Crystal plasticity model with enhanced hardening by geometrically necessary dislocation accumulation

A strain gradient dependent crystal plasticity approach is used to model the constitutive behaviour of polycrystal FCC metals under large plastic deformation.Material points are considered as aggregates of grains, subdivided into several fictitious grain fractions: a single crystal volume element stands for the grain interiorwhereas grain boundaries are represented by bi-crystal volume elements, each having the crystallographic lattice orientations of its adjacent crystals. A relaxedTaylor-like interaction law is used for the transition from the local to the global scale. It is relaxed with respect to the bi-crystals, providing compatibility and stressequilibrium at their internal interface. During loading, the bi-crystal boundaries deform dissimilar to the associated grain interior. Arising from this he! terogeneity, ageometrically necessary dislocation (GND) density can be computed, which is required to restore compatibility of the crystallographic lattice. This effect provides aphysically based method to account for the additional hardening as introduced by the GNDs, the magnitude of which is related to the grain size. Hence, ascale-dependent response is obtained, for which the numerical simulations predict a mechanical behaviour corresponding to the Hall¯Petch effect. Compared to afull-scale finite element model reported in the literature, the present polycrystalline crystal plasticity model is of equal quality yet much more efficient from acomputational point of view for simulating uniaxial tension experiments with various grain sizes.