# Patent

## Title
Compaction of prepreg plies on composite laminate structures

## Abstract

A flexible compaction sheet is used to compact uncured composite plies onto the surface of a ply layup. The compaction sheet includes a peripheral seal that seals the perimeter of the sheet to the surface of the part layup and forms a vacuum chamber over the plies which is evacuated through the sheet to apply compaction pressure to the plies.

## Background
TECHNICAL FIELD
This disclosure generally relates to fabrication of composite structures, especially large scale structures such as aircraft subassemblies, and deals more particularly with a method and apparatus for compacting uncured composite plies on the surface of a composite structure.
BACKGROUND
There is sometimes a need to compact one or more localized plies of composite prepreg on a surface of a composite laminate or ply layup that is considerably larger than the localized plies. For example, in the aircraft industry, local patches of composite plies, sometimes referred to as “doublers”, may be applied to certain areas of large scale subassemblies such as fuselage sections in order to add stiffness, strength or reinforcements for fasteners.
In some cases, it may be most efficient to fabricate the doublers offline and then apply them to the fuselage sections as a kit. However, compacting these doublers to a fuselage section may be difficult because of the substantial force required to achieve the necessary compaction pressures, especially where the doublers are relatively large. For example, a door surround or window belt may require several tons of force that must be reacted against the surface of the structure to achieve proper compaction. A similar problem may exist where it is necessary to compact co-cured stringers on tooling or on surfaces of the structure.
Current solutions for reacting compaction forces are largely mechanical, and may require substantial tooling and equipment to handle the necessary reaction loads. The tooling and related equipment may be relatively heavy and expensive, especially where large scale structures such as wings or fuselage sections are involved.
An alternate method for reacting compaction forces involves sealing a vacuum bag over the entire surface of the structure, however this approach may be time consuming and may not allow other work to be performed in parallel with the vacuum bag compaction process. Moreover, problems may be encountered in holding the doubler or stringer in place during the process of applying the vacuum bag to the structure and drawing a vacuum.
Accordingly, there is a need for a method and related apparatus compacting doublers and other uncured part layups on large scale structures that reduce or eliminate the problems discussed above.
SUMMARY
In accordance with the disclosed embodiments compaction of uncured plies on cured or uncured large scale ply layups is achieved using a portable, reusable compaction sheet that is placed locally over the uncured plies. The compaction sheet has an included seal that seals the sheet directly to the ply layup to form a vacuum chamber around the uncured plies. The compaction sheet is flexible, allowing it to conform to and be sealed against a variety of part structural surfaces, including curved or contoured surfaces and surfaces having irregularities. The compaction sheet may be coupled with a vacuum source which evacuates the vacuum chamber to draw the compaction sheet against the ply layup, thereby compacting the uncured plies. The use of a portable compaction sheet temporarily applied to the ply layup allows pressure to be applied locally to the uncured plies, instead of reacting the compaction load externally or applying a vacuum bag to the entire ply layup.
The disclosed embodiments allow pre-kitted doublers and stringers to be installed and compacted on a ply layup in a single step, while assuring that they do not shift or fall away before they are compacted to the surface of the ply layup.
According to one disclosed embodiment, apparatus is provided for compacting at least one uncured ply against the surface of a ply layup, comprising: a flexible sheet adapted to be disposed over the uncured ply; a seal for sealing the perimeter of the sheet to the surface of the ply layup; and, means for applying a vacuum to the ply and the surface of the ply layup through the sheet. The flexible sheet is substantially gas impermeable and has a face adapted to engage and apply compaction pressure to the ply. Means for applying the vacuum includes a vacuum port on the sheet adapted to be coupled with a vacuum source, and a vacuum plate having a plurality of spatially distributed passageways through which a vacuum may be drawn.
According to another disclosed embodiment, apparatus is provided for compacting an uncured composite layup onto the surface of a ply layup, comprising: a sheet having a face adapted to be placed onto the surface of the ply layup and overlying the uncured layup; means on the sheet for forming a vacuum chamber over the surface of the ply layup and surrounding the uncured layup; and, a vacuum plate connected to the sheet for drawing a vacuum within the vacuum chamber. At least the perimeter of the sheet is sufficiently flexible to allow the sheet to conform to the surface of the ply layup. The sheet may include a breather for allowing vacuum pressure to be applied substantially uniformly over the uncured layup. The sheet may be contoured to generally match curvatures of the ply layup surface.
According to a disclosed method embodiment, compacting an uncured composite layup against the surface of a ply layup, comprises: positioning the uncured layup between a surface of the ply layup and a gas impermeable sheet; forming a gas impermeable seal between the sheet and the surface of the ply layup; and, using the sheet to compact the uncured layup by generating vacuum pressure between the sheet and the surface of the ply layup. The method may further include allowing the sheet to flex as the sheet compacts the uncured layup.
The disclosed apparatus and method satisfy the need for a simple and effective method of compacting local ply patches on a cured or uncured ply layup that uses a minimum amount of equipment and eliminates the need for large scale vacuum bags.
Other features, benefits and advantages of the disclosed embodiments will become apparent from the following description of embodiments, when viewed in accordance with the attached drawings and appended claims



BRIEF DESCRIPTION OF THE ILLUSTRATIONS
FIG. 1 is a functional block diagram of apparatus for compacting plies on a ply layup.
FIG. 2 is a perspective illustration of a typical aircraft fuselage section ply layup, and depicting a typical uncured doubler.
FIG. 3 is a perspective illustration of the face of a compaction sheet, showing the breather network and a vacuum seal.
FIG. 4 is a sectional illustration taken along the line 4-4 in FIG. 3.
FIG. 5 is an illustration similar to FIG. 4 but showing an uncured layup being compacted against the surface of a ply layup.
FIG. 6 is a perspective view showing the compaction sheet positioned on the surface of a ply layup and illustrating a vacuum coupling.
FIG. 7 is a cross sectional view illustrating a compaction sheet having an integrally formed edge seal.
FIG. 8 is a cross sectional view illustrating a compaction sheet having an edge seal insert.
FIG. 9 is a cross sectional illustration of a portion of a curved ply layup and illustrating a compaction sheet contoured to match the curvature of the ply layup.
FIG. 10 is a simplified flow chart of a method for compacting uncured plies on a ply layup.
FIG. 11 is a flow diagram of aircraft production and service methodology.
FIG. 12 is a block diagram of an aircraft.



DETAILED DESCRIPTION
Referring first to FIGS. 1 and 2, the disclosed embodiments broadly relate to apparatus 20 and a related method for compacting one or more uncured plies 22 on the surface 25 of a ply layup 24, which may be uncured or uncured. The uncured composite plies 22, which may be prepreg for example, may form a doubler for stiffening or strengthening an area or feature 26 on the ply layup 24, or may comprise a preformed layup such as a stringer (not shown) that is to be applied to and co-cured with the ply layup 24. The size of the ply layup 24 is typically much greater than that of the plies 22 and thus the plies 22 may be considered as a local “patch” on the ply layup 24. However, while the apparatus 20 will be described in connection with its use for compacting local plies 22 on the surface 25 of a large scale ply layup 24, it is to be understood that the disclosed embodiments may also be used as a portable, drop-in-place compaction bag type device on a standard layup table, which may not require bag sealant, breather, parting film, bag film or the setup time required for these components.
The apparatus 20 broadly comprises a portable and flexible compaction sheet 30 which includes a vacuum coupling 40, a vacuum plate 36, a breather 32 and a seal 34. The vacuum coupling 40 couples the vacuum plate 36 with a vacuum source 35 via a vacuum line 50. The vacuum plate 36 includes passageways 38 therein that allow gas to pass therethrough. The seal 34 engages the ply layup surface 25 and forms a vacuum chamber 45 over the uncured plies 22. The portability of the compaction sheet 30 allows it to be readily moved to a location on the ply layup surface 25 where the uncured plies 22 are to be placed and compacted. Moreover, because the compaction sheet 30 includes a preassembly of the vacuum plate 36, breather 32 and seal 34, the need to assemble these individual components on the ply layup surface 25 for each ply compaction procedure may be eliminated.
Referring now also to FIGS. 3 and 4, the compaction sheet 30 may be formed from a flexible and durable elastomeric-like material such as, without limitation, rubber, urethane, or a fluoroelastomer such as Viton®, such that at least the outer perimeter 31 of the sheet 30 may flex, thereby allowing the seal 34 to better conform to the contour of the surface 25 of the ply layup 24. The seal 34 may also be formed from a flexible, deformable material such as, without limitation, rubber without limitation, rubber, urethane, or a fluoroelastomer such as Viton®. The seal 34 may be attached to the bottom face 28 of the compaction sheet 30 by any suitable means, and extends around the entire periphery 31 of the compaction sheet 30. Alternatively, as will be later discussed, the seal 34 may be formed integral with the compaction sheet 30.
The vacuum plate 36 may be formed of a rigid, flexible or semi-flexible material and is disposed within a central recess 37 in the compaction sheet 30. The vacuum plate 36 includes a plurality of spatially distributed passageways 38 therein through which gas may escape from beneath the compaction sheet 30, as indicated by the arrows 46. The spatial distribution of the passageways 38 assists in assuring that a substantially uniform vacuum will be drawn over the area of the plies 22.
A breather network 32 is connected to the bottom face 28 of the compaction sheet 30, and beneath the vacuum plate 36. The breather network 32 may be positioned flush with the bottom face 28 of the compaction sheet 30 and functions to allow a more uniform application of vacuum pressure to the uncured plies 22 during the compaction process. A base 40 formed of metal or other rigid material is secured to the top of the vacuum plate 36 and includes a vacuum port 44 adapted to be coupled to the vacuum source 35 (FIG. 1) via the vacuum line 50 (FIG. 1). The base 40 includes a manifold 42 that places the vacuum port 44 in communication with the passageways 38 in the vacuum plate 36 and thus forms a collection area for gases escaping through the vacuum plate 36 before they are drawn out through the vacuum port 44. The passageways 38 extend in a direction generally normal to the planes of the plies 22, ensuring that gas may pass substantially unrestricted from the vacuum chamber 45 into the manifold 42.
Although the material from which the compaction sheet 30 is formed may be flexible, the incorporation of the rigid or semi-rigid vacuum plate 36 into the body of compaction sheet 30 provides at least central areas of compaction sheet 30 with sufficient structural support and stiffness such that the shape of the compaction sheet 30 is substantially maintained as the compaction sheet 30 is being moved from location-to-location, and placed at particular locations on the ply layup surface 25.
FIG. 5 illustrates the apparatus 20 having been placed on the surface 25 of the ply layup 24 to compact plies 22 against surface 25. As gas is withdrawn from the vacuum chamber 45 through the vacuum plate 36, the compaction sheet 30 is drawn down by a vacuum pressure toward the surface 25, thereby compressing the seal 34 which seals the vacuum chamber 45. Compaction force is applied by ambient air pressure to the uncured plies 22 through the compaction sheet 30 and the breather network 32 as the gas continues to be evacuated from the vacuum chamber 45 through the vacuum plate 36.
FIG. 6 illustrates the use of a vacuum coupling 55 for coupling the base 40 to the vacuum line 50.
FIG. 7 illustrates an alternate embodiment in which the compaction sheet 30a and the seal 34a are of a one-piece, unitary construction. In this embodiment, the seal 34a may be formed from the same material as the compaction sheet 30a, as by molding the seal 34a and compaction sheet 30a as one piece at the same time.
FIG. 8 illustrates a further embodiment wherein the seal 34b may comprise a material that is the same as or different from that of the compaction sheet 34b, and forms an insert within the compaction sheet 34b, as may be accomplished using well known insert molding techniques.
Referring to FIG. 9, in some applications, a ply layup 24a may have curved or irregularly contoured surfaces 25a. In these applications, components of the apparatus 20a may be formed such that the bottom face 28a of the compaction sheet 30 is curved to match the contours or curvature of the surface 25a. 
Attention is now directed to FIG. 10 which broadly illustrates the steps of a method for compacting the uncured plies 22 against the surface 25 of a ply layup 24. Beginning at step 52, the uncured doubler plies 22 or other single or multi-ply layup is formed, following which the uncured plies 22 are positioned between the face 28 of the compaction sheet 30 and the surface 25 of the ply layup 24, as generally indicated by the numeral 53. In one embodiment, the uncured plies 22 may be positioned between the apparatus 20 and the surface 25 by placing the uncured plies 22 on the face 28 of the compaction sheet 30, as shown at step 54. Then, at step 56, a vacuum may be drawn through the vacuum plate 36 which holds the plies 22 on the face 28 of the compaction sheet 30. With the uncured plies 22 held on the compaction sheet 30, the plies 22 may be placed on the ply layup surface 25 using the compaction sheet 30. Alternatively, as shown at steps 58 and 60, the uncured plies 22 may be manually placed on the ply layup surface 25, following which the compaction sheet 30 is placed on the ply layup surface 25, overlying the uncured plies 22.
After the uncured plies 22 have been positioned between the compaction sheet 30 and the ply layup surface 25, a vacuum is drawn in the vacuum chamber 45 which draws the compaction sheet 30 down against the uncured plies 22 thereby compacting the plies 22 against the ply layup surface 22. During this compaction process, the compaction sheet 30 is allowed to flex, thereby assuring that the seal 34 remains tightly compressed against the ply layup surface 25 to maintain the necessary vacuum pressure.
Finally, at step 64, the vacuum pressure may be released and the compaction sheet 30 may be removed, following which the uncured plies 22 and the ply layup 24 may be co-cured as shown at step 65.
Embodiments of the disclosure may find use in a variety of potential applications, particularly in the transportation industry, including for example, aerospace, marine and automotive applications. Thus, referring now to FIGS. 11 and 12, embodiments of the disclosure may be used in the context of an aircraft manufacturing and service method 66 as shown in FIG. 11 and an aircraft 68 as shown in FIG. 12. During pre-production, exemplary method 66 may include specification and design 70 of the aircraft 68 and material procurement 72. During production, component and subassembly manufacturing 74 and system integration 76 of the aircraft 68 takes place. Thereafter, the aircraft 68 may go through certification and delivery 78 in order to be placed in service 80. While in service by a customer, the aircraft 68 is scheduled for routine maintenance and service 82 (which may also include repair, modification, reconfiguration, refurbishment, and so on).
Each of the processes of method 66 may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and major-system subcontractors; a third party may include without limitation any number of vendors, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on.
As shown in FIG. 12, the aircraft 68 produced by exemplary method 66 may include an airframe 84 with a plurality of systems 86 and an interior 88. Examples of high-level systems 86 include one or more of a propulsion system 90, an electrical system 92, a hydraulic system 94, and an environmental system 96. Any number of other systems may be included. Although an aerospace example is shown, the principles of the disclosure may be applied to other industries, such as the marine and automotive industries.
Systems and methods embodied herein may be employed during any one or more of the stages of the production and service method 66. For example, components or subassemblies corresponding to production process 74 may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft 68 is in service. Also, one or more apparatus embodiments, method embodiments, or a combination thereof may be utilized during the production stages 74 and 76, for example, by substantially expediting assembly of or reducing the cost of an aircraft 68. Similarly, one or more of apparatus embodiments, method embodiments, or a combination thereof may be utilized while the aircraft 68 is in service, for example and without limitation, to maintenance and service 82, including repairs.
Although the embodiments of this disclosure have been described with respect to certain exemplary embodiments, it is to be understood that the specific embodiments are for purposes of illustration and not limitation, as other variations will occur to those of skill in the art.

## Claims

1. A method of compacting an uncured composite layup against a surface of a composite ply layup, comprising:
positioning the uncured layup between a surface of the ply layup and a gas impermeable sheet, the sheet including a breather network positioned substantially flush with a bottom face of the sheet, said bottom face of the sheet positioned beneath a vacuum plate;
forming a gas impermeable seal between the sheet and the surface of the ply layup; and
using the sheet to compact the uncured layup by generating vacuum pressure between the sheet and the surface of the ply layup by drawing the vacuum though said vacuum plate comprising a plurality of spatially distributed passageways extending through a bottom face of said vacuum plate, the bottom face of the vacuum plate facing said uncured layup.

2. The method of claim 1, further including allowing the sheet to flex as the sheet compacts the uncured layup; wherein the breather network is positioned to allow a more uniform application of vacuum pressure to uncured plies during a compaction process.

3. The method of claim 1, wherein positioning the uncured layup between the surface of the ply layup and the sheet is performed by:
placing the uncured layup on the surface of the ply layup, and
then placing the sheet over the uncured layup.

4. The method of claim 1, wherein positioning the uncured layup between the surface of the ply layup and the sheet is performed by:
holding the uncured layup on the sheet, and
then using the sheet to place the uncured layup on the surface of the ply layup.

5. The method of claim 4, wherein holding the uncured layup on the sheet is performed using a vacuum pressure.

6. The method of claim 1, wherein generating the vacuum pressure includes drawing gas through the sheet.

7. The method of claim 1, wherein forming a gas impermeable seal is performed by bringing a seal on the sheet into contact with the surface of the ply layup.

8. The method of claim 1, wherein said passageways are disposed substantially normal to the surface of the ply layup.

9. A method of installing an uncured composite layup on a surface of a composite aircraft part layup, comprising:
providing a flexible sheet;
applying a vacuum through the sheet; said sheet including a breather network positioned substantially flush with a bottom face of said sheet, said bottom face of the sheet positioned beneath a vacuum plate;
using the vacuum applied through the sheet to hold the uncured layup on the sheet;
using the sheet to locate and place the uncured layup on the surface of the part layup;
forming a vacuum chamber between the sheet and the surface of the part layup and surrounding the uncured layup, including forming a vacuum tight seal between the sheet and the surface of the part layup;
drawing a vacuum within the vacuum chamber by evacuating gas within the vacuum chamber through the sheet, the vacuum drawn through said vacuum plate, said vacuum plate comprising a plurality of spatially distributed passageways extending through a bottom face of said vacuum plate, the bottom face of the vacuum plate facing said uncured layup; and,
using the vacuum in the vacuum chamber to compact the uncured layup against the part layup.

10. The method of claim 9, wherein said passageways are disposed substantially normal to the surface of the part layup; and wherein the breather network is positioned to allow a more uniform application of vacuum pressure to uncured plies during a compaction process.