Intermediate and high strain-rate testing of soft materials

Strain-gauged bars are often employed as load cells for direct impact testing of materials and are incorporated within the split Hopkinson pressure bar (SHPB). Low impedance bars (e.g., magnesium or polymer bars) are desirable when testing soft specimens such as various energetic materials and cellular solids. However, due to the rheological properties of polymer bars, wave dispersion and attenuation occurs. For relatively large diameter bars and high frequency waves, geometrical wave dispersion due to radial inertia also occurs. In this paper the spectral finite element method (SFEM) is applied to the SHPB to obtain the stress-strain curves of specimens under investigation by an inverse analysis. The method presented makes use of a higher-order rod approximation, applicable to viscoelastic bars, that accounts for wave dispersion. To demonstrate the technique experimental results for balsa wood using both magnesium alloy and PMMA bars are provided.