Experimental study on damage evolution and failure precursor characteristics of granite under thermal shock cycles

Investigating the damage evolution of surrounding rock under thermal shock cycles is crucial for ensuring the stability of engineering rock masses. This study performed Brazilian splitting tests on granite specimens under varying temperature and cycle conditions, employing acoustic emission monitoring, digital image correlation, and three-dimensional scanning technology. A systematic analysis was conducted on the patterns of damage evolution, failure precursor, and response mechanisms under combined thermal and cyclic loading. Experimental results show that both P-wave velocity and tensile strength degrade significantly with increasing temperature and cycle count, with temperature having a more pronounced effect than cycle count. Notably, damage evolution exhibits a dual-threshold behavior in which degradation accelerates markedly above 400 °C and stabilizes after 5 thermal cycles. Fracture surfaces evolve from initially planar to rugged morphologies, with peak-valley height differences at 600 °C being approximately three times greater than those at 200 °C. Furthermore, based on acoustic emission energy entropy analysis, we introduce a novel failure precursor indicator where the sustained increase and critical surge in average entropy serve as reliable early-warning signals for impending rock failure. These findings establish a solid theoretical basis and practical methodology for damage assessment and instability early-warning systems in high-temperature rock engineering.