Investigation on Fracture Mode and Initiation Mechanisms of Cracks in Sandstone Disk Containing a Preexisting Flaw

Determination of the fracture mode and the initiation mechanism of cracks emanating from preexisting flaws are of great significance for understanding the failure process of rocks. In this research, Brazilian tests were conducted on sandstone disc-shaped specimens containing a single preexisting flaw with different inclination angles, and the development process of cracks was captured by a high-speed image-capture system. To accurately determine the fracture mode and initiation mechanism of cracks in the preexisting flawed specimens, a criterion of the fracture mode was proposed based on digital image correlation analysis and five types of fracture modes were established. The characteristics of the microcracks and the distribution of full-field lateral stress were analyzed by scanning an electron microscope and particle flow code, respectively. The influence of the flaw inclination angle on the fracture mode and the initiation mechanism of cracks were discussed according to the microscopic and macroscopic observation. The results showed that under tensile conditions, all the wing cracks are pure tensile mode (PT-mode) cracks. With the increase of the inclination angle of the preexisting flaw, the dominant effect on the initiation mechanisms of antiwing cracks gradually changes from tensile effect to shear effect.