Secondary afterfailure fractures due to transversely reflected waves

The method of caustics was employed to study dynamic crack propagation and after failure fracture in PMMA plates containing an almost longitudinal internal crack, forming a small angle with the loading axis. In order to initiate fracture a high strength longitudinal stress pulse was applied producing two symmetric transverse crack branches running under mode-I deformation with very high velocities. After complete fracture of the plate, the re-entrant corners between the initial slant crack and the two transverse crack branches become centers of emanation of new cracks under the influence of the transversely reflected stress pulses created during the rupture of the longitudinal boundaries. The secondary crack branches are strongly curved, propagate under mixed-mode conditions and arrest. During a third step of fracture the arrested secondary branches accelerate again forming new mixed-mode cracks, which are curved, zig-zaged and presenting sometimes local branchings.