D. P. H. Hasselman

The Griffith Criterion and the Reversible and Irreversible Fracture of Brittle Materials

On the basis of the concepts of global and local instability of mechanical systems, it is concluded that the Griffith criterion represents a necessary, but not sufficient condition for catastrophic fracture. Instead, a brittle material with a microcrack will have two values of critical fracture stress. The lower critical fracture stress which corresponds to the Griffith criterion for reversible fracture, represents the minimum stress level for slow crack growth by a reversible thermally activated process, i. e the fatigue limit. The higher critical fracture stress corresponds to the stress required for irreversible catastrophic fracture, which requires an energy expenditure in excess of the surface free energy of the new crack surfaces even in the absence of energy dis-sipative processes, such as plastic flow at the crack tip. Over the stress range intermediate of the two critical fracture stresses, the crack exhibits slow crack growth, without requiring the presence of stress-corrosion reactions. It is concluded that the surface free energy of a material cannot be measured by a fracture experiment.

Thermal Fatigue and Its Failure Prediction for Brittle Ceramics

The thermal fatigue behavior of a soda-lime-silica glass subjected to water quench and silicon-nitride subjected to thermal environment of a turbine engine was predicted from data of slow (subcritical) crack growth. A numerical integration technique was developed to calculate the extent of slow crack growth for each thermal cycle over the total duration of the transient thermal stress and temperature, as well as the total number of cycles required for catastrophic failure to occur. Good agreement between the predicted and experimental data was found. The results indicate that, for reliable prediction of thermal fatigue resistance, an estimate of critical flaw-depth based on a statistical (such as the Weibull) theory of brittle fracture is necessary.