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Eliminating lightning strike damage to carbon fiber composite structures in Zone 2 of aircraft by Ni-coated carbon fiber nonwoven veils
Composites Science and Technology 169 (2019) 95–102
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Eliminating lightning strike damage to carbon fiber composite structures in Zone 2 of aircraft by Ni-coated carbon fiber nonwoven veils
T
Yunli Guoa, Yongzheng Xua, Qinglin Wanga, Qi Donga, Xiaosu Yib, Yuxi Jiaa,∗ a b
Key Laboratory for Liquid-solid Structural Evolution & Processing of Materials (Ministry of Education), Shandong University, Jinan, 250061, China National Key Laboratory of Advanced Composites, AVIC Composites Center, Beijing, 100095, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Lightning strike protection A. Carbon fibers B. Electrical properties
By using standardized Waveforms C and D, this study explored the lightning strike protection (LSP) effectiveness of nickel coated carbon fiber nonwoven veils (Ni-CFNVs) on protecting the carbon fiber composite structures in Zone 2 (aircraft lightning zoning). The post-lightning damage was evaluated by visual inspection, ultrasonic scan and residual strength test. Results showed that the Ni-CFNV of 70 g/m2 eliminated lightning strike damage inflicted by both waveforms and it even performed better than the commercial expanded copper foil of 73 g/m2. Therefore, the Ni-CFNV will be a promising alternative to metal mesh for eliminating lightning strike damage to the structures in Zone 2. The remarkable LSP effectiveness of Ni-CFNV can be explained by its integration of high electrical conductivity of Ni-coating and prominent ablation resistance of carbon fiber. The experiments indicate that not only the electrical conductivity, but also the ablation resistance of LSP layer greatly influences the LSP effectiveness.
1. Introduction Lightning strike threatens all structures, especially the aircraft in flight that is unable to be grounded. Lightning plasma possesses the temperature as high as 30,000 K and the pressure up to ten atmospheres [1], which will induce severe damage to aircraft. As it has been reported that a commercial aircraft might be struck once every 3000 flight hours [2], it is vital to eliminate the threat of lightning strike to aircraft. Carbon fiber reinforced polymers (CFRPs) are increasingly adopted in aircraft because of their excellent mechanical and chemical performance, as well as light weight for fuel saving [3]. However, CFRPs present semi-conductive property, which limits their applications due to lightning strikes. On the one hand, the semi-conductive CFRPs can be struck as frequently as lightning receptors [4]. On the other hand, the high electrical resistance of CFRPs will impede lightning current from readily discharging, resulting in electric spark, Joule heat or dielectric breakdown [4–6]. Hence, effective lightning strike protection (LSP) solutions are highly desired for CFRP-based aircraft. On the basis of the experimental [3,7–13] and numerical [14–19] studies of lightning damage behaviors and mechanisms of CFRPs, a few LSP solutions [4–6,20–32] have been proposed, which can be divided into three categories according to the types of materials they rely on: metallic method, non-metallic method and the integrated method [6] ∗
that integrates metallic and non-metallic materials. The metallic methods, such as meshes and foils, are frequently selected for LSP owing to their high conductivity and low cost. Nevertheless, the metals usually cause overweight and galvanic corrosion [5]. The recently developed non-metallic methods, for instance, buckypaper (by Gou et al. [26] and Han et al. [27]), graphene (by Zhang et al. [28] and Wang et al. [29]) and conductive resin (by Hirano et al. [30] and Katunin et al. [31]), are lightweight and incorruptible. However, they hardly yield large and uniform surface layers for aircraft structures due to manufacturing technique and cost. The integrated LSP method usually introduces metallic fibers (wires), metallic nanoparticles or metallic coating into the non-metallic support materials (such as fibers, fabrics and prepregs), in order to achieve the LSP layers with high conductivity but low weight and low cost. The metallic fibers and wires woven into the carbon fabrics are susceptible to galvanic corrosion as metallic mesh. Chakravarthi et al. [22] and Dong et al. [25] proposed nickel-coated CNTs for the LSP of CFRP. However, considering the application to large structures of aircraft, the metallized nanoparticles [22,25] need to be further developed due to their dispersibility and cost. Rehbein et al. [23] used the silvercoated polyamide knitting yarns to increase the conductivity of NCFreinforced CFRP, but the solution might be insufficient to withstand heavy lightning because of the limited content of the conductive knitting yarns. By contrast, the metallizing carbon fibers and fabrics by
Corresponding author. No. 17923, Jingshi Road, Jinan, Shandong Province, China. E-mail address: [email protected] (Y. Jia).
https://doi.org/10.1016/j.compscitech.2018.11.011 Received 9 July 2018; Received in revised form 17 October 2018; Accepted 5 November 2018 Available online 09 November 2018 0266-3538/ © 2018 Elsevier Ltd. All rights reserved.
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2.2. Experimental setup
surface coating may be a workable plan. Firstly, not only the metal coating, but also the carbon fiber is quite conductive. Secondly, the metal coating will be isolated from air and water when encapsulated by the cured resin, eliminating galvanic corrosion [6]. Thirdly, the metalcoated carbon fibers can generate large and uniform LSP layers for aircraft structures easily by woven or nonwoven techniques. Furthermore, since the metal-coated carbon fiber fabric has carbon fiber framework, it has similar modulus and coefficient of thermal expansion with the CFRP base, which contributes to preventing thermal stress, aging, fatigue and cracking of LSP layer and surface paint when the aircraft undergoes the thermal cycling between the high and low altitude flights. However, by now, there has been no available literature that presents a detailed study of the LSP effectiveness of metal-coated carbon fibers. In order to optimize lightning protection, aircraft is divided into three lightning strike zones (Zones 1, 2 and 3) by SAE ARP 5414 [33], on the basis of the possibility of lightning environment that each zone encounters. For evaluating the direct effect of lightning, four standardized current waveforms (Waveforms A, B, C and D) are recommended to simulate the lightning environment by SAE ARP 5412 [34]. According to the standard, the aircraft structures in each lightning strike zone should sustain various standardized waveforms before service. Nevertheless, the standardized waveforms are very difficult to obtain because of the lack of the professional equipment which is usually exclusive to aerospace and military departments [13]. As Zone 2 covers the vast majority of airframe [8,33], the nickelcoated carbon fiber nonwoven veil (Ni-CFNV) is proposed as an LSP layer for the CFRP structures of the zone. According to SAE ARP 5412, the structures in Zone 2 should sustain the lightning current components of Waveforms B, C and D. As the electrical charge transfer of Waveform B (10 Coulomb) is only 5% of the electrical charge transfer of Waveform C (200 Coulomb), if the Ni-CFNV can withstand Waveform C without severe damage, Waveform B will be no longer a threat to the Ni-CFNV. Therefore, Waveforms C and D are enough to verify the LSP effectiveness of the Ni-CFNV, and the lightning strike of Waveform B was not performed in the report. The work aims to investigate the LSP effectiveness and mechanism of Ni-CFNV for protecting the CFRP structures in Zone 2, as well as to clarify a promising LSP solution that integrates metal with nonmetal materials.
The specimens were wrapped by copper foil tape (60 mm in width) in four edges for grounding. Then they were fixed on an epoxy/fiberglass panel by two grounded steel beams that tightly pressed the copper foil tape on two edges of the specimen by clamps (Fig. 2a). A copper rod electrode with a spherical ceramic tip (20 mm in diameter) was fixed over the specimen to inspire lightning arc (Fig. 2a). The gap between the electrode and specimen was about 10 mm, where a copper wire (0.05 mm in diameter) was used to induce the initial arc attaching to the center of the specimen top surface according to SAE ARP 5416 [35]. 2.3. Waveforms of artificial lightning In the study, Waveforms C and D were used to verify LSP effect of Ni-CFNVs on protecting the structures in Zone 2, to which the vast majority of airframe belongs [8,33]. The Waveform C has the charge transfer of 200 Coulomb ( ± 20%) with a current amplitude of (200–800) A, and Waveform D is featured as a peak of 100 kA ( ± 10%) with an action integral of 2.5 × 105 A2s ( ± 20%) [34]. The lightning was artificially simulated by an impulse current generator system in Xi'an Airborne Electromagnetic Technology Co. Ltd, China. The typical Waveforms C and D are presented in Fig. 2b and 2c, respectively. The detailed testing conditions of each specimen are listed in Table 1 and Table 2. Therein, the first capital of specimen labels (C or D) denotes the waveforms to which the specimens subjected and the other capitals and numbers indicate the LSP methods (02S denotes the unprotected CFRP). Besides, t1/t2 and action integral are the parameters of Waveform D as depicted in previous study [7]. 3. Results and discussion As the impulse current waveforms (e.g., Waveform D) that is characterized by high current peak and short duration are more widely applied and reported [3,7–13,23–31] than the continuing ones (e.g., Waveform C), the section begins with Waveform D. 3.1. Lightning strike test using Waveform D
2. Experimental
Waveform D represents a subsequent stroke [34] for assessing the direct effects of lightning of Zone 2. As Waveform D has a current peak of 100 kA, it possesses high voltage to puncture the air and the specimen, causing CFRP delamination, breakdown or piercing, as well as shock wave and thermal impact [36–38]. The visual inspection shows that the unprotected CFRP specimen (D02S, Fig. 3a) suffers the most severe damage (∼75 mm in diameter), which manifests as fiber breakage, ply-lift, resin pyrolysis and surface ablation. However, for DN1 and DN2, the damage is obviously suppressed (Fig. 3c and 3d). More specifically, DN1 only suffers a little carbon fiber damage (∼30 mm in diameter) with a ply angle of 45°, indicating that the 100 kA strike only affects the outmost CFRP ply of DN1. As for DN2 protected by the thicker Ni-CFNV, the carbon fiber damage is eliminated because the Ni-CFNV LSP layer is nearly intact. It is noteworthy that the destroyed LSP layers of both DN1 and DN2 are less than the vaporized expanded copper foil of DC1 (Fig. 3b), although the copper mesh seems more conductive than Ni-CFNVs [5]. If so, it implies that the Ni-CFNVs have better ablation resistance than the expanded copper foil [3]. In order to detect the internal damage of the post-lightning specimens, an ultrasonic scanner system (Tomoscan Focus LT, Olympus, 5 MHz transducer) was used. The B-scan and C-scan depict cross-section (the central section was extracted, i.e., “A-A” in Fig. 4) and in-plane damage projection, respectively. The specimens were detected by the ultrasonic scanner with the damage side (the LSP layer side) down, so that the bottom signal of the B-scan results in Fig. 4 denotes the damage
2.1. Materials and specimens The Ni-CFNVs (20404E series, Technical Fibre Products Ltd, UK) that are originally designed for electro-magnetic interference shielding, are proposed as LSP layers of CFRP. The Ni-CFNVs are a kind of nonwoven made of chopped Ni-plated carbon fibers (Fig. 1a), and the two types of them with different area weights labeled as N1 (the thinner one, 34 g/m2) and N2 (the thicker one, 70 g/m2) are studied. The CFRP preforms were stacked by TR50S15L/YPH-308 unidirectional prepreg (C15000, Dezhou Furun Co., China) in sequence of [45°/ 0°/-45°/90°]2S, with a sheet of Ni-CFNV glued on their top surface by the surface resin of prepreg. The Ni-CFNV was impregnated with the excess resin exuded from the prepreg when the preform was heated and pressurized. After the Ni-CFNV was cured with the prepreg in hot-press process (heating at 80 °C for 0.5 h and then 130 °C for 1.5 h under 0.75 MPa), the laminate was cut into 300 mm × 300 mm specimens (Fig. 1b). For comparison, the unprotected CFRP specimens (labeled as 02S) and the reference specimens (Fig. 1c) protected by a commercial expanded copper foil (ECF; 2CU4-100FA, 73 g/m2, Dexmet Co., USA, labeled as C1) were fabricated as well. The carbon fiber volume fraction of the CFRP laminate is about 59% (ASTM D3171). The thickness of the specimen without LSP layer (02S) is 2.25 mm. The thickness of the specimen with the LSP layer of C1, N1 and N2 is about 2.30, 2.33 and 2.40 mm, respectively. 96
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Fig. 1. Materials and specimens. (a) Ni-CFNV of 34 g/m2; (b) CFRP with Ni-CFNV; (c) CFRP with expanded copper foil.
Fig. 2. Experimental conditions of lightning strike test. (a) Specimen and electrode setup; (b) Typical Waveform C; (c) Typical Waveform D.
side (the LSP layer side) of the specimens. The damage of D02S is about 80 mm in diameter and 1.8 mm in depth (∼2/3 of the total thickness) according to the ultrasonic scan. By contrast, the B-scans (Fig. 4) reveal that the damage depth of DC1, DN1 and DN2 is about 0.2 mm, demonstrating that the expanded copper foil and Ni-CFNVs have successfully prevented the strike damage from penetrating downward. Notably, the damage detected by C-scan, which is derived from the LSP layer and the outmost ply of CFRP laminates, of both DN1 and DN2 is less than that of DC1. Considering that DN2 only appends 70 g/m2 compared with the original CFRP, which is less than DC1 (73 g/m2), the Ni-CFNV might have better LSP effectiveness than the expanded copper foil with the same surface density (weight), at least for 100 kA strike of Waveform D.
Table 1 Testing conditions of the specimens subjected to Waveform C. Specimen
LSP method
Weight (g/m2)
Avg. current (A)
Duration (ms)
Charge (Coulomb)
C02S CC1 CN1
N/A ECF(C1) Ni-CFNV (N1) Ni-CFNV (N2)
0 73 34
−363.94 −372.57 −383.70
504.00 503.60 503.40
183.43 187.63 193.15
70
−344.47
501.00
172.58
CN2
Table 2 Testing conditions of the specimens subjected to Waveform D. Specimen
D02S DC1 DN1 DN2
LSP method N/A ECF (C1) Ni-CFNV (N1) Ni-CFNV (N2)
Weight (g/m2)
Peak (kA)
Waveform t1/ t2 (μs/μs)
0 73 34
−95.373 −98.916 −97.940
14.00/41.80 14.00/42.80 13.75/42.18
2.54 × 10 2.84 × 105 2.73 × 105
70
−98.143
13.75/42.38
2.76 × 105
Action integral (A2s)
3.2. Lightning strike test using Waveform C
5
Waveform C, as the only continuing current waveform specified by SAE ARP 5412, represents the lightning environment that might be caused by the long-duration lightning [34]. Although it is as important as the other standardized waveforms [33–35], the related lightning strike reports are seldom available [23]. Because Waveform C just requires the current amplitude ranging from 200 A to 800 A, its channel voltage is too low to puncture the air gap between electrode and specimen. In the study, an impulse current (∼15 kA) with high voltage but short duration (< 10 μs) was used 97
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Fig. 3. Overhead views of the specimens subjected to Waveform D. (a1-d1) Full scale; (a2-d2) Detail of damage zones.
spark or re-strikes. The thinner Ni-CFNV (N1) has suppressed most of the fiber damage dots that appear on C02S, only leaving a small centered ablated region (Fig. 6c). The B-scan in Fig. 7c indicates that the damage depth of CN1 is about 0.3 mm, which is much less than that of C02S (∼0.8 mm, Fig. 7a). It indicates that the thinner Ni-CFNV presents quite good LSP effectiveness, but not enough. Despite slight surface ablation, the LSP layer, as well as the underlying CFRP, of post-lightning CN2 maintains structural integrity (Fig. 6d) by visual inspection. The ultrasonic scan result (Fig. 7d) makes sure that the thicker Ni-CFNV (N2) has eliminated the threat induced by Waveform C, as no internal damage is detected. The surface ablation of LSP layer appears as superficial resin pyrolysis and partial Ni-coating evaporation, but the carbon fiber framework of the Ni-CFNV stays intact and the Ni-CFNV LSP layer is not pierced, which can be observed clearly via SEM (Fig. 10e–10g). The result demonstrates that the thicker Ni-CFNV not only has high conductivity, but also presents remarkable ablation resistance to withstand lightning plasma, arc, spark and Joule heat of Waveform C. The ablation resistance might be a prominent advantage for LSP, especially for long duration strikes. Moreover, the post-lightning Ni-CFNV of CN2 can still serve as an LSP layer to struggle against the subsequent lightning, though it might not perform as well as before.
before Waveform C to trigger the plasma channel. Waveform C has the largest charge transfer and longest duration among the four standardized waveforms [34], and it is mainly regarded as an indicator of lightning ablation effect [35]. The distinct ablation effect of Waveform C can be seen by comparing the different reactions of CFRP under Waveform C (Fig. 5a) and Waveform D (Fig. 5b). Unlike D02S whose damage centered on a lightning attachment point, C02S suffered lots of irregularly distributed damage dots, where resin was ablated; fibers were broken, cured, tufted and lift-up (Fig. 6a). The damage area of C02S is much larger than that of D02S, which mainly results from the drastic ablative effects of Waveform C. Because of the low voltage and long duration of Waveform C, and the dynamic change of system resistance caused by the ablation of specimens, the plasma channel of Waveform C is unstable, which can be deduced from the fluctuant current amplitude in Fig. 2b. When the fluctuant plasma encounters the semi-conductive CFRP, the arc spurts and sparks, irregularly. Hence, the arc attachments (i.e., damage dots) are randomly scattered. Though the expanded copper foil almost suppresses lightning strike damage to the underlying CFRP, the copper grids are nearly destroyed by Waveform C within a diameter of 150 mm (Fig. 6b). The damaged copper grids are also scattered but nearly connected together. The fibers of outmost ply of CC1 are slightly damaged, which appears as parallel stripes of 45° (e.g., A and B in Fig. 6b2). As the copper grids are almost burned up, the exposed CFRP is vulnerable to the subsequent electric
Fig. 4. Ultrasonic scan results of the specimens subjected to Waveform D. 98
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Fig. 5. CFRP reactions under artificial lightning. (a) Waveform C; (b) Waveform D.
CFNV has metallic crystalline structure. Since the electrical conductivity of nickel (1.44 × 107 S/m) is two or three orders of magnitude higher than that of carbon fiber (104–105 S/m), the conductivity of the Ni-CFNV is dominated by the Ni-coating. In order to study the LSP mechanism, the morphologies of the NiCFNV (Fig. 9) and the Ni-CFNV LSP layer (Fig. 10) were characterized by SEM (Hitachi SU-70). The element linear analysis in Fig. 9b via SEM (EDS) shows that the nickel is coated on the cylindrical surface of carbon fibers, forming a dense metallic shell. The tubular nickel shell encircles the carbon fiber evenly and continuously like a sheath (Figs. 9c and 10a–10d), indicating that the Ni-coated carbon fibers present as a skin-core structure. As the diameters of naked and Nicoated carbon fibers are approximately 7 μm and 7.6 μm, respectively, according to Figs. 9 and 10, the average thickness of the Ni-coating is about 0.3 μm. The content of nickel is about 42 wt% in the Ni-CFNV by comparing the weight loss after the acid treatment (20 wt% H2SO4 for 2 days and 10 wt% HCl for 2 days) with the initial weight of the Ni-CFNV. By nonwoven technology, the Ni-coated carbon fibers form a threedimensional network (i.e. the Ni-CFNV) as they contact, overlap, intersect and intertwine (Figs. 1a and 9a). When the resin infiltrates the Ni-CFNVs and cures, the LSP layers (Fig. 10a and 10c) are formed. The conductivities of the thinner (N1, 80 μm in thickness) and thicker (N2, 150 μm in thickness) Ni-CFNV LSP layers (polished by 0.25 μm diamond) are 9.6 × 103 S/m and 4.1 × 104 S/m, respectively, by fourpoint probe method. The Ni-CFNV layers present the conductivity as high as buckypapers [26,27], but their manufacturing process is simpler and more economical than the buckypapers.
3.3. Residual strength of post-lightning specimens The residual strength of post-lightning specimens was evaluated by three-point flexure test (Fig. 8) according to ASTM D 7264-15. The composite strips of 100 mm × 40 mm for flexural property test were cut out from the most serious damage zone of the post-lightning specimens by milling cutter. The lightning attachment point(s) was usually located in the center of strips (Fig. 8b, bottom). The strips were placed on two supports whose span is 60 mm, with the damage side (LSP layer side) down to suffer tensile failure (Fig. 8b). The non-struck pristine strips (02S, C1, N1 and N2) were tested as the references to calculate the strength and modulus retention (see Table 3). The Ni-CFNVs highly improved the residual strength and modulus compared with the unprotected specimens of C02S and D02S as listed in Table 3. Especially, the specimens protected by the Ni-CFNV of 70 g/m2 present higher strength retention (CN2 as 97.01% and DN2 as 98.33%) than those protected by the expanded copper foil of 73 g/m2 (CC1 as 89.96% and DC1 as 91.79%) under both waveforms.
3.4. Possible mechanism of Ni-CFNV for LSP 3.4.1. High electrical conductivity The remarkable LSP effectiveness of Ni-CFNV, which is verified in Sections 3.1-3.3, indicates the high electrical conductivity of the NiCFNV layers. The Ni-CFNV is made of chopped Ni-coated carbon fibers that consist of carbon fiber and Ni-coating. The X-ray diffraction pattern (by Rigaku Dmax-2500) in Fig. 9d shows that the Ni-coating in Ni-
Fig. 6. Overhead views of the specimens subjected to Waveform C. (a1-d1) Full scale; (a2-d2) Detail of damage zones. 99
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Fig. 7. Ultrasonic scan results of the specimens subjected to Waveform C.
Fig. 8. Three-point flexure test. (a) Setup; (b) Typical flexural failure of DN2.
carbon fiber than by coating insulated fibers, such as polyamide fiber [23]. Actually, the carbon fiber framework itself has certain LSP effect as well, because it can transfer lightning arc and current to discharge, especially after Ni-coating is vaporized (Fig. 10e–10h). Therefore, the Ni-CFNV produces double electrical conductive networks for LSP: the Ni-coating network (dominant) and the carbon fiber framework (assistant).
Table 3 Residual strength and modulus of post-lightning specimens. Specimen
Flexural strength (MPa)
Strength retention
Flexural modulus (GPa)
Modulus retention
02S C1 N1 N2 C02S CC1 CN1 CN2 D02S DC1 DN1 DN2
846.91 814.90 807.83 804.55 665.61 733.07 687.99 780.53 405.23 747.97 727.80 791.09
100.00% 100.00% 100.00% 100.00% 78.59% 89.96% 85.16% 97.01% 47.85% 91.79% 90.09% 98.33%
45.04 42.45 42.26 42.15 31.10 36.87 35.10 41.00 19.14 38.66 37.57 41.44
100.00% 100.00% 100.00% 100.00% 69.05% 86.85% 83.07% 97.26% 42.49% 91.19% 88.92% 98.31%
3.4.2. Good ablation resistance As well known, the carbon fiber has good ablation resistance [15] and can withstand the temperature as high as 3000 °C [16], at which metals have vaporized. In addition, as the melt and vaporization of the nickel will absorb a great deal of energy, the residual heat or temperature is too low to cause sublimation of the carbon fibers. As a result, even if the nickel has melted and vaporized, the carbon fiber framework in the Ni-CFNV still keeps intact after a strike (Fig. 10e–10h). Therefore, the Ni-coated layer shows much better ablation resistance than the copper mesh. With regard to LSP, the ablation resistance can be as important as the electrical conductivity [3], otherwise the LSP layer will be burned up soon. Kamiyama et al. [3] proved that a higher onset temperature of thermal decomposition of CFRP could reduce the lightning strike damage. Similarly, if the LSP layer has a better ablation resistance to withstand the high temperature of the lightning strike, it can keep its structure intact for larger lightning current and longer strike time, consequently increasing its service life. As a result, the LSP layer can discharge more lightning current with less damage. Owing to its good ablation resistance, the carbon fiber framework can form an isolation layer. On the one hand, the isolation layer can prevent lightning plasma, arcs and sparks from penetrating downward to the underlying CFRP, and meanwhile transfer them to nearby Ni-coating network for discharge due to its fairly good conductivity. On the other hand, as the Ni-coating fibers are repeatedly stacked in the thickness direction
To further verify the LSP contribution of Ni-coating, the CFRP specimens with the surface layers of chopped carbon fiber (without Nicoating) nonwoven fabrics (30 g/m2 and 50 g/m2) were fabricated. The conductivities of the two chopped carbon fiber layers are 3.3 × 101 S/m and 2.4 × 102 S/m, which are 2–3 orders of magnitude lower than that of the Ni-CFNV layers. Thus, Ni-coating, rather than carbon fiber, plays a dominant role in lightning current discharge of the Ni-CFNV layers. The Ni-CFNV with the skin-core structure can be divided into two parts: Ni-coating network and carbon fiber framework. Undoubtedly, Ni-coating network plays a dominant role in lightning current discharge due to its high conductivity. Nevertheless, the carbon fiber framework in Ni-CFNV also possesses good electrical conductivity owing to its graphitic structure, so that the lightning current can flow through both the Ni-coating and carbon fibers. Hence, the carbon fiber framework can assist the Ni-coating network in lightning current discharge. For this reason, the higher conductivity can be obtained easily by coating
100
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Fig. 9. Characteristic of Ni-CFNV. (a–c) SEM; (d) X-ray diffraction.
Fig. 10. Characteristic of Ni-CFNV LSP layer. (a–b) Polished top surface, without strike; (c–d) Polished cross-section, without strike; (e–h) Top surfaces, after strike.
was verified by the artificial lightning using standardized Waveforms C (200 Coulomb) and D (100 kA). The experimental results manifested that the Ni-CFNV of 70 g/m2 performed better than the commercial expanded copper foil of 73 g/m2 in withstanding both Waveforms C and D. Hence, the Ni-CFNV can be a promising LSP solution to replace metallic mesh for eliminating lightning strike damage to the CFRP structures in Zone 2. As made of skin-core structured Ni-coated carbon fibers, the NiCFNV consists of two networks for LSP: Ni-coating network and carbon fiber framework. The possible LSP mechanism of Ni-CFNV can be described by the different roles that the two networks play. The Ni-coating network makes the Ni-CFNV more conductive in order to fast discharge lightning current and suppress resistive heat. The carbon fiber framework enhances the ablation resistance of Ni-CFNV to prolong the service life of the LSP layer. Furthermore, the carbon fiber framework can form an isolation layer to prevent lightning plasma, arcs and sparks from penetrating downward to the underlying CFRP laminates. The
(Fig. 10c), the isolation layer might suppress the vaporization of underneath Ni-coating by the mechanism of blocking the releasing channel of nickel vapor (Fig. 10h), prolonging the service life of the NiCFNV. Consequently, the Ni-coated carbon fibers in the bottom of the LSP layer can be hardly damaged unless the upper ones are destroyed. The isolation effect is a distinct advantage of the Ni-CFNV compared with meshes or foils, whose metallic material is continuously distributed in the thickness direction and can be burned up easily once heated by lightning. In summary, the Ni-CFNV integrates the high conductivity of Nicoating network and the ablation resistance of carbon fiber framework, which results in its remarkable LSP effectiveness. 4. Conclusions The Ni-CFNV was proposed as an LSP surface layer of the CFRP structures in Zone 2 of aircraft, and its remarkable LSP effectiveness 101
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study also demonstrates that besides the electrical conductivity, the ablation resistance plays a vital role in the protection effectiveness of the LSP layer.
[18]
Acknowledgments
[19]
This work was supported by the Fundamental Research Funds of Shandong University [2016JC012]; National Key Basic Research Program of China [JCKY2016205B007]; and Special Research Foundation of China Civil Aircraft [MJ-2015-H-G-103].
[20] [21]
[22]
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Eliminating lightning strike damage to carbon fiber composite structures in Zone 2 of aircraft by Ni-coated carbon fiber nonwoven veils
T
Yunli Guoa, Yongzheng Xua, Qinglin Wanga, Qi Donga, Xiaosu Yib, Yuxi Jiaa,∗ a b
Key Laboratory for Liquid-solid Structural Evolution & Processing of Materials (Ministry of Education), Shandong University, Jinan, 250061, China National Key Laboratory of Advanced Composites, AVIC Composites Center, Beijing, 100095, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Lightning strike protection A. Carbon fibers B. Electrical properties
By using standardized Waveforms C and D, this study explored the lightning strike protection (LSP) effectiveness of nickel coated carbon fiber nonwoven veils (Ni-CFNVs) on protecting the carbon fiber composite structures in Zone 2 (aircraft lightning zoning). The post-lightning damage was evaluated by visual inspection, ultrasonic scan and residual strength test. Results showed that the Ni-CFNV of 70 g/m2 eliminated lightning strike damage inflicted by both waveforms and it even performed better than the commercial expanded copper foil of 73 g/m2. Therefore, the Ni-CFNV will be a promising alternative to metal mesh for eliminating lightning strike damage to the structures in Zone 2. The remarkable LSP effectiveness of Ni-CFNV can be explained by its integration of high electrical conductivity of Ni-coating and prominent ablation resistance of carbon fiber. The experiments indicate that not only the electrical conductivity, but also the ablation resistance of LSP layer greatly influences the LSP effectiveness.
1. Introduction Lightning strike threatens all structures, especially the aircraft in flight that is unable to be grounded. Lightning plasma possesses the temperature as high as 30,000 K and the pressure up to ten atmospheres [1], which will induce severe damage to aircraft. As it has been reported that a commercial aircraft might be struck once every 3000 flight hours [2], it is vital to eliminate the threat of lightning strike to aircraft. Carbon fiber reinforced polymers (CFRPs) are increasingly adopted in aircraft because of their excellent mechanical and chemical performance, as well as light weight for fuel saving [3]. However, CFRPs present semi-conductive property, which limits their applications due to lightning strikes. On the one hand, the semi-conductive CFRPs can be struck as frequently as lightning receptors [4]. On the other hand, the high electrical resistance of CFRPs will impede lightning current from readily discharging, resulting in electric spark, Joule heat or dielectric breakdown [4–6]. Hence, effective lightning strike protection (LSP) solutions are highly desired for CFRP-based aircraft. On the basis of the experimental [3,7–13] and numerical [14–19] studies of lightning damage behaviors and mechanisms of CFRPs, a few LSP solutions [4–6,20–32] have been proposed, which can be divided into three categories according to the types of materials they rely on: metallic method, non-metallic method and the integrated method [6] ∗
that integrates metallic and non-metallic materials. The metallic methods, such as meshes and foils, are frequently selected for LSP owing to their high conductivity and low cost. Nevertheless, the metals usually cause overweight and galvanic corrosion [5]. The recently developed non-metallic methods, for instance, buckypaper (by Gou et al. [26] and Han et al. [27]), graphene (by Zhang et al. [28] and Wang et al. [29]) and conductive resin (by Hirano et al. [30] and Katunin et al. [31]), are lightweight and incorruptible. However, they hardly yield large and uniform surface layers for aircraft structures due to manufacturing technique and cost. The integrated LSP method usually introduces metallic fibers (wires), metallic nanoparticles or metallic coating into the non-metallic support materials (such as fibers, fabrics and prepregs), in order to achieve the LSP layers with high conductivity but low weight and low cost. The metallic fibers and wires woven into the carbon fabrics are susceptible to galvanic corrosion as metallic mesh. Chakravarthi et al. [22] and Dong et al. [25] proposed nickel-coated CNTs for the LSP of CFRP. However, considering the application to large structures of aircraft, the metallized nanoparticles [22,25] need to be further developed due to their dispersibility and cost. Rehbein et al. [23] used the silvercoated polyamide knitting yarns to increase the conductivity of NCFreinforced CFRP, but the solution might be insufficient to withstand heavy lightning because of the limited content of the conductive knitting yarns. By contrast, the metallizing carbon fibers and fabrics by
Corresponding author. No. 17923, Jingshi Road, Jinan, Shandong Province, China. E-mail address: [email protected] (Y. Jia).
https://doi.org/10.1016/j.compscitech.2018.11.011 Received 9 July 2018; Received in revised form 17 October 2018; Accepted 5 November 2018 Available online 09 November 2018 0266-3538/ © 2018 Elsevier Ltd. All rights reserved.
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2.2. Experimental setup
surface coating may be a workable plan. Firstly, not only the metal coating, but also the carbon fiber is quite conductive. Secondly, the metal coating will be isolated from air and water when encapsulated by the cured resin, eliminating galvanic corrosion [6]. Thirdly, the metalcoated carbon fibers can generate large and uniform LSP layers for aircraft structures easily by woven or nonwoven techniques. Furthermore, since the metal-coated carbon fiber fabric has carbon fiber framework, it has similar modulus and coefficient of thermal expansion with the CFRP base, which contributes to preventing thermal stress, aging, fatigue and cracking of LSP layer and surface paint when the aircraft undergoes the thermal cycling between the high and low altitude flights. However, by now, there has been no available literature that presents a detailed study of the LSP effectiveness of metal-coated carbon fibers. In order to optimize lightning protection, aircraft is divided into three lightning strike zones (Zones 1, 2 and 3) by SAE ARP 5414 [33], on the basis of the possibility of lightning environment that each zone encounters. For evaluating the direct effect of lightning, four standardized current waveforms (Waveforms A, B, C and D) are recommended to simulate the lightning environment by SAE ARP 5412 [34]. According to the standard, the aircraft structures in each lightning strike zone should sustain various standardized waveforms before service. Nevertheless, the standardized waveforms are very difficult to obtain because of the lack of the professional equipment which is usually exclusive to aerospace and military departments [13]. As Zone 2 covers the vast majority of airframe [8,33], the nickelcoated carbon fiber nonwoven veil (Ni-CFNV) is proposed as an LSP layer for the CFRP structures of the zone. According to SAE ARP 5412, the structures in Zone 2 should sustain the lightning current components of Waveforms B, C and D. As the electrical charge transfer of Waveform B (10 Coulomb) is only 5% of the electrical charge transfer of Waveform C (200 Coulomb), if the Ni-CFNV can withstand Waveform C without severe damage, Waveform B will be no longer a threat to the Ni-CFNV. Therefore, Waveforms C and D are enough to verify the LSP effectiveness of the Ni-CFNV, and the lightning strike of Waveform B was not performed in the report. The work aims to investigate the LSP effectiveness and mechanism of Ni-CFNV for protecting the CFRP structures in Zone 2, as well as to clarify a promising LSP solution that integrates metal with nonmetal materials.
The specimens were wrapped by copper foil tape (60 mm in width) in four edges for grounding. Then they were fixed on an epoxy/fiberglass panel by two grounded steel beams that tightly pressed the copper foil tape on two edges of the specimen by clamps (Fig. 2a). A copper rod electrode with a spherical ceramic tip (20 mm in diameter) was fixed over the specimen to inspire lightning arc (Fig. 2a). The gap between the electrode and specimen was about 10 mm, where a copper wire (0.05 mm in diameter) was used to induce the initial arc attaching to the center of the specimen top surface according to SAE ARP 5416 [35]. 2.3. Waveforms of artificial lightning In the study, Waveforms C and D were used to verify LSP effect of Ni-CFNVs on protecting the structures in Zone 2, to which the vast majority of airframe belongs [8,33]. The Waveform C has the charge transfer of 200 Coulomb ( ± 20%) with a current amplitude of (200–800) A, and Waveform D is featured as a peak of 100 kA ( ± 10%) with an action integral of 2.5 × 105 A2s ( ± 20%) [34]. The lightning was artificially simulated by an impulse current generator system in Xi'an Airborne Electromagnetic Technology Co. Ltd, China. The typical Waveforms C and D are presented in Fig. 2b and 2c, respectively. The detailed testing conditions of each specimen are listed in Table 1 and Table 2. Therein, the first capital of specimen labels (C or D) denotes the waveforms to which the specimens subjected and the other capitals and numbers indicate the LSP methods (02S denotes the unprotected CFRP). Besides, t1/t2 and action integral are the parameters of Waveform D as depicted in previous study [7]. 3. Results and discussion As the impulse current waveforms (e.g., Waveform D) that is characterized by high current peak and short duration are more widely applied and reported [3,7–13,23–31] than the continuing ones (e.g., Waveform C), the section begins with Waveform D. 3.1. Lightning strike test using Waveform D
2. Experimental
Waveform D represents a subsequent stroke [34] for assessing the direct effects of lightning of Zone 2. As Waveform D has a current peak of 100 kA, it possesses high voltage to puncture the air and the specimen, causing CFRP delamination, breakdown or piercing, as well as shock wave and thermal impact [36–38]. The visual inspection shows that the unprotected CFRP specimen (D02S, Fig. 3a) suffers the most severe damage (∼75 mm in diameter), which manifests as fiber breakage, ply-lift, resin pyrolysis and surface ablation. However, for DN1 and DN2, the damage is obviously suppressed (Fig. 3c and 3d). More specifically, DN1 only suffers a little carbon fiber damage (∼30 mm in diameter) with a ply angle of 45°, indicating that the 100 kA strike only affects the outmost CFRP ply of DN1. As for DN2 protected by the thicker Ni-CFNV, the carbon fiber damage is eliminated because the Ni-CFNV LSP layer is nearly intact. It is noteworthy that the destroyed LSP layers of both DN1 and DN2 are less than the vaporized expanded copper foil of DC1 (Fig. 3b), although the copper mesh seems more conductive than Ni-CFNVs [5]. If so, it implies that the Ni-CFNVs have better ablation resistance than the expanded copper foil [3]. In order to detect the internal damage of the post-lightning specimens, an ultrasonic scanner system (Tomoscan Focus LT, Olympus, 5 MHz transducer) was used. The B-scan and C-scan depict cross-section (the central section was extracted, i.e., “A-A” in Fig. 4) and in-plane damage projection, respectively. The specimens were detected by the ultrasonic scanner with the damage side (the LSP layer side) down, so that the bottom signal of the B-scan results in Fig. 4 denotes the damage
2.1. Materials and specimens The Ni-CFNVs (20404E series, Technical Fibre Products Ltd, UK) that are originally designed for electro-magnetic interference shielding, are proposed as LSP layers of CFRP. The Ni-CFNVs are a kind of nonwoven made of chopped Ni-plated carbon fibers (Fig. 1a), and the two types of them with different area weights labeled as N1 (the thinner one, 34 g/m2) and N2 (the thicker one, 70 g/m2) are studied. The CFRP preforms were stacked by TR50S15L/YPH-308 unidirectional prepreg (C15000, Dezhou Furun Co., China) in sequence of [45°/ 0°/-45°/90°]2S, with a sheet of Ni-CFNV glued on their top surface by the surface resin of prepreg. The Ni-CFNV was impregnated with the excess resin exuded from the prepreg when the preform was heated and pressurized. After the Ni-CFNV was cured with the prepreg in hot-press process (heating at 80 °C for 0.5 h and then 130 °C for 1.5 h under 0.75 MPa), the laminate was cut into 300 mm × 300 mm specimens (Fig. 1b). For comparison, the unprotected CFRP specimens (labeled as 02S) and the reference specimens (Fig. 1c) protected by a commercial expanded copper foil (ECF; 2CU4-100FA, 73 g/m2, Dexmet Co., USA, labeled as C1) were fabricated as well. The carbon fiber volume fraction of the CFRP laminate is about 59% (ASTM D3171). The thickness of the specimen without LSP layer (02S) is 2.25 mm. The thickness of the specimen with the LSP layer of C1, N1 and N2 is about 2.30, 2.33 and 2.40 mm, respectively. 96
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Fig. 1. Materials and specimens. (a) Ni-CFNV of 34 g/m2; (b) CFRP with Ni-CFNV; (c) CFRP with expanded copper foil.
Fig. 2. Experimental conditions of lightning strike test. (a) Specimen and electrode setup; (b) Typical Waveform C; (c) Typical Waveform D.
side (the LSP layer side) of the specimens. The damage of D02S is about 80 mm in diameter and 1.8 mm in depth (∼2/3 of the total thickness) according to the ultrasonic scan. By contrast, the B-scans (Fig. 4) reveal that the damage depth of DC1, DN1 and DN2 is about 0.2 mm, demonstrating that the expanded copper foil and Ni-CFNVs have successfully prevented the strike damage from penetrating downward. Notably, the damage detected by C-scan, which is derived from the LSP layer and the outmost ply of CFRP laminates, of both DN1 and DN2 is less than that of DC1. Considering that DN2 only appends 70 g/m2 compared with the original CFRP, which is less than DC1 (73 g/m2), the Ni-CFNV might have better LSP effectiveness than the expanded copper foil with the same surface density (weight), at least for 100 kA strike of Waveform D.
Table 1 Testing conditions of the specimens subjected to Waveform C. Specimen
LSP method
Weight (g/m2)
Avg. current (A)
Duration (ms)
Charge (Coulomb)
C02S CC1 CN1
N/A ECF(C1) Ni-CFNV (N1) Ni-CFNV (N2)
0 73 34
−363.94 −372.57 −383.70
504.00 503.60 503.40
183.43 187.63 193.15
70
−344.47
501.00
172.58
CN2
Table 2 Testing conditions of the specimens subjected to Waveform D. Specimen
D02S DC1 DN1 DN2
LSP method N/A ECF (C1) Ni-CFNV (N1) Ni-CFNV (N2)
Weight (g/m2)
Peak (kA)
Waveform t1/ t2 (μs/μs)
0 73 34
−95.373 −98.916 −97.940
14.00/41.80 14.00/42.80 13.75/42.18
2.54 × 10 2.84 × 105 2.73 × 105
70
−98.143
13.75/42.38
2.76 × 105
Action integral (A2s)
3.2. Lightning strike test using Waveform C
5
Waveform C, as the only continuing current waveform specified by SAE ARP 5412, represents the lightning environment that might be caused by the long-duration lightning [34]. Although it is as important as the other standardized waveforms [33–35], the related lightning strike reports are seldom available [23]. Because Waveform C just requires the current amplitude ranging from 200 A to 800 A, its channel voltage is too low to puncture the air gap between electrode and specimen. In the study, an impulse current (∼15 kA) with high voltage but short duration (< 10 μs) was used 97
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Fig. 3. Overhead views of the specimens subjected to Waveform D. (a1-d1) Full scale; (a2-d2) Detail of damage zones.
spark or re-strikes. The thinner Ni-CFNV (N1) has suppressed most of the fiber damage dots that appear on C02S, only leaving a small centered ablated region (Fig. 6c). The B-scan in Fig. 7c indicates that the damage depth of CN1 is about 0.3 mm, which is much less than that of C02S (∼0.8 mm, Fig. 7a). It indicates that the thinner Ni-CFNV presents quite good LSP effectiveness, but not enough. Despite slight surface ablation, the LSP layer, as well as the underlying CFRP, of post-lightning CN2 maintains structural integrity (Fig. 6d) by visual inspection. The ultrasonic scan result (Fig. 7d) makes sure that the thicker Ni-CFNV (N2) has eliminated the threat induced by Waveform C, as no internal damage is detected. The surface ablation of LSP layer appears as superficial resin pyrolysis and partial Ni-coating evaporation, but the carbon fiber framework of the Ni-CFNV stays intact and the Ni-CFNV LSP layer is not pierced, which can be observed clearly via SEM (Fig. 10e–10g). The result demonstrates that the thicker Ni-CFNV not only has high conductivity, but also presents remarkable ablation resistance to withstand lightning plasma, arc, spark and Joule heat of Waveform C. The ablation resistance might be a prominent advantage for LSP, especially for long duration strikes. Moreover, the post-lightning Ni-CFNV of CN2 can still serve as an LSP layer to struggle against the subsequent lightning, though it might not perform as well as before.
before Waveform C to trigger the plasma channel. Waveform C has the largest charge transfer and longest duration among the four standardized waveforms [34], and it is mainly regarded as an indicator of lightning ablation effect [35]. The distinct ablation effect of Waveform C can be seen by comparing the different reactions of CFRP under Waveform C (Fig. 5a) and Waveform D (Fig. 5b). Unlike D02S whose damage centered on a lightning attachment point, C02S suffered lots of irregularly distributed damage dots, where resin was ablated; fibers were broken, cured, tufted and lift-up (Fig. 6a). The damage area of C02S is much larger than that of D02S, which mainly results from the drastic ablative effects of Waveform C. Because of the low voltage and long duration of Waveform C, and the dynamic change of system resistance caused by the ablation of specimens, the plasma channel of Waveform C is unstable, which can be deduced from the fluctuant current amplitude in Fig. 2b. When the fluctuant plasma encounters the semi-conductive CFRP, the arc spurts and sparks, irregularly. Hence, the arc attachments (i.e., damage dots) are randomly scattered. Though the expanded copper foil almost suppresses lightning strike damage to the underlying CFRP, the copper grids are nearly destroyed by Waveform C within a diameter of 150 mm (Fig. 6b). The damaged copper grids are also scattered but nearly connected together. The fibers of outmost ply of CC1 are slightly damaged, which appears as parallel stripes of 45° (e.g., A and B in Fig. 6b2). As the copper grids are almost burned up, the exposed CFRP is vulnerable to the subsequent electric
Fig. 4. Ultrasonic scan results of the specimens subjected to Waveform D. 98
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Fig. 5. CFRP reactions under artificial lightning. (a) Waveform C; (b) Waveform D.
CFNV has metallic crystalline structure. Since the electrical conductivity of nickel (1.44 × 107 S/m) is two or three orders of magnitude higher than that of carbon fiber (104–105 S/m), the conductivity of the Ni-CFNV is dominated by the Ni-coating. In order to study the LSP mechanism, the morphologies of the NiCFNV (Fig. 9) and the Ni-CFNV LSP layer (Fig. 10) were characterized by SEM (Hitachi SU-70). The element linear analysis in Fig. 9b via SEM (EDS) shows that the nickel is coated on the cylindrical surface of carbon fibers, forming a dense metallic shell. The tubular nickel shell encircles the carbon fiber evenly and continuously like a sheath (Figs. 9c and 10a–10d), indicating that the Ni-coated carbon fibers present as a skin-core structure. As the diameters of naked and Nicoated carbon fibers are approximately 7 μm and 7.6 μm, respectively, according to Figs. 9 and 10, the average thickness of the Ni-coating is about 0.3 μm. The content of nickel is about 42 wt% in the Ni-CFNV by comparing the weight loss after the acid treatment (20 wt% H2SO4 for 2 days and 10 wt% HCl for 2 days) with the initial weight of the Ni-CFNV. By nonwoven technology, the Ni-coated carbon fibers form a threedimensional network (i.e. the Ni-CFNV) as they contact, overlap, intersect and intertwine (Figs. 1a and 9a). When the resin infiltrates the Ni-CFNVs and cures, the LSP layers (Fig. 10a and 10c) are formed. The conductivities of the thinner (N1, 80 μm in thickness) and thicker (N2, 150 μm in thickness) Ni-CFNV LSP layers (polished by 0.25 μm diamond) are 9.6 × 103 S/m and 4.1 × 104 S/m, respectively, by fourpoint probe method. The Ni-CFNV layers present the conductivity as high as buckypapers [26,27], but their manufacturing process is simpler and more economical than the buckypapers.
3.3. Residual strength of post-lightning specimens The residual strength of post-lightning specimens was evaluated by three-point flexure test (Fig. 8) according to ASTM D 7264-15. The composite strips of 100 mm × 40 mm for flexural property test were cut out from the most serious damage zone of the post-lightning specimens by milling cutter. The lightning attachment point(s) was usually located in the center of strips (Fig. 8b, bottom). The strips were placed on two supports whose span is 60 mm, with the damage side (LSP layer side) down to suffer tensile failure (Fig. 8b). The non-struck pristine strips (02S, C1, N1 and N2) were tested as the references to calculate the strength and modulus retention (see Table 3). The Ni-CFNVs highly improved the residual strength and modulus compared with the unprotected specimens of C02S and D02S as listed in Table 3. Especially, the specimens protected by the Ni-CFNV of 70 g/m2 present higher strength retention (CN2 as 97.01% and DN2 as 98.33%) than those protected by the expanded copper foil of 73 g/m2 (CC1 as 89.96% and DC1 as 91.79%) under both waveforms.
3.4. Possible mechanism of Ni-CFNV for LSP 3.4.1. High electrical conductivity The remarkable LSP effectiveness of Ni-CFNV, which is verified in Sections 3.1-3.3, indicates the high electrical conductivity of the NiCFNV layers. The Ni-CFNV is made of chopped Ni-coated carbon fibers that consist of carbon fiber and Ni-coating. The X-ray diffraction pattern (by Rigaku Dmax-2500) in Fig. 9d shows that the Ni-coating in Ni-
Fig. 6. Overhead views of the specimens subjected to Waveform C. (a1-d1) Full scale; (a2-d2) Detail of damage zones. 99
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Fig. 7. Ultrasonic scan results of the specimens subjected to Waveform C.
Fig. 8. Three-point flexure test. (a) Setup; (b) Typical flexural failure of DN2.
carbon fiber than by coating insulated fibers, such as polyamide fiber [23]. Actually, the carbon fiber framework itself has certain LSP effect as well, because it can transfer lightning arc and current to discharge, especially after Ni-coating is vaporized (Fig. 10e–10h). Therefore, the Ni-CFNV produces double electrical conductive networks for LSP: the Ni-coating network (dominant) and the carbon fiber framework (assistant).
Table 3 Residual strength and modulus of post-lightning specimens. Specimen
Flexural strength (MPa)
Strength retention
Flexural modulus (GPa)
Modulus retention
02S C1 N1 N2 C02S CC1 CN1 CN2 D02S DC1 DN1 DN2
846.91 814.90 807.83 804.55 665.61 733.07 687.99 780.53 405.23 747.97 727.80 791.09
100.00% 100.00% 100.00% 100.00% 78.59% 89.96% 85.16% 97.01% 47.85% 91.79% 90.09% 98.33%
45.04 42.45 42.26 42.15 31.10 36.87 35.10 41.00 19.14 38.66 37.57 41.44
100.00% 100.00% 100.00% 100.00% 69.05% 86.85% 83.07% 97.26% 42.49% 91.19% 88.92% 98.31%
3.4.2. Good ablation resistance As well known, the carbon fiber has good ablation resistance [15] and can withstand the temperature as high as 3000 °C [16], at which metals have vaporized. In addition, as the melt and vaporization of the nickel will absorb a great deal of energy, the residual heat or temperature is too low to cause sublimation of the carbon fibers. As a result, even if the nickel has melted and vaporized, the carbon fiber framework in the Ni-CFNV still keeps intact after a strike (Fig. 10e–10h). Therefore, the Ni-coated layer shows much better ablation resistance than the copper mesh. With regard to LSP, the ablation resistance can be as important as the electrical conductivity [3], otherwise the LSP layer will be burned up soon. Kamiyama et al. [3] proved that a higher onset temperature of thermal decomposition of CFRP could reduce the lightning strike damage. Similarly, if the LSP layer has a better ablation resistance to withstand the high temperature of the lightning strike, it can keep its structure intact for larger lightning current and longer strike time, consequently increasing its service life. As a result, the LSP layer can discharge more lightning current with less damage. Owing to its good ablation resistance, the carbon fiber framework can form an isolation layer. On the one hand, the isolation layer can prevent lightning plasma, arcs and sparks from penetrating downward to the underlying CFRP, and meanwhile transfer them to nearby Ni-coating network for discharge due to its fairly good conductivity. On the other hand, as the Ni-coating fibers are repeatedly stacked in the thickness direction
To further verify the LSP contribution of Ni-coating, the CFRP specimens with the surface layers of chopped carbon fiber (without Nicoating) nonwoven fabrics (30 g/m2 and 50 g/m2) were fabricated. The conductivities of the two chopped carbon fiber layers are 3.3 × 101 S/m and 2.4 × 102 S/m, which are 2–3 orders of magnitude lower than that of the Ni-CFNV layers. Thus, Ni-coating, rather than carbon fiber, plays a dominant role in lightning current discharge of the Ni-CFNV layers. The Ni-CFNV with the skin-core structure can be divided into two parts: Ni-coating network and carbon fiber framework. Undoubtedly, Ni-coating network plays a dominant role in lightning current discharge due to its high conductivity. Nevertheless, the carbon fiber framework in Ni-CFNV also possesses good electrical conductivity owing to its graphitic structure, so that the lightning current can flow through both the Ni-coating and carbon fibers. Hence, the carbon fiber framework can assist the Ni-coating network in lightning current discharge. For this reason, the higher conductivity can be obtained easily by coating
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Fig. 9. Characteristic of Ni-CFNV. (a–c) SEM; (d) X-ray diffraction.
Fig. 10. Characteristic of Ni-CFNV LSP layer. (a–b) Polished top surface, without strike; (c–d) Polished cross-section, without strike; (e–h) Top surfaces, after strike.
was verified by the artificial lightning using standardized Waveforms C (200 Coulomb) and D (100 kA). The experimental results manifested that the Ni-CFNV of 70 g/m2 performed better than the commercial expanded copper foil of 73 g/m2 in withstanding both Waveforms C and D. Hence, the Ni-CFNV can be a promising LSP solution to replace metallic mesh for eliminating lightning strike damage to the CFRP structures in Zone 2. As made of skin-core structured Ni-coated carbon fibers, the NiCFNV consists of two networks for LSP: Ni-coating network and carbon fiber framework. The possible LSP mechanism of Ni-CFNV can be described by the different roles that the two networks play. The Ni-coating network makes the Ni-CFNV more conductive in order to fast discharge lightning current and suppress resistive heat. The carbon fiber framework enhances the ablation resistance of Ni-CFNV to prolong the service life of the LSP layer. Furthermore, the carbon fiber framework can form an isolation layer to prevent lightning plasma, arcs and sparks from penetrating downward to the underlying CFRP laminates. The
(Fig. 10c), the isolation layer might suppress the vaporization of underneath Ni-coating by the mechanism of blocking the releasing channel of nickel vapor (Fig. 10h), prolonging the service life of the NiCFNV. Consequently, the Ni-coated carbon fibers in the bottom of the LSP layer can be hardly damaged unless the upper ones are destroyed. The isolation effect is a distinct advantage of the Ni-CFNV compared with meshes or foils, whose metallic material is continuously distributed in the thickness direction and can be burned up easily once heated by lightning. In summary, the Ni-CFNV integrates the high conductivity of Nicoating network and the ablation resistance of carbon fiber framework, which results in its remarkable LSP effectiveness. 4. Conclusions The Ni-CFNV was proposed as an LSP surface layer of the CFRP structures in Zone 2 of aircraft, and its remarkable LSP effectiveness 101
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study also demonstrates that besides the electrical conductivity, the ablation resistance plays a vital role in the protection effectiveness of the LSP layer.
[18]
Acknowledgments
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This work was supported by the Fundamental Research Funds of Shandong University [2016JC012]; National Key Basic Research Program of China [JCKY2016205B007]; and Special Research Foundation of China Civil Aircraft [MJ-2015-H-G-103].
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