Large plastic strains in surface grains

Abstract
Many experimental studies have shown that during FCC single crystals tensile tests, the escape of dislocations through the free surface can be large. This induces more hardening in the bulk than near the free surface. In the framework of continuum mechanics and using very simple plasticity laws, our aim is the evaluation of plastic strains in surface grains. Surface and bulk grains are considered and the escape of dislocations is modelized approximately using perfect plasticity. Finite Element computations show that the effect of the boundary condition σ.n=0 is small when the macroscopic plastic strain is large. But the coupling of the boundary condition and of the perfect plasticity induces very large plastic strains in the surface grains. In this case, the strains are heterogeneous in the surface grains. Finally, the influence of the free surface is compared between large and small macroscopic plastic strains.