This chapter discusses helmet ballistic testing methodologies. It describes helmet design and suspension systems as well as current and proposed clay head forms used in the ballistic testing of helmets.
BALLISTIC HELMET TEST METHODOLGIES
The development of modern military helmets based on aramid fiber composites has been an outstanding success (e.g., Carey et al., 1998). Numerous soldiers and civilian police have been saved from threats that would have defeated earlier metallic helmets (Carey et al., 2000). However, though current protective levels have proved to be well matched to the threats they are designed to protect against, increasing threats on the battlefield, especially from high velocity rifle rounds, will likely require new or modified helmet test methodologies assess the risk of injuries while using improved ballistic protective helmets.
On the battlefield, mobility is often the key to survival. The development of robust test methodologies is crucial to comparing the effect of potential trauma to ergonomic and other trade-offs required for personal protection. The mass of the protection is particularly important, as it may impede mobility.
The standoff between the head and the backface of the helmet in the current helmet systems was designed to be 1.3 cm (0.5 in.) or greater (McManus, 1976). Substantial research has been performed on traumatic brain injury (TBI), but much of the work is not applicable to military threats. For example, TBI may occur from blunt impact during vehicle crashes, falls, and sports impacts. However, there are important physical differences between these lower velocity events and impacts from the backface of military helmets.
The difference in incoming momentum between several representative rounds ranging from 9 mm to 0.50 cal and a typical American football head contact is shown in Figure 7-1. The football impact typically has a much larger transfer of momentum, implying much greater overall head motion and more global internal brain deformation. This difference and the much more localized contact from a helmet backface deformation (BFD) impact raises questions about the applicability of existing head injury criteria.
FIGURE 7-1 Effective momentum of high-rate ballistic impacts at muzzle velocity and low-rate football impact. SOURCE: Cameron Bass, Duke University.
This difference between conventional blunt trauma and ballistic blunt trauma is emphasized by considering typical timelines for ballistic impact. The need to decelerate an incoming round from hundreds of meters per second to zero over a span of centimeters implies a relatively rapid interaction between the head and the deforming helmet. A typical interaction timeline is shown in Figure 7-2.
FIGURE 7-2 Ballistic impact injury timescales. SOURCE: Bass et al., 2003.
The peak impact force occurs approximately 100 µsec after helmet/head interaction. In contrast, automobile impacts and falls typically occur at time scales of 1 msec or greater, often 5-15 msec, which longer by a factor of 10-150. These momentum time scale and rate effects may play a large role in the causation of head trauma.
Finding: The existing helmet test methodologies, including the current Army test methodology, do not relate directly enough to human injury to confidently assess injury risk from back-face trauma to the head. Improving the link between test methodology and human injury is an urgent matter in light of the newer helmet systems with lower areal densities and increased threat velocities.
There are two important injury modes with ballistic protective helmets and/or facial ballistic protection (Figure 7-3). First, penetrating injuries may be incurred because the helmet is defeated by the projectile. Second, impact loading injuries—also known as behind-armor blunt trauma (BABT)—may be caused by translational and/or rotational acceleration of the head. These may occur either from local contact of the deforming undefeated helmet onto the head/underlying skull or from more regional helmet/head contact, with acceleration loads transmitted through the helmet webbing or padding to the skull (Bass et al., 2003).
FIGURE 7-3 Likeness of a deformed personnel armor system for ground troops helmet. SOURCE: Cameron Bass, Duke University.
Generally, the function of the helmet is to prevent penetration and minimize the injurious effects of BABT. Owing to different techniques for assessing penetration and BABT, some methodologies may assess the penetration separately from the BABT.
Existing Human Injury Criteria
Substantial work has been done by the automobile, sports, and occupational biomechanics communities on head and neck injury (Pilkey et al., 2009; Mueller et al., 2008; Fuller et al., 2005). It is clear, however, that much of the work on existing injury tolerance values does not translate well to high-rate, low-momentum transfer events typical of ballistic impact events (e.g., Bass et al., 2003). Therefore, to evaluate the performance of ballistic helmets in protecting the wearer from injury, a test methodology should assess the mechanical performance of the helmet while addressing the associated risks of injury to the head/brain and neck.
The mechanisms that have been proposed for mechanical injury to the brain fall into six general categories: (1) direct contusion of the brain from skull deformation or fracture; (2) brain contusion from movement against rough interior surfaces of the skull; (3) reduced blood flow due to infarction or pressure; (4) indirect (countercoup) contusion of the brain opposite the side of the impact; (5) tissue stresses and strains produced by motion of the brain hemispheres relative to the skull and each other; and (6) subdural hematoma produced by rupture of bridging vessels between the brain and the dura matter (Melvin, 1993). It is hypothesized that the latter three mechanisms are involved in both impact and non-impact head injury (Ommaya, 1985). Using a variety of experimental models, researchers have been able to reproduce some of the above injury mechanisms. However, no satisfactory experimental model succeeds in producing the complete spectrum of brain injury seen clinically and yet is sufficiently well controlled and quantifiable to be a useful model for experimental studies. To improve the experimental models, it is necessary to improve our understanding of the mechanical and directional properties of brain tissue as well as intracranial deformations, relative motions, and interfaces, especially at ballistic impact rates.
Using head-impact models of primates, Gurdjian, Lissner, and associates attributed intracranial damage to skull deformation and change in intracranial pressure (Gurdjian, 1944). Holbourn, on the other hand, using photoelastic models of the head, proposed that head rotational acceleration and the induced shear strains in brain tissue are the causes of diffuse TBI (Holbourn, 1943). Gurdjian et al. later showed that injuries resulting from relative motion between the skull and the brain can be caused by head rotation (Gurdjian, 1954). Ommaya and coworkers, using acceleration models of primates, revised Holbourn’s rotational theory by proposing that there is similar brain injury potential from rapid head rotation and from skull distortion owing to direct impact (Ommaya et al., 1966, Ommaya et al., 1971, Ommaya et al., 1973). Ommaya also proposed the centripetal theory, which states that the distribution of damaging diffuse strains induced by inertial loading decreases in magnitude from the surface to the center of the approximately spheroidal brain mass. Genarrelli and Thibault continued the
investigation into the relative roles of translational and rotational accelerations by using more elaborate experimental models of subhuman primates (Gennarelli et al., 1982; Ommaya and Gennarelli, 1974). They concluded that diffuse injures to the brain occurred only in the presence of head rotational motion. They also investigated the effect of pulse duration on diffuse brain injury and concluded that diffuse brain injuries occurred at lower angular deceleration levels as pulse duration increased (Gennarelli and Thibault, 1982). It was shown that the incidence of unconsciousness with prolonged coma was greatest in purely lateral (i.e., coronal) plane acceleration (Gennarelli et al., 1982). Further analysis of injuries produced in the primate and human head models led Margulies et al. to conclude that in coronal plane rotational acceleration, the critical shear strain associated with the onset of diffuse axonal injury (DAI) was about 10 percent and the rotational acceleration threshold for severe DAI was about 16,000 rad/sec2 (Margulies et al., 1990). It should be noted that in brain tissue, the threshold of functional failure is far lower than the threshold of mechanical failure (Varney and Varney, 1995). In some instances, no penetration is required to produce injury. For example, it has been shown that inertial loading alone to the head can cause DAI, an important cause of fatality due to head injury (Gennarelli and Thibault, 1989).
The early works of Lissner et al. and Gurdjian et al. emphasized skull deformations and intracranial pressure gradients as sources of brain injury. This work resulted in the Wayne State tolerance curve and was the basis for Head Injury Criteria (HIC), defined by the National Highway Traffic Administration in Federal Motor Vehicle Safety Standards (FMVSS) 208 in 1972 (Gurdjian et al., 1964; Lissner et al., 1960). HIC is based on the time history of the resultant translational acceleration of the center of gravity of the head and is currently used to assess head injury potential in automobile crash test dummies (Melvin, 1993). The HIC has been criticized by many investigators as a measure of head injury mainly because it does not distinguish between different types of head injury, nor does it address brain damage due to rotational accelerations. More recent tolerance frameworks for head injury have incorporated rigid body rotations with accelerations (e.g., Newman et al., 2000), but none are widely accepted, and their usefulness with ballistic impact is questionable. Finally, recent mild/moderate brain injury has been characterized based on sports data (Funk et al., 2007); timescale and momentum differences between sports impacts and behind armor ballistic impacts make extrapolations uncertain.
Currently, although some measures based on internal stresses and/or strains have been proposed as the injury criteria for the brain, no improved injury criteria have been developed that are universally acknowledged (Goldsmith, 1981). The accuracy of the proposed measures relies on accurate modeling of the geometry, material properties, and interfaces of the head-brain complex. By using advanced medical imaging techniques, the geometry of a head-brain model can be significantly improved. However, owing to their highly inhomogeneous, anisotropic, and nonlinear nature, no universally acknowledged method of modeling head and neck constitutive properties and interfaces has been
developed, and correlates of traditional impact TBI research with ballistic impact research are uncertain.
Head Injury from Ballistic Impact
Limited research exists on head injury at ballistic rates (e.g., Viano et al., 2004; Bass et al., 2003). In 2003, Bass and coworkers performed a study to develop injury criteria for skull fracture in human heads during ballistic loading of a protective helmet. The effort further assessed experimental measures to quantify the risk of brain injury under impact loading (Figure 7-4). Two series of ballistic impact tests were performed, including tests with various initial test round velocities; these included nine cadaver tests with helmets and 9-mm test rounds and four cadaver tests with various compliant direct impacts. These ballistic impact tests were used to assess the risk of skull fracture and other head injuries from nonpenetrating BABT for military helmets. Skull fractures were produced in five of nine tests, with a single artifactual fracture at a preexisting unhealed craniotomy. Injuries ranged from simple linear fractures to complex combinations of linear fractures and a depressed fracture. Other injuries included bruising of the dura and severe local skin friction injuries.
FIGURE 7-4 Cadaver instrumentation overview. SOURCE: Bass et al., 2003.
This study developed an injury criterion for both test round velocity and cadaver peak pressure. For this injury risk function, there is a 50 per cent risk of skull fracture for a peak impact pressure of 51,200 kPa as measured by the force/strain instrumentation. Using a simple velocity correlation between a dummy and a cadaver, a dummy injury risk function is developed: namely, there
is a 50 per cent risk of skull fracture for dummy peak impact pressure of 15,220 kPa. This injury risk function may be used with a general helmet and the Hybrid III dummy used in previous testing.47
Automobile injury criteria, including the HIC, were not found to be a good predictor of cadaveric injury. Indeed, for all fracture tests, the calculated HIC was well below the injury reference value. Skull fracture from ballistic BABT is an intrinsically high-rate event. Energy is deposited locally, and local skull deformations are significant. Use of HIC requires essentially rigid body motion of the head at a much lower rate than is characteristic of ballistic events. In addition, the risk of neck injury from lateral impact was found to be small for the 9-mm projectile at up to 460 m/sec velocity, and no neck injuries were found in the cadaver tests.
The principal existing neck injury criterion is promulgated by the National Highway Traffic Safety Administration for frontal impact testing. This injury reference value is termed the National Institute of Justice (NIJ) criterion and is based on human cadaver, volunteer, and animal data; it is intended for use with the Hybrid III dummy (Eppinger, 1999). The NIJ criterion is a composite of injury indicators based on a linear combination of neck loads and moments. The loads include neck axial tension and compression, and the moments include neck flexion and extension. The postulated injury levels for these combined loads have been validated using human cadaver, volunteer, and animal subjects. For this injury criterion, a NIJ value of 1.0 represents a 22 percent risk of an abbreviated injury scale value 3 neck injury (Eppinger, 1999).
The series of tests directly used in developing the neck injury criterion used live pigs and human cadavers. The data from these tests suggested that tension was the best predictor of out-of-position neck injuries; however, the tests were limited to tension-extension, which is the primary mode seen in automobile field data. Predominantly lateral impacts may result in significant lateral shear and bending modes that are not represented in the existing injury assessment criterion. An extension of this NIJ method has been proposed by Bass et al. (2000). Additional neck injury criteria have been proposed that include the effect of head supported mass such as helmets and night vision.
As motion of the neck from ballistic impact onto a helmet occurs on a time scale similar to that seen in vehicle crashes or falls, automotive injury criteria are likely applicable. For the low-momentum transfer that occurs from current helmet threats, the risk of neck injury is quite low. Direct measurements of the neck loads associated with the ballistic impact on a helmet from a 9-mm full metal jacket (FMJ) round at various bullet velocities used for human cadaver tests are shown in Figure 7-5 (Bass et al., 2003). Injury assessment values indicate
47In the early 1970s, the automotive industry developed the Hybrid III 50th Percentile Male anthropomorphic testing device (ATD).
very low risk of neck injuries for these scenarios, and no neck injuries were seen in the testing.
FIGURE 7-5 Neck injury assessment value for 9-mm FMJ test round at various velocities into helmeted human cadavers. SOURCE: Bass et al., 2003.
For future threats, estimates for typical incoming rounds based on momentum arguments suggest that the risk of injury is low for 7.62 × 54 mm rounds at muzzle velocity but may become substantial for .50-cal threats at substantial fractions of the muzzle velocity. The potential for neck injury is proportional to the amount of momentum and resulting head velocity. Thus, head velocity is a conservative estimate of the injury potential. An estimate of the potential for neck injury from various potential threat rounds is shown in Figure 7-6. If the average neck injury risk for the current helmet is taken as shown as in Figure 7-5, the risk of neck injuries for a 7.62 × 54 mm threat is less than 0.1 per cent, but the committee estimates it would be greater than 22 per cent for a .50-cal threat at the current areal density.
FIGURE 7-6 Residual head/neck velocity from momentum transfer to the helmet/head system. SOURCE: Cameron Bass, Duke University.
The glaring weakness of current methodologies, especially clay-based methodologies, is a link to human injury. The Army should immediately investigate human response and injury from helmet BABT to provide injury assessment values and dynamic response values to support the creation of a well-validated test methodology for helmet BABT. Some of the work done on skull fracture should be generalized and extended to other potential brain injury modes incorporating existing epidemiological, cadaveric, and animal studies.
Recommendation 7-1: The Army should perform research to define the link between human injury and the testing methodology for head behind-armor blunt trauma.
HELMET DESIGN AND SUSPENSION SYSTEMS
A potentially important aspect of ballistic protective helmet design is the suspension system that provides helmet standoff from the head, an important factor in ballistic protection. A recent study investigated potential backface trauma to the skull using two ballistic protective helmet interior systems, one a webbing-based suspension system and the second a foam-based padding system (Bass et al., 2006). Both interior systems were installed into a current military ballistic protective helmet. The back face trauma risk assessment was performed using the test methodology outlined by Bass et al. (2003). Fifteen helmet systems
were tested, nine with suspension and six with padding. Two types of round were used: 9-mm FMJ rounds and steel right circular cylinders (64 grain) at velocities that resulted in helmet backface contact with the head form.
For the systems tested, there were no statistically significant differences in backface peak force response to test rounds for the suspension and padding systems considered. Further, the results showed no statistically significant difference in the head acceleration response of the head form. These results are consistent with the understood mechanisms of ballistic mechanical response of a helmet/interior system to local deformation under projectile impact for the systems tested. Assessments of the ballistic backface performance of helmet systems with installed suspension systems should be one component of a comprehensive assessment of the engineering and ergonomic trade-offs associated with ballistic protection and other requirements of ballistic protective helmets.
Existing Helmet Test Methodologies
Test methodologies for helmets, like those for assessing the performance of body armor, generally separate penetration and BFD behavior as separate assessment criteria. Although there is an extensive literature on helmet test methodologies, existing standards are largely based on requirements from motor vehicle and sports impacts. Of the 29 helmet test standards listed in the Advisory Group for Aerospace Research and Development Advisory Report on Dummies for Crash Testing (AGARD, 1996), only one, NIJ-0106.01, is intended for ballistic impact. Differences between crash test impact standards and the ballistic standards are emphasized by the effective impact energy as shown in Figure 7-7. The peak impact energy for the ballistic standard is an order of magnitude larger than that for a typical crash. Further, the impact momentum for the ballistic standard is generally far lower than that for the crash test helmet standards. These differences will increase with higher velocity threats, such as rifle threats.
Impact trauma assessment in all of the current blunt helmet standards is based on similar concepts. The principal implicit assumptions are that concussion or head injury is well correlated with skull fracture (reference) and that the head acts as a mostly rigid body so that “mean” acceleration may be associated with skull fracture (Bass et al., 2003). More recent work suggests that the correlation between skull fracture and brain injury is not good (Viano and Lau, 1985). Further, the skull does not generally act as a rigid body for ballistic deformations of the helmet backface, even for handgun rounds. Measurements taken from cadavers with and without skull fracture show no correlation with the Wayne State Tolerance Curve or similar concepts (Bass et al., 2003), as shown in Figure 7-8. Indeed, for all fracture tests, the calculated HIC was well below the injury reference value.
FIGURE 7-7 Impact energy for helmet standards. SOURCE: AGARD, 1996.
FIGURE 7-8 Ballistic (high-rate) skull fracture data vs. impact injury criteria for typical blunt injury. SOURCE: Bass et al., 2003.
The earliest published standard for use with ballistic protective helmets, NIJ Standard 0106.01 (NIJ, 1981), was developed by the Law Enforcement Standards Laboratory of the National Bureau of Standards (now the National Institute of Standards and Technology). The NIJ helmet standard specifies penetration and inertial impact tests of ballistic helmets. The penetration testing is performed using a test round impacting a fixed head form with witness panels located in the midsaggital or midcoronal planes, depending on shot direction. The saggital head form is shown in Figure 7-9. For the impact tests, the test round impacts a rigid head and neck complex mounted on a trolley that translates in the direction of the travel of the test round. An inertial impact acceleration limit of 400 g is specified.
FIGURE 7-9 NIJ sagittal penetration head form. Two other head forms are also used for the helmet tests: one for side (temple) impact and one for crown impact. SOURCE: NIJ, 1981.
Characteristics of the test rounds for each helmet type specified in the standard are shown in Table 7-1. Bullet velocities for different test rounds range from 259 m/sec to 425 m/sec, with energies ranging from 133.1 J to over 900 J.
TABLE 7-1 Characteristics of Test Rounds from NIJ Standard 0106.01
|Helmet Type||Test Ammunitiona||Nominal Bullet Mass (g)||Required Bullet Velocity (m/sec)||Nominal Bullet Energy (J)|
|I||22 LRHV Lead||2.6||320 ± 12||133|
|38 Special RN Lead||10.2||259 ± 15||342|
|II-A||357 Magnum JSP||10.2||381 ± 15||740|
|9 mm FMJ||8.0||332 ± 15||441|
|II||357 Magnum JSP||10.2||425 ± 15||921|
|9 mm FMJ||8.0||358 ± 15||513|
aLRHV, long rifle round nose; and JSP, jacketed soft-point pistol.
Human Injury Criterion
A simple translational head acceleration limit of 400 g is used in the NIJ ballistic helmet standard. It is uncertain whether the impact attenuation test has ever been used for assessing ballistic protective helmets.48 For impact loading injuries, this standard can obscure potentially injurious shocks in some realistic situations and is overly conservative in others (Bass, 2003). Further, typical automobile injury assessment filtering techniques require low-pass filtering to 1650 Hz, although typical timescales of back-face impact occur at similar or
48Personal communication between Kirk Rice, National Institute of Standards and Technology, and Dale Bass, committee member, on August 10, 2010.
higher frequencies, as discussed above. This makes it inappropriate to use the NIJ 0106.01 BFD criterion (impact attenuation) for ballistic impact.
The Army clay head form is an aluminum head form based on the penetration head form specified in NIJ 0106.01, shown in Figure 7-10. The empty spaces on the head form are filled with Roma Plastilina No. 1 (RP #1) clay.
FIGURE 7-10 Army clay head form. SOURCE: Courtesy of Rob Kinsler, U.S. Army Research, Development and Engineering Command/ARL.
This is the same type of clay used to certify ballistic vests. The plastic property of the clay allows it to record BFDs caused by impact of nonpenetrating projectiles during the ballistic testing of hard body armor. Helmet testing standards and practices are derived from body armor testing standards and practices and, as in body armor testing, are based on the use of RP #1 as the test recording medium. As such, they capitalize on existing infrastructure and organizational experience with those test methods, yet they also suffer from all the attendant issues and weaknesses associated with these methods. The test standards are derived from
the observed performance of a sample of helmets rather than experimental evidence relating test outcomes to likely injury.
Finding: It is uncertain how clay response correlates with human head/skull/brain response. Yet, clay response serves as the basis for current clay-based helmet methodologies.
Helmets are subjected to a series of ballistic and nonballistic tests as part of both first article testing and lot acceptance testing. Nonballistic tests include the inspection and verification of various aspects of helmet construction such as edging adhesion, adhesion of the coating, and barcode labeling.49 Ballistic tests assess the helmet’s ability to (1) withstand penetration and (2) not exceed a BFD limit.
In the testing of body armor plates, a “plate penetration” occurs when a round penetrates the soft armor behind the plate. In the testing of helmets, a “helmet perforation” occurs when the helmet is visibly penetrated by a round during the test. Both events are defined in the test procedures and both result in a test failure. Since there is no practical way to determine or measure “degree of penetration,” both the plate and helmet tests must be attribute-based.
Current Aberdeen Test Center (ATC) testing practice is to assess a helmet’s resistance to penetration in terms of penetration and perforation. According to Page and Humiston: 50
Perforation occurs when the threat defeats the sample. This is noted when the threat and/or sample fragments have entered the witness medium…. Penetration occurs when a threat comes in contact with the sample but does not defeat it.
The draft DOT&E Military Combat Helmet Standard for Ballistic Testing (DOT&E, 2010) redefines the penetration and perforation in terms of partial and complete penetration:
A complete penetration shall be defined as complete perforation of the shell by the projectile or fragment of the projectile as evidenced by the presence of that projectile, projectile fragment, or spall in the clay, or by a hole which passes through the shell. …Any fair impact that is not a complete penetration shall be considered a partial penetration.
49Matthew Page and Travis Humiston, ATEC Protective Equipment Division, “Head Protection Testing: Processes, Issues,” presentation to the committee, October 13, 2010.
Resistance to penetration (or V0 testing) is measured as a sequence of five ballistic impacts, one each to the front, rear, left, and right sides of the helmet as well as to the crown of the helmet. Current ATC protocol also tests the V50 ballistic limit using a series of 7 to 14 shots to the five regions of the helmet at varying velocities.51 V50 testing is not part of the currently proposed DOT&E protocol.
Helmet BFD (also referred to as ballistic transient deformation) is assessed using the same sequence of five ballistic impacts, one each to the front, rear, left, and right sides of the helmet as well as to the crown of the helmet. As with current body armor testing practices, the BFD is assessed using a laser scanner and it is defined as the maximum impact depression depth in the clay, as measured from the original clay surface. Under current ATC testing protocol, the BFD can be no greater than 16 mm for crown, left, and right impacts and no greater than 25.4 mm for front and rear impacts.
The many unknowns in the use of clay as a medium make it unclear as to whether the BFD response in clay methodology is appropriate for helmet testing, especially since the mechanical backface response of the head surrogate may govern both penetration and impact tolerance portions of the test.
Recommendation 7-2: The Aberdeen Test Center should ensure the following:
1. Dynamic mechanical strain/deformation response of the head surrogate is similar for both types of loading at loading rates typical of behind-helmet response;
2. Response of the head surrogate is similar to that of the human head;
3. Required head quality control calibration is either performed on the head surrogate itself or is shown to be demonstrably represented by a surrogate for the head itself (i.e., by a sample box filled with clay) in controlled testing using a standard test procedure; and
4. Response of the clay for the low-rate calibration tests is shown to be similar or scalable to the high-rate BFD response of the surrogate in controlled testing using a standard test procedure.
Test range setup is in accordance with ATC Test Operating Procedure 10-2-210 (ATC, 2008). The test is conducted in accordance with NIJ Standard 0106.01 with the following four exceptions:
51Matthew Page and Travis Humiston, ATEC Protective Equipment Division, “Head Protection Testing: Processes, Issues,” presentation to the committee, October 13, 2010.
• Test items may be conditioned as needed.
• Test distances may be altered.52
• The test head form is modified with slots in both the coronal and midsagittal directions, with a depth of approximately 143 mm below the head form crown, with drill points along the headform pillar for use by the laser scanner.
• Striking velocities are calculated according to the U.S. Army Test and Evaluation Command International Test Operating Procedure 4-2-805 in order to determine if a shot is fair (DOT&E, 2010).
Clay Preparation, Conditioning, and Calibration
Helmet testing is based on a head form with slots in both the coronal and midsagittal directions. There is only one head form size, although there are between four and six helmet sizes depending on the type of helmet. The slots in the head form are packed with RP #1 as the recording medium for both penetration/perforation and BFD. As shown in Figure 7-11, the clay is shaped to create a smooth, uninterrupted surface with the headform.
FIGURE 7-11 ATC head form with clay. SOURCE: Rob Kinsler, U.S. Army Research, Development and Engineering Command/ARL.
52Test distances are given in Figure 6 of NIJ Standard 0106.01 (NIJ, 1981).
As described in the ATC Internal Operating Procedures for the Head Protection Testing (ATC, 2010), the clay in the head form is calibrated in a 12 in. × 12 in. × 4 in. plywood-backed box, analogous to the box form used for RP #1 in the body armor plate testing procedure. Up to eight head forms may be conditioned with each box so long as the clay in the box and in the head forms comes from the same lot and the head forms are conditioned within 12 in. of the box (Figure 7-12). Once conditioned, calibration of the box is performed via drop test in which 2.2-lb, 1.75-in.-diameter steel cylinders are dropped from a height of 78.7 ± 0.8 in., into the clay box. The clay is considered to be within calibration if the indentations made by the steel cylinders are all within 1.0 ± 0.1 in., as measured by a digital caliper. The clay head form removed from the oven with the clay box may be used for up to 45 min after the third drop, and the remaining head forms may be used for up to 4 hours from the time of the third drop and for up to 45 min after being removed from the oven.
FIGURE 7-12 Head form clay conditioning by analogy. SOURCE: Kevin Reilly, Program Manager Infantry Combat Equipment, Marine Corps Systems Command, “PM ICE Helmet Testing Overview,” presentation to the committee, August 10, 2010.
Helmets are tested against the following threats:
- Remington 9-mm, 124-gr FMJ projectile;
- 2-gr right circular cylinder (RCC) fragment;
- 4-gr RCC fragment;
- 16-gr RCC fragment;
- 64-gr RCC fragment; and,
- 17-gr fragment simulating projectile.53
Test Item Configuration and Impact Locations
The head form used in the test is mounted on a test fixture that is capable of being rigidly fixed with 6 degrees of freedom to allow for positioning the head form in all required positions. The helmet is mounted on the head form in the as-worn configuration and position, using the helmet’s suspension/retention system to hold it on the head form. Prior to mounting, the helmet is marked to show the desired impact locations. As illustrated in Figure 7-13, the current test uses five preset impact locations that are shot in the following order:
- Crown at the intersection of midsaggital and coronal planes;
- Left side (as facing) on the coronal plane 50 mm above the earflap;
- Right side (as facing) on the coronal plane 50 mm above the earflap;
- Front on the midsaggital plane 85 mm above the edge and at least 1.5 in. from the night vision goggles hole; and
- Back on the midsaggital plane 75 mm above the edge (DOT&E, 2010).
In addition, at least one of the noncrown shots must be at a bolt. A high-speed camera is used to record the bolt shot (to ensure the hit is within 1/2-in. as per the standard), and the FARO laser is used to measure BFD.
53Matthew Page and Travis Humiston, ATEC Protective Equipment Division, “Head Protection Testing: Processes, Issues,” presentation to the committee, October 13, 2010.
FIGURE 7-13 Test impact locations. SOURCE: Linda Moss, Survivability/Lethality Analysis Directorate, U.S. Army Research Laboratory, “Statistical Issues Related to Helmet Testing,” presentation to the committee, August 10, 2010.
The head form and helmet attached to the test fixture are mounted on the test frame shown in Figure 7-14. The helmet is aligned to ensure the target location achieves the required obliquity and, for bolt shots, the high-speed cameras are aligned. The helmet is removed from the head form and the clay surface is scanned with the laser. The helmet is reattached to the head form and the shot taken. The helmet is then removed from the head form and inspected for penetration and perforation. The clay is rescanned with the laser to calculate BFD. A fair hit is recorded if the shot location, obliquity, yaw, and shot velocity are within required limits.
FIGURE 7-14 Test frame. SOURCE: Matthew Page and Travis Humiston, ATEC Protective Equipment Division, “Head Protection Testing: Processes, Issues,” presentation to the committee, October 13, 2010.
H.P. White Laboratory Test Procedure
The helmet testing procedures used by H.P. White Laboratory, a private test laboratory, were developed for testing helmets for law enforcement agencies and have been adapted to the testing of military helmets. The test procedure was developed specifically for bullet penetration or for excessive BFD of the helmet material, and does not include biomechanical shock. It is similar to the one used currently by the Army and is based on the original NIJ 106.01.
The projectile types are described in the NIJ Standard 0101 and the head form used for the deformation studies is described in NIJ Standard 0106. The test procedure evaluates the helmets for penetration or excessive deformation by bullet type, impact velocity, and bullet caliber. Six different threat levels are used in the test: I, IIA, II, IIIA, III, and IV (see Table 7-2). Helmets passing the test are certified only for these six threat levels. A helmet size equivalent to a hat size 7-1/4 is considered standard for the H.P. White test. Four helmets of each design are tested for threat levels I, IIA, II, and IIIA; two helmets are tested for levels III and IV.
TABLE 7-2 H.P. White Laboratory Test Procedure
SOURCE: H.P. White.
The test procedure involves five impacts on each of two helmets with bullets of two different calibers. Helmets are impacted on five sites: front, back, left, right, and top. The helmet is held on the head form only by the chin strap and may be replaced on the head form if it is knocked loose during the test. Shots must be “fair,” i.e., normal to the helmet surface, of the correct velocity, and not yawed more than 3 deg. If the shot is unfair, a second shot can be made at least 3 in. from the first. One fair shot must be made at each of the five locations, and if one penetrates the helmet or if the impact crater in the clay on the head is deeper than 25 mm, the helmet will not be certified.
Tests are made at 70°F after wetting the helmet with water. The shot is made at a distance of 16.5 ft from the helmet. The muzzle velocity (see Table 7-1) is measured at a distance of 8 ft for each shot. After each shot, the helmet is removed from the head form and examined for penetration by the bullet. All locations on the helmet must be tested even if the helmet has failed at one impact site.
BFD is tested on a second helmet. The deformation test uses a head form, Figure 7-15, filled with RP #1 modeling clay. The clay is first qualified by dropping a 2.2-lb, 1.75-in. diameter steel sphere from a height of 78.7 in. onto the clay already mounted in the head form. To be acceptable for the ballistic test, an impression of 25 mm ± 1 mm deep should be left behind. Three drop tests are carried out for each head form prior to shooting the helmet. The depression left is measured after each shot to the nearest millimeter, but the clay in the head form is qualified only prior to the first shot. As with the penetration tests, the deformation tests are carried out until all five locations on the helmet are tested. No reference to the acceptable penetration depth could be found in the H.P. White written procedure.
FIGURE 7-15 H.P. White head form: Only one headform is used for all impact tests. Overall dimensions comply with NIJ-STD-0106.01 (hat size 7¼). Upper portions, including the base plate, are made of 6061-T6 aluminum or equivalent. Lower head form is of USG epoxy #303. SOURCE: H.P. White Laboratory, Inc., 1995.
Hot tests, 120°F, or cold tests, -20°F, can also be carried out by this procedure. Only the helmet is temperature conditioned. The test environment is maintained at 70°F and the tests are to be completed in 30 min. Wet conditioning is not carried out for helmets tested under cold conditions. Penetration of the helmet by any fair shot or an excess of deformation deny the helmet certification.
This test methodology is similar to the current Army methodology and provides a record of BFD in the clay. No displacement limits are specified in the document, and human injury has not been linked with this assessment technique.
A new head form, referred to as the Peepsite head form (Figure 7-16) was developed by the U.S. Army Research Laboratory to avoid potential drawbacks of the NIJ head forms.54 A big shortcoming of the current test head forms is that the clay used to measure the BFD of the helmet upon impact is contained between two solid aluminum parts of the head form (see Figure 7-10).
FIGURE 7-16 Peepsite head forms: different head forms for different shot directions.
54The new head form was demonstrated to the committee by Rob Kinsler, ARL, on October 13, 2010.
This causes three potential problems. One, the solid aluminum will constrain both the outward and inward flow of the clay during the impact, giving a smaller displacement than might actually occur during use of the helmet in combat. Since blunt trauma by the impact is the primary cause of injury to the soldier for a bullet that does not penetrate the helmet, the actual displacement of the back surface of the helmet should be measured with some accuracy. The Peepsite head form reduces this concern by eliminating the metallic petals near the backface impact.
The second problem is that the backface contact can span the aluminum petals, preventing further impact or altering the BFD response and backface signature. Again, elimination of the petals in the Peepsite headform eliminates the potential for helmet/head form interactions to alter the backface response.
The third potential problem arises from the fact that the clay and the helmet have very different temperature characteristics. In such tests the clay is normally heated above room temperature to achieve the desired rheological behavior. Testing on the Peepsite head form, however, is done at room temperature, which means that the rate of cooling of the clay and the aluminum head form will be very different from one another, resulting in thermal gradients and residual strains and stresses in the clay that may affect the impact event.
As in the tests developed by NIJ and H.P. White, five surfaces are tested for impact: left, right, front, back, and crown. Instead of only one impact in each area, three are used, each on different helmets, and the displacement is measured after each impact. The test matrix is illustrated in Table 7-3. Three replicate shots are made for each area, and five threats are tested.
SOURCE: H.P. White.
Clay deformations may be measured by several methods, including manual calipers, digitization arms, digitization arms with laser scanning capability, and purpose-built laser scanners. Since the deformation of the head form is three-dimensional, laser arm and laser scanning systems can be used to maximize the information detail of the deformations.
As there is no link to human injury for this methodology, especially with the current generation of room-temperature clay, the U.S. Army Research Laboratory (ARL) provided results of comparison testing of an existing helmet system against a new candidate helmet system.55 This time of comparison testing is essential for establishing a baseline in the absence of a link to human injury. Additional head form sets have been supplied to five commercial testing laboratories to more widely disseminate the capability for Peepsite head form testing.
Finding: The Peepsite head form reduces or eliminates several potential problems with the National Institute of Justice head form that is used in the current clay test methodology.
Since testing with the clay head forms is based on an unproven assumption that clay deformation is correlated in some way with human injury, an essential prerequisite to the development of the Peepsite head form as a viable test methodology is correlation of the current helmet system performance with deformations in room-temperature clay at desired threat levels. This would provide a benchmark for the clay methodology while human response and injury metrics are developed.
The Peepsite head form and test procedure have clear advantages, including these:
• Use of room-temperature clay;
• Inherent quality control since the drop test procedure uses the head form itself;
• BFD characteristics that are not limited by the petals at the edge of the clay-containing region but are limited only by the characteristics of the clay itself.
Recommendation 7-3: The Army should investigate use of the Peepsite head form currently in development by the Army Research Laboratory with room-temperature clay. This head form and the procedure have potential as a near-term alternative to testing using the National Institute of Justice clay head form tested at elevated clay temperatures.
55Presentation to the committee by Rob Kinsler, ARL, on October 13, 2010.
To simulate the impact response of the human, the automotive industry developed the Hybrid III 50th Percentile Male anthropometric test device (ATD) in the early 1970s. It has since become a validated tool for the evaluation of automotive impacts, and can accommodate a wide range of instrumentation and transducers. It is the required test device for automotive crash testing (DOT, 1998) and is robust enough to perform repeatedly in ballistic environments (Bass et al., 2003).
For ballistic testing, a collaborative effort between Natick labs, Defense Research and Development Canada -Valcartier, and the University of Virginia (UVA) developed a ballistic version of the Hybrid III head augmented with impact pressure sensors (Bass et al., 2003). In what is called the “UVA head form,” shown on the left side of Figure 7-17, instrumentation for the Hybrid III head and neck region consisted of three linear accelerometers and angular rate sensors at the center of the ATD headform and six-axis upper and lower neck load cells. Injury metrics assessed using this headform include the HIC and the NIJ neck injury criteria. With the Hybrid III head form modified to accept the Dynasen pressure sensors, the pressure measurements at various locations were recorded, analyzed, and compared to human cadaver results (Bass et al., 2003).
An injury risk assessment was developed based on cadaver tests with force sensor gauges as shown on the right side of Figure 7-17. This head form has been evaluated by the ARL,56 UVA (Bass et al., 2003), and by Duke University/Applied Research Associates.
FIGURE 7-17 Left, UVA head form; right, risk assessment.
56Presentation to the committee by Dixie Hisley, ARL, on August 10, 2010.
The ballistic load sensing (BLS) head form (Biokinetics and Associates, LP, Ottawa, Ontario, Canada) is an evolution of the UVA head form consisting of two load-measuring head forms based on the International Organization for Standardization J size. One head form measures impact forces to the front and back of the head form, the other measures forces applied to the left and right sides of the head form. Both head forms are mounted on a Hybrid III ATD neck (Figure 7-18).
The BLS head form enables a direct measurement of the dynamic load imparted to the skull by the deformation of a ballistic helmet caused by nonpenetrating projectiles. To measure the force of impact, the BLS head form is equipped with an array of seven piezoelectric load cells residing under hexagonal aluminum pieces. These load cells were positioned directly under the impact site. The load cell arrangement is shown in Figure 7-19. The load cells are covered by a flat rubber sheet to simulate normal skull load distribution response. This head form is not able to record the global dynamic response of the head form.
Originally, the force data were correlated with the injury risk assessment of Bass et al. (2003). However, there is currently no suggested correlation between the BLS force data and injury data. ARL researchers have evaluated this head form and it is the subject of continuing evaluation.57 Additional ballistic impact response data have been collected by TSWG for use in assessing head form response.
FIGURE 7-18 BLS head form. SOURCE: Courtesy, Biokinetics and Associates, Ltd.
FIGURE 7-19 Arrangement and dimensions of load cells in the BLS head form. SOURCE: Courtesy, Biokinetics and Associates, Ltd.
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