Aerospace Fasteners: Use in Structural Applications

Aircraft components need to be selected and manufactured to adequately combat the environment, temperature, loading, compatibility, and so on. When structural materials such as aluminum alloys or fiber-reinforced polymer composites need to be joined in aircraft, the selection of fasteners, bolts, rivets, adhesives, and other methods need to be quantitatively assessed in order that the correct design for the component and joining method is identified. There is a variety of fasteners, bolts, and rivets, made using a variety of materials. Aluminum rivets are often used to join aluminum components in an aircraft. Rivets do not perform well under tension loading, but perform better in shear, thus limiting the application specifically for these purposes. Bolts are designed to clamp material together, and even though the bolt may be adequate to support a particular structure and load requirement, consideration must also be given to the modulus of elasticity and stiffness of the components that are being clamped together. Therefore, an understanding of each of the materials being clamped or joined together is necessary. Bolts manufactured from steel, for instance, have coatings applied in order to help protect them from corrosion. The use of composites translates to a reduced number of rivets and fasteners to be used. Drilling of holes into composites to insert fasteners poses many challenges because the fibers are damaged, a region of high stress concentration may be formed, and the hole is a site for the ingress of water or moisture. The insertion of aluminum fasteners or the contact of aluminum components with carbon fibers creates galvanic corrosion due to the large difference in electrical potential. Titanium alloy (Ti-6Al-4V) is a typical fastener where there is composite joining due to its better compatibility (elimination of galvanic corrosion) and increased strength properties. Substitution of rivets and fasteners for welding is also on the increase in aircraft because laser beam welding (LBW) and friction stir welding both reduce cracking, porosity, and better properties achieved due to deeper penetration, and reduce the heat-affected zone which would typically be undesirable with conventional arc welding such as metal inert gas and tungsten inert gas welding. The shear and compressive stresses are increased, and fatigue cracking, weight, and cost are also reduced as a result of LBW, including the elimination of stresses and corrosion associated with rivets and the elimination of adhesives. Dissimilar metals such as the 7000 series and the 2000 series can be joined with a filler metal compatible to both metals to mitigate galvanic corrosion.