Microstructural variations and damage evolvement of salt rock under cyclic loading

Fatigue tests under different maximum cycling stresses were performed on salt rock specimens to study the microstructural variations and damage evolvement of salt rock under cyclic loading. A nuclear magnetic resonance (NMR) instrument was utilized to record the microstructural variations in the specimens. The results indicated that with an increase in the number of cycles, the maximum pore radius in the salt rock initially increased and then remained constant, whereas the minimum pore radius initially decreased and then increased. The minimum pore size in the salt rock after 12,000 cycles was closely related to the maximum cycling stress. The porosity and permeability of the salt rock exhibited a two-stage change of "rapid increase - slow increase" and "slow increase - rapid increase", respectively, with an increase in the number of cycles. In contrast, the tortuosity presented a two-stage process of "rapid decrease - slow decrease" with an increase in the number of cycles. With an increase in the maximum cycling stress, the porosity and permeability of the salt rock after 12,000 cycles increased nonlinearly, whereas the tortuosity decreased linearly. The permeability exhibited a gradually ascending trend with an increase in the porosity. However, a critical porosity was observed. When the porosity was lower than that of the threshold, the change in porosity had a minor effect on permeability, whereas the effect of porosity on the permeability gradually increased when the porosity exceeded the threshold. The value of critical porosity of the salt rock specimens used in this paper was approximately 0.35%. The fatigue damage evolvement equation of salt rock under cyclic loading was established based on the change in porosity. The damage evolvement process of the salt rock specimens under different maximum stresses was analyzed. The results demonstrated that the fatigue damage evolvement process of salt rock consisted of three stages. However, the damage in the first two stages was relatively small, and the damage increased rapidly only when the salt rock was close to failure.