Experimental study of lightning damage resistance of unprotected and protected stitched warp-knit carbon-epoxy composites

In-flight lightning strike damage to aircraft composite structures may compromise aircraft airworthiness. Hence, it is crucial to incorporate adequate protection systems to mitigate the lightning current. Most lightning strike protection (LSP) techniques involve bonding a metallic conductive layer to the cured laminate exterior. In this study, a novel LSP integration technique was used to develop unitized panels with three different protection layers: pitch carbon fiber paper (PCFP), graphene paper (GP), and copper mesh (CM). Each LSP layer was overlaid on through-the-thickness VectranHT stitched warp-knit multiaxial dry carbon fabric stacks, resin-infused, and oven-cured. A series of lightning strike tests to protected and unprotected stitched carbonepoxy laminates were conducted at a nominal peak current of 150 kA. Visual inspection was used to investigate each panel’s lightning damage resistance, understand the damage mechanisms, and evaluate the surface morphology at the strike locations. The size and severity of the damaged area depended on several factors: the outermost ply fiber direction, the strike location relative to VectranHT and polyester knitting treads, the lightning peak current, and the conductivity of the protection layers. The CM and GP protection layers effectively dissipated the lightning current in-plane and showed no damage to the underlying composite. The degree of lightning damage on an unprotected laminate was significantly lower than for a similar panel with PCFP protection. The presence of VectranHT structural stitches and polyester warp-knitting threads profoundly reduced the size and severity of lightning damage. These threads appeared to promote close contact between adjacent carbon fiber tows, resulting in better in-plane and through-thickness electrical/thermal conductivities and reduced lightning damage.