Extended crystal plasticity theories are well established to study size-dependent hardening of metals. Surface and inner grain boundary conditions play a significant role for crystals at small scales as they affect the dislocation activity and, hence, alter strength and strain hardening behavior. Conventional micro boundary conditions, i.e., microhard and microfree, are unable to capture the underlying physics as they describe ideal and over-simplified surface/interface conditions. In this work, advanced boundary conditions for gradient extended crystal plasticity are introduced to map realistic conditions at external surfaces, interphases, or grain boundaries. They relate the magnitude of plastic slip to surface defect density and slip directions with respect to the surface normal. Characteristic features are highlighted, including the effect of surface yielding and size dependent surface strengthening.
