Improvement of Fatigue Life of Arc Welded T-joints by Local Shape Change Near the Joint Line

Abstract

The objective of this study is to improve fatigue life of T-shape arc welded joint structures which are often used in automobiles. In conventional T-shape joint, ST-joint (Standard T-joint), a vertical part is welded to a flat surface in the horizontal part. In this paper, a new structure, CCT-joint (Concave shape T-joint), is proposed which can reduce stresses around a joint line in car driving. In a CCT-joint, a vertical part is welded to a concave surface in the horizontal part. Fatigue life of a CCT-joint was significantly longer than that of a conventional ST-joint. Results of this study suggest that the drastic improvement in fatigue life was due to stress reduction by the concave shape around the joint line. FEM stress analysis revealed that the maximum stress near the weld line of CCT-joint was reduced to less than half that of the ST-joint. Furthermore, it was found that the locations of the maximum stress were different between the CCT-joint and the ST-joint. This suggests that the crack origin positions of both joints are different, which was also confirmed experimentally.The geometry, hardness, and residual stresses after welding were investigated around the weld toe. The effects of geometry and hardness were small on the fatigue life. The residual stresses were decreased by CCT-joint. The evaluation of the advantage of residual stress reduction by CCT-joint compared with that of stress reduction in car driving will be investigated and discussed in the next step of this study.