Cracks and energy—Criteria for brittle fracture

The energies of cracks are considered in terms of the first and second laws of thermodynamics, and it is concluded that the Griffith criterion provides a necessary but not sufficient condition for crack propagation. Fracture occurs only when the crack tip radius is larger than a critical value that is several times the interatomic spacing. Surface energies calculated from fracture experiments probably give only an upper bound to the true surface energies of solids. The radius of the crack tip is a parameter that cannot be ignored in fracture experiments.