Understanding lightning damage formation in a carbon-epoxy Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) composite

Lightning damage to composite aircraft structures results from a concurrent and sequential interaction between arc discharge multi-physics and the anisotropic electrical and thermal composite properties. In this study, the impact of nominal 50 and 125 kA lightning strikes on damage formation in a carbon-epoxy Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) was investigated. A combination of visual inspection, ultrasonic phased array testing, destructive sectioning, and various microscopy techniques was employed to characterize the damage. Localized damage, including severe matrix decomposition, melting of polyester warp-knitting yarns, fiber splitting, and large-scale delamination, was confined to the immediate vicinity of the lightning attachment point and was accompanied by more diffuse surface damage (i.e., widespread small-scale split fiber tufts and surface primer scorching). The severity and extent of internal damage increased with higher lightning current intensities but were limited to the outermost nine-ply warp-knitted skin stack. Electrically non-conductive through-thickness Vectran^™ stitches and polyester warp-knitting yarns had a profound effect in mitigating lightning damage formation. These results suggest that such through-thickness reinforcement can dramatically enhance the lightning damage resistance and tolerance of composite aircraft structures.