Formalisation of bespoke fatigue approaches to design aluminium-to-steel thin hybrid welded joints

The present research was designed to investigate experimentally and numerically the fatigue behaviour of aluminium-to-steel hybrid-welded joints. It has always been a challenge to weld dissimilar materials because of the significant difference in their mechanical, thermo-physical and metallurgical properties, which causes the formations of hard and brittle intermetallic phases in the welding region. Recently, EWM® Welding has developed a new welding technology (known as coldArc®) to solve this problem which is capable of providing a strong joint on condition that the steel sheet is galvanized. Various welding configurations were manufactured using the coldArc® welding including butt, lap, cruciform and tee welded joints.
Before investigating the fatigue behaviour of the hybrid-welded joints, the static behaviour was investigated to better understand the overall mechanical behaviour of these joints. As far as the static investigation is concerned, the visual examination of the fracture surfaces revealed that, regardless of the geometry of the welded connections, the fracture of the joint always took place in the heat-affected zone (HAZ) on the aluminium side. This inspection indicated that the use of EWM® coldArc welding technology had improved the strength of the hybrid welded joint significantly and removed the problem of having a brittle phase in the welding zone. The results obtained from this investigation show that Eurocode9 (EC9) can also be used to design aluminium-to-steel thin welded joints.
The fatigue lifetime estimation of aluminium-to-steel thin welded joints was carried out using the nominal stresses, effective notch stresses, the Notch Stress Intensity Factors (N-SIFs) and the Modified Wöhler Curve Method (MWCM) in conjunction with the Theory of Critical Distances (TCD). The results of the fatigue investigation showed that for the nominal and effective notch stresses, the available Standards and Codes of Practice (EC9 and the International Institution of Welding (the IIW)) are only suitable for thick welded joints. However, the negative inverse slope of 5 for thin materials suggested by Sonsino0F was seen to provide conservative fatigue life estimations in terms of nominal stresses.
Furthermore, the effective notch stresses and the N-SIF approaches make it evident that the strength of the aluminium alloy used in this investigation was very low compared to the aluminium alloys used in the structural applications and hence another design curves are required. In this context, a FAT of 90 was proposed for the notch stresses to perform an accurate fatigue design of the hybrid-welded joints. For the N-SIFs, a design curve characterised by negative inverse slope equal to 5 million cycles to failure is recommended. Finally, the MWCM was calibrated for the structural details being investigated. The validation process demonstrated a high level of accuracy in estimating the fatigue strength of hybrid welded joints.