U.S. patent application number 17/719101 was filed with the patent office on 2022-07-28 for lightweight sandwich structures and methods of manufacturing the same.
The applicant listed for this patent is HRL LABORATORIES, LLC. Invention is credited to Eric C. Clough, Alicia J. Dias, Jacob M. Hundley, Tobias A. Schaedler.
Application Number | 20220234315 17/719101 |
Document ID | / |
Family ID | |
Filed Date | 2022-07-28 |
United States Patent
Application |
20220234315 |
Kind Code |
A1 |
Hundley; Jacob M. ; et
al. |
July 28, 2022 |
LIGHTWEIGHT SANDWICH STRUCTURES AND METHODS OF MANUFACTURING THE
SAME
Abstract
A method of forming a sandwich structure including at least
partially filling an open volume of an open cellular core with a
sacrificial mold material, consolidating the sacrificial mold
material to form a sacrificial mold, laying up a composite
facesheet on each of at least two surfaces of the open cellular
core, co-curing the composite facesheets by applying a
consolidation temperature and a compaction pressure to the
composite facesheets to form the sandwich structure, and removing
the sacrificial mold. The compaction pressure is greater than a
compressive strength of the open cellular core and less than a
combined compressive strength of the open cellular core and the
sacrificial mold.
Inventors: |
Hundley; Jacob M.; (Thousand
Oaks, CA) ; Dias; Alicia J.; (Boston, MA) ;
Clough; Eric C.; (Santa Monica, CA) ; Schaedler;
Tobias A.; (Oak Park, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HRL LABORATORIES, LLC |
Malibu |
CA |
US |
|
|
Appl. No.: |
17/719101 |
Filed: |
April 12, 2022 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15967037 |
Apr 30, 2018 |
11358350 |
|
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17719101 |
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62527773 |
Jun 30, 2017 |
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International
Class: |
B29C 70/68 20060101
B29C070/68; B32B 3/12 20060101 B32B003/12; B32B 27/38 20060101
B32B027/38; B29C 71/00 20060101 B29C071/00; B29C 33/38 20060101
B29C033/38; B32B 27/06 20060101 B32B027/06 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] This invention was made with Government support under
Contract No. NNC15CA16C awarded by NASA. The Government has certain
rights in the invention.
Claims
1. A sandwich structure comprising: an open cellular core having a
first surface and a second surface opposite the first surface, the
open cellular core defining an open volume; a first composite
facesheet bonded to the first surface of the open cellular core,
the first composite facesheet conforming continuously to the open
cellular core along the first surface; and a second composite
facesheet bonded to the second surface of the open cellular core,
the second composite facesheet conforming continuously to the open
cellular core along the second surface.
2. The sandwich structure of claim 1, wherein the open cellular
core comprises a plurality of interconnected struts arranged in a
lattice structure.
3. The sandwich structure of claim 2, wherein each strut of the
plurality of interconnected struts has a hollow cross-section.
4. The sandwich structure of claim 2, wherein each strut of the
plurality of interconnected struts has a solid cross-section.
5. The sandwich structure of claim 2, wherein the lattice structure
comprises a plurality of repeating unit cells.
6. The sandwich structure of claim 2, wherein the lattice structure
comprises a plurality of repeating half unit cells.
7. The sandwich structure of claim 2, wherein the plurality of
interconnected struts comprise at least one material selected from
the group consisting of metal, silicon carbide, silicon oxycarbide,
alumina, silicon carbonitrile, polymer, ceramic, and combinations
thereof.
8. The sandwich structure of claim 1, wherein the open cellular
core has a density in a range from about 0.02 grams per cubic
centimeter to about 1 gram per cubic centimeter.
9. The sandwich structure of claim 1, wherein each of the first
composite facesheet and the second composite facesheet comprises a
plurality of plies and a matrix around the piles.
10. The sandwich structure of claim 9, wherein at least one sheet
selected from the group consisting of the first composite facesheet
and the second composite facesheet is bonded to the open cellular
core only by excess in material of the matrix of the one sheet.
11. The sandwich structure of claim 9, wherein the matrix comprises
at least one material selected from the group consisting of epoxy,
silicone, urethane, cyanate ester, polyimide, bismaleimide,
acrylate, carbosilane, siloxane, sequisiloxane, and combinations
thereof.
12. The sandwich structure of claim 9, wherein each of the first
composite facesheet and the second composite facesheet further
comprises a fiber reinforcement.
13. The sandwich structure of claim 12, wherein the fiber
reinforcement comprises a at least one material selected from the
group consisting of carbon, glass, alumina, silicon carbide, boron,
aramid, polyethylene, and combinations thereof.
14. The sandwich structure of claim 12, wherein the fiber
reinforcement comprises a fiber reinforcement ply having at least
one configuration selected from the group consisting of continuous
unidirectional fibers, woven fibers, knit fibers, braided fibers,
discontinuous chopped fibers, whiskers, platelets, particulates,
and combinations thereof.
15. The sandwich structure of claim 12, wherein a fiber volume
fraction of at least one sheet selected from the group consisting
of the first composite facesheet and the second composite facesheet
is at least about 65%.
16. The sandwich structure of claim 1, wherein: each of the first
composite facesheet and the second composite facesheet has a
thickness of about 1 mm; and the open cellular core has a thickness
in a range from about 0.5 mm to about 50 mm.
17. The sandwich structure of claim 1, wherein the open volume
extends along three orthogonal axes.
18. The sandwich structure of claim 1, wherein at least one sheet
selected from the group consisting of the first composite facesheet
and the second composite facesheet is curved.
19. The sandwich structure of claim 1, wherein: the open cellular
core comprises a plurality of interconnected struts arranged in a
lattice structure; the open cellular core has a density in a range
from about 0.02 grams per cubic centimeter to about 1 gram per
cubic centimeter; each of the first composite facesheet and the
second composite facesheet comprises a plurality of plies and a
matrix around the piles; each of the first composite facesheet and
the second composite facesheet has a thickness of about 1 mm; and
the open cellular core has a thickness in a range from about 0.5 mm
to about 50 mm.
20. The sandwich structure of claim 19, wherein: the lattice
structure comprises a plurality of repeating half unit cells; the
plurality of interconnected struts comprise at least one material
selected from the group consisting of metal, silicon carbide,
silicon oxycarbide, alumina, silicon carbonitrile, polymer,
ceramic, and combinations thereof; and at least one sheet selected
from the group consisting of the first composite facesheet and the
second composite facesheet is bonded to the open cellular core only
by excess in material of the matrix of the one sheet.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application is a divisional application of U.S. patent
application Ser. No. 15/967,037, filed Apr. 30, 2018, which claims
priority to and the benefit of U.S. Provisional Application No.
62/527,773, filed Jun. 30, 2017, the entire content of which is
incorporated herein by reference.
FIELD
[0003] The present disclosure relates generally to sandwich
structures and methods of manufacturing the same.
BACKGROUND
[0004] Sandwich structures including a pair of facesheets connected
by a low density core are commonly employed in aircraft and
spacecraft due to their increased flexural stiffness and buckling
resistance compared to stiffened plates having an equivalent mass.
In sandwich structures, the facesheets are configured to carry all
in-plane loads and the core transmits shear loads and increases the
effective moment of inertia of the sandwich structure.
[0005] In applications such as space launch vehicles it is
desirable to reduce core mass and increase facesheet stiffness and
strength. Commonly, these properties are achieved by utilizing an
ultralight core material (e.g., having a density less than 0.15
grams per cubic centimeter) and fiber reinforced composite
facesheets (e.g., carbon fiber epoxy). Ideally, these sandwich
structures would be formed by co-curing the facesheets to the core
by laying up polymer impregnated composite plies onto exposed
surfaces of the core and consolidating the plies with the
application of heat and pressure because co-curing can increase the
specific strength and stiffness of the facesheets, eliminate
parasitic adhesive mass in the facesheets, and reduce tolerance
errors for complex assemblies.
[0006] However, related art sandwich structures with lightweight
cores are not formed by co-curing because the pressure utilized to
consolidate the facesheets during co-curing exceeds the relatively
low compressive strength of the lightweight core. Accordingly,
co-curing cannot be utilized with related art methodologies for
forming sandwich structures without damaging the lightweight core.
Accordingly, some related art sandwich structures with lightweight
cores are formed by separately forming and consolidating the
facesheets and then attaching the consolidated facesheets to the
core, which increases the mass and cost of manufacturing the
sandwich structure. Alternatively, related art sandwich structures
may be formed by co-curing the facesheets by consolidating the
facesheets under a reduced compaction pressure (e.g., a sub-optimal
compaction pressure), which limits the performance of the
facesheets to carry in-plane loads and increases the parasitic
adhesive mass of the sandwich structure.
SUMMARY
[0007] The present disclosure is directed to various methods of
manufacturing a sandwich structure. In one embodiment, the method
includes at least partially filling an open volume of an open
cellular core with a sacrificial mold material, consolidating the
sacrificial mold material to form a sacrificial mold, laying up a
composite facesheet on each of at least two surfaces of the open
cellular core, co-curing the composite facesheets by applying a
consolidation temperature and a compaction pressure to the
composite facesheets to form the sandwich structure, and removing
the sacrificial mold. The compaction pressure is greater than a
compressive strength of the open cellular core and less than a
combined compressive strength of the open cellular core and the
sacrificial mold.
[0008] The method may also include placing the open cellular core
in a chamber of a mold before at least partially filling the open
volume with the sacrificial mold material.
[0009] The at least two surfaces of the open cellular core may be
in direct contact with inner surfaces of the chamber.
[0010] The method may also include pressing the at least two
surfaces of the open cellular core into at least one spacer
positioned between the open cellular core and inner surfaces of the
chamber. The at least one spacer masks the at least two surfaces of
the open cellular core from contact with the sacrificial mold
material. The material of the at least one spacer may be silicone,
rubber, closed cell foam, a polymer film, or a combination
thereof.
[0011] The consolidation temperature may be from about 23.degree.
C. to about 180.degree. C.
[0012] The compaction pressure may be from about 0.1 MPa to about
12 MPa.
[0013] The method may also include applying a release agent to the
open cellular core before the at least partially filling of the
open volume with the sacrificial mold material, and masking the at
least two surfaces of the open cellular core against exposure to
the release agent.
[0014] The at least partially filling of the opening volume with
the sacrificial mold material may be performed by pouring under
gravity, filling under vacuum, filling under positive pressure,
sifting powder, compaction of powder, or a combination thereof.
[0015] The sacrificial mold material may be of eutectic salt,
plaster, polyethylene glycol (PEG), polyethylene oxide (PEO),
ceramic spheres, plaster, wax, or a combination thereof.
[0016] Each of the at least two composite facesheets may include
pre-impregnated fiber reinforced polymers.
[0017] Each of the at least two composite facesheets may include a
dry fabric reinforcement layer and a liquid resin on the dry fabric
reinforcement layer.
[0018] The removing of the sacrificial mold may be performed by
burning the sacrificial mold, dissolving the sacrificial mold,
etching the sacrificial mold, fracturing the sacrificial mold,
evaporating the sacrificial mold, melting the sacrificial mold, or
a combination thereof.
[0019] The open cellular core may include a series of struts
arranged in a lattice structure. Each strut of the series of struts
may have a solid cross-section or a hollow cross-section. Each
strut of the series of struts may be a photopolymer waveguide.
[0020] The open cellular core may include foam.
[0021] The open cellular core may include a partially connected
honeycomb structure or a grid architecture.
[0022] A method of forming a sandwich structure according to
another embodiment of the present disclosure includes at least
partially filling an open volume of an open cellular core with a
sacrificial mold material, consolidating the sacrificial mold
material to form a sacrificial mold, laying up a composite
facesheet on each of at least two common surfaces of the open
cellular core and the sacrificial mold, co-curing the composite
facesheets by applying a consolidation temperature and a compaction
pressure to the composite facesheets to form the composite sandwich
structure, and removing the sacrificial mold. The open volume of
the open cellular core extends along three orthogonal axes. The
compaction pressure is greater than a compressive strength of the
open cellular core and less than a combined compressive strength of
the open cellular core and the sacrificial mold.
[0023] The present disclosure is also directed to various
embodiments of a sandwich structure. In one embodiment, the
sandwich structure includes an open cellular core defining an open
volume, a sacrificial mold at least partially filling the open
volume of the open cellular core, and at least two composite
facesheets bonded to at least two surfaces of the open cellular
core.
[0024] This summary is provided to introduce a selection of
features and concepts of embodiments of the present disclosure that
are further described below in the detailed description. This
summary is not intended to identify key or essential features of
the claimed subject matter, nor is it intended to be used in
limiting the scope of the claimed subject matter. One or more of
the described features may be combined with one or more other
described features to provide a workable device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] These and other features and advantages of embodiments of
the present disclosure will become more apparent by reference to
the following detailed description when considered in conjunction
with the following drawings. In the drawings, like reference
numerals are used throughout the figures to reference like features
and components. The figures are not necessarily drawn to scale.
[0026] FIGS. 1A-1F illustrate steps of forming a sandwich structure
without utilizing a spacer layer according to one embodiment of the
present disclosure;
[0027] FIGS. 1G-1H are detail views illustrating steps of forming
the sandwich structure without utilizing the spacer layer according
to the embodiment illustrated in FIGS. 1A-1F;
[0028] FIGS. 2A-2D illustrate steps of forming a sandwich structure
utilizing a spacer layer according to one embodiment of the present
disclosure; and
[0029] FIG. 3 is a flowchart illustrating steps of forming a
sandwich structure according to one embodiment of the present
disclosure.
DETAILED DESCRIPTION
[0030] FIGS. 1A-1H depict steps of a method of manufacturing a
sandwich structure 100 including an open cellular core 101 and
first and second facesheets 102, 103 coupled to the open cellular
core 101 according to one embodiment of the present disclosure.
[0031] As illustrated in FIG. 1A, the method includes a step of
obtaining or manufacturing the open cellular core 101. The open
cellular core 101 defines an open volume (e.g., a porosity) 104. In
one or more embodiments, the open volume 104 of the open cellular
core 101 may extend laterally, longitudinally, and transversely
through the open cellular core 101 (e.g., the open volume 104 may
extend in a thickness direction, a length direction, and a width
direction of the open cellular core 101). That is, in one or more
embodiments, the open volume 104 of the open cellular core 101 is
open along three orthogonal axes. In the illustrated embodiment,
the open cellular core 101 includes a series of interconnected
struts 105 arranged in a lattice structure 106 (e.g., a series of
repeating unit cells or half unit cells). In one or more
embodiments, the open cellular core 101 may include a foam, a grid,
or a partially-connected honeycomb structure. In one or more
embodiments in which the open cellular core 101 includes a series
of interconnected struts 105 arranged in a lattice structure 106,
the struts 105 may be solid or hollow. The open cellular core 101
may include any suitable material depending on the desired
properties of the sandwich structure 100. For instance, in one or
more embodiments, the open cellular core 101 may be made out of
metal (e.g., aluminum, nickel, copper), silicon carbide, silicon
oxycarbide, alumina, silicon carbonitrile, polymer (e.g., acrylate,
methacrylate, thiol, epoxy, urethane, polyimide), ceramic, or any
combination thereof. In one or more embodiments, the open cellular
core 101 may have a thickness T from approximately (about) 0.5 mm
to approximately (about) 50 mm. In one or more embodiments, the
open cellular core 101 may have a density from approximately
(about) 0.02 grams per cubic centimeter (g/cc) to approximately
(about) 1 g/cc. In one or more embodiments, the open cellular core
101 may include hollow nickel struts 105 arranged in a lattice 106
structure having a density of approximately (about) 0.4 g/cc and a
thickness T of approximately (about) 13 mm.
[0032] With continued reference to the embodiment illustrated in
FIG. 1A, the method also includes a step of cleaning the open
cellular core 101 (e.g., surfaces of the struts 105) to remove any
contaminants from the surfaces of the open cellular core 101, such
as particulates, dust, and/or oil. In the illustrated embodiment,
the method also includes a step of applying a release agent (e.g.,
silicone, lecithin, wax, or combinations thereof) to at least a
portion of the open cellular core 101. In one or more embodiments,
the release agent may be applied to surfaces of the open cellular
core 101 (e.g., surfaces of the struts 105) defining the open
volume 104. The release agent is configured to promote or aid in
removal of a sacrificial mold (described below in a subsequent step
of the method) from the open volume 104 of the open cellular core
101. Additionally, in one or more embodiments, the method may
include a step of masking one or more portions (e.g., opposing
upper and lower surfaces 107, 108) of the open cellular core 101
before applying the release agent. Masking one or more portions of
the open cellular core 101 is configured to prevent or protect
these portions of the open cellular core 101 from being exposed to
the release agent during the step of applying the release agent to
the open cellular core 101. In one or more embodiments, the
surfaces 107, 108 of the open cellular core 101 along which the
first and second facesheets 102, 103 will be coupled to the open
cellular core 101 during a subsequent step of the method described
below may be masked against exposure to the release agent.
Otherwise, application of release agent to the surfaces 107, 108
along which the facesheets 102, 103 will be coupled to the open
cellular core 101 might weaken the connection between the
facesheets 102, 103 and the open cellular core 101 (e.g., masking
the upper and lower surfaces 107, 108 of the open cellular core 101
is configured to promote interfacial adhesion between the
facesheets 102, 103 and the open cellular core 101).
[0033] With reference now to the embodiment illustrated in FIG. 1B,
the method includes a step of inserting the open cellular core 101
into a chamber or cavity 109 defined by a mold 110. In the
illustrated embodiment, the mold 110 defines an inlet opening 111
through which sacrificial mold material 115 may be introduced into
the chamber 109 in a subsequent step. Additionally, in one or more
embodiments, the mold 110 may also define an outlet opening. In one
or more embodiments, the mold 110 may be thermally insulated.
[0034] With reference now to the embodiment illustrated in FIG. 1C,
the method includes a step of introducing the sacrificial mold
material 115 into the open volume 104 of the open cellular core 101
and at least partially filling the open volume 104 of the open
cellular core 101 with the sacrificial mold material 115 (e.g.,
infiltrating at least a portion of the open volume 104 of the open
cellular core 101 with the sacrificial mold material 115). In one
or more embodiments, the step of at least partially filling the
open volume 104 of the open cellular core 101 may include
completely or substantially completely filling the open volume 104
of the open cellular core 101 with the sacrificial mold material
115. In one or more embodiments, step of at least partially filling
the open volume 104 of the open cellular core 101 may include
inserting the sacrificial mold material 115 into the chamber 109 of
the mold 110 through the inlet opening 111 and allowing the
sacrificial mold material 115 to flow into the open volume 104 of
the open cellular core 101. In one or more embodiments, the outlet
opening of the mold 110 may be utilized to capture excess of the
sacrificial mold material 115 introduced into the chamber 109 and
the open volume 104 of the open cellular core 101, and/or the
outlet opening may be utilized to capture entrapped air in the open
volume 104 of the open cellular core 101. In one or more
embodiments, the sacrificial mold material 115 may include eutectic
salt, plaster, polyethylene glycol (PEG), polyethylene oxide (PEO),
ceramic spheres, plaster, wax, or combinations thereof. In one or
more embodiments, the sacrificial mold material 115 may be a powder
mixture of ceramic spheres, plaster, and PEG. In one or more
embodiments, the chamber 109 of the mold 110 may be thermally
insulated depending on the type of sacrificial mold material 115
utilized (e.g., the chamber 109 of the mold 110 may be thermally
insulated when the sacrificial mold material 115 is molten eutectic
salt).
[0035] In one or more embodiments, the method may include a step of
completely or substantially completely filling the open volume 104
of the open cellular core 101 with the sacrificial mold material
115. In one or more embodiments, the method may include a step of
completely or substantially completely filling the chamber 109 of
the mold 110 with the sacrificial mold material 115. In this
manner, the chamber 109 of the mold 110 enables integration of the
sacrificial mold material 115 with the open cellular core 101 and
defines the geometry of the combined open cellular core 101 and the
sacrificial mold material 115 (e.g., the chamber 109 of the mold
110 defines the geometry of the parallel core-sacrificial mold
material combination). In one or more embodiments, the step of
introducing the sacrificial mold material 115 into the open volume
of the open cellular core 101 may be performed in any suitable
manner depending, for instance, on the type of sacrificial mold
material 115 utilized and/or the phase of the sacrificial mold
material 115 (e.g., liquid or powder). In one or more embodiments,
the step of introducing the sacrificial mold material 115 into the
open volume 104 of the open cellular core 101 may include pouring
under gravity, filling under vacuum, filling under positive
pressure, sifting and/or compaction of powder, or one or more
combinations thereof.
[0036] The chamber 109 of the mold 110 is at least as large as the
bulk volume of the open cellular core 101. In the embodiment
illustrated in FIGS. 1A-1H, the chamber 109 is sized such that the
upper and lower surfaces 107, 108 of the open cellular core 101
along which the facesheets 102, 103 will be coupled to the open
cellular core 101 are in direct contact with inner surfaces (e.g.,
inwardly facing surfaces) 112, 113, respectively, of the mold
110.
[0037] With reference now to the embodiment illustrated in FIGS. 1D
and 1H, after the step of at least partially filling the open
volume 104 of the open cellular core 101 with the sacrificial mold
material 115, the method includes a step of consolidating the
sacrificial mold material 115 to solidify the sacrificial mold
material 115 into a solid sacrificial mold 116. The solid
sacrificial mold 116, formed by the step of consolidating the
sacrificial mold material 115, is configured to increase the
compressive strength of the open cellular core 101 (e.g., the
compressive strength of the combined solid sacrificial mold 116 and
the open cellular core 101 exceeds the compressive strength of the
open cellular core 101 alone). In one or more embodiments, the step
of consolidating the sacrificial mold material 115 may be performed
in any suitable manner depending, for instance, on the type of
sacrificial mold material 115 utilized and/or the phase of the
sacrificial mold material 115 (e.g., liquid or powder). In one or
more embodiments, the step of consolidating the sacrificial mold
material 115 may include curing (e.g., heating), solidification
(e.g., cooling), compaction (e.g., sintering), and/or evaporation
of a liquid (e.g., a solvent) in the sacrificial mold material 115.
Following the step of consolidating the sacrificial mold material
115, the combined sacrificial mold 116 and the open cellular core
101 may be removed from the chamber 109 of the mold 110.
[0038] In one or more embodiments in which the sacrificial mold 116
is porous, the method may include a step of applying a sealant on
surfaces (e.g., upper and lower surfaces 117, 118) of the
sacrificial mold 116 along which the composite facesheets 102, 103,
respectively, will be laid up in a subsequent step (e.g., a sealant
may be applied to the upper and lower surfaces 117, 118 of the
sacrificial mold 116 that will interface with (e.g., contact) the
composite facesheets 102, 103, respectively). The sealant is
configured to prevent or inhibit the infiltration of excess
adhesive into the porous sacrificial mold 116 during a subsequent
step of co-curing composite facesheets 102, 103 to the open
cellular core 101, and the inhibition of adhesive into the porous
sacrificial mold 116 is configured to aid in the removal of the
sacrificial mold 116 from the open volume 104 of the open cellular
core 101 during a subsequent step of the method described below.
Additionally, in one or more embodiments, the method may include a
step of applying a release agent on the surfaces 117, 118 of the
sacrificial mold 116 along which the composite facesheets 102, 103
will be laid up in a subsequent step (e.g., a release agent may be
applied to the surfaces 117, 118 of the sacrificial mold 116 that
will interface with (e.g., contact) the composite facesheets 102,
103, respectively). The release agent is configured to aid in the
removal of the sacrificial mold 116 from the open volume 104 of the
open cellular core 101 during a subsequent step of the method
described below. In one or more embodiments, the upper and lower
surfaces 107, 108 of the open cellular core 101, along which the
composite facesheets 102, 103 will be attached, may be masked
against exposure to the sealant and/or the release agent applied to
the sacrificial mold 116, which is configured to promote a robust
bond between the composite facesheets 102, 103 and the open
cellular core 101.
[0039] With reference now to the embodiment illustrated in FIG. 1E,
the method also includes a step of laying up the composite
facesheets 102, 103 (e.g. composite plies) on at least two surfaces
(e.g., the opposing upper and lower surfaces 107, 108) of the open
cellular core 101. In the illustrated embodiment, the sacrificial
mold 116 is in parallel with the open cellular core 101 and is in
series with the composite facesheets 102, 103 on the surfaces 107,
108 of the open cellular core 101. In one or more embodiments, the
composite facesheets 102, 103 may be pre-impregnated
fiber-reinforced polymer plies. In one or more embodiments, the
composite facesheets 102, 103 may be dry fabric reinforcement plies
onto which a liquid resin is deposited. In one or more embodiments,
each of the composite facesheets 102, 103 may have a thickness from
approximately (about) 0.1 mm to approximately (about) 13 mm. In one
or more embodiments, the composite facesheets 102, 103 may include
any suitable fiber reinforcement material, such as carbon, glass,
alumina, silicon carbide, boron, aramid, polyethylene, or any
combination or combinations thereof. In one or more embodiments,
the composite facesheets 102, 103 may include a matrix material,
such as epoxy, silicone, urethane, cyanate ester, polyimide,
bismaleimide, acrylate, carbosilane, siloxane, and/or
sequisiloxane. In one or more embodiments, the composite facesheets
102, 103 may include a fiber reinforcement ply having continuous
unidirectional fibers, woven fibers, knit fibers, braided fibers,
discontinuous chopped fibers, whiskers, platelets, and/or
particulates. In one or more embodiments, each of the facesheets
102, 103 may be an approximately (about) 1 mm thick unidirectional
carbon fiber reinforced epoxy composite facesheet with a
quasi-isotropic layup.
[0040] With continued reference to the embodiment illustrated in
FIG. 1E, after the step of laying up the composite facesheets 102,
103 on the surfaces 107, 108 of the open cellular core 101, the
method includes a step of co-curing the composite facesheets 102,
103 onto the surfaces 107, 108 of the open cellular core 101. The
step of co-curing the composite facesheets 102, 103 onto the
surfaces 107, 108 of the open cellular core 101 includes
consolidating the composite facesheets 102, 103. In one or more
embodiments, the step of consolidating the composite facesheets
102, 103 includes applying a consolidation temperature and a
compaction pressure to the composite facesheets 102, 103. In one or
more embodiments, the consolidation temperate applied to the
composite facesheets 102, 103 is from approximately (about)
23.degree. C. to approximately (about) 180.degree. C. In one or
more embodiments, the consolidation temperature may be greater than
approximately (about) 180.degree. C. In one or more embodiments,
the consolidation temperature may be selected depending on the type
of matrix material in the composite facesheets 102, 103 (e.g., a
consolidation temperature greater than 180.degree. C. may be
utilized in the step of consolidating the composite facesheets 102,
103 when the matrix material is a high temperature polymer, such as
bismaleimides and polyimides). In one or more embodiments, the
compaction pressure applied during the step of consolidating the
composite facesheets 102, 103 may be applied by differential
atmospheric pressure (e.g., a vacuum bag), hydrostatic pressure
(e.g., a pressurized bladder), a platen press, and/or an autoclave.
In one or more embodiments, the step of consolidating the composite
facesheets 102, 103 may include applying a consolidation
temperature of approximately (about) 177.degree. C. under vacuum,
and applying a compaction pressure of approximately (about) 1.4 MPa
with a heated platen press to the composite facesheets 102, 103. In
the illustrated embodiment, the step of consolidating the composite
facesheets 102, 103 includes placing the open cellular core 101,
the sacrificial mold 116, and the composite facesheets 102, 103
onto a caul plate 119, and covering the open cellular core, the
sacrificial mold 116, and the composite facesheets 102, 103 with a
vacuum bag 120 that is sealed to the caul plate 119 with vacuum
sealant 121. Additionally, in the illustrated embodiment, the
system for consolidating the composite facesheets 102, 103 on the
surfaces 107, 108 of the open cellular core 101 includes a pair of
rigid hard stops 122 on opposite sides of the open cellular core
101 configured to control the compaction pressure applied to the
composite facesheets 102, 103. In the illustrated embodiment, the
system for consolidating the composite facesheets 102, 103 on the
surfaces 107, 108 of the open cellular core 101 also includes a
breather layer 123 and a peel ply 124 on the upper composite
facesheet 102 configured to facilitate removal of the vacuum bag
120 after the step of consolidating the composite facesheets 102,
103.
[0041] In one or more embodiments, the compaction pressure applied
during the step of consolidating the composite facesheets 102, 103
may be from approximately (about) 0.1 MPa to approximately (about)
12 MPa. In one or more embodiments, the compaction pressure exceeds
the compressive strength of the open cellular core 101, but the
compressive strength of the combined sacrificial mold 116 and the
open cellular core 101 exceeds the compaction pressure. In this
manner, the sacrificial mold 116 is configured to increase the
compaction pressure that may be applied to consolidate the
composite facesheets 102, 103 compared to a related art process in
which the open cellular core 101 is not reinforced by a sacrificial
mold.
[0042] Applying the compaction pressure during the step of
consolidating the composite facesheets 102, 103 is configured to
press excess resin out of the composite facesheets 102, 103 and
thereby increase the fiber volume fraction of the composite
facesheets 102, 103. In one or more embodiments, the fiber volume
fraction of the composite facesheets 102, 103 may be increased to
at least approximately (about) 65% following the step of
consolidating the composite facesheets 102, 103. Additionally, in
one or more embodiments, the excess resin that is pressed from the
composite facesheets 102, 103 by applying the compaction pressure
may flow to the interfaces between the open cellular core 101 and
the composite facesheets 102, 103 and thereby bond the composite
facesheets 102, 103 to the surfaces 107, 108 of the open cellular
core 101. Accordingly, the excess resin that is pressed from the
composite facesheets 102, 103 and bonds the composite facesheets
102, 103 to the surfaces 107, 108 of the open cellular core 101
saves mass that would otherwise have to be applied to the
interfaces between composite facesheets 102, 103 and the open
cellular core 101 if the composite facesheets 102, 103 and the open
cellular core 101 were separately formed and subsequently adhered
together. In this manner, the step of co-curing the composite
facesheets 102, 103 to the surfaces 107, 108 of the open cellular
core 101 by applying a compaction pressure to the composite
facesheets 102, 103 reduces the parasitic adhesive mass of the
sandwich structure 100 compared to related art sandwich structures
that are not formed by co-curing. Additionally, co-curing the
composite facesheets 102, 103 to the open cellular core 101 by
applying a compaction pressure to the composite facesheets 102, 103
is configured to reduce tolerance errors for sandwich structures
100 having complex geometries. For instance, during the step of
co-curing the composite facesheets 102, 103 to the open cellular
core 101 by applying the compaction pressure, the composite
facesheets 102, 103 conform to the surfaces 107, 108 of the open
cellular core 101 because the composite facesheets 102, 103 are
still in a pliable (e.g., pre-cured) state, which enables complex
geometries (e.g., curved facesheets) to be formed in a single step.
In contrast, related art methods of forming a sandwich structure
with complex geometry requires forming the composite facesheets and
the core separately with the desired geometry (e.g., curvature).
Forming the composite facesheets and the core separately requires
additional tooling and increases the chance of assembly
misalignment because the composite facesheets are fully cured
before being attached to the core and therefore cannot conform to
the core during processing.
[0043] With reference now to the embodiment illustrated in FIG. 1F,
the method also includes a step of removing the sacrificial mold
116 from the open volume 104 of the open cellular core 101. In one
or more embodiments, the process utilized during the step of
removing the sacrificial mold 116 may depend, for instance, on the
type of sacrificial mold material 115 and/or the process utilized
during the step of consolidating the sacrificial mold material 115.
For instance, in one or more embodiments, the step of removing the
sacrificial mold 116 may be performed by dissolution of the
sacrificial mold 116 in water or a solvent, etching the sacrificial
mold 116 in an acidic or basic bath, melting the sacrificial mold
116, and/or vaporization or combustion of the sacrificial mold 116
at a temperature greater than the consolidation temperature. In one
or more embodiments, the sacrificial mold 116 may be removed
utilizing heated (e.g., 60.degree. C.) pressurized water. Following
the step of removing the sacrificial mold 116, the open volume 104
defined by the open cellular core 101 of the sandwich structure 100
is free or substantially free of the sacrificial mold 116.
[0044] In an alternate embodiment illustrated in FIGS. 2A-2D, one
or more spacer layers 114 may be introduced in the chamber 109
between the upper and lower surfaces 107, 108 of the open cellular
core 101 and the inner surfaces 112, 113 of the chamber 109 (e.g.,
one or more spacer layers 114 may be inserted into the chamber 109
before inserting the open cellular core 101 into the chamber 109,
or the one or more spacer layers 114 may be applied to the upper
surface 107 and/or the lower surface 108 of the open cellular core
101 before inserting the open cellular core 101 into the chamber
109 of the mold 110). In one or more embodiments, the surfaces 107,
108 of the open cellular core 101 along which the facesheets 102,
103 will be attached are pressed into the one or more spacer layers
114, thereby deforming or penetrating the one or more spacer layers
114 (e.g., the surfaces 107, 108 of the open cellular core 101 may
be pressed into the one or more spacer layers 114 during the step
of inserting the open cellular core 101 into the chamber 109 of the
mold 110). In one or more embodiments, the material of the one or
more spacer layers 114 may include silicone, rubber (e.g., buna
rubber), closed cell foam, and/or a polymer film (e.g.,
polyethylene terephthalate (PET) film). In one or more embodiments,
each of the one or more spacer layers 114 may have a thickness t
from approximately (about) 0.05 mm to approximately (about) 3.5 mm.
In one or more embodiments, the one or more spacer layers 114 may
have a thickness t of approximately (about) 1.6 mm. In one or more
embodiments, the method may include a step of pressing each of the
surfaces 107, 108 of the open cellular core 101 along which the
facesheets 102, 103 will be attached into the one or more spacer
layers 114. In one or more embodiments, fewer than all of the
surfaces 107, 108 of the open cellular core 101 along which the
facesheets 102, 103 will be attached may be pressed into the one or
more spacer layers 114.
[0045] In one or more embodiments in which the surfaces 107, 108 of
the open cellular core 101 are in direct contact with the inner
surfaces (e.g., the inwardly facing surfaces) 112, 113,
respectively, of the mold 110 (embodiment illustrated in
FIGS.1A-1H) when the open cellular core 101 is inserted into the
chamber 109 of the mold 110, the upper and lower surfaces 107, 108
of the open cellular core 101 are coextensive or substantially
coextensive (e.g., co-planar or substantially co-planar) with the
upper and lower surfaces 117, 118, respectively, of the sacrificial
mold 116 (see FIG. 1H). That is, the open cellular core 101 and the
sacrificial mold 116 occupying the open volume 104 of the open
cellular core 101 share a continuous, common outer mold line (e.g.,
continuous, common outer surfaces) following the step of at least
partially filling the open volume 104 with the sacrificial mold
material 115 and consolidating the sacrificial mold material 115
into the sacrificial mold 116. Accordingly, in one or more
embodiments, the composite facesheets 102, 103 may be laid up and
co-cured on the common surfaces (e.g., the shared surfaces) 107,
108, 117, 118 of the open cellular core 101 and the sacrificial
mold 116.
[0046] In one or more embodiments in which the surfaces 107, 108 of
the open cellular core 101 are pressed into the one or more spacers
114 (embodiment illustrated in FIGS. 2A-2D) when the open cellular
core 101 is inserted into the chamber 109 of the mold 110, the one
or more spacers 114 mask off the surfaces 107, 108 of the open
cellular core 101 such that the sacrificial mold material 115 does
not contact the surfaces 107, 108 of the open cellular core 101
when the sacrificial mold material 115 is inserted into the open
volume 104 of the open cellular core 101. Accordingly, in one or
more embodiments, the one or more spacers 114 create a
discontinuous, offset interface (e.g., a gap G in FIG. 2D) between
the surfaces 107, 108 of the open cellular core 101 and surfaces
117, 118, respectively, of the sacrificial mold 116. This
discontinuous, offset interface between the surfaces 107, 108 of
the open cellular core 101 and the surfaces 117, 118 of the
sacrificial mold 116 allows the excess resin pressed from the
composite facesheets 102, 103 during the step of consolidating the
composite facesheets 102, 103 to flow (e.g., wick) into the
portions of the open cellular core 101 unoccupied by the
sacrificial mold 116 and thereby form a finite thickness adhesive
interface between the composite facesheets 102, 103 and the
surfaces 107, 108 of the open cellular core 101.
[0047] FIG. 3 is a flowchart illustrating steps of a method 200 of
forming a sandwich structure including an open cellular core
between two composite facesheets according to one embodiment of the
present disclosure. In the embodiment illustrated in FIG. 3, the
method 200 includes a step 210 of cleaning an open cellular core
defining an open volume to remove any contaminants from surfaces of
the open cellular core. The open cellular core may have any
suitable configuration described above, such as a series of hollow
or solid interconnected struts arranged in a lattice structure, a
foam, a grid, or a partially-connected honeycomb structure.
[0048] In the illustrated embodiment, the method 200 includes a
step 220 of applying a release agent (e.g., silicone, lecithin,
wax, or combinations thereof) to at least a portion of the open
cellular core (e.g., surfaces of the open cellular core defining
the open volume). The release agent is configured to promote or aid
in removal of a sacrificial mold (formed during a subsequent step)
from the open volume of the open cellular core. Additionally, in
one or more embodiments, the method 200 may also include a step of
masking surfaces (e.g., upper and lower surfaces) of the open
cellular core against exposure to the release agent, which is
configured to promote interfacial adhesion between the open
cellular core and facesheets applied to these surfaces of the open
cellular core during a subsequent step.
[0049] With continued reference to FIG. 3, the method 200 also
includes a step 230 of inserting the open cellular core into a
chamber of a mold. In one or more embodiments, the surfaces of the
open cellular core along which the facesheets will be coupled to
the open cellular core are in direct contact with inner surfaces
(e.g., inwardly facing surfaces) of the mold. In one or more
embodiments, the method 200 may include a step 240 of inserting one
or more spacer layers into the chamber before the step 230 of
inserting the open cellular core into the chamber, or the method
may include a step of applying the one or more spacer layers to the
surfaces of the open cellular core before the step 230 of inserting
the open cellular core into the chamber of the mold.
[0050] The method 200 also includes a step 250 of introducing a
sacrificial mold material (115 in FIGS. 1C, 1G, and 2B) (e.g.,
eutectic salt, plaster, PEG, PEO, ceramic spheres, plaster, wax, or
combinations thereof) into the open volume of the open cellular
structure and at least partially filling the open volume of the
open cellular core with the sacrificial mold material 115. The step
250 of introducing the sacrificial mold material into the open
volume of the open cellular core may be performed in any suitable
manner, such as pouring under gravity, filling under vacuum,
filling under positive pressure, sifting and compaction of powder,
or combinations thereof.
[0051] The method 200 also includes a step 260 of consolidating the
sacrificial mold material 115 to solidify the sacrificial mold
material 115 into a solid sacrificial mold (116 in FIGS. 1D, 1H,
2C, and 2D). The step 260 of consolidating the sacrificial mold
material 115 may be performed in any suitable manner depending, for
instance, on the type of sacrificial mold material utilized and/or
the phase of the sacrificial mold material 115 (e.g., liquid or
powder), such as curing (e.g., heating), solidification (e.g.,
cooling), compaction (e.g., sintering), and/or evaporation of a
liquid (e.g., a solvent) in the sacrificial mold material 115.
[0052] In one or more embodiments in which the sacrificial mold 116
is porous, the method 200 may include a step 270 of applying a
sealant on surfaces of the sacrificial mold 116 to prevent or
inhibit the infiltration of excess adhesive into the porous
sacrificial mold 116 during a subsequent step of co-curing
composite facesheets to the open cellular core, which is configured
to aid in the removal of the sacrificial mold 116 during a
subsequent step of the method described below.
[0053] In the illustrated embodiment, the method 200 also includes
a step 280 of laying up composite facesheets on at least two
surfaces (e.g., two opposing surfaces) of the open cellular
structure. The composite facesheets may have any configuration
described above, such as pre-impregnated fiber-reinforced polymer
plies or dry fabric reinforcement plies onto which a liquid resin
is deposited.
[0054] In the illustrated embodiment, the method 200 also includes
a step 290 of co-curing the composite facesheets onto the surfaces
(e.g., the upper and lower surfaces) of the open cellular core. The
step 290 of co-curing the composite facesheets onto the surfaces of
the open cellular core includes consolidating the composite
facesheets by applying a consolidation temperature (e.g.,
approximately (about) 23.degree. C. to approximately (about)
180.degree. C.) and a compaction pressure (e.g., approximately
(about) 0.1 MPa to approximately (about) 12 MPa) to the composite
facesheets. The compaction pressure applied may be applied in any
suitable manner, such as by differential atmospheric pressure
(e.g., a vacuum bag), hydrostatic pressure (e.g., a pressurized
bladder), a platen press, and/or an autoclave. In one or more
embodiments, the compaction pressure may be greater than the
compressive strength of the open cellular core, but less than the
compressive strength of the combined sacrificial mold 116 and the
open cellular core.
[0055] With continued reference to the embodiment illustrated in
FIG. 3, the method 200 also includes a step 300 of removing the
sacrificial mold 116 from the open volume of the open cellular
structure in any suitable manner, such as by dissolution of the
sacrificial mold 116 in water or a solvent (e.g., 60.degree. C.
pressurized water), etching the sacrificial mold 116 in an acidic
or basic bath, melting the sacrificial mold 116, and/or
vaporization or combustion of the sacrificial mold 116 at a
temperature greater than the consolidation temperature. Following
the step 300 of removing the sacrificial mold 116, the sandwich
structure includes an open cellular core defining an open (e.g.,
porous) volume between the two composite facesheets.
[0056] While this invention has been described in detail with
particular references to embodiments thereof, the embodiments
described herein are not intended to be exhaustive or to limit the
scope of the invention to the exact forms disclosed. Persons
skilled in the art and technology to which this invention pertains
will appreciate that alterations and changes in the described
structures and methods of assembly and operation can be practiced
without meaningfully departing from the principles, spirit, and
scope of this invention.
[0057] Although relative terms such as "inner," "outer," "upper,"
"lower," and similar terms have been used herein to describe a
spatial relationship of one element to another, it is understood
that these terms are intended to encompass different orientations
of the various elements and components of the invention in addition
to the orientation depicted in the figures. In addition, it will
also be understood that when a layer is referred to as being
"between" two layers, it can be the only layer between the two
layers, or one or more intervening layers may also be present.
[0058] Additionally, as used herein, the term "about",
"substantially," and similar terms are used as terms of
approximation and not as terms of degree, and are intended to
account for the inherent deviations in measured or calculated
values that would be recognized by those of ordinary skill in the
art. Furthermore, as used herein, when a component is referred to
as being "on" or "coupled to" another component, it can be directly
on or attached to the other component or intervening components may
be present therebetween. Further, the use of "may" when describing
embodiments of the inventive concept refers to "one or more
embodiments of the inventive concept." Also, the term "exemplary"
is intended to refer to an example or illustration.
[0059] Any numerical range recited herein is intended to include
all sub-ranges of the same numerical precision subsumed within the
recited range. For example, a range of "1.0 to 10.0" is intended to
include all subranges between (and including) the recited minimum
value of 1.0 and the recited maximum value of 10.0, that is, having
a minimum value equal to or greater than 1.0 and a maximum value
equal to or less than 10.0, such as, for example, 2.4 to 7.6. Any
maximum numerical limitation recited herein is intended to include
all lower numerical limitations subsumed therein and any minimum
numerical limitation recited in this specification is intended to
include all higher numerical limitations subsumed therein.
* * * * *