaking out under the bonnet of Cortina cars less than 18 months old. The fires which start when cars are at rest after a long high-speed run, are thought to be due to a new air filter system which becomes overheated causing molten plastic to drop onto the exhaust manifolds, where it catches fire. The Mark IV Cortina has been in production for about four years and it could be a freak occurrence affecting recently built models, a spokesman added.

The report may correctly reflect the statement made by Ford, but seems to confuse cause and effect: the molten plastic was likely caused by the fire from the leaking fuel pipe and not vice versa. The comment about a freak incident betrays a lack of analysis of the problem, with the fuel pipes the

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most likely source of the problem. In response to our report, Ford suggested that vandalism with a knife could cause the problem, but there was little evidence of a cut in the braid, let alone the tube of the car we examined. Under a standard then applicable to hose, a pressure test was required for petrol lines of at least 50 psi, but whether or not it had been carried out was unknown. The line itself was under a much smaller pressure, but enough in their tests to produce a leak rate of 10 ml/min: not great but enough to start a fire with further fuel siphoning through from the tank. A recall was carried out, and the repairs were very simple and of low cost: simply replace the affected lines with high quality hose. The faulty tubes had an inner weld line (similar to those found in some faulty injection moulded plastics) that would be impossible to detect without removing the lines and cutting them up to view the inner bores, a good example of a latent defect. It is interesting to note that textile braided tubing of the kind used on the Cortina offers no protection against ozone gas initially, but when in use for some time in a car, becomes soaked in grease and oil, which does form a barrier against the gas. But the best remedy is to use an ozoneresistant rubber such as EPDM or add an anti-ozonant to the compound.

9.5.3 Sir John Gielgud However, there are other causes of fuel line failure, which can also be difficult to detect by visual examination alone, especially in textile covered rubber hose. The problem is ozone cracking, as already discussed elsewhere. And the problem had been encountered in another new model: the Fiat Mirafiori. In fact we had reported on the problem a few months before the Cortina problem. The car involved was bought on 31 January, 1978 with a mileage of 12 342, and had a mileage of 45 051 at the time of the incident, so was not brand new like the Cortinas. The car fire happened rather dramatically, as appropriate to one of Britain’s most distinguished thespians, Sir John Gielgud. He was being chauffeured down the Chelsea embankment at 9.30 am on 18 July, 1981, and while halted at the traffic lights on the Chelsea Bridge road, the chauffeur noticed smoke rising from below the bonnet. With the aid of an extinguisher he managed to control the flames until the London Fire Brigade arrived to douse them completely. The damage was extensive as shown in Fig. 9.31, but it was possible to locate the source of the fire by examining the nature and direction of flame damaged components. Analysis of unattractive and often daunting remains lies at the heart of all forensic work, and often leads on to the cause or causes of the problem, which is the basic motive for all investigators. So what did the remains show? The first task was to identify the major damage to visible parts, such as the large radiator hose in left foreground (A), which

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J E

H I

A

B

D G

F

C

9.31 Fire damage to Sir John Gielgud’s Fiat Mirafiori saloon car.

has almost burnt through near the engine block near the centre of the picture. Just above can be seen the charred remains of the radiator expansion hose next to the radiator filling cap, although an adjacent pipe at B is scarcely affected at all. It was the burnt remains of the fuel hoses (at C) which suggested where the fire had started. The remains consisted of a large diameter rubber tube and a shorter length of narrow diameter thermoplastic tube next to it. A narrow tube in good condition lay at D and one vane of the radiator had melted (not shown in the picture but located at E). One of the plastic covered leads to the sparking plugs (F) had been destroyed: the front lead near G in the picture, together with surface discolouration to the distributor cap. There was also some minor damage to the air intake of the heating system at H, and the battery had suffered some charring and partial melting at J. Further information from the repairer of replacement parts supplied showed that minor damage had also occurred to the fan belt, expansion tank, radiator grille, washer bottle and bonnet. The flames had clearly risen up from the source and been deflected down wards by the bonnet so as to affect more distant parts of the compartment. The remains showed that a liquid spill must have occurred because various parts (such as the distributor cap) below the assumed source of the flames had suffered flame damage. The pattern of damage was of intense heat near to the metal oil filler cap on the engine block, judging by the severe hole burnt in the radiator hose near a metal U-clip holding several pipes. A close-up of this clip (Fig. 9.32) on an intact model showed that it originally held two types of fuel hose. The pattern of damage indicated a flow of flaming petrol from a point very close to the clip, from one or other or indeed both of the fuel lines passing through the clip.

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O

R

N

9.32 Close-up of U-clip at O holding the fuel pipes, R and N. Distributor at lower right.

9.5.4 Fiat fuel lines The fuel pipes from several Mirafiori saloons of similar age to Gielgud’s car were supplied by the insurer for detailed examination, and are shown in Fig. 9.33. The narrow long tube (N) is the fuel return pipe and was taken from a Mirafiori saloon with 21 345 miles on the odometer, as was the plain rubber tank to fuel pump sample (T). The small sheathed sample was taken from another Mirafiori of similar mileage (R). The thermoplastic tube showed one zone at which the surface had been abraded by contact with the sharp metal edge of the U-clip (marked by the paper label at left (in Fig. 9.34) while the longer of the rubber tubes (T) showed deep ozone cracks (similar to those in Fig. 9.35). The external surface of the rubber was unprotected by any textile sheath, unlike the rubber fuel lines of the Cortinas. It exhibited deep ozone cracks oriented at right angles to its length, in such a way as to suggest that the cracks had formed when the tube was bent to a low radius (as one might expect when in place in the engine compartment). The form of such cracks is shown schematically in Fig. 9.36. Closer inspection also showed a smaller number of longitudinal cracks, rather surprisingly, on the underside or compressive part of the tube. The cracks were measured using a depth gauge (a stiff wire) and analyzed, the distributions for depth and length respectively

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N

T

R

9.33 Fuel pipes of Mirafiori saloon car.

a a

9.34 Abrasion of a Rilsan fuel pipe (a–a).

9.35 Ozone cracks in NBR rubber fuel hose: the hose has been bent to open the cracks.

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9.36 Schematic diagram to show formation of ozone cracks under tensile stress.

18 16 14 12 10 8 6 4 2 0

0.5

1.0

1.5

2.0

2.5

5.0

7.5

10.0

2.5 Crack depth (mm)

8 7 6 5 4 3 2 1 12.5 Crack length (mm)

9.37 Histogram of ozone cracks in smooth rubber fuel pipe from Mirafiori saloon: crack depth at top and crack length at bottom. Vertical scale: number of cracks.

being shown in Fig. 9.37. The deepest crack was at about 2.5 mm, compared with tube dimensions ID of 5 mm and OD of 16 mm with a wall thickness of 4.5 mm. The worst crack was thus over half way through the wall. The rubber was identified from simple swelling measurements, where cut sections of hose are dipped into several different organic solvents: the

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degree of swelling shows the type of elastomer present. Identification can also be achieved somewhat faster and more exactly using ATR spectroscopy by pressing a free surface against a selenium single crystal and obtaining the absorption spectrum on an FTIR spectrometer. Ozone gas is produced near electrical circuits, especially where either sparking or silent discharge of current occurs. Rubbers with double bonds in their chains (NR, SBR, NBR especially) are susceptible to cracking because the gas attacks the bonds and cleaves them very quickly. Only very low gas concentrations are needed (parts per billion in air), and so high concentrations of the gas may be expected near to the electrical distribution system (Fig. 9.32). So if the damage was produced by about 20 000 miles car usage, then failure (deepest crack reaches tube bore) would be expected between 30 000 and 40 000 miles, assuming constant ozone production. The plastic tube was identified as Rilsan nylon from its melting point of about 176–180°C, with a density of 1.04 g/cc using a standard sample for comparison. It is a thermoplastic polymer (8) composed of nylon 11 chains plus filler (probably carbon black at low level). The two abrasion zones (Fig. 9.34) were caused by direct contact between the pipe and the sharp edge of the U-clip and the inevitable vibrations from engine movement during driving. The damage (at right in the figure) was measured as 0.5 mm deep in a wall 1 mm thick, so was also significant. The zone was an ellipsoid with major and minor axes measuring 5 mm by 2.5 mm, so difficult to see unless the tube was removed for inspection. The engine temperature after some time running (so at equilibrium) was measured directly with a contact thermocouple at about 70°C, so the U-clip would have been at a similar temperature. Since the damage had penetrated about half the wall thickness, it would fail after the damage had penetrated the tank-pump line. Both kinds of defect represented a considerable risk for users of welltravelled cars such as that used by Gielgud. He clearly travelled a great deal, acting at different theatres around the country, and since the mileage at the time of the accident was about 45 000, it compared with the estimates made from the younger samples (Fig. 9.33). And it also appeared from that analysis that ozone cracking was marginally more likely than abrasion to have caused the fire in Chelsea. The samples from Gielgud’s car were unfortunately much more seriously damaged than those from the Cortina, so were of little help in the final analysis. It was a situation which could only be resolved by a recall by the manufacturer, although the Fiat Motor Company were initially resistant to the claim, as the following timeline shows: 1. 18 July 1978 Gielgud car fire 2. 8 August 1978 Insurer’s motor engineer indicates petrol rather than electrical fire

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3. 21 August 1978 Car repairs approved 4. 29 August 1978 Request to us to investigate 5. 8 Sept 1978 Our report identifies ozone cracking and recommends replacement of all rubber pipes 6. 14 December 1978 Request note to Fiat from insurers asking for payment 7. 25 January 1979 Fiat refuses payment 8. 26 February 1979 Fiat requests proof of claim 9. 12 July 1979 Fiat recalls Mirafiori cars for rubber tubes susceptible to ozone cracking However, the recall eventually went ahead, although the problem with the Rilsan tubes remained. Perhaps Fiat advised mechanics to examine and replace damaged tubes, or alternatively chamfered the corners of the U-clip. Both were simple faults easy to remedy and cheap to rectify. The only surprising feature of the case was why such elementary design mistakes were made in the first place.

9.5.5 A car fire in Ireland Unbeknown to innocent drivers in Eire, many were happily driving around in the 1980s in Mirafioris which were in a dangerous condition. The Fiat Company in Ireland had not carried out a recall, so putting many drivers at risk of immolation. All this and more emerged from a preliminary trial at the Supreme Court in Dublin in the 1990s. But how did we become involved? The story is rather tortuous, but easy to understand. A case of negligence had been lodged in Eire after a car accident on 23 June 1988. That particular car, a Mirafiori saloon, suddenly burst into fire and severely injured two young children. Mrs Murphy, driving the car in Co Carlow, had been returning home from shopping with her two children strapped in the back seat. She stopped in the drive of her house, and removed the ignition key because the house key was attached. She went to her front door and after opening it, went inside to put the kettle on and then turned back to collect the children. As she left the house, she saw smoke in the sky and turning the corner of the house, saw smoke billowing from the partly open sunroof of the car. The interior was full of smoke and she could see an orange glow between the front seats. She fainted, but her sister-in-law pulled the children out, although they both suffered severe and extensive burns from exposure to the flames. She estimated later that she had been gone from the car for less than a minute. The fire seemed to have started in the car interior rather than under the bonnet, so what caused the problem?

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9.38 Remains of the Fiat car in which two children were badly burnt.

The car had been bought second-hand in 1987 with 29 000 miles on the clock, and it had covered about 35 000 miles at the time of the accident. It had received a full service in May 1988. The burnt-out car was examined by several investigators, some of whom created irreparable damage by removing various electrical parts, a shocking revelation given the importance of primary evidence. However, much of the evidence which might have yielded a clue to the origin had been consumed by the fire (Fig. 9.38). One possible cause lay in the electrical circuitry from which a fire could have been started by a short-circuit. According to the investigator who contacted us, short-circuit fires are a common problem in many cars, owing to the nature of car wiring using the chassis as the common earth. Many cars such as the Mirafiori, had live circuits even though the ignition was switched off. Some permanently live circuits had no fuse protection at all. There were known problems with the car heater on the Mirafiori, and one investigator concluded that this was the most likely cause of the fire which gutted the vehicle and injured the children. A short circuit in this component led to ignition and then fire in the dashboard. However, the fuel pipe from the pump in the engine compartment ran through the interior to the tank at the rear (Fig. 9.39). It is possible that it became abraded by contact with a sharp metal edge and leaked, followed by fuel ignition. Where the pipes passed near the doors, for example, could be a zone where repeated vibration could initiate abrasion. So an alternative scenario to an electrical fault involved a liquid leak occurring before the driver stopped, with the vapour ignited by a static electricity charge, a possibility we raised given our experience of the earlier fires. Such static charges are common in dry weather when a motorist steps out because the car is insulated by the tyres, and when the driver touches the ground, a spark will often leap between the car’s metallic body and the driver. Whatever the source of ignition, the interior used many highly flammable

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9.39 Rilsan feed and return fuel pipes inside passenger compartment.

plastics (such as polystyrene and polyurethane) which helped fuel the conflagration.

9.5.6 Murphy infants-v-Fiat spa The Irish Times report of the trial on 12 October 1994 summarized earlier proceedings of the case: . . . the insurers of the father’s car, NEW PMPA agreed to pay substantial damages totalling £815 000 . . . after the then President of the High Court, now Chief Justice, Mr Justice Hamilton, had approved the settlement. This was on the basis that the children had to be compensated by somebody, and should be spared the delays of the legal process.

Whatever the precise cause, the case which came before the court related to the discovery of documents from Fiat spa in Italy, and their knowledge of the Mirafiori problems. Fiat had produced little in the way of documentation, and specifically the fuel line recall held in the UK. Indeed, our own work had only been found by the investigators when they approached the original insurers (Guardian Royal Exchange) who held a reference to our work, but nothing else. Our original report had been removed from their archive since such reports are routinely destroyed after six years, although its existence was recorded in their archive. The investigators approached us and we fortunately not only held the original report but also several samples, and all related documents such as correspondence and the recall notice itself. We provided a copy of the report to the Plaintiff’s solicitor, who then supplied it in the form of an affidavit to the Supreme Court as evidence of a known prior problem with the cars in the UK. The case against Fiat was argued by James Nugent SC, then Attorney-General in the Irish Government, a sign of the seriousness

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of the case. He advocated a so-called ‘striking-out’ motion of any defence Fiat proposed to the charge of negligence in the case of the state of Mrs Murphy’s Mirafiori car owing to their refusal to supply documents regarding the fuel line and other known problems of the model. It was a draconian action proposed by Nugent, which meant that, if approved by the court, that Fiat would have no defence against the Murphy children and would inevitably lose their case when it came to full trial. As the author of the original report, one of us (PRL) was invited over to attend the hearing in the Supreme Court. While flying to Dublin, I was seated near some passengers of Italian origin speaking in English with a solicitor. I thought nothing of it until when watching the court proceedings, I recognised one of the Italians when he appeared on the witness stand, and who turned out to be their head of legal services, a Mr Antonio Scognamiglio. I warned our counsel that the translator was not needed, since the witness could speak and understand English. The witness benefited of course, because he then had twice the time to think of an answer and respond! However, his answers were very evasive, as were those of other Fiat witnesses. Judgment in the case was given the following year by Judge Johnson, and he was quoted in the Irish Times article published on 4 March 1995. He said: the Fiat companies had failed to comply with the discovery orders because of their failure to contact their sources, and he still did not know what documents had been or were in their possession relating to fires which occurred in the Fiat model Mirafiori between January 1st, 1979 and June 23rd, 1988, and reported to the Fiat parent company.

Counsel for the parents, the Murphys, claimed in court that Fiat were engaged in a deliberate and sustained attempt to conceal from the court details of hazards in Mirafiori cars in the 1980s. Further, the paper said, the judge paid tribute to the ‘extraordinary energy’ of Francis X Burke (solicitor to the Murphys) that the failings of Fiat had come to light. He also said that the: two witnesses for the Italian Fiat company demonstrated to him in the witness box an elusiveness, an unwillingness to be pinned down as to what documents precisely they might or might not have had or what they might get when enquiries were made. . . . He found the attitude taken by Fiat . . . towards the recall campaign of Fiat Mirafiori cars to be nothing short of contumely.

The judge went on to strike out the Fiat defence. However, an appeal to the Supreme court was made by Fiat in view of the rather draconian judgment (which effectively deprived Fiat of any defence to the claim). The appeal was successful, but Fiat agreed to a settlement with the insurers. However, there was a sting in the tail for the company.

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9.5.7 Other Mirafiori fires The trial opened up the issue of fires not just elsewhere in Ireland but also in the USA. If there had been problems in 1988 with just one Mirafiori, had there been any earlier cases, as one might expect given such a serious design flaw in the car? There had indeed been several fires in Mirafiori cars, one of which resulted in the death of a solicitor. The accident happened on 1 September 1977 in Kilrush, Co Clare. Damage was so extensive that identification of the victim, Daniel Chambers, was only possible by the survival of a ring and a sandal buckle which he was wearing at the time of the tragic accident. The car was a burnt-out shell although the police investigation concluded that the car caught fire while being driven, and then went out of control. The car had been bought brand new only 11 months prior to the accident, and must have had a low but unknown mileage. Correspondence at the time referred to other fires in Mirafiori saloons. The accident had occurred well before the Gielgud fire, and could well have been caused by the faulty fuel lines, whether by ozone cracking or abrasion of the Rilsan pipe. But further investigations by Burke revealed many more fires in the same model, needless to say, without the help of Fiat. No less than seventeen unexplained fires were known to have occurred on the model in Eire by the time of the trial, seven of which dated to the late 1970s with ten in the 1980s. A fire had even broken out when a motoring correspondent had been test driving a Mirafiori in October 1978. Unfortunately, few had been investigated seriously, but heater fan switches or fuel line problems were suspected in many of them. Further enquiries in the USA (where the model was known as the Brava) revealed a shocking state of affairs. There had been several deaths in Brava models, including the death of Dick Treon in Arizona in 1976 due to a ‘defective hose’. Several recalls had been initiated by the NHTSA, including the heater switches, drive shafts, and finally in 1978, fuel lines. In 1979, a Department of Transportation inquiry was launched to examine underhood fires in several different Fiat models as a result of 22 reports of fires which had probably been caused by: ‘fuel hose deterioration, fuel leaking from faulty carburettor and canister problems and electrical wiring problems.’ At the time, Fiat denied even the existence of a problem. Despite the various recalls in the USA, accidents continued (as in Eire). A number of crashes involving Bravas resulted in fires: not always an unexpected outcome, but if the fuel supply system was known to be flawed, it did not take any leap of imagination to realize that the impact loads developed in crashes could well cause the failure of weak parts (such as the cracked or abraded fuel lines). In a survey by the University of Michigan carried out

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in 1995, they examined the NHTSA archives, and found no less than 161 deaths in Fiats from all causes between the early and mid 1980s. Of these, 58 were fire-related and included six Bravas. The publicity in Dublin regarding the case meant that old cases of Mirafiori fire casualties were re-opened, and increased compensation for the injured persons.

9.5.8 Global markets The case highlights the way a major manufacturer mis-handled a major product liability problem of one of their new designs in the 1970s, and thereby left a trail of injured drivers and worse. The problem could have been solved at the outset by correctly specifying the fuel pipe design, construction and material to avoid what was, and still is a well-known failure mode of many elastomers. The abrasion resistance of Rilsan tubing is also low, especially at engine temperatures. Fiat had a second chance to correct their original errors when the first accidents were reported shortly after launch of the Mirafiori/Brava, but their reporting procedures appear to have been very poor. Here is what Scognamiglio had to say when asked about the Gielgud incident in the 1994 trial in Dublin: Q. When you saw the papers that there was reference to a fire in the United Kingdom on Chelsea Bridge, a vehicle, the property of Sir John Gielgud, did you write to the importer who controlled the market? A. I did not write to the importer of that market but I made a phone call. Q. Who did you telephone? A. I called someone in the Technical Service Department but I cannot recall the name because I didn’t know this person. JUDGE: The Technical Service department in England or Italy? A. In England. He confirmed the name which I believe this person . . . is the name in the Affidavit. This person just confirmed to have seen this car and denied that there was any problem, any manufacturer’s liability . . . . Q. But a claim had been made, a complaint about the car had been made after the fire even though this person thought it was invalid, is that correct? . . . A. I did not investigate any further, I was only curious to find out what had happened. Q. Did you ask him if there were any documents extant in relation to a complaint from or on behalf of Sir John Gielgud? A. No I did not ask whether there were any such documents.

So the head of product liability at Fiat in Italy did not follow up information which was important in pinpointing the design fault in cars for which he was ultimately responsible, especially our report of why such failures were happening at all. The company would pay heavily for that error, but

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the heavier burden would fall on many unfortunate drivers and their passengers. Production of the car ceased in 1985, and the same witness went on to state that all records were destroyed at that time, despite the fact that many second hand Mirafiori’s were being driven around the world. Even the insurers kept a note about a report, even if they didn’t preserve the report itself. Such negligence by the manufacturer has an unhappy way of repeating itself, as knowledge is lost about design defects and potential failure modes. It was, however, fortunate that one investigator kept intact records to support the Plaintiff’s case. It is not necessary to wait for the first fatality, because early failures can give a fair warning of what might occur later, so giving time for a recall and change of design. However, it does mean that any early incident is thoroughly investigated and analyzed. It is poor practice to deny that a problem exists when there is clear evidence of product failure, especially if similar incidents occur across several different countries. It is more likely than not that failures may be under-reported, as, for example, when a garage might spot a cracked fuel line and replace it before any incident occurs at all.

9.5.9 Fires in tunnels The effects of vehicle fires are serious enough when they occur in the open, but in tunnels, they are much more dangerous. Truck fires within the Channel Tunnel in 1996 and September 2008 created great disruption for users as well as structural damage to the tunnel lining, despite being contained. Not only are high temperatures generated but the smoke and gases generated are life-threatening in the confined space. Increasing the ventilation just increases the intensity of the fire, so is counter-productive. Although the fires were successfully contained, the fire which occurred in the Gotthard road tunnel on Friday 24 October 2001 spread rapidly and killed 11 people trapped there (9). They were killed by gas and smoke inhalation, with carbon monoxide the main culprit. The fire started on a truck, and spread rapidly to engulf surrounding vehicles, a problem which frequently occurs in vehicle pile-ups on motorways. It is clear that more improvements in vehicle design are essential if further accidents are to be avoided, electrical systems and fuel systems being a top priority. The costs of implementing only modest changes can often be minimal. Thus changing fuel lines from commodity rubbers like NBR to ozone and fire-resistant elastomers like Viton are small compared with the damage than can occur from fires. But even armoured lines can be at risk if the matrix rubber remains susceptible to ozone attack, as a recent case involving fires on aircraft haulage vehicles at Heathrow showed. Ozone cracking had developed over many months, and the cracks had penetrated through the inner metal reinforcement mesh until they reached

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the bore. The subsequent fuel leak caused several separate fires in a number of such tractors before the basic cause was discovered.

9.6

Stress corrosion cracking of nylon connectors

There are other failure modes found beneath the bonnet, and we have discussed one of them in detail previously (10, 11), where a nylon connector in a Rilsan diesel fuel line fractured by stress corrosion cracking or SCC. The failure eventually caused an extensive leak of diesel fuel, which fell into the roadway rather than igniting, but the spillage was almost as devastating as a fire because diesel fuel will form a treacherous surface after a few minutes. The lighter fractions of hydrocarbon evaporate to leave a slick of the higher fractions of fuel, which comprises a thick and viscous oil. The spillage is almost invisible to oncoming drivers, who can lose their steering when the front wheels hit the slick. The effect is of similar danger as ‘black ice’ where a patch of invisible ice occurs on a road, ready to destabilize moving traffic. Motor cycles are especially at risk, but so are cars, because when they skid they can collide with other vehicles. Drivers will be aware of the problem of black ice because low temperatures give some warning of the problem, and particular spots on roads are particularly risky, such as bridge crossings, where ice formation occurs earlier than elsewhere owing to the rapid cooling of roads on bridges (cooled from above and below). The failure of the diesel return tube occurred in 1997 and led to three accidents, the most serious of which involved a car which skidded on the patch of diesel, and collided with a lorry in the opposite lane. The driver was seriously injured and claimed damages from the driver of the recovery vehicle responsible for the diesel leak (Fig. 9.40). The police quickly traced the vehicle with the broken fuel pipe since they followed the trail of diesel to a local golf course. Ironically, it turned out to be a break-down vehicle

9.40 Recovery vehicle on which diesel fuel line fractured and caused a serious accident.

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from a local garage. They retrieved the fractured fuel return pipe (Fig. 9.41) and organized repairs. The Forensic Science Service in Strathclyde concluded that the fuel pipe had been cut by a knife, so vandalism was suspected. However, the position of the connector was almost completely inaccessible and why cut the connector when the Rilsan fuel pipe itself was much more accessible? The fracture surface (Fig. 9.42) did not show the characteristic parallel marks caused by defects in the edge of a blade, but it did show striations similar to those seen on fatigue fractures. The edge of the surface (lower part in the figure) did show some damage, and X-ray analysis in the ESEM showed traces of sulphur. Since the junction in the fuel pipe was directly below the lead-acid battery, it was inferred that a small leak of battery acid

9.41 Fractured nylon connector (right) compared with intact return tube (left). Arrows show abrasion damage.

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9.42 Fracture surface of nylon connector with origin at lower right.

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initiated degradation at the corner of the joint. Nylon is especially sensitive to sulphuric acid, a strong acid which will hydrolyze it and other polymers rapidly. A crack started as damage progressed from the outer surface into the bulk, perhaps assisted by the internal strains produced by injection moulding. When tested separately, nylon degraded fast and then cracked, while the Rilsan or nylon 12 line proved totally resistant. The small movements of the fuel line when the truck was being driven were probably enough to induce small incremental growth steps in the fracture, as shown by the scanning electron micrograph of Fig. 9.43, until the tube finally parted. Leaking of the fuel will have occurred as soon as the crack penetrated the bore, but the rate will have been low. However, as the cracks grew, the rate will have increased (Fig. 9.44). The question of

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9.43 ESEM of final cusp showing intermittent crack growth.

9.44 Schematic diagram of failure sequence.

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how fast the rate increased was relevant, and it was possible to suggest that the major crack arrest points occurred when the engine was turned off. It was reasonable to suggest that this occurred daily, since many diesel work vehicles are left running for the whole day. Since there were seven such major arrests in the fracture surface, it suggested that the cracks had grown over about seven days, giving the driver ample opportunity to discover and rectify the leak before final failure. As a result, the insurers paid the injured motorist substantial compensation for her serious and at one point, lifethreatening injuries.

9.7

Conclusions

This chapter has highlighted cases of failure of car components which have caused accidents and personal injury ranging from poorly designed tailpacks for motorcyclists, badly made drive belts to the problem of old tyres and fuel line systems in new cars. The accidents which resulted from these components were avoidable, and should have been addressed either by the manufacturers and designers in the cases of the tailpack, drive belt and fuel lines. The drivers should have taken care in ensuring their vehicle was roadworthy in the cases of the tyre and the leaking fuel line on the recovery vehicle. And it is not as if such cases have not occurred before. The design of the Ford Pinto in rear vehicle shunts showed that the petrol tank could be pierced by sharp parts of the transmission system, resulting in vehicle fires, a problem that resulted in several law suits in the USA (12). Ralph Nader described in his book Unsafe at Any Speed published as long ago as 1965 the problems of poor safety design in cars (13), a general problem which has been addressed by most manufacturers in developing safer components such as laminated windscreens, air bags, collapsible steering wheels, radial tyres and the crash-resistant cell for the passenger compartment. The external features of cars, such as wing mirrors, radiator caps and front bumpers have also been redesigned to mitigate injuries to pedestrians in collisions. Safety engineering indeed has become a selling point for manufacturers, although there is still great room for further progress. For example, drivers tend to travel too close to one another, especially during high-speed driving on motorways. The behaviour can lead to the problem of catastrophic pileups during adverse weather (fog, heavy rain, road ice, etc.). A simple radar detector fitted to the fronts of cars would help warn drivers of their danger, but the device has yet to be adopted. The growing use of computer control systems for engine management and other basic functions may encourage manufacturers to adopt more extensive safety warning systems, however. One feature of the new ways of controlling cars is the use of sensors buried in the fuel system, for example.

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Another way in which such technology could be adopted to improve safety would be to have automatic pressure loss sensors to tyres, a device which would help motorists address the problem of under-inflated tyres, for example. Tyre damage increases greatly in such a condition, and increases the chances of a blow-out. In addition, fuel economy falls rapidly because more energy is expended in tyre deformation when tyres are under-inflated. Run-flat tyres have been developed by many tyre makers, but have yet to be adopted widely across the industry, perhaps owing to their expense. There is no doubt that the materials of construction have improved greatly, so that oxygen and ozone cracking are now rarely seen in tyre sidewalls, for example. Fuel lines continue to be a concern simply because any failure could cause leaks which can in turn cause fires or explosions, or a hazard to other road users in the case of diesel leaks onto the roadway. The case of the tractor fuel lines is a case in point, where greater attention to the problem of ozone cracking could have prevented a series of fires on vehicles close to airplanes. The investigation of vehicle accidents is almost a separate discipline in forensic engineering, such is the sheer number of fatalities and injuries from use of our roads (14). It uses methods such as dynamic analysis, trace evidence examination and materials testing to determine the precise cause or causes of accidents to help resolve personal injury actions. The design lessons have frequently been implemented by manufacturers to improve car safety, although the fall in fatalities over the last two decades also reflects the changes in public attitudes and legislative action to control user behaviour.

9.8

References

(1) National statistics available at www.statisics.gov.uk (2) McIntosh, D H, Meteorological Glossary, HMSO (Met Office, 1963) under ‘Beaufort scale’. (3) Naunton, WJS, The Applied Science of Rubber, Edward Arnold (1961). (4) BS ISO 19013-1:2005 Rubber hoses and tubing for fuel circuits for internal combustion engines. (Diesel fuels); BS ISO 19013-2:2005 Rubber hoses and tubing for fuel circuits for internal combustion engines (Gasoline fuels); there are many other standards for all types of hose, including hydraulic hose in a wide variety of industries supplied by British Standards. (5) BEA, Final report on the accident that happened to the Concorde registered F-BTSC operated by Air France on 25 July 2000 at Gonesse (France), (2004); available for download at: http://www.bea-fr.org/docspa/2000/f-sc000725a/ htm/f-sc000725a.html. (6) Information on the events of the accident and the consequences available at: http://en.wikipedia.org/wiki/Air_France_Flight_4590.

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(7) Discussed briefly in Lewis, Peter Rhys, Reynolds, Ken and Gagg, Colin, Forensic Materials Engineering: Case Studies, CRC Press (2003), p 194 ff. (8) Brydson, J, Plastics Materials, Butterworth, 7th edn (1999). (9) Information available at: http://en.wikipedia.org/wiki/Gotthard_Road_ Tunnel. (10) Lewis, Peter Rhys, Reynolds, Ken and Gagg, Colin, Forensic Materials Engineering: Case Studies, CRC Press (2003), p 199 ff. (11) Lewis, Peter R and Hainsworth, S, Fuel Line Failure from stress corrosion cracking, Engineering Failure Analysis, 13 (2006) 946–962. (12) Such as the case of Grimshaw-v-Ford Motor Co (1981) available at: http:// online.ceb.com/calcases/CA3/119CA3d757.htm. (13) Nader, R, Unsafe at Any Speed: The Designed-In Dangers of The American Automobile, Grossman Publishers, New York (1965). (14) Sherman, R M, Point of Impact: Case Studies of Forensic Engineering in Personal Injury Lawsuits, Lawyers and Judges Publishing Co Inc (2001).

10 Consumer products

10.1

Introduction

We are all consumers and so will be familiar with plastic products in our lives, whether as packaging of foodstuffs, enclosures of electrical products, clothing or those many useful gadgets and tools that ensure we can enjoy our leisure time and family life. Plastics seem ubiquitous and close to us because they are often the outward and public face of so many products, from TV sets and telephones to power tools and packaged products. Yet they are also performing a useful function in protecting the working innards, and giving us a way to handle a product easily and with confidence. They all exploit the low density of polymers, their special properties such as insulation against electricity or their low thermal conductivity. Above all, they are easy to mould into shape, especially the complex forms needed for many products. The variety of polymers of all kinds has increased dramatically over the last two decades, varying from high strength fibres such as aramids and commodity fibres (like nylon) to commodity thermoplastics like polypropylene and engineering plastics like polycarbonate. While it gives designers and manufacturers much greater freedom, it also demands that they make the products to a high standard and ensure that the products are safe to use in their expected environment by the consumer. It is also imperative that when stressed, those products are strong and tough and will not fail or fracture suddenly and put the user at risk of injury or worse. Traditional 3-pin plugs are among the most familiar objects, and will often break when dropped, but are easy to replace. However, the enclosures have become more complex, with fitted plugs being provided for most electrical goods, and transformer plugs used widely to supply low voltage power supplies for small consumer products. The enclosures frequently use engineering thermoplastics such as Noryl, a blend of polystyrene and polyphenylene oxide (PPO). The latter polymer provides a material more capable of resisting higher temperatures than polystyrene alone, and has good insulation properties. But processing to shape can be a problem owing to the need for high tool temperatures. Like polycarbonate, large residual strains can form in products moulded into cold tools. It makes moulded products sensitive to sudden brittle failure, and if in a plug, can expose the consumer to mains electricity. Even more serious problems can occur in insulating enclosures for busbar systems, although here it is 396

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electrical workers who are at risk of electrocution. A number of devices are designed to protect individuals from the same hazard, one is the residual current device or RCD, a device which automatically switches the current off if a leak is detected in the external circuit. They are now required fitments for all electrical circuits in new houses, for example. Their development was a major step forward in consumer safety and the mechanism can readily be incorporated in plugs for use outside, for example when powered gardening tools are being used. Problems have occurred not so much in their failure to function correctly (although they are not unknown), as in fighting patent restrictions for new designs of the switches incorporated in the plugs. Plastic electric kettles have become common in homes, replacing all metal kettles heated on an open flame, always a danger to the user, since metals are excellent heat conductors, so the user can be burnt from the very hot exposed metals surfaces. Thermoplastic materials with high melting points (such as polypropylene) can contain boiling water effectively and the polymer has been widely adopted for kettles. The design freedom in adopting an injection moulded shell has allowed devices such as sight tubes to show the user how much water is present, and safety switches are incorporated to switch the current off when boiling has occurred. Dangers can arise, however, when the inner mouldings needed for the switches are poorly made, and when existing intellectual property rights are abused by manufacturers in distant countries. One of the most surprising cases arose when an attachment on a luggage trolley suddenly fractured, and the bungee cord rebounded and the user lost an eye as a result of the impact. The accident was repeated, so raising the issue of the material used, the design and function of the product. The polymer parts were used to attach steel frames to one another, and the same principle is used in a range of consumer products. A frame used to carry bikes on the back of a car failed when the car was moving; the bikes were seriously damaged but fortunately no other cars were involved. Polymers connectors are also used in baby cots, and when one failed, the baby fell from it and broke his arm. Such parts need very careful design and manufacture since they are safety critical.

10.2

Failure of Noryl plugs

The problem was first recognized by electricians working in an old people’s home in early 2000. In the worst cases, the Noryl enclosures (base and cover) cracked and separated, so exposing live parts to the user’s fingers. The cover and base were welded together around the top of the pins to enclose the inner transformer completely (Fig. 10.1). The company managers moved very quickly to rescue the situation by removing known fitted

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10.1 Transformer plug showing innards.

units and started to investigate the problem. They instructed three organizations, The Welding Institute, Nissan Arc Labs in Japan and us to examine the failed plugs and report back to them. The problem was destined to be quite complex, since the plugs were sourced in Japan but parts made and assembled in China. The base and cover were injection moulded in Shanghai. The first task was to establish traceability, and it proved excellent owing to detailed records kept by the company. The date of assembly was established from logos on each plug (Fig. 10.2). Bases and covers from cracked plugs seemed to have been made in just two weeks of moulding in August 1999. There were some missing moulding records, and while waiting for them to arrive, a study of the cracked enclosures was started using microscopy and routine analytical methods for the polymer parts. Quality inspectors, once alerted of the problem also spotted partly cracked plugs, which were sent for inspection. New intact plugs made in 2000 were provided for comparison.

10.2.1 Microscopy One common mode of failure did not involve separation of the base and cover but rather sub-critical cracks from obvious stress concentrators like the holes for the pins in the bases (Fig. 10.3). The corners mated precisely with the sharp corners in the brass pins with a radius of about 0.05 mm, making them good initiators for brittle cracking. Weld lines were also visible in the bases, distinguishable from true cracks at higher magnification (Fig. 10.4). There were also signs of contamination on the outer surfaces with associated brittle cracks (Fig. 10.5 and close-up shown in Fig. 10.6). The total fractures were mainly circumferential on the ultrasonic weld line, itself an obvious weakness (as in most welded products). Although most of the brittle cracks occurred in the base and weld, some were also seen in the

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10.2 Base (B) and casing (A) showing welded joint.

case, mainly cracks running diagonally along one or more of the flat faces next to and often running into the weld itself. The weld cracks were quite brittle with absolutely no sign of ductility whatsoever (Fig. 10.7), showing that fast growth had occurred not just within the weld line but also adjacent to it in bulk material. Examination also revealed at least one poor design practice of sharp interior corners that could seriously weaken the product (especially when the product was dropped). Of the thirteen rejected products we examined first, only four showed no obvious signs of cracking. However, when dropped onto a hard floor from a height of one metre, cracks were produced (the specification called for a drop from 70 cm). Both base and cover had been injection

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10.3 Weld lines and brittle cracks in base moulding of Noryl plug.

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10.4 Close-up of weld line and cracks next to earth pin.

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10.5 Weld lines and cracks associated with surface blooming.

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10.6 Brittle crack within surface bloom.

10.7 Brittle cracks in ultrasonic weld; sharp inner corner also visible.

moulded from pin gates at one point on the sides, and brittle cracks were evident at or near these gates. Frozen-in strain is always greatest at the gates if for any reason cold tools had been used. One of the labs involved, Nissan Arc in Japan, performed liquid exposure tests on the welded enclosures. They dipped the products into a bath of tri-butyl phosphate, a known crazing agent for Noryl. Brittle cracks appeared during these tests, tending to confirm that high levels of frozen strain were present in the rejected batch of products. So the first round of trial inspections had suggested several different theories as the cause of the cracking. They included from the other labs: • • •

poor welding use of undried material loose transformer inside could impact casing and cause failure.

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Forensic polymer engineering

The first possibility was suggested by The Welding Institute (naturally), and the latter by the Japanese workers. Noryl needs good drying before moulding, but only one sample appeared to show the characteristic marks shown by wet polymer. The final suggestion was less plausible, since although the transformer was heavy, it was difficult to see how a loose device could create brittle cracks. To that list we could add our hypotheses. They included: • • • •

material contamination environmental stress cracking faulty injection moulding (low tool temperatures) excess recycling rate.

Recycling is often used by moulding shops but must be limited owing to the build-up of degraded polymer which inevitably builds up with time. ESC was also a possibility if any stress cracking agents had come into contact with the casings.

10.2.2 Material analysis It was perhaps inevitable that one of the labs involved would focus on a material failure, working from the observation of surface contamination on some plugs, for example. They obtained an FTIR spectrum on an ethanol extract of the material which appeared to show the presence of TCP, or tricresyl phosphate in the material. The compound is a well-known fire retardant and is often added to polymers to increase their flammability resistance (Fig. 10.8). The apparent concentration of the compound appeared to be highest in failed plugs, so they concluded that the cracking was caused by excessive levels of added phosphate. In our own tests, we compared the spectra and thermal behaviour of cracked and good plugs, and failed to find any significant differences between them. FTIR spectra were normal when compared with standards, and we could find no traces of phosphate in any samples. DSC showed that the there was a single glass transition point between 139 and 146°C, corresponding to a PPO content of 40–45%. Given the clash of evidence, the Japanese and ourselves were asked to analyze three different raw material suppliers of Noryl to see if there were significant differences in the materials. For example, Nissan Arc claimed to have found high levels of TCP (ca 4.5%) in a Dutch sample compared with only about 0.84% in a Japanese sample (using their ethanol extraction method). We decided to use an independent and more direct method: taking very thin shavings of Noryl direct from plugs. The method has the advantage of eliminating any possible contamination by solvent extraction methods. In fact we only found one difference between the three different

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33.91 4000

3500

3000

2500

2000

1187.1

Flameproof additive (Phosphoric acid ester or TCP)

P-O C-OP

1487.8

1590.6

This indicates water

1500

962.9

C-H

1437.0 1378.7 1302.8 1265.7 1161.9 1130.0 1072.1 1010.4995.3 1024.2 904.9 859.6 830.4 772.6 754.9 687.5 516.3 580.4 517.4

↑ OH

3069.8 ⎧ 2954.2 ⎨ 2920.7 ⎩ 2857.4

3336.4

99.56 %T

P-O-C –1

1000 cm

500

10.8 FTIR spectrum of suspect product Noryl indicating presence of phosphate fire-retardant.

49.05 %T

Discrepancy

0.10 1200 1150 1100 1050

1000 950

900

–1

850 cm 800

X: Noryl (made in Netherlands) Y: Noryl (made in EC) Z: Noryl (made in Japan)

10.9 Comparison of FTIR fingerprints of different Noryl sources.

polymers in the fingerprint region (Fig. 10.9). The Japanese and Netherlands batches of granules were almost identical but differed from the EC batch, a conclusion supported by DSC analysis (Fig. 10.10). The curves were quite different, a Tg being detected at about 100°C in the EC sample, which could be interpreted as phase-separated polystyrene. The FTIR spectra allowed identification of the polymer used, and one sample (no. 25) showed that the cracked cover was EC material, while the intact base was of Dutch or

404

Forensic polymer engineering .75

Heat Flow (mWatts)

.50

Tg = 104.48 °C 100.32 °C 104.55 °C

.25 0.00 –.25 –.50

Japan Netherlands

142.03 °C Tg = 149.86 °C EC 157.40 °C

130.59 °C

–.75

–1.00

Tg = 143.60 °C

156.04 °C

–1.25 –1.50 40

60

80

100 120 140 160 Temperature (°C)

180

200

10.10 Thermograms of different granules compared.

61.81 %T

JAPAN

EC

0.04 1200 1150 1100 1050 1000

950

900

850

cm–1

800

10.11 Comparison of FTIR spectra of plug no 25 (cracked cover only).

Japanese origin (Fig. 10.11). However, a second sample (no. 5) showed the cracked cover to be either Japanese or Dutch, while the intact base was of EC origin (Fig. 10.12). The results did not support the Japanese conclusions. A further test was used to check the FTIR and DSC tests using the independent method of X-ray analysis in the ESEM. The spectra did not show any high levels of phosphorus in any of the samples of granules or cracked and intact products (Fig. 10.13) and detailed analysis showed similar levels (if the figures were reliable) in all the samples, irrespective of origin (Table 10.1). So although contamination of the outer surfaces of a few plugs had been seen, it was likely to have been as a result of accidental and rare spillages

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52.35 %T

0.10 1200 1150 1100 1050 1000

950

900

–1

850cm 800

10.12 Comparison of fingerprints of sample no 5 (cracked cover only).

Spectrum: SAMPLE4A

Range: 10 keV

Total Cnts = 26732 Linear VS = 200

C Kal 0 Ka1

150

100

P NaKa1 AlKa1

Cula

50 CuLb1 CuLa2 CuL11

0.0

AlKb1

1.0

0

2.0

3.0

4.0

10.13 EDX spectrum of a Noryl sample.

Table 10.1 EDX analyses of various raw materials Sample Sample Sample Sample Sample

no. no. no. no. no.

1, 2, 3, 4, 5,

average average average average average

P P P P P

content content content content content

= = = = =

3.22% 3.26% 3.21% 3.01% 4.80%

EC

of an unknown fluid. The spill encouraged ESC because of the high level of chain orientation in the mouldings.

10.2.3 Injection moulding conditions It was now much more important to examine the moulding records from the Chinese company that made the base and cover. Detailed records were

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Forensic polymer engineering

Table 10.2 Injection moulding conditions in China (suspect mould temperatures in bold) Mold condition list Issue date Customer Mould part No. Mould machine No. Machine model No.

99. 6. 20 Da Hong 3R-B1-0074 15 SM150T

99. 11. 16 Da Hong 3R-B1-0074 38 SM150T

99. 6. 8 Da Hong 3R-B1-0186 25 SM180T

99. 11. 22 Da Hong 3R-B1-0186 35 SM180

Part name Material Material No. No. of cavity

Bottom case PPO SE1-701 1×4

Bottom case PPO SE1-701 1×4

Upper case PPO SE1-701

Upper case PPO SE1-701 1×3

90% 280 + − 280 + − 270 + − 260 + − 8 +− 2 17

65 280 + − 15 275 + − 15 270 + − 15 260 + − 10 8 23 + − 5

35 5 120

90% + − 10 C 280 + − 10 C 275 + − 10 C 270 + − 10 C 260 + − 10 C 9 +− 2 17 2 38 + − 4 5 85 + − 10

29 10 98

80 C 300 + − 300 + − 295 + − 275 + − 10 25 10 53 5 80

Cylinder temperature DH 1st step 2nd step 3rd step 4th step Injection time (sec) Cooling time (sec) Rest time (sec) One cycle time (sec) Back pressure (Kg/c) Injection pressure

10 10 10 10

C C C C

C C C C

5 5 5 5

C C C C

Mould die temperature Fixed part Moving part Cooling method Fixed part Moving part

45 + − 5 C 40 + − 5 C

73 + − 3 C 82 + − 3 C

43 41

90 + − 5 C 93 + − 5 C

Water Water

Oil Oil

Water Water

Oil Water

Pre dry Inserting metal

105 C 4 hour No

100 C 4 hour No

102 C 4H No

100 C 6 hours No

sent for examination (Table 10.2). They showed that although most conditions were invariant, there had been some key changes during the period of interest. Most critically, there had been unexplained variations of the tool temperatures for both the base and cover. Although there were no setting sheets from the period in August 1999, when the cracked products had been moulded, the sheets from before and after the period were available. The data showed that the base was always moulded under cooler conditions than the cover, and that the coolest conditions occurred in June 1999, when tool temperatures of 40–45°C were used in two batch runs. In other periods, especially in later months, much higher temperatures were used. In November 1999 they were up substantially to a maximum of 90–93°C, for example.

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What did the manufacturers recommend? Their advice is very specific when it comes to Noryl moulding conditions (1): The best aesthetic and mechanical properties will be achieved when tool temperatures of 80–120°C are used. That advice had clearly not been followed in China for the crucial two weeks production in June, and may indeed have actually been lower in August as well. A further complication was also discussed: the problem of recycled polymer, where runners and scrap products are reground and then added back into the granules ready for fresh mouldings. A high rate of 20% had apparently been used in the factory. The same technical brochure advised that impact strength could be affected deleteriously if high recycling rates are used with Noryl. The problem arises because levels of degraded and low molecular weight polymer will build up in mouldings.

10.2.4 Conclusions We advised the supply company that the problem of cracked plugs had been caused by poor moulding, especially by low tool temperatures, which produced unacceptable levels of chain orientation in the products. We recommended that recommended conditions always be followed and that a new drop impact test be used to assess product quality (up from 70 to 100 cm drop height), as well as good practice adopted in tool design (such as eliminating sharp inner corners). The problem did not recur with new moulding conditions established in late 1999, and it highlighted: • •

the problem of frozen strain in moulded polymers the globalization of production and consumption.

The problem is much wider than appreciated because it is not immediately obvious either to the machine operator since mouldings appear correct and dimensionally sound, or to QC inspectors if not testing mouldings to high standards. The second problem is even more widespread owing to the dispersion of manufacturing around the world, where products are made often thousands of miles from the countries where they are sold. If mistakes occur during manufacture, then the effort needed to correct the problem is magnified by the geographical separation between the maker and the user. Then there is the problem of different labs being consulted in the different countries involved, in this case, China, Japan and Britain. Although only labs in Japan and Britain were involved, they produced quite different conclusions and led to extra research that was not needed in the final analysis. On the other hand, modern communications by fax and email did enable basic data (especially the setting sheets) to be transmitted quickly to the investigators.

408

Forensic polymer engineering

Other labs have investigated not dissimilar problems with other plugs. RAPRA, for example, reported on a widespread failure of such Noryl plugs in a compilation of case studies (2). The plugs also cracked with users, and the problem was so extensive that 40 000 needed replacing. The investigation showed that excessive compressive stresses were used in welding base and cover together, producing a tensile stress acting at right angles to the weld in the final product. All the failures were at or near the weld, unlike the more extensive signs of cracking in the case study already discussed. ESC was induced by wiping the product with a light fluid, and the solution to the problem was straightforward: modify the welding process and eliminate the cleaning step with fluid.

10.3

Failure of Noryl busbar plugs

Both plugs in the previous examples were safety-critical for the end user or consumer, but electricians can also be exposed to risk when inspecting busbar systems. These are the circuits used in buildings to feed ring circuits, and they work with higher currents. The busbars themselves are wide strips or bars of copper to carry the current. A company in Milton Keynes assembled the plugs from mouldings made elsewhere by screwing base and cover together with the necessary wiring. The busbar system they made connects to 13 amp ring mains by the plug rated at 32 amps (Fig. 10.14), and it was these Noryl plugs that started cracking (Fig. 10.15). In the example shown in the photograph, radial cracks had formed in each of the screw sockets that held the plug together. Close inspection in fact showed that a brittle crack had grown from one of the screw holes along the adjacent sharp corner of the base resulting in almost complete separation (Fig. 10.16). Some remnant of ductility was shown by the lighter coloured line at the bottom of the fracture.

10.14 Base unit linked to busbar system with 32 amp plug at extreme right.

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10.15 Busbar plug rated at 32 amp showing brittle cracks at screw holes in base.

10.16 Brittle crack penetration of base from screw hole with remnant of ductility at bottom (white shear lip).

In other examples returned by users, radial cracks in screw bosses had grown into the shell of the plug, endangering the entire product (Fig. 10.17). The very sharp internal corners are noteworthy, although no cracks had formed here. In this failed sample, the crack had grown from the screw boss into the adjacent wall, although the opposite boss had simply cracked radially without further crack growth. The cracks had formed when the self-tapping screws had been used to close the plug, and it was simple to lower the diameter of the screws to try to limit the damage. However, this policy only tackled the symptom rather than the root cause. Several causes could be eliminated, such as ESC, fatigue and abuse in service. DSC and FTIR confirmed that the material was indeed Noryl, although there was evidence for a degree of phase separation in all the

410

Forensic polymer engineering

10.17 Brittle crack growth in cover of plug; intact cover wall at right.

samples (intact and failed plugs). They showed a separate Tg just below 100°C indicative of a distinct polystyrene phase, which would normally be absent. The curves were very similar to the upper curve of Fig. 10.10 for a Noryl sample from the EC.

10.3.1 Quality control Records from the moulders showed that a simple quality test was used on first-off and occasional samples taken during a moulding run: a screw test on the plastic pods. However, brittle cracks may grow slowly and be missed on fresh mouldings. Their setting sheets proved less than informative, but appeared to show that the tools were ‘cold’. That meant high levels of chain orientation, and the possibility of brittle cracks, especially from the screw pods where screws put the polymer under very high stresses locally. Indeed, microscopic examination showed that the brittle cracks seen first (Figs 10.16 and 10.17) had grown from damage produced in the pods where screws cut into the material. The company were advised to check with their suppliers that tool conditions were as recommended by the manufacturers, and the problem was resolved amicably. New designs of plugs in Noryl polymer apparently gave large cost savings by using injection moulding rather than compression or transfer moulding. The walls are comparable with those in conventional 13 amp plugs in thermosetting phenolic resin of about 3 mm. However, transformer plugs have to resist much greater drop loads owing to the weight of the innards, and so have to be more robust to prevent exposure of live leads. As with battery cases, care is needed in maximizing the strength of the product by ensuring processing is of the highest standard and that the geometry avoids serious stress concentrators.

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411

Residual current devices (RCDs)

Reducing the chances of electrocution from electrical products has always been a high priority for designers, although it is only quite recently that devices have become available to stop current flow harming people when there is a leak in the external circuit. The original ‘fuse’ invented by Thomas Edison can only protect equipment, owing to the time to melt the fuse wire: it is easy to be electrocuted in the interval. The protection comes from a residual current device (RCD), which works on electromechanical principles to disarm a live circuit when a leak occurs (3). They come in many forms and are small enough to replace fuse wire in control boxes in houses, for example (they are mandatory in all new houses). External use is possible with a protected plug (Fig. 10.18) so that gardeners and home handymen using electric tools are also protected. The plug (Fig. 10.19) encapsulates a small electromechanical switch which must be primed manually by pushing a button to activate the mechanism (A). When attached to the mains supply, a solenoid within the device holds a short arm which is pushed into it by the activation force. The set of levers is then held by a spring (B) in the activated state, hinged about a fulcrum (C/D). Fig. 10.20 shows the mechanism in outline in the activated state. If a leak in the outer circuit (such as a cut lead on a hedge-trimmer) is detected, the solenoid current drops and the short arm is released and triggers release of the levers by impact against the large lever (C), breaking the contacts at right. The device acts faster than the time needed for electrocution, so the user is fully protected.

10.18 Residual current device fitted into a 13 amp plug: section shows mechanism.

412

Forensic polymer engineering A

C D

B

10.19 Section of an RCD showing mechanical switch.

Actuator assembly

S1

Lever assembly

R4

R2

2R1

R3 S2

S3

M Solenoid assembly

10.20 Section of an RCD showing primed mechanism and balance of forces in the equilibrium state.

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10.4.1 Patent action A monopoly on the principle of the device was held by one company in the 1990s, the device being known as the Powerbreaker. It was challenged by an entrepreneurial New Zealand company when they introduced the device (the Protector) shown in Fig. 10.18. The clash between the patents covering each device hinged on the exact way each operated. The Powerbreaker operated by levers linked together, with the short arm in the solenoid linked mechanically to the other levers in the system. By contrast, the arm in the Protector operates independently and is quite separate mechanically, triggering the mechanism by impact. The dispute came to a head in 1996 in the Patents County Court in London, and we gave evidence on the way the mechanism operated. The basic action that differentiated the two devices was at the heart of the argument, and the mechanics were researched in detail by engineers for the defendants, makers of the Protector. They performed static analysis of the equilibrium of the mechanism when activated, and we confirmed their assumptions regarding spring constants, for example. The mechanism worked in about 30 milliseconds, well below the threshold of about 50 ms for electrocution. It became clear during the course of the action that the Protector worked in a different way from the Powerbreaker, and thus was an improvement patent. Both devices were electromechanical in action, the one working via linked levers only, while the other involved a physically separate arm working by impact. Since the wording of Claim 1 of the earlier patent clearly specified linked levers only, the later development stood outside its boundaries. The final judgment declared the original patent valid but not infringed, vindication for the new design (4). Since then, competition between the two products has ensured a drop in price, so that new RCD plugs now retail for only a few pounds. Prior to the action, they sold for £15 or more. It has meant that consumers will buy and use the plugs much more readily, and so protect themselves from the hazards of cut wires, or water creating unexpected pathways in live equipment. More designs have also entered the market (from the Far East and China), providing yet more competition. Some of the new models use a polycarbonate casing robust enough to withstand the impact blows it will receive in service. However, the plastic mouldings which form the lever arm assembly of the Protector and lookalikes can and do break prematurely. The mechanism fails completely and operation of the ‘test’ button (Fig. 10.18) does not work, and the mechanism cannot be primed into the activated position at all. Since the contact with the mains supply cannot be made, there is no power available at all from such a plug. In other words, the device is failsafe because it has become inoperable and must be replaced for the electricity supply to be tapped safely again. It is a primary aim of all safety devices

414

Forensic polymer engineering

that, should they fail, then the user must be aware of that failure, and that the failed device cannot itself represent any hazard. Not all safety products meet those criteria, a famous example being the first version of the Davy safety lamp. The product relied on a fine gauze covering the open flame to allow methane gas in to burn safely inside the lamp, but yet not allow the flame to escape. Davy found that there is a safe spacing of the squares in the gauze, but if it increased, then the lamp became unsafe. Since the gauze was made of iron, it rusted easily and quickly in the damp environment of collieries, and gauze breakage was common. That made the lamp unsafe, and prompted many to redesign the device to ensure greater safety.

10.5

Failure of kettle switches

Another popular consumer product is the simple kettle for boiling water, until quite recently an all steel or aluminium product used on open flames. The main problem with using metals lies in the risk of scalding the user, either by applying the hand directly to the metal handle or through contact with the container. The plastic electric kettle resolves those problems by using an insulating material like polypropylene for the body and an internal heating coil, and has been very successful in capturing much of the market previously dominated by metal kettles. Safe operation of electric kettles depends on the thermostat switch housed in the base of the product, usually near the cable entry. The designs are protected by patents held by British companies, who have been much concerned by attempts by rogue Chinese manufacturers to copy their designs for the switches and avoid licence fees. The problem of patent evasion is extensive in many other products, especially by Far Eastern manufacturers who may be difficult to locate and sue. However, US and EC manufacturers have been active in pursuing pirates in cooperation with the Chinese government, for example by examining possible infringing products from patent infringement. They can also demonstrate to infringers the dangers of working without a licence because licensing an invention brings positive benefits in terms of knowledge transfer and know-how to the licensee. Unlicensed manufacturers can make switches which are flawed by poor moulding or design practice, for example. We were asked to examine an unlicensed Chinese switch and compare it with a standard switch, especially for the quality of the thermoplastic material used in its construction. DSC showed that both used glass fibre reinforced nylon 66, but different fire retardants had been used in each device. The additive is needed in order to limit damage in the unlikely event of a fire occurring should the switch fail. Inspection of the unlicensed switch showed that the fire retardant in the unlicensed switch acted by producing gas at a relatively low tempera-

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ture. The thermogram of the standard product (Fig. 10.21) showed a single and distinct melting temperature at about 265°C, while the unlicensed switch showed a smaller endotherm at a slightly lower temperature of 260°C and a large decomposition peak above 350°C (Fig. 10.22). The fire retardant starts decomposing while being injection moulded, so creating switches with large voids (Fig. 10.23). Other unrelated defects were seen on the metallic contacts of the device. Such defects could lower the safety margins on a product that consumers rely upon to perform their function safely and efficiently. By cooperating with the lead designers, other manufacturers could benefit greatly in improving their products, a lesson more widely applicable to many other products. Globalization of production carries in its wake responsibilities for ensuring that valid patents are recognized, and there is much experience

2 mW

Glass transition Onset 44.73 °C Midpoint 47.25 °C Integral normalized Onset Peak

–205.10 mJ –26.64 Jg^ –1 254.67 °C 264.65 °C

–50

0

50

100

150

200

250

300

350

°C

0

5

10

15

20

25

30

35

40

min

10.21 DSC thermogram of switch material.

Ar, 80.0 ml/min

Glass transition Onset 101.60 °C Midpoint 114.42 °C

5 mW

Integral normalized Onset Peak

–108.65 mJ –15.30 Jg^ –1 250.07 °C 260.66 °C

–50

0

50

100

150

200

250

300

350

°C

0

5

10

15

20

25

30

35

40

min

10.22 DSC thermogram of Chinese switch showing anomalous endotherms.

416

Forensic polymer engineering

10.23 Defects in unlicensed switch from electric kettle.

which can be accessed by licensing. One example highlighted by earlier case studies concerned mining lamps, and these are made in several countries supplying the mining industry worldwide. The hard practical lessons learnt during their development in the UK has been of great benefit to those new industries, and so avoiding the failures experienced when those products were first introduced in Britain.

10.6

Failure of fittings on luggage carriers

Serious accidents with luggage are unexpected events which can produce traumatic effects, and in some cases personal injury. An example from a previous chapter cited the case of a bag which became detached from the rear of a motorcycle when travelling fast. It met the rear wheel and caused the bike to skid out of control, while the following bike also skidded and the driver lost a leg when he was thrown clear and into the roadside barrier. Bungee cords were used to hold the bag to the bike and they proved to be unsatisfactory in holding the bag down securely. An earlier accident was investigated which also involved bungee cords holding down luggage on a small carrier. The carriers are familiar to all travellers, and consists of a folding metal frame fitted with two wheels at one end, the upper arm of the frame unfolding to form a handle for the user to pull the trolley along. It is locked by a plastic clip. The lower part of the frame folds out to form a platform on which luggage can be placed, and is kept in position by a bungee cord attached to a plastic fitting riveted to the frame. The bungee cord is strained over the luggage and the hooks at either end linked to the frame. The plastic fitting has two feet so that it will rest against the floor when at rest. When not in use, the frame can be folded up compactly for storage (Fig. 10.24).

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10.24 Folded luggage trolley showing fractured plastic part.

10.6.1 First accident An accident occurred in 1985 shortly after the carrier had been bought three days before, and was being used to carry luggage. As one might expect, it had been used for carrying about 30 lbs of luggage, had been folded, and used again. It failed suddenly when the user was in a lift, (so not under any extra loads from movement or impact), the bungee cord sprang back and the broken plastic end hit the user in the eye. Despite several operations he later lost his eye and sued the supplier and manufacturer. The plastic component had broken across the centre line where it was riveted to the metal shaft (Fig. 10.25). The fracture appeared very brittle, and it was the sharp edges to the break which caused the damage to the user.

10.6.2 Fracture and other surfaces Examination of the fracture surface showed several revealing features (Fig. 10.25). The fracture surface could be divided into four zones (K1 to 4 in the figure) separated by the two rivet holes, and a clear crack junction shown by the arrow in the picture. There were also several polished zones near the lower edge of the sample where it abutted the steel frame (D).

418

Forensic polymer engineering

K1

D

K2

K3 D

K4

D

10.25 Macrograph of fracture showing four sub-surfaces (K1–4) and smooth areas (D).

W

G

10.26 Base of plastic part showing abrasion damage, gate (G) and weld lines (W).

The surface was splattered with small spots of white paint, apparently when being stored in an office prior to our examination. The end showed that the prominent blocks had been deeply abraded by contact with the ground, as would be expected since they would have supported the luggage when the trolley was at rest (Fig. 10.26). When the bungee cord was removed, it exposed the gate from which molten polymer would have entered the tool during processing to shape by injection moulding, as well as several weld lines formed around the gate by cooling polymer not fusing correctly. The weld lines were more extensive over other parts of the failed component, and one could be seen aligned and merging with the main fracture surface on the side of the part (Fig. 10.27). The component was moulded into a four cavity tool, and the failed part came from cavity No 4 of the tool. New samples were requested from all cavities, and comparison showed weld lines were present on all the samples. They all showed signs of distortion as well as splay marks on one sample, indicating wet polymer. The sample from cavity 4 showed the most extensive set of weld lines, sink marks and general distortion. The material was high ethylene content (ca 7%) polypropylene, which is normally tough and ductile, but as with all such materials, brittle behaviour can occur from design defects, poor moulding practices, contamination and so on. But

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W

W

10.27 Weld line next to fracture surface at left, isolated weld line at right.

degradation at least could be ruled out, because the FTIR spectra showed no trace of oxidation and DSC showed nothing amiss in the thermal behaviour of the failed or intact components. The data were compared with standard granules of the polymer with good correlation. There was some evidence for a whitened origin in K1, but did not appear in the other parts of the section, so contamination seemed unlikely, but it was a conclusion which was modified after the second accident.

10.6.3 Failure sequence It was most likely that the weld lines had been opened during the riveting operation at assembly which put a high compression load on the component. The open cracks would have been quite invisible to external inspection, since the outer polished areas (D) do not run into the external surfaces, but are hidden inside the device (Fig. 10.25). In any case, this part where it is joined to the frame has the smallest cross-section of the component, so will be the weakest part of the whole component when subjected to bending from the bungee cord. The overall cross section measured 70 by 25 mm, while that of the joint was 70 by 7 mm, so formed only 28% of the total section area. When the cord was fully stretched, it would exert a tensile load of about 5 kgf on the section. All loads would have been concentrated here, where the round holes for the rivets concentrated the local load yet further by about a factor of three. However, the cracks appeared to originate at or near the tips of the open weld lines, so they were the more serious stress raisers in the system. Those open cracks lowered the section even more since they were so large and deep. The deepest weld line in Fig. 10.25 lay near K2 and penetrated the section by about a third, so was the most likely point from which failure of the component started. Once fast fracture started here, the rest of the section parts would quickly follow from their shallower weld lines in a chain

420

Forensic polymer engineering

reaction. The severity of the defects explains why it failed when the trolley was simply standing in a lift, although no doubt the intermittent loads when moving helped to pull the weld lines further apart. The company making the trolley made several points in its defence, mentioning that over 100 000 had been sold into the market without reported failures of this kind. If the accident had indeed been unique, then it pointed to a sudden deterioration of moulding conditions, perhaps s symptom of wear-and-tear in the tool used to make the parts. Such wear is inevitable after long runs, and can be seen by a fall in the quality of the mouldings with time. A well run QC department should spot this kind of problem by preserving original mouldings as well as by careful testing of a sample of parts taken from batch production runs, for example.

10.6.4 Conclusion Since the case was already being contested by the defendant moulding company, an opposing view of the accident was produced by independent engineers. Their report was disappointing for the absence of any serious study of the failed product at all. Rather, it was a critique of our work with many uncorroborated statements. Such reports are not uncommon unfortunately, but if the case had gone to court, would not have withstood scrutiny by an alert cross-examiner. They concluded that the company were not to blame and the failure was attributed to poor assembly by the manufacturer, and abuse by the user. The latter allegation is also very common, but ignored the evidence of failure within three days of purchase and our direct examination of the abrasion evidence from the failed component (Fig. 10.26). The independent engineers seemed to have little knowledge of the way products are moulded and the defects of poor mouldings. It was apparently a unique failure that could be explained by a maverick moulding. Weld lines can vary greatly in position even when in a single batch, and it was the coincidence of several weld lines with the smallest section which produced such a poor component that it failed within a few hours of first use. But shortly afterwards, there was dramatic confirmation of our conclusions, which led to final settlement of the case.

10.6.5 Second accident Alarm bells rang at the insurer’s when an identical trolley failed in exactly the same way and injured another user (Fig. 10.28). The first accident might be a quirk of fate, but a second suggested a serious underlying problem. The risk of yet further incidents rose sharply, a warning no insurer could ignore. Although the effects were less serious, the user was still severely injured when the sharp plastic part sprang back suddenly into her face.

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10.28 Second failed trolley.

Because she was taller than the previous victim, the end hit her in the mouth rather than her eye, but she still needed medical treatment with stitches to close the open wounds. The accident happened four years after the first, in 1989, so was recent and the evidence correspondingly fresher. The failure prompted rather more extensive research into the quality of the material of the mouldings as well as more detailed microscopy, both areas being easier to conduct given earlier experience with these mouldings. It was clearly vital to get to the heart of the matter and so prevent further failures.

10.6.6 Fracture surface The fracture occurred in exactly the same way as the first, although there was no trace of opened weld lines at the base of the section (Fig. 10.29).

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O1

O2

O3

O4

10.29 Fracture surface of second trolley component showing various origins.

10.30 Optical micrographs of origins O1 (upper) and O2 (lower) showing contaminants.

But there were clear and distinct origins (O1–O4) in each of the four sectors marked by convergence of hackles. Close inspection using the optical microscope showed distinct particles at each of the four origins, those from O1 and O2 being shown in Fig. 10.30. The particle at O1 seemed dark in colouration and was surround by a white or grey halo, while the particle at O2 appeared to be a tiny granule of clear or white appearance. It was also surrounded by a lighter halo, as well as black particles and reflective particles, which could actually be seen by the naked eye when turned in the light. SEM was the obvious method to explore their nature. SEM proved very revealing (Fig. 10.31). The particle at O1 was actually a porous material within which was embedded a solid particle with the distinctive fractures of broken glass (conchoidal facets). A survey of the

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×150 100μm WD28

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10.31 Foreign particles at O1 (upper) and near O1 (lower) as seen in the SEM.

adjacent surface showed yet other particles which appeared more rounded, suggesting river worn sand surrounded by a void. Such particles should be susceptible to X ray analysis within the instrument and the resulting very simple spectrum confirmed the attribution of the central particle at O1 (Fig. 10.32). Despite being gold coated, the particle showed the characteristic peaks for silicon and oxygen as required by the formula SiO2 for silica. Other spectra showed more complex combinations of elements, such as those interpreted as alumina-silicates, minerals common in soil and rocks. Such particles weakened the material substantially because they acted as stress concentrators in the material. Voids could be formed as the plastic cooled after moulding since the coefficient of thermal expansion between dissimilar materials, such as glass and organic polymer, is very large. Such voids form around the particle and then act as stress raisers, the exact value of the effect depending on the precise shape of the void. The polymer was extracted with solvent and thin films cast for FTIR spectroscopy and comparison with standards. The tests showed the polymer to be identical to the previous trolley parts. DSC and GPC analysis confirmed the conclusion with an identical melting point and of similar molecular weight to the standard.

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15:36:05

particle Vert= 5000 counts Disp= 1

–100 secs Preset= Elapsed= 100 secs

Si

Au

0

0.120

1 Range= 10.230 keV

2 Integral 0 =

2.640 92994

10.32 EDX spectrum from particle at O1.

10.33 Debris from connector showing large beetle at lower right.

10.6.7 Contamination But the material was obviously badly contaminated, but where did it arise and why? One way of tackling the problem afresh was simply by complete dissolution of a component using hot decalin solvent, the same solvent used to make thin films for spectroscopy. Two samples were analyzed: a small (3 g) part of the failed product and a new intact part. The hot solvent was filtered and washed, and the debris collected for inspection (part of the intact sample debris is shown in Fig. 10.33). Although most of the debris

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collected proved to be carbon black filler, there was one notable exception: a large black beetle (lower right). The failed part was slightly more contaminated than the intact part at a level of about 0.1% by weight. More detailed examination showed the following variety of materials: • • • • • • •

metal shavings metal wire fragments glass particles sand glass fibre fragments plastic film fragments plastic fragments of irregular shape.

The beetle was tentatively identified as being suggestive of the family Staphylinidae or rove beetles, the largest member being the familiar Devils coach horse. So where did this strange collection of debris originate? The metal and wood shavings suggested a toolroom or workshop origin, although wear within moulding machines can also produce metal fragments. The duplex particle at O1 was probably a bitumen seal reinforced with sharp sand, and often used to seal concrete floors, suggesting an industrial factory such as the moulding shop itself. But why? It may be that operatives were encouraged to conserve polymer pellets by sweeping up spillages on the floor and putting them back into the hopper supplying the moulding machines. If that was the case, then it was deeply misguided policy because it weakened the products used in the trolleys and endangered users. The fact that similar contamination was found in both a failed and intact component indicates it was a policy used for some time. But the final irony of the case lay in the design of the trolley. The bungee cord which exerted the pressure on the plastic attachment need not have been tied to that device at all, so in a later modification, it was tied directly to the frame and became independent of the fragile plastic attachment. There was now no chance of the frame failing and the bungee flying up and hitting the user. One wonders why this simple solution was not adopted in the first place and so saved considerable anguish of two injured consumers.

10.7

Failure of ABS joints on bike carriers

Car use has grown greatly during the past few years, and with it the development of a range of accessories enabling leisure products to be transported easily on the roof or rear (Fig. 10.34). In this particular incident, the driver was returning home from a holiday in the Lake District in 1996 when the device suddenly failed and three bikes on the frame were thrown into the road. Fortunately there was little other traffic, and he was able to

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10.34 Bike carrier made of steel frame fitted to car after accident.

10.35 Close-up of clam shell connector on upper frame.

stop and retrieve the bikes before they could cause a serious accident. However, the bikes were badly damaged and he made a claim against his insurers. He said that he had strapped the bikes down to the frame as recommended in the instructions, but that the straps came away when the bikes fell. The insurers approached us to determine the likely cause of the failure. Bike carriers consist of steel frames assembled together and supported by the car (Fig. 10.34). The frames are connected together by two plastic clam shells at joints between the steel parts. A bolt passes through each assembled shell and both steel tubes. The edges of the shells are serrated and mesh with an identical set of teeth in the opposing shell so that the angle of the top bar which supports the bikes can be varied by the user (Fig. 10.35). The picture shows the intact but broken joint on the left-hand side of the top part of the carrier. The opposite joint had also broken but only one half had survived (Fig. 10.34). One of the shells or cups had been lost by dropping onto the road, and was not found. The two top joints had

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been fitted in reverse with the handles on the inside rather than the outside as recommended. FTIR spectroscopy showed that the polymer was ABS, a normally tough material capable of resisting imposed loads, although like all notionally tough materials, can suffer brittle fracture with intense local loading. There appeared to be no degradation of the polymer from the spectrum, and the thermogram appeared normal with a Tg at about 107°C. The packaging for the device stated that the material of the connectors was nylon.

10.7.1 Damaged shells The broken surviving joint was now the focus of attention, despite the fact that part of the opposite joint had been lost and where the failure may have started. Each of the cups comprising the joint were injection mouldings with a gate at one side of the shape (Fig. 10.36). Each cup was reinforced internally by a set of four ribs set at right angles to the semi-circular recess which enclosed the steel shaft to which the shell was attached. There was a central hole for the steel bolt, the shank of which was equipped with a single steel washer. The shank fitted well into the hole with little clearance, leaving no space for rubber bush or any means of absorbing vibrations from the car. However, it was noticed that the long bolt was slightly bent at one end, presumably as a result of the incident. The opposing bolt was also slightly bent as well. The outer edge of each cup comprised numerous triangular teeth, which meshed well with another set on the opposing cup to form a tight joint. The fractured cup showed a main fracture from the root of a tooth, a crack which had grown into the adjacent rib and along the part next to the

10.36 One shell still attached to frame showing serrations at edge and inner ribs.

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steel shaft but missing the central bolt hole (Fig. 10.37). It had branched several times. Part of the cup edge was missing, presumed lost. Another crack had been initiated almost diametrically opposite again at the root of a tooth, and it had also grown into the shell and part way along the shaft part, but halted. The sub-critical crack had presumably grown at the same instant as the main crack and been halted by loss of the main load of the bikes suspended on the frame. The main fracture itself was quite clearly an overload crack, since there was no trace of any striations or other indicators of fatigue (Fig. 10.38). The gate was situated about 90 degrees away from the crack origin, and a faint weld line could be seen near the central bolt hole, but was not associated with any cracks. The opposing cup showed no cracks at all. The single surviving cup from the failed joint opposite also exhibited no cracks but did show some damage to the sharp tips to the teeth. About ten teeth were damaged, probably indicating where the missing cup had jumped over them when the joint failed.

10.37 Brittle fracture of ABS clam shell from origin at tooth corner.

10.38 Origin of fracture from root of tooth in outer edge of shell.

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All the cups were examined for their design and any possible defects. There were no visible defects, although all showed very sharp corners both at the teeth roots and at internal ribs. Those on the roots varied from extremely sharp (ca 0.01 mm radius of curvature) to a larger value of about 0.05 mm. The value changed in a regular way around the circumference, and the tool was probably made by a CNC machine. But the failure in the opposite cup had occurred at a root of about 0.05 mm rather than the lower values.

10.7.2 Stress analysis Knowing the total weight of the three bikes of about 45 kg, it was possible to analyze the loading situation. The total moment acting on the joint allowing for the different distances at which the bikes were suspended was about 95 Nm, reducing to about 89 Nm to allow for the angle of inclination of the bars to the horizontal (20 degrees). So the load on each shell was calculated at about 110 kgf, a large value owing to the leverage effect of the suspended bikes. But that load would have been shared by the teeth edges, so the net effect on any one tooth would have been affected by the localized strain on the teeth at maximum loading. Although the teeth meshed closely under no load, the effect of loading could have concentrated load much more locally. The abrasion marks on the isolated cup was suggestive but not conclusive, since some such damage would be expected when the teeth slipped past one another. What is certain, however, is that the root of the teeth would have acted as serious stress raisers, the exact value depending on the radius of curvature at the root. One estimate using a diagram from Peterson (5) suggested a value as high as Kt ∼ 12, but such was likely to be an over-estimate since surrounding roots would lower the net effect (6).

10.7.3 Conclusions Since the failed cup was missing, it proved impossible to pinpoint the precise failure mode, a not uncommon event in many accidents. However, it could well have failed by fatigue from a root on the missing part. Since the radii there varied greatly, it is possible that the very sharpest roots were present at the position of maximum load and a fatigue crack grew from that point. The entire frame would transmit much of the vibration to which cars are subjected during normal driving, let alone driving on rough roads of which there are many in the Lake District. Although rubber pads were provided where the frame met the car boot lid, there was very little to prevent vibrations from reaching the upper part of the frame and the supported bikes.

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However, could failure to erect the frame correctly have caused failure? There was no evidence that the bolts had slipped round, so was an unlikely possibility. The Italian company who made the frame pointed to this explanation in their response, but it could not explain the fracture of one cup and the likely fracture of the other and its loss from its position. They insisted that they had always used ABS for the part despite the statement on the packaging, and further stated that the design had been approved by an independent test house (TUV in Germany). No details were given, however, and it was unclear what tests had been performed. They did admit that the shells ‘might have some problems’. It was likely that a faulty ABS shell was the cause of the incident, and its design improved very easily by removing harmful and sharp stress raisers at the teeth roots. The driver was compensated for the loss of three bikes.

10.8

Failure of HDPE baby cot latches

The baby cot is a familiar item to all parents, and one expects that all parts are safe when the cot is in use. But one such cot did fail and a 20 month-old child fell and broke his arm as a result (Fig. 10.39). The part which failed

10.39 Failed baby cot with fractured latch at upper right.

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was a plastic latch which was fitted to the top of one of the rails. It was designed to hold the sliding side of the cot in position, but could be released so as to allow access to the interior. The fracture allowed the baby to put pressure on the side, which fell further down since it was supported only at one side, and he toppled over onto the floor. His mother spotted the fractured component after the baby had been rushed to hospital and she returned home to his bedroom.

10.8.1 Broken latch The latch was a simple moulding which possessed a central hole to fit to a boss attached to the cot, yet allowing rotation to hold or release the side (Fig. 10.40). The picture shows the intact latch at left, the fractured boss at right. The platform on which the top rail rests had fractured along the corner with the upright in a totally brittle way. Load on the platform will have put the shank of the latch into tension, which would have been concentrated at the sharp corner moulded into the design. Direct examination of the fracture confirmed that the main and first crack started at the corner about halfway along its length (upper Fig. 10.41). Hackles in the fracture surface led back to a point in the corner, and there

10.40 Intact left-hand latch and broken right-hand latch.

10.41 Comparison of intact and broken latches.

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O1

O2

10.42 Fracture surface showing main crack origin (O1) and second crack origin (O2).

were traces of striations concentric to the point, which suggested a fatigue mechanism (O1). But there was a second crack, which probably started after the first since there was a clear origin (O2) on a wing of the component (lower Fig. 10.41). The junction between the two crack paths was visible as a sharp line in the fracture surface. Since the part where the second crack started would not have been loaded at all heavily when in use, a badly moulded part was likely. It was confirmed by examination of the rear of the parts, where a deep moulding mark was visible (Fig. 10.43). The same pictures also show the gates (G1 and G2) at the side of the moulding where molten polymer entered the tool. Analysis of the polymer using FTIR spectroscopy and DSC showed it to be high-density polyethylene, and there was no significant difference between the intact and fractured samples. The melting point was centred at about 133°C. Although small carbonyl peaks were detected in thin shavings cut from both samples, they were identical in size and not significant.

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G2

10.43 Melt fracture line on failed latch compared with intact latch.

10.8.2 Analysis The failed latch had failed suddenly from the sharp inner corner to the platform holding the rail of the cot, so allowing the baby to fall and break his arm on the floor. The latch failed owing to poor moulding practice or quality control at the injection moulders. Melt fracture lines are seen in products where the melt is either too cool when it enters the tool, or the tool itself is too cold, and is similar to the weld lines which form when cooling melt fronts impinge. The critical fracture started at a very sharp corner (radius of curvature ca 0.2 mm) to the platform supporting the rail and would have been loaded intermittently by the baby leaning and pushing down on the rail in addition to supporting the relatively low static load of the side of the cot (Fig. 10.40). The stress concentration at the sharp corner was exacerbated by the likely presence of a melt fracture line here, formed by cold or cool moulding, producing a net stress raising effect estimated at about five times the

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maximum nominal stress at the origin using standard diagrams of the stress concentration effect (7, 8). Such defects are produced by oscillations in the melt front, an instability produced by the lower temperatures present at the start of a moulding cycle, for example. If an early moulding, the faulty component should have been removed and scrapped, but it unfortunately entered the product stream and was used in the cot. The accident highlighted the problem of correct and unacceptable design practice, both in terms of geometrical design of components and manufacture to shape. While HDPE was an acceptable choice of material, like all nominally tough and ductile polymers, it will behave in a brittle fashion when severe defects are present. Sharp corners are totally unacceptable in modern design practice, and have been the cause of many premature fractures of safety-critical polymer components. But moulding practice, too, demands high standards of quality control, inspection and testing to weed out faulty parts before they enter the marketplace. If they do, then consumers are put at risk of sudden failure of a part they rely upon for safe use of the product. The failure was likely caused by a moulding defect coincident with the corner, so that the product was a maverick, since no widespread failures were reported at the time. Child safety demands that special care is taken with the design of critical components in products used by children, and cots are covered by BS 1753:1987. The standard (9) specifies a strength test for cot latches of 90 N, but fatigue problems are not mentioned. Falls are one of the most common injuries to children accounting for about 10 deaths per year in the UK according to RoSPA (10). Toys, for example, are subject to strict regulation after numerous swallowing accidents, as are car seats. Many safety devices have been developed to protect children from home hazards where most of the accidents occur, but demand good design and manufacture to ensure that they fulfil their intended function. The family involved in this incident were compensated for their distress.

10.9

Conclusions

The present chapter has discussed a number of product failures of consumer products caused by a relatively small number of failure modes. All could have been prevented by appropriate action before the failures occurred, some actions easy, if not trivial, to carry out in practice, others involving awareness of best advice in manufacture. Above all, designers must specify correct procedures after rigorous and meaningful testing of prototypes or early versions of products before launch into the marketplace. It is the user or consumer who has to rely on those products, especially those where their personal safety depends on product integrity and strength.

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The defects identified as the source of the problem can often be difficult to spot before failure occurs, especially where the part has been moulded under poor conditions and the product appears to all intents and purposes, dimensionally correct and fit for its intended purpose. But then it cracks up when only a small pressure or stress is used, such as when a plug disintegrates when pushed into a socket. The investigation that follows is frequently now made more difficult by the widespread locations of those who need to be consulted, the raw materials supplier, the processor, the moulder, the assembler and the consumer in some cases. The chain of production and supply is often very long, making the task of follow-up and analysis convoluted and time consuming. Some simple ideas can be of immense assistance in the early days of investigation, such as identifying when the product was made and if the problem only occurs in a specific batch of products, for example. It assumes complete traceability from identifying features or logos on the product, now an increasing requirement in many standards. That proved possible with the fractured Noryl plugs, and reduced the analytical effort substantially. Since the failed plugs came from a single month of moulding, the records then became a crucial part of the investigation, but only those from adjacent weeks were made available. They pointed strongly to cold moulding as the source of the problem. But then another investigator came to quite a different conclusion as to the source of the problem, and his conclusions had to be checked independently. They turned out to be wrong, and not supported by our evidence. However, contact with the moulders in Shanghai showed that tool conditions had been modified and production resumed with better safeguards in place to prevent a repetition of the problem. Similar moulding problems occurred on the much more dangerous high voltage supplies on busbars, and were resolved quickly by direct liaison with the local moulders. A quite different type of problem arose with a new design of RCD safety plug, which appeared to infringe an old patent. The trial revealed the nature of the device: it was based on mechanical action triggered by an arm held by an solenoid. When the solenoid detected a drop in voltage, the arm upset the equilibrium of a set of activated levers and spring action disconnected the contacts and cut the power to the external supply very quickly. The speed of reaction of such devices is critical to prevent electrocution: it must be lower than 50 milliseconds and the new design offered a new and possibly faster way of reacting to a sudden leak such as caused by a power tool accidentally cutting the leads. The defendants in the action succeeded and the price to the consumer has dropped substantially with the competition between different devices. Another kind of intellectual property problem was illustrated by the introduction of a new kettle switch from China. It infringed UK patents

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and analysis of the polymer used in its construction showed that an unsuitable flame retardant had been added to the bulk polymer. The retardant decomposed during moulding and created large holes in the switch, endangering its function. Encouraging manufacturers to work with patentees under licence gives access to know-how and expertise, so improving product safety. The problem of policing patents worldwide continues. Polymers have been adopted for use as connectors in many consumer products, and three problems were described and analyzed. A small luggage trolley failed twice and injured users in exactly the same way on both occasions. A polypropylene fixing was attached to the base as a prop for the loaded trolley, as well as holding the knotted centre of a bungee cord for holding the luggage in place. The first failure of the fixing led to the loss of an eye when the part fractured suddenly and the bungee rebounded into the user. The fracture was caused by weld lines from poor moulding practice. The second failure injured a woman user, and was caused by particulate contamination, most likely sweepings from the factory floor added to the hopper. The part was poorly designed to resist bending stresses, and was unnecessary anyway. The bungee was attached to the steel frame as a much more stable connection, and the plastic moulding eliminated entirely. A bike frame attached to the rear of a car failed suddenly and three bikes were lost into the road and destroyed. One cup of an upper joint in ABS probably fractured by fatigue from a sharp tooth corner, and the second joint fractured by overload at a similar corner. Sharp corners will weaken any tough polymer, and good practice demands that large radii of curvature are always specified in polymer products. Even if normally unstressed, a sharp corner can cause sudden failure when least expected. A baby cot latch failed in a similar way from a sharp corner in fatigue, and a baby fell from the cot and broke his arm, reinforcing the message about geometric stress raisers in safety-critical products. What more general points can be made about these failures? There is great pressure on component suppliers such as moulders to maximize the return on the large capital investments in machines. But that should not encourage poor control of component quality or poor moulding practice, such as reducing cycle time to maximize production rate. Designers should test products thoroughly before launch of a new device to at least to current standards, and often beyond, simply because many standards set minimum levels of compliance, and are frequently outdated by the time they come to be published. Product testing itself is an art because it is often difficult to determine what stresses and environments a product may encounter in its normal life. But the design should always allow for worst possible loading, and especially fatigue loading, where even a low load applied intermittently can initiate brittle cracks at stress concentrators. The user is

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often unaware of hairline cracks because they may not be visible at all when the product is unloaded, or in an obscure position quite out of sight. There is no doubt that modern communications such as email messaging have improved the feedback loop between interested parties during failure analysis. The web has opened up vast areas of technical information to designers so that product performance can be improved before introduction into the market. Some skill with key words is needed in finding the precise information needed to solve a particular problem, and there is still a general lack of case studies of failed products and materials. Basic knowledge of the role of stress concentrations in promoting premature fracture still seems to be lacking, however, and understanding of the principles of injection moulding is still at a rather primitive level. Wikipedia articles on the problem are a helpful source of information and a starting point for designers seeking help. A final point can be made about the importance of failures to designers. A frequent response is simply outright denial of any responsibility, an attitude which is not helpful to either the complainant or claimant, or the designer and manufacturer. Once a cause or causes are established, it must be addressed if further failures are to be prevented. Insurers in particular will be unhappy if an underlying design defect in a product is found, and not addressed by the designer or manufacturer, because they have to assume responsibility for compensation. Designers should, on the contrary, regard failures as feedback from the market, and re-examine the problem with a positive approach. Only in that way can product design be improved, and indeed, itself become a selling feature.

10.10 References (1) GE Plastics BV, Bergen-op-Zoom, The Netherlands, Technical Manual: Noryl Profile (Jan 2000). (2) Wright, David, Failure of Plastics and Rubber Products, RAPRA Technology Ltd (2001), p 276. (3) BS 7671:2008 Requirements for electrical installations. IEE Wiring Regulations. Seventeenth edition. (4) Lewis, Peter Rhys, Reynolds, Ken and Gagg, Colin, Forensic Materials Engineering: Case Studies, CRC Press (2004), Chapter 13, p 414. (5) Peterson, RE, Stress Concentration Factors, Wiley-Interscience (1974), Figure 15. (6) Peterson, op cit, Figure 13. (7) Peterson, op cit, Figure 205. (8) Young, Warren C, Roark’s Formulas for Stress and Strain, 6th edn, McGrawHill (1989), Table 37, p 740. (9) BS 1753:1983 Safety requirements for children’s cots for domestic use. (10) Further information available from the Royal Society for the Prevention of Accidents website at http://www.rospa.com/.

11 Conclusions

11.1

Introduction: causes of product failure

The cases discussed in previous chapters have many common components, apart from the obvious point that they all involve failed polymer products of one type or another. The various product failures were caused by defects which arose in several different ways, including: • • • •

poor manufacture poor design poor material unexpected environment.

But many of the failures arose through a combination of such problems, where one kind of defect such as cold moulding was exacerbated by another, such as design or geometric stress concentrations. Polymeric materials exhibit a spectrum of physical and chemical properties quite different from conventional materials such as metals, ceramics or glasses, low density perhaps being among the most prominent. It makes polymers good candidates for any product which involves transportation, such as vehicles of all kinds. Ease of processing to shape is a second feature which makes them attractive for replacement of existing components. Owing to their relatively low thermal transitions and their high viscosities, they can be moulded into very complex shapes so that many parts can be replaced by a single part, thus saving substantial manufacturing costs. On the other hand, they are, as a group of materials, flammable and subject to degradation, especially by oxidation from a range of active agents.

11.2

Poor manufacturing methods

While part consolidation can be beneficial in simplifying product designs, there are many caveats. Polymer must flow evenly and smoothly to all parts of the tool cavity during injection moulding, and must be hot enough to prevent the formation of weld lines. Although not always detrimental, when they occur in a zone that is highly stressed in service, they become defects. The entire product may fail when this happens, so great care is needed in eliminating or moving them from critical areas of a moulding. The case of 438

Conclusions

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the oxygen bottle security caps in Chapter 8 is a case in point: widespread failures occurred in storage simply because a weld line moved to the critical part. The device functioned by the cap bending about the ‘living hinge’ when initially attached to a bottle, and it then had to remain in place until the gas was needed at the hospital. If the hinge failed during storage before even being applied, it had failed its main purpose. The problem produced a crisis at the company as thousands of caps failed during storage, and had to be scrapped. A systematic approach to the problem was needed rather than a knee-jerk reaction.

11.2.1 Faulty moulding The fault lay in control of the moulding machines used to make the caps in a single step. There were three gates to the tool, and if equal flow prevailed then the weld line ended up in the hinge. Changes in barrel or tool temperature had clearly changed the flow patterns within the tool, and had to be monitored more closely and accurately to solve the problem. The investigation also produced a simple quality control test to determine the strength of the device after fitment using simple ergonomic ideas. On discovering the problem, the moulding company jumped to the conclusion that the material itself was at fault, but independent analysis showed little difference between the alternative supplies, and pointed to a quite different cause. The polymer concerned in the example was polypropylene, a common material used widely in many different products. With other polymers, however, other types of problem can occur when tool temperatures are below recommended levels. Polycarbonate is just such a material where cold tools can freeze the molecular chains into non-equilibrium conformations so that it makes the product sensitive to brittle cracking when exposed to organic solvents (and many non-solvents).

11.2.2 Assembly problem The problem was encountered by widespread failures of miners’ lamp battery cases underground when in use at the coalface, so endangering the safety of individual workers. At one point in time, the problem was so serious as to bring an entire colliery to a halt for lack of safe lamps for the face workers to use. Against the advice from the material manufacturers, the sub-contractors moulded the cases and tops into cool or cold tools, so producing very high levels of residual strain. When it came to join top to base after insertion of the contents, brittle cracking was encountered. Unfortunately, reject levels on the production line were relatively low, so

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the true extent of the problem was not fully appreciated. Then cracked lamps were found at the collieries, and colliers themselves suffered from failed lights and spilt acid. Cracks were growing slowly through the polymer and lamps passed as fit for purpose after manufacture ultimately failed as the cracks grew through the walls and led to acid leaks. The cracks were initiated when the solvent used to weld the parts together interacted with the polymer to create the cracks. The root cause was traced by examining the transparent parts using polarized light. It revealed that the plastic sheet showed high levels of birefringence caused by chain orientation. It in turn was due to the moulding conditions used to make the components: cold tools effectively quenched the molten polymer when it entered the cavity. When that highly oriented polymer met solvent, cracks were created and grew with time. The solution lay in changing moulding conditions to use hot tools, a move involving some expense because the tools needed modifying to accept hot oil as the circulating fluid. From then on, the failures started to drop away, although other problems were found in the design of the cases.

11.2.3 Medical devices Polycarbonate has grown in its applications, one being in medical devices such as catheter connections. Much new technology has been introduced into hospitals in recent years, and many devices have failed at critical times when being used with patients. A recent example involved moulded connections between catheters, devices designed to allow staff to feed different fluids to poorly patients such as premature babies. The problem of brittle cracking was discovered by nurses when fluids began to escape but there was a more serious problem of bacterial contamination of the lines through penetrating hairline cracks. One such baby became infected after numerous connectors were replaced. Fortunately, the mother had retained one such set of connectors, and examination showed them to be cracked, the cracks penetrating through the wall, although not, apparently, into the line itself. The polycarbonate of the connectors had probably been moulded into cool tools, and when common hospital liquids contacted the outer surfaces, cracks had been initiated. Similar failures had been widespread through the UK and the manufacturer had not investigated the incidents very thoroughly. They had also destroyed the failed samples, so denying other investigators the opportunity. So despite the existing knowledge concerning the problems of moulding polycarbonate, the manufacturer was blissfully unaware of the perils of that course of action, and put patients at risk of infection. The baby who became infected suffered brain damage, but was compensated by a large award.

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Unfortunately, brain damage cannot be reversed, and the young adult will need ongoing assistance.

11.3

Poor design

Both products also showed basic flaws in their design. The battery cases possessed many sharp external corners as well as sharp inner corners, all of which weakened the final product substantially. Most tough polymers exhibit the same problem, known as notch sensitivity: the sharper a notch, the lower the strength of the product. When lamps are dropped, it is the inner lower corners of the boxes which act as notches, and from which brittle cracks will propagate. Sharp corners occur in numerous polymer products, and battery cases are just one example of a practice that is widespread, as many of the cases in previous chapters have already shown. They include: • • • • •

Chapter 3: corner at shoulder of breast tissue expander Chapter 5: screw thread tips on ebonite handles, corners on truck and miners’ lamps battery cases Chapter 6: shoulder of PVC rising main, screw threads of acetal fitting Chapter 8: corners on angle grinders, and stepladders Chapter 10: screw threads on busbar casing, serrations on bike carrier shell, babycatch.

There is another danger with sharp corners: when exposed to liquids, it is where brittle cracks caused by ESC or SCC will often start. Even when the liquid is removed, a meniscus tends to remain at such corners, so increasing the chances of cracking. When exposed to gases like ozone, attack will tend to start at corners simply because any applied stress will be greatest there. Just this was found in the fractured NBR seals from the semi-conductor factory in Japan. It is inexcusable to leave sharp corners in mouldings, and engineering drawings should specify a minimum radius of curvature. However, more often than not, such recommendations are absent, and toolmakers leave external corners on cores very sharp. It is surely good practice to smooth such corners before moulding products, and so strengthen the final component. It is also one of the simplest and cheapest tasks to perform in the toolroom. But holes also represent stress concentrations, and they are often inevitable for making attachments or connections. It is entirely predictable that such holes will be stressed in service if the connection is live, such as the bucket lugs of the failed bucket case of Chapter 5. In this case, the new approach adopted incorporated the lugs with the wall below, so extra support was gained. The new design was expensive, owing to the new tool

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needed, but produced a much safer product and it has been adopted by all manufacturers of such products. The exposed lugs were at the end of the flow path, so weld lines were likely to be formed there, and so weaken the bucket yet further. Other products where cracks were started at holes included the broken catheter tip from Chapter 3, but here the material itself had become embrittled and the catheter failed where the local load was greatest at the edge of one of the bleed holes. Even scratches can prove the undoing of a product. The WC handle discussed in Chapter 8 failed from scratches made by the user on its chromium-plated front. The scratches allowed strong cleaning agents such as bleach to attack the underlying ABS plastic, as well as acting to concentrate the stress locally. A brittle crack grew slowly at each use of the handle. It finally fractured when the crack was too large for the remaining solid material to support the applied load.

11.3.1 Stress concentrations The way in which load is distributed in a component is vital to its performance, and when the load is concentrated at a critical zone, then problems can be expected. Good design during product development should examine the probable load paths by mechanical testing to find those zones and then change the geometry of the design to ameliorate and lessen the stresses there. Often the exercise is very simple, as in the example of the battery cases, where smoothing or radiusing the corners reduces the danger of brittle cracking and so strengthens the final product. More complex component geometries can be tackled either empirically or by using compilations of stress concentration diagrams or relevant equations (1, 2). Finite element analysis can also be useful during prototyping for predicting critical areas of a product (3). The theory underlying stress concentrations is of course not specific to any material, but there are extra precautions needed when designing with polymers. For example, moulded products show an extra level of complexity: it concerns the way flow occurs within the tool cavity. Holes are a particular problem because the flow has to split as it encounters the edge, and then recombine at the further edge. If the polymer melt is at too low a temperature, a weld line forms. If the temperature of the melt is too low then the weld line represents a proto-crack from which a real crack can start when the product is loaded. One important feature of flow analysis (where finite element analysis can also be applied in tools) is its use in designing tools. Detailed information is needed on the grade or molecular weight of polymer being considered, since it will determine the melt viscosity and hence the flow rates into the tool at various temperatures. The presence of sharp corners can affect melt

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flow by constraining the flow. A good example of the problem is the occurrence of high degrees of chain orientation at such obstacles. So the severity of a geometric stress concentration may be increased because oriented polymer will tend to fail earlier than unoriented material. The effect was shown in Fig. 5.41, where a highly birefringent zone occurred next to a belt loop corner. Such stress concentrations are the weakest points in designs, and from which cracks are likely to form in adverse conditions. Such conditions include: • • • • •

overload under a steady stress impact loads fatigue from cyclic loads stress corrosion, or exposure to an aggressive chemical environmental corrosion, or exposure to specific organic fluids.

Elimination or amelioration of stress raising features are therefore the sine qua non of the designer, especially when using polymeric materials.

11.4

Poor choice of materials

Like many other materials, polymers have strictly limited temperature stability owing to glass transition temperatures or melting points, where physical properties like stiffness and strength diminish greatly. The major difference from other materials is the much lower temperatures at which these transitions occur, typically from about 100°C to 250°C for most common polymers. Even at lower temperatures, many polymers will oxidize fairly rapidly, so degradation can be expected, a process that results in lower molecular weight and so lower strength. Polymers must therefore be selected knowing the maximum temperature in which they will operate. One of the most striking cases involving inappropriate choice of a polymer concerned the composite tank discussed in Chapter 4. The tank was designed to store 100 tonnes of water effluent at a temperature in the nineties centigrade, but was built from polyester and chopped glass fibre. The polyester showed a Tg of about 70°C, so it was unsuitable for its purpose. It failed catastrophically about a year after installation. In addition, any distortion of the walls was hidden by an integral bund, so there was no forewarning of the impending failure. But other inappropriate polymers have been used in products where the results of failure can be damaging to the user, such as in knife handles. The example of small cutting knives discussed in Chapter 8 showed that the polymer used in one of them, apparently HIPS, actually showed all the characteristics of polystyrene, a brittle polymer unsuited to highly stressed products. When it failed near the tip, the user suffered a severe cut to his

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finger. Another example used a tough material, polypropylene, but it could itself be cut easily and another accident occurred. The design could have been improved very easily by incorporating a metal insert to prevent the blade slipping. An example of a component which need not have been used at all was the luggage trolley of Chapter 10. A polypropylene moulding was stapled to the hollow steel frame to locate the bungee cord used to secure the luggage. In two cases, it fractured suddenly under load from the cord, causing severe personal injuries to the face and eye. But the device was entirely superfluous since the cord could simply have been knotted around the frame, as indeed it was after these accidents had occurred. The problem can occur with any material whose role is critical, as the example of the poorly selected sealants used in the ducting of the fire brigade training centre in London. Many of the sealants absorbed hydrocarbon oil used to create smoke in the building, became plasticized and semi-liquid. They ran downwards and so broke the seal in the ducting, allowing the smoke to soak into insulation, and a real fire was created. The sealants should have been tested beforehand, and the unsuitable ones rejected for use in this application.

11.5

Environmental stresses

A related category of product failure occurs when materials are affected deleteriously by their environment, a problem which is widespread among all the different types of material available to designers. Corrosion is a very serious problem with steel and many metals, the attack frequently occurring insidiously at inaccessible points where water collects. If the component is loaded heavily, then stress corrosion cracking can occur, with sudden and dramatic results. The fall of the Silver Bridge in West Virginia killed 46 drivers and passengers on cars on the bridge at the time when it fell at midday on 14 December 1967 (4). The suspension bridge had been underdesigned with only two tie bars in each link of the main chains carrying the roadway, so when one cracked, there was nothing to prevent the entire bridge collapsing. It fell completely in less than a minute. The root cause of the cracked tie bar took the form of a 3 mm long crack which had grown over the 40-year life of the structure when it became critical. It had grown from the lower bearing surface of a joint near the top of the chain where water had collected, and grew under the influence of residual stress left in the bar after manufacture. When rust formed in the crack, the expansion exacerbated the stress on the crack tip. Polymers too can suffer from stress corrosion cracking (SCC), and are also sensitive to environmental cracking (ESC), a problem unknown in other material types. SCC is caused by chemical attack on the polymer

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chains, while ESC is essentially a physical effect where organic solvents penetrate the bulk polymer and initiate brittle cracks (5). The fact that polymers can be degraded by a variety of chemicals is of course well known, although the details often remain obscure.

11.5.1 Stress corrosion cracking There are several distinct kinds of SCC depending on both the polymer and the reagent which attacks the material. One of the most common forms of attack occurs when strong acids or alkalis cause hydrolysis of main chains, as exemplified by the cracking of the diesel fuel return pipe of Chapter 9. It was the cause of a road traffic accident and the cracked pipe was initially thought to have been the result of a knife cut by a vandal. But the pipe was buried deep in the engine compartment and difficult to access. The solution to the mystery lay elsewhere. It needed only a small leak of sulphuric acid from a car battery to initiate a brittle crack in the nylon 66 connector, a crack which grew slowly as the line vibrated during use of the vehicle. It took about a week for the fuel line to fracture completely, and the diesel spewed into the road and caused several accidents. The worst of them severely injured a following driver when her car skidded on the diesel patch and careered into the path of an oncoming lorry. The sequence of events was determined by detailed examination of the fracture surface using ESEM, and the theory of SCC confirmed by analyzing the surface for the traces of sulphur left by the acid attack. All so-called condensation polymers (alternatively called step-growth polymers) are made in the first place by linking monomers together via acid and base units, and it is these ester or amide units that are susceptible to hydrolysis in solid products. But there is a wide variation in their stability to acids and base liquids. For example, polycarbonate is stable to concentrated sulphuric and other acids, but highly sensitive to any type of base. Even very weak bases such as sodium carbonate solution will attack the material. Very strong alkalis such as caustic soda, attack the material very rapidly. By contrast, PET is resistant to alkalis but sensitive to acid attack. Copolymers which possess polyester or polyether units, such as Hytrel, are also sensitive to hydrolysis, as the failure of elastomeric washers at numerous locations showed. They were attacked just by water, although the high temperatures of the radiators was decisive in initiating cracks. Those same temperatures also led to crystallization of the material, and so putting further strain on the components. It was a problem that should have been checked by testing the parts before adoption, and there were warnings from the material supplier (DuPont) of the drop in stability at high temperatures. The same problem can occur during moulding at

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the inevitably high temperatures of the molten polymer in the process machinery, with most step-growth polymers at risk unless dried thoroughly before processing. Since hydrolysis involves chain splitting or scission, the molecular weight of the material falls fast, and since mechanical strength is dependent on molecular weight, the strength drops rapidly, too. It can be highly localized at the crack tip, so crack growth is enhanced; surrounding material may remain entirely unaffected, a frequent characteristic of brittle failure in many materials. As with all types of corrosive attack, crack growth is increased by any residual stresses or strains present in the product, often as a direct result of manufacturing methods.

11.5.2 Oxidation and ozonolysis Most polymers of whatever type are susceptible to oxidation from a variety of powerful oxidants. Oxygen is omniscient and oxidation increases as the temperature rises, so polymers exposed to temperatures greater than ambient may be subject to attack. Polymers with secondary or tertiary carbon-hydrogen bonds are especially vulnerable because the free radicals formed at such bonds are more stable, and hence oxidation is favoured there. That means polymers such as polypropylene and polybutylene can be oxidized relatively easily, and anti-oxidants are added routinely at a very early stage in their manufacture. Such speciality chemicals do have a limited life, however. They act by absorbing the damaging free radicals, and the protection depends on the concentration of anti-oxidant present. As the molecules are depleted by reaction, the protection they provide falls, and there may come a time when there is no further product protection whatsoever. That was shown by the very widespread failures of polybutylene pipe in the USA, although they tended to fail after the acetal fittings used to connect the pipes together. Ultra-violet radiation in sunlight can have a similar effect by interacting with the polymer chains to form free radicals, which then react further, usually resulting in chain cleavage and cracking. Even polyethylene, which is otherwise more resistant to oxidation than polypropylene, can suffer and so UV absorbing chemicals must also be added where continuous exposure to sunlight is expected. The failure of road cones and mancabs of Chapter 4 were examples of rotationally moulded products without that protection, and which failed very quickly when used externally, as was intended. A more complex example was given in Chapter 5 of storage batteries used in fork lift vehicles which had been hot welded, but where failure occurred because the oxidized tops were then exposed to strong sunlight during one of the Israeli wars. Another case involving a fractured catheter in Chapter 4 showed that the catheter had been affected by UV exposure at an early

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point in its history, and resulted in premature failure when in use in the hospital. Chlorine even in very dilute form in water is a potent oxidizing agent, which is why it is so efficient in killing harmful bacteria. However, many polymers are equally susceptible to attack by the chemical (as are many metals). The flood caused by a broken acetal fitting (Chapter 6) was traced to chlorine attack after information from the USA revealed expert reports from a class action in Texas. The failure at Loughborough University caused extensive damage to computers because it happened at a weekend, so went undetected for some time. The fracture was old, but showed intermittent crack growth in the threads. The cracks had started from weld lines in the sample, and the failed moulding was probably a maverick because it could not be matched with current production from the mouldings. ESEM/ EDX later confirmed the presence of bound chlorine in the fracture surfaces. The US reports showed that acetal fittings had failed first in new plumbing systems installed in houses across the entire country. The parts failed in water supplies with chlorine levels as low as 1ppm, but had been worst where water temperatures were highest. Polymers possessing double bonds within their main chains are also liable to oxidation because the bond stabilizes the free radical formed when hydrogen is abstracted from the adjacent carbon atom: —CH=CH—CH2— → —CH=CH—CH*— The activated carbon atom can then react further with oxygen to produce a carbonyl group: —CH=CH—CH*— + O2 → —CH=CH—CO— + O* The carbonyl group can be detected using FTIR spectroscopy, a major tool for investigating polymer degradation, as previous cases have demonstrated. But double bonded polymers, especially elastomers which form the majority of the type, are susceptible to another and more aggressive allotrope of oxygen, namely ozone. The gas is formed near electrical equipment from sparking as well as silent discharge of current, but usually at very low concentrations (in ppb). But even such low levels of ozone will attack double bonds very selectively. They split the chain where a double bond occurs, forming carbonyl groups at the new chain ends. Cracks are formed, but can only grow under an applied load, as several cases showed, including failed NBR fuel pipes on Fiat cars and aircraft haulage vehicles, and NBR seals on semi-conductor fabrication units using pneumatic systems. Simply bending the vehicle fuel pipes was enough to promote crack growth, while the diaphragms in the pneumatic seals were flexed by air pressure when the equipment was working. However, preferential

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attack occurred at sharp inner corners, where small applied loads were concentrated. The failures caused fires in the vehicles fitted with the defective rubber hoses, and the manufacturer may have (wrongly) relied on an oil film providing protection. In the other case, it was a change of compressor of different design which caused very low levels of ozone to appear in the pneumatic system. The solution to all such problems either means using an ozone-resistant rubber such as EPDM, or adding an anti-ozonant chemical to counteract traces of the gas.

11.5.3 Environmental stress cracking Unique to polymers, ESC occurs when products are exposed to active organic liquids or even vapours. The problem was encountered on the assembly line where miners’ battery cases were made, cracking occurring when the injection mouldings were exposed to strong solvents being used to make welds between the cases and the lids. The cracks went mainly unobserved by quality inspectors, but appeared of visible size in colliery lamp rooms and at the coal face. Examination of new parts showed high levels of birefringence due to moulding into cold tools, but changing to high temperature tools improved product life greatly. Many other improvements to the design were also made. The problem re-appeared in the 1990s when polycarbonate connectors used in IV lines in hospitals started cracking when in use and exposing patients to the possibility of bacterial and viral infection. One design failed at numerous separate hospitals, probably caused by contact of the outer surface with common cleaning agents such as bleach, a weakly alkaline liquid which also contains chlorine. However, the inner bore could also be attacked by lipid solutions such as Total Parental Nutrition, a synthetic fluid similar to milk for premature babies. It was inferred that the device had been moulded using cool or cold tools so that there was a driving force for crack growth. The explosion of a compressed air line provided a dramatic example of the problem. Although no-one was injured, there was substantial physical damage to the glass factory where the accident occurred. The pipes of the system were made from ABS, which was stressed highly by the air inside. The walls were under high hoop stresses, and when organic fluid entered the pipes, it reacted with the inner bore to produce deep crazes and cracks. When one became critical, the pipe blew apart. The fluid probably came from the compressor, in which an unapproved oil had been used. The problem of ESC is created by surface absorption of the fluid, which swells the polymer to a greater or lesser extent, the degree of swelling depending on the chemical compatibilities of the polymer and the fluid (6).

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Swollen polymers are weaker than solid bulk, so cracks can be formed quickly and grow when the product is under stress or strained internally by residual stresses or strains. Amorphous materials like PMMA, polycarbonate and ABS are more susceptible than semi-crystalline polymers because the crystallites are much more resistant to swelling. However, they are not immune to ESC, and early use of polyethylene in blow moulded containers, for example, could crack when exposed to strong detergents. We were referred the problem in the 1970s when a number of such containers fractured, releasing large quantities of beer. They were cleaned regularly with a very strong detergent, Igepal, which initiated ESC cracks. They grew with time under the influence of blow moulding orientation until sudden failure occurred. The solution to the problem lay in increasing the molecular weight of the raw polyethylene used in their construction. Higher molecular weight grades are always more resistant to ESC since they provide a greater reserve of strength.

11.5.4 Data compilations Data compilations of interactions between different polymers and chemicals are frequently provided by material suppliers, usually in the form of an extended table. The information is purely qualitative only, using the terms ‘no effect’, ‘good-minor effect’, ‘moderate effect – fair’ or ‘severe effect – not recommended’, for example. Given the sheer number of the possible combinations, they can be a useful starting point for guidance to potential users or investigators. However, further information should always be obtained by direct experiment under conditions directly related to those of interest. For example, the effects of temperature are either completely ignored or limited to one or two temperatures, so detailed resistance must be determined experimentally. The tables also ignore the effects on different grades of polymer and, as already seen above, the grade can have a strong influence on cracking. Low molecular weight grades are inevitably weaker than high molecular weight grades. Individual polymers may also show a wide variation in crystallinity depending on their processing history, which is yet another reason for direct analysis. There is usually little information on the effects of changes in concentration of the attacking agent, which is often critical. Dilute reagents will tend to be weaker cracking agents than more highly concentrated reagents for example.

11.6

Access to information

The availability of information concerning the limits of polymers is widely disseminated in the literature and was formerly difficult to access. It was

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Forensic polymer engineering

spread between suppliers’ technical data sheets or brochures, the technical literature and textbooks, although much key information remained in company archives, where it never saw the light of public scrutiny. The acetal and polybutylene failure reports made by the material suppliers in the case of the US plumbing class action is an example of such secrecy, and only exposed by their discovery during court action. Of perhaps even greater concern was the way in which those companies ignored their own detailed analyses of the problem.

11.6.1 Published literature There are problems with all public published information sources, such as the technical literature and textbooks. Because they are paper sources, they have often been edited or even worse, sanitized to exclude significant details. They are frequently out of date, or overtaken by other events, a problem in an area of rapidly developing technology such as polymers. By contrast, metal technology is much more mature, and considerably more information is available to users. Nevertheless, it tends not to discuss problems in terms of case studies, where the context is often critical. The scientific and technical literature is also mainly directed towards theoretical rather than practical ends, so is limited in the help it can give to investigators. There are some notable exceptions, such as the French paper giving details of the UV sensitivity of thermoplastic nylon elastomers mentioned in Chapter 4. There a few journals that are devoted to failure analysis, most notably Engineering Failure Analysis, where case studies are extolled. Papers dealing with failed products do appear from time to time in other materials or engineering journals, but are widely scattered. Another exception is medicine, where failures are reported in detail, although the detail often excludes engineering information of critical importance. There are detailed case studies published by RAPRA, which has taken a lead in publishing their own files in an effort to make users more aware of the problems of polymer degradation (5). But generally the literature neglects discussion or even mention of failures, probably because there is a systemic aversion to publicizing failed products or processes. However, such discussion is vital when major accidents or disasters occur, and we are fortunate that the results of public enquiries are freely available. The NTSB and HSE are publicly committed to disseminating such results because such incidents must be prevented by an understanding of the causes. But many less serious incidents are suppressed or published in places less accessible to those who need to know about a potential problem. Most major accidents are preceded by warning signs or less serious incidents, or in some of the worst examples, by previous accidents. Much of the information is revealed after a major accident when a court case arises,

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such as occurred in the case of the motoway accident in France. There had been several similar accidents, some fatal, when the same design of bag had fallen from the rear of motorbikes and led to seizure of the back wheel. The accidents were in the public domain, but key details seemed to be missing in newspaper reports. When they were all put together with relevant details, however, a picture emerged of a design with serious flaws.

11.6.2 Old problems One other aspect of the problem of data dissemination relates to old problems solved at the time, but which re-appear many years later when the original problem has been long forgotten. A good example is the problem of ozone cracking, a failure mode long known about and even familiar to car drivers before the use of anti-ozonants in rubber products became widespread. Since vehicle tyres are the most important elastomeric product, use of these protective additives suppressed the problem. But it didn’t go away for many other safety-critical products like fuel lines, where fires continue to occur even after warnings of the problem of radial brittle cracks penetrating the bores, as the recent example of aircraft towing vehicles showed. Although much is known about the mechanism of attack, users may be quite unaware of the problem if they have never encountered it before. Small but critical components such as diaphragm valves were also affected when the compressors were replaced by a quite different design. The new machines produced tiny amounts of ozone but which were enough to bring an entire semi-conductor line to a halt. The aggressive gas is also more likely to be present under certain conditions in the atmosphere as a result of local conditions, especially when organic pollutants such as petrol or diesel fumes are acted on by sunlight. So the problem is likely to continue when unprotected elastomeric products are exposed to the gas. The acetal cracking problem is another example of an old problem which reappeared with devastating results. It had been known when two versions of the polymer were commercialized that extra stabilizing methods were needed. One version was end capped to prevent the chain unzipping from the chain ends, while unzipping of the copolymer version was provided by blocking groups of the more stable copolymer unit (7). But the polymer was known at that time to be unstable to hydrolysis, especially in acid solutions, and the fact that chorine dissolves in water to form dilute hypochlorous acid will have alerted later workers to the problem. We know that was the case because of the research reports from the 60s and 70s about the degradation of acetal mouldings when exposed to very low levels of chlorine in a water supply. The problem lay in managements who either did not know of their research or who blithely ignored those internal reports. The

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result was one of the largest compensation claims ever recorded in the USA. So only publicizing the case to future workers will or should prevent further lapses in judgment. One new factor that suggests that those results will not be forgotten lies in a new way of accessing information: the internet.

11.6.3 The internet It is the world wide web that has transformed the information problem for the better in the last decade. The growth has been staggering, both in terms of numbers of people who can access the web and the sheer amount of information presented. The main search engine, Google, provides searches of many different types of information, including searches of books and the technical literature as well as images. The latter is especially useful for finding photographs of failed or fractured products, and was used to find several images of exploded batteries in Chapter 5, for example. They showed very similar effects to the examples discussed there. Many personal website blogs give information on failures experienced by their authors, an incidental but valuable contribution for investigators. Such ‘metadata’ (8) can provide a broader picture of failure than is often possible from their own information. Metadata is the information usually deleted by editors from official reports, but is frequently useful in providing key data not accessible in other ways. Like their corresponding paper literature, company websites praise the positive virtues of their products with little or no mention of the drawbacks or problems. But technical brochures are a good starting point and certainly much technical information is potentially very useful to designers. Case studies in commercial literature always focus on successful products and their development, without giving counter-examples. One notable exception was the ozone attack paper published by a large pneumatic manufacturer in Japan, which unfortunately was not read by one of their competitors! Forensic company websites are exceptional in giving case studies of failed products, but the cases are normally very brief and lack supportive information. They are also mainly restricted to metal products, with non-metals rarely mentioned. Much legal information such as court judgments and class action announcements are available on the web, especially in the USA. They were the source of some of the discussion of the acetal/polybutylene problems of Chapter 6. Government websites are valuable for the many official reports of major accidents and disasters, and include the NTSB, MDHA (formerly the MDA), FDA and HSE. The FDA website is especially interesting for learning about medical device failures, as was discussed in Chapter

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3. All of the recent US government reports of disasters from the NTSB are available for free download, although older reports are difficult to access, presumably because they have not yet been scanned and downloaded to their website. University web sites are a growing source of information on product failure, especially where staff are active in forensic research. Some sites discuss case studies in detail (9), although many more discuss current theoretical work and other current research.

11.6.4 Wikipedia One central source of information is the web-based encyclopaedia, Wikipedia. Founded only in 2001 by Jimmy Wales, the site offers an unprecedented source of information on every possible subject (10). As of 2007, there were 2.7 million articles on the English language version, and the number is still growing at a fast rate. It is an open access source of information, which currently means that any user can edit articles, but the policy means that vandalism is a severe problem. There is also a problem of spam, especially in technical articles, where commercial users attempt to insert links to their own websites. The breadth of the site is awesome, ranging from trivia and ephemera (video game characters, for example) to everything that one might expect in a conventional encyclopaedia like Britannica. Despite the vandalism, the number of mistakes is about the same, but unlike Britannica, those mistakes can be corrected very easily. Ironically, some articles are based on the 1911 edition of Britannica, which is now out of copyright. The mechanics of editing are very easy to grasp, although there are rules to ensure that it is conducted fairly and without bias. Although the quality of some technical articles is very high, the quality of polymer, material and engineering articles is very variable, with many individual polymers barely discussed in any detail at all. One exception is the article for polycarbonate, perhaps because the material is now so widely used across a spectrum of industries. But because Wikipedia is open for anyone to edit, then there is hope of future improvements. Its open access also means that articles are kept up-to-date by its editors, a severe problem of paper-based encyclopaedias. It is a source of information on product failures and forensic disciplines (fire investigation, forensic identification, trace evidence and so on), some of which we have helped to write and edit, so opening up the subject to wider view. One fascinating feature of the articles is the way in which individual terms or phrases can be linked, so that a browser can follow a subject through many different pathways. It means that users have much wider and deeper access to fundamentals, applications and implications than is possible with a paper encyclopaedia. Each article is also supplied

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with external links so you can access other independent sources on a specific topic. There is, however, great scope for improvement in the area of forensics, especially as the amount of information on product failures grows with time and with greater transparency in publicizing poor product performance. There is no doubt that the journal Engineering Failure Analysis has made great strides in publishing revealing and incisive case studies from investigators, and its expansion in recent years reflects the growing desire to publish detailed case studies so as to limit further failures of the same kind, if not eliminate them. It, too, is readily accessible on the web, so that even the most arcane failure problems can be located by search engines using appropriate keywords. The growth of the web encourages minority interests, or ‘long tail’ of information and sources (11), both in commercial sites like Amazon and independent sources like Wikipedia.

11.7

References

(1) Peterson, RE, Stress Concentration Factors, Wiley-Interscience (1974); Pilkey, W, Peterson’s Stress concentration factors, Wiley-Interscience, 2nd edn (1997). (2) Young, WC, Roark’s Formulas for Stress and Strain, 6th edn, McGraw-Hill (1989). (3) Mohammadi, S, Extended Finite Element Method: For Fracture Analysis of Structures, Wiley-Blackwell (2007). (4) National Transportation Safety Board, A Highway Accident Report, Collapse of US 35 Highway Bridge, (Washington: GPO, 1971). (5) Wright, D, Failure of Plastics and Rubber Products: Causes Effects and Case Studies involving Degradation, RAPRA (2001). (6) Brydson, J, Plastics Materials, Butterworth, 7th edn (1999). (7) Barker, SJ and Price, MB, Polyacetals, Iliffe Books/Plastics Institute, London (1970); Vogl, O, Polyaldehydes, Edward Arnold and Dekker (1967). (8) Weinberger, D, Everything is Miscellaneous: The Power of Digital Disorder, Holt, New York (2007). (9) Websites with extensive collections of failure case studies include: http:// technology.open.ac.uk/materials/mem/ and http://www.tech.plymouth.ac.uk/ sme/UoA30/Consulting.htm#Consulting%20Work (10) See the entry at http://en.wikipedia.org/wiki/WIKIPEDIA. Editors can track the status of articles on their own watchlist. (11) Anderson, C, The Long Tail: How Endless Choice is creating Unlimited Demand, Random House (2006).

Index

ABS see acrylonitrile-butadienestyrene acetal, 81, 263, 447, 450 acetal albatross, 263–4 acetal cracking, 451–2 acetal resin, 259, 264 acetal resin fittings and polybutylene pipes failures, 259–68 acetal albatross, 263–4 degradation mechanism, 264–5 fracture, 259–60 literature review, 260, 262 pipe failures, 266–7 recent developments, 267–8 failure modes, 266 fractured, showing contamination from water supply, 262 section of hot water supply which failed and caused flood, 261 acetal/polybutylene problems, 452 acid storage tanks, 171–2 acrylic, 283 acrylic polymers, 303 acrylonitrile, 282 acrylonitrile-butadiene-styrene, 200, 226, 269, 336, 430, 442, 448, 449 dark grey ABS pipe at Immingham storage facility, 254 fractured pipe from compressed air explosion, 256 handle, 328–31 etch pits in lower wing, 331

fracture surface map of handle break, 330 fracture surface of handle with striations, 329 intact and failed handle, 329 scanning electron microscopy, 331 scratches and peeling of chrome coating, 330 joints failure on bike carriers, 425–30 brittle fracture of ABS clam shell, 428 clam shell connector on upper frame, 426 damaged shells, 427–9 one shell still attached to frame showing serrations, 427 origin of fracture from root of tooth, 428 steel-framed carriers fitted to car after accident, 426 stress analysis, 429 pipes and fittings, 250–3 blanking plate showing contamination at centre, 251 contamination of PP pipe from another HCl facility, 251 hydrochloric acid storage facility, 250 Immingham Docks, 250–3 UV spectra from commercial acid and ferric chloride solution, 252 activated carbon, 292 Afridev, 235

455

456

Index

aircraft batteries, 182 alcohols, 131 aldehyde, 287 alumina-silicates, 423 aluminium, 287 aluminium alloy, 338 Amazon, 454 amber, 17 American Society for Testing and Materials, 38 angioplasty and balloon catheters, 112–14 balloon catheter and guide wire, 112 angle grinder, 316–24 another handle failure, 320 British standard for tools, 320 fracture, 320–3 DSC thermogram of material used in handle, 323 matching parts of fracture surfaces, 321 origin at edge of voids in plastic, 322 origin with void at interface with bolt, 323 upper fracture surface showing shrunken interior and large weld line, 322 fracture surface, 317–20 fatigue striations from sharp outer corner of handle guard, 319 grinder handles vs failed handle, 318 handle guard fracture side showing contaminated polymer, 318 handle screw side showing impact damage, 319 fractured handle, 321 handle socket, 317 aniline, 276 ANTEC conference, 102 anti-oxidants, 306 anti-ozonants, 377, 448, 451 aramid, 15, 195, 210, 366, 371, 396 molecular structure, 17 aromatic ring, 15 Arrhenius equation, 265

ASML line, 288 ATR see attenuated total reflectance ATR spectroscopy, 382 attenuated total reflectance, 70, 276 Babb house, 260, 266 baby cot latches, HDPE failure, 430–4 analysis, 433–4 broken latch, 431–2 failed baby cot with fractured latch at upper right, 430 fracture surface showing main crack origin, 432 intact left-hand latch and broken right-hand latch, 431 intact vs broken latches, 431 melt fracture line on failed latch, 433 Bakelite casing, 223 balloon catheter and angioplasty, 112–14 and guide wire for angioplasty, 112 bathing alcohol, 107 battery case battery containers failure, 177–84 degraded by UV attack along weld, 152 failed battery cases, 152–3 failed truck battery cases, 195–8 miner lamp battery casings failure, 198–219 Beer-Lambert law, 76–7 Bell telephone test, 267 bending, 21 benzene, 276 benzene ring, 15 bike carriers, ABS joints failure, 425–30, 426, 436 brittle fracture of ABS clam shell, 428 clam shell connector on upper frame, 426 damaged shells, 427–9 one shell still attached to frame showing serrations, 427 origin of fracture from root of tooth, 428

Index steel-framed, fitted to car after accident, 426 stress analysis, 429 birefringence girder section showing stress raiser at upper corner, 54 polycarbonate set square showing gate at P and weld line at WW, 54 blanking plate, 250–1 bleach, 131, 265 blue cone, 223 BMW, 254 Boltzmann’s constant, 19 bonding, 2–11 covalent bonds, 3–6 carbon structures, 8 co-polymer repeat units, 4 natural rubber cross-linking, 5 repeat units and size in polymers, 4 tetrahedron carbon atom, 7 electrostatic bonds, 6–7 hydrogen bonds, 7–9 nylon 6,6 sheet structure, 9 polymer structure and melting points, 10 van der Waals bonds, 9–11 rotational isomerism, 10 bone glue, 79 Boston molasses disaster, 134–7 causes of failure, 136–7 plan of Boston north harbour showing damage to buildings, 135 scene of devastation after tank collapse, 135 Brava, 387, 388 breast tissue expander close-up cracks at interface, 117 origin, 117 conclusion, 119–20 failure of tissue expander, 114–18 fracture surface map, 118 fractured breast tissue expander, 115

457

loading pattern, 118–19 oblique view fracture in ESEM, 116 fracture showing cusp, 116 origin of main fracture, 117 other cases, 120–1 record of fills, 118 breathing tubes device failure, 124–30 colour variation in set of mouldings, 129 development of sight tube, 124–6 faulty tubes, 126–9 flash marks caused by wet polymer granules, 128 flow lines in moulding shown by shadow, 127 inclusions in moulding, 128 lessons, 130 original PMMA sight tube, 125 polysulphone melt viscosity as function of shear rate, 127 prototype moulded sight tube in polysulphone, 126 section of assembly showing float, 125 trial, 129–30 Britannica, 453 British Oxygen Company, 155 British Standard, 334 British Standard 4994, 169, 170–1, 173–5 British Standards Institution, 38 brittle crack, 148, 204, 245–7, 440 and screw thread, 181 and weld lines in base moulding, 400 discoloration, 49 growth in plug cover, 410 in radiator tank of GF nylon 6,6, 61 in ultrasonic weld, 401 induced by positive plate expansion, 216 on tank near external buttress, 52 penetration of base from screw hole, 409 within surface loom, 401

458

Index

brittle cracking, 332, 369 brittle fracture, 65 ABS clam shell, 428 chair with leg reassembled, 332 drive belts double tooth, 362 proximal end of failed catheter, 96 surface on drive belts, 362–3 bromobutyl, 365 brown calcite, 265 Brunel’s seals, 272 BS 1753:1987, 435 buckets failure, 184–7 close-up of broken lug, 185 LDPE bucket with fractured lug, 184 lug fracture surface, 185 recessed lug for failsafe, 187 weld line formation, 186–7 builder’s putty, 301 bund, 138, 169–70 Burgoynes, 258, 260 butadiene monomers, 282 butyl rubber, 305 calcium, 287 calcium carbonate, 260, 278, 301, 303, 305 calcium stearate, 123 calorimetry, 303, 338 Canberra, 182 Canon line, 288 carbon black, 75–6, 153, 177, 181, 283, 361, 365, 370, 425 carbon fibre composite wing, 281 carbon tetrachloride, 186 carbonyl group, 447 carboxylic acid, 287 Catgut, 121 catheter balloon catheter and guide wire for angioplasty operation, 112 brittle fracture in proximal end of failed catheter, 96 catheter manufacture flow sequence, 100 compact and expanded stents, 113

connector end showing extensive cracking, 108 distal catheter end another catheter, 95 broken, fracture surface map, 97 fractured, showing bleed hole, 95 DSC curves of new and failed catheters, 97 expanded stents in artery acting against fatty deposits, 113 failed catheter, 90–102 accident at childbirth, 92–4 degradation theory, 99–101 ESEM of failed end, 94–6 material and mechanical testing, 96–9 thermoplastic elastomers, 91–2 FTIR microscopy for good and failed catheter, 99 gate showing contamination, 108 Hickman IV line fitted with polycarbonate connectors, 103 inner crack in connector, 109 polycarbonate connectors, 102–14 balloon catheters and angioplasty, 112–14 connector failures, 102–3 connectors premature cracking, 103–6 disclosure, 106 discussion, 111–12 ESC/SCC hypothesis, 110–11 injection moulding, 110 joint expert examination, 107–9 literature, 106–7 section through connector to show internal structure, 104 tip of thermoplastic nylon catheter showing bleed holes, 93 Tuohy needle used for epidural anaesthetic, 93 caustic soda, 123, 445 Celanese Corporation, 263, 264, 268 cellulose, 18 central heating systems, 293–5, 298 chain splitting, 446 chain-growth polymers, 5

Index chairs, manufactured from PP, 331–6 accident chair showing crack growth directions, 335 another failure, 334–5 brittle fracture with leg reassembled, 332 crack growth directions, 333 fractured plastic chairs, 334 litigation, 335–6 material analysis, 333–4 voids from which cracks grew, 333 voids near centre of leg corner, 335 chalk, 301 Challenger disaster, 278–81, 306 effect of internal pressure on joint, 280 leak of flame from booster rocket, 279 O-ring recovery with time and temperature, 281 section field joint on booster rocket, 279 Channel Tunnel, 389 chemical milling, 207 chevron marks, 65, 231 chlorine, 27, 253, 262, 263, 264, 265, 331, 447, 448, 451 chlorobutyl, 365 cholera, 236, 270 chopped strand mat, 153–4 chromatography, 70–2 GPC column, 71 methods for analysis of polymer structure, 73 molecular weight analysis, 72 chromium, 442 CHS see central heating systems CNC machine, 429 cold slug, 62 collagen, 18 Columbia, 280 compressed gas explosion, 253–9 controversy, 258–9 cracked pipe, 254, 256–7 craze nucleation from diagonal contamination, 257

459

inner bore showing sub-critical cracks and main fracture, 256 main fracture surface, 257 fractured ABS pipe, 256 mechanics, 257–8 pneumatic system at glass works, 255 section showing storage tank and main pipe with bends, 255 compression, 21 compression moulding, 48, 294 Concorde, 371–2 condensation polymers see step-growth polymers configuration, 10 conformation, 10 connector end showing extensive cracking, 108 inner crack, 109 internal structure, 104 polycarbonate balloon catheters and angioplasty, 112–14 cracked joint, 104 disclosure, 106 discussion, 111–12 ESC/SCC hypothesis, 110–11 failures, 102–3 fitted in Hickman IV line, 103 injection moulding, 110 joint expert examination, 107–9 literature, 106–7 premature cracking, 103–6 Consumer Laboratories, 236 copper reservoir, 294 corrosion, 444 cotton, 17 covalent bonds, 3–6 carbon structures, 8 co-polymer repeat units, 4 natural rubber cross-linking, 5 repeat units and size in polymers, 4 tetrahedron carbon atom, 7 cracks, 20, 440, 447 crazing, 21 crazy paving cracks, 336, 361, 369 creep, 20, 51–2, 91

460

Index

creep deformation, 65 creep modulus, 234 creep rupture, 20, 65, 142 critical crack, 336, 369, 371 critical fatigue crack, 371 cross-linked polyethylene, 267–8 cross-linked rubbers, 306 Davy safety lamp, 414 decalin, 251, 424 degradation theory, 99–101 flow sequence of catheter manufacture, 100 depth gauge, 379 detergents, 131 Devils coach horse, 425 differential scanning calorimetry, 82–3, 251, 257, 263, 302, 304, 322, 325, 361, 402, 404, 409, 414, 419, 432 digital microscope, 59–60 dimethyl formamide, 80, 81 DMF see dimethyl formamide domains, 91 Dow Corning, 120 drive belts, 359–65 belt remains, 360–2 brittle fracture on double tooth, 362 brittle fracture surface, 362–3 critical tooth pair comparison, 363 failed belt stripped double teeth, 361 stripped single teeth, 361 fragment of rubber left after crack growth, 364 horizontal fatigue striations, 363 other composite belts, 365 sequence of events, 363–5 stripped belt drive vs new belt, 360 drop ball test, 217 drop impact test, 320, 407 DSC see differential scanning calorimetry ductile fracture, 65 DuPont, 264, 299, 445 DVS 2205, 138, 142, 144, 174 Dyneema, 15

E-glass fibre, 173 ebonite, 177 EDX see energy dispersive X-ray analysis elastic modulus, 50 elastin, 18 elastomer, 365 elastomeric seals failure in brakes, 273–8 accident explanation, 277–8 brake piston with fractured rubber seal, 273 cuts in outer lower edge of seal, 275 elastomer analysis, 276–7 fatigue striations, 274 fracture surface, 274–6 one side of fracture surface, 275 scratches in outer land of piston next to seal recess, 276 failure in semi-conductor factory, 281–93 air bearing with critical diaphragm seal, 282 air chemical analysis, 289 brittle crack in diaphragm seal, 284 chasing the problem, 290–2 compressor and air filters on pneumatic line, 291 crisis development, 289 damage to NBR diaphragm seal surface, 283 diaphragm seal fracture surface, 284 failed diaphragm seal, 282–3 fractured O-ring seal, 284 fractures in diaphragm seal, 285 independent analyses, 287–9 more failed seals, 283–7 new design of compressor, 292 ozone sources, 289–90 ozonolysis, 287 plan of fabrication lines, 288 recess of new seal, 282 sub-critical crack next to steel post, 285

Index X-ray emission spectra from normal and ozonised fracture surface, 286 elastomers, 195, 447, 450 properties, 18–19 tensile strength, 120 thermoplastic see thermoplastic elastomers electrocution, 411 electron microscopy, 207 electrostatic bonds, 6–7 Elf Atochem, 92 elongation ratio, 19 energy dispersive X-ray analysis, 59, 62–3, 331, 447 Engineering Failure Analysis, 36, 450, 454 Enron Inc, 247 environmental scanning electron microscopy, 59, 63, 248, 265, 285, 307, 342–3, 391–2, 404, 445, 447 advantage, 286–7 of failed end, 94–6 environmental stress cracking, 30–2, 203, 221, 224, 226, 244, 258, 269, 402, 405, 408, 409, 444, 448–9 hypothesis, 110–11 leaking polycarbonate battery case, 31 EPDM elastomer, 300, 377, 448 ESC see environmental stress cracking ESEM see environmental scanning electron microscopy Espace, 37 ethanol, 402 ethanol extraction method, 402 Ethicon, 123 ethylene, 190, 197, 418 ethylene and chlortrifluoroethylene copolymer, 154 ethylene chloride, 106, 203 ethylene propylene copolymers, 92, 190 extra corporeal membrane oxygenation, 107 extrusion, 47–8 spider lines in poorly mixed pipe, 47

461

Failure Analysis Group, 107 Faraday cage, 292 fatigue, 20, 66–7, 362 behaviour, 131 crack, 336, 338 striations, 318–20, 338 fatigue cracks, 258–9 Federal Drugs Administration Agency, 90, 452 ferric chloride, 252 Fiat cars, 447 Fiat Mirafiori, 377–81, 383–9 Fiat Motor Company, 382–3 fibreglass forensic investigation of failed storage tanks, 153–66 catastrophic failure on Teesside, 154–5 chopped strand mat, 154 composite thermal properties, 164–6 failed parts reassembly, 160–2 fracture locus, 162–3 plant damage, 155–7 usage history, 163–4 wall and base sections, 157–60 fibres aramid molecular structure, 17 mechanical properties, 17 fingerprint region, 303 fingerprint spectroscopy, 76 fingerprinting, 79 finite element analysis, 442 fittings, polyamide failure in ladders, 338–46 abrasion marks on bearing surfaces, 342 another accident, 343–4 broken composite plate, 346 broken stubs on stepladder, 344 combination of stepladder with blue feet, tips and connectors, 340 connectors showing locking mechanism, 340 failed stepladders, 339–42 fracture corner showing matrix depletion, 345

462

Index

fracture near corner showing weld line smooth areas, 343 moulded composite connector riveted together, 341 product design, quality and testing, 345–6 scanning microscopy, 342–3, 344 skid mark where loss of rubber fitting caused slip, 339 stubs fracture surface, 341 flame safety lamps, 223 flax, 17 fluorinated rubber, 272 Ford Cortinas, 374, 376 Ford Pinto, 393 forensic macroscopy, 56 forensic polymer engineering access to information, 449–54 old problems, 451–2 published literature, 450–1 the Internet, 452–3 Wikipedia, 453–4 causes of product failure, 438–54 poor choice of materials, 443–4 poor design, 441–3 stress concentrations, 442–3 components in transport applications, 349–94 drive belts failure, 359–65 failed Rilsan nylon fuel pipes, 374–90 SCC of nylon connectors, 390–3 tailpack failure in motorbike accident, 350–9 tyres failure, 365–74 consumer products, 396–437 ABS joints failure on bike carriers, 425–30 failure of fittings on luggage carriers, 416–25 HDPE baby cot latches failure, 430–4 kettle switches failure, 414–16 Noryl busbar plugs failure, 408–10 Noryl plugs failure, 397–408 RCDs, 411–14 environmental stresses, 444–9

data compilations, 449 ESC, 448–9 oxidation and ozonolysis, 446–8 SCC, 445–6 examination and analysis of failed polymer components, 42–88 ensuring results integrity, 85–6 forensic microscopy techniques, 58–63 mechanical testing, 50–5 molecular analysis of polymer properties, 69–85 processing methods and defects, 42–50 techniques for recording product failures, 55–8 types of product defect, 63–9 failure modes, 19–32 chemical attack, 26–9 environmental stress cracking, 30–2 mechanical failure, 20–6 stress corrosion cracking, 29–30 polymer properties, 11–19 elastomers, 18–19 natural materials, 17–18 polymer storage tanks, 134–75 Boston molasses disaster, 134–7 dealing with the aftermath, 170–3 fibreglass storage tanks, 153–66 PP and HDPE storage tanks, 137–53 reconstructing the events leading to failure, 166–70 setting new standards, 173–5 polymeric medical devices, 89–132 breast tissue expander, 114–21 breathing tubes, 124–30 catheter, 90–102 polycarbonate connectors, 102–14 sutures, 121–4 polymeric pipes and fittings, 226–70 ABS pipes and fittings failure, 250–3 compressed gas explosion, 253–9 gas pipelines failures, 243–9 polybutylene pipes and acetal resin fittings failures, 259–68

Index PVC water piping fracture, 227–35 PVC water pumps failure, 235–43 polymeric seals, 272–308 Challenger disaster, 278–81 elastomeric seals failure in brakes, 273–8 failed elastomeric seals in semiconductor factory, 281–93 silicone mastics failures, 301–6 TPE radiator washers failure, 293–300 poor manufacturing method, 438–41 assembly problem, 439–40 faulty moulding, 439 medical devices, 440–1 product failure, 1–11 bonding, 2–11 non-metallic elements, 2 product failure investigating methods, 32–5 public information sources, 35–40 small polymeric containers, 176–224 battery containers failure, 177–84 buckets failure, 184–7 design improvement to prevent failure, 219–24 exploding batteries, 187–95 failed truck battery cases, 195–8 miner lamp battery casings failure, 198–219 tools and ladders, 310–48 ABS handle failure, 328–31 failed polyamide fittings in ladders, 338–46 failure of chairs manufactured from polypropylene, 331–6 failure of handles in angle grinders, 316–24 failure of security caps for gas cylinders, 324–8 polypropylene hobby knives failure, 311–13 polystyrene components failure in hobby knives, 313–16 swimming pool steps failure, 336–8 Forensic Science Service, 391 Formula I racing cars, 15

463

fossil tree resin, 17 Fourier Transform Infrared spectroscopy, 73, 85, 190, 196, 234, 237, 251, 257, 263, 267, 298, 302, 304, 315, 322, 325, 333, 338, 361, 382, 402, 404, 409, 419, 427, 432, 447 fracture, 20 fretting, 20, 67 friction, 67–8 frozen-in strain, 26, 29, 32, 200, 222, 269 frozen-in stress, 212 FTIR see Fourier Transform Infrared spectroscopy fuse, 411 galling, 67 gas ionisation process, 63 gas pipelines, 243–9 brittle crack caused by steel pipe, 247 pipe from San Juan disaster of 1996, 246 creep rupture curves for MDPE gas lines, 245 Dubois explosion butt fusion weld rupture, 248 debris of house, 247 thermal weld fracture surface, 249 fracture surface, 248–9 Humberto Vidal store after propane gas explosion, 246 Waterloo explosion fractured MDPE pipe close-up, 244 steel main and plastic service pipe junction, 244 gel coat, 158 gel permeation chromatography, 70–2, 99 spectra of inner and outer surfaces of mancab, 151 General Accident, 227 General Electric Plastics, 212 German standard DVS 2205, 38 German Welding Institute, 142 glass fibre reinforced nylon, 414

464

Index

glass transition temperature, 12, 165, 338, 402, 410, 443 gold coating, 61–2 Goodyear, 272 Google, 452 gossamer, 18 GPC see gel permeation chromatography grouts, 273 Guardian Royal Exchange, 385 hackles, 231 handle, ABS failure, 328–31 etch pits in lower wing, 331 fracture surface map of handle break, 330 fracture surface of handle with striations, 329 intact and failed handle, 329 scanning electron microscopy, 331 scratches and peeling of chrome coating, 330 hard rubber, battery containers failure, 177–84 HDPE see high-density polyethylene Health and Safety Executive, 173, 450, 452 heating coil, 414 helium, 193, 195 Hertzian stress profile, 368 Hevea brasiliensis, 17 Hickman line, 105, 108 IV line fitted with polycarbonate connectors, 103 high-density polyethylene, 268, 435 baby cot latches failure, 430–4 analysis, 433–4 broken latch, 431–2 failed baby cot with fractured latch at upper right, 430 fracture surface showing main crack origin, 432 intact left-hand latch and broken right-hand latch, 431 intact vs broken latches, 431 melt fracture line on failed latch, 433

forensic investigation of failed storage tanks, 137–53 Hindenburg airship, 193–5 airship outer fabric, 195 photo of 1937 disaster, 194 HIPS, 204, 443 hobby knives failure of polypropylene-based components, 311–13 accident reconstruction, 311–13 blade rotation, 312 component parts design, 312 power grip and precision grip of knife, 313 small hobby knives various designs, 311 failure of polystyrene components, 313–16 blade fixing, damage and blood stains, 315 damage near tip of knife, 316 damaged plastic top close-up, 315 failed knife vs Stanley knife, 314 knife inspection, 314–16 Hoechst, 264 Hookean solids, 19 hoop stress, 167, 233–4, 258, 262, 266, 277, 371 hot plate method, 248 Humberside Fire Brigade, 188 hydraulic tensometer, 312 hydrazine, 210 hydrocarbon oil, 444 hydrocarbons, 250 hydrochloric acid, 171, 237, 250, 251 hydrofluoric acid, 174 hydrogen, 193, 253 alleged explosion, 220–1 explosions, 190–1 experiments, 190 hydrogen bonds, 7–9 nylon 66 sheet structure, 9 polymer structure and melting points, 10 hydrolysis, 27–8, 99, 173, 299–300, 445–6, 451

Index hydrostatic pressure, 140 hydrostatic stress, 277 hydroxyl ions, 210 hypochlorous acid, 451 hysteresis, 365 Hytrel, 445 Hytrel polyester elastomer, 307 Hytrel washers, 299–300, 308 Igepal, 31, 449 Immingham Docks, 250–3 acid storage facility, 172 reinforced lower walls of composite storage tanks, 172 impact, 22 Inamed Corporation, 120 India Mark II, 235 infrared spectroscopy, 72–6, 98, 302 correlation table, 74 electromagnetic spectrum and interactions with matter, 74 IR spectrum LDPE showing oxidation, 76 MDPE showing oxidation, 75 using ATR, 276 injection moulding, 43–7, 110, 294 conditions, 405–7 in China, 406 critical entanglement molecular weight, 46 moulding cycle, 44 process flow, 43 variation in melt viscosity with shear rate, 45 iron oxides, 260 ISO 9000, 34 jute, 17 kettle switches, 414–16 defects in unlicensed switch from electric kettle, 416 DSC thermogram Chinese switch, 415 switch material, 415 Kevlar, 15

465

Laporte Chemicals, 195 latch see baby cot latches latent defects, 64 LDPE see low-density polyethylene light scattering, 72 lignin, 18 little scratcher, 208 London Fire Brigade, 301, 377 Loss Prevention Bulletin, 37 low-density polyethylene, 184–5 bucket with fractured lug, 184 buckets failure, 184–7 luers, 103 Luftwaffe, 194 luggage carriers fittings, 416–25 first accident, 417 abrasion damage, gate and weld lines, 418 failure sequence, 419–20 folded luggage trolley showing fractured plastic part, 417 fracture and other surfaces, 417–19 fractured subsurfaces and smooth areas, 418 weld line next to fracture surface, 419 second accident, 420–1 contamination, 424–5 debris from connector, 424 EDX spectrum from particle, 424 foreign particles seen in SEM, 423 fracture surface, 421–3 fracture surface showing various origins, 422 origins showing contaminants, 422 second failed trolley, 421 macrophotography, 105 magnesium, 287 maintenance-free batteries, 187, 188 mancabs, 446 brittle crack in mancab, 148 cracked mancab, 148 GPC spectra of inner and outer surfaces of mancab, 151 IR spectra of HDPE mancab materials, 150

466

Index

severe cracking on roof of mancab, 149 mastics, 273, 301 MDPE see medium-density polyethylene mechanical failure, 20–6 axial crazing, 21 load paths, 22–3 hairline cracks, 23 loading patterns, 21–2 schematic craze profile, 22 stress concentration, 23–6 failed hobby knife, 26 flat bar, 24, 25 fractured wedding knife, 26 internal corners in accelerator pedal, 24 mechanical testing, 50–5 composite materials, 52–3 photoelastic strain analysis, 53–5 tensile testing, 50–2 medical devices see polymeric medical devices Medical Devices Agency, 106 Medicines and Healthcare Products Regulatory, 90 medium-density polyethylene, 243–7, 270 cast film, 74–5 MEDWATCH, 37 melt flow index, 45, 197 melting points, 443 Mentor Corporation, 120 metadata, 452 metal technology, 450 methane, 198–9, 223, 245, 253 methanol, 207 methanometers, 223, 224 methylene chloride, 68, 106, 203, 236–7 Michelin, 366 microtome, 70 miner lamp battery casings, 198–219, 439–40 colliery experience, 217–19 increase in battery life, 219 polycarbonate lamp lifetime before design changes, 218

cracks at inner corners in leaking miner’s lamp, 215 destructive examination, 211 etch pit in strained PC, 207 in TEM, 210 failure records after design changes, 219 from two Lancashire collieries, 218 first failures, 200–1 failed mining lamp cases histograms, 201 leaks in polycarbonate case from ESC, 202 moulding conditions, 211–13 and residual strain in cases, 212 new design in polycarbonate, 200 polishing, 207–10 as function of caustic concentration, 209 interferometer traces of scratch polishing, 208 tensile stress-strain curve for sheet polycarbonate, 206 polycarbonate polishing as function of crack size, 209 mechanism, 210 practical applications, 215–17 brittle crack induced by positive plate expansion, 216 crack at belt loop corner, 216 residual strain at belt loop corners, 217 property checks, 206–7 residual strain from gate to end, 211 solvent cracking, 201, 203 strain birefringence, 203–5 brittle cracks around windows, 204 overall pattern, 205 patterns around windows before and after welding, 205 stress concentrations, 213–15 at box inner corner, 214 various designs, 202 Mirafiori see Fiat Mirafiori modified butyl elastomer, 365–6

Index molecular analysis Beer-Lambert law, 76–7 chromatography, 70–2 finger print spectroscopy, 76 infra-red spectroscopy, 72–6 NMR spectroscopy, 79–80 other methods, 80–1 sampling, 69–70 thermal analysis, 81–5 UV spectroscopy, 77–9 monodisperse polymers, 6 motorbike accident alternative theory, 355–7 bungee cords, 354–5 critical speed, 357 deeper analysis, 352–4 seized wheel, 350–2 skid mark analysis, 358 tailpack design, 358–9 tailpack failure, 350–9 NASA, 278, 280, 306 National Coal Board, 200, 207 natural gas, 245 natural rubber, 17–18, 365, 366, 370, 382, 383 NBR see nitrile butadiene rubber NBR diaphragm seal, 307 NBR fuel pipes, 447 NBR seals, 441, 447–8 Neonatal Network, 107 Neoprene, 272 NHTSA, 387, 388 Nissan Arc Labs, 398, 401, 402 nitric acid, 174, 290 nitrile butadiene rubber, 19, 272, 282, 287, 290, 361, 365, 376, 382, 389 nitrogen, 290 nitrogen dioxide, 292 nitrogen gas, 301 nitrogen gas blanket, 306 nitrogen oxides, 290 NMR spectroscopy, 79–80 DMF extract spectrum, 81 polyacrylic acid spectrum, 80 non-metallic elements, 2

467

Noryl busbar plugs, 408–10 base unit linked to busbar system, 408 brittle crack growth in plug cover, 410 brittle crack penetration of base from screw hole, 409 quality control, 410 rated at 32 amp, 409 Noryl casings, 85 Noryl plugs, 397–408, 435 base and casing welded joint, 399 brittle cracks and weld lines in base moulding, 400 in ultrasonic weld, 401 within surface loom, 401 EDX analysed of various raw materials, 405 spectrum of various raw materials, 405 fingerprints of sample No 5, 405 FTIR fingerprints of different Noryl sources, 403 spectra of plug No. 25, 404 spectrum of suspect product Noryl, 403 injection moulding conditions, 405–7 in China, 406 material analysis, 402–5 microscopy, 398–402 thermograms of different granules, 404 transformer plug showing innards, 398 weld line and cracks associated with surface blooming, 400 next to earth pin, 400 notch sensitivity, 441 NTSB, 243, 247, 248, 249, 450, 452 Nujol, 70 nylon, 322, 339, 392, 396, 450 nylon 12, 98, 392 nylon 6, 5, 322, 323

468

Index

nylon 6,6, 52, 81 nylon 66, 5, 322, 323, 414 nylon 66 connector, 445 nylon connectors failure sequence schematic diagram, 392 final cusp showing intermittent crack growth, 392 fracture surface, 391 fractured nylon connector vs intact return tube, 391 recovery vehicle responsible for diesel leak, 390 SCC, 390–3 O-ring, 272, 278, 283 optical microscopy, 59–60, 248, 256, 298, 422 osmometry, 72 overload, 20 Oxfam, 235 Oxford Products Ltd, 359 Oxford Trading Standards, 359 oxidation, 27, 145, 446–8 oxidation cracks, 369 oxidative cracking, 336 oxygen, 27, 266, 290, 423, 446 oxygen cracking, 361 ozone, 27, 287, 290, 292, 369, 377, 440, 447 sources, 289–90 ozone cracking, 361, 369, 373, 377, 387, 389, 451 ozone gas, 377, 382 ozonolysis, 287, 292–.3, 446–8 PAA see polyacrylic acid paraffin oil, 301, 303, 304, 307 Paris equation, 238 Pasminco mine, 220 patent defects, 64 PBT see polybutylene terephthalate Pebax, 92, 102 pendulum impact test, 196 PET see polyethylene terephthalate petroleum ether, 276 PEX see cross-linked polyethylene

phase-separated polystyrene, 403 phenolic resin, 410 phenolics, 253 photo-tendering, 153 pipes and fittings, 250–3 forensic polymer engineering, 226–70 ABS pipes and fittings failure, 250–3 compressed gas explosion, 253–9 gas pipelines failures, 243–9 polybutylene pipes and acetal resin fittings failures, 259–68 PVC water piping fracture, 227–35 PVC water pumps failure, 235–43 Planck’s constant, 72 plastic electric kettle, 397, 414 plasticisation, 304 PMMA see polymethyl methacrylate pneumatic system, 289 Polaroid sheet, 203, 220 polyacrylic acid, 79 polyacrylonitrile, 283 polyamide, 322, 323 failed fittings in ladders, 338–46 polyamide 12 see nylon 12 polyamide 6, 322 polybut-1-ene, 259 polybutadiene, 315 polybutene, 263, 267, 305, 307 polybutene oil, 305 polybutylene, 266, 268, 269, 446, 450 polybutylene pipes and acetal resin fittings failures, 259–68 acetal albatross, 263–4 degradation mechanism, 264–5 fracture, 259–60 literature review, 260, 262 pipe failures, 266–7 recent developments, 267–8 failure modes, 266 polybutylene terephthalate, 45 polycarbonate, 31, 45, 106, 317, 396, 413, 439, 440, 445, 448, 449, 453 connectors cracked joint, 104 fitted in Hickman IV line, 103

Index polishing as function of crack size, 209 polishing mechanism, 210 Polycase, 183, 195 polychloroprene, 272 polydisperse polymers, 6 polyester, 9, 294, 300, 361, 371, 443 polyethers, 294, 300, 302 polyethersulphone, 45 polyethylene, 226, 243, 259, 446 polyethylene terephthalate, 5, 210, 445 polyglactide, 121–4 polyglactin 910 see Vicryl polyimide, 210 polyisobutylene, 305 polymer storage tanks collapsed storage tank on Teesside, 156 composite storage tank before accident, 156 dealing with the aftermath, 170–3 acid storage facility at Immingham Docks, 172 acid storage tanks, 171–2 glass fibre attack, 173 other failures, 172–3 reinforced lower walls of composite storage tanks, 172 standards, 170–1 fibreglass storage tanks, 153–66 base corner showing metal foil still left in joint, 161 base fragments showing junction of base and wall, 159 close-up of side of tank, 156 DSC curves of PP liner, 165 DSC traces of GPR materials, 165 failed section reconstruction with visible bulging in tank wall, 161 history of tank conditions, 164 parts of side wall resembled with crack growth along seams, 163 pipe hole in side wall with gap from delamination, 163 plan of tank, 159 side wall at pipe junction showing delamination around hole, 162

469

side wall with inner liner at top, 160 tank base showing inner lining and composite bottom, 160 tank remnant, 157 tank section, 158 forensic investigation, 134–75 Boston molasses disaster, 134–7 setting new standards, 173–5 plan of submerged thermoplastic storage tank, 146 PP and HDPE storage tanks, 137–53 close-up of centre panel of tank, 139 crack origin showing four phases of growth, 140 discrepancy between DVS 2205 and tank structure, 143 faulty paint tank, 144 schematic of failed tank, 139 submerged sewage tank, 145 tank design schematic with different walls, 142 reconstructing the events leading to failure, 166–70 failure sequence, 168–9 hoop stress, 167 material strength, 167–8 reactions in tank, 168 the bund, 169–70 thermal expansion, 166–7 tank wall inward deformation, 146 test tank complete collapse of walls, 147 ready for testing, 146 start of wall deformation, 147 polymeric medical devices, 89–132 block copolyester microstructure, 92 device failure breast tissue expander, 114–21 breathing tubes, 124–30 catheter, 90–102 connectors, 102–14 sutures, 121–4 polymers, 447 brittle crack in radiator tank of GF nylon 6,6, 61 on tank near external buttress, 52

470

Index

elastomer properties, 18–19 ensuring results integrity, 85–6 evidence shifting, 32–6 records, 33–4 surviving remains, 34–5 witness evidence, 33 failed components examination and analysis, 42–88 ABS mill bobbins replacing wooden version, 69 butt weld showing brittle crack discoloration, 49 damage to outer strands in failed rope vs unaffected cut rope, 42–88, 57 failed bridge bearing caused by abrasion of steel pin by debris on nylon sleeves, 68 failed car radiator tank showing gross distortion, 51 failed rope coil vs new rope, 57 fatigue crack in nylon 6,6 fuel line connector, 64 fatigue striations in ABS vacuum cleaner part, 66 girder section showing stress raiser at upper corner, 54 pipe failure at chemical plant on Teesside, 49 polycarbonate set square showing residual moulding stresses, 54 poorly mixed pipe showing spider lines, 47 forensic microscopy techniques, 58–63 ESEM, 62–3 optical microscopy, 59–60 SEM, 61–2 gold coated fracture surface optical micrograph showing weld line and cold slugs, 62 SEM micrograph, 62 high performance polymer fibres, 15 mechanical properties, 17 molecular structure of aramid fibre, 17 materials mechanical testing, 50–5

composite materials, 52–3 creep and stress relaxation, 51–2 photoelastic strain analysis, 53–5 tensile testing, 50 molecular analysis, 69–85 Beer-Lambert law, 76–7 chromatography, 70–2 fingerprint spectroscopy, 76 infra-red spectroscopy, 72–6 NMR spectroscopy, 79–80 other methods, 80–1 sampling, 69–70 thermal analysis, 81–5 UV spectroscopy, 77–9 natural materials, 17–18 ozone cracks diesel fuel pipe for CHS boiler, 60 from outer corners in numerals, 60 pipes and fittings, 226–70 processing methods and potential faults in materials, 42–50 extrusion, 47–8 injection moulding, 43–7 other moulding methods, 48 other shaping routes, 48–50 properties, 11–19 mechanical properties, 16 melting vs glass transition point, 13 thermal properties, 14 viscoelastic master curve, 12 public information sources, 35–40 disasters, 38–40 event reporting, 36–7 materials and product standards, 37–8 public domain, 37 textbooks, 36 seals, 272–308 Challenger disaster, 278–81 elastomeric seals failure in brakes, 273–8 failed elastomeric seals in semiconductor factory, 281–93 silicone mastics failures, 301–6 TPE radiator washers failure, 293–300

Index techniques for recording product failures, 55–8 forensic macroscopy, 56 radiography, 56–8 visual observation, 55 types of product defect, 63–9 environmental failure, 68–9 fatigue, 66–7 friction and wear, 67–8 mechanical defects, 64–6 see also specific polymer polymethyl methacrylate, 31, 45, 200, 449 axial crazes in beaker, 21 polyolefin, 92 polyphenylene oxide, 396 polypropylene, 10, 28, 151, 179, 182, 190, 193, 195, 196, 197, 199, 250, 251, 259, 268, 310, 333, 347, 375, 396, 397, 414, 418, 436, 439, 444, 446 battery case degraded by UV attack along weld, 152 chalk filled, 332 copolymer with ethylene, 312 exploding batteries, 187–95 failure in chairs, 331–6 failure in hobby knives, 311–13 forensic investigation of failed storage tanks, 137–53 catastrophic failure, 138 causes of failure, 141–2 failed battery cases, 152–3 investigation, 138–40 other problems, 143 paint accident, 143–4 rotational moulded tanks, 144–8 stress concentration, 140–1 UV degradation, 148–52 GPC spectra of inner vs outer surfaces of PP cases, 153 talc-filled, 325 truck battery cases, 195–8 polystyrene, 32, 188, 191, 200, 204, 315, 396, 403, 410, 443 failure in hobby knives, 313–16

471

polysulphone for breathing tubes, 124 melt viscosity as function of shear rate, 127 prototype moulded sight tube, 126 polytetramethylene glycol, 98 polyurethane, 9, 272, 280 polyurethanes, 204 polyvinyl butyral, 349 polyvinyl chloride, 45, 226, 259, 270 water piping fracture, 227–35 water pumps failure, 235–43 polyvinylidene fluoride, 154 potassium hydroxide, 207 powdered anthracite coal, 177, 180 power law, 46 power-precision grip, 313 Powerbreaker, 413 PP see polypropylene prepreg mat, 153 pressurised air systems, 226 Prolene, 122 propane, 253 propane gas, 245 Protector, 413 proto-crack, 277, 342, 442 PTFE tape, 294 PVC see polyvinyl chloride R101 disaster, 195 radiography, 56–8 contact radiographs of fibre orientation in composite polymer, 58 contact radiography, 58 ramie, 17 RAPRA, 36, 253, 260, 296, 408, 450 rate law, 44 rayon, 18, 366, 371 RCD see residual current devices recycling, 402 reflected light microscopy, 59 repeat unit, 3 residual current devices, 397, 411–14, 436 fitted into 13 amp plug, 411 mechanical switch, 412

472

Index

patent action, 413–14 primed mechanism and balance of forces in equilibrium state, 412 residual strain, 203 Rilsan, 392 Rilsan diesel fuel line, 390 Rilsan nylon, 382, 392 Rilsan nylon fuel pipes, 374–90 car fire in Ireland, 383–5 Fiat car remains, 384 Rilsan feed and return fuel pipes inside passenger compartment, 385 Fiat fuel lines, 379–83 fuel pipes of Mirafiori saloon car, 380 histogram of ozone cracks in smooth rubber fuel pipe, 381 ozone cracks formation under tensile stress, 381 ozone cracks in NBR rubber fuel hose, 380 Rilsan fuel pipe abrasion, 380 fires in tunnels, 389–90 first encounters, 374–6 fire-damaged Ford Cortina, 375 fuel hose showing split at extreme right and mark in bore at top, 376 remains of fuel pump inlet hose, 375 global markets, 388–9 Murphy infants-v-Fiat spa, 385–6 other Mirafiori fires, 387–8 Sir John Gielgud, 377–8 fire damage to Fiat Mirafiori saloon car, 378 U-clip holding the fuel pipes, 379 spider lines, 376–7 road cones, 446 RoSPA, 435 rotational moulding, 48 rubber, 362, 366, 372 rubber seals, 306–7 rubber toughened polymers, 25 rubber washers, 294 rust, 236

SAN see styrene-acrylonitrile SBR see styrene butadiene rubber SBS copolymers see styrene-butadiene copolymers scanning electron microscopy, 59, 61–2, 331, 336, 392, 423–4 SCC see stress corrosion cracking scission, 446 screw test, 410 sealants, 306, 444 security caps, for gas cylinders, 324–8 before fitment showing hinges and gates, 326 maximum torque in hand grip of cylinder of various diameters, 328 safety guard for oxygen cylinder, 325 storage failures, 325–7 torque test development, 327–8 weld lines in thin polymer hinges of cap guard, 327 selenium crystal, 276 SEM see scanning electron microscopy semi-conductor factory, failed elastomeric seals, 281–93 shear, 21 Shell, 262, 268 shellac, 17 silica, 7, 423 silicon, 423 silicone, 302 silicone mastics, 301–6 calorimetry, 303 beige sealant single boiling point, 304 brown sealant showing multiple endotherms, 305 paraffin oil thermogram, 304 fire station training building, 301–2 sealant analysis, 302–3 sealant drip from a joint, 302 sealant extensive deterioration, 303 silicone rubber for breast tissue expander, 114–21 silk, 18 Silver Bridge, 444

Index sintering, 48 small polymeric containers, 176–224 battery containers failure, 177–84 aircraft batteries, 182 failures, 178–9 hard rubber tank storage battery, 178 investigation, 179–80 material analysis, 180–1 military batteries, 177–8 patent action, 182–4 plan and section of handle, 180 screw thread and brittle crack, 181 single handle held by single screw to case, 179 stripped thread on aircraft storage battery, 182 thin walled battery container Polycase UK, 183 buckets failure, 184–7 weld line formation, 186–7 design improvement to prevent failure, 219–24 alleged hydrogen explosion, 220–1 damaged case from Pasminco mine, 220 further developments, 223–4 possible crack path map, 221 South African lamps, 221–3 exploding batteries, 187–95 bulging ends of battery case, 192 crack origins from inner corners, 190 critical crack in battery top, 189 exploded car battery, 192 fire brigade incident, 188–9 Hindenburg disaster, 1937, 193–5 hydrogen explosions, 190–1 internal explosion in large leadacid battery case, 189 material quality, 190 personal injury, 191–3 failed truck battery cases, 195–8 cracked prototype truck battery lids, 196

473

first failures, 196–8 sharp inner corners on truck cases, 197 miner lamp battery casings failure, 198–219 colliery experience, 217–19 first failures, 200–1 moulding conditions, 211–13 new design in polycarbonate, 200 polishing, 207–10 practical applications, 215–17 property checks, 206–7 solvent cracking, 201, 203 strain birefringence, 203–5 stress concentrations, 213–15 strain birefringence patterns in cases of British batteries, 222 in cases of South African batteries, 222 SMC, 293 Society of Plastics Engineers, 107 soda-lime-silica glass, 7 sodium carbonate, 445 sodium hypochlorite, 331 sodium hypochlorite solution, 265 solid polybutadiene rubber, 365 Spectra, 15 spider lines, 47, 376–7 Stanley knife, 311, 312, 314, 316 Staphylinidae, 425 steel-rubber bond, 373 step-growth polymers, 5, 445 stepladders, 310, 348 broken stubs, 344 combination with blue feet, tips and connectors, 340 failed, 339–42 stereomicroscopy, 59 storage tanks see polymer storage tanks strain analysis, 53 stress concentration factor, 319 stress corrosion cracking, 29–30, 150, 173, 226, 253, 262, 263, 265, 267, 269, 331, 390, 444, 445–6 hypothesis, 111–12 nylon connectors, 390–3

474

Index

failure sequence schematic diagram, 392 final cusp showing intermittent crack growth, 392 fracture surface, 391 fractured nylon connector vs intact return tube, 391 recovery vehicle responsible for diesel leak, 390 stained fracture in acetal fitting, 30 stress optical coefficient, 204, 212 stress relaxation, 20, 355 striking-out motion, 386 styrene butadiene rubber, 19, 272, 276, 365, 382 styrene-acrylonitrile, 32, 204, 283 styrene-butadiene copolymers, 91–2 microstructure, 92 sulphation ghost, 188 sulphur dioxide, 27 sulphuric acid, 176, 182, 187, 201, 210, 392, 445 sutures forensic investigation of failure, 121–4 fractured test end of Vicryl suture, 123 new suture analysis, 122–3 outcome, 124 possible causes of failure, 123–4 wound opening, 121–2 swimming pool steps, 336–8 broken step, 337 fatigue crack, 336, 338 fatigue striations at upper right near origin, 338 ladder with top step replaced after failure, 337 part of fracture origin, 337 switches see kettle switches T type batteries, 217 tailpack, in motorbike accident, 350–9 alternative theory, 355–7 bungee cords, 354–5 critical speed, 357 damaged bike lock, 354

damaged tailpack, 353 deeper analysis, 352–4 design, 358–9 lead motorbike after accident, 351 reconstruction of tailpack perched on bike rear seat, 352 schematic diagram of bag stability showing forces acting on the bag, 356 seized wheel, 350–2 rear wheel puncture after wheel seizure, 351 skid mark from rear wheel of lead motorbike, 351 sideways slippage, 353 skid mark analysis, 358 teak oils, 301 tensile modulus, 50 tensile strain, 234 tensile strength half-life, 299 tensile stress relaxation, 51–2 tensile testing, 50 tension, 21 tensometer, 98, 234 tetrahydrofuran, 71 tetramethyl silane, 85 The Welding Institute, 398, 402 thermal analysis, 81–5 analysis methods, 82 crystallisation properties of polyethylenes, 85 melting behaviour of various polyethylenes, 84 schematic DSC thermogram, 83 thermal transitions, 82 thermogram of PET from soft drink bottle, 84 thermal expansion, 166–7 thermal failure, 67–8 thermocouple, 33 thermoplastic elastomers, 14, 91–2, 294 direct examination, 298 DSC thermograms of new and failed washers, 298 fatigue striations on fracture surface, 299 hydrolysis, 299–300

Index leaks in CHS systems, 294–5 cracked seals fitted to steel plugs, 296 failed washer as received, 296 radiator with cracked washer, 295 new washers, 294 radiator washers failures, 293–300 simulation experiments, 296–8 extrusion and paint on seal surface of cracked washer, 297 Hytrel washers exposure experiments, 297 polymer extrusion on failed washer, 297 thermal stability to hydrolysis, 300 thermoplastic transformer plugs see Noryl plugs thermoplastics, 5, 176–7, 374 thermosets, 5, 176 Thiokol Corporation, 280, 306 torsion, 22 Total Parental Nutrition, 448 TPE see thermoplastic elastomers Transco, 243 tri-butylphosphate, 401 triboelectrification, 292 tricresyl phosphate, 402 tung, 301 Tuohy needle, 93 Twaron, 15 tyres, 365–74 damage to crown of tyre, 368 failed truck tyre from fatal accident, 367 fraying of tyre cords, 370 modern tyre technology, 371–4 Concorde tyre fragment, 373 Concorde with fuel tanks above landing gear and wheels, 372 failed bearing from Renault Espace, 373 helicopter rotor bearing with multiple rubber layers, 374 piece of titanium strapping which initiated the Paris disaster, 372 oblique view of damaged tyre, 367

475

oxygen and ozone cracking, 369–70 depth of cracking in sidewall, 369 outer section of sidewall, 369 sequence of events, 370–1 truck tyre failure, 366–8 truck remains from fatal motorway accident, 367 ultra-high molecular weight polyethylene, 15 ultraviolet radiation, 28–9, 446 fractured and whitened battery top, 29 ultraviolet rays, 68, 70 unplasticised polyvinyl chloride, 154 Unsafe at any Speed, 393 uPVC see unplasticised polyvinyl chloride US Industrial Alcohol, 136–7 UV absorbent, 35 UV spectroscopy, 77–9, 252 phenolic UV stabiliser structure, 78 spectrum of extracts showing effect of added stabiliser, 78 van der Waals bonds, 9–11 rotational isomerism, 10 Vickers hardness, 298 Vicryl suture, 121–4 fractured test end, 123 village level operation and maintenance, 236 viscoelastic master curve, 11–12 viscometry, 72 visual observation, 55 Viton, 39, 272, 292, 389 fluoroelastomer O-rings, 278, 306 lip seals, 306 VLOM see village level operation and maintenance VOC see volatile organic components volatile organic components, 290 vulcanisation, 272 vulcanised rubber, 374 Warrington tank, 138, 154, 169 failure, 145

476

Index

washers, 293 water piping fracture, 227–35 broken pipe analysis fracture at solvent welded joint, 231 fracture close-up, 231 fractured collar of rising main on sprinkler system, 230 void between pipe and collar at joint fracture, 232 causes of failure, 235 factory crisis, 227–8 adjuster after the incident, 228 air valve fitted 3 months before accident, 229 new air valve fitted to sprinkler system, 229 sprinkler system plan, 230 reconstruction, 232–3 fracture reconstruction to show twist and separation, 233 original photo of fracture, 232 pipe stresses from hydrostatic pressure, 234 stresses on pipe, 233–4 water pumps failure, 235–43 fatigue tests, 238–40 bonded area and unbonded joint tracing, 239 failure by joint pull-out, 239 fatigue crack at corner of solvent welded joint, 239

hand pump, 237 machined PVC problem, 240–2 bag wrapping machine using grey PVC components, 241 long arm showing large gap in joint, 241 outer joint showing gaps, 242 mediation, 242–3 methylene chloride test for PVC water pipes, 238 polished section long arm showing large gap in joint, 241 outer joint showing gaps, 242 rising mains, 236–8 weakest link principle, 87 wear, 20, 67–8 wear process, 67 weld line, 326, 327, 338, 343, 442 welding, 48–50 Welding Institute, 155 whitening, 28 Wikipedia, 437, 453–4 wind tunnel tests, 358 Woolwich Arsenal, 217 wound opening, 121–2 wrapping machine, 240 Y-junctions, 102 zinc, 287 zinc chromate putty, 278