Damage evolution and acoustic emission characteristics of hard rock under high temperature thermal cycles

Understanding the mechanical properties of rocks under high temperature thermal cycles is critically important for deep geotechnical engineering construction. In this study, the mechanical properties and fracture evolution characteristics of Beishan granite after different temperatures of 25–800 °C and thermal cycles were investigated through multiple experiments. The results show that the tensile strength (σ_t) decreased from 7.49 MPa to 0.47 MPa as the temperature increased from 25 ℃ to 800 ℃, and further decreased to 0.43 MPa after thermal cycling. Incremental temperatures led to more active AE events, with AE cumulative events increasing from 25,250 at 25 °C to 99,389 at 800 °C, but AE cumulative events decreased in thermal cycles. The b-value presented higher level fluctuations at T ≤ 400 °C. When T ≥ 600 °C, the b-value before failure decreased and maintained small dramatic fluctuations, but fluctuated upward in the post-peak failure stage, indicating that rock failure changed from brittle failure to plastic failure. The proportion of shear cracks increased substantially with temperature and thermal cycles, from 14.22% at 25 °C to 23.11% at 800 °C. It can be also found that the damage variable (D_AE) increased with temperature under the same condition of stress. The physical and chemical reactions within granite at high temperature led to the initiation, development, and connectivity of numerous microcracks, especially when T ≥ 600 °C. The alternating thermal stress generated by cyclic heating will further promote the increase and propagation of microcracks, thereby exacerbating the damage to granite.