Carbon nanotube protected composite laminate subjected to lightning strike: Interlaminar film distribution investigation

Xinglin ZHANG, Jikui ZHANG, Xioqun CHENG,*, Wenjun HUANG

a School of Aeronautic Science and Engineering, Beihang University, Beijing 100191, China

b Research Institute of Frontier Science, Beihang University, Beijing 100191, China

c AVIC China Helicopter Research and Development Institute, Jingdezhen 333001, China

KEYWORDS

Abstract A kind of interlaminar film with carbon nanotubes inserted into polyether ketone with cardo was used for lightning strike protection of composite laminates.The distribution of the interlaminar film was investigated by experimental and numerical methods. Artificial lightning strike tests were conducted for 12-film carbon nanotube and traditional surface silver coating protected specimens.Then corresponding finite element models(FEMs)were established to analyze the lightning strike effect and validated by the experimental results. Based on the FEMs, the number, distribution and thickness of interlaminar film were investigated in order to obtain equivalent protection effect with the traditional surface silver coating. The results show that only the first two layers were damaged for the surface silver coating protected specimen, while 5 layers were ablated for the 12-film protected specimen.Lightning strike damage area of the laminate protected with 5-film carbon nanotube is almost the same as that of the laminate protected with 12-film carbon nanotube.Compared with traditional surface silver coating protection,one film protection with thickness of 360 μm can make the laminate to obtain equivalent damage depth,54.8%smaller damage area and 58%less additional weight.And reparability of the laminate is better than that of the laminate protected with 5 interlaminar films.

1. Introduction

Carbon fiber reinforced polymer (CFRP) composite materials show a promising future to replace metals in engineering practice for their high specific strength and specific stiffness, excellent resistance to corrosion and fatigue, outstanding directional design ability. Composite materials account for more than 50% by structural weight in the latest commercial airplanes, such as Boeing 787 and A350XWB, and most modern helicopters. According to the statistics, a commercial aircraft suffers from lightning strike once about every 5000 flight hours.1,2Since lightning strike mainly occurs below 6000 m in height, and the flight altitude of helicopter is generally within this height, the possibility of helicopters suffering from lightning strike is greater than that of commercial aircrafts. Lightning strike causes serious damages to CFRP around the lightning attachment points, including burning and vaporization, magnetic force, overpressure and delamination since the electrical conductivity of CFRP is much lower than that of metals. Therefore, lightning stirke may decrease the load bearing capacity of the CFRP structures dramatically.3,4Lightning strike protection is a challenge for the application of CFRP components in aircraft.

Traditional lightning strike protection methods for composite structures mainly applied in engineering practice now are copper or aluminum foil/mesh due to their high electrical conductivity and heat of vaporization.5,6Copper or aluminum foil/mesh is coated on the structural surface or embedded under the surface layer of CFRP. Boeing 787 applies surface interwoven copper mesh and aluminum coating by flame spray for lightning strike protection of CFRP components.7Lightning strike protection effect of traditional copper or aluminum foil/mesh was verified by many researchers.8–11Guo et al.9investigated the lightning strike protection performance of copper mesh. The test results showed that copper mesh can reduce lightning strike damage area and depth of CFRP laminate compared with unprotected laminate. However, some problems exist with traditional metal lightning strike protection methods, such as galvanic corrosion between aluminum and CFRP and great additional weight of copper meshes.12

In recent years, several light-weight and electrochemical compatible materials have been developed for lightning strike protection of composite structures.9,13–20Guo et al.9also explored lightning strike protection effect of lightning Zone 2 of an aircraft with nickel coated carbon fiber nonwoven veils surface.They found that the surface layer with 70 g/m2is more effective than commercial expanded copper foil with 73 g/m2in lightning strike protection. The Ni-coating can conduct current fast and suppress resistive heat. And the carbon fiber framework can enhance the ablation resistance and prolong service life of the lightning strike protection layer.Gou et al.16developed three kinds of surface lightning strike protection layers with carbon nanofibers and nickel nanostrands.The layers show different electrical conductivities with various addition proportions of carbon nanofibers and nickle nanostrands.The test results show that lightning strike protection effect is proportional to the electrical conductivity of surface layer, which is related to the proportions of carbon nanofibers and nickel nanostrands. Kumar et al.17developed a polyaniline-based surface layer for lightning strike protection of composite structures.The surface layer is polyaniline-based with the thickness of 0.25–0.4 mm. The test results show that the CFRP laminates protected by polyaniline-based surface layer can retain 99% of flexural strength after lightning strike with current amplitude of 100 kA.

In order to restrict the lightning strike damage area, some researchers have been trying to improve through-thickness electrical conductivity of CFRP.21–29Kumar et al.24performed lightning strike tests with four CFRP panels with different through-thickness electrical conductivities,which were tailored by a controlled thermal treatment. The experimental results show that the specimen with through-thickness electrical conductivity of 110 S/m can retain 92% of the flexural strength after lightning strike,and specimens with higher electrical conductivity suffer less damages from lightning strike.The electrical conductivities of the polyaniline-based composites developed by Hirano et al.14are 14,800 S/m and 73 S/m in in-plane and through-thickness directions respectively. The residual flexural strength of the polyaniline-based composites after lightning strike at 100 kA remains 90%, but the residual flexural strength remains only 24% for epoxy-based composites without lightning strike protection after lightning strike at 40 kA. Kumar et al.25also developed a kind of multiwall carbon nanotube buckypaper interlayer to increase throughthickness electrical conductivity of composite laminates for lightning strike protection.They found that buckypaper interlayer is an effective lightning strike protection way to keep mechanical properties of the laminates.

Based on the light-weight lightning strike protection film which is made of polyether ketone with cardo (PEK-C) and mixed with carbon nanotubes,28laminate specimens with this carbon nanotube interlaminar films and traditional surface silver coating were manufactured and tested with lightning strike here. Then corresponding FEMs were established to simulate the lightning strike damages, and configuration of the interlaminar film is discussed.The purpose of this work is to reduce the additional weight of lightning strike protection system.

2. Experiment

In order to assess the lightning strike protection effect of the film, carbon nanotube film and traditional surface silver coating protected specimens were fabricated and conducted artificial lightning strike tests.

2.1. Specimens

The CFRP laminate was made of T700/5228 with the dimension of 150 mm,100 mm and 3 mm in length,width and thickness respectively. The stacking sequence was [45/0/-45/90]3Swith the ply thickness of 0.125 mm.Two kinds of systems were introduced to protect the laminates from lightning strike as follows.

(1) For carbon nanotube film protected specimen,a total of twelve carbon nanotube films were inserted into the upper twelve interlayers of specimen in turn.The carbon nanotube film has the thickness of 40 μm and the surface density of 7 g/m2. The additional weight of lightning strike protection system is 1.26 g (84 g/m2). The film is Polyether Ketone with Cardo (PEK-C) and mixed with carbon nanotubes (mass friction: 30 %). The carbon nanotubes are 10–20 nm in diameter and 5–20 μm in length. Fig. 1 shows the image of the film observed by Hitachi S-4800 scanning electron microscope. It can be found that the orientation of the carbon nanotubes is almost random, meaning carbon nanotube film can be regarded as homogenous.

(2) For surface silver coating protected specimen, The protection layer is silver coated non-woven fabric, with the thickness of 60 μm and surface density of 150 g/m2.The additional weight of surface silver coating is 2.25 g(150 g/m2), which is 78.6% bigger than that of twelve carbon nanotube films.

Fig.1 Image of carbon nanotubes in the film(photographed by Hitachi S-4800 scanning electron microscope).

2.2. Experiment conditions

The clamping end of the specimen is 20 mm at the short edge.The current of waveform D described in SAE ARP 54121is applied on the specimen since this research focuses on 2A lightning strike area of aircraft.

Fig. 2 shows the images of damage and C-scan of carbon nanotube film and surface silver coating protected specimens.The ablation areas of the carbon nanotube film and surface silver coating protected specimens are 1095 and 1660 mm2respectively. That means the novel protection method can reduce 34.0%in damage area and 44.0%in additional weight.There are two issues that should be pointed out.

(1) Only the first two layers are damaged for the surface silver coating protected specimen,whereas as many as five layers are ablated for the carbon nanotube film protected specimen.Therefore,the configuration of the carbon nanotube film should be rearranged to restrict damages in the thickness direction.(2) A total of twelve carbon nanotube films are inserted into the laminate. However, only upper 5 plies are damaged for the film protected specimen. The other 7 inserted films contribute little to the lightning strike protection.In other word, the additional weight of the carbon nanotube film protection system can be reduced further.

Fig. 2 Image of damage and C-scan of carbon nanotube film and surface silver coating protected specimens.

Table 1 Material properties of CFRP.

Fig. 3 Boundary conditions of coupled thermal-electrical finite element model.

3. Finite element model

3.1. Model description

The material properties of CFRP are shown in Table 1. The electrical conductivities of the surface protection layer and the carbon nanotube film are defined as 3×106S/m and 6×102S/m in the simulation respectively.

A coupled thermal-electrical finite element model was developed to simulate the lightning strike damage of carbon nanotube film protected laminate in order to reduce the additional weight and through-thickness damage. The boundary conditions of finite element model are presented in Fig.3.The electrical potential of the clamping ends and side surfaces was set as zero,since the clamping ends were earth grounded and the electrical discharge from the side surfaces to the clamping ends was observed during all the lightning strike tests.The thermal emissivity of clamping ends was set as zero,which was 0.9 for other surfaces.The environment temperature was 25°C.

As pyrolysis of epoxy and remarkable stiffness decline of CFRP will take place when the temperature exceeds 300°C,28300°C can be set as lightning strike damage criterion in this paper.

3.2. Model validation

Fig.5 Procedure of light-weight carbon nanotube film lightning strike protection system development.

Fig. 4 shows numerical damage area of twelve-film carbon nanotube and surface silver coating protected laminates. It can be found that: (1) The damage of carbon nanotube film protected laminate extends in the 45° direction, which is in accordance with experimental results (Fig. 2). The damage of surface silver coating protected laminate is similar ellipse,which is similar to test results(Fig.2).(2)The numerical damage area of carbon nanotube film protected laminate are 1275 mm2, with the error of +16.4%. And for surface silver coating protected laminate, the results are 1500 mm2, with the error of -9.6%. In general, the damage shape and size of numerical results agree well with that of experimental results. Therefore, the finite element model established in this paper is acceptable to simulate the lightning strike process.

Fig.4 Numerical lightning strike induced damage areas(red region)of twelve-film carbon nanotube and surface silver coating protected laminates.

Fig. 6 Numerical lightning strike induced damage areas (red region) of twelve and five film protected laminates.

Fig. 7 Numerical lightning strike induced damage areas of carbon nanotube film protected laminates with different film numbers.

(a) As only five carbon nanotube films are damaged in the 12-film carbon nanotube protected specimen, the number of carbon nanotube film should be studied,as shown in Fig.5(a)and(b).The comparison of damage areas of carbon nanotube film protected laminates with different film numbers is shown in Section 4.1.

(b) The interlaminar film is hard to repair once it is damaged by impact or lightning strike. Consequently, the five interlaminar thin films (40 μm) are concentrated to one thick film(200 μm)placed under the top ply of laminate, as shown in Fig.5(b)and(c).The lightning strike protection effect difference between five interlaminar thin films and one thick film is illustrated in Section 4.2.

(c) The thickness of carbon nanotube film is adjusted to apply the equivalent lightning strike protection with traditional surface silver coating,as shown in Fig.5(c)and(d).Section 4.3 reveals the influence of carbon nanotube film thickness on lightning strike induced damage and additional weight.

4. Results and discussions

4.1. Number of carbon nanotube films

The purpose of this study is to develop a kind of light-weight carbon nanotube film lightning strike protection system with the equivalent protection effect with traditional surface silver coating. Fig. 5 shows the development procedure by considering the balance of additional weight, maintainability and protection effect.

According to the experimental results, only five carbon nanotube films are damaged in the 12-film carbon nanotube protected specimen. The inserted film number should be studied to reduce weight. The lightning strike protection effect of carbon nanotube film protected laminates with different film numbers is calculated by finite element model developed in Section 3.

Fig. 8 Numerical electrical potentials and temperature profiles at the time of peak current (14 μs) at lightning attachment point of five thin film and one thick film protected laminates.

Fig. 9 Damage areas of the top five plies at 200 μs of five thin film and one thick film protected laminates.

Fig. 6 compares the lightning strike induced damage area on 5-film carbon nanotube protected laminate with that of 12-film carbon nanotube protected laminate. Fig. 7 shows lightning strike damage areas of carbon nanotube film protected laminates with different film numbers. Simulation results show that the damage area of 5-film carbon nanotube protected laminate is almost same with that of 12-film carbon nanotube protected laminate, as shown in Fig. 6. As the film number increases from 5 to 6,the lightning strike damage area remains unchanged. Whereas as the film number decreases from 5 to 4, the lightning strike damage area increases from 1275 mm2to 1462 mm2. Hence, only the five inserted films close to the laminate surface should be retained in order to reduce the weight of protection system.

4.2. Distribution of carbon nanotube films

For the interlaminar film,it is difficult to recover the electrical conduction ability of the protection system once it is damaged.Therefore, five interlaminar thin films (Fig. 5(b)) are concentrated to one thick film (Fig. 5(c)) under the top ply of laminate in consideration of maintainability. The protection effects of five interlaminar thin films and one thick film are compared in Figs. 8, 9 and 10.

Fig. 10 Numerical lightning strike induced damage areas (red region) of five thin film and one thick film protected laminates.

Fig. 8 shows the numerical electrical potential and temperature profile at the lightning attachment point at time of peak current applied (14 μs). It can be concluded from Fig. 8 that both electrical potential and temperature are same under the 5th layer for these two kinds of laminates.However, for the upper five layers, the electrical potential and temperature of one thick film protected laminate are lower than that of five thin film protected laminate. And for one carbon nanotube film protected laminate, the temperature decreases greatly at the interface of the top ply and the carbon nanotube film. That implies a large proportion of lightning energy is conducted to the ground by the thick carbon nanotube film.

Fig. 9 presents the damage areas of the top five plies at the end of lightning strike of five-film and one-film carbon nanotube protected laminates respectively. It can be found that damage area of each ply of one-film protected laminate is smaller than that of five-film protected laminate except the second ply. For one thick film protected laminate, the lightning strike energy is conducted to the ground by the thick carbon nanotube film under the top layer. This results in bigger damage area of the second ply.

Fig.11 Damage and additional weight comparison between one carbon nanotube film protected laminates with different film thickness and traditional surface silver coating protected laminate.

Table 2 Lightning strike damage situation of laminates with different protection layer.

Fig. 10 compares the lightning strike induced total damage areas of five interlaminar thin film and one thick film protected laminates. It can be found that the lightning strike damage area of one-film carbon nanotube protected laminate is 975 mm2, which is 1275 mm2for five-film carbon nanotube protected laminate and 1660 mm2for traditional surface silver coating protected laminate. The one thick carbon nanotube film can provide the best protection effect in the point of view of the damage area.

4.3. Thickness of carbon nanotube film

Fig.9 implies that four plies are damaged for the one-film carbon nanotube protected laminate. By contrast, only two plies are damaged for traditional surface silver coating protected laminate as shown in Fig. 2. The thickness of the carbon nanotube film should be discussed further to apply the equivalent lightning strike protection effect with traditional surface silver coating.

Fig. 11 and Table 2 compare carbon nanotube film protected laminate with traditional surface silver coating protected laminate in the point of damage area, number of damage plies as well as additional weight.It can be found from Fig. 11(a) that as the film thickness changes from 200 to 360 μm, the damage plies of one thick film protected laminate decrease from 4 to 2.That means one thick film(360 μm)protected laminate could apply the equivalent lightning strike protection effect with traditional surface silver coating in the thickness direction. By contrast, the additional weight and damage area of one thick film (360 μm) protected laminate decrease 58% and 54.8% than traditional surface silver coating protected laminate, as shown in Fig. 11 (b), (c) and (d).

Table 2 also compares one thick film protection with the original twelve interlaminar film protection.The damage area,number of damage plies and additional weight of one thick film (360 μm) protected laminate decrease 41.2%, 60% and 25% respectively. One thick film (360 μm) can also be divided into two thin films inserted into the first two interlayers close to the laminate surface, which will apply the similar lightning strike protection effect.

5. Conclusions

A kind of interlaminar film made of Polyether Ketone with Cardo (PEK-C) mixed with carbon nanotubes was developed for lightning strike protection of composite laminate.Artificial lightning strike tests were conducted for specimens protected by interlaminar carbon nanotube film and traditional surface silver coating.Finite element models were established and validated by the test results. The effects of number, distribution and thickness of the interlaminar film on lightning strike protection were investigated. The following conclusions can be drawn.

When the film number is more than 5, the lightning strike damage area remains almost the same. But when the film number is less than 5, the damage area increases with the film number decrease.

If several films are inserted in the first interlayer continuously, the protected laminate will show better current conduction ability and provide better protection effect than that with films inserted in different interlayers close to the laminate surface discretely.

When the laminate is protected by one film with the thickness of 360 μm in the first interlaminar, equivalent protection effect in the thickness direction can be achieved compared with conventional surface silver coating protection method, and the damage area and the weight increments are 54.8% and 58% smaller than that of the latter method respectively.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.