Evolution of the mechanical and strength parameters of hard rocks in the true triaxial cyclic loading and unloading tests

Rock fracture process is associated with the degradation and mobilization of the mechanical and strength properties of rock. A number of true triaxial cyclic loading and unloading tests were carried out on three types of hard rocks (i.e., granite, marble and sandstone) to investigate the evolution of the rock properties as a function of accumulated damage. In addition to stress and strain measurements, acoustic emission (AE) signals were monitored and used to associate different stages of crack development with different stages of the mechanical and strength parameter evolution. Young's modulus and five strain increment ratios were calculated from the measured increments of stresses and strains in each cycle. It was found that hard rocks exhibited profound deformation anisotropy and directional dilation. A general trend of decreasing volumetric strain reversal stress and peak stress with increasing plastic strains was observed for all the tested specimens. The evolution of the strength properties of rock under true triaxial compression was investigated using a linear 3D hard rock failure criterion and was found to be generally consistent with the cohesion weakening and frictional strengthening concept. The influence of the intermediate principal stress on the evolution of the rock properties was also discussed.