Introduction
Carbon fiber reinforced polymers (CFRP) offer great advantages over conventional materials, mainly due to their high specific strength and stiffness. However, lightning strikes pose a serious natural threat to high rise structures, such as wind turbines, as well as airplanes made of, or containing significant amount of composite materials \cite{abdelal2014nonlinear,ogasawara2010coupled}.
Modeling the lightning current interaction with CFRP is a challenging problem \cite{Smorgonskiy2014Modeling}. Good understanding of the CFRP material properties is the key to precise numerical models \cite{abdelal2014nonlinear}. The electric conductivity has significant influence on current distribution and also on power densities. As an anisotropic material, CFRP has higher conductivity along the fiber direction, but much lower conductivity in the depth and transverse directions \cite{Yu2015A}, which makes the electrical current tending to concentrate on the surface near the lightning attachment point.
A simple constant conductivity assumption might lead to extremely high Joule heat and unrealistic temperature \cite{abdelal2014nonlinear,ogasawara2010coupled}. Although the conductivity is assumed to depend on temperature in many models \cite{abdelal2014nonlinear,ogasawara2010coupled}, it is seldom considered to be relevant to electric field. While several experiments have been performed for electric conductivity under low current and room temperature, the number of investigations dealing with non-linear characteristics of CFRPs is limited \cite{sun2016dynamic}. Chekanov et al. \cite{Chekanov1999Electrical} has observed the breakdown effect of CFRPs with short cut fibers, which have much lower fiber content and also electric conductivity. Sun et al. \cite{sun2016dynamic} compared the dynamic impedance of several CFRP specimens with emphasis on material surfaces. The depth direction conductivity of long fiber reinforced polymers might exhibit non-linearity under high voltage as well, owing to the sandwiched polymer between plies, which requires more detailed investigation.
The objective of this paper is to experimentally study the non-linear current-voltage behavior of CFRP samples along the depth direction. The rest of the paper is organized as follows. Section 2 discusses the method and the test fixture. The obtained data are presented and discussed in Section 3. Conclusions are given in Section 4.