Conclusion
The CFRP resistivity along the depth direction is measured with the 4-probe method using lightning and HEMP impulse generators. Non-linear effects were observed. Breakdown could be caused not only by 6.4/69 us lightning pulses, but also by 20/500 ns HEMP pulses at a slightly higher voltage with much less pulse width and energy, which implies that the breakdown at these pulse widths should be dominated by the electric breakdown mechanism. By analyzing the waveform under HEMP, the electric breakdown is fast. Voltage spikes occur in the leading edge during the first few nanoseconds. The transient resistance (the ratio of voltage to current at every time point) remains almost constant after the spikes. These characteristics are similar to the breakdown of thin polymer films.
All the measurements were performed with currents about 2 orders of magnitude below lightning standards and the experiments were not intended to be destructive. Although information might be acquired for understanding the onset of the lightning interaction with CFRPs, attention should be paid while analyzing the period after high current and energy are applied, especially when gas from the decomposed resin and lightning plasma pressure might influence the resistance between plies. If the conclusions are verified by further investigations, the impedance along the depth direction drops drastically after the electric breakdown. The relationship between the conductivity along the depth direction and the electric field should be taken into consideration, while modeling the interaction of CFRPs with lightning. Numerical models could be used in subsequent studies to better understand the involved mechanisms.