Characteristics of fracture propagation and permeability response of sandstone under cyclic impact effect

To study the fracture and permeability characteristics of sandstone-type uranium ore under cyclic impact, the Split Hopkinson Pressure Bar (SHPB) experiments, CT scanning, 3D reconstruction, permeability testing, and numerical simulations were first conducted, then the dynamic mechanical properties of specimens subjected to different numbers of impacts, pore-fracture parameter evolution, and microscopic seepage characteristics were analyzed. Results show that cyclic impacts cause cumulative damage in the specimens, reducing their dynamic mechanical properties. As the number of impacts increases, energy in the specimens accumulates and releases cyclically. This cyclic accumulation and release of energy lead to a process of crack "expansion, compaction, re-expansion, re-compaction". During the cyclic impact process, small and isolated cracks inside the specimen gradually develop into larger, interconnected fractures. Simultaneously, medium-sized cracks exhibit both faulting and connectivity effects, presenting nonlinear change characteristics. Cyclic impacts induce more damage in the specimens and cause the evolution of crack parameters related to microscopic seepage characteristics. These impacts generate more complex fractures, which create additional pathways for fluid seepage and increase the overall scale of seepage. However, the roughness of the fracture walls and the presence of more throats partially inhibit fluid flow. The simulated permeability is higher than the measured permeability due to factors such as the blockage of mineral debris after impact, confining pressure, and the accuracy of 3D reconstruction. However, the numerical simulation better reflects the microscopic seepage process in the cracks of the specimen after cyclic impact, which is helpful for the study of the non-uniform seepage behaviour of cracks in the sandstone-type uranium mines after blasting.