DAMAGE TOLERANCE ANALYSES OF LASER WELDED "SKIN-CLIP" JOINTS FOR AEROSPACE APPLICATIONS

Future metallic airframes may contain laser beam welded clips (to the skin) between two stringers to achieve lightweight integral airframes. In this study, an investigation involving stress analysis and fatigue testing of skin-clip laser welded T-joints in 6156 T4 aluminium alloy plates was carried out. The thickness of the base plate (skin) was locally reduced between the welded joints by machining to produce "pockets" to obtain weight reduction. The effects of pocket thickness (3.0 and 1.5 mm) and socket width (the width of the full-thickness part between pockets; 11 and 20 mm) as skin-clip joint parameters on stress distributions around the weld region were studied by FE analyses of 400 mm wide plates under tension. Several numerical analyses were performed considering the effect of different locations of the tips of weld toe cracks in the T-joints, and the results obtained were compared with the experimental observations from fatigue tests of such joints in order to describe the fatigue performance of the laser welded skin-clip joints. Pocketed skin-clip joints exhibited weld region protection as a result of a reduction in the stress in the part containing the welded joint. Among the joint configurations analyzed in the absence of weld toe cracking, 3.0 mm pocket thickness with 11 mm socket width (narrowly prepared pocket design) provided almost full protection of the weld toe with little increase in stress on the opposite side of the plate containing the weld detail. However, in the presence of a weld toe crack, a region of stress concentration arose in the pocket itself, in line with the crack tip. This encouraged deviation of the crack path away from the weld toe but, if the socket width was too small, as in the narrowly prepared pocket design, the two stress fields combined and led to an increase in fatigue crack growth rate. In this study, the best fatigue performance was obtained when crack deviation into the base material occurred but remained within the socket area.