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Bond Strength Degradation of Adhesive-
Bonded CFRP Composite Lap Joints
After Lightning Strike
WENHUA LIN, YEQING WANG, SPENCER LAMPKIN,
SRIHARI GANESH PRASAD, OLESYA ZHUPANSKA
and BARRY DAVIDSON
ABSTRACT1
Adhesive bonding to join fiber reinforced polymer matrix composites holds great
promise to replace conventional mechanical attachment techniques for joining
composite components. Understanding the behavior of these adhesive joints when
subjected to various environmental loads, such as lightning strike, represents an
important concern in the safe design of adhesively bonded composite aircraft and
spacecraft structures. In the current work, simulated lightning strike tests are performed
at four elevated discharge impulse current levels (71.4, 100.2, 141, and 217.8 kA) to
evaluate the effects of lightning strike on the mechanical behavior of single lap joints.
After documentation of the visually observed lightning strike induced damage, single
lap shear tests are conducted to determine the residual bond strength. Post-test visual
observation and cross-sectional microscopy are conducted to document the failure
modes of the adhesive region. Although the current work was performed on a limited
number of specimens, it identified important trends and directions for future more
comprehensive studies on lightning strike effects in adhesively bonded composites. It is
found that the lightning strike induced damage (extent of the surface vaporization area
and the delamination depth) increases as the lightning current increases. The stiffness
of the adhesive joints and shear bond strength did not show a clear correlation with the
lightning current levels, which could be due to many competing factors, including the
temperature rise caused by the lightning strike and the surface conditions of the
adherends prior to bonding. The failure modes of the adhesive regions for all specimens
demonstrate a mixed mode of adhesive and cohesive failure, which may be due to
inconsistent surface characteristics of the adherends before bonding. The energy
absorbed during the lap shear tests generally increases as the lightning current increases.
W. Lin, S. Lampkin, & S. G. Prasad, PhD students, Department of Mechanical & Aerospace
Engineering, Syracuse University, Syracuse, NY 13244
Y. Wang (corresponding author, email: ywang261@syr.edu), Assistant Professor, Department
of Mechanical & Aerospace Engineering, Syracuse University, Syracuse, NY 13244
O. Zhupanska, Professor, Department of Aerospace & Mechanical Engineering, University of
Arizona, Tucson, AZ 85721
B. Davidson, Professor, Department of Mechanical & Aerospace Engineering, Syracuse
University, Syracuse, NY 13244
37
INTRODUCTION
Fiber reinforced polymer (FRP) composites have been increasingly popular over the
past decades for their light weight as well as better mechanical performance than their
metallic counterparts. This is particularly true in the aerospace industry where weight is
at a premium. Unlike metallic structural components that can be joined by drilling holes
and applying fasteners or rivets, joining of two FRP composite parts requires additional
caution since the drilled holes induce stress concentrations that often lead to distributed
damage and delamination during normal usage. Joining two FRP composite parts with
metallic fasteners also adds weight and introduces the possibility of galvanic corrosion
between the fasteners and the FRP composites. To solve these challenges, one
promising solution is to join FRP composites with adhesives.
One critical issue of concern with the use of adhesives for joining FRP composites
is the performance and durability of the joint under different environments. Many
studies have been conducted to evaluate the effects of various environmental conditions
on the behavior of adhesively bonded joints [1-4]. For example, Grant et al. [1]
investigated the effects of temperature on the strength of adhesively bonded joints
through single lap shear tests at room temperature, -40 ºC, and +90 ºC. It was found that
at the lower temperature of -40 ºC, the adhesive was stiffer, more brittle, and exhibited
a higher bond strength than at room temperature, while at +90 ºC, the adhesive was
more ductile and exhibited lower bond strength than at room temperature. Ferreira et al.
[2] investigated the loss of static strength and the fatigue behavior for adhesive lap joints
immersed in water of different temperatures for different durations and found that the
specimens immersed in 20 ºC water showed a strength reduction of 30% after a period
of 15-45 days exposure, while for specimens immersed in 40 ºC water, a similar loss of
static strength was observed after less than 15 days of exposure. Moreover, the authors
found that the fatigue behavior was mainly dictated by the water temperature and to a
lesser degree by the exposure time. Bellini et al. [3] investigated the effects of aging
conditions (aging with air, distilled water, and salt water) and temperature (at 25 ºC, 70
ºC, and 120 ºC) on the residual bond strength of two different types of adhesives (i.e.,
AF 163-2K film adhesive and EA 9309NA paste adhesive) and concluded that for the
AF 163-2K film adhesive, different aging treatments did not significantly affect the
bond strength while the temperature greatly affects the bond strength. For example, a
65% reduction in bond strength was found for samples tested at 120 ºC. For the EA
9309NA paste adhesive, different aging treatments resulted in great variability of
adhesive bond strength. For instance, at test temperature of 70 ºC, a 50% increase and a
30% decay in bond strength with respect to the reference were found for specimens aged
with air and distilled water, respectively. Viana et al. [4] conducted bulk water
absorption tests at different moisture conditions to determine the effects of moisture
uptake on two epoxy adhesives (i.e., XNR 6852-1 and SikaPower 4720). It was found
that the adhesives’ glass transition temperatures, strengths and stiffnesses all decreased
with increasing moisture uptake.
Despite the many studies conducted to investigate the performance and durability
of adhesively bonded joints under various environmental conditions, no attempts have
been made to investigate the effects of lightning strikes on bond strength or stiffness.
High energy lightning strike events are low probability events, but when they occur,
their effects can be severe. Previous work [5-9] on lightning strike damage tolerance in
FRP composites showed that lightning strike impacts can cause significant degradations
38
of strength and modulus. For example, a study showed that carbon fiber epoxy
composites experienced reductions of 17.3% and 14.8% in tension and compression
strengths, respectively, and reductions of 6.0% and 7.8% in tension and compression
moduli, respectively, after being struck by a simulated lightning strike electric arc with
30 kA impulse current [10]. Moreover, the wreckage examination of Schleicher ASK
21 destroyed in a lightning strike accident revealed that bonded joints within the
composite wings and in the fuselage were broken along the bondlines leading to the
airframe disintegration [11]. Understanding how adhesively bonded joints respond to
lightning strikes are essential for the development, certification, and repair of composite
structures. To the authors’ knowledge, the current work is the first attempt to study the
bonding behavior for CFRP single lap joints when subjected to simulated lightning
strikes.
EXPERIMENTAL SETUP
Materials and Specimens
The carbon fiber composite coupons used in the current study were made of twill
weave SGP370-8H/8552 carbon/epoxy prepreg manufactured by Hexcel. First, a 304.8
mm by 304.8 mm carbon/epoxy composite plate was fabricated using a
[(±90/±45)4/±90] layup pattern for a total of 9 layers and a final average thickness of 3.4
mm. Note that the layup orientation is with respect to the warp direction of the woven
fabric. The layup took place on a 609.6 mm by 609.6 mm aluminum tool plate with a
304.8 mm by 304.8 mm aluminum caul plate on top and carefully vacuum bagged. After
the layup, the plate was cured in an oven at the recommended cure cycle provided by
Hexcel (no additional pressure was provided other than the vacuum pressure from the
vacuum bag). Next, the plate was cut in half to obtain two 152.4 mm by 304.8 mm plates
of which one of them was cut in half again making two 152.4 mm by 152.4 mm plates.
The two plates were then bonded together with a single lap configuration with a 25.4
mm overlap using HYSOL EA 9394 epoxy paste adhesive and cured under room
temperature in a vacuum bag. Then, the bonded plates were cut using a wet tile saw into
four single lap joint specimens with 645 mm2 (25.4 mm square) area of overlap and a
total length of 279.4 mm and width of 25.4 mm. Note that up to this point, the two
adherends of each specimen had a length of 152.4 mm while the lap shear tests
according to the ASTM D5868 standard [12] requires that the adherends have a length
of 101.6 mm. The extra length was due to the fact that the single lap joint specimens
needed to be sanded on the two ends (see Fig. 1) for reduced contact resistance during
the simulated lightning strike tests. After the simulated lightning strike tests, lengths of
50.8 mm including the sanded regions were cut off and the resultant length of the
adherends were 101.6 mm. After that, HYSOL EA 9309NA epoxy paste adhesive,
which has a higher bonding strength than the EA 9394, was used to bond glass fiber
tabs (25.4 mm by 50.8 mm) onto both ends of single lap joint specimens using a bonding
fixture to ensure the total thickness after bonding the glass fiber tabs on both ends was
consistent with the thickness of the joint region. Due to the low viscosity of the EA
9309NA adhesive, carbon fiber dust (20% by weight of the EA 9394NA adhesive) was
added to ensure a consistent thickness after bonding of the glass fiber tabs.
39
Figure 1. A schematic of the single lap joint specimen (not to scale).
The Simulated Lightning Strike Test
The simulated lightning strike tests were carried out at the High Voltage
Laboratory at Mississippi State University (MSU- HVL) which hosts an in-house
impulse current generator consisted of eight 47 µF capacitors connected in parallel.
Each of the capacitors can store 50 kJ of energy and can be charged to up to 44 kV
enabling the peak discharge impulse current of over 200 kA. Figure 2 shows the
actual setup for the simulated lightning strike tests. To achieve better grounding, all
specimens were sanded on the two ends of the composite specimens (see Fig. 1) to
remove surface epoxy exposing the carbon fiber directly. Braided wires were then
inserted and tightened onto the exposed carbon fiber layer with clamps on both ends
and connected to the ground. The tests specimens were carefully positioned to ensure
that the electrode and hence the lightning strike point was directly below the center
of the bonded region. The four specimens, namely, specimens 1, 2, 3, and 4, were
subjected to four elevated lightning impulse current levels of 71.4, 100.2, 141, and
217.8 kA, respectively. The 217.8 kA impulse current was chosen since this impulse
current represents the most severe lightning waveform A current (i.e., the initial
stroke) recommended by the existing lightning strike testing standards [13, 14], while
the 71.4 kA impulse current represents the approximate lowest current level that
would cause any visually observable damage on the surface of the adhesive joint
specimens as determined during preliminary sacrificial testing.
Figure 2. A photo of the simulated lightning strike test setup for the adhesive composite joint
specimens at MSU-HVL.
CFRP adherends
Adhesive
region
Sanded
region
Sanded
region
50.8 mm 101.6 mm
50.8 mm
101.6 mm
Adhesive Composite
Joint Specimen
Electrode
Grounding
Braided
wires
Bonded
adhesive region
40
Damage Characterization
To document results from the simulated lightning strike tests, photos were taken
with a HD camera to characterize the visible surface damage. Photos were also taken
with the HD camera after the single lap shear tests to characterize the failure modes
of the adhesive regions. After the single lap shear tests, the upper CFRP composite
adherend with lightning strike induced damage was cut along the width and at the
center of the damage for each failed specimen. Since the primary interest is to
determine the interlaminar delamination depth at the lightning strike damage regions,
the cuts were made to expose the cross section at the center of the lightning strike
damage regions without mounting or polishing. The microscopy imaging was then
conducted on the cross section using a Hirox KH-8700 digital microscope with auto
3D tiling function which achieves automatic focusing and joining of several
magnified pictures along the width direction.
Lap Shear Test Setup
The lap shear tests were carried out according to the ASTM D5868 [12] standard
with an MTS testing system using an 89,000 N capacity load cell calibrated at 9,000
N load. The loading rate was 2.54 mm/min. A self-tightening grip was used for the
testing as shown in Fig. 3. The bottom and the top self-tightening grips were aligned
before tightening onto the load frame. This ensured that the specimens did not
bend/warp while positioning the specimen in the fixture. The mesh on the grips which
holds the specimen in the fixture were cleaned and any dirt particles were removed
to reduce any slip between the specimen and the grips. The test specimen was
positioned vertically prior to loading. The bonded glass fiber composite tabs have a
length of 50.8 mm. Each CFRP composite adherend is 101.6 mm long and bonded
with 25.4 mm overlap, making the total length of the single lap joint specimen 177.8
mm (see Figs. 1 and 3).
Figure 3. Example of a positioned testing specimen prior to loading in the lap shear tests.
Self-tightening Grip
Self-tightening Grip
Test Specimen
50.8 mm
101.6 mm
25.4 mm
overlap
101.6 mm
41
RESULTS AND DISCUSSION
Lightning Strike Induced Damage
Figure 4 shows the visual observation of the lap joints after the simulated
lightning strike tests at the bonded region (images show the surface of the upper
CFRP composite adherend that was directly exposed to the lightning arc). From left
to right are specimens 1, 2, 3 and 4, which were subjected to elevated discharge
impulse currents of 71.4, 100.2, 141, and 217.8 kA, respectively. An increasing trend
of the resin vaporization areas can be observed which can be explained by the increase
in the amount of Joule heating produced in the CFRP composites.
Figure 4. Regions of resin vaporization for specimens 1, 2, 3 and 4 (from left to right).
Figure 5. Microscopy images showing the cross sections of the region with resin vaporization and
delamination (in the upper CFRP composite adherends that were directly exposed to the lightning).
Figure 5 shows the cross sections of each specimen along the width and at the center
of the observed surface damage. It can be seen that for specimens 1 and 2, only surface
resin vaporization can be observed, whereas for specimens 3 and 4, interlaminar
delaminations were found between the first and second ply and between the second and
third ply, respectively. In Fig. 5, regions of voids can also be observed. These are
distinguished from the interlaminar delaminations by the following criteria: 1)
42
interlaminar delaminations are continuous from the lightning strike site and are
accompanied by fiber damage; 2) the surrounding fiber and layers that enclose a void
are intact, hence there is no deformation upon a small applied force, i.e., by pinching or
poking, whereas for regions with interlaminar delamination and resin vaporization,
deformations or collapsing could be observed upon applying small forces.
Lap Shear Test Results
The load vs. displacement curves for all test specimens are shown in Fig. 6. It can
be observed that the load increased nonlinearly until approximately 2.5 mm of the
displacement. This nonlinearity is caused by the bending of the specimen due to the
inherent offset in the load path for single lap shear tests. When the displacement
exceeds 2.5 mm, the specimen bends sufficiently so that the grips align, and
thereafter, the load-displacement curve becomes nearly linear. Such nonlinearity has
also been reported in existing studies [15-17] for single lap shear tests. The load drop
to zero occurs immediately after failure. The overall stiffness of the adhesive joints
was calculated using the slope of the linear region of the load-displacement curve
(i.e., between the displacement of 2.5 mm and the displacement-to-failure). Figure 7
shows a comparison of the stiffnesses calculation for the four specimens. It can be
seen that specimen 1, which was subjected to the lowest lightning current of 71.4 kA,
showed the highest stiffness whereas specimens 2 and 3, which were subjected to the
intermediate lightning currents (100.2 and 141 kA, respectively), showed the lowest
stiffness. Specimen 4, which was subjected to the highest lightning current of 217.8
kA, showed a stiffness lower than that of specimen 1 but higher than the stiffnesses
of specimens 2 and 3. From the results of specimens 1, 2, and 3, one may suggest that
increasing the lightning current can significantly reduce the stiffness of the adhesive
joints. However, the result of specimen 4 does not support this conclusion.
Figure 6. Load vs. displacement curve obtained from the lap shear tests for all specimens.
0
1000
2000
3000
4000
5000
6000
7000
8000
00.5 11.5 22.5 33.5 44.5
Load (N)
Displacement (mm)
Specimen 1 (71.4 kA)
Specimen 2 (100.2 kA)
Specimen 3 (141 kA)
Specimen 4 (217.8 kA)
43
As for the displacement-to-failure and failure loads, based on the results for
specimens 2, 3, and 4, one may suggest that the displacement-to-failure increases as
the lightning current level increases, but the data from specimen 1, which was
subjected to the lowest lightning current level, does not support this conclusion. More
tests are required to eliminate the influence of statistical variability and confirm
whether an increase in the lightning current level leads to an increase in the
displacement-to-failure and a decrease in the stiffness. If confirmed, this would imply
that the ductility of the adhesive joints increases with the lightning current level. It
has been reported in the literature that adhesive subjected to a higher temperature
becomes less stiff and more ductile [1].
Figure 7. A comparison of the overall stiffness of the four adhesive joint specimens.
To provide a better understanding on the problem, Figure 8 provides a schematic
illustration of the lightning strike effect on the composite adhesive joint. When a
lightning arc strikes the surface of the adhesive joint (i.e., the upper CFRP composite
adherend), the lightning energy will be dissipated via discharging the high intensity
electric current through the composite material. The free-moving electrical charges
search for the least resistant path through the composite material to form an electric
circuit between the cloud (or the electrode used in the simulated lightning strike tests)
and the ground. Substantial Joule heating will be generated along the conduction path
of the high intensity electric current, leading to a rapid temperature rise in the
composite. Due to the temperature gradient, the heat will flow from the bottom of the
upper CFRP composite adherend to the adhesive layer, potentially affecting the
adhesive properties.
To provide a rough estimate on the temperature, we can refer to the delamination
damage in the upper CFRP composite adherend. As shown in Fig. 5, the most severe
interlaminar delamination occurred between the second and third ply of specimen 4.
It is widely recognized that the delamination in CFRP composites is caused by the
thermal decomposition of the epoxy resin between interlaminar layers [18, 19]. Note
that the onset temperature for the decomposition of epoxy resin is about 300 °C [18,
19]. Since the most severe delamination only reaches the third layer of the 9-layer
2700
2800
2900
3000
3100
3200
3300
3400
3500
Specimen 1
(71.4 kA)
Specimen 2
(100.2 kA)
Specimen 3
(141 kA)
Specimen 4
(217.8 kA)
Stiffness of Ahesive Joints (N/mm)
44
laminate, it is reasonable to expect that the temperature at the bottom of the upper
CFRP composite adherend did not reach 300 °C. For the epoxy adhesive used, the
onset decomposition temperature is also about 300 °C. Thus, it is more than likely
that the temperature in the adhesive layer did not reach the onset temperature for
thermal decomposition. At the same time, it is well known that rapid degradation in
the mechanical properties of polymers occurs above the glass transition temperature
[20-22], which is much lower than the onset temperature for thermal decomposition.
Thus, temperature fields in the adhesive joints subjected to the lightning strike and
associate changes in the mechanical properties will need to be carefully investigated
in the future through experimental tests and numerical modeling.
Figure 8. A schematic showing the lightning strike effect on the composite single lap adhesive joint.
In addition, the load vs. displacement curves were analyzed to determine the shear
strength and the energy absorbed by each specimen. The shear strength was
calculated by dividing the peak load by the overlapping bond area (i.e., 645 mm2)
while the energy absorbed was calculated by finding the area under the load vs.
displacement curve. Figure 9 presents the calculated shear strengths for all
specimens. The magnitudes of these calculated values, as well as for the energy
absorption of the adhesive joints (Fig. 10), are very close to what has been reported
in the literature [15]. The shear strengths of the four specimens presented in Figure 9
do not show a clear correlation with the lightning strike current levels. Rather, the
shear strength drops when the lightning current increases from 71.4 kA to 100.2 kA,
but increases when the current increases from 100.2 kA to 141 kA and from 141 kA
to 217.8 kA. Although lightning strike causes a temperature rise in the composite
specimen and potentially leads to a degradation of the adhesive bond, the bond
strength also depends on other factors, such as the surface preparation prior to
bonding, the consistency in the geometry of the spew fillets of the adhesive joints,
and the uniformity of the mixing of the carbon dust into the adhesive, as well as that
of any associated resulting porosity. As will be evidenced in what follows, in this
study it is likely that the surface preparation was inadequate, as the full surface
preparation procedure described in the ASTM D2093 standard [23] was not followed.
Instead, only surface cleaning of the composite adherends using denatured alcohol
was conducted prior to bonding. As will also be discussed subsequently, it is
additionally possible that inconsistencies in the spew fillets and/or adhesive mixing
also contributed to the observed results.
CFRP composites
Adhesive
Lightning current
Heat flux flowing from top
adherend to the adhesive
Lightning electric arc
45
Figure 9. A comparison of shear strength for all specimens.
Figure 10 shows a comparison of the energy absorbed for all specimens. It can be
observed that the energy absorbed generally increases with increasing lightning
current, except for specimen 1, for which the energy absorbed is much higher than
those for specimens 2 and 3 but is slightly lower than that for specimen 4. For
specimens 2, 3, and 4, the values of energy absorbed show an approximate linear
relationship to the lightning strike currents. The increase of the energy absorbed when
the lightning current increases could be due to the increased ductility of the adhesive
joint, as shown in Fig. 6. The inconsistency between the results for specimen 1 and
those for the remaining specimens could be due to the inadequate surface preparation
of the CFRP composite adherends or inconsistencies in the geometries of the spew
fillets of the adhesive in the various joints. It is also possible that nonhomogeneous
mixing of the carbon fiber dust caused specimen-to-specimen variations in what was
essentially a particulate reinforced composite adhesive layer.
Figure 11 shows photos of the adhesive joints for all specimens after the lap shear
tests. It can be observed that all specimens presented a mixed mode of adhesive and
cohesive failure. No adherend failure (e.g., fiber tear failure or stock-break failure)
was observed. Specifically, specimens 1 and 4 have clear adhesive failures towards
the end of the bonded regions while specimens 2 and 3 have adhesive failures near
the center of the bonded regions. These specimen-to-specimen variations are
consistent with the previous comments regarding problems with the surface
preparation, inconsistencies in the spew fillets, and/or inconsistencies in the adhesive
itself between the four specimens.
0.00
2.00
4.00
6.00
8.00
10.00
12.00
Specimen 1
(71.4 kA)
Specimen 2
(100.2 kA)
Specimen 3
(141 kA)
Specimen 4
(217.8 kA)
Shear Strength (MPa)
46
Figure 10. A comparison of energy absorbed for all specimens.
Figure 11. Failure modes of the adhesive regions for specimens 1, 2, 3 and 4 (from left to right).
CONCLUSION
The current work investigated the bond behavior of CFRP composite single lap
adhesive joints subjected to four elevated level of discharge impulse currents (71.4,
100.2, 141.0, and 217.8 kA). Visual observations and microscopy imaging were
performed to analyze the lightning strike damage, including the resin vaporization
areas and the extent of interlaminar delaminations. Lap shear tests were conducted to
characterize the behavior of the adhesive joint after lightning strike tests.
The exploratory nature of this study clearly limited the conclusions that could be
made. As expected, it was found that the lightning strike damage increased as the
lightning current increases. The effect of the elevated lightning strike current levels
on the shear bond strength and the stiffness of the adhesive joints did not show a clear
correlation, which could be due to the individual or coupled effects caused by
inadequate surface preparation prior to bonding, inconsistent spew fillets, and/or
0
1
2
3
4
5
6
7
8
9
10
Specimen 1
(71.4 kA)
Specimen 2
(100.2 kA)
Specimen 3
(141 kA)
Specimen 4
(217.8 kA)
Energy Absorbed (J)
25.4 mm
47
inconsistencies in the adhesive itself among the four specimens. With the exception
of one specimen, the energy absorbed prior to failure of the lap shear joint generally
increased as the lightning current increased. All specimens presented a mixed mode
of adhesive and cohesive failure. No adherend failures were observed.
This exploratory study was apparently the first of its kind to evaluate the effects of
lightning strike on the behavior of adhesive joints. Future studies will clearly require
more test replicates at each condition, including control specimens not subjected to any
lightning strike. Future work should monitor bond line temperatures during the lightning
strike event (e.g., using a high-speed infrared thermal camera) and investigate the effect
of adherend thickness to see whether the bond line temperature increase is a major
consideration in strength or stiffness loss. This will allow the lightning impulse current
and adherend thickness to be correlated with adherend damage, bond temperature,
strength and stiffness, and joint failure mechanisms, and will lead to an improved
understanding of this important issue.
ACKNOWLEDGEMENT
The authors would like to acknowledge Mr. Kamran Yousefpour and Dr.
Chanyeop Park at the High Voltage Lab of Mississippi State University for their
assistance with the simulated lightning strike tests. W. Lin, S. Lampkin, & Y. Wang
also thank the support from the Oak Ridge Associated Universities.
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