Chapter 36 - Case History: CF-116 Upper Wing Skin Fatigue Enhancement Boron Doubler

Abstract

This chapter focuses on the development of a composite-bonded doubler as an enhancement for the CF-116 upper wing skin. The CF-116 uses a single-wing box structure—six ribs and six spars per side sandwiched between an upper and lower wing skin. The two wing skins are chem milled from Al 7075-T651 plate. Fatigue cracking in a compression load dominated structure such as an upper wing skin is not a common occurrence. However, if the wing experiences a high positive “g” loading, the resultant compressive stress in the wing skin when coupled with the stress concentrations in the fastener holes can result in localized compressive yielding in the form of hole elongation. This compressive yielding is balanced by a tensile residual stress zone that can raise the local mean applied stress level such that a fatigue crack can initiate and grow in a compression dominated environment. The repair process carried out consisted of a metallic repair that met the residual strength, static and fatigue design requirements for the wing skin. A composite fatigue enhancement doubler was to be bonded onto the wing skin to reduce the strain levels in the affected area.