The fracture process in Al2O3 ceramics was studied by Acoustic Emission (AE) source characterization technique and fractography. AE source function, which estimated the size and nucleation rate of microcracking quantitatively, was obtained by deconvolution integral of detected signals with the dynamic Green’s function of elastic medium and response function of measuring system.
Three points bending tests gave the bending strength of 350 MPa and the microcracks nucleation stress of 210 MPa. On the other hand, the plane strain fracture toughness of the present material was evaluated as KIC=5.5 MPa \sqrtm by double-torsion tests.
In the fracture toughness testing of a 1 inch compact tension specimen, intergranular small microcracks, of which sizes were less than 20 μm, were detected by small amplitude AE signals at 80% of final unstable fracture load, and three large microcrackings, of which sizes of 170-190 μm were evaluated by AE source characterization, were observed at over 90% of final fracture load. These large microcrackings correspond to the coalescence of small microcracks and/or transgranular cracking of coarse grains.
It is concluded that final unstable fracture occurs when one of these large microcrackings has grown and reached to the critical size.
