Fatigue performance of welded joints made of high-strength steel under variable amplitude loading conditions and varying mean stress

Abstract The study experimentally and analytically investigates the effects of variable amplitude (VA) load sequence and varying mean stress on the fatigue strength of welded joints made of high-strength steel. The variable amplitude load spectra were constructed according to a Gaussian normal distribution, and the test series of increasing mean stress (constant maximum stress of the VA load) and decreasing mean stress (constant minimum stress) with identical equivalent stresses were implemented. The applied load spectra incorporated a high peak stress in the beginning of the block to further study the overload and residual stress relaxation effects. In addition, a notch-based fatigue assessment approach, the 4R method, was applied to these load cases. Analyses were conducted with the 4R method to analytically compute local elastic–plastic behavior, and the sequential effect of VA load cycles were considered in the mean stress correction. The fatigue test results showed that the mean stress had a significant impact on the fatigue strength of the welded joints in spite of identical equivalent loads. The difference between the mean fatigue strengths (Δ σ c,50% = 130 MPa and Δ σ c,50% = 80 MPa) of the two-test series was approximately 63% with the higher fatigue strength corresponding to low mean stress loading conditions.