Nonuniversality in microbranching instabilities in rapid fracture

The apparent similarity of microbranching instabilities in different brittle materials gave rise to a widely held belief that many aspects of the postinstability physics were universal. We propose that the physics determining the typical length and time scales characterizing the postinstability patterns differ greatly from material to material. We offer a scaling theory connecting the pattern characteristics to material properties (like molecular weight) in brittle plastics like PMMA, and stress the fundamental differences with patterns in glass which are crucially influenced by three-dimensional dynamics. In both cases the present ab initio theoretical models are still too far from reality, disregarding some fundamental physics of the phenomena.