Theory of Static Fatigue in Brittle Solids. Cracks and Energy -Criteria for Brittle Fracture.

摘要

Two studies are included. In the first study, a theory of static fatigue in brittle solids is derived, based on the ideas of Hillig and Charles, but using a different log reaction velocity-stress relationship. The resulting equation agrees well with the functional dependence of failure time on stress found experimentally, and experimental values of the stress-sensitivity factor are consistent with the theory. Variations in failure times with sample history and glass composition can only partly be explained by the theory. In the second study, 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. Thus surface energies calculated from fracture experiments give only an upper bound to the true surface energies of solids. The correct criterion for fracture is Inglis' equation for the stress at the tip of a crack; the radius of the crack tip is a parameter that cannot be ignored in fracture experiments.