  We have studied the propagation of a crack front along the heterogeneous weak
plane of a transparent poly(methyl methacrylate) (PMMA) block using two
different loading conditions: imposed constant velocity and creep relaxation.
We have focused on the intermittent local dynamics of the fracture front for a
wide range of average crack front propagation velocities spanning over four
decades. We computed the local velocity fluctuations along the fracture front.
Two regimes are emphasized: a depinning regime of high velocity clusters
defined as avalanches and a pinning regime of very low-velocity creeping lines.
The scaling properties of the avalanches and pinning lines (size and spatial
extent) are found to be independent of the loading conditions and of the
average crack front velocity. The distribution of local fluctuations of the
crack front velocity are related to the observed avalanche size distribution.
Space-time correlations of the local velocities show a simple diffusion growth
behavior.
