Deformation characteristics of granites at different unloading rates after high-temperature treatment

Naturally cooled granite specimens pretreated at 800 °C were separately subjected to conventional triaxial compression tests and triaxial confining pressure unloading tests under a constant deviatoric stress. Via these tests, the deformation characteristics of granite at different confining pressure unloading rates were explored. The research results showed that the granite specimens were more easily damaged at an intermediate unloading rate than at a high or low unloading rate. The strain–confining pressure compliance results quantitatively revealed that rock failure during unloading was caused by intense radial deformation and volumetric dilatancy. With the increase in the unloading rate, the strain–confining pressure compliance first increased and then decreased. In the confining pressure unloading process, initially, the elastic modulus of the rocks linearly decreased with the decreasing confining pressure. In the case that the confining pressure was unloaded to the critical point where rock failure occurred, the elastic modulus of the rocks dramatically decreased. Additionally, Poisson’s ratio gradually decreased with the reduction in confining pressure. However, approaching the critical point where rock failure occurred during unloading, Poisson’s ratio remarkably decreased. Afterwards, Poisson’s ratio increased to 0.5 and then remained unchanged until the specimens became damaged.