Investigation of fracture properties of mode I fracture in heat-treated granite

The mode I fracture properties of granite under high-temperature conditions were investigated to reveal the influence of different heat treatment temperatures on the fracture toughness, crack propagation behavior, and brittle-ductile transition of granite. Three-point bending tests (TPB) were conducted on granite samples subjected to heat treatments ranging from room temperature to 900 °C. Crack tip opening displacement (CTOD) and strain field distributions were measured using digital image correlation (DIC) technology, while crack morphology was analyzed through 3D reconstruction using a profilometer. The results indicate that the fracture toughness of granite increases as the temperature rises from 20 °C to 100 °C, followed by a significant decrease as the temperature further increases. During the initial loading stage, the crack tip exhibited elastic deformation, and with increasing load, a nonlinear deformation zone developed at the pre-existing crack tip. The fracture process zone (FPZ) expanded until reaching peak load, at which point unstable crack propagation occurred, leading to sample failure. CTODc increased significantly with rising temperatures, especially above 500 °C, suggesting enhanced ductility. Thermal cracking had a pronounced effect on the main crack propagation path, particularly above 600 °C, where increased branching and irregularity in the crack path were observed, along with a notable increase in surface roughness. Crack width and irreversible deformation increased bilinearly with temperature, with an inflection point near 400 °C. Based on the analysis of load-displacement curves, load-CTOD curves, and irreversible deformation under residual stress, the brittle-ductile transition was determined to occur between 500 °C and 600 °C. This study provides new experimental data and theoretical insights into the mechanical behavior of rock materials under high-temperature conditions, with significant academic and engineering implications. The findings have potential applications in predicting rock failure and stability in fields such as geothermal energy development, tunneling, and deep underground mining.