Crack tip plasticity in ionic crystals with the NaCl-type structure

Dislocation structures near the tip of a crack in ionic crystals with the N aCl-type structure were investigated by using an etch-pit technique, photoe lasticity 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 zo nes developed near a crack lip. Firstly, in hulk NaCl crystals, the so-call ed 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 h eld around a crack tip. Secondly. dislocation configurations in MgO thin cr ystals were analyzed using HVEM, where dislocation arrays of the {110}[1 (1 ) over bar0] slip system corresponding to the plastic zone of the 45 degree s -shear-type were formed around a {100} crack. 3-D stress analyses for the crack-dislocation interaction indicates that the dislocations observed con tribute to mainly the mode I shielding to suppress the crack extension.