Simultaneous photographing of rapidly bifurcating cracks on both surfaces of plate specimens with pulsed holographic microscopy

When brittle materials break under external force, fast propagating cracks appear often whose propagation speed is from 200m/s to 2000m/s. The fast propagating cracks suddenly bifurcate into two cracks when the propagation speed is high enough. But the mechanism of the rapid crack bifurcation has not been well understood. In the present study, two optical systems of pulsed holographic microscopy are applied to take photographs of rapidly bifurcating cracks in PMMA plate specimens. The cracks are of the opening mode and propagate at a speed more than 600m/s. The photographs of the cracks are simultaneously taken on the both sides of the plate specimens about 10μs after bifurcation. The photographs show that, in many cases, the shape of branch cracks on one side of a specimen is apparently different from that on the other side of the specimen. The fact indicates that the rapid crack bifurcation in PMMA is of three-dimensional phenomenon. Also shown is that two branched cracks have different crack opening displacements. It means that the bifurcation of the cracks are asymmetric just after bifurcation.