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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.

Environmental issues require auto makers to improve the fuel consumption of motor vehicles. One way of saving energy is to reduce the total weight of the vehicle by using high strength steels. However, with conventional steels, increasing tensile strength (TS) reduces elongation and stretch flange formability. JFE Steel developed a new TS/780MPa grade of hot-rolled high strength steel with excellent stretch flange formability in which the ferrite matrix is strengthened by ultra-fine, nano-sized precipitates. Transmission electron microscopy (TEM) reveled that the precipitates in ferrite grains are TiMoC2 and their average diameter is less than 10 nm. The steel has an extremely high YS/TS ratio and an excellent balance of elongation and the hole expansion ratio, superior to that of TS/780MPa grade dual phase and HSLA steels. Moreover, because of its Si-free alloy chemistry, the new steel has good coatability and can be used as a substrate for galvanized or galvannealed steel sheets.

A method of fatigue life estimation for the spot-welds of vehicle body structures by means of Finite Element Analysis (FEA) was studied. 6 general forces applied to a nugget of spot-weld under multiaxial loads were determined and the Nominal Structural Stress (σns) was calculated from them. It was confirmed that fatigue strength of the spot-welds under various multiaxial loads could be estimated universally by using σns. Based on the theory of elasticity of plates, stress of spot-weld nugget was analyzed. The theoretical equations for determining the principal stress at the nugget edge from6 general forces acting on a nugget were derived. And the principal stress was defined as the σns. The value of σns was determined by FEM that used a solid model and compared with the theoretical calculation value. They agreed quite well. Fatigue tests of DC specimens under various multiaxial loads (shear plus cross tension and tensile shear plus torsion) were conducted.