Dual role of microcracks: toughening and degradation

One of the methods of improving the mechanical properties of ceramics is to introduce a defective structure that acts as a restraint for the propagati on of cracks. In the present study a detailed investigation was carried out by introducing a defective structure in rock to determine if there is any improvement in properties similar to ceramics. Granite was chosen for the i nvestigation, and the microcracks were introduced by a heating and cooling cycle. Uniaxial compression tests have shown that granite thermally treated to 200 degreesC shows the highest strength, and the strength of granite tr eated to 400 degreesC is comparable to that of unheated granite. Both ultra sonic images and acoustic-emission monitoring show that for thermally treat ed samples the stress-induced microcrack and macrocrack nucleation and thei r growth are retarded. The variations in mechanical properties are explaine d based on the concept of toughening and degradation. Uniaxial compression tests on unheated and thermally treated granite samples have clearly establ ished the dual role of microcracks, which operate in the toughening and deg radation mechanisms.