The Analysis of Cracks

Abstract

The object of studies of fretting is ultimately to be able to quantify the rate of fatigue crack growth, and to be able to describe the conditions under which cracks do not arise, or at any rate do not grow. Traditional approaches to the quantification of fatigue have relied on the assumption that the nominal stress in the absence of the crack is sufficient to determine its behaviour. The classical experiments of Wöhler were designed to measure the fatigue life of railway axles under fully reversing stress, which was calculated from elementary bending theory. A graph of the number of cycles to failure against the bending stress became known as an S-N curve (Dieter, 1961), and subsequent experimental work has enabled the influence of mean stress to be taken into account. However, a bulk approach to fatigue under a multi-axial state of stress has never been fully developed (Toor, 1975 a,b). Further, the initiation and propagation elements of fatigue crack growth cannot be separated by this approach.