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.