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Abstract. Recent research indicates that exposure to low amplitude blast waves, such as IED detonation or multiple firings of a weapon, causes damage to brain tissue resulting in Traumatic Brain Injury (TBI) and Post Traumatic Stress Disorder (PTSD). Current combat helmets are not sufficiently protecting warfighters from this danger and the effects are debilitating, costly, and long-lasting. The objective of the present work is to evaluate the blast mitigating behavior of current helmet materials and new materials designed for blast mitigation using a test fixture recently developed at the Naval Surface Warfare Center Dahlgren Division for use with an existing gas gun. The 40-mm-bore gas gun was used as a shock tube to generate blast waves (ranging from 0.5 to 2 bar) in the test fixture mounted on the gun muzzle. A fast opening valve was used to release helium gas from the breech which formed into a blast wave and impacted instrumented targets in the test fixture. Blast attenuation of selected materials was determined through the measurement of stress data in front of and behind the target. Materials evaluated in this research include polyurethane foam from currently fielded US Army and Marine Corps helmets, polyurea 1000, and three hardnesses of Sorbothane (48, 58, and 70 durometer, Shore 00). Polyurea 1000 and 6061-T6 aluminum were used to calibrate the stress gauges. IntroductionThe dynamic response of viscoelastic materials to low-amplitude (<2 bar) blast waves is of interest for protection against blast-induced traumatic brain injury (TBI). Viscoelastic materials have a demonstrated ability to dissipate shock waves and absorb impact energy [1][2][3]. Many of these materials are soft, making them likely candidates for use in helmets to protect the head from lowamplitude blast waves. Sorbothane, for instance, is a soft viscoelastic polymer used in a number of shock and vibration isolation applications including insoles, recoil pads, and vibration dampers to protect delicate components subject to intense mechanical vibrations [3]. The goal of the present work is to evaluate the blast mitigating behaviour of current helmet materials and to determine if blast protection can be improved by incorporating viscoelastic materials, such as Sorbothane, into the helmet pad system.To evaluate materials for blast mitigation, we employed a recently developed test fixture designed for use with an existing gas gun to generate planar blast waves in the range of 0.5 to 2 bar for 2 ms duration [4]. The human threshold for TBI blast injuries is not explicitly defined, but falls in the range of 0.2 to 5 bar for pressure pulses of several milliseconds duration [5]. The experiments performed in this present research represent the low to middle end of this pressure range.The instrumented test fixture, designed for the Naval Surface Warfare Center Dahlgren Division's (NSWCDD) research gas gun [6], provided measurements of blast wave amplitude and velocity as

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Three polyureas with decreasing soft segment molecular weights of 1000, 650, and a 250/1000 blend were molded onto circular steel plates and then impacted with a high speed (275 m/s) conical-shaped steel cylinder. The polyurea layer of the post mortem bilayers was characterized on a molecular level by small angle synchrotron X-ray scattering (SAXS) at the Advanced Photon Source at the Argonne National Laboratory. Analysis revealed that the hard domains of the polyureas with lower molecular weight soft segments reformed and oriented over a greater area of the coating, thus increasing the polymer strain hardening and resulting in visibly less out of plane bilayer deformation. This agrees with the hypothesis that polymer strain hardening is a mechanism that retards necking failure of the metal plate.