Universal features of the microbranching instability in dynamic fracture

We investigate the universality of the microbranching instability, by comparison between the characteristics of a mode I dynamic crack, in both poly(methyl methacrylate) (PMMA) and soda-lime glass. We demonstrate the existence of the instability in glass, and the similarities of its features in both PMMA and glass. In glass, as in PMMA, there is a well defined threshold for the appearance of microbranches. This threshold is higher in glass (0.44C*R) than in PMMA (0.36C R). In both materials, the mean branch length increases linearly with increasing mean velocity of the crack and the branch profile is identical. In both materials the branches are the cause for the observed roughness of the fracture surface where, in glass, surface roughening is accompanied by fragmentation. Finally, a similar three-dimensional-to-two-dimensional transition occurs in both materials at high crack velocities.