Fast fracture in slow motion

We present recent results of fracture experiments in polyacrylamide gels. Polyacrylamide gels are soft polymer materials in which the characteristic sound speeds are on the order of a few meters/sec - thereby slowing down fracture dynamics by 3 orders of magnitude. We first demonstrate the universality of rapid fracture dynamics, comparing dynamics observed in gels with those seen in "classic" brittle materials such as glass. Among the common features are the appearance and form of branching instabilities as well as characteristic attributes of the resulting fracture surface that provide evidence for crack front inertia when translational invariance along the front is broken. We then demonstrate a number wholly new aspects of the fracture process, whose study is only made possible by utilizing the "slow motion" inherent in the fracture of these materials. These include both a new oscillatory instability at about 90% of the Rayleigh wave speed and measurements of the nonlinear zone at the tip of dynamic cracks.