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)

Abstract

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 Hv=582. The fatigue limit of a long crack, σw2, was obtained from that of a notched specimen with a sharp and deep notch whose radius is smaller than the branch point ρo. Using the present and past σw2 data, the effect of the positive mean stress on the ΔKw of the long crack was evaluated by using 3.2HB (≅σB) and σs/ (3.2HB), where ΔKw is the stress intensity factor range for the purpose of predicting the fatigue limit of a metal with an arbitrary crack, σs is the lower yield stress or 0.2% proof stress, σB is the ultimate tensile strength and HB is the Brinell hardness. Although the σw2 decreased with the mean stress, σw2 was cut off by the threshold stress σw2 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 σw2 and σw2 values were approximated with simple formulae.