A multi-scale approach to modelling size effect in crystal plasticity was presented. At the micro-scale, discrete dislocation dynamics coupled with finite element analysis permitted rigorous treatment of a broad range of micro-plasticity problems while making minimal phenomenological assumptions. At the macro-scale, a gradient crystal plasticity model, which incorporated scale-dependence by introducing the density of geometrically necessary dislocations into the expression for mean glide-path length, was used. Bending of micro-sized single-crystal beams was considered and the correspondence between the predictions of both models was made. The macro-scale model did not capture the experimentally observed effect of specimen size upon initial yield stress. Since this effect was incorporated naturally into the corresponding dislocation-dynamics analysis, it was concluded that there was no density-independent size effect in the expression for the strength of slip systems. In separate work on the tensile loading of μm-sized polycrystals, a size effect was identified which depended upon the size and location of Frank-Read sources relative to grain boundaries,. This effect could be generalized within the context of dislocation/interface interactions which were typically missing from current gradient crystal plasticity models and which could, in principle, be used to understand an initial yield size-dependence in single-crystal bending.

Multiscale Modelling of Size Effect in FCC Crystals - Discrete Dislocation Dynamics and Dislocation-Based Gradient Plasticity. F.Akasheh, H.M.Zbib, T.Ohashi: Philosophical Magazine, 2007, 87[8-9], 1307-26