The failure of materials is a complex and complicated process exhibiting broad phenomenology. The fracture of heterogeneous media under slow external loading displays intermittency  and scale invariance in both dynamical and morphological quantities. In Pontuale et al., EPL 101, 16005 (2013) we have shown that the intermittent and self-similar fluctuations displayed by a slow crack during the propagation in a heterogeneous medium can be quantitatively described by an extension of a classical statistical model for fracture. The model yields the correct dynamical and morphological scaling, and allows to demonstrate that the scale invariance originates from the presence of a non-equilibrium, reversible, critical transition which, in the presence of dissipation, gives rise to self-organized critical behaviour.