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
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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).
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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 1.610 to 4.010 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 1.010 to 2.010 S/m. This is
mainly caused by swelling effect of the impregnated &
cured samples.
K 




0

 r 

0

1
  r exp  2d   ,

(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 2.2310 S/m. The values of the in-plane electrical
conductivity of RTM 6 cured samples are lower:
5
between 0.12 and 1.3510 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
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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.
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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.
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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.
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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
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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
210 to 410 S/m. The electrical conductivity of cured
4
4
samples is about 110 to 210 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.
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