Microstructural dependence of fracture energy and toughness of ceramics and ceramic composites versus that of their tensile strengths at 22 degrees C

The microstructural dependence of fracture energy and toughness of ceramics and ceramic particulate, platelet, and whisker composites is compared with the corresponding dependence of their tensile (flexure) strengths at 22 degrees C. These comparisons show that fracture energy and toughness often do not have the same porosity, or grain- or particle-size dependence as strength. This is attributed to the scale of the cracks for measuring fracture energy or toughness often being too large in comparison to the cracks controlling strength. The large cracks reflect crack-microstructure interaction phenomena such as crack-wake bridging and R-curve effects that are not, or are much less, involved in the control of propagation of most strength-controlling cracks. Thus fracture mechanics must account for the scale of the cracks used in measuring fracture mechanics parameters relative to the scale of the cracks controlling the strength behaviour that is to be explained or predicted.