Experimental Studies on Energy Release Rate and Bifurcation Process of Fast Propagating Cracks

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Two kinds of experimental studies were carried out to figure our the bifurcation mechanism of fast propagating cracks.[1] Fast Propagating cracks at bifurcation were photographed with high-speed holographic microscopy. Crack speed was more than 650m/s. The photographing was done within 10μs before and after bifurcation, and the following results were obtained.(1) Crack opening displacements, CODs, are proportional to the distance r from the crack tip before bifurcation. After bifurcation CODs of mother cracks are proportional to r from the nominal tip of the mother cracks.(2) The energy release rate obtained from the CODs increases gradually and continuously across the bifurcation points of the cracks. Discontinuous change of the energy release rate doesn't occur at the bifurcation.(3) The energy flux toward the crack tips also increases gradually and continuously at the moment of crack bifurcation, and doesn't show discontinuous change. This is caused by the fact that the crack dosen't decelerate discontinuously after crack bifurcation.[2] Two optical systems were applied to take photographs of rapidly bifurcating cracks on the both surfaces of the plate specimens, and the following results were obtained.(1) There are some bifurcating cracks whose shape on one surface of a specimen is apparently different from that on the other surface. In these cases, the crack bifurcations are not two dimensional phenomena, but three dimensional ones.(2) In the above cases, some crack branches keep developing and break up the specimens.(3) Even in the case that cracks have similar shape on the both surface of specimens, two crack branches have different opening displacement. This means that bifurcation is not symmetric with respect to the line along the mother crack.[3] The continuous increase of energy release rate across the bifurcation point may be caused by the three dimensional nature of crack bifurcation.