Crack propagation characteristics and fracture toughness analysis of rock-based layered material with pre-existing crack under semi-circular bending

Fracture initiation, fracture propagation, and their interactions with the interlayer interface in rock-based layered material are important in assessing the stability of rock-based structures such as lined tunnels. A numerical model based on the inserted cohesive zone model (CZM) method is established, and further verified against the semi-circular bending (SCB) test to simulate the complex crack initiation and propagation behavior in a rock-based layered semi-circular disk with pre-existing rock crack at different angles. From the results, three typical crack propagation modes are identified: interface fracture, wing fracture from crack tip and wing fracture not from crack tip. At a small crack inclination angle, interfacial crack is formed, while at a larger crack inclination angle, wing crack which initiates deviating from the crack tip appears. It is found that the crack propagation mode is also controlled by the interface strength, and the interface crack is always formed in SCB specimens with lower interface strength. By increasing the interface strength, the length of interface crack gradually decreases and wing crack presents. When the pre-existing rock crack inclines greater than 45 degrees and the interface strength is larger, the wing crack initiating far away from the crack tip merges with the interface crack to be connected propagating path, and the initiation position of wing crack gradually moves toward the crack tip with the further increasing of interface strength. Moreover, it is also found that the crack length ratio and the support span ratio are important influencing factors for crack propagation mode. Increasing the crack length ratio can suppress the formation of interfacial crack. When the crack inclination angle is more than 45 degrees, the wing crack tends to originate at the tip of pre-existing crack as the support span ratio increases. In addition, the effects of crack length ratio, support span ratio, crack inclination angle and Young's modulus ratio on the fracture toughness of layered SCB specimen under different loading modes are also investigated and the corresponding results indicate that these factors remarkably control the fracture mode and fracture resistance of rock-based layered material.