Development of Life Prediction Tool for Damage Tolerant Friction Stir Welded Structures

Friction Stir Welding (FSW) is well suited for joining aluminium alloy components and is increasingly being used by the aerospace industry. The introduction of alternative fabrication technologies can reduce the need for fasteners and creation of lap joints, which are typically the primary locations of crack initiation and multi-site fatigue damage in aerospace structures. The manufacture of airframe structural sections in large modular integral forms results in reduced fabrication time, cost and weight. To insure the safe insertion of more complex integral structures, an accurate understanding of the fatigue crack growth behaviour and the complex crack process in these structures must be developed.  The FSW process introduces residual stresses and alters the material microstructure and properties locally; these effects must be considered in damage tolerance models.  One major factor that must be considered is the effect of residual stresses and mechanical history in the weld zones on fatigue crack growth. Fatigue life estimates are complicated by the fact that residual stresses redistribute during crack growth. A life prediction methodology that will examine fatigue crack growth through a residual stress field in aluminium alloy structures fabricated using FSW will be detailed. Crack closure levels are calculated for different applied loading conditions in the presence of residual stress. The effects and significance of residual stress magnitude at a crack tip and its redistribution as the crack propagates on the estimated crack tip driving force are highlighted. The location of the crack tip relative to FSW and the effect of microstructure on fatigue crack growth are considered. Some of the salient results from the analyses will be compared with experimental results. A life prediction methodology which can accurately account for the effects of residual stress and material variation on fatigue crack growth will make it possible to design lighter and more reliable structures.