Universal R-curve of Crack Propagation Resistance in Ceramic Composites

An analysis of a shape of KR-curves of subcritical crack propagation resistance in ceramic composites is performed on the basis of energy concept of fracture mechanics. Two cases of a broadening of frontal process zone and of crack bridging zone are considered. The existence of “universal K -curve” is proved which is generally described with l’ = F(K’) relationship, where l’ = 1/B is the dimensionless crack increment; F(K’) is a certain function of dimensionless increment of stress intensity factor K’; B is a factor depending on the structure and properties of a material and characterizing the extent of a subcritical crack growth. An analytical expression for F(K’) function have been derived and it is shown that as first approximation “universal K -curve” can be approximated with parabolic law. Examples are R givenfor the composites alumina - silicon carbibe whiskers and lanthanium chromite - chromium as well as for porous zirconia ceramics.