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Description  |
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The present invention relates to aircraft lightning protection systems and,
more particularly, to lightning protection structures for composite
structural components having fasteners.
It is known that the present planned graphite epoxy composite structural
components to be used on aircraft will be subjected to natural lightning
strike discharges. The most severe strikes will be limited to structures
located at or near the aircraft protuberances (wing tips, stabilizer tips,
vertical tips, rudder, elevators, ailerons, some engine cowlings, etc.).
These locations are designated Zone 1 and will be subjected to the initial
attachment of the lightning channel. The initial attachment lightning
strike is characterized by a fast rise, high peak current
(2.times.10.sup.5 amp) and a large energy transfer (2.times.10.sup.6
amp.sup.2 sec). It can create severe structural damage to unprotected
graphite epoxy structure (much more so compared to aluminum structure).
Other parts of the structure will be subjected to lesser discharges,
called swept stroke lightning. These areas are designated Zone 2 and are
located aft of the initial attachment points. Particularly sensitive areas
are integral fuel tanks and pressurized sections. Punctures cannot be
tolerated in either area but fasteners penetrating the skin and protruding
into a fuel tank area have been shown to constitute a fuel ignition source
even without a skin puncture. Unprotected fasteners are a preferred
attachment point for the lightning and spark because the energy cannot be
distributed fast enough into the surrounding graphite epoxy material (due
to its low thermal and electrical conductivity). The test criteria for
Zone 2 also contains a fast rise current but with a 1.times.10.sup.5 amp
peak and an energy transfer of 0.25.times.10.sup.6 amp.sup.2 sec.
Representative of the prior art literature is U.S. Pat. No. 3,755,713,
assigned to The Boeing Company. Knitted wire mesh is utilized in U.S. Pat.
No. 3,755,713 in contrast to preferred embodiments of the present
invention which utilize metal (e.g. nickel) coated or plated graphite
fibers. A decorative layer is shown in the reference patent, thereby
providing insufficient coverage for fastener heads. Further, in U.S. Pat.
No. 3,755,713 there is no insulation between the fasteners and the
structure since the reference structure is fiberglass and, therefore,
electrically nonconductive.
Heretofore, lightning protection material most commonly utilized for
non-metallic structures is aluminum. The aluminum is normally applied as a
flame spray, a woven screen, a foil or a plating. Such prior method works
satisfactorily when the structure is a dielectric such as fiberglass or
Kevlar epoxy. However, aluminum is galvanically incompatible with graphite
epoxy forming the composite structures in accordance with the present
preferred embodiments. If not isolated from the graphite epoxy, the
aluminum will corrode; if isolated, the aluminum loses its effectiveness
as a protection system (no electrical path).
As a consequence of the preceding, it is an object of the present invention
to provide a more compatible protection system for graphite epoxy
structures to prevent future maintenance problems with the protection
system.
It is a further object of the present invention to provide woven outer
layer structures for composite structural components which include
interwoven metal (e.g. nickel) coated or plated graphite fibers.
It is yet a further object of the present invention to provide coating or
plating of fiber tows as a function of protection desired, e.g., Zone 1 or
Zone 2, in the outer layer of tape or fabric for the structural component
requiring lightning protection.
A full understanding of the present invention, and of its further objects
and advantages and the several unique aspects thereof, will be had from
the following description when taken in conjunction with the accompanying
drawing in which:
FIG. 1 is a cross-sectional view of a composite structure including
fasteners showing the present metal (e.g. nickel) coated or plated
graphite fiber lightning protection;
FIG. 2 is a further embodiment of the present invention showing composite
structure and fastener with lightning protection features including metal
(e.g. nickel) coated or plated graphite fibers, fiberglass insert to
prevent lightning currents from entering the graphite epoxy laminate
through a metal (e.g. titanium) fastener and/or use of dielectric strip
material over a row of fasteners or single fastener to prevent lightning
currents from entering a metal (e.g. titanium) fastener;
FIG. 3 is a cross-sectional view of a further embodiment of a composite
structure including metal fasteners having integral lightning protection
wherein the potential path between the skin and substructure has been
interrupted by utilization of a non-conductive angle member;
FIG. 4 is a cross-sectional view of another embodiment of composite
structure including metal fasteners having integral lightning protection
for metal fasteners where utilized in needed regions, e.g., as where the
metal fasteners penetrate aircraft wing skins and into fuel tanks wherein
a conductive angle member is electrically isolated by non-conductive
layers between the angle and the skin and between the angle and the
fastener retainer;
FIG. 5 is a cross-sectional view of a further lightning protection
embodiment of fastener heads in a composite wing skin illustrative of
electrical isolation of a conductive angle member from a conductive
substructure such as a rib/spar web; and,
FIG. 6 is a view in cross section of a composite-fastener assembly
utilizing FIG. 3 and 5 features and includes localized dielectric
protection of fastener heads in the composite, e.g. skin, and isolation of
the skin and substructure, e.g. spar/rib web, with a nonconductive angle
member.
Turning now to FIG. 1, a composite aircraft skin structure comprising a
graphite epoxy laminate 10 is seen to include a protruding-head fastener
20 and a countersunk fastener 22. The present lightning protection system
utilizes a metal (e.g. nickel) coated or plated graphite epoxy tape or
fabric ply 24 comprising individual metal (e.g. nickel) coated or plated
graphite fibers which are integral or woven into the outer layer of tape
or fabric for the structural component requiring protection. Depending on
the protection desired (for Zone 1 or Zone 2), either 100% of the fiber
tows in both the warp and fill direction of fabric 24 can be coated or
plated or a lesser percentage (such as every other tow or every third
tow). Similarly, tape can be metal coated or plated with various amounts
of metal.
To protect against a direct attachment to a fastener 20 or 22 by a
lightning stroke, the fastener holes are counterbored (or both
counterbored and countersunk, see FIG. 1) and the counterbore filled with
a potting compound or resin 26. The exterior surface of the integral
lightning protection system shown in FIG. 1 is coated with primer and
paint 28.
Preliminary test results from a simulated Zone 2 lightning strike were
encouraging in that the current spread out over a 5-inch diameter area and
the damage did not appear to go beyond the first ply of the integral
lightning protection assembly of FIG. 1. In contrast, severe local damage
(material blown away and delaminated) was experienced on the unprotected
part of the graphite epoxy laminate panel structure.
Turning now to FIG. 2, it will be noted that a dielectric (e.g., glass
fiber) insulation layer in the form of insert 42 is utilized between metal
(e.g. titanium) fastener 32 and graphite epoxy laminate 10 forming the
skin structure of the aircraft. Dielectric insert 42 further decreases, in
the present lightning protection system, the likelihood of sparking in
regions of fuel tanks because of lightning. Lightning currents are thus
prevented from entering laminate 10 through metal (e.g. titanium) fastener
32. In addition to providing electrical insulation, dielectric (e.g.
fiberglass epoxy) sleeve 42 should provide a structural "softening" effect
at the fastener hole (i.e. allow some local yielding) and also minimize
the effect of hole imperfections. A further plastic-like strip member 52,
sandwiched between metal (e.g. nickel) coated or plated graphite epoxy
outer ply 24 and primer and paint exterior surface coating 28, extends
over a row (or, as shown here, a single fastener 32).
Metal fastener 32 is shown in FIG. 2 securing graphite epoxy or metal (e.g.
titanium) angle member 62 with graphite epoxy composite structural member
10, the fastening assembly further including metal (e.g.
corrosion-resistant stainless steel) washer member 63 and metal (e.g.
corrosion-resistant stainless steel) shear collar member 65. Dielectric
plastic strip member 52 may comprise a polyester material which is
transparent and available under the trade name Mylar, of the Du Pont
Company of Wilmington, Del., or a polyimide insulation material available
from the Du Pont Company of Wilmington, Del., under the trade name Kapton.
Turning now to the integral lightning protection embodiments for metal
fasteners shown in FIGS. 3 through 6, it will be noted that the same
numerals as used hereinbefore in the description of FIG. 1 and 2
embodiments for similar components are utilized.
The lightning protection protection concept of the embodiment shown in FIG.
3 interrupts the potential electrical path between graphite epoxy
composite skin structure 10 and electrically conductive substructure 162
through utilization of electrically non-conductive angle member 106.
Electrically non-conductive bushing 42, comprising e.g. a glass fiber
insert, has been retained in this embodiment to keep the fastener from
becoming a preferred lightning current path to the lower plies of wing
skin laminate 10. In the embodiment of FIG. 3, it can be seen that the
options for surface conductivity enhancement have been expanded to include
thin aluminum or bronze wires woven into the outer ply of fabric 101 in
lieu of the metal (e.g. nickel) coated or plated graphite fibers. The
counterbore 26 of the embodiments of FIGS. 1 and 2, filled with potting
compound, has been eliminated. The counterbore becomes less desirable,
from a structural standpoint, as the skin 10 gets thinner. Instead, the
lightning current is prevented from flowing into the substructure 162
(spar/rib web) by the use of non-conductive angle member 106. Angle member
106 can be made from fiberglass or Kevlar epoxy or non-conductive fibers
in a polymeric (plastic) matrix. Non-conductive bushing 42 serves to
prevent the fastener from becoming a preferred lightning current path to
the lower plies of the skin laminate which, in turn, would cause the
fastener to overheat and produce local damage to the graphite epoxy skin.
Metal and graphite epoxy outer ply member 101 enables surface conductivity
enhancement, viz. the spread of lightning currents over a large area such
that the current density at the lightning attachment point is reduced
considerably. Structural integrity of the jointed structure is not
compromised since the lightning damage is confined to the outer ply.
Repair and refinishing consists of an aerodynamic smoothing patch and
topcoat, respectively. Metal and graphite epoxy outer ply 101 was tested
utilizing nickel-plated graphite fabric as the outer ply; however, the
metal and graphite epoxy outer ply may also comprise any one of metal
(e.g. nickel) coated or plated graphite fibers, Fiberite aluminum fibers
woven in graphite fabric, or Brochier bronze fibers woven in graphite
fabric. The primary purpose of the integral metal and graphite epoxy outer
ply is to provide surface conductivity enhancement for lightning current
dispersion in contrast to flame spray or Thorstrand which is for lightning
current diversion.
Electrically non-conductive bushing 42, comprising a glass fiber insert, is
bonded into a hole drilled in graphite epoxy skin laminate 10. The bonding
agent used was epoxy resin, lightning testing validation for resistance to
electrical breakdown to the longitudinal fibers of the laminate being
required. The primary purpose of electrically non-conductive bushing 42 is
to prevent the lightning currents from seeking out the longitudinal plies
of skin laminate 10 through the fastener hole parallel to the radial or
axial direction.
Electrically non-conductive structural angle member 106 provides a very
high electrical resistance path between skin 10 and conductive
substructure 162, and angle member 106, during tests, comprised fiberglass
and Kevlar.
In the embodiment of FIG. 4, electrically conductive structural angle
member 62 has been electrically isolated by electrically non-conductive
layers 103 and 104 forming an electrically non-conductive barrier between
angle member 62 and the skin 10 and, also, between angle member 62 and the
fastener/retainer, respectively. Electrically non-conductive bushing 42 is
also utilized for the same reasons as hereinbefore discussed in connection
with the description of the FIG. 3 embodiment.
The FIG. 4 embodiment, in can be observed, shows a variation of the concept
of the embodiment of FIG. 3. Presently, the commonly utilized materials
for non-conductive angle member 106 would comprise either fiberglass or
Kevlar epoxy. Depending on structural loads and environment, these
materials may not be as desirable as graphite epoxy or titanium. Thus, in
this embodiment, the metal or conductive angle member 62 has been retained
but is then electrically isolated from the skin 10 by a non-conductive
layer 103 between skin 10 and angle member 62, and a non-conductive washer
104 between angle member 62 and the fastener retainer (metal nut/collar
and washer 63, 65). The non-conductive bushing 42 has been retained in
this embodiment for the same reason as in the FIG. 3 embodiment.
Electrically non-conductive barrier 103 provides the aforementioned
electrical isolation of two component parts through the faying surface,
thereby precluding flow of any electrical currents between the respective
components.
Electrically non-conductive washer 104 has the purpose of electrically
isolating the fastener retainers (washer and nut) at the structure
retainer interface, thereby precluding conduction of any electrical
currents between the components.
The embodiment of FIG. 5 is illustrative of localized dielectric protection
of fastener heads in graphite epoxy skin 10 and electrical isolation of
electrically conductive angle member 62 from electrically conductive metal
substructure 162 (e.g. rib/spar web). Secondary applied dielectric layer
105 in the form of strips or patches extending over the fastener heads
provides resistance to electrical breakdown from the lightning channel to
the fastener or fastener head. By preventing direct lightning attachment
to the fastener or fastener hole, this layer 105 provides protection
similar to that provided by non-conductive bushing 42, hereinbefore
discussed. That is, the lightning attachment is forced to occur at the
perimeter of the dielectric or further away from the fastener and will
disperse into the laminate through metal and graphite outer ply 101.
It can be recognized that the FIG. 5 embodiment shows yet another variation
of the concept shown in the FIG. 3 embodiment. It may be advantageous,
from a structural standpoint, depending on skin and spar/rib web
thickness, or from manufacturing or assembly considerations, to have the
electrical isolation at the angle-to-spar/rib web interface rather than at
the angle-to-skin interface. FIG. 5 shows the fastener through the angle
and spar/rib web isolated with a non-conductive bushing. The metal or
conductive angle 62 is isolated from the spar/rib web 162 by a
non-conductive layer 103 between the two. The fastener retainer (typically
a metal nut or collar and a metal washer) is isolated from the spar/rib
web 162 by a non-conductive washer 104. The fastener through the skin is
only countersunk flush with the metal and graphite epoxy outer ply 101; no
potting compound is used over the fastener head as in the embodiments of
FIGS. 1 and 2. Localized dielectric protection, in the form of a plastic
film 105, is used to prevent direct lightning strike attachment to the
fasteners in the skin.
The embodiment of FIG. 6 includes, in combination, certain features
hereinbefore described in connection with the embodiment of FIGS. 3 and 5,
viz. localized dielectric protection of fastener heads (and, as also shown
by strip member 52 in FIG. 2, in graphite epoxy composite skin member 10,
and isolation of skin member 10 and spar/rib web 162 by electrically
non-conductive structural angle member 106. The embodiment of FIG. 6 is
seen to include a combination of features from the embodiments of FIGS. 3
and 5. The dielectric 105 on skin 10, in the area of the fasteners,
prevents direct attachment of lightning to the fasteners. The
non-conductive angle member 106 prevents current from a nearby lightning
strike from flowing into the substructure 162. This current could produce
arcing between the angle member 106 and the skin 10 and the angle member
106 and the spar/rib web 162. This conceptual embodiment from a standpoint
may be preferred where non-conductive bushings are objectionable for
either structural or assembly means.
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