Crystal plasticity prediction of Lankford coefficient using the MULTISITE model: influence of the critical resolved shear stresses

The MULTISITE model [1] is based on polycrystalline plasticity and the underlying hypotheses of the model are (i) that the deformation of each grain is significantly influenced by the interaction with a limited number of adjacent grains, and (ii) that local strains deviate from their macroscopic average according to specific “relaxation modes”. The LAMEL model [2] is reformulated into the more general elastic-viscoplastic MULTISITE model permitting various relaxation modes. This model has been validated for cubic materials but hexagonal close-packed (HCP) crystals usually demonstrate larger anisotropy than cubic crystals. The model was used to simulate uniaxial tensile tests performed on rolled sheets made of Ti-6Al-4V. The Lankford coefficients (r) calculated in various directions in the plane of the sheet were analysed. In this study, different grain interaction hypotheses were tested. Besides, it appeared that the value of the critical resolved shear stresses (CRSS) of the different slip system families of the HCP metal had significant effects on the results. Their influence as well as the influence of the strain rate sensitivity parameter was examined.