Quantitative evaluation of the fatigue limit of a metal with an arbitrary crack under a stress controlled condition - Stress ratio R =-1

In this paper, the result from rotating bending fatigue tests of notched specimens are carried out on aluminum cast alloys and high strength steels with H-B similar or equal to 600 are reported. The threshold stress intensity factor range under a stress controlled condition, DeltaK(w), is introduced for the purpose of predicting the fatigue limit of a metal with an arbitrary crack. The DeltaK(w) value of a long crack, DeltaK(wUL), is obtained from the fatigue crack propagation limit sigma(w2) of specimens with a sharp and deep notch; DeltaK(w) increases with crack length, and DeltaK(wUL) is an upper limit of DeltaK(w). Since there are few sigma(w2) data of steels with H-B greater than or equal to 400, the sigma(w2) values are evaluated by the fatigue crack initiation limit predicted using Linear Notch Mechanics and the relation between sigma(w1) and sigma(w2) at the branch point. Using DeltaK(wUL) values of many metals with a long crack, the DeltaK(wUL) values are approximated with a simple formula. Moreover the lower limit value of DeltaK(w) versus the crack length, DeltaK(wLL), is proposed. Then using the DeltaK(w) and DeltaK(wUL) formulae and the DeltaK(wLL) value, the smallness of a fatigue crack is clarified.