Influence of the T-stress on the Crack Bifurcation Phenomenon in Ceramic Laminates

Abstract

The main goal of ceramic laminates designed with residual stresses is to increase the fracture energy of the system during fracture through energy dissipating mechanisms such as crack deflection or crack bifurcation.The angle of crack deflection from the straight propagation direction is influenced mainly by level of the residual stresses in the laminate layers, which was investigated in previous works of the authors. Another key factor playing role in the crack deflection and presence of the crack bifurcation phenomenon is the thickness of the layer to which the crack propagates. This factor has not been studied yet and will be thus essence of this contribution where the combined effect of compressive residual stresses and layer thickness is numerically investigated. Three different multilayer designs of Al2O3-ZrO2 based ceramic materials are modelled using a non-periodic and symmetric architecture (i.e. layers of a given material may not have the same thickness within one specimen, but they are symmetrically distributed with respect to the laminate mid-plane). The volume ratio of both material constituents is kept constant, so that the residual stresses in the different laminates remain unchanged. Varied is only the thickness of the second compressive layer (where the bifurcation phenomenon is observed). A crack terminating perpendicularly on the first interface with the compressive layer is considered as the initial state before application of the four point bending. Using the fracture criterion, based on the Finite Fracture Mechanics, the angle and type of crack propagation (single crack deflection/bifurcation) is investigated. In these cases, the level of the T-stress (in the vicinity of the crack tip) can play a role as a second parameter in the fracture criterion (next to the generalized stress intensity factor) and as a consequence can this term influence the predicted crack propagation behaviour.