Abstract: The dynamics of nucleation of microcracks in the region of a developing macroscopic crack has been studied using scanning electron microscopy during in situ experiment and the acoustic emission method. An explosive-like nucleation of microcracks and the influence of dissipative properties of a material on the size and the rate of redistribution of local stresses of the microcracks have been established. Each of nucleations of microscopic and mesoscopic defects is considered as an act of local testing of the dissipative ability of the system, and the interaction between microcracks is determined by relaxation processes when the ability is sufficient. With deteriorating dissipative properties (with increasing residence time under loading, deformation rate, etc.), an elastic linear interaction becomes possible, and the macroscopic fracture occurs. In the microcrack dynamics, a specific ductile-brittle transition is determined by a time deterioration of the dissipative properties of the material under the action of a mechanical load. Generally, as a large latent energy is stored during deformation, a macroscopic fracture can be caused by any little discrete structural transformation.