Strain-rate-dependent behavior of polymeric foams

In many applications composite sandwich structures with polymeric foam cores are exposed to high energy and high velocity dynamic loadings producing multi-axial dynamic states of stress. The material studied was a closed cell PVC foam, an orthotropic/transversely isotropic material, exhibiting strain-rate-dependent elastic/viscoplastic behavior. The material was characterized at three strain rates, quasi-static (10^-4 s ^-1), intermediate (1 s^-1) and high (10³ s ^-1). Quasi-static and intermediate strain rate tests were conducted in a servo-hydraulic testing machine. High strain rate tests were conducted under strain control using a split Hopkinson Pressure Bar (SHPB) system made of polymeric (polycarbonate) bars. The polycarbonate material has an impedance that is closer to that of foam than metals and results in lower noise to signal ratios and longer loading pulses. It was determined by analysis and verified experimentally that the loading pulses applied, propagated along the polycarbonate rods at nearly constant phase velocity with very low attenuation and dispersion. It was found that the initial elastic properties did not vary with strain rate as opposed the first peak stress which increased noticeably with strain rate.