Effect of loading direction on grain boundary failure under shock loading

We investigate the effect of grain boundary inclination with respect to the loading direction on void nucleation at a boundary, using plate impact experiments on polycrystalline copper. Examination of damaged specimens reveals that boundaries perpendicular to the loading direction are an order of magnitude more susceptible to failure than those parallel to the loading direction. We investigate the mechanisms and reasons behind this experimental observation through molecular dynamics (MD) simulations, as a function of loading direction, in a copper bicrystal. Two extremes of loading directions are considered, either parallel or perpendicular to the grain boundary plane, spanning the range that grain boundaries within a polycrystalline sample will ordinarily experience under uniaxial strain conditions. Using MD simulations, we demonstrate that, during shock compression, the ability of a boundary to undergo plastic deformation is altered measurably by changing the loading direction with respect to the boundary plane. This change in the plastic response of the GB affects the development of stress concentrations believed to be responsible for void nucleation. MD simulations show that boundaries perpendicular to the loading direction do not undergo as much plastic deformation, by dislocation emission, as those parallel to the loading direction. The lack of plastic deformation at the GB, in the perpendicular loading case, can decrease the stress required for void nucleation. The MD results are consistent with experimental observations, and support the contention that plastic response of a grain boundary under shock compression can be a contributing, or even dominating, factor in determining the stress for void nucleation.