Control of crack growth rates and crack trajectories for enhanced fail safety and damage tolerance in welded aircraft structures

The paper describes work that has been undertaken in controlling residual stresses in welded structures and in predicting crack growth rates and crack trajectories under the influence of friction stir weld residual stress fields. Experiments to explore the ability of stressing during welding to modify post-weld residual stress fields associated with the weld are described. They demonstrate that residual stresses can be successfully modified by this treatment. C(T) and ESE(T) samples in three different sizes, together with SEN samples to test crack deviation behaviour were machined from friction stir welded 2198 and 2195 material. Crack growth rates in a range of locations and orientations with respect to the weld were determined. To predict crack growth rates in the welded samples, stress intensity factors associated with the residual stress field were calculated and used to modify the values of the stress intensity factors due to the applied loads. Fatigue crack growth rates were calculated from parent plate data using the Harter T-Method. Crack trajectories were predicted using the maximum tangential stress criterion. Good agreement of model predictions with experiments for all crack paths and residual stresses was obtained. The implications for aircraft fail safety and damage tolerance of a capability to control and predict fatigue crack development in welded aircraft structures are discussed.