Abstract: Particle impact, a new drilling technology, has been applied in drilling and gas and oil exploitation. Also, it is considerably promising as an assisting rock breakage method for the excavation of tunnels in extremely hard rocks. In this paper, an experimental study was conducted to investigate the effects of particle impact number, particle strength, and particle impact velocity on the damage and fracture characteristics of surface crater in extremely hard granite. The three-dimensional morphology, rock fragment distribution, and fracture properties in the crater were quantitatively analyzed. The results indicate that the maximum depth of the crater increases in a parabolic way, while both the volume and surficial area of the crater grow linearly with the impact number. Moreover, the crater volume first increases and then decreases with the particle impact velocity, with a critical velocity of nearly 82.5 m/s. In addition, the mechanism difference of mesoscopic fragmentation inside and outside the crater center causes the double-peak characteristics of average size of fragments. The increase of volume, surficial area, and maximum depth of the crater are in linear relation to the kinetic energy of impacting particles in the double logarithmic coordinate. The fractal dimension variation trend of internal crack distribution in main minerals around the crater under different impact velocities and numbers were obtained through image-processing method. The experimental results manifest that the damage scale of rock crater is enlarged through improving particle velocity or impact number, while the former exerts a more significant effect.

Key words: rock dynamics, particle impact, assisting rock-breakage method, dynamic fragmentation, damage characteristics