Crack-tip shielding by dislocations and fracture toughness in NaCl crystals

Stress modification by dislocations around a crack tip and its effect on fracture toughness in NaCl crystals were investigated using a photoelastic method. NaCl crystals are very brittle and give small values of fracture toughness KIC at and below room temperature. The crystals become fairly ductile to deform over 50% in elongation and their apparent KIC values are increased at elevated temperatures. In crystals stretched at elevated temperatures, the KIC values are increased even at 77 K through a stress modification due to dislocations around a crack tip. The stress modification by dislocations is actually revealed in photoelastic images. A compressive stress field appears near the crack tip when an external load is removed after the introduction of dislocations. The strength of the compressive stress field estimated from a photoelastic image is nearly equal to the increment of KIC values due to dislocations at 77 K. The slip operation on both {110} and {001} planes, which provides five independent slip systems, takes place around a crack tip at elevated temperatures. The slip operation on {110} planes alone providing two independent slip systems is observed even at room temperature where KIC values are small. It is thus evident that the formation of strong kD fields takes place through the operation of a sufficient number of independent slip systems for increasing KIC values.