# Review on the Electrical Resistance/Conductivity of Carbon Fiber Reinforced Polymer
## 2 Structure of CFRP and Its Electrical Conductivity
### Conductivity and Lightning Strike  
#### 3.2.1 Why is CFRP Vulnerable to Lightning Strike?  
Recently, the application scope of CFRPs has been largely expanded. Lightning strikes an aircraft at least once a year on average and seriously threatens the operation safety of aircrafts [71; 72; 73; 74]. CFRPs have high in-plane electrical conductivity but low through-thickness electrical conductivity and can be more easily destroyed by lightning strikes [75; 76]. In case of lightning strikes, a lot of energy is delivered rapidly, so ionized channels expand immediately. In traditional aircrafts, lightning may flow along the aluminum skin without serious damage. However, composite structures with lower conductivity, such as CFRPs, cannot conduct away the large electrical current from a lightning strike and lightning may pass through the aircraft structure, vaporize cables, weld hinges on aircraft surfaces, explode fuel vapors, and generate serious consequences. Damages, which are deeper in laminates with poor conductivity in thickness direction, may lead to the catastrophic accidents of CFRP structures [77; 78; 79; 80]. Electromagnetic interference may affect the functions of electronic and electrical systems too [81]. This has led to more complicated lightning strike protection problems [82].  
In previous studies, [PERSON] explored lightning strike damages on carbon/epoxy specimens under different currents and found that electrical conductivity was a key parameter for developing effective lightning strike protections (LSP) [83; 22]. Hence, it is necessary to further explore the relationship between lightning strike damage mechanisms and electrical conductivity. The effect of a lightning strike can be reduced by diverting electrical currents or designing the CFRP components [84; 22]. These lightning protection measures are also applicable to other vehicles. [PERSON] et al. analyzed the lightning strike behavior in CFRP materials through the impulse current testing [85]. The inhomogeneous and anisotropic electrical property determines the damage types such as fiber failure, delamination or decomposing of polymer matrix. Lately, in order to explain the failure mechanism by means of numerical simulation and experimental testing, the researchers performed a thermal-electrical analysis of CFRP laminates suffered from the artificial lightning striking [86]. The conductivity along the thickness direction is in a linear relationship with temperature. The delamination zones and damage depths obtained from numerical analysis were consistent with the thermal damage area observed from experimental results.  
Many studies into the simulation and experimental methods have been conducted in this field [87]. Through artificial lightning testing on carbon woven fabric/epoxy laminates [88], damage characteristics, models, and mechanisms have been evaluated based on measurement results. With a thermal-electrical model, the ablation damage of carbon woven fabric/epoxy laminates under simulated lightning current was simulated and the electrical conductivity in the thickness direction varied with the pyrolysis degree of composite materials. Through optical visualization experiments with CFRP laminates, [PERSON] explored lightning impulse current discharge phenomena and found that the propagation speed of shock waves was influenced by the thermal and electrical conductivities of specimens [89]. [PERSON] et al. explored quasi-isotropic laminated CFRPs through simulation and experiments and analyzed the lightning strike damage behavior of CFRP structures. Under the peak current of 40 kA, with finite element method (FEM), the damage condition was investigated through thermal and electrical analysis and experimental results were consistent with numerical results [85].