Abstract: The fatigue strengths of welded joints in welded structures are much lower than those of the base metal because there are stress concentrations and tensile residual stresses at weld toes. Ultrasonic peening (UP) treatment and the use of low transformation temperature electrodes (LTTE) are new developed methods for improving the fatigue strengths of welded joints. The fatigue tests of two methods are carried out on both transverse butt welded joints and longitudinal fillet welded joints. The fatigue strengths at 2×106 cycles of the two kinds of welded joints treated by ultrasonic peening are respectively improved by 50% and 71% compared with those of the as-welded condition. The fatigue strengths at 2×106 cycles of the two kinds of welded joints welded with low transformation temperature electrodes are respectively improved by 11% and 41% compared with those of E5015. In addition, their fatigue lives are also increased by from several times to hundred times at different stresses. The test results show that the UP and LTTE are effective methods to improve the fatigue strengths of welded joints. It is suggested that UP is used to improve the fatigue performance of transverse butt welded joints, but it is thought that the LTTE may be particularly suitable for longitudinal welded joints containing high tensile residual stresses since it is not necessary to add another process after welding.

Key words: Fatigue strengths, Low transformation temperature electrode, Ultrasonic peening, Welded joints