Micromechanical Studies of Strain Rate Dependent Compressive Strength in Brittle Polycrystalline Materials

We propose a novel computational model for the high fidelity prediction of failure mechanisms in brittle polycrystalline materials. A three-dimensional finite element model of the polycrystalline structure is reconstructed to explicitly account for the micro-features such as grain sizes, grain orientations, and grain boundary misorientations. Grain boundaries are explicitly represented by a thin layer of elements with non-zero misorientation angles. In addition, the Eigen-fracture algorithm is employed to predict the crack nucleation and propagation in the grain structure. In the framework of variational fracture mechanics, an equivalent energy release rate is defined at each finite element to evaluate the local failure state by comparing to the critical energy release rate, which varies at the grain boundaries and the interior of grains. Moreover, constitutive models are considered as functions of the local microstructure features. As a result, a direct mesoscale simulation model is developed to resolve the anisotropic response, intergranular and transgranular fractures during the microstructure evolution of brittle materials under general loading conditions. A micromechanics-based interpretation for the rate dependent strength of brittle materials is derived and verified in examples of dynamic compression tests. In specific, the compressive dynamic response of hexagonal SiC with equiaxed grain structures is studied under different strain rates.