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 ex tend the fatigue life of cracked metallic aircraft components already in se rvice. Typically for realistic applications, the design of reworks has been undertaken through trial-and-error finite element analyses, guided by simp lified analytical formulations, the aim being to achieve reduced stresses w hile generally restricting the shape boundaries to circular and straight se gments. In the present work, an automated sensitivity-based shape optimizat ion procedure has been developed for the optimal design of free-form rework s and bonded reinforcements and demonstrated through application to a reali stic practical problem, the F/A-18 FS 470 bulkhead. Here, a least-squares o bjective function written in terms of selected stress quantities is used to gether with multiple basis shape vectors to specify allowable shape changes . For the rework option, a unique optimal solution has been determined whic h achieves a region of constant boundary hoop stress which is 27 per cent l ess than for the nominal initial uncracked geometry, even though material r emoval at the critical location is accounted for. Subsequently, for the bon ded reinforcement analyses, two distinct optimal designs were determined co rresponding to the case where either shear or peel stresses in the adhesive layer are minimized. Both the shape of the adhesive layer and the reinforc ement are allowed to vary, and significant improvements over a conventional reinforcement design are obtained as assessed by the reduction in peak str esses. These results indicate that the numerical shape optimization procedu res presented can provide designs of reworks and bonded reinforcements, whi ch offer significant improvements over standard designs.