"Both of these effects increase the risk of the fasteners sparking during a lightning strike, which could be disastrous if it happened near the wings that hold the fuel tanks," said Raskind. "Various strategies have been developed to avoid these issues, including tight-fitting sleeves to deal with the machine-induced micro texture, and dielectric patches to prevent arcing from the fasteners during a lightning strike."
The issues related to lightning strike protection in composite structures have been pretty well handled, said Hempstead. Fasteners for components made of carbon composites must be bonded to a conductive material that can dissipate the charge. A lot of work is going into how to do this for permanent fasteners in more cost-efficient and weight-efficient ways.
Alcoa Fastening Systems has done extensive research to understand how a joint behaves differently with composites versus metallics, said Gurrola. The load distribution on a fastener in a composite versus a metallic joint is very different. The company has designed its newer fasteners to accommodate this difference, and to be optimized for the different behavior of the joint. In addition, since composites aren't set up well to harmlessly dissipate energy from lightning strikes over the surface of the aircraft, fasteners are being designed to do that job.
For example, Alcoa's Flite-Tite pin fastening system provides electrical contact with the structure so electricity can flow better from the composite. Another is the Eddie-Bolt, which installs in a more compatible way because it doesn't have prevailing torque. "Prevailing torque is a characteristic of most threaded fasteners that makes the whole thing want to turn as you torque the nut," said Gurrola. The FC43 is a high-strength structural panel fastener that's compatible with composite or metallic structures. For joining aluminum with composite, the Extended Performance Lockbolt fastening system has a titanium collar.
Alcoa is working on new fastener materials, such as higher-strength titanium alloys. The typical titanium alloy in aerospace fasteners is Ti6Al4V, with an ultimate shear strength of 95ksi. "For most applications, that strength works just fine," said Gurrola. "But as you get into larger diameter fasteners with some areas that need higher strength, we've been looking at 108ksi or higher." Alcoa is also investigating lower-cost alternatives to titanium, and higher-strength aluminum-lithium alloys. Those alloys have two benefits over conventional materials. They can directly replace some standard aluminum alloy fasteners and increase strength, or because of their higher strength, they can replace some steel alloy fasteners.
The shift toward increased use of composite materials means fewer fasteners will be needed per airplane, although inflationary pressures will be greater for aerospace fasteners than for fasteners used in other applications, said Raskind. "The relatively small number of suppliers making fasteners for use with composite materials, the more heterogeneous nature of these products, and the lack of effective substitutes means that price competition for these fasteners will be less intense."
Also because composites require fewer fasteners, the shift toward increased composite use by airplane manufacturers means that fastener suppliers will sell fewer units. The Freedonia Group predicts that lower sales volume may also reduce the economic viability of large R&D projects for these products.