Local Waiting Time Fluctuations Along A Randomly Pinned Crack Front

The propagation of an interfacial crack along a heterogeneous weak plane of a transparent Plexiglas block is followed using a high resolution fast camera. We show that the fracture front dynamics is governed by local and irregular avalanches with very large size and velocity fluctuations. We characterize the intermittent dynamics observed, i.e., the local pinnings and depinnings of the crack front by measuring the local waiting time fluctuations along the crack front during its propagation. The deduced local front line velocity distribution exhibits a power law behavior, P(v) proportional to v(-eta) with eta = 2.55 +/- 0.15, for velocities v larger than the average front speed < v >. The burst size distribution is also a power law, P(S) proportional to S-gamma with gamma = 1.7 +/- 0.1. Above a characteristic length scale of disorder L-d similar to 15 mu m, the avalanche clusters become anisotropic providing an estimate of the roughness exponent of the crack front line, H = 0.66.