Optimization design of lightning impulse shielding effectiveness of aircraft composite skin
2021-03-25
Abstract : In order to optimize the shielding effectiveness of CFRP under copper net, in this paper, shielding effectiveness of magnetic field for time-domain lightning pulse waveform is simulated in 3 different methods. These 3 methods are: single layer copper net covering the outer surface of CFRP; double layer copper net respectively attached to the inner and outer surfaces of CFRP; and connecting the inner and outer double-layer copper net covering the CFRP with fixed-components. Calculation method is time domain transmission line matrix algorithm. Simulation results indicate that there are some differences in shielding effectiveness between 3 kinds of shielding method. The best shielding effectiveness is double-layer copper mesh, followed by double layer copper mesh connected with fasteners, and the shielding effectiveness of single layer copper mesh is the lowest.
Keywords : CFRP lightning pulse shielding effectiveness
Introduction
Carbon fiber composite (CFRP) is widely used in aerospace, manufacturing and other fields due to its high strength, high hardness, light weight and corrosion resistance . CFRP has been widely used in modern aircraft structures. Its appearance makes the aircraft structure weight reduced, environmental protection and energy saving, comfort and safety greatly improved. Although the CFRP has many advantages, its conductivity is far lower than that of traditional metals. Therefore, for the cable harness and other electronic sensitive equipment inside the cabin, the CFRP may not provide good electromagnetic protection, making the electromagnetic wave interfere with the aircraft's driving system and communication system, making it unable to complete takeoff and landing or even normal operation, producing potential safety hazard of aircraft. HPM, LEMP, and HEMP and other external complex electromagnetic environment will cause great challenges to the flight safety of the aircraft. Therefore, it is of great significance to study the electromagnetic shielding effectiveness of aircraft cabin. The aircraft body skin can effectively shield the external interference field and greatly reduce the radiation in the cabin.
Definition of shielding effectiveness
Electromagnetic shielding effectiveness (SE) is the most important parameter to measure the performance of electromagnetic shielding materials，in dB unit. It reflects the attenuation degree of electromagnetic shielding materials to electromagnetic wave，its expression is as below:
are respectively electric field strength before and after shielding, 
are the magnetic field strength before and after shielding. 
are power density before and after shielding.
The shielding effectiveness of the fuselage can be calculated by the above formula. But the shielding effectiveness not only depends on the performance of shielding materials, it is also related to the frequency of the electromagnetic wave, the distance between the radiation source and the shield, and the degree of discontinuity on the shield surface. Therefore, the data obtained from the analytical solution is often far from the actual measured value. Therefore, we use the numerical simulation method to get the results. In this way, the result is closer to the actual measured value than the analytical formula, so it has more practical value.
Setting of simulation parameters and model materials
Considering that good conductor can form higher reflection loss, copper material is set as shielding material of aircraft composite skin[5]. In the simulation of aircraft skin, the worst electromagnetic environment during the flight is considered, that is, the electromagnetic environment when the aircraft is struck by lightning. The set lightning current is the specified lightning current component A in SAE ARP5412B. The peak value of the waveform is at 6.4 μs, total length 300 μs. Considering that the peak value of space electric field and magnetic field reaches in a very short time ( about 10μs )，the electric and magnetic fields of the
subsequent attenuation part have no effect on the shielding effectiveness. So the simulation time is reduced to 20 μs. The complete waveform (300 μ s) and simulation waveform of lightning current are shown in the figure 1a and 1b.
Considering the real situation, the structure of copper net is very complex. If we build the structure according to the actual situation, the number of mesh generation is very large. It will lead to a very long calculation time, even unable to calculate. In fact, the copper net size of CFRP materials is in the order of meters. The amount of meshcells is extremely large. So we use size of 250mm*250mm. In this way, the trend of electromagnetic field in the cabin will not change. It does not affect the result comparison of the three situations, and the number of mesh needed is small, which can greatly shorten the simulation time. The three simulation models are shown in Figure 2. Because the separate lightning current cannot form a circuit, when calculating the shielding effectiveness, no calculation of Ha component (the magnetic field strength without copper net), only compare the component Hb size under three shielding modes. That is, the value of magnetic field distribution.
The physical process of the simulation is as follows: The lightning current is injected into the side of the copper net above the CFRP, the lightning current
injection point is shown on the right end of the blue straight line in Figure 2. The measuring position is 4mm below the center of CFRP, using probe to measure the magnetic field strength at this position. Because the spectrum components of lightning current signal are mostly concentrated in the low frequency band, signal energy over 100MHz is almost negligible, so the simulation frequency range is set at 0-100MHz，the probe position is shown in Figure 3.
The algorithm used here is the finite integration algorithm in time domain, which is widely used in the calculation of large-scale and broadband devices.
Comparison and analysis of simulation results
Figure 4 shows the curve of the magnetic field intensity on the probe with time. It can be seen that the magnetic field intensity of the probe has overcome the peak value. It indicates that the peak signal of lightning current has been completely transmitted to the probe position. Subsequent descent has no effect on shielding effectiveness, so it is not considered in simulation.
Fig. 5 shows the curve of magnetic field intensity at the probe with time under three types of copper net. As can be seen from the figure, the shielding effectiveness of single-layer copper net is the lowest, the second is the
double-layer copper net with fixed component. The shielding effectiveness of double layer copper net without fixed component is the best. Because there is only one layer of copper mesh in single layer copper net, only one reflection loss can be generated for electromagnetic wave. Therefore, the electromagnetic energy through the CFRP is larger than that through the double-layer copper net.
Figure 6 shows the absolute value distribution of the surface current of the copper mesh with fixed component, when there are fixed component connecting the inner and outer copper nets, fixed component are conductors, therefore, lightning current is transmitted from the copper net above the CFRP to the copper net below through fixed component, therefore, the electromagnetic field at the probe position is enhanced.
Figure 7 shows the absolute value distribution of the surface current of the double-layer copper mesh without fixed component under the action of lightning current. When there is no fixed component, because the inner and outer conductors are not connected, the lightning current cannot be transferred from the outer copper net to the inner copper net. Therefore, the electromagnetic field at the probe is relatively weak compared with the fixed component. It can be seen from the above calculation results that at the initial time of lightning current injection（0-2μs）,the shielding effectiveness of three kinds of copper nets is almost the same, difference starts from 2 μs. Among them, the shielding effectiveness of single-layer copper mesh is the weakest, followed by double-layer copper mesh with fixed components, and the effect of double-layer copper mesh without fixed component is the best, the ratio of the average magnetic field strength of three kinds of copper nets is: 1: 0.8: 0.7. In the interior of the aircraft, places with high shielding efficiency are needed, such as cable harness laying area, is suitable for double layer copper mesh without fixed component. Single layer copper net can be used for aisle and other positions which don’t need high shielding efficiency to save costs.
Summary
The current of lightning can be up to 200kA, and the voltage can reach millions of volts，It is one of the safety hazards to be considered during the flight. In this paper, the shielding effectiveness of copper covered on the surface of CFRP in three different ways is simulated. The conclusion is that the shielding effectiveness of single-layer copper network is the weakest, The second is the double-layer copper net with fixed component, the shielding effectiveness of double layer copper net without fixed component is the best. The ratio of the average magnetic field strength of three kinds of copper nets is about 1:0.8:0.7. The specific way to lay the copper net needs to analyze the shielding effectiveness requirements according to the different positions of the aircraft. The single-layer copper net can be used for the positions with low requirements, and the double-layer copper net without fixed component can be used for the positions with high requirements.
