Mechanical properties and energy characteristics of single-fractured composite rock mass with different dip angles under impact load

In rock mass and mining engineering, shock waves induced by engineering disturbances significantly affect rock mass stability. To explore the dynamic mechanical behavior and energy dissipation mechanisms of single-fractured composite rock masses under impact loading, a series of tests were conducted using a split Hopkinson pressure bar (SHPB) system with a 50 mm rod diameter. Specimens containing a single inclined fracture with seven different dip angles and located in different lithological layers were tested. The results show that both peak stress and peak strain exhibit a non-monotonic trend with increasing dip angle α - first decreasing, then increasing, reaching a minimum at α = 45°. This behavior is attributed to enhanced energy concentration and dissipation in the fracture zone, where shear-dominated failure leads to more effective crack propagation and stress redistribution. Moreover, the proportion of crushing energy dissipation is significantly affected by the fracture dip angle, reaching a minimum at α = 45° and a maximum at α = 90°, indicating a transition from shear to tensile failure modes with increasing angle. Lithology also plays a crucial role: grey sandstone specimens absorbed more energy compared to yellow sandstone, implying higher impact resistance due to differences in microstructural cohesion. The evolution of fragment fractal dimension with increasing dip angle follows an "M-shaped" trend, reflecting changes in fragmentation intensity and failure mode. Notably, yellow sandstone tends to produce higher fractal dimensions, with larger mass but smaller volume of powdered debris, indicating more intense fragmentation. This study reveals the coupling effect of fracture dip angle and lithology on dynamic mechanical response and energy evolution, providing new insights into the failure mechanisms of layered composite rock masses under impact loading.