Drilling fastener holes in composite materials is much more difficult than in aluminum or other metals because individual carbon fibers fracture at irregular angles, resulting in numerous microscopic voids. The voids can trap excess sealant, inhibiting the intimate electrical contact between a fastener and the composite structure.
As the cutting tool wears, there is an increase of surface chipping and an increase in the amount of uncut fibers or resin. This condition is referred to as machining-induced micro texture, which is associated with the presence of arcing between the fastener and the composite structure during lightning strike tests.
Lightning protection of composite structures is complex for several reasons: the intrinsic high resistance of carbon fibers and epoxy, the multilayer construction, and the anisotropic nature of the structure. The inherent conductivity of metallic fasteners and the large number of fasteners used in aircraft construction combine to create a condition of high probability of lightning attachment to fasteners. Intimate contact between bare metallic fastener and the composite structure is the best condition electrical current dissipation.
Improved machining techniques could improve some of the machining-induced micro texture and reduce the development of microscopic voids. Fasteners that conform to the inherent machine-induced micro texture offer another approach for reduction of arcing. In tests, Alcoa researchers found that such fasteners decrease the voltage drop across the interface and reduce the dielectric effect caused by the sealant, thus minimizing the possibility of arcing between the sleeve and the composite panel.
Composite structures in aircrafts are more susceptible to lightning damage than metallic ones. Metallic materials such as aluminum are very conductive and able to dissipate the high currents resulting from a lightning strike. Carbon fibers are 100 times more resistive than aluminum to the flow of current. Epoxy, which is often used as a matrix in conjunction with carbon fibers, is 1 million times more resistive than aluminum. The composite structural sections of an aircraft often behave like anisotropic electrical conductors.
Some studies show that, on average, each commercial aircraft in service is struck by lightning at least once per year.
A newly developed Alcoa fastener system consists of a core pin and a conformable sleeve. The company claims the conforming sleeve provides a much better intimate contact between the sleeve and individual carbon fiber as it deforms to fill the microscopic machining-induced voids. As the sleeve deforms into the void, it displaces the entrapped sealant. The insertion of the core pin causes the excess sealant to be extruded outside the sleeve/composite interface to produce an intimate electrical contact with the composite structure. This increased contact surface decreases current density and the voltage drop across the sleeve/composite interface, which leads to more efficient current transfer from fastener to the panel, minimizing the dielectric effect caused by the sealant. The result is a reduced possibility of arcing between the sleeve and the composite panel.
The conforming sleeve can be achieved in a variety of ways, with some more suitable for particular structures. The conformable sleeve could consist of a soft sleeve or a hard base material with a soft conformable coating. The coating can be selected from a group of relatively soft, conductive metallic materials that are known to be galvanically compatible to composite structure. These materials include gold, silver, nickel, copper, and tin. It is also possible to use various alloys.
This article is based on SAE technical paper 2009-01-3240 by Hasim Mulazimoglu and Luke Haylock of Alcoa New Product Development, Aerospace Fasteners.