Investigation on improving fatigue properties of welded joints by ultrasonic peening method

Fatigue strength of welded joints in welded structures are much lower than that of base metals. Many experiments show that the fatigue crack normally initiates at welded toe, so that the fatigue strength can be increased dramatically by peening weld toe.([1,2,3]) Ultrasonic peening made by Tianjin University in China under the financial support of Natural Science Foundation of China is one of the most useful methods to improve fatigue behavior of weld toe due to improving toe geometry, removing defects and modifying the residual stresses distribution in this region. For evaluating the ultrasonic peening performance carried out by our equipment, the fatigue tests were performed on butt and cruciform joints of Q235B steel both in the as-welded and peened condition. Test results are as follows: 1.Both butt and cruciform peened joints show a significant increase in fatigue life under different stress levels (high cycle fatigue). The results show that the fatigue life of the peened weld toe was 20 similar to 30 times as long as the as-welded joints, and in many cases the fatigue cracking initiation was transferred to the base metal instead of the weld toe. 2.The increase in fatigue strength (at 2x10(6) cycles)of the peened Q235B butt welded joints compared to the as-welded joints was 57%,and for the cruciform joints, the increase was 64 similar to 71%. The fatigue strength of both the butt and cruciform welded joints were no lower than that of the base metal. In such cases, weld joints is not the degense location any longer.