Enhanced multifunctional properties of CFRP composites for lightning strike protection

Abstract

Lightning is one of nature's most unpredictable and destructive forces and poses a serious risk to commercial aircraft, which statistically encounter a lightning strike every 1,000 - 10,000 flight hours or approximately once per year. The damage of lightning strikes displays as burnt paint, damaged fiber, removal composite layer in aircraft structures, and degraded or even malfunction of avionic equipment performances. This can severely hinder the airline's operation results in costly delays and service interruptions that are the primary concerns in aircraft structures. Metal sheets are used as primary material in aircraft structures due to their excellent electrical and thermal conductivities, which allow electrical current to travel on the exterior aircraft skin and instantly exit through other extremity points. However, recent developments in aircraft design involving non-metallic skins and lightweight structural concepts have used carbon fiber reinforced polymers (CFRP), which have high orthotropic electrical resistivity and therefore increase aircraft vulnerability to the effects of lightning strikes. A considerable damage could be observed in the low electrical conductivity materials as they have absorbed high electrical energy during lightning strike due to the increase of Joule heating. To develop a new generation of highly conductive carbon composites that address these shortcomings, carbon nanotubes (CNTs) were grown on nickel-coated carbon fiber using chemical vapor deposition (CVD) method at low-temperature growth to create a fuzzy fiber (FF). An additional conductive filler, graphene nanoplatelets (GNPs), was then dispersed on the fuzzy fiber surface to form synergistic physical interactions between two different low-dimensional carbon-based nanostructures. Specimens were fabricated by integrating the FF layer with and without GNPs to the outermost ply of the CFRP composite using vacuum-assisted resin transfer molding (VARTM) method or wet lay-up process with the autoclaved curing. The results reveal a synergistic enhancement in both functional conductivity and mechanical properties. The electrical conductivity of composites with the inclusion of GNPs resulted in approximately 40%, 300%, and 190% enhancement in the fiber, surface, and through-thickness direction compared to the fuzzy fiber composites. Mechanical properties, including flexural, impact, and interlaminar shear stress properties were further enhanced. These results reveal that the presence of GNPs creates more electron transfer pathways, and also promotes a synergistic effect in physical interactions. Altogether, these enhancements provide avenues for future high-performance conductive carbon fiber composites in aircraft structures. Then, a three-dimensional thermal-electrical coupled model based on COMSOL was created to characterize the thermal damage propagation modes in woven CFRP composite with and without an LSP system. FF serving as a carbon-based protection layer was attached to the outermost ply of the CFRP composite to fabricate a fuzzy fiber reinforced polymer (FFRP) composite. CFRP and FFRP composites with temperature-dependent properties were inspected to predict lightning-induced damage resulting from a 20- and 40-kA peak current for 100 μs. The predicted area of thermal damage and the appearance of the composite surface agreed fairly well with post-lightning damage observed from experiments, thus demonstrating the credibility of the numerical model. LSP characteristics were evaluated for a range of CFRP composite properties in the outermost layer by enhancing the functional conductivity of the top layer in the in-plane and out-of-plane directions. The irreversible thermal damage region in the in-plane and thickness directions was dramatically mitigated by enhanced electrical conductivity, whereas a slight reduction of matrix decomposition damage was observed by varying the thermal conductivity. Due to the increased functional conductivity and integration of the lightweight FF carbon-based protection layer into the uppermost composite layer, the depth and area of damage can be limited by decreasing the thermal damage penetration through the underlying composites. These results reveal that a highly conductive FF layer may serve as a lightweight and effective anti-lightning strike layer for protecting the underlying composite.