  The acoustic emission of fracture precursors is measured in heterogeneous
materials. The statistical behaviour of these precursors is studied as a
function of the load features and the geometry. We find that the time interval
between events (precursors) and events energies are power law distributed and
that the exponents of these power laws depend on the load history and on the
material. In contrast, the cumulated acoustic energy presents a critical
divergency near the breaking time. The critical exponent is independent, within
error bars, by all the experimental parameters.
