Mechanical properties of rock salt under combined creep and fatigue

The mechanical performance of rock salt under combined creep and fatigue plays a critical role in the long-term stability and serviceability of underground compressed air energy storage (CAES). In the present study, laboratory tests were performed to investigate the impact of simultaneous creep and fatigue on the hysteresis behavior, deformability, progressive damage, cycling life, and fracture mechanism of rock salt. The strain-time curves undergo a three-phase (transient, steady-state and tertiary phase) S-shaped evolution, which is similar to that observed in pure creep or fatigue tests. It has been demonstrated that the presence of a stress dwelling period does not change the geometric similarity of the cumulative deformation curve, but may accelerate the accumulation rate of inelastic deformation, leading to a reduction in the cycling life of rock salt. The consistency between acoustic emission (AE) counting rate and the volumetric deformation indicates that micro-cracks are primarily generated during the loading period and grow stably during the stress dwelling period in rock salt within a single loading cycle. The cumulative cyclic damage and time-dependent damage in rock salt promote each other, which results in a notable positive interaction between them. The life of rock salt specimens can be reasonably predicted by employing a nonlinear cumulative damage rule that incorporates an interaction term. The logarithm of the deformation rate in the steady-state phase is linearly correlated with the logarithm of the creep or fatigue life, regardless of the loading history, and serves as an alternative approach for predicting the life of rock salt. It seems that the typical failure pattern tends to transform from multiple shearing to axial splitting as the creep load fraction increases. For rock salt in the combined creep and fatigue test, a mixed microcosmic intergranular and transgranular fracture was observed using a scanning electron microscope.