Note: Descriptions are shown in the official language in which they were submitted.
COATED FASTENERS WITH CONFORMING SEALS
Cross-Reference to Related Applications
This application relates to and claims the benefit of commonly-owned, co-
pending
U.S. Application Serial No. 15/059,608 entitled "FASTENERS WITH CONFORMING
SEALS," filed March 3, 2016, which claims the benefit of co-pending U.S.
Application
Serial No. 14/854,223 entitled "FASTENERS WITH COATED AND TEXTURED PIN
MEMBERS," filed September 15, 2015, which claims the benefit of commonly-
owned, co-
pending U.S. Provisional Patent Application Serial No. 62/051,602, entitled
"FASTENERS
WITH COATED AND TEXTURED PIN MEMBERS," filed September 17, 2014. The
present application also claims the benefit of commonly-owned, co-pending U.S.
Provisional Patent Application Serial No. 62/211,250 entitled "CONFORMING
CONICAL
SEAL FOR FASTENERS," filed August 28, 2015.
Technical Field of the Invention
The present invention relates to fasteners and, more particularly, to
fasteners
having coated pin members and conforming conical seals.
Background of the Prior Art
Continuous fiber reinforced composites are extensively used in both primary
and
secondary aircraft components for a variety of applications where light
weight, higher
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strength and corrosion resistance are primary concerns. Composites are
typically
composed of fine carbon fibers that are oriented at certain directions and
surrounded in
a supportive polymer matrix. Since the plies of the composite material are
arranged at
a variety of angles, and depending upon the direction of major loading, the
resultant
structure is typically a stacked laminated structure, which is highly
anisotropic and
heterogeneous. A significant portion of the composite structure is fabricated
as near
net-shape, but is drilled in order to facilitate joining of components using
mechanical
fasteners. Drilling fastener holes in composite does not compare to the
uniformity of
aluminum or steel since individual carbon fibers fracture at irregular angles
and form
microscopic voids between the fastener and the hole. As the cutting tool wears
down,
there is an increase of surface chipping and an increase in the amount of
uncut fibers or
resin and delamination. The composite microstructure containing such defects
is
referred to as "machining¨induced micro texture."
In addition to their machining challenges, composite structures in aircrafts
are
more susceptible to lightning damage compared to metallic structures. Metallic
materials, such as aluminum, are very conductive and are able to dissipate the
high
currents resulting from a lightning strike. Carbon fibers are 100 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. The
composite structural sections of an aircraft often behave like anisotropic
electrical
conductors. Consequently, lightning protection of a composite structure is
more
complex, due to the intrinsic high resistance of carbon fibers and epoxy, the
multi-layer
construction, and the anisotropic nature of the structure. Some estimates
indicate that,
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on average, each commercial aircraft in service is struck by lightning at
least once per
year. Aircraft flying in and around thunderstorms are often subjected to
direct lightning
strikes as well as to nearby lightning strikes, which may produce corona and
streamer
formations on the aircraft. In such cases, the lightning discharge typically
originates at
the aircraft and extends outward from the aircraft. While the discharge is
occurring, the
point of attachment moves from the nose of the aircraft and into the various
panels that
compromise the skin of the aircraft. The discharge usually leaves the aircraft
structure
through the empennage.
The protection of aircraft fuel systems against fuel vapor ignition due to
lightning
is even more critical. 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. It is a well-known fact that fasteners are often the
primary
pathways for the conduction of the lightning currents from skin of the
aircraft to
supporting structures such as spars or ribs, and poor electrical contact
between the
fastener body and the parts of the structure can lead to detrimental fastener
arcing or
sparking.
To avoid the potential for ignition at the fastener/composite structure
interface,
some aircraft use fasteners which are in intimate contact with the fastener
hole.
Intimate contact between bare metallic fasteners and the hole in the composite
structure
has been known to be the best condition for electrical current dissipation.
One
approach to achieve fastener-to-composite hole intimacy is to use a sleeved
fastener.
This approach involves first inserting a close fitting sleeve in the hole. An
interference-
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fit pin is then pulled into the sleeve. This expands the sleeve to bring it in
contact with
the wall of the hole in the composite structure. Although the sleeve
substantially
reduces the gap between the fastener and composite structure, it cannot
eliminate the
small gaps created due to the presence of drilling induced texture across the
composite
inner-hole surface. This machining induced texture also entraps excess
sealant, an
insulating material, inhibiting the intimate contact between the sleeve and
the hole. This
situation becomes even worse as the cutting tool wears, resulting in more and
larger
machining induced defects.
In order to avoid this condition, the current must dissipate through the
carbon
fibers exposed along the inner surface of the fastener hole. If the fastener
is not in
intimate contact with the inside of the hole, the instantaneous joule energy
driven by the
lightning strike leads to plasma formation within the gap that leads to
air/metal vapor
ionization which leads to pressure buildup that blows out in the form of a
spark or hot
particle ejection. 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.
Disclosure of the Invention
In an embodiment, a fastener comprising a pin member including an elongated
shank having a first end, a second end opposite the first end, a cylindrical
shank portion
having an outer surface, a head located at the first end of the elongated
shank, the
head including a bearing surface located on the underside of the head, and a
threaded
portion located at the second end of the elongated shank; and a seal element
attached
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to the pin member and juxtaposed with the bearing surface of the head of the
pin
member. In an embodiment, the seal element is made of copper. In an
embodiment,
the seal element includes a sealing portion having a first side and a second
side
opposite the first side, a lip extending from the first side of the sealing
portion. In an
embodiment, the lip extends angularly from the sealing portion. In an
embodiment, the
seal element includes a tubular portion extending axially from the side of the
sealing
portion. In an embodiment, the seal element includes a thickness in a range of
about 5
microns to about 100 microns. In an embodiment, the pin member includes a
coating.
In an embodiment, the coating is a metallic coating. In an embodiment, the
metallic
coating is selected from the group consisting of gold, silver, and copper. In
an
embodiment, the coating is made from a material having an electrical
conductivity
higher than 20% IACS.
In an embodiment, the head of the pin member is coated with the coating. In an
embodiment, the outer surface of the cylindrical shank portion of the pin
member is
coated with the coating. In an embodiment, the head of the pin member and the
cylindrical shank portion of the pin member are coated with the coating. In an
embodiment, the threaded portion of the pin member and the cylindrical shank
portion
of the pin member are coated with the coating. In an embodiment, the pin
member is
fully coated with the coating.
In an embodiment, in combination, a structure; and a fastener installed within
the
structure, the fastener including a pin member having an elongated shank with
a first
end, a second end opposite the first end, a cylindrical shank portion having
an outer
surface, a head located at the first end of the elongated shank, the head
including a
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bearing surface located on the underside of the head, and a threaded portion
located at
the second end of the elongated shank, and a seal element attached to the pin
member
and juxtaposed with the bearing surface of the head of the pin member. In an
embodiment, the structure includes a composite material. In an embodiment, the
structure is substantially made from the composite material. In an embodiment,
the
structure is partially made from the composite material. In an embodiment, the
structure
includes a metallic material. In an embodiment, the metallic material is
aluminum. In an
embodiment, the structure is made substantially from the metallic material. In
an
embodiment, the structure is made partially from the metallic material.
In an embodiment, a method of making a fastener, comprising the steps of:
providing a pin member including an elongated shank having a first end, a
second end
opposite the first end, a cylindrical shank portion having an outer surface, a
head
located at the first end of the elongated shank, the head including a bearing
surface
located on the underside of the head, and a threaded portion located at the
second end
of the elongated shank; and attaching a seal element to the pin member in a
position
that is juxtaposed with the bearing surface of the head of the pin member. In
an
embodiment, the method includes the step of coating at least a portion of the
pin
member with a coating. In an embodiment, the coating is a metallic coating. In
an
embodiment, the metallic coating is selected from the group consisting of
gold, silver,
and copper. In an embodiment, the coating is made from a material having an
electrical
conductivity higher than 20% IACS. In an embodiment, the coating step includes
coating the head of the pin member with the coating. In an embodiment, the
coating
step includes coating the outer surface of the cylindrical shank portion with
the coating.
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In an embodiment, the coating step includes coating the head of the pin member
and
the cylindrical shank portion with the coating. In an embodiment, the coating
step
includes coating the threaded portion and the cylindrical shank portion of the
pin
member with the coating. In an embodiment, the coating step includes coating
the pin
.. member fully with the coating.
In an embodiment, a method of installing a fastener into a structure,
comprising
the steps of: providing a fastener having a pin member including an elongated
shank
having a first end, a second end opposite the first end, a cylindrical shank
portion
having an outer surface, a head located at the first end of the elongated
shank, the
head including a bearing surface located on the underside of the head, and a
threaded
portion located at the second end of the elongated shank and a seal element
adapted to
be positioned on the pin member such that the seal member is juxtaposed with
the
bearing surface of the head of the pin member; and installing the fastener
into the
structure in an installed position. In an embodiment, the method includes the
step of
coating at least a portion of the pin member with a coating. In an embodiment,
the
coating is a metallic coating. In an embodiment, the metallic coating is
selected from
the group consisting of gold, silver, and copper. In an embodiment, the
coating is made
from a material having an electrical conductivity higher than 20% IACS. In an
embodiment, the coating step includes coating the head of the pin member with
the
coating. In an embodiment, the coating step includes coating the outer surface
of the
cylindrical shank portion with the coating. In an embodiment, the coating step
includes
coating the head and the cylindrical shank portion of the pin member with the
coating.
In an embodiment, the coating step includes coating the cylindrical shank
portion and
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the threaded portion of the pin member with the coating. In an embodiment, the
coating
step includes coating the pin member fully with the coating. In an embodiment,
the
structure includes a composite material.
In an embodiment, the structure is
substantially made from the composite material. In an embodiment, the
structure is
partially made from the composite material. In an embodiment, the structure
includes a
metallic material. In an embodiment, the metallic material is aluminum.
In an
embodiment, the structure is made substantially from the metallic material. In
an
embodiment, the structure is made partially from the metallic material.
In an
embodiment, the method further comprises the step of trimming the seal element
flush
with the structure. In an embodiment, the trimming step includes sanding the
seal
element. In an embodiment, the method further comprises the step of providing
a
metallic mesh on an outer surface of the structure, wherein when the fastener
is in its
installed position, the sealing element of the fastener is in direct physical
and electrical
contact with the metallic mesh. In an embodiment, the seal element includes a
sealing
portion having a first side and a second side opposite the first side, a lip
extending from
the first side of the sealing portion, the lip being in direct physical and
electrical contact
with the metallic mesh. In an embodiment, the metallic mesh is made from
copper.
Brief Description of the Drawings
FIG. 1 is a side elevational view of an embodiment of a pin member having
selected surfaces coated with a material;
FIG. 2 is a bottom perspective view of an embodiment of a seal;
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FIG. 3 is a bottom perspective view of the pin member and the seal shown in
FIGS. 1 and 2, respectively, assembled together;
FIG. 4 is a photograph of an embodiment of an outer surface of the coated pin
member shown in FIG. 1;
FIG. 5 is a photograph of the topography of an outer surface of an embodiment
of the coated pin member shown in FIG. 1;
FIGS. 6 and 7 are photographs of an embodiment of a pin member having a
textured surface;
FIG. 8 is a side elevational view of an embodiment of a pin member having a
.. conforming seal element;
FIGS. 9A and 9B are top plan and side views, respectively, of an embodiment of
a conforming seal element;
FIG. 10 illustrates a screenshot of a stress distribution analysis of an
installed
fastener;
FIG. 11 is a micro-photograph of the cross-section of a standard fastener
installed in a structure;
FIG. 12A is a micro-photograph that illustrates a standard fastener installed
in a
structure, while FIG. 12B is a micro-photograph that illustrates the pin
member and the
seal element shown in FIG 8 installed in a structure;
FIGS. 13A and 13B depict photographs showing pre-sanding and post-sanding
steps of a structure containing a fastener of FIG. 8 installed therein;
FIGS. 14A and 14B are schematic illustrations of the fastener of FIG. 8 before
and after a sanding step, respectively;
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FIG. 15A is a micro-photograph of a standard fastener installed in a structure
with an associated copper mesh, while FIG. 15B is a micro-photograph of a
fastener
shown in FIG 8 installed in a structure with an associated copper mesh;
FIG. 15C is graph and associated photographs corresponding to specific data
points on the graph showing flushness tolerance between the fastener shown in
FIG. 8
and a standard fastener;
FIGS. 16A and 16B are micro-photographs of a conventional fastener installed
in
a structure (40 times and 600 times magnification, respectively), while FIGS.
16C and
16D are micro-photographs of a fastener as shown in FIG. 8 installed in a
structure (25
times and 1000 times magnification, respectively);
FIG. 17A is a photograph showing the effects of lightning damage on a standard
fastener installed in a structure, while FIG 17B is a photograph showing the
effects of
lightning damage on a fastener as shown in FIG. 8 installed in a structure;
FIG. 17C is a micro-photograph showing the effects of lightning damage on a
standard fastener installed in a structure, while FIG 17D is a micro-
photograph showing
the effects of lightning damage on a fastener as shown in FIG. 8 installed in
a structure;
FIG. 18A through 18F illustrate a series of simulation results showing
reduction
of contact resistance and optimized electrical intimacy of the fastener of
FIG.8; and
FIG. 19 is a graph showing electric contact resistivity versus preload force
.. between the fastener shown in FIG. 8, a fastener with a coated pin member,
and an
anodized fastener.
CA 02954898 2017-01-13
Best Mode For Carrying Out The Invention
Referring to FIG. 1, in an embodiment, a pin member 12 includes an elongated
shank 14 having a cylindrical shank portion 16, a head 18 at one end of the
cylindrical
shank portion 16 and a threaded portion 20 at an opposite end of the
cylindrical shank
portion 16. In an embodiment, the head 18 is a countersunk head. In an
embodiment,
the outer surfaces of the head 18, including an underside surface (e.g.,
bearing surface)
21 of the head 18, and the cylindrical shank portion 16 are coated with
coating 22. In
an embodiment, the coating 22 is tungsten. In another embodiment, the coating
22 is
molybdenum. In another embodiment, the coating 22 is a refractory metal, such
as
tantalum or niobium. In another embodiment, the coating 22 is a refractory
ceramic,
such as alumina (A1203), silica (S102) or other metal oxides. In another
embodiment,
only the outer surfaces of the head 18 are coated with the coating 22. In
another
embodiment, only the outer surface of the cylindrical shank portion 16 is
coated with the
coating 22. In an embodiment, the coating 22 lowers electrical contact
resistance and
reduces probability of arc initiation/damage. In an embodiment, the coating 22
includes
a high electrical conductivity (higher than 20% IACS) and be galvanically
compatible to
a structure (e.g., anodic index less than 1.0V) for corrosion resistance.
In an
embodiment, the structure includes a composite structure. In another
embodiment, the
structure includes a metal structure. In another embodiment, the structure
includes a
fiber metal laminate structure.
In an embodiment, the coating 22 is a thin film coating having a thickness in
a
range of about one (1) nanometer to about two-hundred (200) microns. In an
embodiment, the coating 22 is applied by physical vapor deposition. In another
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embodiment, the coating 22 is applied by chemical vapor deposition. In another
embodiment, the coating 22 is applied by a selective additive process. In
another
embodiment, the coating 22 is applied by electroplating. In another
embodiment, the
coating 22 is applied by a spraying process. In another embodiment, the
coating 22 is
applied by cold spraying. In another embodiment, the coating 22 is applied by
thermal
spraying. In another embodiment, the coating 22 is applied by plasma coating.
In
another embodiment, the coating 22 is applied by a sputter deposition process.
In another embodiment, the outer surfaces of the head 18 and the cylindrical
shank portion 16 are textured. In an embodiment, the outer surfaces of the
head 18
and the cylindrical shank portion 16 of the pin member 12 are textured to
conform to the
machine¨induced micro texture inherent in fastener holes drilled in composite
structures, and to provide mechanical interlocking between the pin member 12
and the
composite structure. In an embodiment, the textured pin member 12 excavates
excess
entrapped sealant during installation of the fastener while bringing the
fastener in
intimate contact with the structure, and, thus, lowering the electrical
contact resistance
at the interface. The term "intimate contact" as used herein means that the
textured
outer surface of the pin member 12 is deformed into all or substantially all
of voids
between the pin member and the composite structure. In another embodiment,
only the
outer surfaces of the head 18 are textured. In another embodiment, only the
outer
surface of the cylindrical shank portion 16 is textured.
In an embodiment, the textured surfaces of the pin member 12 are created by
surface reshaping processes, such as media blasting. In an embodiment, the
textured
surfaces of the pin member 12 are grit blasted. In an embodiment, the grit
blasting
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utilizes fine grit glass bead media (100-170 mesh). In an embodiment, the grit
blasting
is performed until there is full coverage of the outer surfaces of the pin
member 12 to be
textured. In an embodiment, the grit blasting is performed for at least one
minute. In
another embodiment, the grit blasting is performed for about one minute. In an
embodiment, the grit blasting step is performed twice. In another embodiment,
the
textured surfaces of the pin member 12 are created by removal processes, such
as
selective electro-etching, laser etching, abrasive blasting, and mechanical
polishing. In
another embodiment, the textured surfaces of the pin member 12 are created by
chemical etching. In an embodiment, the chemical etching utilizes 50/50
hydrochloric
acid (HCI). In an embodiment, the chemical etching is performed for about 30
minutes.
In an embodiment, the pin member 12 is rinsed with distilled water for about
15-20
seconds, and dried with forced, room-temperature air for approximately Ito 2
minutes.
In another embodiment, the surfaces of the head 18 and the cylindrical shank
portion 16 of the pin member 12 are coated and textured by a combination of a
coating
process and a texturing process as described above. In an embodiment, a
combination
of the coating and texturing processes can be used to develop functional
characteristics
of the pin member 12, based on a specific loading pattern thereof. For
example, in an
embodiment, where the preload is high, the texturing/coating is performed to
lower
contact resistance. At locations which carry no preload and are not in
intimate contact
with the composite layer, mitigation of plasma generation and arc
formation/damage is
desired.
In an embodiment, the pin member 12 is part of a fastener that is adapted to
secure a plurality of work pieces of to one another, and is adapted to be
installed within
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aligned holes in such work pieces. In an embodiment, the work pieces are made
of a
composite material. In another embodiment, the work pieces are made of metal.
In
another embodiment, the work pieces are made from a fiber metal laminate. In
an
embodiment, the fastener includes a locking member (not shown in the Figures).
In an
embodiment, the locking member is a nut. In another embodiment, the locking
member
is a collar. In an embodiment, a fastener 10 includes the pin member 12 and a
seal 24
installed on the bearing surface 21 of the head 18 of the pin member 12, as
shown in
FIGS. 2 and 3, and to be discussed in further detail below.
During a lightning strike on an aircraft, the lightning typically attaches to
the head
18 of the pin member 12 first. In an embodiment, the coated and/or textured
pin
member 12 improves contact resistance. In this regard, all solid surfaces are
rough on
a micro-scale and contact between two engineering bodies occurs at discrete
spots
produced by the mechanical contact of asperities on the two surfaces. For all
solid
materials, the true area of contact is a small fraction of the apparent
contact area.
Electrical current lines get increasingly distorted as the contact spot is
approached and
flow lines bundle together to pass through "a-spots". An electrical junction
consists of a
number of contact "a-spots" through which electrical current passes from one
connector
component to the other and is often characterized by electrical contact
resistance of the
interface.
When a fastener is installed in a composite structure using a clearance fit,
the
primary load bearing surface of the pin member 12 as installed is the bearing
surface 21
of the head 18. This is an electrical contact through which it is desired to
pass a high
frequency, high voltage current and is a significant first line of defense to
the lightning
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strike. If the current has a path to flow easily, no arcing and resultant
damage would
occur. The pin or bolt to composite interface can prove to be an inefficient
electrical
contact due to dissimilar materials, presence of electrically insulating films
like aircraft
sealant and/or hard oxide layers on the surface and irregular cut pattern of
the
composite. To allow current to flow easily through the pin/bolt to composite
interface,
the interface contact resistance is desired to be low.
Contact resistance is highly dependent on the applied load on both the
surfaces
that brings them in contact and electrical and mechanical properties of the
material
surface in contact. A soft material at the interface with high electrical
conductivity
lowers the contact resistance, as do higher loads. The load in a pin member
joint is
provided by the preload and is primarily geometry/design dependent. As
described
above, the material coating 22 or texturing on the bearing surface 21 of the
head 18 is
used to both provide a low resistivity material at the contact interface and a
soft
conforming layer for better contact with the structure. Soft materials with
high electrical
conductivity, such as copper, gold, silver or other metals/materials can be
used to lower
contact resistance (see, e.g., the copper seal 24 shown in FIGS. 2 and 3).
The surfaces of the pin member 12, as described above, can also be textured to
enable better intimacy with the surrounding composite layer. As the textured
pin
member 12 is installed, the textured pin member deforms into the small voids
that are
created during drilling of the composite layer. As the textured surfaces
deform into the
voids, they displace the entrapped sealant during fastener installation. The
insertion of
the pin member 12 causes the excess sealant to be extruded outside the pin
member
12/composite interface. Thus, the textured pin member 12 excavates excess
entrapped
CA 02954898 2017-01-13
sealant during installation of the fastener while bringing the pin member 12
in intimate
contact with the composite structure. The finish texture of the pin member's
12 surfaces
is adjusted to provide a surface micro-roughness (Sa) value in order to
increase the
level of conformity and mechanical interlocking. In an embodiment, the surface
roughness (Sa) is greater than 0.5 micron.
As described above, FIG. 1 shows an embodiment of a tungsten coated pin
member 12. In an embodiment, plasma coating was used to deposit tungsten on
the
pin member 12 and achieve a surface roughness (Sa) equal or greater than 7
micron.
FIG. 2 shows the seal 24 and FIG. 3 shows the pin member 12 with the seal 24
installed
on it to promote intimacy with the composite layer on the bearing surface 21
of the head
18. In an embodiment, the seal 24 is frusto-conical in shape, and is sized and
shaped
to fit on the bearing surface 21 of the head 18. In another embodiment, this
can also be
achieved by copper coating the bearing surface 21 of the head 18. In another
embodiment, the seal 24 is a captive washer. In another embodiment, the seal
24 is
coated with a coating. In an embodiment, the coating of the seal 24 includes
the
coating 22.
FIG. 4 shows a photograph of the texture variation of the coated pin member
12,
while FIG. 5 shows the surface topography of the coated pin member 12. In an
embodiment, the coated surfaces of the pin member 12 have an average surface
roughness (Sa) of 7.5 micron. FIGS. 6 and 7 are photographs of the textured
pin
member 12 at 40X and 190X magnification, respectively. As can be seen in FIGS.
6
and 7, the textured pin member 12 exhibits a substantially rough finish. In an
embodiment, the textured pin member 12 provides improved electrical contact
along the
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textured surfaces of the pin member 12, which minimizes the dielectric effect
caused by
the sealant, promotes easier transfer of electric current, reduces the voltage
potential
across the pin member 12/composite interface, and thus enables transfer of
electric
current without any breakdown effects like arcing.
In an embodiment, in a clearance fit hole, there is no preload between the
shank
14 of the pin member 12 and the composite layer, and thus electrical contact
is
relatively poor. Thus, it would be difficult to ensure significant current
flow between the
pin member 12 and the composite layer. In case sufficient currents are not
conducted
by the bearing surface 21 of the head 18, there would be a possibility of
arcing at the
gap between the shank 14 and the adjacent composite layers. Arc formation
under
such conditions typically initiates in the metal vapor itself. The presence of
a high
temperature melting material with high conductivity will ensure that
sufficient metal
vapor is not present to initiate arcing. Even if arcing is initiated, the
volume of plasma
will be low. Higher conductivity will also ensure that current is more easily
passed
between the shank 14 and composite layer if contact is available. As described
above,
in certain embodiments, materials like tungsten, molybdenum, or refractory
metals/ceramics can be used as the coating 22 on the shank 14 of the pin
member 12
to ensure reduction in arc damage. Since lightning strikes generate high
frequency
currents, current would typically flow close to the fastener surface due to
"skin effect".
The coating on the pin member 12 also helps in this respect that a higher
temperature
melting point and high conductivity material would carry most of the current
lowering the
likelihood of fastener melting or plasma generation.
Thus, the coated/textured pin member 12:
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= Improves electrical contact between composite and fastener surface;
= Minimizes fastener arcing during lightning strikes;
= Provides gap filling and mechanical interlocking capabilities;
= Reduces likelihood of plasma formation during arcing around the fastener
shank;
= In case
arcing occurs in the fastener, reduces the volume of plasma generated to
make it easier to be contained.
Coated Fasteners with Conforming Conical Seals
Referring to FIGS. 8, 9A and 9B, in an embodiment, a fastener 110 includes a
pin member 112 having an elongated shank portion 114 with a smooth cylindrical
shank
portion 115, a head 116 at one end of the smooth cylindrical shank portion 115
and a
threaded portion 117 at an opposite end of the smooth cylindrical shank
portion 115. In
an embodiment, the head 116 is a countersunk head. In an embodiment, a locking
member is adapted to be installed to the pin member 112 (not shown in the
Figures). In
an embodiment, the locking member is a threaded nut that engages the threaded
portion 117 of the pin member 112. In another embodiment, the locking member
is a
collar adapted to be swaged into the lock grooves of the threaded portion 117
of the pin
member 112.
In an embodiment, the pin member 112 is fully coated with a coating 119. In an
embodiment, the coating 119 is a metallic coating. In an embodiment, the
coating 119
is a soft, metallic coating. That is, the coating 119 is applied to the
elongated shank
portion 114, including the smooth cylindrical shank portion 115 and the
threaded portion
117, and the head 116, including an underside (e.g., bearing surface 120) of
the head
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116. In an embodiment, the coating 119 is copper. In another embodiment, the
coating
119 is silver. In another embodiment, the coating 119 is gold. In other
embodiments,
the coating 119 is made from a material having a high electrical conductivity,
for
example, a material having an electrical conductivity higher than 20% IACS.
In other embodiments, the coating 119 can consist of any one of the coatings
22
with respect to the embodiment of the pin member 12, which are described in
detail
above.
In another embodiment, the pin member 112 is partially coated with the coating
119. In an embodiment, the coating 119 is applied to the head 116, including
the
underside 120 of the head 116, of the pin member 116. In another embodiment,
the
coating 119 is applied to the head 116 (including the underside 120 of the
head 116)
and to the smooth cylindrical shank portion 115 of the pin member 112. In
another
embodiment, the coating 119 is applied to the smooth cylindrical shank portion
115 of
the pin member 112. In another embodiment, the coating 119 is applied to the
smooth
cylindrical shank portion 115 and the threaded portion 117 of the pin member
112.
In another embodiment, the pin member 112 does not include the coating 119.
Still referring to FIGS. 8, 9A and 9B, in an embodiment, a conforming seal
element 118 is attached to the elongated shank portion 114 and juxtaposed with
the
bearing surface 120 of the head 116 of the pin member 112. In an embodiment,
the
seal element 118 is separate and distinct from the pin member 112. In an
embodiment,
the seal element 118 can be positioned within a hole of a structure and the
pin member
112 can then be inserted into the seal element 118 during installation of the
fastener
110. In an embodiment, the seal element 118 is frusto-conical in shape and
includes a
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CA 02954898 2017-01-13
centrally located, circular-shaped aperture 122 that is sized and shaped to
fit around the
shank portion 114 of the pin member 112 and juxtaposed with the bearing
surface 120
of the head 116 of the pin member 112. In an embodiment, the seal element 118
includes a sealing portion 121. In another embodiment, a lip 123 extends from
one side
of the sealing portion 121. In an embodiment, the lip 123 is angled upwardly
from the
sealing portion 121. In another embodiment, a tubular portion 125 extends
axially from
an opposite side of the sealing portion 121. In an embodiment, the seal
element 118 is
made from copper. In an embodiment, the sealing portion 121 of the seal
element 118
has a thickness in a range of about 5 microns to about 100 microns.
It is noted that all solid surfaces of the pin member 112 and a structure 150
in
which the fastener 110 is adapted to be installed are rough on a microscopic
scale.
Surface micro-roughness consist of peaks and troughs whose shape, variations
in
height, average separation and other geometric characteristics depend on the
details of
the process used to generate the surfaces. Contact between two engineering
bodies
occurs at discrete microscopic spots that are the result of mechanical contact
of
asperities on the two surfaces. For all solid materials, the area of true
contact is a small
fraction of the nominal contact area for a wide range of normal contact loads.
Referring to FIGS. 10 and 11, when mechanical load is exerted through this
contact area, the mode of deformation of contact asperities is elastic,
plastic, or a
mixture of plastic and elastic depending on the local mechanical stresses, and
on the
properties of the material such as the elastic modulus and hardness. In a bulk
electrical
interface where the mating components are metals, the contacting surfaces
often
contain an oxide or other electrical insulating layers. Generally the
interface becomes
CA 02954898 2017-01-13
electrically conductive only when electrically insulation films are displaced
at the
asperities of the contacting surfaces or the potential across the interface
exceeds the
dielectric strength of the electrically insulation film. For the sake of
simplicity in the field
of electrical connectors, the discrete spots are often assumed to be circular.
This
assumption provides an acceptable geometric description of the average contact
spots
where the roughness topographies of the mating surfaces are isotropic. While
this
assumption is acceptable for metallic structures, it becomes invalid when the
mating
surfaces are characterized by directional micro-texture or are clearly
anisotropic in
nature. The true area of contact between a fastener and the surrounding CFRP
.. structure is a very small percent of the nominal area due to the multi-
layered
construction and anisotropic nature of CFRP structures, which further
complicates
quality of the electrical contact between the fastener and surrounding CRFP
structure.
FIG. 12A illustrates a standard fastener installed in a structure (e.g., an
aluminum panel), which shows microgaps between the head of a pin member and
the
.. structure. In an embodiment, with reference to FIG. 12B, the conforming
seal element
118 is adapted to maximize the true area of contact between the fastener
(e.g., the
bearing surface 120 of the head 116 of the pin member 112) and a structure 150
with
minimum mechanical load. In an embodiment, the structure 150 includes a
composite
material. In another embodiment, the structure 150 is substantially made from
a
.. composite material. In another embodiment, the structure 150 is partially
made from a
composite material. In another embodiment, the structure 150 includes a
metallic
material. In an embodiment, the metallic material is aluminum. In another
embodiment,
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the structure 150 is made substantially from a metallic material.
In another
embodiment, the structure 150 is made partially from a metallic material.
In an embodiment, the conforming seal element 118 includes a multi-layer
construction with a relatively soft, yet highly electrically conductive base
layer, which
provides macroscopic conformity, and a softer top layer, which provides
microscopic
conformity.
In an embodiment, a method by which the fastener 110 with the seal element is
installed is described hereinbelow. In an embodiment, with reference to FIGS.
8, 9A,
9B, 13A, 13B, 14A and 14B, the method includes the steps of coating the pin
member
112 with the coating 119 (either fully or partially as described above),
attaching the seal
element 118 to the fastener 110 (e.g., the pin member 112), and installing the
fastener
110 in the structure 150. In an embodiment, the coating step is not included
when the
pin member 112 is not coated with the coating 119 as described above. In
another
embodiment, the seal element 118 can be positioned within a hole of the
structure 150
and the pin member 112 can then be inserted into the seal element 118 during
installation of the fastener 110. In an embodiment, with respect to the
installation step,
a preload to the fastener 110 is provided by the locking member (e.g., nut or
collar), and
a force is exerted on the structure 150 by the pin member 112 with the seal
element 118
positioned between the head 116 of the pin member 112 and the structure 150.
As the
seal element 118 conforms to the inherent micro-roughness between the head 116
of
the pin member 112 and the structure 150, a portion of the seal element 118 is
extruded
upward the edge of the pin member 112 and protrudes above the surface of the
structure 150. With reference to FIGS. 13A, 14B, 14A and 14B, the seal element
118 is
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CA 02954898 2017-01-13
trimmed flush with the surface of the structure 150 by sanding the top of the
seal
element 118 (e.g., proximate to the lip 123) and, if necessary, the structure
150. In an
embodiment, the sanding step is simultaneous with the preparation of the
surface of the
structure 150 for the application of paint 152.
FIGS. 15A and 15B are photographs illustrating the cross-sections of a pin
member without the seal element 118 (FIG. 15A) and the pin member 112 with the
seal
element 118 (FIG. 15B). As shown, the inclusion of the seal element 118 is
provided
along with a copper mesh 154 and improves paint adhesion.
Referring to FIG. 15C, the fastener 110 improves a range of countersink within
the structure 150 over which the connection with the copper mesh 152 is
maintained.
As seen on the graph shown in FIG. 20, the flushness tolerance of the fastener
110,
shown on the left, is wider than a baseline fastener without the seal element,
as shown
on the right.
FIGS. 16A through 16D are photographs showing the difference in pin/CFRP
interface between a conventional fastener without a seal element (FIGS. 16A
and 16B)
and a fastener with the seal element 18 (FIGS. 16C and 16D). Micro-level
conformance
between the seal element 118 and the CFRP structure 50 enhances the current
transfer
from fastener to the structure 150 and reduce arcing.
FIGS. 17A through 17D illustrate the differences in the effects of damage in
aluminum panels of a fastener without the seal element 118 (FIGS. 17A and 17C)
and a
fastener with the seal element 118 (FIGS. 17B and 17D). The seal element 118
increases the electrical intimacy between the fastener 110 and the structure
in the area
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adjacent to the seal element 118. As will be described in more detail below,
this
reduces the magnitude of the electric field near the locking member (e.g., nut
or collar).
With reference to FIGS. 18A through 18F, which illustrate simulation results,
the
seal element 118 reduces contact resistance around the head 116 of the pin
member
112 and results in optimized electrical intimacy. This nanoscale conformity
leads to
improved current transfer into the upper panel of the aircraft structure 150.
During a
lightning strike, the external discharge, which attaches to the head 116, will
tend to
attach to regions having larger electric fields. In the case of the fastener
110 having the
seal element 118, the electric fields are much lower, resulting in so-called
equipotential
surfaces with a flatter field profile. This field flattening effect minimizes
the amount of
structural damage caused by large concentrated flows through sharp edges.
An advantage of the seal element 118 is the large reduction of the charge
buildup between the fastener 110 and surrounding materials within the fastener
assembly. The time dependent electric potential has a lower peak value, which
results
.. in a large reduction of the electric field magnitudes around the bearing
surface of the nut
region. Typically, large fields around the nut region and sharp edges can
result in
dielectric breakdown and edge glow phenomenon. The large reduction in electric
fields
is a direct result of the enhanced current transport.
Referring to FIG. 19, the fastener 110 includes a reduced contact resistance.
Contact resistivity measurements show current transfer improvement with the
fastener
110 having the coating 119 and the seal element 118 over baseline pin members
without the coating 119 and the seal element 118.
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It should be understood that the embodiments described herein are merely
exemplary and that a person skilled in the art may make many variations and
modifications without departing from the spirit and scope of the invention.
All such
variations and modifications are intended to be included within the scope of
the claims.
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