It is well known that fatigue strength of spot weld of high strength steel sheet is not improved, compared with that of low strength steel sheet. In order to predict and improve the fatigue strength of spot weld, it is necessary to clarify the factors which determine fatigue strength. However, the governing factors and the effect of steel grade have been obscure. Therefore, a valid prediction method of fatigue strength of spot weld is desired.
In this study, firstly, small specimens with total length of less than 3mm are taken from each region (base metal, HAZ and nugget) in spot weld of low strength steel sheet (mild steel) and high strength steel sheet (590MPa-grade steel). And then, tensile and high cycle fatigue properties are individually evaluated by newly-developed testing technique. Secondly, finite element analyses of spot-welded joints under cyclic loading are carried out and fatigue limit of the joints are predicted by using the above-mentioned local material strength properties. Predicted result of mild steel is higher than that of 590MPa-grade steel, which coincides with experimental results. Furthermore, it is clarified that fatigue strength of HAZ, which is the crack initiation site of joint, is almost equal in both steels and this may be one of the reasons why the fatigue strength of the joint of 590MPa-grade steel does not exceed mild steel.
