METHOD FOR MANUFACTURING AN ACOUSTIC PANEL WITH SLOPED CAVES

Description

Technical field

[0001] The invention relates to the field of acoustic panels intended for the aeronautical sector. State of the prior art

[0002] A conventional aircraft propulsion system comprises acoustic panels, commonly referred to as "sandwich panels", comprising two skins and an intermediate structure sandwiched between them. The intermediate structure is generally a honeycomb-like structure forming acoustic absorption cavities, or Helmholtz cavities, to attenuate the noise generated by the propulsion system. To this end, the skin intended to be oriented towards the source of the noise is made permeable to air, typically using holes passing through this skin to be able to guide the air within the cavities and thus absorb acoustic energy.

[0003] Generally speaking, the thickness of the panel determines the length of the cavities and therefore their attenuation capacity. In particular, a longer cavity makes it possible to attenuate waves of greater length and therefore lower frequencies.

[0004] To reduce the bulk of the panel while attenuating low frequencies generated by the propulsion assembly, it is known to tilt the cavities in the manner described in documents US3821999, WO92 / 12854 and WO2020 / 152418.

[0005] Such an inclination of the cavities results in a reduction in the compressive strength of the intermediate structure compared to a structure having cavities whose axis is normal to the skins.

[0006] When the panel manufacturing includes automation of the draping operations, the mechanical pressure applied to the intermediate structure by a draping roller is typically of the order of 0.35 MPa. This pressure can reach 1.5 MPa in areas with geometric singularities given the corresponding reduction in the contact surface between the roller and the plies. However, it is estimated that the compressive strength of an intermediate structure whose cavities are inclined at an angle of 45° and which has a density between 67 kg / m 3 and 104 kg / m 3 is typically between 0.2 MPa and 0.5 MPa.

[0007] There is therefore a risk of crushing the intermediate structure during the manufacture of the panel requiring part of the draping to be carried out manually, which increases the manufacturing cost of the panel. Statement of the invention

[0008] The invention aims to provide an acoustic panel comprising an intermediate structure forming oblique cavities at a competitive cost and without increasing the mass of the panel.

[0009] To this end, the invention relates to a method of manufacturing an acoustic panel in accordance with claim 1, comprising: a step of arranging on a mold one or more first plies of a material comprising a resin, so as to form a first skin of the panel, a first step of heating the first plies so as to polymerize the resin, a step of arranging on the first skin an adhesive material, a step of arranging on the adhesive material an intermediate structure comprising cavities which each open on the one hand onto a first surface of the intermediate structure and on the other hand onto a second surface of the intermediate structure, the cavities being oblique with respect to the first and second surfaces, so that the adhesive material is sandwiched between the first skin and the first surface of the intermediate structure, a step of arranging on the second surface of the intermediate structure one or more second plies of a material comprising a resin, so as to form a second skin of the panel,a third step of heating the assembly formed by the first skin, the intermediate structure and the second skin, so as to polymerize the resin of the second plies.

[0010] According to the invention, the method comprises, before the arrangement of the second plies, a second step of heating the assembly formed by the first skin, the adhesive material and the intermediate structure, so as to polymerize the adhesive material.

[0011] The second heating step thus makes it possible to fix the intermediate structure on the first skin and to prevent it from slipping relative to the first skin when arranging the second plies.

[0012] It has been observed that such fixing of the intermediate structure to the first substantially increases the compressive strength of the intermediate structure. Indeed, the intermediate structure being glued to the first skin on one side, the cells which form the intermediate structure have much less latitude to deform and are therefore more rigid.

[0013] More specifically, it has been noted that for an intermediate structure having a density of 67 kg / m 3 and having cavities inclined at 45 degrees relative to the first surface of this intermediate structure, the implementation of said second heating step makes it possible to increase the compressive strength from 0.2 MPa to 1.4 MPa.

[0014] The invention thus makes it possible to automate the draping operations, in particular the arrangement of said second folds on the intermediate structure, and therefore to reduce the manufacturing cost of the panel.

[0015] The invention also makes it possible to obtain such a result without increasing the density of the intermediate structure and therefore the mass of the panel.

[0016] It is preferred that one or more of said first plies and / or one or more of said second plies are arranged using a draping robot.

[0017] Of course, the invention also covers methods in which all the draping operations are carried out manually.

[0018] In one embodiment, the first and third heating steps comprise gas pressurizing the heated elements.

[0019] Said heated elements comprise in this case the first skin of the panel concerning the first heating step and, concerning the second heating step, the assembly formed by the first skin, the adhesive material and the intermediate structure.

[0020] Preferably, the maximum pressure applied during the third heating step is lower than the maximum pressure applied during the first heating step.

[0021] For example, the maximum pressure applied during the first heating step is between six and ten bars and the maximum pressure applied during the third heating step is between two and four bars. Preferably, the second heating step is carried out at a reduced pressure compared to that of the first and third heating steps CH1 and CH3, typically between 0.2 bars and 1.5 bars.

[0022] In one embodiment, one or more of the first, second, and third heating steps are performed within an autoclave, and / or the second heating step is performed under a bladder in an oven or furnace.

[0023] Preferably, all of these heating steps are carried out in an autoclave.

[0024] Alternatively, one or more of said heating steps may be carried out using another heating means such as an oven or stove.

[0025] According to a first variant embodiment, the method comprises a step of piercing the first skin carried out after the first heating step.

[0026] According to a second embodiment variant, the method comprises a step of piercing the second skin carried out after the third heating step.

[0027] These variants can be combined.

[0028] Preferably, the method comprises a step of piercing the first skin carried out after the first heating step, in which crosslinking of the adhesive material is carried out between this piercing step and the second heating step.

[0029] Preferably, each of the cavities of the intermediate structure extends along an axis forming an angle of between 30 degrees and 60 degrees relative to the first surface of the intermediate structure.

[0030] For example, this angle can be equal to 45 degrees.

[0031] In one embodiment, the intermediate structure comprises a metallic material such as aluminum.

[0032] Alternatively, the intermediate structure may comprise an organic material and / or comprising synthetic fibers such as Nomex ® or even thermoplastic such as PEEK.

[0033] In one embodiment, the first plies and the second plies comprise an organic matrix composite material.

[0034] In one embodiment, the intermediate structure has a density of less than 130 kg / m 3< , preferably less than or equal to 86 kg / m 3< .

[0035] Other advantages and characteristics of the invention will appear on reading the detailed, non-limiting description which follows. Brief description of the drawings

[0036] The following detailed description refers to the attached drawings in which: [ Fig. 1 ] is a schematic longitudinal sectional view of an aircraft propulsion unit equipped with acoustic panels; [ Fig. 2 ] is a schematic cross-sectional view of an acoustic panel according to the invention, comprising an intermediate structure having oblique sound absorption cavities; [ Fig. 3 ] is a flowchart for manufacturing an acoustic panel according to a first embodiment; [ Fig. 4 ] is a flowchart for manufacturing an acoustic panel according to a second embodiment. Detailed description of embodiments

[0037] THE Figures 1 and 2comprise a reference frame D1, D2 and D3 defining respectively a longitudinal / axial direction, a circumferential / tangential direction and a radial direction orthogonal to each other.

[0038] It is represented in a simplified way on the figure 1 a propulsion unit 1 for an aircraft comprising a turbomachine 3 and a nacelle 4 extending around a longitudinal central axis 2. In this example, the turbomachine 3 is a dual-flow turbojet.

[0039] Subsequently, the terms “front” and “rear” are considered according to a direction S1 of main flow of gases in the propulsion unit 1 according to the axis 2 which is parallel to the direction D1.

[0040] In a manner known per se, the propulsion unit 1 comprises, from front to rear, an air intake 5, a fan 6, a secondary vein 7 delimited radially inwards by an internal fairing 8 enveloping a gas generator 9 formed by the turbojet 3 and radially outwards by elements of the nacelle 4, and a nozzle 10 for ejecting a primary flow leaving the gas generator 9. The nozzle 10 comprises an ejection cone 11 and an ejection nozzle 12.

[0041] The invention relates more specifically to acoustic panels 20 intended to equip such a propulsion unit 1.

[0042] In this non-limiting example, the propulsion unit 1 comprises several acoustic panels 20 as described below, shown in strong lines on the figure 1These panels 20 include panels 20A forming an internal wall of the air inlet 5, panels 20B forming a part of the internal fairing 8 and panels 20C delimiting the secondary vein 7, panels 20D forming an external wall of the ejection cone 11 and panels 20E forming an internal wall of the ejection nozzle 12.

[0043] In this example, each of the panels 20A to 20E extends circumferentially around the axis 2, forming a ring sector. Thus, in particular, each of the panels 20A extends over a respective circumferential sector so as to together form a ring of axis 2. What has just been described concerning the panels 20A applies by analogy to the panels 20B to 20E.

[0044] A panel 20 according to the invention is shown in section on the figure 2 , according to a cutting plane parallel to directions D1 and D3.

[0045] The panel 20 comprises a first skin 21, a second skin 22 and an intermediate structure 23.

[0046] The intermediate structure 23 has an internal surface 26 and an external surface 27 which are, in the section of the figure 2 , represented as plane and parallel to the direction D1. Of course, the internal surface 26 and / or the external surface 27 may have a curved geometry, both in the section of the figure 2 and / or in other sections.

[0047] In this example, the intermediate structure 23 extends in the circumferential direction D2 so as to form a ring sector.

[0048] The intermediate structure 23 comprises partitions 30 delimiting cavities 31 which extend both radially and axially.

[0049] In this example, the majority of the cavities 31 open on the one hand onto the internal surface 26 and on the other hand onto the external surface of the intermediate structure 23.

[0050] Each of the cavities 31 extends along an axis 32 oblique to the internal 26 and external 27 surfaces.

[0051] In this example, the axis 32 of each of the cavities 31 forms with the surfaces 26 and 27 an angle 33 of approximately 45°.

[0052] Concerning the geometry of the cavities 31, the partitions 30 are in this example configured so that each of the cavities 31 has a hexagonal section, in a plane normal to the axis 32. Alternatively, one or more of the cavities 31 may comprise a triangular, square or other section. The cavities 31 may have any other shape, in particular to avoid telegraphing.

[0053] The first skin 21, also called the internal skin, is arranged radially on one side of the intermediate structure 23 so as to cover the internal surface 26 of this structure 23. The internal skin 21 matching the internal surface 26 of the intermediate structure 23, the aforementioned angle 33 is also formed between the axis 32 of the cavities 31 and the internal skin 21.

[0054] The second skin 22, also called the outer skin, is arranged radially on the other side of the intermediate structure 23 so as to cover the outer surface 27 of the intermediate structure 23.

[0055] In this example, the outer skin 22 is solid while the inner skin 21 comprises openings (not shown) intended to guide air into the cavities 31 in order to absorb acoustic energy.

[0056] In this example, the panel 20 has a thickness 50, or radial dimension, of between 30 mm and 40 mm and the skins 21 and 22 each have a thickness of between 0.2 mm and 2 mm, for example 1.0 mm.

[0057] The panel 20 is preferably manufactured using a method comprising the steps described below.

[0058] In reference to the figure 3 , a draping step DR1 is carried out in which folds are arranged on a mold (not shown) so as to form the internal skin 21 of the panel 20.

[0059] The plies comprise an organic matrix composite material, e.g. carbon fibers, and are pre-impregnated with a polymerizable resin, e.g. epoxy.

[0060] In an alternative embodiment, the folds each comprise juxtaposed ribbons.

[0061] In this example, the folds are arranged on the mold using a draping robot (not shown) which includes a roller for pressing the folds against the mold and against each other.

[0062] The folds thus arranged are placed with the mold in an autoclave (not shown) in order to undergo a first CH1 heating step.

[0063] The autoclave enclosure is placed under gas pressure, in this example at a maximum pressure of between six and ten bars, and the temperature is increased to reach a level of between 100°C and 250°C, and more preferably between 150 and 220°C, so as to polymerize the resin contained in the folds forming the internal skin 21.

[0064] After cooling and hardening of the internal skin 21, a drilling step PE1 is carried out so as to form the aforementioned openings.

[0065] A layer of a polymerizable adhesive material, for example epoxy, is then placed on the internal skin 21 (step CO1) then the intermediate structure 23 (step SI1) so that the adhesive material is interposed between the internal skin 21 and the internal surface 26 of the intermediate structure 23. The adhesive layer is crosslinked (for example by blowing hot air) before the intermediate structure 23 is removed, so as not to obstruct the acoustic openings made in step PE1.

[0066] In this example, the intermediate structure 23 comprises aluminum and has a density of approximately 86 kg / m 3< .

[0067] The assembly formed by the internal skin 21, the adhesive material and the intermediate structure 23 is then placed in the autoclave enclosure in order to carry out a second CH2 heating step.

[0068] The autoclave enclosure is placed under gas pressure, in this example at a reduced maximum pressure which is in this example between 0.2 and 1.5 bars, and the temperature is increased to reach a level between 150°C and 230°C, so as to at least partially polymerize the layer of adhesive material. Given that the pressure is reduced, this step CH2 can alternatively be carried out outside the autoclave, for example under a bladder in an oven or furnace.

[0069] This second CH2 heating step makes it possible to fix the intermediate structure 23 on the internal skin 21.

[0070] A second draping step DR2 is then carried out in which folds similar to those described above are arranged on the external surface 27 of the intermediate structure 23, so as to form the external skin 22 of the panel 20.

[0071] Panel 20 is then placed in the autoclave enclosure to undergo a third CH3 heating step.

[0072] The autoclave enclosure is placed under gas pressure, in this example at a maximum pressure of between two and four bars, and the temperature is increased to reach a level of between 100°C and 250°C, and preferably between 150°C and 230°C, so as to polymerize the resin of the folds forming the external skin 22.

[0073] Of course, the invention is not limited to the preceding description and its subject matter is defined by claims 1 to 10.

[0074] For example, in reference to the figure 4 , the drilling step PE1 of the inner skin 21 can be omitted and a drilling step PE2 of the outer skin 22 can be implemented after cooling and hardening of the outer skin 22 having undergone the third heating step CH3.

[0075] Furthermore, all or part of the draping can alternatively be carried out manually and / or the heating steps CH1, CH2 and / or CH3 can be carried out using a heating means other than an autoclave and / or by using any appropriate tools and pressurizing means.

[0076] In addition, the folds of the inner 21 and / or outer 22 skins may also be made of stacks of different materials, such as for example folds of fiberglass and carbon.

Claims

1. Method for manufacturing an acoustic panel (20), comprising: - a step (DR1) of arranging on a mould one or more first layers of a material comprising a resin, so as to form a first skin (21) of the panel (20), - a first step of heating (CH1) the first layers so as to polymerize the resin, - a step (CO1) of arranging on the first skin (21) an adhesive material, - a step (SI1) of arranging on the adhesive material an intermediate structure (23) comprising cavities (31) which each open out on the one hand on a first surface (26) of the intermediate structure (23) and on the other hand on a second surface (27) of the intermediate structure (23), the cavities (31) being oblique with respect to the first and second surfaces (26, 27), such that the adhesive material is enclosed between the first skin (21) and the first surface (26) of the intermediate structure (23), - a step (DR2) of arranging on the second surface (27) of the intermediate structure (23) one or more second layers of a material comprising a resin, so as to form a second skin (22) of the panel (20), - a third step of heating (CH3) the assembly formed by the first skin (21), the intermediate structure (23) and the second skin (22), so as to polymerize the resin of the second layers, the method comprising, before arranging the second layers, a second step of heating (CH2) the assembly formed by the first skin (21), the adhesive material and the intermediate structure (23), so as to polymerize the adhesive material.

2. Method according to Claim 1, wherein one or more of said first layers and / or one or more of said second layers are arranged by means of a draping robot.

3. Method according to Claim 1 or 2, wherein the first and third heating steps CH1 and CH3 comprise pressurizing the heated elements with gas, the maximum pressure applied during the third heating step being lower than the maximum pressure applied during the first heating step.

4. Method according to Claim 3, wherein the maximum pressure applied during the first heating step CH1 is comprised between six and ten bar and wherein the maximum pressure applied during the third heating step CH3 is comprised between two and four bar, and preferably, the second heating step CH2 is carried out at a reduced pressure relative to that of the first and third heating steps CH1 and CH3, typically between 0.2 bar and 1.5 bar.

5. Method according to any one of Claims 1 to 4, wherein one or more among the first, second and third heating steps are carried out within an autoclave, and / or in that the second heating step is carried out under a bladder in an oven or a furnace.

6. Method according to any one of Claims 1 to 5, comprising a step (PE1) of piercing the first skin carried out after the first heating step and / or a step (PE2) of piercing the second skin carried out after the third heating step.

7. Method according to any one of Claims 1 to 5, comprising a step (PE1) of piercing the first skin carried out after the first heating step, wherein cross-linking of the adhesive material is carried out between this piercing step (PE1) and the second heating step (CH2).

8. Method according to any one of Claims 1 to 7, wherein each of the cavities (31) of the intermediate structure (23) extends along an axis (32) forming an angle (33) comprised between 30 degrees and 60 degrees, for example equal to 45 degrees, relative to the first surface (26) of the intermediate structure (23).

9. Method according to any one of Claims 1 to 8, wherein the intermediate structure (23) comprises a metallic material such as aluminium and wherein the first layers and the second layers comprise an organic matrix composite.

10. Method according to any one of Claims 1 to 9, wherein the intermediate structure (23) has a density of less than 130 kg / m3, preferably less than or equal to 86 kg / m3.