Strain-gradient effects in anisotropic materials

The isotropic single-parameter strain-gradient plasticity model of Fleck and Hutchinson (2001 J. Mech. Phys. Solids 49 2245–71) is generalized in order to account for plastic anisotropy in a finite strain elastic–viscoplastic formulation. Plastic anisotropy is introduced through a phenomenological yield surface, which is described by a homogeneous function of degree 1. The anisotropy affects the plastic work done by the conventional effective plastic strain while the plastic work done by the strain-gradients remains unchanged. A modified measure of the effective stress arises naturally in the formulation. Local plasticity (isotropic and anisotropic) as well as nonlocal isotropic plasticity are special cases of this anisotropic nonlocal plasticity model. Results for an anisotropic nonlocal aluminium containing voids are obtained by a finite element implementation of the model.