Multiaxial fatigue assessment of ColdArc®-welded aluminium-to-steel hybrid joints

This study investigates the multiaxial fatigue behaviour of coldArc®-welded AA6082-T6 aluminium-to-S235 steel tubular joints and evaluates the effectiveness of established fatigue criteria when combined with different stress-analysis approaches. Fatigue tests were conducted under uniaxial, torsional, proportional, and 90° out-of-phase loading, across stress ratios (R) of –1 and 0.1. All fatigue failures initiated from the aluminium side, confirming aluminium as the fatigue-critical region in this configuration. Mean stress effects were modest, and non-proportional loading did not significantly reduce fatigue life, supporting the use of simplified multiaxial fatigue models for this joint configuration. Increasing shear stress contribution consistently reduced fatigue strength, highlighting the influence of biaxiality ratio. Three stress analysis approaches were examined for fatigue life estimation: the Nominal Stress method, the Fictitious Notch Radius approach with a reference fictitious radius of 0.05 mm, and the Theory of Critical Distances − Point Method (TCD-PM). These were used with the International Institute of Welding (IIW), Eurocode 9 (EC9), and the Modified Wöhler Curve Method (MWCM) criteria and benchmarked against experimental dataset. The results show that IIW and EC9, when paired with nominal stresses, provided safe and consistent predictions, local stress-based approaches improved accuracy, and the TCD-PM delivered reliable estimations without requiring notch-radius assumptions. Overall, the combined experimental evidence and comparative estimated fatigue life assessments clarify the suitability and practical selection of multiaxial fatigue assessment methods for aluminium-to-steel welded joints in engineering applications.