Heterogeneities induced by uniaxial compression and resulting errors in material behavior assessment

A numerical investigation on uniaxial compression tests is performed to highlight the heterogeneous nature of the deformation process. Resulting errors on the material behaviors are deduced from the obtained force versus displacement data with the assumption of homogeneous deformation. The numerical study considers a range of strain rates (from 0.01 to 0.5 (s^-1)), Coulomb friction coefficients (up to 0.3), and elasto-viscoplastic power law behaviors. The heterogeneous nature is characterized in terms of sample shape, strain, and strain rate heterogeneities. The results show that the final shape of the sample at a given macroscopic strain is influenced not only by the friction coefficient but also by the material properties. The levels of strain and strain rate heterogeneities in the samples can be significant in some conditions, leading to large errors when exploiting the force versus displacement data with the hypothesis of homogeneous strain. The estimations of the strain rate sensitivity parameters are not significantly affected by the strain heterogeneities, but the errors on the strain hardening parameters can be as large as 40 %. The apparent strain hardening parameter appears to be artificially strain rate sensitive. Being systematically lower than the material strain hardening parameter, when measured at lower strain rates, this underestimation will induce a systematic error in the determination of material properties and should be taken into consideration.