Review of
Department of Defense
Test Protocols for
Combat Helmets

Committee on Review of Test Protocols Used by the DoD to Test Combat Helmets

Board on Army Science and Technology

Division on Engineering and Physical Sciences

NATIONAL RESEARCH COUNCIL
                                 OF THE NATIONAL ACADEMIES

THE NATIONAL ACADEMIES PRESS
Washington, D.C.
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Committee on Review of Test Protocols Used by the DoD to Test Combat Helmets Board on Army Science and Technology Division on Engineering and Physical Sciences

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THE NATIONAL ACADEMIES PRESS • 500 Fifth Street, NW • Washington, DC 20001 NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance. This study was supported by Contract/Grant No. HQ0034-10-D-0003 between the National Academy of Sciences and the U.S. Department of Defense. Any opinions, findings, con- clusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the organizations or agencies that provided support for the project. International Standard Book Number-13:  978-0-309-29866-7 International Standard Book Number-10:  0-309-29866-0 Limited copies of this report are available from Additional copies are available from Board on Army Science and Technology The National Academies Press National Research Council 500 Fifth Street, NW, Keck 360 500 Fifth Street, NW, Room 940 Washington, DC 20001 Washington, DC 20001 (800) 624-6242 (202) 334-3118 (202) 334-3313 http://www.nap.edu Copyright 2014 by the National Academy of Sciences. All rights reserved. Printed in the United States of America

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The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Ralph J. Cicerone is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. C. D. Mote, Jr., is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Harvey V. Fineberg is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy’s purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Ralph J. Cicerone and Dr. C. D. Mote, Jr., are chair and vice chair, respectively, of the National Research Council. www.national-academies.org

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COMMITTEE ON REVIEW OF TEST PROTOCOLS USED BY THE DOD TO TEST COMBAT HELMETS VIJAYAN N. NAIR, University of Michigan, Ann Arbor, Chair CHRISTINE ANDERSON-COOK, Los Alamos National Laboratory, Los Alamos, New Mexico CAMERON R. BASS, Duke University, Durham, North Carolina THOMAS F. BUDINGER (NAE/IOM), University of California, Berkeley MICHAEL J. CUSHING, U.S. Army Evaluation Center (retired), Portland, Maine ROBERT EASTERLING, Sandia National Laboratories (retired), Cedar Crest, New Mexico RONALD D. FRICKER, Naval Postgraduate School, Monterrey, California PETER N. FULLER, Cypress International, Springfield, Virginia RAUL A. RADOVITZKY, Massachusetts Institute of Technology, Cambridge ERNEST SEGLIE, Office of the Secretary of Defense (retired), Kensington, Maryland Staff BRUCE BRAUN, Director, Board on Army Science and Technology NANCY T. SCHULTE, Study Director DEANNA SPARGER, Program Administrative Coordinator NIA D. JOHNSON, Senior Research Associate v

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BOARD ON ARMY SCIENCE AND TECHNOLOGY DAVID M. MADDOX, Independent Consultant, Arlington, Virginia, Chair JEAN D. REED, Independent Consultant, Arlington, Virginia, Vice Chair DUANE ADAMS, Independent Consultant, Arlington, Virginia ILESANMI ADESIDA, University of Illinois at Urbana-Champaign STEVEN W. BOUTELLE, CISCO Consulting Services, Herndon, Virginia MARY E. BOYCE, Massachusetts Institute of Technology, Cambridge EDWARD C. BRADY, Strategic Perspectives, Inc., McLean, Virginia W. PETER CHERRY, Independent Consultant, Ann Arbor, Michigan EARL H. DOWELL, Duke University, Durham, North Carolina JULIA D. ERDLEY, Pennsylvania State University, State College LESTER A. FOSTER, Electronic Warfare Associates, Herndon, Virginia JAMES A. FREEBERSYSER, BBN Technology, St. Louis Park, Minnesota PETER N. FULLER, Cypress International, Springfield, Virginia W. HARVEY GRAY, Independent Consultant, Oak Ridge, Tennessee JOHN J. HAMMOND, Independent Consultant, Fairfax, Virginia RANDALL W. HILL, JR., University of Southern California Institute for Creative Technologies, Playa Vista JOHN W. HUTCHINSON, Harvard University, Cambridge, Massachusetts BRUCE D. JETTE, Synovision Solutions, LLC, Burke, Virginia ROBIN L. KEESEE, Independent Consultant, Fairfax, Virginia WILLIAM L. MELVIN, Georgia Tech Research Institute, Smyrna WALTER F. MORRISON, Independent Consultant, Alexandria, Virginia ROBIN MURPHY, Texas A&M University, College Station SCOTT PARAZYNSKI, University of Texas Medical Branch, Galveston RICHARD R. PAUL, Independent Consultant, Bellevue, Washington DANIEL PODOLSKY, University of Texas Southwestern Medical Center, Dallas LEON E. SALOMON, Independent Consultant, Gulfport, Florida ALBERT A. SCIARRETTA, CNS Technologies, Inc., Springfield, Virginia JONATHAN M. SMITH, University of Pennsylvania, Philadelphia DAVID A. TIRRELL, California Institute of Technology, Pasadena MICHAEL A. VANE, DynCorp International, Lorton, Virginia JOSEPH YAKOVAC, JVM LLC, Hampton, Virginia Staff BRUCE A. BRAUN, Director CHRIS JONES, Financial Manager DEANNA P. SPARGER, Program Administrative Coordinator vi

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Preface Rep. Louise Slaughter (D-NY) wrote to Secretary of The study was conducted under the auspices of the NRC Defense Leon Panetta in June 2012 to express her concerns Board on Army Science and Technology (BAST). The com- that the new protocol for testing Advanced Combat Helmets mittee appreciates the assistance of Bruce A. Braun, director (ACHs) posed “an unacceptably high risk” for such protec- of BAST, and Nancy T. Schulte, study director, for their very tive equipment. In responding to Rep. Slaughter, Dr. Michael effective support in the conduct of this study. It also offers Gilmore, Director of Operational Test and Evaluation its thanks to the BAST staff members who capably assisted (DOT&E) of the Department of Defense (DoD), indicated in information-gathering activities, meeting and trip arrange- that he had requested the National Academies’ National ments, and the production of this report; they include Nia D. Research Council (NRC) to conduct an independent review Johnson, associate research assistant, and Deanna Sparger, of DOT&E’s test protocols. The Committee on Review of senior program assistant. Test Protocols Used by the DoD to Test Combat Helmets Finally, and most importantly, I want to express my appre- was formed to conduct this review. This report is the result ciation to my fellow committee members for all of their work of that study. in developing the findings and recommendations and in pre- The committee held six meetings, including a site visit paring the report. This was an especially collegial group of to the combat helmet test range at the Aberdeen Test Center experts, and I learned a lot from interacting with them. Rob in Maryland. It received presentations from some two dozen Easterling and Ernest Seglie, two of the committee members, entities, including offices within the U.S. Army, the U.S. deserve special mention for their contributions as part of the Marine Corps, and the Special Operations Forces; the Insti- editorial team. I am also grateful to Naveen Narisetty at the tute for Defense Analysis; DOT&E; manufacturers of com- University of Michigan for his work on the numerical studies bat helmets; and the Office of the DoD Inspector General. to examine the robustness properties of test plans. The committee appreciates the assistance offered by Chris Moosmann, a staff member in the DOT&E Office of Live Vijay Nair, Chair Fire Test and Evaluation, in the course of its deliberations. Committee on Review of Test Protocols Used by the DoD to Test Combat Helmets vii

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Acknowledgments This report has been reviewed in draft form by individuals James R. Moran, The Boeing Company, chosen for their diverse perspectives and technical expertise, John E. Rolph, University of Southern California, and in accordance with procedures approved by the National Dean L. Sicking, The University of Alabama at Research Council’s (NRC’s) Report Review Committee. The Birmingham. purpose of this independent review is to provide candid and critical comments that will assist the institution in making its Although the reviewers listed above have provided many published report as sound as possible and to ensure that the constructive comments and suggestions, they were not asked report meets institutional standards for objectivity, evidence, to endorse the conclusions or recommendations nor did they and responsiveness to the study charge. The review com- see the final draft of the report before its release. The review ments and draft manuscript remain confidential to protect of this report was overseen by James O. Berger, NAS, Duke the integrity of the deliberative process. We wish to thank the University. Appointed by the National Research Council, following individuals for their review of this report: he was responsible for making certain that an independent examination of this report was carried out in accordance with Gordon R. England, NAE, E6 Partners LLC, institutional procedures and that all review comments were Karen Kafadar, Indiana University, carefully considered. Responsibility for the final content of Harvey S. Levin, Baylor College of Medicine, this report rests entirely with the authoring committee and William Q. Meeker, Jr., Iowa State University, the institution. ix

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Contents SUMMARY 1 1 INTRODUCTION 8 1.0 Information Gathering, 8 1.1 Summary of the Report, 8 1.2 References, 10 2 EVOLUTION OF COMBAT HELMETS 11 2.0 Summary, 11 2.1 Introduction, 11 2.2 New Materials and Designs, 11 2.3 Recent Developments and Directions, 13 2.4 References, 14 3 THREATS, HEAD INJURIES, AND TEST METHODOLOGIES 15 3.0 Summary, 15 3.1 Introduction, 15 3.2 Historical Patterns of Treatable Injuries, 15 3.3 Threats, 17 3.4 Advanced Combat Helmet Test Methodology and Links to Biomechanics, 20 3.5 References, 23 4 COMBAT HELMET TESTING 25 4.0 Summary, 25 4.1 Introduction, 25 4.2 Ballistic Testing Methodology, 25 4.3 Sources of Test Variation, 27 4.4 Additional Measurement and Testing Issues, 29 4.5 References, 30 5 HELMET PERFORMANCE MEASURES AND TRENDS IN TEST DATA 32 5.0 Summary, 32 5.1 Introduction, 32 5.2 Performance Measures, 32 5.3 Summary of Results from Available Test Data, 34 5.4 Implications for First Article Testing Protocols, 37 5.5 References, 38 xi

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xii CONTENTS 6 FIRST ARTICLE TESTING PROTOCOLS FOR RESISTANCE TO PENETRATION: STATISTICAL 39 CONSIDERATIONS AND EVALUATION OF DOD TEST PLANS 6.0 Summary, 39 6.1 Introduction, 39 6.2 Statistical Considerations in Designing Test Plans for Resistance to Penetration, 39 6.3 Statistical Evaluation of DoD Protocols for Resistance to Penetration, 42 6.4 Examination of Separate Test Plans by Helmet Size, 45 6.5 Post-Test Analysis, 46 6.6 Future Test Protocols: Helmet as the Unit of Test, 46 6.7 References, 47 7 TEST PROTOCOLS FOR BACKFACE DEFORMATION: STATISTICAL CONSIDERATIONS AND 48 ASSESSMENT 7.0 Summary, 48 7.1 Introduction, 48 7.2 Backface Deformation First Article Acceptance Testing Protocols and Their Properties, 48 7.3 Discussion, 51 8 LOT ACCEPTANCE TESTING 54 8.0 Summary, 54 8.1 Introduction, 54 8.2 Lot Acceptance Testing Protocols, 54 8.3 Evaluating Performance: Comparison of Operating Characteristic Curves, 56 8.4 ANSI Standard and the Acceptance Quality Limit, 58 8.5 Using the Helmet as the Unit of Testing, 60 8.6 References, 63 9 CHARACTERIZATION TESTS FOR THE ADVANCED COMBAT HELMET AND FUTURE HELMETS 64 9.0 Summary, 64 9.1 Introduction, 64 9.2 Characterization of the Advanced Combat Helmet Using Existing Test Data, 65 9.3 Expanded Characterization Requiring Additional Data, 65 9.4 V50 Testing, 67 9.5 Comparison with Industrial Practices, 68 9.6 Concluding Remarks, 69 9.7 References, 69 10 LINKING HELMET PROTECTION TO BRAIN INJURY 70 10.0 Summary, 70 10.1 Introduction, 70 10.2 Brain Injuries, 70 10.3 Head and Brain Injury Tolerances, 73 10.4 Brain Tissue Injury: Experimental Results, 74 10.5 Computational Modeling and Simulation, 81 10.6 Mechanical and Constitutive Properties of Tissues, 84 10.7 Conclusion, 85 10.8 References, 86 APPENDIXES A Study Origination Documents 91 B Protocols for First Article and Lot Acceptance Testing 97 C Committee Meetings and Data-Gathering Activities 122 D Test Range Description and the Ballistic Testing Process 124 E Synopsis of Brain Injury Detection Methods 132 F Biographical Sketches of Committee Members 138

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Tables, Figures, and Box TABLES 3-1 Broad Categories of Threats, 16 3-2 Relative Body Surface Area and Distribution of Wounds by Body Region, 17 3-3 Distribution of Wounds by Body Region in Operation Enduring Force (Afghanistan) and Operation Iraqi Freedom (Iraq), 17 3-4 Percentage of Injuries from Gunshot Wounds and Explosions from Previous U.S. Wars, 17 3-5 Distributions of Injury Causes by Body Region, 17 3-6 Representative Standard-Issue Infantry Rifles and Ammunition for Selected Potential Adversaries, 18 3-7 Representative Battlefield Threats/Impact Velocities, 19 4-1 DOT&E First Article Testing Helmet Test Matrix for the Advanced Combat Helmet, 26 5-1 Summary of Resistance to Penetration Test Data, 34 8-1 Sample Sizes for the Army’s Historical Lot Acceptance Testing Protocol for a 9-mm RTP Shell, 54 8-2 Helmet Lot Acceptance Testing Matrix, 55 8-3 Helmet Shot Order Test Matrix for Aramid 9-mm, 55 8-4 Subtest Acceptance Quality Limits (Approximate), 59 8-5 Sample Sizes per ANSI Standard ASQ Z1.4-2008 to Achieve an AQL of 0.4 Percent, 60 8-6 Lot Acceptance Testing Helmet Sampling Rate as Specified in the Lightweight Advanced Combat Helmet Purchase Description, 61 8-7 Switching Rules for Lot Sizes of 1,200 to 3,200 with Acceptance Quality Limit of 0.4, 62 10-1 Categories of Brain Injuries, 71 10-2 Brain Injury Criteria and Median Values for Concussion for Low-Rate Blunt Impact, 76 FIGURES S-1 Operating characteristic curves for the Army’s and the Director of Operational Test and Evaluation’s first article test- ing protocols for penetration, 2 S-2 Further comparisons of the operating characteristic curves for the Army’s and the Director of Operational Test and Evaluation’s first article testing protocols for penetration, 3 2-1 Evolution of helmets from World War I to present, 12 2-2 Helmet multi-pad and four-point retention systems, 13 xiii

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xiv TABLES, FIGURES, AND BOX 3-1 Typical timeline of blast, ballistic, blunt injuries compared to ergonomics-related injuries, 16 3-2 (a) Traumatic brain injury (TBI) hospitalizations by source for battle injuries categorized by regions in Operation Enduring Force/Operation Iraqi Freedom. (b) TBI hospitalizations by combat/noncombat source, 18 3-3 Sagittal headform specified in National Institute of Justice Penetration Standard, 20 3-4 Long linear and depressed skull fractures from nonpenetrating helmet BFD in a human cadaveric model, 21 3-5 Typical potential neck injury locations in adults from impact loading, 22 3-6 Typical blunt brain trauma diagram, 22 3-7 Energy limits for blunt impact injury assessment in AGARD AR-330, 22 4-1 Clay time and temperature effects in the column drop test, 29 4-2 Aberdeen Test Center headform, 30 4-3 New Army “sized” headforms, 31 4-4 Peepsite headforms: five headforms, one for each shot direction, 31 5-1 Illustrative backface deformation (BFD) laser scan, 33 5-2 Backface deformation (BFD) measurements by location for Data Set 1, 35 5-3 Average backface deformation (BFD) as a function of stand-off for Data Set 1, 35 5-4 Backface deformation (BFD) measurements by location for Data Set 2, 36 5-5 Backface deformation (BFD) measurements by location for Data Set 3, 36 5-6 Backface deformation (BFD) measurements by location and helmet size for Data Set 3, 36 5-7 Backface deformation (BFD) measurements by location for Data Set 4, 37 6-1 Operating characteristic (OC) curve for (c = 1, n = 40) test plan, 40 6-2 Operating characteristic curves comparing 1-out-of-40 test plan with 0-out-of-40 and 1-out-of-70 test plans, 40 6-3 Operating characteristic curves of (c = 1, n = 77) plan with the desired risks, 41 6-4 Operating characteristic curve for the legacy (0, 20) test plan, 42 6-5 Comparison of the operating characteristic curves for (0, 20) and (17, 240) plans, 43 6-6 Comparison of the operating characteristic curves for (0, 20) and (5, 96) plans, 44 6-7 Operating characteristic curves for the hybrid plan and comparison to others, 45 6-8 Operating characteristic curves for three plans with n = 60, 45 6-9 Comparison of helmet-level and shot-level test protocols, 46 7-1 Operating characteristic curves for Director, Operational Test and Evaluation, backface deformation (BFD) protocol for the two groups of shot locations, 50 7-2 The two operating characteristic (OC) curves in Figure 7-1 overlaid with the overall OC curve of the backface deformation (BFD) protocol, 50 7-3 Comparison of the three operating characteristic curves in Figure 7-2 with that of the legacy (0, 20) plan, 50 7-4 Operating characteristic curves for the two location groups for the Enhanced Combat Helmet, 51 7-5 Operating characteristic curves for a single 48-shot plan and for five 48-shot plans, 51 8-1 Operating characteristic curves for resistance to penetration for the three Director, Operational Test and Evaluation, protocols by lot sizes, 56 8-2 Comparison of operating characteristic curves for the three Director, Operational Test and Evaluation (DOT&E) lot acceptance testing protocols (black, red, and green) with the Army’s Legacy first article testing (FAT) protocol (blue) and DOT&E’s FAT protocol (orange), 57 8-3 Comparison of operating characteristic curves for the three DOT&E lot acceptance testing protocols (black, red, and green) with an illustrative (1, 60) first article testing protocol (red), 57 8-4 Backface deformation (BFD) operating characteristic curves for the Director, Operational Test and Evaluation (DOT&E) first article testing (FAT) protocol in blue, the original Army FAT protocol in black, and the DOT&E lot acceptance testing (LAT) protocols in red, 58 8-5 Operating characteristic (OC) curves for the illustrative helmet-based lot acceptance testing (LAT) protocol in red compared to the OC curve for the combined resistance to penetration and backface deformation for the Director, Operational Test and Evaluation (DOT&E) LAT protocol in blue, 61 8-6 Switching rules from ANSI/ASQ Z1.4-2008, 62

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TABLES, FIGURES, AND BOX xv 10-1 Linkages between the force of the impact, how the helmet attenuates it, and resulting brain injuries, 71 10-2 Incidence of traumatic brain injury classified by severity for warfighters, 73 10-3 (a) The University of Virginia’s Hybrid III head model used for laboratory simulations and measurements. (b) Biokinetics headform variant of the Hybrid III headform for ballistic impact, 75 10-4 Instrumented cadaver head, 77 10-5 Thresholds for diffuse axonal injury based on nonhuman primate rotational acceleration experiments and scaling through computational modeling to human brain masses of 500 g (thick solid curve), 1,067 g (solid curve), and 1,400 g (dotted curve). Regions to the upper and right of each curve are regions of diffuse axonal injury, 78 10-6 Left: The base of the human skull supports the bottom of the brain and the brain stem that descends through the large orifice in the center known as the foramen magnum. Right: Positron tomography of the uptake of ammonia-13N in the normal pituitary, 80 10-7 Principal strains in simulated brain material from projectile-induced kinetic energy striking a helmet at two angles. Blue is 0 percent, green is 2 percent, and red is >4 percent, 82 10-8 Computational simulations of the protective effect of the Advanced Combat Helmet (center column) and face shield (right column) show a significant attenuation of the transmitted pressure field when compared to the unprotected head (left column), 83 10-9 Experimental determination of brain shear modulus (magnitude of the complex shear modulus) showing wide variance of experimental results from different researchers, 84 10-10 Dependence of shear strain on stress rate shows the importance of correct simulation of the shear stress rate in simulations, 84 D-1a The helmet test range at the U.S. Army Aberdeen Test Center, 124 D-1b Typical test range at set-up for helmet V0 testing, 125 D-2 U.S. Army Aberdeen Test Center headform, 125 D-3 Packing the headform with clay and shaping the clay, 125 D-4 U.S. Army Aberdeen Test Center headform with clay, 126 D-5 Test impact locations, 126 D-6 Pad Configuration for V0 resistance to penetration testing for full cut style helmet (top) or the tactical cut style helmet (bottom), 126 D-7 Helmet mounted on a headform, 127 D-8 Test frame and fixture, 127 D-9 Example of headform showing a penetration as evidenced by the presence of projectile fragments in the clay, 128 D-10 Witness plate headforms for hardware testing, 128 D-11 V50 helmet test mount (left) and associated witness plate (right), 129 D-12 Headform showing indent in the clay as a result of helmet backface deformation, 129 D-13 Faro® scanning laser instrument laser scan arm, 129 D-14 Headform clay conditioning by analogy, 130 D-15 Clay calibration test rig, 130 D-16 Examples of helmet conditioning, 131 E-1 Brain alterations shown on functional imaging without behavioral changes, 134 E-2 Positron tomography image showing sites of inflammation using the tracer 11C-PK11195 with superposition of the positron emission tomography emission on a magnetic resonance imaging anatomical image, 135 BOX 10-1 Glossary, 72

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Acronyms and Abbreviations ACH advanced combat helmet (Army) M medium ANSI American National Standards Institute M&S modeling and simulation AQL acceptance quality limit MICH Modular Integrated Communications ATC U.S. Army Aberdeen Test Center Helmet ATD anthropometric test device MIL-STD military standard MRI magnetic resonance imaging BFD backface deformation mTBI mild traumatic brain injury BTD ballistic transient deformation NHTSA National Highway Traffic Safety DAI diffuse axonal injury Administration DCMA Defense Contract Management Agency NIJ National Institute of Justice DoD Department of Defense NIST National Institute of Standards and DOT Department of Transportation Technology DOT&E Director, Operational Test and Evaluation NRC National Research Council DTI diffusion tensor imaging OC operating characteristic (curve) ECH enhanced combat helmet OEF Operation Enduring Freedom (Afghanistan) FAST Future Assault Shell Technology OIF Operation Iraqi Freedom FAT first article testing FMJ full metal jacket P probability FSP fragment simulating projectile PASGT Personnel Armor System for Ground Troops GSW gunshot wounds PEO-S U.S. Army Program Executive Office Soldier HEaDS-UP Helmet Electronics and Display System – PET positron emission tomography Upgradeable Protection P(nP) probability of no penetration HIC head injury criteria Pr(pen) probability of penetration ICP intracranial pressure R&R repeatability and reproducibility IED improvised explosive device RCC right circular cylinder IG Inspector General RTP resistance to penetration ISO International Standards Organization S small L large SIMon simulated injury monitor LAT lot acceptance testing LWH lightweight helmet (Marine Corps) TBI traumatic brain injury xvii

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xviii ACRONYMS AND ABBREVIATIONS UCB upper confidence bound UVA University of Virginia UHMWPE ultra-high molecular weight polyethylene USSOCOM United States Special Operations WWII World War II Command UTL upper tolerance limit XL extra large