Modern combat helmets improve wearer survivability through preventing ballistic
penetration via delamination or deformation of the shell material. However, this
deformation may impinge upon the skull and present a risk of behind-helmet blunt
trauma (BHBT). While legacy aramid helmets from the late 1970s–1980s inform
current NIJ testing standards, the advent of lower areal density ultra-high
molecular weight polyethylene (UHMWPE) and modern suspensions may incur unknown
changes that present an unknown effect on BHBT risk. This study investigates
whether current standards relying on a peak acceleration threshold and residual
clay deformation may not reflect changes in deformation geometry seen in newer
shell materials and suspension designs. Thirty-eight helmets categorized into
three groups, older aramid, newer aramid (with new suspension), and prototype
UHMWPE (with old suspension), were mounted on Hybrid III ATD head-necks and
impacted on the temporal surface with 9 mm NATO projectiles at velocities
between 388 and 482 m/s. Flash x-ray imaging captured deformation geometry.
Deformations were digitized to measure maximum depth, volume, and surface area
recruited. Multiple linear regression identified the influence of shell material
and suspension on geometry. Results indicated that shell material significantly
affected deformation volume and area (p < 0.05). Notably, UHMWPE exhibited a
qualitatively more “pointed” deformation profile compared to aramid, recruiting
material differently as depth increased. In contrast, suspension type and helmet
design did not significantly influence deformation geometry. These findings
suggest that the BHBT geometry in UHMWPE helmets differs fundamentally from
aramid helmets. As UHMWPE helmets encounter higher-energy threats, current
acceleration threshold or depth-based standards may not fully capture the risk
of skull fracture. To future-proof helmet validation, testing standards should
evolve beyond current standards to include area recruitment and other geometric
measures.
