An equivalent structural stress method for variable-amplitude fatigue assessment of welded joints with load sequence effects

Abstract

This study investigates the fatigue life prediction of welded structures under variable-amplitude loading (VAL) and extends an equivalent structural stress (ESS) method that accounts for both load sequence and mean stress effects. Based on fracture mechanics, the elastic–plastic behavior at the crack tip is considered, with the influence of load sequence incorporated into the ESS framework by means of a generalized Willenborg model. Welding-induced residual stresses (WRS) and plasticity-induced residual stresses (PRS) are incorporated in the development of the VAL-ESS method. Load sequence effects on fatigue damage are quantified through a stress amplitude ratio correction factor and fitted to experimental fatigue data. The VAL-ESS method achieves a high correlation with the master S-N curve under VAL. Further validation using experimental data across varying load conditions, geometries, and surface treatments demonstrates that the VAL-ESS method substantially improves prediction reliability. It achieves higher accuracy and reduced scatter compared with the nominal stress and effective notch stress methods. Finally, the coupled effects of load amplitude sequence and defect size on fatigue behavior are mechanistically interpreted, highlighting the effectiveness of the VAL-ESS approach in capturing the critical features of welded fatigue under VAL.