The Strength of Ceramics under Biaxial Stresses

Abstract

An improved apparatus is described for studying the effects of biaxial stress states on the strength of ceramics. This uses a specimen in the form of a tube. Three pressures can be applied to the tube: inside, outside, and to the end caps on the tube. By selection of a ratio of two of these pressures it is possible to induce selected biaxial stress states. A key feature of the system is that the specimen geometry is kept constant for all stress states. REFEL* silicon carbide has been used as a model material and tubes in three surface conditions have been tested in stress states ranging through, longitudinal tension, equibiaxial tension, hoop tension, to longitudinal compression/hoop tension. Surface conditions comprised: the original shot blasted surface; a single longitudinal notch; an indentation-induced single sharp longitudinal crack. In each case the compression/tension strength is similar to the unidirectional tensile strength but the equibiaxial tensile strength is significantly lower (by ~15%). A quantitative evaluation of the data in terms of existing theories is not possible because these are based on assumptions that are too simple for normal ceramics. Highly non-idealised fracture occurs due to interactions between the fracture process and the material microstructure.