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 HrmBsimeq 600. The fatigue limit of a long crack, sigmaw2, is obtained from that of a notched specimen with a sharp and deep notch whose radius is smaller than the branch point rho0. Using the present and past sigmaw2 data, the effect of the positive mean stress on the dKw of the long crack is evaluated using 3.2 HrmB (simeqsigmarmB) and sigmarmS(3.2 HrmB), where dKw is the stress intensity factor range for predicting the fatigue limit of a metal with an arbitrary crack, sigmarmS is the lower yield stress or 0.2% proof stress, sigmarmB is the ultimate tensile strength and HrmB is the Brinell hardness. Although the sigmaw2 decreases with the mean stress, sigmaw2 is cut off by the threshold stress sigmawe 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 sigmaw2 and sigmawe values are approximated by simple formulae.