Fatigue limit of steel with an arbitrary crack under a stress controlled constant with a positive mean stress

In this paper, push-pull fatigue tests of notched specimens under R = -1, 0 and 0.5 are carried out on annealed 0.1 % carbon steel and quenched-tempered 0.5 % carbon steel with H_B ≃ 600. The fatigue limit of a long crack, σ_w2, is obtained from that of a notched specimen with a sharp and deep notch whose radius is smaller than the branch point ρ₀. Using the present and past σ_w2 data, the effect of the positive mean stress on the δK_w of the long crack is evaluated using 3.2 H_B (≃σ_B) and σ_S/ (3.2H_B), where δK_w is the stress intensity factor range for predicting the fatigue limit of a metal with an arbitrary crack, σ_S is the lower yield stress or 0.2% proof stress, σ_B is the ultimate tensile strength and H_B is the Brinell hardness. Although the σ_w2 decreases with the mean stress, σ_w2 is cut off by the threshold stress σ_we of the plastic deformation at the crack tip. For the quantitative prediction of the fatigue limit of a cracked steel with an arbitrary crack length under a positive mean stress, the σ_w2 and σ_we values are approximated by simple formulae.