Determination of the wave propagation coefficient of viscoelastic SHPB: Significance for characterization of cellular materials

Abstract

Test bars made of viscoelastic materials are frequently employed for the testing of soft materials, using split Hopkinson pressure bar (SHPB) techniques, because of their low mechanical impedance. Determination of the propagation coefficient for such bars is a critical step for the subsequent evaluation of the material properties of the specimen. This propagation coefficient may be determined through experiments or using the analytical solutions if the material properties of the bars are known in advance. Contrary to the case of elastic materials, it is difficult to provide generic properties for such materials as these are dependent on the loading rate, environmental history and manufacturing conditions. Many studies may be found in the open literature reporting numerical values of the identified parameters for various viscoelastic materials evaluated through the wave propagation experiments. However, the observed scatter among such data in the case of individual materials dictates that the published parameters should be used with caution.Two polymethyl methacrylate (PMMA) bars, used as incident and transmitter bar in an SHPB test setup, are being subjected to the wave propagation testing. Longitudinal strains, generated as a result of axial impact of strikers with two different lengths and recorded at the mid-length of the bars, are used to determine the wave propagation coefficient. Propagation coefficients are also evaluated using selected material models of PMMA published in the literature. A considerable scatter is found in the evaluated frequency dependent propagation coefficient. The consequence of using such scattered properties for the bars on the results of the stress–strain response of aluminum foam is being investigated. Although, the evaluated dynamic properties of the tested foam are not considerably influenced in quantitative terms, however qualitative differences are observed.