The Effect of Machining–Induced Micro Texture on Lightning Current Arcing between Fasteners and Composite Structure

Drilling fastener holes in composite is much more difficult than in aluminum or other metallic materials since individual carbon fibers fracture at irregular angles resulting in numerous microscopic voids. These voids can trap excess sealant inhibiting the intimate electrical contact between the 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. Machining–induced micro texture has been shown to be associated with the presence of arcing between the fastener and the composite structure during lightning strike tests.

Lightning protection of composite structure is more complex due to the intrinsic high resistance of carbon fibers and epoxy, the multi-layer 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 a high probability of lightning attachment to fasteners. Intimate contact between bare metallic fastener and the hole in the composite structure has been found to be the best condition for the electrical current dissipation

Improved machining techniques could improve machining-induced micro texture and reduce the development of microscopic voids. Fasteners developed in this study which conform to the inherent machine–induced micro texture offer another approach for reducing arcing between the fastener and the composite hole. The test results presented in this paper indicated that conforming 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.