Introduction
Carbon fiber reinforced polymer (CFRP) composite structures are widely employed as aerospace and automobile components for their superior mechanical properties. Composites used on aircrafts are subject to extreme conditions such as lightning strike. An effective lightning strike protection requires a good understanding of the conduction mechanisms of CFRP.
Another demand for studying the electrical conduction behavior of CFRP comes from the emerging applications of damage sensing utilizing the electrical resistance changes of composite structures. Todoroki et al. {Todoroki:2005ft} {Todoroki:2014jn} published articles on self-sensing CFRPs where delamination cracking and matrix cracking were monitored using the electrical resistance changes of multiple segments on the CFRP plate surface. Xia et al. {XIA:2003fr} proposed analytical models that relates structural changes including fiber breakage and delamination to electrical resistance of carbon composites. In these applications, a good understanding of the fundamental conduction mechanisms is critical for accurate estimation of structural damages.
Laminated composites have strong anisotropic electrical properties due to various dominating conduction mechanisms in different directions. Figure 1 illustrates the electrical conduction mechanisms for carbon composite laminate in three primary directions. The previous papers {Athanasopoulos:2011ea} {Yu:2015jp} {Weber:2004ev} revealed that electric conductivity in the fiber length direction is proportional to the carbon fiber volume fraction and intrinsic resistivity of carbon fibers (see Figure 1(a)). Contact between carbon fibers provide electric conduction pathways in the transverse direction, dominating the conduction behavior {Wang:2013io} (see Figure 1(b)). Investigation of the conduction mechanisms of CFRP is further complicated by the existence of resin rich layer between plies, as shown in Figure 1(c). Many factors may contribute to the formation of resin-rich interface including methods of fabrication and handling issues during layup. In some applications {Sharma:2014gw} {HIRANO:2016cv}, particles to increase toughness are added to the resin system for better interfacial bonding which can result in resin-rich layer, as shown in Figure [REF]. The extra resin rich layer separate carbon fibers from neighboring layers and reduce the number of contact points between layers, thus increasing through-thickness resistivity. The reduction in through-thickness contact points results in localized Joule heating that may change electrical properties of the CFRP laminates, as discussed in later sections.so what prevent it from doing it and thus we all should be