Dynamic Mechanical Properties and Damage Evolution in Granite under Coupled High-Temperature-Impact Cyclic Loading

To study the influence of high-temperature-impact dual-cycle cumulative damage on the dynamic mechanical characteristics of granite, specimens with a height-to-diameter ratio of 0.8 were subjected to high-temperature cycling treatments at 100, 300, and 500 ℃ for 2, 4, and 6 cycles, respectively. The P-wave velocities before and after treatments were measured. Constant-amplitude cyclic impact tests under gas pressures of 0.25, 0.30, and 0.35 MPa were conducted using a split Hopkinson pressure bar (SHPB) system. The effects of temperature, high-temperature cycles, impact gas pressure, and impact cycles on the dynamic behavior of granite were systematically investigated. Based on Lemaitre’s continuum damage constitutive model and the strain equivalence principle, cumulative damage factors for high-temperature and impact cycles were defined, and the critical dual-cycle cumulative damage factor was analyzed. Results indicate that with increasing initial high-temperature cumulative damage and impact gas pressure, the crack morphology of granite during the first impact evolved from single cracks to complex crack networks with increased connectivity, while peak stress decreased and peak strain increased. Significant variations in peak stress and strain were observed between the first and final impacts during cyclic loading. The influence of factors on the cumulative impact damage under high-temperature cycling followed the order: impact gas pressure>temperature>number of high-temperature cycles. The critical high-temperature-impact cumulative damage factor was determined to range between 0.625 and 0.676. These findings provide theoretical support for safety assessment in deep underground resource extraction.