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A method of fatigue life prediction of spot welded joint under multi-axial loads has been developed by fatigue life estimation working groups in the committee on fatigue strength and structural reliability of JSAE. This method is based on the concept of nominal structural stress ( σ ns) proposed by Radaj and Rupp, and improved so that D value is not involved in stress calculation. The result of fatigue life estimation of actual vehicle with nominal structural stress which was calculated through newly developed method had very good correlation with the result of multi-axial loads fatigue test carried out with test piece including high strength steel.

Fatigue strength of typical types of spot-welded joints made of high strength steel sheets was experimentally investigated, as a cooperative program of “Subcommittee on Strength of Spot-Welded Joint” of JSAE. The materials used were rephosphorized high-strength steel sheet (HSS) of 400 MPa class and low-carbon steel sheet (LCS); both were 0.8 mm, 1.2 mm and 1.6 mm thick. The types of loading were tensile-shear, cross-tension and plane bending. The results obtained are as follows; 1) The fatigue strengths of joints of HSS in tensile-shear and plane bending were higher than those of LCS, respectively. But, in cross-tension, the strength of joints of HSS was exceeded by that of LCS beyond 106 cycles. 2) The fatigue strengths of joints of different materials in cross-tension and plane bending were found lower than those of the same materials. 3) The strain range - cycles to fracture (Δε-N) diagrams for the joints of sheets of different steels and thicknesses were obtained.