Crack front dynamics: the interplay of singular geometry and crack instabilities.

When fast cracks become unstable to microscopic branching (microbranching), fracture no longer occurs in an effective 2D medium. We follow in-plane crack front dynamics via real-time measurements in brittle gels as microbranching unfolds and progresses. We first show that spatially local energy balance quantitatively describes crack dynamics, even when translational invariance is badly broken. Furthermore, our results explain microbranch dynamics; why microbranches form along spatially localized chains and how finite-time formation of cusps along the crack front leads to their death.