Mechanical energy relaxation is considered from the viewpoint of physics of fracture. It is shown that the grinding limit is equal to the size of tough-brittle transition. Decreasing the size of particles beyond this size is accompanied by changes in relaxation mechanism from brittle fracture to tough flow. This change leads to substantial growth in structure distortions and may be identified with a deep stage of mechanical activation. To characterize mechanical treatment in a milling device, distribution of impacts by stress is introduced. An equation describing kinetics of activation is deduced.