Investigation of shape optimization for the design of life extension options for an F/A-18 airframe FS 470 bulkhead

Shape reworking and bonded reinforcement are two procedures available to extend the fatigue life of cracked metallic aircraft components already in service. Typically for realistic applications, the design of reworks has been undertaken through trial-and-error finite element analyses, guided by simplified analytical formulations, the aim being to achieve reduced stresses while generally restricting the shape boundaries to circular and straight segments. In the present work, an automated sensitivity-based shape optimization procedure has been developed for the optimal design of free-form reworks and bonded reinforcements and demonstrated through application to a realistic practical problem, the F/A-18 FS 470 bulkhead. Here, a least-squares objective function written in terms of selected stress quantities is used together with multiple basis shape vectors to specify allowable shape changes. For the rework option, a unique optimal solution has been determined which achieves a region of constant boundary hoop stress which is 27 per cent less than for the nominal initial uncracked geometry, even though material removal at the critical location is accounted for. Subsequently, for the bonded reinforcement analyses, two distinct optimal designs were determined corresponding to the case where either shear or peel stresses in the adhesive layer are minimized. Both the shape of the adhesive layer and the reinforcement are allowed to vary, and significant improvements over a conventional reinforcement design are obtained as assessed by the reduction in peak stresses. These results indicate that the numerical shape optimization procedures presented can provide designs of reworks and bonded reinforcements, which offer significant improvements over standard designs.