Evaluation of Critical Stress for Microcracking in Alumina Ceramics by Acoustic Emission

The microfracture process during the bending tests of alumina ceramics was evaluated by the acoustic emission technique. Specimens with different dimensions were used for the bending tests in order to investigate the dependence of the microfracture process on the specimen size. A remarkable point in the 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 at those points is independent of the AE threshold level and specimen size. Using the fluorescent dye penetrant method, it has been made clear that the stress corresponds to the critical stress for the main-crack formation due to the coalescence of microcracks and/or pores. Considering the micro-fracture process, statistical treatment for the strength of brittle materials has been discussed. 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.