Mechanical properties and damage evolution of rock–backfill composite under impact load

The combined failure of rocks and backfills significantly affects the safety of underground mining. Based on the finite difference method (FDM)–discrete element method (DEM) coupled numerical simulation method, a numerical system of a split Hopkinson pressure bar (SHPB) and numerical models of rock–backfill composites were constructed. The mechanical properties, fracture process, microcrack propagation, and damage evolution of rock–backfill composites under impact loading were investigated. The results indicated that the dynamic peak stress and peak strain of the rock–backfill composite decreased at lower cement tailing ratios (CTR). Microcracks first appeared in the low-CTR region of the backfill and gradually spread into the surrounding rock. As the CTR decreased, the rock damage decreased, whereas the backfill damage increased. The rock and low-CTR zone of the backfill underwent coupled failure after instability at their contact interface, resulting in greater fracture penetration in the composite. The damage evolution of the rock–backfill composite in both combination modes occurred in three stages: gradual accumulation, dramatic increase, and deceleration. The layered structure changed the stress distribution in the composite, causing a sharp increase in microcracks in the low-CTR region of the backfill during the gradual damage accumulation stage.