Compression-shear failure of thermally treated granite: insights from digital image correlation analysis

Understanding the temperature-dependent shear failure behavior of rock under compression-shear loads is significant for evaluating the stability of deep surrounding rock with high temperature. In this study, a series of variable angle shear (VAS) tests, were conducted on cubic granite specimens exposed to different high temperatures, observed by two-dimensional digital image correlation (2D-DIC) technique during the tests. The results show that the peak shear stress, the peak shear strain and shear modulus decrease with the shear angle alpha, regardless of the treatment temperature. The peak shear stress increases slightly as the high temperature rises to 400 degrees C, then decrease rapidly. Both the peak shear strain and shear modulus do not change significantly until 400 degrees C and 500 degrees C respectively, then reduce rapidly. The cohesion reduces and the internal friction angle increases with increasing temperature. Based on the principal strain field, the damage evolution and crack extension processes of typical specimen are analyzed. It shows that the cracks initiate from the upper and lower loading points, which further coalesce with each other at the middle part of shear band. The shear angle alpha has a significant effect on the compression-shear failure behavior of granite. The tension failures become less visible with increase of alpha and dominant failure patterns transform from mixed tension-shear failure to shear failure. The ratio of crack initiation force to peak load reduces obviously at 600 degrees C. In addition, the underlying failure mechanism of specimens under compression-shear load is explained by an established mechanical model in the paper.