  The acoustic emission of fracture precursors, and the failure time of samples
of heterogeneous materials (wood, fiberglass) are studied as a function of the
load features and geometry. It is shown that in these materials the failure
time is predicted with a good accuracy by a model of microcrack nucleation
proposed by Pomeau. We find that the time interval $% \delta t$ between events
(precursors) and the energy $\varepsilon$ 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 $E$ presents a critical
divergency near the breaking time $\tau $ which is $% E\sim \left( \frac{\tau
-t}\tau \right) ^{-\gamma }$. The positive exponent $% \gamma $ is independent,
within error bars, on all the experimental parameters.
