Research on dynamic failure mechanism of three-point bending beam with joint

Instability failure of rock masses containing joints has always been a hot topic in rock mechanics. To study the mechanical failure mechanism of rock masses containing internal joint under impact loading, a digital laser dynamic caustic system combined with high-speed photography was used to perform drop hammer impact experiments. The mechanical parameter of dynamic fracture of polymethyl methacrylate (PMMA) beams and physical parameters such as crack extension trajectory and extension velocity were obtained. The extended finite element method was adopted to simulate the propagation process of the internal joint in the PMMA beam, then the von Mises stress distribution and crack extension velocity were obtained. The research results show that: the fracture mode of the three-point bending beam containing internal prefabricated crack is Mode-I; the lower end of the internal crack starts to crack firstly and the initiation toughness shows a tendency to increase and then decrease as the distance between the crack and the specimen boundary ascends; the "unloading stress wave" excited by the prefabricated crack lower end can significantly increase degree of the stress concentration at the crack upper end and promote the cracking. For specimens containing internal crack at different positions, the initiation toughness of the crack upper end increases by 152%, 62%, 88%, 250%, and 176% compared with lower end, respectively. The dynamic failure mechanism of a three-point bending beam containing internal joint has been proposed based on experiment results and numerical simulation. The proposed dynamic failure mechanism facilitates better understanding of failure process of rocks containing joint and for engineering applications related to rock mass fracture.