Fatigue Strength Prediction of Spot-Welded Joints Using Small Specimen Testing

It is well known that fatigue strength of spot weld of high strength steel sheet is not improved, compared with that of mild steel sheet. In this study, the governing factors and the effects of steel grade on fatigue strength of spot weld is investigated. Firstly, small specimens with total length of less than 3mm are taken from the spot weld of mild steel sheet (270MPa-grade) and high strength steel sheet (590MPa-grade). And then, tensile and high cycle fatigue properties are individually evaluated by newly-developed testing technique. Secondly, finite element analyses of tensile-shear specimen of spot-welded joints under cyclic loading are carried out and fatigue limit of the joints are predicted, using the above-mentioned local material strength properties and considering welding residual stresses around spot weld. Predicted results are nearly equal in both steels, which coincides with experimental results. It is found that fatigue strength of HAZ, which is the crack initiation site in joint, of 590MPa-grade steel is higher than that of 270MPa-grade steel. However, residual stress in 590MPa-grade steel is also higher and this may be one of the reasons why 590MPa-grade steel exhibits little improvement in fatigue strength of the joint over 270MPa-grade steel.