Dynamic crack propagation characteristics of media with bedding under an impact load

With the gradual development of mines, tunnels, and other underground constructions, theoretical research on the influence of internal defects in rock structure on rock dynamic fracture behavior and related engineering practices are of great importance. In this paper, a digital laser dynamic caustics experimental system is used to conduct three-point bending drop hammer impact tests on three groups of polymethyl methacrylate specimens with different angles of bedding (30°, 45°, and 60°). The fracture process of the specimens and the shape change process of the dynamic caustic speckle at the crack tip were recorded using a high-speed camera. The characteristics of dynamic stress intensity factors Ⅰ and Ⅱ were obtained, and the crack tip displacement and velocity curves were analyzed. Combined with the discrete lattice spring model (DLSM), the fracture morphology of the specimens was analyzed, and the variation law of the stress field and field at the crack tip was obtained. The transmission and reflection characteristics of stress waves were studied at stratification. Finally, the impact of the fracture characteristic stratification parameters of the medium was analyzed using DLSM. The results show that the fracture characteristics of the specimens, the initiation time of the crack, and the propagation speed of the crack in the bedding plane vary with the bedding angle. With increasing bedding angle, the initiation time of the crack advances, the propagation speed of the crack increases along the weak bedding plane after extending to the bedding plane, and the crack is more inclined to extend along the weak bedding plane to complete specimen fracture. With the crack expansion, the type Ⅱ stress intensity factor appears, and the specimen fracture shows the characteristics of tension–shear composite failure. Before arriving at a particular bedding speed, cracks fluctuate up and down, and attenuation in the aftermath of the bedding generally has lower volatility change; the elastic modulus and bedding thickness affect the dynamic fracture characteristics of the specimens. If the bedding elastic modulus is less than 0.1 GPa, the crack extension in the bedding plane distance increases with the elastic modulus. If it is more than 0.1 GPa, when the bedding for the organic glass bonding effect increases, the crack goes directly through the bedding. The propagation distance of cracks along the weak plane of the bedding increases with the bedding thickness.