Investigation of the fatigue behaviour of the welded joints treated by TIG dressing and ultrasonic peening under variable-amplitude load

Weld toe treatment by ultrasonic peening (IIW.Doc.XIII-1817-00; J. Mech. Strength 21(1999)289; Welding World (3/4)(2001); Welding World (37)(1996)72) or TIG dressing (Trans. Jpn. Welded Soc. 17(2)(1986)3; Int. J. Fatigue 21(6)(1999)587; Metal Construction 19(2)(1984)143; Int. J. Fatigue 20(9)(1998)677) improves the fatigue performance of welded joints and structures significantly. This has been verified by many constant amplitude fatigue tests. However, there is the need to check their benefits for structures subjected to variable-amplitude loading. Therefore, fatigue tests were performed on fillet welded joints in 16Mn steel for three different conditions: as-welded, TIG dressed and after treatment by ultrasonic peening. The beneficial effects of both TIG dressing and ultrasonic peening were found to be less under variable amplitude than under constant amplitude loading. In particular, under constant amplitude loading TIG dressing increased the fatigue strength by 37% and the fatigue life by 2.5 times. In contrast, under variable-amplitude loading the corresponding benefits were 34% increase in fatigue strength and 1.7–1.9 times increase in fatigue life. The improvement in fatigue performance due to ultrasonic peeing depended on the applied stress, being negligible at stresses approaching yield, but greater than that due to TIG dressing in the low stress/high-cycle regime. Under constant amplitude loading, ultrasonic peening increased the fatigue strength by up to 84% and the fatigue life by 3.5–27 times. In contrast, under variable-amplitude loading the corresponding benefits were 80% increase in fatigue strength and 2.5–17 times increase in fatigue life. For either constant or variable-amplitude loading, the improvement in fatigue strength of the welded joints due to ultrasonic peening was greater than that due to TIG dressing, but only in the low stress/high-cycle regime.