Experimental study on the damage and fracture characteristics of granite under the influence of temperature, cooling rate, and prefabricated fissure

Investigating the damage and failure characteristics of rocks under hydro-thermal-mechanical coupling is of great significance for the development of deep geothermal resources. To reveal the influence of temperature, cooling rate, and prefabricated fissure on the damage and failure mechanism of granite, Brazilian splitting tests were conducted on granite specimens containing prefabricated fissure under three temperature gradients (300°C, 600°C, 900°C) and different cooling methods (natural cooling (NC) and water cooling (WC)). Through polarization microscopy and digital image analysis, it was revealed that the development of thermal cracks follows an evolutionary process: intergranular cracks development, intragranular cracks development, and finally intergranular and intragranular cracks interconnect to form a complex thermal crack network. When the heat treatment temperature is below 600°C, the cooling rate is the main controlling factor for strength degradation, above 600°C, temperature and fissure loading angle become the dominant factors. Acoustic emission (AE) and high-speed camera monitoring results show that increasing temperature and cooling rate shift the distribution of AE events from being concentrated in the failure stage to being distributed throughout the entire loading stage, while increasing the fissure angle changes the AE event distribution from unimodal to multimodal. The fissure loading angle is the main factor influencing the propagation of macroscopic main cracks, while temperature and cooling rate further accelerate the development and propagation of secondary cracks. The research elucidates the thermal damage characteristics of granite under multi-factor coupling and provide a theoretical basis for the stability assessment and engineering optimization of deep geothermal reservoirs.