Back face deformation (BFD) is what happens to armor when it is struck with a projectile. On the opposite side of the impact armor can bulge outward resulting in what is known as “Blunt Trauma”.
Ballistic deformation data derived during the “Back face Deformation” portion of testing are recorded to provide the user with information helpful in determining the RELATIVE PERFORMANCE of individual helmet designs and constructions. Inasmuch no known definitive data exists which quantitatively establishes acceptable limits of the magnitude of cranial deformation (H.P. White, 1995).
Modeling of Advanced Combat Helmet Under Ballistic Impact
SOURCE: Department of Mechanical Engineering, Southern Methodist University
Army helmet testing is currently based on the ATC (Aberdeen Test Center) headform—derived from the National Institute of Justice headform with slots in the coronal and midsagittal directions. MIL-STD-662F section 5.2.2 requires that the “Witness” be constructed of aluminum. This standard makes no allowance for head forms constructed from any other materials (Department of Defense Test Method Standard V50 Ballistic Test for Armor, 1997). Additionally, shots are to be in the dead center mass of the plane being tested. Impacts closer to the edges of any material are not as structurally sound. Therefore, for laboratory testing standardization impact is to occur in this region.
Aberdeen Test Center headform SOURCE: NRC (2012)
The slots in the headform are packed with clay as the recording medium for both penetration and BFD. The materials and procedures used to evaluate the back face deformation of the helmets are the same as those used for evaluating the back face deformation of body armor when subjected to ballistic impact as specified by NIJ-STD-0101.
The Department of Testing and Evaluations (DOT&E) protocol specifies BFD thresholds at 25.4 mm for front and back shots and 16 mm for side and crown shots. These appear to be based on historical helmet testing precedent and are not connected to the potential for brain injury. The analysis, however, appears to be based on the presumption that the larger the BFD, the greater the likelihood of serious head injury (Review of Department of Defense Test Protocols for Combat Helmets, 2014).
There are many online videos of individuals, not scientist or lab technicians, conducting “backyard ballistic test” of products and then giving their “expert and qualified” opinion as the the validity or quality of the product. There are too many variable that need to be controlled and monitored in order to achieve accurate results.
Understanding that current U.S. testing standards, both Military, DOT&E as well as NIJ, are based on handgun caliber rounds. A presentation by the Chief Scientist titled, Soldier Protective and Individual Equipment, listed repeating pistols, such as Tokarev (7.62×25-mm caliber) and Makarov (9×18-mm caliber), as emerging threats. However, for insurgent and guerrilla warfare, published data and anecdotal evidence suggest that AK-47 (7.62×39-mm) and other Kalashnikov pattern weapons are the predominant source of ballistic threats in Iraq, Afghanistan, and Somalia (Small Arms Survey, 2012).
In a survey of 80,000 small arms and light weapons seizures, they found that the “vast majority of illicit small arms in Afghanistan, Iraq, and Somalia are Kalashnikov-pattern assault rifles. Other types of small arms are comparatively rare” (p. 6). These weapons and their ammunition are inexpensive and widely available with continuing production and large existing supplies. With current materials technology it has not been feasible to create a helmet to defeat rifle caliber rounds. The helmets would be too heavy and cumbersome for the modern war fighter to perform at peak effectiveness. However, reality is that the threat they face on the battlefield is greater than current testing protocols and methodologies.
In conclusion, current helmet technologies can defeat smaller caliber (handgun) battle field threats, and mitigate ballistic energy from greater sources.
National Academies Press. (2014). Review of Department of Defense Test Protocols for Combat Helmets. Retrieved December 15, 2016
NRC. (2012) Testing of Body Armor Materials: Phase III. The National Academies Press; Washington, D.C.
Department of Defense (1997) Test Method Standards V50 Ballistic Test for Armor. Retrieved December 20, 2016.
Cambridge University Press (2012) Small Arms Survey. Small Arms Survey.; Cambridge, U.K.
H.P. White,. (1995). Bullet Resistant Helmet HPW-TP-0401.01B.
Department of Mechanical Engineering, Southern Methodist University (2015). Modeling of Advanced Combat Helmet Under Ballistic Impact.