Rockfalls commonly undergo successive impacts during downslope propagation, during which the coefficient of restitution (COR) evolves with accumulated damage and fracture state. In this study, repeated free-fall impact tests were conducted on sandstone spheres at different impact velocities. High-speed imaging, image segmentation, and X-ray CT-based three-dimensional reconstruction were combined to quantify COR attenuation, crack propagation, fragmentation threshold, and fragment morphology. The results show that the response of sandstone spheres is characterized by distinct velocity-dependent regimes. At low velocities, most of the incident energy is converted into elastic rebound energy; the specimens remain intact and the COR remains nearly stable. Near the damage threshold, surface cracks are activated and propagate locally, resulting in COR fluctuations. At higher velocities, cracks further propagate and become interconnected, gradually forming through-going fractures. The increased energy consumption associated with crack growth reduces the rebound capacity. Once fragmentation occurs, the intact-sphere rebound mode breaks down, and the response is jointly controlled by residual-block rebound, fragment detachment, and fragment dispersion, leading to a sharp reduction in COR. The fragmentation threshold decreases with increasing impact velocity, whereas the projected fragment area increases and then tends to stabilize, indicating a transition in the fracture-control mechanism from crack propagation to block separation. This study reveals the progressive transition of the energy-dissipation mechanism in sandstone spheres under repeated impacts, from elastic rebound dominance to crack-growth and fragmentation dominance, and provides an experimental basis for COR correction, damage-criterion development, and rockfall trajectory prediction.

Repeated impact,Coefficient of restitution,Rebound attenuation,Crack propagation,Fragmentation threshold