  Statistics and thermally activated dynamics of crack nucleation and
propagation in a two-dimensional heterogeneous material containing quenched
randomly distributed defects are studied theoretically. Using the generalized
Griffith criterion we derive the equation of motion for the crack tip position
accounting for dissipation, thermal noise and the random forces arising from
the defects. We find that aggregations of defects generating long-range
interaction forces (e.g., clouds of dislocations) lead to anomalously slow
creep of the crack tip or even to its complete arrest. We demonstrate that
heterogeneous materials with frozen defects contain a large number of arrested
microcracks and that their fracture toughness is enhanced to the experimentally
accessible time scales.
