  The temporal evolution of mechanical energy and spatially-averaged crack
speed are both monitored in slowly fracturing artificial rocks. Both signals
display an irregular burst-like dynamics, with power-law distributed
fluctuations spanning a broad range of scales. Yet, the elastic power released
at each time step is proportional to the global velocity all along the process,
which enables defining a material-constant fracture energy. We characterize the
intermittent dynamics by computing the burst statistics. This latter displays
the scale-free features signature of crackling dynamics, in qualitative but not
quantitative agreement with the depinning interface models derived for fracture
problems. The possible sources of discrepancies are pointed out and discussed.
