# Review on the Electrical Resistance/Conductivity of Carbon Fiber Reinforced Polymer
## 1 Introduction  
Research and development in the field of materials is largely driven by application demands. Carbon fiber reinforced polymer (CFRP) is a composite material composed of a polymer resin and carbon fibers. CFRP has shown enormous potential in many fields, such as sports and the aircraft industry. Particularly, CFRP is widely applied in automotive and aerospace because of its great potential strength, lightweight, non-corrosion, and excellent fatigue resistance [1, 2]. Over the last two decades, the global demand for carbon fibers has increased rapidly and is expected to reach 140,000 tons in 2020 [3]. In the automobile industry, CFRPs account for 17% of auto weight and the application of CFRPs can reduce auto weight by 30% [4, 5]. For aircrafts, CFRPs account for 50% of the total weight of structural elements and the application of CFRPs can reduce the weight of structural element by 20% [6]. The applications of carbon fibers, especially CFRP, in the civilian aerospace industry are rapidly increasing. CFRP was first applied in astronautics and aeronautics in 1950s [7]. CFRP is applied in primary load-bearing structures, such as wing planks, fuselages, sandwich panel skins [8]. According to Boeing's official website, carbon fiber composites only account for 3% of a Boeing 767's total weight [9]. The proportion in Boeing 777 model increased to 7%. Boeing 7E7 containing about 50% CFRPs can reduce 20% of fuel consumption compared to airliners of the same size [10]. Especially, the wings and fuselage of Boeing 787 Dreamliner was made of CFRP [11]. A350-XWB was mainly made of composite materials, especially CFRPs. In the future, the weight proportion of CFRPs in aircrafts will exceed 50% [12]. In the construction industry, CFRP was first used to strengthen the Ibach Bridge in Switzerland in 1991 [13]. In civil engineering, CFRP is currently being adopted to repair and improve civil structures [14]. The cost of CFRP laminates is higher than that of orthodox metals in aircrafts. However, the application of CFRP is still valuable due to the fuel cost reduction and longer material life cycle. The application of CFRPs will contribute substantially to the reduction of CO\({}_{2}\) emission over the total life cycle of aircrafts [15]. However, the application potential of CFRP is generally reduced by high safety factors. During their service life, due to impacts and fatigue loads on CFRPs, local damages such as delamination may be formed. However, failure prediction is not reliable so far. New problems in these structures made of composite materials remain to be solved [16]. Besides, composites such as CFRP are complex materials exhibiting distinct anisotropic properties [17]. CFRPs have found wide applications in antennas [18], transducers [19], aircrafts [6], automobiles [4], etc. In order to assess damages that occur during the production and the process of using, several non-destructive methods have been proposed, such as electrical measurements [20]. Furthermore, the Joule effect, which is used as an innovative, non-destructive method, has been studied in order to identify interlaminar damages in CFRPs [21]. In addition, CFRPs are expected to solve specific problems for certain applications, such as novel anti-light techniques in the aerospace industry [22]. In all the aforementioned applications, the accurate determination of electrical conductivity of CFRPs is a basic prerequisite. The paper focuses on the electrical conductivity of CFRPs, which shows anisotropy and significantly affects the application performance of CFRPs. In Section 2, we describe the structure and conductive theory of CFRP. Electrical conductivity and its characteristics are emphasized. In Section 3, different techniques for CFRP conductivity measurements are summarized according to different applications, i.e., self-sensing, lightening, and electromagnetic shielding. The applications of CFRP conductivity measurements and techniques are summarized in Section 4.