  Composite materials, with statistically distributed threshold for breakdown
of individual elements, are considered. During the failure process of such
materials under external stress (load or voltage), avalanches consisting of
simultaneous rupture of several elements occur, with a distribution $D(\Delta)$
of the magnitude $\Delta$ of such avalanches. The distribution is typically a
power law $D(\Delta)\propto\Delta^{-\xi}$. For the systems we study here, a
crossover behavior is seen between two power laws, with a small exponent $\xi$
in the vicinity of complete breakdown and a larger exponent $\xi$ for failures
away from the breakdown point. We demonstrate this analytically for bundles of
many fibers where the load is uniformly distributed among the surviving fibers.
In this case $\xi=3/2$ near the breakdown point and $\xi=5/2$ away from it. The
latter is known to be the generic behavior. This crossover is a signal of
imminent catastrophic failure of the material. Near the breakdown point,
avalanche statistics show nontrivial finite size scaling. We observe similar
crossover behavior in a network of electric fuses, and find $\xi=2$ near the
catastrophic failure and $\xi=3$ away from it. For this fuse model power
dissipation avalanches show a similar crossover near breakdown.
