Effect of pretension on fatigue life of stainless steel resistance spot welded joint

In the fatigue test of stainless steel resistance spot welded joints,most of the cracks occurred near the spot welded joints. The reason was ascribed to the residual stress that existed in resistance spot welded joints. In this paper,fatigue tests were carried out on the spot welded specimens with different thickness of 3.5 mm and 4. 5 mm of SUS301 stainless steel,and the median fatigue life under given stress was obtained. Then the fatigue test was carried out again after the residual stress of the spot welded joint was weakened by pretension,and the optimal load interval was analyzed. At the same time,the residual stress of welded and tensile specimens was tested,and the improvement mechanism of fatigue life was analyzed. The results showed that the fatigue life of stainless steel spot welded joints was significantly improved after the pretension load T was loaded. With the increase of T,the improvement degree of fatigue life of the joint also increased,but when T reached more than 55%,the joint would be deformed to affect performance. The optimal selection interval of T was 50% \barF,in which the fatigue life of the joint could be improved from the short and medium life zone to the infinite life zone. After selecting the optimal pretension load,the residual tensile stress in the spot welded joint decreased greatly in the stress direction and could be less than 100 MPa,while the stress in the vertical direction could also be reduced to 1/3.