Effect of thermal treatment on the fracture toughness and subcritical crack growth of granite in double-torsion test

An understanding of the fracture behavior and subcritical crack propagation of hard rock under different temperatures is necessary for the design and stability of nuclear waste disposal projects. The aim of this study was to understand the thermal effect on the fracture behavior of granite, which had been heated at 25 (untreated), 100, 200, 400, and 600 degrees C, respectively. A series of uniaxial compressive tests and double-torsion tests were performed on granite specimens. The three-dimensional digital image correlation technique was used to investigate the strain evolution and fracture pattern in granite specimens during subcritical crack propagation. According to the results, the uniaxial compression strength, Young's modulus and fracture toughness of the granite specimens increased and then decreased with an increase in treatment temperature. These parameters dropped sharply as the treatment temperature exceeded 400 degrees C, mainly because of the thermally-induced destruction of the quartz crystal structure and a generation of wider intergranular fractures. A comparison of the subcritical crack length by theoretical calculations and experimental observations from digital image correlation showed that the two values were consistent. At a treatment temperature of 400 degrees C and 600 degrees C, the velocity of the subcritical crack propagation was accelerated, so the length of the subcritical crack during the relaxation test was 4.14 times and 5.25 times that at 25 degrees C, respectively. The subcritical crack growth and fracture process of the double-torsion tests were visible by the three-dimensional digital image correlation system. A subcritical crack growth index (n) can be used to characterize the susceptibility of the subcritical crack propagation in rock relaxation tests, which helps to evaluate the long-term stability of nuclear waste disposal projects.