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 b oth poly(methyl methacrylate) (PMMA) and soda-lime glass. We demonstra te 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 thres hold is higher in glass (0.44C(R)) than in PMMA (0.36C(R)). In both m aterials, the mean branch length increases linearly with increasing me an 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 b y fragmentation. Finally, a similar three-dimensional-to-two-dimension al transition occurs in both materials at high crack velocities.