On the Fatigue Life of the Single Spot-Welded Joint under the 2-Steps Repeating Loads : Comparison of Results for Mild Steel and Those for High Strength Steel

In this paper, fatigue lives of single spot-welded joints made of mild steel (SPCC) sheets and high strength steel (HT50) sheets of 0.8 mm in thickness are obtained by experiments under the constant amplitude repeating loads and the 2-steps repeating loads in tensile-shear, then fatigue lives for both kinds of steel are compared, and moreover the fatigue life obtained by the experiment is compared with the value estirnated by using Miner's rule. Main conclusions obtained are summarized as follows : (1) The fatigue life for HT50 is generally rather longer than one for SPCC under the 2-steps repeating loads including the peak-overload, which is different from the well-known results under the constant amplitude repeating load. (2) The fatigue life obtained by the experiment under the 2-steps repeating loads including the peak-overload does not as a whole agree with the value estimated by using Miner's rule. (3) From above two conclusions, it is follows that the fatigue life is considerably influenced by the mechanical property in the neighborhood of the spot weld under the 2-steps repeating loads including the peak-overload and cannot be difcussed without the consideration of the load history.