Residual stress effects and fatigue behavior of friction-stir-welded 2198-T8 Al-Li alloy joints

Applications of friction stir welding in a fuselage structure were studied. Samples with two different friction-Stir Weld orientations in the fuselage panel were tested: one is along the fuselage longitudinal direction and the other one is along the fuselage circumferential direction. Then fatigue cracks were investigated that were set in three different types: parallel and perpendicular to friction stir welds and between double welds. Sample geometries were machined from identical welds in order to remove the effect of the weld process on fatigue behavior. Tests were conducted onM (T) specimens with either longitudinal or transverse welds. Cracks growing into or growing away from the weld center, as well as in the nugget zone, were investigated. It is shown that fatigue crack growth for cracks growing away from the center seems similar to that of the parent material; for crack growing in the nugget, crack grows slower than in the parent material; and for cracks starting between double welds, the crack grows slower than in the other two types. The virtual crack-closure technique method was used to calculate stress intensity factor from residual stress (K_res) and effective R ratio in an attempt to explain the experimental findings.