Fracture mechanisms of granite subjected to varied thermal treatments: Insights into cooling-induced failure characteristics

Geothermal energy extraction and unconventional oil and gas production inevitably result in cooling shocks for high-temperature rock masses. In this study, the mode I fracture characteristics and failure mechanisms of granite under different temperatures (20–400° C) and cooling methods (air cooling and water cooling) were investigated. Heat-treated semi-circular bend specimens were subjected to three-point bending tests. A formula for calculating the elastic modulus of SCB specimens was proposed. Advanced characterization techniques, including digital image correlation for fracture process zone (FPZ) and crack tip opening displacement (CTOD) measurement, 3D laser scanning for fracture surface morphology analysis, and scanning electron microscopy for fracture surface microstructure examination, were systematically employed. A 3D heterogeneous numerical model was established to study granite temperature and stress changes during heat treatment. The integrated experimental and numerical results reveal the influence of cooling methods on granite fracture mechanisms. As the heating temperature rose, fracture toughness, strain energy, and energy-release rate decreased, whereas the elastic modulus first grew and then decreased. Compared with air cooling, water cooling caused more significant deterioration to the mechanical properties of granite. At the peak load stage, both FPZ length and critical CTOD grew with heating temperature, with a notable increase at 400 °C. However, the FPZ width did not change significantly. Water-cooled granite consistently exhibited longer FPZs and higher critical CTODs than air-cooled granite. With increasing temperature, the surface roughness coefficient increased, but the tortuosity first increased, then decreased. It was found that water cooling resulted in more tortuous fracture paths and rougher fracture surfaces, as well as lower fracture permeability. These findings can provide theoretical and engineering guidance for enhancing deep energy extraction efficiency.