  We investigate the fracture of heterogeneous materials occurring under
unloading from an initial load. Based on a fiber bundle model of time dependent
fracture, we show that depending on the unloading rate the system has two
phases: for rapid unloading the system suffers only partial failure and it has
an infinite lifetime, while at slow unloading macroscopic failure occurs in a
finite time. The transition between the two phases proved to be analogous to
continuous phase transitions. Computer simulations revealed that during
unloading the fracture proceeds in bursts of local breakings triggered by
slowly accumulating damage. In both phases the time evolution starts with a
relaxation of the bursting activity characterized by a universal power law
decay of the burst rate. In the phase of finite lifetime the initial slowdown
is followed by an acceleration towards macroscopic failure where the increasing
rate of bursts obeys the (inverse) Omori law of earthquakes. We pointed out a
strong correlation between the time where the event rate reaches a minimum
value and of the lifetime of the system which allows for forecasting of the
imminent catastrophic failure.
