# Patent

## Title
Method for manufacturing a stiffened panel of composite material

## Abstract

A process for the production of a stiffened panel made of composite material, includes the stages of depositing the fibers that constitute the panel on a rigid device (50), depositing the at least one stiffener (14), covering the unit with a sealed wall connected to the rigid device (50) on the periphery of the panel, and then polymerizing, and providing—for each stiffener (14)—a rigid counter-mold in contact with the convex surface of the stiffener (14), using—for each stiffener (14)—a core (66) in the form of a flexible and sealed bladder, and subjecting the interior of the bladder to pressure during polymerization for compacting the stiffener (14) and the panel.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for manufacturing a stiffened panel made of composite material.
2. Description of the Related Art
As illustrated in FIG. 1, a stiffened panel 10 comprises, on the one hand, a panel 12 that forms a skin and that consists of fibers that are woven in a resin matrix and, on the other hand, long-member reinforcements 14.
The invention relates more specifically to the long-member reinforcements that comprise a non-flat central part with caps on either side in contact with the panel once assembled, whereby said central part is separated from said panel to be reinforced. By way of example, this reinforcement can have an omega-shaped profile.
In the case of an aircraft, the fuselage comprises, on the one hand, a skin, and, on the other hand, a first series of stiffeners with Omega-shaped cross-sections, flattened against the inside surface of the skin and arranged in a longitudinal direction and also called stringers, and a second series of stiffeners arranged in planes that are perpendicular to the longitudinal direction and also called frames. Thus, the fuselage can come in the form of a stiffened panel or a set of juxtaposed stiffened panels connected to a structure that consists in particular of frames.
According to a first operating mode that is illustrated in FIG. 2, the stiffened panel is produced using a molding device 16 that is rigid in contact with the surface of the panel that comprises the stiffeners.
In this case, this rigid molding device 16 comprises grooves 18 whose profiles are matched to those of the stiffeners 20. Each groove 18 has a central part that corresponds to the central part of the stiffener with—on either side—hollow lateral housings 22 that are matched to the caps of the stiffener.
The depth of the lateral housings 22 is adapted to the thickness of the caps of the stiffener in such a way that the surface of the caps that will be flattened against the skin is essentially at the same level as the surface of the device 16 that will be in contact with the skin. Relative to the width of the lateral housings, there can be a play J between the end of each cap of the stiffener and the edge of the corresponding lateral housing 22.
After the installation of stiffeners, a core 24 is connected at the hollow shape of each stiffener 20.
After the installation of the cores 24, the deposition of fibers of the skin 26 is initiated. This operation can be performed using a draping machine or more generally a fiber-placing machine.
The core 24 is to be rigid enough to withstand compression forces generated by the machine during the deposition of fibers.
Prior to the polymerization, a flexible and sealed bladder 28, which comes into contact with the device 16 on the periphery of the panel, is connected to the unit. Drainage means are inserted between the skin 26 and the sealed bladder 28 to ensure the evacuation of gases during polymerization. Advantageously, a semi-rigid shaping plate 30 is inserted between the skin 26 and the bladder 28. Semi-rigid is defined as the shaping plate 30 being rigid enough to transmit compression forces to the skin and flexible enough to adapt to the radii of curvature of the skin.
Next, the unit is subjected to a polymerization cycle during which the walls of the stiffeners and the skin are compacted so as to reduce the porosity of said walls. Advantageously, each core 24 is placed in a bladder into which a pressurized gas is injected in such a way as to compact the walls of the stiffeners and the walls of the skin facing the core 24.
After the polymerization, the cores 24 are withdrawn.
This operating mode is not satisfactory on the following points:
When the play J is excessive, greater than 0.2 mm, during polymerization a pressure gradient develops perpendicular to this play J, which causes localized surface defects such as deviations of fibers. Furthermore, as illustrated in FIG. 2, perpendicular to the play J, the pressure exerted on the skin is not adequate so that the part can have too high a porosity.
According to another drawback, this pressure gradient generates a surface defect at the outside surface of the panel that has to be corrected by sealing.
According to another drawback, this operating mode requires the use of rigid cores, with dimensions calibrated to those of the recess of each stiffener, difficult to design and to use. Thus, with the stiffeners having different shapes, it is necessary to provide as many cores as shapes of stiffeners. According to another point, these cores are difficult to extract, in particular because of the length of the stiffeners, general curves of the stiffeners, or defects of local shape. In general, these cores are disposable and are dissolved for extracting them.
According to another drawback, a slight mismatch between the surface of the cap of the stiffener and the surface of the adjacent device can develop and produce localized surface defects at the skin.
Finally, according to another drawback, during polymerization, the skin is compacted between a rigid element, namely the device 16, and a semi-rigid element 30, which is sparingly compatible with a homogeneous compacting.
According to another operating mode illustrated in FIG. 3, the stiffened panel is produced using a rigid molding device 32 in contact with the surface of the panel that does not comprise stiffeners.
In a first step, the fibers that form the skin 34 are deposited on the device 32 whose surface in contact with said skin is shaped to the outside surface of the fuselage. This operation can be carried out using a draping machine or more generally using a fiber placement machine.
Next, the stiffeners 36 are arranged on the skin 34 by having taken care to insert a rigid core 38 between each stiffener and the skin.
Prior to the polymerization, a flexible and sealed bladder 40 that comes into contact with the device 32 on the periphery of the panel is connected to the unit. Drainage means are inserted below the flexible and sealed bladder 40 to ensure the evacuation of gases during the polymerization.
Next, a second “comb”-type device that makes it possible to maintain the relative positions of the stiffeners during polymerization is connected. This second holding device comprises, for each stiffener, two elements that are supported on either side of the stiffener on the bladder.
Polymerization is then initiated. The cores 38 should be rigid enough to maintain the shape of the stiffeners during polymerization. They are withdrawn after this phase.
As above, this operating mode is not satisfactory for the following reasons:
Under the effect of pressure and despite the flexibility, the bladder cannot homogeneously compact the zone that corresponds to the end of the caps of the stiffener, taking into account the local discontinuity.
As illustrated in FIG. 3, this zone is subjected to high pressure gradients that cause surface defects at the inside surface of the panel, and even the embedding of the end of the cap in the skin.
Furthermore, this operating mode requires the use of a rigid core with the same drawbacks as those mentioned for the first operating mode.
As above, the pressure gradients also generate surface defects at the outside surface of the panel.
According to another drawback, it is very difficult, and even impossible, to place the flexible bladder in such a way that it exactly follows the profile of a surface that cannot be developed and that comprises in addition stiffeners on a large surface that can reach more than 70 m2.
Finally, according to another drawback, the bladder can be used only for the manufacturing of a single panel and after its use constitutes scrap to be destroyed.
BRIEF SUMMARY OF THE INVENTION
The purpose of this invention is to remedy the drawbacks of the operating modes of the prior art.
According to a first objective, its purpose is to propose a process for manufacturing a stiffened panel made of composite material that makes it possible to simplify the extraction of the cores after polymerization.
For this purpose, the invention has as its object a process for the production of a stiffened panel made of composite material that comprises a panel that consists of fibers that are woven in a resin matrix and to one of the surfaces of which is connected at least one stiffener made of composite material in the form of a section whose cross-section comprises a non-flat central part with caps on either side in contact with the panel once assembled, whereby said central part is separated from said panel to be reinforced, with said process comprising the stages that consist in depositing the fibers that constitute the panel on a rigid device, in depositing said at least one stiffener by inserting a core between each stiffener and the panel, in covering the unit with a flexible and sealed wall connected to the rigid device on the periphery of the panel, and then in polymerizing the unit by subjecting it to pressure, characterized in that it consists in providing—for each stiffener—a counter-mold that is rigid in contact with the convex surface of the stiffener, in using—for each stiffener—a core in the form of a flexible and sealed bladder, and in subjecting the interior of said bladder to pressure during polymerization for compacting the stiffener and the panel.



BRIEF DESCRIPTION OF THE DRAWINGS
Other characteristics and advantages will emerge from the following description of the invention, a description that is provided only by way of example, relative to the accompanying drawings in which:
FIG. 1 is a perspective view of a stiffened panel that illustrates the surface that comprises the stiffeners,
FIG. 2 is a cutaway that illustrates a device that makes it possible to implement a first operating mode according to the prior art, accompanied by a surface pressure diagram of the skin,
FIG. 3 is a cutaway that illustrates a device that makes it possible to implement a second operating mode according to the prior art, accompanied by a surface pressure diagram of the skin,
FIG. 4 is a cutaway that illustrates a stiffened panel that is equipped with a part of the device,
FIG. 5 is a cutaway that illustrates a first variant of a device that makes it possible to implement an operating mode according to the invention, accompanied by a surface pressure diagram of the skin,
FIG. 6 is a perspective view that illustrates the device of FIG. 5 at the end of a stiffener,
FIG. 7 is a cutaway that illustrates in details the lateral edge of a stiffener on the panel during the implementation of the process of the invention,
FIG. 8 is a cutaway that illustrates another variant of a device that makes it possible to implement an operating mode according to the invention, and
FIG. 9 is a diagrammatic perspective depiction of the device of FIG. 8.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A stiffened panel made of composite material 10 comprises, on the one hand, a panel 12 that forms a skin and that consists of fibers that are woven in a resin matrix, and, on the other hand, long-member reinforcements 14.
By way of example, this stiffened panel 10 can be used as part of a fuselage of an aircraft. In this case, the stiffeners are called stringers.
The panel 12 can be flat or have a curved or non-developable profile. Hereinafter, the surface of the panel on which the stiffeners are connected is called the inside surface, with the other surface being called the outside surface. The inside surface consists of zones covered by the stiffeners and uncovered zones.
According to a minimalist variant, the panel can comprise a single stiffener, as illustrated in FIGS. 8 and 9.
Each stiffener comes in the form of a section whose cross-section comprises a non-flat central part with caps on either side in contact with the panel once assembled, whereby said central part is separated from said panel to be reinforced. By way of example, this reinforcement may have the shape of an omega.
As appropriate, the stiffener can have a constant or variable cross-section over its entire length. Likewise, the stiffener may or may not be rectilinear.
The shapes of the panel and stiffeners are not presented in more detail because they can vary based on the constraints applied to the panel.
In a first step, the stiffener(s) is/are produced independently. Several techniques can be considered, in particular the one that is described in the document FR-10.54218.
Advantageously, the stiffeners are partially polymerized in such a way as to be stable over the dimensional plane while allowing a final polymerization with the panel so as to obtain a simple and solid connection between the stiffener and the panel.
Preferably, the stiffener is produced by using a rigid mold in contact with the convex surface of the stiffener.
This rigid mold can be used not only for partial polymerization but also for manipulating the stiffener, with the latter being kept in its rigid mold. Advantageously, the rigid mold will constitute a part of the device that will make it possible to implement the operating mode of the invention as will be presented in detail below.
According to the operating mode of the invention, the first stage consists in depositing the fibers that constitute the panel 12 on a rigid device 50 that comprises an upper surface whose shapes are consistent with those of the outside surface of the panel.
This operation can be automated by using a machine for depositing layers of fibers, for example.
The mode for depositing fibers is not presented in more detail because it is within the scope of one skilled in the art.
Likewise, the types of fibers and resin are determined by one skilled in the art based on constraints applied to the panel.
In a second stage, the stiffener(s) 14 is/are connected to the inside surface of the panel 12 by having taken care to place a core 52 between each stiffener and the panel. Next, a device for holding stiffeners in place is deployed.
For each stiffener, this holding device comprises, on the one hand, a rigid counter-mold 54 that covers the stiffener 14.
According to a variant that is illustrated in FIG. 5, the device comprises, in addition to counter-molds 54, at least one connecting element 56 between two rigid counter-molds 54 in such a way as to couple them together. Preferably, at least one of the connecting elements 56 is connected to the rigid device 50.
According to another variant that is illustrated in FIG. 8, the device comprises, in addition to the counter-molds 54, means for holding them in place that can be identical to those described in the embodiment that is illustrated in FIG. 3.
Advantageously, the rigid mold that has served as a mold during the manufacturing of the stiffener is used as a rigid counter-mold 54. Thus, the stiffener 14 is always held in place in its mold from the installation of the fibers that constitute it up to its total polymerization with the panel. In this way, the stiffener is protected during manipulation phases whereas it is still fragile but not totally polymerized.
According to a first variant of the device of the invention that is illustrated in FIGS. 4 to 7, the rigid counter-mold 54 covers the entire convex surface of the stiffener, with the lateral edges 58, 58′ of the rigid counter-mold 54 not going beyond the lateral edges 60, 60′ of the stiffener. Advantageously, each lateral edge 58, 58′ of the rigid counter-mold 54 is offset toward the central part of the stiffener relative to the corresponding lateral edge 60, 60′ of the stiffener. The advantages obtained by this arrangement will be developed later.
After the installation of the stiffeners 14, at least one plate or band 62 of elastomer is applied on the uncovered inside surface zone(s) as illustrated in FIG. 4.
Preferably, the inside surface is covered by several blades or plates 62 that are juxtaposed with one another. These bands or plates can have different geometric shapes. Hereinafter, band is defined both as band and plate.
These bands 62 are rigid enough to exert an essentially homogeneous pressure on all of their surfaces. However, they can possibly deform by compression in such a way as to adapt to the roughness of the inside surface.
By way of example, the bands 62 are made of Mosite.
The bands 62 have shapes that are adapted in such a way that there is a positive play between the bands and the stiffeners.
According to this variant of the invention, a seal 64 is applied in such a way as to ensure the seal between the rigid counter-molds and the bands.
This seal 64 is able to have two states, a first state that is malleable during its application in such a way that it adapts to the profile of the lateral edges of the stiffeners in such a way as not to allow a vacuum between the panel, the stiffener, the rigid counter-mold and the band, as illustrated in FIG. 5, and a hardened state after the application, in particular during polymerization, to ensure the transmission of compression forces in a manner that is essentially identical to the rigid counter-molds 54 or to the bands 62 that are arranged on either side of the seal so as not to generate a pressure gradient, as illustrated in the diagram that appears in FIG. 5.
The rigid counter-mold(s) 54, the band(s) 62, and the seal 64 form a wall that is connected by any suitable means to the rigid device 50 on the periphery of the panel in such a way as to form a sealed chamber.
According to one embodiment, the seal between, on the one hand, the rigid device 50 and, on the other hand, the rigid counter-mold(s) 54 and/or the band(s) 62 is ensured by using the seal 64.
The seal 64 should adhere sufficiently to the rigid counter-mold(s) 54, to the band(s) 62, and to the rigid device 50 for forming—with the rigid counter-molds 54, the bands 62, and the rigid device 50—a sealed chamber in which the panel and the stiffeners are deployed.
The seal 64 comes in the form of a putty bead, for example silicone.
The rigid counter-molds 54, the bands 62, and the seal 64 can be deposited manually or in an automated manner.
In the presence of several juxtaposed bands 62, the seal 64 can be used to ensure the seal between two adjacent bands 62.
Advantageously, prior to the deposition of the seal 64, it is possible to provide a seal 68 between the lateral edges 60, 60′ of the stiffeners and the panel so as to prevent the seal 64 from being introduced into the gap between cap and skin, as illustrated in FIG. 7. According to one embodiment, it is possible to make the lateral edges 60, 60′ of the stiffeners adhere to the panel 12, by heating by means of, for example, a thermal roller.
According to an advantage obtained by this variant of the invention, the bands and the rigid counter-molds are reusable. Only the bead 64 is lost and constitutes scrap to be destroyed for each stiffened panel produced.
According to another variant of the device of the invention illustrated in FIGS. 8 and 9, the rigid counter-mold 54 covers the entire convex surface of the stiffener 14. For forming a sealed chamber with the rigid device 50, a flexible and sealed wall 55 that covers the panel in the zones that are not covered by the stiffeners and the counter-mold(s) 54 is deposited on the unit. This flexible and sealed wall 55 is connected in a sealed manner to the rigid device 50 all around the panel. Wedges can be provided on the periphery of the panel to limit the compression of its edges.
In the two variants, means are provided for generating a negative pressure in the chamber and for promoting the extraction of gases during polymerization.
Advantageously, drainage means can be inserted between the panel 12 and the rigid device 50 for evacuating the gases during polymerization. With the stiffeners having been shaped and compacted until contraction occurs during a preliminary partial polymerization, there is no gas to remove at the stiffener so that the drainage means do not need to be in contact with the stiffeners.
According to an important characteristic of the invention, a core 66 in the form of a flexible and sealed bladder is arranged between each stiffener 14 and the panel 12, taking into account the presence of the counter-mold 54 that limits the flattening of the stiffener during polymerization. Thus, during polymerization, the inside of the bladder is subjected to pressure for compacting the stiffener and the panel. After polymerization, with the bladder no longer being subjected to pressure, its flexibility allows it to be easily withdrawn from the stiffener.
According to another characteristic, the bladder 66 can be produced from a slightly expandable material to be adapted to different forms of stiffeners. Thus, it is possible to reuse it for the manufacturing of another panel. According to another advantage, even if the stiffeners of the same panel have different shapes, it is possible to use a limited number of bladder shapes to the extent that the bladders of a given shape may be suitable for stiffeners of shapes that are different but close to those of the bladder.
The flexible and sealed wall 55 makes it possible to compact—against the rigid device 50—the panel in the zones beyond the stiffeners.
Perpendicular to the stiffeners, the flexible and sealed wall 55 makes it possible to compact, via the counter-mold 54, the panel and the stiffener against the rigid device 50 at the level of the caps of the stiffener and the stiffener against the bladder 66 at the central part of the stiffener.
The bladder 66 makes it possible to compact the stiffener against the counter-mold and the panel against the rigid device 50 under the stiffener.
In general, the stiffeners protrude at least one of their ends. In this case, the flexible and sealed bladder 66 comes in the form of a flange with at least one protruding end at the level of the protruding end of the stiffener, as illustrated in FIGS. 6 and 9. This arrangement makes it possible to obtain an identical pressure in the bladder 66 and the environment outside of the chamber that is formed by the device, in particular during polymerization. As a result, the pressure inside the bladder is identical to the one that is present in the autoclave during polymerization. Thus, a satisfactory compression of the panel under the stiffeners is obtained.
According to a particular arrangement described in FIG. 6, the bladder 66 has a length that is greater than that of the stiffener in such a way that the end of the bladder exceeds the stiffener. In the same manner, the stiffener 14 has a length that is greater than that of the counter-mold in such a way that the end of the stiffener exceeds the counter-mold.
As illustrated in FIG. 6, it is possible to use a seal 64 for ensuring the seal between the rigid counter-mold 54 and the bladder 66, between the rigid mold 54 and the rigid device 50, and between the rigid device 50 and the bladder 66.
For the installation of this seal, it is necessary that the bladder 66 be flattened against the walls of the stiffener, either by inflating said bladder 66 at a low pressure or by generating a negative pressure between the bladder and the stiffener.
Not being rigid, the bladder 66 can easily be withdrawn after the consolidation of the stiffened panel. In addition, it can be reused and is not disposable, which tends to reduce the amount of scrap.
According to a particular arrangement described in FIG. 9, the counter-mold 54 has a length that is essentially identical to that of the stiffener 14, and the bladder 66 has a length that is greater than the counter-mold 54 and extends beyond the ends of the counter-mold at each end of the stiffener in such a way as to be connected in a sealed manner to the flexible and sealed wall 55 and optionally to the rigid device 50.
According to one embodiment, it is possible to provide frames 70 for holding the bladder 66 in place at each end of the stiffener, with each frame having a profile that is close to that of the stiffener.
According to the variant that is illustrated in FIG. 9, the bladder 66 is protruding at each end of the stiffener.
Advantageously, it is possible to provide a wedge 72 that is inserted between the bladder 66, the stiffener 14, and the panel 12 at the level of each acute angle formed by the stiffener and the panel. According to one embodiment, the wedges 72 are made of Mosite and come in the form of a section with a triangular cross-section.
When the bladder is inflated, it pushes the wedges 72, which thus ensure a satisfactory compacting pressure at the angles.

## Claims

1. A process for the production of a stiffened panel made of composite material that comprises a panel that consists of fibers that are woven in a resin matrix, one of the surfaces of the panel being connected to at least one stiffener made of composite material in the form of a section whose cross-section comprises a non-flat central part with caps on either side in contact with the panel in an assembled state, said central part being separated from said panel to be reinforced, the process comprising:
depositing the fibers that constitute the panel on a rigid device;
depositing said at least one stiffener by inserting a core between each stiffener and the panel;
covering a unit comprising at least the panel, the at least one stiffener and the core with a sealed wall connected to the rigid device on the periphery of the panel; and
polymerizing the unit by subjecting the unit to pressure,
wherein each stiffener is provided with a counter-mold that is rigid in contact with the convex surface of the stiffener, lateral edges of the rigid counter-mold not extending beyond lateral edges of the at least one stiffener,
the core in the form of a flexible and sealed bladder is used with each stiffener,
the interior of said bladder is subjected to gas pressure during polymerization for compacting the stiffener and the panel, and
for each stiffener, the bladder has at least one protruding end at the level of the protruding end of the stiffener to communicate with an outside environment of a chamber formed by at least the panel, the counter-mold, and the bladder to obtain an identical pressure in the bladder and in the outside environment of the chamber.

2. The process for the production of a stiffened panel according to claim 1, wherein for each stiffener, the bladder extends beyond the end of the stiffener.

3. The process for the production of a stiffened panel according to claim 1, further comprising inserting a wedge between the bladder, the stiffener, and the panel at the level of each acute angle formed by said stiffener and said panel.

4. The process for the production of a stiffened panel according to claim 1,
the process further comprises:
covering one or more zones of the panel that are not covered by said at least one stiffener with at least one band having sufficient rigidity to transmit compression forces by allowing play between said at least one band and said at least one stiffener; and
depositing a seal such that said seal, said at least one band, and said at least one rigid counter-mold form said sealed wall, said seal being configured to have two states comprising a first malleable state during application of the seal such that the seal adapts to a profile of the lateral edges of the at least one stiffener, and a hardened state after the application of the seal to ensure the transmission of compression forces in a manner that is essentially identical to the rigid counter-molds or to the bands that are arranged on either side of said seal so as not to generate a pressure gradient.

5. The process for the production of a stiffened panel according to claim 1, wherein for each stiffener, a seal is used to ensure sealing between the rigid counter-mold and the bladder, and between the bladder and the rigid device.

6. The process for the production of a stiffened panel according to claim 1,
wherein a flexible and sealed wall is used to cover the panel, and
for each stiffener, the bladder is connected in a sealed manner to said flexible and sealed wall.

7. The process for the production of a stiffened panel according to claim 1, wherein the bladder is made of an expandable material.

8. The process for the production of a stiffened panel according to claim 1, wherein a mold that has served in its partial polymerization as a rigid counter-mold is used for each stiffener.

9. The process for the production of a stiffened panel according to claim 1, wherein prior to depositing the at least one stiffener, each stiffener is partially polymerized so as to be stable on a dimensional plane while allowing a final polymerization with the panel.

10. The process for the production of a stiffened panel according to claim 2,
further comprising inserting a wedge between the bladder, the stiffener, and the panel at the level of each acute angle formed by said stiffener and said panel.

11. The process for the production of a stiffened panel according to claim 2,
wherein a flexible and sealed wall is used to cover the panel, and
for each stiffener, the bladder is connected in a sealed manner to said flexible and sealed wall.

12. The process for the production of a stiffened panel according to claim 3,
wherein a flexible and sealed wall is used to cover the panel, and
for each stiffener, the bladder is connected in a sealed manner to said flexible and sealed wall.

13. The process for the production of a stiffened panel according to claim 1, wherein when the interior of the bladder is subjected to the gas pressure during polymerization, the gas pressure within the bladder becomes equal to the gas pressure in the outside environment.