Residual stresses, defects and non-destructive evaluation of FSW joints

Abstract

Summary This article describes some of the research activities in progress at the German Aerospace Centre, DLR, on the friction stir welding process. It shows in detail the principal defects that can be found in FSW joints, particularly those defects that are due to incomplete filling, incomplete penetration and root discontinuities. The non-destructive ultrasonic method has been used as a promising technique to detect and characterise any of the imperfections mentioned above. The results obtained are confirmed by both light and scanning electron microscopy (SEM). The second part of the analysis focuses on the quantitative evaluation of the residual stresses using the ‘cut compliance’ method, and the mathematical tools provided by fracture mechanics. Finally, two examples of fatigue life predictions for welded joints are presented that are considered to be significant in light of the above considerations. One concerns the propagation of defects in a specimen FSW weld in the presence of a residual stress field and variable amplitude loads, and the other concerns the prediction of the fatigue behaviour of defective FSW specimens without the influence of residual stresses. The results obtained showed good agreement with the experimental data.