Crack propagation in brittle materials under compressive stresses studied by caustics

Abstract

An experimental study was undertaken concerning the propagation of a slant crack under compression. By using the equations of the deformed shape of a crack and by introducing a correction model in order to prevent the crack lips from incompatible displacements, an estimate of the stress distribution along the crack borders was achieved. It was found that an opening-mode K1 stress intensity factor (SIF) must be introduced at the crack tips in the overall compressive stress field, in order to give the required space for the lip-slip phenomenon, due to shearing, to take place. This local dilatation in the vicinity of the crack tip, together with the lip-slip phenomenon, due to which the initial crack tip is displaced into a new position along the deformed crack borders, causes the out-of-plane propagation of the crack, either towards the largest compressive stress in the case of biaxial stress field, or towards the applied compression in a uniaxial compressive field. A series of experiments on PMMA (perspex) rectangular specimens with pre-existed cracks and slits has been executed and the type of the stress field in the front of the propagated branches is examined by using the method of caustics. It was found that the crack propagation under compression is an interactive process of two conjugate branches, which is strongly influenced by the boundary conditions of the pre-existed discontinuity.