The arrest conditions due to the action of the elastic fields of a dipole of wedge disclinations and a planar shear mesodefect are considered for dislocation cracks propagating under external loading. The regions of stable cracks existence are determined using the method of configurational force in the configuration space of the system parameters (the mesodefect strength vs the Burgers vector of the crack) and the maps of the cracks lengths distribution were plotted at various values of the external stress. The length of the cracks turns out to be comparable with the mean size of fragments and lies in the range of 0.1-0.3 μm at the parameters of the fragmented structure and values of the external stress typical for the stage of material pre-fracture. It is shown, that both in the case of the disclination dipole and in the case of the planar shear mesodefect the regions of instability of cracks shift towards lower dislocation charge of the crack and higher values of the mesodefects strength with an increase in the external stress. It is concluded, that the considered mesodefects can effectively arrest the propagation of cracks in the fragmented structure, thereby providing their accumulation in certain areas of the material and creating the "fracture nuclei".