Recent Developments in Techniques to Minimize Lightning
Current Arcing Between Fasteners and Composite Structure
Hasim Mulazimoglu
Alcoa New Product Development, Aerospace Products
900 Watson Center Road, Carson, CA 90745
Luke Haylock
Alcoa New Product Development, Aerospace Fasteners
3000 W. Lomita Blvd., Torrance, CA 90505
Abstract
Lightning protection of composite structures, which are extensively used on newer composite aircrafts, is more
complex due to the inherent high resistance of carbon fibers and epoxy, the multi-layer construction and the
anisotropic nature of the structure. On the other hand, the intrinsic high conductivity of metallic fasteners and the
large number of fasteners used in aircraft construction combine to create a condition of a high probability of
lightning attachment to fasteners when they are used for joining composite structures. It is well know that lightning
currents may create detrimental ignition sources by attaching to a fastener and flowing through the fastener to
some point, which may result in arcing between fasteners and the composite structure. The danger from the
lightning strikes is particularly worrisome near the composite fuel tanks since fastener arcing can create an
ignition source for fuel vapor.
In this paper, the basic mechanisms pertaining to the fastener arcing are described including the lightning current
flow behavior at fastener/composite interface before discussing how the lightning protection and countermeasure
techniques evolved over the years. In addition, modeling and simulation methods to predict the current flow
pattern between fasteners and composite structure are presented. Finally, the recent techniques to mitigate the
fastener arcing are reviewed in details, including the use of sleeved fasteners for joining carbon fiber composite
structures and how these newly developed fasteners can be incorporated into the design of the aircraft joints.
I.
Introduction
Continuous fiber reinforced composites are seeing
an expanded use in the design of aircraft
components for variety of applications where the
light weight, higher strength and corrosion
resistance are primary concerns. Composites are
typically made up of fine fibers such as carbon or
glass that are oriented at certain directions and
surrounded in a supportive matrix material. Although
a wide variety of matrix materials are commercially
available, elevated temperature cured epoxy resins
are by far the most commonly used. In most
component design, the plies of the composite
material are arranged at a variety of angles
depending on the direction of major loading. This
manufacturing technique produces a stacked
laminated structure which is highly anisotropic and
structurally inhomogeneous. It is well established
that the composite structures in aircrafts are more
susceptible to the lightning damage compared to
metallic structures [1-4]. Metallic materials such as
aluminum are excellent conductor of electricity and
able to dissipate the high currents resulting from a
lightning strike. The high conductive properties of
aluminum allow lightning currents to conduct through
structure with relatively few adverse effects since
most of the lighting current remains on the exterior
skin of the aircraft. On the other hand, lightning
current cannot dissipate fast enough throughout a
composite skin resulting in significant current
penetration into the substructures such as ribs and
spars. This may lead to voltage drops across
connected structures high enough that potentially
can result in arcing. Additionally, since composite
airframes cannot readily conduct lightning current
away, as the traditional metal ones do, they are
more prone to severe damage from lighting if proper
protection methods are not in placed. Carbon fibers
reinforced plastics (CFRP) are 2000 times more
resistive than aluminum to the flow of current.
Similarly epoxy, which is often used as a matrix in
conjunction with carbon fibers, is 1 million times
more resistive than aluminum [1].The main danger
for airplane designers guard against is sparking
inside the wings which serve as the main fuel tank.
Consequently, lightning protection of composite
structure is more complex due to the intrinsic high
resistance of carbon fibers and epoxy, the multilayer construction and the anisotropic nature of the
structure [5]. It is a well established that fasteners
are primary pathways for the conduction of the
lightning currents from skin of the aircraft to
supporting structures such as spars or ribs and poor
fastener arcing or sparking from the lightning strikes.
It is more worrisome if the lightning occurs near the
composite fuel tank since the fastener arcing may
occur, creating a ignition sources for the fueled
volume.
Generally, lightning current and fasteners interact in
two different means. The most common mode is the
direct attachment of the lighting to the exposed head
of the individual fasteners as illustrated in figure 1a.
In this case, the struck fastener experiences very
high levels of current up to 200,000 amps and it
carries the bigger portion of this lighting current than
the neighboring fasteners in the same row and
transfers the current to the surrounding structure. If
the surrounding structure is aluminum, the current in
the struck fastener is usually only 10 to 30 % higher
than the adjacent fasteners and the current density
is typically low enough to cause any sparking at
fasteners [1]. If the skin is made of CFRP, however,
the struck fasteners may experience substantially
more current than the neighboring fasteners due to
a) Direct attachment
electrical contact between the fastener body and the
parts
of
the
structure
can
lead
to
high resistivity of CFRP. Lightning current that flows
along the body of the struck fastener could be high
enough to generate hot particles or gases that may
be ejected from the struck fastener into the fuel tank
thereby creating a hazard. The second mean of
fastener/lighting interaction is that fasteners could be
in the pathway of flowing lightning currents that enter
to the aircraft structure at various points on the skin
other than fasteners as shown in figure 1b. In this
case, the fasteners located in the same row or
nearby share lighting current. Although this lowers
the current density per fastener, it still can be
sufficiently high to cause arcing/sparking of the
fasteners. As a result, there is a potential
susceptibility to sparking/arcing inside the composite
fuel tank from these fasteners as very high lightning
currents can enter the skin and substructures
components of the fuel tank via the fasteners.
Figure 2 illustrates the potential effects of direct
lightning attachment to the head of a fastener
installed on a composite “T” joint resulting in
sparking.
b) Current transfer via fastener
Figure 1: Lightning and fastener interaction modes.
Figure 2: Photographs showing the fastener sparking during lightning test.
One of the methods to improve lightning current
protection of the composite material is to introduce
metal meshes and/or foils near the outer surface of
composite structures during the fabrication of
panels. Aluminum, copper or bronze foils or meshes
are adhered during the process for manufacturing
the composite structures to the outer face which will
receive the direct lightning strike. Current technology
already exists for fabricating panels with this
protection technique [6,7]. Although this technology
assists maintaining the lightning current closer to
external surface of the skin, it does not diminish the
risk of fastener arcing. It is important to emphasize
here that composite parts still need to be drilled in
order to facilitate joining the components using
mechanical fasteners. Drill holes can create
disruption to the flowing lightning current throughout
the CFRP structure and lead to increased current
path and higher resistance around the drill holes.
In addition to their lightning challenges, machining
fastener holes in composite materials can be quite
problematic. While machining ductile materials such
as steel or aluminum results in shearing of the
material and the formation of consistent chips,
machining of composites involves a more complex
mechanism with fracture of the fibers under
compression or bending combined with shearing of
the matrix material [8,9]. Drilling fastener holes in
composite does not compare to the uniformity of
aluminum or steel, which can be expected to remain
consistent from a drill's entry point to its exit.
Individual carbon fibers fracture at irregular angles
and form microscopic voids between the sleeve and
the hole. As the cutting tool wears there is an
increase of surface chipping and an increase in the
amount of uncut fibers or resin and delamination
[10]. The most common defects observed in drilled
composite holes are fiber fractures, fiber pull-outs,
chipping, drill entry and exit delaminations, voids
and pitting [11-14]. The composite microstructure
containing such defects is referred to as machining–
induced micro texture. Although there have been
substantial amount of works done on the effects of
machining induced defects on the mechanical
strength of the composite structures there is not
much research done on the effects of such defects
on the lightning current behavior of a composite
joint, fastener arcing in particular [15-17].Enhanced
machining techniques could improve machininginduced micro texture and reduce the development
of microscopic voids [18, 19].
In recent years, there have been a large number of
techniques developed in order to minimize or
mitigate fastener sparking as the use of composite
material in commercial aircrafts was dramatically
increased. Figure 3 presents the data from our
recent survey of the number of patents issued
and/or applied by decades since 1980. It is obvious
that continuing surge of the issued/applied patents in
this field since the start of the new century coincides
with the beginning of design activities of mostly
composite commercial aircraft programs. These new
research studies are expected to continuity with
similar rapid pace for the next couple of years.
Figure 3: Survey of data on the number of patents issued and/or applied by decades since 1980
In this paper, first the basic mechanisms of the
fastener arcing are described including the lightning
current flow behavior at fastener/composite interface
before discussing how the lightning protection and
countermeasure techniques evolved over the years.
Furthermore, the recent techniques to mitigate the
fastener arcing are reviewed in details, including the
use of sleeved fasteners for joining carbon fiber
composite structures and how these newly
developed advanced fasteners can be incorporated
into the design of the aircraft joints.
Finally, an example of modeling and simulation
methods to predict the current flow pattern between
fasteners and composite structure is presented.
II.
2.1.
Discussion
Macrostructure analysis of lightning damage
When the lightning attaches to a fastener head, the
evidence of lightning damage is typically limited to
the areas surrounding the struck fastener. This is
consistent with the fact that there is a current
conductance path from fastener head into the skin of
the composite laminate. Typical damage to the test
panel after the lightning testing is shown in Figure 4.
As it can be seen in this figure, there is a significant
burn of the composite panel around the fastener and
partial melting of the fastener head at the top side of
the test panel whereas no damage is found at the
collar/nut side. In some cases, there is visual soot
marks radiating from the collar/nut base. The
damage area on the top plate is typically about 25
mm in diameter at the strike zone. The delamination
of the composite and burning of epoxy and copper
mesh are the main types of damage found. Such
damage is expected due to the fact that the
composite panels have a poor electrical conductivity
and can be damaged structurally by high current
flow through the fibers and overheating the epoxy
matrix. Some efforts have been made to quantify
this damage to CFRP panels due to lightning strike
using non-destructive inspection techniques [2,4,
20].
Figure 4: Typical view of the test panels after lightning strike
2.2. Microstructure analysis and arcing phenomena
The sparked fasteners generally exhibit heavy deposits around the base of the nut/collar as shown in figure 5.
These deposits consist principally of polysulfide sealant with small amount of fastener droplets and carbon fiber
particles as determined by energy dispersion spectrometer (EDS) chemical analysis [21]. The presence of
fastener material and fiber particles in these deposits is a clear evidence that these deposits are originated from
arcing between the fastener and the hole surface and carried by the hot gases and ejected from fasteners by the
arcing pressure.
a) Deposit
b) Fastener OD surface
c) Residual Sealant
d) Arcing
Figure 5: SEM micrograph of the collar deposit and arc spots along the fastener outer surface
Another factor that is believed to be playing an
important role during occurrence of fastener arcing
is the drilling induced damages on the machined
fastener hole. Surface topography examination of
the fastener holes showed that average surface
roughness of drilled aluminum hole is 5-6 µin
whereas in composite it is 120-150 µin, indicating
substantially rougher and irregular finish for
composite holes. SEM micrographs taken from the
surface of the representative holes drilled on
unidirectional CFRP are shown in Figures 6 and 7.
As can be seen in these figures, subsurface
delamination and pitting due to fiber pull-outs are
evident along the hole axis resulting from the fiber
fracture during drilling. Delamination and the pitting
were the predominant damage forms observed and
were found to be of about 100 micron in size. As
reported in earlier studies the comparatively low
a) Composite
interfacial shear strength of CFRP results in
delamination along the fiber-matrix interface and
bending induced fiber fracture beneath the drilled
surface [11-14]. Pitting and craters were created on
the worked surface of the matrix of CFRP at
positions shown by where the cutting direction is 45
degrees relative to the extending direction of carbon
fibers. During a previous work [22], it has been
found that such defective hole texture indeed leads
to the arcing between sleeve and the CFRP panels.
Photos presented in Figure 5 illustrate typical arcing
evidence between the fastener and the CFRP
structure. Arc pits are randomly distributed and do
not appear to be related to any geometric feature of
the fastener. Arcing appears to be associated with
residual sealant on the fastener outer surface and
the drilling induced voids along the axis of the
composite hole.
b) Aluminum
Figure 6: Drilling texture of the bore surface of composite versus aluminum
a) Delamination
b) Pitting due to fiber pull-out
Figure 7: SEM micrographs from the surface of the fastener hole drilled on the directional CFRP.
In addition, individual carbon fibers fracture at
irregular angles and form microscopic voids between
the fastener and the hole. As the cutting tools wear
there is an increase of surface chipping and an
increase in the amount of uncut fibers or resin and
delamination. The photographs shown in Figure 8
fastener
illustrate the contact between a conventional
fastener and the composite structure. It shows the
typical voids formed between the fastener and the
composite structure with entrapped sealant, marked
by a red circle in the micrographs.
fastener
Figure 8: SEM pictures taken from the cross section of the installed fastener showing the voids
Although enhanced machining techniques [18,19]
could improve the drilling induced defects, as a
result of the drilling technology available today and
the inherent heterogonous structure of the CFRP,
such micro level damage described above is still
unavoidable in the current manufacturing practice.
2.3 Strategies to mitigate arcing
The protection of aircraft fuel systems against fuel
vapor ignition due to lightning is one of the primary
task for designers and engineers. Since commercial
aircraft contain relatively large amounts of fuel and
also include very sensitive electronic equipment,
they are required to comply with a specific set of
requirements related to the lightning strike protection
in order to be certified for operation [23]. Generally
multilayered approaches are taken to lightning
protection of the fuel tank during the design stages
of aircraft program. Several of these protection
strategies related to the fasteners used in composite
fuel tank applications are discussed in following
sections.
2.3.1 Fastener isolation
Lightning is known to be attracted to external
fasteners on the skin and the struck fastener and
neighboring fasteners carry the lighting current to
the substructures which can spark at the gaps.
Fasteners themselves may also arc while they are
transferring the current. In this protection approach,
the fastener heads are isolated to avoid any direct
lightning attachment and the lightening is diverted
away from the fasteners to other structural members
which can tolerate lighting currents. Since the
introduction of the idea by Amason in 1975 [24],
more works has been done in this area to develop
various methods for electrically isolating the
fasteners
[25-27]. The most common method
involves covering the head of each fastener using a
barrier dielectric patch as described by Covey [27]
and others [28,30]. The dimensions of this patch
which is shown in figure 9, is chosen sufficiently
large enough to cause the lighting current to diffuse
mostly before reaching the fastener thereby
minimizing the risk for fastener arcing. Further
improvement can be achieved by filling the gaps
between the fasteners head and the countersink
hole with dielectric filler material as demonstrated by
Heeter et al. [31] and Winter et al. [32]. Le et al. [33,
34] has also shown that adhesively bonded multilayer arrangements of conductive and dielectric plies
are used to cover metal surface features such as
skin fasteners completely wherein the current from
lightning strike is dispersed away from the fasteners
with the aid of conductive plies over the dielectric
plies.
Additional protection methods described by Pearson
[35] and Bannink Jr. et al.[25,26 ] include further
electrically isolating metallic fasteners from skin.
These are accomplished by applying a dielectric
coating to any surface portions of fasteners that
substantially contact with skin or applying a glass
fiber insert around the fastener shank to fill the gap
between fasteners and composite. It has been
shown that such non-conductive insert keeps the
fastener becoming a preferred lightning current path
and prevent current from entering composite through
metallic fastener. This electrical isolation prevents
arcing between an external skin and metallic
fastener. The protection against arcing provided by
the electrical isolation is in addition to substantially
redundant with protection provided by confining
lightning strikes to portion of the outer surface of the
skin and diffusing current flow from such strikes.
It was also proposed to use of fasteners made
entirely from dielectric materials where the load on
fasteners is sufficiently low [35]. This approach,
however, may not be practical since the load levels
are quite high in most airframe joints, requiring
stronger metallic fasteners.
Figure 9: Use of dielectric patch to isolate fasteners
2.3.2. Enhanced current conduction
Uses of sufficiently large size fasteners and/or large
number of fasteners were suggested by A. Plumer to
lower the current density of individual fastener below
a predetermined arcing threshold [1, 36].
Experimental studies were performed to determine
this threshold current level of typical aerospace
fasteners. These tests indicated that the spark
threshold current levels of individual fastener are
about 5 kA [1]. Current densities in fasteners
installed on aluminum aircraft structures are below
this threshold level due to high conductivity of
aluminum structure and large number of fasteners
employed in those structures thereby sharing the
lightning current more efficiently. It is important to
note that the lightning current tends to concentrate in
the fasteners closest to point of attachment,
especially in the case of fasteners installed in a
CFRP. For this reason additional improvement in
electrical conductivity by using a thicker wire mesh
or strip such as bronze or copper material
overlapping the LSP wire mesh along the rows of
the fasteners was suggested by De La Fuent De
Ana et al. [37]. These thicker mesh/strips (Figure 10)
which are referred as bolt-line mesh are placed up to
a minimum of 50 mm to both sides of the rows of the
fasteners. It has been suggested that bolt-line
meshes allow increasing the exterior metallic cross
section and optimizing electrical continuity around
the fasteners by providing a better bonding between
fasteners and bolt-line. Additionally, bolt-line meshes
improve electrical current dispersion among the
fasteners hence reducing the current density of each
fastener and minimizing the risk of fastener sparking.
Figure 10: Fasteners with bolt-line mesh configuration
As mentioned earlier, drilled holes cause the
disruption of electrical continuity of the LSP mesh. In
order to re-establish the electrical continuity of the
composite skin and to ensure the presence of
electrical contact between fastener heads and mesh,
a metal spraying process was described by
Sanchez-Brunete Alvarez et al. [38]. According to
their description, this process ensures the electrical
continuity allowing that most part of the discharged
current is conducted over the surface, limiting the
current conducted into substructures throughout the
fasteners and reducing the subsequent risk of
sparks or hot spots. Brown et al. [39] has proposed
use of a conductive hot-melt fastener cap that can
be placed over countersunk fasteners for electrical
continuity and smooth surface finish. It was also
claimed that this type of fastener filler permits quick
maintenance and field-level repairs because of
relatively short cure time of the filler material as
compared to the dielectric patches.
2.3.3. Sleeved fasteners
a) Flush head
b) Protruding head
Figure 11: Sleeve fasteners with different head configurations
Sleeved fasteners were specifically developed for
composite application where they are installed with
close-fitting sleeves in the drilled holes to improve
electrical conductivity and fatigue life of joints. Figure
a) after sleeve insertion
11 presents sleeves fasteners with two different
head configurations. After the sleeve is placed in the
hole, the interference-fit pin pulled into the sleeve as
illustrated in figure 12.
b) after completed installation
Figure 12: Installation sequence of sleeved fasteners
This expands the sleeve radially to provide an
interference fit between sleeve and composite and
minimizes the risk of damage to the composite
structure around the holes which can occur if the
interference-fit pins are forced into un-sleeved holes.
Sleeve fasteners do not have to conduct the full
lightning current. Instead, they ease the flow of
current into CFRP since the interference fit provides
more evenly distributed contact between the
fastener and the wall of the hole and distribute
electrical current more uniformly through the wall
and/or internal structure. Since the interference fit
also minimizes the air gap between the fastener and
structure it may reduce or prevent arcing between
the fastener and the wall and/or the internal
structure. This fastener configuration was shown to
reduce the intensity of arcing at fastener/panel
interface and allows increasing the arc threshold
levels of the fasteners installed on composite
structures [21,22]. When the contact area is
increased, the current density through the fastener is
reduced which in turn decreases the intensity of the
arcing at fastener/wall interface and potential of
ejection of any hot particles from the struck
fasteners.
2.3.4. Arc containment
The techniques explained above may prevent
ignition at low to moderate current levels, but at
higher current levels fastener arcing may still occurs.
For this reason, it is generally necessary to employ
as barriers between the arc source and fuel vapor
areas. There has been several different techniques
were reported in literatures[1, 40-43] but the most
common method of containing the arc products
generated at the fasteners is to coat the colar/nut
side of exposed fasteners with fuel tank sealant.
Figure 13 shows the basic principle of arc
containment with the use of sealant.
Figure 13: Arc containment method using fuel tank sealant.
It is important to note that sealant itself does not
mitigate fastener arcing but contains the arc
products, so that they cannot reach to flammable
fuel vapor inside fuel tank.
Another way to reduce this danger is to use a
special type of fasteners that provides a pressure
seal. Examples of this type of fastening systems
were described in several different patents. Morrill et
al. [42,43] has described a washer sealing assembly
for internal lightning protection. The washer may
also include dielectric rings on both sides. When it
used with a nut, the washer seals the fastener hole
and contains any sparking and hot gasses that may
arise along the fastener hole from entering the
structures. Additionally, the dielectric rings
electrically isolates the metal collar or nut from
underlying metal or CFRP substructure thereby
preventing arcing between nut/substructure interface
due to poor contact. Jones [41] has developed a
fastener system comprising separate nut and bolt
portions and a cage member. The nut is captively
mounted inside to cage. When it is seated against a
substructure, cage provides adequate seal and
prevents arc products entering into fuel tank. Covey
[40] has suggested use of a specially design gas
filled-cap around installed fastener. The volume of
gas is sufficiently large enough to absorb arc
products and hot gases that may be present
resulting
from
fastener
arcing.
3.3.5 Conforming fasteners
The newly developed fastener system is a sleeved
fastener consisting of a core pin and a conformable
sleeve. The sleeve OD surface is plated with low
hardness but highly conductive materials such as
gold, silver or nickel. The finish texture of the sleeve
surface is adjusted to provide a surface roughness
(Sa) value greater than 0.34 micron in order to
increase the level of conformity. The sleeves are
specifically designed to conform to the machine–
induced micro texture inherent in fastener holes
drilled in composite. It has been reported that the
conforming sleeved fasteners were spark free during
the lightning testing where they were exposed to 100
kA current [ 21, 22]. Figure 14 illustrates the typical
evidence spark-free conforming fastener detected by
the chamber cameras. It is important note here that
the orange dot on the spark-free fastener
photograph is not an actual spark but it is the
reference light placed in the dark chamber to ensure
the proper operation of the light detection systems.
Figure 14: Photographs showing the spark-free conforming fastener during lightning test
The following photographs shown in Figure 15
illustrate macro level conformance between the
sleeve and the composite structure. Figure 16
shows the imprints of individual carbon fibers clearly
visible on the OD of installed sleeves indicating
micro-level conformance between the sleeve and
a) sleeve head
the panel hole as a result of micro level deformation
of the conforming sleeve. It is believed that the
conforming sleeve provides a much better intimate
contact between the sleeve and individual carbon
fiber as it deforms to fill the microscopic machining
induced voids.
b) sleeve shank
Figure 15: SEM micrographs showing the macro level conformity between the sleeves on the hole
a) perpendicular to fibers
b) tangent to fibers
Figure 16: SEM micrograph showing the carbon fiber imprints on the OD of sleeves
It is suggested that the conformable sleeve deforms
into the small voids that are created during drilling of
the composite. As the sleeve deforms into the void it
displaces the entrapped sealant. The insertion of the
core pin causes the excess sealant to be extruded
outside the sleeve/composite interface hence the
conformable sleeve excavates excess entrapped
sealant during installation of the fastener while
bringing the sleeve in intimate electrical contact with
the composite structure. This increased contact
surface decreases current density and the voltage
drop across the sleeve/composite interface. It is
obvious that the current will only flow through the
graphite fibers embedded inside CFRP structure
since it has a much higher conductivity compared to
the epoxy matrix. Therefore enhanced contact to
graphite fiber provided by confirmed sleeves have a
profound effect on the current transferring capability
of the fasteners. This leads to more efficient current
transfer from fastener to the panel and minimizes
the dielectric effect caused by the sealant thus
reducing the possibility of arcing between the sleeve
and the composite panel. That means that
conforming fasteners can tolerate significantly higher
threshold current than 5 kA. The conforming sleeve
can be achieved in a variety of ways, which some
are more suitable for particular structures. The
conformable sleeve could simply consist of a soft
sleeve such as titanium sleeves while in another
conforming the sleeve may consist of a hard base
material with a soft conformable coating. The
coating can be selected from a group of relatively
soft, conductive, metallic materials which are known
to be galvanically compatible to composite structure.
These materials include gold, silver, nickel, copper
and titanium. It is also possible to use various alloys
and the best material for a particular application
generally involves a trade-off between cost and
performance.
3.3.6. Conductive Fastener coatings
Another important subject to discuss is the effect of
coating on fastener sparking during the event of
lighting strike. Although the best approach for
enhanced electrical current flow is to have metal-tometal contact. This is quite often not practical since
fasteners needs to be coated for installation
purposes and/or for corrosion protection. Since most
of these coatings are dielectric they can be
problematic with respect to current conduction within
the fastener components and/or fastener to
structure. Therefore, there have been some efforts
to develop alternative conductive fastener coating
for further improvement of the current threshold of
fastener systems. Recently, Alcoa has successfully
III.
developed high conductivity low friction coating by
adding a small concentration of carbon nano tubes
(CNTs) [44]. This recent study showed that as low
as 1% carbon nanotube addition reduces the
surface resistivity of typical fastener coating by 10
million times without sacrificing the desired low
frictional properties of the conventional nonconductive coating. It has also been demonstrated
that this newly developed conductive coating can be
easily incorporated to the sleeved fasteners to
further increase the threshold current levels that
fasteners can tolerate without sparking during a
lightning strike event [22].
Concluding Remarks
It is now well established that fasteners installed in
CFRP are more likely to spark than the ones
installed on aluminum structures during the event of
lightning channel attachment to aircraft due to the
inherent high resistance of composite materials and
anisotropic nature of the structure. The present
review has demonstrated that extensive new
developments have already been made and still
more research efforts are being carried out to
mitigate
potentially
hazardous
fastener
arcing/sparking in composite fuel tanks.
The review has attempted to describe the
mechanism and the source of fastener sparking and
outline the contributing factors including the effect of
machining induced micro-texture of the drilled
composite holes. It was explained that fastener
arcing occurs along the fastener/carbon fiber
interface and the drilling induced mico-texture
defects are associated with the occurrence of
fastener arcing. It has shown that new fastener
systems based on conforming sleeve technology
provides an excellent gap filling at both macro and
micro levels and an intimate contact between the
sleeve and the composite structure and makes the
fastener less sensitive to the hole quality.
Consequently, these new fasteners enhance the
current transfer from fastener to structure and
mitigate the fastener arcing as demonstrated during
lightning strike tests. Furthermore, conforming
fasteners with conductive CNT coating are superior
to typical aerospace fasteners with respect to their
lighting current carrying capabilities and they have
the ability to carry a Zone 2A strike of 100,000
amperes without sparking. When these advanced
fasteners are incorporated into joining composite
structures and fuel tanks, they can play an important
role for facilitating to dissipate the lightning strike
energy over the CFRP structures and doing that
without sparking they can offer additional lightning
protection for composite airplanes.
In recent years, numerical analysis techniques
based on finite element analysis (FEA) has been
employed to predict current distribution and
dissipation in mechanically fastened composite
joints [45-49]. However, design validation of fuel
tanks still largely involves actual testing of
representative test panels. In addition, Federal
aviation regulations require that the aircraft fuel tank
system to be designed and verified to prevent the
ignition of fuel vapor [ 23,50]. An example of the use
of FEA technique for predicting the lightning current
distribution within fastener and surrounding structure
is illustrated in Figure 17.
Figure 17: FEA model predicting lightning current distribution within fastener and CFRP structure
1 microsecond after lightning attachment.
In future, it is reasonable to expect that FEA
techniques will be used more widely for designing
composite fuel tanks at least for screening purposes
such as the effects of different joint configurations on
lightning current density and distribution.
REFERENCES
[1] F. A. Fisher, J.A. Plumer and R.A. Perala
‘Lightning Protection of Aircraft’ Lightning
Technologies Inc., second edition, 2004.
[2] P. Feraboli and H. Kawakami “Damage
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CONTACT
Hasim Mulazimoglu, Ph.D., is Manager,
New Product Development Center at Alcoa
Fastenings Systems. He can be reached at
900 Watson Center Road, CA 90745
Tel: 310 847 8100; e-mail:
hasim.mulazimoglu@alcoa.com; web
address:
http://www.alcoa.com/fastening_systems/