In this paper, fatigue tests and finite element analysis are carried out on spot weld-bonded joints of mild steel (270MPa class) and ultra-high strength steel (980MPa class) in order to investigate influence of strength level of base steels on fatigue strength and fatigue fracture behavior of spot weld-bonded joints. From the fatigue tests and finite element analysis, the following results are obtained : (1) The fatigue strength of the spot weld-bonded specimen is higher than that of the spot welded specimen. (2) The fatigue limit of the spot weld-bonded specimens of the ultra-high strength steel is higher than that of the mild steel. (3) The interfacial debonding propagates from the adhesive edge to a nugget edge, and the fatigue crack initiates at the nugget edge in both steels. (4) The fatigue strength of spot weld-bonded specimens is improved because the stress concentration at the nugget edge is reduced by adhesive bonding during large part of fatigue life.
