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

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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.


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


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


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


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


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


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


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


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]


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


xii


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]


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]


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: 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



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


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


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


Part I General issues


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


4


(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


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


6


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.


[1.6]


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


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.



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,


10


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.



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


12


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



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.


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.


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


16


(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.



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


18


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.


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

5.5

35–100 24


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

< 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


96


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


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.


C n

98


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


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.


100


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.



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.


102


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.


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.).


104


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).


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


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



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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)


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)

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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.,



Front impacts

Side impacts

Front boss impacts

Rear boss impacts

Rear impacts

Top impacts

4.13 Schematic of NOCSAE drop test sites.


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



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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.


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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



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


114


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

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