Result

During the test, the short-circuit current setting of the pulse source was increased by 3 dB per test step, that is, \(\sqrt{2}\) times of the previous step. (Due to the finite source resistance shown in Table 1, the monitored actual current increment per step was a little larger than 3 dB.) Each specimen only experienced 1 test cycle. After the current was increased step by step to the maximum and then decreased back to the initial value, the specimen was replaced to avoid severe cumulative damages. Surface damages have never been observed during the whole experiment.
The current and voltage waveforms of a 6.4/69 \(\mu\)s pulse applied to a specimen are shown in Fig. \ref{fig:waveformLemp} (short-circuit output set to 800A). The current waveforms were still in good double exponential shapes, while some voltage distortion could be observed in the first few microseconds.
The plot of the peak voltage versus peak current is shown in Fig. \ref{fig:viLemp}. The V-I curve has an initial high increase rate, while becomes significantly less steep after about 470 V, showing the non-linear effect. The arrows in the figure represent the sequence of test steps. The DC resistance was measured again after each pulse, as shown in the same figure. Compared to its initial value, the resistance drops significantly after breakdown. In a lightning strike, the current is not influenced by the material due to the high impedance of the plasma channel \cite{chemartin2009three}. The decrement of the resistance means much less energy (several order of magnitude) is transferred into the material after breakdown. This result shows that the resistivity should not be regarded as constant in simulations.
For the HEMP 20/500 ns pulse, the waveform with a peak current of 660 A is shown in Fig.\ref{fig:waveformHemp}. The voltage waveform is characterized by a significant number of spikes in its leading edge, while the trailing edge follows an exponential shape. To quantitatively evaluate the voltage-current (V-I) relationship, voltage values were extracted from the peak of exponential fittings of the voltage waveforms, neglecting the voltage spikes. In Fig. \ref{fig:viHemp} shows the peak voltage versus peak current, where a non-linear behavior can be observed as well. The arrows in the figure represent the sequence of test steps. The HEMP pulse is so short that its pulse width is only 0.7 % of the 69 ns lightning WF1. Although the time duration varies by more than two decades, the turning point of the voltage is in the same order of magnitude. This implies that breakdowns relate weakly to the transferred energy but more to the applied voltage or electric field. The dissipated energy within the specimen, integrated according to the Joule’s law \cite{ogasawara2010coupled} from Fig.\ref{fig:waveformHemp}, was found to be 0.4 J, which is even not enough for the temperature of the specimen to increase on average by more than 2 K. Although the temperature rise is not uniform, drastic inhomogeneity might not appear before breakdown, leading to a local high temperature. Hence, it could be inferred that the electric breakdown mechanism rather than the thermal breakdown mechanism dominates.
The electric breakdown is a fast process. Significant voltage spikes exist in the leading edge, as shown in Fig.\ref{fig:waveformHemp}. The spikes could not be measured precisely, since they were so narrow that they had exceeded the 100 MHz probe bandwidth. Moreover, the over-range audible alert of the probe sounded during the experiment, indicating that the actual voltage spike may even approach 6 kV (the upper limit of the probe). Compared to the small voltage spikes in the waveform with low current (Fig. \ref{fig:waveformHempLow}), these spikes should not be subjected to electromagnetic interference, because electromagnetic coupling is linear, which could not change the shape of voltage curves. The transient resistance (the ratio of voltage to current at every time point) remains almost constant after the spike (Fig. \ref{fig:resistance}). Therefore, it seems that conductive channels form just within the spikes, in no more than a few nanoseconds. After that, the channels would provide good conductivity. The resistance measured from the waveform is slightly lower than that measured with DC after the pulse, which might be attributed to the high temperature of the channel during the pulse. Fig. \ref{fig:waveformLemp}, no spike was observed, for the rising edge of the lightning pulse was too slow. Breakdowns happened continually during the rising edge, which formed the flat tops of voltage waveforms.
The above-mentioned characteristics look similar to the breakdown of thin polymer films \cite{Zakrevskii2005Electrical}. The formation of breakdown channels is within a short period of nanoseconds. These channels would keep their conductivity during the rest of the pulse. They are also irrecoverable, and they therefore still exist after the pulse. The breakdown field strength tends to grow slightly as the voltage rise rate increases. If the thin polymer film breakdown theory \cite{Zakrevskii2005Electrical} could be applied to the breakdown of CFRP, the process would be described as below. Upon applying the voltage, the electric field between the plies drives electrons into the polymer. Tunneling takes place where the electric field strength is in excess of its average value, especially where the fibers are closer between plies. The electrons are captured by traps and form space charges. As the space charges propagate toward anode, the electric field near the ply with higher potential will increase. Current amplification initiating local destruction of polymer is a consequence of space charge evolution. The intense current surge causing a breakdown channel between plies is the last stage of the electric field induced destruction of a polymer \cite{Zakrevskii2005Electrical}. The breakdown channels are about 10 \(\mu\)m in diameters. It is believed that destruction of a polymer (the formation of a hollow channel with conducting walls as a result of polymer evaporation and the formation of soot) is the outcome of the Joule heat liberation and heating of the material to high temperatures by an intense surge in the current \cite{Zakrevskii2005Electrical}. As the inter-ply polymers are broken down layer by layer, the voltage leading edge might comprise several narrow spikes.