Materials in Sports Equipment, Volume 2 (Woodhead Publishing in Materials)

Author: A. Subic

77 downloads 1390 Views 23MB Size Report

This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!

Report copyright / DMCA form

DOWNLOAD PDF

Materials in sports equipment

© 2007, Woodhead Publishing Limited

Related titles: Materials in sports equipment Volume 1 (ISBN 978-1-85573-599-6) Improvements in materials technology have produced a signiﬁcant impact on sporting performance in recent years. The relationship between materials technology and design and their effects on sporting performance is the focus of this important handbook. From topics related to the general use of materials in sports – for example, sports surfaces and the behaviour of balls and ballistics, the book goes on to explore in detail the particular requirements of materials for many of the most popular sports including golf, tennis, cycling, mountaineering, skiing, cricket and paralympic sports. This book is an essential text for students on sports technology courses, manufacturers of sports equipment and materials scientists working with new materials with potential for sports applications. Textiles in sport (ISBN 978-1-85573-922-2) Technical developments in the sports clothing industry have resulted in the use of engineered textiles for highly specialised performance in different sports. With highly-functional and smart materials providing a strong focus in the textiles industry, companies are increasingly looking for ‘value-added’ textiles and functional design in sportwear, as well as intelligent textiles which monitor performance using built-in sensors. The combination of clothing function with comfort is a growing market trend, and for all those active in sport, constitutes one of the vital factors for achieving a high level of performance. Written by a distinguished editor and a team of authors from the leading edge of textile research, Textiles in sport is an excellent resource for anyone with an interest in the role of advanced textiles in sports performance. Details of these and other Woodhead Publishing books, as well as books from Maney Publishing, can be obtained by: • visiting www.woodheadpublishing.com • contacting Customer Services (e-mail: sales@woodhead-publishing. com; fax: +44 (0) 1223 893694; tel.: +44 (0) 1223 891358 ext. 130; address: Woodhead Publishing Ltd, Abington Hall, Abington, Cambridge CB21 6AH, England) Maney currently publishes 16 peer-reviewed materials science and engineering journals. For further information visit www.maney.co.uk/ journals.

© 2007, Woodhead Publishing Limited

Materials in sports equipment Volume 2 Edited by Aleksandar Subic

Woodhead Publishing and Maney Publishing on behalf of The Institute of Materials, Minerals & Mining CRC Press Boca Raton Boston New York Washington, DC

Cambridge England

© 2007, Woodhead Publishing Limited

Woodhead Publishing Limited and Maney Publishing Limited on behalf of The Institute of Materials, Minerals & Mining Woodhead Publishing Limited, Abington Hall, Abington, Cambridge CB21 6AH, England www.woodheadpublishing.com Published in North America by CRC Press LLC, 6000 Broken Sound Parkway, NW, Suite 300, Boca Raton, FL 33487, USA First published 2007, Woodhead Publishing Limited and CRC Press LLC © 2007, 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 978-1-84569-131-8 (book) Woodhead Publishing ISBN 978-1-84569-366-4 (e-book) CRC Press ISBN 978-1-4200-6572-5 CRC Press order number WP6572 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 elementary chlorine-free practices. Furthermore, the publishers ensure that the text paper and cover board used have met acceptable environmental accreditation standards. Typeset by SNP Best-set Typesetter Ltd., Hong Kong Printed by TJ International Limited, Padstow, Cornwall, England

© 2007, Woodhead Publishing Limited

Contents

Contributor contact details Preface Introduction

xi xv xvii

Part I

General issues

1

1

Modelling of materials for sports equipment M. Strangwood, The University of Birmingham, UK

3

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2

Introduction Properties of metallic alloys Modelling the properties of metallic alloys Modelling polymeric materials Properties and modelling of composites Modelling sandwich structures Future trends Acknowledgements References

Non-destructive testing of sports equipment: the use of infrared thermography M. P. Luong, LMS CNRS, France 2.1 2.2 2.3 2.4 2.5 2.6 2.7

Introduction Principles of infrared thermography testing Infrared thermography technology Applications: mechanical performance of tennis racket strings Applications: damage detection in leather sports footwear Applications: testing sailcloth for yachts Applications: soccer and long distance walking

3 4 9 15 18 23 31 32 33

35 35 36 40 42 47 52 53 v

© 2007, Woodhead Publishing Limited

vi

Contents 2.8 2.9

3

60

3.1 3.2 3.3 3.4 3.5 3.6

60 61 66 69 72

Introduction Textiles for sports apparel: ﬁbers, yarns and fabrics Finishing and fasteners Testing sports apparel performance Design of sports apparel: thermal performance Design of sports apparel: water resistance and other properties Future trends Sources of further information and advice References

78 81 84 85

Protective helmets in sports S. V. Caswell, George Mason University, USA; T. E. Gould and J. S. Wiggins, University of Southern Mississippi, USA

87

4.1 4.2 4.3

87 88

4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 5

57 57

Materials and design for sports apparel K. B. Blair, Sports Innovation Group LLC, USA

3.7 3.8 3.9 4

Summary References

Introduction Incidence of mild traumatic brain injury in sport Biomechanics and dynamics of head impacts in sport Helmet construction: shell materials Helmet construction: liner materials Helmet safety standards and performance testing Helmet design for particular sports: lacrosse, ice hockey, rugby and football/soccer Future trends Sources of further information and advice Acknowledgements References

89 95 100 104 110 117 117 123 123

Mouth protection in sports T. E. Gould, S. G. Piland, C. E. Hoyle and S. Nazarenko, University of Southern Mississippi, USA

127

5.1 5.2

127

Introduction The development and classiﬁcation of mouth protection in sport

© 2007, Woodhead Publishing Limited

128

Contents 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11

Incidence of orofacial injury in sport Biomechanics and dynamics of dental injury Polymeric materials and fabrication techniques for mouthguards Standards and testing for mouthguards Comfort and ﬁt of mouthguards Future trends Sources of further information and advice Acknowledgements References

vii 131 132 140 145 148 149 150 154 154

Part II Speciﬁc sports

157

6

Design and materials in baseball J. Sherwood and P. Drane, University of Massachusetts–Lowell, USA

159

6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9

159 162 166 176 178 181 182 183 184

7

8

Introduction Ball design and construction Bat design and construction Baseball gloves Protective and other equipment Future trends Sources of further information and advice Acknowledgements References

Design and materials in snowboarding A. Subic and J. Kovacs, RMIT University, Australia

185

7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8

185 186 188 192 195 200 202 202

Introduction Riding styles in snowboarding Snowboard design Materials and their conﬁguration in snowboards Manufacture of snowboards Summary and future trends Acknowledgements References

Design and materials in ice hockey D. Pearsall and R. Turcotte, McGill University, Canada

203

8.1 8.2

203 203

Introduction Skate design

© 2007, Woodhead Publishing Limited

viii

Contents 8.3 8.4 8.5 8.6 8.7

9

11

205 213 217 222 222

Design and materials in ﬂy ﬁshing G. Spolek, Portland State University, USA

225

9.1 9.2

225

9.3 9.4 9.5 9.6 9.7 9.8 9.9 10

Evaluating skate design The design of ice hockey sticks Evaluating ice hockey stick design Summary References

Introduction Performance requirements: hooking and landing the ﬁsh Performance requirements: casting Leaders Flylines Rods Reels Summary and future trends References

228 231 234 236 239 244 245 246

Design and materials in archery B. W. K ooi, Vrije Universiteit, The Netherlands

248

10.1 10.2 10.3 10.4 10.5 10.6 10.7

248 250 255 258 267 268 269

Introduction Modelling bow performance Modelling bow design Modelling bow materials and their properties Summary and future trends Conclusions References

Design and materials in rowing B. K. Filter, Consultant, Germany

271

11.1 11.2

271

11.3 11.4 11.5 11.6 11.7 11.8 11.9

Introduction International regulation of competitive rowing equipment Design of modern rowing boats Materials and technologies for modern rowing boats Materials and technologies for rowing boat equipment Materials and technologies for oars Testing of rowing material Leisure rowing boats and equipment Acknowledgements

© 2007, Woodhead Publishing Limited

271 277 281 286 291 292 294 295

Contents 12

Design and materials in athletics N. Linthorne, Brunel University, UK

296

12.1 12.2 12.3 12.4

296 297 304

12.5 12.6 12.7 12.8 12.9 12.10 13

ix

Introduction Pole vault design and materials Javelin design and materials Design and materials for the shot put, hammer and discus Design and materials for hurdles, starting blocks and shoes for athletes Design and materials for running surfaces and other athletic facilities Design and materials in timing and other equipment Future trends Sources of further information and advice References

307 309 312 314 317 318 318

Design and materials in ﬁtness equipment M. Caine and C. Yang, Loughborough University, UK

321

13.1 13.2 13.3 13.4

321 322 325

13.5 13.6 13.7 13.8

Introduction Market research for ﬁtness equipment The product development process Using materials and processes to improve design of ﬁtness equipment Future trends Sources of further information and advice Acknowledgements References

© 2007, Woodhead Publishing Limited

332 335 337 338 338

Contributor contact details

(* = main contact)

Editor and Chapter 7 co-author A. Subic RMIT University School of Aerospace, Mechanical and Manufacturing Engineering Bundoora East Campus Bundoora, Melbourne VIC 3083 Australia Email: Aleksandar.Subic@rmit. edu.au

Chapter 1

Chapter 2 M. P. Luong LMS CNRS UMR7649 Ecole Polytechnique 91128 Palaiseau France Email: [email protected]

Chapter 3 K. B. Blair Sports Innovation Group LLC 36A Academy Street Arlington, MA 02476 USA Email: kbb@sportsinnovationgroup. com

M. Strangwood Sports Materials Research Group Department of Metallurgy and Materials, The University of Birmingham Elms Road Edgbaston Birmingham B15 2TT UK

Chapter 4

Email: [email protected]

Email: [email protected]

S. V. Caswell* School of Recreation, Health and Tourism 208D Bull Run Hall, PW MS 4E5 George Mason University 10900 University Blvd Manassas, VA 20109 Virginia USA

xi

© 2007, Woodhead Publishing Limited

xii

Contributor contact details

T. E. Gould and J. S. Wiggins Sport and High Performance Materials Program The University of Southern Mississippi School of Human Performance and Recreation Hattiesburg, MS 39406 USA Email: [email protected] [email protected]

Chapter 5 T. E. Gould*, S. G. Piland, C. E. Hoyle and S. Nazarenko Sport and High Performance Materials Program The University of Southern Mississippi School of Human Performance and Recreation Hattiesburg, MS 39406 USA Email: [email protected] [email protected] [email protected] [email protected]

Chapter 6 J. Sherwood* and P. Drane Baseball Research Center Department of Mechanical Engineering University of Massachusetts–Lowell One University Ave Lowell, MA 01854 USA Email: [email protected]

© 2007, Woodhead Publishing Limited

Chapter 7 A. Subic* RMIT University School of Aerospace, Mechanical and Manufacturing Engineering Bundoora East Campus Bundoora, Melbourne VIC 3083 Australia Email: Aleksandar.Subic@rmit. edu.au J. Kovacs Centre for Computational Prototyping (CfCP) Victorian Partnership for Advanced Computing (VPAC) 4 Central Blvd Fisherman’s Bend, 3207 Melbourne Australia Email: [email protected]

Chapter 8 D. Pearsall* and R. Turcotte Department of Kinesiology and Physical Education McGill University 475 Pine Avenue West Montreal Quebec H2W 1S4 Canada Email: [email protected]

Contributor contact details

Chapter 9

Chapter 12

G. Spolek Mechanical and Materials Engineering Department Portland State University P.O. Box 751 Portland OR 97207–0751 USA

N. Linthorne School of Sport and Education Brunel University Uxbridge Middlesex UB8 3PH UK

Email: [email protected]

Email: Nick.Linthorne@brunel. ac.uk

Chapter 10

Chapter 13

B. W. Kooi Faculty of Earth and Life Sciences Vrije Universiteit De Boelelaan 1087 1081 HV Amsterdam The Netherlands.

M. Caine* and C. Yang Sports Technology Research Group Loughborough University Loughborough LE11 3TU UK

Email: [email protected]

Email: [email protected]

Chapter 11 K. B. Filter Adlergestell 207 12489 Berlin Germany Email: [email protected]

© 2007, Woodhead Publishing Limited

xiii

Preface

Today, more people than ever before are participating in sports. With increased interest and participation in sports, and the extensive media coverage of sporting events worldwide, sport has evolved into a global business worth around US$600 billion in total. The world sporting goods market is estimated at US$120 billion retail, with footwear accounting for US$30 billion, apparel US$50 billion and equipment US$40 billion. The sporting goods industry has diversiﬁed over the years to accommodate the different interests and needs of the athletes and also of consumers in general. The industry has also promoted and helped to develop new sports that have in turn served as catalysts for new types of products. The quest for new markets, records and sports supremacy has led to millions of dollars being spent on research in and development of sport techniques and equipment. Athletes are now involved in increasingly complex systems that rely heavily on advanced technologies. New technologies and materials readily adopted from other industries have made sport faster, more powerful and enjoyable. For example, materials such as carbon ﬁbre reinforced polymers, new elastomers, new sandwich and foam structures and high-strength steel, titanium and aluminium alloys developed initially for defence and space applications, have improved sports products dramatically. The sports equipment industry has been exceptionally receptive to new materials and processes, due primarily to the fact that it is less material-cost sensitive than other, more conventional, industry sectors. The price of sports equipment, as in the case of biomedical equipment for example, easily compensates for the cost of the materials used. This is due mainly to the high ‘value-added’ generated through innovation and design whereby the value of the product as perceived by the customer is much higher than the costs involved in making it, especially if the equipment in question enhances the performance of the athlete. Materials in sports equipment Volume 1 introduced this topic and discussed details of advancements in materials and processes used for sports equipment. Also, it provided in-depth coverage of selected equipment used xv

© 2007, Woodhead Publishing Limited

xvi

Preface

in speciﬁc sports. The main objective of this second volume is to expand the body of knowledge in the area by offering a greater insight into some contemporary topics of relevance to the design of modern sports equipment using new and improved materials and structures. Volume 2 combines coverage of recent developments in both advanced materials and novel processing methods which have enhanced the properties of materials and improved the design of individual sporting goods. It provides comprehensive coverage of equipment used for popular sports not addressed by other texts in the ﬁeld. This volume in particular describes in detail the interrelationships between the design intents and materials used, taking into account broader considerations such as life cycle design of sports equipment and sustainability issues in general. The book comprises two distinct parts, the ﬁrst covering general issues of interest to all sports and the second focusing on speciﬁc sports. Speciﬁc sports such as baseball, snowboarding, ice hockey, ﬂy ﬁshing, archery, rowing, athletics, and ﬁtness equipment are covered in detail in individual chapters. I gratefully acknowledge all the authors who have contributed to this book, and also thank Woodhead Publishing for continued support and assistance in the production of this publication. Finally, I hope that the book’s diversity of topics and approaches to the interdisciplinary subject of design and materials for sports equipment will make it an essential reference source for all materials scientists and sports equipment designers and also for manufacturers developing products in this rapidly evolving ﬁeld. Aleksandar Subic

© 2007, Woodhead Publishing Limited

Part I General issues

© 2007, Woodhead Publishing Limited

1 Modelling of materials for sports equipment M. S T R A N G W O O D, The University of Birmingham, UK

1.1

Introduction

The chapters in Part II of this book cover design and materials in particular sports and will emphasise the interrelationship of design and materials for performance. In the ﬁeld of sports equipment, as in all other applications, such as aerospace, automotive and biomedical, it is the combination of materials and design that achieves the requirements speciﬁc to that application. The most suitable materials for the application are therefore those that most completely and readily achieve the mix of properties (mechanical, physical, chemical and non-technical) in the desired shapes and dimensions. In this way ‘sports materials’ do not differ from any other type of ‘material’, but are materials designed for the operating conditions pertinent to sporting applications. Of the particular sports covered in Part II, the operating conditions are between −5 and +40 °C; involve exposure to moisture; and cover a range of strain rates. Additionally, most of the equipment interacts strongly with athletes, whose soft tissue can suffer damage and injury at strains and strain rates that would be negligible for structures such as aircraft or power generation plant. Modelling of materials covers a range of scales and outcomes that relate to different engineering disciplines, which include: (1)

Atomistic or ab initio modelling (Wahn and Neugebauer, 2006) based on interatomic potentials, which can be used to design speciﬁc localised properties, such as doping of semiconductor devices. (2) Analytical models: these operate at the micron to millimetre scale and involve thermodynamics and kinetics for structural changes as well as dislocation motion relating to strength and fracture. They are used in designing material compositions and microstructures to achieve properties over a limited portion of the structure (Ghosh et al., 2002; Robson, 2004). This could be viewed as the ideal or target composition and microstructure for the processed component. 3

© 2007, Woodhead Publishing Limited

4

Materials in sports equipment

(3) Process modelling (Grong, 1994): these models often involve numerical methods, such as ﬁnite element (FE) and computational ﬂuid dynamics (CFD) in order to determine thermomechanical and ﬂuid ﬂow conditions throughout complete components, such as shaped castings or forgings. They give structures and properties which are more average, i.e. do not have the ﬁne-scale resolution of structure possible in (2), but do give variations across full components and can predict defects such as porosity in castings (Lee et al., 2004). (4) Continuum mechanics: these models (also often numerical) are used to deﬁne the properties, e.g. strength and stiffness, required at different positions throughout the component. Design and materials for various applications are assisted through computational modelling based on an iterative combination of (2) to (4) mostly, although the resources of smaller manufacturers may only allow some aspects, e.g. (4), to be carried out. Models are only as good as the data that they use and so, if a full mix of models and data is not available, it is important to understand which of the many database values usually available are appropriate for use in the models. As the range of properties and materials is very wide, this chapter will concentrate on the more common materials (metallic alloys, polymers and polymer matrix composites) and properties (modulus and yield stress) encountered in sporting applications.

1.2

Properties of metallic alloys

Table 1.1 summarises the range of mechanical properties of metallic systems commonly encountered in golf equipment and is typical of the data available in materials handbooks (e.g. Boyer et al., 1994) or online sources, such as www.matweb.com. In general, the density values of the alloys do not vary much, usually because the amounts of alloying elements present are limited, except when the atoms are similar in size (and hence mass) as for Fe and Cr in stainless steels. Titanium-based systems are also an exception due to the greater solubility of elements in titanium, with the three alloys in Table 1.1 showing a 10% variation in density. Of the other alloy systems then, only the addition of Li to Al results in a decrease in density (up to 7% reduction), which also increases the Young’s modulus (by up to 10%). The beneﬁcial improvements in density and modulus are accompanied by strength increases, but at the expense of reduced formability, toughness and easier crack formation. Stiffness, for efﬁcient energy transfer, is important in many sporting applications but, as Table 1.1 indicates, the variation in Young’s modulus (and hence also in shear and bulk modulus) for the alloys is limited; for example, the Young’s modulus of the steels falls below 200 GPa for

© 2007, Woodhead Publishing Limited

Modelling of materials for sports equipment

5

Table 1.1 Summary of typical metallic alloy properties Alloy

Density, ρ (g/cm3)

Young’s modulus, E (GPa)

Yield stress, sy (MPa)

Tensile Ductility strength, (%) UTS (MPa)

C–Mn (mild) steel High-strength steel, e.g. 4340 316 stainless steel Cu–Be Al–Cu Al–Mg Mg–Ti Ti-3 Al-2.5 V Ti-6 Al-4 V Ti-15V-3 Al-3Si-3Cr (Beta titanium)

7.85

210

210–350

400–500

7.85

207

860–1620

1280–1760

7.85

195

205–310

515–620

30–40

8.25 2.77 2.77 1.78 4.50 4.43 4.71

128 73 70 45 105–110 110–125 85–120

200–1200 75–345 130–192 200–220 750 830–1100 800–1270

450–1300 185–485 225–275 260–290 790 900–1170 810–1380

4–60 18–20 7–22 15 16 10–14 7–16

15–35 12

316 stainless steel, which has a change in structure from bcc (ferrite) to fcc (austenite) achieved by the addition of more than 25 at. % alloying element. As for density, the major exception is in the Ti-based system, where the low temperature hcp (alpha) phase has a signiﬁcantly higher modulus (125 GPa) than the higher temperature bcc (beta) phase (80 GPa). Therefore, using composition and heat treatment, the mix of α and β can be altered to control modulus. On elastic loading of a Ti-based alloy containing a volume fraction, Vfa of α and Vbf of β, where Vaf + Vbf = 1, then, if the interfaces between the two phases remain intact, the overall stress, σ, is the same in both phases and the strain, ε, is partitioned between both phases so that the overall strain is the sum of that in α and in β. Thus: et = Vfae a + Vbf e b

[1.1]

σ Vαf σ V βf σ = α + β Et E E

[1.2]

1 V αf V βf = + E t Eα E β

[1.3]

Thus the Young’s modulus of the alloy (E t) can be estimated from the phase balance and the properties (in this case moduli) of the individual phases. The principle of additivity of properties based on a particular boundary condition (usually constant stress or strain) is the ‘Rule of

© 2007, Woodhead Publishing Limited

6

Materials in sports equipment

Mixtures’ and is widely used in materials science (particularly composites) for estimating properties and designing alloy structures. Phase balance is also one of the principal strengthening mechanisms in metallic alloys and so can contribute to the range of yield stress and tensile strength values in Table 1.1; for the titanium-based alloy examples, the strength of α is greater than that for β so that, as the modulus is increased through formation of greater amounts of α, then strength levels also increase. The same effects of phase balance are also seen for mixtures of phases (ferrite, austenite, bainite and martensite) in steels with reduced options for shaping and reduced toughness. ‘Rule of Mixtures’-based approaches are used in these cases, with a number of empirically determined equations relating mechanical properties to microstructural parameters in the literature (Llewellyn, 1992). Phase balance is only one of the ﬁve main strengthening mechanisms active in most metallic alloys. The others are: (1)

Solid solution strengthening – the substitution of solute atoms for solvent in the crystalline matrix results in lattice strains which increase the yield stress of the alloy. Strength levels depend on the amount of the element in solution (Xi) and the mismatch in atom size between solute and solvent (ei) represented as strengthening coefﬁcients, Ki. The strengthening (∆tss) due to increased solute content of an alloy is estimated from equations of the form: ∆τ ss = ∑ Ki X in

n∼1

[1.4]

i

(2)

Precipitation strengthening – as solute levels rise then the solubility product is exceeded (at lower solute levels as mismatch increases) and ﬁne secondary phases precipitate, Fig. 1.1. These either introduce elastic strains or block slip paths in the matrix, both of which increase strength. The strength level increases with increasing volume fractions (Vf) of smaller precipitates (radius r) with the strengthening increment (∆tppt) being given by: ∆τ ppt =

(3)

Gb 1

[1.5]

⎛ 2π V ⎞ 2 r ⎝ 3 f⎠

where G = shear modulus and b = Burgers vector. Reduced grain size – grain boundaries, Fig. 1.1, can also act to block slip paths so that yield stress strengthening (∆tgb) is inversely proportional to the square root of grain size: ∆τ gb = ky d

−

1 2

where ky = Hall–Petch parameter.

© 2007, Woodhead Publishing Limited

[1.6]

Modelling of materials for sports equipment

7

1 µm

1.1 Example micrograph showing ﬁne strengthening particles and grain boundaries.

(4)

Work hardening (cold work) – plastic deformation increases the number of dislocations present, which impedes the progress of other dislocations and so raises the strength (although at the expense of toughness and ductility). Strengthening (∆td) by this effect is given by: ∆τ d = α 1Gb ρd

[1.7]

where a1 = constant ρd = number of dislocations/unit area. In real alloy systems, a number of strengthening mechanisms operate, although one may dominate. The strength level achievable and suitable processing routes for that alloy (which affect the shapes that may be achieved) will depend on the strengthening mechanisms operating which, for simple systems, can be selected on the basis of the phase diagram, Fig. 1.2. Figure 1.2 shows, schematically, the phase diagram for a binary eutectic alloy, which exhibits three classes of alloy, namely solid solution strengthening (A), precipitation hardening (B) and phase balance (C). For alloys with composition similar to A then, the operative strengthening mechanisms are solid solution strengthening, grain size reﬁnement and work hardening; this class of alloy would require shaping by cold working, e.g. drawing, rolling or forging at temperatures below one-third of the absolute melting

© 2007, Woodhead Publishing Limited

8

Materials in sports equipment Alpha

Temperature

Liquid (L)

L+ L + Beta

Alpha A B

Alpha + Beta

C

Composition

1.2 Schematic binary phase diagram for simple eutectic system.

temperature. Alloys of this type, e.g. 316 stainless steel and 5xxx series Al–Mg alloys, are better suited to sheet and wire applications although, from Table 1.1, their overall strengths are limited. Higher strength values are achieved by precipitation hardening, as for 4340 steel and 2124 Al–Cu alloy, which would correspond to region B on the schematic phase diagram. This class of alloys would also be termed age hardening or quench and tempering (for steels) and can be shaped by hot and cold processes so that they can be shaped in thin and thick sections. The behaviour of these alloys means that the ﬁnal stage in processing should be heating to the single phase ﬁeld for long enough to fully dissolve any precipitates, then quenching to room temperature at a rate fast enough to prevent precipitation so that all the alloying elements are retained in solid solution. Section size is governed by heat removal, which is controlled by the quenching medium and heat transport through the metal. The latter is governed by Fick’s equations and can be readily solved, in one dimension, by a ﬁnite difference method (Crank, 1975). More complex two- and threedimensional ﬂow is governed by the two- and three-dimensional forms of the same equations, but these need FE methods for their solution (Grong, 1994). Ferrous systems, and others where allotropic transformations occur, can give a phase change on quenching; for ferritic steels the structure would change from fcc (austenite) to body-centred tetragonal (martensite). This phase transformation can lead to changes in properties and volume so that distortion and cracking (quench cracking) can occur, limiting the quenching rate that can be applied. The distortion and cracking depend on the volume change and the volume fraction of martensite formed; this is dependent mainly on the temperature and stress acting and so can readily be incorporated into an FE code describing the situation.

© 2007, Woodhead Publishing Limited

Modelling of materials for sports equipment

9

After quenching, there is a strong chemical driving force for the excess solute to precipitate, but diffusion is too slow, at room temperature, for this to happen except for certain Al-based alloys, and even then it is still slow. Hence, the alloys are aged or tempered in the two-phase region (120–175 °C for Al-based alloys; 400–550 °C for metastable β-Ti alloys and 350–650 °C for steels). These temperatures are much lower than the dissolution temperature so that the chemical driving force (∆Gch) remains high, resulting in a large number of ﬁne and ﬁnely spaced precipitates. The number and size of the precipitates depend on their nucleation (Iv) and growth (Y) rates, which are both time (t) and temperature (T) dependent: − ∆Gm ⎞ ⎛ − ∆G* ⎞ I v = ν DC0 exp ⎜ exp ⎛ ⎝ RT ⎠ ⎝ kBT ⎟⎠

Υα

D t

[1.8] [1.9]

where ␯D = Debye frequency, C0 = number of nucleation sites per unit volume, ∆G* = activation barrier for nucleation (decreases with decreasing temperature), ∆Gm = activation barrier for diffusion (increases slightly on decreasing temperature), D = diffusivity, R = universal gas constant, kB = Boltzmann’s constant. During the initial stages of ageing/tempering, nucleation dominates so that the number of precipitates increases causing an increase in strength but, in the later stages, growth dominates and the precipitates increase in size (reducing in number). This causes a loss in strength and is known as over-ageing. The effective use of high-strength metallic alloys requires optimisation of this heat treatment through all stages of processing (including machining and joining – see Section 1.4) and in-service. As most sports equipment is used at ambient temperatures, over-ageing in service is usually only a problem in motorsport applications. Eutectic alloys (C in Fig. 1.2) develop at least a two-phase structure directly from the liquid with the mix of phases developing strength (as for α and β phases of titanium above). The eutectics do not readily dissolve without liquation and so these alloys are mostly used in the as-cast state with higher cooling rates during solidiﬁcation reﬁning the eutectic to increase strength levels, although these are not usually as high as wrought age-hardened alloys (Table 1.1).

1.3

Modelling the properties of metallic alloys

The above discussion about the types of metallic alloys, their strengthening mechanisms and processing has been related to a simple phase diagram,

© 2007, Woodhead Publishing Limited

10

Materials in sports equipment

but these are only available for two-component systems, whilst most commercial alloys contain many more elements. The large number of binary alloy phase diagrams and the results of extensive phase measurements have built up sizeable thermodynamic and diffusion databases for most alloy systems. These then allow the stability of different mixes of phases of varying composition to be calculated and total free energy minimised to predict the phase mix and composition for an overall alloy composition as a function of temperature. This can now be readily carried out using commercial packages (Thermo-Calc, MTDATA and ChemSage) as for the example of 4340 steel in Fig. 1.3. This indicates that heating the steel to 800 °C will dissolve the carbides, releasing Cr, Mo and C into solution, but a temperature of 1100 °C is needed to dissolve AlN and release Al and N into solution as well. The energies and compositions of the different phases give the driving forces and composition gradients for nucleation, growth and dissolution of phases so that the rates of precipitate dissolution and precipitation can be calculated from equations 1.8 and 1.9. A simple use of these rates is in an ‘Avrami’ plot (based on separate analyses by Johnson, Mehl, Avrami and Kolmogorov [Kolmogorov, 1937; Avrami, 1939; Johnson and Mehl, 1939]), which plots the characteristic sigmoidal shape to overall amount of transformation (e.g. precipitation) as a function of time at constant temperature, Fig. 1.4. The isothermal ‘Avrami’ equation is given by:

10

3:T-273.15, NP(FCC_A1#1) 4:T-273.15, NP(MNS) 5:T-273.15, NP(FCC_A1#2) 6:T-273.15, NP(ALN) 7:T-273.15, NP(KSI_CARBIDE) 8:T-273.15, NP(CEMENTITE) 9:T-273.15, NP(MC_SHP)

9 8

7 7

NP (*)

7

7

6 5

9

9 7 9

4 3 2 10–3

1 0 200

6 4 5

400

6 5 4

6 66 66 66 6 4 55 5 44 5444 55 4 5

65 4

4 5

4 4 5

3

600 800 1000 1200 1400 1600 Temperature (°Celsius)

1.3 Plot of number of moles of stable phases (NP(*)) as a function of temperature in high-strength 4340 steel.

© 2007, Woodhead Publishing Limited

Modelling of materials for sports equipment

11

Extent of transformation

Impingement

Growth

Nucleation Time

1.4 Schematic isothermal variation of transformation against time showing incubation, growth and impingement.

π Vf = 1 − exp ⎛ − I vΥ 3t 4 ⎞ ⎝ 3 ⎠

[speciﬁc]

[1.10]

Vf = 1 − exp(−kt n)

[general]

[1.11]

The ‘Avrami’ parameters k and n depend on the precise balance of nucleation and growth, which will depend on the exact type of system being modelled and its prior processing. Example values of ‘Avrami’ parameters are tabulated in Christian (1975). By combining the ‘Avrami’ plots at different temperatures, the transformation–time–temperature (TTT) behaviour can be described. Many TTT diagrams were determined experimentally, but they can now be modelled (Lee and Bhadeshia, 1993) to give overall extent of transformation, and they can be used to determine the schedule needed to achieve the desired properties from that alloy. The isothermal relationships can be used to deal with continuous heating and cooling situations by replacing the temperature variation by a series of small isothermal steps and determining the proportion of precipitation/dissolution that would have occurred during that time step at that temperature. This can be used to deﬁne the heating and cooling rates which determine whether or not transformations can occur and would be used, e.g. with FE thermal ﬁelds, to establish the maximum section size or appropriate quenching medium to achieve the desired properties without excessive distortion/cracking. The modelling above will give overall volume fraction, but does not give the size and spacing of individual precipitates, and so the accuracy of the property prediction could be improved by applying the nucleation and growth equations to individual regions in the alloy. This requires a reﬁnement of the mesh from mms to microns and so the computing time would

© 2007, Woodhead Publishing Limited

12

Materials in sports equipment

increase dramatically (days cf. minutes typically) and so this would only be carried out at critical positions, e.g. around welds, or where a small, representative area was fully modelled (local) and this was then used as the repeat unit over the whole of the structure (global). Within the local region the approaches adopted would usually be: (1)

(2)

(3)

Monte Carlo: atoms are assigned a probability of transforming, which allows grain boundaries to be distinguished from grain bulks and favours growth of pre-existing particles over formation of new ones. For each time step, the untransformed atoms are assessed in turn to determine whether they transform during that step or not (simple yes or no decision). If transformation occurs then the probability of transformation of the surrounding atoms for the next time step is modiﬁed (along with any other modiﬁcation for, e.g. temperature change). This approach is particularly suited for grain growth and grain boundary precipitation. Phase ﬁeld: the yes/no decision of the Monte Carlo approach is very good for sharp interfaces, but many interfaces between transformed and non-transformed regions are not sharp but diffuse, e.g. the ‘mushy’ solid + liquid region in solidiﬁcation. The phase ﬁeld method separates the transformed/non-transformed regions by a third phase, whose nature varies gradually from 0 (non-transformed) to 1 (transformed). This was developed for solidiﬁcation with the solid growing at different rates along its length resulting in re-production of the dendritic structure and prediction of the eutectic phase regions. Phase ﬁeld modelling should also be suitable for modelling grain boundary precipitation. Cellular automata: in this approach the structure, e.g. grain or groups of grains, is represented by a number of cells, which are then assigned probabilities of transformation. As for the Monte Carlo approach, each cell is assessed for transformation during each time step. The use of appropriate cells allows more efﬁcient modelling of threedimensional structures than a Monte Carlo approach.

1.3.1 Application of modelling: assessing hardness around a weld A relatively simple application of the modelling principles above is provided by the variation in hardness around a fusion weld in a 5xxx series Al–Mg alloy, as may occur in the welding of bicycle tubes. As noted above, the strengthening mechanisms in this alloy are solid solution strengthening (from the Mg alloying), grain size and work hardening. During fusion welding, the base metal under the heat source (electric arc) is melted, with heat being conducted into the base metal raising the temperature of the

© 2007, Woodhead Publishing Limited

Modelling of materials for sports equipment

13

adjacent material from ambient (far-ﬁeld) to the melting temperature (at the edge of the weld pool). Exposure of the base metal to elevated temperatures can cause loss of work hardening by recovery (low temperatures), recrystallisation, grain growth and ﬁnally dissolution of intermetallic particles (highest temperatures). These processes are driven by the stored energy and grain size in the base metal (determined by the amount of cold work) and the intermetallic particles (determined by alloy preparation). The temperatures at which each process commences can be determined theoretically or practically and the rates for each temperature calculated so that the extent of each process for a given time step at any temperature can be determined. As each process proceeds, then the driving force is consumed and so this is modiﬁed for the next stage, as: dX R −QR ⎞ = const.exp ⎛ ⎝ dt RT ⎠ n

XR = ∑ 0

const. (QR 0 − C (1 − X Ri )) ⎛ Q − C (1 − X R i ) ⎞ exp ⎜ − R 0 ⎟⎠ ⎝ RTi RTi 2

[1.12] [1.13]

where XR = fraction recrystallised, QR0 = initial activation barrier for recrystallisation, QR = current activation barrier for recrystallisation, C = constant. Welding uses a moving heat source and so the thermal ﬁeld has to be modelled using FE methods with a mesh that is ﬁner close to the weld line but gets coarser further away (Fig. 1.5) without loss of precision. This then gives the transient heating and cooling cycle for a given weld heat input (Fig. 1.6), and this is used to give the extents of recovery, recrystallisation, grain growth and intermetallic dissolution, which are then combined to give the alloy strength (sy) from:

1.5 FE meshing of seam weld in 5xxx Al–Mg alloy plates.

© 2007, Woodhead Publishing Limited

14

Materials in sports equipment 600 Experiment_7.27 mm Experiment_9.16 mm Modelling_7.27 mm Modelling_9.16 mm

Temperature (°C)

500

400

300

200

100

0 0.000

5.000

10.000

15.000

20.000

25.000

30.000

35.000

Time (s)

1.6 Transient temperature proﬁles for welding 5xxx Al–Mg alloy plates.

80 MIG-AA5251-H34-Weld8 75

Hardness

70 65 60 55 50

Predicted heat input 0.14 Measured HV200 g middle

45 40 0

5

10

15

20

Distance from fusion boundary (mm)

1.7 Comparison of predicted and measured hardness traces for 5xxx Al–Mg alloy welds.

© 2007, Woodhead Publishing Limited

Modelling of materials for sports equipment sy = s0 − ∆td − ∆tgb + ∆tss

15 [1.14]

where s0 = yield stress of single crystal, annealed pure element. The results are plotted in Fig. 1.7 along with experimentally determined hardness values, showing that the metallic alloy behaviour can be predicted covering (from right to left) recovery (gradual loss of hardness), recrystallisation (steeper loss of hardness), grain growth (around minimum) and increased solid solution strengthening from dissolution of intermetallic particles to the left of the minimum.

1.4

Modelling polymeric materials

In metallic alloys, the mechanical properties are controlled at the atomic level by bonding and movement of dislocations. The long-range order in metallic alloys averages these individual effects and means that properties can be related to micron-scale features (grains and precipitates). This improves the reproducibility of properties and has led to the development of accurate relationships with sizeable databases, which allow the predictive modelling described above. Many pieces of sports equipment utilise polymeric materials that are loaded and unloaded in play, as for the golf ball in Fig. 1.8. The strength and stiffness of polymers is governed at the molecular level by the combination of covalent, hydrogen and van der Waals bonds. Some thermosetting polymers, such as epoxy resins, are three-dimensional macromolecules, which exhibit long-range order and have reproducible, if brittle, properties. Many other polymers, particularly thermoplastics, contain amorphous regions as well as crystalline, which introduce heterogeneities on the micron scale through the size and distribution of the crystallites as well as density, chain length and cross-link variations in the amorphous material. The strength and stiffness of elastomers and other thermoplastics depends on the extent of cross-linking, and this is controlled through additives made to the original polymer formulation and the processing/heat treatment of the polymer, either during processing or in-service. The lower melting temperatures of polymers means that in-service property modiﬁcation is much more extensive than for metallic alloys, e.g. the cores of golf balls harden as they age in the same way as exposed rubber hardens in air. The hardening is due to an increased number density of cross-links (strong covalent bonds rather than the weaker hydrogen, dipole or van der Waals) between chains (Fig. 1.9) impeding their movement. Cross-linking only occurs at certain points, when reactive functional groups from different chains come together with sufﬁcient energy to form a bond. As the extent of cross-linking increases, then chain mobility decreases so that the extent of cross-linking with time exhibits a sigmoidal curve. The development of cross-linking (and

© 2007, Woodhead Publishing Limited

16

Materials in sports equipment

(a)

(b)

1.8 Golf ball ﬁred at 40 m/s at rigid steel target (a) prior to impact, (b) at point of maximum deformation during impact.

crystallisation) in polymers can therefore be modelled using ‘Avrami’ approaches as described above. The rate and extent of cross-linking is dependent on temperature and the amount of additives such as perchlorides and diacrylates (Table 1.2). Detailed modelling of properties at a ﬁner scale is dependent on the local orientations and spacings of the chain segments but, unlike the atoms in metals, the chains have many more degrees of freedom and so number of states. This means that molecular dynamic models are required (Valavala et al., 2007), these can currently model small chain segments, but not with the predictive accuracy seen in metallic alloys.

© 2007, Woodhead Publishing Limited

Modelling of materials for sports equipment

17

Network chain

Cross-links Polymer chains

1.9 Schematic diagram of cross-linking in polymers.

Table 1.2 Typical rubber formulation for a golf ball core in parts per hundred of rubber (phr). Zinc diacrylate and peroxide increase cross-linking Component

Preferred range

Most preferred range

Polybutadiene Zinc diacrylate Zinc oxide Zinc stearate Peroxide Filler (e.g. tungsten)

100 parts 20–35 phr 0–50 phr 0–15 phr 0.2–2.5 phr As desired (2–14 phr)

100 parts 25–30 phr 5–15 phr 1–10 phr 0.5–1.5 phr As desired (10 phr)

1.4.1 Data for modelling polymeric materials Crystalline materials, such as metallic alloys, show linear elastic behaviour with little energy loss for loading and unloading in the elastic regime. The modulus of a metallic alloy is the same at small elastic strains and at strains just below the elastic limit. Most mechanical property data are obtained at low deformation rates (∼2 mm/s), but the strain rate sensitivity of modulus and yield stress for metallic alloys are small, meaning that the low strain rate data can be used in design of sports equipment where deformation rates may be around 50 m/s. Polymeric materials are generally viscoelastic rather than linear elastic with much greater sensitivity of stiffness to strain and strain rate. Although recent work has shown a relationship between energy losses in golf ball materials for the same amount of deformation at low and high strain

© 2007, Woodhead Publishing Limited

18

Materials in sports equipment

rates (Strangwood et al., 2006), this is within a small set of polymeric material and it is not generally possible to extrapolate low strain/low strain rate data to the strains and rates encountered in sporting applications. The strain and strain rate dependence of polymeric deformation mean that FE methods are needed for modelling, but the current lack of databases for mechanical properties under the pertinent conditions reduces the accuracy of these models.

1.5

Properties and modelling of composites

Composites can use a range of matrices (polymers, metals and ceramics) and reinforcements (particles, whiskers, platelets and ﬁbres) (Matthews and Rawlings, 1999), but the majority of sports equipment uses polymer matrix composites with continuous ﬁbre reinforcements. Table 1.3 Table 1.3 Properties of common polymer matrix composite components and systems Component

Density, ρ (g cm−3)

Young’s modulus parallel to ﬁbres, E|| (GPa)

HM carbon ﬁbre HS carbon ﬁbre E-glass ﬁbre S-glass ﬁbre Kevlar 149 Epoxy resin M55J unidirectional carbon ﬁbre/ epoxy (Vf = 0.6) M46J bidirectional carbon ﬁbre/ epoxy (Vf = 0.6) E-glass/epoxy composite (Vf = 0.6) Kevlar1/epoxy composite (Vf = 0.6)

1.95

380

1.75

1 ®

Strength parallel to ﬁbres (MPa)

Strength normal to ﬁbres (MPa)

12

2400

–

230

20

3400

–

2.56 2.48 1.45 1.1–1.4 1.7

76 86 130 3–6 270

76 86 10 3–6 5.5

2000 4600 3000 35–100 1600

– – –

1.7

102

102

573

573

2.1

45

12

1020

40

1.4

76

1380

30

DuPont.

© 2007, Woodhead Publishing Limited

Young’s modulus normal to ﬁbres, EⲚ (GPa)

5.5

35–100 24

Modelling of materials for sports equipment

19

summarises some of the major ﬁbres and matrices available. Within the ﬁbres a wide range of properties are available, e.g. for carbon ﬁbres highmodulus (HM) and high-strength (HS) grades are available, with the different properties being developed during the processing of the ﬁbres. Carbon ﬁbres can be manufactured from pitch, although the majority is derived from polyacrilonitrile (PAN), which is a thermoplastic and can be drawn into ﬁbres as a viscous melt (so retaining its shape). This stage deﬁnes the size (10s to 100s of microns) and shape (round, ovoid or kidneyshaped), which affect the ﬁbre strength and stiffness. After drawing, the ﬁbres are then stretched and heat-treated in three stages in controlled atmospheres with controlled heating rates to drive off the hydrogen and nitrogen and graphetise the remaining carbon. The time and temperature of the ﬁnal graphetisation stage are used to give either high-strength or modulus. Aramid ﬁbres [e.g. Kevlar®(DuPont)] are produced by extruding a solution of the aramid at elevated temperature followed by drawing to the ﬁnal ﬁbre properties. Carbon and aramid ﬁbres are prone to surface damage and so are protected by incorporation into a matrix. This also provides more even load transfer from the external system to the ﬁbres, provided that the ﬁbre/matrix is intact, which means that the liquid matrix must wet the ﬁbres (contact angle <90°) before curing the composite to achieve full cross-linking of the matrix resin and the properties in Table 1.3. The mix of ﬁbre and matrix is purely mechanical (subject to wetting) and so the proportions of each are not limited by the composition or heat treatment as for precipitation in metallic alloys. The limit on the amount of ﬁbre depends on the processing route, but most ﬁbre volume fractions are between 0.5 and 0.75 in order to generate a signiﬁcant effect at the lower end, whilst above the upper limit the number of defects in processing becomes too high. The advantage of composites is that stiffness and strength can be designed from choice of ﬁbre type, matrix type and ﬁbre volume fraction. For unidirectional ﬁbres in a matrix then the ‘Rule of Mixtures’ can be used; the modulus normal to the ﬁbre direction is calculated for equal stress in the components and results in equation 1.3. For elastic stressing parallel to the ﬁbre, then for no interface separation, the ﬁbres and matrix experience the same strain with stress being distributed between the two components. Thus: sc = Vfsf + Vmsm

[1.15]

Ece = VfEf e + VmEme

[1.16]

Ec = VfEf + VmEm

[1.17]

The subscripts c, f and m refer to composite, ﬁbre and matrix respectively. The unidirectional properties are thus easy to calculate or control, but the difference in properties between those parallel and those normal

© 2007, Woodhead Publishing Limited

20

Materials in sports equipment

to the ﬁbres is very large (Table 1.3). Most applications require biaxial properties, which can be achieved by different ﬁbre architectures, i.e. using bidirectional weaves or twills; or by using laminates, i.e. symmetrical stacks of differently oriented unidirectional plies. Weaves (plain or twill) involve threading the ﬁbres over and under each other in an orthogonal array. In plain weaves the ﬁbres are woven individually around each ﬁbre, whilst in twill weave groups of three–ﬁve (typically) ﬁbres are interwoven leading to lower strains and higher ﬁbre volume fractions as penetration of resin to avoid ﬁbre touching and voids is easier. The properties of weaves are isotropic within the plane of the weave (Table 1.3) but are below the average of unidirectional parallel and transverse properties, because a portion of the ﬁbres is out of plane as they weave around each other. The in-plane properties can be used in designing structures with weaves. Laminates can be stacked from weave, but more commonly from unidirectional (UD) plies. The stacks need to be symmetrical in order to avoid warping during processing or service through unbalanced strains in different plies. Stacking of UD plies allows the material modulus and structure stiffness to be varied with orientation, something that is less signiﬁcant in metallic systems where modulus is more isotropic. A sports equipment example would be the shaft of a golf club, which is subjected to bending in the plane of the swing during the player’s down swing but, because the centre of mass of the club head is away from the shaft, a torque is developed that twists the shaft during the down swing. The correct bending stiffness is needed for the golfer’s swing speed to ensure an appropriate dynamic loft at impact and correct launch conditions. If, however, the position of the club head’s centre of mass, but not its mass, changes then the torque changes and the torsional stiffness of the shaft should be changed, but not the bending stiffness. If the degree of twisting changes then the club head will not hit the ball squarely, leading to lateral dispersion. The bending stiffness can be maintained by using UD plies parallel to the shaft axis (0° plies), whereas the use of different thicknesses or angles of off-axis plies (e.g. 35–55°) allows the torsional stiffness to be varied, with a small effect on bending stiffness. The use of off-axis plies means that, unlike the parallel and perpendicular modulus values in a single ply, the modulus is not being measured along a principal axis and simple equations, such as 1.3 and 1.17, cannot be used. Instead, a matrix description of the stiffness is needed to take into account Poisson’s ratio effects and shear loading (involving the in-plane shear modulus, G12). Taking the axes for a UD laminate as 1 (parallel to ﬁbres), 2 (in-plane perpendicular to ﬁbres) and 3 (through thickness) then the stiffness matrix, Q, is given by:

© 2007, Woodhead Publishing Limited

Modelling of materials for sports equipment ⎡ E11 ⎢ 1 − ν 12ν 21 ⎢ ⎢ ν 12 E22 ⎢ ⎢ 1 − ν 12ν 21 ⎢ 0 ⎢⎣

ν 21E11 1 − ν 12ν 21 E22 1 − ν 12ν 21 0

0 ⎤⎥ ⎥ ⎥ 0 ⎥ ⎥ G12 ⎥ ⎥⎦

21

[1.18]

This is then used to determine the strain matrix (εxy) for an externally applied stress system (σxy), where the in-plane stress axes x and y are at an arbitrary angle to the laminate axes 1 and 2. Solutions of these are speciﬁc to particular loading situations and laminate stacking, with examples available in specialised composite mechanics texts (e.g. Matthews and Rawlings, 1999). Simple geometries can be solved easily in code such as MatLab® (The MathWorks Inc.) (particularly suitable for matrix calculations) with specialist software available (e.g. CoDA and Laminate Tools) as the complexity increases, before equations such as 1.18 have to be used as the constitutive equations in full FE models, such as ABAQUS, COMSOL MultiphysicsTM and SYSPLY.

1.5.1 Limits to composite modelling Whichever system is used to model the properties and performance of composites in sports equipment, the accuracy will depend on the data used and assumptions made in the model. As the matrix is a polymeric material, then a strain rate dependence of properties exists, but this will be limited for the ﬁbres themselves. The effects of strain rate are therefore orientation dependent with little effect noted for loading within 10° of the ﬁbre axis (for UD laminates) as most of the load is carried by the ﬁbres (Gilat et al., 2002). However, at larger angles the resin is playing a larger part in the mechanical response and stiffness increases with increasing strain rate from static to impact (1–5 s−1) when the effect has saturated (Tay et al., 1995). Most data for the mechanical properties of composites are either from static/low strain rate testing or from crash studies, e.g. safety compliance for motorsports. Thus, there exists a need for greater stiffness and strength determination at strain rates applicable to sporting activities, e.g. for a golf shaft 0.03–0.065 s−1 (Lee et al., 2002). The second major limitation on composite modelling is a scale issue, where the plies are largely assumed to be homogeneous. During processing of composites the resin matrix goes through a low-viscosity state before cross-linking to a high-viscosity solid. While the resin is in its low-viscosity

© 2007, Woodhead Publishing Limited

22

Materials in sports equipment

state, then movement of the ﬁbres can occur to give ﬁbre-rich and resinrich regions through the structure. These inhomogeneities result in greater scatter in properties and can be increased by the fabrication method. Continuing with the golf shaft example, two main methods are used to manufacture these in carbon ﬁbre composites: ﬁlament winding and sheet lamination. In ﬁlament winding (Fig. 1.10) the ﬁbres are coated with resin and then wound on a rotating mandrel using a moving guide. This results in uniform properties (Fig. 1.11) as more precise ﬁbre placement is possible but, as ﬁbres close to 0° are difﬁcult to wind, very high stiffness values cannot be achieved. Orientation is not such a problem for sheet lamination, where UD pre-preg sheet (parallel ﬁbres impregnated with a partly cured resin to allow storage, cutting – by laser or water jet – and stacking) is used. The cut pre-preg sheet is wrapped around a mandrel in the correct

Rotating mandrel

Resin coated filament

Reel of filament

Reel moves along mandrel length

(a) Filament reel moves along the length of the rotating mandrel to build up the first ply Rotating mandrel

Resin coated filament Successive plies built up in opposing spirals

Reel of filament

Reel moves along mandrel length

(b) Successive plies built up as reel goes back and forth

1.10 Schematic diagram of the ﬁlament winding method of golf shaft production.

© 2007, Woodhead Publishing Limited

Modelling of materials for sports equipment

23

394.0 393.0

Frequency (CPM)

392.0 391.0 390.0 389.0 388.0 387.0 386.0 385.0 –5

45

95 Orientation (degrees)

145

1.11 Variation in fundamental bending frequency around circumference of ﬁlament wound shaft.

orientations (Fig. 1.12) prior to autoclaving in order to displace any air while curing the resin matrix. If air bubbles are trapped between the plies they lower stiffness and act as defects leading to premature failure; nondestructive evaluation of composites is not as well developed as for metallic structures and so detecting these defects is problematic. The sheet lamination technique does result in greater inhomogeneities than the ﬁlament winding method, through resin-rich regions between plies and at ply ends (Fig. 1.13). These resin-rich regions cause a variation in stiffness (and potential strain rate dependence), which can be seen in the variation in fundamental bending frequency around the circumference of the shaft (Fig. 1.14). Modelling of metallic alloys, polymers and composites can be carried out but, in all cases, the models need to be improved to deal with variations in the material on a range of scales.

1.6

Modelling sandwich structures

The thin nature of polymer matrix composite plies means that, for more highly loaded situations, a greater number of plies are required to reduce the mean stress to a level below that needed for delamination or fracture. In the aerospace industry, stiffening stringers have been fabricated in carbon ﬁbre composites using as many as 700 individual pieces co-cured

© 2007, Woodhead Publishing Limited

24

Materials in sports equipment Shaped mandrel

Sheet wrapped around mandrel

(a) Wrapping of innermost unidirectional sheet around mandrel

(b) Oppositely oriented spiral sheets wound around mandrel and first ply several times

(c) Spiral plies wrapped several times around mandrel

(d) Outermost single ply wrapped around mandrel and spiral plies

1.12 Schematic diagrams of the sheet lamination method of golf shaft production.

© 2007, Woodhead Publishing Limited

Modelling of materials for sports equipment

25

Heat and pressure applied to cure resin in die

Die shaped to shaft outer dimensions Uncured loose plies around mandrel inserted into shaped die (e) Insertion and curing of shaft in outer shaped die Outermost ply Gaps at end of ply Innermost ply Spiral ply Mandrel

Gaps at end of ply (f) Section through composite sheet shaft prior to curing in die Outermost ply

Mandrel

Innermost ply Spiral plies

(g) Section through composite shaft after curing – the plies have been compressed and stuck together as the resin achieves full strength and the gaps at the end of the plies have been removed

1.12 Continued

© 2007, Woodhead Publishing Limited

26

Materials in sports equipment

1.13 Resin-rich region at end of ply in commercial carbon ﬁbre composite golf shaft.

Frequency (CPM)

272.0 271.0 270.0 269.0 268.0 267.0 266.0 265.0 264.0 263.0 262.0 261.0 0

20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 Orientation (degrees)

1.14 Variation in fundamental bending frequency around circumference of sheet laminated shaft.

together (Postlethwaite, 1988). In a sporting context then the Callaway® C4 driver head features a fully composite face, which contains 47 separate plies (Fig. 1.15). The use of so many plies in a small mass-produced component requires very good quality control to ensure that the defects noted above for shafts are not manifested. Subsequent heads, e.g. Callaway Fusion, have restricted the use of composites to the more lightly loaded crown and sole (Fig. 1.16), with a metallic, typically β-Ti alloy, face being used. This also helps combat abrasive wear of the face and its grooves (needed for generating backspin during impact with the ball), as composites with epoxy resin matrices are very prone to wear (Hutchings, 1992). The

© 2007, Woodhead Publishing Limited

Modelling of materials for sports equipment

27

Face Different orientation and thickness carbon fibre composite plies

Sole 2 mm

1.15 Section through sole and face of Callaway® C4 driver head showing large number of plies.

Twill weave carbon fibre composite crown Ti-based alloy face

10 mm

1.16 Mixed metallic face/carbon ﬁbre composite golf driver head.

© 2007, Woodhead Publishing Limited

28

Materials in sports equipment

structural performance of these shapes can be estimated from laminate theory coupled, for more complex shapes, with FEA. In dealing with materials properties, then the greater risks of air bubbles (voids) and ﬁbre clumping (leading to resin-rich regions) need to be taken into account. As thickness and the number of plies increase, then scatter in properties increases along with a general decrease in modulus and strength if the proportion of voids increases. The use of higher strength (yield or fracture stress) materials in some applications does not lead to full reduction in mass through down-gauging as the failure criterion changes from plastic deformation/fracture to elastic instability (commonly Euler buckling). Euler buckling is commonly encountered for slender columns or sections under compression for which the failure load (Fbuc) is given by: Fbuc =

π 2 EI L2

[1.19]

where I = second moment of area, L = unsupported column length. Elastic instability is deliberately used in pole vaulting, where the stiffness of the pole can be tailored to the athlete in order that the load exerted when the pole is planted in the box is sufﬁcient to exceed Fbuc so that the pole can ﬂex in order to store strain energy and convert the vaulter’s kinetic energy to potential energy as the pole unbends. The stiffness and Fbuc of the pole are determined by the dimensions (length, outside diameter and wall thickness) of the pole coupled with the modulus of the material. For composite poles, the last feature will be determined by the selection of ﬁbre and matrix type, ﬁbre volume fraction and orientation of the plies. The load applied along the pole is determined by the vaulter through their speed at the end of the runway, the mass of the vaulter and angle of the pole to the horizontal when planted in the box. Elite vaulters will need poles of different stiffness and Fbuc corresponding to their different runway speeds as they warm up through a competition. The simplest pole shape is a parallel-sided, hollow tube for which the stiffness and Fbuc are given by: Stiffness = Fbuc =

48 Eπ 4 (do − di4 ) 64 L3

π 2E ⎛ π 4 (do − di4 )⎞ ⎠ L2 ⎝ 64

[1.20] [1.21]

where do is the tube outer diameter and di its inner diameter. Whilst buckling is essential for the operation of high-performance poles, any buckling must be avoided in many other sporting applications, where it would lead to loss of efﬁciency and/or control. In these situations and

© 2007, Woodhead Publishing Limited

Modelling of materials for sports equipment

29

where dimensions, such as thickness, are not freely variable, then sandwich structures are a low-weight solution. All classes of materials can be used in sandwich structures, but the most common, in sporting applications, consist of thin composite faces separated by a low-density foam core. The composite skins provide high, controllable elastic modulus value surfaces, which resist bending, whilst the core should have a high shear modulus, which prevents lateral movement between the skins and so resists buckling. The stiffness of a sandwich structure can be estimated from its dimensions and the properties of the component materials, e.g. the bending stiffness of a beam is given by: Stiffness =

2 L3 1 + B1Ec btc B2 bcGc

[1.22]

where B1 and B2 = constants, b = beam width, c = core thickness, t skin thickness, Gc = core shear modulus. Expressions of this type can be used in the design of structures using different materials, but they will tend to be an approximation and so their range of applicability needs to be checked. Because the majority of sports equipment is designed against a criterion of high stiffness and low weight, long, slender shapes are prevalent, which are in the range of applicability of equation 1.22. However, if the sandwich structure was a more complex shape, then the values from equation 1.22 may not be applicable as, for shorter spans, shear deformation dominates, lowering effective stiffness. Equation 1.22 is also only applicable for strong, intact bonding between the skin and the core, which can modify the potential beneﬁts of the sandwich material and emphasises the need for good adhesion between the two components. One class of cores is that of honeycombs, which exhibit much higher stiffness for compression along the hexagonal cell axis compared with directions perpendicular to it. Honeycombs are frequently used as low-mass, high-stiffness control surfaces (as in Fig. 1.17, which uses an epoxy-saturated aramid paper (Nomex® – DuPont) honeycomb sandwiched between carbon ﬁbre composite skins) and cabin ﬂooring (Nomex between glass ﬁbre composite skins) in aerospace applications, and in motorsport (Nomex or aluminium, the former for ﬂame retardation, whilst the latter gives greater overall strength levels and energy absorption in impact situations). Whilst the honeycomb provides good shear resistance the contact area between the cell edges and the skin is limited; adhesive joint strength is strongly dependent on contact area as the shear strength of most adhesives is limited. Thus it is essential that an intact adhesive layer exists over the entire contact area between the skin and the honeycomb. As adhesives are applied in a liquid or low-viscosity form, during adhesive curing they will tend to be drawn into the honeycomb cells (Fig. 1.18).

© 2007, Woodhead Publishing Limited

30

Materials in sports equipment

10 mm

1.17 Nomex® honeycomb sandwiched between carbon ﬁbre composite skins in an aircraft control surface.

Glass fibre reinforced polymer skin

Nomex core

Excess resin

2 mm

1.18 Adhesive accumulation at joint between Nomex® honeycomb and glass ﬁbre reinforced polymer skin.

© 2007, Woodhead Publishing Limited

Modelling of materials for sports equipment

31

If the adhesive is added as a ﬁlm between the core and the cured skins, then extra adhesive is needed to compensate for that drawn into the honeycomb cells and so the mass of the component increases. Alternatively, the skins can be cured around the core so that the epoxy resin matrix constitutes the adhesive between the two components. Given the thin nature of carbon ﬁbre composite skins for sports applications, curing the skin on a honeycomb core often results in print-through of the underlying honeycomb pattern onto the skin, which can alter the aerodynamic properties of the surface. The aerodynamic properties of the skin can also be affected because resin, like an adhesive layer, can be drawn into the honeycomb cells resulting in loss from the outer surface. The skin therefore becomes pitted which generally increases aerodynamic drag, whilst risking exposure and damage to ﬁbres resulting in premature failure. This would necessitate using a skin composite with excess resin at the price of increased cost and/or reduced skin modulus. Closed cell rigid foams can be used as alternatives to honeycomb structures with the added advantage that internal cell gas pressurisation can be used to increase shear modulus. The surface cells are still open, however (Fig. 1.19 (a)), allowing resin uptake, so that surface thermo-mechanical modiﬁcation is needed to collapse the open cell walls without excessive bulging (and rupture) of sub-surface cells or permeation of gas (reducing gas pressure and stiffness) (Fig. 1.19 (b)). The combination of time, temperature, pressure and heating rate can be determined by modelling heat transfer into the foam, thermal expansion of cells, internal pressure change and permeation through cell walls. As this is a one-dimensional process, this modelling can be carried out using an iterative ﬁnite difference method similar to that in Section 1.3 for diffusional growth (Crank, 1975; Caton, 2007).

1.7

Future trends

Modelling and design of materials with speciﬁc property mixes in all major classes of materials are possible with greater or lesser degrees of accuracy and complexity, shown here mainly for mechanical properties. Greater implementation of materials modelling within sporting contexts will arise as data more appropriate to sporting equipment (e.g. correct strain rate regimes and damping coefﬁcients relating to athlete perception) are measured for a wider range of materials and sports. Larger and more reliable databases will improve the accuracy of models, whilst a greater understanding of the behaviour of the material in relation to sporting performance will allow greater prediction and design of materials for sports. Finally, modelling techniques and algorithms will need to be developed that can deal with varying materials properties over a range of scales – e.g.

© 2007, Woodhead Publishing Limited

32

Materials in sports equipment

(a)

(b)

1.19 Open cells at surface of closed cell foam structure (a) as-cut, and (b) after thermal treatment (212 °C, ﬁve minutes).

distributions of grain sizes and precipitates sizes or resin-rich regions in composites, to more accurately represent ‘real’ materials rather than ‘model’ materials.

1.8

Acknowledgements

Much of the information in this chapter has been taken from the research activities in the Phase Transformations and Microstructural Modelling and

© 2007, Woodhead Publishing Limited

Modelling of materials for sports equipment

33

Sports Materials Research Groups at the University of Birmingham. The author is grateful to Catherine Caton, Matthew Huntley and Yongjun Wang for their results.

1.9

References

avrami m (1939), Kinetics of phase change. I general theory, J Chem Phys, 7, 1103–12. boyer r, welsch g and collings e w (1994), Materials Properties Handbook: Titanium Alloys, Materials Park, OH, ASM International. caton c j (2007), An Investigation into the Optimisation of Bonding of Sandwich Panels for Carbon Fibre Composite Bicycle Wheels, PhD Thesis, The University of Birmingham. christian j w (1975), The Theory of Transformations in Metals and Alloys, Oxford, Pergamon Press. crank j (1975), The Mathematics of Diffusion, Oxford, Oxford University Press. ghosh g, van de walke a, asta m and olson g b (2002), Phase stability of the HfNb system: from ﬁrst principles to CALPHAD, Calphad, 26(4), 491–511. gilat a, goldberg r k and roberts g d (2002), Experimental study of strainrate-dependent behaviour of carbon/epoxy composites, Compos Sci Technol, 62, 1469–76. grong o (1994), Metallurgical Modelling of Welding, London, Institute of Materials. hutchings i m (1992), Tribology, London, Edward Arnold, 160–64. johnson w and mehl r (1939), Reaction kinetics in processes of nucleation and growth, Trans AIME, 135, 416–25. kolmogorov a (1937), A statistical theory for the recrystallisation of metals, Akad Nauk SSSR, Izv, Ser Matem, 1, 355–62. lee j-l and bhadeshia h k d h (1993), A methodology for the prediction of time– temperature–transformation diagrams, Mater Sci Eng A, 171(1–2), 223–30. lee n, erickson m and cherveny p (2002), Measurement of the behaviour of a golf club during the golf swing, Science and Golf IV, London, E F Spon, 554–61. lee p d, chirazi a, atwood r c and wang w (2004), Multiscale modelling of solidiﬁcation microstructure, including microsegregation and microporosity, in an Al–Si–Cu alloy, Mater Sci Eng A, 365(1–2), 57–65. llewellyn d t (1992), Steels: Metallurgy and Applications, Oxford, Butterworth-Heinemann. matthews f l and rawlings r d (1999), Composite Materials: Engineering and Science, Cambridge, Woodhead. postlethwaite a (1988), Manufacturing: the cutting edge, Flight International, 134(17/9/88), 36–42. robson j d (2004), Modelling the overlap of nucleation, growth and coarsening during precipitation, Acta Mater, 52(15), 4669–76. strangwood m, johnson a d g and otto s r (2006), Energy losses in viscoelastic golf balls, Proc. I Mech Part I:L Materials – Design and Applications, 220, 23–30.

© 2007, Woodhead Publishing Limited

34

Materials in sports equipment

tay t e, ang h g and shim v p w (1995), Empirical strain rate-dependent constitutive relationship for glass-ﬁbre reinforced epoxy and pure epoxy, Compos Struct, 33(4), 201–10. valavala p k, clancy t c, odegard g m and gates t s (2007), Nonlinear multiscale modelling of polymer materials, Int J Sol Struct, 44(3–4), 1161–79. wahn m and neugebauer j (2006), Generalized wannier functions: an efﬁcient way to construct ab-initio tight-binding parameters for group-III nitrides, Phys Status Solidi, 243, 1583–87.

© 2007, Woodhead Publishing Limited

2 Non-destructive testing of sports equipment: the use of infrared thermography M. P. L U O N G, L M S C N R S, France

2.1

Introduction

Sports engineering covers various research themes in design, materials and sport, biomechanics, instrumentation, modelling, mechanics, motion analysis, dynamics, etc. In all these areas, thermal aspects should not be ignored because they help identify different product attributes for better competitive sports performance, style, comfort, safety and enjoyment (Taylor, 1998). New technologies have made sports faster and more powerful and they have also improved the performance, enjoyment, safety and the overall well-being of athletes (Subic and Ujihashi, 2005). This leads to an increasing requirement for appropriate testing (1) to ensure product integrity and reliability, (2) to avoid failure, (3) to aid in better product design, (4) to control manufacturing processes, etc.

2.1.1 Objective Mechanical engineering deals with various types of materials and structural components. The use of thermal phenomena to identify product attributes follows a well-known process: thermal phenomena are identiﬁed and theoretical models are obtained intuitively from experience. These must subsequently be validated: their predictions concerning the application are compared with relevant experimental results. An approach based on temperature change measurement provides a better understanding of the mechanical behaviour of sports goods, equipment and accessories. It allows the detection of physical phenomena which can lead to damage, and it leads to new ideas for the improvement of sports goods and related products. This non-destructive testing technique can effectively evaluate the mechanical performance of products, depending upon the requirements of the athletes. 35

© 2007, Woodhead Publishing Limited

36

Materials in sports equipment

2.1.2 Method The thermal effects due to thermomechanical coupling in solids have been identiﬁed within the general framework of the thermodynamics of irreversible processes based on internal variables. This chapter aims to illustrate the use of infrared thermography as a non-destructive, real-time and noncontact technique to detect, observe and evaluate the evolution of temperature changes caused by the diverse processes of irreversible physical phenomena. The results obtained highlight the advantages of differential infrared thermography. This technique minimises the thermal noise in real environments and thus facilitates the detection, discrimination and interpretation of the diverse thermal phenomena involved in these non-linear coupled thermomechanical effects. Stress and strain concentrations in loaded materials and structural components occur and result in localised forces that are sufﬁcient to promote plasticity and/or inelasticity. In addition to traditional techniques of mechanical strength evaluation, the technique provides a ready evaluation of (1) a threshold or limit of acceptable damage under service loading beyond which the material will be rapidly destroyed, or (2) fatigue resistance under repeated and cyclic excitations or dynamic solicitations. Finally, this approach suggests various potential applications of the thermal scanning technique in diverse sports engineering domains: localisation of dissipative phenomena and rapid evaluation of fatigue limit, non-destructive testing using thermal conduction phenomena and detection of heat sources in sports equipment.

2.2

Principles of infrared thermography testing

Thermomechanical coupling effects have traditionally been neglected in thermal stress analyses. The temperature ﬁeld and the deformation induced by thermal dilation and mechanical loads were solved separately. However, this effect can become signiﬁcant when mass inertia is not negligible, due to the ﬂux of heat generated through the boundary of the body, or if the material is loaded beyond its stable reversible limit. The relevance of coupled thermomechanical analysis has been demonstrated for a variety of problems of mechanical engineering, such as fault analysis, damping of stress wave propagation, deformation localisation after bifurcation and strength softening of material due to the heat generated by repeated plastic deformations. Internal energy dissipation has been recognised by a number of wellknown scientists (Farren and Taylor, 1925; Nguyen, 1980; Maugin, 1992). Carrying out experiments on the cyclic twisting of cylindrical bars, Dillon (1963) identiﬁed the work done to the system by plastic deformation as the major contribution to the heat effect, and proposed an internal dissipation

© 2007, Woodhead Publishing Limited

Non-destructive testing of sports equipment

37

rate -D related to the plastic strain rate. The thermal effect due to thermomechanical coupling at the tip of a moving crack has been investigated within the framework of thermodynamics, taking into account stress and strain singularities (Bui et al., 1987). The heat generated due to plastic deformation causes a large local temperature increase which is expected to affect the selection of failure modes during dynamic fracture and thus to inﬂuence the fracture toughness of the material. Well-developed empirical theories of plastic deformation in metals allowed engineers to predict successfully the behaviour of a variety of structures and machine elements loaded beyond the elastic limit for design purposes. Infrared thermography is a convenient technique for producing heat images from the invisible radiant energy emitted from stationary or moving objects at any distance and without surface contact and without in any way inﬂuencing the actual surface temperature of the objects viewed. A consistent theoretical framework is necessary in order to correctly interpret the thermal images. When restricting the analysis to perfectly viscoelastic–plastic material, this leads to the following coupled thermomechanical equation: . . . [2.1] ρCvT = S:E i − (β:D:E e)T + Kⵜ2T + ρr where ρ (kg m−3) = unit mass, Cv (J kg−1 K−1) = speciﬁc heat at constant deformation, C = ρ Cv the volumetric heat capacity of the material (the energy required to raise the temperature of unit volume by 1°Celsius or 1 Kelvin), T = absolute temperature, S = second Piola–Kirchhoff stress tensor, ‘:’ the tensorial contracted product operator, ‘.’ (dot over a letter) the time derivative operator, Ei = irreversible strain tensor, β = coefﬁcient of the thermal expansion matrix, D = fourth-order elasticity tensor, Ee = elastic strain tensor, K (W m−1 K−1) = thermal conductivity, ⵜ2 = Laplacian operator and ﬁnally r is the heat sources. Since the underlying physical processes are highly diversiﬁed, the modelling is approached from a purely phenomenological point of view. Such an approach can be useful in the interpretation of the energetics of the thermoelastic–plastic behaviour. The classical theory of rate-independent isotropic or kinematical hardening plasticity is considered to be an adequate basis for such modelling as it offers the simplest constitutive model for elastic behaviour of the material while still allowing consistent inclusion of two-way thermomechanical coupling effects. When using internal state variables (IV) that describe structural changes of material, the right-hand side member will be completed by other terms representing the cross-coupling effects (Duszek and Perzyna, 1991). These effects inﬂuence the evolution of temperature through the second-order terms when compared with the internal dissipation term. Their contribution to internal heating during the adiabatic process is small and so they are sometimes neglected.

© 2007, Woodhead Publishing Limited

38

Materials in sports equipment

This coupled thermomechanical equation suggests the potential applications of the infrared scanning technique in diverse engineering domains: localisation of dissipative phenomena, elastic stress measurements, nondestructive testing using thermal conduction phenomena and detection of ﬂuid leakage. Thus the detected temperature change, resulting from four quite different phenomena, must be correctly discriminated by particular test conditions and/or speciﬁc data reduction. This is the main difﬁculty when interpreting the thermal images obtained from experiments under the usual conditions.

2.2.1 Intrinsic dissipation The ﬁrst term on the right-hand side of equation 2.1 deﬁnes the energy dissipation generated by viscosity and/or plasticity. Internal energy dissipation has been recognised by many scientists, and the work done to the system by plastic deformation has been identiﬁed as the major contribution to heat effect. In thermoelastic–plasticity, it is generally accepted that not all mechanical work produced by the plastic deformation can be converted to the thermal energy in the solid. A portion of the work is believed to have been spent in the change of material microscopic structure. The work done, in plastic deformation per unit volume, can be evaluated by integrating the material stress–strain curve. This internal dissipation term constitutes a signiﬁcant part of the non-linear coupled thermomechanical analysis. The quantiﬁcation of this intrinsic dissipation for engineering materials is an extremely difﬁcult task without infrared thermography. The infrared thermographic technique is mainly concerned with differences in temperature (or thermal gradients) that exist in a material rather than the absolute value of temperature. The work, reported in this chapter, considers intrinsic dissipation to be the most accurate indicator of damage manifestation. It highlights the advantages of the infrared thermographic technique, used for the detection and discrimination of this non-linear coupled thermomechanical effect within the framework of a consistent theoretical background. Several applications have been proposed herein for materials testing in sports equipment.

2.2.2 Thermoelasticity The second term in equation 2.1 illustrates the thermoelastic coupling effect. Within the elastic range and when subjected to tensile or compressive stresses, a material experiences a reversible conversion between mechanical and thermal energy, causing it to change temperature. Provided adiabatic conditions are maintained, the relationship between the change

© 2007, Woodhead Publishing Limited

Non-destructive testing of sports equipment

39

in the sum of principal stresses and the corresponding change in temperature are linear and independent of loading frequency. It is the reversible portion of the mechanical energy generated; this thermoelastic coupling term may be signiﬁcant in cases of isentropic loading.

2.2.3 Thermal conduction The third term in equation 2.1 governs heat transference by thermal conduction, in which the heat passes through the material to make the temperature uniform in the specimen. The second-order tensorial nature of the thermal conductivity K may sometimes be used for the detection of anisotropy of heavily loaded materials. It could also be used to discriminate different thermal phenomena generated by different dissipative mechanisms within the tested object due to their delay in conduction (Luong, 2001).

2.2.4 Thermal phenomena The fourth term in equation 2.1 is related to the existence of sources or heat sinks in the scanning ﬁeld. The surface heat patterns displayed on the scanned specimen may result from either external heating, referred to in the literature as passive heating, where local differences in thermal conductivity cause variations on isothermal patterns, or from internally generated heat, referred to as active heating. This term has been recently considered in a non-invasive technique to detect variations in skin temperature and thus to analyse the pharmacodynamic properties of topical corticosteroids using vasoconstriction as a marker (Luong et al., 2000). Several subjective and objective methods are available to categorise the potency of topical corticosteroids on healthy skin. They are based on analysis of the vasoconstriction caused by corticosteroids. The parameters that have been studied involved changes in skin colour. In this proposed work, cutaneous thermal images were recorded in real time. A short-duration test was carried out using a single application of corticosteroid without massage or occlusion. A predetermined 1.2 cm radius template surface was used for all tests in order to standardise the results with those of the standard skin-blanching test that was performed after occlusion according to McKenzie’s protocol. Several skin disorders have been studied using infrared thermography, including allergies, skin infections, burns, tumours and microangiopathy (Di Carlo, 1995). However, when used according to the standard method, this technique has been considered insufﬁciently sensitive to detect slight changes in superﬁcial blood ﬂow induced by topical corticosteroids relative to heat emitted from underlying vascularisation (Aiache et al., 1980).

© 2007, Woodhead Publishing Limited

40

Materials in sports equipment

2.3

Infrared thermography technology

Thermography is one of several techniques used to see the unseen. The distribution of surface temperature is used to assess the structure or the behaviour of what is under the surface. Infrared thermography has become the most well-known type of thermography with the proliferation of infrared scanners and other infrared equipment. This technique produces heat pictures – or infrared thermograms acquired through the infrared portion of the electromagnetic spectrum – from the invisible radiant energy emitted from stationary or moving objects at any distance and without surface contact or in any way inﬂuencing the actual surface temperature of the objects viewed. The temperature rise ahead of a fatigue crack has been measured using a thermographic camera in order to demonstrate the local heating at the tip predicted by Barenblatt et al. (1968). Attempts have been made to measure and characterise the heat generated during the cyclic straining of composite materials (Reifsnider and Williams, 1974). The scanning infrared camera has been used to visualise the surfacetemperature ﬁeld on steel (Luong, 1995), wood, engineering materials and ﬁbreglass epoxy composite samples during fatigue tests (Charles et al., 1975). Recently this infrared thermographic technique has been applied in sport engineering (Luong et al., 1998; Parganin et al., 1998; Luong, 2000).

2.3.1 Scanning radiometer with a unique infrared detector The scanning camera is analogous to a television camera. It uses an infrared detector in a sophisticated electronics system in order to detect radiated energy and convert it into a detailed real-time thermal image in a colour and monochrome video system. Response times are shorter than one microsecond. Temperature differences in the heat patterns are discernible instantly and represented by several distinct hues. The quantity of energy W (W m−2 µm−1), emitted as infrared radiation, is a function of the temperature and emissivity of the specimen. The higher is the temperature, the more important the emitted energy. Differences of radiated energy correspond to differences of temperature. The AGA 782 SW infrared scanner unit employed comprises: •

a set of infrared lens which focuses the electromagnetic energy, radiating from the object being scanned, into the vertical prism; • an electro-optical mechanism which discriminates the ﬁeld of view in 104 pixels by means of two rotating vertical (180 rpm) and horizontal (18 000 rpm) prisms with a scanning rate of 25 ﬁelds per second;

© 2007, Woodhead Publishing Limited

Non-destructive testing of sports equipment •

•

•

•

41

a set of relay optics containing a selectable aperture unit and a ﬁlter cassette unit which focuses the output from the horizontal prism onto a single element point detector, located in the wall of a Dewar chamber; a photovoltaic short waves infrared detector composed of indium antimonide (InSb), which produces an electronic signal output varying in proportion to the radiation from the object within the spectral response 3.5–5.6 µm; a liquid nitrogen Dewar which maintains the InSb detector at a temperature of −196 °C allowing a very short response time of about one microsecond; and a control electronics with preampliﬁer that produces a video signal on the display screen.

Since the received radiation has a non-linear relationship to the object’s temperature, can be affected by atmospheric damping and includes reﬂected radiation from object’s surroundings, calibration and correction procedures have to be applied. Knowing the temperature of the reference, the object’s temperature can then be calculated with a sensitivity of 0.1 °C at 20 °C. The infrared scanner unit converts electromagnetic thermal energy radiated from the tested specimen into electronic video signals. These signals are ampliﬁed and transmitted via an interconnecting cable to a display monitor where the signals are further ampliﬁed and the resultant image displayed on the screen.

2.3.2 Infrared focal plane array camera The infrared image of the scene is directly imaged on the detector matrix. No scanning is needed. All pixels are acquired simultaneously and a highspeed frame rate is achieved. Temperature calibration is done externally by pointing the focal plane array camera to a blackbody of known temperature. This thermography system offers a complete package for real-time temperature measurement and analysis of thermal distribution patterns in dynamic and static applications. The focal plane array technology uses 128 × 128 mid wave infrared (MWIR) detectors operating in the spectral range 3–5 µm with a Stirling cooler, at a thermal resolution as low as 20 mK at 30 °C and a pixel size/pitch 35/40 µm. The camera is operated and controlled from a PC computer running in 14 bits under the industry standard Microsoft Windows.® Tools such as spot reading, regions of interest measurements, proﬁles or histograms can be displayed in the live mode. The software calibration module allows recalibrating the camera output in radiometric units as often as required.

© 2007, Woodhead Publishing Limited

42

Materials in sports equipment

2.3.3 Thermography testing data An active thermographic system with external thermal stimulation is scanned from a single side access. When temperature variations occur on the observed surface, the infrared camera records all data for spatial visualisation of temperature distribution and temporal separation of the heat images thanks to advanced image processing techniques applied to the thermal response. The thermal data processing simply consists of using an image subtraction function between two stages of loading.

2.4

Applications: mechanical performance of tennis racket strings

Damage and failure behaviour of natural gut strings and other types of tennis racket strings in synthetic products are an important consideration for skilled tennis players who should be extremely aware of their equipment’s performance relative to their personal needs and game style. What mechanical performance in string tensions will best ﬁt the player’s requirements? Borg strung his rackets at 80 pounds (367 N) and his strings often broke from this high tension. In contrast, McEnroe chose 48 pounds (220 N) for stringing his rackets. Commonly the recommendations are in a range of about 55–65 pounds (250–295 N). The tension is very important as it has a direct effect on power and control. As a general rule, the looser the tension, say in the 50-pound range, the less control the player has. The racket acts more like a trampoline and the ball may ﬂy off the strings and appear to have more speed. Tighter strings, say in the 60-pound range, will give more control over the shot. Professionals often string their rackets in the 70-pound range. This gives them a lot of control, but the strings break more often. Much engineering research has been conducted to determine optimal string tension for different size rackets made from various types of materials (Groppel et al., 1987). The limit of acceptable damage in tension for most of these tennis strings probably lies between these two values. This work proposes to determine objectively this tension threshold from the material point of view, using an infrared thermographic technique. It may suggest material optimisation of string products for manufacturers. In addition this technique can be used as a non-destructive, non-contact and real-time method for inspection and ready evaluation of stress concentration in strung rackets. This work considers the intrinsic dissipation, generated by plasticity, as a highly sensitive and accurate indicator of damage manifestation. Thanks to the thermomechanical coupling (Luong, 1999), infrared thermography is used to observe the physical processes of damage and to detect the onset and the evolution of damage and failure processes of tennis string when the

© 2007, Woodhead Publishing Limited

Non-destructive testing of sports equipment

43

specimen is subjected to increasing tensile loading. It readily gives a measure of the material damage and permits evaluation of the limit of a progressive damaging process under tension beyond which the material fails. The mechanical properties of tennis racket strings are mainly dependent on its type (natural gut, artiﬁcial gut, nylon, graphite string, synthetic products, etc.), nature and structure. Manufacturers give players a choice of string styles. The higher the number, the thicker the gauge of the string. Consumers can choose 15-gauge, 16-gauge, 17-gauge, 18-gauge and even 19-gauge strings: the lower the gauge number, the lighter the string. Lower gauge strings, like 16, last longer and are good for players who hit with a lot of spin. Thinner gauges of string are livelier and can impart more ball spin but are likely to break sooner. This fact suggests that there is an instantaneous non-linear phenomenon creating large displacements and large deformations due to Coulomb friction at string nodes: wear by fretting. This aspect will be examined hereafter in the case of natural gut. Better players like to use 17-gauge or 18-gauge as this gives them more touch and feel. The effect of varying string tensions is important to skilled players wanting to improve their shot velocity and control (Groppel, 1992). The looser the strings (within the recommended range), the higher the ball velocity after impact (Brody, 1996). When string is loaded, it deforms as a whole in spite of its heterogeneous characteristics and its localised defects. Stress concentrations occur and result in localised forces that are sufﬁcient to promote plasticity and inelasticity. At the structural level, breakdown appears as microcracking and possibly slippage at component interfaces. Failure mechanisms of string specimens subject to tensile loading are readily evidenced by infrared thermography in this work. Uniaxial tensile tests were carried out for two types of tennis strings: natural gut and synthetic ﬁbres.

2.4.1 Natural gut string Gut is a natural animal ﬁbre, manufactured from the smooth muscle portion of sheep or beef intestines through a sophisticated chemical process of washing, bleaching, twisting, drying and reﬁning to ensure strength and uniformity (Babolat, 1996). When loaded in tension (Figs. 2.1 and 2.2a), natural gut string undergoes plasticity (Figs. 2.2b and 2.2c) that is detected by infrared thermography. Subtractions of raw thermal images at tension levels T2 and T3 and a reference tension level T1 evidence a dissipative phenomenon (Figs. 2.2d and 2.2e) between these tension levels that locates the failure of gut string. When tensile loading is applied up to failure, the evolution of dissipation suggests the deﬁnition of a limit of acceptable damage (LAD) that separates low and high regimes of dissipation or damage manifestation (Fig. 2.3).

© 2007, Woodhead Publishing Limited

600

Tension (N)

T2

T3

400

200

T1

0

10 5 Elongation (mm)

0

15

2.1 Natural gut string under tension.

(c) Tension T3

(b) Tension T2

(e) Dissipation between T3 and T1

(d) Dissipation between T2 and T1

(a)

2.2 Tension on natural gut string (a): raw thermal images (b, c) at different tensile levels T2 and T3 preceding failure and their corresponding dissipation localisation between T2 and T1 (d), and T3 and T1 (e) (temperature changes are given in Celsius).

600

Tension (N)

500 400 300 LAD 200 100 0 0

2

4 6 Dissipation (°C)

8

10

2.3 Limit of acceptable damage (LAD) of a natural gut string in tension. © 2007, Woodhead Publishing Limited

Non-destructive testing of sports equipment

45

0.7

Tension (N)

0.6 0.5 LAD 0.4 0.3 0.2 0.1 0 0

2

4 6 Dissipation (°C)

8

10

2.4 Limit of acceptable damage (LAD) of a synthetic ﬁbres string in tension.

2.4.2 Synthetic gut string Material composites have increased the diversity of design and manufacturing for sports products. There are a large variety of synthetic products including nylon, artiﬁcial gut, graphite string, oil-ﬁlled string, etc. Very speciﬁc designs are targeted to match the physical capability of each player. Can their mechanical performance be characterised in term of damage and durability? In the interaction of the ball and the strings, the kinetic energy of the ball is converted into potential energy stored in both ball and string deformation. By storing a larger fraction of the incoming energy in the strings, less is dissipated, and more is returned to the ball’s rebounding kinetic energy (Brody, 1996). The shock vibrations of the wrist joint are transmitted from the racket with an impulse at the impact location and several vibration mode components of the racket frame and strings (Kawazoe et al., 1998). The higher the string tensions, the higher the vibration frequency. This fact inﬂuences the feel or comfort of the arm or hand on impact. As tension increases, the elasticity of these materials degrades. Dissipation occurs and infrared thermography readily detects the manifestation and localisation of damage. As with natural gut, when tensile loading is applied up to failure an LAD is suggested, separating low and high regimes of dissipation or damage (Fig. 2.4).

2.4.3 Dry sliding of natural gut string at nodes In order to produce ball spin in tennis, the player must accelerate the racket head through impact to brush the bottom of the ball: (1) upward for topspin, (2) downward for underspin and (3) sideways for sidespin.

© 2007, Woodhead Publishing Limited

46

Materials in sports equipment

The more vertical (or horizontal) the racket swings (in either direction), the more ball spin will be produced. The amount of spin a player imparts on the ball, combined with a high stroke velocity, generates dry sliding between longitudinal and transverse strings at nodes located in the racket’s effective hitting zone. Relative movements between longitudinal and transverse strings occur depending on the type of ball–racket impact. They lead to tribological phenomena, such as friction, wear, pitting and fretting fatigue. Fretting is a major problem in optimising tennis strings. It is deﬁned as the surface damage induced by small-amplitude oscillatory displacements between strings in contact. This damage can either be wear or crack nucleation, depending on the prescribed forces or the displacement amplitude. Infrared thermography has been used to estimate tribological parameters, such as a frictional temperature rise, the shape and the size of the contact area. We consider a contact problem, in which a moving longitudinal string (9 cm long) is in contact with a ﬁxed transverse string (5 cm long). The two strings were initially stretched at 200 N. An electromagnetic vibrator at a frequency of 1 Hz controls the cyclic motion of the longitudinal string. Force sensors, respectively, measure the normal and tangential contact forces during testing (Fig. 2.5). Infrared thermography readily detects heat dissipation by Coulomb friction at contact location where sliding occurs between longitudinal and transverse gut strings in the following case: initial normal contact force NCF = 50 N.

2.5 Experimental set-up for fretting fatigue testing on natural gut string at nodes.

© 2007, Woodhead Publishing Limited

Non-destructive testing of sports equipment

47

3

Dissipation (°C)

2.5 2 1.5 1 FFL 0.5 0

0

100

200 300 400 500 Tension on string (N)

600

700

2.6 Experimental determination of fretting fatigue limit (FFL) of a natural gut string using infrared thermography.

The experimental data demonstrate that wear phenomena occurring in tennis rackets could become signiﬁcant in long matches such as the men’s ﬁnal of 1988 US open Tennis Tournament between Lendl and Wilander (4 hours, 54 minutes with several thousand strokes). The main parameters identiﬁed are string tension, racket stiffness, effects of spin, hitting power, etc. This work has demonstrated that the dissipativity of tennis string material under tension or frictional loading is highly sensitive and is an accurate predictor of damage. Owing to the thermomechanical coupling, infrared thermography offers the possibility of a non-destructive, non-contact testing of string degradation and damage. It provides a ready evaluation of a limit of fretting fatigue load (Fig. 2.6), beyond which the string will fail in a long match. The opportunities offered by thermal techniques with remote operation and fast surface-scanning rates are particularly attractive for sports equipment.

2.5

Applications: damage detection in leather sports footwear

Before 1924 the whole sports footwear was made in leather, thanks to its ability to absorb the sweating that is 6–8 times higher than during a normal sporting activity. Today, leather is still well present in sports footwear. Annually France produces 320 000 pairs for tennis, basket, training and jogging, 200 000 pairs with staples and toes, 930 000 pairs for touring, 24 000 pairs for riding, and 180 000 pairs for dancing.

© 2007, Woodhead Publishing Limited

48

Materials in sports equipment

2.5.1 Main characteristics of leather Leather is produced by stabilising through tanning the proteins of animal skins that are by-products of the meat industry. While about ﬁve-sixths of all the leather produced is used by the shoe industry, special uses of leather meet many industrial requirements. Through its extensive use in boots, shoes, clothing, straps, harness or handbags, wallets, billfolds, leather novelties, etc., leather has become a most essential commodity for humans. Softness, damage and strength behaviour are important considerations in connection with design or regulations. An additional concern is to produce leather of uniform high quality, for example, which is acceptable to the shoe manufacturer and the customer. Leather is a product of biological origin (Bérard and Gobillard, 1951). It is a very variable and heterogeneous material. The mechanical properties are mainly dependent on inhomogeneous composition and anisotropic structure of skins. The different ﬁbres of the grain and corium layers are interwoven at the boundary line, making it difﬁcult to deﬁne the precise location of this interface. Random interweaving collagen ﬁbres, or bundles containing ﬁbres, form a complex ﬁbre matrix and constitute the ﬁbrous structure of the leather. Mechanically, collagen ﬁbres are strong in the direction of the ﬁbre axes (Mitton, 1945; Kanagy, 1954), and the elongation at failure is about 2%. In the dead animal skin, the ﬁbres are set in a matrix of natural binding agents such as blood, etc. These agents are removed during the tanning process to create an open structure of ﬁbres that is held together by their interwoven arrangement. During the later stages of the tanning process, the pores between the ﬁbres are partially ﬁlled with oils and greases that determine the subsequent behaviour of the leather. The partial ﬁlling of the zones between adjacent ﬁbres produces a porous matrix structure. The degree to which the larger ﬁbres are broken and separated into smaller ﬁbrils has an inﬂuence on the strength and other physical properties of leather. Under applied uniaxial loading this type of structure enables the ﬁbres to rotate and to slide relative to each other. The breaking load measured in any strength test is directly related to the number of ﬁbres that become involved in the test prior to rupture. The tensile strength is higher in the direction parallel to the backbone than it is in the perpendicular direction; but the strains at failure have an inverse directional dependence (Roddy, 1956). On initial monotonic loading for a typical test, the stress increased proportionally with elongation. The mechanism of deformation is due to ﬁbre rotation. Subsequently for large strains, the behaviour is non-linear with the stress, reaching a plateau at failure. Deformation takes place by the combined mechanism of ﬁbre rotation and of slippage between adjacent ﬁbres.

© 2007, Woodhead Publishing Limited

Non-destructive testing of sports equipment

49

2.5.2 Traditional techniques for testing leather Very extensive work has been carried out by the Technical Committee of the Upholstery Leather Group in attempting to set speciﬁcations and to develop methods that can be related realistically to service performance (Compton, 1956; Kronick et al., 1993). Particular tests and speciﬁcations are useful for the automotive industry: breaking strength, elongation (stretch), stitch tear resistance (static and dynamic), tongue tearing, bursting strength, abrasion resistance, ﬂex resistance, etc. Tensile strength was the ﬁrst strength test applied to leather; at the present time, it is more generally applied than any other mechanical tests (Lin and Hayhurst, 1993). Breakage in the tensile test may be considered to result from either the rupture of chains or the slipping of adjacent chains. The reason for the high tensile strength of the native ﬁbres is believed to be their predominantly primary valence chain structure. A large amount of energy is required to break such chains. Stitch tearing test was introduced recently for the evaluation of leather with regard to strength of seams. Devised to simulate the behaviour of stitches in shoes under service conditions, it has become quite popular because only a small amount of leather is required to perform the test and the procedure is simple. The stitch-tearing test depends on the load that may be supported by the leather before a tear begins, and for this reason it is a valuable test in the evaluation of seams. Tongue-tear test is a simple tear test. A piece of leather about 1 inch wide and 6 inches long is split at right angles to the grain and ﬂesh sides for about two-thirds of its length. The end of one of the pieces formed by the split is put in each jaw of the testing machine. Tongue-tear differs from both the tensile strength test and the stitch-tear test in that the strength in the direction perpendicular to the backbone is higher than that in the parallel direction. Burst test gives an average value of the strength in both directions. A specimen of leather 4 by 4 inches is clamped between two corrugated steel plates having an aperture 1 3/8 inches in diameter. This adapter is placed in one jaw of a testing machine. In the other jaw a plunger, hemispherical at the tip, 2 inches long and 1 inch in diameter, is fastened. The force is applied to the plunger so that it moves at the rate of 1 inch per minute. The force required to crack the grain of leather and that required to burst the leather are recorded. The burst test is closely related to performance in shoe making. Pulling the upper over the last applies forces similar to those applied in the test. Since the ﬁrst failure noted is generally cracking of the grain, it may also be used for measuring the tenderness of the grain. Nondestructive and non-contact tests are thus needed to deﬁne leather and leather product properties (1) to establish strength, (2) to optimise design values and (3) to ensure quality control.

© 2007, Woodhead Publishing Limited

50

Materials in sports equipment (a)

Uniaxial tension (kN/m)

25 20 15 10 5 0

0

1

2 3 4 Axial displacement (mm)

5

6

(b) 25

Tension (kN/m)

20 LAD 15 10 5 0 0

0.2

0.4

0.6 0.8 1 Dissipation (°C)

1.2

1.4

2.7 (a) Load–displacement curve of a leather specimen under uniaxial tension. (b) Limit of acceptable damage (LAD) of a leather specimen under uniaxial tension.

2.5.3 Thermography testing of a leather specimen Chrome-tanned calfskin of 0.98–1.02 mm thickness was selected for the uniaxial tension test. Figure 2.7a shows the load–displacement curve of a leather specimen. The Agema SW-782 infrared scanner, incorporating a hightemperature ﬁlter and equipped with a real-time system DISCON (digital infrared system for coloration) converting thermal images into a ten-colour isotherm display, was used to record the heat emission data. This device displays a ten-colour calibrated surface-temperature picture of the specimen. Infrared thermography readily detects intrinsic dissipation announcing the occurrence of damage, preceding the failure of the leather specimen, localises heat dissipation evidencing the mechanism of damage in the leather specimen and depicts the evolution of intrinsic dissipation when loading increases up to failure. Thus it deﬁnes a LAD, separating low and high regimes of dissipation or damage occurrence (Fig. 2.7b).

© 2007, Woodhead Publishing Limited

Non-destructive testing of sports equipment

51

2.5.4 Thermography testing of a leather seam The process of tailoring leather products involves the transformation of a two-dimensional structure (planar sheet fabric) into a three-dimensional form. It necessitates the formation of extreme curvatures by sewing or fusing and pressing. The joining of different pieces of leather components is achieved by sewing with threads. There are many problems associated with the performance of sewing threads during a sewing process, which can reach very high speeds. During the stitching process, thread is subjected to complex kinematic and dynamic conditions. Tensile and bending stresses take place at very high speeds and localised heating may cause reduction of strength of thermoplastic sewing threads by as much as 60%. In addition, the mechanics of the thread structure and their properties in the seams after sewing include seam strength, with stretch and pucker also being extremely important. Current design procedures for leather seams are based upon simpliﬁed assumptions of connection behaviour. The increased use of diverse types of seams by sewing and/or gluing suggests that a more thorough understanding of the behaviour of leather seams would be beneﬁcial for upgrading design procedures. As infrared thermographic scanning offers new information on leather connection behaviour, reﬁnement of design procedures can be made as needed and new, more effective and specialised seam conﬁgurations can be imagined. Infrared thermographic scanning of a typical leather seam, loaded in tension, provides interesting insights into load transfer in seams, improving our understanding of stress distribution within connections, far beyond the elastic domain and up to failure. It is of particular interest that the method allows not only qualitative work such as ﬁnding ﬂaws, defects or weakness zones, but also quantitative analysis of the effects of ﬂaws and defects on

Tension (kN/m)

25 20 15 10 5

LAD

0 0

0.5

1

1.5 2 2.5 Dissipation (°C)

3

3.5

2.8 Limit of acceptable damage (LAD) of a leather seam showing that the sewing technique could be improved (when compared to the LAD of leather specimen).

© 2007, Woodhead Publishing Limited

52

Materials in sports equipment

the strength and durability of leather components. This useful and promising technique offers accurate illustration of crack initiation, the onset of its unstable propagation through the material and/or ﬂaw coalescence. The main interest of this energy approach is to unify microscopic and macroscopic test data. The parameter intrinsic dissipation under consideration is a scalar quantity, easy to evaluate accurately. Subsequently it may suggest multiaxial design criteria, highly relevant for full-scale testing on sport footwear. It permits the deﬁnition of a LAD in leather seams (Fig. 2.8), very useful for the optimisation of the sewing technique.

2.6

Applications: testing sailcloth for yachts

Since the 1950s, sailcloths have been widely improved leading to increased sailing speeds for yachts. Sail makers directed their research towards stronger, lighter and low-stretch sails. In the past, sailcloths were made with ﬂax. Cotton sails then appeared; they were not so strong as the ﬂax ones, but they stretched less and thus held their shape better in the wind. In fact, sail makers used cotton and ﬂax fabrics until shortly after World War II. In the 1950s, newly developed synthetic ﬁbres in polyester and/or nylon replaced natural ﬁbres. In the 1970s, polyester reinforcements appeared. Attempts with sailcloths reinforced with Kevlar® (DuPont) broke down because sail makers were unable to work with such strong, but fundamentally brittle, ﬁbres. The weaves they created turned out to be self-abrasive and they were so strong that they could break a boat’s rigging. Throughout the 1970s and 1980s, designers continued to experiment with Kevlar. In 1980, a boat equipped with a main sail made of Kevlar reinforced laminate won the America’s Cup. In 1983, the Australian winner used Kevlar laminates for all its main sails and jibs. Kevlar remains the most popular ﬁbre for use in racing sail, but other ﬁbres are also chosen. In the mid-1980s, Spectra® (Honey Well International Inc.) ﬁbre, a polyethylene material, appeared. It is not as brittle as other high-performance material and lasts longer. Although it can gradually elongate under load, Spectra is found most frequently in cruising sails. In 1992, carbon ﬁbre was used for the ﬁrst time in sailcloth laminates in preparation for the America’s Cup. Carbon weighs only about 60% as much as Kevlar for the same strength, but it is exceedingly brittle and difﬁcult to incorporate into sail cloth. Throughout the 1980s and into the 1990s, laminate sailcloths improved steadily. The newest fabrics are made from layers of tough ﬁbres and thin plastic ﬁlms. These laminated sails have revolutionised yacht racing. However, even as the mechanical performance of the cloth itself improved, it became increasingly difﬁcult to make seams that would not slip or break. In 1990, Dubois and Baudet invented a novel manufacturing system for moulding laminate sails in one piece: the three-dimensional laminate or 3DL. The outer plastic layers of the sail are taped together to

© 2007, Woodhead Publishing Limited

Non-destructive testing of sports equipment

53

Freeze 19.7 18.2 16.8 15.3 13.8 12.4 10.9 9.4 8.0 6.5 5.0 3.5 2.1 0.6 –0.9 –2.3 –3.8

2.9 Heat image showing a large dissipation at failure of a one-layer Dacron® sailcloth specimen (temperature changes are given in Celsius).

form a membrane large enough for the mould. And between them, the sail cloth ﬁbres run continuously from one corner to another and provide the structure’s backbone. Because the process involves no cutting, the resulting sails are essentially seamless and so less susceptible to stretching. Two specimens of sail have been selected by the National Sailing School (École Nationale de Voile): a woven Dacron® (Invista Inc.) sample and a three-layer composite sample (polyester + Kevlar + polyester ﬁlm). The specimen size is: 10 cm long and 20 cm wide. Each kind of sail is tested in two directions, 0 and 90°, in order to study the isotropic property of the material. They are submitted to a uniaxial tension test. The tension speed is 20 mm/min. The tension test is continuously ﬁlmed with an infrared digital camera. The Agema infrared scanner Thermovision 900 was used to record the heat emission data. The detector is sensitive in a 2–5,6 µm bandwidth and the scan frequency is 30 Hz. Infrared thermography depicts the intrinsic dissipation localisation announcing mechanisms of damage preceding sail failure (Fig. 2.9). In the 0 and 90° directions, the results are nearly the same. The one-layer Dacron sail sample appears quite homogeneous according to the obtained thermal images.

2.7

Applications: soccer and long distance walking

2.7.1 Testing the 1998 World Cup soccer ball The manufacturer claimed that the WC 1998 soccer ball contains hollow acryl-nitryl micro balls of 70 µm in diameter, pressurised at 14 MPa. They are highly compressible and to respond to a great amount of kicking energy. Figure 2.10 depicts a check-experiment on the soccer ball kicked by a steel ball. The elasticity of the collision between soccer ball and steel ball is given by a measure of how much bounce there is or, in other words, how much of the kinetic energy of the colliding objects before the collision

© 2007, Woodhead Publishing Limited

54

Materials in sports equipment

2.10 Check-experiment of energy restitution on a soccer ball.

remains as kinetic energy of the objects after the collision. With an inelastic collision, some kinetic energy is transformed into deformation of the material, heat, sound and other forms of energy, and is therefore unavailable as motion. A perfectly elastic collision has a coefﬁcient of restitution of 1 (ratio of the differences in velocities before and after the collision). Figure 2.11 shows the kicked soccer ball (green), the steel ball striker (red) and a very localised hot zone due to increased pressure in the pressurised micro balls. The role of the pressurised micro balls consists in storing elastically the kicking energy according to the equation: pv = RT

and

h = cpT

[2.2]

In the case of a perfect gas with gas constant R and heat capacity cp, the variables are pressure p, speciﬁc volume v, temperature [K] T and speciﬁc

© 2007, Woodhead Publishing Limited

Non-destructive testing of sports equipment

55

29.20 28.72 28.27 27.79 27.34 26.89 26.41 25.96 25.48 25.03 24.49

2.11 The kicked soccer ball is the big circle at 26.5 °C. The steel ball striker is the mean size circle at 30 °C. A very localised hot zone at nearly 28 °C is due to the increased pressure in the pressurised micro balls (temperature scale is given in Celsius).

enthalpy h. The experiment evidenced a very localised heat change decreasing rapidly. Moreover, several consecutive experiments at the same location conﬁrm that the heat increase remains very localised and that no accumulation of temperature changes occurs on the soccer ball. This fact demonstrates that dissipative behaviour is negligible.

2.7.2 Screening foot trauma from long distance walking Paris–Colmar (a town near Strasbourg, France) is the longest non-stop race-walking competition in the world. To qualify for this race every walker has to participate in a race of 200 km and be capable of walking at least 195 km in 24 hours. Generally this is not enough to succeed in the race so the walker should be able to walk at least 200 km in 24 hours. The selected top 30 walkers are invited for the Paris–Colmar race in June. This race of 530 km starts in the French capital Paris on Thursday at 9 pm. Following the river Marne, walkers pass the ﬁrst night and day, and after about 250 km they must stop for an obligatory three-hour rest. Early on Saturday morning they start the second part of the walk, which becomes hillier. From this time, some of the walkers pull out of the race. After a further day and night, there is a second obligatory stop for one hour. After this short rest, walkers must cover the ﬁnal 100 km in about 12–16 hours. During this period all walkers must clime to an altitude 950 m above sea level and after that walk down close to the Rhine river on the German border. The winner crosses the ﬁnishing line in the lovely town of Colmar in Alsace on Sunday afternoon, with the last tired and happy ﬁnisher arriving early on Monday morning. The average speed of the winner can exceed 8.5 km/h. This average speed includes all stops for massage, etc.

© 2007, Woodhead Publishing Limited

56

Materials in sports equipment

An ofﬁcial deﬁnition of race walking to formally differentiate it from running is as follows: •

•

race walking is a progression of steps so taken that the walker makes contact with the ground so that no visible (to the human eye) loss of contact occurs; the advancing leg must be straightened (i.e. not bent at the knee) from the moment of ﬁrst contact with the ground until in the vertical upright position.

Due to the repetitive nature of walking, plantar fasciitis, the main cause of heel pain, can be considered a repetitive stress injury. With each step, the heel contacts the ground followed by the front of the foot lowering and then the arch, stretching the plantar fascia, a spring-like mechanism. Pain or burning is caused by inﬂammation (predominant process) of the supporting structures of the foot, particularly for those that don’t have enough arch support. This can be readily diagnosed with an infrared camera that evidences abnormal heat increases. Thermography screening of foot, knee, leg traumatisms is performed at race stops in order to detect shin splints, plantar fasciitis, blisters, chaﬁng, Achilles tendonitis, etc. Figure 2.12 shows burning caused by inﬂammation of the supporting structures of the foot. It is clear that infrared thermography provides an efﬁcient aid to sports medicine for injury prevention.

2.12 Burning caused by inﬂammation of the plantar fascia.

© 2007, Woodhead Publishing Limited

Non-destructive testing of sports equipment

2.8

57

Summary

This work has demonstrated that (1) the evolution of heat change facilitates the detection of thermobiological coupling and (2) the dissipativity of engineering materials or structures under solicitations is a highly sensitive and accurate manifestation of damage. Thanks to the thermomechanical coupling, infrared thermography provides a non-destructive, non-contact and real-time test to observe the physical process of concrete degradation and to detect the occurrence of its intrinsic dissipation. It thus readily provides a measure of the material damage and allows the deﬁnition of a limit of acceptable damage and fatigue limit of concrete under load beyond which the material is susceptible to failure. It should be pointed out that the inelastic strain due to compressive loading provides information only on the current geometry while the internal state variables provide information on the internal state and on the microstructural defects. The method allows not only qualitative studies such as ﬁnding ﬂaws, defects or weakness zones, but also quantitative analysis of the effects of ﬂaws and defects on strength and durability of structural components. Several published results demonstrate the versatility of the infrared thermographic technique in various domains of application, provided that we interpret correctly the physical phenomena in a consistent theoretical framework. The main interest of this energy approach is to unify microscopic and macroscopic test data. The parameter intrinsic dissipation under consideration is a scalar quantity, easy to evaluate accurately. Subsequently it may suggest multiaxial design criteria, highly relevant for full-scale testing on engineering structures.

2.9

References

aiache j m, lafaye c, bouzat j and rabier r (1980), Measurements of corticosteroids topical availability by thermography, J Pharm Belg 35, 187–95. babolat d l (1996), Squash, Tennis, Badminton, technical note, Lyon, Babolat. 2–14. barenblatt g i, entov v m and salganik r l (1968), On the inﬂuence of vibrational heating on the fracture propagation in polymeric materials, IUTAM Symp Thermoinelasticity, East Kilbride. bérard j and gobillard j (1951), Cuir et Peaux, Paris, Presses Universitaires de France. brody h (1996), The modern tennis racket in S Haake (ed.), The Enginneering of Sport, Rotterdam, Balkema, 79–92. bui h d, ehrlacher a and nguyen q s (1987), Thermomechanical coupling in fracture mechanics, in H D Bui and Q S Nguyen (eds), Amsterdam, Thermomechanical Coupling in Solids, IUTAM, 327–41.

© 2007, Woodhead Publishing Limited

58

Materials in sports equipment

charles j a, appl j and francis j e (1975), Using the scanning infrared camera in experimental fatigue studies, Exp Mech, 14(4), 133–8. compton e d (1956), Upholstery leather manufacturer, Chem Technol Leather, III (41), 292–305. di carlo a (1995), Thermography and the possibilities for its applications in clinical and experimental dermatology, Clin Dermat, 13(4), 329–36. dillon o w jr (1963), Coupled thermoplasticity, J Mech Phys Solids, 11, 21–33. duszek m k and perzyna p (1991), The localization of plastic deformation in thermoplastic solids, Int J Solids Struct, 27(11), 1419–43. farren w s and taylor g i (1925), The heat developed during plastic extension of metals’, Proc Roy Soc Series A, 107, 422–8. groppel j l (1992), High Tech Tennis, Champaign, Ill, 2nd edn, Leisure Press. groppel j l, shin i s, thomas j a and welk g j (1987), The effects of string type and tension on impact in midsized and oversized tennis racquets, Int J Sport Biomech, 3, 40–46. kanagy j r (1954), Signiﬁcance of the results of some physical tests on upper leather, J Amer Leather Chem Assoc, 8, 112–48. kawazoe y, tomosue r and yoshinari k (1998), Performance prediction of tennis rackets with different racket head size: impact shock vibrations of a racket grip and a player’s wrist joint, The Engineering of Sport, in S Haake (ed.), The Engineering of Sport – Design and Development, Oxford, Blackwell Publishing, 325–32. kronick p, page a and komanowsky m (1993), An acoustic emission study of staking and fatliquor, J Amer Leather Chem Assoc, 88, 178–86. lin j and hayhurst d r (1993), Constitutive equations for multi-axial straining of leather under uni-axial stress, Euro J Mech, A/Solids, 12(4), 471–92. luong m p (1995), Infrared thermographic scanning of fatigue in metals, Nucl Eng Des, 158, 363–76. luong m p (1999), Infrared thermography of macrostructural aspects of thermoplasticity, in O T Bruhns and E Stein (eds), Micro- and Macrostructural Aspects of Thermoplasticity, Dordrecht, Kluwer Academic Publishers, 62, 437–46. luong m p (2000), Infrared thermography of the tensile behavior of natural gut string, in A Subic and S Haake (eds), The Engineering of Sport – Research, Development and Innovation, Oxford, Blackwell Publishing, 423–30. luong m p (2001), Thermomechanical couplings in solids, in X P V Maldague and P O Moore (eds), Infrared and Thermal Testing, Nondestructive Testing handbook, 3rd edn, Vol. 3, Columbus, OH, American Society for Nondestructive testing, 342–7. luong m p, parganin d and loizeau j (1998), Non-destructive detection of damage in soccer balls using infrared thermography, in S Haake (ed.), The Engineering of Sport – Design and Development, Oxford, Blackwell Publishing, 145–52. luong m s, luong m p, lok c, carmi e, chaby g and visieux v (2000), Bioavailability of topical corticosteroids evaluated by differential thermography, Ann Dermatol Venereol, 127(8–9), 701–5. maugin g a (1992), The Thermomechanics of Plasticity and Fracture, Cambridge Texts in Applied Mathematics, Cambridge, Cambridge University Press. mitton r g (1945), Mechanical properties of leather ﬁbres, J Soc Leather Trades Chem, 29, 169–94.

© 2007, Woodhead Publishing Limited

Non-destructive testing of sports equipment

59

nguyen q s (1980), Méthodes énergétiques en mécanique de la rupture, J Mécanique, 19, 363–86. parganin d, luong m p and loizeau j (1998), Infrared scanning of damage in sail materials, in S Haake (ed), The Engineering of Sport – Design and Development, Oxford, Blackwell Publishing, 163–70. reifsnider k l and williams r s (1974), Determination of fatigue-related heat emission in composite materials, Exp Mech, 14(12), 479–85. roddy w t (1956), Chemistry and Technology of Leather, American Chemical Society Monograph 134, New York, Reinhold Publishing Corp. subic a and ujihashi s (2005), The Impact of Technology on Sport, Tokyo, ASTA. taylor p (1998), The economics of the sports products industry, in S Haake (ed.), The Engineering of Sport – Design and development, Oxford, Blackwell Publishing, 3–12.

© 2007, Woodhead Publishing Limited

3 Materials and design for sports apparel K. B. B L A I R, Sports Innovation Group LLC, USA

3.1

Introduction

Sports apparel includes numerous products, many with very speciﬁc performance requirements. These requirements vary from comfort to protection from potentially dangerous environments. When designing sports apparel for a speciﬁc application, there are choices of materials, construction methods, and garment assembly; each step in the process having an impact on the apparel performance parameters. Effective material selection and apparel design then requires a detailed understanding of the performance requirements of sports apparel, as well as a detailed understanding of the components of the garment and how they affect the garment performance. The criticality of the performance of sports apparel is obvious in extreme sports such as mountaineering, where protection from the elements is required simply for survival. In other sports, while survival may not be an issue, the apparel an athlete wears directly affects athlete performance. For example, inadequate thermal protection can cause an athlete to overheat and poor aerodynamic design can cost minutes in timed events. Comfort, while it may not directly affect the performance of an athlete, is also of utmost importance, as apparel that is not comfortable will be distracting to the athlete. This hierarchy of performance requirements can be addressed through apparel design. Apparel design begins at the material selection stage. These materials are then made into ﬁbers, ﬁbers are spun into yarns, yarns are woven into textiles, and textiles are assembled into garments. Decisions at each step in the process affect the performance of the ﬁnal product. The challenge is to properly manage all the levels of material design, from the ﬁber to the garment, as a change at any one of the layers can have a large effect on the performance of the ﬁnal product. This chapter provides a broad overview of the concepts required to address the role of materials in the design of apparel for sports applications. We begin by describing the building blocks of apparel, the materials used 60

© 2007, Woodhead Publishing Limited

Materials and design for sports apparel

61

to build the ﬁbers and yarns, which in turn are woven into textiles, and the subsequent ﬁnishing treatments used on the textiles to add further performance features. The ﬁnal step in apparel construction, fastening, is discussed, as the textiles need to be assembled to create the ﬁnal garment. The fastening systems employed may also add further performance features to the ﬁnal product. Testing methods are considered next as, with any engineering system, the characterization of the performance of the individual materials, as well as the subsystems, for example a fabric or a seam, within the ﬁnal product is essential to being able to predict its performance. The performance features of apparel include thermal regulation, water and moisture management, ﬁt, aerodynamics, and safety concerns. Each of these performance features is considered. The chapter concludes with a discussion of trends and sources for further information.

3.2

Textiles for sports apparel: ﬁbers, yarns and fabrics

The basic building block of apparel is the fabric from which a garment is made. This chapter begins with an overview of some of the basic concepts of textiles and an introduction to the components, processes, and terminology used in the development of textiles.

3.2.1 Fibers The basic unit of a textile is the ﬁber. Fibers are any substance, natural or manufactured, with a length to width ratio of at least 100 to 1 (ASTM) and with suitable characteristics for being processed into fabric (Kadolph and Langford, 2002). This deﬁnition does not encompass many of the attributes of ﬁbers that are needed to create textiles. Like any other engineering material, many characteristics and properties of a ﬁber are considered for their selection in the development of sports apparel. Characteristics include engineering dimensions such as strength and elasticity, and aesthetic qualities such as appearance and feel. In general there are two major classes of ﬁbers: natural and manufactured (Collier and Tortora, 2001). Natural ﬁbers are cellulose-based, proteinbased, or mineral-based. Cellulose natural ﬁbers include cotton and ﬂax, which is the basis for linen. Protein-based ﬁbers typically come from animal hair, with the exception of silk. Asbestos was a common mineral-based ﬁber, but has been phased out of use due to health concerns. While common in casual wear, natural ﬁbers and mineral ﬁbers are now rarely used in sports apparel. Manufactured ﬁbers are those that do not occur as ﬁbers in nature, but instead are created through a manufacturing process. Manufactured ﬁbers

© 2007, Woodhead Publishing Limited

62

Materials in sports equipment

are generally grouped into three classes: regenerated, inorganic, and synthetic (Collier and Tortora, 2001). Regenerated ﬁbers are made from materials that cannot be used in their native form. Examples include rayon and acetate. Inorganic ﬁbers seeing increased use in sports apparel include glass, ceramic and metallic-based ﬁbers. Synthetic ﬁbers, the most common used in sports apparel, include polyester, nylon, and spandex. New manufactured ﬁbers are continually developed and see an increasing use in sports apparel. The many names of manufactured ﬁbers on the market today are often a source of confusion to consumers (Watkins, 1995). The United States Federal Trade Commission acknowledges 21 generic synthetic ﬁbers. These generic classiﬁcations are family names of ﬁbers and begin with a lowercase letter, for example, nylon or polyester. Many manufacturers produce ﬁbers from the same generic substances, but form these substances into different physical structures or treat them with different ﬁnishes. The resulting ﬁbers are given their own trade names. These names are capitalized to distinguish them from the generic ﬁber category. Examples include Dacron® (Invista Inc.) polyester and Fortrel® (Fiber Industries, Inc.) polyester. The Textile Fiber Products Identiﬁcation Act, passed in 1960, requires that, at a minimum, the generic name of a ﬁber be listed on all product labels. The properties of ﬁbers are a signiﬁcant contributor to the performance of textiles used in sports apparel (Kadolph and Langford, 2002). Fibers come in a variety of lengths. Staple ﬁbers are short ﬁbers. Natural ﬁbers, with the exception of silk, are available in staple form. Filaments are long, continuous strands of ﬁber. Filament ﬁbers can be either monoﬁlament, a single ﬁlament as seen in ﬁshing line, or multiﬁlament. Fiber diameter largely inﬂuences the fabric feel or ‘hand.’ Thin ﬁbers tend to result in soft and pliable textiles; thicker ﬁbers result in stiffer and rougher textiles. The ﬁneness of ﬁber is deﬁned as its denier, the weight in grams of 9000 meters of ﬁber. Apparel ﬁbers typically range from one to seven denier. Fibers also have a variety of cross-sectional shapes. This shape affects the luster, bulk, and texture of the ﬁber. The crimp in a ﬁber is a measure of the waves, kinks, or twists in a ﬁber. Fiber crimp increases warmth, absorbency, and skin contact comfort. Blending of different ﬁbers is often done to enhance the performance and improve the aesthetic qualities of the fabric (Watkins, 1995). Fibers are selected and blended in certain proportions so the fabric will retain the best characteristics of each ﬁber. Blending can be done with either natural or manufactured ﬁbers, in a variety of combinations and percentages. Polyester, a very common material in sports apparel, is the most blended manufactured ﬁber. Polyester ﬁber is strong, resists shrinkage, stretching, and wrinkles, is abrasion resistant, and is easily washable, all features required for sports apparel.

© 2007, Woodhead Publishing Limited

Materials and design for sports apparel

63

3.2.2 Yarns The next building block in sports apparel is forming yarns from ﬁbers. Yarns are deﬁned as an assemblage of ﬁbers that is twisted or laid together so as to form a continuous strand that can be made into a textile fabric (Collier and Tortora, 2001). Spinning is the term used to describe the process of combining ﬁbers to result in yarn. As with ﬁbers, ﬁlament and staple are terms used to describe types of yarn. Filament yarns are made from long, continuous ﬁber ﬁlaments. Filament yarns are mono or multiﬁlament. The twisting together of multiple ﬁber ﬁlaments makes multiﬁlament yarns. Staple or spun yarns are made from staple length ﬁbers. While the multiple processes required to produce staple yarns are expensive, the qualities of comfort, warmth, softness, and appearance of staple yarns offset the added expense for sports apparel and other applications. Manufactured ﬁbers that are made as ﬁlaments are often chopped into staple ﬁbers for this reason. The number of constituent parts of the yarn often classiﬁes yarns. A single yarn is made from a group of ﬁlament or staple ﬁbers twisted together. Twisting together two or more single yarns results in a ply yarn. Each of the single yarns involved in this process is called a ply. Twisting together two or more ply yarns makes chord yarns. Yarn twist is an important characteristic of the yarn (Kadolph and Langford, 2002). Twist, speciﬁed by turns per inch, can be either clockwise (Z twist) or counterclockwise (S twist). Increasing the twist in a yarn increases the ﬁber-to-ﬁber cohesion resulting in an increased stiffness. However, excessive twist results in the ﬁbers being perpendicular to the yarn axis resulting in a shear between the ﬁbers, weakening the yarn. The direction and amount of yarn twist helps determine appearance, performance, and durability of both the yarns and the subsequent fabric or textile product. Yarns come in a variety of types, including spun, smooth-ﬁlament, bulk or textured ﬁlament, fancy or novelty, or composite, each with its own particular properties. For example, fabrics constructed of spun yarns look like cotton or wool. These fabrics are typically warm and absorbent due to the large surface area of the yarn. Spun yarns do not snag easily, but will stain. Smooth-ﬁlament yarns result in fabrics that are smooth and luxurious, are cooler, and readily wick moisture. Thus, for sports apparel, spun yarns would be more appropriate for cool weather garments, and ﬁlament yarns are more appropriate for warm weather apparel.

3.2.3 Fabrics A fabric is deﬁned as a ﬂexible planar substance constructed from solutions, ﬁbers, yarns, or fabrics in any combination (Kadolph and Langford, 2002).

© 2007, Woodhead Publishing Limited

64

Materials in sports equipment

The process of combining yarns into fabrics is termed fabrication. The fabrication method contributes to the fabric appearance, texture, performance, and cost. In addition, the fabrication method may determine the name of the resulting fabric, for example, felt or double-knit. The majority of fabrics used in sports apparel are either woven or knit. Woven fabrics Woven fabrics are made with two or more yarns interlaced at right angles to one another (Kadolph and Langford, 2002). Yarns running lengthwise along the fabric are called warp yarns; yarns running crosswise are called weft yarns. This right-angle relationship between the yarns gives woven fabric that is stiffer and more rigid than other fabrications. Fabric count or fabric density is a quantitative measure of the number of warp and weft yarns per square inch of fabric as it comes off the loom, expressed as warp × weft. (Count may increase during subsequent treatments, such as dying, due to fabric shrinkage.) The balance of a fabric is the ratio of warp to weft. A balanced fabric has approximately an equal number of warp and weft yarns. High-count fabrics will be ﬁrm, strong, provide good cover and are typically more wind- and water-repellent. Lowcount fabrics are ﬂexible, permeable, and more pliable. Knit fabrics Knit fabrics are constructed by interloping one or more sets of yarns (Kadolph and Langford, 2002). Common examples of apparel utilizing knit fabrics are socks. Knitting is a more versatile manufacturing process, as entire garments can be manufactured on a single knitting machine, and it is much faster than weaving. However, due to the looping, more yarn is required to manufacture a knitted garment than a comparable woven garment. Thus any cost savings gained in manufacturing speed are offset by the higher materials cost. Knits are comfortable fabrics, as they adapt to body movement. The loop structure contributes to elasticity beyond what is capable of the yarns or ﬁbers alone. A knit fabric is prone to snagging, and has a higher potential shrinkage than a woven fabric. The loop structure also provides many cells to trap air, and thus provides good insulation in still air. Knits are not typically very wind- or water-repellent. Other fabric construction There are a variety of other fabric construction processes, although many typically do not result in end-use applications in sports apparel. Non-woven fabrics are made from ﬁbrous webs, which are bonded by mechanical

© 2007, Woodhead Publishing Limited

Materials and design for sports apparel

65

entanglement of the ﬁbers, or by added resins, thermal fusion, or by chemical processes (Kadolph and Langford, 2002). Fiberﬁll, an insulation used for winter apparel, is an example of a non-woven textile. DuPont has several products in this category, including Thermoloft®, Holloﬁl®, and Qualloﬁl® (Invista Inc.). Composite fabrics are fabrics that include several structural elements that are combined into a single structure (Kadolph and Langford, 2002). A commonly used composite fabrication in sports apparel is the use of laminated ﬁlms for adding performance properties to fabrics. The addition of a urethane ﬁlm to Lycra® (Invista Inc.) spandex/nylon is used to create fabrics for competition swimwear and wind surﬁng suits (Collier and Tortora, 2001). Poromeric fabrics are created by the addition of an extremely thin, porous ﬁlm to a fabric base layer. This ﬁlm is stretched and annealed to provide micropores in the fabric. These micropores are small enough to allow the passage of water vapor out of the apparel, but prevent liquid water or other chemical contaminants from passing through to the skin. Gore-TexTM (W. L. Gore and Associates) (Fig. 3.1), created by applying a ﬂuoropolymer membrane underneath a layer of outer fabric, is waterproof, windproof, and breathable. Fabric properties A wide variety of terms are used to describe the qualities of fabrics. Many of these characteristics are subjective, with no direct correlation to any speciﬁc, or speciﬁc combinations of, fabric material properties. For example, the ‘hand’ of the fabric refers to the tactile sensations or impressions that arise when fabrics are touched, squeezed, rubbed, or otherwise handled

3.1 The Gore-Tex® porous ﬁlm. The pores are 700 times larger than a water vapor molecule and 20 000 times smaller than a water drop. This image is reproduced with the permission of W. L. Gore and Associates, Inc. Copyright © 2006, W. L. Gore & Associates, Inc.

© 2007, Woodhead Publishing Limited

66

Materials in sports equipment

(AATCC). The smoothness of the fabric itself contributes to the fabric hand, as well as the ability of the fabric to compress and recover from compression. Covering power is the ability of a fabric to cover what is underneath it. The end-use of the fabric has an effect on covering power. Fabrics have different covering power if they are wet or dry, if they are worn tight or loose in the ﬁnal garment. Consequently, designers need to be concerned with the use of the fabric in the garment. Other fabric properties include more traditional material characteristics, such as strength, tearing resistance, abrasion resistance, heat and moisture transfer, permeability, etc. These will be discussed in more detail in Section 3.4 below.

3.3

Finishing and fasteners

Typically, the ﬁnal step in the preparation for textiles for sports apparel is a ﬁnishing process. A ﬁnish is any process that is done to ﬁber, yarn, or fabric either before or after fabrication to change the appearance, hand, or performance (Kadolph and Langford, 2002). All ﬁnishing processes add to the cost of the textile. The most common ﬁnishing process is of course dyeing, or adding color to either a yarn or a fabric. Finishes are typically classiﬁed by their expected useful life. A permanent ﬁnish, such as mercerization, lasts the life of the garment. A durable ﬁnish, such as wrinkle resistance, may last the life of the garment, but the effectiveness reduces with age. A temporary ﬁnish usually washes out, and renewable ﬁnishes can be reapplied. While many of these ﬁnishing techniques are common for leisure and sports wear, a few ﬁnishing processes that affect the performance characteristics of sports apparel will be highlighted. A very common ﬁnish is one that affects the way a fabric behaves in wet conditions. Water-repellent fabrics resist wetting. Water applied with enough force will penetrate water-repellent fabrics. Waterproof fabrics will not wet regardless of the amount or force of water exposure. These fabrics tend to be hot and uncomfortable, as they trap moisture and heat next to the body. Waterproof and breathable fabrics typically are constructed using thin ﬁlms and have been discussed in the section on composite fabrics (p. 65). Ultraviolet absorbent ﬁnishes have become more popular recently due to increased interest in protecting athletes from potentially harmful exposure to the sun. Many of these ﬁnishes are a spray coating of metal and resinous substances, applied to closely woven fabric (Collier and Tortora, 2001). Phase change ﬁnishes are added to minimize heat ﬂow through fabrics (Kadolph and Langford, 2002). These ﬁnishing chemicals change from solid to liquid to absorb heat, and from liquid to solid to release heat. The phase change occurs at near-skin temperatures, therefore adding extra cooling or

© 2007, Woodhead Publishing Limited

Materials and design for sports apparel

67

warmth. One such ﬁnish is polyethylene glycol, which is applied to fabrics with a methylol agent (Collier and Tortora, 2001). The resulting ﬁnish is durable and does not wash out, as the ﬁnish is cross-linked on the fabric. As a side beneﬁt, these types of ﬁnishes also increase the ability of the fabric to absorb moisture. Light-reﬂecting ﬁnishes are used for increased safety of athletes during night-time activities. (Collier and Tortora, 2001). Typically, light-reﬂective fabrics are created by the application of microscopic reﬂective beads to the fabric. Man-made technical fabrics retain body odors, as some of the ﬁbers in these garments are oleophilic, and they attract body oils and sweat making it difﬁcult for the microbes from sweat to be washed out of the garment. Thus, antimicrobial ﬁnishes are becoming more prevalent in athletic wear. These ﬁnishes inhibit the growth of bacteria and other odor-causing germs. These ﬁnishes can be metal compounds that are toxic to bacteria or organic substances such as phenols or anilides (Collier and Tortora, 2001). Figure 3.2

3.2 Magniﬁcation of an anti-microbial textile created by adding 99.9 % pure silver coated ﬁbers to the textiles. Image courtesy of X-STATIC® – The Silver Fiber®.

© 2007, Woodhead Publishing Limited

68

Materials in sports equipment

shows an example antimicrobial ﬁber construction created by bonding nearly pure silver to the surface of textiles. Microencapsulated ﬁnishes include a water soluble or other material in capsule form (Kadolph and Langford, 2002). These capsules, between 5 and 50 microns, may contain fragrance, insect repellents, or other additives. The capsules are sprayed onto a fabric and held in place with polyvinyl alcohol or acrylic binder. Forces exerted on the fabric during normal wear rupture the capsules and release the contents. This type of ﬁnish has limited durability.

3.3.1 Fasteners Fasteners are used to assemble pieces of textiles into the ﬁnal form of the garment. The fastening systems employed in a garment are used to permanently combine fabric sections of the garment, as in a sewn seam, to provide entry and exit from the garment, as in a zipper, as well as to provide sizing and ﬁt adjustments, as with a Velcro® (Velcro Industries) wrist band on a sleeve. Perhaps somewhat confusing is the fact that some fasteners such as zippers are fastened to garments by sewing, another type of fastening. Fastening systems have a critical role in deﬁning the garment’s shape and appearance. There are a wide variety of fasteners used in sports apparel. Common fastening methods include stitching and adhesives. In addition, fasteners such as buttons, hooks, snaps, and zippers serve a double purpose of holding the garment together, and also allowing for opening and closing the garment or pockets and ﬂaps on the garment. Each of these fasteners is utilized in clothing to serve a combination of purposes, particularly function and design. In performance sports apparel, the seam in a garment can affect a wide variety of apparel performance parameters. The strength of the seam is a critical performance parameter. Seams also have a large impact on the comfort of a garment. Well-designed and located seams allow apparel to move with the body in a manner appropriate to the sport. Seams that are next to the skin can also cause chaﬁng or irritation as the apparel slides against the body. In outerwear, the effect of the seam on the garment’s ability to protect the wearer from the elements needs to be considered. In some high-performance applications, like skiing, cycling, swimming, and running, seam location can affect the aerodynamic performance of the apparel. Stitching is likely the most common form of joining fabric in apparel and is the process of joining two fabrics together with a thread. While the practice is common, the design of a stitched seam in performance apparel is not simple. Factors that are varied to optimize the performance of a stitched

© 2007, Woodhead Publishing Limited

Materials and design for sports apparel

69

seam include the seam type, thread type and size, the needle type and size, and the number of threads per inch (Watkins, 1995). The strength of the stitched seam needs to be carefully considered. For most garments, stitching should break at stresses just slightly below those at which the material of the garment will tear. If the stitching breaks too easily, the garment will not be as durable as it could be. If the stitching is too strong relative to the fabric, the fabric tears before the stitching, resulting in a difﬁcult repair. It is obvious that stitched seams result in small holes in the fabric that can compromise the performance of waterproof and windproof garments. These holes allow vapor and moisture to pass through the garment unless the seam is protected with an adhesive tape or unless a seam sealer is added to ﬁll the sewing holes. The location and size of stitched seams can be used to inﬂuence the aerodynamic performance of apparel as well. An alternative to stitched seams now becoming quite prevalent in sports outerwear is the use of some form of heat-sealing or welding for seam construction. Thermal welding, radio frequency (RF), or ultrasonic methods are common methods used to create heat-sealed seams. Thermal welding relies on the use of heat and pressure to fuse two fabric components (Watkins, 1995). Heat is applied to melt some portions of the fabric in the seam area, and then pressure is applied to force the seams to adhere. As the process requires melting, some thermoplastic material must be present in the fabric components. RF sealing techniques also rely on heat to seal the seam. However, instead of directly applying heat to the seam, radio waves are directed to the material through a pair of electrodes. The radio waves cause the molecules in the seam area to vibrate, thereby generating the heat required to melt the portions of the constituent fabrics and create the seam, and generate a molecular bond. Ultrasonic welding works in a similar manner, using high-frequency vibrations to heat the fabrics. Typically, heat sealing or welding results in a seam that is stronger than the constituent fabrics. Thus, apparel failures are not easily repaired, as the fabric fails, not just a seam. Should the seam itself fail, it will likely need to be repaired using some form of patch or welding technique. There are a variety of other fasteners used in sports apparel. Zippers, buttons, drawstrings, and Velcro closures are the most common. Fasteners are important components in the performance of the garment, and their selection is a function of garment design and fastener materials selection.

3.4

Testing sports apparel performance

As with other engineering materials, apparel, and its component parts of ﬁbers, yarns, and textiles, has deﬁned performance parameters, guidelines, and standards for their evaluation. Some ﬁber, textile, and apparel properties are similar to other engineering materials, such as elongation, elasticity,

© 2007, Woodhead Publishing Limited

70

Materials in sports equipment

breaking strength, and tearing strength. Like other materials, ﬁbers, textiles, and apparel have unique performance requirements, thus speciﬁc tests and parameters have been deﬁned for textiles. International standards exist for a number of textile and apparel tests, such as those listed by the International Standards Organization (ISO). There are also many regional or national test standards, including those from the European Union (EN, DIN) or the USA (AATCC, ASTM, ANSI). In addition, many retailers have their own standards and tests that often reﬂect the intended use of their product lines. Apparel designers and developers also need to be aware that different areas of the world can have, among other things, vastly different laws about materials that can be in contact with human skin. As many apparel products have a global market, any materials selection must consider the implications from a wide variety of standards and government rules.

3.4.1 Strength parameters Breaking tenacity is the breaking strength of ﬁbers, or the maximum load that can be supported by the ﬁber. For man-made staple ﬁbers, 1 mm length samples are pulled until failure. Breaking tenacity is measured in grams per denier. Extremely low forces are encountered in evaluating ﬁber properties, requiring instrumentation with gram level accuracy. Similar tests for the evaluation of monoﬁlament, multiﬁlament, and spun yarns are common. In addition to measuring the tenacity of these ﬁbers, the elongation, knot breaking strength, and loop breaking strength are often measured. Looping and knotting typically reduces overall strength. Thus, tests performed on looped or knotted samples indicate the brittleness of the yarn. The engineering properties of many yarns are dependent on the rate of application of the applied load as well as the environmental conditions. Consistent testing results require the accurate control of the applied loading and controlling the test environment. Tests are also conducted on textiles. A concern of sports apparel for many sports is the ability of the material used in the apparel to resist tearing. Tear strength is the force required to start or continue a tear in a fabric under speciﬁed conditions. After starting a cut in the center, the opposing edges of the material are gripped in a test ﬁxture and pulled apart. As textiles are not typically uniform materials, the resulting test curve is often quite jagged. Numerical averaging is used to smooth the test data. Test parameters that can be recorded during these tests include tearing strength, peak load, and median load. A related property is the bursting strength of a fabric. As knitted fabrics stretch when pulled in one direction, as in a tensile test, a steel ball or

© 2007, Woodhead Publishing Limited

Materials and design for sports apparel

71

rubber diaphragm is pushed through the fabric sample, causing the fabric to stretch in all directions. The force required to burst through is recorded. Burst strength may be indicative of how a fabric will perform in contact sports where apparel is grabbed and pulled by athletes.

3.4.2 Elongation Tests are required to characterize the performance of yarns made from elastomeric materials such as rubber or synthetics which are used in textiles for their ability to stretch and recover. Typical tests include stretching elastomeric yarns to at least twice their original length. Parameters such as the elongation at speciﬁed load, the load at speciﬁed elongation, or the area under the load–elongation curve may be determined from the test results. Elongation tests are typically conducted at a controlled load rate of 500 mm per minute using samples of 50 mm in length.

3.4.3 Abrasion Sports apparel often is subject to abrasion during use. This can occur between two adjacent pieces of apparel, as under the arm when running, or between the apparel and another object, such as a bicycle seat. Abrasion properties are measured by mounting a piece of test fabric in an abrasion tester. A constant load is applied between the textile and the abrading surface, and the surface is moved back and forth at a speciﬁed speed for a speciﬁed time. The wear of the fabric is observed to a prescribed level, and the test is stopped and the number of cycles is recorded. Similar methods are used to test for pilling. Pilling occurs when ﬁbers of the fabric become released and tangled on the surface of the textile, resulting in small balls of ﬁber on the surface. Contacting other garments of different color during wear or care can cause the pills to collect ﬁbers of other colors.

3.4.4 Seam breaking strength The connection points in apparel need to be evaluated as well. The seam breaking strength of fabrics may be evaluated by grabbing apparel samples cut to include seam lines. The samples are often cut to provide a variety of angles between the applied load and the seam direction. The samples are then pulled apart under controlled conditions, and the failure loads recorded. Similar tests can be devised for testing other fastening systems, such as zippers, buttons, or hook and loop closures.

© 2007, Woodhead Publishing Limited

72

Materials in sports equipment

3.4.5 Performance of laminated fabrics A variety of test methods also exist to evaluate the performance of laminated fabrics, both on performance and on the wear resistance of the laminated coating. In addition, the resistance of layers to shearing may be of interest.

3.4.6 Other parameters Many other factors beyond strength parameters are important in sports apparel. Considerations for biological properties (i.e. the performance of antimicrobial ﬁnishes), color, dyeing, stain resistance, wrinkle resistance, and care requirements are of considerable importance as well. The American Association of Textile Chemists and Colorists (AATCC) lists a variety of testing methods for evaluating these crucial parameters.

3.5

Design of sports apparel: thermal performance

The previous section of this chapter concentrated on the materials components utilized in sports apparel. This section investigates the design principles of performance apparel, and how these design principles impact the selection and use of materials. Sports competitions and sporting activities occur in a wide variety of weather and environmental conditions. Considerable research, design, and development of materials used in sports apparel are focused on creating a comfortable and protective environment for the athlete. Key areas include thermal regulation, moisture transport, water resistance, and wind resistance. The goal of sports apparel is to create a comfortable microclimate, the area next to the skin, for the athlete, often in the presence of extreme conditions. A comfortable microclimate incorporates aspects of ﬁt, thermal management, moisture management, and aspects of the contact of the apparel with the skin. Original performance apparel concentrated on outdoor clothing and was based around the three-layer system. This system has its roots in cold weather military uniforms developed in the era of the Korean War (McCann, 2005). The three-layer system comprises a next-to-skin or base layer with the primary function of moisture management, a middle insulating layer, (i.e. sweater) and a protective outer layer (i.e. waterproof jacket). This three-layer system is still widely used today for outdoor sports, with the components of the system optimized for particular sports. Each of these systems continues to evolve, with newer pieces often combining the functionality of two or more components through the use of advanced materials and construction techniques.

© 2007, Woodhead Publishing Limited

Materials and design for sports apparel

73

Many of the same materials and design concepts originating in the outdoor apparel market are utilized in other sports as well, with sport speciﬁc adaptations for both performance and style. In most sports, the base layer or next-to-skin layer is the only apparel worn for performing in good weather conditions. For inclement weather conditions, additional layers are added as required. The athlete’s body attempts to maintain a core temperature of 37 °C with a body surface temperature of 34–37 °C. A core body temperature change of even a few degrees results in severe health complications. A change of even one degree can result in serious performance degradation (Koehler, 1996). Under normal conditions, there is a balance between the heat generated by the natural metabolism in the athlete and the amount of heat lost to the surrounding environment. If athletes produces more heat than they reject, body temperature will rise; if they reject more heat than they produce, body temperature will fall. An adult male at rest generates about 90 W of waste heat. During exercise, an athlete will generate 600–1000 W of waste heat that needs to be dissipated to the surrounding environment. Thus, except in conditions of extreme cold, where the body is trying to conserve heat, or in extreme heat, where the environment is much hotter than the body, the body is always dissipating heat into the surrounding environment. Competitive athletes impose extreme requirements on their clothing. During training or a sporting event, athletes are operating at elevated metabolic rates, and often need to dissipate large amounts of heat and moisture in the form of sweat. While this certainly has an impact on comfort, the athlete’s performance can also be compromised if the body is not able to adequately dissipate heat or moisture. For example, sweating in hot conditions also dehydrates the body, leading to decreases in maximum oxygen consumption or possibly muscle cramps. Consequently athletic apparel is designed to aid the body in managing heat and moisture. There are four heat transfer mechanisms with which heat can be exchanged between the body and the surrounding environment: conduction, convection, evaporation, and radiation. The ﬁber used in a garment affects the thermal balance of the system in three ways: by resisting conduction of heat, by helping to preserve still air in a garment system, and by handling moisture in a way that promotes desired heating or cooling. In cold environments, even before noticeable shivering occurs, cold muscles have increased activity, ﬁring different motor units without any noticeable movement (Clark and Edholm, 1985). This phenomenon, known as thermal muscular tone, is generally experienced as a feeling of stiffness and increased exertion, sapping energy from a cold athlete that would otherwise be used in competition.

© 2007, Woodhead Publishing Limited

74

Materials in sports equipment

3.5.1 Conduction For most apparel, there is a small microclimate or layer of air between the skin and the apparel. Conductive heat transfer occurs through contact between the air in the microclimate and the athlete’s skin. Conduction also occurs between the microclimate and the inner side of the apparel and between the outside of the apparel and the surrounding environment. The rate at which heat is dissipated is a function of the temperature difference between the layers, the contact area, the thickness of the layer, and the thermal conductivity of the medium. Apparel worn in warm or hot conditions should be constructed of materials that have a high thermal conductivity. Traditionally, hot weather apparel was designed to ﬁt loosely to allow a larger microclimate between the skin and the apparel. However, new products have entered the market that ﬁt tight against the skin in order to aid in the conduction of heat and moisture away from the skin. One mechanism used by the tight ﬁtting garments is to spread localized heat or moisture over a larger surface area to speed cooling. To date, no independent conclusive studies have been published comparing the performance of loose-ﬁt and tight-ﬁt garments. For apparel worn in cold conditions, a low thermal conductivity is desired. Cold weather apparel typically has layers designed to trap additional air, thereby reducing conductive heat transfer. Materials used in the insulating layers are composed of ﬁbers that naturally or synthetically trap air including pile fabrics such as furs and terry cloth, foams, and ﬁberﬁlls. Natural ﬁbers, such as wool, have a rough surface ﬁnish and twist randomly leading to threads that contain many air pockets. Synthetic ﬁbers can also be made with crimps and twists, leading to threads with similar air trapping capability. Other synthetic ﬁbers have hollow tubes, with air trapped inside the ﬁber itself (Watkins, 1995). Often a thick, light-weight (low-bulk density) fabric is an excellent insulator because it is primarily air. This air, however, must be in relatively small pockets where it is protected from movement. A large air-ﬁlled pocket can be subject to convection currents and could actually promote heat loss.

3.5.2 Convection Convective heat transfer results from ﬂuid moving across a surface that carries heat away. For athletes, convective heat transfer occurs directly between the skin in contact with air or water, as well as between the apparel that is in contact with the surrounding environment. Thus, air moving across skin provides convective cooling. The rate of convective heat transfer is a function of the ﬂuid and surface temperatures, the surface area, and the speed of the ﬂow across the surface. To improve convective heat ﬂow,

© 2007, Woodhead Publishing Limited

Materials and design for sports apparel

75

increasing the area of contact either between the athlete or the apparel and the ﬂow, as well as the speed of the ﬂow is important. For cooling, materials selection and apparel designs can focus on allowing air to ﬂow between the body and the apparel. The interface between the fabric and the body should be considered, as this can have an effect on the size and thickness of the layer of the air in the microclimate. Some apparel has textured surfaces to allow for a larger microclimate, while others speciﬁcally include vent panels or mesh sections to promote airﬂow. Hot weather athletic apparel is generally worn loosely to allow air to ﬂow freely in between the skin and the garment, or extremely tightly putting the ﬁbers of the textile in direct contact with the skin. Cold weather apparel is designed to minimize the movement of air, thus preventing convective heat transfer. Air can be trapped by the weave of the textile, or in-between layers of clothing, retaining body heat.

3.5.3 Evaporation For skin surface temperatures above 37 °C, perspiration begins. This perspiration aids in cooling through evaporation. Evaporation is a process by which moisture is transported from the athlete to the surrounding environment. Evaporation is the only method for rejecting body heat when the ambient temperature is equal to or higher than skin temperature (Adams and Iampietro, 1968). The rate of heat transfer is a function of the sweat rate and the heat of vaporization. An athlete’s sweat rate varies from 1.5 to 3.5 liters per hour, which results in up to 2400 W of heat transfer. However, sweat that pools or drips off the body does not contribute to cooling. The evaporative cooling process requires that the sweat evaporate. Materials selected for increasing evaporation are often chosen to assist in transporting sweat away from the skin and spreading sweat across the outer layers of the garment where evaporation can occur (Fig. 3.3). Evaporative heat loss reduces fabric temperature, which allows improved heat conduction from the skin to the fabric. Moisture can be spread through the use of either absorbent or adsorbent ﬁbers. Traditional loose-ﬁtting garments made with absorbent ﬁbers soak up sweat into the fabric ﬁbers themselves, cooling as the fabric dries out. More modern garments are made of adsorbent ﬁbers such as polypropylene, which transports moisture along the surface of ﬁbers, wicking moisture from wet areas to dry areas, thus effectively increasing the area over which evaporation occurs. The pattern and density with which wicking ﬁbers are woven are also key factors in the textile’s ability to wick moisture. For example tightly woven polypropylene textiles can, instead of wicking moisture away, trap water in between the ﬁbers, preventing the garment from drying (Watkins, 1995). In garments made from adsorbent ﬁbers, it is important that the fabric be

© 2007, Woodhead Publishing Limited

76

Materials in sports equipment

3.3 Comparison of hydrophylic and hydrophobic ﬁbers. Image courtesy of Under Armour®.

in direct contact with the skin to minimize any chance of moisture collecting on the surface of the skin. Conversely, in cold weather sports, even though the athlete still sweats, evaporation of this moisture is not desired, as it will cause the athlete to get cold. Many cold weather sports involve periods of intense activity followed by periods of rest. A classic cold weather example is downhill skiing, with intense activity during the descent followed by a cold ride on a ski lift. During the descent, the body sweats but gets little added heat loss since clothing absorbs the moisture, leading to increased body temperature and increased sweating. The environment near the skin in cold weather sports is similar to that in warm weather sports. Thus, many of the same materials and techniques used in hot weather apparel are applied to the base layers of winter clothing. Wicking ﬁbers such as polypropylenes draw moisture from the skin to either the outside of the garment or to a microclimate inside the clothing, which can then be vented. Traditional natural ﬁbers such as wool are still useful, as wool can absorb moisture without losing insulation value, and does not readily give up moisture thus minimizing evaporative heat loss. Outer layers of cold weather sports apparel are typically water and wind resistant, allowing heat and moisture to breathe out

© 2007, Woodhead Publishing Limited

Materials and design for sports apparel

77

of the clothing, while protecting athletes from the sudden chill of snow or wind.

3.5.4 Radiation Radiation is the result of heat transfer due to electromagnetic energy and does not depend on physical contact between the body and its surroundings. The rate of heat transfer is a function of the difference of body temperature and surrounding air temperature, the area of the surface, and the emissivity of the object. Thus, materials used in apparel to reduce radiation are low in emissivity. Both color and texture are particularly important to the heat gain and loss through the transmission of radiant energy. Fabrics that absorb radiant energy need to be good emitters if they are to pass it toward the body efﬁciently and poor emitters if they are to keep the heat from adding to the warmth of the body (Watkins, 1995).

3.5.5 Measuring thermal performance While individually, each of these four mechanisms, conduction, convection, evaporation, and radiation, is easy to explain and understand, their combined effect is quite complex. Even sophisticated models of heat and mass transfer may not adequately model the thermoregulatory system of an athlete and apparel. Thus, much apparel analysis is done through testing. Thermography, which is a method of detecting the infrared radiation emitted by objects, can also be used to evaluate the thermal insulation of garments. Anything above absolute zero emits infrared radiation. Thermographs are generally in color, with the lightest values and warmer hues such as reds and yellows indicating areas of greater loss, and darker values and cooler hues such as blues indicating well-insulated areas where little heat loss occurs. Thermography enables a designer to determine speciﬁc areas of heat loss such as seams quilting lines and garment openings, and to make adjustments in insulation or construction techniques accordingly. Heated mannequins are also used for apparel performance studies. The mannequin is placed in a controlled environment wearing an apparel ensemble. The conditions of the environment, temperature, humidity, and wind, are set to the desired parameters. During the test, power is added to the mannequin to maintain it at a constant temperature. The amount of power consumed over the test is compared to a standard test set of apparel to provide an indication of the thermal performance of the articles of the test apparel. The mannequin can also be kept moist to mimic the human sweating response and its effect on the garment performance. Studies subjecting athletes to controlled environments are also used to measure apparel performance. Sensors for measuring athlete energy

© 2007, Woodhead Publishing Limited

78

Materials in sports equipment

expenditure, skin surface temperature and core body temperature can be used during the test. Sweat rate can be measured by pre- and post-exercise weighing. Such tests have been used to compare performance apparel versus cotton apparel in moderate exercise. In this study, before, during, or after exercise in a moderately warm environmental condition, neither the addition of a modest amount of clothing nor the fabric characteristics of this clothing alters physiological, thermoregulatory, or comfort sensation responses (Gavin et al., 2001).

3.6

Design of sports apparel: water resistance and other properties

The resistance to water is a common design feature of sports and leisure outerwear. Water-resistant textiles depend on surface tension effects to provide protection from liquids. Water molecules are attracted to one another. In the absence of other objects to cling to, water molecules will collect and form droplets. When droplets encounter an object, their behavior depends on the properties of the object’s surface. If the water molecules are attracted to the surface more than each other, the molecules will spread out along the surface forming a small ﬂat pool. If, instead, the molecules are attracted to each other more than the surface, they will remain in a droplet shape, minimizing the contact with the foreign surface. Fibers and weaves in water-resistant textiles are chosen so that water molecules are not attracted to the surface, instead forming droplets that roll off. Even so, pressure from wind or someone sitting on a puddle can force water droplets through openings in the textile. Using a fabric with a tighter weave makes the textile more water resistant by increasing the pressure required to force water into the fabric. A fabric is classed as waterproof if it can repel water at pressures of at least 170 kPa (Watkins, 1995). In addition to the weave of the textile, coatings and barrier membranes are often used to improve the water resistance performance of apparel.

3.6.1 Wind resistance Similar to techniques used to improve the water resistance of apparel, the weave of the textiles and topical treatments or barrier membranes are used to improve the wind resistance of sports apparel. Obviously, a tighter weave in the material improves the wind resistance capabilities of fabrics. However, barrier membranes are becoming more common, allowing manufacturers to create ‘windproof’ jackets that are based on polyurethane ﬂeece fabrics.

© 2007, Woodhead Publishing Limited

Materials and design for sports apparel

79

3.6.2 UV resistance Sports played outdoors expose athletes to the sun’s ultraviolet (UV) rays, putting athletes at risk from sunburns and eventually skin cancer. Texture is as important as color for UV protection in clothing. Aluminum ﬁbers are very reﬂective, and a textile composed entirely of aluminum, such as an emergency blanket, can reﬂect radiation. However, individual aluminum threads and ﬁbers in a textile have little effect on the reﬂectivity of the garment, as the UV rays may reﬂect off the ﬁber onto the skin as easily as they would reﬂect out of the clothing. Today, some sports apparel is rated for UV protection. The textiles used in the apparel, the weaves of the apparel and, in some cases, fabric ﬁnishes all affect the UV rating of the apparel.

3.6.3 Fit Movement is an inherent part of sports; an athlete who can move his/her body faster, with more control or with more force than his/her opponent has an advantage. Athletic clothing that ﬁts poorly may restrict movement, either by limiting the actual range of motion or by making movements less efﬁcient. Traditionally, athletic clothing that ﬁts well has minimized its inﬂuence on the athlete, attempting to replicate the performance of a nude athlete. Modern engineered apparel can actually improve some aspects of performance, such as endurance, circulation, and aerodynamics, beyond that of an unclothed athlete. Understanding the motions speciﬁc to a sport is essential in designing sufﬁcient movement into athletic garments. A movement study can provide important information on where mobility needs to be designed into a garment. Understanding the motion of the athlete is particularly important in apparel that must incorporate special features such as padding or ﬁre protection. Designs should be created on three-dimensional body forms such as those used in the fashion industry as opposed to a two-dimensional paper design. Human skin is incredibly ﬂexible, stretching in excess of 50% in some athletic motions (Kirk and Ibrahim, 1966; Voyce et al., 2005). Clothing made of textiles less ﬂexible than skin must design in extra material to account for this stretching. For athletic apparel, this extra material adds weight and may interfere with motion, but otherwise the clothing would restrict the range of motion of the athlete. Stretch ﬁbers allow sports apparel to stretch with the athlete’s skin. The most popular and widely used stretch ﬁbers are elastane ﬁbers. These petroleum-based ﬁbers are capable of stretching seven times their length and returning to their original length. By combining elastane and other

© 2007, Woodhead Publishing Limited

80

Materials in sports equipment

synthetic or natural ﬁbers into threads, stretchable garments can be made with the same look and function as non-stretch garments. An example is the combination of a wicking polypropylene ﬁber with a small percentage of elastane to create form ﬁtting, sports apparel. Elastane ﬁbers may compose 2–30% of a garment, depending on the amount of ﬂexibility desired (Voyce et al., 2005). Stretch fabrics provide more than unrestricted movement to athletes; they can also improve athletic performance by delaying muscle fatigue. Kraemer et al. (1996, 1998) demonstrated an average 12% increase in power output during repetitive exercise while wearing garments containing elastane ﬁbers. The beneﬁt of these garments is seen primarily in increased endurance, as opposed to an immediate increase in performance. In addition, the beneﬁt of these garments is related to the amount of compression they provide, with too much compression restricting movement (Voyce et al., 2005). Compression garments can be particularly useful in supporting injured muscles, and perhaps preventing similar injuries.

3.6.4 Aerodynamics In some sports, reducing the aerodynamic drag of an athlete may increase performance more than any comfort-related features of apparel. Aerodynamic apparel has typically taken the form of skin suits for sports such as swimming, speed skating, cycling, or alpine skiing, where an athlete’s speed is directly related to victory. The design of an aerodynamic skin suit focuses primarily on fabric texture and seam location. The speed and environment (air or water) that the athlete is operating in, along with the associated body motion, determine what texture and seam locations are used. Typically seams are placed either parallel to the ﬂow, or on the back of the body. Textiles used to control heat and moisture with less than desirable aerodynamic properties are relocated to the back of the body where they will not disrupt the airﬂow. Further, the natural shape of the garment is designed to ﬁt the athlete in the position in which they perform their sport, not in a natural standing position. Designing the garment to conform to the shape of the athlete in competition further eliminates drag-causing wrinkles in the apparel (Brownlie et al., 2004; Kyle et al., 2004).

3.6.5 Safety In all sports, the safety of the athlete should take precedence over all other design constraints. Components of the athlete’s apparel system in contact sports, such as hockey or American football, must protect the wearer from impacts. The clothing worn in modern fencing protects athletes from accidental injury if a blade breaks and becomes sharp. If safety features

© 2007, Woodhead Publishing Limited

Materials and design for sports apparel

81

impede performance, athletes will resist their use. Examples include some American football cornerbacks and wide receivers playing without padding on their legs in order to run faster, professional cyclists and rock climbers who resist the use of helmets they claim are uncomfortable. For acceptance by the end-user, safety equipment must be designed not only for protection but for all of the other performance parameters required in the particular sport. By looking at the entire uniform as a system and considering temperature regulation and ﬁt, safety features can be integrated into the design without sacriﬁcing some aspects of performance.

3.6.6 Regulatory compliance Some clothing must conform to sporting regulations, for both safety and performance reasons. Different rules are written for each sport, and the intent of the rule must ﬁrst be understood before designing a garment. For example the locations of seams are restricted in both downhill skiing and fencing. However, the intent in fencing is to improve safety, while in skiing it is to prevent further aerodynamic development of the skiing skin suits. In some sports, the materials used in garments are also controlled, for example materials used in ﬁreprooﬁng automotive racing suits must meet certain standards speciﬁed by the governing body. Clearly, the governing rules need to be understood prior to materials selection for sports apparel.

3.7

Future trends

Advancements in sports apparel are occurring at a rapid pace. Developments are focused on ﬁt, textile and fabric performance, the integration of electronics into apparel, and the development of smart apparel, capable of interacting with the user and the user’s environment. The ﬁt of apparel is often a function of fashion as much as of performance. Compare the apparel worn by competitive skiers and those worn by snowboarders, both of whom have arguably very similar performance requirements, but drastically different apparel. Advancements are still being made in ﬁt. The development of 3-D body mapping technology allows for athletes to have their entire body scanned and recorded. From this information, detailed information as to the size of the body can be ascertained. Certainly, 3-D body mapping allows for custom ﬁt apparel for the individual athlete. Additionally, recent larger-scale studies have updated the current size information for speciﬁc general populations (size USA). These surveys can be extended to include a broader population or focus on speciﬁc sports-related population segments. From a scientiﬁc perspective, it is possible to further develop models of the microclimates between layers

© 2007, Woodhead Publishing Limited

82

Materials in sports equipment

of apparel by obtaining detailed 3-D body scans of the athlete with each layer of clothing. The development of new textiles continues at a rapid pace. The goal of the developments is to enhance certain performance aspects of textiles. One very interesting approach has been to imitate structures in biology. Years of evolution and adaptation have resulted in biological systems with extremely well-adapted characteristics. One example includes mimicking the surface of sharkskin in suits designed for swimming (Fig. 3.4). The use of this surface was found to reduce the friction drag of the suit in the water (Vizard, 2004). For joining textile components, welded seams are replacing sewn seams in outdoor apparel. Welded seams reduce the weight and bulk of the garment, and of course there are no sewing holes that allow moisture penetration. Currently this technology is reserved for more expensive items, although advances in glue and manufacturing processes should reduce the cost, thereby allowing the technology to be seen in more affordable items. New ﬁnishing technologies are improving fabric performance as well. Many manufacturers are already looking to nanotechnology methods either

3.4 The surface of this swimsuit mimics that of sharkskin to reduce drag. Image kindly reproduced with permission from Speedo Holdings B.V ©2007. All rights reserved.

© 2007, Woodhead Publishing Limited

Materials and design for sports apparel

83

to help reﬁne the ﬁnish material itself or to aid better application of the ﬁnish compounds to the fabric. One example is the use of nanotechnology coatings for stain resistance. Billions of tiny whiskers create a thin cushion of air above the fabric, smoothing out wrinkles and allowing liquids to bead up and roll off the fabric (Fig. 3.5). Nanotechnology has also resulted in the development of wrinkle reducing, advanced moisture wicking, and other performance enhancements for apparel. Research and development will continue in this area, resulting in improved fabric treatment materials and processes. The integration of electronics and textiles has already begun. Many apparel companies now market apparel with touch-sensitive fabric panels that are then connected to personal electronic devices such as mobile telephones and music players. Recently other electronics components, such as heart rate monitors, are being integrated into apparel with textile-based wiring and connectors. As technology in other physiology sensing capabilities develops, these will likely be integrated into apparel as well. Any endurance athlete can appreciate the usefulness of calorie and hydration monitors to help them gauge ﬂuid and calorie intake in training and competition. Textiles including new materials that are able to adapt to their surroundings will allow for better integration of functionality of apparel. Examples

3.5 Example of using a nanotechnology-based treatment for stain resistance which allows liquids to bead up and roll off the fabric. Image courtesy of Nano-TexTM, Inc.

© 2007, Woodhead Publishing Limited

84

Materials in sports equipment

of this technology have already been demonstrated in ski apparel. This new apparel contains molecules in the apparel that move with the skier during normal skiing, but stiffen immediately on impact to protect the skier in the event of a crash (NewScientist.com, 2006). Future advancements will include apparel that can vary its insulation or water repellency based on the environment and the athlete’s heat production. In many sports, often those bound by tradition, advances in performance apparel can be made simply by a detailed study of the sport and the apparel components. One can easily imagine better integration of apparel and padding systems for contact sports. While not high technology based, these simple changes can even impact the way a sport is played.

3.8

Sources of further information and advice

There is no well-deﬁned body of research for materials used in sports apparel. Depending on the topic of interest, one may need to refer to information published in the ﬁelds of textiles, textile design, textile engineering, human performance, or materials science. A few suggested references follow that will allow the reader to explore the next level of some of these topics. There are many sources for further information on textiles, textile design, and textile materials. Understanding Textiles (Collier and Tortora, 2001) and Textiles (Kadolf and Langford, 2002) each provide an in in-depth overview of the basics of textiles. Clothing: The Portable Environment (Watkins, 1995) is a great resource for understanding the interactions and interfaces between clothing and people, and the resulting design issues. Unfortunately the second edition is now out of print, but copies can still be found. Of related interest, Man and His Thermal Environment (Clark and Edholm, 1985) is a good general reference on human performance and response to temperature. For textile and apparel testing information, one can refer to Textile Testing (Collier and Epps, 1998), which covers the methods and techniques used in textile performance testing. Test methods published by the various standards organizations listed below should be consulted for guidelines on particular test protocol. • • • • • •

AATCC, American Association of Textile Chemists and Colorists, www.aatcc.org ANSI, American National Standards Institute, www.ansi.org ASTM International, American Society for Testing and Materials, www.astm.org CEN, European Committee for Standardization, www.cenorm.be CIN, Deutsches Institut fur Normung, www2.din.de ISO, International Standards Organization, www.iso.org

© 2007, Woodhead Publishing Limited

Materials and design for sports apparel

85

Perhaps the most recent collection of research on textiles in sports application appears in Textiles in Sport (Shishoo, 2005). Nanotechnology and smart textiles are also active research areas that will have an impact on the future of sports apparel. There are a variety of conferences and organizations focused on each of these topics, too numerous to list here.

3.9

References

adams t and iampietro p f (1968), Temperature regulation, in Falls H B (ed.), Exercise Physiology, New York, Academic Press, 175–80. brownlie l w, kyle c r, harber e, macdonald r and shorten m (2004), Reducing the aerodynamic drag of sports apparel: development of the Nike Swift sprint running and SwiftSkin speed skating suits, in Hubbard M, Mehta, R D and Pallis, J M (eds), The Engineering of Sport 5, Vol. 1, Shefﬁeld, International Sports Engineering Association, 90–96. clark r p and edholm o g (1985), Man and His Thermal Environment, London, Edward Arnold. collier b j and epps h h (1998), Textile Testing and Analysis, New Jersey, Prentice Hall. collier b j and tortora p g (2001), Understanding Textiles, 2nd edn, New Jersey, Prentice Hall. gavin t p, babington j p, harms c a, ardelt m e, tanner d a and stager j m (2001), Clothing fabric does not affect thermoregulation during exercise in moderate heat, Med Sci Sports Exer, 33(12), 2124–30. kadolph s j and langford a l (2002), Textiles, 9th edn, Englewood Cliffs, NJ, Prentice Hall. kirk w j and ibrahim s m (1966), Fundamental relationship of fabric extensibility to anthropometric requirements and garment performance, Text Res J, 36(1), 37–47. koehler k r (1996), College physics for students of biology and chemistry, http://www.rwc.uc.edu/koehler/biophys/contents.html, last accessed March 17, 2007. kraemer w j, bush j a, bauer j a, triplett-mcbride n t, paxton n j, clemson a l, koziris l p, mangino l c, fry a c and newton r u (1996), Inﬂuence of compression garments on vertical jump performance in NCAA Division I volleyball players, J Strength Cond Res, 10(3), 180–83. kraemer w j, bush j a, newton r u, duncan n d, volek j s, denegar c r, canavan p, johnston j, putukian m and sebastianelli w j (1998), Inﬂuence of a compression garment on repetitive power output production before and after different types of muscle fatigue, Sports Med Train Rehabil, 8(2), 163–84. kyle c r, brownlie l w, harber e, macdonald r and nordstrom m (2004), The Nike Swift Spin cycling project: reducing the aerodynamic drag of bicycle racing clothing by using zoned fabrics, in Hubbard M, Mehta R D and Pallis J M (eds), The Engineering of Sport 5, Vol. 1, Shefﬁeld, International Sports Engineering Association, 118–24. mccann j (2005), Material requirements for the design of performance sportswear, in Shishoo R (ed.), Textiles in Sport, Cambridge, Woodhead, 44–69.

© 2007, Woodhead Publishing Limited

86

Materials in sports equipment

NewScientist.com (2006), US and Canadian skiers get smart armour, http://www. newscientist.com/article.ns?id=dn8721&feedId=online-news_rss20, last accessed March 17, 2007. shishoo r (2005), Textiles in Sport, Cambridge, Woodhead. Size USA, The US National Size Survey, http://www.tc2.com/what/sizeusa/index. html, last accessed March 17, 2007. vizard f (2004) The Olympian’s New Clothes, Scientiﬁc American On-Line, http:// www.sciam.com/article.cfm?SID=mail&articleID=000902AC-487A-1112B89C83414B7F4945, last accessed March 17, 2007. voyce j, dafniotis p and towlson s (2005), Elastic textiles, in Shishoo R (ed.), Textiles in Sport, Cambridge, Woodhead, 204–32. watkins s (1995), Clothing: The Portable Environment, Ames, I A, Iowa State University Press.

© 2007, Woodhead Publishing Limited

4 Protective helmets in sports S. V. C A S W E L L, George Mason University, USA; T. E. G O U L D and J. S. W I G G I N S, University of Southern Mississippi, USA

4.1

Introduction

The physical, emotional, and ﬁnancial costs associated with head injury in sport can be great. Head injury in sports can take several different forms that range from obvious structural damage of the skull, mandible, and soft tissue to the more subtle traumatic brain injury. Regardless of injury type, most head injuries in sport typically result from biomechanical forces generated within the body from either a head-to-object collision (e.g., playing surface or another player) or projectile-to-head collision (e.g., ball or stick). Perhaps the most problematic head injury related to sport is mild traumatic brain injury (MTBI) or sport-related concussion (SRC) because of the insidious nature of its symptoms and inherent sequelae (e.g., post-concussion syndrome and second impact syndrome). The First International Symposium on Concussion in Sport deﬁned MTBI as a pathophysiological metabolic process affecting the brain induced by traumatic biomechanical forces (Aubry et al., 2002). These biomechanical forces can be generated via direct insult to the head or neck region or by forces encountered elsewhere on the body that are transmitted to the head (McCrory et al., 2005). Unfortunately, uncertainty remains regarding the precise etiology (McCrory et al., 2005) and best methods to prevent MTBI. The prevention of head injury in organized sport has long been an aim of the medical and scientiﬁc communities. Head protection in sport was ﬁrst developed due to frequent serious injuries subsequent to participation in American football during the late 1800s (Hoshizaki and Brien, 2004). Since that time, innovations in helmet design and materials, and the establishment of performance and safety standards have improved the reliability, comfort, and protective capabilities of sport helmets. Nonetheless, despite vast improvements in helmet technology, the potential for serious head injury in sport remains an unfortunate reality. For a more comprehensive exposition of the history of protective helmets in sport, the authors refer the reader to a review by Hoshizaki and Brien (2004). 87

© 2007, Woodhead Publishing Limited

88

Materials in sports equipment

The primary objective of this chapter is to provide a broad overview of protective helmets in sport, their composition, and manufacturing processes. Traditionally, the efﬁcacy of protective helmets in sport has been investigated independently from both the materials science and human performance disciplines. In this chapter, the authors attempt to integrate the areas of biomechanics, kinesiology, sports medicine, and materials science in the effort to discuss the interface that occurs when materials (e.g., helmet) are used to prevent injury in humans. It is hoped that, through a better understanding of this interface, the physical, emotional, and ﬁnancial costs associated with head injury in sports may be abated.

4.2

Incidence of mild traumatic brain injury in sport

Worldwide participation in an extensive array of recreational activities, including organized sport, is increasing. For example, in the USA, a 2003 report by the National Federation of High Schools (NFHS) and the National Collegiate Athletic Association (NCAA) indicated that participation in organized sport is at an all-time high with over 7.5 million participants (Bray, 2004; NFHS, 2004). With greater participation in sport worldwide, the incidence of MTBI is also likely to increase. Unfortunately, obtaining a valid estimate of the global incidence of MTBI is difﬁcult due to a lack of uniform access and quality of healthcare and reporting mechanisms. However, review of the epidemiological data leaves little doubt that MTBI has become a signiﬁcant international public health issue. Tagliaferri et al. (2006) examined national studies from European countries and reported an aggregate hospitalized plus fatal MTBI incidence rate of about 235 cases/100 000 Europeans. North America was similar with an estimated annual incidence of 110 cases/100 000 Canadians (Gordon et al., 2006) and 503 cases/100 000 Americans (Bazarian et al., 2005). Furthermore, a 2003 report to congress by the National Center for Injury Prevention and Control (NCIPC), part of the Centers for Disease Control and Prevention, estimated that sport-related injuries accounted for 20% (306 000) of the 1.5 million traumatic brain injuries in the USA (NCIPC, 2003). Research conducted by Delaney et al. (2004) supports these estimates, reporting that sport-related MTBI accounted for 17 008 (ice hockey), 86 697 (soccer), and 204 802 (football) hospital emergency department visits in the USA from 1990 to 1999. More recently, Bazarinan et al. (2005) reported that bicycles and sports accounted for 26.4 % of MTBI cases in children aged 5–14 in the USA. In addition to its human toll, the health care costs of MTBI have been estimated at nearly $17 billion annually in the USA alone (National Center for Injury Prevention and Control, 2003). For a more comprehensive discussion of the management of sport-related concussion, the authors refer the reader

© 2007, Woodhead Publishing Limited

Protective helmets in sports

89

to a position statement by the National Athletic Trainers’ Association (Guskiewicz et al., 2004).

4.3

Biomechanics and dynamics of head impacts in sport

The biomechanics of head injury are complex and vary depending on the sport and equipment. A signiﬁcant effort to understand these complexities has been undertaken in American football. Sponsored by the National Football League (NFL) Committee on Mild Traumatic Brain Injury, the research ﬁndings were published in a 12-part series of articles by Pellman et al. (2003a). Among the most signiﬁcant ﬁndings of this series were the results from the reconstructed laboratory impacts, which demonstrated the average impact speed of an NFL player at V = 9.3 ± 1.9 m/s. The ﬁndings of the NFL study additionally noted that 71 % of impacts to struck players were to the helmet shell between 45 deg and 135 deg with 80% above the head center of gravity (CG). The resultant change in velocity (∆V) subsequent to a helmeted impact in American football was shown to be 7.2 ± 1.8 m/s versus 5.0 ± 1.1 m/s for concussed and non-concussed players, respectively. Helmeted impacts to the ground had the highest ∆V and the most severe head response because of rebound with the ground. Additionally, peak head acceleration has been calculated at 98 ± 28 g for concussed and 60 ± 24 g for non-concussed players at an estimated average time interval of 15 ms. Further, the NFL study revealed that the best correlates of MTBI in struck players are (in descending order) the head injury criterion (HIC: R = 0.70), the severity index (SI: R = 0.68), and ∆V (R = 0.63) (Pellman et al., 2003a). As a result of the NFL study, 31 head-to-head or head-to-playing surface impacts were reconstructed in a laboratory setting from game video. Figure 4.1 illustrates the locations of the initial impacts for striking and struck players. Additionally, impacts to the struck players were categorized by quadrant and level as shown in Fig. 4.2. The reconstructed results indicated that, for striking players, 57% of impacts were from level +Q4, which indicates that NFL players are creating initial impact with the crown of the head (i.e., spearing). This type of impact can cause catastrophic injury not only to the struck player, but also to the head and neck of the player creating the contact (Pellman et al., 2003b). It is rare in any helmeted sport to have an impact where the CG of both masses impact one another in a straight line. The NFL data support this position by noting that 76% of facemask impacts were to levels −Q1 to −Q3 (below CG) and 79% of helmet shell impacts were to levels +Q2 to +Q4 (above CG). Additionally, a more recent study by Delaney et al. (2006) also demonstrated that side/temporal area impacts to the head or helmet are

© 2007, Woodhead Publishing Limited

90

Materials in sports equipment

(A) Struck players

Concussion (Head to head) No injury (Head to head)

Concussion (Head to ground) No injury (Head to ground)

(B) Striking players

No injury (Head to head)

4.1 Locations of initial helmet contacts for struck (A) and striking (B) players. (Pellman et al., Concussion in professional football: reconstruction of game impacts and injuries, Neurosurgery, 53(4): 799–813. Permission to reprint from Lippincott, Williams and Wilkins.)

the most probable area resulting in concussion for both football and soccer. These types of eccentric impacts are partially responsible for the angular acceleration seen in helmeted sports. Research reports have indicated that concussions regularly occur at rotational accelerations greater than 7000 radians/sec. Extrapolation from the NFL study indicates that diffuse axonal injuries might occur at rotational accelerations greater in magnitude (16 000 radians/sec) than previously thought. However, research has indicated that rotational acceleration is only moderately correlated (R = 0.56, P = 0.031) and that rotational velocity did not signiﬁcantly correlate (R = 0.31, P = 0.210) with MTBI (Pellman et al., 2003a). Moreover, it is interesting to note that none of the ‘striking’ players were clinically diagnosed with MTBI

© 2007, Woodhead Publishing Limited

Protective helmets in sports

91

Top view A impact (36%)

R 90°–135°

R 45°–90°

R 135°–180° R 0°–45°

Hybrid III cg

X

Peak head Acc. 78.5±18.1g 48.8°±18.0°

L 0°–45°

L 135°–180° L 45°–90°

L 90°–135° Y

Side view Y +Q4 A impact (36%)

+Q3 d +Q2

Hybrid III cg

+Q1

d/4

X –Q1 Peak head Acc. –5.4°±5.2° –Q2 –Q3

4.2 Classiﬁcation of impact quadrants from top and side view. (Pellman et al., Concussion in professional football: reconstruction of game impacts and injuries, Neurosurgery, 53(6): 1328–1341. Permission to reprint from Lippincott, Williams and Wilkins.)

© 2007, Woodhead Publishing Limited

92

Materials in sports equipment

while 25 of the 31 ‘struck’ players were diagnosed with MTBI (Pellman et al., 2003b).

4.3.1 Dynamics of head impacts When two helmeted head masses collide, they give rise to an impulsive force. Linear momentum must be conserved. However, when the helmeted masses separate, the total kinetic energy before impact is generally not equal to the total kinetic energy after the impact (inelastic collision). Additionally, post-impact velocity can be predicted through Newton’s observation that the relative velocity after impact is proportional to the relative velocity before impact, which is termed the coefﬁcient of restitution represented by the equation: e = −(v1− v2)/(u1 − u2)

[4.1]

where u1 and u2 are the velocities of the two bodies before impact and v1 and v2 are the velocities after impact. This coefﬁcient represents the degree to which the collision approaches perfect elasticity or perfect inelasticity. Elasticity is a property of a material which tends to return to its original size or shape when deformation forces are removed. Likewise, the elastic limit of a material is the point at which an elastic material can no longer return to its original size or shape and it takes on a permanently deformed shape. The force delivered in a helmet-to-helment collision can be determined from Newton’s second law of motion, which states that the time rate of change in linear momentum of a body is equal to the resultant force acting upon the body, and has the same direction as the resultant force. This law is represented in the equation: F = ma

[4.2]

where F is the resultant force acting on a body (newtons), m is the mass of the body (kg) and is constant, and a is acceleration (m/s2). Various upper limits for serious head injury or concussion have been posited between 5 and 10 kN. The results of the Pellmen studies indicate that the peak force of concussion may be lower than once expected, averaging only 4.4 ± 1.2 kN (980 ± 280 lb) to the head. In a typical helmeted head mass impact scenario, the two masses approach one another with a certain velocity and have a kinetic energy (Ek) given by the following equation: EK = 1/2mV2

[4.3]

where EK is the kinetic energy measured in joules (J), m is the mass of the body (kg), and V is the velocity of the body (m/s). Given the average velocity of an American professional football impact at 9.3 m/s, estimates from

© 2007, Woodhead Publishing Limited

Protective helmets in sports

93

the Pellman study indicate that concussions regularly occur at Ek values averaging approximately 118 J, but for those players struck without subsequent concussion the Ek values averaged 57 J. If such a large amount of energy were not dissipated to acceptable levels through the helmet cushioning system, bony and soft tissue injury would likely occur. Of additional importance in the physics of impact is linear momentum. Linear momentum is represented by the equation: P = mv

[4.4]

where P is the linear momentum measured in kg*m/s, m is the mass of the body (kg), and v is the velocity of the body (m/s). In general, helmet collisions in sports deal with a delivered force that varies over a period of time known as impulsive force (see Fig. 4.3). The area under the force–time curve is termed the impulse and is calculated using the integral: t2

I = ∫ Fdt

[4.5]

t1

where I is the impulse (N s), F is the impact force (N), and dt are inﬁnitesimal time increments (s). From Newton’s second law, we can derive the linear impulse–momentum theorem. This theorem states that the resultant linear impulse of a force in a given direction is equal to the change in linear momentum, in the same direction, of the body on which this force acts. This theorem is represented by: t2

∫ Fdt = mv

2

− mv1

Force

t1

Time

4.3 Example of an impulsive force.

© 2007, Woodhead Publishing Limited

[4.6]

94

Materials in sports equipment

where the left-hand of the equation represents the linear impulse (N s) and the right-hand of the equation represents the linear momentum (kg m/s). As discussed in Section 4.3.1, the biomechanics of head impact in American football have been calculated at ∆V = 7.2 ± 1.8 m/s for concussed players. Therefore, the estimated linear impulse or linear momentum experienced, on average (given above velocity and a 6.05 kg mass for head + helmet), in an American football helmeted impact would be 43.5 N s (43.5 kg m/s). The ability to control this transfer of momentum from the helmet to the skull (and brain) is of paramount importance with regard to the prevention of MTBI.

4.3.2 Energy absorption process During the point of impact, the rigid outer shell responds with the initial energy absorbing mechanisms by deﬂecting and spreading the impact over a larger area. This spreading effect helps to delocalize the impact energy and transfer the loads into the foam liner that continues to further deform and spread the impact energy, thereby effectively reducing and dissipating the shock and properly decelerating the athlete’s head. More speciﬁcally, from a mechanical perspective, energy absorption is achieved in two different ways: by extending the time course of the impact event or by dissipating the energy. The amount of dissipated energy controls the amount of momentum (∆P) that is transmitted from the impacting projectile to the skull (see equation 4.4). It has already been demonstrated that an average NFL player impact time interval will typically be about 15 ms. However, a smaller area under the F(t) impact curve coupled with a longer impact time will typically reduce the amplitude of the transmitted force. However, if the head is decelerated over a longer period of time, it must necessarily travel further. Thus, reducing the peak force in this way requires the presence of an inﬁnite thickness of compressible material. However, the thickness of liner material is strictly controlled by various national and international performance standards. Where the thickness of material is inadequate, the phenomenon of ‘bottoming-out’ occurs because the stiffness of the material increases abruptly once it has been compressed through most of its thickness. The helmet must have a ‘comfortable’ thickness while limiting the maximum amount of force which can be transmitted. These constraints deﬁne the upper limit of how rigid the elastomeric cushioning material should be. Regardless, both the energy adsorbed and the length of the impact must be effectively controlled by the viscoelastic properties of the elastomeric liner material used.

4.3.3 Viscoelastic properties of material selection The material selections for the construction of sports helmets are segmented and driven by numerous factors. Primarily, material selection is

© 2007, Woodhead Publishing Limited

Stress (σ)

Protective helmets in sports

95

Loading >

< Unloading

Strain (ε)

4.4 Typical viscoelastic hysteresis loop. Note: the shape of the loop depends on the rates of loading and unloading.

determined by the sport, economics, the anticipated shock severity, and probable impact repetitions during sports applications. As their name suggests, viscoelastic materials combine two different properties. The term ‘viscous’ implies that a material deforms, or ﬂows, when exposed to an external force. The term ‘elastic’ implies that once a deforming force has been removed from a material then it will return to its original conﬁguration in contrast to pure viscous materials (e.g., ﬂuid), in which deformation involves a permanent rearrangement of the ﬂuid molecules. Therefore, a material used in a protective helmet for use in sport must possess the characteristics of a viscous liquid and an elastomeric solid (i.e., viscoelastic) such that the material can lessen the linear impulse side or the linear momentum side of equation 4.6. The viscoelastic properties of materials are usually examined by means of stress/strain behavior. With viscoelastic materials, a ‘hysteresis’ loop is formed (see Fig. 4.4), and the area within the loop represents the energy lost which dissipates as heat. This energy dissipation behavior in part explains why viscoelastic materials are good shock absorbers. There are several important factors that affect the properties of viscoelastic materials including rate of deformation and temperature. Therefore, when selecting materials for protective sport helmets, it is important to examine the viscoelastic material properties at an appropriate loading rate and end-use temperature.

4.4

Helmet construction: shell materials

Contemporary helmet constructions are represented by multiple-impact university and professional level football helmets on the upper-end of cost

© 2007, Woodhead Publishing Limited

96

Materials in sports equipment

Table 4.1 Relative performance selection criteria for helmet shell construction materials Material

Thermal stability

Impact strength

Chemical resistance

UV stability

Dimensional stability

Cost

Polycarbonate ABS Polypropylene Polyethylene

High Med–high Med Low

High Med–high Low–med Low

Low–med Med–high High High

Med–high Med–high Med–low Low

High High Med–low Low

High Med Low Low

and performance to single-impact youth bicycle and skateboarding helmets represented at the lower-end of the cost and performance spectrum. The materials incorporated into the various constructions are directly related to the cost–performance needs for the helmets in application. Regardless of the application and economics, helmets universally consist of an injection molded rigid polymer outer shell combined with a polymer foam liner energy dissipation system. The aforementioned deﬂection and energy transfer mechanisms from the shell into the liner are critical for proper performance and protection. Consequently, the selection of materials and construction of the helmet will directly inﬂuence the performance of the protection. Polymer shell materials for most athletic applications typically fall into three categories for performance. Polycarbonate (PC) shells offer the highest level of energy absorption and impact resistance for helmet shells and are exclusively used in high-quality helmets for all sports and for most multiple-impact and high-impact helmet applications such as football helmets. High-impact modiﬁed acrylonitrile–butadiene–styrene (ABS) resins are often used as the shell material for lower repetition mediumimpact helmet applications such as general-purpose ice hockey, lacrosse, and baseball batting headgear. Single high-impact, high-volume and lowercost general-purpose youth helmets such as those sold for bicycling and skateboarding are typically constructed from polyoleﬁns, including polyethylene (PE) and polypropylene (PP) materials. Some relative selected properties associated with helmet shell material performance and selection criteria are summarized in Table 4.1.

4.4.1 Properties and manufacturing of polycarbonate Polycarbonate is generally considered to be the highest impact strength thermoplastic commercially available polymer, within reasonable economics, and is the material of choice for all high-quality multiple-impact highenergy absorption sports helmets. The unusually tough PC thermoplastic is available under the trade names of Lexan® (General Electric) and

© 2007, Woodhead Publishing Limited

Protective helmets in sports

97

Makrolon® (Bayer). Polycarbonate is produced by the interfacial polymerization of bisphenol-A and phosgene as depicted in Fig. 4.5. Its toughness and high-impact strength are typically associated with energy absorbing mechanisms likewise associated with (aromatic) phenyl ring spin deformations that take place along the backbone of the polymer. Polycarbonate provides the necessary robustness and impact strength for shells in multiple high-impact sport applications while providing the necessary toughness for performance and dimensional stability in cold-weather, hot-weather and moist-weather environments. They are aesthetically attractive materials with high-gloss and color stability. Limitations for PC materials in sport helmet applications include notchsensitivity. Failures in plastic parts that are notch-sensitive frequently originate at a discontinuity in the structure, such as a hole, thread, notch, groove, or scratch (Inberg and Gaymans, 2002). When designing loadbearing parts, it is essential to know how the material will respond to load-concentrating discontinuities. Proper design can eliminate or reduce stress concentrators and therefore minimize problems. A thorough understanding of PC notch-sensitivity and proper design and placement of secondary drilled holes, vents, snaps, and any other modiﬁcations of the as-molded shell is necessary for performance durability. Injection molding of PC shells requires proper drying of the resin prior to meltprocessing, proper placement and design of injection molding gate and runners, proper melting and residence time within the injection molding process, and a complete understanding of mold-ﬁlling dynamics to

O

CH3 HO

OH

C

+

C

Cl

CH3

Cl

Phosgene

Bisphenol-A –2HCl

O

CH3 O

C

O

CH3 Polycarbonate

4.5 Basic chemical structure of bisphenol-A polycarbonate.

© 2007, Woodhead Publishing Limited

C n

98

Materials in sports equipment

minimize or eliminate weld-line formation within the molded part (Bayer, 1995). Additionally, part packing and cooling dynamics during the molding process can lead to residual stresses within the molded product that can ultimately have a detrimental effect on the performance of the shell in application. Failures and a loss of PC helmet aesthetics can also be caused by many solvents and chemicals due to the relatively low chemical resistance for the polymer (Al-Saidi et al., 2003). Care should be taken to minimize the exposure of PC to acids, alkalis, hydrocarbons, and solvents. A shell that has been properly or improperly manufactured will contain various levels of molded-in-stresses. When these stressed regions of the shell are exposed to certain chemicals, possibly from exposure to cleaning solutions or postmolding spray coating operations, the result could lead to microscopic or macroscopic cracks (notches) in the shell that ultimately lead to a catastrophic failure of the material upon impact. Knowledge of PC chemical resistance and the speciﬁc chemical composition of any cleaners, paints, or sprays should be taken into consideration prior to exposure of the helmet to these types of chemicals. Cleaning of PC helmets should be limited to soap and water scrubs. In general, painting of a PC helmet should only be considered when necessary and under the strict recommendations of the helmet manufacturer.

4.4.2 Properties and manufacturing of acrylonitrile–butadiene–styrene (ABS) High-impact ABS is lower cost than PC and has many of the desirable properties for helmet shell materials. It has slightly lower impact strength and is therefore not as useful in multiple high-energy impact applications such as collegiate and professional football helmets, but it is adequate for many other sport helmet shells including ice-hockey, lacrosse, and baseball batting helmets where the severity and frequency of impacts are not as great as those experienced in high-level football athletics. The polymer is dimensionally stable, tough, aesthetic, and has better chemical resistance than PC. ABS is generally less notch-sensitive than PC and less likely to fail from molded-in-stresses incurred during the injection molding process. ABS is a terpolymer, and the relative amounts of the three monomers incorporated during the polymerization process will determine the ultimate physical properties for the material. Acrylonitrile is associated with strength and thermal stability, butadiene with impact and low-temperature toughness, and styrene with dimensional stability, color, and lower cost. The development of an impact material for applications such as sport helmets will have the proper ratio of monomer content to adequately perform in

© 2007, Woodhead Publishing Limited

Protective helmets in sports H CH2

H C C

C m N

C

H

H

H

C

C

H

Acrylonitrile

H

99

H CH2

C p

n

Butadiene

Styrene

4.6 Basic chemical structure of poly(acrylonitrile-butadiene-styrene) terpolymer.

H

H

C

C

H

H

Polyethylene

n

H

H

C

C

H

CH3

n Polypropylene

4.7 Basic chemical structure of polyoleﬁns.

the ﬁeld. The polymerization process for ABS will determine the various block-lengths and sequences for the polymer. The chemical structure for ABS is shown in Fig. 4.6.

4.4.3 Properties and manufacturing of polyoleﬁn materials High-volume and lower-cost shells used in general-purpose and safety helmets for youth bicycle riding, skateboarding, etc. are often constructed from PE or PP (ConsumerSearch, 2006). These polyoleﬁn materials are inexpensive, reasonably dimensionally stable, easily molded, notchinsensitive, and have the added beneﬁt of high resistance to most chemicals, cleaners, and solvents. Ultrahigh molecular weight (UHMWPE) is the material of choice for PE shells since the long chain lengths of over 100 000 ethylene units ‘intertangle’ causing physical crosslinks leading to increased tensile strength, impact strength, fatigue, abrasion resistance, and other important physical properties for shell materials. Polypropylene materials have similar properties and higher thermal stability than most PE materials. Although polyoleﬁns will not dissipate energy to the level of PC or ABS, both PP and UHMWPE are excellent material choices for general-purpose youth sporting activities because they are robust, chemically stable, notchinsensitive, and simply better-suited for youth activities. The chemical structures for PE and PP are shown in Fig. 4.7.

© 2007, Woodhead Publishing Limited

100

Materials in sports equipment

4.5

Helmet construction: liner materials

Materials selected for the construction of helmet liners are critical for the sport and use of the helmet in application. Whereas the helmet shell must be rigid, dimensionally stable for outdoor use, and have adequate strength to initiate and spread energy dissipation into the foam cushioning system, the foam system must efﬁciently accept transfer of energy from the shell and is ultimately responsible for the bulk energy absorption and protection of the athlete. Foam physical properties vary greatly in density, resiliency, and energy absorption characteristics, and designers must consider the severity and frequency of impacts to properly select the foam material to pass standardization approval for a given helmet application. In general, the foam cushioning system must absorb the peak g energy forces without ‘bottoming-out’ leading to any residual impact energy being transferred directly into the head of the athlete. When considering cushioning foams, there are two broad categories generally deﬁned as ‘resilient’ or ‘crushable’.

4.5.1 Multiple-impact resilient foam systems Resilient foams can be thought of as energy absorbing springs. They are able to dissipate energy over a broad area, sustain multiple mid- to highenergy impacts, and return to their original shape and energy absorbing potential almost immediately after the sustained impact. One must note that even the highest quality and most resilient energy absorbing foams will slowly lose their energy absorbing properties with time and impacts during the expected lifetime of the helmet. Therefore, manufacturer and performance standards committee recommendations for foam cushioning replacement must be carefully followed to properly protect athletes. The most common and high quality foam used in high-energy impact applications is the family of polyurethane (PU) foams that literally surround society in a ubiquitous fashion for human cushioning. Furniture cushions, automotive seat cushions, and carpet backing cushions all rely on the energy absorbing, forming and deformation, and high resiliency properties of PU foams for properly cushioning human–material interfaces. A broad range of PU formulations and raw-material supplies are abundant, well known, and easily fabricated at relatively low scost. Other families of polymers that represent resilient foam applications in helmet cushioning include polyvinyl chloride (PVC), PE, and PP. These foams might be selected if the application requires long levels of moisture contact, since one concern for PU systems is associated with potential hydrolytic degradation with long-term moisture. Another factor for selecting alternate foam systems, especially polyoleﬁn types, would be for weight reduction purposes.

© 2007, Woodhead Publishing Limited

Protective helmets in sports

101

When considering high-impact resilient foam cushioning, foam density becomes a critical material design criterion. Foam density is simply the weight per given volume of foam, typically measured in kg/m3, and gives an indication to the cell structure of the foam material. High-density foams contain smaller gas voids and tend to have more of a ‘closed-cell’ construction (meaning the gas-bubbles formed during the polymerization process remain closed or locked into the cured foam), while low-density foams have more of an ‘open-cell’ or sponge-like construction where the cured polymer material has continuous interconnected passageways throughout the structure.

4.5.2 Multiple-impact dual-density foam systems The ‘dual-density’ resilient foam cushioning system found in many baseball batting helmets is a perfect example of good resilient foam management in a multiple-impact helmet construction. In this example, a thicker layer of open-cell and low-density PU foam is used directly against the human head. This low-density foam is light-weight, conforms to ﬁt nicely to a variety of head shapes, and the open-cell nature of the foam helps to wick sweat from the athlete keeping him or her comfortable. This low-density resilient foam is adequate for relatively minor multiple head impacts most often occurring during a baseball game, such as bumping your head while sliding into a base, being tagged by a glove, or being brushed by a pitch. A thinner strip of heavier high-density closed-cell foam is placed directly between the thicker low-density foam and the helmet shell to absorb the less frequent high-energy impact, such as a fastball pitched directly to the head. In this dual-density high-impact scenario, the low-density foam will bottom-out into the high-density PU foam that will take over the energy absorption and provide the ultimate protection in cushioning system. A picture of the dual-density PU foam cushioning system is shown in Fig. 4.8. Shell

Low-density foam

High-density foam

4.8 Dual-density polyurethane foam batting helmet cushioning system.

© 2007, Woodhead Publishing Limited

102

Materials in sports equipment

4.5.3 Multiple-impact two-stage foam/mechanical systems Dual-density cushioning systems are not unique to baseball batting helmets, nor are they limited to PU foam cushioning. Cushioning systems that respond differently to multiple-impact forces at varying degrees of impact are constructed in many ways. Similar two-stage cushioning capacity as that described for the dual-density batting helmet example, some football helmets incorporate an air bladder to help dissipate secondary, higher energy impact forces. These bladders are typically constructed from a thermoplastic elastomer polymer, such as a thermoplastic polyurethane (TPU) or ﬂexible PVC. The bladders are normally designed as a series of sealed tube structures that cushion through bladder deﬂection and airdisplacement mechanisms. Some helmet air-cushioning bladders incorporate an air-pressure inﬂation device for prescribing cushioning at elevated pressures while other air-bladder cushions perform at zero (atmospheric) pressure and function as a result of deﬂection and displacement. A picture of a zero-pressure air-bladder for high-energy football helmet impacts is shown in Fig. 4.9. Mechanical secondary high-impact cushioning systems incorporated into football helmets have been the subject of more recent development. SKYDEX® energy absorbing cushioning was introduced into professional and collegiate football helmets in the Schutt DNA® helmet in 2006. This mechanical system is based on a series of inverted hemispheres that act as mechanical springs during an impact. The level of cushioning support can be adjusted by the size, placement, thickness, and material incorporated into the hemisphere design. The materials used are typically some form of thermoplastic elastomer that has high energy return. During an impact, the inverted hemispheres deﬂect against each other during the energy

4.9 Football helmet bladder air-cushioning system.

© 2007, Woodhead Publishing Limited

Protective helmets in sports Neutral

Impact load

103

Energy return

4.10 SKYDEX® mechanical cushioning system.

Shell Foam SKYDEX®

4.11 Twin-sheet thermoformed mechanical cushioning system.

absorbing event and then return to their original position helping to dissipate the total energy of the impact. The deﬂection mechanism of energy absorption is depicted in Fig. 4.10. The SKYDEX® mechanical cushioning system is incorporated into the helmet as a secondary energy absorbing system and lies between a layer of ﬂexible PU foam and the PC shell of the helmet. The system is typically manufactured by injection molding the elastomeric hemispheres followed by an RF-welding operation to attach the inverted molded hemispheres to each other, or by twin-sheet thermoforming operations where the cushioning system is manufactured into a ﬁnished bladder. The advantage of injection molding is better control and variation of hemisphere wall thickness for ‘tuning’ the cushioning system, while the twin-sheet thermoforming method is lower cost and eliminates the secondary RF-welding process. Figure 4.11 shows a twin-sheet thermoformed mechanical secondary highimpact cushioning system of inverted hemispheres, and the incorporation of this system into a football helmet.

4.5.4 Single-impact crushable foam systems In contrast to multiple high-energy impact cushioning systems such as those discussed above, crushable foams are used in single high-energy impact event helmets, such as those used in youth general-purpose bicycling and skateboarding helmets. The advantage of the crushable foams is low-weight and economics. The primary crushable foam system used in these helmets is expanded polystyrene (EPS), or StyrofoamTM (Dow Chemical Co.).

© 2007, Woodhead Publishing Limited

104

Materials in sports equipment

These systems are designed to sustain a single high-energy impact and rendered insufﬁcient for adequate safety and energy absorption after exceeding a particular impact threshold (Landro et al., 2002). The reason for this limitation is due to a permanent set and possible microfracturing of the EPS foam that occurs at the impact threshold. EPS foam manufacturing technology is a relatively low-pressure and low-cost process, but the technology has been in production for decades and has a high level of reproducibility for quality control. In the process, a matched die tool set that creates an internal cavity for the desired shape of the ﬁnal EPS part is ﬁlled with unexpanded EPS beads. These beads, available from numerous suppliers, are the consistency of ﬁne-grained sand. The tool set is heated, normally by simple steam, and as the beads heat up they begin to expand and ﬁll the tool cavity. Foam density is simply controlled by the amount of unexpanded beads that are initially charged into the cavity. As the beads expand, they eventually ﬁll the cavity and compress against each other creating a bond between the individual expanded polystyrene particles. This compaction, based on the amount of the initial charge, controls the EPS foam density. Typical foam densities for singleimpact helmet constructions are in the 2–3 kg/m3 range.

4.6

Helmet safety standards and performance testing

There are several organizations internationally that regulate the performance speciﬁcations for materials used in protective sports headgear. In the USA, the American Society for Testing and Materials (now ASTM International) has standards for various types of materials used in sporting equipment, surfaces, and facilities. Of particular importance, ASTM regulates the impact strength of a material. Impact strength is typically deﬁned as the amount of energy required to fracture a specimen subjected to a speciﬁc shock loading under impact. Alternative terms are impact energy, impact value, impact resistance, and energy absorption. It is essentially an indication of the toughness of a material and associated with the behavior of material subjected to shock loading in bending, tension, or torsional modes. The quantity usually measured is the energy absorbed in breaking the specimen in a single blow, as in the Charpy Impact Test (ASTM, 2006a), Izod Impact Test (ASTM, 2006b), and Tension Impact Test (ASTM, 2006c), and reported strengths are generally in the form of energy per area, such as Joules/cm2. Additionally, the F08 Technical Committee of ASTM deals directly with performance speciﬁcations of headgear and other sports equipment in various American recreational and organized sports (see Table 4.2). The National Operating Committee on Standards for Athletic Equipment (NOCSAE) is the organization that regulates testing performance for protective headgear in several American sports (see Table 4.3).

© 2007, Woodhead Publishing Limited

Table 4.2 Sports covered by ASTM Technical Committee F08 on sports equipment and facilities Subcommittee

Subject area

F08.10 F08.12 F08.15 F08.16 F08.17 F08.22 F08.23 F08.24 F08.25 F08.26 F08.30 F08.51 F08.52 F08.53 F08.54 F08.55 F08.57 F08.63 F08.64 F08.65 F08.66 F08.67

Bicycles Gymnastics and wrestling equipment Ice hockey Archery products Trampolines and related equipment Camping softgoods Tennis courts and track surfaces Paintball and equipment Recreational basketball equipment Baseball and softball equipment Fitness products Medical aspects and biomechanics Miscellaneous playing surfaces Headgear and helmets Athletic footwear Body padding Eye safety for sports Playground surfacing systems Natural playing surfaces Artiﬁcial turf surfaces and systems Sports facilities Pole vault

Table 4.3 Sports and equipment covered by NOCSAE performance standards Standard

Subject area General Standards

ND01-06m06 ND021-98m05a ND081-04m04 (DRAFT) ND100-98m03 ND101-00m03

Standard Drop Test Method and Equipment Used in Evaluating the Performance Characteristics of Protective Headgear Standard Projectile Impact Testing Method and Equipment Used in Evaluating the Performance Characteristics of Protective Headgear, Faceguards or Projectiles Standard Linear Impactor Test Method and Equipment Used in Evaluating the Performance Characteristics of Protective Headgear and Faceguards Troubleshooting Guide for Test Equipment and Impact Testing Equipment Calibration Procedures Football

ND002-98m05 ND003-96m03 ND004-96m06 ND005-96m03

Standard Performance Speciﬁcation for Newly Manufactured Football Helmets Laboratory Procedural Guide for Certifying Newly Manufactured Football Helmets Standard Performance Speciﬁcation for Recertiﬁed Football Helmets Laboratory Procedural Guide for Recertifying Football Helmets Baseball/Softball

ND22-06m06 ND023-98m03

Standard Performance Speciﬁcation for Newly Manufactured Baseball/Softball Batter’s Helmets Laboratory Procedural Guide for Certifying Newly Manufactured Baseball/Softball Batter’s Helmets

© 2007, Woodhead Publishing Limited

Table 4.3 (cont.) Standard

Subject area Baseball/Softball

ND072-04m05a

Standard Performance Speciﬁcations for Newly Manufactured Baseball/Softball Batter’s Helmet Mounted Faceguard ND24-06m06 Standard Performance Speciﬁcation for Newly Manufactured Baseball/Softball Catcher’s Helmets with Faceguards ND025-98m03 Laboratory Procedural Guide for Certifying Newly Manufactured Baseball/Softball Catcher’s Helmets with Faceguards ND027-04m05 Standard Performance Speciﬁcation for Newly Manufactured Youth Baseballs ND026-04m04b Standard Performance Speciﬁcation for Recertiﬁed Baseball/ Softball Batter’s and Catcher’s Helmets Ice Hockey ND030-04m04a ND031-04m04 ND035-04m04 ND032-04m04a ND033-04m04

Standard Performance Speciﬁcation for Newly Manufactured Hockey Helmets Laboratory Procedural Guide for Certifying Newly Manufactured Hockey Helmets Standard Performance Speciﬁcation for Newly Manufactured Hockey Face Protectors Standard Performance Speciﬁcation for Recertifying Hockey Helmets Laboratory Procedural Guide for Recertifying Hockey Helmets Lacrosse

ND041-05m05

Standard Performance Speciﬁcation for Newly Manufactured Lacrosse Helmets with Faceguards ND045-04m04b Standard Performance Speciﬁcation for Newly Manufactured Lacrosse Face Protectors ND042-04m04 Laboratory Procedural Guide for Certifying Newly Manufactured Lacrosse Helmets ND043-04m05 Standard Performance Speciﬁcation for Recertiﬁed Lacrosse Helmets ND044-04m04 Laboratory Procedural Guide for Recertifying Lacrosse Helmets ND049-05 Standard Performance Speciﬁcations for Newly Manufactured (DRAFT) Lacrosse Balls Polo ND050-04am04 ND051-03m03 ND055-03m05 ND056-03m03

Standard Performance Speciﬁcation for Newly Manufactured Polo Helmets Laboratory Procedural Guide for Certifying Newly Manufactured Polo Helmets Standard Performance Speciﬁcation for Helmet Mounted Polo Eye Protection Laboratory Procedural Guide for Certifying Newly Manufactured Eye Protectors for Polo Headgear Soccer

ND090-06m06a ND091-03m03

Standard Performance Speciﬁcation for Newly Manufactured Soccer Shin Guards Laboratory Procedural Guide for Certifying Newly Manufactured Soccer Shin Guards

© 2007, Woodhead Publishing Limited

Protective helmets in sports

107

4.6.1 Linear impact performance testing The biomechanics of head injury have already been discussed. This section will address the industry standards used to determine whether protective helmets meet standard performance characteristics. With regards to linear impact testing, we will speciﬁcally discuss the NOCSAE’s helmet testing protocol as it relates to American football. One of the basic elements of this equipment testing system includes a NOCSAE-speciﬁc headform with a triaxial accelerometer mounted at the headform’s center of mass and sealed in glycerin to promote a bioﬁdelic response. The headform is attached to a carriage assembly that is guided in a free fall by two cables allowing the headform to precisely impact into a 1/2 inch test modular elastic programmer (MEP) pad that has a speciﬁc durometer PU surface. See Fig. 4.12 for a schematic of this test. The speciﬁc testing protocol calls for the sample helmet to be subjected to a series of impacts over six speciﬁc and one random helmet locations (see Fig. 4.13). At the front and side impact locations, the drop height starts at 91 cm and ends at 152 cm. All other impact locations to be tested are from a drop height of 152 cm (see Table 4.4). Two major standardized criteria have been developed that assess a helmet’s ability to protect humans against head trauma: the Gadd Severity Index (also known as just Severity Index or SI) and the Head Injury Criterion (HIC). The NOCSAE’s performance speciﬁcations utilize the SI and require that no single impact, regardless of location or drop height, should exceed a SI of 1200 when the time between successive impacts is 75 ± 15 seconds. The equations for these SI and HIC performance criteria are given below: SI = ∫ [a(t )]2.5dt

[4.7] 2.5

⎡ 1 t2 ⎤ HIC = ⎢ adt ⎥ (t2 − t1 ) ∫ ⎣ t2 − t1 t1 ⎦

[4.8]

Table 4.4. Matrix of NOCSAE drop heights and locations for new helmets. All drop heights are in inches (cm) and must be within ±1/8″

Ambient temp

High temp

Front

Side

36 48 60 60

36 48 60 60

(91) (122) (152) (152)

(91) (122) (152) (152)

60 (152) 60 (152)

© 2007, Woodhead Publishing Limited

Front boss

Rear boss

Rear

Top

Random

60 (152) 60 (152)

60 (152) 60 (152)

60 (152) 60 (152)

60 (152) 60 (152)

60 (152) 60 (152)

108

Materials in sports equipment

Wall

H Installation must be plumb and allow 72 inches free fall.

H

17 inch minimum

Wall

4.12 Linear impact schematic.

4.6.2 Projectile impact performance testing The majority of this chapter has dealt with impact operationally deﬁned as helmet-to-helmet or helmet-to-surface. Brain injury and sport-related concussion can also be the result of a high-velocity, low-mass projectile impact to the helmet as is the case with sports that incorporate balls, e.g.,

© 2007, Woodhead Publishing Limited

Protective helmets in sports

Front impacts

Side impacts

Front boss impacts

Rear boss impacts

Rear impacts

Top impacts

4.13 Schematic of NOCSAE drop test sites.

© 2007, Woodhead Publishing Limited

109

110

Materials in sports equipment (a)

(b)

(c)

(d)

(e)

(f)

4.14 Projectile impact schematic. This is an exemplary system; any system that provides the required test parameters is acceptable. (a) = air reservoir; (b) = air solenoid; (c) = loading breech; (d) = interchangeable barrel; (e) = velocity measurement sensor; (f) = head form – fully adjustable 3 axis and rotation.

baseball/softball, hockey, lacrosse, etc. Therefore, the NOCSAE developed a standard for projectile impact testing that again incorporates the NOCSAE-speciﬁc headform mounted to a linear bearing table. An air cannon is used to project the desired implement to the necessary velocity by adjusting the psi. The resultant head acceleration is taken by the same method as the linear impact testing method. To ensure accurate test velocity, the projectile is exposed to two velocity traps at set distances from the cannon to capture a reliable measure of velocity. See Fig. 4.14 for a schematic of this test.

4.7

Helmet design for particular sports: lacrosse, ice hockey, rugby and football/soccer

4.7.1 Lacrosse The sport of men’s lacrosse, the oldest and among the fastest growing team sports in North America, is a fast-paced game played on a ﬁeld similar in size to football ﬁeld. The physical act of body checking is permitted in men’s lacrosse. Players use a stick to catch, carry, pass, and ultimately shoot a hard elastomeric ball toward an opponent defending a netted goal. The sport incorporates both the high-mass, high-velocity body-to-body collisions typical other helmeted sports and the low-mass, high-velocity object-to-body impact from the sport apparatus (i.e., lacrosse stick or ball). Mandatory protective gear for mens’ lacrosse includes a helmet with full face guard and a mouthpiece (Hinton et al., 2005). The 2004–2005 NCAA Injury Surveillance Report revealed that the majority of lacrosse injuries result from player-to-player contact followed by player-to-ground and player-to-apparatus (stick or ball) contact. In

© 2007, Woodhead Publishing Limited

Protective helmets in sports

111

addition, the report also indicated that 11.5% game injuries in lacrosse result in MTBI (NCAA, 2004). Hinton et al. (2005) reported similar incidence rates in secondary school male lacrosse players ﬁnding MTBI to be the second most prevalent injury (10%). In addition, the most common causes of MTBI were legal body-to-body or object-to-body contact (44%). To help reduce the risk for head injury, male lacrosse players are required to wear a helmet. Since the 1990s, there have been signiﬁcant advances in lacrosse helmet design. Traditional lacrosse helmets were typically offered in small to extra-large sizes and were heavy and bulky in design, lacked adequate ventilation holes, and ﬁt loosely on the player’s head. Contemporary lacrosse helmet designs closely resemble bicycle helmets in appearance. Most contemporary designs incorporate dual-density cushioning systems (see Fig. 4.15) and are lightweight, ventilated, and provide a tighter and more comfortable ﬁt than older model lacrosse helmets. Unfortunately, limited independent research has been published examining the effectiveness of various lacrosse helmet designs. Caswell and Deivert (2002) examined four popular lacrosse helmets models and demonstrated decreased abilities in all helmets to attenuate repetitive linear impacts.

4.15 Example of contemporary lacrosse headgear.

© 2007, Woodhead Publishing Limited

112

Materials in sports equipment

Additionally, Caswell reported some SI values for traditional helmet to exceed 1500 indicating failure of the 1990 NOCSAE lacrosse helmet standard. The current NOCSAE lacrosse helmet performance criteria (NOCSAE DOC (ND) 041-05m05) are similar to football specifying that no impact, regardless of location or drop height, should exceed an SI of 1200.

4.7.2 Ice hockey Ice hockey is a fast-paced sport involving intentional and unintentional collisions. Skated players use a stick to receive, stick-handle, pass, and ultimately shoot a hard elastomeric puck toward an opponent defending a netted goal. The sport incorporates both the high-mass, high-velocity (i.e., body checking) and low-mass, high-velocity (i.e., stick or puck) impacts similar to other helmeted sports. However, unlike other helmeted team sports ice hockey is played on hard slippery surface enclosed by immovable boards approximately 1 m high with vertical extensions of thick glass or Plexiglas® (Atoyina) extending from them. The incidence of MTBI in professional ice hockey is reported to be increasing (Biasca et al., 2002), and it may be under-reported in youth hockey (Williamson and Goodman, 2006). A recent study of collegiate ice hockey players in the USA reported collisions with an opponent (32.8%) or the boards (18.6%) as the causes for more than half of all injuries. Furthermore, MTBI (18.6%) was the most common injury (Flik et al., 2005). Similarly, Cuputo and Mattson (2005) examined the incidence of injury in non-contact adult ice hockey leagues and found the most common anatomic region injured was the head/neck/face (35%). Biasca et al. (2002) analyzed video tapes of 40 professional ice hockey players who sustained MTBI and found the most common mechanisms to be: (1) a direct blow to the head; (2) a direct ﬂow to the face or jaw; or (3) a directed blow to the chin. To reduce the obvious risks of injury, various forms of mandatory protective equipment are worn in ice hockey, including a helmet. Ice hockey helmets ﬁrst came into mandatory use in Sweden during the early 1960s due to an insurance study demonstrating the escalating risks of serious head injury in ice hockey (Biasca et al., 2002). In the mid-1960s the Canadian Amateur Hockey Association (CAHA) and the Amateur Hockey Association of the United States (AHAUS) made helmets mandatory equipment for all non-adults (Hoshizaki and Brien, 2004). These early helmets were typically constructed of leather lined with felt. In 1969, following the death of two helmeted teenage Canadian hockey players from closed head injury, a technical committee from the Canadian Standards Association (CSA) was formed with the purpose of approving helmets. In the 1970s leather

© 2007, Woodhead Publishing Limited

Protective helmets in sports

113

4.16 Example of contemporary ice hockey headgear.

and felt, were replaced by formed plastic shells – either PE, PC, or ABS types – and foam liners – either vinyl nitrile (VN) or ethylene vinyl acetate (EVA) types) that provided improved energy absorption and ﬁt. In 1975 all CAHA players were required to wear CSA approved helmets. In recent years improvements in hockey helmet shell and liner construction have been made. Contemporary helmet materials are typically composed of an ABS shell and urethane or PP foam liner (see Fig. 4.16) having a thickness of about 16 mm (Biasca et al., 2002). At present, several standards exist for ice hockey head and face protection (CEN, CSA, ASTM, NOCSAE, and ISO). In 2003, a unifying international standard (ISO, 2003) was published, specifying performance requirements and test methods for head and face protection for use in ice hockey and endorsed by International Ice Hockey Federation.

4.7.3 Rugby Rugby is a physical sport where repetitive collisions between players and playing surfaces occur regularly. The tackle is reported to be associated with approximately 50% of all rugby injuries (McIntosh, 2005). Several research reports indicate that up to 40% of all rugby injuries may be accounted for by MTBI (Gerrard et al., 1994; Bird et al., 1998; Finch et al., 2001; Marshall and Spencer, 2001; McIntosh, 2005). Thus, this sport has seen a signiﬁcant increase in headgear models offered as well as in the number of youth players that are wearing headgear (>60%). Further, in countries like Japan headgear use has become mandatory (Wilson, 1998). In a study by Knouse et al. (2003), it was demonstrated via drop testing that a Hybrid III headform with a rugby headgear properly ﬁtted to it could

© 2007, Woodhead Publishing Limited

114

Materials in sports equipment

decrease derived SI values by more than 200% and 50% for the parietallateral and occipital sites respectively. However, it is important to note that, although rugby headgear works well with respect to its intended purpose of decreasing lacerations and abrasions, several research reports still do not support the use of rugby headgear to reduce the risk of MTBI (McIntosh and McCrory, 2000; McIntosh et al., 2000; McIntosh and McCrory, 2001). One possible explanation for this recommendation is that the drop height performance standard for rugby headgear (30 cm = 2.4 m/s) may not accurately represent the true average impact velocity (7.0 m/s) during typical rugby matches, thereby overestimating the safety capability of the headgear. Additionally, all rugby headgear must conform to International Rugby Board (IRB) standards which include: foam thickness of <1 cm, density of foam <45 kg/m3, and no less than 200 g or greater than 550 g from a 30 cm drop height on impact testing. The reason that the lower threshold value is set at 200 g in rugby is because headgear performance that would result in peak acceleration less than 200 g may cause players to use their head more when tackling thereby increasing the potential risk for neck injury. In general, rugby headgear is designed in a laminate construction style with the inner layer composed of a PE liner. The outside layer generally contains a high-density, closed-cell PU foam (varies by manufacturer and trademark) in either a continuous, sectional, or honeycomb conﬁguration (see Fig. 4.17). Comfort features include ear holes, frontal and parietal

(a)

(b)

4.17 Example of contemporary rugby headgear liner (a) and exterior shell (b).

© 2007, Woodhead Publishing Limited

Protective helmets in sports

115

ventilation holes, and a chin strap. Rugby headgear is donned and doffed through the use of an expandable posterior lace-up closure and generally comes in sizes S–XL (varies by manufacturer).

4.7.4 Football/soccer Football, also referred to as soccer, is a demanding collision-type sport in which players are at risk for injury. Worldwide an estimated 200 million people participate in soccer. The incidence of MTBI among athletes participating in soccer is comparable with that of ice hockey and football (Delaney, 2004). Soccer athletes have also been estimated to experience peak impact accelerations higher than hockey and football players (Naunheim et al., 2000). Estimates indicate that 5–22% of American high school and collegiate soccer injuries each year are head injuries (Barnes et al., 1998; Covassin et al., 2003). Two primary mechanisms of head impacts exist in soccer, ball-to-head and head-to-hard-object. Unique to soccer is the intentional use of the head to direct and gain control of the ball (Barth et al., 2001; Kirkendall et al., 2001). During each ‘header’ athletes are instructed to contract their neck musculature to stabilize the head during impact with the ball and reduce head movement (i.e., acceleration) (Lynch and Bauer, 1996; Bauer et al., 2001). Researchers have suggested that repetitive blows to the head from heading the soccer ball may have deleterious effects on cognitive functioning (Sortland and Tysvaer, 1989; Tysvaer and Storli, 1989; Tsyvaer and Lochen, 1991; Matser et al., 1999; Witol and Webbe, 2003). Boden et al. (1998) reported head-to-ball collisions as the mechanism accounting for 24% of MTBI in elite collegiate soccer players. However, others estimate player collisions and falls as responsible for 74% of all football injuries. Therefore, rather than heading of the ball, MTBI in soccer is more likely to be due to forces imparted to players from head-to-head, head-to-body, head-to-ground, or head-to-goalpost impacts (Jordan et al., 1996; Guskiewicz et al., 2002; McCrory, 2003). The Consumer Product Safety Commission (CPSC) in the USA reported head-to-player impacts as the mechanism accounting for 40% of head injuries in soccer. In response, manufacturers have developed and marketed protective headgear to reduce players’ risk of sustaining a head injury during football. In general, most football headgear is composed of PU, EVA, or PE closedcell foam or a combination of foams and plastic that is covered by either a fabric or a vinyl coating (varies by manufacturer and trademark) (see Fig. 4.18). Football headgear generally ranges in size from S–XL (varies by manufacturer) and is worn similar to a headband and typically afﬁxed at the occiput. Typical comfort features include frontal and parietal ventilation holes. A recent study by Withnall et al. (2005) reports that football

© 2007, Woodhead Publishing Limited

116

Materials in sports equipment

4.18 Example of contemporary football/soccer headgear.

headgear is effective in reducing head-to-head impacts. However, the effectiveness of soccer headgear to attenuate forces from heading a ball remains uncertain (Broglio et al., 2003; Naunheim et al., 2003; Withnall et al., 2005). As such, the only headgear standard (ASTM, 2006d) addresses head-to-hard-object impacts rather than ball-to-head impacts.

4.7.5 The role of proper ﬁt Although the helmet is generally accepted as necessary protective equipment for many collision sports and recreational activities, simply wearing one does not guarantee protection. Similar to other protective devices, proper helmet ﬁt is necessary to ensure optimal protection. Most equipment manufacture and sport-speciﬁc governing bodies provide guidelines for proper helmet selection and ﬁt. Research has demonstrated that improper helmet ﬁt may increase the risk of head injury in bicycling (Rivara et al., 1999). However, Parkinson and Hike (2003) reported that 96% of children and adolescents studied wore bicycle helmets in poor condition and/or of inadequately ﬁt. Depending on the sporting activity, helmets may be ﬁt differently. For example, American Football has established detailed ﬁtting procedures that are performed by a qualiﬁed professional, such as a certiﬁed athletic trainer (ATC) or equipment manager, who ensures proper ﬁt for each player. Other collision sports like ice hockey and lacrosse also provide guidelines for helmet ﬁt. However, attention to proper helmet ﬁt may be less customized.

© 2007, Woodhead Publishing Limited

Protective helmets in sports

117

Table 4.5 General composite ﬁber reinforcement properties Material

Cost

Young’s modulus

Tensile strength

Impact strength

Density

Glass Kevlar1 Carbon Polyester Epoxy

Low Med High Low High

Med–high Med–high Very high Low Med–high

Med–high High Very high Low–med High

Med–high Very high High Low High

Very high Med Med–high Med Med

1 ®

DuPont.

4.8

Future trends

Future trends in helmet shell and cushioning technologies will focus on reducing weight and improving energy absorbing capacity of the constructions. One technical proliferation that is certain to enter the sports helmet arena is the development of composite shell materials and manufacturing technologies. Motorcycle helmets that must pass stringent (Snell) impact thresholds are almost exclusively made from polyester-based ﬁberglass materials. Fiberglass shells in sports, although possible, are generally too dense and heavy for athletic wear. Lightweight advanced composite materials were developed for use in military helmets that also require high-impact and blast-level protection are fabricated with carbon-ﬁber and Kevlar® (DuPont) ﬁber epoxy matrix materials. It is these materials that will most likely play a signiﬁcant role in future athletic gear. The materials are very high strength and lightweight and, in fact, provide the highest level of strength-to-weight ratio for materials available in contemporary materials science. The limitations and barriers preventing these materials from entering sports have been economics. Composite fabrication technologies are labor-intensive, require post-ﬁnishing operations, and have prohibitive cure times that limit their utility for high-volume sporting goods. As composite materials and manufacturing technologies progress, and as economics for these materials improve, the eventual cost–beneﬁt thresholds for use in sport helmet shells will be met. Some relative and fundamental properties for composite reinforcement ﬁbers are given in Table 4.5.

4.9

Sources of further information and advice

Sources of further information and advice are given in Table 4.6 starting on page 118.

© 2007, Woodhead Publishing Limited

118

Table 4.6 Further sources of information Mild traumatic brain injury education, treatment, and research Website

Description

American College of Sports Medicine

www.acsm.org/

The largest sports medicine and exercise science organization in the world committed to the diagnosis, treatment, and prevention of sports-related injuries and the advancement of the science of exercise.

National Athletic Trainers’ Association

www.nata.org/

An association dedicated to the enhancement of the quality of health care provided by certiﬁed athletic trainers to a physically active population.

The American Academy of Neurology

www.aan.com/professionals/ index.cfm?a=0&fc=1#

A medical specialty society established to advance the art and science of neurology, and thereby promote the best possible care for patients with neurological disorders.

The National Institute of Neurological Disorders and Stroke

www.ninds.nih.gov/disorders/tbi/tbi.htm

A leading supporter of biomedical research on disorders and injuries of the brain and nervous system.

© 2007, Woodhead Publishing Limited

Materials in sports equipment

Agency

Safety education organizations Description

Bicycle Helmet Safety Institute

www.bhsi.org/

A non-proﬁt program acting as a clearinghouse and a technical resource for bicycle helmet information.

Consumer Product Safety Commission

www.cpsc.gov/

The U.S. Consumer Product Safety Commission is charged with protecting the public from unreasonable risks of serious injury or death from consumer products.

Lids On Kids

www.lidsonkids.org/snowsports-safety.asp

The National Ski Areas Association (NSAA) developed this site to help educate parents about the beneﬁts and limitations of helmets.

Safe Kids Worldwide

www.safekids.org/

Global network of organizations whose mission is to prevent accidental childhood injury, a leading killer of children 14 and under.

The Snell Memorial Foundation

www.smf.org/

Non-proﬁt organization dedicated to research, education, testing and development of helmet safety standards.

World Health Organization Helmet Initiative

www.whohelmets.org/

Promotes the use of helmets as a strategy for preventing head injuries caused by bicycle or motorcycle crash or fall.

© 2007, Woodhead Publishing Limited

119

Website

Protective helmets in sports

Agency

120

Table 4.6 (cont.) Sport helmet standard setting and research organizations Website

Description

ASTM International

www.astm.org

Formerly known as the American Society for Testing and Materials ASTM International is one of the largest voluntary standards development organizations in the world.

British Standards Institution

www.bsi-global.com/index.xalter

The national Standards Body of the UK and develops standards and standardization solutions to meet the needs of business and society.

Canadian Standards Association

www.csa.ca/Default.asp?language=english

A non-proﬁt standardization body in Canada.

Certottica

www.certottica.it/e_casc_mp.htm

Italian Institute for the certiﬁcation of helmets, head-protectors, and other personal protective equipment.

European Committee for Standardization

www.cenorm.be/cenorm/index.htm

The European Committee for Standardization. Founded by the national standards bodies in the European Economic Community and EFTA countries.

International Organization for Standardization

www.iso.org/iso/en/ISOOnline.frontpage

A non-proﬁt standardization body in Canada.

© 2007, Woodhead Publishing Limited

Materials in sports equipment

Agency

The association provides education to the public regarding the standardization and uniﬁcation of industrial standards.

Le CRITT Sport Loisirs

www.critt-sl.com/index.php

A standardization body in France.

National Institute for Sports Science and Safety

www.nisss.org/

A non-proﬁt research and education institute for the prevention of sports injuries and related musculoskeletal research through the study of injury mechanisms and protective sports equipment.

National Operating Committee on Standards for Athletic Equipment (NOCSAE)

www.nocsae.org/

Non-proﬁt organization established to commission research on and, where feasible, establish standards for protective athletic equipment.

Safety Equipment Institute

www.seinet.org/

Non-proﬁt organization established to administer non-governmental, third-party certiﬁcation programs to test and certify a broad range of safety and protective products.

Oslo Sports Trauma Research Center

www.klokavskade.no/ ostrc.asp?s=main&lang=en

Norwegian University of Sport & Physical Education research center. Founded with the objective is to prevent injuries in sports through a long-term research program focusing on risk factors, injury mechanisms, and injury prevention methods.

© 2007, Woodhead Publishing Limited

121

www.jsa.or.jp/default_english.asp

Protective helmets in sports

Japanese Standards Association

122

Table 4.6 (cont.) Sport helmet standard setting and research organizations Website

Description

A Protective Headgear Manufacturers’ Association

www.phma.org/

A voluntary, non-proﬁt organization of manufacturers of headgear for nonmotorized sports. PHMA was established in 1994 with a mission to help reduce the risk of head injury through education, promotion of headgear use, and support of programs that may lead to further reduction of head injury.

RIH Orthopaedic Foundation Test Facility

biomed.brown.edu/Medicine_Departments/ ORTHOPAEDICS/RIHOF/RIHhelmet.html

RIH Orthopaedic Foundation Test Facility works in close conjunction with the Bioengineering Laboratory at Brown Medical School provides a resource for the Department of Orthopaedics and for private industry.

Standards Australia

www.standards.com.au/

A non-government standardization body in Australia.

The Hockey Equipment Certiﬁcation Council (HECC)

www.hecc.net/

A non-proﬁt independent certiﬁcation body for amateur hockey equipment.

Wayne State University (WSU) Bioengineering Center

www.bioengineeringcenter.org/home/ labs/sports/

Bioengineering Center, the Sport Injury Biomechanics Lab capable of evaluating all types of athletic personal protective equipment.

© 2007, Woodhead Publishing Limited

Materials in sports equipment

Agency

Protective helmets in sports

4.10

123

Acknowledgements

We would like to thank Mr Mike Oliver, executive director/legal council for the National Operating Committee on Standards for Athletic Equipment and Lippincott, Williams & Wilkins for allowing us to reprint ﬁgures.

4.11

References

al-saidi l f, mortensen k and almdal k (2003), ‘Environmental stress cracking resistance. Behavior of polycarbonate in different chemicals by determination of the time-dependence of stress at constant strains,’ Polym Degrad Stabil, 82, 451–61. astm (2006a), D6110-06 Standard Test Methods for Determining the Charpy Impact Resistance of Notched Specimens of Plastics, West Conshohocken, PA, ASTM, International. astm (2006b), D256-06a Standard Test Methods for Determining the IZOD Pendulum Impact Resistance of Plastics, West Conshohocken, PA, ASTM International. astm (2006c), D1822-06 Standard Test Method for Tensile Impact Energy to Break Plastics and Electrical Insulating Materials, West Conshohocken, PA, ASTM International. astm (2006d), F2439-06 Standard Speciﬁcation for Headgear Used in Soccer, West Conshohocken, PA, ASTM International. aubry m, cantu r, dvorak j, graf-baumann t, johnston k, kelly j, lovell m, mccrory p, meeuwisse w and schamasch p (2002), ‘Summary and agreement statement of the ﬁrst International Conference on Concussion in Sport, Vienna 2001,’ Brit J Sport Med, 36, 6–10. barnes b c, cooper l, kirkendall d t and mcdermott p t (1998), ‘Concussion history in elite male and female soccer players,’ Amer J Sport Med, 26, 433–8. barth j, freeman j, broshek d, and varney r (2001), ‘Acceleration–deceleration sports related concussion: The gravity of it all,’ J Athl Training, 36, 253–6. bauer j, thomas t, cauraugh j, kaminski t and hass c (2001), ‘Impact forces and neck muscle ativity in heading by collegiate female soccer players,’ J Sport Sci, 19, 171–9. bayer (1995), Injection Molding Guide, Bayer Corporation Product Literature. bazarian j j, mcclung j, shah m n, cheng y t, ﬂesher w and kraus j (2005), ‘Mild traumatic brain injury in the United States,’ 1998–2000, Brain Injury, 19, 85–91. biasca n, wirth s and tegner y (2002), ‘The avoidability of head and neck injuries in ice hockey: an historical review,’ Brit J Sport Med, 36, 410–27. bird y, waller a, marshall s, alsop j, chalmers d and gerrard d (1998), ‘The New Zealand Rugby Injury and Performance Project, V: epidemiology of a season of rugby injury,’ Brit J Sport Med, 32, 319–25. boden b p, kirkendall d t and garrett w e (1998), Concussion incidence in elite college soccer players, Amer J Sports Med, 26, 238–41.

© 2007, Woodhead Publishing Limited

124

Materials in sports equipment

bray c (2004), 1982–2001 NCAA Sports Sponsorship and Participation Report, National Collegiate Athletic Associations, http://www.ncaa.org/library/research/ participation_rates/1982–2001/index.html, last accessed March 17 2007. broglio s p, j u y-y, broglio m d and sell t c l (2003), The efﬁcacy of soccer headgear, J Athl Train, 38, 220–24. caputo p and mattson d j (2005), Recreational ice hockey injuries in adult nonchecking leagues: a United States perspective, J Sport Sci Med, 4, 58–65. caswell s v and deivert r g (2002), Lacrosse helmet designs and the effects of impact forces, J Athl Training, 37, 164–71. consumersearch (2006), Bicycle helmets consumer report, http://www. consumersearch.com/www/sports_and_leisure/bicycle-helmet-reviews/fullstory. html, last accessed March 17 2007. covassin t, swanik c b and sachs m l (2003), Epidemiological considerations of concussion among intercollegiate athletes, Appl Neurropsychol, 10, 12–22. delaney j s (2004), Head injuries presenting to emergency departments in the United States from 1990 to 1999 for ice hockey, soccer, and football, Clin J Sport Med, 14, 80–7. delaney j s, puni v and rouah f (2006), Mechanisms of injury for concussions in university football, ice hockey, and soccer – a pilot study, Clin J Sport Med, 16, 162–5. ﬁnch c, mcintosh a and mccrory p (2001), What do under 15 year old schoolboy rugby union players think about protective headgear? Brit J Sport Med, 35, 89–94. ﬂik k, lyman s and marx r g (2005), American collegiate men’s ice hockey – an analysis of injuries, Amer J Sport Med, 33, 183–7. gerrard d, waller a and bird y (1994), The New Zealand rugby injury and performance project, II: previous injury experience of a rugby-playing cohort, Brit J Sport Med, 28, 229–33. gordon k e, dooley j m and wood e p (2006), Descriptive epidemiology of concussion, Pediatr Neurol, 34, 376–8. guskiewicz k m, marshall s w, broglio s p, cantu r c and kirkendall d t (2002), No evidence of impaired neurocognitive performance in collegiate soccer players, Amer J Sport Med, 30, 157–62. guskiewicz k, bruce s, cantu r, ferrara m, kelly j, mccrea m, putukian m and valovich mcleod t (2004), National Athletic Trainer’s Association Position Staqtement: Management of Sport-Related Concussion, J Athl Training, 39, 280–97. hinton r y, lincoln a e, almquist j l, douguih w a and sharma k m (2005), Epidemiology of lacrosse injuries in high school-aged girls and boys: a 3-year prospective study, Amer J Sport Med, 33, 1305–14. hoshizaki b t and brien s e (2004), The science and design of head protection. Neurosurgery, 55, 956–65. inberg j p f and gaymans r j (2002), Polycarbonate and co-continuous polycarbonate/ABS blends: inﬂuence of notch radius, Polymer, 43, 4197–205. iso (2003), ISO 10256: 2003, Head and face protection for use in ice hockey, Geneva, International Standards Organization. jordan s e, green g a, galanty h l, mandelbaum b r and jabour b a (1996), Acute and chronic brain injury in United States National Team Soccer players, Amer J Sport Med, 24, 205–10.

© 2007, Woodhead Publishing Limited

Protective helmets in sports

125

kirkendall d, jordan s and garrett w (2001), Heading and head injuries in soccer, Amer J Sport Med, 31, 369–86. knouse c l, gould t e, caswell s v and deivert r g (2003), Efﬁcacy of rugby headgear in attenuating repetitive linear impact forces, J Athl Training, 330–35. landro l, sala g and olivieri d (2002), Deformation mechanisms and energy absorption of polystyrene foams for protective helmet, Polym Test, 21, 217–28. lynch j and bauer j (1996), Acute head and neck injuries, in Garrett W, Kirkdall D and Contigulia S (eds), The U.S. Soccer Sports Medicine Handbook, Baltimore, MD, Williams and Wilkins, 81–5. marshall s and spencer r (2001), Concussion in rugby: the hidden epidemic, J Athl Training, 36, 334–8. matser j t, kessels a g, lezak m d, jordan b d and troost j (1999), Neuropsychological impairment in amateur soccer players, JAMA, 282, 971–3. mccrory p r (2003), Brain injury and heading in soccer, Brit Med J, 327, 351–2. mccrory p, johnston k, meeuwisse w, aubry m, cantu r, dvorak j, graf-baumann t, kelly j, lovell m and schamasch p (2005), Summary and agreement statement of the 2nd International Conference on Concussion in Sport, Prague 2004, Clin J Sport Med, 15, 48–55. mcintosh a s (2005), Rugby injuries, Med Sport Sci, 49, 120–39. mcintosh, a s and mccrory p (2000), Impact energy attenuation on performance of football headgear, Brit J Sport Med, 34, 337–41. mcintosh a s and mccrory p (2001), Effectiveness of headgear in a pilot study of under 15 rugby union football, Brit J Sport Med, 35, 167–9. mcintosh a, mccrory p and comerford j (2000), The dynamics of concussive head impacts in rugby and Australian rules football, Med Sci Sport Exer, 32, 1980–84. ncaa (2004), National Collegiate Athletic Association (NCAA) Injury Surveillance Report, Indianapolis, IN, National Collegiate Athletic Association. nfhs (2004), NFHS 2003-04 high school athletics participation survey, Indianapolis, IN, National Federation of State High School Associations. naunheim r s m d, standeven j p, richter c m d and lewis l m m d (2000), Comparison of impact data in hockey, football, and soccer, J Trauma, 48, 938–41. naunheim r s, ryden a, standeven j, genin g, lewis l, thompson p and bayly p (2003), Does soccer headgear attenuate the impact when heading a soccer ball? Acad Emerg Med, 10, 85–90. ncipc (2003) Report to Congress on Mild Traumatic Brain Injury in the United States: Steps to Prevent a Serious Public Health Problem, Atlanta, GA, Centers for Disease Control and Prevention. parkinson g and hike k (2003), Bicycle helmet assessment during well visits reveals severe shortcomings in condition and ﬁt, Pediatrics, 112, 320–23. pellman e i, viano d c, tucker a m and casson i r (2003a), Concussion in professional football: location and direction of helmet impacts – part 2, Neurosurgery, 53, 1328–41. pellman e j, viano d c, tucker a m, casson i r, waeckerle j f, maroon j c, lovell m r, collins m w, kelly d f, valadka a b, cantu r c, bailes j e and levy m l (2003b), Concussion in professional football: reconstruction of game impacts and injuries. Neurosurgery, 53, 799–814.

© 2007, Woodhead Publishing Limited

126

Materials in sports equipment

rivara f p, astley s j, clarren s k, thompson d c and thompson r s (1999), Fit of bicycle safety helmets and risk of head injuries in children, Inj Prev, 5, 194–7. sortland o and tysvaer a (1989), Brain damage in former association footall players: an evaluation by cerebral computed tomography, Neuroradiology, 31, 44–8. tagliaferri f, compagnone c, korsic m, servadei f and kraus j (2006), A systematic review of brain injury epidemiology in Europe, Acta Neurochir, 148, 255–68. tsyvaer a and lochen e (1991), Soccer injuries to the brain. A neuropsychologic study of former soccer players, Amer J Sport Med, 19, 56–60. tysvaer a t and storli o v (1989), Soccer injuries to the brain. A nerurologic and electroencephalographic study of active football players, Amer J Sport Med, 17, 573–8. williamson i j s and goodman d (2006), Converging evidence for the underreporting of concussions in youth ice hockey, Brit J Sport Med, 40, 128–32. wilson b (1998), Protective headgear in rugby union, Sports Med, 25, 333–7. withnall c, shewchenko n, wonnacott m and dvorak j (2005), Effectiveness of headgear in football, Brit J Sport Med, 39, i40–i48. witol a d and webbe f m (2003), Soccer heading frequency predicts neuropsychological deﬁcits, Arch Clin Neuropsych, 18, 397–417.

© 2007, Woodhead Publishing Limited

5 Mouth protection in sports T. E. G O U L D, S. G. P I L A N D, C. E. H OY L E and S. N A Z A R E N K O, University of Southern Mississippi, USA

5.1

Introduction

Sports participation across the globe is at an all-time high with estimates in the USA approaching 7 million high school (NFHS, 2004) and one-half million college level (Vicente, 2006) participants involved with organized sport. An increase in athlete participation creates a concomitant increase in athlete exposure to risk of injury (including orofacial injuries). Potential orofacial maladies include fractures to bones of the face and jaw. Likewise, teeth can be chipped, fractured, dislocated, concussed, and/or avulsed. The direct mechanisms that cause these injuries may also be associated with brain concussion or sports-related mild traumatic brain injury (MTBI) (Miller and Truhe, 1991; Wojtys et al., 1999; Aubry et al., 2002; Guskiewicz et al., 2004). It has been suggested, although not scientiﬁcally proven, that impact forces to the jaw may be transmitted to the base of the brain via the mandibular condyles resulting in concussive insult (Miller and Truhe, 1991; McCrory, 2001). Regardless of mechanism, dental trauma is a serious and global public health problem that mostly affects the youth and participants in organized sport. Additionally, dental trauma carries with it both signiﬁcant short-term and long-term injury sequelae (e.g., root resorption). In the USA, estimates of dental rehabilitation costs as a result of sport-related orofacial injury have been estimated at $15 000 per tooth over the span of a lifetime for athletes who lose a tooth (Gutman and Gutman, 1995; Woodmansey, 1997). These types of injuries carry with them physical, emotional, and ﬁnancial costs. The primary objective of this chapter is to provide a broad overview of mouth protection in sport, their various compositions, and manufacturing processes. The most frequent approach to the investigation of protective mouthpieces has been independent approaches from either the human performance or materials science disciplines. In this chapter, the authors attempt to integrate the areas of biomechanics, kinesiology, sports medicine, and materials science in the effort to discuss the interface that occurs when materials (e.g., mouthguards) are used to prevent injury in humans. It is hoped that, through a more comprehensive multidisciplinary 127

© 2007, Woodhead Publishing Limited

128

Materials in sports equipment

understanding of the human–material interface, the physical, emotional, and ﬁnancial costs associated with dental trauma may be abated.

5.2

The development and classiﬁcation of mouth protection in sport

Sport-speciﬁc intra-oral dental appliances were originated in the late 1800s as a way to reduce the prevalence of lip lacerations in boxing. These appliances have been referred to as: shields, gumshields, defensors, protectors, shields, guards, and most currently as mouthguards. However, the originators of these devices used the term ‘mouthpiece’ to describe their appliances. It appears that the mouthpiece has independent origins in both the USA and the UK. In the UK, Woolf Krause, has been credited with placing gutta percha strips over the upper incisors of boxers as early as 1892. His son, Phillip F. Krause, has been credited with making the protective ‘gumshields’ for British boxer Ted ‘Kid’ Lewis. Lewis used his ‘gumshield’ in his famous series of ﬁghts with US boxer Jack Britton in the early 1900s. Additionally, Lewis is credited as the ﬁrst professional boxer to use a mouthpiece (Reed, 1994). In the USA, the ﬁrst protective mouthpiece has been credited to Thomas A. Carlos (a Chicago dentist) in 1916, followed closely by E. Allen Frankel (a Chicago dentist and boxing judge) in 1919 (Carlos TA, 1938; Reed, 1994). For a more thorough review of the origin and history of the mouthpiece, please see a review by Reed (1994). As the movement of sport grew and diverse opportunities for activities arose, clinicians became aware of the need for appliances that served to provide more than just simple protection against soft tissue facial injuries. Various materials, beginning with gutta percha, were introduced into the formation of these devices (Reed, 1994) and evolved over the decades into the use of multiple types of polymers. Examples of polymers that have been used in the construction of mouthguards include (most frequent to least frequent): polyethylene–polyvinylacetate copolymers (EVA), acrylics (methyl-methacrylates), silicone rubbers, polyvinylchlorides (PVC), natural rubber, and polyurethane (PE) elastomers (Going et al., 1974; Tran et al., 2001). Typical options available to today’s athletes revolve around similar materials but different, cost driven, applications. These applications include the three basic forms of appliances: stock, boil and bite, and custom. As with all forms of protective devices, bias began to form by manufacturers and clinicians on the actual beneﬁts and properties of used materials and mouthguard products; thus scientiﬁc investigations into these issues began to surface in literature in the 1960s and are ongoing. Indeed, the use of mouthguards has been mandated in high-risk sports, such as football, since the early 1960s (Miller and Truhe, 1991). However, the past several decades have failed to produce industry standards related to design, type of materi-

© 2007, Woodhead Publishing Limited

Mouth protection in sports

129

als used, and methods for application. Hence, a paucity of evidence-based research exists to support the efﬁcacy of the modern athletic mouthguard or mouthpiece.

5.2.1 Classiﬁcation of mouthguards There are two major classiﬁcation systems for mouthguards. In the USA, the classiﬁcation system is given in the American Society for Testing and Materials (ASTM) International Designation: F 697-00 (2006) titled ‘Standard Practice for Care and Use of Athletic Mouth Protectors’. In Australia, the classiﬁcation system is given in the Standards Australia International (SAI) document: HB 209–2003 titled ‘Guidelines for the Fabrication, Use and Maintenance of Sports Mouthguards’. See Table 5.1 for a comparison of the ASTM and SAI mouthguard classiﬁcation systems. Type III and stock appliances require no specialized ﬁtting by the user. They are considered ‘off-the-shelf’ appliances that are simple rigid trays offering the most basic form of cost-effective protection. Type I (class 2) and mouth-formed appliances are generally referred to as ‘boil-and-bite’ mouthguards. These appliances are advances upon the stock in that they require some ﬁtting to the user by heating and immediately wearing, which provides a small level of customization to the teeth and gums. Type I (class 1) and custom-made mouthguards (as well as laminated mouthguards) are the most highly recommended appliances by clinicians. These types of custom mouthguards require ﬁtting by a trained sports dentist. Moulds of the mouth are cast and EVA materials, or laminated combinations of EVA, are thermoformed (via a vacuum or air pressure machine) over the mould to produce a congruently ﬁtted mouthguard (ASTM F 697-00, 2006a; SAI

Table 5.1 Comparison of ASTM and SAI mouthguard classiﬁcation systems ASTM classiﬁcation system1

SAI classiﬁcation system2

Type I – Thermoplastic type Class 1 – Vacuum-formed

Custom-made Single layer vacuum-formed Single layer pressure-formed Laminated–air pressure Bimaxillary Mouth-formed

Class 2 – Mouth-formed Type II – Thermosetting type Class 1 – Mouth-formed Type III – Stock type 1 2

Last updated in 2006. Last updated in 2003.

© 2007, Woodhead Publishing Limited

Stock

130

Materials in sports equipment

HB 209–2003, 2003). Despite recommendations by sports dentists and other sports medicine professionals, stock (Type III) and mouth-formed mouthguards (Type 1, class 2) are the most commonly used appliances, probably due to their low expense and availability. Thermosetting type mouthguards are relatively new to the sports dental appliance culture and are still gaining in popularity. See Fig. 5.1 for pictures of the various mouthguard types.

(a)

(b)

(c)

(d)

(e)

5.1 Pictures of various mouthguard types: (a) Type III or stock, (b) Type I, class 2 or mouth-formed, (c) Type I, class 1 or vacuumformed, (d) Type II or thermosetting type, (e) Bimaxillary.

© 2007, Woodhead Publishing Limited

Mouth protection in sports

5.3

131

Incidence of orofacial injury in sport

It has become a common understanding that participation in physical activities for sport or recreation places an individual at greater risk for physical injury. In the USA, 50 million Americans were reported to have experienced a medically treated injury during the year 2000 (Corso et al., 2006). These injuries are associated with lifetime costs greater than 400 billion US dollars. Although these statistics represent total injuries reported during that time frame, a portion can be attributed to participation in sport or recreational activities. Woodmansey reported that 39% of all dental injuries in the USA occur in sport-related accidents (Woodmansey, 1997). However, the speciﬁcs related to the true incidence of orofacial injury are difﬁcult to establish due to the paucity of overall studies and the typical anecdotal and retrospective nature of the available work. This lack of good scientiﬁcally based information is consistent across continents and was highlighted by Bastone, Freer, and McNamara in an epidemiological study of dental trauma in Australia (Bastone et al., 2000). Also, ﬁndings and conclusions from completed studies are commonly limited by inconsistent deﬁnitions of particular injuries or speciﬁcity of subgroups evaluated. This makes it difﬁcult to generalize across a variety of populations. In a 10-year study of cranio-maxillofacial trauma comprising 9543 individual cases, over 30% were shown to be the direct result of organized sporting activities. This particular study grouped ‘play’, which would include unorganized sporting activities, in a separate category collapsed with injuries resulting from ‘activities of daily living’; thus the associated injury rate could actually be higher (Gassner et al., 2003). When viewed through the retrospective lens, studies have reported orofacial injury incidence rates from 11 to 60% of total incurred injuries. Though limited by recall and subjectivity biases, these estimates of injury occurrence are directly related to the estimated cost of immediate and lifetime care. Additionally, lifetime dental rehabilitation costs for a sport-related avulsed tooth have been reported to extend as high as $15 000 US dollars per tooth (Gutman and Gutman, 1995; Woodmansey, 1997). National and international direct costs associated with general traumatic dental injury have been estimated (all in US dollars) at $870 million (US), $32 million (Ontario), $24 million (Denmark), $196 (UK), demonstrating a potential global strain on respective economic resources (Locker and Maggirias, 2004). A 15-year study at a high school in Hawaii on dental injury reported 19 492 total injuries with 56 (<1%) being dental-related (Beachy, 2004). Though lower than other retrospective studies indicate, the important issue is demonstrated in the fact that dental injuries occurred across a wide spectrum of sporting activities (e.g., American football, judo, wrestling, basketball, soccer, water polo, baseball, swimming, volleyball, and track and ﬁeld). This trend has also been identiﬁed internationally (see Table 5.2).

© 2007, Woodhead Publishing Limited

132

Materials in sports equipment

Table 5.2 International dental injury incidence (retrospective studies) Study

Country

Sports

Total participants

Total reported dental injury

Total %

Keçeci et al. (2005)

Turkey

Taekwondo Handball Volleyball

162

32

20

Lieger et al. (2006)

Switzerland

Handball Ice hockey Basketball Soccer

267

119

45

Levin et al. (2003)

Israel

Basketball Soccer Cycling Martial arts Swimming Roller blading Tennis

943

249

27

Ferrari et al. (2002)

Brazil

Martial arts Hockey Basketball Handball Soccer

1029

295

28.7

In 2002, an ecologic study evaluating injury rates in two contact sports (i.e., rugby and American football) demonstrated that rugby athletes were three times more likely to incur an orofacial injury compared to American football athletes during a competitive season (Marshall et al., 2002). Some sports, like American football, mandate a face shield. The application of this equipment may greatly reduce the prevalence of dental trauma. Evidence for variation of injury incidence among sports has been provided by two American studies (see Table 5.3) These longitudinal studies, the ﬁrst collecting data among high school aged sporting participants for 21 months and the second collecting data for a 15-year period from the same age group, provide more robust representations of true dental injury rates. However, cross-sectional study across international borders, across sporting applications and between athletes with and without mouthguard protection has been inadequate.

5.4

Biomechanics and dynamics of dental injury

Prior to undertaking a formal discussion regarding the chemistry, processing, and fabrication of contemporary mouthguard materials, it is important

© 2007, Woodhead Publishing Limited

Mouth protection in sports

133

Table 5.3 United States dental injury incidence (prospective longitudinal studies) Study

Country

Sports

Flanders et al. (1995) 21-month study

United States

Basketball Football

Beachy (2004) 15-year study

United States

Baseball Basketball (w) Basketball (m) Football Judo Soccer (w) Soccer (m) Softball Swimming Track and ﬁeld Volleyball Water polo Wrestling (w) Wrestling (m)

Total participants/ dental injury

Rate of injury exposure(*) or session(†)

120/41 820/1

18.3/10,000* 1.4/10,000*

1139/8 1174/2 1057/6 2438/7 103/1 1317/3 1420/13 864/3 797/1 2780/1 974/1 149/1 51/1 1141/8

0.079/1000† 0.02/1000† 0.064/1000† 0.029/1000† 0.189/1000† 0.031/1000† 0.127/1000† 0.049/1000† 0.016/1000† 0.004/1000† 0.014/1000† 0.019/1000† 0.243/1000† 0.083/1000†

to discuss the biomechanics, the viscoelastic nature, and the energy dissipation process of contemporary mouthguard materials. A sound understanding of these concepts is warranted in order for one to properly select a material platform for mouthguards.

5.4.1 Tooth biomechanics As previously discussed, mouthguards have traditionally been designed to keep the soft tissue of the lips and cheeks from coming into direct contact with the sharp edges of the teeth, thereby reducing the likelihood of soft tissue laceration and hematoma. Additionally, mouthguards are intended to stop violent contact of the upper and lower teeth by holding the jaws apart, thereby acting as a shock absorber. Primarily, mouthguards redistribute the traumatic forces applied at the point of contact to the frontal aspect of the teeth and spread them over a larger surface area. Clinically, this process has been termed ‘splinting’ of the teeth and is more comprehensively addressed in Fig. 5.2. Secondarily, the viscoelastic behavior (discussed in greater detail in Section 5.4.2) of the mouthguard material at the point of contact allows it to dissipate some of the impact energy at the molecular level and decrease the amount of momentum of motion transferred to the substrate tooth–bone complex (see Fig. 5.3).

© 2007, Woodhead Publishing Limited

External force

External force

External force

(a)

(b)

(c)

5.2 Splinting mechanism. (a) An external force is introduced to the unprotected (i.e., no mouthguard) anterior aspect of the dentition. (b) The external force focalizes at one point and transfers its momentum to the tooth–bone complex causing fracture or luxation. (c) With the mouthguard in place, the external force is diffused across the dentition essentially ‘splinting’ the teeth together.

A

A

B

5.3 Material absorption and energy dissipation. Left scenario. The external force (A) is introduced to the unprotected anterior aspect of the tooth resulting in a limited amount of energy dissipation. Right scenario. The external force (A) is directed at the protected anterior aspect of the tooth. The protective thickness of the mouthguard material (B) compresses, dissipates a certain amount of energy as heat, and transfers the remaining energy to the substrate tooth. The same mechanism occurs when there is a violent contact to the tip of the teeth resultant to a violent collision between the upper and lower dentitions.

© 2007, Woodhead Publishing Limited

Mouth protection in sports

135

5.4.2 Head biomechanics Not previously discussed, several author(s) have made claims that mouthguards can do more than just protect the teeth and oral soft tissue. One highly referenced article by (Stenger et al. 1964) claimed that mouthguard use could abolish Menière’s disease and decrease the incidence of nerve root compression, brachial plexapothies (i.e., burners), dizzy spells (vertigo), and repeated concussions (MTBI). This study has been criticized by at least one scholar (McCrory, 2001) noting that the study had low numbers (5–10 cases) that did not allow for statistical analysis. Additionally, the Stenger study lacks the ability to make meaningful interpretation because it was not a prospective randomized controlled clinical trial. However, the Stenger study was the ﬁrst to radiographically show that mouthguards altered mandibular position in situ by separating the mandibular condyles from their respective fossa (Stenger et al., 1964). Another author has claimed that mouthguards can attenuate the force transmitted to the cranial vault from a blow to the chin (Hickey et al., 1967). The researchers measured differences in intracranial pressure subsequent to a chin impact of cadaveric models both with and without mouthguards. This is a popular theory among the sports dentists who believe that a mouthguard can prevent incidence and severity of sport-related MTBI. A more comprehensive explanation of this unproven theory is illustrated in Fig. 5.4. However, at least one scholar (McCrory, 2001) has criticized this study, noting that the skull was ﬁxed in the experimental model and would not likely reproduce normal kinematic human motion. Additionally, McCrory noted that there are signiﬁcant differences in cadaveric and live human skulls, which could have accounted for the study’s outcomes (McCrory, 2001). Regardless, the Hickey (1967) article addressed sport-related MTBI as if it were a pure linear acceleration/deceleration focal injury where the measurement in changes to intracranial pressure would be warranted. Contemporary consensus in the literature is that sport-related MTBI is a diffuse brain injury that affects normal brain metabolism. Because MTBI is a diffuse injury, it can affect any number of the multiple brain centers. Therefore, not just a single measurement (i.e., intracranial pressure), but a multi-faceted (including neurocognitive, balance, and self-report symptoms) measurement approach must be taken. Another common anectdotal claim made regarding mouthguards is that they can decrease the amount of rotation (see Fig. 5.5) and transmitted forces to the cranial vault (see Fig. 5.6) that can occur subsequent to an impact event to the chin. Although there is some modicum of clinical evidence that mouthguards are effective at reducing the amount and severity of dental injury, there is no corpus of sound scientiﬁc literature to date that

© 2007, Woodhead Publishing Limited

D

B A

C

5.4 Mandibular distraction and recoil space. (A) No mouthguard is in place and there is normal occlusion of the dentitions. (B) No mouthguard is in place and the mandibular condyles are fully seated in their respective fossa during normal occlusion. (C) A properly ﬁtted mouthguard is in place and the upper and lower dentitions are separated by a thickness (3–4 mm) of the mouthguard material. (D) A properly ﬁtted mouthguard is in place and the mandibular condyles are distracted from their respective fossa, thereby creating what is commonly referred to as a ‘recoil’ space.

D

C

B

B

A

A

5.5 Schematic of decreased rotation. Left scenario. (A) An impulsive force is introduced to the chin. (B) With no mouthguard protection in place, the full momentum of motion is transferred from the chin into the cranium, thereby causing cranial rotation (C) to take place. Right scenario. (A) Same as previous scenario. (B) With mouthguard protection in place, the viscoelastic behavior of the material allows some of the energy to dissipate as heat. The remaining momentum is then transferred to the cranium with less resultant cranial rotation (D) taking place. © 2007, Woodhead Publishing Limited

Mouth protection in sports

Energy transferred to base of cranial vault C

Energy transferred to base of cranial vault B

A

137

A

5.6 Schematic of energy transmission to the cranial vault. Left scenario. (A) An impulsive force is delivered to the chin. (B) With no mouthguard protection in place, the full energy is transferred to the soft tissue structures at the base of the cranial vault. Right scenario. (A) Same as previous scenario. (C) With mouthguard protection in place, impact forces to the chin are attenuated by the viscoelastic properties of the mouthguard material.

demonstrates mouthguards as effective appliances for any type of cranial or MTBI injury including transfer of energy into the cranial vault (Labella et al., 2002; Wisniewski et al., 2004).

5.4.3 Behavior of viscoelastic materials Viscoelastic materials, as their name suggests, combine two different properties. The term ‘viscous’ implies that they deform slowly when exposed to an external force. The term ‘elastic’ implies that, once a deforming force has been removed, the material will return to its original conﬁguration. In contrast, pure viscous ﬂuids involve deformation followed by a permanent rearrangement of the ﬂuid molecules. The mechanical properties of materials are usually examined by means of stress/strain (or load/deformation) behavior. For purely elastic materials, loading and unloading ‘stress versus strain’ curves (lines) are superimposed. For viscoelastic ones they form a ‘hysteresis’ loop. The area within the loop represents the energy lost which dissipates as heat. This energy absorption behavior in part explains why viscoelastic materials are good shock absorbers. A further important property of viscoelastic materials is that their mechanical properties depend on the rate at which they are deformed. The stiffness of the material increases with the loading rate. Therefore, not one stress/strain curve but a whole family of curves represent the deformation behavior at different deformation rates. Viscoelastic behavior also depends on temperature because material stiffness changes with temperature. In selecting materials for clinical application, it is important to know what will be the corresponding

© 2007, Woodhead Publishing Limited

138

Materials in sports equipment

viscoelastic response at an appropriate loading rate and temperature. Viscoelastic response of material is typically analyzed in terms of complex modulus of elasticity: E = E′ + iE″

[5.1]

where real E′ (storage) and imaginary E″ (loss) components of complex modulus E, as well as their ratio, termed tan δ: tan d = E″/E′

[5.2]

are experimentally determined via measuring the stress response to the applied oscillatory deformation. Naturally, E′, E″, and tan δ are all the function of temperature (T) and oscillation frequency (ω). The amount of energy (Ψ) dissipated per unit volume during the period of oscillation is proportional to the loss modulus (E″), Ψ = p · (e0)2 · E″

[5.3]

where e0 is the amplitude of oscillatory deformation. With regards to the amplitude of E′ and E″, at a given oscillation frequency w, E′ rapidly decreases with temperature in the range corresponding to the glass transition (Tg) reaching the so-called rubber elasticity plateau well above Tg. Additionally, E″ reaches the maximum at Tg and then decreases with temperature as well, but it does so more slowly than E′.

5.4.4 Energy dissipation in viscoelastic materials From a clinical standpoint, one of the most important properties of viscoelastic materials is their ability to reduce the magnitude of impact forces. This is achieved in two different ways: by extending the time course of the impact event and by absorbing the energy. The amount of absorbed energy controls the momentum of motion (∆P) which is transmitted from the impacting projectile to the jaw. In turn, ∆P can be expressed as the integral of the transmitted force over the impact time represented in the equation: τ

∆p = ∫ F (t )⋅ dt

[5.4]

0

A schematic of a typical impact curve (F(t)) is shown in Fig. 5.7. The typical impact time associated with orofacial injury varies from about 10 to 100 msec. When the area under the F(t) impact curve is smaller and the impact time (t) is longer, the amplitude of the transmitted force will typically be smaller. Assuming small deformation behavior associated with the compression mode of loading the mouthguard, both the energy absorbed and the length of the impact must be effectively controlled by the visco-

© 2007, Woodhead Publishing Limited

139

Impact force

Mouth protection in sports

Impact time

5.7 Typical impact curve.

elastic properties of the elastomeric material used. As the value of E″ increases so too does the amount of energy absorbed. Likewise, the smaller that E′ is, the longer the impact time will be. However, if the mouthguard is decelerated over a longer period of time, it must necessarily travel further. Thus, reducing the peak force in this way requires the presence of an inﬁnite thickness of compressible material. Where the thickness of material is inadequate, the phenomenon of ‘bottoming-out’ occurs. This results in the stiffness of the material increasing abruptly once it has been compressed through most of its thickness. For the best oral comfort, the thickness of the mouthguard has typically ranged from 3–4 mm. Further, the mouthguard must have a ‘comfortable’ thickness, while limiting the maximum amount of force which can be transmitted. These constraints deﬁne the upper limits of the rigidity for the elastomeric material that will be selected.

5.4.5 Desired dynamic properties of materials The primary performance parameter associated with mouthguard performance is the absorption and dissipation of energy by the polymer material. Other than performance parameters, the service life of the material and its wearer comfort are also important. These properties can be inﬂuenced by the exact selection of material thickness and type. Presently there is no consensus among researchers as to the beneﬁts or detriments of each type and thickness of material in mouthguard design. Therefore, it is difﬁcult to apply a given set of criteria in developing new polymer materials which overcome deﬁciencies which exist currently. Particularly noteworthy is the

© 2007, Woodhead Publishing Limited

140

Materials in sports equipment

fact that, in no case in the literature, have any investigations been conducted which attempt to correlate the mechanical and physical engineering properties of polymer mouthguard materials at approximate core body temperature. In order to exhibit optimum shock absorbance, viscoelastic properties of elastomeric materials used for mouthguard applications should be ‘tuned’ in such a way that at rest, while in the mouth at about 37 °C, they have to resemble the properties of E″ and E′ corresponding to the rubber–elasticity plateau. Additionally, when subject to impact (the corresponding oscillation frequency of the typical impact is in the range of 50–150 Hz), the material must be in close proximity to the resonance conditions at Tg from the rubbery (high-temperature) side of the frequency/temperature dependence. Optimization of this sort has never been made with regards to elastomeric materials currently used for mouthguard applications.

5.5

Polymeric materials and fabrication techniques for mouthguards

While it is essential to design the macroscopic conformal structure of mouthguards, it is equally important to consider the basic chemical structures of the polymer materials used in their construction. Indeed, mouthguards have been manufactured from a wide variety of elastomeric or viscoelastic materials (see Section 5.2 for examples). It should be noted that PVC, which in the past has been used extensively to fabricate mouthguards, is currently banned for use in Europe, likely due to its potential toxicity under the performance requirements of mouthguards (Patrick et al., 2005). Various plasticizers and ﬁllers have also been used with these polymers (Westerman et al., 2002a, b, c) in attempts to ﬁnd a combination which modiﬁes the energy absorbing properties of the base polymer systems and subsequently offers better protection in an actual in vivo environment. However, this type of approach is less than desirable since plasticizers can leach out from the material, thereby potentially increasing the risk for health considerations. Therefore, it is essential to comprehensively discuss the chemical, mechanical, and physical attributes associated with contemporary polymer materials.

5.5.1 Chemical properties There are two prominent energy absorbing materials that are particularly useful for mouthguard fabrication: polyurethanes and a particular vinyl copolymer (ethylene plus vinyl acetate – denoted EVA) that is by far the major material found in virtually almost all mouthguards in current use. Both classes of polymers, polyurethanes and EVA copolymers, are used in wide variety of materials. For example, EVA copolymers (see general

© 2007, Woodhead Publishing Limited

Mouth protection in sports

141

chemical structure in Fig. 5.8) are used as adhesives, ﬁlms/bags for food packaging, plastic hoses, and functional rubber-type materials. Polyurethane elastomers (see general chemical structure in Fig. 5.9) can be used as shoe insoles, helmet liners, automobile dashboard liners, acoustic dampening, seat pads, carpet padding, packaging materials, and vibration dampening components (Hiles, 1989). Strictly from a performance standpoint, polyurethanes are quite suitable for mouthguard applications since they readily conform to a variety of substrates (teeth) and have a distinctive ability to absorb large impact energies efﬁciently. In practice, however, they are not used to the extent of other mouthguard materials such as EVA, most likely because they are not easily and economically fabricated in a clinical setting (due to more stringent thermoforming requirements). Additionally, polyurethanes cannot be fabricated from the isocyanate monomers in clinical facilities or in the ﬁeld due to serious toxicity/safety issues. Due to their availability, easy processability, and adequate mechanical properties, EVA copolymers are the most widely used material for mouthguard fabrication. However, in considering the exact combination of ethylene and vinyl acetate repeat units in the chain, it is important to consider the effect of vinyl acetate concentration upon the extent of crystallinity and

CH2

CH2

CH2

n

CH

m

O C

O

CH3

5.8 General chemical structure of EVA copolymer.

HO

OH

+

Polyol

HO

CH2 OH X

OCN

OC

Isocyanate

O

O N H

CH2

N CO H

NCO

CH2

Diol chain extender

O O

+

CH2

OC X

O N

CH2

H

Polyurethane

5.9 General chemical structure of polyurethane elastomers.

© 2007, Woodhead Publishing Limited

NC H

142

Materials in sports equipment

the melting point of the crystallites. Basically, as the concentration of vinyl acetate in the copolymer increases, the size of the crystallites and concomitantly the crystalline melting point are reduced. It is obvious that the best combination between a purely amporphous structure, as would be found for pure polyvinylacetate, and a highly crystalline structure with large crystals and high melting point, as would be found for pure polyethylene, involves a critical combination of ethylene and vinyl acetate. Most functional EVA copolymers have about 10–60% vinyl acetate. The small crystallites basically behave as physical cross-links at temperatures below the crystalline melting temperature, which generally ranges from 60 to ∼100 °C depending upon the exact content concentrations of each component as well as the polymer molecular weight. It is noted that the material clarity, ﬂexibility, toughness, and solvent solubility all increase with increasing vinyl acetate content. Most commercially available mouthguards are made of ∼28% VA units (Tran et al., 2001), where the copolymer is in the rubbery state at room temperature and the crystalline melting point is near 70 °C.

5.5.2 Mechanical and physical properties The utility of EVA copolymers is a direct result of their two-part morphology near room temperature consisting of crystalline regions, called crystallites and formed from ethylene unit crystallization, and amorphous regions comprising both polymerized vinyl acetate units and uncrystallized ethylene units (see Fig. 5.10) This distinctive two-phase morphology, where the crystallites serve as junction points for the uncrystallized elastomeric chains, provides a material that is elastomeric, ﬂexible, clear, and tough (i.e., it is

Crystallite Polymer chain

5.10 Pictorial of EVA molecular structure.

© 2007, Woodhead Publishing Limited

Mouth protection in sports

143

not prone to fracture upon impact, compression, mechanical deformation, or stretching). The two-phase morphology essentially deﬁnes EVA copolymers as true viscoelastic materials capable of energy storage, release, and localized viscous movement simultaneously when in the rubbery state. At low temperatures where EVA is below its glass transition temperature, the EVA exhibits glass-like characteristics (i.e., it is hard, rigid and not easily deformed). For pure polyethylene as well as for virtually all commercial EVA copolymers, the transition from a rubbery to a glassy material (the glass transition), which is relatively independent of the concentration of vinyl acetate in the EVA sample, is below 0 °C. Thus, in mouthguard applications, EVA clearly exists as a rubbery– viscous material that is capable of both storing/returning and dissipating impact energy at, or slightly above, room temperature. The rubbery material, which exists at a temperature between the glass transition temperature and the crystalline melting point, is ﬂexible and pliable. It is also capable of retaining its shape upon deformation either by mastication (chewing) or upon intercepting a sudden impact (to the mouth). The energy of an impact will be distributed in two ways. First, the material can store and return the energy as a rubber by a reversible deformation process due to a chain extension/reversion that is entropically driven. Second, the material can dissipate energy in a viscous process by undergoing an irreversible molecular motion. The partition between these two processes determines the response of the EVA copolymer to impact. While EVA copolymers are elastic, they are also relatively hard and approved for use in food contact, which is conducive to their use in mouthguard applications.

5.5.3 Fabrication techniques Since a copolymer between ethylene and vinyl acetate (EVA) is by far the most commonly used mouthguard material worldwide, its chemistry in relationship to processing, macromolecular structure, and performance parameters is discussed in greater detail. As already discussed, at a given temperature, an EVA copolymer can exist as a glass, rubber, or viscous melt. At room temperature, EVA copolymers are rubbery materials that can be stretched to several hundred percent without failure. In order to process EVA copolymers and thermoform them to the contour of teeth, it is essential to heat the stock mouthguard material above its crystalline melting temperature, which for all EVA materials is less than 100 °C and typically around 65–70 °C. The exact melting point temperature is determined by the concentration of vinyl acetate in the copolymer, i.e., the higher the vinyl acetate, the lower the crystalline melting point of the ethylene crystalline units. The actual crystalline melting point is dictated by a variety of factors including the size of the crystallites and their surface free-energy.

© 2007, Woodhead Publishing Limited

144

Materials in sports equipment

These in turn depend upon the ethylene–vinyl acetate compositional balance in the copolymer. Once the particular EVA copolymer sample is heated above the melting point, there are no longer crystalline regions and thus the barrier to melt ﬂow that involves the viscous ﬂow of linear EVA polymer units is removed. At a critical temperature above the crystalline melting point, the softening point at which temperature the EVA melt strength is severely reduced, EVA ﬂows in a time frame that allows it to create a conformal layer around a substrate which can either be teeth directly or a positive mold of the teeth. As the EVA temperature decreases, crystallites are again formed below the crystalline freezing point, thus locking in the conformal structure and returning the mouthguard to the rubbery state. A typical EVA copolymer used to fabricate mouthguard materials will have approximately 28% vinyl acetate (Tran et al., 2001). These EVA copolymers have a Tg below 0 °C, a melting temperature near 70 °C, and a melt ﬂow temperature a few degrees higher than 70 °C. The crystalline freezing point is typically near 40–50 °C. These temperatures (glass transition, crystalline melting point, softening temperature, crystalline freezing temperature) thus deﬁne processing temperatures and the temperature range for which the material will be useful. In considering the use of a particular polymer material for mouthguard fabrication, a wide variety of mechanical and physical properties must be optimized to ensure optimum performance in a real setting. The literature-based general consensus is that the properties listed in Table 5.4 Table 5.4 Literature-based consensus of principles and properties for mouthguard materials Principles

Properties

• • •

Shore A @ 37 °C Liner – 40 to 60 Shell – 55 to 85

• • • • • • •

Easy to manipulate Resistant to damage from hot water Sufﬁcient elastic modulus to reduce stress beneath material at point of impact Sufﬁcient rigidity to distribute forces over a larger surface area of the dentition Sufﬁcient toughness to resist cutting by biting Resistant to fracture under sudden impact Resistant to water absorption in order to prevent tainting by mouth ﬂuids Ability to withstand normal cleaning compounds Resistant to low pH Tasteless and odorless

© 2007, Woodhead Publishing Limited

Water absorption @ 37 °C <0.5 % Impact test @ 23 °C Not less than 70 % absorbed Tear strength @ 37 °C Not less than 200 N/cm

Mouth protection in sports

145

Table 5.5 Additional mouthguard material potential considerations Processability of material in a clinical or ﬁeld setting

Ability of material to conform to the tooth structure accurately

Optimum thickness needed for meeting performance standards

Ability to fabricate with pigments and determine mouthguard color

Tensile strength

Percent elongation at break

Cost

Wearer comfort and ‘feel’

Low toxicity via reduction of any additives or impurities such as plasticizers that may leach out

Product degradation products upon mastication or immersion in aqueous or saliva environment

must all be taken into account when choosing a particular mouthguard material (Greasley et al., 1998; Tran et al., 2001; Craig and Godwin, 2002; Westerman et al., 2002b; Patrick et al., 2005). In addition to these measurable mechanical and physical properties of a mouthguard material, there are additional considerations that are often as important as the above standard materials properties that should also be considered (see Table 5.5). The properties listed above must be considered in the context of how mouthguards are constructed and their ultimate performance characteristics.

5.6

Standards and testing for mouthguards

As previously discussed in Section 5.2.1, there are two major governing bodies that produce standards for the classiﬁcation, fabrication, use, and maintenance of mouthguards. In the USA, the standards system is given in the ASTM Designation F 697–00 (ASTM, 2006a). In Australia, the standards system is given in the SAI document HB 209–2003 (SAI, 2003). ASTM guidelines place several limitations on the use of mouthguards as protective intra-oral devices. The most noted limitation is that mouthguards should be properly ﬁtted by a sports dentist. Additionally, mouthguards should be cleaned daily in cold or lukewarm water, paying special attention to avoiding excessive heat or cold. Mouthguards should be replaced when splits appear or resiliency is lost. The Australian standard is more detailed including design speciﬁcations on labial, occlusal, and palatal surface thicknesses. The labial ﬂange should extend within 2 mm of the vestibular reﬂection while the palatal ﬂange

© 2007, Woodhead Publishing Limited

146

Materials in sports equipment

Table 5.6 Clinical fabrication speciﬁcations for athletic mouthguards Full enclosure of the maxillary teeth to the distal surface of the second molar for high contact sports

Full enclosure of the maxillary teeth to the distal surface of the ﬁrst molar for low contact sports

∼2–3 mm thick on labial aspect

∼3 mm on occlusal aspect

∼2 mm on the palatal aspect

Edge of labial ﬂange rounded

Edge of palatal ﬂange tapered

Adjusted occlusion for even contact

Palatal ﬂange should extend about 10 mm above gingival margin

Labial ﬂange should extend to within 2 mm of the vestibular reﬂection

should extend about 10 mm above the gingival margin. Additional clinical principles for the fabrication of mouthguard materials can be found in Table 5.6. The Australian guidelines are probably the most detailed at this time and have adopted literature-based recommendations for the material properties of hardness, water absorption, impact testing, and tear strength. (See the ‘Properties’ column of Table 5.4 for speciﬁc ranges of property values.) Therefore, it is important to discuss the standardized procedures involved in testing a material for suitability as a mouthguard material.

5.6.1 Testing for hardness The durometer hardness of mouthguard materials in the USA is measured according to ASTM guideline D2240-05 ‘Standard Test Method for Rubber Property – Durometer Hardness’. The hardness measuring device has a hardened steel Type A indentor that conforms to ASTM guidelines for shape (Tip = 0.031 ± 0.001 mm; Taper = 35° ± -41 °; Shaft = 1.40 ± 0.005 mm) and is attached via a spring mechanism to a scale reading from 0–100. Mouthguard materials are fabricated into test specimen sheets that are 6 mm in thickness (plied or solid) and at least 12 mm in length and width. Prior to testing, the durometer device is calibrated and the specimens are placed on a surface that is hard and as close to the horizontal level as possible. The indentor is introduced to each test specimen from the vertical position as quickly as possible without shock and held for one second. The maximum hardness reading is taken and recorded from ﬁve locations and the median hardness value reported. All hardness testing is generally made at approximately room temperature 23 ± 2 °C (73.4 ± 3.6°F) (ASTM, 2005a).

© 2007, Woodhead Publishing Limited

Mouth protection in sports

147

5.6.2 Testing for impact In the USA, the capacity of mouthguard materials to dissipate energy is typically measured using an impact test very similar to the ASTM guideline D6110-06 ‘Standard Test Methods for Determining the Charpy Impact Resistance of Notched Specimens of Plastics’, but modiﬁed as reported in the literature (Craig and Godwin, 2002). The impact measuring device is ﬁtted with a hardened steel Charpy style tip that conforms to ASTM guidelines for shape (Taper = 45 ± 2°; radius = 3.17 ± 0.12 mm) that is afﬁxed to the pendulum apparatus. The pendulum is ﬁtted with a weight set capable of producing a maximum potential energy of 11.3 joules at a 61 cm drop height. Prior to testing, the impact measuring device is calibrated and the test specimens are supported against two steel anvils and stacked to achieve the test dimensions. One modiﬁcation to the test is that the impact tup that is afﬁxed to the pendulum device will be raised to a height (61.28 mm), where 113 Ncm (1.1 joules) of force will be introduced to the test specimen at impact as compared to a large joule force in a typical ASTM D6110-02 test. Another modiﬁcation includes stacking 4 mm thick ﬁlms on a substrate material (i.e., usually steel) instead of using a notched plastic specimen. The impact hammer is released from test height ﬁve times and the energy absorbed by each new test specimen at initial impact will be measured with mean value reported. All impact tests are made at approximately room temperature 23 ± 2 °C (73.4 ± 3.6°F) (ASTM, 2006b). It is important to note that, at this time, the aforementioned modiﬁed impact test is the only performance standard that is directly associated with a material’s ability to prevent injury. However, it should likewise be noted that, in any collision event where one wants to minimize injury, the momentum of motion that is transferred (PT) to the impacted material needs to be controlled. Therefore, the best case scenario of transferred momentum of motion is PT = mv for a perfectly inelastic collision. Conversely, in a perfectly elastic collision, the PT will be PT = 2mv. Since the best case scenario will always only be PT = mv, it becomes important then to characterize the F(t) impact curve for reasons already described in Section 5.4.4 ‘Energy dissipation process’. From a fundamental physics viewpoint, the modiﬁed impact test does not fully characterize a material’s ability to control transferred force. Therefore, future experimentation in this area needs to focus on the type of instrumentation that will garner F(t) information such as is accomplished with accelerometers.

5.6.3 Testing for moisture absorption To test the rate at which water diffuses into the material, the water absorption rates of mouthguard materials are measured in the USA according to

© 2007, Woodhead Publishing Limited

148

Materials in sports equipment

ASTM guideline D570-98 (2005) ‘Standard Test Method for Water Absorption of Plastics’. The test specimens are fabricated into 50.8 mm (2 in.) diameter disks with a 3.2 mm (1/8 in.) thickness and weighed to the nearest 0.001 g. Test specimens can be machined, sawed, or sheared from the sample so as to have smooth edges free from cracks. The cut edges should be smoothed by ﬁnishing with No. 0 or ﬁner sandpaper or emery cloth. The specimens are conditioned by being placed in an oven at 50 °C ± 3 °C for 24 hours. The conditioned specimens are then completely immersed into distilled water (23 ± 1 °C) for two hours. At the end of two hours, all surface water is wiped off and the specimen weighed to the nearest 0.001 g. The sample is then immediately be replaced into the distilled water for the remaining 24 hour test period when another weighing takes place. Again, the specimen is replaced into the water and left for seven days. The percent increase in weight is calculated (% = wet weight − conditioned wet/ conditioned weight × 100) and reported (ASTM, 2005b).

5.6.4 Testing for tear strength To measure the force required to completely rupture across the width of the material, the tear strength of mouthguard materials is measured according to the ASTM guideline D-624-00e1 ‘Standard Test Method for Tear Strength of Conventional Vulcanized Rubber and Thermoplastic Elastomers’. The tear strength (Ts) of thermoplastic elastomers is expressed by the formula Ts = F/d where F = the median maximum force, N, of the die (type C in this case) and d = the thickness of the test specimen in meters. It is important to measure the resistance of a material to tearing because many conventional thermoplastic elastomers fail in service due to generation and/or propagation of tears. To determine the Ts, a tensile testing machine is used to place an uninterrupted, constant rate of tensile force on the test specimen. Three test specimens are prepared to conform to tear die shape C and the force placed on the test tabs pulls perpendicular to the ﬂow direction of the material. The median test value is reported from tests that are made at approximately room temperature 23 ± 2 °C (73.4 ± 3.6°F) (ASTM, 2000).

5.7

Comfort and ﬁt of mouthguards

The perceived comfort of the mouthguard by the wearer is absolutely critical and of paramount importance for compliant usage. Non-compliance

© 2007, Woodhead Publishing Limited

Mouth protection in sports

149

has also been shown to be related to bulkiness, stability, hardness, dyspnea, speaking difﬁculty, oral dryness, and nausea (McClelland et al., 1999; Brionnet et al., 2001, Westerman et al., 2002b). Being individually crafted by licensed dentists, custom-fabricated mouthguards can address some of the aforementioned issues when they are properly fabricated and adjusted. McClelland et al. (1999) noted that perceived comfortability ratings of a mouthguard increased when the palatal peripheries were tapered and the labial and buccal peripheries were rounded. By tapering of the palatal periphery, the lingual bulk can be controlled. Likewise, by shaping the margins, it is possible to affect comfort without any sacriﬁce to the protective ability of the device. A more detailed list of clinical mouthguard fabrication speciﬁcations is located in Table 5.6. Unfortunately, usage of these types of appliances is restricted by availability and expense. Most of the expense of custom-fabricated mouthguards is absorbed in the clinician’s fabrication time and not in material costs. Although some research has established that mouthguards protect the teeth and mandible from injury, there is still a dearth of research that supports best current practices regarding the fabrication process and materials involved. In the USA, the Academy for Sports Dentistry (ASD) created a position statement for a ‘Properly Fitted Mouthguard’ that was approved in December of 1998. The goal of the position statement was, and is, to deﬁne an athletic mouthguard, to discern the appropriate criteria for fabrication and adaptation of an athletic mouthguard, and to disclose the properties of the ﬁnished end-product. The ASD uses the same deﬁnition of an athletic mouth protector as deﬁned by ASTM F697-00 (ASTM, 2006a). Additionally, the ASD has mandated that the single word ‘mouthguard’ be replaced with the term ‘properly ﬁtted mouthguard’. The connotation carried with this mandate is that an improperly ﬁtted mouthguard (Types I and II) will compromise its own ability to protect the teeth and jaw structures. The ASD position statement further denotes the considerations that must be taken into account when a dentist is ﬁtting a mouthguard, which are consistent with criteria in Table 5.6. The ASD recommends that the ﬁtting of a mouthguard be performed only under the supervision or direction of a sports dentist so that the end-product will comply with established parameters.

5.8

Future trends

In recent years, there has been interest in developing new materials that can absorb impact energy efﬁciently, reduce the momentum of impact transferred to the substrate and, at the same time, be readily fabricated. A large variety of chemistries have emerged since the 1980s

© 2007, Woodhead Publishing Limited

150

Materials in sports equipment

that are cured by application of UV light. In a ﬁrst attempt at producing an easily fabricated mouthguard, a photocurable resin based upon a diacrylate urethane monomer was reported to be effective in fabricating a mouthguard using a light source (Ranalli and Guevara, 1992). While acrylates have traditionally been used in most industrial photocuring processes, it has been demonstrated recently that thiol–ene systems, which are based on chemistry that was developed in the 1980s but never exploited to its full extent except in the manufacture of optical connectors, cure rapidly in air to give thermoset polymer materials with properties unmatched by virtually any type of polymer system (Hoyle et al., 2004). Formulations based upon unique combinations of thiols, multifunctional enes, and acrylates have been shown to cure rapidly upon exposure to light to give conformal thermoset materials with impact properties that exceed those of traditional mouthguard materials (Hoyle et al., 2006). The future of mouthguards may well beneﬁt from this unique method of fabricating custom ﬁt materials. Since the 1990s, several different design approaches have been attempted to increase the mouthguard’s ability to attenuate impact forces. Westerman et al. (1997, 2002a, c) examined the effect of air inclusions in EVA, noting a signiﬁcant decrease in transmitted forces up to a 10 kN threshold. Bulsara and Matthew (1998) used a popular polyurethane material called Sorbothane® as an intermediate layer between two layers of EVA. They found that the Sorbothane layer may function as a force attenuating device. Patrick et al. (2005) hypothesized that harmful forces to the teeth could be reduced by a placing a highly compliant material between two semi-rigid layers of materials. In this study, four lamination variations were tested with different thickness and types of materials including EVA, polymethylmethacrylate (PMMA), and silicone rubber. The author(s) concluded that the heterogeneously laminated appliances deformed less when impacted thereby reducing harmful effects. Although these engineering improvements have contributed to mouthguards that demonstrate some level of increased protection, the physical design options for mouthguards appear more ﬁnite than the investigation of nearly an inﬁnite amount of materials options. Exploration of new material platforms for the athletic mouthguards is nearly non-existent. Thus, there remains a strong need for new material platform options that can provide better comfort, splinting, and force attenuation abilities when compared with currently used systems.

5.9

Sources of further information and advice

Sources of further information and advice are given in Table 5.7 starting on page 151.

© 2007, Woodhead Publishing Limited

Table 5.7 Further sources of information Dental trauma education, treatment, and research Description

American Dental Association

http://www.ada.org/

An association started in 1859 with a long history of commitment to the public’s oral health, ethics, science and professional advancement of dentistry.

British Dental Association

http://www.bda.org/

An organization that develops policies to represent dentists working in every occupational setting in the UK.

Canadian Dental Association

http://www.cda-adc.ca/

An organization that develops policies to represent dentists working in every occupational setting in Canada.

Academy of General Dentsitry

http://www.agd.org/

An association founded a forum for general dentists interested in exchanging ideas related to the dental needs of patients.

Academy for Sports Dentistry

http://www.sportsdentistry-asd.org/

An association founded in 1983 as a forum for dentists, physicians, athletic trainers, coaches, dental technicians, and educators interested in exchanging ideas related to the dental needs of athletes at risk to sports’ injuries.

American College of Sports Medicine

www.acsm.org/

The largest sports medicine and exercise science organization in the world committed to the diagnosis, treatment, and prevention of sports-related injuries and the advancement of the science of exercise.

National Athletic Trainers’ Association

www.nata.org/

An association dedicated to the enhancement of the quality of health care provided by certiﬁed athletic trainers to a physically active population.

© 2007, Woodhead Publishing Limited

151

Website

Mouth protection in sports

Agency

Table 5.7 (cont.)

152

Safety education organizations Website

Description

Consumer Product Safety Commission

www.cpsc.gov/

The U.S. Consumer Product Safety Commission is charged with protecting the public from unreasonable risks of serious injury or death from consumer products.

World Health Organization

www.who.org/

Promotes the use of safety equipment as a strategy for preventing head and jaw injuries caused by sports participation.

Mouthguard standard setting and research organizations Agency

Website

Description

ASTM International

www.astm.org

Formerly known as the American Society for Testing and Materials ASTM International is one of the largest voluntary standards development organizations in the world.

Standards Australia

www.standards.com.au/

A non-government standardization body in Australia.

British Standards Institution

www.bsi-global.com/index.xalter

The national Standards Body of the UK and develops standards and standardization solutions to meet the needs of business and society.

Canadian Standards Association

www.csa.ca/Default.asp?language=english

A non-proﬁt standardization body in Canada.

International Organization for Standardization

www.iso.org/iso/en/ISOOnline.frontpage

A non-proﬁt standardization body in Canada.

© 2007, Woodhead Publishing Limited

Materials in sports equipment

Agency

Mouthguard standard setting and research organizations Website

Description

Japanese Standards Association

www.jsa.or.jp/default_english.asp

The association provides education to the public regarding the standardization and uniﬁcation of industrial standards.

National Institute for Sports Science and Safety

www.nisss.org/

A non-proﬁt research and education institute for the prevention of sports injuries and related musculoskeletal research through the study of injury mechanisms and protective sports equipment.

Wayne State University (WSU) Bioengineering Center

www.bioengineeringcenter.org/home/labs/ sports/

Bioengineering Center, the Sport Injury Biomechanics Lab capable of evaluating all types of athletic personal protective equipment.

Agency

Website

Description

National Collegiate Athletics Association

http://www.ncaa.org/wps/portal

Purpose is to govern intercollegiate competition in a fair, safe, equitable and sportsmanlike manner.

National Federation of High Schools

http://www.nfhs.org/

Purpose is to build awareness and support while establishing consistent standards and rules for high school sports competition.

US Lacrosse

http://www.uslacrosse.org/safety/ mouthguards.phtml

A position statement on the proper use of mouthguards in the sport of lacrosse.

Sport governing organizations

Mouth protection in sports

Agency

153

© 2007, Woodhead Publishing Limited

154

Materials in sports equipment

5.10

Acknowledgements

The authors would like to thank Ms. Askim Senyart, doctoral student, and Huanyu Wei, PhD, post-doctoral student, from The University of Southern Mississippi School of Polymer Science for their laboratory contributions. Additionally, we would like to thank Dr. Dennis Ranalli, DDS, Assistant Dean, University of Pittsburgh Dental School for his collaborative efforts with our Sports and High Performance Materials program at Southern Miss.

5.11

References

astm d 624 (2000), Standard Test Method for Tear Strength of Conventional Vulcanized Rubber and Thermoplastic Elastomers, West Conshohocken, PA, ASTM International. astm d 2240 (2005a), Standard Test Method for Rubber Property – Durometer Hardness, West Conshohocken, PA, ASTM International. astm d 570 (2005b), Standard Test Method for Water Absorption of Plastics, West Conshohocken, PA, ASTM International. astm f 697-00 (2006a), Standard Practice for Care and Use of Athletic Mouth Protectors, West Conshohocken, PA, ASTM International. astm d 6110 (2006b), Standard Test Methods for Determining the Charpy Impact Resistance of Notched Specimens of Plastics, West Conshohocken, PA, ASTM International. aubry m, cantu r, dvorak j and et al. (2002), Summary and agreement statement of the 1st International Conference on Concussion in Sport, Vienna 2001, Clin J Sport Med, 12, 6–11. bastone e b, freer t j and mcnamara j r (2000), Epidemiology of dental trauma: a review of the literature, Aust Dent J, 45, 2–9. beachy g (2004), Dental injuries in intermediate and high school athletes: a 15 year study at Punahou School, J Athl Train, 39, 310–15. brionnet j m, roger-leroi v, tubert-jeannin s and garson a (2001), Rugby players’ satisfaction with custom-ﬁtted mouthguards made with different materials, Community Dent Oral Epidemiol, 29, 234–8. bulsara y r and matthew i r (1998), Endod Dent Traumatol, Forces transmitted through a laminated mouthguard material with a Sorbothane ins, 14, 45–7. carlos t a (1938), The mouthguard in use, Oral Hyg, 28, 1580–81. corso p, ﬁnkelstein e, miller t, ﬁebelkorn i and zaloshnja e (2006), Incidence and lifetime costs of injuries in the United States, Inj Prev, 12, 212–18. craig r g and godwin w c (2002), Properties of athletic mouth protectors and materials. J Oral Rehabil, 29, 146–50. ferrari c h and medeiros j m f (2002), Dental trauma and level of information: mouthguard use in different contact sports, Dent Traumatol, 18, 144–7. ﬂanders r a and bhat m (1995), The incidence of orofacial injuries in sports: a pilot study in illionis , J Amer Dent Assoc, 126 (April), 491–6. gassner r, tuli t, hachl o, rudisch a and ulmer h (2003) Cranio-maxillifacial trauma: a 10 year review of 9543 cases with 2 1067 injuries, J Craniomaxillofac Surg, 31, 51–61.

© 2007, Woodhead Publishing Limited

Mouth protection in sports

155

going r, loehman r and chan m s (1974), Mouthguard materials: their physical and mechanical properties, J Amer Dent Assoc, 89, 132–8. greasley a, imlach g and karet b (1998), Application of a standard test to the in vitro performance of mouthguards, Brit J Sport Med, 32, 17–19. guskiewicz k m, bruce s l, cantu r c, ferrara m s, kelly j p, mccream, putukian m and valovich mcleod t c (2004), National Athletic Trainers’ Association Position Statement: Management of Sport-Related Concussion, J Athl Train, 39, 280–97. gutman j l and gutman m s (1995), Cause, incidence, and prevention of trauma to teeth, Dent Clin North Amer, 39, 1–15. hickey j, morris a, carlson l and seward t e (1967), The relation of mouth protectors to cranial pressure and deformation, J Amer Dent Assoc, 74, 735–40. hiles m (1989), Energy absorbing polyurethane composite article, US patent 480849. hoyle c e, gould t e, piland s g, wei h and senyart a f (2006), RadTech Report, 20, 12–17. hoyle c e, lee t y and roper t (2004), Thiol-enes: chemistry of the past with promise for the future, J Poly Sci, 42, 5301–38. kececi a d, çetin c, eroglu e and batdar m l (2005), Do custom-made mouth guards have negative effects in aerobic performance capacity of athletes? Dent Traumatol, 21, 276–80. labella c r, smith b w and sigurdsson a (2002), Effect of mouthguards on dental injuries and concussions in college basketball, Med Sci Sport Exerc, 34, 41–4. levin l, friedlander l d and Geiger s b (2003), Dental and oral trauma and mouthguard use suring sport activities in Israel, Dent Traumatol, 19, 237–42. lieger o and von arx t (2006), Orofacial/cerebral injuries and the use of mouthguards by professional athletes in Switzerland, Dent Traumatol, 22, 1–6. locker d and maggirias j (2004), Costs of traumatic dental injury in Ontario, Health management and Epidemiology Report No. 19, Toronto, On, University of Toronto. marshall s w, waller a e, dick r w, pugh c b, loomis d p and chalmers d j (2002), An ecologic study of protective equipment and injury in two contact sports, Int J Epidemiol, 31, 587–92. mcclelland c, kinirons m and geary l (1999), A preliminary study of patient comfort associated with customized mouthguards, Brit J Sport Med, 33, 186–9. mccrory p (2001), Do mouthguards prevent concussion? Brit J Sport Med, 35, 81–2. miller m and truhe t e (1991), Mouthguard use should be encouraged for many sports, J Dent, 11, 21–2. nfhs (2004) NFHS 2003-04 high school athletics participation survey. Indianapolis, IN, National Federation of State High School Associations. patrick d g, van noort r and found m s (2005), Scale of protection and the various types of sports mouthguards, Brit J Sport Med, 39, 278–81. ranalli d n and guevara p a (1992), A new technique for the custom fabrication of mouthguards with photopolymerized urethane diacrylate, Quintessence Int, 23, 253–5. reed r v (1994), Origin and early history of the dental mouthpiece, Brit Dent J, 176, 478–80.

© 2007, Woodhead Publishing Limited

156

Materials in sports equipment

sai hb 209–2003 (2003) Guidelines for the fabrication, use and maintenance of sports mouthguards, Sydney, Standards Australia International. stenger j, lawson e, wright j and ricketts j (1964), Mouthguards: protection against shock to the head, neck and teeth, J Amer Dent Assoc, 69, 273–81. tran d, cooke m s and newsome p r h (2001), Laboratory evaluation of mouthguard material, Dent Traumatol, 17, 260–65. vicente r (2006) 1981–82 to 2004–05 NCAA Sports Sponsorship and Participation Rates Report, Indianapolis, IN, National Collegiate Athletic Associations. westerman b, stringfellow p m and eccleston j a (1997), An improved mouthguard material, Austral Dent J, 42, 189–91. westerman b, stringfellow p m and eccleston j a (2002a), Beneﬁcial effects of air inclusions on the performance of ethylene vinyl acetate (EVA) mouthguard material, Brit J Sport Med, 36, 51–3. westerman b, stringfellow p m and eccleston j a (2002b), EVA mouthguards: how thick should they be? Dent Traumatol, 18, 24–7. westerman b, stringfellow p m, eccleston j a and harbrow d j (2002c), Effect of ethylene vinyl acetate (EVA) closed cell foam on transmitted forces in mouthguard material, Brit J Sport Med, 36, 205–8. wisniewski j f, guskiewicz k, trope m and sigurdsson a (2004), Incidence of cerebral concussion associated with type of mouthguard used in college football, Dent Traumatol, 20, 143–9. wojtys e m, hovda d, landry g, boland a, lovell m, mccrea m and minkoyy j (1999), Concussion in sports, Amer J Sport Med, 27, 676–87. woodmansey k f (1997), Athletic mouthguards prevent orofacial injury, J Amer Coll Health, 45, 179–85.

© 2007, Woodhead Publishing Limited

Part II Speciﬁc sports

© 2007, Woodhead Publishing Limited

6 Design and materials in baseball J. S H E RW O O D and P. D R A N E, University of Massachusetts–Lowell, USA

6.1

Introduction

Baseball has long been considered America’s pastime and is played around the world. A sport that started as a children’s game has been around since the beginning of the nineteenth century. Some historians believe the origins of the game date back to the days of the pharaohs in Egypt. The basic equipment, i.e. the bat, the glove and the ball, has principally not changed since the early nineteenth century. However, the ‘tools of the game’ have evolved as players have recognized changes that could improve their performance for ﬁelding and batting and as engineers and scientists have developed an understanding of the physics of the game and introduced advances in material performance for the equipment. The ball began in sandlots as anything that could be hit by a stick and evolved into a wound ball. Baseballs used today are still wound in a very similar manner as they have been for the last century. The baseball bat started as a stick in the sandlot and has evolved from different solid woods to the current selection of ash and maple woods, ﬁber-reinforced composites, aluminum and combinations of two or more of the aforementioned material choices. The equipment used in the game has expanded beyond the bat, the ball and the glove to equipment that provides for the protection of the players. This chapter will discuss the current construction, design and understanding of the equipment used in the game of baseball along with a historical basis for how the equipment has evolved to its current state as well as potential future directions. This chapter describes the basic rules and history of baseball for those unfamiliar with the game, discusses the construction of the baseball, both wood and non-wood bats, and gloves, and then ﬁnishes with a cursory discussion on the protective equipment associated with the game and a commentary on future trends. 159

© 2007, Woodhead Publishing Limited

160

Materials in sports equipment

6.1.1 How baseball is played The object of the game of baseball is fairly simple. However, the ofﬁcial rule book is fairly extensive and addresses many different scenarios that can occur in a game. The basic play of baseball involves two teams of nine players. One team’s players are ‘in the ﬁeld’ at strategically distributed locations while a single player from the opposing team is ‘at bat.’ One player ‘in the ﬁeld’ ‘pitches,’ meaning throws, the ‘baseball,’ which is a ball with stitches in its leather cover having a diameter of about 2.9 inches, across ‘home plate’ to another player on the same team. Home plate is identiﬁed as an 18-inch wide location at a corner of the ﬁeld. The player ‘at bat’, better known as the batter, stands next to ‘home plate’ and tries to hit the ball with a ‘bat’, a round stick about 33 inches long and no more than 2 3/4 inches in diameter. If the batter hits the ball into the ﬁeld, then he/she tries to run around the ‘bases,’ three speciﬁc locations around the ﬁeld that form the four corners of a square with ‘home plate’ as the ﬁrst corner. The player running the bases is called ‘out’ if he/she is tagged by a ﬁelder holding the ball before getting to one of the bases. The player running may stop at any base and wait for another player to move him/her along safely to the next base. The team ‘at bat’ is trying to get as many ‘runs,’ a measure of how many players are able to run around all the bases and return to ‘home plate’ without being called ‘out.’ The teams switch places after the team ‘in the ﬁeld’ records three outs. An out can also be recorded if the batter gets three strikes. A strike occurs when the batter swings at the ball but either misses the ball completely or hits the ball into an out-of-play section in the ﬁeld or fails to swing at a pitch that was ‘right over the plate.’ Once both teams have played both ‘in the ﬁeld’ and have been ‘at bat,’ it is called an ‘inning.’ A game ends after nine ‘innings,’ and the winner is declared to be the team which has scored more ‘runs.’ This description covers some of the most basic attributes of the game. To understand more details of the game, both critical on a day-to-day basis as well as the more obscure rules, the reader should read the rule book for the particular league. Both Major League Baseball (MLB) and the National Collegiate Athletic Association (NCAA) have their rule books posted to their websites, www.mlb.com and www.ncaa.org, respectively.

6.1.2 The development of baseball Though baseball is credited by some as being invented in Cooperstown, New York, in 1839 by Abner Doubleday, the game resembles that of some previously existing children’s games in Europe. During the early 1840s, organized baseball teams developed in New York. With the formation of new teams, baseball became a gentleman’s game and proceeded to spread

© 2007, Woodhead Publishing Limited

Design and materials in baseball

161

ﬁrst through the Northeastern portion of America and then continued to expand west as modes of transportation evolved. The ﬁrst professional baseball team was formed in 1869 and was called the Cincinnati Red Stockings. College baseball began in 1879. During the twentieth century, baseball spread throughout all age groups and social classes making it America’s pastime. The baseball used in Major League Baseball has had essentially no changes in its design for over a century. The current baseball is constructed from a cork and rubber pill surrounded by three layers of wool and then by cotton thread coated in cement and ﬁnally wrapped in two leather pieces stitched together. The construction of youth baseballs can be much simpler and softer with a more cushioned homogeneous material like cork, foam or rubber being used as the inside of the ball. In most cases, the baseball has a circumference of about 9 1/8 inches and weighs about 5 1/8 ounces. Baseball bats originally were made of wood. Currently, ash and maple are the most common woods for baseball bats, and the weights of the bat are generally less than 36 ounces. During the 1920s and 30s, the era of Babe Ruth, the typical bat weighed more than 36 ounces. Additionally, the wood types varied, and hickory was a very common choice. Babe Ruth was said to have swung a bat as heavy as 56 ounces in his early years (Adair, 1994). Hillerich and Bradsby, commonly known as Louisville Slugger, and Worth Sports started developing and selling aluminum bats made from aluminum alloys in 1970. Aluminum bats were developed in cooperation with Little League, which by this time had established itself as the governing youth baseball organization throughout the world. In 1971, aluminum bats were ﬁrst used in Little League games (Little League, 2007). Three years later, aluminum bats were allowed for use in the College World Series (SportsLine, 2003). Aluminum bats were initially a cost-effective alternative to the problem of wood-bat breakage. While an aluminum bat would cost more than its wood counterpart, the aluminum bat would last many more at bats than a wooden one, thereby helping to reduce the operating costs for a team. These ﬁrst aluminum bats are alleged to have performed like wood bats with respect to the batted-ball speed, but had the advantage of being more durable and could be produced lighter than wood. However, this alleged wood-like performance is hearsay as no performance testing is known to exist to support this observation. During the early 1990s, aluminum bat manufacturers learned to exploit the potential of aerospace-quality high-strength aluminum alloys and began to develop aluminum baseball bats with increased batted-ball speeds over their wood counterparts. This period of aggressive development continued until 1998, when the NCAA introduced regulations to limit the performance of non-wood bats. These

© 2007, Woodhead Publishing Limited

162

Materials in sports equipment

regulations were possible with the introduction of testing methodologies that could measure batted-ball bat speeds off bats within 1%. Composites were also introduced into the game during the late 1990s. Though essentially still cost-effective today relative to using wood bats, some non-wood bats can sell for more than $375 US a piece.

6.2

Ball design and construction

The baseball is pitched, hit, caught and thrown and therefore is the centerpiece of the game of baseball. For over a century, the design of the baseball used in games has had very little change. All baseballs consist of a pill as the core of the ball. The pill is wound with three layers of wool yarns, a layer of cotton thread and wrapped with a hand-stitched leather cover. The current major league, minor league, college and high school baseballs are constructed of the same materials that have been used since the invention of the methodology for making the current pill center in 1925 by Milton Reach. Most baseballs are currently manufactured in China, with the signiﬁcant exception of the major league baseball which is currently assembled in Costa Rica from materials made in various parts of the USA. Figure 6.1 shows the layers of the current Major League Baseball. Practice baseballs used at the youth level are produced by both the large baseball companies as well as small companies that come up with a new idea, and therefore

6.1 View of the cross-section of a standard baseball.

© 2007, Woodhead Publishing Limited

Design and materials in baseball

163

these baseballs are constructed almost anywhere and from a variety of materials.

6.2.1 Construction of the baseball Baseballs are constructed with what is called a cushioned cork-center. This object is called the pill. The pill core is a small round ball of cork that may have been reinforced with a bonding agent to hold the pieces of cork together under the stresses of impact. This cork ball is typically then wrapped in two layers of rubber. The current process of making the pill was invented by Milton Reach of Spalding in 1925 when he was issued a US patent for the technology. The pill is then wrapped with one or more wool windings. The professional baseball has three separate windings, a gray 4-ply winding, followed by a white 3-ply winding, and followed by a gray 3-ply winding. Each of the professional ball windings is about 85% natural wool and about 15% synthetic material. Different college, high school and youth baseballs that have wool windings may have different numbers of windings and different quantities of natural wool. Wool is the material of choice for the windings because it has ‘memory.’ When wool is crushed under a load, it quickly rebounds to its original size upon removal of the load. Synthetic yarns do not necessarily quickly return to their original size after the load is removed – they may creep. A ball constructed with a relatively high percentage of natural wool will have a better durability than a low-wool content ball, because the natural wool will recover better than its synthetic counterpart and will allow the ball to recover to its original size, shape and properties faster and more times. Additionally, the ball made with a higher percentage of natural wool may actually have a lower compression, i.e. be softer, because synthetic wool can be stretched to a high tension and therefore end up being wrapped tighter. The liveliness of the baseball is inﬂuenced by the materials used in its construction and the response of these materials to the environment. The pill is a rubber-matrix composite with cork particles. As the volume fraction of cork increases, the liveliness of the baseball decreases. The response of the wool windings is very sensitive to the moisture content. The coefﬁcient of restitution (COR) of the baseball increases as the moisture content of the ball decreases. As the tension in the wool windings is increased, the hardness of the ball increases, and as a result the liveliness of the ball increases. During the manufacturing process, the respective weight and diameter of the ball are required to be within speciﬁed tolerances as each layer of material is added. These speciﬁcations ensure that the COR performance of the ball will be in the allowable range for its respective league.

© 2007, Woodhead Publishing Limited

164

Materials in sports equipment

6.2.2 Ball construction in Major League Baseball Major League Baseball Rule 1.09: The ball shall be a sphere formed by yarn wound around a small core of cork, rubber or similar material, covered with two strips of white horsehide or cowhide, tightly stitched together. It shall weigh not less than ﬁve nor more than 5 1/4 ounces avoirdupois and measure not less than nine nor more than 9 1/4 inches in circumference. (MLB, 2007)

The current major league baseball is manufactured by Rawlings Sporting Goods Co. and is assembled in Costa Rica. The ball consists of a cushionedcork pill, surrounded by three wool windings, and a fourth thin winding of cotton thread. The fourth winding of the ball is coated in a thin layer of cement to hold everything together. The two cowhide pieces are then sewn together by hand with 108 stitches to cover the windings. The seams of the major league ball are rolled to compress them to be essentially even with the leather cover of the baseball. The Major League Baseball name, logo and commissioner’s signature are then printed on each baseball before being shipped to the major league ballparks. Decorative stitching and logos are used for the All-Star game and World Series. Major League Baseball regulates many aspects of the construction of the baseball used in its games. These speciﬁcations include weight and circumference, not only of the overall baseball, but of each layer, the pill, the 1st winding, 2nd winding, 3rd winding, 4th winding, and the cement coating. The weight and thickness of the cover are also regulated. The natural wool content of each of the wool windings is speciﬁed. The pill performance is also regulated with a bounce test. Additionally, and possibly most importantly, the COR, the performance, of the ball is speciﬁed. Table 6.1 identiﬁes the various values and ranges for the parts of the Major League ball.

6.2.3 Ball construction in college and high school baseball The baseballs used in the NCAA games are made by several manufacturers, including but not limited to Diamond, Rawlings, Worth and Wilson. The Rawlings’ R100 and R200 baseball models are now being made with a new layer between the cement-coat and leather-cover layers. This new layer is being called the EIT (Extra Inning Technology) layer. The EIT layer is intended to make the balls less susceptible to environmental changes. The extra layer is a thick rubber-like coating that is non-porous and will not allow the moisture level of the air to affect the moisture content of the wool. It is well known that the COR performance of a baseball is very sensitive to moisture content. As the moisture content drops, the COR increases. The EIT layer ensures that the COR of the baseball remains constant over a range of relative humidities.

© 2007, Woodhead Publishing Limited

Design and materials in baseball

165

Table 6.1 Speciﬁcation of the MLB baseball Component

Attribute

Speciﬁcation

Pill

Weight Diameter Bounce

0.80–0.90 oz 1.365–1.385 in 32–38 in

1st winding

Weight Circumference Natural wool content

2.812–2.938 oz 7.365–7.563 in ∼85 %

2nd winding

Weight Circumference Natural wool content

3.312–3.438 oz 7.937–8.063 in ∼85 %

3rd winding

Weight Circumference Natural wool content

4.062–4.188 oz 8.687–8.813 in ∼85 %

4th winding

Weight Circumference

4.375–4.438 oz 8.750–8.813 in

Cement coating

Weight Circumference

4.437–4.500 oz 8.750–8.813 in

Cover

Weight Thickness

∼0.625 oz 0.046 to 0.056 in

Stitches

Count Seam Height

108 Even

Overall baseball

Weight Circumference COR

5.000–5.250 oz 9.000–9.250 in 0.514–0.578

Diamond’s new DriCoreTM technology helps prevent the baseball from absorbing water and weight in wet ﬁeld conditions. If a ball is subject to extreme water conditions, DriCore technology will accelerate the drying time, making the ball playable sooner (Diamond Sports, 2007). The college baseball has raised seams. These seams give two beneﬁts to the pitcher over the rolled seams of the major league baseball. First, they give the pitcher a ridge to hold the ball better than they can when using the rolled seams. Second, the raised seams inﬂuence the aerodynamic response of the baseball and can change the ‘movement’ of the ball relative to the rolled-seam baseball. The baseballs produced for high schools are very similar to the college baseball. The rules and speciﬁcations are about the same as they are for

© 2007, Woodhead Publishing Limited

166

Materials in sports equipment

college. The natural wool content may be less in the high school ball to make the ball less expensive. Higher natural wool content can help the durability of the baseball, but it also makes the ball more expensive.

6.2.4 Ball construction in Little League Baseball Little League baseballs are very similar in design to the high school and college baseballs. The natural wool content in many models is signiﬁcantly less than in the collegiate baseball. One special-construction ball used in Little League is known as the Reduced Injury Factor (RIF) baseball. This baseball is constructed of a polyurethane ball with a synthetic cover. RIF baseballs are rated by level, with 10 being the hardest and 1 being the softest. The level 10 baseball has a compression rating of 75 lb. The level 5 baseball has a compression rating of 40 lb. The typical professional and collegiate baseballs have compression values between 200 and 300 lb, respectively, for a 0.25-inch compression.

6.3

Bat design and construction

The baseball bat has been around as long as the game of baseball. It is likely that, before the beginning of organized baseball in the early 19th century, the bat was crudely made of almost any piece of wood that could be swung to hit the ball. David Block, author of ‘Baseball Before We Knew It’, identiﬁes that one of the earliest records of rules for youth playing sports was recorded in a German book printed in 1796 and describes a bat that is about two feet long and has a four-inch ﬂat face at the hitting end (Block, 2005). The early rules of baseball in the USA in the 1840s did not identify speciﬁcations about the equipment used. In 1854, the Rules of the Massachusetts Game Town Ball speciﬁed various rules about the equipment. Rule 2 stated that the bat must be round and must not exceed two and a half inches in diameter in the thickest part. It must be made of wood, and may be of any length to suit the Striker (Baseball Almanac, 2007). Town Ball is not the same game as baseball but, being a predecessor of the game and being played around the same time as when baseball was being developed, the equipment would likely have been instrumental in helping to establish the equipment that baseball would use. In 1885, the rules for Major League baseball stated that one side of the bat could be ﬂat (Baseball Almanac, 2007). In 1895, the rules for Major League baseball changed to state that the bats were permitted to be up to 2 3/4 inches in diameter and up to 42 inches long. In 1975, a player could be suspended for three days if they were to hit a fair ball with a doctored or ﬁlled bat (Baseball Almanac, 2007).

© 2007, Woodhead Publishing Limited

Design and materials in baseball

167

Major League Baseball Rule 1.10: (a) The bat shall be a smooth, round stick not more than 2 3/4 inches in diameter at the thickest part and not more than 42 inches in length. The bat shall be one piece of solid wood. NOTE: No laminated or experimental bats shall be used in a professional game (either championship season or exhibition games) until the manufacturer has secured approval from the Rules Committee of his design and methods of manufacture. (b) Cupped Bats. An indentation in the end of the bat up to one inch in depth is permitted and may be no wider than two inches and no less than one inch in diameter. The indentation must be curved with no foreign substance added. (c) The bat handle, for not more than 18 inches from its end, may be covered or treated with any material or substance to improve the grip. Any such material or substance, which extends past the 18 inch limitation, shall cause the bat to be removed from the game (MLB, 2007).

In 1970, aluminum bats were introduced to the amateur game. Aluminum and composite bats have since become the bats of choice for use in youth and amateur baseball. With the introduction of these types of bats, the depth of engineering design has increased signiﬁcantly. During the 1990s, aluminum and then composite bat manufacturers began to make non-wood bats that were higher performing than their wood counterparts. In response to the perception that the performance of non-wood bats was increasing each year, the NCAA decided to investigate the situation in 1998. Based on the ever-increasing offensive statistics and lab measurements of batted-ball speeds using a hitting machine, the NCAA found the need to regulate the performance of non-wood baseball bats. In January 1999 as the result of a Bat Summit in July 1998 with manufacturers and scientists, the NCAA restricted the length to weight difference to be no greater than 3 units, meaning that the bat’s weight in ounces minus the bat’s length in inches could be no less than minus 3. In September 1999, the NCAA implemented a certiﬁcation process for all non-wood bats that required each bat model/length/weight combination to be tested for its respective performance, and that the measured performance be below a speciﬁed limit. The limiting value was set by the highest performing 34-inch long wood bat. The performance was measured in the lab using a machine to swing a bat into a moving baseball. In 2001, the NCAA implemented a minimum Moment of Inertia (MOI), a measure of the resistance to swinging the bat. This minimum MOI restricted the bats from getting any easier to swing than they already were. In November 2005, the NCAA revised its certiﬁcation protocol without changing its impact. This all inclusive document can be found on the NCAA website, www.ncaa.org. In 2000, the National Federation of High Schools (NFHS), the organization that governs most of High School sports in the USA, adopted to follow the NCAA regulations for games under its jurisdiction.

© 2007, Woodhead Publishing Limited

168

Materials in sports equipment

During the same period of time as when the NCAA and the NFHS were implementing regulations, Japan’s governmental oversight organization implemented a series of regulations to help to ensure reasonable durability and performance of the bats used in amateur baseball in Japan. The Japanese bat compliance testing does not use a dynamic test to measure bat performance as is used in the USA. The testing in Japan is a series of static tests which include a three-point bend test to quantify the force required to break the handle and a barrel-ring test to measure the effective stiffness of the barrel. The efﬁciency of the bat–ball collision is a function of the hoop frequency in hollow-barrel bats. Because the hoop frequency is directly related to the stiffness of the barrel, this barrel-ring compression test is an effective static test for controlling batted-ball speeds. To perform the Japanese barrel compression test, the barrel of a bat is cut into three 50 mm-long rings. Then, a force is applied on each ring until the deformation of the ring reaches 0.2Do, where Do is the maximum original outer diameter of the ring and until the applied force reaches 10 000 N. If the ring breaks without reaching 0.2Do deformation and 10 000 N applied force, the bat is considered to fail the test. From the load–deﬂection curve obtained from this test, the load corresponding to a residual deformation of 0.02Do, which is called ‘offset load,’ is found as shown in Fig. 6.2. For an aluminum baseball bat, the offset load must be equal to or greater than 7500 N to pass the Japanese standard (Nabeshima and Sherwood, 2004).

Load

Offset load

0.02 D0

D0: Maximum outer diameter of ring before deflection

Deflection

6.2 Offset load from load–deﬂection curve.

© 2007, Woodhead Publishing Limited

Design and materials in baseball

169

The 1990s also brought some regulations of the performance of nonwood bats used in Little League baseball. A Bat Performance Factor (BPF) test was implemented. This test uses an air cannon to ‘pitch’ a baseball at a stationary bat. The bat is held in a ﬁxture that is mounted on a freely spinning turntable. The rebound speed of the bat is measured, and using a conservation of momentum approach, the BPF of the bat is calculated. A wood bat exhibits a BPF of about 1.0. The Baseball Research Center at the University of Massachusetts–Lowell is currently the certiﬁcation center for all non-wood bats used by the NCAA, NFHS and Major League Baseball players. While Major League does not allow anything but solid wood bats to be used in its games, Major League does allow non-wood bats to be used by its lower-level (Minor League) teams. These non-wood bats must perform comparable to solid wood in how they hit, swing and break. The reason for allowing the use of non-wood bats in the minors is to reduce expenses. The engineering challenge is to ﬁnd the right combination of materials to meet these bat performance requirements.

6.3.1 The wood bat The solid wood bat is allowed by every rule in every league that governs baseball. Some people consider baseball with bats made of anything but wood a disgrace to the game. In the current game of baseball, wood bats are primarily used by players in professional baseball or those who aspire to play in professional baseball. The ﬁrst wood bats were made using hickory and white ash. Both wood species had been used extensively for handles on tools such as axes and hammers. Because of the good impact performance of these woods for tools, it was only natural to consider them for making baseball bats. Because ash is not as dense at hickory, players elected to use ash bats as they were easier to swing. This choice of ash is a compromise because the more mass that is in the bat, the more momentum there is available to deliver to the ball during the collision. However, if the bat cannot be swung fast enough to make contact with the baseball, then the extra mass brought to the collision is meaningless. The majority of the northern white ash used for bats is harvested in eastern Pennsylvania and upstate New York. Most wood bats are turned on a lathe from a solid piece of white ash or hard maple. Prior to Barry Bonds’ homerun-record breaking season in 2001 using a hard rock maple bat, almost all wood bats were made of northern white ash. As a result of the 2001 homerun record, the popularity of maple has spread throughout major league players. Today, over half of the bats used at the major league level are made of hard rock maple, even though maple bats can cost 50% more than ash bats in the same proﬁle.

© 2007, Woodhead Publishing Limited

170

Materials in sports equipment

Some scientiﬁc studies have been made to compare ash and maple bats. With respect to elastic moduli, the two woods are very comparable. Maple is relatively denser than ash. Consequently, to have equal weight and equal length, ash and maple bats require that the maple bat be smaller in diameter. There is no measurable difference in the hitting performance for the two woods. Ash bats tend to ﬂake after repeated hitting. Maple bats do not exhibit this ﬂaking behavior, and are thus promoted as being more durable. While maple is more durable with respect to ﬂaking, maple is more prone to have pin knots than ash is and, therefore, maple is more prone to breakage than ash. It has been observed that a maple bat tends to explode into multiple pieces when it breaks, while an ash bat tends to just crack or break into two pieces. With respect to the future of wood bats, beech and birch and red oak are being considered as other alternatives to northern white ash. Table 6.2 identiﬁes some of the properties for different woods that are currently used or being considered to be used for producing wood bats. Table 6.3 identiﬁes some generic material properties for generating models to predict the behavior of wood bats. Figure 6.3 shows a variety of handle designs. The early bats had relatively thick handles, some nearly two inches, in comparison to today’s bats, which typically have a diameter less than one inch. The early bats also had a range Table 6.2 Material properties of woods at 12 % moisture content used to make baseball bats (Forest Products Society, 1999) Wood type

Speciﬁc gravity

Modulus of elasticity (×106 lbf/in2)

Modulus of rupture (lbf/in2)

Ash, blue Ash, white Beech, American Birch, yellow Hickory, mockernut Maple, sugar Oak, northern red

0.58 0.60 0.64 0.62 0.72 0.63 0.63

1.40 1.74 1.72 2.01 2.22 1.83 1.82

13 800 15 000 14 900 16 600 19 200 15 800 14 300

Table 6.3 Typical wood bat properties (Vedula, 2004) Young’s modulus (lbf/in2)

Density (lb/in3)

Poisson’s ratio

Shear modulus (lbf/in2)

E1

E2

E3

r

Pr1

Pr2

Pr3

G1

G2

G3

25E5

9E5

1.7E5

0.026

0.027

0.044

0.067

1E5

3.4E5

1.3E5

© 2007, Woodhead Publishing Limited

Design and materials in baseball

171

6.3 Wood bat proﬁles past and present.

of knob shapes. One bat in Fig. 6.3 has a ball knob and another has a ﬂat knob at its base, as is seen on contemporary bats, but also has a ‘second knob’ on the handle. The two knobs were for registering each hand’s position along the bat. Today, many players use white medical tape to customize the shape of the knob to a desired shape. With respect to the future of bat handles, two new geometries have been recently introduced. One design patented by Giant Project, Inc. conforms to the shape of the hand as the ﬁngers wrap around the bat. An example bat is shown in Fig. 6.3 and exhibits the knob at approximately a 45° angle with respect to the axis of the bat as opposed to the standard 90° angle seen in bats today. The design claims to address the persistent problem of injury to the hook of the hamate (the wrist bone in line with the 4th and 5th ﬁngers) due to swinging a bat. Mattingly Baseball has developed a

© 2007, Woodhead Publishing Limited

172

Materials in sports equipment

34 22 L 2

52 W 24 18 30 26 2

28 34

20 38

6.4 V-GripTM developed by Mattingly Baseball (Mattingly, 2007).

V-GripTM as shown in Fig. 6.4. The grip is promoted as a natural shape for teaching young players how to hit correctly and to ensure that seasoned players maintain their optimal performance when batting.

6.3.2 The aluminum bat Aluminum bats were initially introduced as a cost-effective alternative to wood bats. Aluminum bats have helped to improve the game of baseball for many amateur players, but have also caused a need for organizations to implement regulations to maintain the balance between offense and defense and thereby maintain the integrity of the game. Aluminum bats are much more durable and can be made lighter than wood bats. The increased durability feature of aluminum bats make them cost-effective for teams, schools, parents and players. The lighter bat in comparison to a comparable-length wood bat results in a reduced swing weight, which allows youth players learning how to play the game to swing the bat easier and hence increase the probability of making contact with the ball and of getting more balls into play. This probably allows children learning to play the game to enjoy the game more than they might with a wood bat. When aluminum bats were ﬁrst introduced in the 1970s, they were hollow and relatively thick walled. Very few design changes were made until the 1990s when aircraft grade aluminum alloys were introduced to the manufacturing process. These relatively lightweight, high-strength alloys allowed bat manufacturers to decrease the wall thickness of their aluminum bats, which led to aluminum bats outperforming wood by a signiﬁcant margin. The new alloys enabled manufacturers to make bats that were much lighterweight than wood bats and therefore much easier to swing. The reduction

© 2007, Woodhead Publishing Limited

Design and materials in baseball

173

Table 6.4 Typical aluminum bat properties (Vedula, 2004) Young’s modulus (lbf/in2)

Density (lb/in3)

Poisson’s ratio

10 × 106

0.1

0.33

Table 6.5 Example of end cap properties (Vedula, 2004) Young’s modulus (lbf/in2)

Density (lb/in3)

Yield stress (lbf/in2)

Poisson’s ratio

37 000

0.096

12 950

0.33

in wall thickness also created a ‘trampoline effect’, which can lead to higher batted-ball speeds. Some of the aluminum alloys currently used in aluminum bats are C405, 7046, 7050 and alloys including scandium. Tables 6.4 and 6.5 identify some material properties for C405 aluminum and a typical plastic end cap, respectively. Titanium bats were also introduced in the 1980s. However, the battedball speeds off these bats were observed to be much greater than wood. Thus, the alloy was quickly banned. The trampoline effect refers to the fact that, with a hollow bat, the barrel of the bat will compress local to the impact location during the bat–ball collision and some of that energy can be returned to the ball. With a solid wood bat, there is essentially no deformation of the barrel, and a large amount of deformation in the ball. When the ball deforms, a large amount of energy is lost to internal frictional forces, and up to 75% of the ball’s initial energy can be lost in a collision with a wood bat (Russell, 2006). When a hollow bat deforms, some of the energy is stored in the barrel and returned to the ball. Therefore, for a high performing bat, it is desired to minimize ball deformation and to maximize the amount of energy stored in the bat and transferred back to the ball. The efﬁciency with which the bat can transfer energy back to the ball depends on the hoop stiffness of the barrel of the bat. The reason the titanium bats were so high performing was because they had much lower hoop stiffness than the existing aluminum bats (Russell, 2006). Aluminum bats also have an advantage over wood bats in that they can be made much lighter. A lighter bat can be swung faster and also gives the player more control over the swing. The moment of inertia of a bat, sometimes referred to as the ‘swing weight,’ is usually signiﬁcantly lower for aluminum bats than wood bats. A lower swing weight is not always beneﬁcial

© 2007, Woodhead Publishing Limited

174

Materials in sports equipment

though: reducing the swing weight also reduces the amount of mass ‘seen’ by the ball during the collision. In an attempt to control the performance of aluminum bats, some governing bodies have set minimum weights and moments of inertia for each length bat. These regulations are meant to make aluminum bats feel more like wood bats and to bring the player swing speeds down closer to swing speeds comparable with wood bats. The bending vibrations in a bat can also affect the batted-ball performance. Any energy that goes into bending vibrations is not transferred back to the ball and therefore decreases the performance of the bat. For this reason, more ﬂexible bats tend to be slightly lower performing than highstiffness bats, especially when impacted away from the sweet spot. It has been hypothesized that some players can actually increase their bat speed by ‘whipping’ the bat. If players are able to accomplish this, then a more ﬂexible handle may actually be beneﬁcial. The effect of handle stiffness differs from player to player.

6.3.3 The composite bat Composite materials have made their way into many sports. During the 1990s, some bats began to be made of more than one piece, including different materials in different layers, and used composite materials. The beneﬁt of composite constructions is that the best features of each material can be utilized in different locations along the length of the bat and through the thickness of the bat to achieve a desired mechanical behavior. There are three main categories of bats using multiple materials and/or pieces in different parts of the design. The ﬁrst category is to use different materials for the different regions of the bat, e.g. one material for the handle and another material for the barrel. The second category of this composite design is to use different materials in different layers of the bat. The third category is to use multiple pieces of the same material. An example of a bat in the ﬁrst category would be the metal-wood bat, which due to the wood barrel has the performance, hitting characteristics and sound of a wood bat, while the aluminum handle is more durable than wood. If the wood barrel ﬂakes or cracks, a new wood barrel can be ﬁtted to the aluminum handle. Another example in this category is the Hoosier three-piece bat that utilizes three different types of wood bonded together to get a balance of weight, solid-wood construction and durability. The handle of the bat is ash, the hitting area of the bat is hickory and the tip of the barrel is maple. The ash handle is for strength like a solid ash bat. The hickory is for durability of the hitting surface. The soft maple is to compensate for the increased density of the hickory over that of ash. An example of the second classiﬁcation of composite construction is the Louisville Slugger Air Attack bat, which has an air bladder within the barrel

© 2007, Woodhead Publishing Limited

Design and materials in baseball

175

of an aluminum bat. The air bladder allows the aluminum walls to be thinner than they could be without the bladder. Easton makes ‘core’ technology bats that have a woven ﬁberglass or combination ﬁberglass and carbon fabric bonded to the inside wall of the barrel. Like the air bladder, this design allows for the aluminum walls to be thinner than they could be without the fabric. The third classiﬁcation is the bonding of multiple pieces of the same material. The bamboo bat is made from strips of bamboo, which is not actually a wood and is technically classiﬁed as a grass. The bamboo strips are then laminated together into a block and turned down to the proﬁle of the bat as is done with a wood billet. Likewise, strips of ash can be used to make laminated bats. The lamination process allows for the smaller pieces of wood that are not big enough to make a solid wood bat to still be used for making a bat, thereby taking advantage of a natural resource that might otherwise be disposed of as waste. The Easton Connection bats are an example using two pieces of the same material, one to make the handle and another piece for the barrel. This two-piece concept has a ‘rubber’ connecting piece and is promoted as isolating the hands from the ‘sting’ of the bat. The beneﬁt to the future of composite bats is being able to tailor material properties along the length of bat. Composites offer the bat engineer many more design choices than are available using aluminum, wood or even combinations of wood and aluminum. Another advantage of composite bats is that to set up a composite bat manufacturing facility requires far less capital investment than is required for an aluminum bat facility. The downside of resin–matrix composite bats is that a bat design can require signiﬁcant trial and error to achieve the desired performance and durability. Despite this obstacle, the future of non-wood bats looks to belong to the composite bat technology.

6.3.4 The future design of aluminum and composite bats The future design of aluminum and composite bats is fairly limited. The bats have restrictions on size, weight and performance. Many recent design changes have occurred in the feel of the bats in the hands of the batter. Some of these design changes have focused on creating a bat made of two or more pieces that are joined with a connection that dampens the vibrations created from the impact with the ball. This type of design makes a wide range of impacts not severely sting the hands of the batter. One problem with this design approach is that the player may not be able to recognize a good hit from a bad hit. Traditional wood bats, as well as most single-piece aluminum bats, cause a sting to the players hands when the ball impacts locations on the bat that excite the 1st and 2nd bending modes.

© 2007, Woodhead Publishing Limited

176

Materials in sports equipment

It is only at the node locations that these vibrational modes are not excited. The nodes of these two modes are both within an inch of the sweet spot location on the barrel of the bat. The sweet spot is the best performance location on the bat that will result in the highest batted-ball speed. Therefore, when the impact takes place around these vibrational nodes, the batter feels little to no sting and can identify that he/she made contact at the location on the bat that he/she should have for the best performance. If the ball makes contact on a different location on the bat, the batter is able to recognize from the vibrations sent to his/her hands that he/she needs to make better contact the next time. The bats that reduce the vibrations which the batter feels on a bad hit make the sport more comfortable for the batter, but may not be the best training aid for improving the player’s swing.

6.4

Baseball gloves

Two types of gloves are worn by the players. One type of glove is used for catching the ball in the ﬁeld. The other glove type is worn to protect the player’s hands when batting. There are many styles of baseball gloves to suit the different conditions under which the different position players ﬁeld the ball.

6.4.1 Gloves for catching the ball In the very early years of baseball, there was no use of any covering for the hands for catching the baseball in the ﬁeld. In 1870, the ﬁrst professional catcher put on several thin, half-ﬁngered buckskin gloves to protect his hands after he caught eight games in nine days. He was heckled by the opposing team and the fans for being a sissy. A major evolution in the glove occurred in 1883 when Providence shortstop Arthur Irwin fractured a ﬁnger. He covered his bandage with an oversized buckskin glove and added padding. By the 1890s, almost all players were wearing gloves. Catchers typically stuffed their gloves using feathers or cotton or any other available material to their liking. Slowly the baseball glove developed to protect the hands and increased in size for ease in catching the baseball. Figure 6.5 shows some early 20th century glove designs. There are different styles of gloves for the different positions in the ﬁeld. The typical glove is made of leather that is padded, has extended ﬁngers and has a web between the index ﬁnger and thumb. The outﬁelder gloves are typically larger than the gloves worn by the inﬁelders. The smaller glove allows the inﬁelders to catch the ball, retrieve the ball and throw it to the desired base very quickly. The catcher’s mitt is much more padded and is spaded to be able to grab and/or trap the ball to keep it from getting away.

© 2007, Woodhead Publishing Limited

Design and materials in baseball

177

6.5 Early baseball gloves.

Major League Baseball rule 1.12: The catcher may wear a leather mitt not more than thirty eight inches in circumference, nor more than ﬁfteen and one half inches from top to bottom. Such limits shall include all lacing and any leather band or facing attached to the outer edge of the mitt. The space between the thumb section and the ﬁnger section of the mitt shall not exceed six inches at the top of the mitt and four inches at the base of the thumb crotch. The web shall measure not more than seven inches across the top or more than six inches from its top to the base of the thumb crotch. The web may be either a lacing or lacing through leather tunnels, or a center piece of leather which may be an extension of the palm, connected to the mitt with lacing and constructed so that it will not exceed any of the above mentioned measurements. (MLB, 2007) Major League Baseball rule 1.13: The ﬁrst baseman may wear a leather glove or mitt not more than twelve inches long from top to bottom and not more than eight inches wide across the palm, measured from the base of the thumb crotch to the outer edge of the mitt. The space between the thumb section and the ﬁnger section of the mitt shall not exceed four inches at the top of the mitt and three and one half inches at the base of the thumb crotch. The mitt shall be constructed so that this space is permanently ﬁxed and cannot be enlarged, extended, widened, or deepened by the use of any materials or process whatever. The web of the mitt shall measure not more than ﬁve inches from its top to the base of the thumb crotch. The web may be either a lacing, lacing through leather tunnels, or a center piece of leather which may be an extension of the palm connected to the mitt with lacing and constructed so that it will not exceed the above mentioned measurements. The webbing shall not be constructed of wound or wrapped lacing or deepened to make a net type of trap. The glove may be of any weight. (MLB, 2007)

© 2007, Woodhead Publishing Limited

178

Materials in sports equipment

Major League Baseball rule 1.14: Each ﬁelder, other than the ﬁrst baseman or catcher, may use or wear a leather glove. The measurements covering size of glove shall be made by measuring front side or ball receiving side of glove. The tool or measuring tape shall be placed to contact the surface or feature of item being measured and follow all contours in the process. The glove shall not measure more than 12″ from the tip of any one of the 4 ﬁngers, through the ball pocket to the bottom edge or heel of glove. The glove shall not measure more than 7 3/4″ wide, measured from the inside seam at base of ﬁrst ﬁnger, along base of other ﬁngers, to the outside edge of little ﬁnger edge of glove. The space or area between the thumb and ﬁrst ﬁnger, called crotch, may be ﬁlled with leather webbing or back stop. The webbing may be constructed of two plies of standard leather to close the crotch area entirely, or it may be constructed of a series of tunnels made of leather, or a series of panels of leather, or of lacing leather thongs. The webbing may not be constructed of wound or wrapped lacing to make a net type of trap. When webbing is made to cover entire crotch area, the webbing can be constructed so as to be ﬂexible. When constructed of a series of sections, they must be joined together. These sections may not be so constructed to allow depression to be developed by curvatures in the section sides. The webbing shall be made to control the size of the crotch opening. The crotch opening shall measure not more than 4 1/2″ at the top, not more than 5 3/4″ deep, and shall be 3 1/2″ wide at its bottom. The opening of crotch shall not be more than 4 1/2″ at any point below its top. The webbing shall be secured at each side, and at top and bottom of crotch. The attachment to be made with leather lacing, these connections to be secured. If they stretch or become loose, they shall be adjusted to their proper condition. The glove can be of any weight. (MLB, 2007)

6.4.2 The batting glove The grip a baseball player has on the bat is critical in controlling the bat to make contact with the ball. If the bat slips in the hands, it can signiﬁcantly affect the direction and speed of the batted ball. Batting gloves help the batter to grip the bat and also provide protection for the hands from the sting of the bat when hitting and subsequently as a runner when sliding into a base. An increasing number of players now wear the gloves inside their baseball gloves to provide a better ﬁt and to reduce the sting. The batting glove is made of relatively thin leather or a synthetic material. The palm of the glove may have ridges of material added to the basic glove for giving additional gripping potential.

6.5

Protective and other equipment

The game, starting as a gentleman’s game, has progressively gotten faster and therefore the potential for injury has increased. This increase in potential risk has been counteracted by the introduction and subsequent

© 2007, Woodhead Publishing Limited

Design and materials in baseball

179

evolution of equipment to protect the players and spectators. Many rules and pieces of equipment have been introduced at the youth levels. Masks and/or helmets have protected catchers and umpires and batters for many years. Some equipment in the ﬁeld like nets and backstops has protected spectators. Additionally, there are newer devises like breakaway bases that have protected the base runners from injury. Without these protective measures, the game of baseball could be much more dangerous today than when it began more than 100 years ago.

6.5.1 Protective equipment for the catcher The catcher was the ﬁrst player on the ﬁeld that needed protection from getting injured during play. The catcher has a padded mitt, glove, for his/her hands, but he/she is crouched right behind the plate where he/she can get hit by the pitch, a foul tip, and even occasionally the bat. The ﬁrst catcher’s mask was modeled after a fencing mask in 1876. This mask was marketed by Spalding and was constructed of a steel frame with padding made from ‘imported dog skin’ according to the Spalding catalog (Rosciam, 2007). The mask changed slightly in design and was reﬁned over the years to primarily give the catcher the protection without blocking the peripheral vision. In the 1970s, a throat protector became much more popular. In the late 1990s, the hockey goalie style mask started becoming popular. This design protects the entire head within a polycarbonate or acrylonitrile–butadiene– styrene (ABS) polymer shell with less of a ﬂat front that causes the ball to glance off easier. The mask is less obstructive of peripheral vision (Rosciam, 2007). These masks may become more popular in the near future, but they are slightly heavier and cannot be ripped off to catch the pop-up. The organization that tests and certiﬁes helmets and other protective equipment is the National Operating Committee on Standards for Athletic Equipment (NOCSAE). Forms of chest protectors have also existed for the same period. The chest protectors started smaller and would have been worn under the uniform in the early years before 1884. In 1885, chest protectors began being worn outside the uniform and were constructed of fur stuffed sheepskin. The modern version of the chest protector uses shock-absorbing polyfoam. Shin guards have also been part of the catcher’s uniform since the early 1900s and have not changed much in design with the exception of becoming hinged to allow more ﬂexibility. The guards were developed from those used by cricket players and originally were constructed from rods of light cane held together by fabric. The modern shin guards are made from molded plastic (Rosciam, 2007). Umpires wear many of the same pieces of protective equipment as catchers to avoid injury from the same problems. Because the umpire is

© 2007, Woodhead Publishing Limited

180

Materials in sports equipment

positioned directly behind the catcher, they may only wear the facemask and some padding depending on their style.

6.5.2 Batting helmets Batting helmets were developed in 1959 by Dr Creighton J. Hale, then Director of Research for Little League Baseball. The batting helmets became required in 1983 in professional baseball after several players were hit by pitches that resulted in head injuries. Prior to that, players would often use plastic inserts in their baseball caps. The modern batting helmet is constructed of a high impact polymer shell, often ABS, and may include some thermoplastic elastomer regions. Some youth models have a wire face mask similar to a catcher’s mask. Some players, especially pitchers and inﬁelders, additionally wear these same or similar helmets while they are in the ﬁeld to avoid injury from batted balls. Major League Baseball rule 1.16: A Professional League shall adopt the following rule pertaining to the use of helmets: (a) All players shall use some type of protective helmet while at bat. (b) All players in National Association Leagues shall wear a double ear ﬂap helmet while at bat. . . . (d) All catchers shall wear a catcher’s protective helmet, while ﬁelding their position. (e) All bat/ball boys or girls shall wear a protective helmet while performing their duties. . . . (MLB, 2007)

6.5.3 Bases The bases that form the baseball diamond have changed over the years. The newest change has been the breakaway base that was originally patented in 1979 by Roger Hall. In 1877, bases were made of canvas and were 15 inches by 15 inches. Eventually, these canvas bases were attached to poles in the ground. The base is typically hooked onto a support that is anchored in the ground using concrete. This anchoring ensures the base remains in its prescribed location on the ﬁeld. Base runners slide into the bases and occasionally can injure their ankles, wrists or ﬁngers when they hit the base too hard. The breakaway bases that have been accepted by many governing organizations are designed to snap off a base when enough force is exerted that injury might occur. In 1885, home plate could be made of whitened rubber or marble. 1887 saw the rule change stating that home plate could only be made of rubber (Baseball Almanac, 2007). To reduce injuries to batters running to ﬁrst base and to players covering that base, some amateur leagues use a double-bag for ﬁrst base. Rather than both the batter and the ﬁelder running to touch the same bag, each player has a separate bag. This double-bag base reduces the potential for the players to collide.

© 2007, Woodhead Publishing Limited

Design and materials in baseball

181

6.5.4 The backstop The spectators attending a baseball game must always be aware of the foul balls and homeruns entering the spectator areas. Foul tips that send the baseball ﬂying back and into the stands can be very fast and potentially dangerous. The backstop is typically a fence or net that protects the spectators from these quick and hard to judge foul balls. Spectators must always be aware that baseballs and bats can ﬂy into areas where they may be viewing the game.

6.6

Future trends

The game of baseball is a game where statistics are kept with respect to every aspect of the game. These statistics allow team administrators and baseball fans to compare the talents of one player against another or the overall performance of one team relative to another, whether of the same or different eras. As a result, baseball is a game of tradition, and any changes in the balls or the bats used in the game can introduce a bias to these statistics. As a consequence, changes in the bats and balls rarely occur at the Major League level without a good reason as to why the change should occur. Changes in bats and balls are more likely to occur in the amateur leagues. With respect to bats, the evolution from wood to aluminum occurred in the 1970s. Because of limitations being placed on aluminum bat performance with respect to batted-ball speed and swing weight and amateur leagues showing interest in pushing the non-wood bat performance levels back to being equal to wood, it is anticipated that future bat designs will be dominated by ﬁber-reinforced composite materials. These composites give the bat designer a wide range of choices for material behavior to allow for a bat design that will be very comparable to that of wood. These composite bat designs will need to also exhibit the improved durability over wood that aluminum bats currently demonstrate. With respect to protective gear, the game has come a long way since its introduction in 1839. As mentioned previously, it was not until the 1870s that players began to wear gloves in the ﬁeld to protect their hands. The use of protective wear has now extended to ‘coats of armor’ for the catchers and to batting helmets for the players. Next to the catcher, the pitcher is the most vulnerable player on the ﬁeld. While a batted-ball causing injury to a pitcher is rare, it nonetheless does happen. In wood-bat leagues, a piece from a broken bat could potentially hit a pitcher. Considering that a pitcher is very off balance after releasing the ball, the combination of these events puts the pitcher in a compromised position. Thus, the extension of protective equipment to include a pitcher’s helmet could be a consideration for the future of baseball.

© 2007, Woodhead Publishing Limited

182

Materials in sports equipment

6.7

Sources of further information and advice

This chapter has included an overview of the design and construction of equipment used in the game of baseball. There are many sources of information and details about the design restrictions implemented by regulatory organizations, the history of the construction of the equipment, and research into the science of what actually takes place during the game. Some of the major sources of this information are identiﬁed in this section. Some of the major governing bodies that regulate professional, amateur and youth baseball are identiﬁed. Baseball is played in nearly 100 different countries. Each country would have its own organizations that regulate baseball for their countries and levels of competition. The following list identiﬁes some of the major baseball governing organizations around the world.

International • • •

International Baseball Federation (IBAF) – www.baseball.ch/index2. html World Baseball Classic (WBC) – http://ww2.worldbaseballclassic. com/2006/index.jsp Little League Baseball – www.littleleague.org

US • • •

Major League Baseball (MLB) – www.mlb.com National Collegiate Athletic Association (NCAA) – www.ncaa.org National Federation of State High School Associations (NFHS) – www. nfhs.org

Japan • • • •

Nippon Professional Baseball (NPB) – www.npb.or.jp Japan High School Baseball Federation – www.jhbf.or.jp All-Japan College Baseball Federation Japan Student Baseball Association

Different research, both historical and scientiﬁc, is being conducted and maintained by different organizations. The scientiﬁc research is being performed by many researchers in the ﬁelds of Engineering and Physics. Organizations like the Baseball Hall of Fame and Society for American Baseball Research have also been involved in retaining the history both in artifacts and by analyzing data that has been recorded about the game since its inception. Some of the major organizations and individuals who have con-

© 2007, Woodhead Publishing Limited

Design and materials in baseball

183

tributed to the scientiﬁc knowledge base are identiﬁed in the following list. • • • • • • • • • • • • • • •

UMass Lowell Baseball Research Center (UMLBRC) – mechanical. uml.edu/umlbrc Society for American Baseball Research (SABR) – www.sabr.org National Baseball Hall of Fame – www.baseballhalloffame.org The Baseball Hall of Fame (Japan) – http://english.baseball-museum. or.jp/ International Sports Engineering Association (ISEA) – www. sportsengineering.org ASTM International (ASTM) – www.astm.org Dr Alan Nathan (Physics, University of Illinois) – http://www.npl.uiuc. edu/~a-nathan/pob/ Dr Daniel Russell (Mechanical Engineering, Kettering University) – http://www.kettering.edu/~drussell/bats.html Dr Lloyd Smith (Mechanical Engineering, Washington State University) – http://www.mme.wsu.edu/~ssl/index.htm Dr Robert Adair (Physics) – The Physics of Baseball Dr Michael Carroll (Engineering, Rice University) Dr Keith Koenig (Aerospace Engineering, Mississippi State University) – http://www.ae.msstate.edu/pages/koenig.php Dr James Ashton-Miller (Mechanical Engineering, University of Michigan) Dr Kenneth Johnson (Physics, Southern Illinois University) Dr Robert Watts (Mechanical Engineering, Tulane University), Dr Terry Bahill (Systems and Industrial Engineering, University of Arizona) – Keep your Eye on the Ball: The Science and Folklore of Baseball

The manufacturers have their own, often proprietary, knowledge of the design and construction of equipment used in baseball. Some information they use for marketing their products can be found on their brochures and websites as well as the stores and websites that sell their products. The Sporting Goods Manufacturer Association, www.sgma.com, is an international trade organization to which many sporting good companies belong and helps promote sports throughout the world and unite the companies for common goals.

6.8

Acknowledgements

The authors of this chapter would like to thank Major League Baseball, Jim Gates of the National Baseball Hall of Fame and Rawlings Sporting

© 2007, Woodhead Publishing Limited

184

Materials in sports equipment

Goods for helping to contribute to the collection of information presented in the chapter. Portions of the Ofﬁcial Baseball Rules appearing in this document have been reprinted by special permission of the Ofﬁce of the Commissioner of Baseball. The copyright in the Ofﬁcial Baseball Rules is owned and has been registered by the Commissioner of Baseball. Any additional material contained in this document has not been endorsed by the Commissioner of Baseball.

6.9

References

adair r (1994), The Physics of Baseball, New York, HarperCollins, 108–11. block d (2005), Baseball before we knew it: A search for the roots of the game, Lincoln, NB, University of Nebraska Press. baseball almanac (2007), Baseball Almanac, Miami, FL, , last accessed 17 March 2007. diamond sports (2007), Diamond Sports, Cypress, CA, , last accessed 17 March 2007. forest products society (1999), Wood Handbook: Wood as an Engineering Material, Madison, WI. Forest Products Laboratory, US Department of Agriculture, Forest Service. little league (2007), Little League chronology, <www.littleleague.org/about/ chronology.asp>, last accessed 17 March 2007. mattingly (2007), Mattingly Baseball, Whiteland, IN, , last accessed 17 March 2007. mlb (2007), Major League Baseball, New York, , last accessed 17 March 2007. nabeshima s and sherwood j (2004), Comparison of the performance of U.S. and Japanese aluminum baseball bats in M Hubbard, R D Mehta and J M Pallis (eds), The Engineering of Sport 5, Volume 2, Shefﬁeld, International Sports Engineering Association, 73–9. rosciam c (2007), Encyclopedia of Baseball Catchers, , last accessed 17 March 2007. russell d (2006), Daniel Russell, Kettering University, Flint, Michigan, , last accessed 17 March 2007. sportsline (2003), CWS History – 1974, CBS SportsLine, Fort Lauderdale, FL, , last accessed 17 March 2007. vedula g (2004), Experimental and ﬁnite element study of the design parameters of baseball bats, Master’s Thesis, Baseball Research Center, University of Massachusetts Lowell. watts r g and bahill a t (1990), Keep your Eye on the Ball: The Science and Folklore of Baseball, New York, W.H. Freeman and Co.

© 2007, Woodhead Publishing Limited

7 Design and materials in snowboarding A. S U B I C and J. K O VAC S, RMIT University, Australia

7.1

Introduction

Snowboarding is a relatively young sport, having been formally admitted as an Olympic Games event only recently, in 1998 at Nagano. Nevertheless, this is a modern sport with one of the highest levels of participation, also widely considered as the winter sport of choice of the younger generation. Snowboarding evolved as a blend of many inﬂuences and features from skiing, skateboarding and even surﬁng. This is reﬂected in the design of equipment and apparel, as well as in the riding styles and attitudes. The ofﬁcial snowboarding discipline includes the half-pipe, parallel giant slalom and snowboarding cross-event competitions for both men and women. Similar to skateboarding, the half-pipe event involves acrobatic manoeuvres and tricks performed in the tube, which is carved out of snow. The parallel slalom is based on the ski version, but with competitors racing side-by-side to the ﬁnish line. The snowboard cross-event is held on an alpine terrain with a range of challenging obstacles and jumps, making it similar in style to the BMX for example. In terms of the general styles of riding, snowboarding is divided into freestyle, freeride and alpine snowboarding. Riding styles have a profound effect on the design of snowboards. This chapter will present the key design features of snowboards, including a detailed treatment of the materials and manufacturing processes used. The focus here is on the structural design of snowboards, because the structure and associated materials have a direct effect on the weight of the board and its responsiveness to the input of the rider (and the terrain). Typically, a lightweight responsive snowboard will take less energy to ride and, if designed properly, will have the inherent snap energy (‘liveliness’) required for advanced performance. Other equipment, such as bindings and boots, will not be covered here as the scope of this chapter is primarily focused on snowboard technology. 185

© 2007, Woodhead Publishing Limited

186

Materials in sports equipment

7.2

Riding styles in snowboarding

Riding style plays a critical role in snowboard design and manufacture. Riding conditions determine the desired characteristics and type of equipment that a rider will require. The riding styles are characterized by the preferred terrain, their speciﬁc equipment and type of competitions. Freeride snowboarding, or all-mountain riding, is the most general style of snowboarding with the most versatile equipment. It involves riding almost any terrain, ranging from open terrain to backcountry chutes, and encompasses ground manoeuvres, jumps and a limited amount of aerial tricks. In general, it is considered the most suitable style for beginners. As a result, freeride snowboarding is the most popular style and the corresponding equipment is the most common in the shops and on the slopes. Freeride snowboards are relatively long and stiff boards with a narrow width, a deep side cut and a directional shape. They also feature a long nose and an upturned tip and tail. In addition, the stance width is offset to the rear of the board to facilitate riding in deep powder. Essential freeride equipment includes soft boots and strap or ﬂow-in bindings. Freestyle snowboarding is considered the most spectacular and wild riding style. It focuses on jumps and a vast variety of aerial manoeuvres, such as twists, turns and grabs. Snowboarding movies typically showcase freestyle riding and freestyle competitions, including such events as the half-pipe and quarter-pipe. In general, freestyle boards are not likely to perform well outside of their speciﬁc riding conditions, as the majority are designed speciﬁcally for the half-pipe and rail slide. Freestyle snowboards are typically shorter, softer, wider and lighter than their freeride counterparts, in part to aid manoeuvrability. They also sport an upturned tip and tail and have twin tips, that is, the tail and tip are of an identical shape. Essential freestyle equipment includes soft boots and strap or ﬂow-in bindings. Alpine snowboarding, or freecarving, focuses on speed, carving turns and racing, and is the least common riding style. It is modeled on parallel giant slalom ski racing, in which skiers maneuver around gates or poles. Due to the high-speed nature of the descent and slalom turns, the carving style is less suited to beginners than freeride and freestyle riding. In general, rider’s crossing over from skiing are more likely to prefer the alpine riding style, as it is somewhat analogous to skiing. Alpine snowboards are generally narrow, long and stiff to increase stability and edge-hold at high speeds. It should be noted that alpine boards are not suitable for freestyle manoeuvres. Essential alpine equipment includes hard boots and plate bindings. An example of each riding style is shown in Fig. 7.1.

© 2007, Woodhead Publishing Limited

Design and materials in snowboarding

Freestyle

Freeride

187

Alpine

7.1 Snowboard riding styles (www.abc-of-snowboarding.com/ snowboardstyles.asp).

7.2.1 Loading conditions Dynamic loading conditions encountered during riding induce various bending and twisting forces on the board. The core and reinforcing layers are the structural backbone of the board, acting together to withstand the various stresses. There are four key loading conditions that a snowboard may experience: longitudinal shear loading, transverse shear loading, torsional loading and compressive loading. Longitudinal shear loading is applied to the core along its longitudinal axis approximately midway between the rear binding region and the tail end of the core. This corresponds to landing a jump that causes the tail end of the board to bend upwards. Transverse shear loading is applied to the core approximately midway between the longitudinal axis and the edge of the board. This corresponds to executing a hard turn on edge that causes the top edge to bend upwards along an axis that is parallel to the longitudinal axis. Torsional loading is applied to a centre section of the core between the front and rear binding regions off the longitudinal axis. This corresponds to the rider initiating and exiting a turn that causes the board to twist along the longitudinal axis. The front section of the board twists in one direction about the longitudinal axis, whilst the rear section of the board twists in the opposite direction about the longitudinal axis. A graphical representation of the key loading conditions that occur during riding is shown in Fig. 7.2. Also, an additional pre-load is applied by the rider at the forward and rearward inserts. A compressive load is applied to the binding regions when the board is bent due to the loading conditions described in the ﬁrst two points above or under the weight of the rider.

© 2007, Woodhead Publishing Limited

188

Materials in sports equipment Bends upwards Tail

Transverse shear load

Tail Rearward inserts Transverse axis

Tip

Longitudinal axis Tail

Bends upwards

Longitudinal shear load Tip Transverse Edge' axis Edge Longitudinal axis R2

Torsional load

Tail

Rearward inserts

Forward inserts

Heel Transverse axis

Tip Waist Shovel

Longitudinal axis

R1

7.2 Typical dynamic loads that are experienced during riding. Figure adapted from: US Pat. 6,520,530 B1.

7.3

Snowboard design

A snowboard is a thin, multi-layered composite structure. In general, it includes a tip, a tail and opposed heel and toe edges. The width of the board typically tapers from the tip and tail towards the central region of the board (the waist), facilitating turn initiation and exit and edge grip. The design features that are common to all snowboard types are shown in Table 7.1, while Fig. 7.3 gives a graphical representation. The geometry of a snowboard is governed by ﬁve key parameters, which can be seen in

© 2007, Woodhead Publishing Limited

Design and materials in snowboarding

189

Table 7.1 Design features common to all snowboard types Tip/tail

The forward and rearward section of the board

Shovel/heel

The upturned ends of the board at the tip and tail

Waist

The width of the board measured from edge to edge at the centre of the board along the longitudinal axis

Forward/rearward inserts

Threaded holes on the top surface of the board to which the bindings are attached. The holes are tapped through the core to increase strength

Contact points

Located at the heel and shovel where the board ﬁrst touches the snow when laid on its edge

Camber

The curved nature of the contact area, also known as the running length, of the board. Snowboards typically have a camber at their mid-length for assisting board resilience

Tip Shovel Contact points Forward inserts

Waist

Upturned ends

Camber

Rearward inserts

Heel Tail

7.3 Common snowboard design features. Figure adapted from: US Pat. 6,309,586 B1.

Table 7.2, and a graphical representation of the design features governing snowboard geometry is shown in Fig. 7.4. There are two types of snowboard manufacturing techniques, sandwich construction and cap construction. Sandwich construction uses plastic strips bonded to the side of the core to protect it from the elements. This requires more material and is heavier, but can be easier for homebuilders to deal with.

© 2007, Woodhead Publishing Limited

190

Materials in sports equipment

Table 7.2 Key design parameters governing the snowboard’s shape Running length

The distance along the central axis of the board between the front and rear contact points

Effective edge

The distance along the central axis from the indent point of the tip to the indent point of the tail

Sidecut radius

The radius between the front and rear contact points along the central axis of the board which can be expressed as a quadratic function

Stance width

The distance between the forward and rearward inserts

Nose/tail length

The distance from the forward and rearward contact points to the tip of the board

Sidecut radius

Stance width Effective edge

Running length

7.4 Design features that govern snowboard geometry. Figure adapted from: US Pat. 6,309,586 B1.

Cap construction has a tapered edge to the core, and the top reinforcement and topsheet wrap around the taper and meet the base. It is lighter and its strength is comparable to a typical sandwich section. A graphical representation of each type of snowboard cross-section is shown in Fig. 7.5. A range of design features are common to both types of board construction. The torsion box core provides the board with its structural strength. The vertical sidewalls support edging forces of the snowboard when carving turns and thus deﬁne the board edging strength. In addition, they also protect the reinforcement ﬁbres of the torsion box core from abrasions resulting from side impacts.

© 2007, Woodhead Publishing Limited

Design and materials in snowboarding Upper structural layer

Topsheet

Core

Topsheet Sidewall

Core

Rubber layer Stainless steel edge

Upper structural layer Torsion box

191

Lower structural layer Running board

Lower structural layer

Rubber layer Running board

Stainless steel edge

7.5 Cross-sections of sandwich and cap type snowboards. Figures adapted from: US Pat. 5,782,482 and US Pat. 5,769,445.

The sidewall provides protection for the core. While carving a turn or stop, it is common to encounter an object with the edge of the board which imparts a localized force on the vertical composite sidewall. In addition, it increases the edging strength and overall rigidity of the board. Thus, the sidewall also provides protection for the core in addition to increasing the edging strength and overall rigidity of the board. For cap construction, the upper structural layer and topsheet form the sidewall. Edging strength is primarily related to the strength of the vertical composite sidewalls of the torsion box construction formed around the base core. Edging strength determines the ability of the board to cut and hold an edge against a slope under forces of a turn or stop. Due to the difference between the thermal expansion coefﬁcients of the metal edges and the ﬁbreglass sheet, thin rubber sheets are placed between the two to act as a thermal expansion interface. In addition, the rubber provides dampening by buffering shear forces. The rubber directly overlies the steel edge. In general, a snowboarder desires various degrees of longitudinal and torsional rigidity depending on the conditions and style. Longitudinal rigidity characterizes the board’s ability to bend along its length, whilst torsional rigidity describes the board’s ability to ﬂex and twist about its longitudinal axis. For downhill speed, a stiff board is generally preferred wherein the long and torsional stiffness are limited. In contrast, a soft board having increased long and torsional stiffness is desirable for performing tricks and manoeuvring among moguls and bumps. Torsion box cores have had limited degrees of freedom (DOF) between longitudinal and torsional rigidity. A successful snowboard design must meet a number of design requirements, both objective and subjective. Objective design requirements determine the product’s ultimate effectiveness or suitability for the given task, while subjective requirements are not as well deﬁned and may be interpreted differently by each individual customer. Typical objective and

© 2007, Woodhead Publishing Limited

192

Materials in sports equipment

Table 7.3 Objective design requirements for a snowboard Riding style

Be it freestyle or freeride, it determines the desired performance characteristics of the board

Dimensions

Primarily length and width of board for different riding styles and terrains, sex, weight, height and stance of riders

Strength

Determined primarily by the quality and type of materials used in construction (especially the reinforcement ﬁbres)

Stiffness

Bending and torsional stiffness of board determined primarily by the structure and materials, as well as by the riding style

Weight

Lightweight boards have increased manoeuvrability

Cost

Low-, mid- or high-end boards

Manufacturing method

Sandwich construction or injection-moulding

Table 7.4 Subjective design requirements for a snowboard Aesthetics

Colours and texture: Targeted towards a particular demographic Graphics: Identiﬁes the size and type of board and provides additional graphic features

Ergonomics

Relates geometry (dimensions and shape) and weight of board to speciﬁc rider proﬁle and preferences

Feel

Subjective perception of snowboard design performance by individual rider (usually based on the perceived weight and response to riders input)

subjective design requirements for a snowboard are shown in Tables 7.3 and 7.4, respectively.

7.4

Materials and their conﬁguration in snowboards

In general, the multi-layer conﬁguration of a snowboard includes a core, top and bottom reinforcing layers that sandwich the core, a top cosmetic

© 2007, Woodhead Publishing Limited

Design and materials in snowboarding

193

Topsheet

Upper structural layer

Core Lower structural layer

Running base

7.6 The multi-layer conﬁguration of a sandwich type snowboard. Figure adapted from: burton.com/tech.

layer and a bottom gliding surface that is typically formed from a sintered or extruded plastic. The multi-layer conﬁguration of a sandwich type snowboard is shown graphically in Fig. 7.6. The core runs along the length of the board, with the widest section abutting the base. Its weight, thickness and durability determine the riding characteristics of the board. Most cores are constructed from vertically laminated strips of wood since a single plank of wood contains knots and irregularities that can affect its strength and ﬂexural performance. By using a variety of strips, the effects of such irregularities are negligible. A mixture of hardwood and softwood can be used to provide a balance between high strength and light weight. The width of the strips affects the core’s torsional stiffness; many thin strips increase resistance but increase weight due to a higher proportion of resin used. Common types of wood used in construction include spruce, ﬁr, ash and maple. Additionally, honeycomb panelling can be used to reduce weight. Honeycomb structures also exhibit excellent vibrational damping characteristics. Aluminium honeycomb is created by bonding strips of thin aluminium sheet, which are then drawn out. It is usually treated with corrosion inhibitor to reduce its susceptibility to water. Suitable cell size for snowboards is around 3 mm, with a density of around 65 kg/m3.

© 2007, Woodhead Publishing Limited

194

Materials in sports equipment

Sandwich designs use a vertical sidewall to protect the core and increase the edging strength of the board. In general, ABS (acrylonitrile–butadiene– styrene) is used as it has excellent temperature and impact resistance. The upper and lower structural layers bolster the structural rigidity of the snowboard. The upper structural layer surrounds the top and, in some conﬁgurations, the sides of the core. The structural layer is constructed of a thermoplastic having multi-directional, high-modulus reinforcing ﬁbreglass embedded in the thermoplastic. Fibreglass is a high-modulus reinforcement that is impregnated with a thermoset resin. In general, the layers are positioned such that the ﬁbres run parallel and perpendicular to the longitudinal axis of the board. This enhances the board’s longitudinal and torsional rigidity. In addition, it can be used to form a protective layer around the core. The topsheet extends over the top and, in some conﬁgurations, the sides of the core and the top structural layer. It serves two functions; it protects the upper structural layer of the board from abrasion and UV light (prolonged exposure can break down epoxy bonds), and it covers the board graphics. In general, a thin layer polyethylene glass is used as it is capable of bonding to epoxy, is lightweight, transparent and temperature resistant. Finally, metal edges, generally stainless steel, wrap around the full perimeter of the board, providing a hard gripping edge for board control over snow and ice. The edges have an L-shaped cross-section, which resides along the bottom plastic layer with the lower portion sandwiched between the lower ﬁbreglass sheet. Due to the difference between the thermal expansion coefﬁcients of the metal edges and the ﬁbreglass sheet, thin rubber sheets may be placed between the two to act as a thermal expansion interface. In addition, the rubber provides dampening by buffering shear forces. It should be noted that the edges have a higher coefﬁcient of friction and mass in comparison to the running base and hence their area should be kept to a minimum. The base layer commonly comprises a high-density plastic know as ultrahigh molecular weight polyethylene (UHMWPE) that provides a smooth, low-friction bottom layer that greatly enhances the riding experience. UHMWPE is a thermoplastic polymer that is classiﬁed as a liner homopolymer, that is, all its monomers are the same. Its molecular chain can consist of as many as 200 000 repeated ethylene units. The polymer’s molecular weight is typically less than 50 000 g/mol. For a polymer to be classed as ultra-high, its molecular weight must be greater than 2 000 000 g/mol. UHMWPE molecular weight is around 6 000 000 g/mol. The extremely high molecular weight of UHMWPE material yields several extraordinary properties that are perfect for snowboarding applications. These include superior abrasion resistance, chemical resistance, the highest known impact

© 2007, Woodhead Publishing Limited

Design and materials in snowboarding

195

Upper/lower structural layer Topsheet

Core

Running base

Sidewall Steel edges

7.7 Three-dimensional section view of a snowboard sandwich structure.

Table 7.5 Material properties for sandwich laminated snowboard Layer

Material

Topsheet Structural layers Core Sidewall Running base Steel edges

Polyurethane E-glass ﬁbreglass Wood ABS UHMW polyethylene Steel

Properties E (GPa)

r (kg/m3)

m

1.5 80 14 1.7 0.82 210

940 2500 500 1050 790 7800

0.47 0.26 0.26 0.49 0.16 0.33

strength for any thermoplastic and excellent mechanical properties, even in cryogenic conditions. Outstanding properties also include a low coefﬁcient of friction (∼ 0.15) and non-stick and self-lubricating properties which aid wax retention. The three-dimensional section view of a sandwich type snowboard structure with different layers is shown in Fig. 7.7, and the material properties corresponding to each of these respective layers are given in Table 7.5.

7.5

Manufacture of snowboards

There are two main types of snowboard manufacturing techniques: injection-molding and laminate construction. The laminate method typically

© 2007, Woodhead Publishing Limited

196

Materials in sports equipment

uses a wood core which has a structural function and is placed inside layers of stiffening material of various kinds, placed between a base and a topsheet. Injection-moulded boards use a mould into which the upper and lower external parts of the snowboard is placed. The polyurethane foam is then injected between the upper and lower layers. Most modern snowboards are produced by laminating several materials together to form a ‘sandwich’ structure. This consists of sandwiching multiple layers together using resins and industrial adhesives, which are then placed in a press. Laminated boards have set the standard for the snowboard industry in terms of aesthetics and performance. The boards are strong enough to withstand the loads applied by the rider and ﬂexible enough to absorb loading experienced during manoeuvres. However, laminated boards experience several disadvantages. The laminated layers are held together by adhesives. The layers tend to separate over prolonged use. Furthermore, ice and snow can penetrate into the cracks between adhered layers and destroy the structural integrity of the board, effectively delaminating the layers and the metal edges. The inserted screw threads also depend on the strength and quality of the adhesives. The threads may loosen and spin with the rider. Critically, modern laminating techniques are very time consuming and environmentally harmful, with several hours of manual labour required to adhere each individual layer while shaping the ﬁnished product. The construction of a laminated snowboard consists of three main steps: preparation, lay-up and ﬁnishing. The ﬁrst step of the preparation process is to glue together a variety of types of wood to suit the required bending and torsional characteristics of the respective riding style, as shown in Fig. 7.8(a). The base is then cut to size using a template and a router. The rough-side edges are slightly rounded off so that the radius on the steel edges will ﬁt accurately. The insert holes are drilled and countersunk (Fig. 7.8(c)), with the inserts pre-bonded to the core with some of epoxy. It should be noted that it is critical to plug the holes with wax for cold cures or grub-screws for hot cures. The sidewalls are then bonded to the core with epoxy, as can be seen in Fig. 7.8(c). The core is then sanded, since a rougher surface is desirable in order to create a better bond. Finally, the upper and lower ﬁbreglass sheets are cut to size. Around 25 mm of overhang should remain to ensure that the entire edge is ﬁlled with ﬁbre. Board lay-up begins when all of the preparation stages are complete; all components are dry and clean. The running base is placed in a press, as can be seen in Fig. 7.9(a). The resin used in the lay-up process can then be mixed, using around 400 g per batch. A composite includes reinforcement ﬁbres and a matrix binder. The reinforcement ﬁbres provide the composite with its primary tensile strength, whereas the matrix binder serves to molecularly intercouple the reinforcement ﬁbres within its own polymerized, crosslinking structure. The matrix binder is an epoxy-resin mixture in a 50 : 50

© 2007, Woodhead Publishing Limited

Design and materials in snowboarding

(a)

197

(b)

(c)

7.8 Preparation process for manufacturing of a laminated snowboard (grafsnowboards.com).

(a)

(f)

(b)

(c)

(e)

(d)

7.9 Lay-up process (grafsnowboards.com).

ratio, and the curing agent comprises a hardener. The epoxy and curing agent are combined together. The stainless steel edges are then bonded to the running base. The edges are cut to length. It should be noted that annealing will allow the edges to be bent around the perimeter of the base. Drops

© 2007, Woodhead Publishing Limited

198

Materials in sports equipment

of glue are placed on the edge’s teeth, as shown in Fig. 7.9(b), at regular intervals, after which they are clamped in place until the glue cures. The top layer of the running base can then be coated with epoxy resin. The lower structural layer is then placed onto the epoxy-coated base layer. It should be noted that these layers require a signiﬁcant curing period (an average of ten minutes is required for curing) and are difﬁcult to work with. The entire base must be covered in resin, as shown in Fig. 8.9(c), ensuring that there are no creases, and that there are some material overhangs around the edges of the base. The snowboard structure is then developed by applying another coat of epoxy to the top of the structural layer (Fig. 7.9(d)). The bottom layer of the core is then fully coated in epoxy resin, after which it is placed on top of the lower structural layer, as shown in Fig. 7.9(e), paying careful attention to accurate central positioning with respect to the lower layers. Finally, the bottom layer of the topsheet is coated in epoxy resin and placed on top of the upper structural layer, as shown in Fig. 7.9(f). The press is then closed. An example of a typical snowboard manufacturing press is shown in Fig. 7.10. Features include an adjustable press with a 3 HP air compressor, a band saw and edge grinder for the ﬁnishing process. The ﬁnal stage of the manufacturing process is ﬁnishing. It encompasses the removal of the board from the mold to the tuning of the completed board. After the required cure time has elapsed, the press is opened and the board is carefully extracted, as can be seen in Fig. 7.11(a). Noting that it may have stuck to the press at certain locations, it should not be forced out as the epoxy resin does not reach full strength for about two weeks. If delamination occurs at this stage, the structural integrity of the board could

7.10 TF-1000 snowboard production press. (www.snowboardmaterials.com/pages/TF_2000_kit.htm).

© 2007, Woodhead Publishing Limited

Design and materials in snowboarding

199

be greatly compromised. The topsheet and base should be checked for smoothness and any problems, such as rough spots. At this stage, the board should be scrapped if any major problems are found. A bandsaw can be used to cut away the excess resin and ﬁbre, known as mould ﬂash, as shown in Fig. 7.11(b). The remaining resin can be removed using a coarse ﬁle, as shown in Fig. 7.11(c), which produces a clean and shiny ﬁnish. After the resin has fully cured, a sharp blade can be used to cut off seeped resin. Any holes should be ﬁlled with thickened epoxy-resin, as bubbles or small porosities in the resin can lead to delamination if any water is able to penetrate the core. The insert holes, which were blocked with grub-screws during the preparation stage, are exposed via drilling, as shown in Fig. 7.11(d). A ﬁne drill bit can be used to create the initial tap, after which an Allen key can be used to remove the grub-screws. Finally, a wide drill bit

(b)

(a)

(d)

(c)

7.11 Finishing process (grafsnowboards.com).

© 2007, Woodhead Publishing Limited

200

Materials in sports equipment

is used to create a countersunk rounded edge around each thread. The topsheet should also be tested for any air voids simply by tapping it gently with a ﬁnger. Voids can be carefully ﬁlled by drilling two small holes on both sides and injecting epoxy resin using a syringe. The topsheet can then be polished to remove any ﬁne scratches. The completed board is run through a grinding machine in order to polish the base. The board is passed through until the base is determined to be free from resin and any rough spots. The base is then textured using a special diamond-dressed stone to cut a pattern of ﬁne grooves and to polish the edges. The grooves further promote the wax-retaining properties of the UHMWP running base. A thin layer of wax can then be applied using a hot iron. Finally, the base is polished with a soft cloth to create a glossy shine. A major disadvantage of the lamination method of snowboard manufacturing is the time and cost associated with manufacturing multi-layer laminated snowboards to meet speciﬁc aesthetic and performance requirements. The injection-moulding process offers a signiﬁcant decrease in time and cost in comparison. In the injection-moulding process, the upper and lower layers of the board are positioned in a jig, with respect to the desired core proﬁle (see Fig. 7.12). A low-density PVC foam is then injected between the upper and lower structural layers to form the core. Advantages of the injection-moulding technique include a reduction in the overall weight of the board, due to the use of PVC foam for the core. Disadvantages include a lack of mechanical bond strength between layers, which can lead to a reduction in board stiffness over time. Snowboard manufacture introduces additional hurdles into the injection and co-injection molding process, due to the incorporation of non-moulded components, namely the stainless steel edges. This necessitates the need to adjust for shrinkage of the polymer as it cools in the mould.

7.6

Summary and future trends

The chapter introduced snowboarding technology and the different structures and materials used in board design and manufacture. Two particular structures, sandwich and cap type, have been described in detail. In general, the multi-layer conﬁguration of a snowboard includes a core, top and bottom reinforcing layers that sandwich the core, a top cosmetic layer and a bottom gliding surface or base. The core represents the ‘guts’ of the snowboard as it creates life or liveliness in the board, also described as the snap energy. Wood is the preferred material for cores in most competitive snowboards. Reinforcement ﬁbres, such as ﬁbreglass and carbon ﬁbre, provide strength to the board structure. All board bases are made from polyethylene (PE) plastic using either extrusion or sintering process.

© 2007, Woodhead Publishing Limited

Design and materials in snowboarding

Insert

201

Upper half cavity

Snowboard

Insert

Insert

Lower half cavity

7.12 Exploded view of a snowboard injection-moulding press. Figures adapted from: US Pat. 6,349,961 B1.

Sintered bases have a higher molecular weight than extruded bases resulting in better glide performance and wax retention. The chapter introduced two types of manufacturing techniques for snowboards: injection moulding and laminate construction. As most modern snowboards are produced by laminating several materials together to form a sandwich structure, this method was described here in greater detail. At present most renowned snowboard manufacturers aim to produce lightweight boards with enhanced responsiveness. It is likely that this practice will continue with boards becoming increasingly versatile and used across different riding styles and terrains. This will be achieved primarily by using new advanced materials such as polymer-reinforced ﬁbres and even cellular materials. Also, the development and application of new types

© 2007, Woodhead Publishing Limited

202

Materials in sports equipment

of structures can be expected in order to control bending and torsional stiffness in a more effective way while enhancing the board responsiveness. These characteristics will further enhance the board on-snow performance, such as tracking, turning, stopping, stability control and manoeuvrability in general. Following the trends from other sporting goods manufacturing sectors (such as sports shoe manufacturing), it is expected that mass customization will become the norm in snowboard design and manufacture in the future as well. This will allow customers to specify some of the key design features and options that they personally prefer in snowboards. This may involve specifying the length and width of the board (based, for example, on personal weight, height and stance), selecting the desired colours and graphics, specifying personalized designations and signage on the board, etc. This will require the development of more advanced automated design and manufacturing processes and technologies. The sport of snowboarding is continuously evolving; hence it is very likely that the associated technology will continue to evolve as well in the future, perhaps more rapidly than in other more traditional sports.

7.7

Acknowledgements

Much of the information in this chapter has been taken from the research in snowboard design undertaken at RMIT University by the Sports Engineering research group. The authors of this chapter would also like to acknowledge the resources used in the discussion of manufacturing processes that have been adopted from a range of snowboard manufacturers, in particular Burton Corporation, Graf, K2, Jumbo Snowboards and others. These resources have been invaluable in the preparation of this chapter.

7.8

References

1. brennan s m (2003), Modelling the mechanical characteristics and on-snow performance of snowboards, PhD Thesis, Stanford University. 2. sutton e b (2000), Better snowboards by design, Proceedings of IMECE: International Mechanical Engineering Congress & Exposition, Orlando, FL, 1–5. 3. dodge d j, smith r p and ﬁdrych p j (2003), The Burton Corporation, Core for a gliding board, US Patent 6,520,530 B1, Feb. 18. 4. colley d, mcroskey d and mjelde o (2001), Jumbo Snowboards LLC, Use of co-injection molding to produce composite parts including a molded snowboard with metal edges, US Patent 6,309,586 B1, Oct. 30. 5. graf d (2005), Resources For Building Snowboards, Snowboard preparation/layup/ﬁnishing, www.grafsnowboards.com. 6. History of Snowboarding, www.sbhistory.de. 7. Technical Snowboarding Information, www.transworldsnowboarding.com.

© 2007, Woodhead Publishing Limited

8 Design and materials in ice hockey D. P E A R S A L L and R. T U R C O T T E, McGill University, Canada

8.1

Introduction

The origins of ice hockey date back to the 1880s in Canada and Europe; since then, it has evolved into a fast-paced game with international appeal. In addition to its growing popularity, hockey has become increasingly sophisticated in terms of technological innovations, equipment design, and improvements in training, coaching, and game strategies (Pearsall et al., 2000, Haché, 2002). Due to the specialized environmental conditions (e.g. low surface friction), ice hockey requires a unique skill set. These skills can be subdivided into general categories of skating, shooting, and checking. The intent of this chapter is to examine the mechanical aspects of skates and sticks.

8.2

Skate design

The modern skate design has evolved primarily as a result of trial and error on the part of designers in the ice hockey industry. Figure 8.1 denotes the basic features and appearance of the modern skate. Most ice hockey skates are designed using both leather and synthetic materials. Designers desire to optimize durability, performance, comfort, and ﬁt for the skater. The amount of each type of material used, including materials such as Kevlar® (DuPont) and graphite, depends on the quality of the skate. Recent designs include a moulded hard plastic boot, which offers good protection against blows from sticks and pucks and other hard objects. This type of skate can also provide a great deal of support for the skater’s ankles. Some recent designs using hard moulded skates respect the foot anatomy more closely and may offer better kinesthetic awareness during skating manoeuvres. The skate has a history that originates from activities unrelated to the sport of ice hockey. During the age of the Renaissance it has been documented that individuals used carved animal bones strapped to their boot soles as skates. Old paintings dating back to the 1400s depict individuals skating on 203

© 2007, Woodhead Publishing Limited

204

Materials in sports equipment Achilles’ tendon guard Eyelets for skate laces

Toe box Boot

Blade Blade holder

8.1 Depiction of typical modern ice hockey skate.

ice with this early skate model (Koning et al., 2000). Some of these early skating scenes were observed in Scandinavia. By the late 1880s the use of bones as blades had given way to metal blades strapped to a wooden slab ﬁtted to the sole of the boot. Further modiﬁcation included a complete metal blade which increased weight but decreased skating speed (Minkoff and Simonson, 1994). This was one of the ﬁrst skate design modiﬁcations, showing that changes to skate construction affected skating performance. Weight was decreased with the implementation of tubular skate blades in the 1950s and by the 1960s and 1970s blade ends were covered, increasing safety, and the use of composite plastics like polyethylene resins, carbonates, and ﬁberglass with metal blade assemblies resulted in a further decrease in mass facilitating skating speed and manoeuvrability (Pearsall et al., 2000). Thus, these previous design changes demonstrate that the alteration of variables such as the blade hollow, sharpness, geometry, as well as the materials used to construct the skate boot, can have a profound effect on the performance capabilities of the athlete (Minkoff and Simonson, 1994). For example, by decreasing the radius of the skate’s blade, the turn radius will decrease. In addition, the sharpness of the skate blade affects the ease of push off and stopping ability. A sharper blade makes it easier to push off but with the tradeoff of a more difﬁcult stop (Pearsall et al., 2000). Players will often customize radius and sharpness parameters of their skates to suit the perceived needs and demands of the positions that they play. However, there has been relatively little research conducted to elaborate on the effects of changes in skate design and skating performance.

© 2007, Woodhead Publishing Limited

Design and materials in ice hockey

205

In contrast, research on running shoes has shown that an optimal shoe design supports the movement pattern speciﬁc to the activity, therefore reducing muscular activity and increasing comfort (Reinschmidt and Nigg, 2000). Thus, by creating a shoe that is speciﬁc to the sport and its demands (i.e. movement patterns), energy expenditure could in theory be reduced and performance could then be optimized (Reinschmidt and Nigg, 2000). It is not possible to make the same bold statements with respect to ice hockey skate design since research aimed at systematically evaluating design changes and their effects on kinematic and kinetic skating performance characteristics is still in its infancy. We have conducted a number of recent studies in an effort to begin this process of characterization of the impact of speciﬁc skate design changes on comfort, ﬁt, and performance of ice hockey skates. Most of the work done in ice hockey has been what one might call ‘reverse engineering’ since many products have been conceived by intuition and evaluated post hoc. In order for a skate design to provide optimal function it would seem logical that a number of conditions must be met when evaluating the design. For skate design to be considered optimal, a skate must: • • • • • •

permit adequate kinaesthetic sense of joint position and limb orientation; avoid pinching of sensitive soft tissue areas overlying muscles and neurovascular structures; accommodate geometric anthropometrics and orientation of bony structures; provide for effective anterior posterior and medial lateral alignment stability and range of motion; provide for effective fore foot and rear foot leverage for controlled blade movement; accommodate the restriction of joint movements and coupled foot– ankle–knee–hip chain coordination.

8.3

Evaluating skate design

Not all of these aspects of skate design have been evaluated. However, we have evaluated the impact of skates on a skater to determine the effect the skate has on the kinematics of the skating stride. Pearsall et al. (2000) and Hoshizaki et al. (1989) have pointed out that the use of different materials (leather, polyethylene shells, lacing) and the construction of the skate boot (sewing, gluing, material orientation and layers) can alter stiffness characteristics which in turn may lead to an increased support but may also potentially restrict movement during skating tasks. For example, a high cut boot provides medial/lateral support during turning tasks but, depending

© 2007, Woodhead Publishing Limited

206

Materials in sports equipment

on the skate construction, could restrict plantarﬂexion and dorsiﬂexion during striding and push-off events. Two approaches have been pursued to evaluate modiﬁcations in skate design and its impact on skating kinematics. Electrogoniometers have been used (Fig. 8.2) to measure dorsi/plantarﬂexion (DF/PF) and inversion/ eversion (INV/EV) in barefoot conditions while wearing skates. Two- and three-dimensional video analysis have also been used to corroborate the results. The range of motion in plantarﬂexion is restricted when the foot is placed in a standard ice hockey skate compared to a barefoot condition. We measured the range of motion in dorsi/plantarﬂexion and inversion/eversion in the barefoot condition and compared this to the same motion in a standard skate. As can be seen in Fig. 8.3 dorsi/plantarﬂexion was restricted when wearing a skate. Related to this, we also conducted a pilot study investigating the effect of modifying skate construction to eliminate some movement restrictions during forward skating. This modiﬁcation consisted of removing the Achilles’ tendon protector. The pilot study showed an increase in dorsi/ plantarﬂexion (22° modiﬁed versus 15° standard skate) but a decrease in inversion/eversion (7° versus 12°) with the modiﬁed skate (Fig. 8.4).

8.2 Skater instrumented with electrogoniometers.

© 2007, Woodhead Publishing Limited

90 80

ROM (degrees)

70

Barefoot One90

60 50 40 30 20 10 0 IN/EV

PF/DF

8.3 Range of motion in inversion/eversion or dorsi/plantarﬂexion with and without a hockey skate (Dewan, 2004). Comparison of plantar/dorsi flexion –5 Mod Reg

(a) –10

Angle

–15

–20

–25 Comparison of inversion/eversion

–10

Mod Reg

(b)

Angle

–15

–20

–25 0

10

20

30 40 50 60 70 Percentage of stride

80

90 100

8.4 Range of motion in subject skating with a modiﬁed skate and a regular skate for (a) plantar/dorsiﬂexion and (b) inversion/eversion. © 2007, Woodhead Publishing Limited

208

Materials in sports equipment

However, only one subject was evaluated in this pilot study and further observations are needed to conﬁrm this result. Another study using a ‘minimalist’ type of test skate with no support above the malleoli and similar to our study showed that a variety of skate models were able to displace the ankle joint to the same extent as the test skate (Hoshizaki et al., 1989). This ‘test skate’ also removed the Achilles’ tendon protector, therefore removing what would be considered to be movement restrictions of the ankle. Unlike our results, this study revealed similar range of motion in the test skate compared to the standard skate with dorsi/plantarﬂexion range of motion comparable at about 15–20° (Hoshizaki et al., 1989). Differences in test method may have accounted for the disparity of observed range of motion; that is, the latter study used two-dimensional ﬁlm analysis of external markers that would not necessarily detect foot/ankle motion internal to the boot. Thus, as presently constructed, the standard hockey skate does introduce some movement restriction in the ankle joint, and it may be possible to modify it further to allow a greater range of motion. However, range of motion is but one factor that relates to skating performance. The task of skating in ice hockey is very complex since it includes a variety of skating skills. Many distinctly different yet essential manoeuvres are necessary to be able to perform efﬁciently in the context of a game situation. In addition, changing mobility in the ankle has implications for the coupling and coordination of the kinematics of the entire body, especially hip, knee, and ankle chain. We have also used these electrogoniometers to determine the range of motion found during skating on-ice in elite ice hockey players. These results are presented in Table 8.1 and provide a summary of ranges of motion during starts, stops, and steady state forward skating. As can be seen from these results the range of motion is different depending on the task undertaken by the skaters.

Table 8.1 Summary range of motion data (degrees) for forward skating following a parallel start (Pearsall et al., 2001) Phase

Skate

Eversion

Inversion

Total ROM

Plantar

Dorsi

Total ROM

Parallel start

Lead foot Rear pushoff

2.6 5.6 6.1 0.4 4.1

8.0 2.0 6.9 3.1 −0.1

10.6 7.5 13.0 3.5 4.1

5.9 2.7 3.1 −3.1 −2.2

15.8 10.2 17.4 10.6 10.7

21.7 12.9 20.5 7.5 8.5

Steady state Parallel stop

Inside edge Outside edge

© 2007, Woodhead Publishing Limited

Design and materials in ice hockey

209

Electrogoniometric data have been corroborated by two-dimensional video analysis (McCaw and Hoshizaki, 1987), revealing a total displacement of about 20° of dorsi/plantarﬂexion in the ankle during forward skating. We have also conducted recent kinematics studies using a skating treadmill and video analysis (Fig. 8.5) to determine three-dimensional skating kinematics during forward skating. This evaluation of forward skating on a skating treadmill has shown that the total range of motion in dorsi/plantarﬂexion was about 27° with about 22° of inversion/ eversion motion (Upjohn, 2006). Thus, the few studies that have been conducted have reported a similar range of motion in the ankle in standard skates. A previous investigation also examined the range of motion during the acceleration phase from a start to maximum striding speed during forward skating (Dewan, unpublished Master’s thesis). An interesting ﬁnding from this study was that the range of motion was smaller in the ﬁrst several strides compared to the range of motion achieved during steady-state skating (Fig. 8.5). Taken together, these results support the notion that the dynamics of the skating stride change depending on the nature of the skating task undertaken. The ankle therefore does not seem to be using the full anatomical range of motion available to it during skating in ice hockey regardless of whether motion is restricted at that joint. The range of motion observed during skating in ice hockey must not be based solely on anatomical restrictions but also on the kinematics of the skating stride. The kinematics used presently may also be dependent on the skate design, and further

Ankle: sagittal plane

5 PF

0 CONST A1 A2 A3

Angle

–5

–10 DF

–15 –20 0

10

20

30 40 50 60 70 Percentage of stride

80

90 100

8.5 Range of motion in ﬁrst three strides (A1, A2, A3) compared to steady-state skating.

© 2007, Woodhead Publishing Limited

210

Materials in sports equipment

investigations are required to determine the impact of design changes on skate dynamics.

8.3.1 Analysing and improving the stiffness properties of skates One explanation for not using the full range of motion may be related to the generation of appropriate forces both for support and for power generation during striding. Thus, the skate is likely used as a lever that helps support and generate force and, because of the dynamics of skating, it is possible that this precludes the use of the full anatomical range of motion of the ankle joint. For example, one related aspect of interest to athletes and consequently to the sporting equipment industry in recent years has been the stiffness of the skate boot construction. It has been a common perception that a stiff skate offers better support and, with the right materials, also offers better protection. We constructed a jig (Fig. 8.6) that makes possible the measurement of the stiffness of the skate during the execution of simulated on-ice tasks (Turcotte et al., 2001). In this study the jig was used to measure the load in Newton versus the extension in cm (stiffness/ rigidity modulus) in various movements including dorsi/plantarﬂexion, eversion, inversion, and medial (MT) and lateral torsion (CT) movements.

8.6 Jig for measurement of skate stiffness properties.

© 2007, Woodhead Publishing Limited

Design and materials in ice hockey

211

The results of these studies conﬁrmed that it was possible to manufacture a ‘stiffer’ skate, which was the goal of the equipment maker for whom we constructed the jig. As can be seen in Table 8.2, skates designed to be stiffer (B5000) were indeed stiffer than the other prototype models (A90, B1000). However, several questions emerged from this study. Was such an increased stiffness truly desirable? What effect does increasing stiffness have on the ﬁt and comfort of the skate? What are the possible advantages of increasing stiffness of the skate boot from a performance perspective? We have attempted to answer these questions with a series of experiments using a pressure sensing technology (piezo-resistive sensors, FSA Inc.). One such study examined the measured pressures (kPa) in various areas of the skate to document where in the skate high pressures were experienced by athletes wearing the skates. Mapping of pressure patterns in different areas of the foot in this manner allowed us to determine if the skate ﬁt was appropriate and was suggestive of possible design changes that would improve ﬁt by decreasing overall pressure hot points. A common problem of athletes is having a comfortable ﬁt without the skate being too ‘loose’ so as to negatively affect a perceived snug ﬁt of the skate. This study enabled us to map pressure patterns and document which areas of the foot exhibit the greatest and lowest pressure points with the present conventional modern skate design. Such a study has important implications for determining the ideal ﬁt for comfort and appropriate skate boot ﬁt (Gheorgiu et al., 2004). In addition, the effect of ﬁtting varying skate size on pressure points and perception of comfort of the subjects wearing these skates were also examined (Gheorgiu et al., 2004). As can be seen in Fig. 8.7 the perception of pressure was negatively related to the perception of comfort. In addition to comfort and ﬁt issues that can be examined with such technology, it is also possible to confer performance advantages of skate construction. We have recently completed a series of studies looking at the interface between the skate and foot and pressure generation. Pressures

Table 8.2 Comparison of stiffness of different skate models (Turcotte et al., 2001) Skate model

EV

INV

PF

DF

MT

LT

B5000 A90 B1000

46.1 ± 14.5 31.5 ± 10.5 24.7 ± 8.2

35.6 ± 11.3 26.8 ± 8.9 27.7 ± 9.2

100.2 ± 31.7 89.9 ± 30.0 69.6 ± 23.2

79.8 ± 25.2 67.5 ± 22.5 54.2 ± 18.1

40.9 ± 12.9 31.8 ± 10.6 26.1 ± 8.7

31.1 ± 9.8 29.3 ± 9.8 22.8 ± 7.6

© 2007, Woodhead Publishing Limited

Materials in sports equipment Standing(St) Eversion(E)

Seated(Se) Plantarflexion(P)

Measured pressure (kPa)

140

10

*E > all other *P > I,Se *St > Se

120

*D > Se

9 8

*D > E.Se,St *I > E,Se,St *P > St

100 80

Dorsiflexion(D) Inversion(I)

7 6

*I > Se,St

5

60

PSI

212

4

*E > S

3

40

2 20

1 0

0 Medial 1st maleolae metatarsal

Instep

Achilles tendon

5th Lateral maleolae metatarsal

Heel

Area

8.7 Pressure patterns in different areas inside the skate boot. 120 100

Recreational left turn Elite left turn

Recreational right turn Elite right turn

KPa

80 60 40 20 0 1 10 19 28 37 46 55 64 73 82 91 100 1 10 19 28 37 46 55 64 73 82 91 100 Turn completion (%) Turn completion (%)

8.8 Pressures exerted during right and left turns in elite and recreational skaters.

were measured in various areas of this skate/boot interface during the execution of a variety of skills such as tight turns and forward and backward crossovers as well as forward skating. By comparing elite and recreational level skaters we were able to show that skilled skaters can take advantage of the new conventional skate design. It appears that a greater leveraging of the lateral and medial parts of the skate is possible during the execution of turning motions if the skater is skilled enough to take advantage of the skate’s construction properties. This is exempliﬁed in Fig. 8.8 showing that the greater pressures were generated in the lateral calcaneous inside the skate boot during the execution of tight turns. Elite skaters were

© 2007, Woodhead Publishing Limited

Design and materials in ice hockey

213

able to generate higher peak pressures as well as higher pressures during both left and right turn initiation (McGrail, 2006). Therefore, based on the differences seen here between elite and recreational skaters, it would seem that our ﬁndings support the notion that the stiffness properties of the skate are an important aspect of the skate design. The stiffness of the skate in the medial and lateral parts of the boot offers much needed support during the execution of skating maneuvers and provides a way of levering forces to support more rapid and powerful execution of these movements. Good skaters seem capable of taking advantage of the special features built into the design of the top-line skate models.

8.4

The design of ice hockey sticks

The stick is so named because it was originally made solely of wood. The ice hockey stick is the tool used to control the puck’s position and movement both while skating and passing/receiving, in offensive and defensive positions. The hockey stick is an extension of the hockey player’s arm. The stick has to match the player’s size, strength, preferred carrying (and shooting) side and playing style. The ice hockey stick has speciﬁc features (Figs 8.9–8.11). These include the following: • •

shaft – the straight, handle portion of the hockey stick; blade – lowermost, curved portion of the hockey stick, which is used for puck control and projection; • toe – the furthermost end of the blade; • heel – the angled portion of the hockey stick where the blade meets the shaft – the beginning of the blade; • hosel – the socket or neck portion of the lower shaft of a hockey stick, into which the blade is inserted; • lie – the angle formed between the blade and the shaft when the blade is ﬂat on the ice. A #5 is a lie angle of 45°. Each increment up or down corresponding to a change of 1.5°. Higher numbers indicate a smaller Butt Lie angle Blade Shaft

Hosel

Maximum shaft length 63'' (162 cm)

8.9 The basic components of a hockey stick.

© 2007, Woodhead Publishing Limited

214

Materials in sports equipment

Hosel

B C'

Top edge

D' Mid line

A

Toe Heel

C'

Bottom edge

D'

8.10 The basic components of a hockey stick blade.

X

Maximum blade curvature depth ½'' (1.3 cm)

Maximum blade length 12½'' (32 cm)

X D'

C' Y Z

Blade width 2''–3'' (5.1–7.7 cm)

D'

C'

8.11 The basic components of a hockey stick blade curvature.

© 2007, Woodhead Publishing Limited

Design and materials in ice hockey

•

•

215

angle between the blade and the shaft, while smaller numbers indicate a larger angle. Lie angles are typically rated on a scale from 4 to 8. As a general guide, lower lie angle sticks are used for players who skate low to the ice and carry the puck out in front whereas lies of 7 and 8 are for players who skate upright and carry the puck close to their skates; blade curve (pattern) – refers to the shape of the curve in the blade provided during manufacturing – blades are pre-curved for either a left or right side; butt end – the top (proximal) end of the shaft, towards where the player’s top hand is located.

Several stick dimensions are delimited by regulations; for example, maximum stick and blade length as well as blade width and maximum depth of blade curvature. In terms of optimal length when wearing skates, a general guide is for the upright hockey stick (toe blade to butt) to extend upward in the range of the chin or slightly below (7.5 cm or 3 inches). Blade curves are classiﬁed (e.g. heel-, mid-, or toe-curve) based on the location of the origin of the curve when the blade is laid ﬂat on the ice and viewed directly from above (Figs. 8.10 and 8.11). Various forms of blade twist or torsion patterns along the blade’s long axis exist. Hockey sticks are identiﬁed as ‘left’ or ‘right,’ referring to the direction of blade curve. The curvature of the blade allows greater control of the puck both in terms of stick handling possession and shooting accuracy (lift and spin). Conversely, blade curve compromises shooting and passing with the backhand.

8.4.1 Stick materials and construction The materials and construction of hockey sticks have changed considerably since their origin in the 1880s. Initially, whole sticks were cut in one piece from wood timbers or bolts. Local industries arose in conjunction with existing carpentry and furniture stores. Wood stocks were harvested such that the form and grain followed the general stick and blade shape. Hard woods, such as rock (or cork) elm were ﬁrst used given their durability, though they tended to be heavy (about a kilogram) and stiff. Top end sticks were free of knots or grain irregularities. The shapes of wood bolts were then modiﬁed by combination of steam and clamps, then kiln dried to approximately 8% moisture content. Blade ends were then dipped in varnish to prevent cracking (Dowbiggin, 2001). Such steamed sticks were prone to gradually reverting to their original shape in wet and warmer conditions; consequently, players often used electrical tape around their blade ends to reduce warping (a tradition persisting to this day for different reasons). As elm wood stocks became scarce, alternative woods and

© 2007, Woodhead Publishing Limited

216

Materials in sports equipment

construction techniques evolved c. 1920. Softer woods such as white ash became popular substitute materials due to their plentiful availability, decreased weight (less than 800 g), and greater shaft ﬂex. In addition, to overcome grain limitations in acceptable wood bolts, two-part sticks were developed, consisting of a blade inserted and glued into a mortise joint on the heel of the shaft (Dowbiggin, 2001). By the 1950s, three-part sticks were developed consisting of a separate heel joint about 20 cm long glued to the base of the shaft. By the 1960s and 1970s, laminates (for example, 14–21 multi-ply wood ‘wafer’ or ‘sandwich’ strips held together with epoxy) and ﬁbreglass composites became more and more prevalent. These newer materials diversiﬁed the available properties of stick ﬂexibility and responsiveness as well as permitting greater consistency in mechanical properties. To further lighten sticks (to approximately 600 g), some manufacturers hollowed the shafts. Wood hybrid sticks were introduced with blades and/or shafts of wood (aspen or birch) cores wrapped in ﬁbreglass or Kevlar/aramid, reinforced cloth (or laminates) and resin. Variations in weaving patterns, laminate densities, and epoxy resins have allowed manufacturers greater control over engineering the mechanical behaviour of the stick. Aluminium shafts with wood laminate or plastic blade inserts were also introduced in the 1980s but failed to hold a substantial market share, due primarily to problems with vibration and ringing in players’ hands and they were superseded by the next generation of materials: graphite (carbon ﬁbre) composites in the 1990s. Composite sticks became popular, despite being substantially more expensive than wood laminate models, because they offered further lightness and material fatigue resistance to bending. The major criticism of composites is their susceptibility to brittle fracture. In terms of composite sticks, various construction techniques are used such as resin transfer molding (RTM), pre-impregnated (pre-preg), fullwrap, and sandwich structures. RTM involves dry ﬁbres placed, then compressed, while resin and a catalyst are injected under low pressure. Pre-preg involves carbon or ﬁbreglass ﬁbres placed in a mold impregnated with resin. This process uses less resin than the RTM method, which can make prepreg blades and shafts lighter. Full-wrap refers to the outer layers of the blade or shaft being wrapped with carbon ﬁbres. Lastly, the sandwich structure involves ﬁbres being layered over each other but not wrapped. The orientation, number of layers, the continuity of layers, and the assembly process modulate the stick’s bending and torsional stiffness. Composite sticks may be constructed as true one-piece or fused (twopiece) products. The former is one continuous structure from shaft to blade. The latter consists of a separate blade and shaft that are joined together during the manufacturing process and superﬁcially appear as a one-piece stick. Since separate blades are inserted into the shafts, the hosel portion

© 2007, Woodhead Publishing Limited

Design and materials in ice hockey

217

of a fused stick is a solid structure that has increased torsional stiffness compared to a ‘true one-piece’ stick. Stick blades can be replaced on aluminium alloy or composite sticks by heating the shaft at the end of the stick and removing the blade.

8.5

Evaluating ice hockey stick design

The stiffness, or ﬂex, of a stick’s shaft is important in determining control and performance. The general dimensions of the stick’s cross-section are rectangular (3.2 cm × 1.9 cm or 1.25″ × 0.75″), corresponding to the major and minor axes respectively. Variation in materials and construction will determine the shaft’s (i.e. beam’s) effective stiffness. Bending primarily occurs about the stick’s major axis. No industrial standard exists to describe quantitatively shaft stiffness. In general, most stick shafts come in ﬂexes of medium, stiff, or extra stiff relative categories. Beginning players typically use a light stick with a medium stiffness rating whereas larger and stronger players typically choose a stick with a stiffer ﬂex. There is much variation in stick preferences; indeed many professional players will choose low-ﬂex sticks. Shaft stiffness may be determined in a number of ways. The simplest form is to apply known loads then measure the induced bend or rotational displacement (Marino, 1998; Simard et al., 2004). Stiffness may be expressed as the coefﬁcient of rigidity, which corresponds to the applied force divided by the stick’s displacement. For instance, if a blade were to bend 5 cm under a load of 160 N, the resulting coefﬁcient of rigidity would be 160/5 = 32 N/cm (Marino, 1998). More sophisticated material testing systems can be used not only to control the magnitude of load (or displacement) perturbations but also to assess the effects of loading rate or fatigue properties due to multiple load cycles. A typical means to determine shaft (beam) stiffness is a three-point bending test with central and/or cantilever loading. Deﬂections over the stick’s length characteristically can range up to 5 cm without fracture.

8.5.1 Performance testing Since the primary objective in ice hockey is scoring goals against the opposing team, ice hockey skills are predominantly goal-oriented, with the timing and organization of movements a secondary function of this pursuit (Pearsall et al., 2000). As such, the function of the hockey stick and the skills associated with it are of great importance to the overall success of a player or team. In fact, a survey of over 900 National Hockey League (NHL) scouting reports ranks three skills associated with the stick (i.e. shooting/scoring, puck control, and passing) in its top ten list of skills/

© 2007, Woodhead Publishing Limited

218

Materials in sports equipment

attributes in both forwards and defensive players (Renger, 1994). Of these skills, shooting and scoring were ranked most important in forwards, while puck control was ranked most important in defensive players. Players commonly use a wide variety of shots during a typical game situation. Since the ability to shoot with optimal velocity is a decisive factor in the overall performance of a player, the distinguishing feature between shots is often their velocity (Lariviere and Lavalle, 1972). To date, at least six different approaches have be used to quantify shot velocities – they are: impact velocity (Alexander et al. 1963, 1964), average velocity (Roy, 1974; Roy et al., 1974; Doré and Roy, 1976; Roy and Doré, 1976), instantaneous velocity (Chau et al., 1973; Sim and Chau, 1978), maximal velocity (Doré and Roy, 1978), radar (Pearsall et al., 1999; Wu et al., 2003), integration of accelerometer data (Villaseñor-Herrera et al., 2006). Based on these analyses, the slap shot consistently demonstrated the highest shot velocity (Table 8.3), while the wrist shot was deemed the most accurate shot (Alexander et al., 1963; Nazar, 1971; Pearsall et al., 1999; Wu et al., 2003). Table 8.3 Summary of puck velocities (km/h) reported in various studies (adapted from Pearsall et al., 2000) Author(s)

Method

Velocity Age

Slap

Wrist

Skate Stand Skate Stand Alexander et al., 1963 Alexander et al., 1964 Cotton, 1966 Furlong, 1968 Chau et al., 1973 Roy et al., 1974 Roy and Doré, 1976 Doré and Roy, 1976 Sim and Chau, 1978 Pearsall et al., 1999 Meng and Zhao, 2000 Wu et al., 2003

Ballistic

Impact

Adult

127

Ballistic

Impact

Varsity

121

Stop watch Cine Cine Sound

Average Instant Average Average

Adult Professional Adult Junior B Pee-wee

100 175 132 89

Sound

Average

Adult Adult

104

Cine

Max

High school

150

Max

Adult Varsity

200

Radar Cine

Instant

Elite adult

Radar

Max

Varsity Recreational Varsity

105 95 121

Recreational

80

Villaseñor-Herrara Accelerometer et al., 2006

© 2007, Woodhead Publishing Limited

Max

111

117

97

114 90 110 92 69 96 97

108 87

90 163 143 81

81 132 64

Design and materials in ice hockey

219

To determine the optimal stick design, materials and construction, various biomechanical studies involving human performance analyses have been conducted. Some of the proposed mechanical factors of importance during the slap shot are: (1) lower (distal) shaft velocity prior to puck contact, (2) pre-loading of the stick, (3) stick stiffness characteristics, and (4) puck– blade contact time (Doré and Roy, 1976; Hoerner, 1989; Marino, 1998). However, the precise relationships between these mechanical properties of the stick and shot performance have not been extensively investigated. Some examples of fundamental studies will be presented. Roy and Doré (1973, 1975, 1976) completed a series of studies involving the ﬁrst comprehensive kinetic analyses of various shots in ice hockey (e.g. slap, wrist, and sweep shots) with high-speed ﬁlm (200 Hz) and several strain gauges attached to the shaft and blade of the stick. Using multiple strain gauges placed along the shaft, the authors calculated forces exerted by the top and bottom hands and back side of the blade during the impulse (i.e. loading) phase. In their initial study, dynamic characteristics were obtained using high-speed ﬁlming and a series of eight strain gauges placed along the shaft and blade. Values obtained included: impulse phase, shaft deﬂection during impulse, puck velocity, and blade velocity during impulse. Using this conﬁguration the authors were able to calculate the location of the forces exerted on the stick during a slap shot. Recorded forces were approximately 20–30 N at the blade locations, 30–60 N at each of the top hand locations, and 50–80 N at each bottom hand location for one subject. However, there was a substantial amount of inter- and intra-subject variability during the slap shot trials. In a later study using 12 strain gauges (Doré and Roy, 1978), maximum forces tended to occur when the puck left the blade, at the top and bottom hands, and were determined to be 13–33% less in the ﬂexible shaft stick for each respective hand. Another study of note was conducted by Sim and Chau (1978) using cinematographic analysis to measure puck velocity and stick angular velocity. The authors also embedded force plates into the ice to measure vertical ground reaction forces during the skating slap shot, although no details were provided as to how this was accomplished. Puck velocities ranged from 150 km/h for high-school aged players to 200 km/h for college and professional players. Angular stick velocity ranged from 20 to 40 radians/s and vertical ground reaction forces ranged from 1.5 to 2.5 times player body weight. Pearsall and colleagues (1999) also examined the role of stick shaft stiffness in six elite male hockey players. Initial ground reaction forces, stick deformation, and puck velocity were measured using a force platform, highspeed ﬁlming system (480 Hz), and radar gun, respectively. Shafts with four different stiffness properties (i.e. medium, stiff, extra, and pro stiff) were tested. Vertical ground reaction forces were relatively low compared to

© 2007, Woodhead Publishing Limited

220

Materials in sports equipment

those of Sim and Chau (1978), ranging from 120 to 130 N (approximately one-ﬁfth of body weight), while anterior posterior forces were substantially less (16–25 N). Total contact time was 60 ms, as compared to 90 ms reported by Roy and Doré (1976). The highest puck velocity was associated with the medium stiff shaft (108.2 km/h), while the lowest velocity was associated with the extra stiff shaft (105.9 km/h). Peak stick deﬂection angle reached 20° (Fig. 8.12). From this study, signiﬁcant biomechanical execution differences existed between subjects. The authors conclude that subject parameters are likely to have more inﬂuence on slap shot performance than shaft stiffness. This supposition is further supported by subsequent work by Wu and colleagues (2003), which concluded that player skill and technique are primary determinants of slap shot performance. In a brief abstract, Baroud and associates (1999) reported preliminary ﬁndings to determine the amount of mechanical energy stored and returned in the shaft during pre-loading and impact, as well as the inﬂuence on puck velocity. A three-dimensional ﬁnite element model of a wooden hockey stick was created to quantify the deformation and displacement of the stick. Overall, the stick exhibited non-symmetric bending behaviours in the zand y-directions due to different bending stiffness and acting forces. The

8.12 Representative trial of a slap shot: shaft linear and angular displacement (1000 Hz). Such analysis assists in identifying dynamic bending behaviour and location of the shaft ﬂex point, i.e. point of greatest curvature (Haché, 2002).

© 2007, Woodhead Publishing Limited

Design and materials in ice hockey

221

maximum displacement occurred just under the lower hand and generated total deformation energy of 11 J. The authors noted that the deformation energy translated into an 11.3 m/s increase in puck velocity for the particular stick modeled. As such, the authors speculate that stick performance could be substantially improved by altering stick shape and construction material so as to maximize the kinetic energy return to the puck. Woo (2004) was the ﬁrst to examine the three-dimensional kinematics of the stationary slap shot in both elite and recreational hockey players. An electro-magnetic tracking device (60 Hz) was used to determine the kinematics of the players’ torso, arms, and hockey stick (Fig. 8.13). Increased shot velocity was attributed to an increased translational acceleration of the stick by the elite group, as compared to the recreational group who used more rotational acceleration. The elite group also demonstrated less variation in stick movement path and a more proximal-to-distal kinematic chain sequence than the recreational group. A recent study by Villaseñor-Herrera and colleagues (2006) also examined the energy storage and transfer during the pre-loading and impact stages (i.e. ‘recoil’ effect) of the stationary slap shot. Both elite and y b 1.16 m

0.46 m

1.93 m

x z

c a

8.13 Example swing path for ELITE group. Mean peak blade positions are indicated along each axis. Start, top of swing, and impact events are labeled a, b, and c respectively. Dotted lines indicate the path of the blade and butt ends of the stick during backswing, while the solid lines represent downswing.

© 2007, Woodhead Publishing Limited

222

Materials in sports equipment

recreational subjects had their slap shots evaluated through the simultaneous recording of high-speed video (1000 Hz) and a tri-axial accelerometer embedded into a hockey puck. Puck velocity proved to be inﬂuenced by skill level, blade–puck contact time, and stick bending energy, but not puck acceleration. The elite and recreational groups demonstrated a mean puck velocity of 120.8 km/h and 80.3 km/h, respectively. Puck velocity was highly correlated to both stick bending energy and total puck contact time.

8.6

Summary

This chapter has examined the mechanical aspects of skate and stick design and construction; in particular, methods for evaluating the physical interactions between equipment and player were presented as were representative biomechanical estimates. From the above information, it should be obvious that optimizing product development must involve co-ordinated human factors analysis.

8.7

References

alexander j f, haddow j b and schultz g a (1963), Comparison of the ice hockey slap and wrist shots for speed and accuracy, Res Quart, 34, 259–66. alexander j f, drake c j, reichenbach p j and haddow j b (1964), Effect of strength development on speed of shooting of varsity ice hockey players, Res Quart, 35, 101–6. baroud g, stefanyshyn d and bellchamber t (1999), Performance enhancement of hockey sticks using numerical simulations, XVIIth International Society of Biomechanics Congress, Calgary, Ab, 827. chau e g, sim f h, stauffer r n and johannson k g (1973), Mechanics of hockey injuries, New York, American Society Of Mechanical Engineers, 143–54. cotton c (1966), Comparison of ice hockey wrist, sweep, and slap shots for speed, MSc thesis, University Of Michigan, Ann Arbour, MI. dewan c j (2004), Biomechanics of the foot and ankle during ice hockey skating, MSc thesis, McGill University. doré r and roy b (1976), Dynamometric analyses of different hockey shots, In Komi P (ed.), Proceedings of the Fifth International Congress of Biomechanics, Baltimore, MO, University Park Press, 277–85. doré r and roy b (1978), Inﬂuence de la rigidité des bâtons sur la cinématique et la cinétique des tirs au hockey sur glace, Technical Report Ep 78-R-5, Montréal, QC, Ecole Polytechnique De Montréal. dowbiggin b (2001), The Stick: A History, A Celebration, An Elegy, Toronto, ON, Macfarlane, Walter, & Ross, 27–37, 65. furlong w b (1968), How science is changing hockey: 80 mph mayhem on ice, Popular Mechanics, February 110–14. gheorgui c, pearsall d j and turcotte r a (2004), Quantifying ﬁt in ice hockey skate boots, International Society of Biomechanics of Sport (ISBS), August 14–18, 509.

© 2007, Woodhead Publishing Limited

Design and materials in ice hockey

223

hache a (2002), Shooting, Physics of Ice Hockey, Baltimore, MD, Johns Hopkins University Press, 33–47. hoerner e f (1989), The dynamic role played by the ice hockey stick, in Castaldi C R and Hoerner E F (eds) Safety In Ice Hockey, ASTM STP 1050, Philadelphia, PA, ASTM International, 154–63. hoshizaki t b, kirchner g and hall k (1989), Kinematic analysis of the talocrural and subtalar joints during the hockey skating stride, in C R Castaldi and E F Hoerner (eds), Safety in Ice Hockey, ASTM STP 1050, Philadelphia, PA, American Society for Testing and Materials, 141–9. koning j, houdijk h, groot g and bobbert m f (2000), From biomechanical theory application in top sports: the klapskate theory, J Biomech, 33, 1225–9. lariviere g and lavalle h (1972), Evaluation du niveau technique de joueurs de hockey de categorie moustique (technical evaluation of young hockey players), Mouvement, 7, 101–11. marino g w (1998), Biomechanical investigations of performance characteristics of various types of ice hockey sticks, in Riele H J and Vieten M M (eds), Proceedings of XVI International Symposium on Biomechanics in Sports, Konstanz, Germany, 184–7. mccaw st, hoshizaki t b (1987), A kinematic comparison of novice, intermediate, and elite ice skaters, in M Jonsson (ed.), Biomechanics X-B, Il Champaign, Human Kinetics, 637–42. mcgrail s (2006), Dynamic foot pressure analysis of an ice hockey tight turn, MSc thesis, McGill University. meng x and zhao y (2000), Biomechanical analysis of four shooting techniques in ice hockey, in Hong Y and Johns D P (eds), Proceedings of XVIII International Symposium on Biomechanics in Sports, Hong Kong, 317–20. minkoff j v g and simonson b g (1994), Ice Hockey; Sports Injuries: Mechanisms, Prevention and Treatment, Baltimore, MD, Williams & Williams, 397–444. nazar p r (1971), Comparison between the curved blade and straight blade hockey sticks on shooting velocity and accuracy in university varsity ice hockey players, MA, University Of Minnesota. pearsall d j, montgomery d l, rothsching n and turcotte r a (1999), The inﬂuence of stick stiffness on the performance of ice hockey slap shots, Sports Eng, 2, 3–11. pearsall d j, turcotte r a and murphy s d (2000), Biomechanics of ice hockey, in Garrett W E and Kirkendall D T (eds), Exercise And Sport Science, Philadelphia, Pa, Lippencott, Williams & Wilkins, 675–92. pearsall d, turcotte r, lefebvre r, bateni h, nicolaou m, montgomery d and chang r (2001), Kinematics of the foot and ankle in forward ice hockey skating, in XIX International Symposium on Biomechanics in Sports, ISBS, San Francisco, June 20–26, 78–81. renger r (1994), Identifying the task requirements essential to the success of a professional ice hockey player: a scout’s perspective, J Teaching Phys Educ, 13, 180–95. reinschmidt c and nigg b m (2000). Current issues in the design of running and court shoes, Sportvel Sportschad, 14, 71–81. roy b (1974), Les lancers au hockey: retrospective et prospective biomechanique (hockey shots: retrospective and future biomechanics), Mouvement, 9, 85–9.

© 2007, Woodhead Publishing Limited

224

Materials in sports equipment

roy b and doré r (1973), Facteurs biomechaniques caracteristiques des different types de lancers au hockey sur glace (biomechanical factors of the different types of shots in ice hockey), Mouvement 9, 169–75. roy b and doré r (1975), Incidence des characteristiques des batons de hockey sur l’efﬁcacite gestuelle des lancers (Inﬂuence of hockey stick characteristics on the efﬁciency of shots), Ingenieur, 306, 13–8. roy b and doré r (1976), Kinematics of the slap shot in ice hockey as executed by players of different age classiﬁcations, in Komi P (ed.), Proceedings of the Fifth International Congress on Biomechanics, Baltimore, MD, University Park Press, 286–90. roy b, doré r, parmentier p h, deroy m and chapleau c (1974), Facteurs biomechaniques caracteristiques de differents types de lancers au hockey sur glace (Biomechanical characteristics of different types of ice hockey shots), Mouvement, 9, 169–75. sim f h and chau e v (1978), Injury potential in modern ice hockey, Amer J Sports Med, 6, 378–84. simard e, roy e, martin g, cantin h and therrien r (2004), Static and dynamic characteristics of composite one-piece hockey sticks, in Lamontagne M, Robertson D G E and Sveistrup H (eds), Proceedings of XXII International Symposium on Biomechanics in Sports, Ottawa, 515–8. turcotte r a, pearsall d j and montgomery d l (2001), An apparatus to measure stiffness properties of ice hockey skate boots, Sport Eng, 4, 43–8. upjohn t (2006), Three-dimensional kinematics of the lower limbs during forward hockey skating, MSc thesis, McGill University. villaseñor-herrera a, turcotte r a and pearsall d j (2006), Recoil effect of the ice hockey stick during a slap shot, J Appl Biomech, 22, 200–9. woo t k (2004), Three dimensional kinematic analyses of the stationary ice hockey slap shot: elite versus recreational, MSc thesis, McGill University. wu t-c, pearsall d j, hodges a, turcotte r, lefebvre r, montgomery d l and bateni h (2003), The performance of the ice hockey slap shot and wrist shots: the effects of stick construction and player skill, Sports Eng, 6, 31–40.

© 2007, Woodhead Publishing Limited

9 Design and materials in ﬂy ﬁshing G. S P O L E K, Portland State University, USA

9.1

Introduction

There is a common idyllic perception of ﬂy ﬁshing: a gentleman in tweed coat, a peaceful river meandering through green meadow, and a rhythmic waving of the ﬂyline through the air. However, the sport itself is quite demanding and anglers are not there just to look good. They expect to catch ﬁsh and want to use ﬂy ﬁshing equipment that gives them the best opportunity. And, as in most sports, they turn to technological solutions to meet these high expectations. Generally, three requirements are placed on ﬂy ﬁshing equipment: casting, ﬂy manipulation, and ﬁsh landing. Since the same equipment is used for all three functions, performance in one area may compromise that in others. So product design and material selection must be prepared to compromise while delivering the best overall performance. Before focusing on ﬂy ﬁshing, let’s consider ﬁshing equipment in general. These are the basic components of any ﬁshing system: 1. 2. 3.

4.

5.

Hook and line – The hook snares ﬁsh in the mouth, while the line connects the hook to the ﬁsherman. Attraction for ﬁsh – The bait (food) or lure (simulated food) is attached to the hook. Weight –A heavy weight is attached to the line or lure to assist in casting and to sink the bait or lure to a ﬁshy depth; ﬁshing with equipment that employs a weight is called terminal gear ﬁshing or, simply, gear ﬁshing. Fishing rod – A long, slender ﬁshing rod has two functions: • assist in casting or reaching to ﬁsh; • assist in protecting line from breakage during a tug-of-war battle between the ﬁsherman and quarry. Reel – A reel has a spool and some way to turn the spool to collect the line. The reel has several functions: • store line; • smoothly feed line off of the spool during cast; 225

© 2007, Woodhead Publishing Limited

226

Materials in sports equipment • •

retrieve line smoothly to impart motion to lure; provide mechanical braking (drag) to wear down ﬁsh during ﬁght.

9.1.1 The process of ﬁshing •

•

•

Casting – An angler ﬂings the ﬁshing lure far out into river, lake, or sea. The caster starts with the lure near the tip of the rod, and the rod aimed behind. Then the caster accelerates the rod forward and releases the lure at a point with desired velocity and direction. The casting motion is similar to throwing a rock or baseball. When the lure is released, its weight carries it out in projectile motion, pulling the line along with it. For long casts, the line must be very light and feed smoothly off of the reel and freely through the rod guides. By virtue of its length, the rod acts as a lever to increase lure velocity during the casting motion: a longer rod produces a greater velocity and more casting distance. Retrieving lure – The lure simulates food (small ﬁsh, frogs, insects, etc.) due to its spinning, wobbling, or waving motion through the water. The reel is used to slowly retrieve the lure through water to impart this required action. Landing ﬁsh – When the ﬁsh strikes the lure and is impaled by the hook, it attempts to escape by pulling in the opposite direction. Even though its dead weight may be lower than the breaking strength of the line, it is still capable of breaking the line through application of a dynamic load by lunging. To prevent this line breakage, the line must be strong but, if line strength arises from large diameter, that effect increases line weight and stiffness, which inhibits effective casting. Instead, the rod serves as shock absorbing spring that protects relatively fragile line.

9.1.2 The distinctive characteristics of ﬂy ﬁshing Fly ﬁshing is similar to gear ﬁshing in many ways. Both use the same basic equipment: rod, line, lure, reel, and hook. Several differences do arise, though, that require specialized design and materials. Here are the primary distinctions of ﬂy ﬁshing. •

•

Fishing ﬂy – A ﬂy is made of feathers, fur, yarn, and other lightweight materials. In some cases, the ﬂy actually ﬂoats on the water’s surface by using buoyant materials, surface tension, or a combination. In any case, the ﬂy is very light and is incapable of being cast directly. Flyline – No weight is attached to the line; rather, the casting weight is the line itself, to which the ﬂy is attached. A cast is performed through a series of false casts whereby the line is extended a small amount with each stroke, requiring several strokes to fully extend the line.

© 2007, Woodhead Publishing Limited

Design and materials in ﬂy ﬁshing

227

•

Leader – The ﬂy is connected to the ﬂyline with a lightweight leader or tippet material. Because the ﬂyline is heavy, its large size is easily seen by a ﬁsh. So the leader isolates the heavy line from the ﬂy to reduce visibility. • Fly rod – Fly rods tend to be much longer and more ﬂexible than those rods used for gear ﬁshing. While it is often perceived that this combination provides a much more sporting opportunity for the ﬁsh, that is not the reason for the ﬂy rod’s design. The length and ﬂexibility are inherent for a rod capable of casting a ﬂyline. • Fly (lure) motion – The reel is not used to retrieve the ﬂy. Instead, the caster strips line by hand to impart a swimming ﬂy motion. To most directly connect motion of the line in the caster’s hand to motion of the line at the ﬂy, rod ﬂexibility is eliminated by pointing the rod directly at the ﬂy. Control of ﬂy motion sacriﬁces shock absorption capability, which can lead to leader breakage if the ﬁsh strikes the ﬂy abruptly. • Dry ﬂy ﬁshing – In a special type of ﬂy ﬁshing, the ﬂy remains dry as it ﬂoats on a river’s surface, simulating an insect. This is very common in trout ﬁshing. For realism, the ﬂy must ﬂoat naturally with no drag. This is best accomplished by a combination of two factors. Firstly, the leader must be very ﬂexible so river current action on the leader is not transmitted to the ﬂy. Secondly, the angler must manipulate line during the drift to ensure that the line and ﬂy are moving at same speed, accomplished with line ‘mending’ techniques to lift and ﬂip the line without moving the ﬂy. A long ﬂy rod is required to provide the reach necessary to mend. • Reel – The reel is involved with neither casting nor ﬂy retrieval. Line to be cast is stripped off the reel prior to casting; it lies in loose coils at the caster’s feet. During ﬂy retrieval, hand stripping of line returns loose coils to the caster’s feet. So, the reel serves to store line during transport, to store extra line during ﬁshing, and to provide drag or braking force during ﬁsh ﬁghting. A ﬂy ﬁshing system is composed of distinct components as illustrated in Fig. 9.1. A ﬂy ﬁsherman purchases these components separately and assembles them to meet the performance requirements of the particular type of ﬁshing. To achieve system performance, each angler must understand component performance and integration of components. This chapter on design and materials of ﬂy ﬁshing equipment ﬁrst discusses the performance requirements of the integrated ﬂy ﬁshing system as required for ﬁsh ﬁghting and for casting. Fly retrieval tends to be less demanding of the equipment and more demanding of the angler, so it is not explicitly included in this work. Once the system requirements have been concerned, design of individual components is discussed.

© 2007, Woodhead Publishing Limited

228

Materials in sports equipment

Rod Flyline

Leader Fly

Reel

9.1 Components of the ﬂy ﬁshing system.

9.2

Performance requirements: hooking and landing the ﬁsh

Two fundamental objectives deﬁne ﬂy ﬁshing. 1.

2.

Hook the ﬁsh – The ﬁrst step is for the angler to entice a ﬁsh to strike the ﬂy, which includes a hook. To succeed, the angler must be proﬁcient in casting the ﬂy, retrieving the ﬂy so it looks realistic, and in recognizing and reacting to the strike. Generally this requires signiﬁcant skill and patience because the majority of ﬁshing time is spent in this activity. On the basis of time spent, ﬂy ﬁshing equipment should be optimized for this objective, but it is not. Land the ﬁsh – The hooked ﬁsh, ﬁghting for its life, draws on all of its strength, guile, and acrobatics to escape while the angler works it close enough to be netted. This is very exciting and rewarding, for the angler anyway, with an immediate sense of gratiﬁcation. Overall, though, very little ﬂy ﬁshing time is spent on this activity. However, the importance of ﬁghting the ﬁsh is very high, so equipment performance must generate success.

So what causes failure, the inability to land the ﬁsh? First, the ﬁsh becomes unhooked. If the hook is barely engaged in soft tissue, it can pull through the mouth tissue and, second, the hook can back out if the line is not kept taut throughout the battle. The angler tries to respond to ﬁsh movement by raising and lowering the rod to maintain line tension throughout the battle. However, the ﬁsh is ﬁghting for its life and tries many different tactics and maneuvers to escape the hook. For example, a hooked ﬁsh often jumps into the air shaking its head. It is very rare for a ﬁsh to do this unless it is hooked, but when it is hooked, this maneuver may prevent the angler from keeping the line taut, so the momentary slack may be sufﬁcient for the hook to back out of the ﬁsh’s mouth, and the ﬁsh escapes. Prevention of either type of hook disconnection failure depends on angler skill (and some luck). Anglers learn, through experience, not to apply too much line tension to pull a hook out while applying enough to sustain a taut line and avoid slack.

© 2007, Woodhead Publishing Limited

Design and materials in ﬂy ﬁshing

229

A second type of failure is one that the angler can control, namely, the equipment breaks. Since all components are connected in series, breakage of any component at any place leads to a lost ﬁsh. The weak link in system deﬁnes system strength. An angler tries to select system components to match the size of quarry. However, ﬁsh weight alone is not necessarily the limiting factor. For example, a ﬁsh that lunges during battle imparts a dynamic load that may exceed the static load capability of the weak link. So, the angler assembles the ﬂy ﬁshing system recognizing this possibility, and includes enough inherent ﬂexibility to minimize the impact of the dynamic load. The ﬂy ﬁshing system, relative to ﬁghting the ﬁsh, comprises ﬁve main parts, as shown in Fig. 9.1. 1.

Fly – The ﬂy is made of a steel hook with soft materials attached that make it look like something to eat. The hook used in ﬂy ﬁshing is essentially the same as that used for any other kind of ﬁshing, except that the hook shank between the eye and the bend tends to be longer to allow space to attach the feathers and fur. Hooks are sharp to easily penetrate a ﬁsh’s mouth. Made of steel and being small, they are quite strong and stiff. Normally, hooks do not break during ﬁsh ﬁghting. 2. Leader – A leader attaches the ﬂy to the ﬂyline. The main purpose of the leader is to provide a low-visibility spacer so that the ﬁsh does not associate the visible ﬂyline with the food (ﬂy). Reduced visibility is accomplished by using a small-diameter, ﬂexible line made of transparent materials. Nylon, copolymer nylon, and ﬂuorocarbon are the most commonly used materials. When ﬁghting a ﬁsh, the leader is loaded in pure tension so its ability to withstand the load without breakage is deﬁned by its tensile breaking strength. It also stretches during this loading so its stiffness is controlled by the length and tensile elastic modulus. Normally, the leader has very small diameter and relatively low breaking strength; hence, it tends to be the weakest link in the ﬂy ﬁshing system during ﬁsh ﬁghting. 3. Flyline – During casting, the ﬂyline provides the weight that is cast out into the water. Hence, while still simply a piece of ﬁshing line like the leader, its diameter is much larger. Manufactured in layers to provide speciﬁc casting and ﬂoating/sinking characteristics, the core of each ﬂyline is strong. Even though it is longer than the leader, its larger cross-section renders it stiffer in tension during ﬁsh ﬁghting. 4. Fly rod – Perhaps the most critical component in the ﬂy ﬁshing system during ﬁsh ﬁghting is the ﬁshing rod. While the hook–leader–ﬂyline connected system is purely in tension, the ﬂy rod load varies as the angler raises or lowers it. The ﬂyline is fed through rod guides to the reel, which essentially allows the line to apply transverse loading on the rod when it is lifted but no load on the rod when it is lowered.

© 2007, Woodhead Publishing Limited

230

5.

Materials in sports equipment During ﬂy retrieval, to simulate swimming prey, the angler lowers the rod, pointing it directly at the ﬂy. In that orientation, the rod supplies neither stiffness nor strength to the system; it simply directs the path of the line. However, when a ﬁsh is hooked, the angler raises the rod so that the line pulls transversely to the rod, bending it. In that position, bending stresses certainly build up so the rod choice must be adequately strong to resist breakage. More importantly, though, is the low bending stiffness of the ﬂexible rod. During the ﬁght with a lunging ﬁsh, the rod acts as a very soft spring to absorb the motion without exceeding the breaking strength of the leader, the weak link. Reel – A ﬂy reel stores extra line and, if a large ﬁsh makes long runs, pays out line during the ﬁght with a preset drag force. The reel, though, makes no overall contribution to the system strength and stiffness.

Relative strength

Figure 9.2 shows the relative strength and stiffness of the four components participating in ﬁsh ﬁghting; the reel does not contribute. For strength, the

40 35 30 25 20 15 10 5 0 Fly

Leader

Flyline

Bent rod

Fly

Leader

Flyline

Bent rod

Relative stiffness

1 0.8 0.6 0.4 0.2 0

9.2 Relative strength and stiffness of ﬂy ﬁshing system.

© 2007, Woodhead Publishing Limited

Design and materials in ﬂy ﬁshing

231

leader represents the weak link although adequate rod strength cannot necessarily be assumed. Hook strength and ﬂyline strength tend to be sufﬁcient. In stiffness, or its reciprocal ﬂexibility, the bent rod completely controls the overall system stiffness. Based on this functional evaluation, then, the key performance characteristic of leader materials is strength and for rods is bending stiffness.

9.3

Performance requirements: casting

The purpose of ﬂycasting is, of course, to deliver the ﬁshing ﬂy into the home of the ﬁsh. Because the ﬂy is quite light and ‘bushy’ (large air drag), it cannot be cast directly from the rod as a heavier lure might be. Instead, the ﬂy is attached to a rather massive ﬂyline and the ﬂyline is then cast; the ﬂy simply goes along for the ride. The caster’s role in all of this is to use the rod to impart adequate ﬂyline speed to propel the ﬂy. Flyﬁshers use a variety of different casts depending on the ﬁshing conditions and their equipment. Of these many casts, probably the most common is the overhead cast. To perform this cast, the angler pays out the line to the desired length by false casting, where the ﬂy is not allowed to touch the water. A backcast is then made to extend the entire line behind the caster. With the line initially straight in the backcast as illustrated in Fig. 9.3, the caster applies force and torque to the base of the rod to cause the rod tip and attached ﬂyline to accelerate. As the rod straightens, the rod tip and the ﬂyline achieve their maximum velocity. When the rod begins to ﬂex forward, the rod tip velocity decreases and the ﬂyline travels free of the rod

THE FORWARD CAST

9.3 Illustration of the forward cast in ﬂy ﬁshing (used with permission, Mark Sussino).

© 2007, Woodhead Publishing Limited

232

Materials in sports equipment

motion at the horizontal component of the rod tip velocity. However, since the end of the line is attached to the rod tip, that end will have essentially zero velocity. Thus, the attached line is stationary while the remaining line is traveling, and a loop is formed at the interface between these two segments of line. Since the relative length of each portion of line will change during the cast, the loop will travel down the line like a wave until it reaches the free end of the line carrying the ﬂy. The loop unrolls, the line straightens, and the cast is complete. During the forward stroke of the cast, the rod is used as a spring (energy storage) and as a lever to develop the high tip speed necessary for effective casts (Mosser and Buchman, 1980; Spolek, 1987a). Due to the rod’s inherent ﬂexibility and the forces imparted by the caster, the rod exhibits large deﬂections during casting. Figure 9.4 is a multiple exposure photograph of a forward cast that shows the various stages of rod ﬂexure. As the cast begins, the caster accelerates the rod and the attached ﬂyline. The dynamic loading causes the rod to ﬂex toward the rear. The rod then straightens with the maximum tip and line speed. As the rod overshoots, the line travels free of the rod and the loop is formed. During the acceleration phase, line acceleration causes a tip load on the rod while the acceleration of the rod itself acts as a distributed load. The combination of these two loads tends to cause the rod to be displaced with a curvature that closely represents the mode shape of the ﬁrst fundamental frequency of the rod; this is illustrated in Fig. 9.4. If the rod were sub-

9.4 Multiple exposure photograph of the cast (used with permission, Ed Mosser).

© 2007, Woodhead Publishing Limited

Design and materials in ﬂy ﬁshing

233

sequently decelerated with a motion similar to the acceleration stroke, the rod motion would basically emulate the ﬁrst mode of vibration of a tapered cantilever beam. The maximum tip speed would depend on the ﬁrst mode natural frequency of the rod and the amplitude of vibration. The amplitude depends on the acceleration loading, a function of the caster’s ability to accelerate the rod and line. A long rod with low mass and high natural frequency yields a high tip speed. Once the line loop forms, the line and ﬂy travel independently of the rod. As the traveling portion of the line shortens, the moving mass decreases and the velocity increases, much like a bullwhip (Bernstein et al., 1958). The ﬂyline is tapered, which increases or decreases the build-up of kinetic energy in the traveling line, effectively controlling the cast (Spolek, 1986; Robson, 1990; Gatti-Bono and Perkins, 2004; Wang and Werely, 2004). Rod performance requirements include: •

•

Distance – Ability to cast a long distance is often at a premium. In river ﬁshing, greater distance allows the angler to cast to more likely spots that ﬁsh would hold in the current. This is especially important on large rivers. During lake ﬁshing with ﬂy retrieval, such as ﬁshing from boat and covering water in all directions, the area covered is proportional to the casting distance squared. So long-distance casting, again, provides premium performance. Distance is controlled by line speed at completion of cast, similar to projectile motion. Line speed is directly proportional to rod frequency (Spolek, 1993b). So high frequency of ﬂy rod is a design objective for casting distance. Accuracy – Fish in rivers sit in feeding lanes and only eat insects and other prey that ﬂoat through the feeding lane. A caster must be able to place an artiﬁcial ﬂy directly in a feeding lane to be successful. In lake ﬁshing, casting a ﬂy too close to a feeding ﬁsh may spook it but too far away and the ﬁsh will not see the ﬂy. In all casting, accuracy of ﬂy placement is critical. During casting, the angler moves their arm, wrist, and hand to move the rod in the desired direction. In theory, if the caster’s grip on the rod butt is accurately directed, then the entire rod will follow. The ﬂyline follows the rod tip; the ﬂy is attached to the line and is carried along. So accuracy of ﬂy placement is controlled by the path of the line during a cast, but line direction is set when the rod tip achieves maximum speed in cast direction. Once the rod tip decelerates (see Fig. 9.4), the line continues independent of rod. To sum up the ﬂy placement process: caster aims the rod butt; the rod tip is directed by the butt; the line is directed by the rod tip; the ﬂy is carried by the line.

© 2007, Woodhead Publishing Limited

234

•

Materials in sports equipment

Fly rods are inherently ﬂexible and long. The rod tip does not necessarily travel in a straight line, instead exhibiting out-of-plane whirling (Spolek, 2004). This whirling tendency can be controlled by rod design and construction relative to a rod’s spine. So control of rod spine is a design objective. Ease of use – Fly casting is repetitive, with several false casts typically required prior to the actual cast to deliver the ﬂy. False casts are used to dry a ﬂoating ﬂy or to extend the amount of line somewhat with each false cast. In any case, the casting motion is repeated several times for each delivered cast. The force required to accelerate and then decelerate the rod and attached line with each casting stroke, moved through the arm motion distance, adds up to a lot of work. Casters become fatigued. To minimize the work required in a long day of casting, a ﬂy rod should have low mass. So rod mass is a design objective. Line performance requirements include:

•

•

Rod loading – The mass of the line is required to load the rod during the acceleration phase of the cast. Stiff rods require more line mass to properly deﬂect them so the energy stored in the bent rod can be delivered to the line with unloading or straightening (Fig. 9.4). So, lines must be designed with speciﬁc total mass. Taper – Lines are tapered to place more or less mass close to the tip of the line, which travels the furthest and fastest during casting. For greater distance, line motion needs to be less affected by air drag. For delicate presentation, the tip of the lines should carry less mass. These factors affect line design.

9.4

Leaders

The purpose of the leader is to connect the relatively heavy ﬂyline to the ﬂy. Its performance requirements are high strength, ﬂexibility, and low visibility. Throughout history, woven horsehair lines were used to connect hooks to lines, but their only redeeming feature was ﬂexibility; they were relatively weak and quite visible. As ﬂy ﬁshing developed, anglers discovered silkworm gut that was far better than horsehair in all respects (Schullery, 2005). From early 1700s until the mid-1900s, silkworm gut leaders were used extensively and almost exclusively for ﬂy ﬁshing. Highquality gut exhibited tensile strength greater that 20 000 psi. Nylon leaders ﬁrst appeared in the 1940s and soon displaced gut because of higher strength, somewhat lower visibility, lower cost, and less maintenance. Reﬁnements to the nylon polymer through copolymerization have dramatically increased the tensile strength of nylon used for leaders. Nylon leaders currently dominate the ﬂy ﬁshing market share because of their overall performance.

© 2007, Woodhead Publishing Limited

Design and materials in ﬂy ﬁshing

235

Since the 1990s, leader materials manufactured from ﬂuorocarbon have appeared on the market. The primary motivation to move from nylon to ﬂuorocarbon leaders is visibility. While both nylon and ﬂuorocarbon are transparent, the nylon’s index of refraction is 1.62 while that of ﬂuorocarbon is 1.42. When submerged in water with index of refraction of 1.33, the transmitted light through the ﬂuorocarbon is bent less than nylon, reducing the visibility. This is illustrated in Fig. 9.5. When water conditions are clear or ﬁsh are easily frightened by the sight of the leader, anglers often will switch from nylon leader to ﬂuorocarbon leaders. However, other properties of ﬂuorocarbon affect this decision: • ﬂuorocarbon has lower ﬂexibility than nylon (Spolek and Spolek, 2001; Spolek, 2003b), reducing the ability of the ﬂy to accurately mimic the natural prey; • ﬂuorocarbon’s density is much higher than that of nylon – this difference beneﬁts wet ﬂy ﬁshing where the ﬂy is meant to sink, but hampers dry ﬂy ﬁshing where the ﬂy and leader should ﬂoat or, at worst, the leader should sink very slowly; • ﬂuorocarbon’s price is much higher than nylon. A comparison of the properties of nylon and ﬂuorocarbon leader materials is given in Table 9.1. The ﬂy ﬁshing leader is usually tapered to smoothly transmit the kinetic energy of the line to the ﬂy as the cast unrolls. Leaders for trout ﬁshing, for

AIR

WATER

NYLON

FLUOROCARBON

9.5 Comparison of nylon and ﬂuorocarbon leader visibility, in air and in water. Materials have the same diameter.

© 2007, Woodhead Publishing Limited

236

Materials in sports equipment

Table 9.1 Leader properties Property

Nylon copolymer

Fluorocarbon

Speciﬁc gravity Tensile strength (ksi) Elongation at failure (%) Flexural stiffness (ksi) Leader–ﬂy knot strength (%) Leader–leader knot strength (%) Index of refraction Cost (US$/30 m)

1.14 167 20 330 92 67 1.62 $6

1.81 171 20 430 91 72 1.42 $14

example, typically taper from 0.020 inch at the butt end down to 0.005 inch at the tip end. Commercially tapered leaders are produced by variable diameter extrusion or by selective chemical removal during the drawing process. Braided leaders accomplish the same thing by braiding several ﬁne strands of nylon and gradually reducing the number of strands (braided leaders, like rope or cable, offer greater ﬂexibility than solid leaders of the same cross-section). Many do-it-yourself ﬂy ﬁshers make their own tapered leaders by knotting together leader segments with progressively smaller diameters. While this approach allows customized tapers, each knot introduces a potentially weaker connection. In use, leaders must be knotted to the ﬂy or to other pieces of leader material, as with the tapered leader or simply to extend the leader length. Knot strength creates broad controversy among users and very little hard evidence. Laboratory studies have been performed (Spolek, 1988b, 2003a) to measure the average strength of different types of knots commonly used. The results are compiled in Table 9.1 and indicated that knots used for ﬂy–leader connection provide about 90% of the base material strength while leader–leader knots give about 70% strength. As illustrated in Fig. 9.2, the leader’s weak link role is exacerbated by knots.

9.5

Flylines

A ﬂyline’s main purpose is to provide the weight required to cast a ﬂy. During the forward cast, the rod accelerates the line which then has adequate momentum to either extend the amount of line through the rod guides or to carry the light but air-resistant ﬂy to its target, or both at once. A rod is rated by the weight of line that it can easily or most effectively cast, so ﬂylines are produced with a speciﬁed mass. Speciﬁcally, the mass of the ﬁrst 30 feet deﬁnes the line size; for example, a 5-weight line has 140 grains of mass in the ﬁrst 30 feet of line; a 6-weight, 160 grains; a 7-weight,

© 2007, Woodhead Publishing Limited

Design and materials in ﬂy ﬁshing

Tip Front taper

Belly

Rear taper

237

Running line

9.6 Flyline taper.

180 grains; and so on. Flyline mass is the ﬁrst measure of line performance. The next performance measure of the ﬂyline is strength. Clearly, the line must be strong enough to withstand the tug of the hooked ﬁsh without breaking in tension. Normally that is not a problem. Since the line is relatively massive, many materials supply strength greater than that of the leader material for the ﬂyline cross-section used. Typical ﬂylines have breaking strength in the 20–40 pound range, with leader strength of perhaps half of that value. However, in some ﬂy ﬁshing venues, especially in salt water, very heavy leaders are used that might exceed the ﬂyline strength and the ﬂyline itself will break, surprising the angler who assumed that ﬂylines simply do not break in use (Richards, 2005). Flylines are tapered, with different tapers allowing better distance casting or more accurate casting. The basic parts of the tapered ﬂyline are shown in Fig. 9.6. The lengths of the relative portions of the line are varied by design. For example, the weight forward line has a relatively short belly section compared to the double taper line. By making the length of the front taper very long, the line supplies a cast with greater accuracy and delicacy of ﬂy delivery but not as good performance during windy conditions, which call for a line with a very short front taper. So taper design can vary dramatically. Finally, ﬂoating and sinking lines are available to allow either dry ﬂy ﬁshing or wet ﬂy ﬁshing, but not both with a single line. The overall speciﬁc gravity (SG) of the construction material dictates whether a particular line will ﬂoat or sink and, if it sinks, it will sink at a faster rate if the speciﬁc gravity is larger. As the speciﬁc gravity for a particular line increases, its diameter must decrease to maintain its overall mass. The rod designed to cast a 6-weight line must still carry the same 160 grains whether the line is intended to ﬂoat or sink, so the designer must adjust diameter.

9.5.1 Flyline manufacture Early ﬂylines were woven of silk. With a loose weave, air would become entrapped in the line. The surface of a silk line was treated with a wax or oil coating that prevented water penetration, retaining the air ﬁlling and allowing the line to ﬂoat. Without surface treatment, the silk line would sink. So a single line could be used for dry ﬂy or wet ﬂy ﬁshing by treating

© 2007, Woodhead Publishing Limited

238

Materials in sports equipment

or not treating the surface. Silk lines, though, required regular maintenance and would weaken with time and use. Modern ﬂylines employ two distinct parts, a core and a coating; this is illustrated in Fig. 9.7. The core is typically a multiﬁlament nylon braid of 16 strands. Braiding supplies ﬂexibility while the total cross-section provides tensile strength. Nylon strands have tensile strength of about 150 ksi and 1.15–1.20 SG. With air entrapment in the braiding, the overall speciﬁc gravity of the core is very close to 1.0. So the core essentially supplies the ﬁnished ﬂyline’s strength. A polymer coating – polyvinylchloride (PVC) – extruded over the outside of the core supplies all of the ﬂoating or sinking properties required by the angler. Line diameter is adjusted, depending on the inherent speciﬁc gravity, to meet the overall mass (line weight) requirement and tapering. Speciﬁc gravity is adjusted by additives to the PVC with its inherent 1.2–1.3 SG. To make the line ﬂoat, tiny bubbles of air are entrained in the PVC (Fig. 9.7(b)). The ability to uniformly distribute bubbles of consistent size continually challenges ﬂyline manufacturers. Even when that process works well, it still does not deliver uniform ﬂotation. The braided core has uniform diameter, but the coating diameter varies with tapering. Largediameter sections have greater buoyancy because the coating/core ratio is larger. Uniform ﬂotation requires new technologies that yield non-uniform speciﬁc gravity.

(a)

(b)

9.7 (a) Cross-section of a modern ﬂyline. (b) Tiny air bubbles help the line to ﬂoat (used with permission, Darrell Wilson).

© 2007, Woodhead Publishing Limited

Design and materials in ﬂy ﬁshing

239

Sinking ﬂylines pose the opposite problem. High speciﬁc gravity of the PVC coating, up to 2.0 or more, is achieved by imbedding tungsten powder (Fig. 9.7(a)). Flylines are designed and manufactured to sink at different rates. For lake ﬁshing where the ﬂy should remain at a constant, shallow depth, the sink rate should be low, nominally 1–2 inch per second (ips). For deeper lake ﬁshing, 3–5 ips is used while for swift rivers, 7 ips and above is needed. Wide ranges of tungsten powder concentration are needed to produce the requisite speciﬁc gravity. As an example, for the same diameter line, a SG of 1.1 is needed for 1 ips while SG of 3.0 is needed for 7 ips (Havstad, 1979). Line tapers confound the situation. Line segments with the same speciﬁc gravity will sink at different rates based on diameter. Havstad (1979) measured that the sink rate is proportional to D0.689. For the line shown in Fig. 9.6, the belly of the line sinks faster than the tip which prevents the desired straight line connection between the angler and the retrieved ﬂy. The effect is partially overcome by manufacturing ﬂylines with variable density; the speciﬁc gravity of the tip is higher than that of the line’s belly. These lines are labeled as ‘progressive tapers’ by manufacturers, but accurate control of the tungsten powder concentration is difﬁcult. Current tungsten powders are limited in the maximum speciﬁc gravity that can be achieved and, because of that, fast sinking lines (5 ips and above) cannot be manufactured in low line weights (below 180 grains). In addition to the complications of manufacturing lines of all different weights and sink rates, the surface coating of the line is separately controlled. Firstly, the surface must have low friction to allow easy ﬂow through the steel guides on the rod. Different proprietary formulations are regularly being developed. Then, the actual texture of the surface is perceived to affect the line’s ability to shoot through the guides, and an array of dimples, ripples, and other rough surfaces are produced to appeal to anglers who read the trade magazines. Finally, the surface must be durable. Repeated ﬂexing can cause fatigue cracks. While cracks do not affect line strength, per se, they can allow water to penetrate into the core and change the effective speciﬁc gravity of the line. As can be surmised, the array of combinations of line weights, tapers, sink rates, and surface ﬁnishes leads to a huge number of options. It has been estimated that about 2000 different ﬂylines are currently produced (Jaworowksi, 2005). Since ﬂylines retail for $US 50–60 each, most anglers compromise on performance and own a handful of lines.

9.6

Rods

Let’s start with a brief history of ﬁshing rods. The earliest rods were undoubtedly used because they allowed more ﬁsh to be caught than could be done with hand lines. Rods essentially provided two functions: (1) they

© 2007, Woodhead Publishing Limited

240

Materials in sports equipment

allowed the angler to reach out to gently and accurately place the bait in front of the ﬁsh; and (2) they protected the relatively fragile horsehair lines from breaking by absorbing the shock of a large ﬁsh lunging for freedom. No casting was involved. Rods were made of readily available materials that were long (reach) and ﬂexible (shock-absorbing). Green saplings or bamboo canes worked just ﬁne. When casting became necessary or desirable, the simple requirements of a rod changed. Fish ﬁghting ability was still necessary, of course, but the rod also had to be able to cast bait or lure a great distance with little effort. Rod builders began experimenting with different materials. Solid hardwood rods made of greenheart and lancewood began to appear as the instruments of choice in the mid-1800s. As ﬂy ﬁshing became popular, these rods were lengthened and lightened as much as possible to allow the casting of the low-weight ﬂylines. In the late 1800s, hardwood rods were replaced by those manufactured from bamboo cane that was split into strips and then glued together. By cutting and gluing the strips, the bamboo’s high-strength outer ﬁbers could be condensed into a more compact package, yielding a stiffer and lighter rod. Early bamboo rods used Calcutta cane, but then superior properties were discovered for the cane from the Tonkin region of China. Different strip conﬁgurations were tried as well: 4-strip, 5-strip, 6-strip, and 8-strip; 6-strip proved to be best. In the early 1900s the cane itself was modiﬁed: hardened by heat treating or impregnated with glues. Modern technology changed all of that when synthetic materials were developed that could be formed into useful ﬁshing rods. In the 1940s, rods were ﬁrst made of steel and, later, beryllium–copper; these rods were too heavy. But when ﬁberglass was ﬁrst tried in 1948, the rod manufacturing industry permanently changed. The ﬁrst ﬁberglass rods were of solid construction, like the Shakespeare Wonder Rod. Later, hollow, tubular designs were developed and, eventually, graphite appeared in the 1970s to capture the entire rod market as we know it today. Investigation into quantiﬁcation of ﬂy rod performance has clearly demonstrated that the mechanical behavior of a ﬂy rod can be completely described by two parameters: stiffness and frequency (Spolek, 1987b). These two factors were chosen because they have direct correlation to the two main demands placed on the rod during its use: ﬁsh ﬁghting capacity and casting effectiveness. When ﬁghting a ﬁsh, the rod absorbs the shock of a lunging ﬁsh and prevents the line from breaking; it puts a static demand on the rod. The energy absorbing ability of a rod is characterized by its stiffness. One therefore selects a rod with the appropriate stiffness for the size of ﬁsh being sought: high stiffness for large salmon, low stiffness for small trout. The rod’s frequency, on the other hand, reﬂects its ability to cast, which is the dynamic activity of ﬂy ﬁshing. During a typical forward cast, the rod is loaded by the

© 2007, Woodhead Publishing Limited

Design and materials in ﬂy ﬁshing Bamboo

4.00

1.00

0.00 1850

1900

1950

Graphite

Fiberglass

Solid wood

Frequency (Hz)

3.00

2.00

241

2000

Year 9.8 Evolution of ﬂy rods.

caster’s arm stroke, but it unloads on its own and at its own speed. The speed at which it unloads is dictated by the fundamental natural frequency of the rod. A rod with a higher frequency will produce greater tip velocity, hence line speed, to overcome air friction and cast longer distances. Over the past 130 years or so, ﬂy rods have evolved with the discovery or development of new materials. A series of rods from different vintages, hence materials, were laboratory tested to measure the stiffness and frequency (Spolek, 1988a). Over time, the fundamental frequency of rods with comparable stiffness increased continuously with each material change (Spolek, 1995). This evolution is illustrated in Fig. 9.8. As can be seen, rods constructed of greenheart and lancewood exhibit a frequency in the range of 1.0–1.3 Hz, rods built of bamboo show about 1.6–2.4 Hz, ﬁberglass about 2.6 Hz, and rods made of graphite composite display 2.4 –3.7 Hz. The recent increase in frequency during the graphite phase has occurred as graphite ﬁber elastic modulus has increased from about 30 Mpsi (low modulus) to about 45 Mpsi (medium modulus) to 60 Mpsi (high modulus) since the 1980s. Measurements have shown that each step to higher modulus graphite ﬂy rods has produced rods that deliver higher line speed (Spolek and Morris, 1993a; Spolek, 1993b).

9.6.1 Rod design Essentially all modern ﬂy rods are hollow, tubular, and tapered. Additionally, the rod’s wall thickness varies along its length to reduce overall weight and maintain uniform stresses during bending under load. Graphite ﬁbers, aligned with the rod’s axis, are the primary structural material because of their attractive stiffness-to-weight ratio.

© 2007, Woodhead Publishing Limited

242

Materials in sports equipment

The graphite or carbon-reinforced plastic (CRP) rod is built by wrapping several layers of pre-preg over a mandrel, as illustrated in Fig. 9.9. The pre-preg is a cloth-like material with graphite ﬁbers oriented primarily in one direction so that they are aligned along the length of the ﬁnished rod. The cloth is cut to shape so that, when rolled onto the mandrel, it provides the taper and wall thickness desired. The steel mandrel provides the shape and taper for the rod blank inner dimensions, and the pre-preg shape yields the outer dimensions. During construction, the wedge of scrim is ﬁrst tacked lightly to the mandrel. It is next rolled very tightly so that a few layers are built up. Finally, the entire assembly is sealed and heat-cured. The mandrel is removed and the blank is essentially ﬁnished. During curing, the epoxy resin of the scrim melts to form a solid substrate ﬁlled with oriented graphite ﬁbers. Since the graphite ﬁbers give the rod its stiffness and strength, they must be aligned with the length of the rod to carry to load as the rod is bent. If the rod has more ﬁbers, it is stronger and stiffer. So to make the rod butt stiff, more layers of scrim are wrapped around the mandrel in the butt section. The rod tip is very ﬂexible because fewer layers are applied. The relative stiffness is also affected. Rods manufactured this way invariably contain a spine. The spine is deﬁned as the rod orientation, when rotated about its long axis, that gives preferential bending. During wrapping of the pre-preg layers, partial wraps produce some sections with more thickness than others, leading to localized increase of the moment of interia and bending stiffening. An illustration of how partial layers of pre-preg affect the wall thickness is given in Fig. 9.10. Of the various sections shown in Fig. 9.10, the sections with 3 wraps and 31/2 wraps will both demonstrate no preferential bending or spine; the

Mandrel

CRP pre-preg

9.9 Manufacture of CRP ﬂy rod.

© 2007, Woodhead Publishing Limited

Design and materials in ﬂy ﬁshing

3 Wraps

3 ¼ Wraps

243

3 ½ Wraps

Increasing wall thickness

9.10 CRP overlap to achieve variable wall thickness.

9.11 Cross-section of a tyical graphite ﬂy rod.

section with 31/4 wraps, on the other hand, will exhibit a maximum spine effect (Spolek, 2004). A cross-sectional view of an actual graphite rod is shown in Fig. 9.11, where the layer overlap is clearly obvious. The magnitude and orientation of the spine has been shown to be directly attributable to the this layer overlap effect (Spolek, 2005). Rod spine, in and of itself, is not problematic. During assembly, the spine of the rod is located manually (Clemens, 1987) and the guides are located so that the pull on the line during ﬁsh ﬁghting is aligned in the direction of the preferred bending. Problems can arise, though, during casting. Since the spine varies with the amount of layer overlap, its magnitude and orientation vary along the length of the rod. The standard method for locating the spine identiﬁes the spine orientation mid-shaft, However, the dynamic behavior of the rod depends on the spine orientation right at the butt, the point of

© 2007, Woodhead Publishing Limited

244

Materials in sports equipment

SPINE

SPINE

SPINE

9.12 Rod tip whirl due to spine misalignment.

greatest bending curvature. If the spine at the butt is misaligned with the plane of casting, then the rod tip will whirl and casting accuracy is severely compromised. An illustration of this effect is shown in Fig. 9.12, where a slight misalignment of the spine relative to the casting direction can induce a large whirl of the rod tip during oscillation. Rod designers seek to control this effect by adjusting the amount of pre-preg overlap at the rod butt.

9.7

Reels

A ﬂy reel is used to store extra line and to provide mechanical braking (drag) to wear down ﬁsh during ﬁghting. The most common type has a single action with no gears to increase speed or confer mechanical advantage. Essentially the reel consists of two components: the frame and the spool. The frame attaches to the ﬂy rod, surrounds the spool for protection, and supports the axle. The spool rotates on the axle and is turned by a handle attached to its side. A drag system is common, supplying a constant torque resisting the line being pulled off, as a ﬁsh would do during a run, but the drag is unidirectional so that no resisting torque engages during line retrieval. The main performance criteria are: spool capacity; smooth and even action during drag and retrieval; close ﬁt between spool and frame to prevent line pinching or cutting; and lightweight. Fly reels are manufactured of metal and composites, with aluminum being the most common material. Low-cost reels are made of stamped steel parts screwed together in the frame with a diecast aluminum spool; these frames tend to loosen and wobble. Somewhat better reels use diecast aluminum frames which are lighter than steel and do not have screws to loosen.

© 2007, Woodhead Publishing Limited

Design and materials in ﬂy ﬁshing

245

Molded polymer composite graphite frames are also used with aluminum spools and provide comparable performance to diecast aluminum frames. One of the main problems arising with these designs is the ability to maintain close tolerances. To ensure that the rim of the spool does not rub on the frame during rotation, relatively large gaps are left between them because of manufacturing tolerances. If the ﬂyline or leader slips into that gap during spool rotation, it can easily be pinched causing damage. To reduce this problem, most high-end reels are machined from bar stock aluminum with computer numerical control (CNC) machines to provide a close tolerance ﬁt between the spool and frame. Reel spools need smooth rotation on the axles, so all use some sort of bearing surface. Typically the axles are made of polished stainless steel. Teﬂon® (DuPont) or bronze bushings, pressed onto the spool housing, can provide low-cost yet reliable bearing surfaces. For higher-end reels, the spools are supported by a pair of ball bearings or roller bearings. Spool drag is supplied through a variety of designs. For low drag forces, a simple pall and clicker mechanism is used. Most reels, though, use a conventional drum or disk brake conﬁguration with adjustable brake pad force. Brake pads are made either of cork or plastic, commonly Delrin® DuPont. Reels designed for large salt water ﬁsh, where brake heating can cause failure, use a turbine that rotates in a viscous bath to supply the brake force. All reels include a clutch or ratchet mechanism to engage the drag only as line is stripped out but to override the drag as line is retrieved. Since the line is wound onto the spool from inside out, the diameter of the effective spool changes as line is fed out. However, the drag mechanism supplies a constant torque, so greater line force is required to overcome the drag torque as the spool empties. A drag set for a full spool may lead to a broken leader as it empties and the force increases. To minimize this effect, many reels currently in production use a ‘large arbor’ conﬁguration. This spool design is much wider and larger in diameter, so less diameter change occurs as the line is stripped. Well-made large arbor reels, machined from bar stock aluminum and including a disk drag system, retail for about US$150.

9.8

Summary and future trends

As has been shown, ﬂy ﬁshing equipment has been designed to compromise the strength requirements of landing a large ﬁsh without breakage and the need for a light, responsive casting system. Rods must have high frequency but low stiffness, implying low mass. Lines need adequate mass to load the rod but small diameter to cut through the air. Leaders have to be strong in tensions but very ﬂexible in bending and invisible when submerged in water. Reels should be very light, well-ﬁt, and provide uniform drag force. These demands are very challenging.

© 2007, Woodhead Publishing Limited

246

Materials in sports equipment

Table 9.2 Properties of ﬂy ﬁshing components Component

Features

Material

Cost

Rod

Strong in bending, high frequency

Carbon (graphite) Reinforced plastic

US$200–600

Reel

Close tolerance ﬁt between spool and frame

Machined bar stock aluminum

US$100–300

Floating line

Tapered

Braided nylon core, air bubble impregnated PVC coating

US$50–60

Sinking line

Tapered, different sink rates available

Braided nylon core, tungsten powder impregnated PVC coating

US$50–60

Dry ﬂy leader

Tapered

Copolymerized nylon

US$4–5

Wet ﬂy leader

Tapered

Fluorocarbon

US$8–10

Currently, most anglers assemble systems from components to best ﬁt the size and type of ﬁsh they seek, their personal skills, and budget. Typical system components for fresh water ﬂy ﬁshing are listed in Table 9.2. Trends discussed here are likely to continue. For rods, materials that deliver a higher effective elastic modulus will allow higher frequency for longer casts. But material strength must be maintained to prevent rod fracture during extreme bending. For ﬂylines, the most important development will be in sinking lines. To produce smaller diameter lines with more uniform sink rates, better control over distribution of tungsten powder in the PVC coating must be attained. With local control of tungsten loading, speciﬁc gravity of 5.0 and above can allow production of lines with different sink rates in different parts of the line. Leader materials will continue to move toward ﬂuorocarbon because of its very low visibility, but the inherent stiffness of ﬂuorocarbon will need to be reduced to broaden its appeal for a wider range of ﬁshing methods.

9.9

References

bernstein b, hall d a and trent h m (1958), On the dynamics of a bull whip, J Acoustic Soc Amer, 30(12), 1112–15.

© 2007, Woodhead Publishing Limited

Design and materials in ﬂy ﬁshing

247

clemens, dale (1987), Advanced Custom Rod Building, New York, Winchester Press, 22–6. gatti-bono c and perkins n c (2004), Predicting the Motion of a Fly Line During and Overhead Cast, in Hubbard M, Mehta R D and Pallis J M (eds), The Engineering of Sport 5, Volume 2, International Sports Engineering Association, 340–6. havstad james (1979), Sink rate of sinking lines, The Flyﬁsher, Fall, 35– 42. jaworoski Ed (2005): Understanding ﬂy lines, Fly Fisherman, 36(3), 48–50, 67–8. mosser Ed and buchman william (1980), The dynamics of a ﬂycast, Flyﬁsher, 13(4), 5–9. richards bruce (2005), Scientiﬁc Anglers Corporation, personal communication. robson john m (1990), The physics of ﬂycasting, Amer J. Phy, 58(3), 234 – 40. schullery paul (2005), Fishing with guts, American Angler, 28(5), 60–2. spolek graig (1986), The mechanics of ﬂycasting: the ﬂyline, Amer J. Phys, 54(9), 832–6. spolek graig (1987a), The mechanics of ﬂycasting equipment, in R Rekow, J G Thacker and A G Erdman (eds), Biomech Sport – A 1987 Update DE-Vol. 3, New York, American Society of Mechanical Engineers, 23–6. spolek graig (1987b), Where the action is: part I, The American Fly Fisher, 13(4), 7–11. spolek graig (1988a), Where the action is: part II, The American Fly Fisher, 14(1), 2–9. spolek graig (1988b), Testing new tippet materials, Fly Fisherman, 19(5), 60–1. spolek graig and morris, skip (1993a), Fly rod action quantiﬁed, Fly Fisherman, 25(1), 42–5. spolek graig (1993b), Fly rod performance, in Simon B (ed.), Advances in Bioengineering: 1997 International Mechanical Engineering Congress and Exposition, Dallas, TX, Nov 16–21 (BED), New York, American Society of Mechanical Engineers, 251– 4. spolek graig (1995), Fly-rod evolution, Fly Fisherman, 27(1), 12–16. spolek g a and spolek r g (2001), Measurement of bending stiffness in ﬂexible ﬁshing leaders, Proceedings of the SEM Annual Conference on Experimental and Applied Mechanics, Portland, OR, June 4 –6, 498–501. spolek graig (2003a), Fluorocarbon vs monoﬁlament, Fly Fisherman, 34(5), 46–7, 59. spolek graig (2003b), Modern leader materials – how ﬂexible are they? http:// ﬂyﬁsherman.com/skills/gstippetsiffness/, last accessed 28 March 2007. spolek graig (2004), Fly rod spines, in Hubbard M, Mehta R D and Pallis J M (eds), The Engineering of Sport 5, Volume 2, International Sports Engineering Association, 319–25. spolek graig (2005), Measurement of ﬂy rod spines, Proceedings of the SEM Annual Conference on Experimental and Applied Mechanics, Portland, OR, 7–9 June. wang g and werely n m (2004), Analysis of ﬂy ﬁshing rod casting dynamics, in Hubbard M, Mehta R D and Pallis J M (eds), The Engineering of Sport 5, Volume 2, International Sports Engineering Association, 333–9.

© 2007, Woodhead Publishing Limited

10 Design and materials in archery B. W. K O O I, Vrije Universiteit, The Netherlands

10.1

Introduction

In essence the bow proper consists of two elastic limbs, often separated by a rigid middle part, the grip or riser. This is illustrated in Fig. 10.1. The bow is strung or braced by fastening a string between both ends of the limbs. After an arrow is set on the string, the archer pulls the bow from braced situation into full draw. While the bow is held in place the bowstring is pulled to the anchor point on the archer’s face. This completes the static action in which potential energy is stored in the elastic parts of the bow. After aiming, the arrow is released. The force in the string accelerates the arrow and transfers part of the available energy as kinetic energy to the arrow. Meanwhile the bow is still held in its place and the archer feels a recoil force in the bow hand. After the arrow has left the string, the bow returns to the braced position because of damping. The action of the bow before the arrow departures from the string is called interior ballistics. Exterior ballistics deals with the ﬂight of the arrow through air. A number of disciplines are involved in the design of the bow. 1.

Material science, is a discipline in which processability, strength, fracture mechanics, fatigue, maintenance and resistance to atmospheric action are subjects of interest. Bowyers (manufacturers of archery equipment) have to cope with such issues as the inﬂuence of temperature changes on the performance of the bow and, of course, the availability of materials. 2. The working of the bow as a mechanical device can be described by mathematical formulations of physical laws, e.g. Newton’s law, but also constitutive relationships such as the Euler–Bernoulli beam-equation where the elastic or Young’s modulus is the proportionality constant between exerted force and the resulting elongation (see Gordon, 1978). 3. The success of the archer depends largely upon his or her skill because, when mechanical systems are used to launch arrows, the point of impact is approximately the same for each shot. Small deviations may occur 248

© 2007, Woodhead Publishing Limited

Design and materials in archery

249

Limb

String Grip

Stabilizers

Limb

10.1 The bow consists of two elastic limbs separated by a rigid grip. A string is strung between the tips of the limbs. The stabilizing elements consist of a top stabilizer, a front stabilizer that projects forward toward the intended target, and a V-bar.

due to differences in the mass of the arrow or wind velocity/direction. Thus an archer will try to perfect a technique that produces a repeatable shot, and he or she must gain experience in coping with changing weather conditions. In the 1930s bows and arrows became objects of study by scientists and engineers, for example by Hickman, Klopsteg and Nagler (Hickman et al., 1947; Klopsteg, 1963, 1987). In previous papers (Kooi and Sparenberg, 1980; Kooi, 1981, 1983, 1991a, b) the mechanics of the different types of bow – non-recurve, static-recurve and working-recurve – were studied. In addition, the design of the bow was dealt with from a historical point of view, following the change of the shape and materials used in history (Kooi, 1994). Here, we will concentrate on the design of modern bows seen at target archery events such as the Olympic Games, and especially on the role of the choice of the materials. The model for the dynamics of the bow and arrow is described in Section 10.2. All design parameters are charted accurately and quality coefﬁcients are deﬁned. Besides real number parameters, there are three functions of

© 2007, Woodhead Publishing Limited

250

Materials in sports equipment

the length co-ordinate along the limbs, i.e. bending stiffness, mass distribution and geometric shape of the unstrung bow, giving rise to an intricate design process. For a discussion of the mathematical modelling of the problem, and the derivation and use of the numerical techniques, the reader is referred to earlier papers (Kooi and Sparenberg, 1980; Kooi, 1981, 1983, 1991a, b, 1994). The subject of Section 10.3 is the design and construction of the bow where the technological properties of the materials used to make bows are taken into account and the manner in which these materials are applied in bows is considered. Dimension analysis yields two useful quantities. One is the energy storage capacity per unit of mass of the material. For a particular bow–arrow combination, the allowable amount depends on the failure mode (fracture, delamination, buckling, fatigue, creep) of the limbs. The other is a dimensionless shape-factor of the cross-section of the limbs. This measures the usage of the limb material in bending. Similar considerations hold for the stretched string. Assumptions with respect to a uniform loading along the limbs yield relationships between the design parameters. This reduces the dimension of the design parameter space. Constraints imposed by technological requirements, such as strength, set bounds in the design parameter space. Differences in the performances of various types of bows can be explained by taking these feasible regions into account. Kooi (1994) showed that the type of the bow and the properties of the material used are strongly related and that the application of better materials and better usage of these materials contributes most to the improvement of the bow. Moreover, one of the most important parameters is the mass of the arrow. In practice there is a constraint either on the mass itself, or on the ratio of the arrow mass and mass of the bow limbs. In Section 10.4 we discuss the materials used for the different parts of the bow: the limbs, the arrow, the string and the riser or grip. Besides model predictions, we present also experimental results for a Greenhorn Comet TD 350, 68″ 30# working-recurve bow. The experiments are described in Tuijn and Kooi (1992).

10.2

Modelling bow performance

The bow is assumed to be symmetric with respect to the line of aim. It is placed in a Cartesian coordinate system (x ¯ , ¯), y the line of symmetry coinciding with the ¯x-axis and the origin O coinciding with the midpoint of the bow. We assume the limbs to be inextensible and the Euler–Bernoulli beam theory to be valid. The total length of the bow, measured along it, ¯ . The bending stiffness ¯ W(s¯ ) and mass per unit of length is denoted as 2L ¯ V(s¯ ) are both functions of the length coordinates measured along the bow from O.

© 2007, Woodhead Publishing Limited

Design and materials in archery

251

In Fig. 10.2(a) the unbraced situation (without string) is shown. The geometry of the bow is described by the local angle q0(s¯ ) between the elastic line and the ¯y-axis, the subscript 0 indicates the unstrung situation. Gener¯ 0 is half the ally the middle part of the bow, called the grip, is stiff. Then L length of the grip and 2mg denotes its mass. The bow is braced by applying a string, see Fig. 10.2(b). The length of the unloaded string is denoted as 2l¯0, its mass by 2m ¯ s. We assume that the mater¯s. ial of the string obeys Hooke’s law; longitudinal stiffness is denoted as U Note that the length of the string determines the brace height, denoted as ¯H ¯ |, and therefore only one of these parameters is a design parameter. |O So-called non-recurve bows have contact with the string only at their tips, ¯ ). In the static-recurve bow the outermost parts of the then always (s¯ = L limbs are stiff. These parts are called the siyah or ear. The ﬂexible part between the ear and the grip is called the working part of the limb. With a working-recurve bow the parts near the tips are elastic and bend during the ﬁnal part of the draw, see Fig. 10.2(c). When drawing such a bow, the length of contact between string and limb decreases gradually until the point where the string leaves the limb, denoted as (s¯ = ¯sw), coincides with the tip ¯ ). It remains there during the ﬁnal part of the draw. (s¯ = L ¯(b ¯ ), into a partly drawn position where the The bow is pulled by a force, F ¯ . To each bow belongs a value b ¯ middle of the string has the ¯x-coordinate b ¯ ¯ ¯ = b1 = |OD| for which it is called fully drawn. Values of variables in this situ¯(|O ¯¯ ation are indicated by a subscript 1. The force F D|) is called the weight ¯ ¯ of the bow and the distance |OD| is its draw. The weight is generally noted in pounds (lbs) at a draw length of 28 inches, e.g. #30 @28. Which means, at a full draw of 28 inches (0.7112 m), the force required to hold the bowstring at this length will be 30 lbs (about 133 N). This 28 inches is measured as 26.25 inches (0.6668 m) from the nock of the arrow to the position of the arrow

y

y

s=L

y

s=L

s = L = sw

s = sw

θ0(s)

θ(s)

s

O (a)

K(b)

α(b)

x

O (b)

H

x

F (b) O (c)

F (b) b

10.2 Three situations of the working-recurve bow: (a) unbraced, (b) braced, (c) partly drawn.

© 2007, Woodhead Publishing Limited

D

x

252

Materials in sports equipment

rest plus 1.75 inches (0.0445 m). The arrow, represented by a point mass 2m ¯ a, is propelled towards its target by releasing the drawn string at time ¯t = 0 and holding the grip of the bow at its place. During acceleration at some speciﬁc point in time t the string touches the belly side of the limb again. The kinetic energy associated with the movement of the string can be assessed using a simpliﬁed model in which we assume that the string remains straight between the points of attachment at the tip of the bow and the nock of the arrow. The resulting distribution of the kinetic energy along the string indicates that -31 of the mass of the string should be added to the arrow after which the string can be taken without mass. Then the acceleration force ¯ E acting on the arrow plus the added mass of the string -31 ¯ ms is deﬁned by . ¯ ¯ a + -31 ¯ ms)c¯ t≥0 [10.1] E = 2(m . ¯ is the dimensionless arrow speed (the dot indicates differenwhere ¯c(t¯ ) = b ¯ (t) the coordinate of the middle of the tiation with respect to time) and b string where the arrow nock ﬁts to the string. The arrow leaves the string when the acceleration of the midpoint of the string becomes negative. This moment is denoted as ¯t l and its velocity at that moment is called muzzle velocity or initial velocity of the arrow, which is denoted as ¯cl. A shorthand notation for a bow and arrow combination is introduced as follows ¯ (L ¯ ,L ¯ 0,W ¯ (s¯ ),V ¯ (s¯ ),q0,m ¯ s,m ¯H ¯ | or ¯l 0;|O ¯¯ ¯(|O ¯¯ B ¯ a,m ¯ g,U ¯ s,|O D|,F D|),m ¯ b)

[10.2]

where ¯ mb is the mass of one limb excluding the mass of the grip and includ¯¯ ¯(|O ¯¯ ing the mass of the ears. The quantities |O D|, F D|) and ¯ mb are taken as the elements of a dimensional base in a dimensional analysis. The ﬁrst factor is limited by the length of the bowman’s arms, from the left hand fully extended to the right hand, drawn back beside the right shoulder. The second one is linked up with the bowman’s muscle. In addition it depends on the ability of the ﬁngers of the right hand to control the string, to hold it during aiming and release it at the right moment. The third factor is a limitation of the strength of the materials used. Quantities with dimension are labelled by means of a bar ‘¯ ’ and quantities without the bar are the associated dimensionless quantities. ¯(|O ¯¯ D|) and Note that the two latter parameters, the weight of the bow F the mass of the limbs ¯ mb, have been added to the list artiﬁcially. This implies that both functions W(s) and V(s) are constrained. Both functions, the bending stiffness ¯ W(s¯ ) and the mass distribution ¯ V(s¯ ), are considered to be ¯ (L ¯ 0) and ¯ ¯ (L ¯ 0) of the length the product of the functions ¯ W(s¯ )/W V(s¯ )/V ¯ 0) and coordinate ¯s into R (the set of real numbers) and parameters ¯ W(L ¯ ¯ V(L0) with dimensions. These resulting functions together with the function q0(s) make the bow a distributed parameter structure. Thus the values of the functions W(s) and V(s) for s = L0 are already ﬁxed by two constraints.

© 2007, Woodhead Publishing Limited

Design and materials in archery

253

The ﬁrst constraint, relating to weight, is an implicit relationship between ¯(|O ¯¯ a number of parameters, of which W(s) is one, and the weight F D|) = 1 of the bow. The second constraint is L

mb = 1 = ∫ V ( s) ds

[10.3]

L0

10.2.1 Quality coefﬁcients The purpose for which the bow is used has to be considered in the deﬁnition of a cost-function that should be optimized to obtain the ‘best’ bow. It appears, however, to be very difﬁcult to deﬁne such a cost-function uniquely. Therefore we introduce a number of quality coefﬁcients that can be used to judge the performance of a bow and arrow combination. We will now deﬁne three dimensionless quality coefﬁcients. First, the static quality coefﬁcient q is given by q=

A and OD F ( OD )

A=∫

b = OD

b = OH

F (b ) d b

[10.4]

¯ is the energy stored in the elastic parts of the bow, the working where A parts of the limb and the string, by deforming the bow from the braced position into the fully drawn position. For a bow with a larger q, the draw force at half draw-length is larger than its value for a bow with a smaller q, where the draw force for both bows is the same at full-draw. Second, the shooting efﬁciency h deﬁned by ma c l2 [10.5] A where ¯cl is again the initial velocity. This initial velocity of the arrow then follows from equations 10.4 and 10.5, being

η=

cl =

qη OD F ( OD ) = ν dbv ma mb

[10.6]

¯ bv and the dimensionless initial velocity where we introduced a factor d referred to as v according to dbv =

OD F ( OD ) mb

and ν =

qη ma

[10.7]

respectively. This dimensionless initial velocity v is taken as the third quality coefﬁcient. It is determined by the other two coefﬁcients, q by equation 10.4, h by equation 10.5, and the dimensionless mass of the arrow ma.

© 2007, Woodhead Publishing Limited

254

Materials in sports equipment

Observe that, by deﬁnition, the quality coefﬁcients q,h and v mentioned above are dimensionless. This means that the quality coefﬁcients measure the quality of the bow and arrow combination under comparable conditions; the same weight, draw and mass of the limbs. In Fig. 10.3 the predicted static force draw (SFD) curve F(b) and the dynamic force draw (DFD) curve E(b) for the Greenhorn working-recurve bow are compared. All characteristic parameter values and the predicted dimensionless quality coefﬁcients are given in Table 10.1. F, E

1

E

F

0

1

b

10.3 Static force draw curve, F(b) and dynamic force draw curve, E(b), for the Greenhorn bow. Table 10.1 The characteristic parameter values for the Greenhorn bow and Easton 1616X75 arrow, and the predicted dimensionless quality coefﬁcients. The actual weight and draw length are slightly smaller that their nominal values Dimensionless |OD| |OH| |GD| |GH| F(|OD|) mb ma ms q h v

© 2007, Woodhead Publishing Limited

With dimension 1 0.281 1.08 0.377 1 1 0.0629 0.0222 0.434 0.729 2.23

¯ ¯D | |O ¯ ¯H | |O ¯ ¯D | |G ¯ ¯H | |G ¯F (|O ¯ ¯D |) ¯ mb ¯ ma ¯ ms ¯A m c¯ l

0.597 0.168 0.645 0.225 123.6 0.1457 0.0183 0.0065 32.9 72.9 51.39

m m m m N kg kg kg J % m/s

Design and materials in archery

255

The mass of the string gives the following approximation for the maximum efﬁciency max(η) ≈

ma ma + 13 ms

[10.8]

see equation 10.1. There is no simple expression for the amount of kinetic energy involved in the limbs and its mass. Klopsteg (1943) introduces a ¯ h by virtual mass K ¯ = (m ¯ h) ¯c 2l A ¯a + K

[10.9]

¯ h represents one-third of the (half ) string mass -1 ¯ The parameter K ms plus 3 an unknown added mass, accounting for the kinetic energy of one limb. In ¯ h also accounts for the excess of elastic energy in the limbs and the fact, K string at arrow exit compared with the undrawn, braced situation. The ¯ h is a phenomenological quantity that cannot simply be identiparameter K ¯ h can be ﬁed with the physical properties of the bow. The virtual mass K determined by measuring the velocity as a function of arrow mass. Experi¯ h is a constant for a speciﬁc bow. Tuijn mentally Klopsteg showed that K and Kooi (1992) conﬁrmed this for the working-recurve Greenhorn bow ¯ h = 4.6 g per limb thus 9.2 g in total. Table 10.1 shows the charand it was K acteristic parameter values for the Greenhorn bow and Easton 1616X75 arrow together with the predicted dimensionless quality coefﬁcients. Kooi and Sparenberg (1980) showed that, theoretically, a shooting efﬁciency of 100% could be obtained if the limbs of the bow are taken to be rigid with all the elasticity to be concentrated in two elastic hinges and an inextensible string without mass. This simple model shows the principle of the bow very clearly; because of geometrical constraints the velocity of the relatively heavy limbs at arrow exit is small, while the very light string is connected to the fast moving arrow. This implies that the kinetic energy of the moving parts of the bow at arrow exit is relatively small and therefore almost all energy available is transferred as kinetic energy to the high-speed light arrow. The results (Kooi, 1994) show that the modern working-recurve bow is a good compromise between the non-recurve bow and the static-recurve bow. The recurve yields a good static quality coefﬁcient q and the light tips of the limbs give a high efﬁciency h.

10.3

Modelling bow design

The limbs of the bow can be considered to be a beam of variable crosssection ¯ D(s¯ ) made out of one material with density r ¯ and Young’s modulus ¯ E . We neglect damping and rotary inertia and the inﬂuence of the normal

© 2007, Woodhead Publishing Limited

256

Materials in sports equipment

and shear forces. Assumption of the Euler–Bernoulli hypothesis yields the ¯ (s¯ ,t¯ ) and the normal following relationship between the bending moment M stress s¯ (s¯ ,t¯ ,r¯ ), with r¯ the distance from the neutral axis, which passes through the centroid of the cross-section:

σ (s , t , r ) =

M ( s , t )r

∫

[10.10]

2

D( s )

r dD

Kooi (1994) showed that it is acceptable to assume that the maximum bending moment for each ¯s as a function of time ¯t occurs in the fully drawn ¯ b the elastic energy in the limbs of the situation. Then we have again with A bow in a fully drawn situation Ab = mb

2∫

L

L0

1 (σ 1( s , r ))2 dDds E

∫2

∫

L

L0

where C ( s ) = ∫

D( s )

[10.11]

ρC ( s )d s

¯ (s¯ ) = ¯ ¯C V(s¯ ) dD is the area of the cross-section. Thus r

is the mass distribution along the limb. The stress s¯ 1 is the resulting normal stress due to the bending moment in the fully drawn bow, indicated by the subscript 1. Two additional useful quality coefﬁcients are deﬁned by L

σ2 δ bv = w , aD = 2ρ E

∫ ∫ L0

1 2

1 2

(σ 1( s , r ))2 dDd s L

[10.12]

σ w2 ∫ C ( s )d s L0

where s¯ w is the working stress of the material, and the quantity ¯d bv is the amount of energy which can be stored per unit of mass in the material. Then, when ¯ E and r ¯ are constant Ab = 2aDδ bv mb

[10.13]

The maximum value for the dimensionless coefﬁcient aD equals 1 in the homogeneous stress-state s¯ 12(s¯ ,r¯ ) = s¯ 2w for all ¯s and r¯ . However, aD is generally smaller than 1 for two reasons. First, the stress in the ﬁbres near the neutral axis is smaller than the working stress and this reduces the coefﬁcient aD. Suppose the stress in the outermost ﬁbers equals the working stress for all ¯s . In this case the limb is called uniformly stressed, and then we have for each cross-section due to bending

© 2007, Woodhead Publishing Limited

Design and materials in archery

σ 1( s , r ) = σ w

r e (s )

257 [10.14]

where ¯e is the largest of the two distances between the outermost ﬁbers and the neutral axis. For a uniformly stressed bow, aD is maximum for the given ¯ D(s¯ ). If the material has the same strength in tension and compression, it will be logical to choose shapes of cross-section in which the centroid is at the middle of the thickness of the limb, equal to 2e¯(s¯ ). Then the dimensionless coefﬁcient aD reads

aD =

∫

I (s ) ds (e ( s ))2

L

L0

∫

L

L0

[10.15]

C ( s )d s

where I¯(s¯ ) is the moment of inertia of the cross-section with respect to the neutral axis. The quantity ¯ E I¯(s¯ ) = ¯ W(s¯ ), already introduced in Section 10.2, is the distribution of the bending stiffness along the limb. When the limbs of the bow have geometrically similar cross-sections along the limb and for a uniformly stressed bow, we have aD = aD where aD(s¯ ), deﬁned by

α D( s ) =

I (s ) C ( s )(e ( s ))2

L0 ≤ s ≤ L

[10.16]

is constant, i.e. independent of ¯s . For a rectangular cross-section with the neutral axis at the centre, we have

αD =

width × height 3 / 12 = 0.333 width × height × (height / 2)2

In modern bows synthetic plastics are reinforced with glassﬁbre (a combination of glass and a polymer) or carbon. So, in composite bows not only are better materials used, but they are also used in a more proﬁtable manner. For composite bows we deﬁne equivalent quantities for the Young’s modulus and density so that it has the same mechanical action with respect to bending as the simple bow made out of one kind of material. If these magnitudes are substituted in the product aD¯d bv this product can be substantially larger than for simple wooden bows. Second, the tensile and compressive stresses in the fully drawn bow in the outermost ﬁbres of the limb, in the back and the belly respectively, may be less than the working stress. The working stress is equal to the yield point or the ultimate strength (the maximum stress a material withstands when subjected to an applied load) divided by a factor of safety. For target shooting, safety also determines the quality of the bow. With a low factor of safety, high stresses are allowed and this yields light limbs and generally

© 2007, Woodhead Publishing Limited

258

Materials in sports equipment

a high efﬁciency h and high arrow speed ¯cl. So speed and safety are oppositely related and a compromise has to be found.

10.4

Modelling bow materials and their properties

In this section we discuss the materials used for the different parts of the bow. Materials science encompasses the study of the structure and properties of any material and the tailoring of these properties for speciﬁc uses.

10.4.1 Limb materials The limbs are the most critical parts of the bow. They store the energy that can be imparted to the arrow. Kooi (1994) has shown that the shapes of the limbs and their materials are related. We introduced a classiﬁcation of the bows based on the shapes of the limbs and consequently also the materials as follows. •

•

•

Non-recurve bows – simple bows made out of one piece of wood, straight and tapering towards the ends, have been used by early peoples in Africa, South America and Melanesia. Ancient composite bows, e.g. the angular bow used in Egypt and Assyria, consisted of layers of several kinds of organic materials, wood, sinew and horn. The simple straightend bow and the English longbow, and the composite angular bow are non-recurve bows. Static-recurve bows – some Tartar, Chinese, Persian, Indian and Turkish bows are static-recurve bows. These bows are also composite bows. In the braced situation the string rests upon string-bridges. These stringbridges are ﬁtted to prevent the string from slipping past the limbs. When such a bow is drawn, at some moment the string leaves the bridges and has contact with the limbs only at the tips. Generally these bows are composite bows. Working-recurve bows – today almost all modern bows seen at target archery events such as the Olympic Games are working-recurve bows. The core of these modern composite bows is made of wood, for instance, maple, with layers of synthetic plastics reinforced with glassﬁbre or carbon on both sides. Nowadays also honeycomb/foam cores are used.

When we neglect the elastic energy stored in the string, the equation for the initial velocity and the kinetic energy of the arrow follow from cl = 2

q η aDδ bv Ab ma

and ma c 2l = 2 mb

q ηaDδ bv Ab

[10.17]

respectively. Hence, the initial velocity of an arrow depends on the ratio of the static quality coefﬁcient q and the amount of elastic energy stored

© 2007, Woodhead Publishing Limited

Design and materials in archery

259

in the fully drawn limbs, Ab, on the ratio of the efﬁciency h and the dimensionless mass of the arrow ma and ﬁnally on the product aD¯d bv. The kinetic energy is linearly dependent on the mass of the bow, mb, whereas it is independent of the mass of the arrow. In Table 10.2 an estimation of ¯d bv for some materials used in making bows is given. Better materials (larger ¯d bv) or a better use of the materials (larger aD) for a certain draw-length bow weight allow lighter bow limbs and therefore a higher efﬁciency h. Elliot (2002) mentions three main types of limb of working-recurve bows are mentioned. •

Laminated wood and glassﬁbre – wood/glass limbs perform well in areas where the temperature and humidity stay constant; however, wood is prone to stretching and warping when heat and humidity vary signiﬁcantly. • Laminated wood and glassﬁbre and some carbon ﬁbre layers – carbon ﬁbre layers help strengthen the limb and reduce the tendency to twisting. • Carbon ﬁbres and a core made out of some hard foam or ceramics – modern carbon/foam limbs are highly impervious to climatic changes, and are therefore the most consistent. A honeycomb/foam core provides consistent limbs resisting twist to a high degree, especially at the limb tips where they are weakest. The following additional features of the materials are important for the performance, especially for target shooting: relative immunity of the mechanical properties to temperature and humidity variations, no tendency to follow the string associated with creep and no hysteresis (no inner friction between limb panels). The materials should give the limbs a high durability, high torsion-resistance and should provide stable limb movement and good speed balance. The archers need more speed, for a given

Table 10.2 Mechanical properties and the energy per unit of mass ¯d bv for some materials used in making bows (see also Gordon, 1978) Material

¯s w (N/m2 107)

¯E (N/m2 1010)

¯r (kg/m3)

¯d bv (Nm/kg)

Steel Sinew Horn Yew Maple Glassﬁbre

70.0 7.0 9.0 12.0 10.8 78.5

21.0 .09 .22 1.0 1.2 3.9

7800 1100 1200 600 700 1830

130 2500 1500 1100 700 4300

© 2007, Woodhead Publishing Limited

260

Materials in sports equipment E, P

E

tl

t

P

10.4 Acceleration force E and recoil force P for the working-recurve bow. Moment of arrow exit is t = tl where the acceleration force is zero.

draw weight. Smoothness means consistency, which minimizes the inﬂuence of a mistake and means better arrow grouping, that is better accuracy in shooting. A measure for the smoothness is the recoil force the archer feels in his or her bow hand. For the Greenhorn bow the predicted recoil force denoted by P is depicted in Fig. 10.4, together with the acceleration force E(t) deﬁned in equation 10.1 acting on the arrow, both as function of time. The arrow leaves the string when E = 0; the time at which this occurs is denoted by t = tl. After arrow exit in Fig. 10.4 the acceleration force E is the force acting on the added mass representing the string. As can be seen, E oscillates around zero for t = tl.

10.4.2 Arrow materials Wooden arrows are mainly used for traditional archery. Later glassﬁbre shafts became popular for target shooting and nowadays aluminium and carbon are used. The arrow must be strong enough to withstand the acceleration force. Liston (1988) discusses the structural strength of the arrow. The accelerations involved in the propulsion of the arrow are enormous; for a 23 kg (50 lb) bow and an arrow with mass 0.02 kg the acceleration after release is about 12 500 m/s2 = 1250 g, where g is the gravitational acceleration. The shaft of is able to withstand the compression force. However, the arrow bridges the distance between the middle of the string and the grip. Therefore it is a long slender beam and the arrow buckles and bends but does not collapse. The fact that the arrow will bend when shot is unavoidable. In Fig. 10.5 a three-dimensional picture shows the position of the arrow with respect to

© 2007, Woodhead Publishing Limited

Design and materials in archery

261

y

Elastic limb

String

Rigid grip O

G

Arrow

H

D

x

z

10.5 Static deformation of the Greenhorn working-bow: the unbraced bow (box), braced bow H and fully drawn bow D (both solid and some intermediate situations (dashed lines). Point O denotes the place where the elastic limbs are ﬁxed to the rigid grip and G denotes the position of the arrow rest, which is the pressure button. The position of the arrow before release is also shown.

the bow. Launching the arrow, the string slips off the three ﬁnger tips and in this way the nock of the arrow is moved swiftly sideways. Hence by its inertia the arrow will bend. This phenomenon is associated with the ‘Archer’s Paradox’. Different physical and mathematical models dealing with the interaction between the bow and the arrow have been described and analysed (Kooi, 1998a; Zanevskyy, 2001). In Fig. 10.6 we show experimentally measured transverse displacements of the arrow during the release of the arrow until its nock passes the grip. These deﬂection curves are measured by Pe˛kalski (1987, 1990) and taken from the ﬁlm made with the camera viewing the archer from above. Also predicted shapes by a model developed by Kooi and Sparenberg (Kooi and Sparenberg 1997; Kooi, 1998b) are shown. The arrow is modelled as a slender beam (the shaft) with two particles (the nock and the arrowhead). The bow used by Pe˛ kalski (1987,1900) was a Hoyt Pro Medalist T/D, 66 inch, 34 lbs bow. The parameter values for the bow and arrow are given in Table 10.3. The DFD curve shown in Fig. 10.3 belonging to the

© 2007, Woodhead Publishing Limited

262

Materials in sports equipment

6 ms 8 ms

Horizontal deflection (m)

0.015

10 ms

4 ms

0.01

12 ms 0.005

D 0

2 ms

0 ms

G H

0.645

0.225

O 0

Forward displacement (m)

10.6 Deformation of arrow: high-speed ﬁlm experimental data (dashed curve) after Pe˛kalski (1987, 1990), calculated with mathematical model derived in Kooi and Sparenberg (1997) (solid curve), every 2 ms after release, until the arrow nock passes the grip. Pe˛kalski used the Hoyt Pro Medalist T/D, 66 inch, 34 lbs bow and the Easton 1714X7 (Aluminium 7178) arrow. Table 10.3 Values for the parameters of the Easton 1714X7 (Aluminium 7178) arrow. The outside diameter d is measured in 64ths of an inch and is designated by the ﬁrst two numbers of the shaft code: e.g. d = 17/64 inch. The wall thickness is measured in thousandths of an inch and is designated by the last two digits on the shaft code: e.g. g = 0.014 inch. The ﬂexural rigidity equals the moment of inertia of cross-section of arrow shaft × elastic modulus, that is 7.11.010 × 0.0037 10−8 = 2.6 Nm2 Parameter

Unit

Value

Interpretation

¯d ¯g ¯ C ¯r ¯I ¯E ¯l a ma 2¯ ¯ man ¯ mat

m m m2 kg m−3 m4 Nm−2 m kg kg kg

0.00675 0.000356 0.0723 10−4 2.82 103 0.0037 10−8 7.1 1010 0.67 0.0188 0.0014 0.004

External diameter of arrowshaft Wall thickness of arrowshaft Area of cross-section of arrowshaft Mass density Moment of inertia Modulus of elasticity Length of arrow Mass of arrow Mass of nock Mass of arrowhead

Greenhorn Comet TD 350, 68 inch, 30 lbs was scaled using the weight ¯(|O ¯¯ ¯¯ D|) = 143 N, draw |O D| = 0.584 m and efﬁciency h = 0.76. The value for F the ﬂexural rigidity supplied by the manufacturer equals ¯ E I¯ = 2.6 = 7.11010 −8 2 × 0.0037 10 Nm . The mass of the Easton 1616X75 (Aluminium 7075)

© 2007, Woodhead Publishing Limited

Design and materials in archery

263

arrow mentioned in Table 10.1 is 0.0183 kg while the mass of the 1714X7 (Aluminium 7178) arrow Pe˛ kalski used is 0.0188 kg. Using parameter values for different arrow simulations showed that the nock of the arrow slaps against the pressure point and this is undesirable. Hence the correct arrow bending properties must be selected to ensure that the arrow neither bends too much, nor too little so that the rear end of the arrow snakes around the grip. In archery, ‘spine’ is used as a measure for the ﬂexibility of the arrow. To measure the spine of an arrow, for instance, one that is 29 inches long, in a three-point bending test a bob weighing 1.94 lb (8.63 N) is hung in the centre of the arrow supported at two points separated by a distance (the span) of one inch less than the arrow length, in our example 28 inches (0.7112 m). The deﬂection measured in inches is the spine of the arrow. The shaft selection charts provided by the arrow manufacturers can be used by an archer to determine the best arrow with the correct spine depending on the weight and draw length of the bow. The model proposed (Kooi and Sparenberg, 1997; Kooi, 1998b) for the arrow ﬂight, make it possible to do tuning by simulation on the computer. In the model all parameters have a clear mechanistic interpretation. Therefore arrow clearance can be predicted (see Fig. 10.6). We conclude that the deﬂection in the horizontal plane and the stresses remain small. Consequently there is no breakage during the process of shooting. Of course, the arrow still has to withstand the forces when it hits and enters the target mat. It is important that the archer is able to choose very similar arrows of nearly the same mass, spine and air drag to shoot in one ‘end’ (often six arrows). So the manufacturing processes of arrows should deliver uniform spine between all arrow shafts of the same size, meet strict weight tolerances and surpass stringent straightness requirements. Elliot (2002) mentions the following arrow materials. •

•

Indoor target archery – aluminium is the most popular arrow choice for indoor shooting. The arrow is formed from an extruded aluminium tube, typically heavier than carbon shafts since the strength comes entirely from the thickness of the material. They are generally half as expensive as carbon arrows. Aluminium arrows can also be easily cut to length. Outdoor target archery – carbon or aluminium/carbon mix is the usual arrow of choice for the long distances of outdoor shooting.

Although there are pure carbon arrows available, arguably the most popular arrows of choice are formed from a combination of aluminium and carbon. This mixture provides the beneﬁts of lightness with strength. The arrows have an inner aluminium alloy core tube and an outer wrap of carbon ﬁbre and epoxy-resin matrix. However, a small ‘diameter’ thickwalled carbon-ﬁbre core and a micro-wall alloy metal jacket can also be

© 2007, Woodhead Publishing Limited

264

Materials in sports equipment

used, giving a more consistent spine, straightness and weight compared to all-carbon arrows. This surface provides also easy removal from tough, high-density target materials (Easton, 2006). Arrows are also ‘barreled’, i.e. they have a larger diameter at the centre than at the ends. This makes the surface area lower and so improves performance in windy conditions. Carbon arrows can be light and this gives high speed. However, a light arrow may be more prone to the effects of crosswinds.

10.4.3 String materials The same reﬂections on energy storage capacity can be given for the materials of the string. To reduce the mass of the string, strong materials should be used which are able to store large amounts of elastic energy for a given mass. Except for the loop with which it is fastened to the bow, the material throughout the cross-section of the string is uniformly stressed. The maximum force determines, together with the strength of the material, the minimum mass of the string. Computer simulations show that maximum force does not occur in the fully drawn situation. So, there is a relationship ¯ s, ¯ ms, and the strength of the material used between the string parameters, U for the string, and the maximum force in the string. This force is not known from static calculations, so an initial guess has to be made, which must be checked after the dynamic calculations. The acceleration forces E(t), equation 10.1, and the force in the string K as a function of time t are shown in Fig. 10.7. The maximum force in the string occurs after arrow exit and it is larger than in the fully drawn situation. An initial guess for the maximum force is about seven times the weight

E, K

K

E

tl

t

10.7 Acceleration force E(t) and string force K for the working-recurve bow. Moment of arrow exit is t = tl where the acceleration force is zero.

© 2007, Woodhead Publishing Limited

Design and materials in archery

265

of the bow. However, neither internal or external damping nor hysteresis are taken into account. In reality there is always internal and external damping which forces the bow and string to return to the braced situation. This reduces the actual force in the string for large t and shows that the number seven from Fig. 10.7 is conservative. We refer to Halpin (2003) for a description of bowstring materials. Ancient materials are ﬁbres from animals: rawhide, sinew, intestine, hair and silk; or plants: yucca, cannabis, nettle and cotton. The Belgian strings made of long-ﬁbred Flemish ﬂax have been famous. Before the 1940s manmade ﬁbres were already developed. Popular man-made ﬁbres were and are Dacron® (Invista Inc.), Kevlar® (DuPont), Twaron® (Teijin Twaron BV), Fast Flight introduced by Brownell & Company made from Spectra® (Honeywell) ﬁbre and Dynaﬂight made from Dyneema® (DSM). Dacron is a polyester which was developed in the late 1950s by Dupont and is still in use today. Dacron is long lasting and it is slow (see Table 10.4, after Tuijn and Kooi (1992)). The slow speed is due to the excessive stretch on each shot. The stretching does have the advantage that it is kinder to the bow limbs and risers and is therefore ideal for bows with wooden risers or limbs. Kevlar is an aramid ﬁbre, and these ‘Liquid Crystal Polymers’ ﬁbres are strong due to the unidirectional nature of the molecules. It is susceptible to moisture and so must be waxed carefully. Fast Flight is a high modulus polyethylene (HMPE) ﬁbre. The ultra-long chain polyethylene ﬁbre strings are popular today. It proved to be superior to Kevlar (see again Table 10.4). The ﬁbre is very long lasting, is not susceptible to moisture, and it can be twisted as much as required. These are the lightest and fastest of the materials available today. Because it has no wax it is a little lighter than an equivalent string made from other materials.

Table 10.4 The velocity c¯ l and the efﬁciency h for different strings released from a rack with the modern working-recurve bow. The ma = 18.1 g arrow mass is 2¯ String

ms (g) 2¯

c¯ l (m/s)

h

Dacron1 Kevlar2 Twaron3 Fast Flight4

6.55 6.48 6.53 6.45

49.73 51.25 51.68 52.09

0.70 0.738 0.75 0.762

1®

Invista Inc. DuPont. 3® Teijin Twaron BV. 4® Brownell and Co. 2®

© 2007, Woodhead Publishing Limited

266

Materials in sports equipment Table 10.5 Velocity as a function of the number of strands. Fast Flight string on working-recurve Greenhorn bow Number of strands

ms (g) 2¯

c¯ l (m/s)

16 14 12 10

6.45 5.82 5.28 4.64

51.88 51.43 51.62 51.76

In Table 10.4 experimental data are given where different string materials were used. The bow was mounted in a rack and a release aid was used. The results indicate that the Dacron strings give a distinctly lower velocity. This is in agreement with its lower modulus of elasticity (Kooi, 1983, p.119). Apparently the Fast Flight strings are best. The force to elongate a string has been determined for Dacron and Kevlar: for Dacron it is about 22 N per % of elongation per strand and for Kevlar 70 N per % per strand. Twaron too is known for its high stiffness. Elliot (2002) gives the following data for strength/stretch (creep) properties for the string materials. For Dacron – strength per strand: 225 N, stretch: 2.6%; for Kevlar – strength per strand: 318 N, stretch: 0.8%, and for Fast Flight – strength per strand: 455 N, stretch: 1.0%. With almost all modern materials used in the manufacture of bowstrings, the strength is well in excess of the maximum tension applied to the string. However, a weaker and therefore lighter string yields a higher efﬁciency h and initial velocity v (see equation 10.8). If safety allows, a decrease of number of strands makes the string less stiff (generally this implies a lower speed) but also lighter (and this is advantageous). Hence there is a tradeoff. However, Tuijn and Kooi (1992) prepared a Fast Flight string of 16 strands and removed consecutively two, four and six of its strands. Table 10.5 shows that the differences are small. Obviously the two effects, light–ﬂexible versus heavy–stiff, neutralize each other. For safety reasons it is best to use the manufacturer’s recommended number of strands in the bowstring for the weight of the bow. If too few strands are used it is likely that some ‘creep’ will occur, particularly on the high poundage bows at high temperatures.

10.4.4 Riser materials Finally we discuss the construction of the riser or grip. Since the riser is held in place during shooting, the mass 2m ¯ g does not appear in the mathematical model as long as the recoil force between the grip and the bow hand is positive. The archer does not hold or grip the bow; it is held in place by

© 2007, Woodhead Publishing Limited

Design and materials in archery

267

pressure on the bow hand at the pivot point. Avoiding a ﬁrmly held bow minimizes torque when the arrow is released, which would result in an erratic arrow ﬂight. The mass is only important when the motion of the bow is calculated after the bow has left the bow hand. Elliot (2002) calls the riser the ‘heart’ of the bow. Until approximately 1970, most risers were made out of wood, often combinations of different types of wood, or a composite of materials such as wood, horn, sinew, ﬁsh glue and so on. The advent of modern materials (carbon arrows, Fast Flight strings) has meant that wooden risers could no longer cope with the strain placed upon them. Therefore metal-handled and carbon/foam material risers have been developed. The majority of modern recurve (Olympic) bows are forged and then CNC machined: computer numerically controlled machining services use machines that are fast, repeatable and programmable, and which can function while unattended, making it possible to manufacture parts quickly and efﬁciently. At both sides of the riser, limbs are attached, often such that the position of the limbs can be adjusted. Also stabilizers are mounted to the riser (see Fig. 10.1), and these compensate for the archer’s shooting faults involving the bow hand. Finally, in order to get a centre-shot bow, a section is cut away in the middle of the riser. In this window an arrow rest is mounted to support the arrow and guide it during the shooting.

10.5

Summary and future trends

Nowadays, measurement apparatus like the Bow Force Mapper is available to measure the SFD curve and an Arrow Chronograph to measure the velocity of the arrow (Easton, 2006). Via an interface with a computer the measured data can be stored on a computer and post-processed with software Shaft Selector software (see Easton, 2006). The computer program calculates the kinetic energy of suggested arrows, the bow static and dynamic efﬁciency and arrow selection recommendations. The use of these analysis facilities signiﬁcantly improves bow and arrow selection. We consider only bows of traditional design, consisting of a bent stick with a grip in the middle. We do not deal with newer developments in which mechanical devices such as pulleys, cams and springs are used to improve the design (Arinson, 1977; Mullaney, 1978). As an example we mention the popular ‘compound bow’. A physicist Claude Lapp reportedly built the ﬁrst compound bow in 1938. This bow has pulleys with eccentric bearings at the end of the rather stiff elastic limbs. Due to the mechanisms (cams) about half-draw length, the draw force is maximum and becomes minimum at full-draw (Easton, 2006). Since the archer aims in that position, a low draw force improves aiming and a steady release.

© 2007, Woodhead Publishing Limited

268

Materials in sports equipment

Kooi (1994) analysed a bow that resembled the bow described and shot by Hickman et al. (1947) which had a very large static equality coefﬁcient q = 0.81. However, its efﬁciency is low, h = 0.417, and therefore the overall performance is disappointing. Mullaney (1978) describes the ‘Buckle-bow’ designed by the Dutch engineer G.J. Scholten. Two light rigid limbs are attached by a hinge to the grip. At the belly side a steel buckle spring is place between the limb and the grip that stores the energy. When this bow is drawn the string buckles and, as a consequence, the drawing force does not increase during this ﬁnal part of the draw. This gives the bow a large static quality coefﬁcient q. The weight of the bow was 100 N. Shooting a 0.016 kg arrow, the measured efﬁciency was 81.8 %, that is higher then the 72.9% efﬁciency given in Table 10.1 where the mass of the arrow is heavier, namely 0.0183 kg; therefore shooting a lighter 0.016 kg arrow would give an efﬁciency smaller than 72.9%. However, the use of a mechanism prevented acceptance of this bow despite its better mechanical performance (the smoothness in shooting is not reported unfortunately). Nowadays bow manufacturers are still looking for limb designs that make it possible to store more energy (higher q) with the same weight and draw length. The so-called ‘ellipse’ double curved limbs are shown in Greenhorn (2006). It is claimed: ‘in comparison with the traditional recurve shaped limbs, the double curved limbs deliver a lot more energy and are much smoother drawing’. It is challenging to design a bow with a force draw curve with a hump in the middle but without the use of mechanisms: that is, with conventional limbs. In order to get a hump in the force draw curve the stiffness of the limbs should diminish during the draw. This would happen when the crosssection of the limbs changes shape (for instance the height diminishes when the load on the limb increases) so that the moment of inertia of the crosssection with respect to the neutral axis diminishes when the bow is drawn. What is needed is a core material for the limbs that is able to withstand shear forces but at the same time one that shrinks when loaded. The same loading characteristics can be achieved by applying materials for the layers on the belly- and back-side, with a non-linear but elastic stress–strain relationship.

10.6

Conclusions

In this chapter we have given explanations for facts and claims made by bow manufacturers. This shows that mathematical modelling is a helpful tool in the archery research for the design of new bow equipment. We have demonstrated the need to formulate the bow design problem in terms of structural optimization theory. Mathematical formulation helps

© 2007, Woodhead Publishing Limited

Design and materials in archery

269

with the systematic identiﬁcation of all the design parameters. Simplicity assumptions make the design process surveyable. This allows us to clarify the differences in the performance of bows of different makes by the introduction of technological constraints. Introduction of technological constraints and realistic assumptions about the cross-section of the limbs yield a relationship between the distribution of the mass and bending stiffness along the limbs. This reduces the dimension of the design parameter space. The quality coefﬁcients of the modern bow are only slightly better than those of the historical bows. Materials used in modern working-recurve bows can store more deformation energy per unit of mass than materials used in the past. Moreover the mechanical properties of these materials are more durable and much less sensitive to changing weather conditions. This contributes most to the improvement of the modern bow. Developments in materials technology will make the production of lighter and stronger bows as well as lighter and stronger arrows possible. Arrow speed will increase giving better accuracy over the longer distances.

10.7

References

arinson r b (1977), The compound bow: ugly but effective, Machine Design, Nov., 38– 40. easton (2006), http://www.eastonarchery.com/. elliot m (2002), Reference guide for recurve archers, Grange and Balbardie archery clubs, http://www.archersreference.pwp.blueyonder.co.uk, last accessed 17 March 2007. gordon j e (1978), Structures or Why Things Don’t Fall Down, Harmondsworth, Pelican. greenhorn (2006), http://www.greenhorn.be/. halpin r w (2003), Bowstring materials, J Soc Archer–Antiquaries, 46, 54–61. hickman c n, nagler f and klopsteg p e (1947), Archery: the Technical Side, Redlands, CA, National Field Archery Association. klopsteg p e (1943), Physics of bows and arrows, Amer J Phys, 11, 175–92. klopsteg p e (1963), Bows and Arrows: a Chapter in the Evolution of Archery in America, Technical report, Washington, Smithsonian Institute. klopsteg p (1987), Turkish Archery and the Composite bow, Manchester, Simon Archery Foundation. kooi b w (1981), On the mechanics of the bow and arrow, J Eng Math, 15(2), 119–45. kooi b w (1983), On the Mechanics of the Bow and Arrow, PhD thesis, Rijksuniversiteit Groningen. kooi b w (1991a), The ‘cut and try’ method in the design of the bow, in Eschenauer H A, Mattheck C and Olhoff N (eds), Engineering Optimization in Design Processes, Vol 63 Lecture Notes in Engineering, Berlin, Springer-Verlag, 283–92. kooi b w (1991b), On the mechanics of the modern working-recurve bow, Computat Mech, 8, 291–304.

© 2007, Woodhead Publishing Limited

270

Materials in sports equipment

kooi b w (1994), The design of the bow, Proc Kon Ned Akad v Wetensch, 3, 283–309. kooi b w (1998a), The archer’s paradox and modeling, a review, Hist Technol, 20, 125–37. kooi b w (1998b), Bow-arrow interaction in archery, J Sport Sci, 16, 721–31. kooi b w and sparenberg j a (1980), On the static deformation of a bow, J Eng Math, 14(1), 27–45. kooi b w and sparenberg j a (1997), On the mechanics of the arrow: Archer’s Paradox, J Eng Math, 31(4), 285–306. liston t l (1988), Physical laws of Archery, San Jose, CA, Liston & Associates. mullaney n (1978), Buckle bow . . . a new twist, Archery World, 27(2), 38–61. pe˛kalski r (1987), Modelling and simulation research of the competitor–bow–arrow system (in Polish), PhD thesis, Academy of Physical Education, Warsaw. pe˛kalski r (1990), Experimental and theoretical research in archery, J Sport Sci, 8, 259–79. tuijn c and kooi b w (1992), The measurement of arrow velocities in the students’ laboratory, Eur J Phys, 13, 127–34. zanevskyy i (2001), Lateral deﬂection of archery arrows, Sports Eng, 4, 23–42.

© 2007, Woodhead Publishing Limited

11 Design and materials in rowing B. K. F I LT E R, Consultant, Germany

11.1

Introduction

Rowing has a very long history with many different forms taking place worldwide. However, the origins of competitive rowing boats go back to the work boats of the professional watermen, especially from the rivers Tyne and Thames in England. Derived from the nature of their daily labour – bringing a load or a passenger to the required destination faster than other watermen – by the early nineteenth century professional watermen were already holding regattas using their own boats. When working rowing boats lost their role as a result of the introduction of steam boats, competitive and pleasure rowing spread, especially in universities and among the wealthy. In this period, the sport of ‘amateur rowing’ meant the exclusion of the waterman and other manual workers. Four developments enabled the evolution of traditional work boats into modern racing shells: • • • •

the the the the

outrigger; sliding seat; swivel oarlock; inside keel.

The racing shell, with an inside keel, gunwhales from spruce, shell and saxboard from cedar wood and shoulders from ash, oak or maple wood (see Fig. 11.1) remained for nearly 100 years with little change until new technologies in the materals ﬁeld enabled the invention of new composites. The Olympic Games in Munich in 1972 saw for the ﬁrst time competition between different composite shells made by different boat builders, and over time a sort of ‘arms race’ in boat building began.

11.2

International regulation of competitive rowing equipment

In 1979, the International Federation of Rowing Associations (FISA) formed a technical board to monitor and control the development of 271

© 2007, Woodhead Publishing Limited

272

Materials in sports equipment

Seat Saxboard

Rail

Gunwhale

Step in board Cross-ledge Diagonal stiffness ledge

Shoulder

Keel rider

Shell Keel

11.1 Cross-section of traditional racing shell.

rowing materials. The FISA rules concerning equipment for rowing focus on fairness of competition and the safety of the athletes taking part. In Sections 11.2.1–11.2.4 indented material is taken from the 2006 FISA Rulebook, parts 1 and 2.

11.2.1 The deﬁnition of rowing Rule 1 – Rowing, Boats, Regattas Rowing is the propulsion of a displacement boat, with or without coxswain, by the muscular force of one or more rowers, using oars as simple levers of the second order and sitting with their backs to the direction of movement of the boat. Rowing on a machine or in a tank which simulates the action of rowing in a boat is also considered as rowing. In a rowing boat, all load bearing parts including the axes of moving parts, must be ﬁrmly ﬁxed to the body of the boat, but the rower’s seat may move along the axis of the boat. A rowing regatta is a sporting competition consisting of one or more events divided, if necessary, into a number of races, in one or more classes of boats for rowers divided, as a general rule, into different categories of sex, age or weight.

11.2.2 Boats and construction Rule 31 – Free Construction The construction, design and dimensions of boats and oars shall, in principle, be unrestricted subject to the limits laid down in Rule 1, paragraphs 1 and 2,

© 2007, Woodhead Publishing Limited

Design and materials in rowing

273

and Rule 58. Nevertheless the Council of FISA may, in the Racing Bye-Laws, impose appropriate requirements.

Bye-Law to Rule 31 – Boats and Equipment 1. Requirements for racing boats: 1.1 The bows of all boats shall be ﬁtted with a white ball shape, minimum diameter 4 cm, in hard rubber or a hard material. If this is an external part it shall be ﬁrmly afﬁxed to the bow of the boat, if an integral part of the hull construction it shall afford equivalent protection andvisibility to a ball. 1.2 All boats and oars shall comply with the requirements set out in the Bye-Laws to Rule 41, below (name, symbol, etc.). 1.3 During racing, no wireless transmission equipment shall be used, whether for sending or receiving. 1.4 No substances or structures (including riblets) capable of modifying the natural properties of water or of the boundary layer of the hull/ water interface shall be used. 1.5 To avoid accidents arising from capsizing, all boats shall be equipped with stretchers or shoes that allow the competitors to get clear of the boat without using their hands and with the least possible delay. 1.6 The edges of blades must have a minimum thickness throughout as follows: – oars 5 mm, – sculls 3 mm. This thickness shall be measured 3 mm from the outer edge of the blade for oars and 2 mm for sculls. 1.7 The opening of the coxswain’s seat must be at least 70 cm long and it must be as wide as the boat for at least 50 cm. The inner surface of the enclosed part must be smooth and no structure of any sort may restrict the inner width of the coxswain’s section. 1.8 All boats used in eights events at World Rowing Championships, Olympic regattas, Olympic qualiﬁcation regattas, Regional Games and Continental Championships and at all International regattas shall be in a minimum of two sections, with no section longer than 11.9 metres. 1.9 At World Championships, Olympic Regattas or RowingWorld Cup regattas, the Council may require crews to carry on their boats such equipment as it considers desirable for the better promotion of the sport of rowing (e.g. mini cameras) provided that such equipment is identical for all boats in a race. 1.10 Boats constructed or delivered after 1 January 1998 must have a production plaque or equivalent visible and permanently afﬁxed inside the boat, up to 50 sq cm in area, on which is written the name and address of the boat builder, its mark or logo, the year the boat was constructed, the average weight of the crew for which the boat is designed, and the weight of the boat on construction or upon delivery.

© 2007, Woodhead Publishing Limited

274

Materials in sports equipment 1.12

Length of Boats – Minimum length of racing boats – The minimum overall length of a racing boat shall be 7.20 metres. This will be measured from the front of the bow ball to the furthest aft extent of the boat, which may include an extension beyond the hull. If an extension is used it will be ﬁrmly afﬁxed to the stern and terminate in a 4 cm ball as described in Bye-Law to Rule 31: 1.1. If a boat cannot be correctly aligned because it is less than the minimum overall length, the Starter may exclude the crew from the race. Rule 32 – Boat Weights All boats used at Olympic regattas or qualiﬁcation regattas, World Championships, Rowing World Cup regattas, Regional Games and Continental Championships and all international regattas shall be of deﬁned minimum weights. Bye-Law to Rule 32 – Boat Weights 1. Minimum boat weights are the following: 1x–14 kgs, 2x–27 kgs, 2− –27 kgs, 2+ –32 kgs, 4x–52 kgs, 4− –50 kgs, 4+ –51 kgs, 8+ –96 kgs 2. The minimum weight of the boat shall include only the ﬁttings essential to its use; in particular – riggers, stretchers, shoes slides, seats and hull extensions. The minimum weight shall not include the oars or sculls, the bow number, any sound ampliﬁcation equipment and loudspeakers or any other kind of electronic equipment.

11.2.3 FISA guideline for safe rowing equipment Since 1 January 2007 the boatbuilder’s plaque, as described above in Bye-Law to Rule 31, must include additional information as described below. 1.11

Boats constructed or delivered after 1 January 2007 must also show on the production plaque (in 1.10 above) whether the boat meets ‘FISA’s Minimum Guidelines for the Safe Practice of Rowing’: ‘A boat when full of water with a crew of average weight equal to the design weight stated on the boat’s production plaque, seated in the rowing position should ﬂoat such that the top of the seat is a maximum of 5 cm below the static waterline.’

FISA guidelines indicate that the proper wording to be included on the plaque, in the same font and size as the other required information, is: ‘This boat meets or exceeds the minimum ﬂotation guideline’

or ‘This boat does not meet the minimum ﬂotation guideline’

© 2007, Woodhead Publishing Limited

Design and materials in rowing

275

FISA has devised a special ﬂotation test to be carried out by boat manufacturers for each model/size of boat to establish that any boat meets the guideline on the plaque.

11.2.4 Innovation Rule 58 – Fairness – Innovation 1. Signiﬁcant innovations in equipment including, but not limited to, boats, oars, related equipment and clothing, must meet the following requirements before they are allowed for use in FISA International Regattas; including World Championships and Olympic Games: a. They must be available to all competitors (no exclusive patents); b. The costs involved must be reasonable; c. There must be equal chances for all competitors; d. They must be safe and environmentally sound. 2. The innovation must be submitted to the FISA Executive Committee for evaluation. If it is judged to meet the above conditions and is approved for use, it must be readily available for all competitors by 1 January in order to be authorised for use in International regattas that year. Crews with unapproved innovations shall not be allowed to compete. The Executive Committee has the sole authority to decide all matters under this Rule including whether an innovation is signiﬁcant, whether it is readily available, whether the costs are reasonable and whether it is safe and environmentally sound.

11.2.5 Rowing boat classes for Olympic and World Championship competitions •

Scull boats – two oars are used, one in each hand. Scull boats can be single scull (1 person), double scull (two person) or quadruple scull (four person). • Sweep boats – one oar is used and the rower holds it with both hands. Sweep boats can be: pair (2−); coxed pair (2+); four (4−); coxed four (4+); eight (8+).

11.2.6 Adaptive rowing events Indented material is taken from the 2006 FISA Adaptive Rowing Regulations. Rules 29 and 30 cover World Championship adaptive boat classes and Paralympic Games boat classes: World Rowing Championships include the following adaptive events: Legs, trunk and arms, Mixed (LTAMix) – 4+ (LTAMix4+) Trunk and arms, Mixed (TAMix) – 2x (TAMix2x)

© 2007, Woodhead Publishing Limited

276

Materials in sports equipment

Arms only, Men (AM) – 1x (AM1x) Arms only, Women (AW) – 1x (AW1x) In these Mixed adaptive events half of the rowers in a crew shall be men and half shall be women. In the 4+, the coxswain may be of either sex.

Rules 31 and 32 set out regulations for boats and equipment and for boat weights.

Regulation, Rule 31 – Boats and Equipment 1. General Aspects (a) The use of FISA Standard Adaptive boats is mandatory for all adaptive 1x and 2x events. (b) All boats used in the 4+ event shall be stern-coxed. (c) At the Paralympic Games regatta, the use of FISA Standard Adaptive boats provided by FISA is mandatory for all events. 1.1 The Council shall determine the design and speciﬁcations of FISA Standard Adaptive boats and any changes thereto. The design and speciﬁcations shall be a part of these Regulations. Those parts of the Standard Adaptive Boats which are not speciﬁed in these Regulations may be modiﬁed subject to these Regulations and subject to Rule 58. 1.2 No changes in the standard design and speciﬁcations of FISA Standard Adaptive Boats shall be made except in the year following the Paralympic Games. 1.3 The minimum weight of FISA Standard Adaptive boats shall be as speciﬁed in these Regulations (Regulation, Rule 32: Boat Weights). 2. Standard Adaptive 4+ The FISA Standard Adaptive 4+ used at the Paralympic Games regatta is a stern-coxed boat. The design and speciﬁcations shall be stipulated by FISA. 3. Standard Adaptive 2x The FISA Standard Adaptive 2x has a ﬁxed seat and may have stabilising pontoons. The hull, the pontoons where ﬁtted, and the seat ﬁxing are part of the Standard speciﬁcations. The design and speciﬁcations shall be stipulated by FISA. The seat itself and the rigger design of the standard 2x are not restricted. The TA2x boat shall have a seat to which the athlete is strapped at the hips to ﬁx the pelvis so that the rower is not able to use the foot stretcher for leverage. The method of strapping shall be of a design which allows immediate release with single hand movement in case of emergency. 4. Standard Adaptive 1x The FISA Standard Adaptive 1x has a ﬁxed seat and must have stabilising pontoons. The pontoons must be ﬁxed in position so that when the rower is

© 2007, Woodhead Publishing Limited

Design and materials in rowing

277

seated in the balanced boat both pontoons shall be horizontal and shall, at a minimum, touch the water. The hull, the pontoons and the seat ﬁxing are part of the Standard speciﬁcations. The design and speciﬁcations shall be stipulated by FISA. The seat itself and the rigger designs are not restricted. The seat design and its manner of use must meet the following requirements: The design of the seat of the A1x is unrestricted except that it must be compatible with the Standard seat ﬁxing. In order to ensure that the arms only aspect of the A1x boat class are fully met, the A1x boat shall have a high seat back to which the athlete is strapped so that only the arms and shoulders can move during rowing. The strap should be at the level of the diaphragm, directly below the nipples or breasts, and be tight enough to restrict any trunk movement without causing breathing problems. The method of strapping shall be of a design which allows immediate release with single hand movement in case of emergency. Further: 1. All Adaptive TA2x and A1x boats must have a quick single-action footrelease system located within easy reach of the ﬁxed seat to assist rowers to release their feet in case of capsize or accident. 2. For the A1x, any hand strapping must be able to be released immediately by quick mouth action and abdomen strapping by single quick hand action. Regulation, Rule 32 – Boat Weights In addition to the requirements of Rule 32, the minimum weights for Adaptive boats shall include pontoons where used. The minimum weights of Adaptive boats are: 4+ 51 kg 2 × 36 kg 1 × 22 kg

11.3

Design of modern rowing boats

Traditional boat design was, for nearly 100 years, a matter of experience and copying of successful designs. During preparation for the 1936 Olympic Games the ﬁrst tank tests with rowing boats were carried out. Modern design is based on a mixture of experience, tank tests and the use of 3D computer programs. Copying still takes place because copyright or patents for hydrodynamic shapes are not possible. Traditional design took into account the different weights of crews in the same boat class by building the same design so that it sat higher or lower in the water to maintain a suitable waterline. Modern design creates special boats for each category of rowers, heavy-weight men, light-weight men, heavy-weight women and light-weight women, with an optimum hydrodynamic shape for each degree of displacement.

© 2007, Woodhead Publishing Limited

278

Materials in sports equipment

11.3.1 Hydrodynamic design Low frictional resistance depends upon minimising the wetted area; this can in theory be achieved by a boat with semi-circular cross-sections and by reducing the length of the boat to the point where wave drag begins to increase dramatically. However, the fact that the boat must have adequate transverse as well as longitudinal stability limits the extent to which these desirable features can be realised. With regard to transverse stability – in ship building this is the ability of a ship to right itself from a tilt – rowing boats are unique in that they have no positive static stability; in other words, the boats would be happier upside down. This is because, with a crew sitting in them, rowing boats have an exceptionally high centre of gravity. The transverse level results from the mass inertial forces of the system, which includes the rowers, boat and oars. This is especially true of the oars because they are comparable to the balancing pole of a tightrope-walker. The potential to control a racing boat is mainly based on the technical performance of the crew, but it is also important that the negative transverse stability is not increased beyond the point where even crews with the highest technical ability are not able to level the boat. Typical cross-sections of Olympic gold medal rowing boats from 1968 and 2004, including the length-to-breadth (L/B) and breadth-to-draught (B/D) ratios, are illustrated in Figs 11.2 and 11.3, respectively. These examples indicate the direction which rowing boat design has taken in recent times. Compared to the 1968 boat the negative transverse stability of the 2004 boat B is increased by 10% and the total water resistance decreased by 1.3%. With the same propulsion, resulting from crews with the same performance, the 2004 boat has an advantage of 16 m over the 2000 m racing distance.

11.2 Cross-section of 1968 Olympic gold medal pair boat; L/B ratio = 28.45 and B/D ratio = 3.4.

© 2007, Woodhead Publishing Limited

Design and materials in rowing

279

11.3 Cross-section of 2004 Olympic gold medal pair boat; L/B ratio = 28.24 and B/D ratio = 2.9.

0.29 L/B = 28.24 B/D = 2.90 L/B = 25.00 B/D = 3.03

–MBG (m)

0.27

L/B = 28.45 B/D = 3.403

0.25

L/B = 26.35 B/D = 3.185

0.23

0.21 116

117

118

121 123 126 Resistance (N)

127

128

11.4 The relationship between negative transverse stability and water resistance.

Figure 11.4 illustrates the relation between negative stability and water resistance with some examples of boats (2×; 2−), including the two from Figs 11.2 and 11.3. The measure of stability is the metacentric height (MbG). The metacentric height is the distance on the midship line between the centre of gravity of the crewed boat and the point where this line is cut by the buoyancy force of a tilted boat. In all ‘normal’ systems the centre of gravity lies under the metacentre. This means that the counteracting forces of gravity and bouyancy produce an uprighting moment, and the metacentric height has a positive value. In the case of rowing boats designed for racing, the centre of gravity lies above the metacentre, so the two forces produce a capsizing moment. The metacentric height has a negative value.

© 2007, Woodhead Publishing Limited

280

Materials in sports equipment e

50 mm (min) a d c

DECK CWL (construction waterline)

b

11.5 Boat cross-section showing parameters which can be varied.

Table 11.1 Measurement parameters for Fig. 11.5 a b c (from 1× to 8+) d e (1×) (2×) (2−) (2+) (4×) (4−) (4+) (8+)

14–17 cm 15–20 cm 7–10 cm 22–26 cm 79–81 cm 78–80 cm 85–87 cm 86–88 cm 78–80 cm 84–86 cm 85–87 cm 83–85 cm

Future reduction in hydrodynamic resistance needs very close cooperation between experts in hydrodynamics and biomechanics and top coaches and athletes.

11.3.2 Other factors in design The basic factors for rigging are the seat height over the waterline and the lengthways position of the swivel. These are derived from displacement calculations based on the average weight of the crew that the boat

© 2007, Woodhead Publishing Limited

Design and materials in rowing

281

has been built for and on balancing the centre of gravity and the centre of buoyancy. All other features can be changed to adapt to individual body size and the requirements of different rowing techniques. Figure 11.5 is a rowing boat cross-section, showing where measures can be adapted. The accompanying Table 11.1 gives the parameters within which each can be varied.

11.4

Materials and technologies for modern rowing boats

It is the task of any technological research to ﬁnd the optimum solution for a particular need. The materials and technologies used today in boat building were not originally developed for this purpose, but mainly for the aerospace industry. This industry has a particular need for new materials with higher strength-to-weight ratios, and this has driven the development of sandwich composite technology which now has worldwide application. Simply stated, a composite is a combination of different materials into a single structure to take advantage of the best properties of each. Steel-reinforced concrete and ﬁbre-reinforced plastics are composites, but they are not the hi-tech sandwich composites with the highest strength-to-weight ratio in use for rowing boats. Sandwich composite structures use a low-density, shear-resistant core sandwiched between high-strength, high-modulus skins of ﬁbre-reinforced plastics (FRP) materials. The skins are bonded to the core to prevent displacement between the layers when the composite is subjected to different loads. Together they create a synergistic structure with physical properties greater than the sum of their parts.

11.4.1 Matrix, reinforcement and core materials Liquid resins are mainly used as matrix materials. These convert, through a chemical reaction during the curing process, to a thermoset solid. The matrix resins have the task of supporting the reinforcing ﬁbres and dispersing the forces affecting the structure into the ﬁbres. In this respect it is very important that the ultimate elongation of the matrix is higher than the ultimate elongation of the ﬁbres in order to avoid breakage and tears of the structure. For very light-weight boats, epoxy resins are standard. The properties of resins used for boat building are shown in Table 11.2. Fibres used for reinforcement have different physical properties and different prices. As with different resins, while most ﬁbres have very similar densities there are other important differences between them. The properties of ﬁbres used in boat building are shown in Table 11.3.

© 2007, Woodhead Publishing Limited

282

Materials in sports equipment

Table 11.2 General overview of resins used in boat building Physical properties Heat deﬂ. temp. (°C) Unsaturated polyester Vinyl ester resins Epoxy resins Epoxy resins post-cured 8 h 60 °C

Price index

Purpose

Training boats and leisure sports

Ultimate elongation (%)

70

3

1

100

5

2

120 140

7 12

6 6

Boats for racing

Table 11.3 General overview of ﬁbres used in boat building Physical properties

Price index

Purpose

73

1

Mainly boats for training and leisure sports, in combination for racing boats

Density (g/cm3)

Ultimate elongation (%)

E modulus (Gpa)

Glassﬁbre

2.6

3

Aramid HM (Kevlar1 49) (Twaron2 HM) Carbon ﬁbre HT

1.45

1.9

120

5

Mainly boats for racing, in combination

1.78

1.4

235

6

with glassﬁbre also for training and leisure sports

1® 2

DuPont. Teijin Twaron BV.

Carbon ﬁbres have become the most widely used reinforcement material for hi-tech composites. The higher the degree of carbonisation of the basic viscose or acrylic ﬁbres, the higher their strength will be. However, the energy needed to reach the carbonisation temperature of 3000 °C means that costs are correspondingly high. The right combination of ﬁbres and resins is always a compromise between the optimum stiffness–weight ratio, the impact strength and, of course, the price.

© 2007, Woodhead Publishing Limited

Design and materials in rowing

283

Core materials for sandwich composites used in boat building are mainly honeycombs made from aramid paper and hard foams of polyvinylchloride (PVC), polyurethane (PU) or acrylic foams. The same stiffness of a sandwich composite can be achieved using either a honeycomb with a density of 60 kg/m3 or a foam with a density of 90 kg/m3; the impact strength of the foam sandwich is higher, but the boat will be heavier.

11.4.2 Wet laminating technology Most boats are built using wet laminating technology with curing at room temperature. Combining with post curing at medium temperature (50– 60 °C) results in laminates with good strength properties. Matrix materials are mainly epoxy-resins with different hardeners for curing modiﬁcations. Wet laminated composites have limited strength properties when heated up. The temperature limit, Tg is highest after post-curing up to 100 °C. In certain circumstances, such as transport or storage with long exposure to sunlight and heat, the pressure of ﬁxing belts can cause plastic deformation of the shell. For reinforcement, combined glass–aramid–carbon fabrics with different textures and weights per square metre are used. Woven carbon tapes with a high density of ﬁbres are used for additional reinforcement in all directions where needed. It is likely that wet laminating technology will continue to be used in rowing boat building for racing at the lower level and for training and recreational boats because of the reasonable price of the product. The following shows a sample calculation for the inﬂuence of boat weight on the racing speed of a given crew: An 8 + (boat A) built from the same mould with the same stiffness, but with higher weight caused by cheaper materials and technology, can be produced up to 30% cheaper than a hi-tech boat (boat B) built at or under the FISA weight limit. Displacement A (∆2): boat 110 kg + crew and oars 761 kg = 871 kg Displacement B (∆1): boat 96 kg + crew and oars 76l kg = 857 kg

Using a formula derived from the tank tests of the ‘Versuchsanstalt für Wasserbau und Schiffbau Berlin’ Report no. 52/10 and an average racing speed vA = 5.8 m s−1 for the heavier boat (i.e. a time over 2000 m distance of 5 min 44.4 sec), the speed of the lighter boat vB can be calculated from the following. The speed difference vdiff between boat A and boat B is given by:

© 2007, Woodhead Publishing Limited

284

Materials in sports equipment 6 ⎛ ∆2 ⎞ vdiff = vA × ⎜ 1 − ∆ 1 ⎟⎠ ⎝ 6 ⎛ 871 ⎞ = 5.8 × ⎜ 1 − 857 ⎟⎠ ⎝

= 5.8 × (1 − 1.0027) = 5.8 × − 0.0027 = −0.016 m s−1 vB is then given by vA − vdiff: vB = 5.8 − 0.016 = 5.784 m s−1 The time difference over the racing distance is 0.9 sec which makes an advantage of 5.2 m for the lighter boat over the 2000 m racing distance. It is obvious that, in international competitions at the highest level, lighter boats will be used. For intermediate crews with lower technical ability the light boat can be a disadvantage because it does not tolerate mistakes and the average speed can decrease in these circumstances.

11.4.3 Pre-preg technology The most advanced technology in many respects is pre-preg. Here one works with ﬁbres that are machine pre-impregnated with modiﬁed resins. The material is ﬂexible and sticky so it is easy to build up laminates without using wet resins. Its beneﬁts are that the work is cleaner and the process ensures that the ﬁbres contain the right amount of resin. The greatest advantage of the pre-preg technology is the potential to use layers of unidirectional (UD) carbon ﬁbres. Optimal strength properties of the composite in all directions of load are achieved with a lay-up of a calculated amount of UD ﬁbres in the direction required. Sample directions are (0°) for longitudinal stiffness, diagonally crossed (45°) for torsion stiffness and at right angles (90°) for transverse stiffness. The Tg of pre-preg composites depends on the type of resin system and a curing temperature of at least 120 °C. Figure 11.6 shows the construction of a boat using pre-preg technology. Advanced pre-preg technology enables the production of racing boats, complying with the FISA minimum weight limit, in accordance with the FISA Guidelines for Safety in Rowing with regard to the ﬂotation requirements, i.e. the boat can still be rowed when full of water and is self-bailing through the rowing motion (see Fig. 11.7). The future of hi-tech racing boat building lies in advanced pre-preg technology. Moreover, a hurdle for the introduction of this technology is the high investment needed, particularly for specialist workshops with appro-

© 2007, Woodhead Publishing Limited

Design and materials in rowing

285

Deck construction Clear coat Uni-directional pre-preg carbon fibre (multiple layers) Nomex honeycomb core Uni-directional pre-preg carbon fibre (multiple layers)

Epoxy used to bond deck to hull

Hull construction Uni-directional pre-preg carbon fibre (multiple layers) Nomex honeycomb core Uni-directional pre-preg carbon fibre (multiple layers) Primer Paint

11.6 Boat construction using pre-preg technology (Nomex® DuPont).

11.7 Photo showing bailing outlets on self-bailing boat.

priate air conditioning, heat-resistant moulds and industrial ovens or autoclaves for the curing procedure.

11.4.4 Moulds Moulds for boat building are generally negative moulds taken from positive plugs shaped exactly to the contour of the boat. The basic mould used for

© 2007, Woodhead Publishing Limited

286

Materials in sports equipment

11.8 Mould for shell including saxboard.

11.9 Mould for inside structure.

all technologies is the shell including the saxboard. Racing boats in wet lay-up technology are assembled into a so-called monocoque structure with prefabricated parts for decks, shoulders, bulkheads and supports between shell and deck. Heavier training boats in wet lay-up and ultra-light racing boats in prepreg technology have a second mould for the complete inside structure with the decks simply glued together during ﬁnal assembling. The transverse stiffness normally achieved with shoulders is realised in the pre-preg boats with additional layers of UD carbon in a 90° direction. Shell and inside moulds are illustrated in Figs 11.8 and 11.9, respectively.

11.5

Materials and technologies for rowing boat equipment

The body of the boat is completed with parts which are afﬁxed with screws to enable removal for transport or adjustment. This is illustrated in Fig. 11.10.

11.5.1 Riggers The function of the rigger or outrigger is to keep the position of the swivel oarlock at a suitable distance from the middle of the boat in the transverse direction (see (e) in Table 11.1).

© 2007, Woodhead Publishing Limited

Design and materials in rowing

287

11.10 Part of the cockpit with removable parts.

The traditional rigger or outrigger is an inert gas-welded construction of aluminium (Al) tubes with different dimensions and alloys with the strength properties of Al Mg5 and higher. The riggers and other aluminium parts used in rowing boats are generally anodised for protection against corrosion. They are screwed at the outside of the saxboard and need a shoulder or rib inside the boat to provide the stiffness needed. The two-stay carbon rigger is ﬁxed to the boat in the same way as the traditional rigger. The advantages are less weight and a swivel adjustment where the force during the stroke is directed through the centre of the supporting tube with no bending moment at the pin of the swivel. Traditional riggers need a backstay from the top of the pin to the saxboard to support the pin against bending during the stroke. Sufﬁcient stiffness of riggers and swivel pins is very important because they have to resist the highest load of all parts in a rowing boat. In a race the pressure of the oar at the swivel during the stroke phase of the rowing cycle is on average 1000 N, and at can rise to 1500 N at the start of a race. Elastic deformation has two main disadvantages: ‘force gets lost for the propulsion of the boat and a bended pin changes the angle of the blade – the pitch – to a negative so that the blade goes too deep under water – in the worst case the rower catches a crab’. The Al tube rigger and the carbon two stay rigger are illustrated in Fig. 11.11.

© 2007, Woodhead Publishing Limited

288

Materials in sports equipment

(a)

(b)

11.11 (a) Al tube rigger. (b) Carbon two-stay rigger.

(a)

(b)

11.12 (a) Al wing rigger. (b) Carbon wing rigger.

Wing riggers are mounted to ﬂanges at the tops of the saxboard on both sides of the boat. Advantages are the omitted shoulders and the opportunity to move the rigger in a longitudinal direction to correct the centre of gravity when crew members have very different body weights. Disadvantages are restricted space for the rower’s feet under the rigger or the hands moving above the rigger. Wing riggers are welded aluminium constructions made from tubes or special proﬁles or carbon ﬁbre constructions laminated in special moulds. The Al wing rigger and the carbon wing rigger are illustrated in Fig. 11.12. The price of all carbon rigger constructions is generally higher than that of Al constructions. All kinds of riggers bear the same swivel oarlocks with

© 2007, Woodhead Publishing Limited

Design and materials in rowing

289

different dimensions either for sculling or sweep rowing and made of thermoplastic materials like nylon which are injection moulded. The material for swivel pins and other bolts, screws and nuts used in rowing boats is stainless steel with the main alloy contents being max. 18 % Cr, 10 % Ni, 3 % Mo, 2 % Mn and 1 % Si.

11.5.2 Stretchers and seats Stretchers and seats are the parts of the equipment supporting the body of the rower in the boat. The footplate or stretcher ensures that the rower’s feet are ﬁxed so that he or she stretches their legs against the stretcher when pulling the handle of the oar. The direction of the load on the stretcher is opposite to that on the swivel, the latter being in the direction of the bow. The load on the stretcher is, on average, 20% less than that on the swivel. The difference is the force for the propulsion of the boat. The stretcher or footplate is adjustable in the length direction for adaptation to different lengths of leg, using rails which are connected to the boat. Stretchers in boats which are used by different rowers are mostly equipped with heel rests and belts, the rowers using normal sport shoes. For competitive rowing at a higher level, the stretchers are equipped with special light rowing shoes, similar to running shoes used in track and ﬁeld events. In addition to the length adjustment, modern stretchers have an adjustable angle, normally between 42–45°, and the shoes are mounted on plates which are adjustable in height. The parts of a modern stretcher are detailed in Fig. 11.13.

Shoe plate Shoe screw Footstretcher Clamp Aluminium tube

Shoe clip for heel tie-down

Angle adjustment T-bolt Footstretcher keel track

11.13 Parts of the modern stretcher.

© 2007, Woodhead Publishing Limited

Tube end T-bolt

Footstretcher side track

290

Materials in sports equipment

To prolong the stroke, the seat moves along the axis of the boat on rails made from special extruded Al proﬁles between 720 and 800 mm long. The rails are adjustable in the length direction on the rail deck for adaptation to different rowing techniques, and they are raised by 15 mm from stern to bow end. The raised rails prevent the rower from slipping off the seat during the stroke when he or she stretches their legs under pressure and when the boat ﬂoats deeper with the bow because of the movement of the crew. The rails have different track width related to the different breadth of the boats from 1× to 8+. Narrow tracks can pinch the calves and avoid total stretching of the legs at the end of the stroke. This disadvantage can be avoided with the advanced pre-preg technology in fours and eights where reinforced saxboards instead of shoulders enable wider tracks where the calves ﬁt between. The seat has two main parts, the carriage with the axles and roller bearing wheels which are connected to the seat horns and the seat plate. The seat horns have safety hooks which grip under the rail proﬁle to keep the wheels safely in the half round tracks of the rails. The most recent trend is for height-adjustable seat horns to enable individual adaptation of the seat height. The seat plate is adapted to the buttocks with holes which have different distances for the seat bones of women (wider) and men. The materials used are nylon or acrylonitrile–butadiene–styrene (ABS) for the wheels with inserted stainless steel roller bearings, axles and seat horns from Al and seat plates shaped from cedar wood or moulded from carbon composites as the latest developments. Figure 11.14 shows the parts of the modern rowing boat seat. Carbon seat plate

Seat axle Adjustable seat horn

Ball bearing wheel

11.14 Parts of the modern rowing boat seat.

© 2007, Woodhead Publishing Limited

Design and materials in rowing

11.6

291

Materials and technologies for oars

The oars for sculling (one oar in each hand) and for sweep rowing (one oar in both hands) are similar, differing only in their dimensions.

11.6.1 Basic construction The oars consist in principle of the shaft with handle and blade. For nearly 100 years the optimal stiffness–weight relation of the shafts was achieved with selected woods such as, for example, Canadian spruce. Starting from solid wood, development progressed to hollow, bamboo-like structures of glued ledges. Since 1978 reliable oars from high-tech composites have been in use for competitions. The shafts are now conical tubes with an inclination of about 100 : 1. The use of pre-preg materials enables the application of UD carbon ﬁbres to a mould core of steel in calculated amounts and directions to achieve the required stiffness–weight relation of the tube. The pressure needed during the curing procedure is achieved using plastic shrink tapes wound around the layers of pre-pregs. The development of the blades has been mainly focussed on the shape. For nearly 100 years different sizes of delta-shaped blades with the widest breadth at the end have been in use. At the European Championships in 1959 in Macon France the German 8+ won with shorter blades which had their widest breadth more to the middle. These so-called Macon Blades dominated rowing until 1992. Research in the USA and Germany led to asymmetric blade shapes, shorter and wider than all earlier blades. Also the composite sandwich structures already in use enabled shapes without the limitations imposed by the use of wood. Since the 1992 Olympic Games the so-called Big Blades have been used. Typical blade shapes are shown in Fig. 11.15. Additional ﬁttings at the shaft are the sleeve and the collar. The sleeve prevents the oar from wearing out when turning in the oarlock and has a ﬂat area to provide the right pitch during the stroke when the shafts are tubes. The collar prevents the oar from slipping in the length direction through the oarlock. It is adjustable so that the relation of the inboard and outboard lever of the oar can be changed; for example, under strong head wind conditions the inboard lever can be prolonged to keep the load at the handle in a normal range.

11.6.2 Main dimensions Average dimensions (in cm): • •

Length of sculls 299 cm with inboard lever 87 cm. Length of sweep oars 383 cm with inboard lever 115 cm.

© 2007, Woodhead Publishing Limited

292

Materials in sports equipment

11.15 Blade shapes: (a) delta-shaped blades; (b) Macon Blades; (c) Big Blades.

• For oars with Big Blades the outboard lever and with that the total length, is reduced by 8 cm. • The inboard lever overlaps the distance from the pin to the middle of the boat, the spread by 8 cm sculling and 30 cm for sweep rowing.

11.7

Testing of rowing material

There are three main parameters determining the quality of rowing boats: the shape, the weight and the stiffness. When a customer orders the right boat type for a speciﬁc purpose, the boat will be delivered with the desired shape and weight from the manufacturer. There are some basic tests which can be used to measure the stiffness of a boat and its main load-bearing parts, such as the riggers and shoulders. Figure 11.16 shows an apparatus for testing longitudinal stiffness. The boat is ﬁxed in normal position on two supports and a weight is applied at a point half way between. The deﬂection f1 can then be measured. Table 11.4 gives the half-way distance L between the supports and he maximum deﬂections f1 for different classes of boat. Figure 11.17 shows an apparatus for testing the stiffness of riggers and shoulders. With the boat held in a sling, a pulling force is applied at either end of the rigger and the resulting deﬂections f2, are measured on both sides. The results can be interpreted as follows:

© 2007, Woodhead Publishing Limited

Design and materials in rowing

293

200 N (20 kg) L

L

f

11.16 Apparatus for testing the longitudinal stiffness of a boat.

100 N (10 kg)

100 N (10 kg)

f1

f2

11.17 Apparatus for testing the stiffness of riggers and shoulders.

Table 11.4 Typical measures for the test method of Fig. 11.16 for different types of boat Type of boat

L

f1 (mm) (max)

1× 2× 2− 2+ 4× 4− 4+ 8+

2500 3500 3500 3700 4500 4500 4500 6300

10 8 8 8 7 7 7 10

• f2 < 5 = stiff • f2 > 5 艋 10 = medium • f2 > 10 = soft For oars there is a basic stiffness test for the shafts. In theory an absolutely stiff shaft should be optimal for the transmission of forces. However, that would overtax the rowers’, sinews, joints and muscles, so oars with different stiffness parameters are available.

© 2007, Woodhead Publishing Limited

294

Materials in sports equipment

11.8

Leisure rowing boats and equipment

Although high level competitive rowing commands the most attention, only a small number of rowers are engaged in it. More people are engaged with tour rowing, recreational and ﬁtness rowing, offshore rowing or machine rowing. In general boats for all kinds of leisure rowing are more stable than racing boats, and they need no special skills for balance. Many competitions are held for all kinds of leisure boats, mostly over longer distances.

11.8.1 Traditional European tour rowing boats These boats are descendants of historic racing boats with an outside keel. They were originally clinker built, having external planks from cedar wood overlapping downwards and secured with clinched copper nails. With the introduction of water-resistant plywood, originally for the aircraft industry, a version with a racing boat like shell was developed. Today such boats also have composite shells. The differences between them and racing shells are the outside keel and the rules for the main dimensions and the minimum weight limits. Measurements of the most-used type of boat for tour rowing and longdistance races, the 4+/4×+ are as follows: • • •

length overall breadth overall breadth of the construction waterline (measured 0.125 m above the deepest part of the keel plank) • minimum weight

11.00 m 0.78 m 0.33 m 80.00 kg

11.8.2 Recreational and ﬁtness rowing There are many types of singles on the market, mostly used by people who are not members of special rowing clubs. Shorter and wider than normal racing boats, these boats are easy to row and to transport on private motor cars. A typical example of a single which is used for recreational rowing is shown in Fig. 11.18.

11.8.3 Offshore rowing Offshore rowing has mostly regional importance and has historic roots. Rules for the boats are similar to those for European tour rowing boats but with additional safety requirements. There are boats with sliding seats, as for example the French ‘bateaux de mer’, or ﬁxed seat boats, like the Australian ‘surf rowing boats’.

© 2007, Woodhead Publishing Limited

Design and materials in rowing

295

11.18 Recreational rowing boat.

11.8.4 Machine rowing Indoor machine rowing has found worldwide application as a special kind of ﬁtness rowing for all. For competitive rowers it has become the favourite kind of winter training besides tank rowing. Rowing federations have annual championships in so-called ergometer rowing. For example, Concept II Indoor Rower, in which the load at the handle generated through air resistance via a ﬂywheel is very common. Other systems work with water, frictional or electric resistance. All modern rowing machines have monitors which show the performance, stroke rate, time over distance and other parameters.

11.9 •

• •

•

Acknowledgements

The content of the chapter ‘Design and materials in rowing’ is based on the results of my own research and design work in the area of rowing materials. The opening facts about the history of rowing are based on C Dodd, The Story of World Rowing, London, Stanley Paul and Co., 1992. Photos of boats and boat parts are taken from products of the Hangzhou Flying Eagle Boat Co., Ltd, China, in co-operation with Wintech Racing USA, and are based on own design. I thank my friends David Thomas and Ed Hofmeister from Wintech Racing USA for their co-operation and assistance in the arrangement of photos and images.

© 2007, Woodhead Publishing Limited

12 Design and materials in athletics N. L I N T H O R N E, Brunel University, UK

12.1

Introduction

Athletics has been described as many sports within a sport. There are 24 events in the Olympic competition programme, and these events may be grouped into sprints, middle-distances, long-distances, hurdles, relays, walks, jumps, throws, and multi-events. To excel in a particular event an athlete must be genetically endowed with an appropriate body size. The athlete must then undertake physical training to develop the required mix of strength, speed, and endurance; and spend many hours practising the technical and tactical skills of the event. Athletes who are successful in the international arena usually specialise in just one event. Most athletics competitions are held under the rules and regulations of the International Association of Athletic Federations (IAAF), which was founded in 1912 and has about 220 member nations. Of interest to the sports engineer and sports scientist are the restrictions on the design of the competition facilities, the design of the athlete’s equipment, and the techniques that the athlete may employ. An unstated but underlying philosophy in athletics is that the outcome of a competition should be determined by the physical and technical abilities of the athlete, and not by differences in the quality of the athletes’ equipment (Julin, 1992). Most of the rules for the competition arena and the athlete’s equipment are ‘proscriptive’ in that the material, construction, and dimensions are speciﬁed in detail and to high precision; whereas the rules for the athlete’s technique are ‘restrictive’ in that they usually specify what is forbidden, rather than specifying how the movement must be performed. Like many other sports, athletics places a strong emphasis on tradition and the historical continuity of record performances. The IAAF has been reluctant to approve technological innovations that change the nature of the event or which are aimed solely at improving the athlete’s performance. In contrast, the governing body has welcomed innovations that reduced the incidence of injury to athletes and judges, reduced the incidence of judging errors, or enhanced the enjoyment of spectators. Financial costs are also 296

© 2007, Woodhead Publishing Limited

Design and materials in athletics

297

considered when deciding whether to permit a technological advance. The IAAF endeavours to make athletics a ‘globally accessible sport’, with high participation rates by all peoples of all nations. An important aspect of retaining the universal appeal of athletics is through minimising the cost of participation; particularly the cost of competition facilities, competition implements, and judging equipment. Figure 12.1 shows the trends during the twentieth century towards better athletic performances. However, in most events, very little of the overall improvement has been due to innovation in the design of sports equipment or the materials used. The main causes of improvement are socio-economic factors such as increased leisure time, increased professionalism of sport, state-supported sports systems, and the increased participation rate of women. Some of the improvement in performance may also be due to better coaching and training methods, particularly in strength training and cardiovascular training, and advances in sports medicine that have prolonged athletic careers. Figure 12.1 clearly shows temporary declines in performance due to World War I and World War II. In some events, particularly for women, there is also a noticeable decline in performance starting in 1989. This is due to the demise of the organised sports systems in the Eastern European nations and the more expansive drug testing programmes that were introduced following the disqualiﬁcation of Ben Johnson at the 1988 Olympic Games. This chapter looks at examples of innovation in design and materials under six main themes; (1) pole vault, (2) javelin throw, (3) other throwing implements and equipment, (4) hurdles, starting blocks and shoes, (5) running surfaces and other athletic facilities, and (6) timing and other equipment.

12.2

Pole vault design and materials

The most notable example of innovation in athletics implements is the ﬂexible ﬁbreglass pole. In the early 1960s performances rapidly improved when the relatively rigid poles made from steel or bamboo were superseded by highly ﬂexible poles made of ﬁbreglass. Pole vaulting with a highly ﬂexible pole looks spectacular, but the mechanics of pole vaulting and the advantages of the ﬂexible pole are not always appreciated by the viewer (Linthorne, 2000). Pole vaulting is mainly about converting running speed into height above the ground. In mechanical terms the aim of the vaulter is to generate maximum kinetic energy in the run-up, and then convert as much of this energy as possible into the gravitational potential energy of the athlete’s body at the peak of the vault. The athlete uses a long pole to achieve this energy conversion. Once the pole is planted into the ground (actually a take-off box that is sunk into the ground), the athlete and pole

© 2007, Woodhead Publishing Limited

298

Materials in sports equipment 28

Time (s)

26 WWI

24

WWII

Women

22 Men 20 1900

1920

1940

(a)

1960 Year

1980

Distance (m)

8

2000

Men

7 WWII

Women

WWI

6

5

1900

1920

1940

(b)

1960 Year

1980

2000

22 Men

Distance (m)

20 WWII

18 WWI

16

Women

14 12 10 8 1900

1920

1940

(c)

1960 Year

1980

2000

12.1 Historical trends in athletic performances for men and women. Data points are the performances by the tenth-best athlete in the year: (a) 200 m, (b) long jump, (c) shot put. Performances in these three events have not been greatly inﬂuenced by innovation in equipment design or materials. © 2007, Woodhead Publishing Limited

Design and materials in athletics

299

rotate about the box, gradually transferring the athlete’s horizontal speed into height above the ground. The faster the run-up, the longer the pole the athlete can rotate to vertical and the higher the athlete can vault. However, the transfer of kinetic energy to gravitational potential energy is not the only important energy transformation. When the pole is planted into the take-off box, the athlete experiences a sharp jarring action, and so some of the athlete’s run-up kinetic energy is dissipated in the athlete’s body as heat. Also, during the vault the athlete uses muscular energy to lift his/her body up, and so adds to the height cleared. The ﬂexible ﬁbreglass pole owes it success to its inﬂuence on these last two energy transformations. The main advantage of a ﬂexible pole is that it reduces the shock experienced by the vaulter when the pole is planted in the take-off box (Linthorne, 2000). The vaulter therefore loses less run-up kinetic energy and so is able to rotate a longer pole to vertical. Mathematical models also suggest that the optimum take-off angle with a ﬁbreglass pole is lower than with a more rigid pole, and so the vaulter retains more run-up kinetic energy because he/she does not have to spring up as much at take-off. When vaulting with a ﬁbreglass pole the vaulter is able to clear a slightly greater height above his/her hand grip. This is probably because the athlete is able to place him or herself in a better mechanical position to add muscular energy to the vault. Modern pole vaulting poles are hollow columns constructed from ﬁbreglass or a mix of ﬁbreglass and carbon ﬁbre. These poles may be bent by over 170° without breaking and are able to store an amount of elastic strain energy that is equivalent to about one-half of the athlete’s run-up kinetic energy (Linthorne, 2000; Nielson, 2006). Pole vaulters do not need a highly ﬂexible pole to successfully perform a pole vault (a rigid pole will do), but they can achieve a considerably greater height through choosing a pole with an appropriate stiffness. Most experienced pole vaulters bring several poles to a competition. Before each jump the athlete selects a pole of appropriate length and stiffness to suit their physical capabilities and the environmental conditions (wind, etc.). With a ﬂexible pole the athlete must take account of the timing of the storage and return of energy in the bending pole. The pole vaulter wants a highly ﬂexible pole so as to minimise the shock on his/her body during the pole plant and take-off (and hence minimise the loss of kinetic energy), but not so ﬂexible that the pole returns the stored energy to the athlete too late. The athlete must select the pole stiffness so that the pole ﬁnishes its recoil at about the time when the pole has rotated to vertical (Hubbard, 1980; Ekevad and Lundberg, 1995). If the pole is too ﬂexible the peak of the vault will be located beyond the crossbar, and if the pole is too stiff the peak will be achieved in front of the crossbar. The optimum pole length and stiffness is different for each athlete, and depends on the athlete’s run-up speed, body weight, vertical reach, and vaulting

© 2007, Woodhead Publishing Limited

300

Materials in sports equipment

technique. Male vaulters tend to use longer and stiffer poles than female vaulters. Typical pole lengths are 4.90–5.40 m for elite male vaulters, and 4.30–4.60 m for elite female vaulters. The IAAF competition rules state that the pole may be of any length or diameter and constructed from any material or combination of materials (IAAF, 2006). Originally, pole vaulters used solid wooden poles made of ash, ﬁr, spruce, or hickory (Ganslen, 1979). However, these poles were relatively heavy and so were not conducive to producing a fast run-up. By the early 1900s most good vaulters were using bamboo poles. A bamboo pole is hollow and therefore much lighter than a solid pole of equivalent structural strength. A lighter pole allows the vaulter to achieve a faster run-up. The bamboo pole also had greater ﬂexibility than the other types of wooden pole, and so helped absorb some of the shock when the pole was planted in the ground. Unfortunately, bamboo poles were not particularly durable; they could be damaged or broken when over-stressed during vaulting or through rough handling. Durable and slightly ﬂexible poles made from Swedish steel became available in the 1940s, and by the 1950s steel was the most popular material among the world’s best vaulters. Lightweight aluminium poles were introduced in the mid 1930s, but these poles were less ﬂexible than steel or bamboo poles and so did not see widespread use among the leading pole vaulters.

6 Men

Height (m)

5 WWII WWI 4

Fibreglass pole Women

3 1900

1920

1940

1960

1980

2000

Year

12.2 Historical trends in pole vault performances for men and women. Data points are the performances by the tenth-best athlete in the year. Note the sudden improvement in performance in the early 1960s when the ﬁbreglass pole was adopted. The pole vault for women was introduced only recently and so the event is still developing.

© 2007, Woodhead Publishing Limited

Design and materials in athletics

301

Experimentation with ﬁbreglass poles began in the late 1940s, but it took several years before construction techniques were sufﬁciently advanced that a durable product with consistent bending properties was produced. The early 1960s saw widespread adoption of the highly ﬂexible ﬁbreglass pole and this resulted in a revolution in performance standards. In the three years from 1961 to 1964 the world record increased by 48 cm (10%). Figure 12.2 shows the increase in pole vault performance over the last century. Most of the improvement is due to the socio-economic and other factors mentioned in Section 12.1. However, superimposed on this trend we can clearly see a sudden increase in performance in the early 1960s.

12.2.1 Construction of a ﬁbreglass pole Fibreglass poles are constructed from woven ﬁbreglass cloth that is impregnated with epoxy-resin. The pole is heated during construction so that the resin bonds the layers of glass together to form a composite material. The stiffness, weight, and recoil speed of the pole are determined by the resin properties, the ﬁbre properties, the orientation of the ﬁbres, and distribution of the ﬁbres along the length of the pole. The two types of ﬁbreglass used in pole vaulting poles are E-glass and S-glass. S-glass is slightly lighter, has a greater modulus of rigidity, and is more expensive than E-glass. Eglass is used in some of the less expensive poles that are intended for use by less experienced athletes. The poles for these athletes are relatively short, and so the weight of the pole is not a limiting factor to performance. S-glass is more commonly used for the relatively longer poles used by good athletes, where a lighter pole can signiﬁcantly improve performance by allowing a faster run-up. A ﬁbreglass vaulting pole is constructed over a heated metal mandrel, which is removed after construction. Different-sized mandrels are used according to the desired length and diameter of the pole. A pole that is made using a larger diameter mandrel has a thinner wall thickness for a given pole stiffness, and hence will be lighter (Burgess, 1996). Any given athlete will have a preferred pole diameter, depending on the size of their hand, that allows a comfortable grip on the pole. Poles designed for female vaulters and junior vaulters are usually made using smaller mandrels. Most ﬁbreglass poles are constructed from three separate layers of woven ﬁbreglass cloth. For the bottom layer, a narrow strip of ﬁbreglass cloth is wound in a spiral around the mandrel. When a pole is bent during a vault, the material on the far side of the pole is stretched and the material on the near side is compressed. The original circular shape of the pole tends to become oval-shaped and the compression side of the pole has a tendency to collapse inwards. The ﬁbres in the bottom spiral layer of the pole are

© 2007, Woodhead Publishing Limited

302

Materials in sports equipment

mostly orientated perpendicular to the long axis of the pole, thus giving the pole its ‘hoop strength’, or resistance to change in shape. The second layer of ﬁbreglass cloth for the pole is a ‘full body wrap’; which is a rectangular piece of ﬁbreglass cloth that is the length of the pole. Most of the ﬁbres in this layer are aligned parallel to the long axis of the pole, thus giving the pole its resistance to lateral deﬂection or ‘bending strength’. A pole will have a speciﬁed number of complete wraps of cloth around the its circumference, and the number of wraps will determine the stiffness of the pole. The third layer of ﬁbreglass cloth is called the ‘sail piece’. The purpose of this layer is to set the distribution of glass ﬁbres along the length of pole, and hence its strength proﬁle. A pole that has a uniform distribution of ﬁbreglass along its length will experience greatest stress and lateral deformation at a point midway along its length. However, such a pole is heavier than is necessary. A better design that minimises the weight of the pole is to taper the distribution of ﬁbreglass along the length of the pole so that there is more towards the centre and less towards the ends. This will give a more uniform distribution of lateral bending strength along the length of the pole. For a pole made from a material with uniform density and Young’s modulus, the equation for the optimum distribution of material is a sine function (Burgess, 1996). Real poles are not designed so that the maximal bending stress is exactly the same along the length of the pole. The sail piece is usually in the shape of a trapezoid, which is then wrapped around the pole several times. The geometry of the sail piece and its position on the pole determine where the pole has the greatest bend. Some elite vaulters specify to the manufacturer the desired location of the pole bend so as to give a better match to their vaulting technique. After the three layers of ﬁbreglass are set on the pole, the pole is cured under high temperature and pressure to get the epoxy-resin to ﬂow in the ﬁbreglass cloth. Pole vaulting poles are not perfectly straight; they are deliberately made with a slight curvature. This ‘pre-bend’ makes the initial stiffness of the pole lower, and so reduces the energy lost when the vaulter plants the pole into the take-off box. The pre-bend is set in the pole by orienting the mandrel horizontally and supporting it at each end when curing the pole. The horizontal mandrel sags under gravity, and so gives the pole a slight bend. The mandrel is also slightly tapered to allow easier removal after curing of the resin. Fibreglass poles therefore have a smaller diameter towards the grip end of the pole. Pole manufacturers adjust the stiffness of a pole through varying the amount of ﬁbreglass cloth in the body wrap and through the shape of the sail piece. Even so, they always perform a test measurement of the lateral bending stiffness of the ﬁnal product. A static ﬂex test is performed by supporting the ends of the pole and then loading the pole at the midpoint with a known weight (usually 50 lbs). The central deﬂection of the pole is

© 2007, Woodhead Publishing Limited

Design and materials in athletics

303

measured and recorded on the pole. When selecting poles, most elite athletes will specify to the manufacturer their desired pole lengths and ﬂex ratings, and sometimes a preferred mandrel size. Most poles weigh between about 1.5 and 3 kg. At ﬁrst glance such a small weight would appear to have only a relatively small detrimental effect on the speed that the vaulter can attain in the run-up. However, the vaulter holds the pole towards one end, and so it is the ‘carry weight’ of the pole that is important. The carry weight is the force the vaulter must exert on the pole to hold the pole in a horizontal position, and it can be many times greater than the actual weight of the pole. For example, a 5.0 m ﬁbreglass pole used by an elite male pole vaulter has a carry weight of 170 N, or about seven times the actual weight of the pole (Nielson, 2006). Exerting such a large force to hold the pole inhibits the vaulter’s natural sprinting action. The technique of starting the run-up with the pole pointing vertically upwards, and then steadily lowering the pole as the vaulter approaches the take-off, is deliberately used to minimise the detrimental effect of the pole weight on the vaulter’s run-up speed. Even so, any reduction in a pole’s carry weight is welcomed by the athlete. Pole manufacturers prefer to use materials that minimise the carry weight of the pole for any given pole length.

12.2.2 Carbon ﬁbre poles Carbon ﬁbre has recently been used to produce lighter poles. The best material for a pole is one that maximises the ratio: (specific strength )2 (specific stiffness)

=

(σ / ρ )2 (E / ρ )

=

σ2 ρE

[12.1]

where s is the maximum allowable bending stress, E is the Young’s modulus, and r is the density of the material (Burgess, 1996; Wegst and Ashby, 1996). The best practical materials for a vaulting pole are carbon ﬁbre reinforced plastic, followed by glass-reinforced plastic. Figure 12.3 shows the stress– strain curves for carbon ﬁbre, S-glass, and E-glass. Carbon ﬁbre and glass ﬁbre have a similar density, but carbon ﬁbre has a steeper stress–strain curve than glass ﬁbre, so less material is required in a pole of equivalent strength. An important design constraint on a pole is that the maximum allowable bending stress of the pole must not be exceeded during the deformation of the pole. The failure strains of glass ﬁbres and carbon ﬁbres are sufﬁciently high that it is possible to construct a pole that can withstand a very large deformation. Carbon ﬁbre has been used in some poles since the early 1990s. In the poles made by Gill Athletic, carbon ﬁbre is currently used only in the body wrap section of the pole. The carbon ﬁbre maintains the mechanical

© 2007, Woodhead Publishing Limited

304

Materials in sports equipment

Stress (MPa)

3000

Carbon

S-glass

2000 E-glass 1000

0 0

1

2

3

4

5

6

Strain (%)

12.3 Stress–strain curves for glass ﬁbre and carbon ﬁbre.

properties of the pole, but reduces the weight by about 15–25% over a pole made from S-glass. Although lighter than ﬁbreglass poles, carbon ﬁbre poles have not yet been universally adopted by the world’s best pole vaulters and so there is no discernible inﬂuence of the pole on the historical trend of pole vault performances (see Fig. 12.2).

12.3

Javelin design and materials

The javelin throw can trace its origins as a sport back to the Olympic Games of ancient Greece. In the modern event, the javelin must be thrown using one hand only, without the aid of a sling or other throwing device. Because an athlete can generate a greater release speed with a lighter implement, the competition rules in the throwing events always specify a minimum weight for the implement. In the javelin throw the minimum weight is 800 g for men and 600 g for women. In the ﬁrst half of the twentieth century many of the best throwers used javelins made of Finnish birch (Isaacs, 1992). The two basic design principles were to minimise the cross-sectional area of the javelin so as to minimise aerodynamic drag, and to make the javelin as stiff as possible so as to minimise aerodynamic losses arising from vibration of the javelin. Nowadays javelins are constructed from steel, aluminium alloy, or carbon ﬁbre. A modern javelin differs from the early designs in that it has a much larger cross-sectional area. Dick Held is credited with introducing the ‘aerodynamic’ javelin in the 1950s. His experiments led to the realisation that it is better for the javelin to have a larger surface area to augment the javelin’s

© 2007, Woodhead Publishing Limited

Design and materials in athletics

305

ﬂight capacity through producing a greater lift. Dick’s brother Bud set a world record using his aerodynamic javelin in 1953. Ever since, the rules governing the ﬂight-enhancing capabilities of the javelin have been subject to considerable debate and controversy, and they have been tightened and changed several times. The aerodynamic behaviour of a javelin is relatively complex. When a javelin is launched with an angle of attack, the javelin acts as an airfoil and so generates lift as well as drag. If the javelin is asymmetrical in shape the centre of pressure of the javelin (the point through which the aerodynamic forces act) does not coincide with the centre of mass. The aerodynamic forces then create a moment about the centre of mass, causing the javelin to pitch up or down depending on whether the centre of pressure is ahead or behind the centre of mass. Wind tunnel tests on javelins in the 1970s revealed that there were three equilibrium points in the pitching moment curves of a typical javelin (Terauds, 1972; Hubbard and Rust, 1984). The centre of pressure is ﬁrst behind, then in front of, and then again behind the centre of mass as the angle of attack is increased. The rules governing the dimensions of the javelin were substantially changed in 1986. At the time, the world record in the men’s event was about 100 m, which was making it increasingly difﬁcult to hold the event within the conﬁnes of a standard athletics stadium. However, this was not the main reason for the change in rules, as is often reported. The main factor that motivated the change was that in many throws the javelin was landing nearly ﬂat, placing large pressure on judges to determine whether the throw was valid or not. (A throw is valid if the tip of the metal head of the javelin strikes the ground before any other part of the javelin.) Also, the pitchingmoment characteristics of most javelins were unstable in yaw, causing erratic throws which endangered athletes and ofﬁcials (Hubbard, 1989). The javelin was redesigned by shifting the centre of mass forward by 4 cm, while constraining the dimensions of the rear section of the javelin to effectively prohibit the centre of pressure from being moved forward as well (Borgström, 2000; Bremicker, 2000). The new design guaranteed that the pitching-moment proﬁle of the javelin was monotonically decreasing, without ever attaining a positive value. As a result, the nose-down pitching moment now lasts throughout the ﬂight, giving a greater incidence of tipﬁrst landings. The new speciﬁcations severely restrict the ﬂight-enhancing effects of the javelin, and the possible variations in javelin construction are now much more limited. Tests using javelin launching machines suggest that distances achieved with the redesigned javelin should be about 5 m less than those achieved under the previous rules. A drop in performance of about this magnitude is clearly evident in the historical record of competition performances (see Fig. 12.4).

© 2007, Woodhead Publishing Limited

306

Materials in sports equipment 100

80

Men

Distance (m)

1986 60

Aerodynamic javelin 1999

40 Women 20 1900

1920

1940

1960

1980

2000

Year

12.4 Historical trends in javelin throw performances for men and women. Data points are the performances by the tenth-best athlete in the year. The aerodynamic javelin was developed in the mid 1950s. Note the decrease in the men’s performances in 1986 when the rules governing the dimensions of the javelin were changed. A slight decrease in women’s performances is also evident following a similar rule change in 1999.

The high incidence of ﬂat landings was also a problem in the women’s event. In 1991 the dimensions of the women’s javelin were changed so as to specify a minimum diameter for the shaft. However, the location of the centre of mass of the javelin was not changed. While this change in speciﬁcations produced fewer ﬂat landings, they were not completely eliminated. The centre of mass was put forward by 3 cm in 1999, and this has been sufﬁcient to create valid landings in most throws. As in the men’s event, slight reductions in throwing distance arising from the rule changes are evident in historical record of competition performances (see Fig. 12.4). Most javelins experience a small lateral oscillation during ﬂight (Hubbard and Bergman, 1989; Macari Pallis and Mehta, 2003). Lateral oscillation is detrimental to performance as it increases the aerodynamic losses. Many athletes attempt to minimise the magnitude of oscillation by using a straight line pull on the javelin during the launch phase; but this is very difﬁcult or even impossible to achieve. Most of the best athletes prefer to use a stiff javelin that minimises any lateral deﬂection generated during the launch. The best javelins are also designed so that if an oscillation is produced, it is quickly damped out. The javelins used by most of the leading athletes are constructed from carbon-reinforced ﬁbreglass, but these implements

© 2007, Woodhead Publishing Limited

Design and materials in athletics

307

are relatively expensive. The majority of lesser athletes use javelins made from aluminium alloy. In the early 1990s, several world records were set by athletes using a javelin designed by the former Olympic Champion, Miklós Németh. This javelin featured surface roughness on the tail to reduce aerodynamic drag. However, the view of the IAAF was that surface roughness was a feature that was incorporated into the javelin purely to enhance its performance relative to previous designs. The javelin was therefore banned, and all performances achieved with the implement were retrospectively disqualiﬁed. More recently, some manufacturers have incorporated an internal stiffening bar into the shaft of the javelin to reduce oscillation. These designs have also been banned.

12.4

Design and materials for the shot put, hammer and discus

The shot put is probably derived from the Highland Games event of thrusting or ‘putting’ a heavy stone for maximum distance. Under IAAF competition rules the shot may be constructed from solid iron, brass, or similar hard metal. Nowadays, most of the better implements are made of stainless steel. The competition rules allow a range of about 15% in the diameter of the implement. A smaller diameter shot does not travel substantially farther than a large diameter shot that has the same launch speed. There is a negligible difference in range arising from differences in aerodynamic drag between the smallest and largest implements. Most elite athletes prefer to use a large diameter shot, claiming that they like the ‘feel’ of the implement. Some say the large shot gives them a ‘larger area to push on’, and so allows them to achieve a greater release speed. The original implement used in the hammer throw was a sledge hammer like those used in ironworking and mining. Nowadays, the hammer head is a spherical ball of steel, and the wooden handle has been replaced by a steel wire that is attached to a special grip. Unlike the shot put, the diameter of the hammer head has a substantial inﬂuence on the distance achieved (Dapena and Teves, 1982; Hubbard, 1989; Dapena et al., 2003). Aerodynamic drag is considerably more substantial in the ﬂight of the hammer than in that of the shot, mainly because the velocities are roughly twice as great. Also, the projected area of the hammer and the effective drag coefﬁcient are both about 50 % larger. Dapena et al. (2003) calculate that aerodynamic drag reduces the range of a 84 m throw by about 3.8 m (4.5 %). Most elite throwers deliberately select a hammer that is close to the minimum diameter permitted by the competition regulations. The length of the hammer wire has a very strong inﬂuence on the release velocity of the hammer. It is well known that the release velocity of a

© 2007, Woodhead Publishing Limited

308

Materials in sports equipment

hammer is determined by the angular velocity of the athlete–hammer system just before release and by the distance from the axis of rotation of the athlete–hammer system to the centre of mass of the hammer. A longer implement therefore results in a greater release velocity, and so most throwers deliberately use a hammer whose length is as close as possible to the maximum permitted by the competition regulations. In major competitions the athletes are provided with a selection of certiﬁed implements from different manufacturers. Even so, competition ofﬁcials must be vigilant in spotting implements that become illegal through damage suffered during the competition or through deliberate tampering by an athlete. For example, at the Sydney 2000 Olympic Games, all the athletes in the ﬁnal decided to use one particular hammer out of the range of implements on offer. The model of hammer in question had a design defect in its handle that caused the hammer to become stretched by 9 mm (Wilson et al., 2006). The defect was quickly noticed by the athletes, but not by competition ofﬁcials until the implements were remeasured after the competition was over. Changes were subsequently made to the competition regulations for the dimensions of the hammer handle in an endeavour to improve the reliability of hammer handles.

12.4.1 Design and materials for the discus The competition implements used in the Olympic Games of ancient Greece were constructed from bronze and varied in weight from 1.5 to 4 kg (Quercetani, 2000). The modern discus has been standardised at 2 kg for men and 1 kg for women, and the dimensions of the implement are tightly speciﬁed. The materials used in the construction of the discus are chosen on the basis of durability and cost. A discus usually has sides made of aluminium, ﬁbreglass, or wood and a rim made of steel, bronze alloy, or brass alloy (Macari Pallis and Mehta, 2003). Although the minimum weight of the discus is speciﬁed, there are no restrictions on how the mass must be distributed within the discus. The moment of inertia of the discus has a strong inﬂuence on performance, and so the athlete must make an appropriate choice of implement. The ﬂight of a discus is strongly inﬂuenced by aerodynamic forces. The discus is a symmetric airfoil, and when launched with a small angle of attack it generates aerodynamic lift which prolongs its ﬂight. Unfortunately, the aerodynamic forces acting on a spinning discus also produce torques which tend to pitch the discus upwards and sideways (Soong, 1976; Frohlich, 1981; Hubbard, 1989). However, the gyroscopic properties of the spinning discus may be used to stabilise its ﬂight and minimise the adverse effects of these aerodynamic torques. Elite throwers prefer an implement that has most of the mass concentrated in the rim, hence giving the discus a large moment of inertia. Elite

© 2007, Woodhead Publishing Limited

Design and materials in athletics

309

throwers can launch such a discus with a relatively high spin and therefore a large angular momentum. A discus with a larger angular momentum has greater resistance to changes in the orientation of the discus that arise from the aerodynamic forces that act on the discus during its ﬂight. A discus used by an elite thrower usually has a rim weight of about 80–92% of the total weight of the discus, and will be rated by the manufacturer as ‘ultra-high spin’ or ‘very high spin’. Less capable athletes prefer to use a discus with a lower moment of inertia. Such a discus has about 70–75% of the total weight of the discus in the rim, and will be rated as ‘low spin’. The differences in an athlete’s preferred moment of inertia for the discus are probably due to differences in the launch speed that the athlete can generate. As well as maximising the launch speed of the discus, the thrower would like to maximise the angular momentum of the discus at the instant of release so as to give the discus the greatest stability in ﬂight. At the end of the release phase of the throw, the discus is spun off the index ﬁnger or the middle ﬁnger of the throwing hand, spinning clockwise when viewed above for a right-handed thrower. A discus with a large moment of inertia will be more difﬁcult to get spinning during the release phase, and so will have a lower spin at the instant of release. For any given release velocity there is probably an optimum moment of inertia that allows the thrower to produce the greatest angular momentum in the discus. The greater strength of elite throwers allows them to generate a higher speed in the throwing hand, and hence impart a high rate of spin to a discus with a larger moment of inertia. Some coaches have suggested that a discus with a larger moment of inertia remains in contact with the thrower’s hand for a little longer, and so the athlete is able to generate a slightly greater launch speed.

12.5

Design and materials for hurdles, starting blocks and shoes for athletes

The design of the barriers used in the hurdles events has shown an evolution in design towards minimising the risk of injury to the athlete. The hurdle barriers of the nineteenth century were solid sheep fences that crossed over several lanes and were staked rigidly into the ground. Under such conditions the athletes were primarily concerned with making a safe clearance rather than a biomechanically efﬁcient clearance to maintain running speed. Performances improved at the start of the twentieth century following the introduction of the movable individual hurdle in the form of an inverted T. Even so, the physical penalty to striking a hurdle varied considerably between meets because of differences in the weight and construction of the hurdle. The modern L-shaped hurdle and set of design speciﬁcations was introduced in 1935. The hurdle must now be constructed in such a way that it

© 2007, Woodhead Publishing Limited

310

Materials in sports equipment

will overturn if a force equal to a weight of between 3.6 and 4.0 kg is applied to the top edge of the crossbar. This change has lessened the chance of injury and consequently frees the athletes of much of the psychological hindrance. Because men, women and junior athletes compete over hurdles of different heights, most modern hurdles are adjustable in height and have movable counterweights in the base so that the required overturning force may be obtained for each hurdle height. Some of the more advanced hurdle designs automatically move the counterweight in response to the height adjustment. At least one company manufactures a hurdle that has a bevelled edge on its base so that the crossbar of the hurdle is directly above the tip-over fulcrum. This ensures that the crossbar does not rise when the hurdle is hit or toppled. Modern hurdles are constructed from lightweight materials such as aluminium so as to make it easier for competition staff to place and remove the hurdles from the track. Splinter-proof polycarbonate crossbars have now replaced the more dangerous wooden crossbars. Several commentators have suggested that the women’s hurdle should be higher than is speciﬁed by the competition regulations (Etcheverry, 1993; Stein, 2000). Under the present rules the women’s event places less emphasis on hurdling skill and more on sprinting ability than the men’s event. The men’s hurdle is about 57% of a typical athlete’s standing height, but the women’s hurdle is only about 50% of an athlete’s standing height. Increasing the height of the women’s hurdle to about 0.91 m would raise the technical demands of the women’s event and give parity with the men’s event.

12.5.1 Starting blocks Starting blocks were introduced in the late 1920s and were used in the Olympic Games for the ﬁrst time at the 1948 Games in London. Previously, athletes either had no starting aids or dug starting holes in the ground for their feet. Modern starting blocks usually have a heavy metal base and two adjustable foot pedals about 15 cm wide. The longitudinal spacing and angle of the foot pedals may be adjusted to the athlete’s preferred setting. In most starting blocks the foot pedals have a ﬁxed lateral spacing of about 10 cm. However, this spacing may not be optimal. The footfalls of most elite sprinters have a lateral spacing of about 40 cm for the ﬁrst few strides out of the blocks; reducing to about 17 cm at full speed sprinting (Ito et al., 2006). This suggests that starting blocks should have a wider foot pedal spacing, allowing the athlete to produce a more direct linear exit from the blocks, with the feet landing directly ahead of the initial position of the feet in blocks. In recent experiments, athletes using blocks that were spaced laterally by about 20 cm produced faster times to the 20 m mark than those using blocks with the usual 10 cm spacing (Parry et al., 2003). Some manu-

© 2007, Woodhead Publishing Limited

Design and materials in athletics

311

facturers have now produced starting blocks that have very wide foot pedals which permit athletes to select their preferred lateral foot spacing.

12.5.2 Running shoes For many athletes, running shoes are their most important piece of kit. Under IAAF regulations the purpose of the athlete’s shoe is to give protection and stability to the foot and provide a ﬁrm grip on the ground. The shoe must not be constructed so as to give the athlete any additional assistance, and no spring or device of any kind may be incorporated in the shoe. The design of running shoes has shown a steady evolution towards minimising the weight of the shoe. A lighter shoe reduces energy consumption during a distance event, gives a quicker acceleration and a higher top speed in a sprint, and allows a greater vertical take-off speed to be produced in a high jump or long jump. The stiffness of the baseplate of a sprint shoe can have a signiﬁcant effect on performance (Stefanyshyn and Fusco, 2004). Adidas produces a ‘performance plate’, which is a rigid carbon ﬁbre plate that is inserted into the sole to stiffen the shoe under the metatarsal joints. The energy that is normally dissipated during the initial bending of the metatarsal joints at touchdown is stored in the carbon ﬁbre plate and returned to the athlete during the toe-off phase of the stride. Experiments on sprinters running over 20 m showed an improvement of just over 1% with a stiffening plate in their shoes.

12.5.3 High jump and long jump shoes In the high jump, the design of the athlete’s shoe is believed to have a small but signiﬁcant inﬂuence on performance. Yuri Stepanov (URS) set a world record of 2.16 m in 1957 using a take-off shoe that had a 2–4 cm thick sole (Lawson, 1997; Hymans, 2003). The most obvious advantage of a built-up shoe is that the athlete’s centre of mass is higher above the ground at takeoff and so the height of the jump is correspondingly increased. Taken to the extreme, an athlete could wear what is essentially a pair of stilts and then simply step over the crossbar. The IAAF viewed the built-up shoe as giving ‘unfair assistance’ to the athlete, and it was banned shortly after being introduced. However, Stepanov’s record was allowed to stand. Since 1958, the thickness of the sole of the high jump shoe has been restricted to 13 mm. The restriction on the thickness of the sole also applies to long jump shoes. In the long jump the athlete wishes to project their body for maximum distance beyond the take-off line, and it is well known that the range of a projectile is greater the higher the launch position is above the landing

© 2007, Woodhead Publishing Limited

312

Materials in sports equipment

position. In the long jump the angles of take-off and landing are usually around 20–30°, and so the athlete can be expected to jump about 2 cm farther for each 1 cm increase in take-off height. It therefore seems appropriate to place a restriction on sole thickness for long jump shoes as well as for high jump shoes. Some shoe manufacturers have optimised the design of the sole within the restrictions that have been placed on the thickness. Adidas produce a long jump shoe that is described in their promotional literature as having a ‘negative heel design’. The sole thickness is tapered, from thickest under the ball of the foot down to thinnest under the heel. The aim is to give the athlete an effectively longer take-off leg during the take-off. This advantage is maximised by making the difference between the length of the leg at touchdown (with the landing on the heel) and the instant of take-off (with the ball of the foot) as great as possible. A longer take-off leg may give a biomechanical advantage by allowing the athlete to generate a greater takeoff velocity, and hence produce a longer jump.

12.6

Design and materials for running surfaces and other athletic facilities

Until the late 1960s, most major athletic competitions were held on surfaces of grass or cinders. Synthetic running surfaces were approved for competition by the IAAF in 1964, and the ﬁrst major international competition to be held on a synthetic surface was the 1968 Olympic Games in Mexico City. The main beneﬁt of a synthetic track is that it is ‘all weather’, with a constant performance standard under all environmental conditions. Unlike cinders and grass tracks, it is relatively unaffected by rainfall. Synthetic running surfaces are durable and require less maintenance, but have a high initial capital outlay. The running track, aprons, and runways of a typical athletics arena are constructed in a series of layers (Buccione, 1999). The foundation layer usually consists of a ballast of loose stone, which is then overlaid with a binder layer of asphalt. The top layer is the synthetic running surface, which comes in four main types: pre-fabricated, solid polyurethane, sandwich system, and porous rubber crumb. Two of the more notable surfaces are Rekortan® M99 by APT, which has a base layer of polyurethane-bound rubber granules overlaid with a layer of polyurethane and rubber granules, and Sportﬂex Super X by Mondo, which is a pre-fabricated surface of calendered, vulcanized, and stabilised polyisoprenic rubber. The performance of sports surfaces is measured by a battery of tests, including tests of resilience, deformation, spike-resistance, skid-resistance, and sliding behaviour. The most important performance test for a running surface is the measurement of resilience, and the IAAF has set an approved

© 2007, Woodhead Publishing Limited

Design and materials in athletics

313

range of resilience for tracks intended for international competitions. The standard device for measuring the resilience of an athletics track is the ‘Artiﬁcial Athlete Berlin’, in which a 20 kg weight is dropped a distance of 55 mm onto a 3 kg measurement foot that incorporates a spring with a spring constant of 2000 MN/m (Kolitzus, 1984; Walker, 2003). The force– time curve is recorded during the bounce of the mass, from which the energy loss and force reduction of the surface may be calculated. Hard tracks are believed to produce fast times for sprinters, but they are associated with an increased risk of leg strain injuries, particularly in long distance athletes. Mondo’s Sportﬂex Super X is a favourite competition surface with many athletes, particularly sprinters and jumpers. Mondo surfaces have been installed in many of the venues used for major international competitions. This type of surface has an underlying honeycomb structure of small airﬁlled deformable cells. The sidewalls of the cells are angled at about 45° in the direction of running, which gives the track a greater stiffness to the horizontal–vertical forces generated by sprinters, while at the same time presenting a softer vertical surface desired by distance athletes. Also, with a Mondo surface it is not necessary for the athlete to have penetrating spikes in their shoes. Instead, special cone- or pyramid-shaped spikes that merely deform the running surface are used. Mondo claim that nonpenetrating spikes reduce the energy loss at each footfall, and thus increase the athlete’s running speed.

12.6.1 Safety equipment Considerable energy continues to be expended in efforts to make the competition environment safer and to reduce the risk of injury to athletes, ofﬁcials, and spectators. The most obvious examples of safety equipment are the throwing cages used to capture wayward implements in the discus throw and hammer throw. The IAAF speciﬁes the dimensions of the throwing cages, including the minimum strength of the cage netting (Laruel et al., 2004; Wilson et al., 2006). The cage must be designed so as to stop an implement moving at a speed equal to that generated by an elite thrower, without allowing the implement to ricochet or rebound back towards the athlete or over the top of the cage. The modern landing mat has played a vital but sometimes over-looked role in the progress of the high jump and pole vault (Guy, 1994). Without it, the Fosbury Flop and the uninhibited landings of today’s pole vaulters would not be possible. Originally, the landing areas in the high jump and pole vault were just piles of sand, sawdust, or wood shavings. These were replaced in the late 1950s by mounds of loose foam rubber, and later by the integrated pits constructed from several large pieces of low-stiffness

© 2007, Woodhead Publishing Limited

314

Materials in sports equipment

polymer foam that we see today. The energy absorption of the landing mat is a direct consequence of the foam structure, in that air can be expelled from the mat, and the cell walls of the foam can collapse (Mills, 2003). Landing mats usually have a PVC cover to reduce damage to the foam from the UV component of solar radiation. The IAAF speciﬁes the minimum dimensions of the landing pits for both the high jump and pole vault.

12.6.2 Visual design Good design in athletics also includes the visual characteristics of the equipment and its inﬂuence on safety and performance. Obvious examples include the top beam of the steeplechase hurdle, which under IAAF regulations must be painted in black and white stripes (or other contrasting colours) to aid visibility and thereby reduce the risk of injury to the athletes. In the high jump and pole vault the crossbar is often painted in colours that help the athletes sight the bar, and in the long jump and triple jump the take-off board must be painted white to help the athletes sight the board and so reduce the number of invalid jumps. Recently, the IAAF speciﬁed that the plasticine indicator board must be a contrasting colour to the takeoff board. (The plasticine indicator board is a 10 cm wide board that is placed after the take-off board to assist the judges in determining if the athlete has overstepped the take-off line.) In competitions that used a white indicator board, many athletes were targeting it as if it were part of the take-off board and so tended to overstep the take-off line (Linthorne, 2005). An indicator board that is a contrasting colour to the take-off board is expected to minimise the number of invalid jumps and so improve the appeal of the event to spectators.

12.7

Design and materials in timing and other equipment

Accurate timing is essential for the recognition of record performances, especially in the sprint events. Initially, timing in athletics competitions was not particularly precise; most hand-held stop watches could only be read to a ﬁfth of a second. The ﬁrst Olympic Games to use timing to a tenth of a second was the 1912 Games in Stockholm. Fully automatic electronic timing to 0.01 seconds has been used in the Olympic Games since 1932, but the electronic times were not reported as the ofﬁcial time until the 1972 Olympic Games. Fully automatic electronic timing has been mandatory for the recognition of record performances since 1974. Electronic timing is usually about 0.23 s slower than hand timing because of the reaction time of the time-keepers to the starting gun and the rounding of the time to the

© 2007, Woodhead Publishing Limited

Design and materials in athletics

13

315

Women

Time (s)

Electronic timing 12

11

Men

10

1900

1920

1940

1960

1980

2000

Year

12.5 Historical trends in 100 m performances for men and women. Data points are the performances by the tenth-best athlete in the year. Electronic timing to 0.01 s has been mandatory for the recognition of record performances since 1974.

next highest tenth of a second. This difference is clearly seen in the historical trend of 100 m performances (see Fig. 12.5). Video-based electronic timing systems have now replaced earlier systems that required the capture and developing of photographic images. In a fully automatic electronic timing system that is approved by the IAAF, the timing system is started by a sound signal or electrical signal from the starting gun. The delay between the report of the gun and the starting of the timing system must be less than 0.001 s. The leading edge of the ﬁnish line is recorded by a video camera through a narrow vertical slit in front of the camera lens. This produces a continuous image of the athletes as they cross the ﬁnish line. The photo-ﬁnish image is synchronised with the timing system, and the times and placings of the athletes are determined by moving a cursor on the photo-ﬁnish image. Performances can be determined to an accuracy of 0.001 s, and the more sophisticated timing systems can provide almost instantaneous results to television and to the stadium information system.

12.7.1 Starting guns The purpose of the starting gun is to give the athletes a fair start and to trigger the timing system. Seiko has developed a ‘silent gun’ which eliminates the disadvantage that is usually given to the outside lanes. In a normal starting system the athletes respond to the sound produced by the starter’s

© 2007, Woodhead Publishing Limited

316

Materials in sports equipment

gun. However, the athletes that are farther away from the starter are at a disadvantage because of the ﬁnite speed of sound. The speed of sound is about 350 m/s, and so 0.01 s is added to the race time for each 3.5 m between the starter and the athlete. In a silent gun the starting gun itself makes no sound. Instead, the gun sends an electrical signal to the speakers on the athlete’s starting blocks, which then produce a sharp sound. All the athletes therefore hear the starting signal at the same time, and the signal is heard almost as soon as the gun is ﬁred. The silent gun has been used at recent World Championships, but not at the Olympic Games because the ofﬁcial suppliers of the timing systems have not developed such a device (Julin and Dapena, 2003).

12.7.2 False start detection At major competitions, including the Olympic Games and World Championships, the athletes must use starting blocks that are linked to an approved false start control apparatus. If the force exerted by the athlete on the blocks exceeds a certain threshold before 100 ms after the gun is ﬁred, the athlete is deemed to have committed a false start and the starter is alerted by a tone through a set of headphones. The false start time threshold is based on the assumption that the minimum physiological reaction time to an auditory signal is at least 100 ms, but this assumption has recently been questioned (Pain and Hibbs, 2007). The threshold force required to trigger a false start (typically about 200 N) depends on the design of the blocks. Unfortunately, continual changes in technology and subtle reﬁnements in equipment speciﬁcations make it difﬁcult to compare start time data from different competitions. However, on average, female sprinters record slower reaction times on starting blocks than male sprinters, even though no gender bias is evident for reaction times in general (Martin and Buoheristians, 1995). The difference is believed to be due to the fact that male athletes are usually heavier and stronger than female athletes. A stronger athlete is able to generate a force that crosses the threshold sooner. Also, a heavier athlete exerts more force on the blocks in the set position, and so is closer to the threshold force. Therefore, male athletes appear to react faster than female athletes when measured using starting blocks.

12.7.3 Wind gauges The wind can have a strong effect on athletic performances, and records in some events will only be recognised if the assistance from the wind is not deemed to be excessive. Excessive wind assistance was originally based on the subjective opinion of the referee, sometimes using observations of the

© 2007, Woodhead Publishing Limited

Design and materials in athletics

317

movements of nearby trees, a hand-held cloth or handkerchief, or smoke from a small ﬁre to determine the direction and strength of the wind. Since 1936 the wind must be measured using a calibrated wind gauge, and a record performance will not be accepted in the sprints, hurdles, and horizontal jumps if there is a following wind of more than 2.0 m/s. The most common type of wind gauge is the tube propeller anemometer, which consists of a lightweight and freely rotating propeller inside a tube about 10 cm in diameter and 40 cm long. The tube is placed parallel to the direction of running, and a 10 s wind measurement is taken during the event. (A 5 s measurement is used in the horizontal jumps.) A tube propeller anemometer gives a reliable reading in a steady wind. However, under ﬂuctuating conditions the intrinsic inertia of the propeller leads to a time lag of several seconds while it adjusts to the change in wind velocity. The short measurement times required in athletics mean that ofﬁcial wind readings taken with this type of device can be in error by up to 50% (Vanuytven, 1994). A more accurate and more expensive device is now mandatory for major championships and international competitions. The device uses ultrasonic sound waves to measure the wind speed. A sound wave travelling through a stationary medium has a characteristic velocity (e.g. 343 m/s at 20 °C). Moving air from the wind will add to this characteristic velocity. In an ultrasonic wind gauge, time of ﬂight measurements of short bursts of sound are made between small transmitters and receivers about 15 cm apart. The system measures the sound travel time in both directions to compensate for temperature, humidity, and air pressure effects.

12.8

Future trends

The preceding discussions should give the impression that opportunities for the innovative sports engineer to improve safety and event presentation in athletics are reasonably good. However, the conservative nature of the underlying philosophy of athletics leads one to be less optimistic about opportunities for developing equipment to enhance performance. Even so, many equipment manufacturers put considerable effort into optimising the design of their equipment within the often quite tight boundaries set by the competition regulations. A product that gives a performance advantage to the athlete, however small, will have an advantage in the sports equipment market. From the viewpoint of the sports engineer and sports scientist, it is important to recognise that the athlete’s body should also be considered as a machine that can be engineered and optimised. The performance of the human machine may optimised through talent identiﬁcation, where athletes of appropriate body size and body type are recruited to the sport; through

© 2007, Woodhead Publishing Limited

318

Materials in sports equipment

athletic training, where the performance of the human machine is improved through strength training and physical conditioning; and through technique coaching, where the movement patterns of the human machine are optimised for its given set of physical dimensions and physical capabilities. Consideration of the athlete as a machine is an under-used concept, and one that should lead to better athletic performances in the future. Until now, innovative athletic techniques have been almost exclusively developed by athletes and coaches, and not by sports scientists and sports engineers. In the future, scientists and engineers should look harder at discovering new athletics techniques. However, one must always be wary of developing a technique that will be banned because it changes the fundamental nature of the event or is dangerous. Current athletic techniques should also be optimised for the individual athlete. The optimum body size, level of physical conditioning, and technique for an event may be identiﬁed through mathematical modelling, and then veriﬁed with experimental studies on athletes.

12.9

Sources of further information and advice

Any scientist or engineer working in the sport of athletics needs to become familiar with the competition rules. These are contained in the IAAF rule book, which may be obtained from the IAAF website (http://www.iaaf. org/). A good understanding of the history of athletic techniques and equipment may be obtained by reading Track and Field Omnibook by Ken Doherty, and A History of Modern Track and Field Athletics by Robert Quercetani. Alphonse Juilland presents some radical ideas on the future of athletics in his book, Rethinking Track and Field. Most of the major sports equipment manufacturers have a website with details about their products. Useful websites include those for Gill Athletic (http://www.gillathletics.com/), UCS (http://www.ucsspirit.com/), Dima Sport (http://www.dimasport.fr/), Nordic Sport (http://www.nordicsport.se/), and Nemeth Javelins (http://www.nemethjavelins.hu/eindex.htm). For information about synthetic running tracks see the Mondo website (http://www. mondoworldwide.com/index.cfm?lingua=it), and for information about electronic timing systems see the websites for Omega Electronics (http:// www.omega-electronics.ch/) and Finish Lynx (http://www.ﬁnishlynx.com/).

12.10 References borgström a (2000), The development of the javelin, New Stud Athl, 15(3/4), 25–8. bremicker e (2000), Why did the senior javelin speciﬁcation have to be changed, New Stud Athl, 15(3/4), 29–31.

© 2007, Woodhead Publishing Limited

Design and materials in athletics

319

buccione r (1999), Synthetic surfaces in athletics, Track Technique, 103, 3279–83. burgess s c (1996), The design optimisation of poles for pole vaulting, in Haake S (ed.), The Engineering of Sport, Rotterdam, Balkema, 83–90. dapena j and teves m a (1982), Inﬂuence of the diameter of the hammer head on the distance of a hammer throw, Res Quart, 53(1), 78–85. dapena j, gutiérrez-dávila m, soto v m and rojas f j (2003), Prediction of distance hammer throwing, J Sports Sci, 21(1), 21–8. doherty k (1985), Track and Field Omnibook, 4th Edn, Los Altos, CA, Tafnews Press. ekevad m and lundberg b (1995), Simulation of smart pole vaulting, J of Biomech, 28(9), 1079–90. etcheverry s g (1993), A proposal to change the women’s hurdles events, New Studies in Athletics, 8(2), 23–26. frohlich c (1981), Aerodynamic effects on discus ﬂight, Amer J Phys, 49(12), 1125–32. ganslen r v (1979), Mechanics of the Pole Vault, 8th edn, St Louis, MO, John Swift. guy a (1994), Technology in athletics, New Stud Athl, 9(4), 9–11. hubbard m (1980), Dynamics of the pole vault, J Biomech, 13(11), 965–76. hubbard m (1989), The throwing events in track and ﬁeld, in Vaughan C L (ed.), Biomechanics of Sport, Boca Raton, FL, CRC Press, 213–38. hubbard m and bergman c d (1989), Effect of vibrations on javelin lift and drag, Int J Sports Biomech, 5(1), 40–59. hubbard m and rust h j (1984), Simulation of javelin ﬂight using experimental aerodynamic data, J Biomech, 17(10), 769–76. hymans r (2003), Progression of World Best Performances and Ofﬁcial IAAF World Records, 2003 edn, Monaco, IAAF. iaaf (2006), IAAF Handbook, 2006–2007, Monaco, IAAF. isaacs t (1992), The javelin controversy, in Matthews P (ed.), Athletics 1992: The International Track and Field Annual, Windsor, Berkshire, Burlington Publishing, 127–9. ito a, ishikawa m, isolehto j and komi p v (2006), Changes in the step width, step length, and step frequency of the world’s top sprinters during the 100 metres, New Stud Athl, 21(3), 35–6. juilland a (2002), Rethinking Track and Field: The Future of the World’s Oldest Sport, Milan, SEP Editrice. julin a l (1992), The unwritten axioms of athletics, in Matthews P, Athletics 1992: The International Track and Field Annual, Windsor, Berkshire, Burlington Publishing, 125–6. julin a l and dapena j (2003), Sprinters at the 1996 Atlanta Olympic Games did not hear the starter’s gun through the loudspeakers in the starting blocks, New Stud Athl, 18(1), 23–7. kolitzus h j (1984), Functional standards for playing surfaces, in Frederick E C (ed.), Sports Shoes and Playing Surfaces, Champaign, IL, Human Kinetics, 98–118. laruel b, wilson d and young r (2004), Hammer throw safety cages, New Stud Athl, 19(1), 47–51. lawson g (1997), World Record Breakers in Track and Field Athletics, Champaign, IL, Human Kinetics.

© 2007, Woodhead Publishing Limited

320

Materials in sports equipment

linthorne n p (2000), Energy loss in the pole vault take-off and the advantage of the ﬂexible pole, Sports Eng, 3(4), 205–18. linthorne n (2005), Color of the plasticine indicator board in the horizontal jumps, Track Coach, 171, 5466–8. macari pallis j and mehta r d (2003), Balls and ballistics, in Jenkins M (ed.), Materials in Sports Equipment, Cambridge, Woodhead, 100–25. martin d e and buoheristians j f (1995), Inﬂuence of reaction time on athletic performance, New Stud Athl, 10(1), 67–79. mills n j (2003), Foam protection in sport, in Jenkins M (ed.), Materials in Sports Equipment, Cambridge, Woodhead, 9–46. nielson d (2006), Athletics outstanding performer – The vaulting pole, http://www. pvei.com/documents/Vaulting%20Pole.htm last accessed 18 March 2007. pain m t g and hibbs a (2007), Sprint starts and the minimum auditory reaction time, J Sports Sci, 25(1), 79–86. parry t e, henson p and cooper j (2003), Lateral foot placement analysis of the sprint start, New Stud Athl, 18(1), 13–22. quercetani r l (2000), A History of Modern Track and Field Athletics, (1860–2000), Men and Women, Milan, SEP Editrice. soong t c (1976), The dynamics of the discus throw, J Appl Mech, 43(12), 531–6. stefanyshyn d and fusco c (2004), Increased shoe bending stiffness increases sprint performance, Sports Biomech, 3(1), 55–66. stein n (2000), Reﬂections on a change in the height of the hurdles in the women’s sprint hurdles event, New Stud Athl, 15(24), 15–9. terauds j (1972), A comparative analysis of the aerodynamic and ballistic characteristics of competition javelins, PhD Thesis, University of Maryland. vanuytven e (1994), Ultra sonic wind measurement device, New Stud Athl, 9(4), 41–4. walker c (2003), Performance of sports surfaces, in Jenkins M (ed.), Materials in Sports Equipment, Cambridge, Woodhead, 47–64. wegst u g k and ashby m f (1996), Materials selection for sports equipment, in Haake S (ed.), The Engineering of Sport, Rotterdam, Balkema, 175–84. wilson d, guy a and matrahazi i (2006), Hammer cage and hammer developments, New Stud Athl, 21(3), 43–9.

© 2007, Woodhead Publishing Limited

13 Design and materials in ﬁtness equipment M. C A I N E and C. YA N G, Loughborough University, UK

13.1

Introduction

The ﬁtness equipment industry comprises home and commercial equipment sales. Companies servicing each of these sectors typically divide their product lines into three distinct yet complementary product ranges: (1) cardiovascular equipment, used for endurance training (e.g. treadmills, cycle ergometers), (2) resistance equipment, used for strength and power training (e.g. multi-gyms, weights benches), and (3) associated accessories (dumbbells, skipping ropes, etc.). The home sector constitutes approximately 80% (by sales) of the total ﬁtness equipment market, having grown by 241% from 1990 to 2000 (SGMA, 2005). However, it is extremely fragmented with typically many small companies only servicing local territories. The major ﬁtness equipment brands such as Life Fitness, Precor® and Technogym® generate turnovers up to US$300 million and predominantly focus on product development for the commercial sector. The commercial sector is consequently dominated by a small number of companies, each competing across multiple territories.

13.1.1 Absence of product differentiation Unlike most other categories in the sporting goods sector, innovation and differentiation in new product development are often not prioritised within the ﬁtness industry. Perhaps inﬂuenced by the largely conservative attitudes of consumers, the term ‘iteration’ better illustrates the most common method of ﬁtness product development. Particularly in the home sector, companies often expand their product line by selecting from a portfolio of equipment options offered by a set of common, typically Far Eastern, manufacturers. This lack of in-house innovation explains the lack of product differentiation often encountered in the market place. Whilst brand is ordinarily an important factor of distinction, it becomes markedly less so 321

© 2007, Woodhead Publishing Limited

322

Materials in sports equipment

in the ﬁtness market due to the lack of differences in the product ranges marketed.

13.1.2 The value of innovation When innovative new products do come to market, they can often enjoy remarkable success. Introduced in 1995, the Bowﬂex® machine uses tensioned rods to generate resistance during exercise. Sales grew from just US$21.5 million in 1997 to over US$585 million in 2002. New and innovative products such as this often use direct marketing channels, commonly through television infomercials. Once ﬁtness products become more established in the market, traditional points of sale such as specialty ﬁtness stores all-purpose sporting goods stores and mail-order catalogues are more common.

13.1.3 Chapter overview This chapter reviews the design process as it relates to ﬁtness equipment. The ﬁtness industry market is ﬁrst examined, providing insight into typical consumer trends and attitudes. Elements of the ideation process are then described using case studies from within the Sports Technology Group at Loughborough University and from Progressive Sports Technologies Limited (a Loughborough University spinout). Topics covered include methods of concept development, embodiment design, tools for assessing design, mass manufacture, target price points and aesthetic styling. In particular, a special focus is given to design facilitated by improved materials and processes. Probable future trends in the ﬁtness industry are also identiﬁed, whilst the ﬁnal section of the chapter details sources of further information.

13.2

Market research for ﬁtness equipment

Rising concerns about health and prevention of injury or illness have contributed to the growth of the ﬁtness industry since the 1990s. Market reports from Tekes estimate that the US ﬁtness market generated nearly US$6.0 billion in equipment sales in 2000 (Kratzman and Stamford, 2002). While the sporting goods sector overall demonstrated a 3.2% growth in 2005 compared to the previous year, exercise equipment rose by 6% (SGMA, 2005). According to a report from Market & Business Development (Feedback Research Services, 2003), the health and ﬁtness market in the UK was estimated at approximately US$6.2 billion (£3.5 billion) in 2003 with a 6% growth rate forecast from 2004 to 2008. Demand for health and ﬁtness

© 2007, Woodhead Publishing Limited

Design and materials in ﬁtness equipment

323

equipment is still growing, even in mature territories such as Germany where the number of ﬁtness studios has increased from 225 in 1980 to 5700 in 2005 (Kietz, 2005). The TEKES technology review 2002 for ‘The U.S. Fitness Industry Market – Overview and Entry Strategies’ (Kratzman and Stamford, 2002) identiﬁes general trends in today’s market: 1. the majority of participants are still not technically advanced; 2. how or why exercise beneﬁts the body is not well understood by participants; 3. purchases are based upon ease of use and positive personal association; 4. participants are easily bored with repetitive exposure to the same exercise.

13.2.1 Participation rates and sales of current equipment In 2006, there were an estimated 41.1 million gym and health club users in the USA, the world’s biggest market. By comparison, there were 8.7 million users in the UK (Mintel, 2003). Within the last six years, there has been massive growth in the participation rate and sales of treadmills, ellipticals, recumbent cycles, free weights and abdominal machines. Conversely, participation and sales of rowing machines, stationary (upright) cycles, stair-climbers, aerobic riders and ski machines have declined (see Table 13.1).

Table 13.1 Fitness equipment participation trends: US population, six years of age or older, at least once per year (thousands) Fitness equipment exercise

2004

Six year % change (1998–2004)

Free weights Weight/resistance machines Abdominal machine/device Rowing machine Stationary cycling (upright bike) Stationary cycling (recumbent bike) Treadmill exercise Stair-climbing machine exercise Aerobic rider Elliptical motion trainer Cross-country ski machine exercise

52 056 30 903 17 440 7 303 17 889 11 227 47 463 13 300 2 468 15 678 4 155

+26.2 +37.2 +5.5 −2.4 −13.8 +65.8 +28.0 −28.5 −58.0 +305.9 −39.5

© 2007, Woodhead Publishing Limited

324

Materials in sports equipment

Growth in machines like the elliptical and recumbent cycle can be attributed to the desire of (typically older) users to reduce the impact and stress on their joints during exercise. Increases in treadmill and free weights use reﬂect the signiﬁcant and growing minority of consumers who are increasingly health-aware and realise the potential beneﬁts when complementing cardiovascular workouts with strengthening exercises. Considerable growth in sales (and perhaps use) of abdominal machines is accredited to the singular role of infomercials that target consumers who are particularly susceptible to impulse-purchasing and look to quickly improve target areas of their body. It is interesting to note that abdominal toning rates consistently as the number one priority for both male and female exercisers.

13.2.2 User demographics Whilst the most devoted groups of exercisers view in-home ﬁtness as an extension of their gym-based workouts, less committed exercisers typically see it as a less costly, more ﬂexible alternative (Mintel, 2003). These alternate views lend themselves to markedly different equipment preferences amongst home users. Less-committed exercisers whose primary concerns are about diet, combating weight gain and (lack of) exercise favour inexpensive and intuitive equipment such as abdominal trainers, recumbent bikes, yoga mats and hand-weights, with instructor-led video and DVD exercise programming often being followed. In contrast, those committed to frequent exercise and healthy eating are more likely to purchase more sophisticated equipment, for example, multi-gyms, which require knowledge of technique and frequent use to maximise effectiveness. With respect to gender, women tend to use equipment geared toward aerobic ﬁtness and muscle-toning, whereas men prefer strength-based and muscle-building equipment (Mintel, 2003). Examining ﬁtness equipment use amongst different age groups, the 55-and-over age group comprises a major percentage of participation numbers. Since 1987, health club membership has increased by 127% overall, but for people aged 55 and older, membership has increased over 343% (American Sports Data, 2006). Over half (55%) of the 33 million current US club members are adults aged 40 years and older (American Demographics, 2001). This increase in participation is due to the continuation of active lifestyles, increased social acceptability of gym membership, increased information linking exercise to the promotion of health and the production of more user-friendly equipment. In 2000, this group spent more than $1.0 billion on home exercise equipment and accounted for the highest membership levels in health clubs (7.4 million) in the USA (Feedback Research Services, 2003).

© 2007, Woodhead Publishing Limited

Design and materials in ﬁtness equipment

13.3

325

The product development process

The product development process for ﬁtness equipment follows that employed for the majority of consumer products. Broad stages include: (1) market research, (2) creation of a product design speciﬁcation (PDS), (3) conceptual design, (4) embodiment design, and (5) design for mass manufacture. Each stage of the process is described below in greater detail with speciﬁc examples provided.

13.3.1 Primary and secondary research The initial stage of the development process establishes the need and identiﬁes the target market associated with a product. Two methods of gathering information are used: primary research and secondary research. Primary research refers to a direct access of information via focus groups, questionnaires, interviews and observational research. Secondary research involves indirect collection of information from journals, databases, reports, patent databases and such like. Together, these methods are used to gain a clearer understanding of the target market, customer demographics, environment of use, purchasing trends and key consumer motivators. Knowledge of underlying physiology, potential health beneﬁts, competing products and existing patents is also crucial at this stage.

Primary research case study: Reebok® Deck The initial insight which formed the Reebok Deck (a reconﬁgurable exercise platform) concept was obtained from direct observation of equipment abuse in the ﬁtness studio. The challenge for professional ﬁtness instructors is to provide new exercise routines on a weekly basis. Once instructors reach the ‘designed usage’ limit of equipment, ‘creative use’ begins, often involving the abuse or adaptation of equipment. Speciﬁc to the Deck development, the following observations were concluded: • • • •

aerobic exercises classes increasingly use resistance training activities (emergence of new market); step platforms are often used as benches and adapted for use in the incline position (creative misuse of product); step platforms are used for anchoring resistance tubes and chords (creative misuse of product); use of equipment accessories during exercise classes (dumbbells, bands, balls, etc.) result in clutter;

© 2007, Woodhead Publishing Limited

326 • •

Materials in sports equipment

storage system is needed for additional equipment accessories; setting up and breaking down time for instructors increases with additional equipment.

These initial observations were captured to conceive the Deck concept. During product development, further user observation was carried out by video taping exercise classes and reviewing the footage. This provided a number of additional insights which became design features of the ﬁnal Deck product. These observations include the following: • • •

step platforms and separate risers cause difﬁculty for users in assembly and adaptation of the step height; separate risers increase the number of parts to store, set up and break down; varying level of incline from seated position to low incline for abdominal exercises is beneﬁcial.

Using the insight gained through observational research, the Deck was designed to include a step platform and risers as a single assembly. An adjustable support surface with independently movable risers allows numerous platform heights and incline levels to be achieved. This same support surface can be adjusted to attain various seating angles to perform a variety of resistance training activities. An inner cavity allows storage for dumbbells and elastic resistance bands.

Primary research case study: dumbbell focus group The convening of a targeted focus group can often prove invaluable. Whilst developing a range of novel hand dumbbells aimed at women, the design team at Progressive Sports Technologies recruited a range of female exercisers, half novice, half expert. A diverse array of existing products was reviewed by the group. Many insights ensued, but all are consistent with the overarching ﬁnding, namely that novice users prefer products that are overtly feminine, regardless of functionality, whilst expert users prioritise features and usability over styling. This sort of insight is often difﬁcult to quantify but is often nonetheless useful in informing choices made during the product development process. Focus groups implemented at different times in development process serve different roles and can be used to both inform and verify product design choices. Secondary research case study: patent search Often, a pre-existing patent on an identical or similar product can severely alter (or stop) the course of its development and commercialisa-

© 2007, Woodhead Publishing Limited

Design and materials in ﬁtness equipment

327

tion. This case study serves to illustrate the importance of performing a comprehensive patent search before embarking upon rigorous development. A seemingly novel weight training system was designed that comprised a set of small, medium and large dumbbells (see Fig. 13.1). Each dumbbell was capable of being used independently as a training weight but also engaged with at least one other member of the set to generate a range of training masses. The set of dumbbells could additionally be combined with a further like set of dumbbells also comprising small, medium and large dumbbells to further vary the available range. Standard on-line patent searches of ‘worldwide’ databases indicated that no potentially infringeable patents had been ﬁled. However, once a patent application for the concept was ﬁled the patent examiner identiﬁed a very similar Russian patent that had yet to be translated and digitised into a searchable database. Upon discovering the Russian patent the development team elected to discontinue their commercialisation plans. In this instance the development team were somewhat unlucky not to have discovered the pre-existing Russian patent earlier. This example merely serves to highlight the importance of frequent and comprehensive patent searching in active areas of product development.

33'

30' 32'

31'

31

32 33 35 34

30

13.1 Stackable dumbbell patent WO 2004098722.

© 2007, Woodhead Publishing Limited

328

Materials in sports equipment

13.3.2 The product design speciﬁcation (PDS) Using information gained in the initial investigation, a PDS is established that lists the objectives for design. It is not intended to limit creativity or deﬁne a product, but instead describes how a product must perform by associating quantitative measures with a performance metric. Although the PDS is created at the beginning of the development process, it can be continually updated as the project progresses. Of particular importance to ﬁtness products are metrics relating to aesthetics, costs, ergonomics, environmental factors, installation, product life-span, maintenance, manufacture, material, performance, quality, reliability, safety, standards and testing. At this stage, a clear distinction should be made as to whether the design is focused on the home or commercial environment due to signiﬁcantly different price points, standards and packaging associated with each market. In general, commercial ﬁtness equipment tends be more costly and robust with higher levels of functionality than its home counterpart.

PDS case study: Reebok® Gripmasters Leather weight training gloves are typically bulky, being heavily stitched with extensive padding in the palm area. They frequently protect the palm but not the ﬁngers and result in localised discomfort and sweating. They also tend to become slick after repeated use, reducing the hand grip force that can be applied during exercise. Thus a new weight training glove was designed that aimed to provide protection and grip to areas of the hand in contact with handles and bars whilst simultaneously minimising the material associated with the glove itself. The following PDS (Table 13.2) and corresponding design (Fig. 13.2) was produced from this design brief.

Table 13.2 PDS of Gripmasters Protect only the surface of the hand in contact with the bar Reduce the non-protective material to a minimum Cushioning thickness should exceed current best Grip (coefﬁcient of static friction) should exceed current best % of total area used for stitching should be minimal

© 2007, Woodhead Publishing Limited

Optimise Minimise 3 mm 0.70 5%

Design and materials in ﬁtness equipment Finger loop entry

329

Zoned protection

Minimal stitching High cushioning properties

Minimal material

Nylon-laminated neoprene rubber for increased grip

13.2 Gripmasters and speciﬁed features targeted by the PDS.

13.3.3 Conceptual design Translation of the PDS to a working concept is achieved through concept generation. Divergent thinking processes and techniques can be used to explore many different solutions. Although partial or generalised solutions are often developed ﬁrst, these ideas are later reﬁned to include a more detailed design proposal. Complex design briefs can be subdivided into constituent components to simplify the generation of solutions. For instance, typical divisions of an exercise machine include separate conceptual design processes for the resistance mechanism, movement mechanism and electronics integration. Once a large body of ideas is generated, a systematic method for concept selection is often used. This may include rating and ranking matrices, classiﬁcation trees, low-level prototypes, evaluation of strengths and weaknesses, group decision and intuition. In particular for ﬁtness equipment, the production of low-level prototypes is invaluable for examining the exercise work envelope, product footprint, user-interference and potential issues in design. Design case study: low-level prototyping of the Reebok Deck Low-level prototyping is an easy and extremely useful method for studying shapes and conﬁgurations of a product. Particularly for equipment that is greatly affected by ergonomics of use, prototypes often give invaluable insight to developers at crucial stages of the design process. During initial development stages of the Reebok Deck, multiple full-scale foam models of the support body and risers were created to determine the optimal

© 2007, Woodhead Publishing Limited

330

Materials in sports equipment

conﬁguration and folding geometries of the structure. These iterative concepts helped establish a ﬁnal form that allowed the support risers to be folded under the main body of the platform to enable multiple heights and inclines to be achieved.

13.3.4 Detailed design During the detailed design stage of development, major subsystems and interfaces are deﬁned. Component geometry, materials, and tolerances are also established. Tools for assessing design such as computer-aided design (CAD), ﬁnite element analysis (FEA), ergonomics software and motion analysis are used extensively to critically analyse part geometry, construction and possible failure modes. Liaison with manufacturers early in this stage of the development process is crucial as piece-part production processes, limitations in manufacturing and cost restrictions greatly inﬂuence numerous details of design. Manufacturing issues relating speciﬁcally to ﬁtness products are discussed in Section 13.3.5. Although aesthetic styling of ﬁtness products has not traditionally been a design focus, greater attention to this aspect of design has prevailed in recent years. Since the popularity of a product line generally results in a ﬂood of comparable models (often termed ‘copy cat’ or ‘me-too’ products in the trade) from competitors, careful attention to aesthetics is one way of differentiating one product from another. Styling cues taken from the automotive and electronics industry are often used for ‘inspiration’ to convey a sense of quality and luxury. Aesthetic styling can be applied in all manners from streamlining the basic shape of metal extrusions to detailing the side rail inserts of treadmills. The application of colour detailing to home ﬁtness products has also been popular as consumers take pleasure in the selection of a ‘customised’ piece of equipment. Further integration of the home market has meant that innovative storage solutions for large and bulky equipment are desirable. Consumers increasingly require home ﬁtness equipment to integrate within their living space rather than being isolated to a speciﬁc work out area such as the spare bedroom or garage. Commensurately manufacturers have moved towards more organic forms and less industrial styling to enable high-end equipment to mirror the characteristics valued in contemporary furniture.

13.3.5 Manufacturing In an attempt to achieve low costs and high margins, traditional manufacturing methods for ﬁtness equipment have included metal casting, extrusion, welding, bending, stamping and injection moulding. These

© 2007, Woodhead Publishing Limited

Design and materials in ﬁtness equipment

331

processes are suitable for high-volume production. Since these particular manufacturing methods are so widely used, their capabilities and limitations are well understood. However, deviating from traditional manufacturing methods is not inadvisable as it is likely to ensure product differentiation. Due to the low cost of manufacturing in Asia (mainly China and Taiwan), the majority of ﬁtness industry manufacturing resides in these territories. Working with overseas manufacturers is thus typical for many ﬁtness equipment brands and requires continuous communication along with intermittent visits to the factories themselves to ensure that the processes, materials and quality of product are acceptable. Major ﬁtness equipment brands often employ staff within third-party factories to co-ordinate production activities and to oversee quality control processes. The desire for low cost and absence of brand premium in the home sector also dictates that typically only the most commonly available materials are used in production. Steel box extrusions are commonly used in the framework of most resistance machines. Casings and housings for components are often made from standard polymers such as polypropylene and nylon.

Price points The price points for ﬁtness equipment differ according to intended usage, i.e. commercial versus home and via route to market. At the highest level, specialty ﬁtness dealers sell home equipment in the US$1300–US$3000 range and commercial products in the US$2000–US$10000 range. Full line sporting goods dealers sell home equipment that ranges in price from US$600–US$1200. Department stores, mail-order catalogues and mass merchandisers will often only carry low-end equipment with maximum price points of US$500–US$1000. Manufacturing costs are approximately 25–30% of the retail price, or closer to 15% if selling via costly media channels such as is typical with infomercials.

Traditional outsourced versus innovative in-house manufacture There are some good examples of companies that have elected to keep manufacture in-house and to strategically acquire cutting edge manufacturing facilities to enable progressive product development. Whilst not alone in this regard, the Italian company, Technogym, has a particularly strong record of in-house research and development and high-quality manufacture.

© 2007, Woodhead Publishing Limited

332

Materials in sports equipment

13.4

Using materials and processes to improve design of ﬁtness equipment

Development of novel ﬁtness equipment can be driven by innovation in materials, construction, testing and design. These factors of consideration inﬂuence both the overall form and function of ﬁtness products.

13.4.1 Case study: handheld dumbbell technology Traditional handheld dumbbell technology has stayed relatively constant over the years. Aside from small differences in dumbbell shape and material, the overall form has resulted in a distinct dumbbell for each different desired weight class. For those who are constrained by space and cost but still look to have a relatively wide range of dumbbell weights, there are few options from which to choose. Using innovation in design, Progressive Sports Technologies Ltd designed a set of three stacking dumbbells that can be used individually or in any conﬁguration to create a unique set of weights (see Fig. 13.3). This innovative, yet simple, design allows low-cost metal-casting manufacturing to be used to provide an intuitive product for consumers.

concept 1

concept 2

+ • Recognisable ‘dumbbell’ format • Weights mesh together with detent • All weights held in the hand during exercise. • Intuitive mode of adjustment. • Reduced variation in grip size – • Current proposal cannot acommodate all three weights together.

+ • Soft, ergonomic profile to each weight • Weights slide together with detent or... • Weights could snap together-over-moulded rubber coating providing retention. • All weights held in the hand during exercise • Intuitive mode of operation – • variation in grip size

concept 3

+ • Soft, pebble-form, tactile weights • Weights cradled in the handsecured by the hand strap. • Hand strap pulls through to hold weights in situ-positive detent • All weights held in the hand during exercise. – • Less intuitive mode of operation

13.3 Design concept generation of stackable handheld dumbbells.

© 2007, Woodhead Publishing Limited

Design and materials in ﬁtness equipment

333

13.4 Reebok Vector Bench layered laser-cut construction.

13.4.2 Case study: weights bench design Design can also be facilitated by innovation in construction. In order to fully change the aesthetic of a standard weights bench, laser cutting was used as the method of construction for the Reebok Vector Bench (see Fig. 13.4). Sheets of aluminium were successively cut and assembled to form a Reebok Vector logo shape. The ﬁve layers assemble with surfaces parallel to each other, separated by spacers. The top plane provides a stable surface to mount the fully adjustable bench and surface. This unique method of construction can be used to produce a variety of desired aesthetics in materials including polycarbonate, heavy-gauge plastics, aluminium and steel. The layered construction distinguishes the vector bench from other conventional weights bench designs and enables strong brand promotion with the product form.

13.4.3 Case study: testing techniques for the Reebok Deck Innovative testing techniques can aid in the reﬁnement of design. In the early stages of development for the Reebok Deck, a multi-dimensional exercise platform used to perform both aerobic and resistance workouts, CAD models of the design were imported into ergonomics software SAMMIE CAD. This enabled optimal seating dimensions and back support angles to be determined relative to the expected user populations.

© 2007, Woodhead Publishing Limited

334

Materials in sports equipment

Clearance angles for a variety of exercises were also examined for obstructions in the working environment. The use of ergonomics software allowed small structural design changes to take place prior to the construction of costly prototypes or tooling. Ergonomics software and other such simulation programs are becoming increasingly prevalent for design reﬁnement due to the huge savings in cost versus the relatively small investment in time.

13.4.4 Case study: materials for the Orbitor In many cases, advancements in materials and their associated technologies steer the development of new products. Coupled with the use of highperformance (and often costly) materials is the use of FEA. Standard prototyping materials and methods may not always give sufﬁcient information during the design process. Thus FEA is an extremely useful way of reducing the time and cost of development. Researchers within the Sports Technology Research Group at Loughborough University designed and prototyped a new resistance training device for potential mass manufacture called the Orbitor (Hodgkins, 2004). Most traditional weight stack machines restrict movement to a single plane and use weight stacks as resistance, resulting in uneven inertial loading throughout the course of exercise. To facilitate a more comprehensive movement envelope, the Orbitor was designed to enable multi-axis, free motion resistance training. Free movement and resistance in all directions of use are enabled by a vertical telescoping member, whose base is mounted onto a two-axis orientation device. User interface is achieved by a three-degree-of-freedom handle which focuses all axes origins at the centre of the grip. Adjustable resistance is provided by three pneumatic cylinders that provide controllable resistance by use of a frictional valve unit with the ability to be upgraded to a positioning, negative force movement system. Elements of the two-axis orientation device and handle assembly were subjected to high shear forces due to large impact forces and repetitive use. To combat this problem, a special class of martensitic stainless steels was selected for highly loaded orientation components because of its high yield strength (983 MPa) and tensile strength (1185 MPa). Components in the vertical motion mechanism and handle assembly that were required to be low mass used an aluminium alloy that was treated to give a greater yield strength (500 MPa), tensile strength (570 MPa), and shear strength (350 MPa) then standard aluminium alloys. Finite element analysis was used extensively during development stages of the Orbitor to ensure that the design could manage loads applied by the pneumatics and the user (see Fig. 13.5). Analysis of critical components (two-axis orientation device, vertical motion

© 2007, Woodhead Publishing Limited

Design and materials in ﬁtness equipment Model name: Grientate FEA Study name: Rotate Pix type: Static Narial stress-Plot1 Deformation Scale: 39784B

335

High stress area von Mises (N/mm2) 700.0 641.7 583.3 525.0 466.7 408.3 350.0 291.7 233.3 176.0 116.7 58.3 0.0

Rotation

Bearing restraints

13.5 Orbitor CAD model (left) and FEA of the orientation mechanism (right).

mechanism) facilitated alterations during the design process, ensuring component optimisation.

13.4.5 Case study: test rigs Innovation in the design of ﬁtness equipment can also be led by research rather than design. Often, standard equipment is not sufﬁcient for certain topics of investigation. Since mass manufacture is not ordinarily an issue in this case, many custom-built testing rigs have been created to fulﬁl a speciﬁc need. For example, a six-degrees-of-freedom upper body exercise rig was built by researchers within Loughborough University’s Sports Technology Research Group to investigate a wide variety of multidirectional exercise sequences. This innovative piece of equipment allows independent resistance in each of three main axes of movement. The creation of this test rig allowed new upper body exercises for aerobic ﬁtness to be performed and monitored for experimental testing.

13.5

Future trends

In a market traditionally characterised by lack of product differentiation and correspondingly tight proﬁt margins, there exist promising avenues for future growth. Likely future trends look to increase exercise effectiveness and sustainability through methods of user engagement, health monitoring, exercise guidance and personal assessment.

© 2007, Woodhead Publishing Limited

336

Materials in sports equipment

13.5.1 Gaming and electronics Instances of user engagement have successfully integrated television or gaming technologies into ﬁtness products. This trend looks to continue with the falling cost of electronic components and will likely be used in attempts to encourage children to exercise. These so called ‘distraction technologies’ can help with boredom or lack of motivation in traditional exercise equipment. Further developments are likely to incorporate elements of inertial and vibrational feedback to the user. Audio and visual enhancements of the gaming environment, as well as the ability to compete against different users, may further engage users during exercise. Highly popular gaming systems such as Microsoft’s Xbox®, Sony’s PlayStation®, and the Nintendo WiiTM further blur the line between exercise and entertainment by engaging users in creative and active ways. These types of consoles appeal to a large sector of the population who have close interaction with gaming interfaces. Advancements in audio technology are also likely to continue in the future. While exercise stimulation (through speed of music) and performance feedback are currently available through music player and shoe integration, future avenues may exist where this audio feedback will be available embedded within the ﬁtness machines themselves.

13.5.2 Health monitoring and personalised exercise Audio feedback of exercise performance highlights a greater trend of providing users with detailed information about their bodies and how exercise aids in increasing ﬁtness and overall health. Advances in electronics will allow measures such as weight, body fat percentage and heart rate to be monitored and displayed in real time via remote sensors integrated within the exercise machine. Many users feel that, with more information about the state of their bodies, they will be able to make more informed decisions about how they exercise; however, it is likely that machines will have the ‘intelligence’ to adjust work out proﬁles automatically thus reducing further the requirement for user intervention. Perhaps the culmination of machine and programming technologies lies in the total customisation of an exercise system. Future exercise machines may look to use custom workouts and a user-friendly virtual reality interface to guide customers throughout the course of an entire exercise session. Integrated computer/memory systems built into the machine not only monitor the type and number of exercises completed, but also guide users to exercise with correct body positioning and speeds to ensure an effective workout. Technologies such as these are appealing to novices and frequent

© 2007, Woodhead Publishing Limited

Design and materials in ﬁtness equipment

337

exercisers alike and may be able to accurately replicate a wide range of ‘real’ activities.

13.6

Sources of further information and advice

There are numerous sources of further information available regarding the ﬁtness industry. Key trade and professional bodies include the Sporting Goods Manufacturers Association (SGMA), International Health, Racquet, and Sportsclub Association (IHRSA) and National Strength and Conditioning Association (NSCA). Each year these associations host a number of key worldwide events such as the SGMA Spring Market, International Trade Show for Sports Equipment and Fashion (ISPO), IHRSA’s International Convention and Trade Show and the International Trade Show for Fitness and Wellness (FIBO). These events serve to showcase new products, demonstrate innovative technologies, uncover current trends and provide contacts for companies and distributors. There are a limited number of books that can be consulted speciﬁcally related to ﬁtness equipment design, including Sports and Fitness Equipment Design edited by Ellen Kreighbaum and Mark Smith. On-line book retailers may also provide relevant digital articles for purchase relating to strategy planning, entry markets and target-demographic design of ﬁtness equipment. Additional market and consumer information can be found through SGMA on-line market reports (www.sgma.com) or through specialist market intelligence consultancies such as Mintel Group Limited, Internet Home Alliance and Tekes. Links on websites for larger trade bodies such as IHRSA and SGMA also give up-to-date information regarding upcoming events, current industry news, job opportunities and further points of contact. Information about current products available in the market can be found on individual websites for companies. Websites for Nautilus, Precor, Technogym, Life Fitness, Reebok Fitness Ltd and other major manufacturers of ﬁtness equipment provide direct information regarding product lines, speciﬁcations, comparisons, technology overview and customer support. Patent information regarding speciﬁc mechanisms and technologies can be found on websites for the European Patent Ofﬁce and the US Patent and Trademark Ofﬁce. For in-depth information regarding research studies, physiological data, design methodology, new exercise test methods, comparison studies of ﬁtness equipment, a variety of refereed journals are available. The most prominent of these journals include the British Journal of Sports Medicine, European Journal of Applied Physiology, Journal of Applied Physiology, Journal of Exercise Physiology, Journal of Physiology, Journal of Strength

© 2007, Woodhead Publishing Limited

338

Materials in sports equipment

and Conditioning Research and Medicine and Science in Sports and Exercise.

13.7

Acknowledgements

Much gratitude is extended to the team at Progressive Sports Technologies Ltd and members of the Sports Technology Research Group at Loughborough University for their contributions to this chapter. A special thanks is given to Phil Hodgkins for his work on the Orbitor and Ross Weir for his input regarding the Reebok Deck, Vector Bench, Gripmasters, and stackable dumbbell concepts.

13.8

References

american demographics (2001), Grandpa gets ﬁt: over half of health/ﬁtness club members are 40 or older, American Demographics, 1 November. american sports data (2006), Health and Fitness Business – an interview with Henry Lauer, http://www.americansportsdata.com/pr-leisuretrends.asp, last accessed 18 March, 2007. feedback research services (2003), Fitness products and service markets, 6(5), http://www.feed-back.com/dec03ezine.htm, last accessed 18 March, 2007. hodgkins p (2004), Orbitor: development of a free motion, multi-axis resistance training device, MSc thesis, Loughborough University. kietz b (2005), German sporting goods market: opportunities for U.S. exporters, Washington, U.S. Department of Commerce and Department of State, December. kratzman v a and stamford f (2002), US Fitness Industry Market Overview and Entry Strategies, Helsinki, Tekes National Technology Agency. kreighbaum e and smith m a (1995), Sports and Fitness Equipment Design, Champaign, IL, Human Kinetics. mintel (2003), In-home Fitness, Leisure Intelligence, October 2003. sgma (2005), The State of the Industry: SGMA’s Annual Report on the US. Sporting Goods Market, Washington, Sporting Goods Manufacturers Association.

© 2007, Woodhead Publishing Limited

Materials in Sports Equipment, Volume 2 (Woodhead Publishing in Materials)

Materials in Sports Equipment, Volume 2

Materials in Sports Equipment

Building Decorative Materials (Woodhead Publishing in Materials)

Materials in Sports Equipment (2003)

Thermal Barrier Coatings (Woodhead Publishing in Materials)

Biomedical Composites (Woodhead Publishing in Materials)

MIG Welding Guide (Woodhead Publishing in Materials)

High Temperature Superconductors (Woodhead Publishing in Materials)

Bioactive Glasses: Materials, Properties and Applications (Woodhead Publishing in Materials)

Sustainability of Construction Materials (Woodhead Publishing in Materials)

Sintering of Advanced Materials (Woodhead Publishing in Materials)

Corrosion Control in the Aerospace Industry (Woodhead Publishing in Materials)

Friction in Textile Materials (Woodhead Publishing in Textiles)

Nanolithography and Patterning Techniques in Microelectronics (Woodhead Publishing in Materials)

Welding and Joining of Magnesium Alloys (Woodhead Publishing in Materials)

Corrosion of Magnesium Alloys (Woodhead Publishing in Materials)

Superplasticity and Grain Boundaries in Ultrafine-Grained Materials (Woodhead Publishing in Materials)

Tribology of Natural Fiber Polymer Composites (Woodhead Publishing in Materials)

Hydrometallurgy: Principles and Applications (Woodhead Publishing in Materials)

Corrosion of Magnesium Alloys (Woodhead Publishing in Materials)

Cellular Response to Biomaterials (Woodhead Publishing in Materials)

Biomaterials, Artificial Organs and Tissue Engineering (Woodhead Publishing in Materials)

Mineral Wool: Production and Properties (Woodhead Publishing in Materials)

Electrospinning for Tissue Regeneration (Woodhead Publishing in Materials)

Woodcarving Tools, Materials & Equipment

Advances in marine antifouling coatings and technologies (Woodhead Publishing in Materials)

In situ Characterization of Thin Film Growth (Woodhead Publishing in Materials)

Thermal and moisture transport in fibrous materials (Woodhead Publishing in Textiles)

Materials for Energy Efficiency and Thermal Comfort in Buildings (Woodhead Publishing Series in Energy)

Creep and Fatigue in Polymer Matrix Composites (Woodhead Publishing in Materials)

Materials in Sports Equipment, Volume 2 (Woodhead Publishing in Materials)

Materials in Sports Equipment, Volume 2

Materials in Sports Equipment

Building Decorative Materials (Woodhead Publishing in Materials)

Materials in Sports Equipment (2003)

Thermal Barrier Coatings (Woodhead Publishing in Materials)

Biomedical Composites (Woodhead Publishing in Materials)

MIG Welding Guide (Woodhead Publishing in Materials)

High Temperature Superconductors (Woodhead Publishing in Materials)

Bioactive Glasses: Materials, Properties and Applications (Woodhead Publishing in Materials)

Sustainability of Construction Materials (Woodhead Publishing in Materials)

Sintering of Advanced Materials (Woodhead Publishing in Materials)

Corrosion Control in the Aerospace Industry (Woodhead Publishing in Materials)

Friction in Textile Materials (Woodhead Publishing in Textiles)

Nanolithography and Patterning Techniques in Microelectronics (Woodhead Publishing in Materials)

Welding and Joining of Magnesium Alloys (Woodhead Publishing in Materials)

Corrosion of Magnesium Alloys (Woodhead Publishing in Materials)

Superplasticity and Grain Boundaries in Ultrafine-Grained Materials (Woodhead Publishing in Materials)

Tribology of Natural Fiber Polymer Composites (Woodhead Publishing in Materials)

Hydrometallurgy: Principles and Applications (Woodhead Publishing in Materials)

Corrosion of Magnesium Alloys (Woodhead Publishing in Materials)

Cellular Response to Biomaterials (Woodhead Publishing in Materials)

Biomaterials, Artificial Organs and Tissue Engineering (Woodhead Publishing in Materials)

Mineral Wool: Production and Properties (Woodhead Publishing in Materials)

Electrospinning for Tissue Regeneration (Woodhead Publishing in Materials)

Woodcarving Tools, Materials & Equipment

Advances in marine antifouling coatings and technologies (Woodhead Publishing in Materials)

In situ Characterization of Thin Film Growth (Woodhead Publishing in Materials)

Thermal and moisture transport in fibrous materials (Woodhead Publishing in Textiles)

Materials for Energy Efficiency and Thermal Comfort in Buildings (Woodhead Publishing Series in Energy)

Creep and Fatigue in Polymer Matrix Composites (Woodhead Publishing in Materials)

Recommend Documents