Characterization of polymeric foams under multi-axial static and dynamic loading

An orthotropic polymeric foam with transverse isotropy (Divinycell H250) used in composite sandwich structures was characterized under multi-axial quasi-static and dynamic loading. Quasi-static tests were conducted along principal material axes as well as along off-axis directions under tension, compression, and shear. An optimum specimen aspect ratio of 10 was selected based on finite element analysis. Stress-controlled and strain-controlled experiments were conducted. The former yielded engineering material constants such as Young's and shear moduli and Poisson's ratios; the latter yielded mathematical stiffness constants, i. e., C_ij. Intermediate strain rate tests were conducted in a servo-hydraulic machine. High strain rate tests were conducted using a split Hopkinson Pressure Bar system built for the purpose. This SHPB system was made of polymeric (polycarbonate) bars. The polycarbonate material has an impedance that is closer to that of foam than metals. The system was analyzed and calibrated to account for the viscoelastic response of its bars. Material properties of the foam were obtained at three strain rates, quasi-static (10^-4/V^-1), intermediate (1 s^-1), and high (10³s^-1) strain rates.