Damage and failure mechanism of pre-statically loaded rock under coupled impact disturbance and strain energy release

Deep underground engineering often subjects surrounding rock to complex conditions involving high in-situ stress, accumulated strain energy, and dynamic disturbances from activities such as mining. Understanding the damage and failure mechanisms under these coupled static-dynamic and energy-release conditions is crucial for predicting and preventing dynamic hazards like rockbursts. In this study, a series of experiments were conducted using a self-developed multi-strain-rate dynamic-static load superposition and energy release test system. This system simulated far-field strain energy accumulation and release through gas compression and expansion, while impact loads were applied by drop hammer. The damage process was characterized using mechanical parameters, fractal dimension analysis of fractures, and full-field strain measurement. The results indicate that the pre-peak mechanical behavior of rock is dominated by the superposition of static and dynamic loads, whereas the post-peak stage is governed by strain energy release. Strain energy release significantly amplifies the dynamic effects of rock failure, leading to more complex crack patterns (evolving from single shear to X-shaped conjugate and localized dispersed failures) and increased post-peak strain rates. Discrete element numerical simulations revealed that compressive stresses dominate the internal stress field, but tensile stresses control the instability-driven failure. Higher impact loads and strain energy magnitudes result in sparser contact force chains, non-uniform load transfer after local instability, and a rapid decline in bearing capacity. Furthermore, a quantitative relationship was established among input energy density, dissipated energy density, and fractal dimension, providing an index to evaluate rock fragmentation degree and dynamic effects under force-energy coupling.