Fundamental Concepts for Formulating Fatigue Strength Diagrams of Notched Metals

Abstract

In the present paper, two fundamental concepts are discussed for formulating fatigue strength diagrams of notched metals. The first is a hypothesis of cyclic plastic adaptation that reflects mechanical behavior/property inherent in a persistent slip band. From this hypothesis, an equivalent cyclic stress ratio , which is the corresponding parameter between the cyclic stress condition of a notched and un-notched specimen, is derived. is extended to multi-axial cyclic stress conditions by use of the potential stress defined by Mises', Kawamoto's criterion, etc. The fatigue strength of a notch specimen is diagramed as a relation between and the notch root stress range . As a result, the fatigue strength is characterized into the two types of and , which can be replaced by the fatigue strength in the surface layer of the un-notch specimen and by the threshold stress of the crack specimen with the same crack depth as the notch depth, respectively. The second idea is how to express parametrically notch size factors based on a notch behavior/property map. For the type of , the notch size factor is expressed by a power function of a square root of a product of a notch root radius and a notch depth , and for the type of as a power function of the notch depth . These factors can be also applied to multi-axial cyclic stress conditions.