Shift of S-N curves with stress ratio

The fatigue strength of transverse butt-welded joints of SB410, SPV490 steels and SUS304-HP stainless steel was evaluated experimentally at R = -1, 0 and 0.5 and by σmax = σy tests. The yield strengths for the SB410, SPV490 and SUS304-HP were 284, 579 and 275 MPa. The fatigue strength was found to decrease at the higher R values, even up to around 0.8 in the case of the SPV490. In the case of SB410, the maximum stress at R = 0.5 usually exceeded the yield strength, and some of the fatigue strengths were lower than those obtained from the σmax = σy tests. The fatigue limit was slightly higher compared with that from the σmax = σy tests. In the case of the SUS304-HP joints, the maximum applied stress at R = 0.5 was higher than the yield strength even at the fatigue limit. Therefore, some of the fatigue strengths at R = 0.5 were lower than those from the σmax = σy tests. An empirical formula for the fatigue limit as a function of R, which should be applicable to stress-relieved welded structures, was derived.