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-breaking 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 fragmentation characteristics of surface impact crater in extremely hard granite. The three-dimensional morphology, rock fragments, and mineral fragmentation properties in the impact crater were quantitatively analyzed. The results indicate that the maximum depth of the impact crater increases in a parabolic way, while both the volume and surficial area of the impact crater grow linearly with the impact number. Moreover, the impact 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 difference in mesoscopic fragmentation mechanism inside and outside the crater center causes the double-peak characteristics of average size of rock fragments. The volume, surficial area and maximum depth of the impact crater increase linearly with the kinetic energy of impacting particles in the double logarithmic coordinate system. The fractal dimension variations of internal crack distribution in main minerals around the impact 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 and impact number, while particle velocity exerts a more significant effect than impact number does.

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