Rotorcraft Fastener Electrical Contact Analysis for Optimal Carbon Fiber Composite Lightning Strike Protection

In this paper we investigate the importance of electrical contact efficiency between fasteners and skin-level expanded metal mesh and how it influences lightning current energy transport. Since carbon fiber reinforced polymer composites (CFRPs) are electrically anisotropic and typically exhibit relatively low conductivities which are directionally distinct, aerospace manufacturers frequently use metallic foil layers or an expanded metal mesh on outer surfaces of composite structures for lightning strike protection. Due to irregular topology and associated micro-texture of machined holes in composites it is shown non-uniform interface surfaces between the fastener and composite layers reduces electrical continuity, thereby impeding current flow as a result of increased contact resistance levels. Furthermore, lightning strike experiments have indicated that metal mesh intimacy around the perimeter of the fastener head directly affects overall electromagnetic response of the rotorcrafts outer surface when subjected to lightning currents due to impedance modification of the electrical network. The effect of electrical continuity between the fastener head and mesh strand contact points is calculated using a Multiphysics model to demonstrate the effects on current load distribution and fusing lifetime of the individual strands. Additionally, total number of fastener-mesh strand contact points and cross sectional area of each strand are shown as being important parameters which ultimately determine the amount of lightning current efficiently transferred across the rotorcraft skin surface.