In the framework of nonequilibrium evolution thermodynamics, the influence of additive fluctuations on the kinetics of structural defects under severe plastic deformation has been studied. The applied method is a new one for the description of fragmentation modes and corresponding self-organization processes. It is found that a fragmented metallic specimen demonstrates a self-similar behavior, which results in the formation of a grain structure with various grain sizes. Such a behavior takes place provided that the probability distribution for the grain boundary density has a power-law dependence. A comparison of the results obtained in the Itˆo and Stratonovich forms demonstrates the absence of qualitative changes in the behavior of the system.