Nonlinear Mechanical Responses and Fatigue Damage Mechanisms of Roadside Backfill Materials Under Static‐dynamic Combined Loads

ABSTRACT The long‐term stability of roadside backfill structures in deep coal mining is critically threatened by complex static‐dynamic combined loads, yet the multiscale fatigue damage mechanisms remain elusive. This study aims to elucidate the nonlinear mechanical responses and damage evolution of backfill materials by establishing a high‐fidelity 3D discrete element model that integrates micro‐CT scanning reconstruction with a nonlinear parallel‐bonded stress corrosion (NPSC) model. The simulation results indicate that the proposed framework accurately reproduces the hysteresis curve characteristics and irreversible strain accumulation, with peak strength prediction errors controlled within 0.08% ∼ 4.31%. A critical dynamic amplitude threshold of 4 ∼ 5 MPa was identified, exceeding this limit triggers a transition from stable two‐stage damage accumulation to accelerated three‐stage failure. Mesoscopic analysis reveals that fatigue cracks preferentially initiate at weak matrix‐aggregate interfaces and propagate to dismantle the aggregate–aggregate load‐bearing skeleton. Additionally, the coupling effect of high static pre‐load was found to significantly promote crack propagation, thereby diminishing the material's safety margin against subsequent dynamic disturbances. These findings provide theoretical support for optimizing mining intensity and support strategies to enhance the durability of deep underground infrastructure.