Fundamental Concepts for Diagraming/Formulating Fatigue Strength of the Notched and Un-notched Specimen

(Background, Significance and Perspective of the Approach)

Abstract

More than ten years, an author has been developing a study for diagraming and formulating fatigue strength of notched and un-notched specimens. As a result, two fundamental and important concepts have been proposed; a hypothesis of cyclic plastic adaptation and a size effect criterion of a yield-zone-partitioning type. The former describes microscopic actual cyclic stress behavior at a fatigue slip band. From this concept, an equivalent cyclic stress ratio R_EQ and equivalent mean stress σ_meanEQ are derived as a parameter correlating a cyclic stress condition of the notched specimen with that of the un-notched specimen, and as a result a fatigue strength diagram is formulated with a consistent equation for a large-sized notch type and un-notch type specimen. The latter concept of the size effect is formulated based on a normalized yield zone growth curve drawn from a notch root yield zone model (NRYZ model) that has been devised/constructed based on Dugdale model, and as a result a subordinate parameter that concretely quantify the size effect and inserted in the equation of the fatigue strength diagram has been derived. In the present paper, background, significance and perspective of the study are described with reviewing the outline of approach.