CRITERION for FRACTURE TRANSITION to CRITICAL STAGE

The paper presents analysis of the data obtained in laboratory investigations of deformation of rocks by acoustic emission and X-ray microtomography. We show that the defect accumulation occurs in fundamentally differing manners during loading. At first defects are generated randomly and have a specific size determined by a typical structural element of a material (for example, a grain in granite). Then the defects the sizes of which are not dictated by the material structure are generated. The interaction between these defects gives rise to critical defects which are capable of self-development. In all probability, a sample breakdown results from the evolution of the ensemble of critical defects. We show that the fracture stages can be distinguished by the type of the energy distribution function of acoustic emission signals. At the first stage the distribution is approximated by an exponential function, while the second stage is characterized by a power-law function which points to a self-organized criticality state. This approach allows an early prediction (at early stages of deformation) of the spatial region in which a fault can be formed.