Structural stress concepts are widely used in fatigue life prediction of spot welds and seam welds in vehicle structures. For fatigue life prediction of welded joints based on the structural stress methods, it is necessary to obtain applied force ranges versus fatigue life of the welded specimens. Then, the force ranges versus fatigue life information is converted to structural stress ranges versus fatigue life (S-N) of the joints. The structural stress ranges versus the fatigue life curve of the welded specimens becomes the material fatigue property of the welded joints to predict fatigue life of joints in vehicle structures. While converting the applied load ranges to the structural stress ranges, linear elastic finite element analysis (FEA) is used. Therefore, the applied load ranges are considered as the responses of linear elastic deformation even though the load ranges consist of the linear elastic deformation and plastic deformation. As results the structural stress ranges of the welded joints are reduced in S-N curve. This study introduces simple techniques for spot welded joints to include the plastic deformation effect on the structural stress calculation without performing elastic-plastic finite element analysis. Fatigue test results of spot welded joints for lap-shear and coach peel specimens of advanced high strength steels (AHSS) and mild steels were used. The corrected structural stress methods showed better correlation with the test results.

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