Acoustic emission is a spontaneous radiation of elastic waves in solids and solid structures during some irreversible processes, such as formation and development of brittle cracks, plastic deformation, dry friction, etc.. The processes of formation and growth of cracks are accompanied by the radiation of elastic waves into the solid containing the crack and into the surrounding space; the intensity of the radiation is sometimes so great as to be perceptible to the ear. The practical importance of this effect lies mainly in two aspects: it can be used for prediction of catastrophic failure of important engineering structures and also for research into solid state physics and fracture mechanics. The former aspect is especially important since radiated elastic waves provide information about developing cracks, which present the greatest danger.
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