In stress-strain relationship on the compression process of low-density porous materials, nonlinear deformation is observed with the plateau region where much strain is occurred without stress increasing. This phenomena is useful to design the shock absorber in various machines, however it also causes difficulties in the deformation analysis of the materials because of its nonlinearities. In this paper, some types of uniaxial compression testings are conducted for inspecting the

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... ation behavior of the materials in compression process. Here, we use a kind of polystyrene foam as a typical porous material with closed cellular structure. In the results of uniaxial compression testings with some constant rates, it is shown that the plateau stress, which is the stress level of the plateau region, has the strain-rate dependency. In the compression testing which transmits compression rates for the mechanical inspection of the dependency, it is revealed that the dependency relates to the behavior of well-known overstress phenomena. Therefore, the viscoplastic constitutive equation presented by Perzyna is adopted to simulate the uniaxial compression process for the representation of the dependency. Compared results of testings and simulation, it is indicated that the overstress phenomenon in the compression process of the material can be simulated by using the viscoplastic constitutive equation, and this technique has the ability to inspect the dynamic deformation process of the machines in which are implanted the materials.