Experimental study on mechanical properties of thermal-damage granite rock under triaxial unloading confining pressure

The triaxial tests on post-high-temperature granite samples (25℃~900℃) under unloading confining pressure are performed to study the deformation characteristics, parameters and failure patterns by using the servo-controlled rock test system. The results show that the granite rocks undergoing a certain temperature (300℃) resist the lowest level of confining pressure and are the most prone to damage. Based on the ratios of strain increment to confining pressure decrement, it is quantitatively revealed that the unloading damage is caused by the intense radial deformation and volume expansion. The ratios of strain increment to confining pressure decrement all increase firstly and then decrease with the increase of temperature and reach the maximum value at 300℃. The deformation modulus gradually decreases during unloading by 33.20%~59.11% between 25℃~900℃, and the higher the temperature is, the more it decreases. The decreasing tendency is a quadratic polynomial relation with the volume strain. The Poisson's ratio increases gradually during unloading by 164.96%~274.03% between 25℃~900℃, and the higher the temperature is, the more it increases. The increasing tendency is linearly related to the volume strain. The post-high-temperature granite samples fail in the pattern of axial splitting under uniaxial compression with multiple transfixion cracks. And the rocks damage with cutting-through shear failure under triaxial compression. It is most complicated under triaxial unloading. The local shear failure with a large angle at the normal temperature turns into cutting-through with the increase of temperature, and returns to local shear failure again when the temperature reaches 900℃.