Fracture behavior of thermally treated granite under compression-shear loading

Understanding the fracture behavior of rock after thermal treatment is important in the deep rock engineering, such as nuclear waste disposal and geothermal energy exploration. In this work, to investigate fracture properties of thermally treated rock under compression-shear loading, a series of variable angle shear (VAS) tests were performed on shear-box (SB) granite specimens exposed to temperatures from 25 degrees C to 600 degrees C. The results indicate that the mode II fracture toughness KIIC and shear modulus G of granite increase with temperature up to 300 degrees C, and then decrease. The deformation evolution of specimens was analyzed using a two-dimensional digital image correlation (2D-DIC) technique. It is found that wing cracks at all temperature firstly initiate from the upper notch tip prior to 40% of peak load. The fracture mechanism of wing crack is identified as mixed mode I-II fracture via a displacement analysis, while dominant mechanism varies at different positions along wing crack paths. The influence of wing crack on overall fracture behavior was explored in detail and the load condition for wing crack initiation was verified based on the general maximum tangential stress (GMTS) criterion. In addition, the influence of temperature on the roughness of mode II fracture surfaces was investigated based on 3D optical scanner and fractal theory. The results show that the roughness of fracture surfaces firstly decreases from 25 degrees C to 300 degrees C then increases with temperature. There is a negative correlation between the KIIC and the roughness of fracture surfaces. Finally, a more rigorous strategy is proposed to assess the reliability of new methods for estimating KIIC.