Effect of strain rate sensitivity in visco-plastic modeling

Many crystal plasticity models use a rate sensitive law for its convenience. The rate sensitivity can be determined experimentally and input in the models. From a numerical point of view, high values of the strain rate sensitivity parameter m favor easy convergence of these models, but when m is fixed arbitrarily, which is in practice done for slightly rate sensitive materials, the physical meaning of m is lost. The present study shows that the flow stress σ, slip systems activity γs, strain anisotropy and simulated texture are strongly dependent on m. Precautions should be taken when predicting the mechanical behavior of polycrystals. To simulate nearly rate insensitive materials with models using a rate sensitive law, the use of the reference shear strain γ˙0 equal to the imposed strain rate is necessary but not sufficient. It is also necessary to use a high strain rate sensitivity parameter (m = 0.05) to correctly evaluate the material parameters, i.e., the critical resolved shear stresses (CRSS). This study also provides a relationship between the CRSS determined for different values of strain rate ɛ˙, m and γ˙0, so that CRSS determined in different studies can be compared.