Crystal plasticity modeling of the cyclic behavior of polycrystalline aggregates under non-symmetric uniaxial loading: Global and local analyses

When a geometry is cyclically loaded under a mean stress or strain, incremental strain ratcheting or mean stress relaxation are observed. Experiments shows that for metallic materials there is non-zero mean stress as well as saturation of macroscopic strain ratcheting, and most macroscopic models produce both quantities in excess. Little attention has been paid to model such phenomena using polycrystal aggregates especially going up to the regime of cyclic mechanical stability. In this paper it will be shown that the interaction between different grains is enough to cater for such complex phenomena using crystal plasticity models for FCC crystals. Light will be shed on how different regions accommodate each other and how the classical definition of constant rate strain ratcheting or a zero mean stress is nearly impossible in a virtual polycrystal. More importantly, it will be shown that even if there is a macroscopic stable hysteresis stress strain loop, local stabilization is not guaranteed. The distribution of different constitutive quantities within a polycrystal are also analyzed which give a new insight into what is happening inside a polycrystal. Specifically, within a polycrystal, accumulated plasticity divides into two parts, and it is postulated that this division of plasticity gives the material its capability to retain its cyclic mean stress or a saturating ratcheting strain. Two numerical tests to detect strain ratcheting will also be presented. To go up to asymptotic values, as well as to get an unbiased solution a strictly rate-independent model will be used