Phenomenological modelling of rate-dependent plasticity for high strain rate problems

The results of two related theoretical investigations for large-scale computations of elasto-plastic deformations at ultrahigh strain rates are summarized in this paper. The first effort concerns the development of a phenomenological constitutive model for finite deformation elasto-plasticity which includes the effects of thermal softening, strain hardening, rate-dependence, as well as the noncoaxiality of the plastic strain rate and the stress deviator, and the incorporation of this model in a large-scale explicit finite-element code. The second effort involves the investigation of localized deformations and shear banding at high strain rates. It is shown that the constitutive model considered, together with standard quadrilateral finite elements with one point integration (piece-wise constant strain and stress fields), can nicely produce the observed intense localized deformations without recourse to any special elements. The results are illustrated in terms of the shear localization observed in uniaxial extension, and of void collapse under uniaxial compression. In addition, the effect of the noncoaxiality of the plastic strain rate and the stress tensor is included in the model, and its influence on strain localization at high strain rates is investigated.