Predicting small-offset yield surfaces with polycrystal plasticity

The small-offset yield surfaces of aluminium deformed in uniaxial tension, and free-end torsion to various finite strain levels are predicted using a binary tree-based polycrystal plasticity model. The inelastic response of the model grains is taken to obey rate-independent plasticity, and anelasticity. The substructural state of each grain, comprised of dislocation densities, slip system hardness, backstress, and friction stresses are evolved during the deformation. Model parameters are algorithmically fitted to the measured yield surfaces of aluminium 1100 after uniaxial tensile deformation, as reported in the literature. With the same parameters, the model accurately captures the subsequent yield surfaces after free-end torsion also. Analysis of the model parameters reveals that coplanar interactions are mostly responsible for the sharp curvature at the nose of the yield surface. Also, anelastic strains, aided by backstress, are found to be essential to explain the large experimental Bauschinger effect.