Experimental Study of Micro/Macro Damage and Failure Mechanism of Granite Subjected to Different Impact Velocities and Numbers

Rockfall typically involves repeated impacts that induce progressive damage and fragmentation in rock masses. To investigate the mechanism governing this process under different impact velocities, a series of controlled impact tests were conducted using a newly developed compressed gas-driven rock impact apparatus. This study systematically examined the effect of impact velocities and number on rock damage, distinguishing between internal damage (<10.0 m/s) and local failure (10.0 m/s–20.0 m/s). At the internal damage level, uniaxial compression tests with acoustic emission monitoring were employed to analyze the macro-mechanical properties and micro-failure processes of granite. At the local failure level, the repeated impact number required to transition from localized to complete failure was recorded, and polarizing microscopy was used to characterize microstructural evolution. The results show that damage and failure mechanisms are strongly influenced by both impact velocity and repeated impact number. Specifically, higher impact velocities and repeated impacts promote a shift toward brittle failure, with threshold behaviors observed at 5.0 m/s (fourth impact) and 7.5 m/s (third impact). A quantitative analysis further correlates impact conditions with mechanical degradation and energy evolution, providing insight into the underlying processes controlling rockfall fragmentation.