2019 International Conference on Lightning and Static Electricity (Wichita, US) HYBRID LIGHTNING PROTECTION OF CFRP STRUCTURES WITH CNT NON-WOVEN MATS 1 2 2 3 Christian Karch , Matan Alper , Yehoshua Yeshurun , and Johannes Wolfrum 1 Airbus Defence and Space GmbH, 85077 Manching, Germany 2 Tortech Nano Fibers, Hanassi Herzog St., Koren Industrial park, Ma’alot Tarshiha, 24952 Israel 3 Bundeswehr Research Institute for Materials, Fuels and Lubricants, 85435 Erding, Germany christian.karch@airbus.com ABSTRACT The present study shows how a hybrid lightning protection measure made from Tortech’s CNT nonwoven mats and Dexmet expanded copper foils can be designed to effectively mitigate lightning damage in underlying composite structures. First of all, the used CNT non-woven mats are briefly described. Then, measured results of electrical conductivity of CNT non-woven mats with the FourPoint and the Gobin’s magnetic probe method are presented. It is shown that the in-plane electrical conductivity of pure and epoxy-resin impregnated CNT 5 mats reaches the values of about 10 S/m, one to two orders of magnitude higher than that of the currently available CNT bucky papers based on CNT powders. Thereafter, EMI shielding effectiveness measurement results for CNT mats and reference samples are presented. The results show that CNT mats in various configurations present very high EM attenuation over a very wide range of frequencies from 100 KHz to 40 GHz. In addition, the normalized areal density attenuation results are presented and show the potential of CNT non-woven mats to be used as a lightweight EMI shielding solution. Afterwards, the TGA procedure is used to assess the thermal decomposition of pristine and cured sample with RTM 6 epoxy resin. The differences of the thermal decomposition in air and nitrogen atmosphere and the influence of chemical treatment are presented. The TGA analysis confirms the temperature resilience of CNT mats for oxidation processes up to 450° C. Moreover, the CNT samples are chemically characterized by EDX spectroscopy. The EDX analysis indicates that about 10 wt. % of elemental iron remains as catalytic residue. However, it has been shown that other post treatments can reduce the residual iron content to about 1 wt %. Finally, a parametric study is performed to assess effect of the high current lightning strikes on coated and protected CFRP samples. The CFRP samples are protected by CNT non-woven mats and by hybrid lightning protections made from CNT non-woven mats and expanded copper foils. For comparison CFRP reference samples protected by expanded copper foils only are assessed as well. The preliminary analysis of lightning induced damage shows that the designed hybrid protection measure can effectively mitigate lightning-induced damage in underlying composite structures. This parametric study demonstrates that it may be possible to tailor lightweight hybrid lightning protection measures based on CNT non-woven mats as an effective alternative to traditional purely expanded copper protection layers. KEYWORDS: CNT, Lightning Strike Protection, Direct Effects, CFRP ACRONYMS AND SYMBOLS: AI CFRP CNT ECF EDX EMI HC HV SEdb TGA Action Integral Carbon Fiber Reinforced Plastic Carbon nanotube Expanded Copper Foil Energy-Dispersive X-Ray Spectroscopy Electromagnetic Interference High Current High Voltage Shielding Effectiveness [dB] Thermogravimetric Analysis 1 INTRODUCTION The lightning induced damage of protected and coated CFRP test samples with different lightning protection measures is determined experimentally. The high current lightning tests are carried out in accordance with the applicable aviation standards EUROCAE ED-84 [1], ED-91 [2] and ED-105 [3]. The mechanical damage of protected CFRP laminates generally depends on the dielectric coating thickness / surface weight, as exemplarily shown in Fig. 1. These CFRP laminates were protected with expanded copper 2 foil having surface weight of 73.3 and 195.3 g/m , respectively. It is clearly shown that the damage area increases with increasing surface weight of the dielectric coating / polyurethane paint. Moreover, the mechanical damage occurs already during the early stage / the rise time of the transient lightning current component [4]. The damaged area of the CFRP structure was determined with ultrasonic A- and C-scans. Fig. 1 indicates that at moderate thicknesses of the dielectric coating < 1mm the B (intermediate) and C* (continuing) lightning current components do not have significant effects on the size of the damage / delamination area. This is another direct indication that the mechanical 85.1 2019 International Conference on Lightning and Static Electricity (Wichita, US) damage of protected CFRP structures is caused mainly by the transient lightning current component A or D. It has been demonstrated that the contributions of continuing current components C or C* or the intermediate lightning component B to the thermomechanical damage is rather negligible [4]-[7]. Therefore, in the present study the HC lightning tests of the CFRP samples are performed using the transient lightning current component D (100 kA / 250 kA²s in < 500 µs) for the lighting current zone 2A only. The CFRP samples are painted with polyurethane paint system. The targeted value of the total thickness of the primer and topcoat is approximately 400 µm, since significant delamination damage of CFRP samples is expected for this dielectric coating thickness when insufficient lightning protection is used, see Fig.1. Fig. 2. Tortech's CNT non-woven mat with dimensions of 90×200 cm. In the button right corner the typical nano morphology of the mats is presented. 2.1 Manufacturing Procedure Fig. 1. Damage area of protected CFRP laminates as a function of polyurethane paint thickness [5]. The basics of physical processes and numerical models that describe the transient lightning loads on coated and protected CFRP structures can be found in Ref. [4]. 2 CNT NON-WOVEN MATS The structure of single CNTs is locally (essentially) hexagonal, but the two-dimensional hexagonal layers are rolled into long single or multi-wall cylindrical structures. The single CNTs are extremely strong, and usually tend to aggregate in the form of bundles of CNTs. The tubes of carbon are usually only a few nanometres in diameter, and can range from less than a micrometre to a few tens of micrometres in length. However, through a unique manufacturing process, developed by Tortech Nano Fibers [8], the length of the carbon nano tubes can reach one millimetre, see Fig. 2. Their unique molecular structure results principally in extraordinary macroscopic properties, including high tensile strength, high electrical conductivity, high ductility, etc. However, it should be pointed out that electrons from neighbouring structures, impurities as well boundaries influence the electronic band-structure of this 2D structure and degrade its superior electrical and others properties. Until now, the industry standard delivery for CNTs has been mainly in a powder form with a number of recognized drawbacks including agglomeration problems when used in epoxy resins, as well as low aspect ratio (~1:1000) and thus low thermal and electrical conductivity of the cured CNT enhanced epoxy resins. In the last few years, Tortech Nano Fibres, a CNT development and manufacturing company have developed a new industrial manufacturing process to fabricate flexible, Ultra-Long CNT non-woven mats that have a rather high aspect ratio of up to 1:100,000 [8]. This high aspect ratio is in fact the main basis for the rather good data of electrical, thermal and mechanical properties of Tortech's CNT non-woven mats. Tortech's technology is a novel patented process, originally developed at Cambridge University. The manufacturing process is a continuous gas phase catalytic reaction (FFCVD) between a floating catalyst and a carbon source. The process enables the formation of a continuous and robust non-woven mats and wires made of Ultra-Long Carbon Nanotubes (ULCNT) that could be handled and used without any addition of binders, see figure below. Fig. 3. Tortech's CNT non-woven mat for industrial application (mat width 90 cm). 85.2 2019 International Conference on Lightning and Static Electricity (Wichita, US) This manufacturing process has been scaled up to an industrial level by Tortech Nano Fibers to allow high throughput production of CNT non-woven mats and wires. 2.2 In-Plane Electrical Conductivity For the characterization of electrical and thermogravimetric properties of the CNT non-woven mats about 60 samples with different thicknesses and sizes are manufactured. Some of the samples are chemically post-treated to enhance their electrical conductivity. Roughly half of the samples were impregnated and cured with RTM 6 epoxy resin in order to assess the influence of the epoxy resin on the macroscopic electrical properties and thermal stability. 2.2.1 Four-Point resistivity Measurements Due to the limitations of the Two-Point resistivity measurement procedure, the Four-Point measurement approach is used here. This approach is the most widely used method for resistivity measurements reducing the effect of test lead resistance. The in-plane electrical conductivity of different CNT samples was determined from the measured value of the electrical resistance. Three measurements were performed for each pristine and RMT 6 epoxy resin impregnated & cured sample. The measured in-plane electrical conductivity is generally larger in spinning direction than in perpendicular direction; approximately 30 % for pure and less than 10 % for samples impregnated and cured with RTM6 epoxy resin. The Four-Point resistance measurements demonstrate that the in-plane electrical 5 5 conductivity reaches values of 1.610 to 4.010 S/m. The impregnation and curing procedure with RTM 6 epoxy resin reduces the in-plane electrical conductivity 5 5 of the samples to about 1.010 to 2.010 S/m. This is mainly caused by swelling effect of the impregnated & cured samples. K 0 r 0 1 r exp 2d , (2) and J1(R) is the first-order Bessel function, while d is the thickness of the conducting screen, z is the distance 2 2 between both magnetic loops, and = √(λ -β ). The SE of a conducting barrier in low frequency range can be approximated by the function SEdB 0 20 log 1 j f fLF , (3) where the cut-off fLF (critical frequency fc) is given by fLF d Ra fc R a Re . (4) The constant α is approximately /9 [14]; the lowfrequency behaviour is mainly determined by the sheet resistivity Re and the radius Ra of the excitation loop. The measured values of the surface resistivity Re show that the in-plane electrical conductivity of pure CNT nonwoven samples reaches quite high values of about 0.56 5 to 2.2310 S/m. The values of the in-plane electrical conductivity of RTM 6 cured samples are lower: 5 between 0.12 and 1.3510 S/m. This decrease is most likely related to the swelling of the CNT mats during the impregnation and curing procedures that increases the contact resistances between the single CNTs. 2.2.3 Shielding Effectiveness Measurements Two shielding effectiveness tests were performed on several samples of non-woven carbon nanotube mats and composites, and some reference metal and commercial EMI shielding samples, see Table 1. Table 1: Areal density and thickness of tested samples. 2.2.2 Gobin’s Probe Method This method is based on measurement of the normal magnetic field with and without a planar conducting sample using two magnetic loops [9]. By measuring this magnetic field in the presence of the sample and without sample while keeping the same distance between excitation and receiver loop, the magnetic shielding effectiveness can be determined. This procedure is quite simple, contactless and do not require any special sample preparation. The magnetic SE of the sample can be evaluated as [9], [10] SEdB 20 log10 1 4 r 0 0 where 0 J1 R exp 0 z d 2 1 K 0 J1 R exp 0 z 0 d d 2 2 , (1) All samples were tested in a frequency range between 100KHz and 1GHz, and some selected samples were additionally tested for high frequency range of 1GHz to 40GHz. The measurements were performed by two different measurement setups (ASTM D4935 below 1 GHz, and Marvin et al 'Absorber Box' Method above 1 85.3 2019 International Conference on Lightning and Static Electricity (Wichita, US) Attenuation [dB] GHz).The measurements were performed at Eurofins York's Castleford EMC Laboratory [11], [12]. In general, the EMI shielding performance of pristine CNT mats and composites is high in a wide range of frequencies from 100 KHz to 40 GHz, see Fig. 4 and 5. 100 90 80 70 60 50 40 30 20 10 0 1 10 Frequency [GHz] Brass Mesh Ref 2Cu4-100FA - Copper Mesh - 107.4 gsm CNT mat - 30 gsm CNT mat - 60 gsm Aluminium Foil - 172 gsm Fig. 4. High frequency (1GHz-40GHz) attenuation tests results. 95 properties / the effective electrical conductivity of the CNT non-woven mats should be further increased. In the high frequency range (1GHz-40GHz), Tortech's CNT mats present high attenuation (>60 dB) that might compete with thin aluminium foils and out-performs commercial expanded copper mesh. The measurements of samples providing more shielding 2 (aluminum and 60 g/m CNT non-woven mat) have been limited by the dynamic range of the test system. In the low frequency range, commercial metalized EM shielding materials were tested compared to different CNT configurations. These different CNT configurations, CNT 11gsm, CNT oxidation 30 gsm, presented performance that competed in performance against commercial samples, while the particular hybrid CuCNT system presented extremely high attenuation at a 2 very low areal density (19 g/m ). Normalizing the measured shielding effectiveness by the areal density of the samples allows a consideration of the weight of material in each sample. The normalization by areal density exhibits the efficiency of the tested material and the ratio between the performance and the used weight. In Fig. 4 it is obvious that the best attenuation-to-areal density ratio was received for the hybrid Cu-CNT sample while still maintaining very high absolute attenuation (78 dB). Attenuation [dB] Areal Density normalized Attenuation [(dB*sqm)/gr] 85 75 65 55 45 35 0 1 10 Frequecny [MHz] 100 1.000 Al foil (dynamic range) - ~40gsm 2Cu4-100FA - Copper Mesh - 107.4 gsm CNT-Cu mat - 19gsm nickel/Cu plated PET fabric - 74gsm CNT mat - Oxidation - 30 gsm nickel coated carbon non-woven - 36gsm blackened silver-plated S.S woven mesh - 70gsm CNT mat - 11 gsm nickel coated carbon non-woven - 10gsm Fig.5. High frequency (1GHz-40GHz) attenuation tests results. The areal density of the mats has an effect on the level of attenuation, especially at low frequencies, where the skin depth increases. In order to achieve better performance at very low frequencies, the electrical 4,5 4,0 3,5 3,0 2,5 2,0 1,5 1,0 0,5 0,0 46 78 39 93 63 51 59 59 64 55 84 Fig. 6. Areal density normalized attenuation of the tested CNT and commercial samples. All of the red columns presented in Fig. 6 above are different Tortech CNT configurations, and all of the blue columns are the reference commercial samples. The purple marked values above each column represent the absolute SE attenuation of the corresponding sample. 85.4 2019 International Conference on Lightning and Static Electricity (Wichita, US) The measured values are in good agreement with Schelkunoff's SE theory model for the entire tested frequency range, and confirm the validity of the measurement procedure. To conclude, CNT non-woven mats can be embedded in any composite structure to provide excellent EMI shielding performance with minimal weight addition. 2.3 TGA and EDX For the TGA several, different RTM 6 impregnated and cured as well as pristine CNT non-woven samples were used. All measurements were conducted under air or inert nitrogen atmosphere, with a sample weight of about 3-6 mg [15]. The heating rate was chosen to 10 and 50 K/min, and the test temperature ranged from the ambient to 1000 °C. The decomposition and TGA measurements of samples in air are plotted Fig. 7. a) weight loss, and can be simply identified as the peaks in the derivative of the weight loss as a function of temperature, see Fig.7b. The DTG results demonstrate that most of the carbon materials of the pristine CNT mats oxidize in a rather narrow temperature range centred at the peak oxidation temperature (well above 600 °C). The obtained results in nitrogen atmosphere show that temperature-induced decomposition of chemically untreated CNT non-woven mats is rather low, below 0.5 ‰ / °C, up 1000 °C. Moreover, the TGA and DTG curves of cured CNT samples in air atmosphere indicate that the CNT network stabilize and even delay the decomposition of the cured RTM 6 epoxy resin. The EDX spectroscopic analysis was performed for the pristine, pure and thermal treated samples. A high concentration, approx. 10 % by weight, of iron beside of carbon atoms can be observed in pure pristine samples. Moreover, the elements sulphur and oxygen are present in a very lower concentration. After thermal posttreatment, an increase of oxygen share and a slight decrease of sulphur share can be observed. The iron (and sulphur) atoms are the remnants of the gas-phase catalytic reaction used for the creation of the CNTs / CNT network. If necessary, a further post treatment can reduce the residual iron content from approx. 10 % to about 1 wt %. 3 b) Fig. 7. a) Decomposition curves for CNT and RTM 6 impregnated and cured samples. b) DTG curves. As expected the decomposition and DTG curves are shifted to higher temperatures for higher heating rates. The residual mass of CNT network can be attributed to the metal catalysts used to manufacture the carbon nanotubes, as well as the oxidation products of these catalysts. The oxidation decomposition temperatures of the CNT network are defined as the points of maximum HIGH CURRENT TEST The high current tests were performed at the Department of Electrical Apparatus and Switchgear of the Technical University Ilmenau. The main research areas are switching devices and systems engineering, high-voltage technology and impulse and lightning protection [13]. Transient high amplitude currents with extreme short rise time, even below few μs can be generated by their low inductive coaxial pulse generator. The upper limit of decay time to half of the crest amplitude is about 350 μs. The maximum driving voltage of the generator is slightly above 15 kV, the maximum action integral of the generated current pulse is limited to about 10 MJ/. The experimental setup for the high-current lightning tests is designed in accordance with the requirements of EUROCAE standards [1]-[3]. The test panels with dimensions of 400×400 mm were clamped in a circumferential frame made from brass, see Fig. 8b. The used CFRP test samples ([+45/-45/0/90/90/0/-45/+45]) are characterized in more detail in the Table 2. Different protection measures including hybrid ECF / CNT non-woven mats [18] were applied on the top of the CFRP samples. Finally, the test samples were painted with an epoxy resin primer and a polyurethane paint topcoat. The targeted value of the thickness of the primer (topcoat) was 50 (350) µm. The measured total thickness values are also listed in the Table 2. 85.5 2019 International Conference on Lightning and Static Electricity (Wichita, US) a) A simplified equivalent circuit of the coaxial pulse generator is shown in the figure below. b) Fig. 9. Equivalent circuit of the coaxial HC generator. Fig. 8. a) View of the measuring cabin with high-current pulse generator b) High current test-set up. Table 2: Test matrix. No. 1 2 3 4 5 6 7 8 9 10 11 CNT mat [g/m2] 43 35 47 57 2 × 29 2 × 27 2 × 29 47 ECF mat [g/m2] 175,0 195,3 107,4 107,4 107,4 107,4 107,4 107,4 107,4 107,4 - ECF ECF-175 (3M) [16] 3Cu7-100FA [17] 2Cu6-100FA [17] 2Cu6-100FA [17] 2Cu6-100FA [17] 2Cu6-100FA [17] 2Cu6-100FA [17] 2Cu6-100FA [17] 2Cu6-100FA [17] 2Cu6-100FA [17] - Paint [µm] 360 380 385 420 385 390 390 360 360 390 360 3.2 Results 3.1 Calibration The transient current waveform D has been calibrated using protected and coated CFRP test specimens during previous lightning tests campaigns. At begin of this test campaign two test trials were performed additionally. The following (averaged) current waveform parameters were obtained: Amplitude: 94,0 kA; min. 8,2 kV driving voltage (Target: 100 kA ± 10 kA) AI: 6 2 0,33×10 A s 6 2 6 2 (Target: 0,25×10 A s ± 0,05×10 A s) Pulse: 14/51 µs (Target: duration < 500 µs) The inductance of the HC generator LG is only about 300 to 500 nH. The lowest resistance value RG of the HC generator is about 39 mΩ. Rd and Cd are elements at the output of the generator used to damp overshoots. REnt is used as contact protection to discharge the capacitor Cd. The parameters of the lightning current waveform are mainly fixed by the parameters of the coaxial HC generator. However, it should be pointed out the magnitude and the shape of the applied current waveform are influenced by the resistance RS and less by the inductance LS of the sample and by the arc discharge behaviour as well. This means that the parameters of the HC pulse generator have to be appropriately adapted to the resistance of sample / lightning protection system, to the inductance of the sample, and finally to the dielectric coating of the sample in order to maintain the parameters of the required current waveform. The increase of the total impedance of the test specimen decreases generally the amplitude of the current waveform and broadens its pulse width. In extreme cases a high dynamic behaviour of the arc root caused mainly by thick dielectric coatings and low surface resistance value of lightning protection measures causes instabilities of the arc discharge and influences curve shape of the current waveform and the magnitude of the action integral. In the following the high-current tests are summarized and few representative results are presented in detail. The standard full lightning current waveform rises to its peak value IP and falls, appreciably slower, ultimately back to zero by definition. The rising part of the impulse voltage is referred to as the front, the maximum as the peak and the decreasing part as the tail. The various lightning impulse voltages are identified in the test specifications by e.g. the front time T1, the time to halfvalue T2, the action integral AI [1]. The selected waveform parameters of the applied current waveform D are exemplarily given in Table 3 for the lightning tests no. 1 to 3. These parameters are derived from the recorded current loads, see Fig. 10. 85.6 2019 International Conference on Lightning and Static Electricity (Wichita, US) Table 3: Reference samples: lightning load parameters. No. 1 2 3 Driving Voltage [kV] 8,2 8,2 8,2 IP [kA] AI [kAs2] T1/T2 [μs] 94 95 91 334 337 315 14,0/50,5 14,1/49,4 14,3/51,3 the visible surface damage of the coating / the ECF on the sample protected by the 3Cu7-100FA ECF is much smaller than the damaged surface on the sample protected by the lighter 2Cu6-100FA ECF. a) As expected the peak current values decreases with increasing effective surface resistivity of the applied ECF. Moreover, a slight broadening of the current wave shapes with increasing surface resistivity of the applied ECF can be observed as well. b) Fig. 10. Recorded current loads. A time-integrated image of the transient lightning current load, test no. 2, is shown exemplarily in the figure below; the sparking of the (partially) burned materials, mainly copper, can be clearly seen on this picture. Fig. 12. Damage of coating / ECF on the top of the loaded samples. a) Test no. 2: 3Cu7-100FA. b) Test no. 3: 2Cu6-100FA. The parameters of the lightning current loads no. 4 to 11 are summarized in the table below. Table 4: Test samples: lightning load parameters. No. 4 5 6 7 8 9 10 11 Fig. 11. Time-integrated record of a lightning load. The damaged area of the coting / the ECF depends strongly on the arc root radius of the plasma discharge. However, it can be shown that the arc root radius depends on the effective resistivity of the applied lightning protection measure (ECF) and increases (nonlinearly) with increasing effective resistivity values [4]. This is nicely demonstrated in the Fig. 12; the area of Driving Voltage [kV] 8,2 8,2 8,2 8,2 8,2 8,2 8,2 8,4 IP [kA] AI [kAs2] T1/T2 [μs] 90 90 89 90 90 89 87 74 309 316 309 309 306 303 297 241 13,2/51,8 13,3/51,3 13,1/52,0 13,3/51,6 13,2/51,2 13,4/51,5 13,7/52,1 15,9/59,6 The values of the peak current and the action integral for the samples with a hybrid protection layer are generally a bit smaller than those of the reference 85.7 2019 International Conference on Lightning and Static Electricity (Wichita, US) samples. However, in case of the sample no. 11 protected by a CNT non-woven mat only these values are significantly lower. Moreover, the width of the current waveform applied on the sample no. 11 is broadened considerably. This is a clear indication that the surface resistivity of the applied CNT non-woven mat is significantly lower than the values of the hybrid protection systems made from low-weight hybrid protection systems. As shown in Fig. 13 the area of the burned paint and CNT non-woven mat on the top of the sample no. 11 is rather small. However, it can be seen that there is a carbon fibre breaking of the first unidirectional CFRP ply. This finding implies that the single CNT non-woven mat cannot carry the current lightning load of 100 kA and a blast of the epoxy resin occurred due to diffusion of the applied lightning current into the CFRP laminate. a) the CFRP laminate / carbon fibres of the first CFRP ply. This is exemplarily shown in Fig. 14 for the samples no. 4 and 7. For these samples single CNT non-woven 2 mats with surface weight between 53 and 57 g/m were applied. It is remarkable to see that in these cases there is a rather large area of burned / splintered dielectric coating. Moreover, the area of the burned / splintered coating is considerably generally larger than that of the melted / burned copper from the applied ECF. In case of the reference samples protected by ECF only both areas are almost identical, see Fig. 12. In case of samples no. 8 to 10 with hybrid lightning protection system made from the lightweight 2Cu6-100FA ECF and two layers of the CNT non-woven there is no visible mechanical damage of the CFRP laminate / breaking of carbon fibres of the first CFRP ply. However, as shown in Fig. 15 there is an indication of thermal damage / damaged epoxy resin spot at the centre of these samples. This is probably due to long dwelling time of the arc root and therefore longer thermal loading due to direct heat from the plasma channel and generated Joule heat at this point. a) b) c) Fig. 13. a) Surface damage of sample no. 11. b) Detailed view (90×90 mm) of the damaged CFRP ply / broken carbon fibres. The samples with hybrid lightning protection system made from the lightweight 2Cu6-100FA ECF and one layer of the CNT non-woven mats withstand the lightning current waveform 2A without visible damage of Fig. 14. Image of surface damage. a) Sample no. 4. b) Sample no. 7. 85.8 2019 International Conference on Lightning and Static Electricity (Wichita, US) a) of the lightweight CNT non-woven mats that are chemically compatible with thermoset and thermoplastic materials. The obtained results indicate the high potential of pristine and/or hybrid Cu/CNT non-woven mats of performing as a lightweight broadband composite EMI shielding solution. Furthermore, high current lightning tests were carried out on CFRP test samples with different lightning protection measures. The corresponding lightning load parameters and relevant environmental conditions were recorded. High resolution picture were taken to visualize the lightning induced damage of the loaded samples. Typical lightning induced damage, which manifest themselves in varying degrees, are c) Fig. 15. Image of surface damage. a) Sample no. 8. b) Sample no. 9. 4 burning and splintering of the dielectric coating, melting and evaporation of the expanded copper foil as well as of the CNT non-woven mats, local damage of the CFRP structure, in particular fibre tearing. The preliminary visual analysis of the of the CFRP panels after lightning tests show that cracking through the entire CFRP sample occurred for the sample which was protected by CNT non-woven mats only. However, the visual analysis of lightning induced damage of samples protected by a combination of low-weight ECF and a single layer of CNT non-woven mats ECF indicates that this hybrid protection measure might be an option to the standard expanded copper foil solution. A reliable comparatively evaluation of the lightning induced damage cannot be performed using the summarized visible damage only. For a definitive comparatively evaluation at least non-destructing X-ray and ultrasonic tests of the loaded samples are necessary and are planned. ACKNOWLEDGEMENTS SUMMARY AND OUTLOOK The Four-Point DC resistance and Gobin’s probe measurements show that the in-plane electrical conductivity of CNT mats reaches the values of about 4 4 210 to 410 S/m. The electrical conductivity of cured 4 4 samples is about 110 to 210 S/m. For future application the use of either pressure assisted processes or pre-impregnated CNT non-woven mats is recommended to avoid volume swelling effects, and therefore the avoid the increase of effective surface resistivity of the cured CNT mats. The TGA analysis confirms the temperature resilience of CNT non-woven mats for oxidation processes up to 450 °C. Moreover, the dense CNT fabric stabilizes and delays significantly the thermal decomposition of the cured RMT 6 epoxy resin. The TGA results in nitrogen atmosphere show that temperature-induced decomposition of pristine CNT non-woven mats is very low, below 0.5 ‰ / °C, up 1000 °C. The performed shielding effectiveness measurements demonstrate rather high shielding effectiveness values The work was partially performed within the project CNT Based Materials for EMI Shielding and LSP. Financial support from German MoD under Contract No. E/E210/AG008/GF057 is gratefully acknowledged. REFERENCES [1] [2] [3] [4] [5] 85.9 ED-84, Aircraft Lightning Environement and Related Test Waveforms, European EUROCAE, Paris1997 ED-91, Aircraft Lightning Zoning Standards, EUROCAE, Paris 1998 ED-105, Aircraft Lightning Test Methods, EUROCAE, Paris 2005 C. Karch, A. Arteiro, P.P. Camanho, Modelling mechanical lightning loads and damage in carbon fibre-reinforced polymers, IJSS, 2018, https://doi.org/10.1016/j.ijsolstr.2018.12.013 W. Wulbrand and C. 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Kowalska 143, 51-424 Wroclaw, Poland Dexmet Corporation, 22 Barnes Industrial Road South, Wallingford, CT 06492, US, www.dexment.com C. Karch, B. Lenczowski, Y. Yeshurun, and J. Wolfrum, Structural Component: Hybrid Lightning Protection of CFRP/GFRP Structures with CNT non-woven mats, P701179-EP-EPA / P40301-US, 2016 85.10