A NEW METHOD OF FRACTURE-TOUGHNESS DETERMINATION IN BRITTLE CERAMICS BY OPEN-CRACK SHAPE-ANALYSIS

Improvement of the fracture toughness of high-quality ceramics remains one of the most important goals in materials development. An associat ed problem is the accurate measurement of fracture toughness in such b rittle or semi-brittle ceramics, particularly in small samples encount ered in material development. Previously used methods relying on measu rement of the size of fracture mirrors, the indentation load and crack length in Vickers hardness-induced cracking, and a variant of similar techniques, have all been less than satisfactory in discriminating qu antitative differences among materials. A hitherto unused technique of inferring the fracture toughness in samples from measurements of open -crack flank displacements, which we have developed, avoids most of th e theoretical and experimental difficulties of other methods. While it is somewhat intensive in terms of evaluation and requires high resolu tion of open cracks, the technique is fundamentally the soundest of al l techniques and is capable of furnishing discriminating results. We p resent results of its application to the measurement of some model mat erials such as soda-lime glass, single-crystal silicon, alumina, and a reaction-bonded silicon nitride whose porosity would ordinarily prese nt difficulties with other methods.