Scaling and Correlation of Time Recurrence in Fracture Systems

Fracture in heterogeneous materials occurs as the culmination of progressive damage due to loading conditions or harsh environments. Damage growth is accompanied by the spontaneous release of stored strain energy in the form of transient elastic waves (acoustic emission, or AE) [1-3]. Numerous investigations established that AE phenomenon varies while materials experience evolving damage [4-6], being concentrated in different frequency and amplitude ranges at different stages of damage process. In particular, the observed frequency drops in the late stages suggest the transition from micro-cracking regime to large cracks formation, which eventually leads to the material failure. Recently, a distinction between high-frequency and low-frequency AE, the latter called elastic emission or ELE, has been proposed [6]. ELE are defined as AE whose half-wavelength is greater than the maximum size of propagation medium. While high-frequency AE are pure vibration modes of the body, including longitudinal (P-), shear (S-) and surface (Rayleigh) waves, due to micro-crack growth [4, 7] , ELEs are quasi-rigid body vibrations due to large crack growth. They result from the specimen flexibility and the constraints (specimen-platen contact with friction during laboratory fracture tests). The propagation of macrocracks, revealed by large stress drops, is preceded by bursts of radially expanding dilatational pulses generated by crack opening.