Curved stiffener for a turbine engine nacelle and method for producing such a stiffener

Description

Technical Field

[0001] The present invention relates to a curved stiffener for a turbomachine nacelle, in particular a curved stiffener composed of a resin-impregnated fiber reinforcement. The present invention also relates to the method of manufacturing such a stiffener. Prior art

[0002] The nacelles are generally designed to house a dual-flow turbojet engine capable of generating, on the one hand, a flow of hot gases (also called primary flow) from the turbojet engine's gas generator, and, on the other hand, a flow of cold air (called secondary flow) from the fan and circulating outside the turbojet engine, through an annular passage generally called a secondary vein. Both flows are ejected from the turbojet engine through the rear of the nacelle.

[0003] In order to maintain the structure of the nacelle, one or more curved stiffeners, generally circumferential, may be provided to reinforce the structure of the latter. Composite stiffeners comprising multi-oriented fiber reinforcements, and composed of a number of layers of unidirectional fibers, assembled by a stitching in the thickness, called NCF according to the English sign for "Non-Crimp-Fabric" are known. This type of reinforcement comprises a plurality of parallel fiber layers, and each layer of fibers has fibers oriented in a different orientation from the fiber orientations of the fiber layers immediately adjacent to it. This type of stiffener is obtained by stitching the plurality of fiber layers together flat, draping this reinforcement over a stiffener mold, then impregnating it with resin, then crosslinking the resin impregnated in the fibers.An example is disclosed in patent application EP 2,774,854.

[0004] However, this type of stiffener cannot include a layer of fibers oriented along the length direction of the stiffener. Indeed, the curvature of the stiffener generates a different length between the head of the stiffener and its feet, and the fibers generally used in the manufacture of composite stiffeners do not have sufficient elongation capacity to absorb this difference. The presence of a layer of fibers along the length direction of the stiffener therefore causes defects when the fiber reinforcement is draped onto the stiffener mold in shape, for example the appearance of folds in the fiber layers.

[0005] As a result, the strength of such stiffeners is lower. Indeed, the impact of corrugations on the structural strength of the stiffener is a point of particular interest.

[0006] There is therefore a real need for textile reinforcement adapted to the mechanical strength requirements of the stiffener as well as to the forming methods, and capable of overcoming the aforementioned defects. Statement of the invention

[0007] The present disclosure relates to a curved stiffener for a turbomachine cowl composed of a resin-impregnated fiber reinforcement, the stiffener comprising a head in the form of a ring portion, two feet in the form of a ring portion concentric with the head and having a radius larger than a radius of the head, and two inclined portions, each connecting one of the feet to a lateral side of the head, the fiber reinforcement extending in a length direction, a width direction, and a thickness direction, the dimension of the fiber reinforcement in the length direction being larger than the dimension of the fiber reinforcement in the width direction, and the dimension of the fiber reinforcement in the thickness direction being the smallest of the dimensions of the fiber reinforcement, the fiber reinforcement having a central portion configured to form the head, two end portions arranged on either side of the central portion and configured to form the feet,two connecting portions connecting the central portion to the end portions and configured to form the inclined portions, the fibrous reinforcement comprising a stack of a plurality of layers of fibers stacked in the thickness direction and extending over the entire width of the fibrous reinforcement, each layer of fibers having a plurality of fibers parallel to each other extending in a direction inclined relative to the length direction, the fibers of two consecutive layers of fibers in the thickness direction have directions inclined relative to each other, and the layers of fibers are sewn together by sewing threads extending in the length direction, and wherein the fibrous reinforcement comprises a layer of reinforcing fibers sewn with the layers of fibers by the sewing threads having a plurality of fibers parallel to each other extending in the length direction,the layer of reinforcing fibers extending at least over the central part, over a width smaller than the width of the fiber reinforcement.,

[0008] In this presentation, it is understood that the dimension of the fiber reinforcement along the thickness direction is very much smaller than the dimension of the fiber reinforcement along the length and width directions.

[0009] In certain non-limiting examples, when the stiffener is placed in a turbomachine nacelle, the thickness direction corresponds to the radius direction, the length direction corresponds to the circumferential direction of the nacelle, and the width direction corresponds to the axial direction of the nacelle. In these examples, it is understood that the ring-portion-shaped head is concentric with the two ring-portion-shaped feet, in a given plane perpendicular to the axial direction of the nacelle and passing through the stiffener, and the radius of the head is less than the radius of the feet. However, other configurations may be envisaged, in which the length direction would correspond, for example, to the axial direction or an intermediate direction between the axial direction and the circumferential direction when the stiffener is placed in a nacelle.Furthermore, the stiffener of the present invention could be used in parts other than turbomachine nacelles.

[0010] Such a stiffener including a layer of reinforcing fiber oriented in the length direction (in other words, in the 0 degree direction relative to the length direction) sees its strength and stiffness increase. Indeed, the presence of reinforcing fiber makes it possible to better withstand the stresses that the stiffener could undergo, particularly when this layer is located at the head of the stiffener. For example, such a stiffener makes the nacelle able to more easily take up compression and tensile stresses in the radius direction.

[0011] Furthermore, the configuration of this embodiment makes it possible to include a layer of reinforcing fibers in the length direction of the fiber reinforcement, and therefore of the stiffener, without defects appearing during draping. This aspect is guaranteed by the fact that the layer of reinforcing fibers extends over a width less than the width of the stiffener.

[0012] This solution makes it possible to obtain a stiffener with good performance and the fiber reinforcement of such a stiffener can be fully draped in a single draping step. Indeed, the reinforcing fiber layer can be sewn with the other fiber layers in a single sewing step. It therefore appears that no retouching step of the fiber reinforcement, which could be planned before or after draping, is necessary to obtain a functional stiffener.

[0013] In some embodiments, the layer of reinforcing fibers extends over a portion of the bonding portions.

[0014] In this configuration, the fibers of the reinforcing fiber layer protruding from the connecting portions will facilitate the centering of the fiber reinforcement when draping the fiber reinforcement over the stiffener mold. Indeed, once placed on the mold, these fibers protruding from the connecting portions will have a slightly higher tension than the fibers placed on the central portion. In this way, the fiber reinforcement will be encouraged to position itself so as to evenly distribute the fibers protruding from the connecting portions on either side of the central portion. The fiber reinforcement is therefore more easily centered on the stiffener mold.

[0015] In some embodiments, the reinforcing fiber layer is disposed on the plurality of fiber layers at one end of the fibrous reinforcement along the thickness direction.

[0016] In this configuration, the manufacture of the fiber reinforcement is facilitated.

[0017] In some embodiments, the stiffener comprises at least one additional reinforcing fiber layer disposed in the stack of fiber layers.

[0018] In this configuration, the stiffness of the stiffener is increased. The stiffness of the stiffener can then be controlled based on the number of additional reinforcing fiber layers included in the fiber layer stack.

[0019] In some embodiments, the stiffener comprises chain stitching disposed at the ends of the fiber reinforcement in the width direction.

[0020] In this configuration, the cutting of the fiber reinforcement placed on the stiffener mold is facilitated.

[0021] In some embodiments, the fiber layers and the reinforcing fiber layers are sewn together by knit-type seams.

[0022] The use of knitted seams promotes the deformability of the fiber reinforcement.

[0023] In some embodiments, the tension of the stitching threads is lower in the binding portions than the tension of the stitching threads in the central portion.

[0024] In this configuration, the bonding portions of the fiber reinforcement are able to undergo greater deformations than the central part of the fiber reinforcement. In particular, this configuration facilitates the draping of the fiber reinforcement while limiting the defects that could appear during draping.

[0025] In some embodiments, the distribution of stitches in the central portion is denser than the distribution of stitches in the connecting portions.

[0026] In this configuration, the reinforcing fiber layer is stitched with other fiber layers in a more robust way, which helps in the stability of the fiber reinforcement.

[0027] In some embodiments, the stitching threads sewn onto the end portions and the connecting portions have an excess length.

[0028] In this configuration, the extra length of the stitching threads allows for easier deformation of the end portions and the connecting portions. This makes it easier to drape the fiber reinforcement over the stiffener mold.

[0029] This disclosure also relates to a nacelle comprising a stiffener as defined previously.

[0030] This disclosure also relates to an aircraft engine comprising the nacelle as defined previously.

[0031] The nacelle and the aircraft engine have the advantages previously explained.

[0032] The present disclosure further relates to a method for manufacturing a curved stiffener as defined above comprising a sewing step in which the fiber reinforcement is formed by sewing together the plurality of fiber layers and the reinforcing fiber layer(s), a draping step in which the fiber reinforcement is arranged on a stiffener mold, an impregnation step in which the fiber reinforcement arranged on the stiffener mold is impregnated with resin and a polymerization step in which the resin impregnated in the fiber reinforcement is solidified.

[0033] Such a process makes it possible to obtain a stiffener having the advantages mentioned above.

[0034] The above-mentioned features and advantages, as well as others, will become apparent from reading the following detailed description of examples of embodiments of the device and the proposed method. This detailed description refers to the attached drawings. Brief description of the drawings

[0035] The attached drawings are schematic and are intended primarily to illustrate the principles of the presentation. [ Fig. 1A ] There figure 1A represents a curved stiffener according to a first embodiment of the curved stiffener. Fig. 1B ] There figure 1B represents a curved stiffener according to a second embodiment of the curved stiffener. Fig. 1C] figure 1C represents a curved stiffener according to a third embodiment of the curved stiffener. Fig. 2 ] There figure 2 represents a partial view of a fibrous reinforcement according to a first embodiment, during a sewing step. Fig. 3 ] There figure 3 represents a partial view of a fiber reinforcement according to a second embodiment. Fig. 4A-4B ] THE figures 4A et 4B represent the fiber reinforcement and the stiffener mold at different times of the layup. Fig. 5 ] There figure 5 schematically represents a sewing thread of the fibrous reinforcement. Fig. 6 ] There figure 6 schematically represents the steps of the process of forming the stiffener of the figure 2 . [ Fig. 7 ] There figure 7 schematically represents an aircraft engine comprising a nacelle including a stiffener according to the embodiment. Description of the embodiments

[0036] In order to make the disclosure more concrete, an example of a device is described in detail below, with reference to the attached drawings. It is recalled that the invention is not limited to this example.

[0037] There figure 7 schematically represents an aircraft engine 1000 comprising a nacelle 100. The nacelle 100 comprises a cowl including a curved stiffener 10 according to a first embodiment of the invention.

[0038] There figure 1A represents the curved stiffener 10 according to a first embodiment of the invention. The stiffener 10 is composed of a resin-impregnated fiber reinforcement 20 and comprises a head 12 in the form of a ring portion of radius R0, two feet 16 in the form of a ring portion concentric with the head 12, of radius R1, and two inclined portions 14, each connecting one of the feet 16 to a lateral side of the head 12. Furthermore, the radius R1 of the two feet 16 is greater than the radius R0 of the head 12 when the nacelle 100 has a circular section. Thus, the stiffener 10 extends over an arc of a circle measuring angle α (alpha). This shape of the stiffener 10 is called an omega shape. In particular, it is observed that the head 12 and the inclined portions 14 taken independently have a trapezoid shape. In some examples, the angle α can extend up to 360 degrees.

[0039] There figure 2 represents a partial view of the fibrous reinforcement 20 according to a first embodiment. The fibrous reinforcement 20 extends in a length direction, a width direction, and a thickness direction. These three directions define an orthogonal reference when the fibrous reinforcement 20 is arranged on a flat surface.

[0040] The dimension of the fiber reinforcement 20 along the length direction is larger than the dimension of the fiber reinforcement 20 along the width direction, and the dimension of the fiber reinforcement 20 along the thickness direction is the smallest of the dimensions of the fiber reinforcement 20. It is understood that the dimension of the fiber reinforcement 20 along the thickness direction is negligible compared to the other two dimensions of the fiber reinforcement 20. In other words, the fiber reinforcement 20 extends mainly over two dimensions.

[0041] Furthermore, once the stiffener is arranged in a turbomachine nacelle, the thickness direction corresponds to the radius direction of the turbomachine, the length direction corresponds to the circumferential direction of the nacelle, and the width direction corresponds to the axial direction of the nacelle.

[0042] On the fiber reinforcement 20, a central portion is defined, configured to form the head 12 of the manufactured stiffener. Two end portions are also defined, arranged on either side of the central portion, configured to form the feet 16, and two connecting portions connecting the central portion to the end portions and configured to form the inclined portions 14.

[0043] As shown in the figure 2 , the fibrous reinforcement 20 comprises a stack of a plurality of layers of fibers 22 stacked along the thickness direction. Each layer of fibers 22 extends over the entire width of the fibrous reinforcement 20. Furthermore, each layer of fibers 22 is composed of a plurality of fibers parallel to each other. The fibers of the layers of fibers 22 extend in a direction inclined relative to the length direction and the fibers of two consecutive layers of fibers 22 along the thickness direction have directions inclined relative to each other.

[0044] In the example of this embodiment and in a non-limiting manner, the fibers of the successive layers of fibers 22 in the thickness direction have angles of -45° (or 135°), 90°, 45°90°, 45°, 270° and 225° respectively with the length direction.

[0045] The fiber reinforcement 20 further comprises a layer of reinforcing fibers 24. The layer of reinforcing fibers 24 is arranged on the plurality of fiber layers 22, at one end of the fiber reinforcement 20 in the thickness direction. In other words, the layer of reinforcing fibers 24 is arranged above the fiber layers 22 in the thickness direction.

[0046] The layer of reinforcing fibers 24 has a plurality of fibers parallel to each other extending in the length direction, that is to say they have an angle of 0° with the width direction, the layer of reinforcing fibers 24 extending at least over the central part, over a width smaller than the width of the fiber reinforcement 20.

[0047] In this example and in a non-limiting manner, the fiber reinforcement is composed of NCF (Non-Crimp-Fabric) carbon fibers. It is then understood that the fibers of the fiber layers 22 and the fibers of the reinforcing fiber layer 24 are carbon fibers. However, glass fibers or hybrid fibers could also be used.

[0048] As shown on the figure 2 , the entire fiber layers 22 and the reinforcing fiber layer 24 are sewn together by sewing threads extending along the length direction. For example, these seams are knit-type seams.

[0049] Furthermore, in the embodiment of the figure 2 , by way of non-limiting example, a chain-type seam is provided at the ends of the fibrous reinforcement 20 in the width direction. The chain seams also extend in the length direction and delimit the width of the fibrous reinforcement 20.

[0050] There figure 3 represents a partial view of a fiber reinforcement according to a second embodiment. In this embodiment, the fiber reinforcement 20 comprises at least one additional layer of reinforcing fibers 24' arranged in the stack of fiber layers 22. In the embodiment of the figure 3 , the additional reinforcing fiber layer 24' is identical to the reinforcing fiber layer 24. It therefore comprises a plurality of fibers oriented in the length direction and extends over the central portion of the fiber reinforcement 20. Furthermore, in certain embodiments, the reinforcing layer 24 may be provided above, below or inside the stack of fiber layers 22. Similarly, the additional reinforcing fiber layer 24' may be provided above, below or inside the stack of fiber layers 22.

[0051] In certain other examples, the additional reinforcing fiber layer 24' is of a different nature than the reinforcing fiber layer 24. For example, the additional reinforcing fiber layer 24' may be composed of glass fibers.

[0052] It is understood that in this embodiment, the fiber reinforcement 20 may comprise a plurality of layers of additional reinforcing fibers 24' in order to adjust the stiffness of the stiffener to the needs of the user. Furthermore, for the same purpose, the thicknesses of the additional reinforcing fiber layer(s) 24' may be different from each other and / or different from the thickness of the layer of reinforcing fibers 24. Similarly, the nature of the fibers of the layers of additional reinforcing fibers 24' may be of different natures from each other and / or different from the nature of the layer of reinforcing fibers 24.

[0053] THE figures 4A et 4B represent the fiber reinforcement 20 which is placed on a stiffener mold 40 at different times during the draping. The figure 4A represents the fiber reinforcement 20 positioned to be draped over the stiffener mold 40. The figure 4B represents the fibrous reinforcement 20 draped over the stiffener mold 40.

[0054] The stiffener mold 40 has a shape similar to the shape of the stiffener and includes a head portion 42, foot portions 46, and connecting portions 44 connecting the foot portions 46 to the head portion 42.

[0055] The fibrous reinforcement 20 is configured to cooperate with the mold 40, that is to say to match the shape of the mold 40. More particularly, the central part, the connecting parts and the end parts of the fibrous reinforcement 20 are configured to cooperate respectively with the head part 42, the connecting parts 44 and the foot parts 46.

[0056] The head portion 42 extends over a width I in the width direction while the layers of reinforcing fibers 24 of the fibrous reinforcement 20 extend over a width L in the width direction, the width I of the head portion 42 being less than or equal to the width L of the layer of reinforcing fibers 24. Thus, the layers of reinforcing fibers 24 extend over the head portion 42 and, in the present non-limiting example, extend partially over the connecting portions 44.

[0057] Furthermore, in order to facilitate the draping illustrated in the figures 4A et 4B , the tension of the sewing threads is lower in the connecting portions than the tension of the sewing threads in the central portion. Also, the distribution of the sewing stitches in the central portion is denser than the distribution of the sewing stitches in the connecting portions.

[0058] In other words, the fibrous reinforcement 20 is more easily deformable and more flexible at the connecting portions and the end portions than at the central portion.

[0059] To this end, the figure 5 schematically represents a sewing thread 30 of the fibrous reinforcement 20. The sewing thread 30 extends in the length direction. Sewing stitches having two through parts 32, forward and reverse, and a sewing loop 34 are provided regularly. The sewing stitches are spaced by a pitch P in the length direction and a gauge J in the width direction.

[0060] The total length of thread used for a stitch, including the length of the through portions 32 and the length of the sewing loop 34, is variable depending on the tension that one wishes to apply at the stitch. In particular, this length may have an excess length compared to the thread length of a conventional stitch. This excess length is calculated using the following formula. 1 : 2 P . R 1 R 0 + P . R 1 R 0 2 + J 2 − 2 P − P 2 + J 2

[0061] There figure 1B represents a stiffener 10' according to a second embodiment of the invention. The characteristics identical to the first embodiment are omitted.

[0062] In this embodiment, the stiffener 10' comprises two trapezoid-shaped portions 101' and 102' as previously described for stiffener 10 of the figure 1A , according to the first embodiment. In particular, the trapezoid shape comprises a small base corresponding to the head, a large base and sides corresponding to the inclined portions.

[0063] The trapezoidal shape of the portion 101' has an angle α1 between its large base and its sides while the trapezoidal shape of the portion 102' has an angle α2 between its large base and its sides. Furthermore, the width a of the head of the stiffener 10' of the second embodiment is preserved over its entire length.

[0064] The configuration of the figure 1B allows the shape of the 10' stiffener to be adapted according to structural and dimensional constraints that may be imposed by the 100 nacelle.

[0065] There figure 1C represents a 10" stiffener according to a third embodiment. Features identical to the first embodiment are omitted.

[0066] In the third embodiment, the stiffener 10" comprises a head 12", connecting portions 14" and legs 16". In this embodiment, the legs 16" extend inward of the stiffener 10" along the width direction. In other words, the legs 16" extend below the head 12" along the thickness direction.

[0067] There figure 6 schematically represents the steps of the method for forming the stiffener according to the first embodiment. This method also applies to the second and third embodiments. The method for manufacturing the stiffener successively comprises a sewing step E1, a draping step E2, an impregnation step E3 and a polymerization step E4.

[0068] In the sewing step E1, the fiber layers 22 and the reinforcing fiber layers 24 are sewn together to form the fiber reinforcement 20. As explained previously, the seams are of the knitted type. Furthermore, during this operation, the chain-type seams are sewn at the ends along the width direction of the formed fiber reinforcement 20.

[0069] In the draping step E2, the fiber reinforcement 20 is draped over the stiffener mold 40 so that the fiber reinforcement 20 takes the form of a curved stiffener. The mold 40 may be of the male type as illustrated in the figure 4A , or female type. A female type stiffener mold 40 corresponds to a mold whose shape is complementary to the shape of a male type stiffener mold 40. The male type stiffener mold 40 is of generally convex shape while the female type stiffener mold is of generally concave shape.

[0070] In the second and third embodiments, suitable molds are used during the draping step E2.

[0071] In the impregnation step E3, the fiber reinforcement 20 placed on the mold 40 is impregnated with resin. This impregnation can be carried out by infusion of resin in a sub-pressurized environment (pressure less than 1 bar), using a rigid mold and a flexible membrane or by resin transfer molding (also called RTM, in accordance with the English acronym for Resin Transfer Molding) in a super-pressurized environment (pressure greater than 1 bar), using two rigid molds. In this example, the resin is of low viscosity.

[0072] In the polymerization step E4, the fibrous reinforcement 20 placed in the mold 40 and impregnated with resin is polymerized in an oven. In this non-limiting example, the fibrous reinforcement is maintained at 120°C for 5 hours.

[0073] Finally, a stiffener 10 is obtained once it has been demolded from the stiffener mold 40.

[0074] Although the present invention has been described with reference to specific exemplary embodiments, it is obvious that modifications and changes may be made to these examples without departing from the general scope of the invention as defined by the claims. In particular, individual features of the various illustrated / mentioned embodiments may be combined in additional embodiments. Therefore, the description and drawings should be considered in an illustrative rather than restrictive sense.

[0075] It is also obvious that all the characteristics described with reference to a method are transposable, alone or in combination, to a device, and conversely, all the characteristics described with reference to a device are transposable, alone or in combination, to a method.

Claims

1. A curved stiffener(10) for a turbomachine cowl composed of a resin- impregnated fibrous reinforcement (20), the stiffener (10) comprising a ring portion shaped head (12), two ring portion shaped feet (16) concentric with the head (12) and having a radius greater than a radius of the head (12), and two inclined portions (14), each connecting one of the feet (16) to a lateral side of the head (12), the fibrous reinforcement (20) extending along a length direction, a width direction and a thickness direction, the dimension of the fibrous reinforcement (20) along the length direction being greater than the dimension of the fibrous reinforcement (20) along the width direction, and the dimension of the fibrous reinforcement (20) along the thickness direction being the smallest of the dimensions of the fibrous reinforcement (20), the fibrous reinforcement (20) having a central portion configured to form the head (12), two end portions disposed on either side of the central portion and configured to form the feet (16), two connecting portions connecting the central portion to the end portions and configured to form the inclined portions (14), the fibrous reinforcement (20) comprising a stack of a plurality of layers of fibers (22) stacked along the thickness direction and extending on the entire width of the fibrous reinforcement (20), characterized in that each layer of fibers (22) has a plurality of fibers parallel to each other extending along a direction inclined relative to the length direction, the fibers of two consecutive layers of fibers (22) along the thickness direction have directions inclined relative to each other, and the layers of fibers (22) are sewn together by sewing threads extending along the length direction, wherein the fibrous reinforcement (20) comprises a layer of reinforcing fibers (24) sewn with the layers of fibers (22) by the sewing threads having a plurality of fibers parallel to each other extending along the length direction, the layer of reinforcing fibers (24) extending at least on the central part, over a width smaller than the width of the fibrous reinforcement (20).

2. The stiffener (10) according to claim 1 wherein the layer of reinforcing fibers (24) extends on part of the connecting portions.

3. The stiffener according to any of claims 1 or 2, wherein the layer of reinforcing fibers (24) is disposed on the plurality of layers of fibers (22) at one end of the fibrous reinforcement (20) along the thickness direction.

4. The stiffener (10) according to any of claims 1 to 3, comprising at least one additional layer of reinforcing fibers (24') disposed in the stack of the layers of fibers (22).

5. The stiffener according to any of claims 1 to 4, comprising chain stitch seams disposed at the ends of the fibrous reinforcement (20) along the width direction.

6. The stiffener (10) according to any of claims 1 to 5, wherein the layers of fibers (22) and the layers of reinforcing fibers (24, 24') are sewn together by knit-type stitches.

7. The stiffener (10) according to any of claims 1 to 6, wherein the tension of the sewing threads (30) is lower in the connecting portions than the tension of the sewing threads (30) in the central portion.

8. The stiffener (10) according to any of claims 1 to 7, wherein the distribution of the sewing stitches (32) in the central portion is denser than the distribution of the sewing stitches (32) in the connecting portions.

9. The stiffener according to any of claims 1 to 8, wherein the sewing threads (30) sewn on the end portions and the connecting portions have an excess length.

10. A nacelle (100) comprising a stiffener (10) according to any of claims 1 to 9.

11. An aircraft engine (1000) comprising a nacelle (100) according to claim 10.

12. A method for manufacturing a curved stiffener (10) according to any of claims 1 to 9 comprising: a sewing step (E1) in which the fibrous reinforcement (20) is formed by sewing together the plurality of layers of fibers (22) and the layer(s) of reinforcing fibers (24), a draping step (E2) in which the fibrous reinforcement (20) is disposed on a stiffener mold (40), an impregnation step (E3) in which the fibrous reinforcement (20) disposed on the stiffener mold (40) is impregnated with resin, a polymerization step (E4) in which the resin impregnated in the fibrous reinforcement (20) is solidified.