Mechanical Properties and Energy Evolution of Granite Under Graded Constant-Amplitude Cyclic Loading

To clarify the mechanical response and energy evolution of granite under cyclic disturbance in underground engineering, triaxial graded constant-amplitude cyclic loading–unloading tests were carried out under confining pressures of 6–15 MPa. The stress–strain behavior, residual strain, deformation modulus, energy characteristics, and damage evolution were analyzed. The results indicate that increasing confining pressure significantly improves peak strength and restrains irreversible deformation. Residual strain occurs in every cycle and decreases rapidly after the first cycle before tending to stabilize; compared with that at 6 MPa, the first-cycle residual strain is reduced by 4.31%, 6.62%, and 9.91% at 9, 12, and 15 MPa, respectively. The hysteresis loops evolve from sparse to dense distributions, suggesting progressive compaction and adjustment of pre-existing defects from a macroscopic mechanical perspective. The loading and unloading deformation moduli both increase with confining pressure, with the unloading deformation modulus consistently exceeding the loading deformation modulus. The total input, elastic, and dissipated energies all increase with stress level and confining pressure, whereas the energy dissipation ratio decreases from 15.00% at 6 MPa to 11.35% at 15 MPa in the first cycle. Higher confining pressure therefore suppresses damage-related energy dissipation and promotes elastic energy storage. The damage variable increases with cycle number but is significantly inhibited at higher confining pressures. These results provide experimental support for evaluating the stability of granite in underground rock structures subjected to cyclic loading.