Experimental Studies on Bifurcation Process of Fast Propagating Cracks in Araldite B and Homallite 100

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Experimental studies were carried out to figure out the bifurcation of fast propagating cracks in Araldite B and Homallite 100, and the following results were obtained.[1]Fast : Propagating cracks in Araldite B and Homallite 100 were photographed by high-speed holographic microscopy at bifurcation. Crack speed was more than 400m/s.(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 point 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 doesn't decelerate discontinuously after crack bifurcation.[2]Two optical systems of pulsed holographic microscopy were applied to take photographs of rapidly bifurcating cracks in Homallite 100 and Araldite B on the both sides of plate specimens simultaneously.(1)Bifurcating cracks have different crack shapes on the both side of a specimen. Thus it can be said that the crack bifurcation is not two-dimensional phenomenon, but three-dimensional one.(2)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 above mentioned results are the same as those on cracks in PMMA. Consequently the following two can be said to be common to the three materials, Araldite B, Homallite 100 and PMMA.(1)The energy release rate obtained form COD measurement is continuous across the bifurcation point.(2)Crack bifurcation is not two-dimensional but three-dimensional phenomenon.