Experimental study on microscopic damage characteristics of granite under cyclic dynamic disturbances

To investigate the microscopic damage characteristics of rock materials under cyclic dynamic disturbance, granite was chosen as the research object, and the split Hopkinson pressure bar (SHPB) was used to outperform the cyclic impact test. Besides, with the nuclear magnetic resonance (NMR) technique, the porosity, crosswise relaxation time T₂ distribution and magnetic resonance image (MRI) of the rock sample were obtained. The results show that within the range of the rock's elastic limit, the porosity reduces after the dynamic disturbance. The initial and final transverse relaxation time T₂ shortens, which indicates the process of cyclic disturbance causes the formation of small pores, where the size and number of large pores decrease. The decrease of porosity does not mean that the number of all kinds of pores decreases; on the contrary, the number of small pores increases, and the decreased number of large pores is the root cause which leads to the decrease of porosity after dynamic disturbance.