Lightweight Ballistic Composites: Military and Law-Enforcement Applications [2 ed.] 0081004060, 9780081004067

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Lightweight Ballistic Composites

Related titles Advanced Fibrous Composite Materials for Ballistic Protection (ISBN 978-1-78242-461-1) Fatigue of Textile Composites (ISBN 978-1-78242-281-5) Protective Clothing: Managing Thermal Stress (ISBN 978-1-78242-032-3)

Woodhead Publishing Series in Composites Science and Engineering: Number 71

Lightweight Ballistic Composites Military and Law-Enforcement Applications

Second Edition

Edited by

Ashok Bhatnagar

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Woodhead Publishing is an imprint of Elsevier The Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH, UK 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, USA The Boulevard, Langford Lane, Kidlington, OX5 1GB, UK Copyright © 2016, 2006 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/ permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Disclaimer All statements, data and other information contained in this book are based upon the research, experience and opinions of the individual contributing authors, who are responsible for the statements and data they have chosen to present. This information is not related to and does not represent the opinion of any of the several employers of the contributing authors and editor, in particular Honeywell International. Although the information is believed to be accurate, it is presented without guarantee or warranty of any kind, express or implied, and does not relieve the user from the responsibility of carrying out its own tests and experiments. The user assumes all risks and liability for use of the information and results obtained. Statements or suggestions concerning the use of materials and processes are made without representation or warranty that any such use is free of patent infringement and are not recommendations to infringe any patents. The user should not assume that all safety measures are indicated herein or that other measures may not be required. 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 ISBN: 978-0-08-100406-7 (print) ISBN: 978-0-08-100425-8 (online) For information on all Woodhead Publishing publications visit our website at https://www.elsevier.com/

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This book is dedicated to all the law-enforcement and military personnel who put their lives at risk on a daily basis to save civilian lives.

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Contents

List of contributors Woodhead Publishing Series in Composites Science and Engineering Preface 1

2

3

High-performance ballistic fibers and tapes T. Tam, A. Bhatnagar 1.1 Introduction to high-performance fibers and tapes 1.2 High-performance ballistic fibers and tapes 1.3 UHMWPE fibers 1.4 Aramid fibers 1.5 UHMWPE tape/ribbon 1.6 Ballistic fiberglass 1.7 High-modulus polypropylene fiber 1.8 Recycling of ballistic fibers and converted products Acknowledgment References Additional information

xiii xv xix 1 1 3 6 15 24 29 32 35 37 38 38

High performance fabrics and 3D materials D.J. Carr, C. Crawford 2.1 Introduction 2.2 Fiber types 2.3 Composite fiber architectures 2.4 Failure mechanisms 2.5 Conclusions Useful sources of further information References

41

Nonwoven and crossplied ballistic materials G.A.T. Webster 3.1 Introduction 3.2 Protective materials, devices, and end-use requirements 3.3 Fiber selection criteria for ballistic-resistant materials 3.4 Variations of fiber forms

55

41 42 43 47 48 49 49

55 62 64 67

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3.5 3.6 3.7 3.8 4

5

6

Filament layup composites Historical uses of nonwoven ballistic-resistant fabrics Methodologies for use of nonwoven ballistic-resistant fabrics Future directions for nonwoven fabric applications References

Ballistic threats: bullets and fragments A. Helliker 4.1 What is the threat? 4.2 Small arms ammunition 4.3 Fragments 4.4 Projectile and target interaction 4.5 Summary Acknowledgments References

76 78 79 84 84 87 87 88 103 108 113 113 113

International ballistic and blast specifications and standards Phil Gotts 5.1 Introduction 5.2 Why are there armor test methods and/or standards? 5.3 General definitions used in test methods and standards 5.4 Threat regimes for personal armor test methods and standards 5.5 Threat regimes for vehicle armor test methods and standards 5.6 Personal armor user communities 5.7 Personal armor law enforcement test methods and standards 5.8 Personal armor military test methods and standards 5.9 Personal armor general purpose test methods and standards 5.10 Vehicle armor user communities 5.11 Vehicle armor civilian test methods and standards 5.12 Vehicle armor military test methods and standards 5.13 General ballistic material test methods and standards 5.14 Approach to use when there are no suitable standards or methods 5.15 Issues with contents of some standards 5.16 The possible future of armor test methods and standards 5.17 Summary Glossary References Annex 5-A: Definitions

115

Lightweight composite materials processing D. Dixit, R. Pal, G. Kapoor, M. Stabenau 6.1 Introduction 6.2 Ballistic fibers 6.3 Quality control of ballistic materials 6.4 Various international ballistic specifications/standards

157

115 115 116 116 117 119 119 126 130 133 133 135 137 140 142 143 144 144 145 146

157 157 167 176

Contents

6.5 6.6 6.7 6.8 6.9 6.10 6.11 7

8

9

ix

Processing of ballistic materials Evaluation of molded articles Transportation and storage of ballistic material Durability of the products in field Recycling and disposal of prepregs Ballistic helmets Handheld riot shields Bibliography

Personal armor E. Lewis, D.J. Carr 7.1 Introduction 7.2 Body armor 7.3 Helmets 7.4 Face and eye protection 7.5 Neck protection 7.6 Pelvic protection 7.7 UK Virtus body armor system 7.8 Future developments Useful sources of further information References

179 206 207 209 210 210 214 214 217 217 217 221 222 223 224 226 226 227 227

Durability of high-performance ballistic composites N. Bhatnagar, N. Asija 8.1 Introduction 8.2 Ballistic materials 8.3 Durability of ballistic fibers and materialsdtest protocols 8.4 Tests for assessing durability of converted productsdtest protocols 8.5 Role of specification on durability 8.6 Effects of processing on durability 8.7 Other ballistic products 8.8 Effects of secondary manufacturing processes: machining, trimming, and finishing 8.9 Perception of ballistic threats on durability 8.10 Effects of transportation and storage of ballistic materials on durability References

231

Vehicle armor R.A. Ash 9.1 Introduction 9.2 Vehicle armor 9.3 Ballistic materials 9.4 Processing composite armor panels

285

231 236 245 250 257 262 272 274 276 278 280

285 287 292 298

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9.5 Threats and ballistic test standards 9.6 Armor design process 9.7 Summary References 10

11

12

13

302 306 308 309

Testing of armor systems J. Cronin, R. Kinsler, J. Allen 10.1 Types of armor 10.2 Types of tests 10.3 Velocity measurements 10.4 Penetration testing 10.5 Helmet system performance evaluations 10.6 Environmental and usage considerations 10.7 Soft armor panels Bibliography

311

Numerical analysis of ballistic composite materials S.D. Rajan 11.1 Introduction 11.2 Major ballistic materials 11.3 Finite element analysis as a design tool 11.4 Finite element modeling of ballistic packages 11.5 Concluding remarks References

327

Design, manufacture, and analysis of ceramic-composite armor L. Bracamonte, R. Loutfy, I.K. Yilmazcoban, S.D. Rajan 12.1 Introduction 12.2 Ceramics as an armor material 12.3 Manufacture of ceramics 12.4 Finite element analysis of a ceramics-based ballistic package 12.5 Concluding remarks References

349

Ceramic-faced molded armor B. Quéfélec, M. Dartois 13.1 Introduction to ceramic-faced lightweight armor 13.2 Types of ceramics 13.3 Shapes of ceramics 13.4 Composite backings 13.5 Fabrication of ceramic-faced armor 13.6 Testing of ceramic-faced armor References

369

311 311 313 314 318 320 325 326

327 327 331 338 345 346

349 351 353 358 365 365

369 369 373 378 382 386 390

Contents

14

Materials, manufacturing, and enablers for future soldier protection J.Q. Zheng, S.M. Walsh 14.1 Introduction 14.2 New directions in head protection 14.3 New material developments in torso and related body armor 14.4 Novel exoskeleton development 14.5 The disruptive potential of robotically deployed materials to enhance soldier protection 14.6 Summary Acknowledgments References

Index

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393 393 395 413 425 429 433 434 434 439

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List of contributors

J. Allen

Intertek, Cortland, NY, United States

R.A. Ash

Honeywell International Inc., United States

N. Asija

Indian Institute of Technology Delhi, New Delhi, India

A. Bhatnagar

Honeywell International Inc., United States

N. Bhatnagar

Indian Institute of Technology Delhi, New Delhi, India

L. Bracamonte

MER Corporation, Tucson, AZ, United States

D.J. Carr Cranfield University at The Defence Academy of the UK, Shrivenham, United Kingdom C. Crawford Cranfield University at The Defence Academy of the UK, Shrivenham, United Kingdom J. Cronin

HP White Laboratory, Inc., Street, MD, United States

M. Dartois D. Dixit

TenCate Advanced Armour France S.A.S.

MKU P Ltd, Kanpur, India

Phil Gotts

Phil Gotts Consulting Ltd, Ipswich, Suffolk, United Kingdom

A. Helliker

Cranfield University, Shrivenham, United Kingdom

G. Kapoor

MKU P Ltd, Kanpur, India

R. Kinsler

HP White Laboratory, Inc., Street, MD, United States

E. Lewis

Defence Equipment and Support, Ministry of Defence, United Kingdom

R. Loutfy

MER Corporation, Tucson, AZ, United States

R. Pal MKU P Ltd, Kanpur, India B. Quéfélec

TenCate Advanced Armour France S.A.S.

S.D. Rajan

Arizona State University, Tempe, AZ, United States

M. Stabenau T. Tam

MKU GmbH, Sittensen, Germany

Honeywell International Inc., United States

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List of contributors

G.A.T. Webster S.M. Walsh

Army Research Laboratory, Adelphi, MD, United States

I.K. Yilmazcoban J.Q. Zheng

Auburn University, Auburn, AL, United States

Sakarya University, Turkey

Program Executive Office e Soldier, US Army

Woodhead Publishing Series in Composites Science and Engineering

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Thermoplastic aromatic polymer composites F. N. Cogswell Design and manufacture of composite structures G. C. Eckold Handbook of polymer composites for engineers Edited by L. C. Hollaway Optimisation of composite structures design A. Miravete Short-fibre polymer composites Edited by S. K. De and J. R. White Flow-induced alignment in composite materials Edited by T. D. Papthanasiou and D. C. Guell Thermoset resins for composites Compiled by Technolex Microstructural characterisation of fibre-reinforced composites Edited by J. Summerscales Composite materials F. L. Matthews and R. D. Rawlings 3-D textile reinforcements in composite materials Edited by A. Miravete Pultrusion for engineers Edited by T. Starr Impact behaviour of fibre-reinforced composite materials and structures Edited by S. R. Reid and G. Zhou Finite element modelling of composite materials and structures F. L. Matthews, G. A. O. Davies, D. Hitchings and C. Soutis Mechanical testing of advanced fibre composites Edited by G. M. Hodgkinson Integrated design and manufacture using fibre-reinforced polymeric composites Edited by M. J. Owen and I. A. Jones Fatigue in composites Edited by B. Harris Green composites Edited by C. Baillie Multi-scale modelling of composite material systems Edited by C. Soutis and P. W. R. Beaumont Lightweight ballistic composites Edited by A. Bhatnagar

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Advanced fibrous composite materials for ballistic protection Edited by X. Chen Lightweight composite structures in transport Edited by J. Njuguna Structural health monitoring (SHM) in aerospace structures Edited by F-G. Yuan Dynamic deformation and fracture in composite materials and structures Edited by V. Silberschmidt Advanced composite materials for aerospace engineering Edited by S. Rana and R. Fangueiro Lightweight ballistic composites: Military and law-enforcement applications, Second edition Edited by A. Bhatnagar

Preface

After service as both a soldier and a police officer, Laurence Tobin spent 30 years as a scientist with the UK Ministry of Defence working exclusively on the research and development of military and police protective clothing, followed by a further 13 years running a consultancy, which he wound up in mid-2015. He is an ex-chairman of a NATO study group on ballistic test methods for personal armor and a Franco-British study group on the evaluation of ballistic-protective textiles. He has been a member of NATO, British, and European committees related to the specifications and evaluation of lightweight armor and the development of future small arms weapons. He was a founding member of the NATO Behind Armour Blunt Trauma group and the founder of the Personal Armour Systems Symposium, which is held every 2 years in a different NATO country. It is not uncommon for the authors of books or articles on body armor to start by briefly describing the history of armor development, but even the most dedicated historian cannot know when the first item of clothing was worn which was specifically intended to provide protection to the wearer against attacks by weapons and missiles. What we can be fairly certain about, though, is that when attackers realized that their weapons were not being so effective because of this protective clothing, they concentrated on making more effective weapons. The body armor wearer then had to think how he could give himself a level of protection against these more powerful weapons. The obvious initial conclusion would have been that he needed more of the material that his current armor was constructed from, so his armor increased in weight. The cycle of improvements in armor followed by improvements in weapons followed by improvements in armor continued until the armor wearer probably came to the conclusion that if his armor was too heavy he could not easily defend himself and he would be an easy target for an attacker who aimed at the unprotected parts of his body. It is still the case now as it was then, that for any particular material, as you use more of it you get more protection, but as you continue to add weight the armor eventually becomes so heavy that it affects your efficiency so badly that you may have been safer without it. If you design it so that it covers as much of the body as possible, then not only are you increasing the weight, but you are starting to restrict your freedom of movement. And is it better to protect all of the body with a particular level of protection or, at the same weight, to protect only the most important parts of the body but with a higher level of protection? So the problem that the armor designer had to solve was how to provide the maximum possible protection at the lowest possible weight. At least

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4000 years after early humans set this problem, the contributors to this book, each in his or her own way, are still working on it. Since the introduction of the mild-steel Adrian helmet by the French in 1915, followed by the British Brodie helmet, which was also initially made from mild steel until the harder, 12% manganese Hadfield steel was adopted in 1916, steel remained the standard material for making military helmets until the 1970s when, following experimental designs, the US PASGT helmet was introduced in 1980 followed by the UK Mk6 helmet in 1986. To the developers of those early steel helmets it would have been like science fiction to talk about the construction of helmets from a few layers of woven textile which would offer much greater protection against ballistic missiles than the equivalent weight of steel. Woven nylon textiles had been used in the manufacture of fragment-protective body armor for years, actually resulting in a nylon composite parachutist’s helmet in the United Kingdom, but the more advanced aromatic polyamides, which had started appearing in the late 1960s, soon became the material of choice. Together with the resin that bound them together, these materials were the first composites to be used in what we could loosely call “modern” ballistic-protective clothing. Composite panels using almost exactly the same construction process are now used not just in personal armor systems but in vehicles, aircraft, and naval vessels. Most scientists and engineers who have worked in this area of research and development have at some time found it amusing to see photographs of British soldiers testing the level of protection provided by those early steel helmets against nonballistic impact by running headfirst into a brick wall. We now test helmets against that type of impact by striking instrumented helmeted head forms under carefully controlled conditions, under which the impact can be examined using high-speed photography, the transmitted force can be measured by an accelerometer, and even, as described in a later chapter, the extent of back-face deformation can be measured. The contributors to this book explain how much further we have advanced, not just in the evaluation of helmets, but in all areas of lightweight armor construction and particularly in the use of composite materials. At about the same time that textile-based composites were being developed for helmet production, a greater use of those same textiles in body armor became a major area of research. To the surprise of most members of the general public who know little of military combat situations, it is the case that most military casualties in most combat scenarios are caused by fragmenting devices, not bullets. These relatively light and flexible materials, nylon first again and then aramids, provided excellent levels of protection against fragments, but unfortunately, if you do happen to be hit by a bullet, there is a much higher probability that it will kill you rather than simply to make you a casualty. Textiles alone are not enough to provide protection against the more powerful so-called “high-velocity” bullets. It is probably worth keeping in mind while you are reading these valuable contributions to current research that velocity is only one of the factors that have to be considered when calculating the potential danger from any particular bullet design, but the expression “high velocity” (HV) is a term which is generally, if misleadingly, used and understood by researchers to describe the more powerful small arms ammunition which is usually, but not always, fired from a rifle. To a limited extent, the relatively light fiber-based materials and similar

Preface

xxi

materials which have been derived from them do offer protection against the bullets which traditionally have been more likely to be used against a police officer than a soldier, but defense against HV bullets, which are now seen to be a threat to both soldiers and police, currently requires the use of a hard surface. The most effective hard plates have tended to be constructed from ceramics, which distort most types of bullet on impact, and absorb energy when that impact causes the plate to break up. When they break, the ceramic plates tend to produce their own potentially lethal fragments so they also need a backing plate constructed from a different material, usually a laminated composite similar to those which are used in helmet construction, which tends to keep most of the broken pieces in situ. As the rear face of the plate moves backward toward the wearer, the composite delaminates and the process of delamination absorbs energy and reduces the spread of any fragments that manage to get through. The composite also makes a contribution toward the attenuation of the transmitted force, which may cause trauma to the wearer even when the projectile is stopped by the armor. While there is no evidence that serious injury has ever been caused in this way by a nonpenetrating projectile, the potential for such injuries is obvious and is partly illustrated in this book by showing the significant influence of material architecture on back-face deformation and the level of ballistic protection. In a later chapter which is entirely about the armor which is used in vehicles, aircraft, and naval vessels, you will see this same principle of using composite spall liners, sometimes known as spall shields, to focus the spray of penetrating fragments which are produced by the initial strike, thereby greatly reducing the risk of injury to the occupants. Helmet wearers though, and particularly the occupants of armored vehicles, also suffer low-level nonballistic impacts to the head, and it is arguable whether delamination of a composite helmet under low-level impact is necessarily a good thing if that delamination produces an area of relative weakness in relation to protection from ballistic projectiles. This too is discussed in later chapters, along with the research which is being conducted into both new types of ceramics and novel laminated composites. The various permutations of textile, ceramic, and laminates that can be used in armor construction demonstrate how a combination of materials or a composite material can be more effective than any one component on its own, a point which is made by the authors of following chapters too. A problem that has always existed and to which this book may at least provide a partial solution is how to describe to somebody outside this field of research and development just how effective any particular armor design is likely to be. “Bulletproof” is a meaningless description unless qualifying remarks are added, and nothing can be guaranteed to give 100% protection against any possible threat. It is possible to use numerical modeling techniques to simulate combat situations and to estimate the reduction in casualties by the use of different designs of armor, but this all appears fairly abstract to the nonexpert and it has to be admitted even by specialists in the field of combat simulation that they can provide only rough estimates. You will find described here methods of predicting the likely behavior of materials using numerical analysis, but at some stage actual ballistic testing is always necessary. But how, exactly? Intuitively it may seem reasonable just to shoot at a sample of the armor and see whether the projectile completely penetrates it. But would it be as effective

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against a different projectile? Would it have been as effective against a second shot even using an identical projectile? How far away should the shooter be, and how do you evaluate the effectiveness of armor against fragments, no two of which are identical? After having considered all that, what if blast is likely to be the major threat? How do you measure the likely level of protection against that? A considerable amount of research and much international collaboration have gone into devising commonly accepted test methods for both personal armor and vehicles and these too are described in these pages. I started these notes by talking about the introduction of steel helmets in 1915 and now, 100 years later, helmets manufactured from composites are used throughout the world, and research into how to improve those composites through either manufacturing techniques or the use of novel materials continues. Largely because of the modern equipment that is worn on or near the head and other equipment that the modern soldier carries, you will read that some consider that the way forward is not to continue thinking about helmets in isolation but as part of a soldier protection system. You will learn about the sophisticated methods of, for example, calculating impact velocities during testing, the techniques which are used to reduce possible behind-armor injuries, and even how to judge whether complete penetration of the target material occurred. You will see some of the remarkable similarities between the ways that composites are used both in relatively lightweight body armor and in heavy vehicles. You will learn of the studies that are being conducted into the durability of composites and the end items in which they are used. Although fiber-based materials are still important, they are not always necessarily still used in the form of woven textiles, and in the following pages you will read not only of other versions and applications of the materials that I have briefly described but also of alternatives like ultrahigh-molecular-weight polyethylene and some of the proposed protective materials of the future. Having personally spent nearly half of those 100 years involved in the research and development of ballistic-protective materials and the end items in which they are used, I continue to be impressed by the amount of effort which is still expended with the aim of protecting the members of our armed services and our police officers against ballistic attack and the effects of blast. If my brief summary has been enough to encourage you to learn more about the construction, use, and evaluation of lightweight ballistic composites, then all this and more is described in the following pages by some of the leading researchers and developers of lightweight composites in the world in the year 2016. Laurence Tobin

High-performance ballistic fibers and tapes

1

T. Tam, A. Bhatnagar Honeywell International Inc., United States

1.1

Introduction to high-performance fibers and tapes

High-performance ballistic fibers and ballistic tapes are engineered for lightweight ballistic fabrics, composites, and other industrial applications. These are generally used for niche life-saving products such as flexible body armor, molded breastplates, and molded ballistic helmets and panels for armoring helicopters, military cargo planes, the hulls of navy ships, high-speed coast guard boats, and military ground vehicles. Some of the industrial applications of high-strength fibers and tapes include cut-resistant gloves, premium fishing lines, large fishing nets, ropes, sail cloth, and a host of other applications.

1.1.1

Requirements for high-performance fibers and tapes

To achieve high-performance fibers and tapes with exceptional tenacity and modulus properties, there are at least three necessary requirements: 1. The molecule must be highly oriented in the fiber axis direction. 2. The molecular weight or the molecular chain length must be very high. 3. The fiber must be highly crystalline with few defects.

There are generally two approaches in manufacturing high-performance fibers to meet the above criteria. One can start with a highly oriented chemical rigid-chain, rod-like polymer (Fig. 1.1) such as aramid (lyotropic) or liquid crystal (thermal tropic). The relatively low-molecular-weight liquid crystal rigid-chain polymer is spun into fiber, and the resulting fiber is “solid-state polymerized” to a high molecular weight with drawing and annealing processes. The spinning of aramid fibers is an example of this approach. On the other hand, one can start with an ultrahigh-molecular-weight, flexible, long-chain, randomly coiled polymer like ultrahigh-molecular-weight polyethylene (UHMWPE) (Fig. 1.2). Since the ultrahigh-molecular-weight polymer cannot be melt-spun (the polymer will decompose before it will flow at the melting temperature), the polymer is dissolved in a solvent to form a dilute solution that is then spun into filaments. In this dilute solution, the ultrahigh-molecular-weight polymeric chain is “uncoiled” and the spun filaments are subsequently formed into a network called a gel. By this “gel-spinning” method, a long-chain molecule with a loosely connected network Lightweight Ballistic Composites. http://dx.doi.org/10.1016/B978-0-08-100406-7.00001-5 Copyright © 2016 Elsevier Ltd. All rights reserved.

2

Lightweight Ballistic Composites

Figure 1.1 Random rods of polymers (Bhatnagar, 2006).

Figure 1.2 Random coils of polymers (Bhatnagar, 2006).

xerogel fiber can be made. The xerogel fiber can be drawn into a highly oriented, highly crystalline, high-performance fiber via specially developed drawing techniques. High-performance UHMWPE fibers like Spectra® or Dyneema® fibers are examples of these processes.

1.1.2

Manufacturing of high-performance fibers

In general, high-performance fiber manufacturing requires unique, relatively high-cost processes such as the gel-spinning process for UHMWPE fibers. The gel-spinning process involves dissolving the polymer in a first solvent to “disentangle” the UHMWPE polymer into a dilute solution (eg, 10% solid). The dilute polymer solution is spun with a melt-spinning-type process, forming a solvent-containing gel fiber upon quenching and optionally extracting the first solvent with a second solvent, followed by drying or evaporating the second solvent from the solvent-rich fiber to form a solid fiber. The solid fiber is then drawn at least once or in several steps to develop its high-strength, high-modulus, and highly oriented structure. Fabricating aramid fibers also require a solvent-based process to dissolve the “rigid” aromatic polyamide polymer chain followed by a “dry-jet” wet spinning process. The high-temperature melt-spinning of the liquid crystal polymer requires an annealing and drawing process to develop its molecular weight for strength, which increases the manufacturing cost compared with the conventional melt-spinning processes of nylon, polyester, polypropylene, etc.

High-performance ballistic fibers and tapes

3

Owing to the high cost and solvent-recovery steps of the gel-spinning process, an alternative process to make high-performance UHMWPE tape (fiber) was developed using a compression and sintering process followed by slitting and drawing to develop its strength. However, this compressed/sintered UHMWPE tape/fiber has significantly lower tenacity (about 50%) than its gel-spun counterpart but a reasonable modulus.

1.2

High-performance ballistic fibers and tapes

The high-performance ballistic fibers and tapes are different from high-performance structural fibers, such as glass and carbon fibers, in many aspects. For applications in which both ballistic performance and structural performance are required, a compromise is usually achieved. This chapter will focus on high-performance fibers and tapes which are used for ballistic applications only.

1.2.1

UHMWPE fibers

The UHMWPE fiber is a type of polyolefin fiber. The fibers are made up of extremely long chains of polyethylene, which are aligned in the same direction. Each chain is bonded to the other with many van der Waals bonds. This provides the superior physical properties attractive for a number of military and industrial applications. The UHMWPE fiber polymer chains can attain an orientation greater than 95% and a level of crystallinity of up to 85%. The weak bonding between olefin molecules allows local thermal excitations to disrupt the crystalline structure and therefore UHMWPE fibers have lower heat resistance than other high-strength fibers. The melting point of UHMWPE fibers is around 144e152
C and, generally, UHMWPE fibers are not used at temperatures exceeding 80e100
C for long periods of time. However, the UHMWPE fibers maintain performance at below 50
C. Owing to the molecular structure of UHMWPE fibers, they exhibit surface and chemical properties that are rare in high-performance polymers and do not absorb water readily. For the same reason, skin does not interact with it strongly, making the UHMWPE fiber surface feel slippery. The UHMWPE fibers are resistant to water, moisture, most chemicals, ultraviolet (UV) radiation, and microorganisms. The density of gel-spun UHMWPE fibers is 0.97 g/cm3.

1.2.2

Aramid fibers

Aramid fibers are human-made fibers having molecules that are characterized by relatively rigid polymer chains. These molecules are linked by strong hydrogen bonds that transfer mechanical load very efficiently, making it possible to use chains of relatively low molecular weight with much higher tenacity and elastic modulus.

4

Lightweight Ballistic Composites

The aramid fibers have a high degree of orientation, similar to the UHMWPE fibers. The fibers are known for high strength, good impact and ballistic properties, low flammability, no melting point, and good resistance to chemicals and abrasion. The density of aramid fibers varies from 1.44 to 1.46 g/cm3.

1.2.3

UHMWPE tapes/ribbons

UHMWPE thin tapes and ribbons are made by solid-state extrusion of special-grade low-entangled UHMWPE polymers. The molecular structure after solid-state extrusion and drawing is not perfectly aligned as achieved by the gel-spinning process. This results in lower performance compared to UHMWPE tapes. UHMWPE tapes and ribbons exhibit low shrinkage, high abrasion, high strength and modulus, and excellent chemical resistance. The main features of UHMWPE tapes and ribbons are high dimensional stability, low creep resistance, translation efficiency (polymer molecular weight vs tape molecular weight), and ease of surface modification for higher adhesion, and increased UV stability. The density of UHMWPE tapes and ribbon is 0.97 g/cm3.

1.2.4

Ballistic fiberglass

Glass fibers (commonly referred to as fiberglass) are made of various types of crushed glass depending upon the fiberglass use. The crushed glass contains silica with varying amounts of oxides of calcium, magnesium, and sometimes boron. For fiberglass applications, care is taken during manufacturing to achieve a very low level of defects. Fiberglass filaments are manufactured by a pultrusion process. In the manufacturing process, large furnaces gradually melt the sand, limestone, kaolin clay, fluorspar, colemanite, dolomite, and other minerals into liquid form. The liquid is then extruded through platinum bushings, which are bundles of very small orifices (typically 5e25 mm in diameter for E-glass and 9 mm for S-glass). Just after the pultrusion process, when the filaments become solid, a sizing (coating) with a chemical solution is applied through a spray process. The coated and solid fibers are then combined into bundles to provide a roving. The two most common types of glass fiber used in ballistic applications are E-glass, which is aluminoborosilicate glass with less than 1% w/w alkali oxides, mainly used for glass-reinforced plastics, and S-glass (aluminosilicate glass without CaO but with high MgO content), with high tensile strength. The density of E-glass is 2.58 g/m3 and S-glass is 2.46 g/m3.

1.2.5

Carbon fibers

The raw material for manufacturing carbon fiber, also referred as graphite fiber or CF, is called the precursor. About 90% of carbon fibers manufactured are made from polyacrylonitrile. The process involves melt extrusion followed by pyrolysis. The balance

High-performance ballistic fibers and tapes

5

is made from rayon or petroleum pitch. During the fiber manufacturing process, a variety of gases and liquids are used. Some of these materials react with the fiber and other materials are designed not to react or to prevent certain reactions with the fiber. The carbon fibers are about 5e10 mm in diameter and composed mostly of atoms. Carbon fibers are not ballistic fibers because carbon fibers and composites reinforced with carbon fibers are brittle in nature. However, in certain applications, single or multiple layers of carbon fabric are used to provide structural integrity, repeated compression improvements, and other benefits. The density of carbon fibers is 1.88 g/m3.

1.2.6

Other fibers

There are a number of other fibers which can be combined with the high-performance ballistic fibers to meet specific performance needs or higher specific values.

1.2.6.1

High-modulus polypropylene fibers

High-modulus polypropylene (HMPP) fibers are manufactured using a unique hot melt-spinning process designed to crystallize filaments while the polymer is in a highly relaxed, highly disoriented state. This permits high draw ratios and efficient chain orientation to be achieved in the subsequent drawing operation. The drawn HMPP fibers have high levels of crystallinity and orientation, but the density of the HMPP fibers is about 0.67 g/cm3, which is well below the density of industrial polypropylene in the amorphous state (0.85 g/cm3).

1.2.6.2

Ceramic fibers for ballistics

Ceramic fibers were designed and developed for applications in which the composite matrix/resin temperature can go, for example, as high as 1000
C in a corrosive and oxidizing environment. The ceramic fibers are made from precursor fibers or a very thin tungsten-core wire. Materials like boron and silicon carbide vapors are deposited onto a red-hot precursor moving very slowly. Some of the ceramic fibers are large-diameter monofilaments. The ceramic fibers show high-strength and high-modulus properties in both tension and compression applications. In compression, unidirectional boron composite stresse strain curves are linear to failure (400,000 psi failing stress) and exhibit a modulus of 30 million psi. Because ceramic fibers have large diameters, prepreg tapes formed from the fibers are usually unidirectional only. The ceramic fibers are uniquely suited to handle the high-temperature consolidation conditions of titanium and ceramic matrix composites. Only limited quantities of ceramic fibers are manufactured annually but production can be rapidly expanded to meet new demands.

6

Lightweight Ballistic Composites

For ballistic applications, including reinforcing ceramic tiles, prepregs are crossplied and cured using an autoclave. There are a number of other fibers which can be combined with the highperformance ballistic fibers to meet specific performance needs or higher values.

UHMWPE fibers (Prevorsek, 1996)

1.3 1.3.1

Chemical structure and morphology of UHMWPE fibers

The chemical structure of polyethylene is the simplest repeating molecular unit of CH2 as shown in the schematic below: ðCH2  CH2 Þ UHMWPE generally refers to molecular weights higher than 1 million (8 intrinsic viscosity (IV)) to 5e6 million (30 IV). Depending on the polymerization technique, the structure or the morphology of the UHMWPE polymer may have different features. The polymer morphology and structure have a great impact on the gel-spinning processes, the ultimate fiber morphology, and the final physical properties of the UHMWPE fibers. For example, some UHMWPE polymers have different particle sizes and shapes (Fig. 1.3, cauliflower) as viewed by scanning electron microscopy (Figs. 1.3 and 1.4). When magnified, the individual UHMWPE polymer powder particles show that there is a fibril structure between the “gaps” within the particles themselves. These fibrils are speculated to be highly oriented structures within the polymer particle which may have an elevated melting point in comparison with the rest of the bulk particles.

#2a 5.0 kV x100 100 µm

Figure 1.3 Particle sizes and features of individual UHMWPE polymer particles.

High-performance ballistic fibers and tapes

7

#1b 5.0 kV x2000 5 µm

Figure 1.4 Fibril morphology between the UHMWPE polymer particles.

The particle size, particle size distribution, and morphology have great impact on both fiber processing and the properties of the fibers. Some UHMWPE polymers show different features compared to others. For example, Figs. 1.5 and 1.6 show more uniform particle size and particle size distribution. There is no fibril observed in the gaps within the particles. Naturally, different processing techniques will be required to maximize the potential fiber properties from other polymer types.

#3a 5.0 kV x100 100 µm

Figure 1.5 Uniform UHMWPE particle size and particle distribution.

8

Lightweight Ballistic Composites

#2d 5.0 kV x2000 5 µm

Figure 1.6 No fibril structure within UHMWPE polymer particles.

1.3.2

Gel-spinning process

With the extremely high molecular weight of UHMWPE polymers, the UHMWPE polymers cannot be melt-spun like convention nylon or Polyethylene terephthalate (PET). The UHMWPE polymer will degrade before it can flow. As a result, the gel-spinning process has been developed to handle the UHMWPE polymer (Fig. 1.7). There are two general types of gel-spinning processes: one-solvent systems and two-solvent systems. In a one-solvent gel-spinning process, a solvent such as decalin is used to disentangle the UHMWPE polymer to form a solution having a polymer concentration up to about 15%. The polymer solution will behave like nylon or PET melts at

Suspension UHMWPE

Continuous extrusion/solutions

Metering pump

Spinneret

Figure 1.7 UHMWPE polymer fiber gel-spinning process (Bhatnagar, 2006; Van Dingenen, 2001).

High-performance ballistic fibers and tapes

9

elevated temperatures and can be spun through a spinneret using conventional melt-spinning equipment. After the fiber solution exits the spinneret, it is passed through an evaporation chamber in which the solvent is flashed off to form a gel fiber with a limited amount of solvent remaining in the “solid precursor fiber.” This solid precursor fiber (tenacity about 20 g/denier) can then be drawn in a drawing apparatus wherein residual solvent may be evaporated. During the drawing process, the polymer molecules are aligned, enhancing the tensile strength of the fiber. In an alternate version of this process, the solution fiber is extruded through the spinneret, passed through an air gap, and then quenched in liquid bath to form the gel fiber. The gel fiber then will be drawn in a heated oven in which the solvent is evaporated and the polymer molecules are oriented to develop the high-strength high-modulus (stiffness) fibers. In a two-solvent system, a low-molecular-weight solvent, such as mineral oil, wax, or paraffin wax, is used as the first solvent to disentangle the UHMWPE molecules to make a solution having a polymer concentration up to about 15%. At elevated temperatures, the solution can be melt-spun with conventional melt-spinning equipment. After extrusion, the solution fiber is quenched in a liquid bath to form a gel fiber, optionally stretched, and then the spinning solvent is extracted with a second low-flash-point solvent. During the solvent extraction step, the low-molecular-weight solvent (eg, mineral oil) is replaced with a second solvent. The yarn with the second solvent is then dried and optionally stretched to form a “solid fiber.” The solid fiber is then drawn in different stages with a different drawing apparatus to maximize the fiber tensile properties. The high cost of the gel-spun processes (Fig. 1.8) can be attributed to:

11 12

10 A

13

38

15 14

22

D

16

16

25

20 28

19

18

24 23

27

E B 45 C

33 32

47

37

31 30

41

40 52

56

F

58

63 61

54 55

52

68

65

62

Figure 1.8 Schematic of gel-spinning process (Bhatnagar, 2006).

66

72

10

Lightweight Ballistic Composites

1. The concentration of the polymer in solution is low. For example, assuming a 10% concentration, one has to process 100 lb of solution material to get 10 lb of polymer (fiber). 2. The solvent used during the gel-spinning process must be recovered. Assuming a 10% concentration, one must process 90 lb of solvent to get 10 lb of fiber.

As a result, a less expensive way of making UHMWPE fibers using a nonsolvent process has been developed. This disruptive technology will be discussed later in the UHMWPE tape session.

1.3.3

Morphology of UHMWPE fibers

There are several steps during the gel-spinning process leading to the final UHMWPE fiber morphology. In the solution, the UHMWPE molecules become disentangled. The solution is then spun through a spinneret just like in a conventional melt-spinning process. The spun solution is then quenched, forming a loosely connected network called a gel. After quenching or cooling of the solution into gel fibers, the loosely entangled molecules of the gel fibers can be drawn at a very high draw ratio. Fig. 1.9 shows various stages from spinning of the solution into gel fibers to drawing into high-performance fibers. During the extraction or evaporation step of the solvent, the gel fiber could be drawn further. Like most high-performance fibers, the UHMWPE fiber contains microscopic and macroscopic fiber morphology. A scanning electron micrograph (SEM) of Spectra® fiber is shown in Photo 1.1. The SEM shows regular micro- and macrostructures (see Fig. 1.10). The longitudinal structure of the fibrils consists of microfibrils having a proposed structure as shown in Fig. 1.11, in which nearly perfect crystals are covalently linked through a relatively small amorphous domain. This microfibrillated structure is far from the perfect uniaxial fiber structure in Fig. 1.12 and thus the strength of the UHMWPE fiber, while 15 times stronger than steel, is still far from the theoretical strength of the covalent CeC bonds. It is speculated that an increase in the number of “extended-chain” molecules that span the amorphous domain would increase both strength and modulus. The potential is certainly there to further advance the properties of the UHMWPE fibers. Fig. 1.13 represents a proposed model for the macrofibrils. Because amorphous matter also exists between the microfibrils, the structure appears to be a composite of near perfectly oriented crystalline microfibrils embedded in an amorphous matrix. However, there are extended-chain molecules that can bridge through several layers of the “amorphous” region. It is speculated that the more of this type of “bridging” molecule or, as called by a new term, extended-chain tie molecule, the stronger and more dimensionally stable the UHMWPE fiber will be.

High-performance ballistic fibers and tapes

11

(a)

(b)

Breaking of clusters

Discontinuity

Newly formed fibril

Constriction

(c)

T = 100–133ºC Ea = 50 kJ/mol

T = 133–143ºC Ea = 150 kJ/mol

T = 143–150ºC Ea = 300–600 kJ/mol

Figure 1.9 Morphology of UHMWPE during various stages of production (Bhatnagar, 2006).

12

Lightweight Ballistic Composites

Photo 1.1 Microfibration of UHMWPE fibers. Spectra PET

Fiber

Kevlar

Kevlar

Macrofibrils 100–150 nm

Microfibril 6–10 nm Spectra Extended molecules 0.5–1.0 nm

Figure 1.10 Micro- and macrofibrillar structure of PET, aramid, and UHMWPE fibers (Bhatnagar, 2006).

High-performance ballistic fibers and tapes

13

To scale

Figure 1.11 Proposed longitudinal structure of Spectra® microfibrils (Bhatnagar, 2006).

Reduced scale

Figure 1.12 Perfect uniaxial fiber structure assumed in the calculations of theoretical strength (Bhatnagar, 2006). Interfibrillar amorphous phase

Crystallites

Intrafibrillar amorphous phase

Figure 1.13 Model showing crystallites and amorphous phase (Bhatnagar, 2006).

1.3.4

Physical properties of UHMWPE fibers

The UHMWPE fiber properties are listed in Table 1.1. As the gel-spinning and drawing technology evolves with time, fiber properties improve to dovetail different end uses. As a result, there are different grades of commercial UHMWPE fibers. In short, the new-generation product tends to be in lower denier per filament, with higher tenacity and higher modulus (Fig. 1.14).

14

Lightweight Ballistic Composites

Properties of UHMWPE fibers (United States Patent US 8,361,366, 2013)

Table 1.1

Highly drawn, high-performance fibers

Yarn property

a

Tensile strength, g/denier (GPa)

37.5e70.0a (3.21e5.99)a

Initial modulus, g/denier (GPa)

1320e2000a (113e171)a

Density, g/cm3

0.97

Estimated. All values are exemplary.

New spectra 40

Spectra 2000

Tenacity (g/d)

Spectra 1000 30

Spectra 900 ‘S’ glass

20

K - 129 aramid

K - 29 aramid

HT graphite

‘E’ glass

HM graphite

10 Steel 0 0

50

100 150 Tensile modulus (g/d)

200

250

Figure 1.14 Tensile strength and tensile modulus of high-performance fibers.

1.3.5

Ballistic application of UHMWPE fibers

UHMWPE fiber-based woven and unidirectional (UD) crossplied materials have been developed for soft, hard, and vehicle armor and a host of other lightweight composite applications. Soft ballistic vest materials are designed particularly for use in flexible vests for law enforcement and military personnel. The range of materials provides the highest ballistic protection against handgun bullets and fragments. Hard ballistic UHMWPE materials are available for molded ballistic inserts and helmets to protect against both handgun and rifle bullets and fragments.

High-performance ballistic fibers and tapes

15

UHMWPE has also been used in numerous vehicles and body armor products, including vests, helmets, and inserts, by a rapidly growing number of end users since the early 1990s.

1.4

Aramid fibers

Aramid fibers, like nylon fibers, are polyamides derived from aromatic acids and amines. Figs. 1.15 and 1.16 illustrate nylon 6 and nylon 6,6 polymers, which have flexible chains between the amide groups. Figs. 1.17 and 1.18 illustrate meta-aramid (Nomex®) and para-aramid (Kelvar®) polymers, which have aromatic chains between the amide groups that give these fibers their unique properties. Because of the stability of the aromatic rings and the added strength of the amide linkages, due to conjugation with the aromatic structures, aramids exhibit higher tensile strength and thermal resistance than the aliphatic polyamides (nylons). The para-aramids (trade name Kevlar® and Twaron®) based on terephthalic acid and p-phenylene diamine (PPD-T), or p-aminobenzoic acid, exhibit higher strength and thermal-resistance properties than those with the linkages in the meta positions on the benzene rings (trade name O O NH

H 2O

*

N

n

*

Figure 1.15 Structure of nylon 6. O

O HO

O–

OH O

O– O

H3 N +

H 2N

NH3+

NH2 Heat & vacuum O *

H N N H

O

Figure 1.16 Structure of nylon 6,6.

NH

Figure 1.17 Nomex® structure.

NH

CO

CO

n

*

16

Lightweight Ballistic Composites

NH

NH

CO

CO

Figure 1.18 Structure of aramid fiber.

Nomex®). The greater degree of conjugation and more linear geometry of the para linkages, combined with the greater chain orientation derived from this linearity, are primarily responsible for the increased strength. The high impact resistance of the para-aramids makes them popular for first-generation “bullet-resistant” body armor. Aramid fibers can be chopped into staple form to make felt for applications such as chain saw-protective garments, or they may be blended with other fibers for other end uses. Aramid fiber is lyotropic. It is solution-spun and it melts at a lower temperature than a thermotropic liquid crystal fiber.

1.4.1

Dry-jet wet aramid fiber spinning

The aramid solution is spun by a process called dry-jet wet spinning (Fig. 1.19). In this process, an anisotropic solution of PPD-T is extruded through an air gap into a coagulation bath as shown in Fig. 1.19. The resulting yarn after coagulation is washed and dried. Spin dope Spinneret

Transfer line Spinning block Air gap

Container Filaments

Coagulating liquid

Spin tube

Tube

P

Pump

Rotating bobbin Guide O

Container

Figure 1.19 Schematic diagram of the dry-jet wet spinning process for aramid fibers (Bhatnagar, 2006).

High-performance ballistic fibers and tapes

17

Spinneret

Orientation

Partial deorientation Air gap

Reorientation

Quench water bath

Figure 1.20 Orientation through the capillary die: elongation and shear flow (Bhatnagar, 2006).

The keys to the dry-jet wet spinning method to orient the anisotropic molecule are shear orientation and elongation flow through the spinneret capillaries, as is represented graphically in Fig. 1.20. In addition, the “relaxation” of the molecule after exiting the capillary is kept at a minimum by filament tension or attenuation in the air gap and through the coagulation bath as the filament is precipitated into the highly oriented crystalline fiber. This fiber is also heat treated under tension to increase its modulus.

1.4.2

Aramid fiber structure and morphology

Aramid fibers contain several levels of microscopic and macroscopic morphology. A brief discussion of each is given below using individual fibers as a starting point.

1.4.3

Skin core fibril structure

When aramid fiber is subjected to tensile testing, its typical fracture mode is generally a fibrillated-type failure represented in the following figures. This fracture mode represents a highly ordered lateral fiber structure. The proposed failure mode is shown in Fig. 1.21 with a skin core structure as in Figs. 1.22 and 1.23.

18

Lightweight Ballistic Composites

Figure 1.21 Failure mode of aramid fiber (Bhatnagar, 2006).

1.4.4

Fiber fibrillar structure

Aramid fiber fibrillates easily upon abrasion, especially in the direction perpendicular to the fiber axis. In fact, almost all highly oriented fibers like UHMWPE (such as Spectra® fibers) are easily fibrillated. Aramid fibers are easily fibrillated because the macromolecules are held together only by weak van der Waals forces and/or weak hydrogen bonding. Fig. 1.24 is a proposed model of the fibrillar structures for most of the highly oriented performance fibers. The individual fibrils are the load-bearing elements for the fiber, whereas the tie molecule is the load-bearing element for the conventional fibers. The width of the fibrils is about 600 nm and they are up to several centimeters long. Drilling a layer down on the fibril structure, each “column” of Fig. 1.24 is called a fibril. On each of the fibrils, the straight line represents a PPD-T molecular chain. In most of the chain ends, bends are contained in an alternating “defect” or amorphous layer. These defects or amorphous layers are the weak links in the fiber structure.

High-performance ballistic fibers and tapes

19

Crack propagation path Core

Fiber axis

Skin

Surface Core

Skin

Figures 1.22 and 1.23 Aramid fracture morphology showing long tails fracture mode (Bhatnagar, 2006).

Fibril Ordered lamella Detect zone

Fiber axis

Tie point

° 6000 A

Figure 1.24 Fibrillar structure model of aramid fiber (Bhatnagar, 2006).

20

Lightweight Ballistic Composites

However, some of the PPD-T chain can be oriented and extended to bridge several amorphous or defect layers. This unique “extended-chain tie molecule” should give satisfactory fiber strength.

1.4.5

Pleat structure

Aramid fiber has a unique feature when observed under a cross-polarized microscope light field, featuring transverse bands (Fig. 1.25). However, these transverse bands are diminished when the filament is under tension (Fig. 1.26). This leads to the hypothesis that aramid fiber has a pleated structure (Fig. 1.27). The occurrence of a pleat sheet structure in aramids is not well understood. To explain the formation of the pleated structure, it has been hypothesized that during the coagulation of the aramid fiber the skin is first formed and is subjected to attenuation stress on a spun filament. This allows the “core” of the fiber to relax and form pleats at a uniform periodicity. The formation of the pleat structure gives the fiber an inherent elongation or elasticity. That may be the reason that, when aramid fiber is under stress, the transverse bands diminish as observed under the microscope.

Figure 1.25 Cross-polarized microscope light field featuring transverse bands.

High-performance ballistic fibers and tapes

Figure 1.26 Diminishing transverse bands under stress.

Figure 1.27 Pleat structure model of aramid fiber (Bhatnagar, 2006).

21

22

1.4.6

Lightweight Ballistic Composites

Crystalline structure

Aramid fiber has a highly crystalline, highly ordered molecular structure. Wide-angle X-ray diffraction (Fig. 1.28) shows no amorphous halo indicating a highly crystalline fiber. There is a pair of sharp rings in the equatorial scan indicating that the fiber may contain a few percent unoriented crystals. Northolt and Van Aartsen assumed a centered monoclinic (pseudo-orthorhombic) unit cell and proposed a crystal lattice model of PPD-T. The top view of Fig. 1.29 is a projection of top-view parallels of the c-axis. There are two repeat units of PPD-T per crystal lattice, one at each corner of the crystal lattice and one at the center. The lower view is a projection parallel to the a-axis. It reveals the phenylene rings of the PPD-T repeat unit in the bc plane of the unit cell and its corresponding bonds. The crystal lattice dimensions are a ¼ 7.80 Å, b ¼ 5.19 Å, and the fiber axis c ¼ 12.9 Å. The a angle ¼ 90 degrees.

1.4.7

Ballistic application of aramid fibers

Threats to military and law enforcement have multiplied in recent years, creating the need for protection against armor-piercing bullets and improvised explosive devices. Today, both woven aramid fabrics and UD crossplied materials provide greater protection, more comfort, and advantageous performance/weight ratios for military, police, and other law enforcement people than older aramid materials. A number of civilians who face ballistic threats such as prison guards, cash carriers, and private people benefit from aramid fiber-based composites. Table 1.2 provides some typical properties of aramid yarns. Aramid-coated fabrics are extensively used for manufacturing military helmets and providing spall liners inside military vehicles. Aramid fibers are used in the armoring of police and civilian vehicles while keeping in mind their maneuverability. Even tanks and other military vehicles can be made lighter and safer with aramid fiber composites. The aramid fiber composites can reduce the weight of armored vehicles by 30e60% compared to steel. Aramid fiber ballistic solutions exist for a number of threat levels, ranging from direct fire and shell fragments to high explosives.

Figure 1.28 X-ray photograph of aramid fiber (Handbook of Textile Fiber Structure, 2009).

High-performance ballistic fibers and tapes

23

a

b

1/4

1/4

1/4

c

b

1/4

1/4

Figure 1.29 PPD-T crystal lattice by Northolt (Handbook of Textile Fiber Structure, 2009).

Table 1.2

Typical properties of aramid yarns (Bhatnagar, 2006)

Yarn property

Standard fibers

High-modulus fibers

Tensile strength, g/denier (GPa)

23.0e26.5 (1.97e2.27)

18.0e26.5 (1.54e2.26)

Initial modulus, g/denier (GPa)

550e750 (47e64)

950e1100 (81e94)

Elongation, %

3.6e4.4

1.5e2.8

Density (g/cm )

1.44

1.44

Moisture regain, % 25
C, 65% RH

6

1.5e4.3

3

24

1.5

Lightweight Ballistic Composites

UHMWPE tape/ribbon

Typical gel-spun UHMWPE fiber requires a solvent system to dilute and disentangle the extremely long chain molecules, thus enabling a drawing process to highly orient the molecular chain for increased tenacity and tensile modulus. The gel-spinning process is expensive but is capable of producing extremely high-tenacity and high-modulus fibers. On the other hand, a lower cost, nonsolvent-based, compression or sintering process may be used to make UHMWPE tapes/ribbons/fibers, which was developed by Nippon Chemical in the 1990s (called the Milite process). This nonfibrous tape/fiber has high modulus (about 1400 g/denier) but a comparatively lower tenacity (20 g/denier). Currently, there are several companies investing into research and development resources to further enhance the physical properties of this nonfibrous, nonsolvent UHMWPE process.

1.5.1

UHMWPE polymer for tape/ribbon

Just like other methods of forming UHMWPE fibers, the compression, sintering nonsolvent process needs specially tailored UHMWPE morphology to reach its highest potential strength. In general, the less entangled the polymer chain in the polymer, the better it is for the compression/sintering nonsolvent process.

1.5.2

Extrusion and pressing process

Fig. 1.30 is a schematic representation of the compression and sintering process. By selecting the right polymer morphology, the polymer powder is first compacted into a thick sheet at below or near the melting point of UHMWPE, followed by calendaring it into a thinner sheet. The sintered thin sheet is then subject to further calendaring and drawing in one or more steps. The sheet at this stage could be as wide as 12 in. or more and can be wound up on a package for further drawing. Since it is difficult to draw a sheet of sintered UHMWPE, the sheet is generally silted into a ribbon, as narrower ribbons can be effectively drawn/stretched. There are several published patents detailing the equipment and processing steps of making nonsolvent-based tape/ribbon. As shown in Figs. 1.31 and 1.32, the polymer powder is fed and dropped down to an “endless” moving belt (24). The powder is then compressed and compacted under a weight (26) into a cohesive sheet. The sheet is subjected to one or more calendaring or roll extrusion (“rolltrusion”) steps and/or drawing under heat to further reduce its thickness and at the same time develop partial orientation in the machine direction. The tape can then be wound up into a package as an interim product, which can be slit into narrow ribbons for further drawing. While this nonsolvent process is of lower cost, the tenacity of the tape is about 20 g/denier, but with a respectable modulus of about 1400 g/denier.

High-performance ballistic fibers and tapes

25

Polymer powder

Draw

Slit

UHMWPE tape

Figure 1.30 UHMWPE polymer compression and sintering process for tape/ribbon (Game Changing Technology, 2008).

1.5.3

Drawing of the slit tape/ribbon

The drawing of the tape is accomplished by a multiple-stage drawing process schematically shown in Fig. 1.33. In fact, multiple stages of drawing are used in most methods of forming UHMWPE tape/fiber to develop its high strength properties. In this case, the sheet from the package is first slit into a narrow ribbon about 3/8 to 2 in. wide. The tape/ribbon is then drawn over a long heater plate by passing the tape back and forth over the heater plate surface. Resistance time, strain rate, drawing temperature, and tension are all important variables during this process which are typically proprietary to each individual fiber manufacturer.

26

Lightweight Ballistic Composites

Raw material 22

26 28

30

32

36

34

37

43

43

24

33 38

31

42

Figure 1.31 Compaction, sintering, and rolltrusion steps of the UHMWPE tape process (United States Patent 8,236,119, 2012). 32

30

28

34 36

37 40

33

38

Figure 1.32 Additional rolltrusion and drawing steps before winding (United States Patent 8,236,119, 2012). 44

46

48

50

52

54

56

58

60

62

Figure 1.33 Tape drawing stages (United States Patent 8,236,119, 2012).

1.5.4

High-tenacity and high-modulus fibrous tape/ribbon

To have the best, a tape with tenacity and modulus, Honeywell International, Inc., has developed a one-step drawing and tape converting process as outlined in US Patent 8,236,119. This process starts with a gel-spun UHMWPE precursor fiber that is drawn

High-performance ballistic fibers and tapes

27

50 100 10

20

51

101 102

80

30

31

60

70

32

20

60

Figure 1.34 High-tenacity and high-modulus fibrous tape/ribbon process (United States Patent 6,277,773, 2001).

in a heated oven to enhance its tensile properties. The UHMWPE fiber is then compressed into a tape/ribbon. The schematic in Fig. 1.34 shows the multiple-filament yarn (10) being fed by a set of rolls (20) (feed step). The fiber is drawn under tension to develop its yarn strength between rolls 30, 31, and 32. Roll 32 also compresses the fiber to convert it into a flat tape. The tape is then transported out by a set of rolls (60) (take-up rolls) with the speed determined by the desired draw ratio. This particular process allows the tape to retain most of the fiber strength during the compression step. The tape/ribbon strength is about double, or more than that of tapes obtained through the nonsolvent, nonfibrous sintering process.

1.5.5

Morphology of UHMWPE tape/ribbon

There is an obvious visual difference between a nonfibrous sintering process tape and a fibrous tape converted from a gel-spun fiber. Figs. 1.35e1.38 show comparisons of the

Figure 1.35 Drawn nonfibrous UHMWPE tape/ribbon.

28

Lightweight Ballistic Composites

Figure 1.36 Drawn/fused/pressed fibrous tape/ribbon.

Figure 1.37 Nonfibrous tape/ribbon showing tape/ribbon nonuniformity.

nonsolvent UHMWPE tape made by a sintering process to the fibrous tape. It appears that the surface is not smooth, not uniform, and not homogeneous, as if the polymer particles are still intact. On the other hand, the tape made by gel-spun fiber via the drawn/fused/compressed process is homogeneous and smooth.

High-performance ballistic fibers and tapes

29

Figure 1.38 Drawn/fused/pressed showing uniformity.

1.5.6

Differential scanning calorimetry characteristics of nonfibrous tape vs fibrous tape

Owing to the differences in the processing of the nonsolvent, nonfibrous tape and the fibrous tape, the nonfibrous tape has a lower melting point component at 138.5
C. It is speculated that the sintering process melts part of the UHMWPE surface causing “adhesion” of the particles, resulting in this lower melting point component. On the other hand, the drawn/fused/pressed process fibrous tape does not have the lower melting point component, allowing it to retain most of its original tenacity.

1.5.7

Ballistic application of UHMWPE tape/ribbon

The biggest difference between a UHMWPE tape/ribbon and a UHMWPE fiber is the aspect ratio. In general, the aspect ratio of the tape/ribbon is at least 3:1 instead of a round fiber. It is speculated that this high aspect ratio may be the reason a lower-tenacity fibrous tape could have a higher ballistic performance than the fibers from which it is fabricated with a comparative strength.

1.6 1.6.1

Ballistic fiberglass Raw materials

The primary component of glass fiber is silica, but it also includes varying quantities of feldspar, sodium sulfate, anhydrous borax, boric acid, and many other materials. The

30

Lightweight Ballistic Composites

Raw materials Limestone Silica sand Boric acid Fluorspar Clay Coal

Hopper

Binder formulation Platinum bushings

Tank Screw feeder Automatic controls

Binder applicator

Mixer Hopper

High-speed winder

Figure 1.39 Glass fiber manufacturing (Fiberglass).

raw materials are weighed according to the desired product recipe and then blended well before their introduction into the melting unit. The weighing, mixing, and charging operations may be conducted in either batch or continuous mode (Fig. 1.39).

1.6.2

Glass melting and refining

In the glass-melting furnace, the raw materials are heated to temperatures ranging from 1500 to 1700
C (2700e3100
F) and are transformed through a sequence of chemical reactions to molten glass. The furnaces are generally large, shallow, and well-insulated vessels that are heated from above. In operation, raw materials are introduced continuously on top of a bed of molten glass, where they slowly mix and dissolve. Mixing is effected by natural convection, by gases rising from chemical reactions, and, in some operations, by air injection into the bottom of the bed. Glass-melting furnaces can be electric, gas-fired, or oil-fired. Electric furnaces are currently used only for wool glass fiber production because of the electrical properties of the glass formulation.

1.6.3

Textile glass fiber spinning

Molten glass from either the direct melting furnace or an indirect marble-melting furnace is temperature regulated to a precise viscosity and delivered to forming stations. At the forming stations, the molten glass is forced through heated platinum bushings containing numerous very small openings to form fibers. The continuous fibers emerging from the openings are drawn over a roller applicator, which applies a coating of a water-soluble sizing and/or a coupling agent. The coated fibers are then gathered

High-performance ballistic fibers and tapes

31

and wound into a spindle. The spindles of glass fibers are next conveyed to a drying oven where moisture is removed from the sizing and/or coupling agents.

1.6.4

Fiberglass structure and morphology

Fiberglass is an amorphous material that is neither solid nor liquid. Fiberglass does not possess either the crystalline structure of solids or the flow characteristics of liquids. Chemically, fiberglass comprises primarily a silica (SiO)2 backbone in the form of (eSiO4e)n groups. Since silica by itself requires an extremely high temperature for liquefaction and fiber spinning, modifiers are utilized to reduce glass temperatures to workable levels as well as obtaining molten-glass viscosities suitable for fiber spinning. Table 1.3 lists typical properties of fiberglass.

1.6.5

Applications of fiberglass

Fiberglass, either in woven form or in UD crossplied form, is not used for flexible body armor applications because of its relatively low ballistic resistance against handgun bullets. Fiberglass for molded armor applications typically is provided with a starch finish, which provides tailed bonding between the fiberglass and later applied resins for achieving high ballistic performance without shattering or too much delamination. For example, fiberglass yarns (generally called rovings) are often used to weave 2  2 basket-weave fabrics wherein the fabric is coated with a phenolic/polyvinyl butyral (PVB) resin system. Both autoclave and hydraulic press molding can be used for converting fiberglass/ phenolic/PVB prepregs into molded ballistic panels. Depending upon the type of metal and metal thickness in a vehicle, molded fiberglass spall liners may be designed for military vehicles. For certain armor-piercing bullets, a ceramic is typically added to the front of the panel facing the armor-piercing bullet. On impact, the ceramic blunts and in some cases tumbles the bullet, and the molded fiberglass backing behind the ceramic absorbs the leftover kinetic energy of the bullet and fragments of the ceramics and bullets.

Table 1.3

Typical properties of fiberglass (Fiberglass)

Yarn property

E-glass

S-glass

Tensile strength (GPa)

3.4

4.5

Initial modulus (GPa)

72e80

87e90

Elongation (%)

3e4

5.4

3

2.55

2.49

Density (g/cm )

32

1.7

Lightweight Ballistic Composites

High-modulus polypropylene fiber (Elizabeth Cates, 2015)

HMPP fiber is a melt-spun fiber based on highly oriented polypropylene. These fibers are characterized by high toughness, excellent chemical resistance, and low density. Innegra™ S from Innegra Technologies is the only commercially available HMPP fiber at the time of writing.

1.7.1

Manufacturing process

HMPP fiber is spun from molten polymer in an extrusion process (Fig. 1.40). The rheological limits of the fiber melt-spinning process place certain practical limits on the molecular weight of the polymers used, in contrast with the gel-spinning process used to produce UHMWPE fibers. The molten polymer is quenched to a solid shortly after exiting the spinneret. As with most of the high-performance fibers, the characteristic structure of the fiber is developed by drawing the filaments to increase the crystallinity and alignment of the polymer crystals within the fiber.

1.7.2

Structure of fiber

HMPP fibers crystallize under tension, producing a microfibrillar structure similar to UHMWPE and para-aramid fibers. During the drawing process, the transition of the polymer from the lower density amorphous structure to the higher density crystalline structure results in the formation of voids in the fiber as the polymer chains reorient. The resulting void content creates a fiber with a bulk density lower than the polymer density (Fig. 1.41). Wide-angle X-ray (WAXS) of HMPP fibers clearly shows the high degree of crystallinity and orientation of the polymer chains, with crystallinity levels over 70% and Herman’s orientation function over 0.7. The crystalline phase is the thermodynamically favored a-monoclinic form. Crystallite size is estimated to be around 100 Å based on WAXS measurements (Fig. 1.42). Metering pump Spinneret Extruder

Quench

Figure 1.40 Manufacturing process of HMPP.

Drawing process

High-performance ballistic fibers and tapes

S4800 5.0 kV 7.8 mm × 4.00 k SE(M) 3/18/2008

33

10.0 mu

Figure 1.41 Scanning electron micrograph of HMPP fiber cross section showing microfibrillar structure with voids. Fiber axis is horizontal in image.

Figure 1.42 Wide-angle X-ray of HMPP fiber.

34

Lightweight Ballistic Composites

1.7.3

Properties

1.7.3.1

Tensile properties

The tensile properties of HMPP fall between those of high-performance fibers and commodity fibers. The predicted ultimate tensile strength of polypropylene is substantially lower than that of polyethylene, so this difference in tensile properties is expected. The higher elongation at break, relative to high-performance fibers, gives the HMPP fibers a higher degree of toughness. This is especially evident upon cryogenic exposure of the fibers, in which they have proven to be resistant to cryofracturing for structural examination. Properties of HMPP fiber are given in Table 1.4.

1.7.3.2

Thermal properties

In examination of the thermal properties of HMPP by differential scanning calorimetry (DSC), multiple endotherms may be observed, with the initial peak melting range of 160e164
C and a higher melting endotherm range around 171e175
C. This higher melting endotherm has been attributed variously in the literature to a more perfect crystal structure, which is dependent on the isotacticity of the base polymer, or to crystalline transformation from the a1-monoclinic C2/c space group to the higher order a2-monoclinic P21/c space group, where the polymer chain helices pack more compactly. It is unclear if the higher endotherms measured by DSC are truly an attribute of the fiber or if they are a result of recrystallization of the polymer on the time scale of the DSC scans. Regardless, the DSC scans of HMPP fiber tend to yield sharper, more intense peaks than those of conventional polypropylene or even high-tenacity polypropylene fibers.

1.7.3.3

Chemical and moisture properties

HMPP is a hydrophobic material with very low moisture regain of 50

Steel

7.8

2.8

200

1.4

e

e

e

Spectra 900

0.97

2.4

70

4

e

150

16

Spectra 1000

0.97

3.1

105

2.5

e

150

16

Vectran

1.4

2.85

65

3.3

30

PBI

1.4

0.4

5.6

30

15

550

41

Polyester

1.38

1

15

20

0.4

260

17

PIPD, poly[2,5-diimidazo(4,5-b:40 ,50 -e) pyridinylene-1,4(2,5-dihydroxy)phenylene]; PBI, Poly[2,20 -(m-phenylen)-5,50 -bisbenzimidazole]; LOI, limiting oxygen index (the minimum percentage of oxygen in an atmosphere for a burning event to be self-sustaining under well-defined conditions).

Lightweight Ballistic Composites

Density (g/cm3)

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159

These ballistic fibers are used in soft as well as hard armor applications. The applications for hard armors are in life-saving vests (generally used to protect against rifle fire and for blunt trauma reduction) and land, air, and sea vehicles. In every case weight is of the utmost importance. It is not hard to see that lightweight armor is easier to carry for the individual using it; it requires less power and consumes less fuel, which are beneficial for the environment and economic footprint of a vehicle. Hard armor composites made from para-aramid and UHMWPE make the armor lighter and thus have become the backbone of modern hard armor applications.

6.2.1

Ballistic fabrics

The efforts in the development of fibrous armor were accelerated with the introduction of a rigid rod-type polymer fiber, p-aramid, by DuPont, Inc., in the 1970s, with trade name Kevlar®. With the invention of gel-spun high-modulus polyethylene (HMPE) fiber manufacturing technology, fibers were commercialized by Honeywell (Allied Fibers). These p-aramid and UHMWPE fibers are mostly used worldwide to manufacture ballistic fabrics. Depending upon the manufacturing process ballistic fabrics may be categorized in the following ways: • • •

Woven Unidirectional laminated Nonwoven felt

6.2.1.1

Woven fabrics

Woven fabric is characterized by warp and weft yarn. Warp yarns are those which run in the direction of the fabric length and weft yarns run in the direction of the fabric width (Fig. 6.1). Woven fabric is produced on a loom via interlacing of these warp and weft yarns. Depending upon the pegging plan the design of woven fabric can be varied to suit the end-use application. Normally ballistic woven fabrics come with a 1  1 plain, 2  1 twill, or 2  2 matt configuration. Weft

Warp

Figure 6.1 Schematic representation of woven fabric.

Selvedge

160

Lightweight Ballistic Composites

Uncoated fabrics The uncoated fabrics mainly comprise woven fabric. The woven ballistic fabrics are made from ballistic filament yarns which have high tensile strength compared to staple yarn and are suitable for the field of ballistic application. In ballistics, the most common woven configuration is 1  1 plain weave (Fig 6.2(a)). The cover factor of the plain-woven configuration is highest compared to other configurations. The impenetrability factor of the 1  1 plain-woven configuration is found to be highest. The impenetrability factor is measured in terms of weave impenetrability as a function of the cover factor. Fig. 6.2 shows some configurations which are used for ballistic applications. In general, the performance of ballistic woven fabric depends on the following: 1. 2. 3. 4. 5. 6. 7. 8. 9.

Physical properties of the filament Filament denier Filament strength Twist level in the filament Weave design Cover factor Interfacial frictional behavior of fibers and yarns Damage to yarn during weaving operation Postweaving operation

In Fig. 6.3, the projectile penetration is resisted at the weave point of the fabric. The stacking of woven fabric layers resists and stops the projectile from penetrating the stacked fabric layers. Fibers with high tenacity and higher strain to failure have the capacity to absorb more energy per unit volume before failure and these are ideal candidates for ballistic products. During impact of a projectile on a woven configuration, the filament is caused to slip from its position in the fabric, which can be reduced by lamination or coating of the ballistic fabric. The coating/lamination increases the frictional properties and restricts the movement of the woven filament yarn and hence gives better ballistic performance against penetration of a projectile.

Figure 6.2 Configurations of fabrics commonly used in composite materials. (a) Plain weave, (b) satin weave, and (c) twill weave.

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161

Figure 6.3 Projectile penetrating through woven ballistic fabric.

Coated fabrics or prepregs Coated fabrics come with several benefits which limit the use of woven ballistic fabric in its raw state. During the impact of a projectile, the filaments of woven fabric slip out of position owing to low frictional behavior between fibers and yarns and the high impact energy of the projectile. Although the yarn strength is high, its utilization factor during the impact is hampered because of the low frictional behavior between fibers and yarns. To improve the frictional characteristic of woven fabric a coating is applied onto the woven fabric to improve its utilization factor. The coating keeps the yarns from moving out and, second, it holds the yarns together. Application of coating increases the engagement of more yarns upon projectile impact as slippage of yarns is minimized, and it also better distributes the stress wave from the contact point, and as a result performance is increased. By numerical simulations, Roylance has shown that enhancement of friction force between yarns increases the dispersion of stress waves which improves the ballistic performance. The same fact is also shown by Briscoe and Motamedi experimentally and Duan by using finite element simulations. Thermoset-coated fabrics (film and resin coated) New ballistic materials are characterized by more uniformity and lower defect levels with the incorporation of prepreg, coating, and resin film technologies, and the result is higher yields, lower cost, and consistent ballistic performance of the end product. Thermoset resin-coated woven fabric shows remarkable improvement upon ballistic impact compared to only woven fabric. This has resulted in the design and development of lightweight, low cost, and yet improved ballistic products.

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Lightweight Ballistic Composites

The fabrics coated with thermoset resins come in the form of prepregs which while curing become a thermoset product. Normally prepreg layers are stacked and cured to get the final product. The most common thermoset resins in ballistic composites are modified phenolics, epoxies, and polyesters. The application of thermoset resins onto conventional woven aramid fabrics improves the interfacial adhesion between fiber/yarn and resin, which results in less slippage of yarns upon ballistic impact. Owing to less slippage, a higher number of yarns is engaged during ballistic impact and the energy required to fail is also increased. Thus the product becomes more resistant to incoming threats. Prepregs are produced via two main processes: (1) the hot-melt process (Fig. 6.4) and (2) the solvent-dip process (Fig. 6.5).

Figure 6.4 Simplified schematic of hot-melt process.

Heated oven Paper or poly interleaf Nip rolls Reinforcement

Looping carrier for horizontal oven

Solution resin

Figure 6.5 Schematic diagram of solvent-dip process.

Prepreg

Prepreg wind-up roll

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163

Thermoset resin-coated ballistic materials are generally used to manufacture hard armors and helmets. Depending on the fiber and the application, the prepreg resin content varies, but typically for ballistic application it is kept under 20% by weight. Thermoplastic-coated fabrics (film and resin coated) Thermoplastic ballistic prepregs have excellent shelf life, have a low cost of storage, and are easy to transport compared to the thermoset-coated material. The thermoplastic prepregs can be remolded in the desired shape and size by the application of heat and pressure at comparatively lower cost. Below are the classes of thermoplastic resins which are commercially used for coating applications of structural composites: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Polyolefins Styrenics Vinyls Acrylics Fluoropolymers Polyesters Polyamides (nylons) Polyimides Polyethers Sulfur-containing polymers

For ballistic applications urethane, polyethylene, and synthetic rubber are most commonly used. Current thermoplastic resins for ballistic application have the following properties: • • • • • • • • • •

Low molecular weight Low modulus Low strength High elongation to failure Lower viscosity Lower melting and softening temperature High energy absorption characteristics Do not absorb moisture Good chemical resistance Excellent shelf life at room temperature

Thermoplastics are also available in many forms for ballistic prepregs. These forms include adhesive in powder form, pellet form, and thick liquid form as well as diluted ready-to-use form. The prepreg resins for ballistic materials contain the adhesive composition in a mixture with additives like organic solvent or an aqueous medium. Solvent-based resins offer many benefits that include: • • • •

Excellent wetting characteristics High solid content Higher prepreg rate Lower prepreg temperature

164

• • •

Lightweight Ballistic Composites

High and uniform fiber coverage Excellent moisture resistance Long-term storage

The aqueous-based resins offer flowing benefits and include: • • • • • •

Environmentally friendly No disposal and recovery required Unlimited volume can be used Cost-effective to dilute Easy to store Easy to transport

With aqueous-based resin formulations the removal of water from the prepregs may require additional heat energy to facilitate secondary operations such as film lamination and molding.

6.2.1.2

Ballistic crossplied unidirectional materials

In woven fabric there are many crossover points in its structure that restrict the traverse deflection of the yarn during projectile impact. If the weave density is higher, the traverse deflection will be restricted more, and if the weave density is less, it may allow penetration of the projectile. This is a major drawback of woven ballistic material. To overcome this problem unidirectional (UD) laminated technology was developed. UD laminated technology consists of a specific layup or ply arrangement based on several design criteria imposed on it. Ply arrangement in any laminated fabric refers to the fiber orientation of consecutive plies in a laminate with respect to a reference coordinate system. A schematic ply orientation (Fig. 6.6) is given, which shows the fiber directions with respect to a reference axis. In crossplied UD material each fiber layer, called the lamina, is oriented in cross-direction to the previous layer. The fiber layers are bonded by a thermoplastic or thermoset resin matrix to form the final laminated fabric. The strength and stiffness of an UD laminate material depend quality product on the orientation of the ply with reference to the load direction. A ply direction such as 0 degrees reacts to axial loading, whereas 90 and 45 degrees react to side and shear loading, respectively. Ballistic UD materials generally consist of two, four, six, etc., plies with 0/90 degrees or 0/90/45 degrees orientation depending on requirements. This type of orientation helps to get maximum traverse deflection and as well the higher fiber density in each ply reduces the probability of projectile penetration. Armor solutions produced with UD fabrics are generally lighter in weight with respect to conventional woven material systems. Nowadays most of the manufacturers have adopted this technology to produce lightweight composite material for ballistic application.

6.2.1.3

Ultrahigh-molecular-weight polyethylene tapes

UHMWPE tapes are UHMWPE powder made into sheets using a technique called a solid-state technology process. UHMWPE tapes show characteristics similar to those of UD material and show good impact resistance against projectiles.

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165

90 degree +45 degree

0 degree

– 45 degree

Figure 6.6 Schematic diagram of unidirectional ballistic fabric.

UHMWPE tape is lighter in weight compared to p-aramid fabric and has the following characteristics: • • • • • •

Light weight Good strength Good impact resistance Electromagnetically transparent Good thermal conductivity Environmentally stable

Tapes are generally used to make radomes or in medical applications, impact-resistant panels, lightweight armor such as the liner behind steel or ceramics, spall liners in vehicles, etc. Armor panels made of UHMWPE tapes outperform aramid solutions at a competitive price.

6.2.1.4

Ballistic felts

Felts have shown promising results in the area of ballistic protection and therefore are utilized in the manufacture of body armor. The general practice among armor industries is to use ballistic felt made of aramid fiber by the needlepunch technique. Needlepunch technology is simpler than weaving and a variety of properties can be obtained by varying the fabrication process parameters. In this technology, continuous ballistic fibers are cut into smaller fibers, carded, and randomly oriented by a crosslapper (Fig. 6.7) to form an isotropic sheet. The sheets are then associated by a set of barbed needles (Fig. 6.8), which push some fibers downward through the sheet of randomly oriented fiber felt. Other than barbed needles there are triangular or four-pointed star-shaped cross-section-type needles, which are also used to produce ballistic felts. Felt engages fragments in a better way than a conventional woven ballistic fabric does. Ballistic felt with low areal density is competitive for any fragmentation armor.

166

Lightweight Ballistic Composites

Figure 6.7 A schematic diagram of a crosslapper. http://www.tatham-uk.com/crosslapper-mct1w21.

Stripper plate

Needle

Bed plate Point

Figure 6.8 Schematic diagram of the felt manufacturing process with a barbed needle.

The action mechanism of felt during a ballistic impact is not largely known, though it is understood that the mechanism is highly dependent on fiber-to-fiber friction, widely known as a “stickeslip” phenomenon. The ballistic performance of a ballistic felt mainly depends on the fiber properties and fabrication method adopted. Fiber

Lightweight composite materials processing

167

properties include tenacity, elongation at break, work of rupture, and fiber denier. The fabrication method of a needlepunched felt is also very important to obtain optimum ballistic performance and includes type of needling, density of needling, angle of needling, crimped/uncrimped, length of fiber, thickness of felt, and angle of ply. Laible et al. have shown that fabrication methods play a significant role in the performance of ballistic felt. Here are a few factors: Density of needling: Low needling density during fabrication increases the ballistic performance of the material. Crimped/uncrimped configuration of fibers: Crimped versus uncrimped configuration shows an abnormal relationship although it is essential to have a certain degree of crimping in the structure to facilitate the production process. Length of fibers: Fiber length has a relationship proportional to the ballistic performance of the felt in a range of 0.5e2.5 in. The ballistic performance of felt is not significantly improved beyond this range. Thickness of felt: The performance of felt is directly proportional to the thickness of the felt when all other factors remain the same.

Other than these factors, the type of needling, angle of needling, and angle of ply play unimportant or minor roles in the performance of the felt.

6.3

Quality control of ballistic materials

Quality means: • •

Meeting the needs of the customer Meeting the international standards

Quality is not only for one inspection; we have to make it habit, and to maintain the quality up to the mark throughout the entire time span we need a system, known as quality control. This system helps us to maintain quality and continuous improvement in the quality. Quality control is a process that is essential to set the level of quality of the products and services offered by a company. This control includes the actions necessary to verify and control the quality output of products and services. The overall goal of quality control includes: • • • •

Meeting the customer’s requirements Product satisfaction Fiscal soundness Dependable output

Quality is important to a company for: • • • •

Name recognition or branding Maintaining a position against competitors Customer satisfaction Reducing the cost of replacement of defective products

168

Lightweight Ballistic Composites

6.3.1

Why quality control is essential for manufacturing a quality product

Quality control consists not only of product and service but also of how well an organization works as a whole, within the organization and in the marketplace. Quality is an ongoing effort that must be consistent and improve every day. Every organization or business can benefit by using quality control for their products or services, within the internal organization and interacting in the marketplace. Quality control is very much essential to sustain a competitive presence in the market.

6.3.2 •

•

•

•

Physical properties

Visual inspection Visual inspections of prepreg ballistic composites are simple. They are low cost, consume minimum time and effort, and can provide valuable information about the quality of the prepregs without going through expensive testing. During visual inspection, the ballistic prepreg material is unrolled and passed in front of a set of lights. Any color change, fiber missing, fluctuation in resin content, or impurities are apparent to the naked eye and can be marked, recorded, and flagged. Such tests can usually be quantitative using the Gardner color scale (ASTM D1544) and refractive index (ASTM D542-50). Total prepreg weight Checking the per-unit area weight of incoming and outgoing ballistic prepreg material against the production specifications can provide useful data without going through destructive testing. A few samples of prepregs are cut from the prepreg roll and weight variation is recorded and checked against the specification. A weight variation within 2% is usually considered as good prepreg material. Resin and fiber content Washing the resin completely from the prepreg material can provide information both about the resin and about the ballistic fiber content of the ballistic prepreg material. The washing solvents are usually an industrial solvent, such as acetone, methyl ethyl ketone, toluene, or other commercial solvent. The test samples, as small as 15 cm2, are washed three or four times with fresh solvent and finally oven dried, and the leftover fibers are weighed to provide both resin content and fiber content. Resin content (%) ¼ ((initial sample weight  final dry fiber weight)/initial sample weight)  100. Fiber content (%) ¼ (final dry fiber weight/initial sample weight)  100. If the prepreg has partially cured or the prepreg resin is a blended resin, this technique may not work. Volatile content During the manufacturing of ballistic prepregs, resins are usually diluted to achieve a low resin content. To achieve this goal, solvent-based and aqueous-based resins are diluted. Although a majority of solvents are driven off during prepreg manufacturing, it is good practice to check the volatile content of prepregs. Small samples of prepregs are cut from the prepreg roll. The samples are heated in a circulating-air oven kept at 100e150 C. After some set time the samples are taken out and cooled to room temperature and weight loss is calculated. Volatile content (%) ¼ ((initial weight  dry weight)/initial weight)  100. Usually, three or more samples are tested for a single test. Other similar tests are ASTM D3539, Military Standard (MIL)-G-83410 (USAF), and MIL-R-7575.

Lightweight composite materials processing

•

•

Specific gravity The specific gravity (or density) of ballistic prepreg material is usually specified and may be indicative of batch quality and process control for certain prepreg materials. For molded parts and prepregs which are not soluble in certain chemicals, ASTM D792 (“Specific Gravity and Density of Plastic by Displacement”) and ASTM D1505 (“Density of Plastic by the Density-Gradient Technique”) are used to measure specific gravity of prepregs and molded parts. Flow test This is a common test for structural composite prepregs in which resin content is fairly high and resin viscosity is not very high. However, owing to low content in ballistic prepregs this test may not have sufficient resin to flow under heat and pressure. This is especially true for thermoplastic resin, of which the viscosity is fairly high once all the solvent is driven off during the prepreg operation.

6.3.3 •

•

169

Instrumental and spectroscopy methods

Differential scanning calorimetry The differential scanning calorimeter (DSC) test (Figs. 6.9 and 6.10) is simple and requires a very small amount of ballistic prepregs to confirm the quality of the prepregs. DSC is used to observe fusion and crystallization events as well as glass transition temperature. DSC can also be used to study oxidation as well as the chemical reaction. Infrared or thermal technique Infrared or thermal techniques utilize differences in heat flow due to the presence of defects within the chemical structure of the material. The material is first heated. As the material is heated and cooled, the surface temperature is observed through the use of a sensitive infrared device (radiometer). Each material has a unique infrared wavelength. A typical infrared test is shown in Fig. 6.11.

Oven Sample crucible Reference crucible

Inlet: sample gas

PT100 Heating Cooling

Reference temperature

Purge gas

Inlet: purge gas

Thermocouples Temperature difference

Inlet: cooling gas

Figure 6.9 Differential scanning calorimetry. http://www.intechopen.com/books/applications-of-calorimetry-in-a-wide-context-differentialscanning-calorimetry-isothermal-titration-calorimetry-and-microcalorimetry/thermal-analysisof-phase-transitions-and-crystallization-in-polymeric-fibers.

170

Lightweight Ballistic Composites

DSC of fiber-reinforced composite prepreg

Heat flow (mW)

10.5

#1 Structural prepreg #2 Ballistic prepreg 1 #3 Ballistic prepreg 2 #4 No catalyst

#1

10.0

#2 #3 #4

9.5 9.0 8.5 8.0

2.0

0.0

4.0 6.0 Time (min)

8.0

10.0

Figure 6.10 Differential scanning calorimetry test on prepregs. CIPET Lucknow, U.P, India Time stamp = Thursday, May 21, 2015 13:58:30

Transmittance %

100 95

2

2404.049 0.000 3344.894 0.412

2280.288 0.000

910.124 39.459 757.631 32.

3789.934 0.005

698.395 6

3303.151 0.079

90

1597.112 88.570 2848.067 48.398 1450.614 1.127 963.943 138.826

85 2916.905 491.941

3500

3000

2500

2000

1500

1000

500

Wavenumber 660

Figure 6.11 The general analysis of styrene butadiene rubber with halloysite nanotubes by Fourier transform infrared spectroscopy (FTIR). Project submitted on Studies on Styrene Butadiene Rubber/Halloysite Nanotubes nanocomposite submitted by Rohit Patle (MTech). • •

Gaseliquid chromatography • Used for testing the purity of a particular substance • Used for separating and identifying the various components of a mixture (Fig. 6.12). Thin-layer chromatography Thin-layer chromatography (TLC) is used to separate nonvolatile mixtures. The sample is applied on a plate, and a solvent mixture (known as the mobile phase) is drawn up the plate via capillary action (Fig. 6.13). Because different analytes ascend the TLC plate at different rates, separation is achieved.

Lightweight composite materials processing

171

Detector

Sample injector Proportionating valve

Data system

Injection port Flow controller GC column

Zero air

Carrier gas

Column oven Hydrogen

Figure 6.12 The general flowchart of gas chromatography. Lid

Paper Solvent front

Solvent

Figure 6.13 Thin-layer chromatography. https://commons.wikimedia.org/wiki/File:Chromatography_tank.png. •

•

High-pressure liquid chromatography High-pressure liquid chromatography is used to separate both polar and nonpolar compounds, to identify each component, and to quantify each component. It relies on pumps to pass a pressurized liquid solvent containing the sample mixture through a column filled with a solid adsorbent material. Each component in the sample interacts slightly differently with the adsorbent material, causing different flow rates for the different components and leading to the separation of the components as they flow out the column. Scanning electron microscopy analysis Scanning electron microscopy (SEM) analysis is conducted on ballistic prepregs to check the fiber distribution, fiber packing density, any damage to the fiber during the weaving and prepregs process, and resin distribution within the prepregs. The analysis can also be utilized to understand any impurity in the prepregs. Sample size is relatively small, but provides information at the microlevel. SEM is a variation tool for research and development purposes while designing ballistic prepregs for specific applications.

172

6.3.4

Lightweight Ballistic Composites

Ballistic testing of materials

Ballistic armor performance may be determined by either of two evaluation methodologies: ballistic resistance testing or ballistic limit testing. Ballistic resistance testing evaluates the performance with respect to predetermined performance requirements. Ballistic limit (V50) testing determines the limits of performance. The selection of which type of test is to be employed is determined by the reason for conducting the test. In either case, the detailed procedures of the test are what ensure the reliability and repeatability of the results of testing. The performance of stab-resistant armor is determined by testing to predetermined performance requirements. As with ballistic performance testing, the details of the test are what ensure the reliability and repeatability of the results of testing.

6.3.4.1

Ballistic resistance methodologies

Ballistic resistance testing of armor is conducted to evaluate the pass/fail performance of armor with respect to predetermined performance specifications/requirements. This type of testing will not determine the margin by which a sampling passes those requirements nor, if it fails, the margin of failure. The basic procedures for ballistic testing are the same whether the target is a bullet or fragmentation threat. As a minimum the procedures of ballistic resistance testing must include: 1. Description of the test sampling material coupon versus operational assembly and the number and size(s) of the test samples in the sampling 2. The distribution of the samples over the full spectrum of tests of the standard 3. The ballistic threat to be used in testingdcaliber, bullet type/construction, bullet weight, impact obliquity, and velocity of the impact of that threat with the sample 5. Pretest conditioning of test sample 6. Test environmental conditions 7. Sample backing (if any) and its calibration 8. Acceptable limit of bullet stability (yaw) 9. Acceptable limits of obliquity of impact 10. Definition of fair/unfair shots 11. Required number and location of fair shots on each sample 12. Whether refixturing of sample between shots is permitted 13. Range setup, including mounting of the sample 14. Methodology of velocity determinations 15. Precise definition of penetration including methodology for determinations of penetrations 16. Statement of whether spall constitutes penetration 17. Precise definition of deformation, including methodology for determination of deformation 18. Level of acceptable posttest operability of an assembly 19. Data and reporting requirements 20. Ownership and disposition of tested samples

Ballistic resistance testing is well suited to any material coupon or assembly evaluation requiring only a pass/fail conclusiondproduct demonstrations, marketing, field demonstrations, lot acceptance, etc.

Lightweight composite materials processing

173

Increasing probability of failure (penetration)

1.00

Test data 0.50 Probit,

langlie

0

V50

OSTR and bruceton methods

Variable stimulus (projectile velocity)

Figure 6.14 Armor penetration vs projectile velocity. OSTR, one-shot test response.

Because its findings are limited to pass/fail conclusions, ballistic resistance testing is of limited (if any) value in comparing the performance of differing designs or changes in the same design. For these quantitative purposes, ballistic limit (V50) testing is better suited.

6.3.4.2

Ballistic limit (V50) testing

V50 testing is one of four similar testing methods used to determine the probability of penetration of ballistically resistant materials, all of which were derived for the testing of devicesdnot necessarily armordwhich are consumed in a single test trial of a nonquantifiable reaction to a variable stimulus, ie, a match ignites or does not, a fuse functions or does not, etc. A multiplicity of identical test samples are subjected to a variable stimulus, the “go/no-go” results of which are used to establish a curve with respect to the full range of the variations of the stimulus. The differences in the four methods are procedural and, while the results of each are similar, the reliability of those results is a reflection of the complexity and sophistication of the procedure. These methods are frequently used to establish the probability of penetration of armor as a function of projectile velocity (Fig. 6.14). Adapting these methods to the evaluation of ballistically resistant materials is, in the main, impractical and of academic value only. The control of the stimulusdin the case of armor testing that stimulus is projectile velocitydto precisely, predetermined values is a requirement which cannot be achieved in armor testing without excessive and costly expenditures in ammunition and armor samples.

6.3.4.3

Probit method

Ten firings are conducted at each preselected narrow velocity range. The results of each group of 10 firings are analyzed to determine the number of penetrations, which, when expressed as a percentage, is used to establish a point of the curve. The number of points necessary to establish the curve is a reflection of the required level of confidence and the range of velocities and/or penetration probabilities to be examined.

174

6.3.4.4

Lightweight Ballistic Composites

Langlie method

This method was derived to produce the entire range of results (curve) with a minimum of trials; however, when adapted to armor testing this is largely illusionary, inasmuch as many firings are not usable owing to noncompliance with velocity requirements. The initial firing is conducted at the midpoint of the velocities of predicted 100% and 0% probabilities of penetration. Subsequent firings are conducted at precise, mathematically predetermined velocities based on an analysis of the results of firings to that point. Firings continue until a preselected stopping point is reached, usually 20 usable firings and/or five shot-to-shot reversals within a predetermined zone of mixed results. Measuring the velocity with the required precision is difficult, resulting in many unusable firings.

6.3.4.5

One-shot test response method

The one-shot test response method is a more sophisticated variation of the Langlie method requiring more than one trial at the same velocity as the Langlie method, which requires only one usable shot at each velocity. All of the negative considerations of the Langlie methoddexcessive ammunition and armor costs and procedural and analysis complexitydare amplified by this method; however, the results are more highly refined.

6.3.4.6

Bruceton method

This method may be used to develop the full range of results, but is the least suitable for that purpose, inasmuch as it was derived to focus on the area of 50% probability of penetration. The procedures are less complex and projectile velocities need not be controlled with the same precision as the other methods. The initial firing is conducted at the expected velocity to produce a 50% probability of penetration. All subsequent firings vary by a fixed amount until an even number of trials (2, 4, 6, 8, or 10) is obtained within a predetermined total velocity variation (usually 60, 90, or 125 ft/s), one-half of which (50% probably) are penetrations. The practicality, low cost, and usefulness of this method are the basis for the extensive, universal use of the MIL-SD-662 V50 method of armor testing, which is a specialized case of the Bruceton method. For a more complete discussion of these methods, their procedures, and their strengths and weaknesses consult MIL-STD-331A, “Military Standard, Fuse and Fuse Components, Environmental and Performance Tests for,” dated October 10, 1987. Ballistic limit (V50) testing, as widely used to evaluate the limits of armor performance, is an adaptation of the Bruceton method, which was originally derived for the testing of devicesdnot necessarily armordwhich are consumed in a single test trial of a nonquantifiable reaction to a variable stimulus. Testing standard MIL-STD-662F, “Ballistic Test for Armor,” dated December 18, 1997, is the most comprehensive adaptation of the Bruceton method to armor testing. The requirements of MIL-STD-662F define the procedures to be followed to establish the limits of performance of a sample of armor in terms of the precise velocity of

Lightweight composite materials processing

175

impact which will produce 50% penetrations. The shot-to-shot velocities of the test are intentionally varied (increased and decreased) until equal numbers of penetrations and nonpenetrations are produced within a narrow overall range of velocity. The average of the velocities of these equal numbers of penetrations and nonpenetrations is termed the V50 of that sample. When properly conducted and reported a V50 test is at once a measure of the performance of the armor, self-evaluating, and a reflection of the physical consistency of the test sample. The confidence level of the V50 is inversely related to the narrowness of the range of the velocities used to compute the Vsq. A 9-mm bullet at 2 ft/s, which would not penetrate a sheet of paper, averaged with the same bullet of a second, penetrating shot at 5000 ft/s would yield a V50 of 2500 ft/s. Disallowing extreme velocity variations, 5000 ft/s in this example, eliminates distortions of this nature. Depending on the required confidence level, maximum allowable velocity variations of 60, 90, 125, and 150 ft/s are frequently specified. However, if the sample lacks homogeneity, those inconsistencies may render attainment of a V50 within the specified range of velocities impossible. In such cases the results are termed inconclusive and ignored. The range of velocities used to compute the V50 is often reported as the “range of results.” Variations within the test sample, such as thickness or hardness, may produce apparent inversions in logic if a lesser velocity shot penetrates when a higher velocity shot does not. When this occurs, the lower velocity of the penetrating shot is subtracted from the higher velocity of the nonpenetrating shot and the difference reported as “range of mixed results.” Often ignored, the range of mixed results is a reflection of the consistency of the makeup of the test sample. For example, should a high-velocity shot impact a harder location and not penetrate, while a lower velocity shot impacts a softer spot and does penetrate, the magnitude of the range of mixed results provides a measure of this inconsistency. Minimum procedures of a V50 test must include: 1. Descriptions of the test sampledsize and number of the material coupon (note that V50 testing of armor assemblies is rarely conducted inasmuch as variations in configuration conflict with the sample homogeneity, which is an assumption of V50 testing) 2. The distribution of the samples over the full spectrum of tests of the standard 3. The ballistic threat to be used in testingdcaliber, bullet type/construction, and bullet weight 4. Pretest conditioning of samples 5. Test environmental conditions 6. Backing (if any) of the test sample and its calibration 7. Acceptable limits of bullet stability (yaw) 8. Acceptable limits of obliquity of impact 9. Definition of fair/unfair shots 10. Required minimum number of penetrations and nonpenetrating velocities to be used in computation of V50 11. Whether refixturing between shots is permitted 12. Maximum allowable variation in velocities used to compute V50 13. Maximum number of shots allowable on one sample 14. Range setup, including mounting of the sample 15. Methodology of velocity determinations

176

16. 17. 18. 19. 20.

Lightweight Ballistic Composites

Precise definition of penetrations, including methodology for penetration determinations Statement of whether spall constitutes penetration Statement of whether residual velocities of penetrations are to be determined Data to be recorded and reported Ownership and disposition of tested samples

V50 testing is best suited to any purpose requiring a comparative evaluation such as engineering and development, comparing the performance limits of two or more differing armoring materials or the effects of environmental extremes, or modifications of the same armoring material. Because the procedures of V50 testing are based on the assumption that the test sample is homogeneous, V50 testing is of limited value in evaluating the performance of armored assemblies with configuration variationsdseams, weldments, subassemblies, etc.dfor which ballistic resistance testing is well suited.

6.4

Various international ballistic specifications/standards

Some of the most referred to and used test ballistic standards are:

6.4.1

MIL-STD-662F and Standardization Agreement (STANAG) 2920

The standard provides general guidelines for procedure, equipment, physical conditions, and terminology for determining the ballistics of metallic, nonmetallic, and composite armor against small arms projectiles. The ballistic test procedure described in this standard determines the V50 ballistic limit of armor.

6.4.2

National Institute of Justice Standard-0101.04 for law enforcement vests/hard armor plates

One of the most widely used standards is the US National Institute of Justice (NIJ). NIJ Standard-0.0101.04 was issued in September 2000. Since its introduction, it has been used as a reference by a number of South American, European, and Asian countries. The standard establishes the minimum performance requirement and test method for the ballistic resistance of personal body armor for protecting the human torso against handgun and rifle gunfire. The standard also lays out criteria for acceptance of the armor vest in terms of labeling, test sequence, workmanship, tractability, and labeling. The ballistic-resistance body armor in this standard is classified into seven levels (Table 6.2) Types I, IIA, II, and IIIA provide increasing levels of protection from handgun threats. Type III and IV armor, which protects against high-powered rifle rounds, is for use only in tactical situations.

Table 6.2

NIJ Standard-0101.04 perforation and back-face signature performance test summary Test round

Test bullet

Bullet weight

Reference velocity (±30 ft/s)

BFS depth maximum

Shots per panel

Shots per threat

Total shots required

I

1

.22 caliber LR LRN

2.6 g 40 gr

329 m/s (1080 ft/s)

44 mm (1.73 in.)

6

24

48

2

.380 ACP FMJ RN

6.2 g 95 gr

322 m/s (1055 ft/s)

44 mm (1.73 in.)

6

24

1

9 mm FMJ RN

8.0 g 124 gr

341 m/s (1120 ft/s)

44 mm (1.73 in.)

6

24

2

40 S&W FMJ

11.7 g 180 gr

322 m/s (1055 ft/s)

44 mm (1.73 in.)

6

24

1

9 mm FMJ RN

8.0 g 124 gr

367 m/s (1205 ft/s)

44 mm (1.73 in.)

6

24

2

357 Mag JSP

10.2 g 158 gr

436 m/s (1430 ft/s)

44 mm (1.73 in.)

6

24

1

9 mm FMJ RN

8.2 g 124 gr

436 m/s (1430 ft/s)

44 mm (1.73 in.)

6

24

2

44 Mag JHP

15.6 g 240 gr

436 m/s (1430 ft/s)

44 mm (1.73 in.)

6

24

III

1

7.62 mm NATO FMJ

9.6 g 148 gr

838 m/s (2780 ft/s)

44 mm (1.73 in.)

6

12

12

IV

1

.30 caliber M2 AP

10.8 g 166 gr

869 m/s (2880 ft/s)

44 mm (1.73 in.)

1

2

2

Special

*

*

*

*

44 mm (1.73 in.)

*

IIA

II

IIIA

*

48

Lightweight composite materials processing

Armor type

48

48

* 177

Panel, front or back component of typical armor sample; sample, full armor garment, including all component panels (F and R); threat, test ammunition round by caliber; BFS, back-face signature. (* These items must be specified by the user. All of the items must be specified.) Any special requirement for a level of protection other than one of the above standards should specify the exact test rounds, and indicate that this standard shall govern in all other respects.

178

Lightweight Ballistic Composites

Table 6.3

Hard armor classification

Armor type

Test ammunition

Bullet massa

Barrel length

Bullet velocity

I

22 LRHV lead

2.6 g (40 gr)

15e16.5 cm (6e6.5 in.)

320  12 m/s 1050  40 ft/s

38 Special RN lead

10.2 g (158 gr)

15e16.5 cm (6e6.5 in.)

320  12 m/s 1050  40 ft/s

357 Magnum JSP

10.2 g (158 gr)

10e12 cm (4e4.75 in.)

381  15 m/s 1250  50 ft/s

9 mm FMJ

8.0 g (124 gr)

10e12 cm (4e4.75 in.)

332  12 m/s 1250  40 ft/s

357 Magnum JSP

10.2 g (158 gr)

15e16.5 cm (6e6.5 in.)

425  15 m/s 1395  50 ft/s

9 mm FMJ

8.0 g (124 gr)

15e16.5 cm (6e6.5 in.)

358  12 m/s 1175  40 ft/s

44 Magnum SWC

15.55 g (240 gr)

14e16 cm (5.5e6.25 in.)

426  15 m/s 1400  50 ft/s

9 mm FMJ

8.0 g (124 gr)

24e26 cm (9.5e10.25 in.)

426  15 m/s 1400  50 ft/s

IV

7.62 mm 308 Winchester FMJ

9.7 g (150 gr)

56 cm (22 in.)

838  15 m/s 2750  50 ft/s

V

30-06 AP

10.8 g (166 gr)

56 cm (22 in.)

868  15 m/s 2850  50 ft/s

Special

As specified

As specified

As specified

As specified

IIA

II

IIA

AP, armor piercing; FMJ, full metal jacket; JSP, jacketed soft point; LRHV, long rifle higher velocity; RN, round nose; SWC, semiwadcutter a Five bullets per test for type I, type IIA, type II, type IIIA, and type IV, except one bullet for type V armor.

6.4.3

NIJ Standard-0108.01

A number of lightweight ballistic armor materials are now available that ARC designed to protect against small-caliber handguns and high-powered rifles (Table 6.3). These include handheld riot shields, armored clipboards used by police, armored buildings for security guards, police check posts, and temporary housing for military and peacekeepers and occupants of a vehicle. Such armored materials can be fabricated from metals, ceramics, transparent glazing, fabrics, felts, and fiber-reinforced composites.

Lightweight composite materials processing

6.4.4

179

NIJ-0101.06 Standard for armor vests and hard armor plate

The NIJ-0101.06 standard is a revised version of NIJ-0101.04, dated September 2000. This standard is better than the NIJ 2005 Interim Requirements, dated September 2005; NIJ-0101.04 standard; and all other revisions and addenda to the NIJ-0101.04 standard. There are three main reasons that describe why NIJ-0101.06 is superior to NIJ-0101.04: • • •

Increased performance against current threats in the law enforcement community Better reliability of armor Improved durability of armor can simulate daily wear and tear conditions and evaluate performance of armor after years of use through environmental condition test

Armor products complying with NIJ-0101.06 are more reliable as the testing procedures are more stringent and performance evaluation simulates daily wear conditions (Table 6.4).

6.4.5

NIJ Standard-0106.01 for ballistic helmets

For ballistic helmets, NIJ Standard-0106.01 is an equipment standard being used worldwide and developed by the Law Enforcement Standards Laboratory of the National Bureau of Standards (United States). The purpose of this standard is to establish performance requirements and methods of test for helmets intended to protect the wearer against gunfire. The previously said (Table 6.5) threat levels for helmets are out of context in current warfare, as level IIIA helmets with back-face signature (BFS) are widely in demand nowadays, so the HPW 401.02 standard is also very much important for testing of the helmets with BFS measurement (Table 6.6). The major causes of a ballistic armor failure both for law enforcement and for the military are due to: 1. Testing against wrong ballistic threats and not paying attention to clamping and clay conditions 2. Ballistic design without considering the material and ballistic test fluctuation 3. Inadequate controls for materials 4. Poorly controlled ballistic fiber, weaving, or crossplying manufacturing techniques 5. Wrong application of ballistic materials

6.5

Processing of ballistic materials

6.5.1 6.5.1.1

Raw materials Ceramic-based raw materials

Aluminum oxide Aluminum oxide (Al2O3) has very good mechanical properties and is comparatively cheaper than silicon carbide (SiC) or boron carbide (B4C). Al2O3 provides excellent

Classifications of armor according to NIJ-0101.06 standard

180

Table 6.4

Test variables

Shots per panel

New armor test velocity* m/s

Remington 23558

355  9

373  9

4

44

2

6

11.7

Remington 23686

325  9

352  9

4

44

2

6

9 mm FMJ RN

8

Remington 23558

379  9

398  9

4

44

2

6

.357 Magnum JSP

10.2

Remington 22847

408  9

436  9

4

44

2

6

.357 SIG FMJ FN

8.1

Speer 4362

430  9

448  9

4

44

2

6

15.6

Speer 4453 or 4736

408  9

436  9

4

44

2

6

US/NATO m80 ammunition

847  9

e

6

44

0

6

US military

878  9

e

1 to 6

44

0

1 to 6

IIA

9 mm FMJ RN

8

.40 S&W FMJ

.44 Magnum SJHP III

7.62-mm NATO FMJ

9.6

IV

.30-caliber M2 AP

10.8

Bullet manufacture

* Mentioned velocity to be measured at 2.5 m  25 mm from the target. BFS, back-face signature.

Lightweight Ballistic Composites

Conditioned armor test velocity* m/s

Bullet mass (g)

Test bullet

IIIA

Maximum BFS depth (mm)

Hits per panel at 30 or 45 degree angle

Hits per panel at 0 degree angle

Armor type

II

Performance requirements

Test summary Test variables

Performance requirements

Helmet type

Test ammunition

Nominal bullet mass

Suggested barrel length

Required bullet velocity

Required fair hits per helmet part

Permitted penetrations

I

22 LRHV lead

26 g, 50 gr

15e16.5 cm, 6e6.5 in.

320  12 m/s, 1050  40 ft/s

4

0

38 Special 1 RN lead

10.2 g, 158 gr

15e16.5 cm, 6e6.5 in.

259  15 m/s, 850  50 ft/s

4

0

357 Magnum JSP

10.2 g, 158 gr

10e12 cm, 4e4.75 in.

381  15 m/s, 1250  50 ft/s

4

0

9-mm FMJ

80 g, 124 gr

10e12 cm, 4e4.75 in.

332  15 m/s, 1090  50 ft/s

4

0

357 Magnum JSP

10.2 g, 158 gr

15e16.5 cm, 6e6.5 in.

425  15 m/s, 1395  50 ft/s

4

0

9-mm FMJ

80 g, 124 gr

10e12 cm, 4e4.75 in.

358  15 m/s, 1175  50 ft/s

4

0

IIA

II

Lightweight composite materials processing

Table 6.5

181

182

Table 6.6

Ballistic projection/threat levels

Level

Test ammunition Caliber

(a) I

Bullet Weight (gr)

.22 LR

Required bullet

40

Velocity (ft/s)

Required shots Penetration

Deformation

Type

Minimum

Maximum

(b)

(b)

Lead

1050

1100

5

5

158

RN lead

850

900

5

5

9  19 mm

124

FMJ

1175

1225

5

5

.357 Magnum

158

JSP

1250

1300

5

5

9  19 mm

124

FMJ

1175

1225

5

5

.357 Magnum

158

JSP

1395

1445

5

5

9  19 mm

124

FMJ

1400

1450

5

5

.44 Magnum

240

SWC-GC

1400

1450

5

5

III

7.62  51 mm

150

Ball, M80

2750

2800

5

5

IV

.30-06

166

AP

2850

2900

5

5

V

Special category (c)

e

e

e

5

5

IIA

II

IIIA

Duplicate of ballistic threats and velocities as specified by NIJ-0101.03, “Ballistic Resistance of Police Body Armor.” One shot in each quadrant and helmet crown. This procedure may be used to test the resistance of other bulleted ammunition or fragment-simulating projectiles conforming to MIL-P-46,593A or drawing HPW-02-010-00.

a

b c

Lightweight Ballistic Composites

.38 Special

Lightweight composite materials processing

183

impact resistance, chemical resistance, abrasion resistance, and high-temperature properties. The drawback of Al2O3 over SiC or B4C for armor applications is the higher density of Al2O3 compared to SiC or B4C. Because of the high density of Al2O3, the strength-to-weight ratio of Al2O3 is lower than that of SiC or B4C. For example, typical properties and an image of Al2O3 in armor applications are given in Table 6.7 and Fig. 6.15.

Table 6.7

Properties of Al2O3 in armor applications

Properties

Unit

Value

Purity

%

98

Density

g/cc

3.8

Color

e

White

Water absorption

%

0

Flexural strength

MPa

330

Compressive strength

MPa

2500

R45N (GPa, 1000 g)

81 (142)

Vickers hardness Coefficient of thermal expansion at Sonic velocity Fracture toughness

25e1000 C

6 

10 / C

8.2

m/s

10,200

pffiffiffiffi MPa m

Courtesy: Carborundum Universal Limited, www.cumi-murugappa.com.

Figure 6.15 Image of hexagonal Al2O3 ceramic tile used in armor application.

4 to 5

184

Lightweight Ballistic Composites

Silicon carbide SiC has very high hardness and a high strength-to-weight ratio, which makes SiC suitable for armor applications. It has a high chemical bonding property, which is why it shows high wear resistance and high impact resistance. The service temperature of SiC is very high (approximately 1650 C). For example, typical properties and an image of sintered SiC are given in Table 6.8 and Fig. 6.16.

Table 6.8

Properties of SiC in armor applications

Parameters

Unit

Values

Density

g/cm3

3.10

Purity

%

>98

Medium grain size

mm

4e6

Hardness

Knoop (1000-g load)

2800

Bending strength

400

Fracture toughness

MPa pffiffiffiffi MPa m

Elastic modulus

GPa

410

Weibull factor

3.2

8  C)

102.6 W/(m  C) at 200 C

Thermal conductivity

W/(m

Thermal expansion coefficient

106  C

4.02 (at 700 C)

Highest working temperature

C

1650

Flexural strength

MPa

380

Compressive strength

MPa

3900

Courtesy: Hexoloy, Carborundum, www.saint-gobain.co.in.

Figure 6.16 Image of hexagonal SiC ceramic tile used in armor application.

Lightweight composite materials processing

Table 6.9

185

Properties of B4C in armor applications

Density (g/cm3)

Young’s modulus (GPa)

Fracture toughness, Kic (MPaOm)

Hardness (Knoop, kg/mm2)

Flexural strength (MPa)

Porosity rate%

2.5

460

2.5

3200

410

250 mm) or 3D-shaped parts are more difficult and expensive to source, as a specific tool will have to be generated. In general, to counter this limitation, alumina can be purchased as a mosaic (“precut”) of smaller elements, faceted/beveled to match 3D shapes. This material is mostly used for vehicle-protection applications or cost-efficient personal protection inserts.

13.2.2 Silicon carbide or nitride Silicon carbide offers good weight reduction compared to alumina, but its price limits its use (Fig. 13.1; Table 13.2). Silicon carbide has good overall ballistic properties. Like its alumina counterpart, it can be used against all threats. Care should be taken in assembling it with other materials as its surface is in general very smooth. Poor bonding might be problematic for multihit requirements, as elements or pieces of ceramic close to the first shot might displace or even fall out should the confinement be insufficient or poorly designed (Yadav and Ravichandran, 2003; Reddy et al., 2008). SiC is mostly produced through reaction-bonding or hot-pressing processes. Shapes and element size limitations are similar to the ones for alumina, described above.

Figure 13.1 SiC ceramics.

372

Lightweight Ballistic Composites

Table 13.2

a

Silicon carbide properties (3M Technical Ceramics)

Average density

(g/cm3)

3.1e3.3

Water absorption

%

0

Hardness (Vickers)

MPa

2000e2600

Fracture toughness

MPaOm

2e5a

Flexural strength

MPa

350a

Compressive strength

MPa

2500

E-modulus

GPa

z380e450a

Cost index

NA

3e5

Depends on the manufacturer or process.

SiC is mostly used for personal or aircraft protection, for which weight constraints tend to be tighter than for land vehicles, even though some are equipped with SiC protection systems.

13.2.3

Boron carbide

Boron carbide is among the hardest materials known to humans (Table 13.3). It has a very good performance/weight ratio, yet is also one of the most expensive. In contrast to Al2O3 and SiC, some B4C grades show lower efficiency against tungsten-core ammunition. As for SiC, bonding can be challenging and the same shape limitations apply. Boron carbide is mostly used for high-end personal protection and aerospace applications, for which weight is critical.

Table 13.3

Boron carbide properties (Technical-Ceramics)

Average density

(g/cm3)

2.4e2.5

B4C content

%

98.5e99.5

Water absorption

%

0 2

z2800e3200

Hardness (Vickers)

kg/mm

Fracture toughness

MPaOm

z2.5e3

Flexural strength

MPa

z400e420

E-modulus

GPa

z450

Cost index

NA

5e10

Ceramic-faced molded armor

13.3

373

Shapes of ceramics

13.3.1 Flat tiles Flat tiles are available in various shapes and sizes (Fig. 13.2). They usually range from around 20 to 250 mm length/width and 3 to 20 mm thickness, even though much larger and thicker elements are available.

13.3.1.1 Thin and thick tiles Very thin alumina tiles can display bending due to the important shrinkage during the firing process (w15%). Also, variations in thickness or geometry can induce locally different expansions and cause dimensional tolerances not to be matched (Figs. 13.3 and 13.4). The geometry of the element is important for multihit and overall resistance. When subjected to vibrations or shocks, a panel made of smaller elements will tend to spread its deformation onto each of them, limiting the stress, the ceramic elements acting like scales. Large ceramic elements, however, will sustain stress concentration and might break. In general, the larger the ceramic element, the better the ballistic resistance for single shots (fewer junctions), but the worse for multiple hits (multiple shots on the same element) and shocks, and vice versa.

Figure 13.2 Several types of ceramic shape.

374

Figure 13.3 Thin hexagons.

Figure 13.4 Thick tile.

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375

Figure 13.5 20-mm-width hexagons.

13.3.1.2 Small, large, and monolithic tiles Thickness of the ceramic is one of the main factors of ballistic efficiency for single shots. For multiple close shots, most ceramics must be carefully dimensioned so that two shots are not placed within the same ceramic element (Fig. 13.5). Depending on the physical properties of the ceramic, backing, and bullet, this rule may not always apply. For example, thick and hardened metallic hulls are less sensitive to “blunted-only” projectiles (partially destroyed perforator owing to a hit on an already comminuted ceramic element), whereas fiber backings might be perforated. Nonetheless, it is also possible to increase the efficiency of the backing (typically by increasing the thickness or changing to a higher grade) to counteract this phenomenon. Ceramic thickness can also be increased to improve overall efficiency. Also, adding space between the ceramic strike face and the hull/backing can provide good results, as the fragments generated will spread over a larger area. The challenge is to achieve the greatest weight and/or cost efficiency by adjusting all those parameters. When multihit protection is not required, especially for large-caliber ammunition, large ceramic elements are often preferred, as they perform well while offering a cost-efficient solution. Another type of ceramic tile is used for personal protection. Monolithic tiles offer good performance as most personal protection standards do not require very tight multihit protection. Yet, specific care has to be taken during the design of the confinement, as these elements are quite thin in regard to their length and width. Thus, they can easily be damaged if handled without care should the confinement and/or the backing be too thin or underdimensioned (Sarva et al., 2007).

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The know-how of the ceramic manufacturer is a key factor in ensuring the good dimensional and mechanical homogeneity of the ceramic.

13.3.2

Shaped ceramics

Apart from the flat elements mentioned earlier, some more complex designs are also available in the market. For instance, spherical or cylindrical elements can also be sourced. Their main advantage is their relative ease of use when covering a complex shape: owing to their small size, it is easier to lay them around inserts or holes or on thin strips, where tiles would have to be tailor-made. Also, for the same reason, they tend to be more resistant when subjected to vibrations or shocks. Specific care should nonetheless be taken to confine them, as their round shape makes it difficult to ensure a large bonding surface with rigid confinement. Also, the larger the pellets, the larger the holes in between them, which might be problematic as soft-core projectiles can be channelized when hitting this area, creating a jet which can be aggressive against soft backing material. Owing to their size, pellets are in general not as good as tiles for single shots, but do not exhibit a similar loss in performance when subjected to multiple hits, if well dimensioned and confined efficiently. Some pellet/sphere geometries are still patent pending as of 2015.

13.3.2.1 Spherical elements For a given panel thickness, spheres would in theory reduce the ceramic weight by as much as 50% (Fig. 13.6). Yet, the thickness of the ceramic, when using spheres, needs

Figure 13.6 Ceramic balls.

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377

to be increased, which makes their weight efficiency questionable depending on the threat. Spheres also have the disadvantage of giving quite variable results for some threats, because of the uneven thickness of ceramic across the panel. Yet, they offer the advantage of being very easy to lay when covering cylindrical surfaces (one curvature), as they always are in contact at a point with the others. Some manufacturers advertise the use of multiple-stack concepts to counteract the uneven thickness distribution, others have developed concepts in which pellets are not bonded, capitalizing on the spheres’ capacity to trickle and rearrange after impact, advertising “self-healing” systems.

13.3.2.2 Cylinders Cylinders offer a good alternative to spheres (Fig. 13.7). Even if they share similar limitations, their thickness is nonetheless relatively homogeneous over the surface of the panel. This aspect limits the variability of the results and generally improves the performance. Their availability in small diameters offers good multihit properties but also limits their efficiency for single hits. The production process generally limits their thickness to between half the diameter and one diameter. Cylinders are thus more dedicated to panels requiring protection against tight multiple hits.

13.3.2.3 Multicurvature Monolithic ceramic can be used in various shapes and sizes. It can commonly be found in 250
300 mm and multicurved shapes. Multicurved monolithic inserts are the most commonly used shape for personal protection applications (Fig. 13.8). Some suppliers of finished inserts use the ceramic as a “tool” for shaping the backing and/or confinement to ensure tolerances of the finished product. This process generally occurs under vacuum or in an autoclave. Comparing the ballistic performance of armor systems made of monolithic ceramic or by assembling arrays of tiles, monolithic inserts, in general, provide better performance at a lower weight. Armor system performance is defined by the properties and thicknesses of the various components and the quality of the bonding.

Figure 13.7 Ceramic cylinders.

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Figure 13.8 Monolithic torso and side inserts.

13.4

Composite backings

Backing is the second most important component of an armor system, ceramic being the first. Ballistic backings are composites, but differ from the widely used structural composites. Fibers for backings offer high mechanical resistance combined with good elongation at break and shear resistance. The main difference with structural composites is that fibers need to be able to delaminate, as a large part of the energy under impact will be absorbed during this phase. The former argument explains the fact that carbon, even though efficient as a structural composite, is not extensively used as a backing material, because of its very limited elongation at break and sensitivity to shear. The latter justifies why backings have in general a low resin content (commonly under 20% in weight) and do not use high-resistance resins (phenolic or polyethylene favored over epoxy, for example). This probably led to the (slightly exaggerated) sayings that “a good ballistic composite is a poor structural one” (and vice versa) and that “resin is a dead weight for ballistic backings.” It should also be noted that soft-core ammunition (mostly lead) or “semi-AP” (typically mild-steel-core ammunition) can be stopped just with pure fiber solutions and does not require the use of ceramics. AP ammunition will always require ceramic or steel-faced solutions.

13.4.1

Fiberglass

Fiberglass is extensively used in ballistic protection as it is very cost efficient. Yet, it also has a limited ballistic performance/weight ratio. Owing to this fact, it is difficult to use it as a pure fiber solution, even for soft-core ammunition. Nonetheless, fragments being less discriminating threats, it can successfully be used as a spall liner. Also, fiberglass has good secondary properties and shows good stability when exposed to chemicals, fire, heat, or humidity (Fig. 13.9).

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Figure 13.9 Glass fiber fabric.

There are various types of fiberglass, E-glass, R-glass, S2-glass, and S-glass being the most widespread grades encountered on the market. E-glass is very cost efficient yet has the lowest ballistic performance/weight ratio. R, S2, and S are higher grades with improved performance, yet their price is also increased, roughly by 100% (for a given weight). Yet, as the higher grades perform better, less material is needed to achieve the same performance against fragments. When considering this element, price increases by only 50% (for a given ballistic performance). Fiberglass, when used as a spall liner, is often combined with phenolic resins, but other resin systems, such as thermoplastics, are available. Fiberglass for ballistic use is commonly offered in a thick roving (plain weave) format, coated with a phenolic resin at around 20% (weight ratio).

13.4.2 Woven aramid fabrics Aramid offers improved performance over glass (Fig. 13.10). It also shows good secondary properties, similar to glass, when exposed to heat or fire. The polymeric nature

Figure 13.10 Aramid fabric.

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of aramid fabric makes it more sensitive to some chemical exposures and care should be taken to protect it from ultraviolet light (UV) and humidity (risk of hydrolysis). This is the reason some Ministry of Defence standards require the panels’ edges to be coated or the soft packs to be sealed. Aramid prices are higher, roughly four times that of E-glass (for a given weight). Yet, its improved ballistic properties against fragments require less weight, and, for a given performance, the price would roughly be doubled. The most widespread types of aramid are the ones described in Military Standard MIL-DTL-62474, classes C and D being extensively used for spall liners or backings, coated with 20% (in weight) phenolic resins. It should be noted that aramid shows limited efficiency at stopping soft-core rifle ammunition, as well as mild-steel cores, when used as a “stand-alone” (not combined with other materials).

13.4.3

UHMWPE woven fabric

UHMWPEs (or PEs) offer very good ballistic properties. They are mostly offered in unidirectional formats and are marginally used in woven types (Fig. 13.11).

13.4.4

UHMWPE unidirectional fabric

UHMWPEs in unidirectional (UD) format offer very good ballistic performance, combined with ceramics or metals or as a stand-alone. To ease handling, PE UD tapes are in general combined with low-density polyethylene or polyurethane-based resins, in a 0 /90 orientation (Advanced Fibers datasheets).

Figure 13.11 High-performance polyethylene woven fabric.

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A wide range of PEs is available on the market, with different ballistic performance/ weight ratios (Vargas-Gonzalez et al., 2011). They range from PEs having efficiency and price similar to those of aramid to advanced materials that are extremely costly, yet have highly superior performance. As an average, PE is six times more expensive than E-glass (in weight) but its outstanding performance against fragments limits its cost to two and a half times that of E-glass for a given performance. UD PE can be used as a stand-alone to efficiently stop soft-core ammunition as well as mild-steel cores. UD PE offers good secondary properties and virtually no sensitivity to UV compared to aramid. Yet, PE being a material with a relatively low melting point (w140 C), its performance starts degrading when exposed to heat above 60 C and specific materials need to be added to ensure good flame-retardant properties. The plies require high pressure to be bonded together efficiently, and depending on the threat, ultrahigh pressure might even be required to provide the best performance. It should be noted that as some patents held by the historical manufacturers are expiring, more and more UD PEs are becoming available on the market, with variable performance and quality but also prices starting at a fraction of the price given previously. Care should be taken when benchmarking/using low-cost materials, as the areal density of the laminate can vary, potentially leaving some areas vulnerable.

13.4.5 Aramid unidirectional fabric Aramid UDs are also available (Fig. 13.12). They do not exhibit the same ballistic performance as PEs, though. They are available as plies to be used in (soft) ballistic vests but can also be found with resins allowing them to be pressed into hard armor.

Figure 13.12 Gold Flex.

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13.5

Lightweight Ballistic Composites

Fabrication of ceramic-faced armor

Pressure is needed to assemble all the materials together. Thus, the three main processes for fabricating hard armor panels are vacuum bag, autoclave, or press. Commonly, vacuum bagging reaches a pressure of 1 bar, autoclaving can achieve 3e20 bars, while presses can achieve 200 bars and above. Other processes such as hydroclave, pressureless wet layup, or adhesive bonding also exist, yet being more “exotic.” PE backings achieve higher performance with ultrahigh pressure, but the other materials do not require such process. Vacuum bagging for ballistic protection is equivalent to the technique used in the conventional composites industries. All the elements are placed together under 1 atm until the various materials are cured, with or without temperature. The pressure developed during vacuum bagging is not sufficient to process backings and liners: even though possible, low-resin-ratio materials processed under low pressure will tend to delaminate massively when subjected to (multiple) impacts. The autoclave is used for processing ceramic, confinement, and sometimes backings at once. The advantage of the process is its ability to develop evenly distributed pressure around the parts. This allows, for inserts, the ceramic to be used as a “tool” for the backing to deform and adjust to the ceramic shape. Also, the relatively high pressure achieved in the autoclave gives a good compaction rate for the various elements. Care should be taken if the “ceramic as a tool” technique is used, as even 1 bar of pressure can break the ceramic if not supported or if the backing shape is too different from that of the ceramic. As a reminder, all autoclaves need nitrogen-injection or oxygen-depletion systems for the internal pressure to go above 10e12 bars. Above this pressure, the oxygen density is high enough for corrosion to occur or even detonate in the event of an electrical discharge or thermal runaway, for example. High-pressure presses are mostly used for compaction of liners and backings. If multicurvature parts need to be produced, a specific and generally expensive tool is needed. For assembling ceramic and backings in a flat format, high pressure is not necessarily required.

13.5.1

Personal protection

Ceramic is widely used for personal protection because not only does it come at significantly reduced cost (compared to 100% fiber systems using high-performance polyethylene (HPPE) fibers) but also it can stop AP ammunition at a relatively light weight. Indeed ceramic can break the steel-core bullet tip, in contrast to a 100% fiber solution. The monolithic shape is the most commonly used design. It leads to the lightest body armor inserts. However, the main disadvantage is shock resistance and its low multi-impact capabilities. The manufacturers’ know-how permits them to improve the ceramic shock and multihit resistance by using suitable backing and bonding processes.

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Figure 13.13 Standard ceramic armor and LIBA (light improved ballistic armor).

13.5.1.1 Ceramic-faced hard molded armor backing As explained above it is not possible to recommend a “best armor material.” One should take into account all the ballistic requirements, as well as the manufacturing capability, weight, and cost (Fig. 13.13). Every lightweight advanced component, ceramic or fiber, suffers from one of several disadvantages, which must be balanced by using other components: • • • • •

Hot-pressed and pressureless sintered ceramics are lightweight but brittle materials. Confinement of the ceramic is especially important for dense brittle materials (Carton and Roebroeks). HPPE is the lightest fiber, but sensitive to temperature and deformation. Aramid fiber shows an interesting weight versus cost compromise, but does not offer the best protection against high-speed lead-core ammunition. Monolithic ceramics offer the best absorption energy, but are more fragile than tiles. Ceramic pellets offer the best multihit and shock protection, with overall a weight increase.

13.5.1.2 Ceramic-faced flexible armor backing In the past development of flexible armor became a matter of high interest. Several concepts exist but apparently are not widely used.

13.5.2 Vehicle armor 13.5.2.1 Ground vehicle Most military vehicles are designed with armored hulls, using high-hardness steel or aluminum. Yet, owing to the higher weight efficiency of composite armors over metals, the levels of protection required continue to increase, while the weight of the hull remains stable, the difference being absorbed by add-on armors. Some vehicle manufacturers have even made the choice of reducing their hull mass (and thus base protection), using the efficiency and versatility of composite armors to achieve the desired protection with lower weight. The advantage of a light vehicle is the reduced base price, limited maintenance costs (limited wear and tear), air-cargo capability, enhanced fuel consumption, and better handling (roll over) (Fig. 13.14).

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Figure 13.14 Examples of ground vehicles.

There are quite a large number of standards defining the levels of protection for military vehicles, the most common ones being NATO STANAG 4569 AEP-55, VPAM APR 2006, MIL-STD 662, and EN 1522. Those standards define threat, ranging from small-caliber handguns (such as soft-core 0.22 LR), shoulder-fired AP rounds (among which is the notorious tungsten-core 7.62
51 AP8/M993), and anti-material rounds (12.7
99 mm APM2 or 14.5
114 APIB32) to antitank rounds (30
173 mm APFSDS-T). Metal add-ons can be used to enhance protection as weight is not always considered critical. Yet, the highest performance/weight efficiency is usually achieved with ceramic/composite armors, with the notable exception of high-angle attacks. In such a configuration, the threat tends to ricochet on steel but this phenomenon is more difficult to achieve on softer materials, such as aluminum, titanium, or ceramic/composite solutions. Some standards specify multihit (multiple shots placed close to each other), which requires the ceramic element size to be adjusted. The 600 BHN steels and harder are also brittle; thus some manufacturers provide perforated metallic add-ons, which limit the crack propagation and enhance the resistance to multihit. Vehicle ceramic/composite protection usually includes alumina ceramic, glass fiber, and aramid.

13.5.2.2 Airplanes and helicopters For aircraft, weight is of critical importance (Figs. 13.15 and 13.16). Thus, the materials used are often more advanced than for vehicles. Silicon carbide and boron carbide are usually selected, in combination with carbon and high-end-grade UHMWPE, pressed at ultrahigh pressure.

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Figure 13.15 Helicopter. Praveen (permissions) has identified source.

Figure 13.16 Airbus A400M.

Secondary property specifications are also very stringent, requiring a careful design to ensure fire retardancy, limited smoke emission, vibration resistance, limited back-face deformation in case of impact, etc. The distance of engagement is much greater, which leads to lower velocities at impact, and multihit is rarely requested. This also changes the design as a low-velocity threat has a different behavior on impact compared to a high-velocity one (Billon, 2007). The level of protection is in general lower, as aircraft are less likely to be exposed to direct fire from large-caliber AP rounds, and weight constraints are such that only critical personnel and equipment are protected.

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Figure 13.17 Boat and zodiac.

13.5.2.3 Boat and ship armor For large boats, weight distribution is important owing to the risk of capsizing if the center of gravity is too high. Thus, such a platform often combines highperformance and cost-efficient protections, placed in different locations. UHMWPE under high pressure is often chosen, as this material barely suffers from hydrolysis (unlike aramid), and armored steel needs to be protected carefully against rust due to the environment. For the higher AP threat level, alumina ceramic offers good performance at an affordable price. Unlike ground vehicles, boats’ hulls rarely use armored steel or aluminum (Fig. 13.17). Yet, the structure is in general thick enough to be used as a part of the ballistic solution. Multihit is rarely required because of the distances of engagement, yet, it is not uncommon for the threats to be of very large caliber (40 mm and above).

13.6 13.6.1

Testing of ceramic-faced armor Ammunition

Table 13.4 lists common types of ammunition (see also Fig. 13.18). The ammunition commonly listed in the standards can be divided into subjective categories: soft core, mild-steel core, and AP (Kneubuehl). Soft-core ammunition can efficiently be stopped with a pure fiber solution, as the impact will deform the projectile, which will then mushroom and be stopped by the layers. Ultrahigh-pressure PE can reliably stop this ammunition at a fourth to a fifth of the required weight of armored steel. Standard AK47 Kalashnikov rounds use a mild-steel core. These can also reliably be stopped with PE, as the core is reasonably soft and lightweight. Yet, a slight variation in the hardness of the core can modify the weight of the PE required. Required weight compared to steel is around 50%.

Ceramic-faced molded armor

Table 13.4

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Types of ammunition

Type of ammunition

Examples

Handgun

9
19 mm, 0.357 Magnum, 0.44 Magnum

Hunting (rifles)

12 gauge, 0.300 Winchester, 7.64

Military (rifles)

5.45, 5.56
45 mm (M193), 7.62
39 mm (eg, Kalashnikov)

Military (machine gun)

12.7
99 mm, 14.5
114 mm, fragments (20 mm FSP)

Military (cannon)

20
120 mm, 25
137 mm

FSP, fragment-simulating projectile.

Figure 13.18 Several types of ammunition.

AP rounds require hard material to defeat the high-hardness core. The core itself is generally made of hardened steel (around 650 BHN) but can also be made of tungsten carbide or depleted uranium. Ceramic backed by fiber, or ceramic add-ons on metallic hulls, are very effective at fragmenting and stopping those threats. Depending on the threat, the required weight compared to armored steel ranges from 35% to 50%. Antitank and anti-material rounds are less often requested than their shoulder-fired counterparts. For those very large caliber threats, the required weight of metallic or ceramic/composite armor to stop them at the normal angle of incidence becomes prohibitive. Thus, vehicles are specifically designed to ensure protection only in the frontal area, which is often sloped to deflect the threat rather than stopping it.

13.6.2 Testing standards and methods For personal protection, the main ballistic standards are the NIJ, VPAM, and HOSDB. Their specificity is the measure of the dynamic back-face deformation to ensure that the trauma to the body of the wearer will remain limited and not possibly lethal, even if all shots are stopped.

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For ground vehicles, STANAG 4569 AEP-55, VPAM APR 2006 Ballistic Standard APR 2006, and MIL-STD 662F Standard are commonly requested. Those standards also list threats of greater caliber or penetrating power than shoulder-fired ammunition, such as artillery shells, autocannon, improvised explosive devices, hand grenades, and TNT charges. Aircraft protection levels are defined in MIL-STD 662F and MIL-PRF 46103, but ground vehicle standards are also used as references. Requirements for aircraft protection have the specificity of including long distance of engagement and thus lower velocities. It is not uncommon for the protection to be designed to stop threats at 400-m distance or farther. To simulate such distances during the testing, the gunpowder charge is reduced and/or a powder with a slower burn rate is used. Here are some general recommendations regarding ballistic testing. Even if standards define quite accurately the ammunition used, it can happen that the penetrating power of an ammunition varies depending on its supplier or batch. Thus, it is recommended to ensure a safety margin in the protection to cover this possibility. It is normal that the bullet impact point may vary from its targeted point. In general and particularly in the case of a low-accuracy round, it is recommended, when performing tight multihit shots, that the shot locations be indicated on the panel in advance. Also, instead of one shot of calibration (used to align the laser designator and the impact point), more can be performed so that the laser can be pointed at the center of the group. It will be faster and more efficient to perform the tests as per a predetermined pattern than to try to recalibrate target/impact points after each shot (statistically, the multihit will be tightened if the previous impact location is used as a reference for the next shot). Yaw measurement is important, especially for a high sectional density threat (long, heavy, and thin cores). A perfectly aligned shot (0 degrees incidence) can be much more aggressive than one incoming with a 5-degree angle, especially on armored steel. If shots are performed at an angle (to simulate roofs, for example), it is recommended, especially for a large-caliber threat, that the angle be checked regularly during the tests. The energy and vibrations upon impact could move the frame holding the target, potentially voiding the trials.

13.6.2.1 Personnel protection The specificity of the ballistic test for personal protection is mainly linked to the presence of a large plasticine or gelatin block to support the plate and/or vest (Fig. 13.19). This support is selected to measure back-face deformation, called trauma. Such media are not reliable simulators of the human body, but are mainly a simple tool to compare the energy absorption of the various products. It is very important that care be brought to the composition and conditioning of this plasticine, frequently used by most laboratories. Some specifications incorporate standard tests near edges to ensure that the ceramic surface really corresponds to the protected surface, which is not obviously the case from one product to another. This is an important step to improve the quality of ceramic armors, which can be manufactured to stop bullets mainly in the center, with no multihit capability.

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Figure 13.19 Insert tested against plasticine block.

13.6.2.2 Ground vehicle and aircraft protection Vehicle armor samples are in general approximately 500
500 mm (Fig. 13.20). Coupons are held in a frame so that the front and back are left accessible. A 0.5-mm aluminum witness plate is maintained at 150 mm from the back of the sample. Its role is to ensure there are no potentially harmful fragments generated during impact.

Figure 13.20 Large-caliber tests.

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If no multihit pattern is requested, some standards allow multiple samples to be provided. This can be used so that only a few shots are placed in each target, to limit the loss of protection due to increasing degradation of the target by previous shots. If temperature conditioning is requested, time becomes a critical factor as the target is coming back to room temperature. It is recommended that the shot locations be marked prior to thermal conditioning of the samples (also, frost makes marking difficult). As tests are in general costly, it is also recommended to always plan a backup solution, should a failure occur. It can, for example, be a set of various panels or add-ons or thicker liners. If a ceramic add-on is used as a strike face for metal, it is recommended that the distance between shots be measured on the metallic element. Damaged areas on ceramic tend to be larger and more uneven than on metals, leading to measurement uncertainties. STANAG 4569 defines a multihit pattern as each shot being placed in the corners of a parallelogram of 25e45 mm width and 100e120 mm length. For most ammunition, this (realistic) configuration makes only two shots critical (the second one of each tight pair). For nonofficial research or dimensioning testing, multihit shots can be placed on a line, spaced 25e45 mm from one another. This will make all shots except the first one critical, and potential problems will be detected twice as fast (and twice as cheaply).

References 3M Technical Ceramics Advanced Ceramic Datasheets. http://technical-ceramics.3mdeutschland. de/en/products/3m-ballistic-vehicle-armor.html. Advanced Fibers Datasheets, http://www.dsm.com/products/dyneema/en_GB/product-tech nologies/ud.html. Bao, Y., Su, S., Yang, J., Fan, Q., 2002. Prestressed ceramics and improvement of impact resistance. Materials Letters 57 (2). Billon, H.H., 2007. A New Method for Calculating the Critical Penetration Velocity (V0) (DSTOeTNe0791). Carton, E., Roebroeks, G., 2015. Testing Method for Ceramic Armor and Bare Ceramic Tiles. In: 39th International Conference and Exposition on Advanced Ceramics and Composites. Franzen l’t, R.R., Orphal, D.L., Anderson Jr., C.E., 1997. The influence of experimental design on depth-of-penetration (DOP) test results and derived ballistic efficiencies. International Journal of Impact Engineering 19. Holmquist, T.J., Johnson, G.R., 2005. Modeling Prestressed Ceramic and its Effect on Ballistic Performance. International Journal of Impact Engineering 31. Holmquist, T.J., Rajendran, A.M., Templeton, D.W., Bishnoi, K.D., 1999. A Ceramic Armor Material Database, (Tardec 19990506 017). Kneubuehl, B.P., December 2002. Ballistic Protection. Morgan Advanced Materials, Advanced Ceramic Datasheets, http://www.morgantechnical ceramics.com/products/product-groups-industry/defence-products.

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MIL STD 662F Standard, everyspec.com/MIL-STD/MIL-STD-0500e0699/download.php? spec¼MIL-STD-662F.006718.pdf. Reddy, P.R.S., Madhu, V., Ramanjaneyulu, K., Balakrishna Bhat, T., Jayaraman, K., Gupta, N.K., 2008. Influence of polymer restraint on ballistic performance of alumina ceramic tiles. Defence Science Journal 58. Sarva, S., Nemat-Nasser, S., McGee, J., Isaacs, J., 2007. The effect of thin membrane restraint on the ballistic performance of armor grade ceramic tiles. International Journal of Impact Engineering 34. Vargas-Gonzalez, L., Walsh, S.M., Wolbert, J., February 2011. Impact and Ballistic Response of Hybridized Thermoplastic Laminates, ARL-MR-0769. VPAM APR2006 Ballistic Standard, http://www.vpam.eu/fileadmin/Pruefrichtlinien_ AKTUELL/2009-05-14_APR2006_englisch.pdf. Yadav, S., Ravichandran, G., 2003. Penetration resistance of laminated ceramic/polymer structures. International Journal of Impact Engineering 28.

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Materials, manufacturing, and enablers for future soldier protection

14

J.Q. Zheng 1 , S.M. Walsh 2 1 Program Executive Office e Soldier, US Army; 2Army Research Laboratory, Adelphi, MD, United States

14.1

Introduction

The United States has historically developed head and torso/lower body protection that anticipated and was appropriate for the kinds of threats soldiers were likely to encounter as part of their mission and operational environment. Fig. 14.1 is a brief but representative history of major developments in US body armor, to include protection of the head, torso, and other areas of the body. In World War I, for example, a typical threat was an overhead bursting munition that would release potentially lethal fragments above the soldier. Awareness of the type and behavior of the threat was thus the impetus driving the use of steel and a wide-brim helmet design to manage this type of threat. By World War II, improvements in the type of steel useddas well as the overall design of the helmetdgave rise to the M1 “steel pot” design. The M1 helmet evolved to meet the prevalent fragmentary threats from WWII munitions, and variations of the M1 helmet included the introduction of polymeric materials such as nylon and Doron in the helmet liner; the helmet remained in use well through the Vietnam era. US head protection innovation was relatively modest until the end of the Vietnam era, when the Personnel Armor System for Ground Troops (PASGT) helmet was introduced. The PASGT was truly revolutionary not only for its pioneering use of aramid fiber-based composites (known at the time as Kevlar®) but also for early analytic and casualty reduction assessments and approaches that helped inform the rationale for the design of the helmet ensemble. Since then, head protection has come to define a much broader range of technologies and approaches than the typical and familiar soldier’s “helmet.” Future concepts (eg, >2025) include building on improvements in helmet/body ensembles across multiple materials length scales and introducing mass-efficient functionalities to broaden and enhance soldier survivability. Materials have obviously played a critical role in the type of protection developed for various types of threats and overall soldier operational needs and requirements. The transition from metals (eg, steel) to wholly organic materials (eg, aramid-reinforced polyvinyl butyral (PVB) phenolic resin and ultrahigh-molecular-weight polyethylene (UHMWPE) composites) has been motivated by the desire for more mass-efficient

Lightweight Ballistic Composites. http://dx.doi.org/10.1016/B978-0-08-100406-7.00014-3 Copyright © 2016 Elsevier Ltd. All rights reserved.

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Lightweight Ballistic Composites WW I

Brigandine Plate steel armor torso plates, helmets

WW II

Steel armor torso plates, helmets ballistic nylon & steel plate torso, side, groin armor AD = 30 Ib/ft2

Vietnam

Ballistic nylon vest doron and ceramic plates AD = 11.0 Ib/ft2

Persian gulf

PASGT kevlar vest & helmet supplemental AI203 plate AD = 8.5 Ib/ft2

Korea

Nylon, doron, steel vest & helmet AD = 20 Ib/ft2 GWOT

IBA spectra, kevlar vest & helmet B4C & SiC torso & side plates AD = 6.5 Ib/ft2

Figure 14.1 Brief history of US materials and designs for protection of dismounted soldiers.

ballistic protection. Specifically, demand for helmet materials innovation is driven by the need for effective resistance to penetration from evolving threats, the need to minimize the maximum dynamic deflection associated with nonpenetrating ballistic impacts, and the need to minimize total shell weight. Protection against fragmenting munitions has long been the primary standard, and careful quantification and characterization of fragments of various grain sizes has enabled the development of standards to not only ensure “first article” and lot acceptance quality but also to provide a baseline to compare the performance gains offered by new materials. The 9 mm (handgun) is also a legacy requirement, and an excellent example of a rare but influential requirement that can skew overall helmet properties because of the unique mass and back-face deformation attributes that accompany a bullet of this type. More recently, the first-ever US-defined level of small arms bullet protection has been included in the specifications for the enhanced combat helmet (ECH); the ECH aggressively exploits significant advances in both UHMWPE composite materials and a new generation of manufacturing technologies. The traditional approach in the development of head (“neck up”) and body (“neck down”) ballistic protection has relied primarily on developing new materials and designs that provide effective resistance against contemporary and representative threats. This includes consideration of the ergonomic aspect of designing body armor systems to allow for a reasonable balance of protection and comfort. Fig. 14.2, though extreme, is an example of the type of loads the soldier might carry on a given mission. Much of this weight is parasitic until it is needed (eg, water, ammunition, body armor, batteries). More recently, concerns have been raised about placing additional armor on the soldier in an effort to increase ballistic resistance against more aggressive threats or provide more area of coverage or a combination of both. This increased protection translates

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Figure 14.2 Soldier-borne loads including materials, equipment, and powered devices.

into both physical and, potentially, cognitive burdens for the soldier, depending on the complexity of the devices and systems he or she must interface with under highly variable and demanding operating conditions. Mitigation of physical and cognitive burdens while ensuring robust protection against legacy and emerging threats requires new approaches in materials, design, and disruptive concepts to ensure the survivability of the dismounted soldier. Exploiting the inherent properties of a range of both organic and inorganic materials has been the traditional but effective means of minimizing the weight burden associated with personal protection equipment.

14.2

New directions in head protection

The primary means of protecting the head from ballistic threats has historically been a helmet (Dean, 1918; Alesi et al., 1975; Walsh et al., 2005, 2006a,b, 2007). However, with the advance of new electronic equipment, design tools, and other technologies, the term “head protection” is evolving into a more complex suite of integrated capabilities that enable improved soldier survivability and capability. For example, the development of eye and face protection coupled with the desire to integrate active hearing protection, displays, night-vision devices, and chemo-bio protection has forced a rethinking of how to reconcile weight and design of the head protection “system.” The Natick Soldier RDEC-led Helmet and Electronics and Display SystemeUpgradable Protection (HEaDS UP) program, illustrated generally in Fig. 14.3, was a cross-Department of Defense (DoD) effort to identify, develop, and demonstrate risk-reduction strategies for ensuring added head-borne capability and

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Figure 14.3 The HEaDS UP program: a holistic approach for balancing new levels of protection and capability in head-borne systems. Image courtesy of Natick Soldier RDEC.

improved protection without deleteriously degrading soldier comfort, performance, and effectiveness. The added device aspect of HEaDS UP, together with a desire to address higher levels of fragment and small arms protection, reemphasized the need for lighter, more resilient ballistic materials. Furthermore, HEaDS Up enabled entirely new concepts (eg, the Vertical Load Offset System (VLOS)) for improved protection and capability supported in part by the shoulders and torso. More recently, an effort led by Program Executive Office (PEO) Soldier has taken a more comprehensive approach to overall protection of the soldier, to include simultaneous consideration of head, torso, and other modes of body-worn protection. This initiative is called the Soldier Protection System (SPS). A competitive process has been used to find the most effective combination of materials, concepts, and capabilities subject to the extensive SPS requirements. Fig. 14.4 illustrates some of the novel designs under consideration for enabling new levels of integrated head protection and capability. There is significant research occurring in government, academic, and industrial sectors to improve all aspects of head protection in the context of the types of threats, operations, and environments the individual soldier may encounter. This collaborative research “ecosystem” is shown in Fig. 14.5. The army, as well as other parts of the DoD, has established the underlying materials, armor mechanics, and processing science needed to advance and critically assess head protection. These include the use of numerical tools and modeling, state-of-the-art testing, and visualization techniques such as flash X-ray, digital image correlation, and high-speed imaging. As shown in

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Figure 14.4 Soldier Protection System prototypes to significantly improve comprehensive head, eye, and mandible protection. Image courtesy of PEO Soldier.

Fig. 14.6, the current trend is to establish a richer set of research tools that address not only the types of threats for future head protection requirements, but also the underlying science needed for properly coupling synthetic and biological materials in response to these imposed ballistic and blast threats. Such tools will enable insight that can inform new materials development as well as new head protection design, injury criteria and assessment, and ergonomic device integration and anticipate the challenges associated with the process and manufacture of next-generation materials and components.

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Figure 14.5 Academic, industrial, and government collaboration.

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Figure 14.6 Head protection research thrusts.

14.2.1

A comprehensive material/process/property approach for improving head protection

Constituent material selection has historically been the most influential factor in determining the mass, ballistic resistance, and back-face deformation (BFD) signature in a helmet configuration. However, the trend toward materials like UHMWPE has highlighted the significance of processing parameters on material morphologies, bulk mechanical properties, and ballistic performance as shown in Fig. 14.7. A process/ property/performance correlation research strategy has been developed by the Army Research Laboratory (ARL) and is designed to characterize the complex relationship of performance parameters with intrinsic materials properties, especially for classes of materials known to exhibit a high degree of dependence of variation with their processing history. For example, recent micrographs reveal the influence of pressure and temperature in UHMWPE materials, including the extrusion of fibers and matrix, that affect the resulting volume fractions and thickness of the laminate (Fig. 14.8). Efforts to establish a comprehensive process/property/performance relationship rely on several factors, including the use of state-of-the-art characterization equipment to observe features developed in the materials. Such characterization allows for a direct correlation of processing conditions with the evolution of morphologies in the resulting material. In the case of polymer composites, this could include observation and characterization of fiber, matrix, and void content and their distributions, fractions,

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Figure 14.7 New microstructures and morphologies to enable new properties.

and prevalence depending on constituent materials and processing conditions. However, it is critical that an acceptable level of confidence in the observed materials and process artifacts be obtained before concluding process/property/performance outcomes. For example, UHMWPE is especially challenging when subjected to optical microscopy characterization. For example, optical microscopy may not fully capture the

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Figure 14.8 Micrograph of a laminate section taken from an untrimmed edge of a panel after manufacture at 270 F and 10 ksi. Lateral extrusion of individual layers is visible along the right edge of the specimen. This extrusion of fiber and matrix results in changes in layer thicknesses and fiber volume fractions.

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complete details of UHMWPE microstructure. These limitations include the preparation phases of the sample, such as the unintentional smearing of the polyurethane matrix during polishing. The attraction of X-ray micro-computed tomography (micro-CT), by contrast, is the ability to directly visualize voids, polyethylene fibers, and the polyurethane matrix in situ as illustrated in Fig. 14.9. The challenge with using CT in UHMWPE is that polyethylene and polyurethane have similar densities and therefore similar X-ray mass attenuation coefficients. This leads to similar contrast between the fibers and the matrix, and contrast is typically improved when scanning the sample at a lower voltage and increasing the exposure time to compensate for the reduced signal. The large difference in the density and attenuation between air and the polymeric materials helps distinguish voids within a given sample. Generally, two types of voids have been observed in UHMWPE composites, smaller, spherical-shaped voids and elongated voids in the direction of the fibers, as illustrated in Fig. 14.10 (Omasta et al., 2015; Sietins et al., 2015). Both void types are primarily located between the 0 /90 interfaces, and their size and quantity have been correlated with imposed processing conditions. Scanning parameters can influence the clarity and contrast of the images as well as the degree of scanning artifacts, and thus could potentially influence the quantitative results; examples of problematic scanning artifacts are shown in Fig. 14.11 and include ring artifacts, background noise, and distortions of features due to pixel shifts. These artifacts can be especially problematic in UHMWPE scans; however, prudent selection of volumes of interest, decreased source-detector spacing, and minimizing movement during the scan can improve the quality of results. Thermoplastic ballistic composites (including UHMWPE and aramids with thermoplastic matrices) have demonstrated much higher sensitivity to temperature and pressure conditions. The result can be significant variations in BFD (Fig. 14.12(a)) and

Voids Polyethylene fibers

Polyurethane matrix

Figure 14.9 Micro-computed tomography image indicating voids in black, polyethylene fibers in dark gray, and polyurethane matrix in light gray.

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Figure 14.10 Spherical-shaped voids (left) and voids elongated in the fiber direction (right).

ballistic performance (Fig. 14.12(b)) of UHMWPE ballistic laminates. It should be noted that not all UHMWPE materials exhibit this type of sensitivity, and hence the need for more insight into the source and degree to which end-item ballistic response depends on upstream processing variables. Nevertheless, Fig. 14.12 also suggests caution when processing laminate “hybrids” constituted from different UHMWPE material grades (eg, combinations of Spectra® 3136 and 3130, Dyneema® HB2 and HB80, or various types of Dyneema® and Spectra®). Fig. 14.13 provides a comparison of 17-gr V50 performance for various monolithic aramid and UHMWPE ballistic-grade composites. In addition to materials and process-induced variations, variations in geometry (eg, flat plate versus helmet) can also influence bulk ballistic response. There are multiple methods for processing thermoplastic materials, including autoclaving (heat and pressure applied by gas), matched metal tooling/presses, and variants such as heat tables with sealed vacuum bags. The degree to which these distinct processes can influence ultimate ballistic properties developed in the final composite is shown in Fig. 14.14. One possible method for screening potential candidate materials for future helmet applications is the correlation of V50 with BFD. The goal is to identify materials that have high V50 with low BFD compared to the incumbent state-of-the-art materials. Fig. 14.15 illustrates a series of data points for a variety of UHMWPE

Figure 14.11 Artifacts in scans of UHMWPE composites. (a) Ring artifact, (b) background noise, and (c) distortions due to pixel shifts.

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Figure 14.15 V50 and back-face deformation (BFD) correlation to identify promising new grades of ballistic materials for helmet applications.

composite materials, and there is a clear trend toward the desired combination of V50 and BFD with new fiber and “tape” formats compared to earlier UHMWPE materials (eg, HB26). While UHMWPE materials have clearly demonstrated superior ballistic mass efficiency over aramid materials in certain helmet and body armor applications, it is important to note that there are still instances in which the latter type of material is desirable. For example, aramids exhibit far greater stability in properties over a wide temperature range. As such, advances in linear polymers (to include aramids, UHMWPE, and other materials) are encouraged and welcomed in the future. Often when a piece of ballistically protective equipment is described, it is done so in terms of one or more of the materials that constitute the equipment. For example, a helmet is described as an aramid composite (eg, Kevlar® or Twaron®) or a UHMWPE composite (eg, Dyneema® or Spectra®). Certainly, constituent materials such as high-performance ballistic fibers and matrices with varying levels of compliance are key determinants of ultimate system attributes such as weight, ballistic performance, and thickness. However, recent research has reemphasized the importance of considering material architecture as well. As defined here, material architecture accounts for the geometric arrangement of the constituent material at different length scales. On a macro scale this could include 0 /90 and other relative ply orientations, but in the future it could include the deliberate and repeatable organizing of matter on

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disruptively smaller length scales (eg, UHMWPE tapes, graphene, and other two-dimensional (2D) polymeric membrane materials).

14.2.2 New insight into the influence of laminate architecture on ballistic performance Fig. 14.16 depicts classical orientations for unidirectional ply materials including (Fig. 14.16(a)) unidirectional, (Fig. 14.16(b)) 0 /90 , and (Fig. 14.16(c)) 45 . Materials such as Dyneema® and Spectra® ballistic composites can be supplied in various configurations, including rolls with purely unidirectional orientation and cross-plied product as well. The advantage of the purely unidirectional materials is the ability to explore other types of laminate configurations which may modify bulk laminate response without increasing weight or volume. One example in which such flexibility in ply-to-ply configuration of ballistic material has been critical is in the development of new and innovative methods for reducing BFD without severely reducing ballistic resistance. Fig. 14.17 shows a controlled geometry comparison between a 0 /90 Dyneema® laminate and a hybrid laminate wherein relative ply orientation is varied at discrete locations in the laminate. The results shown in Fig. 14.17 compare a conventional 0 /90 UHMWPE laminate with a new “X-Hybrid” architecture developed by the ARL (Vargas-Gonzalez and Walsh, 2010, 2011; Vargas-Gonzalez et al., 2011a,b); as shown, there is a dramatic reduction in BFD with little reduction in the V50 of the bulk material system (V50 testing is an accepted and objective measure of ballistic integrity, and minimization of BFD is critical to both helmet and torso body armor). The benefit of using the X-Hybrid architecture has been demonstrated with both Dyneema® and Spectra® composite material systems. More fundamentally, the X-Hybrid configuration suggests there are modes of organizing materials to improve performance in critical personnel protective equipment systems. It also suggests that encouraging more of the material to participate in the defeat of a projectile could lead to an improved balance between ballistic resistance and the dynamic and residual BFD that accompanies a ballistic event. Fig. 14.18 clearly illustrates this type of “plate participation.” The images shown in Fig. 14.18(a) are the classic response of planar

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Figure 14.16 Classical macroscale ply configurations used in ballistic helmets and body armor. (a) Unidirectional laminate. (b) [0 /90 ] laminate. (c) 45 laminate.

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Figure 14.17 Significant influence of material architecture on V50/BFD performance (error bars shown).

deformation from a projectile at the center of the 0 /90 laminate, whereas Fig. 14.18(b) illustrates the X-Hybrid response to the same type of ballistic event. Note the degree to which the X-Hybrid laminate becomes more fully engaged, thereby helping reduce the maximum BFD. By contrast, the 0 /90 configuration is much more localized and rapidly deforms around the local area of ballistic impact, as evidenced in Fig. 14.18(a); there the primary fibers associated with the impact are activated but relatively little of the other portions of the plate participate in the early stages of impact. Fig. 14.19 provides a quantitative measure of the cross-sectional displacement of both the 0 /90 and the X-Hybrid laminates. More recently, Vargas-Gonzalez and Gurganus (2015) have demonstrated another potential benefit of the X-Hybrid. As shown in Fig. 14.20, there is a dramatic reduction in the transmitted pressure using the X-Hybrid architecture (compared to the 0 /90 baseline). This observation could potentially lead to new methods for mitigating adverse pressure waves induced by nonpenetrating bullets and blast waves.

14.2.3

Insight enabled by modeling of helmet materials and laminates

The damage and failure induced by low-energy impact on polymer composites is a key driver in determining the structural integrity and durability of ballistic helmets. Modeling is especially useful in describing phenomena such as fiber failure in tensile

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Figure 14.18 Displacement maps generated as a function of time after impact for (a) 0 /90 and (b) the ARL X-Hybrid.

mode, compressive matrix failure, and delamination between adjacent plies. Such modeling can aid in the development of better engineered matrix and fiber materials and configurations to balance high-velocity (ballistic impact) with low-velocity (blunt impact) resistance. Modeling and simulation are increasingly providing insight into the complex response of materials to various types of extreme loading conditions, including low velocity blunt impact and high velocity (ballistic) impact. The effect of damage upon low-energy impact of polymeric composites Batra et al. (2012) has been examined and such tools could help assess materials and configurations for ballistic and durability performance of laminate composites. Models rely on improving the descriptions of the constituent materials. For example, a comparative study exploring the tensile properties of three different types of high-performance fibers (Gao et al., 2011) provides material input for more complex models. Similarly, tools measuring the small-strain response of ballistic fibers (eg, Kevlar® 129) have provided insight into how fibers deform under various loading conditions (Lim et al., 2009, 2010, 2011). Modeling of composite laminates subjected to ballistic impact is critical to both helmet and torso body armor applications. Fig. 14.21 is a model developed by

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Figure 14.21 Comparison of model prediction for and experiment on ballistic impact on composite laminate.

the ARL (Zhang et al., 2015). The model uses LS-DYNA® and captures details of the delamination as the spherical projectile impacts and penetrates the composite laminate. The top image is obtained from a CT scan of posttest panels, and the bottom image is the LS-DYNA® result. The CT scan shows the final deformation, while the model result shows the shapes corresponding to peak BFD. After peak BFD, the panels rebound and oscillate before equilibrium. The BFD and delaminations are larger in the simulation results; however, the remaining thickness of intact composite, delamination sizes, and locations are close to the experimental data.

14.2.4

Manufacturing processes as enablers for improved protection

Ballistic materials such as Kevlar®, Twaron®, Dyneema®, and Spectra® share a very common “roll” format. Manufacturers serving multiple market sectors, including commercial and other military applications, prefer shipping rolls of materials because of economies of scale, ease of transport, and ease of storage. Ply material supplied in this fashion is often referred to as “sheet goods” and allows the user some flexibility on what steps to select in transforming “as-received” materials into final items. Often, there are intermediate steps, and perhaps one of the most critical is the preforming step. In the manufacture of ballistic helmets, especially woven aramid fibers with PVBephenolic resin matrices, the traditional preform approach is to “cut and dart” the sheet goods into a shape so they are more amenable to being formed into a hemisphere (eg, helmet shell). This type of preparation is shown in Fig. 14.22(a). Cutting and darting have been successful in the production of the PASGT and ACH (army combat helmet) and the

Figure 14.22 Various helmet preform forming methods: (a) cut and dart, (b) Tepex® process, (c) uncut sheet forming.

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relatively simple nature of the process allows the helmet manufacturer a desirable and affordable level of capability. However, the disadvantage of this approach is that cutting immediately reduces both the structural and the ballistic integrity of the ply material and results in unwanted waste, and the process is subject to variability due to the hand-intensive preparation involved. To remedy the problems associated with cutting and darting, other methods have been developed. DuPont introduced the Tepex® process, which was novel for a variety of reasons including early use of thermoplastic-based woven ply materials and a thermoforming process that shaped stacked sheet goods into partially or fully formed near-helmet shells. Depending on weave type, materials, and forming factors, this process was used to make shells with varying levels of “wrinkling” due to the physics of transforming a sheet good into a hemisphere. The process was effective but subject to end-use acceptance issues (eg, US helmets have strict cosmetic requirements, which made some of the process-induced wrinkles intolerable). Also, the relatively low pressures associated with the process and the use of venerable materials like woven aramids limited the ballistic mass efficiency. Still, the process was an early demonstration of the potential of thermoforming of thermoplastic composites into ballistic shells, as shown in Fig. 14.22(b), and it is a relatively affordable and reliable process for producing such items. It has found success in some European military and private sectors. A logical progression of preform development would include the use of both the incumbent and the emerging classes of UHMWPE materials, such as Dyneema® and Spectra® composites. These materials offer higher ballistic mass efficiency for contemporary threats of interest, but they are in some cases two to three times more expensive than aramid composites. Also, they tend to be unidirectional crossplied instead of woven, and the thermoplastic matrix is often used as the binder. This allows consideration of near-net forming into complex shapes, such as a helmet preform as shown in Fig. 14.22(c). There is one significant issue with this type of forming process: though these types of “uncut” preforms have proven superior to cut preforms for certain requirements, the forming process produces “dog ears” as illustrated in Fig. 14.22(c). This results in scrap, which is only multiplied by the successive layering of plies and repetitive shell fabrication. Scrap rates can vary from 15% to 35% or more depending on overall performance needs.

14.2.5 Moving beyond composite sheet goods Preforming methods used in the manufacturer of all US helmets since 1980 have been effective. This is due in large part to the careful performance requirements that are developed based on understanding, insight, and extensive process/performance correlations borne out by experiment. However, new computational and manufacturing tools are emerging, together with advances in other areas (eg, computer control, precise placement and actuation, additive manufacturing) that enable entirely new modes of preform and helmet shell manufacture. For example, it is possible to begin to form a hemisphere directly from tows using the filament-winding process. The attraction of such a process is obvious: moving directly from fiber tows to a near-net shape could

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Figure 14.23 (a) Near-net tow placement concept for hemispherical geometry. (b) Prototype device for buildup of tows on hemispherical tool. (c) Buildup of near-net shell.

reduce cycle time, reduce waste of expensive raw materials, and reduce human touch labor. However, there are limitations to such a process. These include the ability to achieve relative tow orientation and, of course, the singularity (ie, hole) introduced by the filament-winding mandrel. Other challenges include the ability to deposit thermoplastic resins on tows and the bulk ballistic efficiency of such a preform compared with cut and uncut process methods. Filament winding is, albeit relatively simple, a process that enables tow placement with variable orientation of the tow relative to the mandrel (tool). However, the term “tow placement” has come to include far more complex technologies, such as thermoplastic-based carbon fiber placement for wings on commercial and military aircraft. Thus, efforts to explore this same basic ideadthe near-net placement of thermoplastic tow material on complex geometric surfacesdmakes sense when considering future helmet and body armor systems. The supportive arguments for tow placement include no wrinkling (typically associated with sheet goods being formed into hemisphere) and reduced cost, waste, and labor in transforming constituent tow material directly into a complex, near-net preform. Fig. 14.23(a) illustrates a prototype machine specifically for forming near-net preforms from tows of UHMWPE. The device, as shown in Fig. 14.23(b), was built and successfully demonstrated to produce successive layers of tows without wrinkling. Tow placement, as with ply-to-ply layup, offers the potential to hybridize with lower-cost materials, such as aramid fibers. Fig. 14.23(c) demonstrates the ability to deposit and build up a hemispherical shell from tows of Kevlar®.

14.2.6

The influence of manufacturing on preform fabrication

Tow placement is an excellent example of the important relationship between materials and processes. The inherent attributes of the materials (eg, a tow of UHMWPE or aramid fibers) allow the development of processes to exploit these attributes (eg, a process for conformal placement of tows over a hemispherical surface). It is for this reason that investments by both the commercial and the military sectors in process and manufacturing technologies for preform development have enabled the maturation of techniques for assembling ballistic materials in helmet, body armor, and other

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Figure 14.24 Machine to reliably automate preforms for helmet manufacture. (a) Machine schematic. (b) Thermally bonded assembly of thermoplastic preforms. (c) Near-net preform.

personnel-protective equipment. Often, the military sector requires specialized capability for which there is no readily available processing equipment to properly exploit new materials and new material architectures. Thus, DoD programs such as Army ManTech and Defense-wide Manufacturing Science & Technology allow for transitioning across the “valley of death” from promising, laboratory-scale technology to fielded items such as helmets with new levels of ballistic protection. For example, Fig. 14.24 illustrates a machine that helped transition from hand labordwhich can introduce variabilitydto a reliable automated system for assembling both “cut” and “uncut” preforms for ballistic helmet fabrication. Risk-reduction processes enabled by ManTech have enabled the development and demonstration of “first of a kind” machines which have had proven impact on the US industrial base to produce new levels performance in helmet and body armor systems.

14.3

New material developments in torso and related body armor

The concept of applying armor to the human body is evidenced both historically and geographically in cultures around the world. There are numerous historical examples that provide clear evidence of exploiting readily available as well as unique or synthesized materials to resist a known threat. There are also early examples of design and fabrication for rationally fashioning the materials in such a manner as to make them more wearable by the user. For example, archaeological discoveries in Alaska have uncovered bone armor used by inhabitants nearly 1000 years ago; the bones are arrayed sequentially to provide, albeit imperfect and irregular, flexible coverage of the torso. Similarly, medieval “chain mail” armor was a highly innovative concept wherein a mesh of metal provides both the protection and the relative comfort needed. The notion of segmented body armor made from modern steel in World War I demonstrates a level of flexibility without compromising the continuity of coverage of the torso (Dean, 1918). While these now antique approaches toward body armor are well known, they are nonetheless consistent with and representative of materials and design philosophies still in use today, ie, balancing awareness of contemporary threats with wearable materials capable of mitigating the threats.

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Generally, the type of threat divides body armor materials into two general categories: soft and hard materials for “neck-down” protection (to include the torso, deltoids, and, in some instances, groin and extremities). A contemporary summary of ballistic body armor materials and mechanisms is provided by David et al., (2008a,b, 2009). An example of soft body armor includes the improved outer tactical vest (IOTV), which has been composed of woven ballistic fibers to arrest defined fragmentary threats. Examples of hard body armor have included steel plates designed to partially conform to the torso, combinations of steel and ballistic nylon and Doron, and eventually, the introduction of ceramic plates to replace steel as the primary material to engage and degrade the projectile (eg, small arms bullets of the 7.62-mm class). The primary drivers of new body armor materials include the need to arrest more aggressive threats, the need to ensure that nonpenetrating threats do not induce trauma from excessive BFD, and the need to maintain an overall system weight that is still tolerable by the user. State-of-the-art body armor is unique in that it uses one of the hardest ceramic materials commercially available (boron carbide) and a backing made from one of the lightest commercially available composite materials (UHMWPE). Contemporary body armor systems are designed to allow insertion of hard plates into the soft vest, as shown in Fig. 14.25. The hard plates generally are classified as ESAPI (enhanced small arms protection inserts) and XSAPI (X-threat small arms protective inserts). This flexibility allows the users to upgrade or downgrade their level of protection as needed, which can include shifting from ESAPI to XSAPI or adding deltoid and groin protection. Variations include “plate carriers,” which are often preferred when a high degree of mobility in extreme environments is required. The desire to protect soldiers is paramount, but a balance must be maintained so that the soldier is not impeded from doing his or her mission. Body armor is essentially parasitic weight; it contributes nothing to the soldier’s operational effectiveness until the moment it is required to resist a potentially lethal threat. In addition, body armor is only one of many items the dismounted soldier must carry on his or her person.

Figure 14.25 Soft vest carrier and hard ceramic plate insert.

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It should be noted that while the overall average total soldier load has remained the same for the soft and hard body armor ensembles, the amount of protection and capability has increased dramatically. The army has made considerable investment in understanding human performance, including scenarios in which varying levels of ballistic protection and equipment are borne by the soldier in moderate and extreme environments. This includes both objective (eg, motional analysis, modeling, and physiological monitoring) and subjective assessments (survey responses from soldiers). Cumulatively, this insight has the potential to better inform future soldier protection concepts and define the demands for new materials to deliver multiple levels of performance subject to an acceptable weight (or areal density) penalty. Fig. 14.26 is an illustration of the army’s future approach to soldier system development; it encourages striking a reasonable balance between lethality, survivability, and mobility. The “traditional” approach of increasing survivability has been to develop better material, better designs, and, on occasion, increased coverage and material to upgrade resistance to defined threats. The term “survivability” is far more encompassing, and recognizes that ballistic protection is only one factor that contributes to overall soldier survivability. The ability to neutralize the threat (lethality) or rapidly deploy to a more favorable position (mobility) also influences survivability; similarly, lethality and mobility benefit from advances in overall survivability. As discussed previously, the army is developing the SPS. The SPS implicitly recognizes the integrated balance needed between competing equipment, capability, protection, and weight allocations. Soft body armor generally employs fibers that are mechanically assembled into a stable, highly flexible fabric. The fabrics can be woven or integrated with unidirectional materials. Aramid fibers (eg, Kevlar® and Twaron®) have been effectively used in soft body armor materials and protection systems. Considerable research has Figure 14.26 “Iron triangle”d a balance of soldier survivability, lethality, and mobility.

Survivability

Mobility

Lethality

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been focused on the fibers themselves (Lim et al., 2009, 2011). The durability of soft armor fiber and fabrics is critical, given the extreme environments and operational conditions in which soldiers are likely to deploy. This could include exposure to extreme heat and cold, moisture/water, and abrasion. Tests are often conducted at conditions more extreme than soldiers will encounter. For example, the influence of water at 100 C for 34 days on properties of various aramids (eg, strength, strain at failure, and modulus) was studied by Obaid et al. (2011). Scanning electron microscopy images reveal unique failure characteristics for AuTx®, Kevlar®, and Twaron® fibers (Gao et al., 2011); fibrillation is observed but varied for the three fiber types, as are the fracture surfaces. Such insight is critical to identifying and promoting failure mechanisms that are more favorable for developing ballistically resistant fiber materials. Recent research has addressed other material and testing aspects of soft body armor. Gogineni et al. (2012) have studied the effects of ballistic impact on Twaron® CT709, a plain-weave aramid material suitable for soft body applications. Similarly, models have been developed to provide a correlation between dynamic strength response of Kevlar® KM2 when subjected to ballistic impact (Grujicic et al., 2011). Recently, the degradation of AuTx® yarns which have been subjected to multiple ballistic impacts has been studied (Hudspeth et al., 2014). This insight reveals the integrity of soft armor materials subject to impact by multiple ballistic fragments. Equally important are the means by which the ballistic response of soft body armor materials are characterized under controlled testing. Often, the type of testing fixture can influence the measurement and thus an awareness and characterization of such effects is critical to confidently and consistently assessing current and future soft armor materials. For example, Zhang et al. (2008) have studied the influence of frame size, type, and clamping pressure on the bulk response of soft body armor systems. The sensitivity to each of these variables was modeled, allowing for a rationale to help guide appropriate test fixture configurations and clamping conditions. More advanced materials could potentially emerge in the future that enable improvements in both soft and laminate-based ballistic materials. Koziol et al. (2007) give a relative comparison of commercial organic ballistic materials, as well insight into carbon nanotubes (CNTs). Materials like CNTs and graphene are exciting because they could theoretically enable “leap-ahead” performance over the best available UHMWPE and polymer composite materials. They also suggest more command over desired microstructures, consistent with the notion of materials by design. However, significant challenges in the manufacture, scale-up, and consistency of these materials temper expectations that they will emerge any time soon. Nevertheless, materials like CNTs and graphene are potential enablers for advances not only in ballistic properties but in other combinations of highly desirable properties as well.

14.3.1

The influence of informed design in soft body armor systems

Ultimately, fibers are configured into fabrics or textiles of various types that constitute the soft body armor ensembles. Fig. 14.27 shows some of the most recent advances in

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Figure 14.27 Improved outer tactical vest (IOTV). (a) Generation II, (b) generation III, and (c) female IOTV. Image courtesy of Natick Soldier RDEC.

soft body armor systems, which include (Fig. 14.27(a)) generation II and (Fig. 14.27(b)) generation III of the IOTV, as well as (Fig. 14.27(c)) a version of the IOTV designed appropriately for the female population. Fabric formats introduce unique performance characteristics that influence the ultimate “system” level response of items like the IOTV. Thus considerable research has been and is being conducted to provide better insight into the response of these soft armor fabrics under a variety of conditions. For example, researchers have developed a three-parameter model of viscoelasticity that described fabric behavior under ballistic impact (David et al., 2008a). This model predicts good correlation with experimental results at low strain rates but at much higher strain rates the correlation degrades and deviates, thus highlighting the unique issues with modeling high-strain-rate behavior of viscoelastic ballistic fabrics. Bulk fabric response subject to high-velocity ballistic impact is another area in which models are providing increasingly predictive capability that can inform fabric design. Other factors can influence the performance of ballistic fabrics in addition to fiber type, yarn configurations, weave styles, and matrix content (Gopinath, 2012; Prat et al., 2012). For example, insight into the degradation of yarns from a variety of causes (eg, environmental, number of ballistic impacts, aging, etc.) is critical to understanding how fielded ballistic fabric response might change with time, use, and exposure to varying conditions. While these research efforts benefit the ballistic performance of soft armor, consideration of novel materials that increase flexibility and fluid and thermal transport to enable more comfortable soft armor is also warranted.

14.3.2 The potential benefits of spider silk in soft armor applications The trend in the development of new and lighter ballistic materials, both organic and inorganic, is to scale promising synthetic materials developed from complex chemistries to stable, repeatable, and relatively affordable manufacturing processes. However, there are notable exceptions. For example, spider silk exhibits remarkable tenacity,

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competitive with some of the venerable grades of synthetically produced aramid fibers (Rengasamy et al., 2005; Elices et al., 2007). Research to understand and encourage the production and consistency of such materials produced by living organisms (such as the Argiope trifasciata and Nephila spiders) is active and intriguing, informing the “bio-inspired” research needed to understand and potentially enhance these material morphologies. In addition to the high tenacity of as-spun spider fibers, researchers (Huang et al., 2012; Liu et al., 2014) have observed unique thermal properties and integrity of spider silk, suggesting potential stability under various environmental conditions. The inherent ability of some spider silks to enable thermal transport (Xu et al., 2014) and biocompatibility suggests methods for enhancing comfort while delivering a defined level of ballistic resistance to certain threats (eg, adversely accelerated metal fragments and sand particles). A contemporary example is the use of silk materials in the development of groin protection to mitigate penetration and subsequent infection. Further research could produce fibers and material blends that preserve ballistic integrity but greatly enhance flexibility, breathability, and overall comfort of soft armor materials.

14.3.3

Novel approaches to hard armor materials

The current materials used in the manufacture of contemporary US body armor are state of the art and among some of the most mass-efficient passive materials for defeating aggressive small arms threats. While fabrics and monolithic composite laminates are often sufficient for providing acceptable levels of fragment resistance, lightweight protection against small arms bullets (eg, 7.62 mm) requires the use of “hard” ceramic materials (generally with a composite backing material). Research and development efforts have focused on understanding why materials like B4C and SiC ceramics, as well as UHMWPE composites, perform so well under extreme (eg, high strain rate) loading (Ghosh et al., 2012). New modeling and experimental tools are helping identify the subscale material mechanisms that allow these ceramics and polymer composites to resist pressures in excess of 30 GPa. The development of residual stresses in SiC (Munn et al., 2011) and the macroscopic characterization of B4C and B4C/SiC composites under high-pressure events has been studied by Salamone et al. (2013a). Fig. 14.28 is a micrograph of a SiC ceramic immediately after exposure to a ballistic impact. Of particular interest is the classical “comminuted zone,” which can be seen as the essentially undisrupted region immediately beneath the ballistic strike area. This comminuted zone is often a critical indicator of whether a certain type of SiC ceramic will exhibit high ballistic resistance. While B4C and SiC have been the traditional materials of choice for hard-plate ceramic body armor, there are trends in exploring new processes and materials to enable more mass-efficient defeat mechanisms. For example, evidence suggests more controlled blending of B4C and SiC could be an enabler for weight reduction, but such blending of distinct ceramics introduces both material formulation and processing challenges (Salamone et al., 2013b, 2014). Another promising set of materials includes “alternate borides” such as B6O and AlB12. B6O, shown in Fig. 14.29, is noteworthy because in a static hardness test it was found to be 30% harder than the current

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Figure 14.28 Comminuted zone in SiC ceramic during ballistic impact.

(a)

(b)

Material

Density (g/cc)

Hardness at 500 gm load

Al2O3

3.92

14 GPa

SiC AIB12

3.22

23 GPa

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25 GPa

B4C

2.52

27 GPa

B6O

2.58

35 GPa

(c)

Figure 14.29 (a) Comparison of properties of alternate borides. (b) B6O icosahedron. (c) B6O synthesized by the Army Research Laboratory.

B4C; hardness is often one of several indicators of good ballistic performance. However, static hardness and dynamic hardness are different, and research is ongoing to make B6O into a more stable ceramic for evaluation. Fig. 14.29(b) illustrates the icosahedron grain of B6O, and Fig. 14.29(c) is B6O made at the ARL. Raman spectroscopy has been used to gain insight into hot-pressed B6O (Machaka et al., 2011). These insights into B6O morphologies aid in the development of process/property/ performance of this novel but marginally evolved materials for future ballistic ceramic applications. AlB12 is also a very interesting alternate boride; micrographs of AlB12 are shown in Fig. 14.30. Although it is slightly denser than B4C, AlB12 is theorized to have a different defeat mechanism compared to B4C; this defeat mechanism could potentially enable resistance to more aggressive threats than current B4C performance. In addition to advances in ceramics, body armor also benefits from organic backing materials such as Dyneema® and Spectra®. Ceramics are typically hard and brittle and subject to fracture; ballistic composites like UHMWPE laminates have high tenacity and resistance to penetration. It is the combination that makes current body armor so effective: the ceramic does the primary work on degrading the projectile and

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Figure 14.30 AlB12 micrographs showing (a) edge and interior and (b) micrograph detail.

preventing penetration, while the composite supports the ceramic during the ballistic event and “catches” the debris from the violent mitigation of the projectile. Thus, there is an equal desire to find and develop ultralight polymer and molecular structures that exhibit high strength and stiffness.

14.3.4

Novel processes and inspection technologies for ceramic/composite-plate body armor

Similar to helmet fabrication, processing of materials and the manufacture of body armor plates influence both the performance and the cost of body armor. New processing techniques and manufacturing methods are enabling more efficient body armor assembly. Fig. 14.31 illustrates some of critical process steps in the fabrication of the ceramic plate; Enhanced forming and process cycles enabled new ceramic compositions

Subscale processing and testing enabled optimization of composite backing

Figure 14.31 Ceramic and composite processes for body armor.

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Figure 14.32 Computed tomographic image of bullet-induced damage in a ceramic/composite flat plate. Image used with permission of IIeVI, Inc.

these include precision blending of powders, extrusion into green body material, and dimensionally controlled hot processing. Similarly, automated ply assembly and ceramic/composite integration allow for subscale testing of promising new grades of inorganic and organic materials. Once test configurations of new ceramics and ballistic composites are fabricated, they can be ballistically tested. Characterization methods such as CT can provide insight on how the dissimilar materials perform after exposure to a ballistic projectile. For example, Fig. 14.32 is a CT image of a flat plate composed of B4C and UHMWPE; it is possible to observe the fracture of the ceramic and the deboning and delamination of the composite. One example in which such flexibility in ply-to-ply configuration of ballistic material has been critical is in the development of new and innovative methods of reducing BFD without severely reducing ballistic resistance. Fig. 14.33 shows a controlled geometry comparison between a 0 /90 Dyneema® laminate and a hybrid laminate wherein relative ply orientation is varied at discrete locations in the laminate. The

(a)

(b)

28 mm

16 mm

Figure 14.33 Significant influence of material architecture on back-face deformation induced by a small arms projectile with (a) 0 /90 backing and (b) ARL X-Hybrid backing.

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Figure 14.34 Dissimilar Materials Assembly System. (a) Robotic arm-based play assembly. (b) Dissimilar materials assembly system (DMAS) concept image. (c) Actual and operational DMAS system.

configuration shown in Fig. 14.33 is representative of the backing materials used in ceramic/composite body armor, wherein it is necessary to fully arrest a small arms projectile but not allow the BFD to exceed a defined specification (eg, 44 mm for typical ESAPI body armor systems). Fig. 14.33(a) shows the response with the 0 /90 laminate, and Fig. 14.33(b) shows the response using a novel ply-orientation process known as “X-Hybrid.” As shown in Fig. 14.33(b), there is a dramatic reduction in BFD with little reduction in the V50 of the bulk material system. The ARL X-Hybrid architecture demands a very specific and precise sequential layup of plies. Fig. 14.34(a) is an early robotic-based ManTech machine that allows a high degree of flexibility in the type and orientation of ply assembly, and ensures both precision and automated repeatability of successive ply placement to form backing materials for ceramic/composite body armor applications. Bridging the gap between lab-scale methods of assembly and processes that can be scaled up to production is a critical goal of ManTech initiatives. Often there is the burden of risk in investing in the type of unique, highly specialized equipment needed to successfully make this transition. One strategy has been to deploy the ManTech program to develop these unique processes and to ensure government ownership of the machine designs. This strategy allows for maximum technology transfer with multiple commercial vendors and contractors. For example, the Dissimilar Materials Assembly System shown in Fig. 14.34(b) (schematic) and Fig. 14.34(c) (full-scale prototype machine) is a more evolved and production-ready version of the machine shown in Fig. 14.34(a) and allows for integration of various types and grades of ballistic material sheet goods. First-article testing and lot-acceptance testing are critical to ensuring the quality and performance of as-manufactured ceramic/composite hard-plate body armor systems. However, once these body armor plates are fielded and issued to soldiers, ensuring plate integrity after repeated use and exposure to extreme operational conditions can pose a serious challenge. Although they have significant ballistic resistance, B4C, SiC, and various combinations of these materials still exhibit the brittle behavior that makes them vulnerable to damage. The results of a development program initiated by PEO Soldier of the US Army enabled a technology known as Smart Body Armor®. This sensing technology has been integrated into the ceramic plate to provide a real-time assessment of ceramic plate integrity. It features a user-friendly interface and has been correlated with experimental data that have demonstrated its accuracy

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Figure 14.35 Smart Body Armor® technology. (a) Key to access sensor tape. (b) Sensor tape on perimeter of double-curved ceramic plate. (c) Real-time retrieval of data and assessment of plate condition. Photos and diagrams are the copyrighted works of TenCate Advanced Armor USA, Inc., and/or its affiliated companies. Used with permission.

for early detection of possible flaws that may have developed in the ceramic while in use. Fig. 14.35 illustrates the concept wherein a sensor “key” (Fig. 14.35(a)) connects to a sensor tape located on the perimeter of a ceramic plate (Fig. 14.35(b)) and the assessment can be made rapidly by the user (Fig. 14.35(c)). If a flaw is detected, the plate can be removed from service and subjected to a more traditional and comprehensive nondestructive evaluation (NDE) assessment (eg, CT) or it can simply be replaced.

14.3.5 Moving beyond discrete materials for soldier protection: “materials multifunctionality by design” Considerable effort is continually expended in the development of lighter ballistic materials to shed weight from individual-protective equipment. This includes the development of polymer fibers, fabrics, and composites as well as inorganic materials such as ceramics. It has come to include active research in new generations of materials such as CNTs, graphene, and other 2D polymers. The relatively recent discovery and development of graphene have excited a diverse array of interest in the potential of such materials to provide disruptive improvements in electrical, mechanical, ballistic, and other desirable material properties. The broad promises and challenges of graphene, including early experimental and theoretical work, are widely described in the scientific literature. Of note are some of the experimental and analytical advances made in assessing graphene as a potential ballistic barrier material. For example, Lee et al. (2014) have developed an experimental infrastructure that measured a specific penetration energy in a multilayered configuration of graphene that was approximately 10 times that of steel. Wetzel et al. (2015) used analytical membrane theory to predict the ballistic penetration behavior of graphene, finding that graphene has the potential to exhibit ballistic penetration performance similar to that of textile-based Kevlar® but

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at masses 10e100 times lower than the latter and venerable linear polymer material. While Wetzel et al. readily acknowledge their model does not explicitly account for brittle fracture, their model nevertheless provides a predictive tool that can enable assessment of other, perhaps more exciting, 2D polymer structures and the organization of their molecules to give new and disruptively enhanced performance in ballistic and other desirable properties. Often, a unique property of graphene (eg, high electrical or thermal conductivity) is described (Wan et al., 2014; Brownson et al., 2011) rather than an integrated array of distinct but desirable multifunctional properties. Described to a far less extent is the potential of organizing matter at the atomistic level to deliver not a single desirable bulk property (eg, high stiffness) but a suite of properties (eg, high stiffness, power storage, ballistic resistance, sensing, etc.) simultaneously. In essence, the goal would be to develop multifunctional materials characteristics on very small length scales. Gibson (2010) provides a survey of the benefits and challenges associated with various types of multifunctional materials. For example, O’Brien et al. (2011) have demonstrated and developed assessment tools for structural capacitors; the composite structure can support required mechanical loads while storing power in the same host “multifunctional” composite. Multifunctional materials approaches on different length scales could comprehensively inform future soldier materials and system designs. The current approach toward soldier materials focuses on discrete systems such as helmet shells or torso plates. A 10% reduction in weight from either of these systems may be only a modest reduction in the overall weight of all the items a soldier carries on his or her person. As shown in Fig. 14.36, the future development of tools and processes to enable what the authors define as “materials multifunctionality by design” could provide a means to deliver the desired set of performance requirements and soldier system capabilities by sharing part or all of the same host materials rather than assembling discrete material components. Concomitant advances in related areas such as additive manufacturing on increasingly

Multifunctional approach to soldier materials capability Total soldier system volume

Total soldier system volume

Discrete approach to soldier materials capability

Total soldier system mass

Ballistic protection Power storage Communication Sensing

Total soldier system mass

Figure 14.36 Multifunctional materials by design as enabler for overall soldier system weight reduction.

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smaller length scales could yield entirely new methods for developing a new generation of highly mass-efficient multifunctional materials.

14.4

Novel exoskeleton development

The DoD, including the Defense Advanced Research Projects Agency and the army, have initiated research and development efforts to explore the potential of both passive and powered (active) exoskeletons. The motivation is multifold but the general intent is to overcome human limitations and upgrade the desired functionality and capabilities in such a manner as to allow the soldier significant advances in applied strength, endurance, and protection. There are many challenges to fully realizing the potential of such a concept, not the least of which is the ergonomic design that effectively and beneficially enables (and does not hinder) the human (ie, soldier). Robust human/ machine interfacing under both routine and extreme conditionsdto include scenarios in which the physical elements of passive or active exoskeletons may be damaged, nonfunctional, or otherwise compromiseddwill be key to evolving viable concepts. In the case of powered exoskeletons, the need for an efficient, compact power source is arguably the most critical technology gap; advances in sensors, actuators, lightweight structural materials, computer control algorithms, and device design are all fairly mature to make practical advances in powered exoskeletons conceivable. It is equally conceivable that early powered exoskeletons might be required only in unique missions in which overwhelming upgrades in soldier strength and protection may be needed for relatively short periods of time. Such scenarios would allow early introduction and adoption of these types of technologies.

14.4.1 Motivation for localized passive exoskeleton concepts The traditional approach to improving soldier-borne weight management is to revisit both materials and ergonomic designs that could make systems like helmets and body armor lighter and more comfortable and mitigate trauma from both low-velocity and ballistic impacts (Kulkarni et al., 2013). To expand the possible solution space for both material-based ballistic protection and added device capability (eg, night-vision systems, active hearing protection, etc.), the concept of locally supporting the helmet by means of infrastructure specially designed to couple the helmet to the shoulders has been explored by the ARL. In essence, the concept is a localized exoskeleton that allows the static load associated with the helmet weight and any other devices or equipment mounted on the head or helmet to be “offset” onto the shoulders. While this is not a weight-savings approach, the intent is to make head-borne loadsdespecially excessive loadsdtolerable for longer periods of time by the soldier. The concept, known as VLOS, provides the means of offsetting part, or all, of the head-borne weight on the shoulders. Such an approach allows for the possibility of helmets with a larger gap between the skull and the helmet interior to improve impact protection; similarly, more ballistic material (ie, a heavier helmet) can be tolerated to enable higher protection from small arms threats.

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The development of Vertical Load Offset System prototypes

The development of VLOS began with conceptualization and early prototyping, as shown in Fig. 14.37(a). Initially, the goal was to provide not only a means of supporting the weight of the helmet on the shoulders but also a design that would allow adverse impact insults to the helmet to be partially or fully redirected away from the helmet and head and onto the shoulders. Owing to the response times needed to effectively engage such a system without significantly limiting the ability to move the head as freely as possible, it was decided to focus the VLOS function explicitly on relieving static head-borne loads. Elements of the VLOS design include, but are not limited to, degrees of freedom (ability to move head in any and all directions such as front/back/ side/twist/turn/diagonally), avoiding impairment of vision or interface issues with equipment (eg, body armor, weapons), easy don/doff, and minimized snag hazard. A further constraint, for the purposes of this research, was to ensure that the concept could be retrofit to fielded helmets such as the ACH. This limited innovations in attachments to the shoulders as well as features in the incumbent helmet, but nevertheless provided an opportunity to explore this novel concept in the context of currently used and accepted individual soldier protection equipment. After early concept generation and design development of the basic elements of the VLOS, intermediate prototypes were built to evaluate both the potential and the limitations of this type of mechanism as shown in Fig. 14.37(b). The mechanism itself comprises a spring arm, which by design, material selection, width, thickness, and span length can be adjusted to provide the desired level of reactive force to offset the load of the helmet onto the shoulders. Human factors evaluation (HFE) included both in-lab (controlled environment), as shown in Fig. 14.37(c) and external (natural environment) data collection. Fig. 14.38(a) shows motion analysis equipment used to capture and quantify relative head movement in both seated and weapon-sighting scenarios (the latter is shown in Fig. 14.38(b)). Fig. 14.38(c) demonstrates evaluation of the VLOS during movement through a heavily wooded environment and Fig. 14.38(d) captures performance during ground crawl over difficult terrain. Collectively, these HFE data identified strengths and weakness of the VLOS that enabled informed and beneficial modification of the VLOS design.

(a)

(b)

(c)

Figure 14.37 VLOS development. (a) Initial concept image. (b) Early VLOS prototype. (c) VLOS user interfacing.

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(c)

427

(b)

(d)

Figure 14.38 Vertical Load Offset System evaluation in representative natural environments. (a) Motional analysis of VLOS. (b) Weapons interfacing with VLOS. (c) Traversing densely wooded environment with VLOS. (d) Crawl with VLOS.

14.4.3 The evolved Vertical Load Offset System prototype The most challenging aspect of the VLOS conceptdor any localized exoskeleton connecting the head to the body in some mannerdis the motion of the head and neck relative to the shoulders and torso. More specifically, the VLOS must not become a liability to the soldier wherein it impedes his or her ability to perform critical functions or has significant potential to injure or incapacitate the soldier. Hence, considerable effort was focused on identifying a design that provides the load-offsetting function while minimally impeding the motion of the head and neck. VLOS improvement included lower profile and less overall protrusion from the axis of rotation, two quick-release mechanisms in case of a snag hazard or the need to don/doff rapidly, and a balanced design that allowed effective offset of eccentric head-borne mass

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Figure 14.39 Improved Vertical Load Offset System based on human factors evaluation.

(such as bulky night-vision equipment). The evolved VLOS used lightweight carbon spring arms and specially designed joints to both permit and limit linear and rotational load transfer through the joints. Perhaps most important, however, was the introduction of added degrees of freedom by means of a slider mechanism, shown in Fig. 14.39; it also demonstrated a substantial improvement in freedom of head movement in all directions as a result of this modification. Cumulatively, these improvements allowed uninterrupted head movement of the VLOS and helmet system while effectively offsetting the loads onto the shoulders.

14.4.4

The potential of torso-based load sharing for improved head protection

Although very preliminary, it is clear from the HFE studies that there exists the opportunity to improve both blunt-impact and ballistic protection for the individual soldier by considering new devices and designs that allow load sharing from the head to the body. The ability of VLOS to offset the load onto the body allows, for example, support and stabilization of helmets with slightly larger diameters to improve impact resistance from both low-velocity (blunt) impact and dynamic BFD from small arms projectiles. Similarly, the ability of the VLOS to enable improved tolerance of heavier loads provides a means of boosting ballistic protection with thicker (heavier) composites, or making additional head-borne devices more acceptable. VLOS research thus has two possible future paths for head protection, both equally viable. The first path

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Head accelerometer - z-acceleration Baseline ACH pads only ACH pads with load shunted to shoulders

350 310 270

Acceleration (g)

230 190 150

Injury limit

110 70 30

–10 –50 0

0.0005

0.001

0.0015 Time (s)

0.002

0.0025

0.003

Figure 14.40 Comparison of conventional helmet impact and load-shunting impact. ACH, army combat helmet.

is to refine the current VLOS to provide a highly accepted and desirable load-offset function. The second path is to consider alternative load paths from the head to the body that would allow direct “shunting” of head-borne loads onto the lower body (eg, shoulders, torso, pelvis, or some combination of these). A simplified model was created to compare two distinct cases. In the first case a helmet is attached to the head in a conventional manner; there is no contact with the torso other than that naturally provided by the neck. By contrast, the second case uses structural elements to couple the helmet to the shoulders. As shown in Fig. 14.40, when both helmets are exposed to the same load, there is considerable mitigation of the acceleration with the load-shunting concept compared to the helmet-only case. The model serves as a prelude to the potential of further research into the benefits (and issues) of load shuntingdin whole or in partddirectly to the shoulders.

14.5

The disruptive potential of robotically deployed materials to enhance soldier protection

Traditionally, advances in protection for the dismounted soldier have come from consistently revisiting helmet and torso armor designs that are worn by the soldier. Factors such as evolving threats and the limits of human endurance and load carriage on the head and body often inform the development and exploitation of new materials. As discussed earlier, protection is only one of several items carried on the soldier’s person and excessive body armor (to include helmets and vital torso protection) can

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Figure 14.41 Integrated approach for developing robotic-based augmentation of soldier protection.

actually degrade soldier survivability if it impedes soldier mobility and lethality, or both. Material advances in current armor systems tend to lead to evolutionary increases in performance once the transition to and use of the new material has matured. A revolutionary “leap forward” in soldier protection should consider technologies unrelated to, but that could be enablers for soldier protection. One potential example is robotic and unmanned platforms that could deploy novel armor and other materials to enhance overall soldier survivability. Current autonomous systems research and development is largely focused in areas such as control, human/machine interfacing, remote surveillance and reconnaissance, precision delivery of lethality, and “mules” for delivery of supplies, ammunition, water, and equipment (Bruemmer et al., 2005; Cassenti et al., 2009). An effort initiated by the ARL known as Robotic Augmented Soldier Protection (or RASP) seeks to develop the science, materials, and rationale necessary to effectively exploit autonomous systems as a means of enabling soldier protection; this is shown generally in Fig. 14.41.

14.5.1

The Robotic Augmented Soldier Protection concept

The RASP approach has included the conceptualization, design, and fabrication of early “discovery demonstrators.” A discovery demonstrator is essentially a working prototype that integrates the basic tenets of the RASP concept, which include use of the robotic platform to enable mobility and deployment of passive and adaptive materials and systems to deflect, degrade, disrupt, or defeat ballistic projectiles and adverse blast-induced stress waves. Fig. 14.42 is a discovery demonstrator known as Personal Upgradable ProtectioneExperimental (or “PUP-E”). PUP-E is a small robotic platform which can operate under the control of the soldier or autonomously and can deploy a shield between the soldier and the threat as shown in Fig. 14.42.

Materials, manufacturing, and enablers for future soldier protection

(a)

431

(b)

Figure 14.42 Personal Upgradable ProtectioneExperimental: remotely deployable ballistic shield to augment soldier protection. (a) Personal Upgradable Protection-Experimental (PUP-E) concept in undeployed state. (b) PUP-E in deployed state.

(a)

(b)

Figure 14.43 Near-net single-curved ultrahigh-molecular-weight polyethylene plates for deploying ballistic protection from unmanned systems. (a) Retracted UHMWPE material for PUP-E shield. (b) Deployed UHMWPE materials for PUP-E shield.

A natural choice for robotic shield material is a polymer composite, such as UHMWPE. The materials and processing technologies developed for helmets and backing materials for body armor can be easily adapted for the robotic shield, as shown in Fig. 14.43.

14.5.2 Dual-use potential: optionally manned platforms for material transport and protection Autonomous systems are already being explored as a viable means of “unburdening” the soldier by providing the ability to transport material to support a soldier or squad.

432

Lightweight Ballistic Composites

Vehicle can operate in manned and unmanned mode (to shadow dismounted soldier) Transparent armor UHMWPE armor

Figure 14.44 A near-term concept for introducing autonomous-based soldier protection, mobility, and transport of material.

As shown in Fig. 14.44, an all-terrain vehicle has been modified so it can operate in either a manned or an unmanned mode and tow a trailer with equipment and supplies. The trailer has been designed to deploy a ballistic shield on demand; shield deployment could be based on autonomous sensing of a hostile presence, detected threats, gunfire, or soldier’s command. Fig. 14.45 is an early but fully functioning prototype

Figure 14.45 Discovery demonstrator of dual-use trailer/protective shield concept.

Materials, manufacturing, and enablers for future soldier protection

433

Protected soldier is able to neutralize threat

Autonomously deployed smoke screen Deployed ballistic shield

Soldiers are shielded during hostile situation

Figure 14.46 Robotic shield provides protection in scenarios in which the soldier is highly vulnerable.

which offers optionally manned ability to transport material and deploy significantly higher ballistic protection for the dismounted soldier in critical scenarios.

14.5.3 Developing the rationale and scenarios for robotically deployed soldier protection There are multiple challenges in developing robotic-based soldier protection. These include defining the scenarios in which such an approach would be of high value. For example, mission environments in which the soldier is highly exposed, as shown in Fig. 14.46, might be an early and relevant use of the RASP concept. Other challenges include developing the performance and operational rationale for this concept, as well as the level of autonomy and human/robot teaming needed to make it effective. The RASP concept is not meant to replace traditional armor worn by the soldier, but sensibly augment it in the face of evolving threats, missions, and operational environments. Robotic platforms could enable unique and interesting opportunities for exploiting both organic and inorganic materials to deliver disruptive new modes of soldier protection in the future.

14.6

Summary

The traditional approach to individual protection has been to identify the types of threats and environments the soldier will encounter and use this assessment to inform the selection, development, and integration of materials into effective, ergonomically acceptable systems borne on the soldier’s person. These materials are generally passive

434

Lightweight Ballistic Composites

in nature and often include combinations of dissimilar materials to deliver the required penetration resistance against ballistic projectiles as well as mitigating the associated BFD. A highly successful example of exploiting new material grades and novel manufacturing processes is the ECH, which delivers >35% increase in frag protection over the ACH and is the first-ever US ballistic helmet with a defined small arms capability. Materials such as graphene and related variants, as well as multifunctional materials across multiple length scales, hold potential promise as a significant advance over current polymer fiber- and ceramic-based ballistic materials. While this venerable materials approach to soldier protection is effective and likely to be revisited with new materials and processes as they mature, the future appears to be the development of a much broader approach, which can be better described as “soldier survivability.” Protection is only one of several capabilities that define survivability. Other forms of increasing survivability could include, but are not limited to, minimizing soldier detection, enhancing situational awareness, introducing new passive and powered exoskeletons, and, potentially, exploiting stand-off and other unique capabilities made possible by robotic and autonomous systems. The need for new, lighter, and more disruptive materials and processes will persist but their required characteristics may be informed differently as the broader concept of soldier survivability evolves.

Acknowledgments The authors gratefully acknowledge contributions from the following: Jason Cain, Timothy Zhang, Sikhanda Satapathy, Lionel Vargas-Gonzalez, Thomas Plaisted, James Campbell, Dan Baechle, Jared Gardner, Jennifer Sietins, Brian Scott, Jerry LaSalvia, Chris Hoppel, Rachel Ehlers, Matt Burkins, Bill Harper, Angela Boynton, Don Lee, Ben Fasel, Rob Dilalla, Virginia Halls, and Suzanne Horner. The authors would also like to acknowledge Eric Wetzel and Shane Bartus, who provided ARL Soldier Protection Research Area leadership and support for many of the ARL technologies discussed. Finally, the authors also acknowledge support from Program Executive Office-Soldier US Army, Army Research Laboratory, Natick Soldier Research and Development Center, Army ManTech Office, OSD Defense-Wide Manufacturing Science & Technology Office, II-VI, Inc., 3M Ceradyne, CoorsTek, TenCate, Creative Engineering, Mentis Sciences, and Accudyne Systems, Inc.

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Index ‘Note: Page numbers followed by “f” indicate figures, “t” indicate tables.’ A AATCC 100. See Antibacterial Finishes on Textile Materials: Assessment of Abrasion, 324 resistance, 252 Accessory ballistic panels, 146 ACH. See Advanced combat helmet (ACH); Army combat helmet (ACH) Acrylonitrile butadiene styrene (ABS), 243 Add-on armor, 291e292 Advanced combat helmet (ACH), 211 AEP-2920. See Allied Engineering Publication-2920 (AEP-2920) AEP-55 volume 1e3, 135e137 Aging studies, 232e233 Air temperature, 247 Air transport, 207e209, 279 Airbus A400M, 385f Aircraft protection, 388e390 Airplanes, 384e385 Allied Engineering Publication-2920 (AEP2920), 128e130 Altitude, 323 Alumina, 269, 349e351 Aluminum armor, 293 Aluminum oxide (Al2O3), 179e183, 369e371 in armor applications, 183t ceramic tiles, 294f hexagonal, 183f properties, 370t American helmet, 211 American Society for Testing and Materials (ASTM), 132 Ammunition, 87, 103, 386e387, 387f, 387t components, 88, 89f bullet, 89e90 cartridge case, 90e91

primer, 91 propellant, 91 Amour-piercing incendiary, 97e98, 98f Angle of incidence, 146 Anti-materiel rounds, 387 Antibacterial Finishes on Textile Materials: Assessment of, 324 Antiballistic fabrics, 185e188 Antimicrobial, 324 Antipersonnel effect, 87 Antitank rounds, 387 AP. See Armor piercing (AP) Apache combat helicopter, 289f API. See Armor-piercing incendiary (API) Appliqué armor, 146 Aramid, 328 aramid-coated fabrics, 22 fabric, 185e186, 379f types, 64e65 unidirectional fabric, 381 Aramid fibers, 3e4, 15, 16f, 37, 237e238, 295 ballistic application, 22 in continuous filament form, 68f crystalline structure, 22 diminishing transverse bands under stress, 21f dry-jet wet aramid fiber spinning, 16e17, 16f failure mode, 18f fiber fibrillar structure model, 18e20, 19f fracture morphology, 19f nylon 6, 15f pleat structure model, 20, 21f PPD-T crystal lattice, 23f properties, 23t skin core fibril structure, 17 in staple form, 67f

440

Aramid fibers (Continued) structure and morphology, 17 Areal density, 146 Areal densityebody armor, 325 ARL. See Army Research Laboratory (ARL) ARL X-Hybrid architecture, 422 Armor, 115, 146 carrier, 147 conditioning, 147 panel, 147 sample, 147 systems, 327, 349e351 test methods/standards, 115e116 Armor materials, 349e351. See also Body armor materials ceramics as, 351e352 manufacture of ceramics, 353e358 select material properties, 350t Armor piercing (AP), 96, 369 bullets, 287 Armor system testing. See also Composite backings; Fabrication of ceramicfaced armor; Testing of ceramic-faced armor armor types, 311 development testing, 312 environmental and usage considerations, 320 carriers, 324e325 environmental exposure, 320e323 head forms used in ballistic testing of helmets, 319t helmet system performance evaluations, 318e320 penetration testing, 314e318 soft armor panels, 325e326 test types, 311e313 velocity measurements, 313e314 Armor vests, NIJ-0101. 06 Standard, 179 Armor-piercing incendiary (API), 118 Army combat helmet (ACH), 410e411 Army ManTech and Defense-wide Manufacturing Science & Technology, 412e413 Army Research Laboratory (ARL), 398 “As-received” materials, 410e411 ASTM. See American Society for Testing and Materials (ASTM)

Index

ASTM B117. See Standard Practice for Operating Salt Spray (Fog) Apparatus ASTM D1171. See Standard Test Method for Rubber DeteriorationeSurface Ozone Cracking Outdoors or Chamber ASTM D3776. See Standard Test Methods for Mass Per Unit Area (Weight) of Fabric ASTM F392. See Standard Practice for Conditioning Flexible Barrier Materials for Flex Durability ASTM G155. See Standard Practice for Operating Xenon Arc Light Apparatus for Exposure of Non-Metallic Materials ASTMD3884. See Standard Test Method for Abrasion Resistance of Textile Fabrics Asymmetric warfare, 285e286, 286f Australian helmet, 212e213, 213f Autoclave, 263, 264f, 299f machine, 197f process, 382 processing, 196e198, 263e264 AUTODYN, 359e360 Automated tap test, 258 B Back-face deformation signature (BFD signature), 398 Back-face signature (BFS), 121, 147, 179, 256, 342 Backing, 378 material, 147 fixture, 147 Ballistic acceptance test, 147 Ballistic application of aramid fibers, 22 UHMWPE fibers, 14e15 UHMWPE tapes/ribbons, 29 Ballistic coefficient, 148 Ballistic composite materials, 327e331 aramid, 328 fiberglass, 329 finite element analysis as design tool, 331e338 modeling of ballistic packages, 338e343 UHMWPE materials, 329e331, 331f

Index

Ballistic crossplied unidirectional materials, 164 Ballistic fabrics, 159, 239e240 ballistic crossplied unidirectional materials, 164 ballistic felts, 165e167 ballistic impact deformation, 240f coated unidirectional and woven fabrics, 242e243 felt manufacturing process, 166f UHMWPE tapes, 164e165 woven fabrics, 159e164, 159f, 241e242 Ballistic fiberglass, 4 applications, 31 glass fiber manufacturing, 30f glass melting and refining, 30 raw materials, 29e30 structure and morphology, 31 textile glass fiber spinning, 30e31 Ballistic fibers, 157, 236 aramid fibers, 237e238 ballistic fabrics, 159e167 durability, 245e246 bonding resins, 248 effects of moisture and heat, 247 exposure to industrial chemicals, 248 laminated film on, 249e250 UV sensitivity, 246e247 M5Ò fibers, 238e239 polybenzobisoxazole fibers, 236e237 properties, 158t UHMWPE fibers, 239 Ballistic helmets, 210 American helmet, 211 Australian helmet, 212e213, 213f French helmet, 212, 212f NIJ Standard-0106. 01 for, 179 Russian helmet, 213e214 United Kingdom helmet, 211e212 Ballistic impact, 148 Ballistic inserts, 200 Ballistic limit (BL), 148e149 testing, 173, 255 protocol for, 256e257 Ballistic materials, 236e245, 410e411. See also Nonwoven ballistic materials ballistic fabrics, 239e243

441

ballistic fibers, 236e239 ballistic products, 272e274 ballistic threats on durability, 276e278 ballistic unidirectional crossplied materials, 243e245 ceramics, 293e294 composite materials, 294e298 durability, 245e246 bonding resins, 248 effects of moisture and heat, 247 exposure to industrial chemicals, 248 laminated film on, 249e250 UV sensitivity, 246e247 effects of processing on durability, 262 durability of level III single-panel vs. level IV breastplates, 268e272 processing equipment, 263e267 processing method, 263 processing panels, 267e268 raw materials, 262e263 effects of secondary manufacturing processes, 274e276 international ballistic specifications/ standards, 176e179 metal armor, 292e293 polymeric materials, 231 processing of helmet shell, 202e206 methods and equipment, 192e198 molds for processing ballistic composites, 198e200 of panels, 200e201 panels for level IV, 201e202 raw materials, 179e191 trimming and finishing of products, 206 quality control, 167 ballistic testing of materials, 172e176 instrumental and spectroscopy methods, 169e171 for manufacturing quality product, 168 physical properties, 168e169 shear thickening suspensions, 234e235 specification on durability, 257e261 tests for assessing durability of converted products, 250 ballistic testing of materials, 255e256 chemical properties, 254e255 physical properties, 250e254 protocol for ballistic limit tests, 256e257

442

Ballistic materials (Continued) transportation and storage, 278 air transport, 279 ground transport, 279 long-distance shipment and storage, 278e279 sea transport, 279 thermoplastic prepregs, 207e209 thermoset prepregs, 207 Ballistic nylon, 6 fabrics. See Plain-woven nylon 6 Ballistic panel, 149 Ballistic products curved components durability, 272e273 helmet shells durability, 272e273 helmet specifications, 273e274 Ballistic protection systems, 157 Ballistic resistance, 149 methodologies, 172e173 Ballistic testing of armor products, 311 of materials, 172 ballistic limit testing, 173 ballistic resistance methodologies, 172e173 Bruceton method, 174e176 Langlie method, 174 one-shot test response method, 174 Probit method, 173 test setup, 343f Ballistic threats on durability, 276 fragments, 277 handgun threats, 276 IEDs, 277e278 rifle bullets, 276e277 fragments, 103e108 projectile and target interaction, 108e113 small arms ammunition, 88e103 threat, 87 Ballistic TipÒ, 96f Ballistic unidirectional crossplied materials, 243 aramid fiber-based materials, 243e244 UHMWPE fiber-based materials, 245 Baseline ballistic limit, 149 Bespoke test specifications, 141 BFD signature. See Back-face deformation signature (BFD signature)

Index

BFS. See Back-face signature (BFS) BL. See Ballistic limit (BL) Black tracer, 97 Blast, 117 lung, 117 Blended-fiber constructions, 80e81 Blending process, 71e72 Blunt-tipped bullets, 57 Boats, 288, 386 Body armor, 41, 149, 217e221, 231, 235e236, 262, 349 analysis, design, and manufacture of, 342e343 plate, 344f Body armor materials, 413 ballistic impact on TwaronÒ CT709, 416 benefits of spider silk in soft armor applications, 417e418 ceramic/composite-plate body armor, 420e423, 420f contemporary body armor systems, 414 influence of informed design in soft body armor systems, 416e417 iron triangle, 415f materials multifunctionality by design, 423e425, 424f novel approaches to hard armor materials, 418e420 soft vest carrier and hard ceramic plate insert, 414f Boiling fiber test, 255 Bomb suit integrity test, 132 Bonding resins, 248 Boron carbide (B4C), 179e183, 185, 269, 294, 349e351, 369e370, 372 in armor applications, 185t hexagonal, 185f Boron silicon carbide (BSiC), 349e351 BP. See Bulletproof (BP) Breastplates, 267 British researchers, 78 British Royal Air Force, 55e56 British Standard EN 1063, 303 Bruceton method, 174e176, 315 BSiC. See Boron silicon carbide (BSiC) BSPs. See Bullet-simulating projectiles (BSPs) Bullet, 89e90 bending, 110

Index

jacket stripping, 110 mushrooming, 110 protection, 42 surrogates, 143 testing with, 103 types, 91e102 Bullet-protective plates, 41 Bullet-simulating projectiles (BSPs), 143 Bulletproof (BP), 58 jackets, 257e258 Buried mines, 105 Bursting strength, 251 C C-IED. See Counter-IED (C-IED) Canadian Standards Association (CAN/ CSA), 132 Carbon fibers (CF), 4e5, 188 Carbon nanotubes (CNTs), 351, 416 Carriers abrasion, 324 antimicrobial, 324 closure wearout, 325 fading and colorfastness, 325 machine washability, 324 Cartridge case, 90e91 CAST. See Centre for Applied Science and Technology (CAST) CBA. See Combat Body Armour (CBA) Centre for Applied Science and Technology (CAST), 313 Ceramic-composite armor systems, 369 armor materials, 349e351 ceramics as, 351e352 manufacture of ceramics, 353e358 select material properties, 350t finite element analysis of a ceramics-based ballistic package, 358e364 Ceramic-faced breastplates processing, 201 Ceramic-faced flexible armor backing, 383 Ceramic-faced hard molded armor backing, 383 Ceramic-faced molded armor ceramic-faced lightweight armor, 369 composite backings, 378e381 fabrication, 382e386 testing, 386e390 Ceramic(s), 293e294, 419e420 as armor material, 351e352

443

balls, 376f ceramic-based raw materials, 179e185 ceramic/composite-plate body armor, 420e423, 420f composite armor, 149 fibers for ballistics, 5e6 inserts, 263 manufacture, 353e358 materials, 369 for armor applications, 353 perforated armor configurations, 356f shapes, 373f flat tiles, 373e376 shaped ceramics, 376e377 small, large, and monolithic tiles, 373 thin and thick tiles, 373 tiles, 271 types, 369e372 aluminum oxide, 370e371 boron carbide, 372 silicon carbide, 371e372 Cermet, 357 Certification of compliance, 149 CF. See Carbon fibers (CF) Chisel nose (CN), 106 fragment-simulating projectile family, 107f Chronograph, 149 Civilian armored vehicles, 133 Closed-die molding process, 264 Closure wearout, 325 CN. See Chisel nose (CN) CNTs. See Carbon nanotubes (CNTs) Coated fabrics, 161e164. See also Uncoated fabrics DSC test on, 170f infrared test on, 170f recycling and disposal, 210 Coated unidirectional and woven fabrics, 242e243 Coefficient of thermal expansion (CTE), 357e358 Colorfastness, 325 Combat Body Armour (CBA), 218 Complete penetration MIL-STD-662F, 153 NIJ-0101. 04, 153 Compliance test group, 149 Compliance Testing Program (CTP), 149

444

Composite, 157. See also Experimental characterization of materials; Raw materials armor, 150 materials, 41, 294, 338e339 ballistic fibers and tapes, 295e297 prepregs, 297e298 resins, 297e298 Composite backings, 378. See also Fabrication of ceramic-faced armor; Testing of ceramic-faced armor aramid unidirectional fabric, 381 fiberglass, 378e379 UHMWPE unidirectional fabric, 380e381 woven fabric, 380 woven aramid fabrics, 379e380 Compression molding, 195e196, 264 press, 299f resistance, 319 Computed tomography scanning (CT scanning), 271e272 Computer-based numerical tools, 358e359 Confocal microscopy, 233 Constitutive modeling, 335e338 double cantilever beam experimental setup, 336f failure modeling, 338 strain-rate effects, 337e338 Contact molding, 193 Contemporary body armor systems, 414 Conventional approaches, 62e63 Conventional technologies, 62e63 Core erosion, 113 Counter-IED (C-IED), 224e225 Crosslapper, 166f Crosslapping, 73e74 modern type of crosslapper, 74f Crossplying. See Crosslapping Crystalline phase, 32 structure, 22 CT scanning. See Computed tomography scanning (CT scanning) CTE. See Coefficient of thermal expansion (CTE) CTP. See Compliance Testing Program (CTP)

Index

Curved components durability, 272e273 Cylinders, 377, 377f D DCB tests. See Double cantilever beam tests (DCB tests) Defence Science and Technology Laboratory (Dstl), 143 Defense Advanced Research Projects Agency, 425 Deformation, 150 Delamination, 44e46 tests, 339e341 Department of Defense (DoD), 126, 395e396, 425 Department of Transportation (DOT), 132 Depth-of-penetration test (DOP test), 361, 363f Design validation testing, 312 Detail Specification Cloth, Duck, Textured Nylon, 324 Differential scanning calorimeter test (DSC test), 169, 169f test on prepregs, 170f Direct heating systems, 198 Discovery demonstrators, 430 of dual-use trailer/protective shield concept, 432f Dissimilar Materials Assembly System, 422f Distance/timer devices, 314 DoD. See Department of Defense (DoD) DOP test. See Depth-of-penetration test (DOP test) DOT. See Department of Transportation (DOT) Double cantilever beam tests (DCB tests), 332 mode-I test, 333e335 Dry-jet wet spinning, 16 aramid fiber spinning, 16e17, 16f DSC test. See Differential scanning calorimeter test (DSC test) Dstl. See Defence Science and Technology Laboratory (Dstl) Dual-use potential, 431e433 Dynamic weight distribution system (DWD system), 226 DyneemaÒ composite, 400e401, 404e405, 411, 419e420

Index

E E-glass, 4 ECBA. See Enhanced Combat Body Armour (ECBA) ECH. See Enhanced combat helmet (ECH) EFP. See Explosively formed projectile (EFP) Ejector pins, 266 Ends per inch (EPI), 63 Energy, 57 absorption and dissipation, 76e77 Enhanced Combat Body Armour (ECBA), 218, 219f Enhanced combat helmet (ECH), 393e394 Enhanced small arms protection inserts (ESAPI), 414 Environmental exposure altitude, 323 flexing, 323 humidity, 321e322 maritime, 323 temperature, 320e321 vibration, 322e323 weathering, 322 EOD. See Explosive ordnance disposal (EOD) EOS. See Equation of state (EOS) EPI. See Ends per inch (EPI) Epoxy prepolymer chemical structure, 188f Epoxy resins, 188e189 Equation of state (EOS), 329e330, 359e360 Ergonomics, 273 Error, 341 ESAPI. See Enhanced small arms protection inserts (ESAPI) Experimental characterization of materials, 332. See also Compositedmaterials close-up image of speckled surface, 334f compression test, 335f DCB mode-I test, 333e335 simple compression and loading/unloading tests, 333 simple tension and loading/unloading tests, 332e333 stressestrain curve, 334f Experimental techniques, 331e332 Explicit FE analysis, 331e332

445

Explosive ordnance disposal (EOD), 41, 119 Explosively formed fragment. See Natural fragment Explosively formed projectile (EFP), 118 Extended-chain tie molecule, 10, 18e20 Extrapolated BFS (R2), 343 Extrusion process, 24 Eye protection, 222e223 F Fabrication of ceramic-faced armor, 382. See also Composite backings; Testing of ceramic-faced armor personal protection, 382e383 vacuum bagging for ballistic protection, 382 vehicle armor, 383e386 Fabrics, 328 formats, 417 Face protection, 222e223 Fading, 325 Failure mechanisms, 47e48 modeling, 338 Fair hits MIL-STD-662F, 150 NIJ-0101. 04, 150 NIJ-0108. 01, 150 Fair impact, 150 FE simulations. See Finite element simulations (FE simulations) Felt manufacturing process, 166f Felting needles, 75f Fiber, 42e43. See also Glass fibers (GF) components, 63 composite fiber architectures, 43e44 hybrids, 45 manipulation of through-thickness, 46 particulate reinforcement, 46 ply orientation, 45 reinforcement structure, 44e45 three-dimensional reinforcements, 46e47 through-thickness reinforcement, 45e46 fibrillar structure model, 18e20, 19f forms, 67 filament, 69e71 nonwovens creation methods, 69

446

Fiber (Continued) staple fiber, 71e75 staple fiber yarns, 68e69 selection criteria for ballistic-resistant materials, 64 aramid types, 64e65 fiber candidates for future use, 66e67 linear polyethylene types, 65e66 PIPD fiber, 66 Fiber-reinforced polymer composites (FRP composites), 231e232 Fiberglass, 4, 329, 378e379 applications, 31 fiberglass-reinforced polymer composites, 187 structure and morphology, 31 Fibril, 18e20 Fibrous tape, nonfibrous tape vs., 29 Filament, 69 layup composites, 76 flexible armor, 77 level III filament layup armors, 77e78 parallel filament layup with resin reinforcement, 69e70 stitchbonding process, 70e71 winding, 412 Film-based resins, 298 Finite element analysis of ceramics-based ballistic package, 358e364 bullet finite element model, 361f DOP test, 361, 363f example, 364 material models, 360e361 model calibration, 361e364 as design tool, 331e338 constitutive modeling, 335e338 experimental characterization of materials, 332e335 Finite element modeling of ballistic packages, 338e339 analysis, design, and manufacture of body armor, 342e343 convergence check and extrapolated solution, 345t convergence study, 344t design experiment for double cantilever beam model, 342t double cantilever beam model, 340f

Index

response metrics for body armor panel, 345t verification tests, 339e342 Finite element simulations (FE simulations), 329e330 First-article testing, 422e423 FLAK. See Fliegerabwehrkanonen (FLAK) Flame resistance, 320 Flammability test, 132 Flat tiles, 373e376 Flat-nose bullets (FN bullets), 276 Flechettes, 101, 102f Flexibility, 326 Flexible armor, 77 Flexible body armor, 151 Flexing, 323 Flexural properties, 252e253 Flexural test, 252e253 Fliegerabwehrkanonen (FLAK), 55e56 Flotation, 323 Flow test, 169 FMJ. See Full metal jacket (FMJ) FN bullets. See Flat-nose bullets (FN bullets) FORTRAN code, 342e343 Fragment-simulating projectiles (FSPs), 106e108, 127, 151, 211, 277, 302 Fragment(s), 103, 277 primary, 103e104 protection, 81 secondary, 104 testing with fragments fragment-simulating projectiles, 106e108 variability of fragments, 105e106 types of devices, 104e105 Fragmentation, 117, 127 fragmenting munitions, 104e105 test, 132 Frangible bullets, 98 Fraying, 206 Freezeethaw phenomenon, 234 French helmet, 212, 212f FRP composites. See Fiber-reinforced polymer composites (FRP composites) FSPs. See Fragment-simulating projectiles (FSPs)

Index

Full metal jacket (FMJ), 92, 92f, 123, 276 full-metal-jacketed bullet, 151 Functional finishes, 275 Future soldier protection, 393 head protection, new directions in, 395e413 new material developments in torso and related body armor, 413e425 NOVEL exoskeleton development, 425e429 potential of robotically deployed materials, 429e433 resistance against contemporary and representative threats, 393e394 soldier-borne loads, 395f US materials and designs for protection of dismounted soldiers, 394f G Gel, 1e2, 10 Gel-spinning method, 1e2, 65e66 General ballistic material test methods and standards, 137 NIJ-0108. 01 Standard, 138 VPAM APR 2006, 138e139 VPAM PM 2007, 139 GF. See Glass fibers (GF) Glass and aramid thermosets, 300 glass-based composites, 44 glass-melting furnace, 30 melting and refining, 30 Glass fibers (GF), 4, 37, 42e43, 187, 295. See also Fiber fabric, 379f manufacturing, 30f Grains (gr), 88 Graphite fiber. See Carbon fibers (CF) Grinding, 36 Gripping system, 333f Ground transport, 207, 279 Ground vehicles, 290e292, 383e384, 384f protection, 389e390 H Hand-layup method, 192e194, 193f Handgun threats, 276 Handheld riot shields, 214 HAPs. See Hard armor panels (HAPs)

447

Hard armor, 42, 151 approaches to hard armor materials, 418e420 hard armor plate, NIJ-0101.06 Standard, 179 Hard armor panels (HAPs), 257 Hard materials, 414 Head and torso/lower body protection, 393 Head protection academic, industrial, and government collaboration, 396e397, 397f influence of laminate architecture on ballistic performance, 405e406 influence of manufacturing on preform fabrication, 412e413 manufacturing processes as enablers, 410e411 material/process/property approach, 398e405 modeling of helmet materials and laminates, 406e410 moving beyond composite sheet goods, 411e412 new directions in, 395e396 research thrusts, 398f test, 132 HEaDS UP program. See Natick Soldier RDEC-led Helmet and Electronics and Display SystemeUpgradable Protection program (HEaDS UP program) Heat and pressure control, 198 Helicopters, 384e385, 385f Helmet(s), 42, 221e222 preform forming methods, 410f shell durability, 272e273 processing, 202e206 specifications, 273e274 system performance evaluations, 318e320 HFE. See Human factors evaluation (HFE) High-hard steel (HH steel), 327e328, 351 High-modulus polyethylene (HMPE), 159 High-modulus polypropylene fibers (HMPP fibers), 5, 32, 66 manufacturing process, 32, 32f properties, 34e35 structure of fiber, 32

448

High-performance ballistic fibers and tapes, 1, 3. See also Ultrahigh-molecularweight polyethylene (UHMWPE) aramid fibers, 3e4, 15e22 ballistic fiberglass, 4, 29e31 carbon fibers, 4e5 ceramic fibers for ballistics, 5e6 high-performance fibers manufacturing, 2e3 HMPP fibers, 5, 32e35 random rods of polymers, 2f recycling of ballistic fibers and converted products, 35e37 requirements for, 1e2 High-performance fibers, 157, 262 High-performance polyethylene fibers (HPPE fibers), 65e66, 382 chemical structure, 65f woven fabric, 380f High-pressure match-die molding, 264 presses, 382 High-pressure liquid chromatography (HPLC), 171 High-strength woven fabrics, 328 High-tenacity and high-modulus fibrous, 26e27 High-velocity bullets, 117 High-volume high-pressure molds, 199 low-pressure permanent mold, 199e200 HMPE. See High-modulus polyethylene (HMPE) HMPP fibers. See High-modulus polypropylene fibers (HMPP fibers) HO CAST 2015, 122 HO CAST 47/11, 125e126 Hollow-point bullet (HP bullet), 93e94, 93f ammunition, 93e94 Hollow-point bullets, 57 Home Office Scientific Development Branch (HOSDB), 120, 313 Publication No. 39/07 2007, 120e122 HOSDB. See Home Office Scientific Development Branch (HOSDB) Hot-melt process, 162f HP bullet. See Hollow-point bullet (HP bullet)

Index

HPLC. See High-pressure liquid chromatography (HPLC) HPPE fibers. See High-performance polyethylene fibers (HPPE fibers) Human factors evaluation (HFE), 426 Human-made yarns, 67 Humidity, 321e322 moisture absorption test, 254 Hybrids, 45 I ICP emission. See Inductively coupled plasma emission (ICP emission) IEDs. See Improvised explosive devices (IEDs) Impact velocity. See Strike velocity Improved outer tactical vest (IOTV), 414, 417f Improvised explosive devices (IEDs), 103e104, 118, 217, 277e278, 302 Improvised explosive devices, 105 In-conjunction armor, 151 plate, 151 In-service testing, 120 Incineration, 36 Indirect heating systems, 198 Inductively coupled plasma emission (ICP emission), 254 Industrial chemicals exposure to, 248 Informed design influence in soft body armor systems, 416e417 Infrared spectroscopy (IR spectroscopy), 233 Infrared techniques, 169 InnegraÔ, 66 Inserts, 152, 200 Integral armor, 152 Interlaminar shear properties, 253e254 Internal Security (IS), 218, 222 International ballistic and blast specifications and standards, 115, 176 amendments by user communities, 141 armor test methods/standards, 115e116 ballistic projection/threat levels, 182t bespoke test specifications, 141 BFS performance test, 177t classifications of armor, 180t

Index

future of armor test methods and standards, 143 bullet surrogates, 143 continued use of irrelevant standards, 143 general ballistic material test methods and standards, 137e139 hard armor classification, 178t issues with, 142 obsolete threat levels, 142e143 reality vs. repeatability, 142 MIL-STD-662F 2920, 176 NIJ Standard-0101 04, 176 perforation, 177t NIJ Standard-0108.01, 178 NIJ-0101.06 Standard, 179 for ballistic helmets, 179 personal armor general purpose test methods and standards, 130e133 law enforcement test methods and standards, 119e126 military test methods and standards, 126e130 test methods and standards, 116e117 user communities, 119 STANAG 2920, 176 suitable test method, 140 test methods, 116, 140e141 vehicle armor civilian test methods and standards, 133e135 military test methods and standards, 135e137 test methods and standards, 117e118 user communities, 133 International Organization for Standardization (ISO), 247 IOTV. See Improved outer tactical vest (IOTV) IR spectroscopy. See Infrared spectroscopy (IR spectroscopy) Iron triangle, 415f Irrelevant standards, 143 IS. See Internal Security (IS) ISO. See International Organization for Standardization (ISO)

449

J Jacketed hollow-point bullet, 152 Jacketed soft point bullet. See Semijacketed soft-point bullet JohnsoneHolmquist model (JH1 model), 359e360 K KE. See Kinetic energy (KE) KED. See Kinetic energy density (KED) KevlarÒ, 237e238, 238f, 393 fibers, 65 hydrolytic degradation, 238f KevlarÒ 129, 406e410 UHMWPE, 187f Kinetic energy (KE), 132 Kinetic energy, 56, 297 Kinetic energy density (KED), 108e109, 132 L Laminated film on ballistic materials durability, 249e250 Langlie method, 174, 315 Large tiles, 373 Large-caliber tests, 389f Laser trimming, water jet vs., 275 Lead bullet. See Semijacketed soft-point bullet Leather, 55 Lethality, 415 Level III single-panel durability, level IV breastplates vs., 268 composition of ceramics, 268e269 crack arrester, 270e271 durability of adhesives, 269 wrapping to constrain ceramics, 271 X-ray of ceramic bonding, 271e272 Light improved ballistic armor (LIBA), 383f Lightweight ballistic composites body armor, 41 composite fiber architectures, 43e47 failure mechanisms, 47e48 fiber types, 42e43 Lightweight ceramic body armor, 353 Lightweight composite materials processing. See also Ballistic materials ballistic fibers, 157e167 ballistic helmets, 210e214 composite, 157

450

Lightweight composite materials processing (Continued) durability of products, 209 handheld riot shields, 214 molded articles evaluation, 206e207 recycling and disposal of prepregs, 210 Linear polyethylene types, 65e66 Localized passive exoskeleton concepts, motivation for, 425 Long rifle (LR), 118 Long-distance shipment and storage, 278e279 Lot-acceptance testing, 422e423 Low-velocity bullets, 116e117 Low-volume molds, 199 LR. See Long rifle (LR) LS-DYNA, 338e339, 406e410 M M1 “steel pot” design, 393 M1 helmet, 393 M5 PIPD fiber, 66f M5Ò fibers, 238e239 Machine washability, 324 Manipulation of through-thickness, 46 Manufacturer’s quality testing (MQT), 120 Maritime, 323 Mat formation methods, 72e74 Material architecture, 404e405 Material models, 360e361 Material/process/property approach, 398e405 artifacts in scans of UHMWPE composites, 401f influence of pressure on ballistic performance, 403f of temperature and molding pressure, 402f micro-CT image, 400f microstructures and morphologies, 399f spherical-shaped voids, 401f voids elongated in fiber direction, 401f Materials, 393e394 multifunctionality by design, 423e425, 424f Matrix comprises, 262 phase, 157

Index

matsum ASCII file, 339 Metal armor, 292e293 Micro-computed tomography (micro-CT), 398e400 MIL-DTL-32439. See Detail Specification Cloth, Duck, Textured Nylon MIL-STD-662. See Military Standard-662 (MIL-STD-662) MIL-STD-662F. See Military Standard662F (MIL-STD-662F) Military armored vehicles, 133 Military helmet trimline, 221 Military Standard-662 (MIL-STD-662), 315 Military Standard-662F (MIL-STD-662F), 130, 146e156, 388 MIL-STD-662F 2920, 176 Military standards, 56, 59 test protocols from, 56f Milite process, 24 Mk7 combat helmet, 222f Mobility, 415 MOD. See UK Ministry of Defence (MOD) Model calibration, 361e364 Moisture detection, 261 Mold(s) cavity, 264 design attributes, 264e267 heating, 265 molded articles evaluation, 206e207 for processing ballistic composites, 198e200 Monolithic armors, 314 breastplates processing, 200e201 shape, 382 tiles, 373 torso and side inserts, 378f MQT. See Manufacturer’s quality testing (MQT) Multicurvature, 377 Multicurved composites, 200 Multifunctional materials approaches, 424e425 Multipiece all-ceramic armor, 354e355 Multipiece ceramic armor, 353e354 Multipiece perforated ceramic armor, 356f Muzzle velocity, 152 Muzzle-loaded weapons, 87

Index

N NA. See National Authority (NA) Natick Soldier RDEC-led Helmet and Electronics and Display SystemeUpgradable Protection program (HEaDS UP program), 395e396, 396f National Authority (NA), 128 National Institute of Justice standards (NIJ standards), 59, 119, 176, 247, 302e303, 317, 342 ballistic testing shot patterns for, 61f NIJ 0108. 01, 302e303 NIJ Standard-0101.04, 122e123, 147, 149e150, 153e155 NIJ Standard-0101.06, 123e124, 146e147, 149, 151e156 NIJ Standard-0106.01, 124e125 NIJ Standard-0117.00, 131e133 NIJ-0101. 04/06, 147, 151e152, 154 NIJ-01018.01 Standard, 151 NIJ-0108.01 Standard, 138, 152e153, 302e303 National Institute of Standards and Technology (NIST), 232 National Nonwovens, 79 National Voluntary Laboratory Accreditation Program, 317 NATO AEP 55 STANAG 4569, 303e305 Natural fragment, 103 Natural rubber latex (NRL), 275 Near-net single-curved ultrahigh-molecular-weight polyethylene plates, 431f tow placement concept, 412f Neck protection, 223e224 Needle board, 75 Needlepunching, 63e64, 74e75 Neutron radiography, 261 Neyer method, 316 NIJ standards. See National Institute of Justice standards (NIJ standards) NILECJ. See US National Institute of Law Enforcement and Criminal Justice (NILECJ) NIST. See National Institute of Standards and Technology (NIST)

451

NMR spectroscopy. See Nuclear magnetic resonance spectroscopy (NMR spectroscopy) NomexÒ structure, 15, 15f Nonballistic tests, 258 automated tap test, 258 moisture detection, 261 neutron radiography, 261 radiography, 260e261 thermography, 261 ultrasonic inspection, 258e260 visual inspection, 258 Noncrimp fabrics, 46 Nonfibrous tape, 28f fibrous tape vs., 29 Nonplanar armor, 153 Nonspalling (NS), 139 Nontraditional machining, traditional machining vs., 274e275 Nonwoven ballistic materials. See also Ballistic materials ballistic testing shot patterns for NIJ Standard, 61f fiber selection criteria for ballistic-resistant materials, 64e67 filament layup composites, 76e78 future directions for, 84 law enforcement armor needs, 59e60 leather, 55 methodologies for use, 79 blended nonwoven in fragment defeat, 82f blended-fiber constructions, 80e81 fragment protection, 81 level IIIA baseline test, 83t multiple layering of single fibers, 79e80 nonwovens and conventional materials combinations, 81e83 single-fiber components, 79 tests by US Army, 81 military armor requirements, 58e59 modern armor, 55e56 NIJ Standard 0101. 06, 61t protective materials, devices, and end-use requirements, 62 conventional approaches, 62e63 unconventional nonwovens approaches, 63e64

452

Nonwoven ballistic materials (Continued) scientific basis of armor construction, 56e58 STANAG 4569, 60t uses of nonwoven ballistic-resistant fabrics, 78e79 variations of fiber forms, 67e75 Nonwovens and conventional materials combinations, 81e83 Nonwovens creation methods, 69 Novel exoskeleton development, 425. See also Robotically deployed materials evolved VLOS prototype, 427e428 motivation for localized passive exoskeleton concepts, 425 potential of torso-based load sharing for improved head protection, 428e429 VLOS prototype development, 426, 426f NRL. See Natural rubber latex (NRL) NS. See Nonspalling (NS) Nuclear magnetic resonance spectroscopy (NMR spectroscopy), 255 Numerical modeling of composite materials, 331e332 Nylon, 43, 64, 191 Nylon 6, 15, 15f Nylon 6, 6, 15, 15f salt, 43 O Obliquity, 153 angle, 153 Obsolete threat levels, 142e143 Office of Law Enforcement Standards, National Institute of Standards and Technology (OLES NIST), 119 One-shot test response method, 174 Open tip match ammunition (OTM ammunition), 94e95, 95f Opening process, 71e72 Osprey plates, 219, 220f Osprey system, 221 Overmatch, 153 testing, 290

Index

P P-BFS. See Perforationeback-face signature (P-BFS) PAN. See Polyacrylonitrile (PAN) Panels processing, 200 ceramic-faced breastplates processing, 201 for level IV, 201e202 monolithic breastplates processing, 200e201 Para-aramid(s), 328 fabricereinforced composites, 42e43 fibers, 157, 217e218 chemical structure, 64f para-aramid-based composites, 44 Parallel filament layup, 69e70 Partial penetration MIL-STD-662F, 153 NIJ-0101. 04, 153 Particulate reinforcement, 46 PAS. See Publicly Available Specification (PAS) PASGT. See Personnel Armor System for Ground Troops (PASGT) PASGT helmet. See Personal Armor System Ground Troops helmet (PASGT helmet) PASS. See Personal Armour Systems Symposium (PASS) PBO. See Polybenzobisoxazole (PBO) PBP. See Portable ballistic protection (PBP) PC strip. See Polycarbonate strip (PC strip) PCD. See Polycrystalline diamond (PCD) PEEK. See Polyether ether ketone (PEEK) PEI. See Polyetherimide (PEI) Pelvic protection, 224e225 Penetration, 153 partial penetration MIL-STD-662F, 153 NIJ-0101. 04, 153 testing, 314e318 Penetration evaluation (V50), 318 of armor, 315 test, 123, 128, 130, 138e139, 156 PEO Soldier. See Program Executive Office Soldier (PEO Soldier) Perforation, 153 Perforationeback-face signature (P-BFS), 124, 255e256

Index

Personal armor, 41e42, 116, 217. See also Vehicle armor body armor, 217e221 eye protection, 222e223 face protection, 222e223 future developments, 226e227 general purpose test methods and standards, 130 NIJ Standard-0117. 00, 131e133 helmets, 221e222 law enforcement test methods and standards, 119 HO CAST 2015, 122 HO CAST 47/11, 125e126 HOSDB Publication No.39/07 2007, 120e122 NIJ Standard-0101.04, 122e123 NIJ Standard-0101.06, 123e124 NIJ Standard-0106.01, 124e125 military test methods and standards, 126 AEP-2920, 128e130 MIL-STD-662F, 130 STANAG 2920 edition 2, 127e128 STANAG 2920 edition 3, 128e130 neck protection, 223e224 pelvic protection, 224e225 threat regimes for test methods and standards, 116 blast, 117 fragmentation, 117 high-velocity bullets, 117 low-velocity bullets, 116e117 UK Virtus body armor system, 226 user communities, 119 general users, 119 law enforcement, 119 military users, 119 Personal Armor System Ground Troops helmet (PASGT helmet), 211 Personal Armour Systems Symposium (PASS), 143 Personal protection, 382 ceramic-faced flexible armor backing, 383 ceramic-faced hard molded armor backing, 383 Personal Upgradable ProtectioneExperimental (PUP-E), 430, 431f Personal weapons, 87

453

Personnel Armor System for Ground Troops (PASGT), 393, 410e411 Personnel protection, 388 Petaling, 154 Phased-array inspection, 260 Phenolics, 189e190 Phenylene terephthalamide (PPTA), 330 PIPD. See Poly(2,6-diimidazo[4,5-b4’,5’-e] pyridinylene-1,4(2,5-dihydroxy] phenylene) (PIPD) Plain-woven nylon 6, 6 fabrics, 43, 64 Plasma emission spectrometry, 254 Plastic tip ammunition, 95e96 Plate armor, 42 Plate inserts, 154 Pleat structure model, 20, 21f Ply orientation, 45 Police protective equipment, 60 Polishing and plating of mold, 267 Poly(2,6-diimidazo[4,5-b4’,5’-e] pyridinylene-1,4(2,5-dihydroxy] phenylene) (PIPD), 66 Polyacrylonitrile (PAN), 188 Poly(p-phenylene-2,6-benzobisoxazole) (PBO). See Polybenzobisoxazole (PBO) Polybenzobisoxazole (PBO), 232, 236e237 mechanism of oxazole ring opening in, 233f Polybenzobisoxazole fibers, 236e237 Polycarbonate strip (PC strip), 249e250 Polycrystalline diamond (PCD), 206 Polyesters, 189 chemical structure, 189f Polyether ether ketone (PEEK), 191, 248 Polyetherimide (PEI), 243 Polyethylene, 65e66 resins, 298 Polymer composite, 431 Polypropylene, 191 Polyurethane (PU), 248 Polyvinyl butyral (PVB), 44, 191, 393e394 Portable ballistic protection (PBP), 126 Postpressing composite armor, 301 PPTA. See Phenylene terephthalamide (PPTA) Precompliance testing, 312e313 Precursor, 4e5 Preneedling, 75

454

Pressing process, 24 Pressure, 382 Pretest parameters, 247 Primary blast, 117 Primary fragmentation, 117e118 Primary fragments, 103e104 Primer, 89, 91 Probit method, 173 Processing composite armor panels, 298e299 compression molding press, 299f glass and aramid thermosets, 300 postpressing composite armor, 301 UHMWPE fiber-based thermoplastics, 300 Profiles. See Views Program Executive Office Soldier (PEO Soldier), 396 Projectile, 89e90 deformation, 109 bullet bending, 110 bullet jacket stripping, 110 bullet mushrooming, 110 core erosion, 113 projectile fracture, 113 factors affecting projectile penetration, 108e109 fracture, 113 and target interaction, 108e113 factors affecting projectile penetration, 108e109 yaw, 156 Propellant, 91, 154 Proper venting, 266 PU. See Polyurethane (PU) Public Safety Bomb Suit Standard, 130 Publicly Available Specification (PAS), 302 PAS 300, 134e135 Pulse-echo test equipment, 260f ultrasonic inspection, 259e260 Punches per square inch fabric, 75 Punching, 154 PUP-E. See Personal Upgradable ProtectioneExperimental (PUP-E) PVB. See Polyvinyl butyral (PVB) Pyrolysis, 36

Index

Q Quality control, 167 Quaternary blast, 117 R Radio-frequency (RF), 261 Radiography, 260e261 RASP approach. See Robotic Augmented Soldier Protection approach (RASP approach) Raw materials, 262. See Composited materials antiballistic fabrics, 185e188 ceramic inserts, 263 ceramic-based, 179e185 high-performance fibers, 262 matrix comprises, 262 resins, 188e191 RCC. See Right circular cylinder (RCC) Recycling of ballistic fibers and converted products, 35 aramid fibers, 37 glass fibers, 37 UHMWPE fibers and tapes, 36e37 Reference bullet velocity, 154 velocity, 154 Regulatory compliance testing criteria, 312e313 Reinforcement structure, 44e45 Reinforcing phase, 157 Relative humidity (RH), 232, 247 Request for information (RFI), 307 Resin reinforcement, parallel filament layup with, 69e70 Resins, 188e191 Retest, 154 RF. See Radio-frequency (RF) RFI. See Request for information (RFI) RH. See Relative humidity (RH) RHA-grade steel. See Rolled homogeneous armor-grade steel (RHA-grade steel) RHC. See Rockwell hardness “C”; (RHC) Rifle bullets, 276e277 Right circular cylinder (RCC), 106 Rigid armor, 151 Robotic Augmented Soldier Protection approach (RASP approach), 430e431

Index

Robotic-based augmentation, 430f Robotically deployed materials, 429e430. See also Novel exoskeleton development dual-use potential, 431e433 RASP approach, 430e431 rationale and scenarios, 433 robotic shield, 433f robotic-based augmentation, 430f Robust human/machine interfacing, 425 Rocket-propelled grenades. See Ruchnoi Protivotankovyi Granatamyot (RPGs) Rockwell hardness “C” (RHC), 107 Rolled homogeneous armor-grade steel (RHA-grade steel), 292, 327e328, 351 Rotary-and fixed-wing craft, 289 Round-nose bullet, 154 Rovings, 31 Ruchnoi Protivotankovyi Granatamyot (RPGs), 118 Russian helmet, 213e214 S Sabot, 154 Saboted light armor penetrator rounds (SLAP rounds), 102 Safety systems, 197 SAP. See Soft armor panel (SAP) SAPI. See Small-arms-protective insert (SAPI) SBS tests. See Short beam shear tests (SBS tests) Scanning electron microscopy analysis (SEM analysis), 171 Scouring, 275 SCRDE. See Stores and Clothing Research and Development Establishment (SCRDE) Sea transport, 279 Sea transportation, 207 Second World War (WWII), 106 Secondary blast, 117 Secondary fragmentation, 117e118 secondary fragments, 104 Secondary manufacturing process effects, 274 coatings, 275e276

455

traditional vs. nontraditional machining, 274e275 water jet vs. laser trimming, 275 SEM analysis. See Scanning electron microscopy analysis (SEM analysis) Semijacketed hollow-point bullet, 154 Semijacketed soft-point bullet, 152, 154 Semiwadcutter, 155 Shaped ceramics, 376e377 cylinders, 377, 377f multicurvature, 377 spherical elements, 376e377 Sheet goods, 410e411 Shield technology, 70 Ship armor, 386 Short beam shear tests (SBS tests), 253e254 Short-term durability, specification on, 257e258 Shot-to-edge distance, 155 Shot-to-shot distance, 155 Shotgun ammunition, 100e101 Silicon boron carbide (SiB4C), 185 Silicon carbide (SiC), 184, 269, 349e351, 369e372 in armor applications, 184t ceramics, 371f hexagonal, 184f properties, 372t Silicon nitride. See Silicon carbide (SiC) Simple compression and loading/unloading tests, 333 Simple tension and loading/unloading tests, 332e333 Single curved composites, 200 Single panel molding, 267e268 Single-fiber components, 79 Skin core fibril structure, 17 SLAP rounds. See Saboted light armor penetrator rounds (SLAP rounds) Small arms, 155 Small arms ammunition, 155 bullet testing with, 103 types, 91e102 components, 88e91, 89f 0.223-inch HP boat tail match, 88f Small arms protective insert plates (SAPI plates). See Ballistic inserts Small tiles, 373

456

Small-arms-protective insert (SAPI), 353f Smart Body ArmorÒ technology, 423f Soft armor. See Flexible armor Soft armor panel (SAP), 257 areal densityebody armor, 325 flexibility, 326 Soft ballistic vest materials, 14 Soft materials, 414 Soft-core ammunition, 386 Soft-point round (SP round), 94, 94f Soldier Protection System (SPS), 396, 397f Soldier-borne loads, 395f Solid fiber, 2, 9 precursor fiber, 8e9 Solid-state nuclear magnetic resonance, 255 Solvent-dip process, 162f SP round. See Soft-point round (SP round) Spaced armor, 155 Spall liners, 290e291 Spalling (S), 139, 155 Specific gravity, 169 Specification on durability nonballistic tests, 258e261 on short-term durability, 257e258 Spectra fiber, 296f Spectra helmet, 212 SpectraÒ ballistic composite, 400e401, 404e405 SpectraÒ composite, 411, 419e420 Spherical elements, 376e377 Spherical-shaped voids, 401f Spider silk benefits in soft armor applications, 417e418 Spine protection test, 132 SPS. See Soldier Protection System (SPS) STANAG 2920. See Standardization Agreement 2920 (STANAG 2920) STANAG 4569, 390 AEP-55, 388 edition 2, 135 Standard AK47 Kalashnikov rounds, 386 Standard Practice for Conditioning Flexible Barrier Materials for Flex Durability, 323 Standard Practice for Operating Salt Spray (Fog) Apparatus, 322 Standard Practice for Operating Xenon Arc Light Apparatus for Exposure of Non-Metallic Materials, 322

Index

Standard shot pattern, 139 Standard Test Method for Abrasion Resistance of Textile Fabrics, 324 Standard Test Method for Rubber DeteriorationeSurface Ozone Cracking Outdoors or Chamber, 322 Standard Test Methods for Mass Per Unit Area (Weight) of Fabric, 325 Standardization Agreement 2920 (STANAG 2920), 127e130, 176 Standardized test methods, 311 Staple fiber, 71 blending process, 71e72 crosslapping, 74 mat formation methods, 72e74 needlepunching, 74e75 opening process, 71e72 yarns, 68e69 “Stickeslip” phenomenon, 165e167 Stitchbonding process, 70e71 Stores and Clothing Research and Development Establishment (SCRDE), 107e108 Strain wave velocity, 76 Strain-rate effects, 337e338 Strike face, 155 Strike velocity, 155 Striking device, 155 Structural armor, 152 Submersion, 323 Survivability, 415 T Target base line, 155 Tearing strength, 250e251 Teijin Aramid, 65 Teijin Twaron. See Teijin Aramid Temperature, 320e321 Tensile strength, 250 Terminal ballistics, 156 Tertiary blast, 117 Test conditions, 247 Test methods and standards, 116 Test sample, 156 Test series, 156 Testing of ceramic-faced armor. See also Armor system testing; Composite backings; Fabrication of ceramicfaced armor

Index

ammunition, 386e387, 387f, 387t ground vehicle and aircraft protection, 389e390 personnel protection, 388 testing standards and methods, 387e390 Textile glass fiber spinning, 30e31 Textile-based materials, 156 Thermal oxidation, 36 Thermal techniques, 169 Thermography, 261 Thermoplastic(s), 190e191 prepregs, 207e209 resins, 298 thermoplastic-coated fabrics, 163e164, 243 Thermoset prepregs, 207 resins, 188e190, 297e298 thermoset-coated fabrics, 161e163, 243 Thick tiles, 373, 374f Thin hexagons, 374f Thin tiles, 373 Thin-layer chromatography (TLC), 170, 171f Threat, 87 Three-dimensional reinforcements, 46e47 ThreeD woven architectureJ3J, 241e242 Through-thickness reinforcement, 45e46 Through-transmission ultrasonic device, 259f inspection, 259 Titanium armor, 293 Titaniumetitanium diboride composite (TieTiB2 composite), 357 TLC. See Thin-layer chromatography (TLC) Torso, 413 Torso-based load sharing for improved head protection, 428e429 Tow placement, 412 Tracer, 97 Traditional machining, nontraditional machining vs., 274e275 Trauma, 57 Trauma inserts, 152 Trauma packs, 152 Trauma plates, 152 Tumbling, 247 TwaronÒ CT709, 416 Two-solvent system, 9

457

U UBACS. See Under Body Armour Combat Shirt (UBACS) UD materials. See Unidirectional materials (UD materials) UD PE. See Unidirectional polyethylene (UD PE) UH-60 medical evacuation helicopter, 289f UHMWPE. See Ultrahigh-molecular-weight polyethylene (UHMWPE) UK Health and Safety Executive, 197 UK Ministry of Defence (MOD), 217e218 UK Virtus body armor system Virtus head subsystem, 226 Virtus torso subsystem, 226 UL 752. See Underwriters Laboratory 752 (UL 752) Ultraconcealable multi-threat body armor, 328 Ultrahigh-molecular-weight polyethylene (UHMWPE), 1, 42, 65e66, 140, 186e187, 327e331, 331f, 349e352, 369, 393e394, 400e401, 414 fibers, 3, 36e37, 157, 217e218, 239, 295e296 ballistic application, 14e15 ballistic laminates, 400e401 chemical structure, 6e7 fiber-based thermoplastics, 300 gel-spinning process, 8e10, 9f materials, 401e404 micro-and macrofibrillar structure, 12f microfibration, 12 morphology, 6e7, 10, 11f particle sizes and features, 6f physical properties, 13 properties, 14t tapes/ribbons, 4, 24, 36e37, 164e165, 296e297, 297f ballistic application, 29 drawing of slit, 25 extrusion process, 24 high-tenacity and high-modulus fibrous, 26e27 morphology, 27e28 nonfibrous tape vs. fibrous tape, 29 nonfibrous tape/ribbon, 28f pressing process, 24 UHMWPE polymer for, 24

458

Ultrahigh-molecular-weight polyethylene (UHMWPE) (Continued) unidirectional fabric, 380e381 woven fabric, 380 Ultrasonic bond tester inspect, 260 Ultrasonic inspection, 258e260 Ultraviolet (UV), 231 light, 231, 295, 379e380 sensitivity, 246e247 Ultraviolet radiation (UVR), 218 Uncoated fabrics, 160 Unconventional nonwovens approaches, 63e64 Under Body Armour Combat Shirt (UBACS), 223 Undermatch, 156 Underwriters Laboratory 752 (UL 752), 303 Unidirectional materials (UD materials), 236 fabrics, 242 laminated technology, 164 Unidirectional polyethylene (UD PE), 381 Uniform abrasion test, 252 United Kingdom helmet, 211e212 Upper prediction limit (UPL), 121 US Army Air Corps, 55e56 US Army Natick Labs, 78 US National Institute of Law Enforcement and Criminal Justice (NILECJ), 317 Usage conditions, 320 User communities, amendments by, 141 UV. See Ultraviolet (UV) UVR. See Ultraviolet radiation (UVR) V V50. See Penetration evaluation (V50) Vacuum and oven processing, 195, 263 Vacuum bagging, 263e264, 382 for ballistic protection, 382 process, 194, 194f Variability of fragments, 105e106 VectranÒ, 66 Vehicle armor, 41e43. See also Personal armor aircraft carrier, 288f airplanes and helicopters, 384e385 armor design process, 306e308 asymmetric warfare, 285e286, 286f ballistic materials, 292e298 boat and ship armor, 386

Index

boats, 288 civilian test methods and standards, 133 PAS 300, 134e135 VPAM BRV 2009, 134 VPAM ERV 2010, 134 conventional force on force, 286f ground vehicles, 290e292, 383e384, 384f markets, 287 military test methods and standards, 135 AEP-55 volume 1e3, 135e137 STANAG 4569 edition 2, 135 military vehicles, 285 processing composite armor panels, 298e301 rotary-and fixed-wing craft, 289 threat regimes for test methods and standards, 117 blast, 118 fragmentation, 117e118 medium-caliber ammunition, 118 small arms, 118 threats and ballistic test standards, 302 British Standard EN 1063, 303 NATO AEP 55 STANAG 4569, 303e305 NIJ 0108. 01, 302e303 UL 752, 303 VPAM and PAS, 305e306 UH-60 Medevac, 306f user communities, 133 Velocity measurements, 313e314 Vereinigung der Pr€ ufstellen f€ ur angriffshemmende Materialien und Konstruktionen (VPAM), 134, 302 BRV 2009, 134 ERV 2010, 134 and PAS, 305e306 VPAM APR 2006, 138e139, 388 VPAM PM 2007, 139 Verification tests, 339e342 Vertical Load Offset System (VLOS), 394e395, 425 evaluation in representative natural environments, 427f evolved VLOS prototype, 427e428 Improved VLOS based on human factors evaluation, 428f prototype development, 426, 426f

Index

Vibration, 322e323 Vietnam War, 56 Views, 271e272 Vinylester resins, 190 Virtus head subsystem, 226 Virtus torso subsystem, 226 Visual inspection, 168, 258 VLOS. See Vertical Load Offset System (VLOS) Volatile content, 168 VPAM. See Vereinigung der Pr€ufstellen f€ur angriffshemmende Materialien und Konstruktionen (VPAM) Vproof test, 123 W Wad cutter, 98, 99f Warp, 241, 242f Water jet, laser trimming vs., 275 WAXS. See Wide-angle X-ray (WAXS) Wear face, 156 Weathering, 322 Weaving, 62e63 Weft, 241, 242f Wet-layup method. See Hand-layup method Wide-angle X-ray (WAXS), 32, 33f Witness plate, 156

459

Woven aramid fabrics, 379e380 Woven fabrics, 76, 159e164, 159f, 241e242 Wrought-steel armor, 292 WWII. See Second World War (WWII) X X-Hybrid configuration, 405e406 process, 421e422 X-rays. See Radiography X-threat small arms protective inserts (XSAPI), 414 Y Yarn, 63 slippage and seam strength, 252 Yaw, 156 Z z-binder. See z-direction binder z-direction binder, 46 Zodiac, 386f ZylonÒ, 236e237