Testing of Body Armor Materials Phase III (2012) / Chapter Skim
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7 Helmet Testing
7 Helmet Testing
Pages 137-168
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From page 137... ...
. 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.
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From page 138... ...
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.
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From page 139... ...
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.
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From page 140... ...
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.
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From page 141... ...
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)
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From page 142... ...
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 -142
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From page 143... ...
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.
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From page 144... ...
NIJ Value FIGURE 7-5 Neck injury assessment value for 9-mm FMJ test round at various velocities into helmeted human cadavers.
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From page 145... ...
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.
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From page 146... ...
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.
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From page 147... ...
skull fracture data vs. impact injury criteria for typical blunt injury.
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From page 148... ...
. The NIJ helmet standard specifies penetration and inertial impact tests of ballistic helmets.
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From page 149... ...
320 12 I 22 LRHV Lead 2.6 133 259 15 38 Special RN 10.2 342 Lead 381 15 II-A 357 Magnum 10.2 740 JSP 332 15 9 mm 8.0 441 FMJ 425 15 II 357 Magnum 10.2 921 JSP 358 15 9 mm 8.0 513 FMJ a LRHV, long rifle high velocity; RN, 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.
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From page 150... ...
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.
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From page 151... ...
…Any fair impact that is not a complete penetration shall be considered a partial penetration. 49 Matthew Page and Travis Humiston, ATEC Protective Equipment Divi sion, "Head Protection Testing: Processes, Issues," presentation to the committee, October 13, 2010.
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From page 152... ...
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.
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From page 153... ...
. Clay Preparation, Conditioning, and Calibration Helmet testing is based on a head form with slots in both the coronal and midsagittal directions.
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From page 154... ...
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.
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From page 155... ...
, and the FARO laser is used to measure BFD. 53 Matthew Page and Travis Humiston, ATEC Protective Equipment Division, "Head Protection Testing: Processes, Issues," presentation to the committee, October 13, 2010.
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From page 156... ...
Army Research Laboratory, "Statistical Issues Related to Helmet Testing," presentation to the committee, August 10, 2010. Test Process The head form and helmet attached to the test fixture are mounted on the test frame shown in Figure 7-14.
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From page 157... ...
H.P. White Laboratory Test Procedure The helmet testing procedures used by H.P.
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From page 158... ...
White. The test procedure involves five impacts on each of two helmets with bullets of two different calibers.
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From page 159... ...
FIGURE 7-15 H.P. White head form: Only one headform is used for all impact tests.
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From page 160... ...
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)
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From page 161... ...
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.
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From page 162... ...
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.
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From page 163... ...
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)
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From page 164... ...
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.
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From page 165... ...
10-2-210 Ballistic Testing of Hard Body Armor Using Clay Backing. Aberdeen Proving Ground, Md.: Aberdeen Test Center.
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From page 166... ...
2010. Standard for Lot Acceptance Ballistic Testing of Hard Body Armor.
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From page 167... ...
1981. Current controversies in the stipulation of head injury criteria, Letter to Editor.
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From page 168... ...
1995. Brain injury without head injury.
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