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Finite element models, i.e. 2D honeycombs and 3D tetrakaidecahedron models, have been established to study defect-effect on the creep behavior of the cellular materials. Simulation results indicated that relative density have dominant effects on the steady state creep rate of the cellular materials. The creep behavior of the cellular materials is significantly correlated with the fraction of missing struts. A theoretical equation is proposed to predict the creep rate of cellular materials as a function of the ratio of different types of missing struts. The capability of the theoretical model was examined by simulating the creep behavior of two types of numerical models. The simulation results revealed that the theoretical model gives a good description of the creep response.

Three different types of tests, i.e. uniaxial compression, shear and combined shear-compression have been conducted on PMMA (polymethyl methacrylate) at different loading rates in order to investigate its failure behavior under different loading conditions. The experiment results show that the material exhibits the rate-dependent failure and post-failure behavior. Macroscopic failure loci of PMMA were was determined experimentally in the shear-normal stress space. Further, a macroscopic failure criterion was proposed to predict the material's failure locus as a function of the loading rate. The capability of the failure criterion was examined by simulating the failure behavior of the material at different load rates. The simulation results revealed very good agreement between the experimental data and the responses determined from the proposed failure criterion.