Changes in shear properties of granite fractures subjected to cyclic heating and air-cooling treatments

The effects of cyclic heat treatments on the fracture shear behaviors are rarely reported. To enhance our understanding, granite fractures having almost the same roughness were first exposed to cyclic heating at 400 °C and air-cooling treatments, and then direct shear tests were performed under four levels of normal loading. The influences of thermal cycles on roughness degradation and shear properties are analyzed. The roughness degradation in the joint roughness coefficient and the three-dimensional (3D) roughness metric exhibit linear increasing tendency with increasing thermal cycles. Typical fracture shear properties, including cohesion and friction angle, peak and residual shear strength, peak and residual shear displacement, and initial and secant shear stiffness, fluctuate generally within the first 10 thermal cycles, followed by gradual decreasing tendencies. The thermal effect on the shear properties become weaker as the number of heat treatments increases from 10 to 80. Nonuniform expansion and shrinkage of mineral grains after thermal treatments produce micro-cracks within the rock matrix and on the rock surface, suggesting that asperities are easier to be sheared-off. Thermal alteration in fracture peak-shear strength could be attributed to the deterioration in rock strengths and the mismatch in opposing fracture walls. The observations would provide better insights into rock friction after high temperatures in geothermal energy exploitation.