Microstructure‐Strength Properties in Ceramics: II, Fatigue Relations

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Abstract
The study of crack‐size effects in aluminas and other selected ceramics in Part I is here extended to dynamic fatigue properties. Controlled flaws are used to measure the fatigue response in the large‐crack (indentation‐controlled) and small‐crack (microstructure‐controlled) regions. It is demonstrated that the “microstructural driving forces” responsible for the R‐curve behavior are readily accommodated into existing indentation fracture theories of fatigue strengths. The modified theory provides well‐defined solutions for the strengths in terms of stressing rate and indentation load. Two load‐invariant quantities, relating to the exponent and coefficient in an assumed power‐law crack velocity function, are sufficient to define the entire data set for a given material, at all stressing rates and loads. This is demonstrated graphically by reducing such data sets onto universal fatigue diagrams. The data for sapphire do not coincide with those for the poly crystalline aluminas, suggesting again that it is the grain‐boundary structure which holds the key to the fracture properties in the latter. From the standpoint of reliability, the study emphasizes the need to account for microstructural effects when extrapolating to the domain of naturally occurring flaws. In this context, the adjustable quantities obtained from the dynamic fatigue data fits emerge as appropriate design parameters.

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