  We derive here a linear elastic stochastic description for slow crack growth
in heterogeneous materials. This approach succeeds in reproducing
quantitatively the intermittent crackling dynamics observed recently during the
slow propagation of a crack along a weak heterogeneous plane of a transparent
Plexiglas block [M{\aa}l{\o}y {\it et al.}, PRL {\bf 96} 045501]. In this
description, the quasi-static failure of heterogeneous media appears as a
self-organized critical phase transition. As such, it exhibits universal and to
some extent predictable scaling laws, analogue to that of other systems like
for example magnetization noise in ferromagnets.
