Abstract：The mechanism of the fracture coalescence between two flaws is studied under uniaxial dynamic loading. The specimens made of sandstone-like modeling material contain two pre-existing flaws with different geometry distribution including echelon and co-planar arrangement pattern. 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 from static one under different geometric setting of the flaws. Furthermore，the inertia effect of the fracture propagation is revealed under dynamic loading，the growth of the wing fracture and secondary co-planar 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 from 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. In virtue of the explanation，the second cause of the mode II shear fracture under earthquake is released.
