  Crack formation is conventionally described as a nucleation phenomenon
despite the fact that the temperatures necessary to overcome the nucleation
barrier are far too high. In this paper we consider the possibility that cracks
are created due to the presence of frozen disorder (e.g. heterogeneities or
frozen dislocations). In particular we calculate the probability for the
occurrence of a critical crack in a quasi two-dimensional disordered elastic
system. It turns out that this probability takes the form of an Arrhenius law
(as for thermal nucleation) but with the temperature T replaced by an effective
disorder temperature T_eff which depends on the strength of the disorder. The
extension of these results to d=3 dimensions is briefly discussed.
