Predicting the fatigue life of pristine and pre‐corroded friction stir welded joints

PurposeFriction stir welding (FSW) is simple, clean and cost effective joining technology which allows high‐quality joining of materials that have been traditionally troublesome to weld conventionally without distortion, cracks or voids such as high‐strength aluminium alloys. Since FSW has been identified as “key technology” for primary aerospace structures, the recent FAR regulations for damage tolerance and fatigue evaluations of aircraft structures require fatigue life predictions for this specific joint type also in the presence of corrosion. The purpose of this paper is to give an overview of the prediction of small coupon fatigue lives of thin section friction stir welded butt and T‐joints.Design/methodology/approachParticularly, as a special application, widespread fracture mechanics software will be used to predict the fatigue life of FSW joints and to obtain SN curves. The engineering approach will start from an easy definition of the damage affecting the fatigue life of any of the previously mentioned cases (inclusions, tool markings, corrosion pits) and will move through affordable fracture mechanics solutions. Particularly, a first step in predicting the fatigue life of complex friction stir welded structures will be taken by combining the FEM code with the fracture mechanics software in the prediction of the FSW T‐joints.FindingsThe calculations are in very good agreement with the experimental results once the following basic assumptions are done: the welded material is treated as base material; particle inclusions and welding imperfections are treated as initial flaws while predicting the life of polished and un‐polished (including the T‐joints) FSW material, respectively, and the entire fatigue life was comprised of crack propagation; pitting and inter‐granular corrosion are treated as a single corrosion damage source and the model surface crack comprehends this damage; and the several corrosion‐damaged areas of the specimen surface are simulated with a single semi elliptical surface crack having the dimensions of the deepest and the widest corrosion damage area.Originality/valueA simple engineering approach which is based on a relatively solid background and which is checked against fatigue test data for various FSW test specimens was developed: it may provide a practical and reliable basis for the analysis of fatigue tests of integral structures in the presence of corrosion attack, by using widespread fracture mechanics principles.