Chapter 2: Evolution of Combat Helmets

Chapter 2 describes the changes in design and materials, from those used in World War I to today’s ACH. One of the key advances was the development of aramid fibers in the 1960s, which led to today’s Kevlar-based helmets. The DoD is continuing to invest in research to improve helmet performance, through better design and materials as well as better manufacturing processes.

Chapter 3: Threats, Head Injuries, and Test Methodologies

A variety of threats lead to head injuries in the battlefield. Since World War II, the predominant threats have been from the following: fragmentation and ballistic threats from explosions, artillery, and small arms fire; blunt trauma caused by translation from blast, falls, vehicle crashes, and impact with vehicle interiors and from parachute drops; and exposure to primary blasts. Key findings in this chapter indicate the following:

•   Wounding from an explosive source (e.g., fragmentation from bombs, mines, and artillery) dominates all wounding, including bullets.

•   Nonbattle causes, including blunt traumatic injuries, produced nearly 50 percent of the hospitalizations for traumatic brain injury in Iraq/Afghanistan.

•   There is no biomechanical link in the current test methodology between the backface deformation (BFD) assessment and head injuries from behind-helmet deformation.

There is a need to revise test methodologies to focus on the dominant threats. The current protocol addresses primarily rounds from 9-mm pistol fire, which is a relatively small contributor to soldier injuries. It is also important to develop better understanding of the scientific connection between head injuries and the performance metrics used in current test methodology.

Chapter 4: Combat Helmet Testing

Chapter 4 describes how combat helmets are tested. It includes a brief summary of the testing process, a description of the test threats, and a discussion of the various sources of variation in the testing process.

Chapter 5: Helmet Performance Measures and Trends in Test Data

A helmet’s protective capabilities are evaluated on the basis of two primary test measures: resistance to penetration (RTP) and BFD. These are formally defined, and their limitations are discussed in this chapter. RTP data available to the committee indicate that the probability of penetration of a helmet shell by a 9-mm bullet, fired under specified conditions, is on the order of 0.005 or less. Available BFD data show that the probability of exceeding the BFD thresholds is also around 0.005 or less. The distributions of the BFD data also demonstrate significant differences among helmet sizes and shot locations. Some of the performance differences among helmet sizes may be attributed to the test process, such as headforms and stand-offs. Many others are likely to be due to the differences in the geometry of helmet shells, molds, manufacturing processes, and other factors. In fact, helmets of different sizes are intrinsically different products. Based on this, Recommendation 5-5 proposes changes to DoD’s test protocols. This is one of the major recommendations in the report.

Chapter 6: FAT Protocols for Resistance to Penetration: Statistical Considerations and Evaluation of DOD Test Plans

The test protocols for Army helmets were originally based on a requirement of zero penetrations in 20 shots (5 shots on 4 helmets). The DOT&E protocol replaced this legacy plan with a requirement of 17 or fewer penetrations in 240 shots (5 shots on each of 48 helmets). The helmets spanned four sizes and were tested in four different environments. The 0-out-of-20 (0, 20) plan and DOT&E’s 17-out-of-240 (17, 240) plan have comparable performance if the probability of penetrating a helmet shell on a single shot is around 0.10. As noted in the Chapter 5, available data indicate that these penetration probabilities are around 0.005 or less. Near this value of penetration probability, both plans have a 90 percent or higher chance of passing the test, so the manufacturer’s risk 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 to 0.05, DOT&E’s (17, 240) plan still has a 95 percent chance of acceptance. This provides little incentive for the manufacturer to sustain current penetration levels. The (0, 20) plan, on the other hand, has only a 38 percent chance of acceptance. Thus, the (17, 240) plan may have the unintended effect of leading to a reduction in helmet penetration resistance. In the absence of a link between penetration probability and human injury, there is no scientific basis for setting a limit on the penetration probability. In such a circumstance, the committee’s view is that the objective of a new test plan should be to provide assurance that newly submitted helmets are at least as penetration resistant as current helmets. Chapter 6 also proposes appropriate criteria for selecting test protocols and illustrates their use through several plans.

Chapter 7: Test Protocols for Backface Deformation: Statistical Considerations and Assessment

The original Army protocols for BFD were based on binary (0-1) data. The BFD measurement at each location was compared against its specified threshold, and the outcome was scored as a “1” (failure) if it exceeded its threshold. This

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