Crack Tip Plasticity in Ionic Crystals with the NaCl-Type Structure

Abstract

Dislocation structures near the tip of a crack in ionic crystals with the NaCl-type structure were investigated by using an etch-pit technique, photoelasticity and high voltage electron microscopy (HVEM). The characteristics of those dislocation structures are reviewed and their effect on the local stress intensity factor is discussed. We focused on two kinds of plastic zones developed near a crack tip. Firstly, in bulk NaCl crystals, the so-called hinge-type structure of slip bands were formed along {110} planes ahead of a {100} crack. The effect of crack tip shielding due to dislocations was demonstrated using a photoelastic method visualizing the internal stress field around a crack tip. Secondly, dislocation configurations in MgO thin crystals were analyzed using HVEM, where dislocation arrays of the {110}〈1\\bar10〉 slip system corresponding to the plastic zone of the 45°-shear-type were formed around a {100} crack. 3-D stress analyses for the crack-dislocation interaction indicates that the dislocations observed contribute to mainly the mode I shielding to suppress the crack extension.