Head Impact Modeling to Support a Rotational Combat Helmet Drop Test

Abstract

Introduction The Advanced Combat Helmet (ACH) military specification (mil-spec) provides blunt impact acceleration criteria that must be met before use by the U.S. warfighter. The specification, which requires a helmeted magnesium Department of Transportation (DOT) headform to be dropped onto a steel hemispherical target, results in a translational headform impact response. Relative to translations, rotations of the head generate higher brain tissue strains. Excessive strain has been implicated as a mechanical stimulus leading to traumatic brain injury (TBI). We hypothesized that the linear constrained drop test method of the ACH specification underreports the potential for TBI. Materials and Methods To establish a baseline of translational acceleration time histories, we conducted linear constrained drop tests based on the ACH specification and then performed simulations of the same to verify agreement between experiment and simulation. We then produced a high-fidelity human head digital twin and verified that biological tissue responses matched experimental results. Next, we altered the ACH experimental configuration to use a helmeted Hybrid III headform, a freefall cradle, and an inclined anvil target. This new, modified configuration allowed both a translational and a rotational headform response. We applied this experimental rotation response to the skull of our human digital twin and compared brain deformation relative to the translational baseline. Results The modified configuration produced brain strains that were 4.3 times the brain strains from the linear constrained configuration. Conclusions We provide a scientific basis to motivate revision of the ACH mil-spec to include a rotational component, which would enhance the test’s relevance to TBI arising from severe head impacts.

Brain strain rate response: addressing computational ambiguity and experimental data for model validation

“…The time-accumulated peaks of and were determined and labelled as and , respectively. Scheme 2 has been previously used to extract the principal strain rate and/or the tract-oriented strain rates in the brain [23, 25, 26, 41-44] and other biological tissues (e.g., leg muscles [45-48], tongue [49, 50], myocardium [51-53]).…”

Neurotrauma Prevention Review: Improving Helmet Design and Implementation

“…Prior work has highlighted the blunt impact standard limitation to linear head acceleration [ 25 ], with the need to focus on rotational head motion, the likely mechanism contributing to diffuse axonal injury (DAI) [ 25 – 28 ]. Military specification (mil-spec) requires a blunt impact acceleration limit testing for pass/fail criteria of the ACH, but does not require rotational component testing [ 27 ]. Blast-induced TBI (BTBI) is also a mechanism of combat-induced diffuse axonal injury where blast waves cause rotational forces on the brain to induce DAI.…”

Neurotrauma Prevention Review: Improving Helmet Design and Implementation

“…24 Prior work has highlighted the blunt impact standard limitation to linear head acceleration, 25 with the need to focus on rotational head motion, the likely mechanism contributing to diffuse axonal injury (DAI). [25][26][27][28] Military-specification (mil-spec) requires a blunt impact acceleration limit testing for pass/fail criteria of the ACH, but does not require a rotational component testing. 27 Blast induced TBI (BTBI) is also a mechanism of combat induced diffuse axonal injury where blast waves cause rotational forces on the brain to induce DAI.…”

“…[25][26][27][28] Military-specification (mil-spec) requires a blunt impact acceleration limit testing for pass/fail criteria of the ACH, but does not require a rotational component testing. 27 Blast induced TBI (BTBI) is also a mechanism of combat induced diffuse axonal injury where blast waves cause rotational forces on the brain to induce DAI.…”

“…The ACH is equipped with high-strength Kevlar 129 fibers, housed in a 7.8 mm thick composite shell. 2,29 Previous literature has highlighted the efficacy of the ACH head protection in reduced likelihood of blast-induced mild TBI (mTBI), where levels of protection increase with peak blast exposure, 27 as well as protection against blast-induced intracranial pressure (ICP) increases and brain strains. 30,31 Although there is increased overall protection against blast exposures, helmet design still has limitations.…”