EFFECTS OF LOW-ENERGY DISLOCATION-STRUCTURES ON CRACK-TIP SHIELDING IN IONIC-CRYSTALS

Dislocation structures near the tip of a crack and its effect on the l ocal stress intensity factor in NaCl crystals are investigated by usin g the etch pit technique and the photoelastic method. In the early sta ge of slip activity, shielding-type dislocations are introduced from a crack tip and their arrangement indicates the presence of a dislocati on-free zone. By using the photoelastic method, it is surely corrobora ted that those dislocations cause crack tip shielding to raise the val ue of fracture toughness in the early stage of dislocation generation. On the other hand, when slip activities are increased around the crac k tip, dislocations of antishielding type as well as shielding type ar e introduced to produce slip bands consisting of high density dislocat ions and the dislocation-free zone disappears, suggesting that the inc rease in the activity of dislocations does not necessarily contribute to raising the value of fracture toughness in NaCl crystals. Neighbori ng dislocations of shielding and antishielding type mutually screen th eir long-range stress fields, so that their introduction indicates the formation of a low energy dislocation structure (LEDS). Thus, LEDS fo rmation may have a marked influence on fracture behavior in NaCl cryst als.