Universal aspects of dynamic fracture in brittle materials

We present an experimental study of the dynamics of rapid fracture in brittle amorphous materials. In this study we utilize a class of model materials, polyacrylamide gels, in which the relevant sound speeds can be reduced by 2-3 orders of magnitude compared to "standard" brittle materials. We first demonstrate that dynamic fracture in polyacrylamide gels has characteristic features which are identical to those observed in well-studied brittle materials, such as brittle plastics and glasses. These features include the existence of a critical velocity beyond which frustrated crack branching occurs (Fineberg [1], Sharon [2]) and the profile of the micro-branches formed. We then examine the behavior of the crack fronts, which are the 1D fronts defined by the leading edge of a propagating crack. During fracture, a crack front can be locally perturbed by either an externally introduced inclusion or, dynamically, by the generation of a micro-branch. Comparison of the behavior of the excited fronts in both gels and in soda-lime glass reveals that, once again, many aspects of the dynamics of these excited fronts are the same in both materials. These include both the generation of coherent, localized waves ([Morrissey [3], Ramanathan [4], Sagy [5], Sharon [6]) ("front waves") which propagate along the crack front as well as the appearance of crack front inertia. Crack front inertia is embodied by a "memory" effect of the crack front in which directed roughly periodic lines of spatially localized micro-branches are generated. These lines are aligned in the direction of propagation (Fineberg [7], Sharon [8]) and the spacing between successive microbranches is proportional to their width (in the direction normal to the propagation direction). This scaling is identical in both glass and gels.