Investigation of the characteristics of rock fracture process zone using coupled FEM/DEM method

During the failure process of quasi-brittle materials such as rock, a fracture process zone (FPZ) can easily develop ahead of a crack tip, significantly affecting the strength and stability of a structure. Therefore, it is very important to investigate the characteristics of FPZ under different conditions. In this study, a coupled 3D FEM/DEM model with zero-thickness cohesive elements is firstly developed using ABAQUS software and then used in a series of three-point bending tests to investigate the effects of specimen size, grain size, material ductility and loading rate on the size and shape of FPZ. The numerical results show that as the specimen size and grain size increase, the length and width of FPZ gradually increase, whereas the shape of FPZ is nearly unchanged. As the material becomes more ductile with increasing kn/knp by changing the peak traction at a fixed fracture energy, the fracture toughness gradually increases. In contrast, when the material becomes more ductile with increasing kn/knp by changing the fracture energy with a fixed peak traction, the fracture toughness gradually decreases. Regardless of whether the increased material ductility is obtained by changing the peak traction or the fracture energy, the influence of the material ductility on the size and shape of FPZ is consistent. Both the length and width of FPZ markedly increase when the material becomes more ductile, especially the FPZ length. The material ductility weakly influences the shape of FPZ. Furthermore, the loading rate affects only the length and shape of FPZ. As the loading rate increases, the length becomes significantly longer and the shape of FPZ becomes narrower.