Thermal mechanical coupling fracture performance of granite under true Mode III loading: experimental realization and mechanism analysis via novel torsional testing

Accurately understanding the Mode III (tearing-mode) fracture behavior of rocks at elevated temperatures is essential for assessing the stability of engineered geostructures such as deep geothermal reservoirs. This study investigates the Mode III fracture characteristics of granite under thermo-mechanical coupling conditions, addressing a significant yet underexplored area in rock fracture mechanics relevant to deep geothermal applications. A novel torsion testing system was developed to conduct pure Mode III fracture experiments on square cross-section specimens with annular cracks at temperatures up to 800 °C. Finite element analysis confirmed a nearly pure shear stress state with negligible tensile components, validating the true Mode III crack propagation observed along the original notch plane. The results reveal a three-stage degradation of Mode III fracture toughness (K_IIIC) with increasing temperature: a gradual decline up to 200 °C, accelerated reduction between 200–400 °C, and stabilization above 600 °C, correlated sequentially with microcrack initiation, mineral dehydration, and quartz phase changes. At room temperature, the fracture toughness hierarchy K_IIC > K_IIIC > K_IC was quantified with ratios of K_IIC / K_IC = 2.33 and K_IIIC / K_IC = 1.81, respectively. Morphological transitions from brittle cleavage to plastic deformation features and thermally smoothed surfaces illustrate a brittle-to-ductile shift across temperatures. A normalized crack length threshold (2a/W ≥ 0.4) is established to ensure pure Mode III failure. Comparative validation using red sandstone confirms the reliability of the method, showing only 1.22 % deviation from existing results. This work establishes fundamental criteria for evaluating fracture behavior in deep geothermal environments under coupled thermal-shear loading.