This study investigates the factors influencing the fatigue strength of welded joints made of ultra-high-strength steel sheets (980 MPa grade) and high-strength steel sheets (440 MPa grade) for automotive chassis components. Although the application of high-strength materials does not necessarily improve the fatigue strength of welded joints, the underlying mechanisms remain unclear. This research focuses on the effects of stress concentration and microstructural characteristics near the weld toe. Experimental results reveal that, although the hardness near the fusion line in the 980 MPa grade joint is comparable to that of the base material, its fatigue strength is lower. In contrast, the 440 MPa grade joint exhibits a fatigue strength consistent with its hardness. This study proposes a correction factor based on microstructural parameters, which is incorporated into a fatigue crack initiation model proposed by Toyosada et al., resulting in improved agreement between calculated and experimental fatigue life results. Overall, the findings underscore the critical roles of microstructural characteristics and stress concentration in determining the fatigue performance of welded joints.
