Size effects in a power law creeping layer under compression or shear, and implications for deformation mechanisms of lithium films

Abstract

The axisymmetric compression of a power law creeping metallic sandwich layer of micron-scale thickness is analysed. Account is taken of the elevation in flow strength due to the presence of a spatial gradient in plastic strain rate. Numerical and analytical solutions reveal that the average compressive traction is enhanced by a combination of strain rate gradients and plastic constraint. A similar size effect is predicted for simple shear of the creeping sandwich layer. The difference in responses for compression and shear is traced to the different profiles of shear strain rate through the thickness of the layer. The sensitivity of compressive and shear strengths to the choice of higher-order boundary condition is explored, and good agreement with recent experiments on compression and shear of a thin sandwich layer of lithium is achieved by assuming fully clamped higher-order boundary conditions and a material length scale on the order of 3−5μm in the strain gradient-based creep theory.