Simulations of microcracking in the process region of ceramics with a cell model

Microcracking around a macrocrack and the consequential toughening in polycrystalline ceramics are simulated. The objective is to check the hypothesis that the suppression mechanism for opening of off-side microcracks does not work in certain ceramic materials because cracks open up, assisted by residual stresses, at a much reduced load in some grains, while, still encountering a high resistance to crack growth at the grain boundaries. A two-dimensional cell model of a polycrystalline material is investigated. Each cell represents one grain. The load-deformation law for the cell is assumed to contain two load peaks. The first peak is associated with microcrack nucleation in the grain, while the second peak is related to the resistance that the microcrack meets at the grain boundary. The cells are included in a finite element model. Grain to grain variations, for instance due to residual stresses, are taken into account by a Weibull distribution of the first load peak. Results from the simulations show that variations of the propensity for microcrack nucleation between different grains constitute a major factor responsible for the generation of microcrack clouds. Such cloud formation would otherwise be impeded by unloading effects from central microcracks. In addition, and in accordance with observations, the simulations also show high fracture energies (compared to what would be expected for a typically brittle material), as well as a period of stable crack propagation.