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

Abstract

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.