Research on crack initiation mechanism and fracture criterion of rock-type materials under compression-shear stress

With the focus on the failure of single-cracked rocks under compression-shear stress, the form and the critical conditions of crack initiation by adopting theoretical derivation and numerical experiments methods are researched in this article. Maximum circumferential stress criterion and modified Griffith's criterion are utilized to derive the expressions for crack initiation angle and fracture criterion. Moreover, the PFC3D, numerical simulation software, is used to verify the results of theoretical research and the research on the generation process of shear fracture zone and the number of microcracks and their distribution law. The results show that the crack initiation angle o(0) and critical conditions of crack initiation correlate with some factors, such as crack inclination angle , coefficient of friction mu, and the ratio of normal stress to shear stress sigma(1)/tau(x). When beta is less than or equal to 90 degrees, the deviation between the theoretical result of crack initiation angle and actual result is small; when the value of Mode I fracture toughness K-IC is accurate, the crack criterion derived is reliable when beta is less than or equal to 120 degrees. The friction between cracks restricts the compression-shear failure on the surface of crack. The ratio of shear stress required for crack initiation to peak shear stress in the model decreases with the normal stress increased. At the crack initiation stage, when the increase in microcracks is relatively slow, the shear fracture zone is thin and when the increase in microcracks becomes rapid, shear fracture zone becomes thick at the peak stage. The shear strength is proportional to the number of microcracks in the model.