Formulation of Fatigue Strength Diagrams of Notched Material: Consideration on the Factors for Estimating Notch Size Effects

摘要

Equations expressing fatigue strength diagrams of notched materials have to explain both of material dependence and notch size dependence of notch effects. From such a viewpoint, two concepts have been introduced into the study. The first is a hypothesis of cyclic plastic adaptation that reflects mechanical behavior/property peculiar to persistent slip bands, and the parameter of an equivalent cyclic stress ratio that is derived from the hypothesis and correlates very well the fatigue strength diagrams of materials including large-sized notches. The second idea is introduction of some notch size factors into the equation of the fatigue strength diagram in order to take a size effect into consideration. The concrete forms of these size factors are determined independently in a parametric style based on the distribution of the fatigue strength σ_(w1) and σ_(w2) that are drawn on a notch behavior map. At the beginning, the notch size factors are determined empirically by trial and error rather than analytically, though they are done based on a systematic way to some extent. However, the subsequent research has proved that the notch size factors can be determined analytically as numerical expressions based on a notch root yield zone model, which is extended from Dugdale Model. In the present paper, how to determine the notch size factors is described as a main subject.