Influence of high temperature treatment on tensile strength and fracture evolution of coarse-grained granite under Brazilian splitting

High temperature induces thermal damage of granite and deteriorates its bearing capacity. To investigate the deteriorating characteristics and mechanism of granite subjected to high temperature, a series of Brazilian splitting tests were conducted on granite discs after thermal treatment with various temperatures and cycles. The results indicate that high temperature is the crucial factor that decreases the mass, longitudinal wave velocity (VP) and tensile strength of granite. Thermal damage based on VP can better reflect the generation of thermally induced cracks and the expansion of granite structure. Temperature and thermal cycling number significantly impact the number, type, magnitude, and distribution of the cracks throughout the splitting process. Specifically, tensile cracks dominate the fracture process, while shear cracks increase rapidly near the peak load, coalescing into large-scale fractures. The spatial distribution of acoustic emission (AE) events becomes more dispersed as temperature elevates, while the number of AE events decreases with increasing thermal cycles, due to the enlargement of the porous structure in rock. This enlargement further reduces both the occurrence areas and propagation paths of AE signals. Elevated high temperature alters the redistribution of the equivalent stress within the granite disc and leads to the shift of strain concentration zone from two ends to the centre of the disc, expanding into a strip-like shape and penetrating the disc. Temperature prominently influences the fracture characteristics of granite. Elevated temperature induces numerous thermal cracks within the rock, and alternating thermal stress caused by thermal cycling also stimulates the propagation and connectivity of micro-cracks and further results in the expansion of porous structure of granite, promoting transition of fracture properties from brittleness to ductility. These findings help in understanding how thermal cycling at high temperatures deteriorates the physical and tensile fracture behaviour of rocks, providing valuable insights for associated thermal engineering problems.