The acoustic emission analysis of the crack processes in alumina

The crack processes in single-phase alumina specimens have been investigated by acoustic emission (AE) analysis with regard to the increase of the crack resistance. During loading of a notched specimen, individual AE signals are observed at first, which are probably due to the generation of microcracks in a process zone around the notch. At higher loads signal clusters are found, which should be due to the coalescence of microcracks. By these coalescence events the main crack is formed. At macroscopic crack propagation most AE events are located within the crack tip zone. However, up to about 20% of all events are located within the crack flank zone behind the crack tip. Thus, it can be concluded that there is an energy dissipation at the crack tip at beginning of the loading, which determines the starting value of the crack resistance. At macroscopic crack propagation crack flank interactions contribute to the increase of crack resistance, too. However, it cannot be decided from AE if the contribution of the process zone at the crack tip or of the crack flanks in the wake of the crack tip play the major role in increasing the crack resistance.