Review Crack growth in elastically damaged materials

Brittle, polycrystalline and polyphase materials such as ceramics and fibre-reinforced brittle composites contain residual thermo-mechanical stresses from manufacturing. These stresses are concentrated at sites of microstructural inhomogeneities such as grain and phase boundaries. The nucleation and growth of microcracks can minimize the local micro-strain energy density; thus, the local, residual stresses can act as nuclei for microcracks. The density of nuclei, statistically distributed within the material, depends on grain size, i.e. the distance between nuclei, with defined values of micro-strain energy density, is material specific. Stress-induced microcracking can act as an attractor for elastic damage at the local scale to produce a process zone that acts as a sink of strain-energy release on a larger scale, for example, the process zone at a crack front. It can be shown that the stress-rate dependent growth of local damage follows a power law which quantifies strengthening and softening during slow crack growth, prior to catastrophic crack extension. The damage-induced zone, produced by the release of strain energy on the local scale, can shield the macrocrack and grow to a critical value at the failure load. The influence of the microstructure on damage will be quantified and related to sub-critical and critical crack extension in brittle materials.