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 local stress intensity factor in NaCl crystals are investigated by using the etch pit technique and the photoelastic method. In the early stage of slip activity, shielding-type dislocations are introduced from a crack tip and their arrangement indicates the presence of a dislocation-free zone. By using the photoelastic method, it is surely corroborated that those dislocations cause crack tip shielding to raise the value of fracture toughness in the early stage of dislocation generation. On the other hand, when slip activities are increased around the crack tip, dislocations of antishielding type as well as shielding type are introduced to produce slip bands consisting of high density dislocations and the dislocation-free zone disappears, suggesting that the increase in the activity of dislocations does not necessarily contribute to raising the value of fracture toughness in NaCl crystals. Neighboring dislocations of shielding and antishielding type mutually screen their long-range stress fields, so that their introduction indicates the formation of a low energy dislocation structure (LEDS). Thus, LEDS formation may have a marked influence on fracture behavior in NaCl crystals.