This chapter provides the study context and statement of task. It also describes the scope of the study and includes summaries of the various chapters in the report.
In June 2012, Rep. Louise Slaughter (D-NY) sent a letter (Slaughter, 2012)1 to Secretary of Defense Leon Panetta expressing concerns that a recent modification to the standard for ballistic testing for the Advanced Combat Helmet (ACH) posed “an unacceptably high risk” for such protective equipment. She urged that ballistics testing procedures be modified.
The July 13, 2012, response to Rep. Slaughter (Gilmore, 2012)2 was made by J. Michael Gilmore, Director of Operational Test and Evaluation (DOT&E), the principal staff assistant and advisor to the Secretary and Deputy Secretary of Defense for operational test and evaluation and live-fire test and evaluation matters. He expressed the view that the revised test protocol for the ACH is “better in several ways that the previously used protocol while being designed to demonstrate the same level of protection (probability of perforation) and also the same level of certainty of our knowledge of the level of protection.” However, he also noted that DOT&E was requesting that the National Research Council conduct a study to review the revised protocol for testing military combat helmets. This report is the result of that request. Following is the statement of task.
The National Research Council will establish an ad hoc committee to consider the technical issues relating to test protocols for military combat helmets and prepare a report. The committee will examine the testing protocols along the following lines:
• Evaluate the adequacy of the Advanced Combat Helmet test protocol for both first article testing and lot acceptance testing, including its use of the metrics of probability of no penetration and the upper tolerance limit (used to evaluate backface deformation).
• Evaluate the appropriate use of statistical techniques (e.g., rounding numbers, choosing sample sizes, or test designs) in gathering the data.
• Evaluate the adequacy of the current helmet testing procedure to determine the level of protection provided by current helmet performance specifications.
• Evaluate procedures for the conduct of additional analysis of penetration and backface deformation data to determine whether differences in performance exist.
• Evaluate the scope of characterization testing relative to the benefit of the information obtained.
The committee held six meetings. The first was held in Aberdeen, Maryland, and included a site visit to the combat helmet test range at the Aberdeen Test Center. The second through sixth meetings were held at the Academies’ facilities in Washington, D.C., and Woods Hole, Massachusetts. A total of 18 presentations were received from the following entities:
• Offices within the United States Army, the Marine Corps, and the Special Operations Forces
• Manufacturers of combat helmets
• Office of the Department of Defense (DoD) Inspector General
The titles of the presentations are listed in Appendix C.
The report contains 10 chapters and several appendices. This is an introductory chapter. Summaries of the remaining chapters are given below.
1The text of Rep. Slaughter’s letter to Secretary Panetta is found in Appendix A.
2The text of Director Gilmore’s letter to Rep. Slaughter is found in Appendix A.
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1 Introduction This chapter provides the study context and statement of tance testing, including its use of the metrics of probabil- task. It also describes the scope of the study and includes ity of no penetration and the upper tolerance limit (used summaries of the various chapters in the report. to evaluate backface deformation). In June 2012, Rep. Louise Slaughter (D-NY) sent a let- • Evaluate the appropriate use of statistical techniques (e.g., rounding numbers, choosing sample sizes, or test ter (Slaughter, 2012)1 to Secretary of Defense Leon Panetta designs) in gathering the data. expressing concerns that a recent modification to the stan- • Evaluate the adequacy of the current helmet testing pro- dard for ballistic testing for the Advanced Combat Helmet cedure to determine the level of protection provided by (ACH) posed “an unacceptably high risk” for such protective current helmet performance specifications. equipment. She urged that ballistics testing procedures be • Evaluate procedures for the conduct of additional analysis modified. of penetration and backface deformation data to deter- The July 13, 2012, response to Rep. Slaughter (Gilmore, mine whether differences in performance exist. 2012)2 was made by J. Michael Gilmore, Director of Opera- • Evaluate the scope of characterization testing relative to tional Test and Evaluation (DOT&E), the principal staff the benefit of the information obtained. assistant and advisor to the Secretary and Deputy Secretary of Defense for operational test and evaluation and live-fire 1.0 INFORMATION GATHERING test and evaluation matters. He expressed the view that the revised test protocol for the ACH is “better in several ways The committee held six meetings. The first was held in that the previously used protocol while being designed to Aberdeen, Maryland, and included a site visit to the combat demonstrate the same level of protection (probability of per- helmet test range at the Aberdeen Test Center. The second foration) and also the same level of certainty of our knowl- through sixth meetings were held at the Academies’ facili- edge of the level of protection.” However, he also noted that ties in Washington, D.C., and Woods Hole, Massachusetts. DOT&E was requesting that the National Research Council A total of 18 presentations were received from the following conduct a study to review the revised protocol for testing entities: military combat helmets. This report is the result of that request. Following is the statement of task. • Offices within the United States Army, the Marine Corps, and the Special Operations Forces The National Research Council will establish an ad hoc • Manufacturers of combat helmets committee to consider the technical issues relating to test • Office of the Department of Defense (DoD) Inspector protocols for military combat helmets and prepare a re- General port. The committee will examine the testing protocols along the following lines: The titles of the presentations are listed in Appendix C. • Evaluate the adequacy of the Advanced Combat Helmet test protocol for both first article testing and lot accep- 1.1 SUMMARY OF THE REPORT The report contains 10 chapters and several appendices. 1The text of Rep. Slaughter’s letter to Secretary Panetta is found in Ap- This is an introductory chapter. Summaries of the remaining pendix A. chapters are given below. 2The text of Director Gilmore’s letter to Rep. Slaughter is found in Ap- pendix A. 8
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INTRODUCTION 9 Chapter 2: Evolution of Combat Helmets by a 9-mm bullet, fired under specified conditions, is on the order of 0.005 or less. Available BFD data show that the prob- Chapter 2 describes the changes in design and materials, ability of exceeding the BFD thresholds is also around 0.005 from those used in World War I to today’s ACH. One of or less. The distributions of the BFD data also demonstrate the key advances was the development of aramid fibers in significant differences among helmet sizes and shot locations. the 1960s, which led to today’s Kevlar-based helmets. The Some of the performance differences among helmet sizes may DoD is continuing to invest in research to improve helmet be attributed to the test process, such as headforms and stand- performance, through better design and materials as well as offs. Many others are likely to be due to the differences in the better manufacturing processes. geometry of helmet shells, molds, manufacturing processes, and other factors. In fact, helmets of different sizes are intrinsi- Chapter 3: Threats, Head Injuries, and Test cally different products. Based on this, Recommendation 5-5 Methodologies proposes changes to DoD’s test protocols. This is one of the major recommendations in the report. A variety of threats lead to head injuries in the battle- field. Since World War II, the predominant threats have been from the following: fragmentation and ballistic threats Chapter 6: FAT Protocols for Resistance to Penetration: from explosions, artillery, and small arms fire; blunt trauma Statistical Considerations and Evaluation of DOD Test caused by translation from blast, falls, vehicle crashes, and Plans impact with vehicle interiors and from parachute drops; and The test protocols for Army helmets were originally based exposure to primary blasts. Key findings in this chapter on a requirement of zero penetrations in 20 shots (5 shots indicate the following: on 4 helmets). The DOT&E protocol replaced this legacy plan with a requirement of 17 or fewer penetrations in 240 • Wounding from an explosive source (e.g., fragmenta- shots (5 shots on each of 48 helmets). The helmets spanned tion from bombs, mines, and artillery) dominates all four sizes and were tested in four different environments. wounding, including bullets. The 0-out-of-20 (0, 20) plan and DOT&E’s 17-out-of-240 • Nonbattle causes, including blunt traumatic injuries, (17, 240) plan have comparable performance if the probability produced nearly 50 percent of the hospitalizations for of penetrating a helmet shell on a single shot is around 0.10. traumatic brain injury in Iraq/Afghanistan. As noted in the Chapter 5, available data indicate that these • There is no biomechanical link in the current test penetration probabilities are around 0.005 or less. Near this methodology between the backface deformation value of penetration probability, both plans have a 90 percent (BFD) assessment and head injuries from behind- or higher chance of passing the test, so the manufacturer’s risk helmet deformation. is small, as it should be. However, if there is a 10-fold increase in the penetration probability from the current level of 0.005 There is a need to revise test methodologies to focus on to 0.05, DOT&E’s (17, 240) plan still has a 95 percent chance the dominant threats. The current protocol addresses primar- of acceptance. This provides little incentive for the manufac- ily rounds from 9-mm pistol fire, which is a relatively small turer to sustain current penetration levels. The (0, 20) plan, on contributor to soldier injuries. It is also important to develop the other hand, has only a 38 percent chance of acceptance. better understanding of the scientific connection between Thus, the (17, 240) plan may have the unintended effect of head injuries and the performance metrics used in current leading to a reduction in helmet penetration resistance. In the test methodology. absence of a link between penetration probability and human injury, there is no scientific basis for setting a limit on the Chapter 4: Combat Helmet Testing penetration probability. In such a circumstance, the commit- tee’s view is that the objective of a new test plan should be to Chapter 4 describes how combat helmets are tested. It provide assurance that newly submitted helmets are at least includes a brief summary of the testing process, a description as penetration resistant as current helmets. Chapter 6 also of the test threats, and a discussion of the various sources of proposes appropriate criteria for selecting test protocols and variation in the testing process. illustrates their use through several plans. Chapter 5: Helmet Performance Measures and Trends in Chapter 7: Test Protocols for Backface Deformation: Test Data Statistical Considerations and Assessment A helmet’s protective capabilities are evaluated on the basis The original Army protocols for BFD were based on of two primary test measures: resistance to penetration (RTP) binary (0-1) data. The BFD measurement at each location and BFD. These are formally defined, and their limitations are was compared against its specified threshold, and the outcome discussed in this chapter. RTP data available to the committee was scored as a “1” (failure) if it exceeded its threshold. This indicate that the probability of penetration of a helmet shell
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10 REVIEW OF DEPARTMENT OF DEFENSE TEST PROTOCOLS FOR COMBAT HELMETS original plan was based on 20 shots; if no BFD measurements Chapter 9: Characterization Tests for ACH and Future exceeded their limit, the demonstration was successful. In this Helmets sense, it was similar to Army’s legacy protocol for RTP. The The committee was tasked to “evaluate the scope of char- DOT&E protocol expanded the number of shots to 240 and acterization testing relative to the benefit of the information used the continuous measurements together with an assump- obtained.” The term “characterization” is broad and is used tion that the data are normally distributed. Specifically, the in different ways in different contexts. However, DOT&E plan compared the 90 percent “upper-tolerance limits” com- provided additional information to elaborate on this task. puted at 90 percent confidence level (90/90 rule) with their Most of the issues raised by DOT&E that relate to this task thresholds for the corresponding location on the helmet. As are addressed in this chapter. Chapter 9 also describes addi- noted in Chapter 5, available BFD test data show that the tional characterization tests that are needed. Some of these probability of BFD exceeding its limits is quite small—on are intended for future helmet designs. A number of these the order of 0.005. As this chapter observes, DOT&E’s BFD additional tests have been discussed in earlier chapters and protocol has about a 90 percent chance of accepting the hel- are repeated here because they can be viewed as being related met design, even if there is an order of magnitude increase to characterization studies. These include the following: eval- in the exceedance probability (from 0.005 to 0.05). This uating helmet performance across a more realistic, broader weakens the incentive for manufacturers to produce helmets range of threats; assessing the effect of aging; understanding that are at least as good as current helmets with respect to the relationship between helmet offsets and helmet protec- BFD. In addition, the DOT&E protocols are based on an (a tion; and conducting gauge repeatability and reproducibility priori untestable) assumption of normality and the complex studies to understand the different sources of variation in the notion of an upper tolerance limit. Recommendation 7-1 test process and possibly providing opportunities to reduce proposes that DOT&E’s protocol for BDF data be changed. some of the variation. Chapter 9 also includes a discussion of This change has the added advantage that the BFD protocol current V50—the velocity at which complete penetration and would exactly parallel the RTP protocol and would be easy partial penetration are equally likely to occur—testing and an for designers and manufacturers to understand and interpret. alternative methodology as well as a discussion of industrial However, it is important that, after testing, the continuous practices in characterizing process capability. BFD measurements be analyzed to assess the actual BFD levels and monitor them for changes over time. Chapter 10: Linking Helmet Protection to Brain Injury Chapter 8: Lot Acceptance Testing The relationships between helmet deformation and brain injury are not well known. Most of the studies in biomechani- Lot acceptance testing (LAT) is used to ensure that manu- cal engineering and medicine are related to sports and vehicle facturers continue to produce helmets that conform to contract collisions, and these investigations are based on a different specifications. A random sample of helmets is selected from range of stresses and stress rates from those encountered in the production lot, and the helmet shells, as well as hard- the battlefield. The aim of Chapter 10 is to present informa- ware, are tested according to the LAT protocol. The number tion on what is known, and the gaps in knowledge, about the of helmets in the protocols is determined from an American linkage between brain injury and current battlefield threats. National Standards Institute (ANSI) standard, and they vary The major finding is that helmet protection from penetration by lot size. Chapter 8 examines the operating characteristic and BFD greater than a particular value does not protect the (OC) curves for DOT&E’s LAT plans and compares them brain from occurrence of many categories of tissue injury. with FAT protocols in the Army’s legacy plans and DOT&E’s This chapter discusses recommendations that can help focus plans. The OC curves for the LAT plans for the different lot research, including determination of the prevalence of revers- sizes can vary a lot, indicating that the manufacturer’s and ible declines in hormonal function years after brain trauma government’s risks can be quite different across lot sizes. and acceleration of research in computational modeling and This is primarily due to the different sample sizes (number simulation that can show shear stress fields associated with of helmets and number of shots) as determined from an ANSI the known spectrum of threats and the protective capabilities standard. Further, DOT&E’s first article testing (FAT) proto- of helmets. cols are considerably less stringent (higher probabilities of acceptance for the OC curves) than their corresponding LAT protocols. This is counter to the philosophy that it should be 1.2 REFERENCES more difficult for manufacturers to pass FAT than LAT. This Gilmore, J.M. 2012. Letter from J. Michael Gilmore, Director of Operation- issue can be addressed if DOT&E makes changes to the (17, al Test and Evaluation, to Representative Louise M. Slaughter, July 13. 240) FAT protocol as discussed in Chapters 6 and 7. Chapter Slaughter, L.M. 2012. Letter from Representative Louise M. Slaughter to 8 also proposes using binary data for BFD LAT protocols, Secretary of Defense Leon Panetta, June 26. to make them consistent with the recommendations for FAT. Finally, the committee examines the properties of LAT pro- tocols based on helmets as the unit of testing.