Acoustic emission at failure in quasi-brittle materials

Development of failure in quasi-brittle materials is associated with microcracks, which release energy in the form of elastic waves called acoustic emission. From three types of tests - diametral compression, flexure and indentation - on specimens of high strength concrete, locations of acoustic emission were observed to form in the critically stressed region in a more or less diffuse manner for stress below the material's strength. However, as damage increased and peak stress was reached, a coalescence or localization of damage occurred and the zone formed a distinct volume, the so-called intrinsic process zone. Typically, the zone was similar in length for specimens of various size. Thus, it appeared that the acoustic emission technique was able to identify a characteristic length of the quasi-brittle material.