Experimental study of the strain rate dependence of a synthetic gel for ballistic blunt trauma assessment.

The mechanical characterization of a polymer gel used as reference backing material for blunt ballistic impact interpretation is performed at room temperature from quasi-static (0.002s-1) up to high strain rates (1500s-1). As very high strain tensile tests (350%) are conducted, an appropriate gripping device and particular strain measurement techniques are used, as well as high strain compressive tests (80%) based on retro lighting imaging. One major challenge is to carry out reliable compressive tests at high strain rates with polymeric split Hopkinson pressure bars using high-speed imaging and specific signal processing software. These mechanical tests provide a primary response to the strain rate dependence of the hyperelastic material behavior. Indeed, the material exhibits a higher stress response when the strain rate increases. Moreover, dynamic compression tests highlight a larger radial strain propagating along specimen axis with higher strain rates. This preliminary study on the characterization of the gel's mechanical behavior, constitutes an interesting step for an evaluation of human surrogate material. The extensive constitutive law can therefore be implemented for numerical simulations, with an aim of impact biomechanics analysis and body armor assessment.