Method for forming a sound attenuation structure provided with s-shaped cells

Abstract

The invention relates to a method for forming an acoustic resonator panel for an aircraft propulsion assembly nacelle, the acoustic panel comprising cells with transverse internal obstacles in order to lengthen the path travelled by the sound waves. The method comprises: - a step (600) of forming a perforated metal sheet; - a step (610) of stacking a plurality of thermoplastic cellular cores and at least one perforated sheet, two consecutive cellular cores being separated by a perforated sheet; - a step (615) of compacting the stack; and - a step (620) of thermoplastic welding.

Description

[0001] Description

[0002] Title of the invention: Method of forming an acoustic attenuation structure having S-shaped cells

[0003] Technical Field

[0004] The present invention relates to the general field of acoustic attenuation structures. It relates more particularly to acoustic attenuation structures used to reduce noise produced in aircraft engines such as in gas turbines or their exhausts.

[0005] Prior art

[0006] The challenges of commercial aviation regarding the reduction of fuel consumption and the ecological impact lead to a rethinking of aircraft engine architectures. Two characteristics are particularly studied for their influence on the performance of the propulsion system: the bypass ratio (ratio of the air flow passing through the secondary flow to the air flow passing through the primary flow of the engine) and the drag induced by the propulsion system.

[0007] It has been proven and shared by the entire industry that an increase in the bypass ratio improves engine efficiency (reduces its consumption and reduces its harmful emissions). This increase in the bypass ratio is achieved in practice by increasing the diameter of the turbomachinery fan. However, this increase in the diameter of the secondary flow induces an increase in the same proportions of the diameter of the casing and the nacelle. If the architecture of the fan module is not redesigned, it is easy to understand the negative impact of the increase in drag induced by the propulsion unit having larger external surfaces in contact with the air. One of the paths studied therefore leads to shortening and thinning the external casing and the nacelle of the engine to reduce its mass and drag.

[0008] On current engines, acoustic attenuation is partly achieved using acoustic panels placed at the casing and nacelle level. To gain compactness on the external casing, the acoustic treatments themselves will therefore have to become more compact. Acoustic panels are mechanical elements similar to honeycomb boxes that reduce the noise emitted by the engine. The shape of the honeycombs and the thickness of these panels are studied in order to reduce the noise pollution of the engine as much as possible, particularly during certain operating phases such as takeoff and landing. Noise reduction is an even more important issue around airports and neighboring towns.

[0009] In future architectures, two new difficulties arise: the large diameter fan will rotate less quickly and will therefore generate lower frequency sound waves and the structural casing will no longer completely cover the engine and will therefore no longer be able to effectively confine noise.

[0010] Acoustic attenuation structures typically consist of an acoustic surface plate or skin permeable to the acoustic waves that are to be attenuated and a reflective solid plate or skin called a "closing plate", a cellular body being arranged between these two walls. The cellular body is generally constituted by a set of partitions, for example in the shape of a honeycomb. As is well known, such structures form Helmholtz-type resonators that make it possible to attenuate acoustic waves in a certain frequency range. Acoustic attenuation structures of this type are described in particular in documents US 5,912,442 and GB 2,314,526. However, the acoustic attenuation structures previously described can only absorb a very limited frequency range.

[0011] With such a structure, the absorbed frequency f is of the order of c / 4e with e the thickness of the honeycomb and c the speed of sound. Or conversely, the processing thickness necessary to process a frequency f is of the order of c / 4f.

[0012] Thus, if the frequency of the noise to be treated decreases, the required treatment thickness increases. Typically, in the absence of grazing flow effects, a 30 mm thick cell body is suitable for attenuating frequencies close to 2000 Hz, and a 70 mm thick cell body is suitable for attenuating frequencies close to 880 Hz. The thickness of the cell body corresponds to the distance between the permeable acoustic skin and the impermeable acoustic skin. In other words, the acoustic length of the cells is approximately equal to the height of the alveolar core.

[0013] This attenuation constraint works against reducing the thickness of the structural casing and its surface area to satisfy mass and drag reduction.

[0014] However, it is desirable to produce sound attenuation structures that largely address low frequencies, while exhibiting satisfactory performance in the medium and high frequencies, for example in the case of a slow-moving engine fan that produces low frequencies and harmonics. In addition, the size and mass of the sound attenuation structure should preferably be limited, for example when it is mounted on an aircraft.

[0015] To expand the absorbed frequency range, it is known to superimpose two cellular bodies, preferably each having a honeycomb structure of different sizes to handle different frequencies. We speak of DDOF structures in English for "Double Degree Of Freedom" (or also 2DOF) for acoustic panels having two distinct superimposed cellular bodies, and SDOF structures in English for "Single Degree Of Freedom" for acoustic panels with a single cellular body.

[0016] However, this solution with two stacked cellular bodies has limitations for the treatment of low frequencies. Indeed, to reduce the lowest frequencies, it is necessary to use very thick cellular bodies. Thus, the acoustic attenuation structure comprising two stacked cellular bodies to treat both low and high frequencies will be relatively bulky.

[0017] Different solutions exist to treat low frequencies while limiting the increase in thickness: the use of a cone introduced into the cells, the inclination of a classic honeycomb or the manufacture of labyrinth or S-shaped cells.

[0018] EP 3 676 825 proposes an acoustic panel structure with a cell body having cells having an S-shaped structure therein to increase the distance traveled by sound within the cell. The acoustic panels described in this document comprise cells having therein at least one partial obstacle extending from the inner wall of the cell and preventing the wave from propagating directly along the direction in which the cell extends, otherwise the direction of the thickness of the cell body.

[0019] Statement of the invention

[0020] The main aim of the present invention is therefore to propose a method for manufacturing an acoustic panel with resonators for an aircraft propulsion unit nacelle, the acoustic panel comprising cells with transverse internal obstacles to lengthen the path traveled by the sound waves, the method making it possible to facilitate the production of the honeycomb cores of the panel and thus to reduce the cost and manufacturing time of the acoustic panel.

[0021] According to an object of the invention, there is proposed a method of manufacturing an acoustic panel with resonators for an aircraft propulsion unit nacelle, the acoustic panel comprising adjoining acoustic cells which form a honeycomb core, each cell extending along an axis of acoustic propagation of the sound waves and comprising inside the cell, at least one partial obstacle which extends transversely with respect to the axis of acoustic propagation and which forms an internal passage off-center with respect to the center of the cell to increase the length of the path traveled by the sound waves through the cell, the method of manufacturing the acoustic panel comprising:

[0022] - a step of forming at least one perforated metal mesh in a regular pattern to form said passages, the center of each passage being separated from the center of the adjacent passages by a distance corresponding to the width of the acoustic cells, and each passage having a size smaller than the size of an acoustic cell measured in a plane perpendicular to a main direction, the main direction being parallel to the acoustic propagation axis,

[0023] - a step of stacking along the main direction a plurality of cellular cores made of thermoplastic resin and at least one perforated metal mesh, two successive cellular cores being separated by a perforated metal mesh, each cellular core comprising a plurality of cells joined in a plane orthogonal to the main direction and forming a portion of acoustic cell of said cellular core in the main direction, and each cell of a cellular core being opposite a passage of a mesh, said passage and the central axis of the cell not being aligned along the main direction,

[0024] - a compaction step during which a compaction pressure is applied in the main direction on either side of the stack obtained following the stacking step,

[0025] - and a thermoplastic welding step carried out while maintaining the compaction pressure.

[0026] The method according to the invention thus makes it easy to industrialize the manufacture of such an acoustic panel, in particular thanks to the production of the alveolar core from a stack of the different parts forming the obstacles and the enclosures of the alveoli or acoustic cells.

[0027] Thermoplastic welding allows for local melting of the thermoplastic material and thus fusion of the honeycomb cores with each other and with said at least one perforated fabric.

[0028] The compaction pressure keeps the honeycomb cores pressed against the perforated metal mesh(es) during thermoplastic welding and thus facilitates and promotes the welding of the elements together. The compaction pressure is maintained while the honeycomb core cools.

[0029] The alveoli, or acoustic cells, can have sections, in a plane orthogonal to the direction of acoustic propagation of the sound waves, of round, hexagonal or other shape.

[0030] Thermoplastic honeycomb cores can be formed by continuous thermoforming technology.

[0031] For each cell of the alveolar core, the perforated passages made in the metal mesh are offset from the central axis of the cell to force the sound wave to follow a non-linear path and therefore longer than if the cell were free of transverse obstacles inside its enclosure. The alveolar cores can be made of resin of the PAEK (Polyarylether ketone), PPS (Polyphenylene sulfone), PSU (Polysulfone), PC (Polycarbonate), PA (Polyamide), PP (Polypropylene), PEI (Polyether imide) type.

[0032] In a first embodiment of the manufacturing method according to the invention, the thermoplastic welding step comprises pulling said at least one perforated metal mesh in at least one direction perpendicular to the main direction, the tensioning making it possible to stretch the mesh so that it is flatter and does not form creases.

[0033] In a second embodiment of the manufacturing method according to the invention, the thermoplastic welding may be induction welding or resistive welding.

[0034] In the case of induction welding, the stack forming the honeycomb core is introduced into a magnetic field which causes said at least one perforated metal mesh to heat up and which results in local melting of the thermoplastic material and thus fusion of the honeycomb cores with each other and with said at least one perforated mesh.

[0035] The perforated wire mesh could be made of stainless steel with good magnetic properties.

[0036] In the case of resistive welding, an electric current is applied to said at least one perforated metal mesh to heat the mesh(es) by Joule effect and cause local melting of the thermoplastic material and thus fusion of the honeycomb cores with each other and with said at least one perforated mesh.

[0037] In a third embodiment of the manufacturing method according to the invention, the method may further comprise, prior to the stacking step, a step of impregnating said at least one perforated metal mesh with a thermoplastic resin or a step of bonding a pure thermoplastic film to the or each perforated metal mesh.

[0038] The preliminary impregnation step allows thermoplastic resin to be applied to the perforated metal mesh to improve the welding between the stages of the honeycomb core. Preferably, the thermoplastic resin used for impregnation is the same as that in which the honeycomb cores are made.

[0039] In a variant, rather than impregnating said at least one perforated metal mesh with a thermoplastic resin, the method may further comprise, prior to the stacking step, a step of impregnating the honeycomb core with a thermoplastic resin or a step of bonding a pure thermoplastic film to the honeycomb core.

[0040] In a fourth embodiment of the manufacturing method according to the invention, the thermoplastic welding is preferably carried out at a temperature between the glass transition temperature and the melting temperature of the thermoplastic cellular cores for amorphous thermoplastics, and at a temperature close to the melting temperature of the cellular cores for semi-crystalline thermoplastics.

[0041] The temperature is controlled by the electric current applied to said at least one perforated metal mesh in the case of resistive welding (typically with a power density of 5 to 50 W / cm 2 ), whereas in the case of magnetic welding, the temperature is controlled by the magnetic power (the efficiency depending on the choice of the inductor).

[0042] For semi-crystalline thermoplastics (PAEK for example), it is necessary to be above the glass transition temperature but this is generally not sufficient. It is necessary to go towards temperatures close to the melting temperature or even slightly above. Welding is carried out so that heating remains localized at the interface so that the rest of the cells remain at a temperature lower than the glass transition so as not to collapse the assembly when pressure is applied.

[0043] In a fifth embodiment of the manufacturing method according to the invention, when the acoustic panel comprises at least two perforated metal meshes, the passages of two successive perforated metal meshes along the main direction are preferably not aligned along the main direction. Having passages that are never aligned two by two successively in the main direction makes it possible to maximize the path traveled by the sound wave between its entry into the cell and its exit.

[0044] In a sixth embodiment of the manufacturing method according to the invention, the method may further comprise, prior to the stacking step, a step of forming the cellular cores with cells having a width of between 0.95 and 2.5 cm and a height of between 5 and 100 mm, the cells of the same cellular core all having the same width and all the same height.

[0045] In a seventh embodiment of the manufacturing method according to the invention, the step of stacking the cellular cores and said at least one perforated metal mesh preferably comprises the formation of a stack of between 15 and 200 mm in height in the main direction and typically 60 mm.

[0046] In an eighth embodiment of the manufacturing method according to the invention, the passages made during the step of forming said at least one perforated metal mesh preferably all have the same shape with a first dimension in a first direction longer than a second dimension in a second direction, the first direction being orthogonal to the second direction, and the first and second directions being orthogonal to said main direction.

[0047] The passages can thus have a rectangular shape, an oblong shape or an oval shape.

[0048] In a ninth embodiment of the manufacturing method according to the invention, the step of stacking the cellular cores and said at least one perforated metal mesh preferably comprises the formation of a stack having, in the main direction, a first face and a second face, the method further comprising a closure of the first face of said stack by an acoustically reflective skin, and a closure of the second face of said stack by an acoustically transparent skin. In a variant, the acoustically porous skin may be formed by a stretched perforated metal mesh with a perforation pattern possibly different from the other perforated metal meshes of the acoustic panel.

[0049] In a tenth embodiment of the manufacturing method according to the invention, the step of forming at least one perforated metal mesh preferably comprises the use of a mesh having an acoustic resistance at 105 cm / s of at least 100 rayls cgs, or 1000 Pa.s / m.

[0050] If the perforated wire mesh that forms the obstacles inside the cells is too porous, the sound wave may not follow the non-linear path.

[0051] In an eleventh embodiment of the manufacturing method according to the invention, the thermoplastic resin may be accompanied by fillers or short fibers, which provides greater stiffness and mechanical resistance.

[0052] In a twelfth embodiment of the manufacturing method according to the invention, the compacting pressure is preferably between 10 and 200 bars.

[0053] In a thirteenth embodiment of the manufacturing method according to the invention, each passage may be formed by a plurality of adjacent orifices. Each passage is produced by a plurality of perforations or orifices, the passages being distributed in a regular pattern on the perforated metal mesh.

[0054] Brief description of the drawings

[0055] [Fig. 1] Figure 1 is a schematic sectional view illustrating a nacelle equipped with a plurality of acoustic panels according to the invention;

[0056] [Fig. 2] Figure 2 is a schematic cross-sectional view illustrating cells of one of the acoustic panels of Figure 1 equipped with two obstacles forming baffles;

[0057] [Fig. 3] Figure 3 is a schematic cross-sectional view illustrating cells of one of the acoustic panels of Figure 1 equipped with three obstacles forming baffles; [Fig. 4] Figure 4 is a schematic perspective view with cutaway illustrating a hexagonal cell of one of the acoustic panels of Figure 1 equipped with two obstacles forming baffles;

[0058] [Fig. 5] Figure 5 illustrates an exploded view of a honeycomb core of an acoustic panel of Figure 2 according to an embodiment of the manufacturing method of the invention.

[0059] [Fig. 6] Figure 6 is a flowchart of a method of manufacturing one of the acoustic panels of Figure 1 according to an embodiment of the invention.

[0060] Description of the embodiments

[0061] Identical or similar elements are identified by identical reference signs throughout the figures.

[0062] In the description, the terminology longitudinal, vertical and transverse will be adopted without limitation with reference to the trihedron L, V, T indicated in the figures.

[0063] The expressions “front” and “rear” will also be used without limitation in reference to the lower part and the upper part respectively of figures 2 to 5.

[0064] Figure 1 shows a nacelle 10 equipped with a plurality of acoustic panels 12 with acoustic attenuation resonators shown schematically in strong lines. Some or all of these, or other acoustic panels may be totally or partially equipped with honeycomb cores according to the invention.

[0065] The acoustic panels 12 are designed to attenuate the noise emitted by the components which are housed in the nacelle 10, such as an engine or a fan (not shown). According to exemplary embodiments of the invention described here, the acoustic panels 12 are integrated into an air inlet shroud 14, into the secondary vein 16 and into an ejection nozzle 20.

[0066] With reference to Figure 2, which illustrates a first example of an embodiment of an acoustic panel 12, the acoustic panel 12 comprises successively, from front to back along the longitudinal axis L, a front acoustic skin 22 which is acoustically porous to sound waves, a honeycomb core 24, and a rear skin 26 which is solid and therefore acoustically reflective.

[0067] The front acoustic skin 22 and the rear skin 26 extend parallel to each other and transversely, that is to say in the transverse direction T which is orthogonal to the longitudinal direction L.

[0068] The front acoustic skin 22 has a plurality of perforations 27, or a permeability formed by a mesh, which are adapted to allow sound waves to penetrate into the alveolar core 24.

[0069] The alveolar core 24 comprises a plurality of acoustic cells 28, or alveoli 28, which are joined to each other in the transverse direction T and the vertical direction V to form a hollow structure such as, for example, a “honeycomb”. The direction of the longitudinal axis L corresponds to the direction of acoustic propagation of a sound wave entering an acoustic cell 28 via the acoustic skin 22.

[0070] Each cell 28 is delimited by a peripheral enclosure 30 extending substantially parallel to the longitudinal direction L from the acoustic skin 22 to the rear skin 26. The shape of the cell 28 may be of hexagonal cross-section, as can be seen in FIG. 4, or of rectangular, or square, or circular shape, or any other geometric shape.

[0071] Also, each cell 28 extends along a main longitudinal axis L, corresponding overall to an axis of propagation of the sound waves, from a front end 32 of the cell 28 resting on the acoustic skin 22, to a rear end 34 resting on the rear skin 26.

[0072] It will be noted that the cells 28 are acoustically independent. The term “acoustically independent cells” refers to cells whose enclosure 30 does not significantly propagate acoustic waves from one cell 28 to another. These terms refer to cells separated by sealed walls or walls perforated with one or a few small and limited number of orifices whose function is mainly to facilitate the evacuation of liquids that may penetrate into the cells. These orifices are preferably two to four in number with a unitary section of the order of 1 to 4 mm. 2, and located in the enclosure 30 of the cells, in the immediate vicinity of the rear end 34 of the acoustic core against the rear skin 26. The obstacles are said to be opaque to acoustic waves, but can however be provided with a drainage device for the evacuation of liquids, preferably made by one or two orifices per obstacle and with a section of the order of 1 to 2 mm 2 each.

[0073] As can be seen in the exemplary embodiment illustrated in FIG. 2, each cell 28 comprises at least a first partial obstacle 36 and even, in this example, a second partial obstacle 38 which extend generally in the transverse direction T from the enclosure 30 of the cell 28. In other words, the obstacles 36 and 38 extend in a plane perpendicular to the main axis of the associated cell 28, the main axis of the cell 28 being coincident with the direction of the longitudinal axis L.

[0074] In addition, each obstacle 36, 38 has a free end edge 40 which delimits a passage 42 with the wall 30 opposite, to allow the passage of sound waves which penetrate into the associated cell 28.

[0075] The obstacles 36, 38 are offset in depth along the main longitudinal axis L of the associated cell 28.

[0076] The obstacles 36, 38 are substantially opposite, that is to say that the first obstacle 36 extends from a first hooking edge 41a on the left side of the wall 30, according to FIGS. 2 and 4, and the second obstacle 38 extends from a second hooking edge 41b on the opposite right side of the wall 30, to form a baffle intended to increase the length of the path traveled by the sound waves through the associated cell 28.

[0077] In addition, the length of each obstacle 36, 38 is adapted so that the obstacles 36, 38 partially overlap in a longitudinal projection view on a surface perpendicular to the longitudinal direction.

[0078] Thus, the sound waves follow a sinuous path between the obstacles 36, 38, from the front end 32 to the rear end 34 of the associated cell 28. This sinuous path is therefore longer than the straight-line distance between the two end faces 32 and 34.

[0079] As illustrated by arrow F in Figure 2, sound waves follow a winding path that has an apparent length greater than the length of a straight path.

[0080] It is found that a cell 28 of 30 millimeters longitudinal thickness which includes two obstacles 36 and 38 extending over approximately two-thirds of the section of the associated cell 28, is equivalent to a cell without obstacle of 64 millimeters longitudinal thickness, in terms of noise attenuation with respect to a given frequency.

[0081] According to another exemplary embodiment shown in Figure 3, which is similar to the example shown in Figure 2, each cell 28 comprises a third obstacle 44, the three obstacles 36, 38, 44 have dimensions such that two successive obstacles in the longitudinal direction have a cumulative surface area greater than the section of the cell and a projected surface area covering the entire section. In other words, the obstacles are arranged to impose a sinuous path on the sound waves which travel through the associated cell 28, as shown by arrow F.

[0082] It is noted that a cell 28 of 30 millimeters of longitudinal thickness which comprises three obstacles 36, 38 and 44 each extending over approximately two thirds of the section of the associated cell as illustrated in figure 3, the successive obstacles being attached on opposite walls, is equivalent to a cell without obstacle of 70 millimeters of longitudinal thickness, in terms of attenuation of the noise with respect to another frequency considered.

[0083] The cells 28 are made of thermoplastic composite material and the obstacles of metallic material. Also, the obstacles 36, 38 can be welded to the material forming the cells 28.

[0084] In Figure 5 is illustrated an exploded view of a honeycomb core 24 of an acoustic panel 12 of Figure 2 according to an embodiment of the manufacturing method of the invention. The honeycomb core 24 comprises thermoplastic honeycomb cores 110, 120 and 130 and perforated metal meshes 102 and 103.

[0085] More particularly, the alveolar core 24 comprises a first alveolar core 110, a second third alveolar core 120 and a third alveolar core 130, having respectively a cell height Hn0, H120, HI 30 , the height being measured in the longitudinal direction L. The three heights Hn0, H120, HI 30 may be equal or different. In the example illustrated in Figure 5, the height Hno of the first cellular core 110 is less than those of the second and third cellular cores 120 and 130, these two cellular cores 120 and 130 having an equal cell height.

[0086] Each cellular core 110, 120 and 130 comprises an upper longitudinal end, denoted respectively 110a, 120a and 130a, a lower longitudinal end, denoted respectively 110b, 120b and 130b, and enclosure portions, denoted respectively 111, 121 and 131, delimiting partial cells, denoted respectively 112, 122 and 132, extending, in the longitudinal direction L, between the upper longitudinal end 110a, 120a, 130a, and the lower longitudinal end 110b, 120b, 130b of the corresponding cellular core 110, 120, 130.

[0087] The cellular cores 110, 120 and 130 have partial cells 112, 122, 132 having a width of between 0.95 and 2.5 cm and a height of between 5 and 30 mm, the cells of the same cellular core all having the same width and all the same height.

[0088] The first perforated metal mesh 102 comprises, in the longitudinal direction L, a first longitudinal end 102a and a second longitudinal end 102b. The first longitudinal end 102a faces the lower longitudinal end 110b of the first cellular core 110 and the second longitudinal end 102b faces the upper longitudinal end 120a of the second longitudinal core 120.

[0089] The first perforated metal mesh 102 comprises perforations 102c forming passages for sound waves. The number of perforations 102c corresponds to the number of cells 28 of the alveolar core 24, and therefore to the number of partial cells 112, 122, 132 of each alveolar core 110, 120, 130.

[0090] Each perforation 102c is spaced from another adjacent perforation 102c by a length equal to the width of a cell 112, 122, 132 measured in a plane comprising the transverse direction T and the vertical direction V.

[0091] The second perforated metal mesh 103 comprises, in the longitudinal direction L, a first longitudinal end 103a and a second longitudinal end 103b. The first longitudinal end 103a faces the lower longitudinal end 120b of the second cellular core 120 and the second longitudinal end 103b faces the upper longitudinal end 130a of the third longitudinal core 130.

[0092] The second perforated metal mesh 103 comprises perforations 103c forming passages for sound waves. The number of perforations 103c corresponds to the number of cells 28 of the alveolar core 24, and therefore to the number of partial cells 112, 122, 132 of each alveolar core 110, 120, 130.

[0093] Each perforation 103c is spaced from another adjacent perforation 103c by a length equal to the width of a cell 112, 122, 132 measured in a plane comprising the transverse direction T and the vertical direction V.

[0094] Considering the longitudinal direction L, the perforations 103c of the second perforated metal mesh 103 and the perforations 102c of the first perforated metal mesh 102 are off-center. Thus, no perforation 103c of the second perforated metal mesh 103 is aligned with a perforation 102c of the first perforated metal mesh 102.

[0095] The perforations 102c and 103c of the first and second perforated metal meshes have elliptical shapes in the example illustrated in Figure 5.

[0096] In Figure 6 is presented a flowchart of a method for manufacturing an acoustic panel 12 of Figure 1 according to an embodiment of the invention with a honeycomb core 24 having a structure corresponding to that illustrated in Figure 5. In a first step 600, the first perforated metal mesh 102 and the second perforated metal mesh 103 are formed, each from a metal mesh perforated in a regular pattern to form said passages with elliptical passages. The metal meshes used to manufacture the perforated metal meshes 102 and 103 have an acoustic resistance at 105 cm / s of at least 100 rayls cgs, or 1000 Pa.s / m.

[0097] In a second step 605, a first honeycomb core 110, a second honeycomb core 120 and a third honeycomb core 130 are formed from thermoplastic resin.

[0098] In a third step 610, the second perforated metal mesh 103 is stacked successively from front to back in the longitudinal direction L on the third cellular core 130, then the second cellular core 120 on the second perforated metal mesh 103, then the first perforated metal mesh 102 on the second cellular core 120, and finally the first cellular core 110 on the first perforated metal mesh 102. The stacking is adjusted so that the partial cells 111, 121, 131 are aligned in the longitudinal direction L, that is to say so that each set of three partial cells 111, 121 and 131 forms a cell 28, and so that each perforation 102c and 103c is inside a cell 28.

[0099] The third step 610 may comprise the addition of a solid rear skin 26 at the rear of the stack, and the addition of an acoustic skin 22 at the front of the stack in the longitudinal direction L. The addition of these two skins may also be carried out after the thermoplastic welding of the fifth step.

[0100] The stack thus obtained has a height measured in the longitudinal direction L of between 15 and 200 mm and typically 60 mm.

[0101] In a fourth step 615, a compacting pressure is applied in the longitudinal direction L to the stack forming the honeycomb core 24 obtained in the third step 610 or to the acoustic panel 12. The compacting pressure is preferably between 2 and 200 bars. In a fifth step 620, thermoplastic welding is carried out while maintaining the compacting pressure.

[0102] Thermoplastic welding is induction welding or resistive welding. It allows for local melting of the thermoplastic material and thus fusion of the honeycomb cores 110, 120, 130 with each other and with the perforated metal meshes 102 and 104.

[0103] In the case of induction welding, the stack forming the honeycomb core 24 is introduced into a magnetic field which causes the perforated metal meshes 102 and 103 to heat up.

[0104] In the case of resistive welding, an electric current is applied to the perforated metal meshes 102 and 103 to heat the meshes by Joule effect.

[0105] Thermoplastic welding is carried out at a temperature between the glass transition temperature and the melting temperature of thermoplastic honeycomb cores.

[0106] The temperature is controlled by the electric current applied to said at least one perforated metal mesh in the case of resistive welding (typically with a power density of 5 to 50 W / cm 2 ), whereas in the case of magnetic welding, the temperature is controlled by the magnetic power (the efficiency depending on the choice of the inductor).

[0107] The fifth thermoplastic welding step may include pulling the perforated metal mesh in the transverse direction T and / or the vertical direction V.

[0108] Thermoplastic welding ends with cooling of the stack after which the assembly formed by the stack is welded together.

Claims

Claims

1. Method for manufacturing an acoustic panel (12) with resonators for an aircraft propulsion unit nacelle, the acoustic panel (12) comprising adjoining acoustic cells (28) which form a honeycomb core (24), each cell (28) extending along an axis (L) of acoustic propagation of the sound waves and comprising, inside the cell (28), at least one partial obstacle (36, 38) which extends transversely with respect to the axis (L) of acoustic propagation and which forms an internal passage (42, 102c, 103c) off-center with respect to the center of the cell (28) to increase the length of the path traveled by the sound waves through the cell (28), the method for manufacturing the acoustic panel (12) comprising: - a step (600) of forming at least one perforated metal mesh (102, 103) in a regular pattern to form said passages (42, 102c, 103c), the center of each passage (42, 102c, 103c) being separated from the center of the adjacent passages (42, 102c, 103c) by a distance corresponding to the width of the acoustic cells (28), and each passage (42, 102c, 103c) having a size smaller than the size of an acoustic cell (28) measured in a plane perpendicular to a main direction (L), the main direction (L) being parallel to the acoustic propagation axis (L), - a step (610) of stacking along the main direction (L) a plurality of cellular cores (110, 120, 130) made of thermoplastic resin and at least one perforated metal mesh (102, 103), two successive cellular cores (110 and 120 or 120 and 130) being separated by a perforated metal mesh (102, 103), each cellular core (110, 120, 130) comprising a plurality of cells (112, 122, 132) joined in a plane orthogonal to the main direction (L) and forming a portion of acoustic cell of said cellular core (24) in the main direction (L), and each cell (112, 122, 132) of a cellular core (110, 120, 130) being opposite a passage (42, 102c, 103c) of a perforated metal mesh (102, 103), said passage (42, 102c, 103c) and the central axis of the cell (28) not being aligned along the main direction (L), - a compaction step (615) during which a compaction pressure is applied in the main direction (L) on either side of the stack obtained following the stacking step, - and a step (620) of thermoplastic welding carried out while maintaining the compacting pressure.

2. The method of claim 1, wherein the thermoplastic welding step (620) comprises pulling said at least one perforated metal mesh (102, 103) in at least one direction perpendicular to the main direction (L).

3. Method according to one of claims 1 or 2, further comprising, prior to the stacking step (610), a step of impregnating said at least one perforated metal mesh (102, 103) with a thermoplastic resin or a step of bonding a pure thermoplastic film to the or each perforated metal mesh.

4. Method according to one of claims 1 or 2, further comprising, prior to the stacking step, a step of impregnating the cellular core with a thermoplastic resin or a step of bonding a pure thermoplastic film to the cellular core.

5. Method according to one of claims 1 to 4, in which the thermoplastic welding is carried out at a temperature between the glass transition temperature for amorphous thermoplastics, and at a temperature close to the melting temperature of the cellular cores for semi-crystalline thermoplastics.

6. Method according to one of claims 1 to 5, wherein the thermoplastic welding is induction welding or resistive welding.

7. Method according to one of claims 1 to 6, wherein when the acoustic panel (12) comprises at least two perforated metal meshes (102, 103), the passages (102c, 103c) of two successive perforated metal meshes (102, 103) in the main direction (L) are non-aligned in the main direction (L).

8. Method according to one of claims 1 to 7, further comprising, prior to the stacking step (610), a step (605) of forming the cellular cores (110, 120, 130) with cells (112, 122, 132) having a width of between 0.95 and 2.5 cm and a height of between 5 and 100 mm, the cells of the same honeycomb core all having the same width and all the same height.

9. Method according to one of claims 1 to 8, in which the step of stacking (610) the cellular cores and said at least one perforated metal mesh comprises the formation of a stack of between 15 and 200 mm in height in the main direction.

10. Method according to one of claims 1 to 9, in which the passages (102c, 103C) produced during the step (600) of forming at least one perforated metal mesh all have the same shape with a first dimension in a first direction longer than a second dimension in a second direction, the first direction being orthogonal to the second direction, and the first and second directions being orthogonal to said main direction.

11. Method according to one of claims 1 to 10, in which the step of stacking (610) the cellular cores and said at least one perforated metal mesh comprises the formation of a stack having, in the main direction (L), a first face and a second face, the method further comprising a closure of the first face of said stack by an acoustically reflective skin, and a closure of the second face of said stack by an acoustically transparent skin.

12. Method according to one of claims 1 to 11, in which the step (600) of forming at least one perforated metal mesh (102, 103) comprises using a mesh having an acoustic resistance at 105 cm / s of at least 1000 Pa.s / m.

13. A method according to any one of claims 1 to 12, wherein the thermoplastic resin is accompanied by fillers or short fibers.

14. Method according to one of claims 1 to 13, in which the compacting pressure is between 10 and 200 bars.

15. A method according to one of claims 1 to 14, wherein each passage is formed by a plurality of adjacent orifices.

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