Formalisation and Validation of Multiaxial Fatigue Assessment Framework for Hybrid Aluminium-to-Steel Welded Joints

The multiaxial fatigue assessment of welded joints is a critical challenge in structural engineering, particularly for hybrid aluminium-to-steel welded joints. While extensive research exists for steel and aluminium welded joints, no established methodologies currently exist for hybrid joints. This study aims to develop a validated multiaxial fatigue assessment framework for hybrid aluminium-to-steel welded joints by integrating experimental testing, stress analysis approaches, and multiaxial fatigue criteria to address existing knowledge gaps.

A comprehensive review and quantitative assessment were conducted on standard fatigue design methodologies from Eurocodes and the International Institute of Welding (IIW), as well as stress-based critical plane criteria. These criteria were evaluated using various stress analysis approaches under constant amplitude (CA) and variable amplitude (VA) loading conditions. Findings revealed that standard fatigue design methods are generally conservative, while the Modified Wöhler Curve Method (MWCM) demonstrated the highest predictive accuracy under VA loading.

To characterise the fatigue behaviour of hybrid joints, experimental fatigue tests were conducted on aluminium-to-steel welded specimens manufactured using EWM ColdArc® welding technology. Failure analysis revealed that fatigue cracks consistently initiated from the aluminium side, primarily at the weld toe, weld root, or heat-affected zone (HAZ), and occasionally within the weld itself. Crack initiation was shear stress-driven, while crack propagation followed normal stress directions.

A comprehensive validation of multiaxial fatigue criteria confirmed that the IIW approach delivered the most reliable and accurate fatigue life estimations for the hybrid joints across all stress analysis methods. The MWCM was effective with localised stress analysis methods but unsuitable for nominal stress in out-of-phase loading.

This study establishes a scientifically validated multiaxial fatigue assessment framework for thin hybrid aluminium-to-steel welded joints, contributing to fatigue-resistant design, structural safety, and practical engineering recommendations for real-world applications requiring weight reduction and fatigue performance.