Quantitative Evaluation of Fatigue Limit of a Metal with an Arbitrary Crack under a Stress Controlled Condition (2nd Report, Steel under a Positive Mean Stress)

In this paper, push-pull fatigue tests of notched specimens under R = — 1, 0 and 0.5 were carried out on annealed 0.1% carbon steel with the Vickers hardness Hv = 105 and quenched-tempered 0.5% carbon steel with H_V = 582. The fatigue limit of a long crack, σ _W2 was obtained from that of a notched specimen with a sharp and deep notch whose radius is smaller than the branch point ρ _0. Using the present and past σ _W2 data, the effect of the positive mean stress on the ΔK_w of the long crack was evaluated by using 3.2H_B (≃ σ _B) and σ _s/(3.2H_B), where ΔK_w is the stress intensity factor range for the purpose of 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 decreased with the mean stress, σ _W2 was cut off by the threshold stress σ _W2 of the plastic deformation at crack tip. For the quantitative prediction of the fatigue limit of a cracked steel with an arbitrary crack length under a positive mean stress, he σ _W2 and σ _we values were approximated with simple formulae.