Abstract
In the present paper, fatigue strength of a notched specimen subject to multi-axial cyclic stress is formulated as a function of an equivalent cyclic stress ratio (named as R_<EQ> -ratio). R_<EQ> -ratio is derived from a hypothesis of plastic adaptation that reflects micro-mechanical behavior of a persistent slip band, and it is proposed as a corresponding parameter between cyclic stress conditions of notched and un-notched specimens subject to multi-axial cyclic stress. R_<EQ> -ratio is given as function of a theoretical stress concentration factor K_t and a nominal cyclic stress ratio RN. A notch-root stress range Δσ_<NR> of the multi-axial stress condition is defined based on Kawamoto's Fatigue Criterion. Fatigue strength is plotted on R_<EQ> -Δσ_<NR> diagram. As a result, the fatigue strength σ_<w1> and σ_<w2> are clearly distinguished. A notch size effect is considered on the basis of the notch behavior map and a size effect factor is introduced for each of σ_<w1> and σ_<w2> . Consequently, both fatigue strength diagrams of σw1 and σw2 are formulated.
