Effect of repair welding-induced residual stresses on fatigue crack growth and life of ductile cast iron: Application to a full-scale wind turbine hub

Repair welding of cast iron components is widely employed to restore structural integrity in large-scale systems such as wind turbine hubs. However the brittleness and susceptibility of cast iron components to crack initiation and combined effect of weld bead geometry and Residual Stresses (RSs) on Fatigue Crack Growth (FCG) highlights the need for a thorough understanding and potential optimization of the process. This study develops an integrated experimental-numerical framework to elucidate the FCG behaviour of repair-welded ductile cast iron, explicitly accounting for RS and geometric effects. FCG tests were conducted on welded specimens extracted from three different areas, namely Weld Metal (WM), Heat-Affected Zone (HAZ), and Base Metal (BM) to determine material-specific crack growth parameters. A coupled thermo-mechanical finite element model is used to predict the RS fields induced during single- and multi-pass repair welding, followed by three-dimensional FCG simulations incorporating semi-elliptical surface defects and elliptical embedded cracks under the influence of RS. Parametric analyses are performed to evaluate the effects of weld bead removal, number of passes, and inter-pass temperature on the RS evolution, Stress Intensity Factors (SIFs), and synthetic S-N curves. The experimental results show the WM exhibits the lowest threshold SIF range, while the BM shows the highest. The numerical results indicate that multi-pass welding with controlled inter-pass temperature reduces RS magnitudes by up to 25%, whereas weld bead removal improves the fatigue life by mitigating local stress concentration. The developed numerical framework is applied to a large-scale wind turbine hub to demonstrate its predictive capability, with the results showing that RS can entail a twofold reduction of the fatigue life. The proposed methodology provides a robust basis and highly cost efficient means for optimising repair welding parameters to enhance fatigue performance in service-critical cast iron structures.