Fatigue assessment of laser beam and friction stir welded joints made of aluminium

Abstract

Available fatigue recommendations are typically developed for arc-welded joints. Some of them may need to be adjusted to fit fatigue life of joints manufactured with non-conventional welding techniques since they show different mechanical properties. Laser beam and friction stir welded butt and overlap joints made from aluminium are addressed in this paper focused on a comparison between different fatigue assessment methods. For an evaluation, S-N data from published papers was collected. Additionally, fatigue tests on laser beam and friction stir welded overlap joints were carried out. For a fatigue assessment, the nominal stress, notch stress and effective stress approach were applied. The comparison of the endurable nominal stresses showed a comparatively high fatigue strength of the tested friction stir welded overlap joints in comparison to literature data. The notches at the interface of the overlap joints were observed to have an asymmetric geometry compared to the common symmetric one, frequently found in literature, leading to a lower stress concentration. A comparison between endurable nominal stresses and the classes FAT 28 for butt welded and FAT 12 for overlap welded joints resulted in a conservative design for the majority of specimens. An evaluation based on notch stresses lead to partly non-conservative results that could be explained by the mild notches present at the laser-beam welded butt joints. For the effective stress approach, an evaluation of the micro-structural length was performed and a FAT-value is proposed.