Geometrical effects in fatigue: a unifying theoretical model

This paper presents a theory of fatigue behaviour in materials which encompasses two areas of the subject-the behaviour of cracks and the behaviour of notches-and which also accounts for size effects in these two types of geometrical feature. The basis of this theory is well established, and has been used by others to explain notch behaviour; it involves consideration of the elastic stresses over a critical distance ahead of the feature. In this paper it is shown that, by applying the same approach to cracked bodies, one can find the critical distance for a given material, avoiding the use of an empirically-derived value. This approach correctly predicts the fatigue limits of bodies containing both cracks and notches and also predicts the experimentally-observed effects of feature size on fatigue limit. It may be extended to consider differences between tensile and bending test results and the differing effects of surface treatments. The implication of this work is that there is no fundamental difference between the fatigue limit of an uncracked body-for which crack initiation is necessary-and that of a body which already contains a crack. The results are of practical value because the approach can easily be extended to cover geometrical features of any shape and size, such as occur in engineering components. (C) 1999 Elsevier Science Ltd. All rights reserved.