Energy dissipation and fracture characteristics of composite layered rock under dynamic load

Six combinations of layered composite rocks were prepared using sandstone, dali rock, and granite. The composite rock specimens underwent a dynamic impact test using the separated Hopkinson pressure rod test system, and the failure patterns of the specimens were recorded using high-speed cameras. The dynamic fracture mode, wave impedance effect, and energy dissipation nature of these composite rock specimens were analyzed, and the relationship between their kinetic energy and fracture energy was explored. The discrete lattice spring model was used to simulate the dynamic fracture process of the composite rock specimens, and the stress wave propagation characteristics and stress and damage evolution nature of the composite specimens were analyzed. The results show that the dynamic fracture characteristics of composite rock materials are strongly influenced by the uppermost and lowermost layer materials. When the dynamic cracking toughness of the material in the lower layer is low, the crack can maintain a high propagation speed and requires a short time from initiation to expansion to the rock cemented surface. The upper layer material has a greater influence on the stress conduction of the composite rock specimen. The overall transmission capacity depends on the upper layer material such that the greater its density, the more conducive it is to wave transmission and the better the stress conduction. The greater the difference in the densities of the lower and upper layer materials, the greater the difference between the stresses at the upper and lower ends of the rock cemented surface. Wave impedance has a significant effect on the propagation behavior of the stress wave. The propagation speed of the stress wave in the composite specimen is influenced by the porosity and density of the material. The larger the wave impedance, the faster the propagation speed of the stress wave, the larger the transmission coefficient, and the higher the energy transmitted. When energy is dissipated, the density, kinetic energy, and fracture energy of the composite rock specimen are influenced by the densities of the materials in the upper and lower layers. If the lower layer material is unchanged, a higher density of the upper layer material results in a smaller density and fracture energy when the energy is dissipated, yielding more kinetic energy when the specimen is completely fractured. When the upper layer material remains unchanged and the density of the lower material is increased, the cutting tip is more likely to crack, and the density and fracture energy are smaller when the energy is dissipated.