Abstract: Compared with rock fragment by static stress, there is a stress transition from compression to tension in the specimens under dynamic loading, e.g. the compression stress will be changed to tension during wave propagation. Based on damage theory and FE method, rock fragmentation mechanism induced by a drill bit subjected to dynamic loading is investigated. To exclude influence of reflection wave from the boundary, the viscoelastic boundary is considered in the simulation. Firstly, the validation of the viscoelastic boundary in homogeneous and heterogeneous media is examined. Then, the response of rock with different heterogeneities under dynamic loading is analyzed. The results show that: in elastic situation, when the rock is loaded, the area, which locates the edge of contact area between the drill and rock and near the free surface, is the tension sphere, and the highest tension lies in former. The highest shear stress lies near the contact surface but exists a distance. If the homogeneity is high, the rock fragmentation process can be divided into three phases, e.g. Hertz cracks, radial cracks and lateral cracks. As Hertz crack propagation, the stress by confinement becomes smaller; the process of wave propagation is changed and the emergency of tension is the key of crack initiation and growth. When the homogeneity is low, fracture pattern shows a wide spectrum, in other words, shear failure of proportion is increased and the failure mechanism becomes complex. The results are also noted, as the granular rock, if the size and proportion of grain can’t be negative; the homogeneity of grain and matrix should also be considered. When the granular rock is subjected to dynamic loading, the crack can across the grain or along the grain-matrix boundary. In all, the tension stress during the wave propagation is the key of crack initiation and growth.

Key words: rock fragmentation, numerical simulation, viscoelastic boundary, granular rock