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Epoxy Resin with Carbon Nanotube Additives
for Lightning Strike Damage Mitigation of
Carbon Fibe
r Composite Laminates
SPENCER LAMPKIN, WENHUA LIN,
MOJTABA ROSTAGHI
-CHALAKI, KAMRAN YOUSEFPOUR,
YEQING WANG and JONI KLUSS
ABSTRACT1
Carbon fiber composites are paving the way for light weight, high strength structures
in the aerospace industry. While the benefits of carbon fiber composites are
undeniable, they also have their drawbacks. Crippling damage due to lightning strikes
is one of them. The current solutions to reducing the damage due to lightning includes
adding expensive and heavy copper mesh into the laminate. A potential solution to this
problem would be to add a lightweight conductive additive to the epoxy resin instead
of a copper mesh. Carbon nanotubes are chosen as the additive to create an electrically
conductive resin matrix and hence increase the overall electrical conductivity of the
composite. Increasing the conductivity will decrease the damage owing to a faster
dissipation of lightning-strike-induced Joule heating, and more importantly increase
the residual strength. In this work, we quantified the increase in conductivity through
measuring the electrical resistance using the four-probe method. Results showed that
the electrical resistance of the sample with carbon nanotube additives is 31% lower
than the one with no additives when the same resin system is used. In addition,
lightning strike tests have also been carried out with both samples using an artificially
generated waveform A impulse current with a peak of 100 kA. The current results
showed no visible damage to both the samples with and without CNT additives in the
epoxy. Time-resolved camera images taken for the lightning strike tests showed that
S. Lampkin, Department of Aerospace Engineering & Advanced Composites Institute,
Mississippi State University, Mississippi State, MS 39762
W. Lin, Department of Aerospace Engineering, Mississippi State University, Mississippi State,
MS 39762
M. Rostaghi-Chalaki, Department of Electrical and Computer Engineering & High Voltage
Laboratory, Mississippi State University, Mississippi State, MS 39762
K. Yousefpour, Department of Electrical and Computer Engineering & High Voltage
Laboratory, Mississippi State University, Mississippi State, MS 39762
Y. Wang (corresponding author, email: yw253@msstate.edu), Department of Aerospace
Engineering & Advanced Composites Institute, Mississippi State University, Mississippi State,
MS 39762
J. Kluss, Department of Electrical and Computer Engineering & High Voltage Laboratory,
Mississippi State University, Mississippi State, MS 39762
the lightning current may have conducted through the metallic grounding device
owing to the small planar sample size (i.e., 6 by 6 inches), which resulted in
unsatisfactory test results.
INTRODUCTION
Composite materials are finding greater applications in modern aerospace, automotive,
construction industries and especially in aerospace industries due to its significant
weight reduction capability while still be able to provide same or even better strength
than certain metallic materials. On the other hand, composite materials are notorious
for their inability to conduct electricity adequately. While, for example, a carbon fiber
reinforced polymer (CFRP) composite may have conductive carbon fiber constituent,
the insulating epoxy matrix creates a very difficult environment for the distribution of
electricity. This failure to efficiently conduct the extreme electricity can affect the
structural integrity of the laminate when struck by a violent electromagnetic discharge
such as a lightning strike. Studies had shown that modern aircrafts are subjected to one
lightning strike per 1000 to 2000 flight hours which equates to one lightning strike per
year for commercial airliners [1]. Without good lightning strike protection (LSP)
systems, the damage caused by lightning strike could be detrimental to aircrafts as can
be seen in many lightning strike experimental studies [3-8] with different composite
materials. For example, Miller and Feraboli [7] tested 60 unprotected carbon
fiber/epoxy specimens and Hirano et al. [8] tested unprotected graphite/epoxy
composite panels. In addition to experimental studies, numerous attempts have also
been made to understand the lightning strike damage mechanisms through simulation
models for composite materials [9-18].
Traditional methods of LSP for composite materials involves the use of expanded
metal films (EMF) on top of composite laminate surface to help safely conduct the
lightning strike, which greatly compromises the weight reduction benefit of using
composite materials. Numerous studies had been done to investigate the effectiveness
of various lightning strike protection methods. Mall et al. [19] tested five distinct
nanocomposite LSP mechanisms and found a strength reduction in the composite
panels ranging from 75% to 30%. Gou et al. [20] investigated the LSP system by
applying a layer of carbon nanofiber paper and found significant reduction in lightning
strike damage. Kumar et al. [21] developed a polyaniline-based all-polymeric adhesive
layer for LSP and were able to maintain 99% residual strength after lightning strike
tests. Chakravarthi et al. [22] investigated the use of nickel coated single-walled
carbon nanotubes (Ni-SWNTs) for LSP and also found a great reduction in lighting
strike damages.
The current work studies the effect of the conductive resin system with carbon
nanotube (CNT) additives on LSP. With the CNT additives, the overall conductivity
of the composite panel increases hence reducing the lightning strike damages. In
contrary with the work done by Chakravarthi et al. [22] where the carbon nanotubes
were first sprayed onto the carbon fiber plies followed by resin transfer molding
(RTM) process, the resin used for this test was specially formulated with premixed
CNT purchased from Soller Composites.
MANUFACTURING METHODS
Four composite panels were fabricated for this test each comprising of the same
stacking sequence. Two of them were fabricated using the same resin Adtech 820/824
of which one of them was subjected to the artificial lighting strike test at the High
Voltage Laboratory at Mississippi State University (MSU-HVL). For the remainder of
the four panels, one of them was fabricated using the Adtech 820/824 resin containing
CNTs and the other one was fabricated using West Systems resin. The one with the
Adtech 820/824 resin containing CNTs was also subjected to the lightning strike test
at MSU-HVL.
The CFRP composite panels were fabricated using the wet layup method. Ten layers
of carbon fiber oriented in a [90/45/90/45/90]s pattern was laid up on an aluminum
plate one by one. Each layer was coated in epoxy, then vacuum bagged. The vacuum
bag was continuously worked on until the vacuum bag achieved an interior pressure of
at least -25 in/hg. Once the desired interior pressure was attained, the samples were
held at that pressure while the epoxy cured. The panels were cut with a wet tile saw to
get rid of their rough edges after they were fully cured. Finally, about half an inch of
surface was sanded down along all four edges for improving the electrical
conductance between the contact of the CFRP panel and the grounding plate used in
the impulse generator for the lightning strike test.
Figure 1. A visual example of what goes into performing a wet layup.
Materials
The carbon fiber used to fabricate the four panels was HexForce 282. This fabric has a
plain weave pattern and 3k tow. Two of the panels were fabricated using the Adtech
820/824 epoxy without CNT additives. The epoxy used for one of the other two panels
was a two-part system called Adtech 820/824 with CNTs infused in the resin and the
other panel used a two-part system called West System 105/206. The reason for
fabricating the additional panel with a different resin system was to study the effect of
resin on lightning strike damages. After the curing of the resin, all panels were cut
down to a size of 6x6 inch.
EXPERIMENTAL PROCEDURE
The procedure for this project started with measuring the electrical conductivity of
each CFRP composite sample using a four-probe method. After analyzing the data, a
determination was made as to whether the change in conductivity was enough to
justify a simulated lightning strike from the impulse generator. The experimental
procedure was designed to limit any unnecessary use of the impulse generator by
examining the conductivity before the lightning strikes. Each sample panel was
carefully examined after the lightning strikes and the damage was appropriately
inspected and documented.
Four-Probe Measurement Method
The four-probe method was employed to measure the surface resistance of the
composite using the Fluke 8846A Digital Precision Multimeter. The setup can be seen
in Fig. 2. Resistance was found by sending a current across a surface and the voltage
was measured. Using the values of current and voltage, the multimeter calculated the
resistance and displays the value. The four-probe measurement method was chosen
over the two-probe because of its increase in accuracy and stable data. The four-probe
measurement method hardly fluctuated in results, while a two-probe measurement
method yielded values that were continuously changing and too far from each other to
be correct due to the effect of contact resistance.
Figure 2. CFRP composite sample prepared for four-probe resistance measurements.
The samples were tested using three rows with four nickel conductive paint (MG
Chemicals Super Shield Nickel) patches to increase the surface area and accuracy of
the tests. The paint is highly conductive with a volume resistivity of 0.0042 Ω•cm.
Due to the high conductivity of the paint, the effect of the nickel paint on the surface
resistance of the CFRP composite panel is ignored. The three rows were positioned
across the composite, as seen in Fig. 3, so three sets of values of resistance could be
calculated and averaged for the composite.
Figure 3. Illustration of the layout for the four-probe measurement method.
Simulated Lightning Strike Tests
Lightning strikes pose a large risk for unprotected composites. The rapid electrical
discharge has nowhere to go and will cause significant damage in a confined area.
Forces within the laminate can delaminate layers and can rupture through the fibers
[3]. MSU-HVL hosted our simulated lightning strike tests. It has an high current
impulse generator capable of +200 kA Waveform A current surges. For the current
experimental samples, a discharge of 100 kA was used. The samples, with the sanded
edges, were grounded to a sheet of steel using braided conductive wires, two copper
strips and two steel strips. The braided wire was wrapped around the edges and the
copper and steel strips are fastened into the sheet of steel with nuts and bolts. The test
apparatus was placed on top of the impulse generator face down and struck.
V
A
Nickel
conductive
paint
Figure 4. CFRP composite sample prepared for lightning strike tests.
RESULTS
Three samples were subjected to the simulated lightning strike, two of the samples
were with Adtech 820/824 resin of which one containing CNTs, and the other one was
with West System resin.
Four-Probe Measurement Results
The CFRP composites with CNT additives in the epoxy performed significantly better
than the samples without CNT additives. The CNT epoxy sample yielded an average
surface electric resistance of 0.0296 Ω while the plain epoxy sample yielded 0.0429 Ω
with the Adtech resin system, as shown in Fig. 5. The resistance underwent a
reduction 31% when CNT was added to the resin sample. The West System resin
yielded an average surface electric resistance of 0.1327 Ω. This shows a decrease in
electric resistance in the CNT infused resin when compared to the plain epoxy resin.
The conductive nature of the CNTs allowed the electricity to flow through the
composite more effectively.
Simulated Lightning Strike Results
Figure 6 shows the lightning current waveform during the lightning strike test for the
CFRP panel with Adtech 820/824 resin and without CNT additives. It can be seen that
the impulse current generated reached a peak amplitude of over 100 kA within 20 μs
time interval. Slight damage was observed for this specific test and it was much
smaller than expected. One explanation is that the lightning current could have been
conducted by the metallic grounding fixtures around the CFRP panel owing to the
small size sample (6×6 inch) that were used in the test. Some frames captured for the
lightning strike induced flames right after the impulse current is shown in Fig. 7. In
comparison with some frames captured in a different test for a large size CFRP panel
(9×9 inch, without CNT additives in the epoxy) where the flame was confined near the
surface of the panel (see in Fig. 8), it can be seen that the flames were widespread
randomly.
Figure 5. Chart of the electrical resistance data across the composite’s surface.
Figure 6. Lightning strike waveform for CFRP panel with Adtech 820/824 resin and without
CNTs for the lightning strike test at MSU-HVL.
0.0317 0.0280 0.0290
0.0496
0.0375 0.0416
0.1370
0.1200
0.1410
0.0000
0.0200
0.0400
0.0600
0.0800
0.1000
0.1200
0.1400
0.1600
Top Middle Bottom
Resistance (Ω)
Adtech_CNT Adtech West System
Figure 7. Time-resolved camera images of the lightning strike induced flame for the 6×6 inch
CFRP panel with Adtech 820/824 resin and without CNTs.
Figure 8. Time-resolved camera images of lightning strike induced flame for a 9×9 inch CFRP
panel (without CNT additives in the epoxy).
Effect of Resin System:
As mentioned earlier, the insulating resin reduces the overall electrical conductivity of
the composite panels. The effects of two different resin systems can be seen in Fig. 9.
Delamination and fiber pullout can be observed for the CFRP panel with West System
105 resin whereas the CFRP panel with Adtech 820 resin suffered slight surface
damages. But as stated, the impulse current might not have entirely passed through the
latter panel but partially conducted by the grounding fixtures instead.
Effect of CNT Additives:
The effect of CNT additives can be seen in Fig. 10. For both CFRP panels fabricated
using the same Adtech 820 resin with and without the CNT additives, they exhibited
almost no visible damage on the sample surface. The panel with CNT additives in the
epoxy showed a slight area of delamination. These damages were not expected and the
damage in the CFRP panel without CNT additives is not consistent with many of the
reported experimental data. As shown in Fig. 7, the lightning strike current was
conducted through the metallic grounding device owing to the small size of the panel
(not through the CFRP composite panel), which has resulted in the unsatisfactory test
results and extreme small damage.
Specimen
Cathode
Before test
0.23 s 0.27 s 0.30 s 0.33 s 0.40 s
Before test 0.20 s 0.24 s 0.27 s 0.30 s 0.35 s
Specimen
Cathode
Figure 9. Visual observations of the damage in carbon fiber composite laminates after 100 kA
lightning strike pulsed current waveform A impact: (a) sample with Adtech 820 resin, (b)
sample with West System resin.
Figure 10. Visual observations of the damage in carbon fiber composite laminates after 100 kA
lightning strike pulsed current waveform A impact: (a) sample with Adtech 820 resin with
carbon nanotube additives, (b) sample with plain Adtech 820 resin.
CONCLUSION
This project has shown the relevance of carbon nanotube technology as means for
lightning strike protection. Despite the unsatisfactory performance of the lightning
strike tests due to the limited size of the fabricated CFRP panel, the carbon nanotube
infused epoxy was effective at increasing the electrical conductivity and potentially
mitigating lightning strike damage to a CFRP composite. The implication of this
Delamination
& Fiber pullout
Slight damage
CFRP panel w.
Adtech 820 resin
CFRP panel w. West System 105 resin
Slight damage
Slight damage
CFRP panel w. Adtech 820 resin
CFRP panel w. Adtech 820 resin
w. CNT additives
research can be applied directly to the aerospace industry as a way to cut down on
lightning prevention weight. Replacing the heavy copper mesh with carbon nanotube
infused epoxy will save fuel cost and limit repair time from lightning strike damage.
As lessons learned, larger sizes of CFRP panels, preferably 9×9 inches, will be
fabricated in further work to eliminate the grounding fixture current conducting issue
as well as to satisfy the requirements to carry out flexural tests to determine the
residual strength after lightning strike tests.
ACKNOLEDGMENT
The authors would like to acknowledge Mr. Gregory Stewart (Aurora Flight Science,
Columbus, Mississippi, USA) for providing us the epoxy samples with carbon
nanotube additives for experimental testing.
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