Biaxial Fatigue of Laser Beam and Friction Stir Welded Components

Abstract

Cruciform specimens made from both FSW T-joints produced by EADS in Suresnes and LBW T-joints delivered by Alenia were fatigued in the DLR biaxial test rig. To resemble the loading situation of stiffened fuselage structures, proportional loading without any phase shift, but with different load ratios between the loading components in both directions was applied. Regardless of the welding process and the biaxial load ratio applied, natural crack initiation and subsequent crack growth were governed by the sharp stiffness gradient caused by introducing the stringer. Cracks initiated and propagated at run-in and run-out locations in a direction perpendicular to the weld seam. The shortest fatigue life was observed for uniaxial loading in stringer direction. An additional stress component perpendicular to the joint line reduced deformation in welding direction before crack initiation and diminished crack opening at maximum load after crack initiation both resulting in a higher number of cycles to failure. Characteristics of a crack propagating from a saw-cut along the weld seam were not markedly deteriorated as compared to crack growth in the base material. Similar to single stringer panels, increasing the load ratio also increased the number of cycles to failure for FSW clip-stringer structural members, but additionally gives a different location of the fatal crack.