  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 which trigger a rich burst activity, by measuring the local waiting
time fluctuations along the crack front during its propagation. The local front
line velocity distribution deduced from the waiting time analysis exhibits a
power law behavior, $P(v) \propto v^{-\eta}$ with $\eta = 2.55 \pm 0.15$, for
velocities $v$ larger than the average front speed $<v>$. The burst size
distribution is also a power law,
  $P(S)\propto S^{-\gamma}$ with $\gamma=1.7 \pm 0.1$. Above a characteristic
length scale of disorder $L_d \sim 15 \mu m$, the avalanche clusters become
anisotropic, and the scaling of the anisotropy ratio provides an estimate of
the roughness exponent of the crack front line, $H=0.66$, in close agreement
with previous independent estimates.
