Sliding effect on branch crack

Abstract

Brittle materials (ceramics, rocks and oce) may contain a distribution of small, grain-sized, cracks. When loaded in compression, these cracks propagate stably until they interact to give final structure. In order to understand the branching process from defects in geological conditions, a set of branch fractures, formed under uniaxial loading at the tip of a pre-existing oblique slot, was studied in PMMA (polymethyl methacrylate) plates. In actual experimental fracture of brittle plate, an elliptical slit is cut in a plate of photoelastic material. Here, the slits were made sharper than in the previous method in order to decrease the applied stress necessary to cause crack growth. The growth of branch cracks was slow and could be stopped or started at will. Most of the cracks could be stopped before they reached the loaded ends of the PMMA plates. It was found that : 1.- Two classical branch cracks, formed from the tip of a central preexisting oblique slot in PMMA plate submitted to an uniaxial loading, are symmetrical with the defect center and rejoined the direction of the applied compression ; 2.- As for an open slot (no friction), the wing cracks were initiated at a certain distance from the initial crack tip ; 3.- For a given friction coefficient, this distance and the branching crack angle (which is less than 90°) increased with the slot-loading axis angle. The study of the stress field linked to an elliptical slot under pure shear conditions is in good agreement with our experimental observations.