A gradient plasticity grain boundary yield theory

•A gradient crystal plasticity theory based on the gradient of an equivalent plastic strain is proposed.•The theory includes a grain boundary yield criterion.•A numerically efficient implementation is outlined.•The proposed model is able to capture the size effect in micro-specimens.•The material parameters are successfully fitted to the experiments of copper micro-wires from literature. A strain gradient crystal plasticity theory is presented that accounts for the resistance of grain boundaries against plastic flow based on an interface yield condition. This theory incorporates the previously presented numerically efficient visco-plastic treatment by the gradient of an equivalent plastic strain ∇γeq in Wulfinghoff and Böhlke (2012). The finite element implementation is discussed and the three-dimensional numerical model is fitted to experimental data of polycrystalline copper micro-tensile tests. The size dependent yield strength is reproduced notably well.