Cracking mechanism and acoustic emission characteristics of granite under varying thermal effects

Understanding the thermo-mechanical behavior of granite under high-temperature conditions is crucial for the safe and efficient development of deep geothermal resources. This study revealed the physico-mechanical properties, cracking mechanism and fracture precursor of granite via compression experiments with acoustic emission (AE) monitoring. Results indicated that 500 °C is the critical temperature for the brittle-ductile transition of granite, characterized by progressive degradation of P-wave velocity, Young's modulus, and uniaxial compressive strength (UCS) with increasing temperature, while peak strain shows an inverse trend. As the temperature and number of thermal cycles increase, the physico-mechanical parameters, AE characteristics and the failure modes show a strong temperature dependence, whereby the temperature is categorized into "slight weakening range (25-200°C)", "stable weakening range (200-500 °C)" and "significant weakening range (≥ 500 °C)". Thermally induced cracks are mainly composed of extrusion cracks from mineral thermal expansion mismatch, and intergranular cracks along weakened grain boundaries. In addition, the high energy rate AE event can be regarded as the fracture precursor, but it is significantly affected by the thermal effect. This study aims to determine the optimal temperature, cooling method, and number of thermal cycles needed for geothermal resource extraction.