EVALUATION OF MICROFRACTURE PROCESS IN ALUMINA BY ACOUSTIC EMISSION

Abstract Microfracture process during the bending tests of alumina ceramics were evaluated by acoustic emission technique. Specimens with different dimensions were used for the bending tests in order to investigate the dependence of microfracture process on the specimen size. A remarkable point in AE generation pattern of each specimen, at which both AE events and energy increased rapidly, was observed before the final unstable fracture. It is important that the apparent stress, σc, at those points were independent of the AE threshold level and specimen size The locations of AE sources, i.e. microcracks, were determined using the differences of arrival times between 2 ch transducers. Those were distributed widely before the stress was lower than σc, and they were concentrated on the point which would become the origin of final unstable fracture. Using the fluorescent dye penetrant method, fracture process on the surface of bending specimens were observed. Those results demonstrated that the stress, σc, corresponds to the critical stress for the maincrack formation due to the coalescence of microcracks and/or pores. Consequently, it was concluded that the critical stress can be the new evaluation parameter, which is equivalent to yield strength in metals, for ceramic materials.