7 - Properties of Hard and Soft Viscoelastic Polymers Under Blast Wave Loading

Abstract

In the present work the properties of hard and soft polymers are measured under blast wave loading at 0.5, 1.3, and 2.1 bar. A new target assembly was designed for an existing gas gun to perform these experiments. The hard polymers are Kevlar KM2 and polyurea 1000; the soft polymers are Zorbium 83i and 110i, and Sorbothane 30, 50, and 70. Kevlar KM2, Zorbium 83i, and Zorbium 110i are used in the Lightweight Marine Corps and Army Advanced Combat Helmets. The remaining four materials were investigated for comparison with these materials and for possible inclusion in future helmet designs. Bilayer attenuation experiments indicate that inclusion of polyurea 1000 and Sorbothane 30 in the helmet may improve attenuation. Longitudinal blast wave velocity versus particle velocity was determined for each polymer. The largest increase in wave velocity occurred for Sorbothane 30 and Zorbium 83i and 110i. Stress–strain relations were also determined. The maximum μstrain values for Kevlar KM2, polyurea 1000, and the Sorbothane polymers were 100–200, whereas the values for the more compressible Zorbium foams were 20,000–60,000. As the typical blast wave risetime was 1 μs, strain rates were 100–200 s-1 for the harder polymers, and 20,000–60,000 s-1 for the Zorbium foams. The stress–strain relations were used to determine the blast energy absorbed at the three incident blast pressures for each polymer. At 2.1 bar blast pressure, polyurea 1000 and Kevlar KM2 absorbed the least at 2 and 11%, respectively, and Zorbium 83i and 110i absorbed the most at 25 and 46%, respectively. As blast wave input and output stresses were measured for the polymer targets, it was possible to determine an attenuation coefficient for the decrease in blast amplitude as a function of polymer thickness. Kevlar KM2 showed the least attenuation (56% for a 6.35 mm thickness) and Zorbium 83i showed the most attenuation (89% for a 6.35 mm thickness). These new measured properties can be used to improve blast wave simulations of the helmet-head system.