In this paper, quasi-static loading tests and fatigue tests are carried out on spot welded joints of mild steel (270MPa class) and ultra-high strength steel (980MPa class) sheets in order to investigate the influence of strength level of base steels on fatigue strength and fracture mechanism of spot welded joints. Under quasi-static loading, a spot welded part is more liable to be a fracture-initiation site in the ultra-high strength steel than in the mild steel, and as a result fracture behavior of the ultra-high strength steel is considerably affected by a spot welded part as compared with that of the mild steel. Although the fatigue strength of spot welded joints is higher in the ultra-high strength steel than in the mild steel in low cycle fatigue regime, the difference in fatigue strength decreases in high cycle fatigue regime and the fatigue limit is almost the same in both steels. Fracture morphology of spot welded joints under cyclic loading depends on the load level in the ultra-high strength steel, but not in the mild steel. From the comparison between S-N curves for spot welded joints and notched base steels, the dependency of fatigue strength of spot welded joints on base steels is attributed to the notch-sensitivity to fatigue strength of the base steels.
