Static–dynamic reloading degradation and brittle-to-ductile failure transition of cyclically pre-damaged sandstone: A multiscale experimental study

Rock masses in underground engineering commonly experience cyclic disturbances induced by blasting, excavation, and seismic loading, which can generate persistent internal damage and strongly affect their subsequent reloading behavior. To clarify the unified mechanism governing the static and dynamic responses of cyclically pre-damaged rock, red sandstone specimens were first subjected to cyclic loading-unloading under different upper limit cyclic stresses and then tested under uniaxial compression and SHPB impact loading. Acoustic emission (AE), digital image correlation, fragment fractal analysis, and scanning electron microscopy were combined to characterize the multiscale evolution of deformation and failure. The results show that cyclic pre-damage progressively reconstructs the internal crack network and degrades the load-bearing skeleton of the rock, causing exponential reductions in static peak strength and elastic modulus and linear attenuation of dynamic mechanical parameters. AE activity is systematically enhanced, the pre-peak quiet period shortens, and burst signals occur earlier with increasing initial damage. Moreover, the sensitivity of dynamic parameters to pre-damage becomes stronger at higher strain rates, indicating a strain-rate amplification of damage weakening. Multiscale observations further demonstrate that cyclic pre-damage alters the energy dissipation pathway and drives the failure mode from brittle-dominated fracture to ductile-dominated instability. These findings provide a unified physical framework for evaluating the residual stability and dynamic disaster potential of disturbed rock masses.