U.S. patent application number 11/154522 was filed with the patent office on 2006-12-21 for composite reinforcement of metallic structural elements.
This patent application is currently assigned to The Boeing Company. Invention is credited to Leanna M. Micona, Gary R. Weber, Willard N. Westre, Mark S. Wilenski.
Application Number | 20060283133 11/154522 |
Document ID | / |
Family ID | 36968871 |
Filed Date | 2006-12-21 |
United States Patent
Application |
20060283133 |
Kind Code |
A1 |
Westre; Willard N. ; et
al. |
December 21, 2006 |
Composite reinforcement of metallic structural elements
Abstract
A selectively reinforced hybrid metal-composite structural
element can include a metal element and a composite material. The
composite material can be bonded to the metal element by an
adhesive layer including a polymer matrix using a radiation curing
process, resulting in insubstantial or negligible residual stresses
at the bond line between the metal element and the composite
element. The structural element also can include a metal closeout
cap to provide a barrier from a corrosive atmosphere, and the
adhesive layer can encapsulate the composite element to provide a
corrosion-resistant barrier between the composite element and the
surrounding metal.
Inventors: |
Westre; Willard N.;
(Bellevue, WA) ; Micona; Leanna M.; (Seattle,
WA) ; Wilenski; Mark S.; (Seattle, WA) ;
Weber; Gary R.; (Kent, WA) |
Correspondence
Address: |
BAKER & HOSTETLER, LLP;FOR BOEING COMPANY
WASHINGTON SQUARE, SUITE 1100
1050 CONNECTICUT AVENUE, N.W.
WASHINGTON
DC
20036
US
|
Assignee: |
The Boeing Company
|
Family ID: |
36968871 |
Appl. No.: |
11/154522 |
Filed: |
June 17, 2005 |
Current U.S.
Class: |
52/837 |
Current CPC
Class: |
E04C 2003/0452 20130101;
E04C 3/29 20130101; Y10T 156/1064 20150115 |
Class at
Publication: |
052/729.1 |
International
Class: |
E04C 3/30 20060101
E04C003/30 |
Claims
1. A reinforced hybrid structural element, comprising: a metal
element; and a composite ply bonded to the metal element using
radiation.
2. The structural element of claim 1, further including an adhesive
layer at least partially cured using radiation, wherein the
adhesive layer bonds the composite ply to the metal element.
3. The structural element of claim 2, wherein the adhesive layer
includes a polymer matrix.
4. The structural element of claim 2, wherein the adhesive layer
includes a plurality of glass fibers.
5. The structural element of claim 2, wherein the adhesive layer
encapsulates the composite ply in order to form a
corrosion-resistant barrier between the metal element and the
composite ply.
6. The structural element of claim 1, wherein the composite ply is
at least partially cured using radiation.
7. The structural element of claim 1, wherein the composite ply is
located at a high-stress area of a structural element cross section
under a loading type which the structural element is designed to
carry.
8. The structural element of claim 1, wherein the radiation is an
electron beam.
9. The structural element of claim 1, wherein the composite ply
includes a plurality of reinforcing fibers having a general fiber
orientation that is substantially aligned in a direction of a
loading which the structural element is designed to carry.
10. The structural element of claim 1, wherein the metal element
includes at least one metal chosen from the following: aluminum,
titanium and iron.
11. The structural element of claim 1, wherein the structural
element further comprises an I-beam.
12. The structural element of claim 1, wherein the structural
element further comprises a seat track configured to support an
airplane seat.
13. A method of manufacturing a reinforced hybrid structural
element, comprising the steps of: providing a metal element; laying
up a composite ply over at least a partial surface of the metal
element; and at least partially curing a polymer matrix using
radiation in order to bond the composite ply to the metal
element.
14. The method of claim 13, further comprising the step of placing
an adhesive layer over at least the partial surface of the metal
element, wherein the adhesive layer includes the polymer matrix and
the step of laying up further includes laying up the composite ply
over the adhesive layer.
15. The method of claim 14, further comprising the step of
encapsulating the composite ply with the adhesive layer in order to
form a corrosion-resistant barrier between the composite ply and
the metal element.
16. The method of claim 14, wherein the step of placing further
includes placing a plurality of glass fibers over at least the
partial surface of the metal element.
17. The method of claim 13, wherein the step of curing further
includes maintaining a temperature at an interface between the
metal element and the polymer matrix within 75 degrees Celsius
(approximately 135 degrees Fahrenheit) of a design application
temperature.
18. The method of claim 13, wherein the step of curing further
includes maintaining a temperature at an interface between the
metal element and the polymer matrix below 120 degrees Celsius
(approximately 248 degrees Fahrenheit).
19. The method of claim 13, wherein the step of curing further
includes actively cooling the structural element.
20. The method of claim 13, further comprising the step of at least
partially curing the composite ply using radiation.
21. The method of claim 13, wherein the step of curing further
includes using an electron beam.
22. The method of claim 13, further comprising the step of removing
a portion of the metal element in order to create a space for the
composite ply.
23. The method of claim 13, wherein the step of laying up further
includes locating the composite ply at a high-stress area of a
structural element cross section under a loading type which the
structural element is designed to carry.
24. The method of claim 13, wherein the step of laying up further
includes: laying up a resin and a plurality of reinforcing fibers,
wherein the reinforcing fibers have a general fiber orientation;
and substantially aligning the fiber orientation in a direction of
a loading which the structural element is designed to carry.
25. The method of claim 13, wherein the step of laying up further
includes laying up a precured composite ply.
26. The method of claim 13, further comprising the step of
overlaying the composite ply with a metal closeout cap in order to
protect the composite ply from a corrosive environment.
27. The method of claim 13, wherein the metal element includes at
least one metal chosen from the following: aluminum, titanium and
iron.
28. A reinforced hybrid structural element, comprising: a metal
element; a composite ply; and means for bonding the composite ply
to the metal element, wherein the means for bonding is at least
partially cured using radiation.
Description
FIELD OF THE INVENTION
[0001] The present disclosure relates generally to structural
reinforcement. More particularly, the present invention relates to
selective reinforcement of metallic structural elements using
composite materials.
BACKGROUND OF THE INVENTION
[0002] Structural elements are used to maintain the structural
integrity of a wide variety of structures, including, for example,
bridges, buildings, airplanes, trains, sea vessels and propellers.
A structural element for a particular application generally can be
either selected from existing types of commonly available
structural elements or specially designed to suit the needs of an
application.
[0003] In either case, the cross-sectional shape of the structural
element often is selected or designed to include more material at
locations in the cross-section that will experience greater stress
under the design loading. Thus, structural elements can be designed
to carry specific types of loads. For example, common structural
elements designed to be especially robust under bending loads
include I-beams, L-shaped beams, C-shaped channels, and the
like.
[0004] In addition, structural elements can be made from many
different materials, including metals, metal alloys, composite
materials, and the like. Generally, a material for a particular
structural element is selected because of specific material
properties that meet the requirements of a particular application.
For example, a metal may be chosen for a particular structural
element designed to endure multiple types of loading in various
directions, because of the isotropic properties of metals. As
another example, if weight is of concern in the design of a
structural element, a composite material may be chosen because of
the specific strength or the specific modulus of elasticity--that
is, the tensile strength to specific gravity ratio, or the modulus
of elasticity to specific gravity ratio--of composites. Weight
savings can be particularly advantageous in certain applications;
for example, in aircraft applications weight improvements can
increase fuel efficiency, which reduces the cost of operation and
increases the range of the aircraft. Thus, for a particular
application, a specific cross section and material combination can
be selected for a structural element in order to produce certain
characteristics.
[0005] Nevertheless, in some applications the properties of more
than one type of material may be desirable. For example, a
combination of a metal structure and a composite structure may be
desirable in an application designed for multiple or complex
loadings where the primary loading type is known and the weight of
the structural element is a concern. However, when composite
materials have been bonded to metal elements using conventional
thermal composite matrix curing techniques, in at least some
instances the differential thermal contraction between the metal
element and the composite material following the relatively
high-temperature curing has resulted in significant residual
stresses at the bond interface between the metal element and the
composite material, weakening the structure and essentially
defeating the purpose of bonding the two materials.
[0006] Accordingly, it is desirable to provide a method and
apparatus that combines a composite material with a metal element
to form a hybrid structural element that can at least to some
extent provide acceptable residual stresses at the bond interface
between the composite material and the metal element.
SUMMARY OF THE INVENTION
[0007] The foregoing needs are met, to a great extent, by the
present invention, wherein in one aspect an apparatus is provided
that in some embodiments bonds a composite material to a metal
element using a radiation curing process in order to form a hybrid
structural element.
[0008] In accordance with one aspect of the present invention, a
reinforced hybrid structural element can include a metal element
and a composite ply. In addition, the composite play can be bonded
to the metal element using radiation. The structural element can
optionally include an adhesive layer that is at least partially
cured using radiation in order to bond the composite ply to the
metal element.
[0009] In accordance with another aspect of the present invention,
a method of manufacturing a selectively reinforced hybrid
structural element can include the steps of providing a metal
element and laying up a composite ply over at least a partial
surface of the metal element. In addition, the method can include
at least partially curing a polymer matrix using radiation in order
to bond the composite ply to the metal element. The method can
optionally include the step of placing an adhesive layer that
includes the polymer matrix over at least the partial surface of
the metal element; in this case, the step of laying up also can
include laying up the composite ply over the adhesive layer.
[0010] In accordance with yet another aspect of the present
invention, a reinforced hybrid structural element can include a
metal element and a composite ply. Furthermore, the structural
element can include means for bonding the composite ply to the
metal element at least partially cured using radiation.
[0011] There has thus been outlined, rather broadly, certain
embodiments of the invention in order that the detailed description
thereof herein may be better understood, and in order that the
present contribution to the art may be better appreciated. There
are, of course, additional embodiments of the invention that will
be described below and which will form the subject matter of the
claims appended hereto.
[0012] In this respect, before explaining at least one embodiment
of the invention in detail, it is to be understood that the
invention is not limited in its application to the details of
construction and to the arrangements of the components set forth in
the following description or illustrated in the drawings. The
invention is capable of embodiments in addition to those described
and of being practiced and carried out in various ways. Also, it is
to be understood that the phraseology and terminology employed
herein, as well as the abstract, are for the purpose of description
and should not be regarded as limiting.
[0013] As such, those skilled in the art will appreciate that the
conception upon which this disclosure is based may readily be
utilized as a basis for the designing of other structures, methods
and systems for carrying out the several purposes of the present
invention. It is important, therefore, that the claims be regarded
as including such equivalent constructions insofar as they do not
depart from the spirit and scope of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a cross-sectional view illustrating a hybrid
metal-composite I-beam according to a preferred embodiment of the
invention.
[0015] FIG. 2 is a cutaway perspective view of the hybrid
metal-composite I-beam of FIG. 1.
[0016] FIG. 3 is a cross-sectional view illustrating a hybrid
metal-composite track member for supporting and constraining
airplane seats.
[0017] FIG. 4 is a flow chart illustrating steps that may be
followed in accordance with one embodiment of the method or
process.
[0018] FIG. 5 is a representative radiation curing system.
DETAILED DESCRIPTION
[0019] Some embodiments in accordance with the present invention
provide a hybrid metal-composite structural element that includes a
metal element and a composite material bonded together using
radiation curing. The structural element can include an adhesive
layer between the metal element and the composite material to bond
the composite material to the metal element and to act as a
corrosion-resistant barrier between the metal and the composite
element.
[0020] Composite materials including a polymer matrix are generally
fabricated by one of three types of processes. The first type is a
chemical curing process in which a two-part epoxy matrix is
combined and a spontaneous chemical reaction occurs between the
components of the two parts, resulting in cross-linking between the
polymer molecules. The second type is a thermal curing process used
with thermosetting or thermoplastic matrices in which an elevated
temperature causes cross-linking or consolidation of the polymer
molecules. The third type is a radiation curing process, in which
radiation--typically in the form of ultraviolet rays, X-rays or an
electron beam--is applied to a polymer matrix to induce a
cross-linking reaction between the polymer molecules.
[0021] As used here, radiation can include electromagnetic
radiation or a charged particle beam. For example, radiation can
include gamma rays, ultraviolet rays or X-rays in a wavelength
range from approximately 10 femtometers (fm) to approximately 380
nanometers (nm). In addition, for example, radiation can include an
electron beam or a cation beam with an energy level up to 10
megaelectron volts (MeV).
[0022] When a composite material is bonded to a metal by way of a
thermal curing process, bonding between the composite material and
a metal takes place at a relatively high temperature--typically
between 250 and 350 degrees Fahrenheit. Because the coefficients of
thermal expansion of metals compared to those of typical composite
materials are relatively high, as the structural element cools down
after the curing cycle a significant differential thermal
contraction occurs between the metal and the composite
material--that is, the metal contracts significantly more than the
composite material contracts. Thus, residual stresses develop at
the bond interface between the metal and the composite material. As
a result, for example, reinforcement of aluminum structural
elements with thermally-cured composites has not previously been
efficient or feasible, because of the differential thermal
expansion between the aluminum and the composite material.
[0023] In contrast, radiation curing of composite materials can
result in relatively small temperature increases in the composite
material--for example, the temperature increase can be as low as or
lower than five degrees Fahrenheit. Two categories of radiation
curing have become known in the art. The first category includes
free radical polymerization, in which a matrix including, for
example, acrylate monomers can be exposed to radiation, resulting
in cross-linking between the acrylate monomers. The second category
includes cationic polymerization, wherein special photoinitiators
such as, for example, iodonium salts, can be included in the matrix
in order to generate cations or protons when radiation is applied.
The cations or protons can act as an initiator to cause a
cross-linking reaction.
[0024] Since radiation curing processes result in such a small
temperature increase, a composite material can be bonded to a metal
structural element without significant differential thermal
contraction. As a result, radiation curing enables the use of
composite materials containing high strength resins to reinforce
metal structural elements. Selective reinforcement of a metallic
structural element with a composite material can increase the
strength as well as the stiffness of the structural element.
[0025] Because of the minimal temperature increase during the
radiation curing cycle, the residual stresses at the bond interface
between the metal element and the composite material is negligible
or insubstantial. Thus, a high-specific-strength and
high-specific-modulus-of-elasticity structural element is produced
having properties of both a metallic structural element and a
composite structural element. The invention will now be further
described with reference to the drawing figures, in which like
reference numerals refer to like parts throughout.
[0026] An embodiment of the present invention can include a
structural element such as the I-beam 10 shown in FIG. 1. The
I-beam 10 can include a web member 12, an upper cap member 14, and
a lower cap member 16. (The terms "upper" and "lower" are used here
solely for the purpose of distinguishing the two cap members from
each other. These terms do not imply a specific orientation of the
structural element in a particular application.)
[0027] The I-beam 10, or other structural element, can include a
metal element 18, containing a metal, such as aluminum, copper,
silver, magnesium, titanium, tungsten, iron, nickel, etc.; or a
metal alloy, such as an aluminum alloy, titanium alloy, brass,
bronze, steel, etc. The metal element can include, for example, the
web member 12 and portions 20, 22 of the cap members 14, 16. The
I-beam 10 also can include a reinforcing composite ply 24 located
in the upper cap member 14, a reinforcing composite ply 26 in the
lower cap member 16, a metal closeout cap 28 on the upper cap
member 14, and a metal closeout cap 30 on the lower cap member 16.
The composite plies 24, 26 can be located in the cap members 14, 16
approximately where the compressive and tensile stresses in a beam
under a bending load are highest. In general, a composite material
can be located at a selected, relatively high-stress location of a
metallic structural member cross section in order to provide
selective reinforcement of the structural member.
[0028] The composite plies 24, 26 can be cured by a radiation
process, for example, an electron-beam curing process. Although a
preferred embodiment includes an electron-beam curing process,
other embodiments can include an ultraviolet radiation, X-ray, or
other radiation curing process. The minimal temperature rise during
the radiation curing cycle results in negligible or insubstantial
residual stresses at bond interfaces 32 between the metal portion
20 of the upper cap member 14 and the composite ply 24, and between
the metal portion 22 of the lower cap member 16 and the composite
ply 26. As a result, this configuration combines the properties of
the metal element 18 and the high specific strength and specific
modulus of elasticity, or stiffness, of the composite plies 24,
26.
[0029] FIG. 2 shows a cutaway perspective view of the I-beam 10 in
FIG. 1. As shown in FIG. 2, the I-beam 10 can have a portion of the
metal element 18 removed to create a space 34 where the composite
ply 24 can be placed. For example, a space 34 can be machined out
of a metal element 18 by way of milling, drilling, or the like. In
FIG. 2, the space 34 has been machined out of the metal portion 20
of the upper cap member 14. In other embodiments, the composite ply
can be placed on the external surface of the metal element, or the
space can be created during the fabrication of the metal element,
for example, using a forging or die process.
[0030] In addition, in some embodiments an adhesive layer 36 can be
placed over the surface of the metal element 18 that interfaces
with the composite ply 24. The adhesive layer 36 can include a
polymer matrix such as, for example, an epoxy. In an alternative
embodiment, the adhesive layer 36 can be formed from a resin that
forms the matrix phase of the composite ply 24. Furthermore, a side
adhesive layer 38 can be included if the metal portion 20 of the
cap member 14 extends upward along the side of the composite ply
24.
[0031] Then, the composite ply 24 can be laid up over the adhesive
layer 36 in order to reinforce the I-beam 10. The composite ply 24
can include a matrix phase reinforced by fibers, such as carbon,
graphite, boron, silicon carbide, silicon nitride, aluminum oxide,
magnesium oxide, fused silica, zylon, aramids, nylon, asbestos, or
the like. The matrix phase can include a thermosetting polymer such
as, for example, an epoxy, bismaleimide (BMI), polyimide, cyanate
ester, polyester, silicon, or phenolic resin.
[0032] In some embodiments, the I-beam 10 can also include an upper
adhesive layer 40 and a metal closeout cap 28, which can protect
the composite ply 24 from a corrosive environment. In addition, the
metal closeout cap 28 can provide an impact-resistant surface to
protect the composite ply 24 from impact damage.
[0033] Some metals, such as aluminum, for example, can be highly
corrosive when in contact with a composite material, such as carbon
or graphite fiber. Thus, in order to extend the life of the
composite ply 24, the adhesive layer 36, 38, 40 can provide a
corrosion-resistant barrier between the composite ply 24 and the
metal element 18 to protect the composite ply 24 from the corrosive
effects of the metal. Thus, as shown in the lower cap member 16 of
FIG. 2, the composite ply 26 can be fully enclosed, or
encapsulated, by an adhesive layer 42 that envelopes the composite
ply 26.
[0034] In addition, the adhesive layer 36, 38, 40, 42 can include
glass fiber reinforcement, for example soda-lime glass, pyrex
glass, E-glass, S-glass or astroquartz fibers. The addition of
glass fiber reinforcement to the adhesive layer 36 can be
especially beneficial toward providing a corrosion-resistant
barrier between the metal element 18 and the composite ply 24, 26,
because when coupled with the various polymer matrices, the glass
fiber can provide physical isolation which can reduce or eliminate
the possibility of galvanic corrosion between the composite and
metal.
[0035] In addition, the thickness provided by glass fiber
reinforcement in the adhesive layer 36, 38, 40, 42 can provide an
additional physical barrier between the metal element 18 and the
composite ply 24, 26. For example, as little as five grams per
square meter of randomly-oriented glass fibers in an epoxy matrix
can provide a robust corrosion-resistant barrier. This barrier can
be very thin--for example ten thousandths of an inch or less, or
even vanishingly thin--and still provide substantial corrosion
resistance.
[0036] The adhesive layer 36, 38, 40, 42, as well as the composite
ply 24, 26, can be cured by a radiation process, for example, an
electron-beam curing process. Once again, although a preferred
embodiment includes an electron-beam curing process, other
embodiments can include an ultraviolet radiation, X-ray, or other
radiation curing process. Once again, the minimal temperature rise
during the radiation curing cycle results in negligible or
insubstantial residual stresses at bond interfaces 32 between the
metal portion 20 of the upper cap member 14 and the adhesive layer
36, 38, 40, and between the metal portion 22 of the lower cap
member 16 and the adhesive layer 42. As a result, this
configuration also combines the isotropic properties of the metal
element 18 and the high specific strength and specific modulus of
elasticity of the composite plies 24, 26.
[0037] The reinforcing fibers in the composite ply 24, 26 can be
generally aligned in a general fiber orientation, which can be
aligned with the stresses caused by the loading type for which the
structural element is designed. For example, in one embodiment the
fibers in the composite ply 24, 26 can be axially aligned in
parallel with a longitudinal centerline of the I-beam 10. In an
alternative embodiment, the fibers can be axially aligned normal to
the longitudinal centerline of the I-beam 10. Similarly, in another
embodiment the fibers can be oriented at approximately forty-five
degrees from the longitudinal centerline in order to provide
strength in shear loading.
[0038] Furthermore, in order to impede crack propagation through
the composite ply 24, 26 the reinforcing fibers can be axially
aligned at relatively small angles on both sides of the
longitudinal centerline of the I-beam 10. For example, in yet
another embodiment the fibers can be oriented at approximately five
degrees from each side the longitudinal center line. Moreover, in
various embodiments multiple composite plies 24, 26 can include
fiber orientations aligned at different angles in order to provide
strength for complex or multiaxial loadings.
[0039] The composite ply 24, 26 can include a composite fabric, or
a composite tape. In addition, the composite ply 24, 26, optionally
including a composite tape or composite fabric, can be
pre-impregnated with a resin before being laid up on the metal
element 18. Furthermore, the composite ply 24, 26 can be precured,
that is, the composite ply 24, 26 can be partially or fully cured
before being laid up on the metal element 18, and the adhesive
layer 36, 38, 40, 42 can be cured using radiation to bond the
precured composite ply 24, 26 to the metal element 18.
[0040] An alternative embodiment of a structural element
selectively reinforced with a composite ply is shown in FIG. 3.
This embodiment is an example of an application-specific
selectively reinforced hybrid metal-composite structural element.
The structural element shown in FIG. 3 is a seat track 44 designed
to support and constrain airplane passenger seats. The seat track
44 includes a track member 46 which is connected to two channels
48, 50 by two support members 52, 54. The track member 46, the
channel members 48, 50 and the support members 52, 54 are made of a
metal, for example, titanium.
[0041] In addition the seat track 44 includes reinforcement
elements 56, 58 along each of the channels 48, 50. In this
embodiment, metal is not removed from the metal element to create a
space for the composite plies. Instead, the composite plies are
bonded to the exterior surface of the metal element. The
reinforcement elements 56, 58 are bonded to the metal channels 48,
50 by way of a radiation curing process, optionally including an
adhesive layer between the reinforcement elements 56, 58 and the
metal channels 48, 50. The reinforcement elements 56, 58 provide
substantial strength to prevent compression crippling of the seat
track channels 48, 50 by limiting the deflection of the channels
48, 50. This configuration can provide a relatively lightweight,
high-strength seat track 44.
[0042] Additional alternative embodiments of the present invention
can include structural members of virtually any shape or form. For
example, a sine wave beam, which includes a sine wave-shaped web
member and cap members similar to an I-beam, can include selective
reinforcement along the lower or upper cap member by a composite
material to increase the beam's resistance to deflection.
[0043] In addition, alternative embodiments can include a
reinforcement element that is a hybrid laminate of alternating
layers of a metal foil and a fiber-reinforced composite material.
For example, the reinforcement element can include a
titanium-polymer hybrid laminate, such as that disclosed in U.S.
Pat. No. 6,114,050, entitled TITANIUM-POLYMER HYBRID LAMINATES, to
Westre, et al., issued Sep. 5, 2000, the disclosure of which is
hereby incorporated by reference in its entirety.
[0044] An embodiment of the present invention can further include a
process or method for manufacturing a selectively-reinforced hybrid
metal-composite structural element. A flow chart illustrating steps
that can be included in an embodiment are shown in FIG. 4. The
process starts in step 60 and in step 62 a metal element can be
provided. As discussed above, the metal element serves as the core
of the structural element, and can be formed or machined to provide
any suitable cross-section, in accordance with the loading types
the structural element is designed to carry. The metal element can
be produced using any suitable means, such as stamping, forming or
machining. However, fabrication of the metal element is optional,
and the metal element can be optionally procured from another
source.
[0045] In step 64, a portion of the metal can be removed from the
metal element in order to provide a space where the composite
reinforcement element can be located. The metal can be removed by
any suitable means, including a machining process, such as milling
or drilling, a mechanical process, a heat process, an
electrochemical process, or the like. Typically, the metal is
removed from an area of the structural member cross-section that
will carry a relatively high stress when the structural element is
under an anticipated type of loading.
[0046] In step 66 an adhesive layer can be placed over a surface of
the metal element 18 where a composite ply is to be located. As
discussed above, the adhesive layer can include glass fiber
reinforcement, for example soda-lime glass, pyrex glass, E-glass,
S-glass or astroquartz fibers. The adhesive layer can form a simple
planar layer over a surface of the metal element, or the adhesive
layer can also include a side or sides. As further discussed above,
the adhesive layer can also include an upper layer in order to
provide a corrosion-resistant layer around the composite ply,
effectively encapsulating the composite ply in order to isolate the
composite ply from the environment.
[0047] Next, a composite ply is laid up on the metal element in
step 68. As discussed above, the composite ply can include a resin
and a fiber reinforcement, such as graphite or aramid fibers.
Alternatively, step 68 can include laying up multiple composite
plies, one over another. In addition, the reinforcement fibers of a
composite ply can be oriented in a direction of a loading type
which the structural element is designed to carry. For example, the
fiber orientation can be aligned parallel to a longitudinal
centerline of the structural element, at approximately forty-five
degrees from the longitudinal centerline, normal to the centerline,
or at five degrees from each side of the center line.
[0048] In embodiments that include multiple composite plies, the
individual plies can include fiber orientations in differing
directions. For example, an embodiment can include a zero-degree
ply, wherein the fiber orientation is approximately parallel to the
longitudinal centerline of the structural element; a ninety-degree
ply, wherein the fiber orientation is approximately normal to the
longitudinally centerline; a forty-five-degree ply, wherein the
fiber orientation is approximately forty-five degrees from the
centerline; and a five-degree ply, wherein the fiber orientation is
approximately five degrees from the centerline of the structural
element; or any combination of these or other fiber-orientation
plies.
[0049] In step 70, the adhesive layer and the composite ply can be
cured using a radiation process, for example, an electron-beam
curing process. Nevertheless, as explained above, even though a
preferred embodiment includes an electron-beam curing process,
other embodiments can include an ultraviolet radiation, X-ray, or
other radiation curing process. Alternatively, the composite ply
can be precured, and the adhesive layer can be cured using
radiation.
[0050] As an example, a representative radiation curing system is
shown in FIG. 5, including a radiation generator 72 and a
manufacturing tool 74 configured to hold the structural element 10.
For example, the radiation generator 72 can include an ultraviolet
radiation source, an X-ray machine, or an electron beam generator,
or "electron gun." Examples of electron beam generators suitable
for use with some embodiments of the invention include the
Rhodotron series of compact, high power electron beam accelerator
systems, such as the TT 100, TT 200, TT 300 and TT 1000 models,
which operate at outputs of 3-10 MeV in a power range from 35 kW to
700 kW, manufactured by IBA Technology Group of Louvain-la-Nueve,
Belgium.
[0051] Furthermore, in some configurations, the radiation generator
72 can be configured to move over the areas to be cured on the
structural element 10, which can be held stationary by the
manufacturing tool 74. In other configurations, the structural
element 10 can be moved under a stationary radiation generator 72
by the manufacturing tool 74.
[0052] As discussed above, in some embodiments the radiation curing
cycle can result in a minimal increase in the temperature of the
composite ply, for example, as low as five degrees Fahrenheit or
less, resulting in a negligible or insubstantial residual stress at
the bond interface between the metal element and the composite ply
or adhesive layer. Nevertheless, in other embodiments, the
radiation curing can occur at a higher temperature that is below
the temperature range of conventional thermal curing, for example,
below approximately 120 degrees Celsius (approximately 248 degrees
Fahrenheit). Alternatively, the temperature of the metal element
and the composite ply or adhesive layer during the radiation curing
cycle can be maintained within an acceptable range--for example,
within 75 degrees Celsius (approximately 135 degrees
Fahrenheit)--of the intended design application temperature, that
is, the temperature at which the structural element is designed to
be used.
[0053] In an alternative embodiment, the adhesive layer cure can be
initiated with a radiation process, initially forming a relatively
weak bond between the metal element and the composite ply, and the
curing process can be completed by a thermal process at an elevated
temperature without inducing thermal distortion, since the initial
bonding between the metal and the composite ply has occurred during
the radiation curing. Similarly, in other embodiments, the adhesive
layer and the composite ply both can be initially cured by a
radiation process, and the curing process can be later completed by
a thermal process. Furthermore, in various embodiments the
composite ply, the adhesive layer and the metal element can be
actively cooled during the curing process, for example, with a fan,
water, or the like.
[0054] Then, in step 76, a metal closeout cap can be optionally
overlaid upon the composite ply. As discussed above, the metal
closeout cap can provide an impact-resistant shell, as well as
provide a barrier to protect the composite ply from a corrosive
environment. In this case, the composite ply can be fully
enveloped, or encapsulated, by the adhesive layer in order to
provide a corrosion-resistant barrier between the composite ply and
the metal element. Additionally, the metal closeout cap can be
bonded to the metal portion of the structural element. The process
ends in step 78.
[0055] The many features and advantages of the invention are
apparent from the detailed specification, and thus, it is intended
by the appended claims to cover all such features and advantages of
the invention which fall within the true spirit and scope of the
invention. Further, since numerous modifications and variations
will readily occur to those skilled in the art, it is not desired
to limit the invention to the exact construction and operation
illustrated and described, and accordingly, all suitable
modifications and equivalents may be resorted to, falling within
the scope of the invention.
* * * * *