MICROBRANCHING INSTABILITY AND THE DYNAMIC FRACTURE OF BRITTLE MATERIALS

We describe experiments on the dynamic fracture of the brittle plastic , PMMA. The results suggest a view of the fracture process that is bas ed on the existence and subsequent evolution of an instability, which causes a single crack to become unstable to frustrated microscopic bra nching events. We demonstrate that a number of long-standing questions in the dynamic fracture of amorphous, brittle materials may be unders tood in this picture. Among these are the transition to crack branchin g, ''roughness'' and the origin of nontrivial fracture surface, oscill ations in the velocity of a moving crack, the origin of the large incr ease in the energy dissipation of a crack with its velocity, and the l arge discrepancy between the theoretically predicted asymptotic veloci ty of a crack and its observed maximal value. Also presented are data describing both microbranch distribution and evidence of a new three-d imensional to two-dimensional transition as the ''correlation width'' of a microbranch diverges at high propagation velocities.