GRAIN-SIZE AND POROSITY DEPENDENCE OF CERAMIC FRACTURE ENERGY AND TOUGHNESS AT 22-DEGREES-C

A review of the fracture energy and toughness data for dense ceramics at 22 degrees C shows maxima commonly occurring as a function of grain size. Such maxima are most pronou need for non-cubic materials, where they are often associated with microcracking and R-curve effects, esp ecially in oxides, but often also occur at too fine a grain size for a ssociation with microcracking. The maxima are usually much more limite d, but frequently definitive, for cubic materials. In a few cases only a decrease with increasing grain size at larger grain size, or no dep endence on grain size is found, but the extent to which these reflect lack of sufficient data is uncertain. In porous ceramics fracture toug hness and especially fracture energy commonly show less porosity depen dence than strength and Young's modulus. In some cases little, or no, decrease, or possibly a temporary increase in fracture energy or tough ness are seen with increasing porosity at low or intermediate levels o f porosity in contrast to continuous decreases for strength and Young' s modulus. It is suggested that such (widely neglected) variations ref lect bridging in porous bodies. The above maxima as a function of grai n size and reduced decreases with increased porosity are less pronounc ed for fracture toughness as opposed to fracture energy, since the for mer reflects effects of the latter and Young's modulus, which usually has no dependence on grain size, but substantial dependence on porosit y. In general, tests with cracks closer to the natural flaw size give results more consistent with strength behaviour. Implications of these findings are discussed.