日本金属学会誌

J. Japan Inst. Metals, Vol. 49, No. 9 (1985),
pp. 707-713

Fracture Process during Fracture Toughness Testing in Al2O3 Ceramics by AE Source Characterization

Teruo Kishi1, Shuichi Wakayama2, Yasuo Shinozaki3, Yutaka Kagawa4 and Eiichi Nakata5

1Faculty of Engineering, The University of Tokyo, Tokyo
2Graduate Student, The University of Tokyo, Tokyo
3Asahi Glass Co., Ltd., Yokohama
4The Casting Research Laboratory, Waseda University, Tokyo
5Nakata Seisakusho, Tokyo

Abstract:

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 μ m, were detected by small amplitude AE signals at 80% of final unstable fracture load, and three large microcrackings, of which sizes of 170--190 μ m 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.


(Received 1985/1/10)

Keywords:

alumina ceramics, fracture toughness, microcracking, acoustic emission, acoustic emission source characterization


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