New Crack Initiation Model For Open-Flawed Rock Masses Under Compression-Shear Stress

Many tests and numerical calculation results have demonstrated that the relationship between the strength of a rock mass with an open single-flaw and the flaw angle (fi)is not a symmetrical U-shaped distribution at fi = 45 degrees, as calculated using the slide crack model. In this study, based on a comparison between previous tests by other investigators and the slide crack model, uniaxial compression tests were conducted on sandstone specimens containing a single flaw. The peak strength (ac), initiation stress (aci), crack initiation, and failure of the specimens were analyzed in detail. In contrast to the sliding crack model, in which cracks in all the specimens initiate in the form of wing cracks, two other modes of crack initiation were observed for specimens with fi = 0 degrees -40 degrees: the anti-wing and middle cracks. The results indicated that the compressive stress (ay) on the flaw surface contributes significantly to the crack initiation process. Therefore, a crack initiation criterion is proposed: when the sum of the I-type stress intensity factor (KI) generated by the shear stress (z) and ay attains the fracture toughness (KIC), cracks will initiate from the tip of the pre-existing flaw (when ay > z, crack initiation is in the form of an antiwing crack, otherwise it is a wing crack). When the tensile stress (at) on the flaw surface reaches the tensile strength of the rock before KI at the crack tip reaches KIC, cracks in the specimen will initiate in the form of middle cracks.