The mechanism of the fracture coalescence for intermittently cracked rock mass is studied under uniaxial dynamic loading. The specimens made of sandstone-like modeling material contain three pre-existing flaws with different geometry distribution. Through the comparisons of the fracture propagation length

coalescence mode of the fractures and strength increase of the pre-cracked specimens under static and dynamic loading

the dynamic response of the fracture coalescence is found different with static loading under different geometric setting of the flaws. Furthermore

the inertia effect of the fracture propagation and coalescence is revealed under dynamic loading

that is to say

the growth of the secondary co-planar fracture and inclined fracture tends to the original propagation direction

and the immediate coalescence is taken place easily between two pre-existing flaws. But the fracture coalescence path is winding under static loading

which is different with dynamic loading. So

the inertia effect of the fracture propagation is regarded as the main cause of the strength increase of the brittle material under slow to medium strain rate. Moreover

the multiple coalescences among the flaws lead to the progressive failure for cracked specimens.