Cyclic Impact Response and Size-Induced Fracture Transitions of Safety Isolation Layers under Coupled Loading

This study systematically investigates the influence of isolation layer thickness on rock fracture and damage evolution under dynamic-static coupled loading. Cyclic impact tests were conducted on granite specimens using a Split Hopkinson Pressure Bar (SHPB) system under varying levels of axial prestress. DIC techniques and FEM-DEM numerical simulations were employed to reveal the effects of layer thickness on crack propagation, fracture mode transition, and energy dissipation. A "fracture life-cycle" framework was established, encompassing crack initiation, stress concentration, accelerated propagation, ultimate failure, and residual damage stages. Results indicate that thinner isolation layers absorb more energy per unit volume and exhibit a higher proportion of shear cracks, enhancing crack connectivity and significantly increasing the risk of through-going failure. In contrast, thicker layers are prone to brittle fracture, with cracks typically not propagating along the principal axis, resulting in lower impact tolerance. Overall, both thin and thick isolation layers demonstrate limited impact resistance, whereas layers of intermediate thickness show optimal energy absorption and crack propagation control, making them suitable as safety isolation layers.