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

摘要

In this paper, push-pull fatigue tests of notched specimens under R = -l, 0 and 0.5 are carried out on annealed 0.1 percent carbon steel and quenched-tempered 0.5 percent carbon steel with H_B approx = 600. The fatigue limit of a long crack, sigma_(w2), is obtained from that of a notched specimen with a sharp and deep notch whose radius is smaller than the branch point rho_0. Using the present and past sigma_(w2) data, the effect of the positive mean stress on the DELTA K_w of the long crack is evaluated using 3.2H_B (approx = sigma_B) and sigma_S/(3.2H_B), where DELTA K_w, is the stress intensity factor range for predicting the fatigue limit of a metal with an arbitrary crack, sigma_S is the lower yield stress or 0.2 percent proof stress, sigma_B is the ultimate tensile strength and H_B is the Brinell hardness. Although the sigma_(w2) decreases with the mean stress, sigma_(w2) is cut off by the threshold stress sigma_(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 sigma_(w2) and sigma_(we) values are approximated by simple formulae.