Slip distribution and fracture in magnesium oxide

It is shown that as-cleaved magnesium oxide may go through several distinguishable stages of deformation prior to fracture. The exact mechanism of fracture depends on the stage at which it actually occurs. In all cases fracture occurs by a dislocation configuration localizing strain energy near a sub-critical microcrack. This supplements the energy available from the overall elastic strain field during crack propagation, and so assists fracture. Particular dislocation configurations may be induced artificially by sprinkling as-cleaved crystal surface with carborundum powder. It is shown that under these conditions a mechanism can occur which is consistent with one proposed by Orowan in 1934.