Acceleration and deceleration of unzipping supershear fracture propagating along a straight perforation line consisting of small-scale cracks

We experimentally investigate two-dimensional dynamic fracture propagation along a straight perforation line containing multiple small-scale cracks. If there exists only one single perforation line in a brittle polycarbonate specimen, dynamic fracture due to application of external quasi-static loading seems propagating unidirectionally in an “unzipping” way along the perforation line, simply linking the edges of the cracks without diverting and branching. However, a closer look into the photographs experimentally obtained by a high-speed video camera shows that fracture propagation is not always unidirectional and its speed also fluctuates back-and-forth between subsonic and supershear levels, i.e. the speed can be lower or higher than the relevant shear wave speed of the specimen during propagation. Generally, the propagation speed becomes maximum just before the fracture leaves the edge of a preexisting small-scale crack and it is low just before the fracture approaches the edge of the next crack in the perforation line. Furthermore, the fluctuation of fracture propagation speed at supershear levels may generate multiple Mach (shock) wavefronts with different Mach angles.