Numerical Prediction of the Impact of Residual Stresses on the Fatigue Life of Welded S355 Steel Plates Using a Phase-Field Approach

The welding process used in the manufacture of offshore wind turbine foundations generates significant residual stresses, which can strongly influence fatigue performance. A better understanding of their magnitude, distribution and relaxation is therefore important for improving fatigue life predictions. This study investigates the influence of welding residual stresses on the fatigue resistance of offshore wind turbine monopile structures, with particular focus on S355 double V-groove multi-pass butt welds. A two-stage finite element procedure was used to replicate the welding process, with a thermal model first predicting the temperature dissipation for each weld pass, followed by a mechanical model to determine the resulting residual stress field. This field was then introduced as an initial condition within a phase-field fatigue model incorporating several empirical residual stress relaxation relationships. The phase-field model was calibrated against the SLIC as-welded mean S-N curve. The results showed that, in the absence of relaxation, welding residual stresses reduced the predicted fatigue life by a factor of approximately 4 to 8, depending on the applied stress range. When residual stress relaxation was included, the damaging effect was reduced and the predicted S-N slope became dependent on the selected relaxation formulation. Removing the residual stress field after back-calibration increased the predicted fatigue life by up to approximately four times. The effectiveness of representing residual stresses through an equivalent R-ratio was also assessed, with R = 0.5 providing a conservative approximation, while values closer to R = 0.25 gave better agreement when residual stress relaxation was considered.