ACOUSTIC PANEL FOR AN AIRCRAFT TURBOMACHINE CASING

Abstract

The invention relates to an acoustic panel (2121-2) for a casing (200) of an aircraft turbomachine (100), comprising a sandwich structure, said structure comprising a cellular material (300) interposed between a first layer (302) of composite material on which a lower surface of the core rests and a second layer (304) of composite material comprising perforations (PF304) and arranged on an upper face of the cellular material and opposite its lower surface, characterized in that the panel further comprises at least one non-woven strip (400) containing a fibrous and porous network which comprises at least carbon fiber scraps (402) having a length less than or equal to 160 mm, and a binder ensuring the cohesion of the fibers between them and the holding of the strip, said strip covering at least a portion of the upper skin. Figure for abstract: Figure 3A

Description

Title of the invention: ACOUSTIC PANEL FOR A CASING OF AN AIRCRAFT TURBOMACHINE Technical field of the invention

[0001] The present invention relates to an acoustic panel for a casing of an aircraft turbomachine, a casing comprising this panel, a turbomachine comprising the casing, a panel on a casing, an assembly comprising at least this tooling and a panel, and a method of manufacturing this panel. Technological background

[0002] Generally speaking, a turbomachine 100, in particular an aircraft turbomachine, as illustrated in [Fig.lA], comprises from upstream to downstream (i.e. in the direction of flow of the gas flows F), a fan 110, one or more compressors 102 12, a combustion chamber 104, one or more turbines 106i, 1062 and an ejection nozzle 108 for the combustion gases leaving the turbine(s).

[0003] [Fig. 1B] schematically and partially illustrates a fan 110 of a turbomachine. The fan 110 comprises a bladed wheel 112 which is surrounded by a fan casing 200, also called a retention casing due to its function of retaining the blades in the event of their rupture, or in the event of debris entering the fan.

[0004] With reference to [Fig. 1C], a fan casing 200, and more generally a casing, typically comprises an annular envelope 202, which may be, for example and in a non-limiting manner, made of metallic material or of composite material from woven fibers embedded in a polymeric resin. Preferably, the annular envelope is made of composite material. This annular envelope 202 has an axis of revolution A and extends around the fan blades 112 of the turbomachine. The casing 200 comprises an annular fixing flange 204b 2042 at each of the axial ends of the annular envelope 202. These flanges 204b 2042 are used to fix the casing 200 to annular walls of a nacelle which surrounds the turbomachine to form an aircraft propulsion assembly.

[0005] The casing 200 may be connected by the flanges 204b 2042 on the one hand to an air inlet sleeve 4a located upstream of the fan casing 200, and on the other hand, to an intermediate casing shroud 4b located downstream of the fan casing 200, as illustrated in [Fig.lD]. The fan casing 200 also comprises upstream acoustic panels 212i and downstream acoustic panels 2122.

[0006] Still with reference to [Fig.lD], an abradable annular cartridge 208 may be positioned on an internal annular surface S202 of the casing 202 of the housing 200, between the upstream acoustic panels 212i and the downstream acoustic panels 2122. This abradable cartridge 208 may comprise an annular layer 210 of abradable material and an annular panel or support 206 supporting the annular layer 210, the support 206 being fixed to the casing 202. This annular abradable layer 210 is intended to wear in a controlled manner during operation. As for the panel 206, it may, for example, be solid or have a honeycomb structure.

[0007] The acoustic panels 212b 2122 contribute to reducing the acoustic emission of the turbomachine.

[0008] The acoustic panels 212b 2122 are generally in the form of a sandwich structure as illustrated in [Fig.lE].

[0009] The sandwich structure comprises a cellular material 300 called the “core” comprising C3Oo acoustic cells and, interposed between a first layer 302 of composite material called the lower “skin” on which a lower surface of the core 300 rests and, a second layer 304 called the upper “skin” of composite material comprising PF304 perforations and arranged on an upper face of the cellular material 300 and opposite its lower surface.

[0010] The acoustic panel 212b 2122 comprises three characteristic dimensions: - Ln: thickness of the upper skin; - Wn: diameter of bores / perforations; and - d or Wc: Size of the acoustic cell

[0011] The dimensions Ln, Wn, d or Wc make it possible to define the acoustic performance and more precisely the frequency range that one seeks to eliminate in order to reduce noise emissions.

[0012] However, current acoustic panels, having a sandwich structure as described above, may have disadvantages including: - a long manufacturing cycle time. For example, and in a non-limiting manner, a panel intended for the external envelope (called internal fixed structure or IFS) of an aircraft nacelle requires at least one baking operation lasting several hours; - the need to use an adhesive at the lower / outer skin - core interface.

[0013] Furthermore, the type of architecture shown in [Fig.lD] provides acoustic attenuation over a reduced frequency range as illustrated in [Fig.lF] for example.

[0014] The quality factor of the structure is defined by the ratio fres / Af between the resonance frequency fres and the bandwidth Af. The higher this ratio, the more selective the structure. This means that the structure is only effective for a limited number of frequencies.

[0015] Furthermore, the upper skin 304 (also called septum) which can be formed at from a few composite plies or layers can systematically present a “telegraphing” defect or wave effects, characterized by the appearance of undulations and which can be detrimental to the aerodynamic performance of the turbomachine.

[0016] Just like the upper skin 304, the cores can deform when a bend is applied, particularly a bend of the upper skin as mentioned above.

[0017] This deformation DFi, DF2 as illustrated in [Fig.2A], can also be detrimental to the acoustic performance of the panels. In particular, this can lead to a loss of performance and / or a shift in the frequencies to be inhibited or attenuated.

[0018] In extreme cases, for example for the attenuation of several natural frequencies, a two-degree-of-freedom acoustic structure or 2-DOF acoustics is often realized.

[0019] [Fig.2B] illustrates an example of a 2 DOF structure. The structure consists of a superposition of two panels with different cell sizes.

[0020] In [Fig.2B], the first panel comprises the lower and upper skins 302i and 3022 respectively, and the core 300i.

[0021] The second panel comprises the lower and upper skins 3022 and 304 respectively, and the core 3002.

[0022] In this example, the upper skin 3022 of the first panel is also the lower skin of the second panel. The sizes of the cells C30o are different and smaller on the second panel. The cores 300i and 3002 can also have a thickness adjusted according to the need.

[0023] Nevertheless, the attenuation of several frequencies requires the superposition of several acoustic cells (for example of the Nida type), which results in an increase in the thickness of the panel and therefore a greater bulk.

[0024] One solution to reduce the bulk of acoustic panels is to use thermosetting or thermoplastic open or closed cell foam in the sandwich architecture of the panels.

[0025] However, this solution has disadvantages including: a reduction in acoustic (frequency range), mechanical and thermal performance, a high manufacturing cost and the absence of eco-responsibility.

[0026] It is therefore urgent to design panels (or structures enabling the design of panels) from materials with a smaller footprint without reducing their acoustic and / or mechanical and / or thermal performance, while respecting environmental and cost constraints.

[0027] The use of composite materials for the production of components for fitting out an aircraft cabin is well known. Such a material comprises, in general, fibers embedded in a polymer matrix. It is, for example, known to use glass fibers, carbon fibers, etc. Other materials are also known for the production of such components, such as aluminum or plastic.

[0028] Consumption of carbon fibers has increased significantly over the last ten years. Production has adapted to meet the growing demand.

[0029] Mainly used in composite structures, carbon fibers are widely used in the production of woven composite material parts in the aeronautical field. In particular, the woven composite blades of turbojets.

[0030] During their manufacture, the layers of warp and weft threads are released as a preform of the composite material part is created, in order to achieve the different desired thicknesses. When the preform is released from the loom, a cutting operation is carried out. The cut carbon threads then become scraps from the weaving process.

[0031] Thus, the carbon fiber scraps, as illustrated in [Fig.2C], represent a potentially significant volume across all production plants.

[0032] In the present application, carbon fiber scraps are understood to mean carbon fibers which have a length less than or equal to 100 mm.

[0033] The carbon fiber scraps have mechanical and / or acoustic and / or thermal performances that are still intact, because they have not been stressed. They can therefore be reused for the manufacture of new parts in composite materials.

[0034] However, until recently, carbon fiber scraps were practically not recovered. Indeed, although sectors for reprocessing such carbon fiber scraps have developed, they are saturated with demand, particularly from the automotive industry.

[0035] Carbon fiber scraps are therefore not highly valued and outlets for the reuse of such fibers are still not very developed. They are mostly pyrolyzed then ground into powder constituting fillers for primers, paints or thermoplastic materials.

[0036] Climate change is a major concern for many legislative and regulatory bodies around the world. Indeed, various restrictions on carbon emissions have been, are being, or will be adopted by various states.

[0037] In particular, an ambitious standard applies both to new types of aircraft and those in circulation requiring the implementation of technological solutions in order to make them compliant with current regulations. Civil aviation has been mobilizing for several years now to make a contribution to the fight against climate change.

[0038] Technological research efforts have already made it possible to significantly improve the environmental performance of aircraft.

[0039] The Applicant takes into consideration the impact factors in all phases of design and development to obtain less energy-intensive, more environmentally friendly aeronautical components and products whose integration and use in civil aviation have moderate environmental consequences with the aim of improving the energy efficiency of aircraft.

[0040] The research and development work supported focuses in particular on lightening the devices, in particular through the materials used and the lightened on-board equipment.

[0041] It was therefore envisaged to recover carbon fiber scraps in order to set up a new sector. In this regard, research and development efforts have made it possible to develop a semi-finished recycling product based on carbon fiber scraps and a process for manufacturing such a product.

[0042] The product is in the form of a roll of a strip or a non-woven layer comprising scraps of carbon fibers and a binder ensuring the cohesion of the fibers between them and the holding of the strip.

[0043] However, to date, very few measures aimed at exploiting such a product, in particular for the manufacture of acoustic panels, have been undertaken. The invention therefore aims to provide an acoustic panel, for an aircraft casing, obtained by recycling carbon fiber scraps and which makes it possible to overcome at least some of the aforementioned problems and constraints.

[0044] To this end, the invention is the result of technological research aimed at very significantly improving the performance of aircraft and, in this sense, contributes to reducing the environmental impact of aircraft. Summary of the invention

[0045] There is therefore proposed an acoustic panel for a casing of an aircraft turbomachine, comprising a sandwich structure, said structure comprising a cellular material called "core" interposed between a first layer of composite material called lower "skin" on which a lower surface of the core rests and a second layer called upper "skin" of composite material comprising perforations and arranged on an upper face of the cellular material and opposite its lower surface, characterized in that the panel further comprises at least one non-woven strip containing a fibrous and porous network which comprises at least scraps of carbon fibers having a length less than or equal to 160 mm, and a binder ensuring the cohesion of the fibers between them and the holding of the strip, said strip covering at least a portion of the upper skin.

[0046] The manufacture of the acoustic panel comprises at least the arrangement of the non-woven strip, obtained at least from the recycled carbon fiber scraps, on the upper skin of the sandwich structure.

[0047] This porous non-woven strip will introduce new vibration modes and thus broaden the frequency range (or bandwidth) for which the panel is effective.

[0048] Thus, thanks to the porosity of the non-woven strip and its association with the sandwich structure of the acoustic panel, it is possible to increase the range of acoustic frequencies to be attenuated and thus improve or optimize the performance of the acoustic panel.

[0049] The invention also allows a saving in cost and mass. Indeed, if the consolidation of the non-woven strip is carried out using thermoplastic and / or mutually miscible materials, it is not necessary to use an adhesive at the interface of the strip and the upper skin. This saves on cost, the use of the adhesive can be eliminated as well as the manufacturing operations associated with the use of the adhesive (elimination of the adhesive film and associated secondary operations).

[0050] The invention also allows a reduction in the ecological footprint relating to the manufacture of acoustic panels because it uses recycled materials.

[0051] The stacking of these carbon fibers creates a fibrous and porous network, which makes it possible to achieve acoustic / phonic attenuation but also to improve the mechanical performance of the panel and damage tolerance, for example by increasing the stiffness of the sandwich structure of the panel. The panel according to the invention is therefore self-stiffened.

[0052] Furthermore, the addition of the non-woven strip on the upper skin makes it possible to have an ad-hoc surface which improves the aerodynamic performance of the turbomachine, the presence of the strip preventing the appearance of the “telegraphing” phenomenon at the level of the upper skin and / or the core.

[0053] The invention also allows for a total lightening of the structure. Indeed, the non-woven strip obtained from carbon fiber scraps is as efficient as current state-of-the-art solutions (glass fibers in particular), for a lower thickness (or density) (no superposition of acoustic panels). There is therefore less bulk in the structure.

[0054] The invention may further comprise one or more of the following optional characteristics, in any technically possible combination: - the binder is made of thermoplastic or thermosetting material; - the strip further comprises thermoplastic fibers; - the non-woven strip is placed on an external face of the upper skin; - the strip further comprises projecting portions housed in the per- drilling; - The panel further comprises another non-woven strip arranged on an internal face of the upper skin and opposite its external face, the internal strip being connected to the external strip via the projecting portions of said external strip; - the strips arranged respectively on the inner face and the outer face of the upper skin have identical or different volume / mass fractions of fibers; - the binder is a thermoplastic and for example PE, PP or a PE-PP copolymer;

[0055] The invention also relates to a casing comprising at least one panel as described above.

[0056] The invention also relates to a turbomachine comprising at least one casing as mentioned above.

[0057] The invention also relates to a method of manufacturing an acoustic panel as described above, the method comprising at least: a. a step of depositing a non-woven strip on a portion of the upper skin; b. compression of the assembly formed by the sandwich structure and the strip non-woven. Brief description of the figures

[0058] The invention will be better understood with the aid of the following description, given solely by way of example and with reference to the appended drawings in which: - [Fig.1A] is a schematic representation of a simplified view of a turbomachine according to the prior art, - [Fig.lB] is a schematic representation of an axial and partial sectional view of a fan of an aircraft turbomachine, according to the prior art, - [Fig.lC] is a schematic representation of a perspective view of a fan casing, according to the prior art; - [Fig. 1D] is a schematic representation of a partial axial sectional view of a fan casing, according to the prior art; - [Fig.lE] is a schematic representation of a sandwich structure of an acoustic panel, according to the prior art; - [Fig.lF] is a schematic representation of an acoustic gain curve for the sandwich structure of [Fig.lE], according to the prior art; - [Fig.2A] is a schematic perspective view of an open cellular material exhibiting curvature; - [Fig.2B] is a schematic perspective view of an acoustic panel type 2 DOF; - [Fig.2C] is a schematic perspective view of carbon fiber scraps; - [Fig.2D] is a schematic perspective view of a non-woven strip of carbon fiber scraps; - [Fig.3A] is a schematic representation of a sandwich structure of an acoustic panel, according to a first variant of the invention; - [Fig.3B] is a schematic representation of the evolution of the microstructure of a non-woven strip as a function of the density of carbon fiber scraps; - [Fig.3C] is a schematic representation of the acoustic gain curve for the sandwich structure of [Fig.3A] as a function of the porosity of the non-woven strip; - [Fig.4A] is a schematic representation of a sandwich structure of an acoustic panel, according to a second variant of the invention; - [Fig.4B] is a schematic representation of a sandwich structure of an acoustic panel, according to a third variant of the invention; - [Fig.4C] is a schematic representation of the acoustic gain curves for the sandwich structure of [Fig.1E] and for the three sandwich structure variants of Figures 3A, 4A and 4B; - [Fig.5] is a schematic representation of the method of manufacturing an acoustic panel, according to the invention; Detailed description of the invention

[0059] Although the following description refers to acoustic panels for a turbomachine casing, those skilled in the art will understand that this is in no way limiting and that the invention can be applied for the production of semi-structural or non-structural parts, in the aeronautical field (in particular for interiors, cabins and seats in particular, or for IFS type assemblies, etc.), but also in the broader field of transport (automobile, nautical, railway) in order to obtain an improvement in acoustic performance.

[0060] With reference to Figures 3A, an acoustic panel for a casing of an aircraft turbomachine according to the invention will now be described.

[0061] Structural elements similar to those of the prior art according to Figures 1A-2B bear the same numerical references.

[0062] As illustrated in [Fig.3A], representing a partial view of a first variant of the acoustic panel, the acoustic panel 212b 2122 comprises a sandwich structure. The sandwich structure comprises a cellular material 300 called “core” comprising C3Oo acoustic cells and, interposed between a first layer 302 of composite material called the lower “skin” on which rests a lower surface of the core 300 and, a second layer 304 called the upper “skin” of composite material comprising PF304 perforations and arranged on an upper face of the cellular material 300 and opposite its lower surface.

[0063] The upper skin has an internal face Sint facing the cells C300 of the core and an external face Sext opposite the internal face Sint.

[0064] The panel 212b 2122 further comprises at least one non-woven strip 400 covering an external face Sext of the upper skin 304 of the panel.

[0065] The non-woven strip 400 comprises at least one fibrous and porous network which comprises at least scraps of carbon fibers 402 having a length less than or equal to 100 mm. Preferably the carbon fibers have a length of between 10 and 25 mm.

[0066] In this first variant, the non-woven strip preferably has a thickness ei of between 1 mm and 50 mm. For example, and in a non-limiting manner, the thickness ei may be of the order of 10 mm.

[0067] The non-woven strip 400 may also comprise a binder or a binding material to ensure the cohesion of the fibers between them and the holding of the strip.

[0068] The binder may be a thermoplastic or a thermoset.

[0069] The binder may be in the form of particles and / or fibers, for example of a resin-based thermoplastic type, such as polyethylene (PE), polypropylene (PP) or in a PE-PP copolymer. The binder may also be a sizing or coating with thermoplastic resins.

[0070] As for the thermosetting binder, it can be, for example, and in a non-limiting manner, polybismaleimides (BMI) or epoxy resin.

[0071] The strip 400 may further comprise thermoplastic fibers, such as poly(phenylene sulfide) (PPS), polyetherimides (PEI), polyetheretherketone (PEEK), polycarbonate (PC), polyethersulfone (PESU) and / or polyphenylsulfone (PPSU).

[0072] These thermoplastic fibers preferably have a glass transition temperature higher than that of the material intended to form the binder.

[0073] The non-woven strip can be presented in different states depending on the parts or the final application. Indeed, an interior aeronautical part (for example a cabin / a seat) will not have the same acoustic needs and associated thermomechanical constraints as a nacelle IFS.

[0074] Thus, one can distinguish between unconsolidated, partially consolidated and fully consolidated bands.

[0075] An unconsolidated band is characterized by the absence (or very few) of elements cohesive or binding agents between the fibers. The web then appears as a tangle of fibers.

[0076] To obtain a partially consolidated non-woven strip, the carbon fibers (or the binder, for example a resin) are heated sufficiently to melt and bond to the other impregnated fibers. The fibers are then in the form of stacked layers with very little contact at the interfaces between adjacent layers. A weak but sufficient compression is then applied to this stack of fiber layers to ensure the bond between the fiber layers. In the end, the strip obtained is weakly compacted, of low density and high porosity for a given frequency range.

[0077] The fully consolidated non-woven strip is obtained with the same principle as a partially consolidated non-woven strip, but with accentuated transformation conditions (time, pressure, higher temperature). As a result, the density is increased and the porosity is lower than that of a partially consolidated strip. The crystallinity rate of the resin is higher, which can lead to obtaining a Young's modulus or a higher rigidity and a better suitability for finishing.

[0078] [Fig.3B] illustrates examples of unconsolidated strips. Strip 400a having a lower porosity (or a higher density) than strip 400b (lower density).

[0079] [Fig.3C] illustrates an example of curves illustrating the acoustic gain obtained with the panel of [Fig.3A] as a function of the density or porosity of the non-woven strip 400.

[0080] Curve G1 corresponds to the gain of a non-consolidated non-woven strip (therefore with very high porosity) and curve G2 to that of a partially consolidated strip (therefore with relatively low porosity). We thus observe a better gain for a partially consolidated strip.

[0081] Thus, depending on the density (or porosity) of the band used, the gain can be improved for specific optimization. The density of the band makes it possible to modify the frequency and amplitude of a signal.

[0082] Preferably, the non-woven strip used in the 212x2 acoustic panel is partially consolidated, i.e. the strip has low porosity.

[0083] The acoustic optimization of the panels can also depend on the mass rate or the porosity of the core 300. It is therefore possible to significantly improve the acoustic gain of the panels by adjusting the porosity of the non-woven strip and that of the core.

[0084] Advantageously, those skilled in the art will understand that the positioning of a fibrous and porous non-woven strip 400 on the upper skin 304 of the structure of the panel 212x 2 makes it possible to improve the acoustic performance of the panel. acoustics 212i_2.

[0085] For a strip having, in particular, zones with different porosities or densities, local acoustic attenuation and / or adjustment adapted to the porosity or density of each zone can be carried out. Thus, by controlling the homogeneity of the porosity or density of the strip, it is also possible to broaden the absorption spectrum of the frequencies and therefore significantly improve the acoustic gain of the panels.

[0086] In another variant of the invention illustrated in [Fig.4A], the non-woven strip further comprises projecting portions housed in the perforations PF304 of the upper skin 304.

[0087] In this configuration, the non-woven strip has an effective thickness e2 which is considered to be equal to the sum of the thickness ei of the non-woven strip of the variant of the panel shown in [Fig.3A] and the thickness of one of the projecting portions housed in the perforations.

[0088] The effective thickness e2 in this second variant of the acoustic panel is of the order of ten millimeters, for example, and in a non-limiting manner, 12mm.

[0089] In a third variant illustrated in [Fig.4B], the acoustic panel 212b 2122 further comprises another non-woven strip 4002 (or internal strip) arranged on the internal face Sint of the upper skin 304. The internal strip 4002 is connected to the non-woven strip 400i arranged on the external face of the skin (or external strip) via the projecting portions of said external strip 400i.

[0090] In this configuration, the non-woven strips 4002, 400i arranged respectively on the internal face Sint and the external face Sext of the upper skin may have identical or different volume / mass fractions of carbon fibers.

[0091] The effective thickness e3 of the strip in this configuration is the sum of the thickness e2 of the non-woven strip illustrated in [Fig.4A] and the thickness of the strip arranged on the internal face Sint of the upper skin 304. Preferably, the thickness of the strip arranged at the internal face is identical to that of the strip arranged on the external face, i.e. equal to eb

[0092] In some embodiments, the thickness of the strip disposed at the inner face is different from that of the strip disposed on the outer face.

[0093] The architectures described above have a structure similar to that of a 2DOF type architecture as illustrated in [Fig.2B] but with reduced bulk.

[0094] [Fig.3B] illustrates an example of the acoustic gain curve obtained as a function of the variants of the acoustic panel of Figures 3A, 4A and 4B.

[0095] The acoustic gain of the panel of [Fig.3A] is represented by curve (a) and ca- characterized by the Afa bandwidth.

[0096] Similarly, the acoustic gains of the panels of [Fig.4A] and 4C are represented respectively by curves (b) and (c) and characterized by the bandwidths Afb and Afc.

[0097] It is observed that the addition of the non-woven strip 400 makes it possible to increase the bandwidth Af or the absorption spectrum of the frequencies and therefore to obtain a significant acoustic gain. Thus, the quality factor obtained with the acoustic panel according to the invention decreases in part as a function of the addition of the non-woven strip.

[0098] Compared to an acoustic panel of the prior art, as illustrated in [Fig.lD], and a gain curve of which is illustrated in [Fig.lE], the acoustic panel 212i, 2122 of [Fig.3A] (first variant) makes it possible to achieve a better gain for high frequencies. However, for low frequencies the performances are quite close.

[0099] Thus, depending on the architecture chosen, the acoustic gain can be significantly improved with an increase in the frequency range towards high frequencies or an increase in the frequency range towards low and high frequencies.

[0100] The invention makes it possible to achieve a very broad attenuation of the acoustic frequency range (greater than 82%) unlike acoustic panels having a sandwich structure similar to that illustrated in [Fig.lE] (the performances of the two technologies are combined).

[0101] Furthermore, with the architecture of the acoustic panel shown in [Fig.4B], two strips having carbon fiber densities can be used at the upper skin 304 in order to improve the acoustic performance of the panel 212b 2122 and / or to target several frequencies. This architecture also makes it possible to increase the mechanical / vibrational performance of the panel structure, in particular due to an increase in stiffness due to the presence of the internal and external strips.

[0102] The invention also relates to a casing 200 comprising an acoustic panel 212i_2 as described above.

[0103] The invention also relates to a turbomachine 100 comprising a casing according to the invention.

[0104] The invention also relates to a method of manufacturing an acoustic panel 212i, 2122 for a casing (200) of an aircraft turbomachine as described above.

[0105] The manufacturing method comprises at least: a. a step of depositing a non-woven strip 400 on a portion of the upper skin 304; b. compression of the assembly formed by the sandwich structure and the strip non-woven 400.

[0106] This manufacturing method can be applied to the acoustic panels described in Figures 3A and 4A.

[0107] In the case of the variant described in [Fig.4B], the manufacturing method further comprises a prior step of pre-consolidation and / or consolidation of the upper skin 304 and the non-woven strip 400 to form a first block, the strip being arranged on the internal and external faces and in the perforations PF304 of the internal skin.

[0108] The block formed by the upper skin 304 and the non-woven strip 400 is then assembled with the core 300 and the lower skin 302 by gluing to form the sandwich structure which will subsequently be compressed.

[0109] In the case where a thermosetting binder is used to produce the non-woven strip, the manufacturing method may further comprise a step of cooking the assembly.

[0110] Advantageously, those skilled in the art will understand that the use of non-woven strips obtained from recycled materials such as carbon fiber scraps makes it possible to improve or optimize the performance of acoustic panels by increasing (or widening) the range of acoustic frequencies to be attenuated.

[0111] The stacking of these carbon fibers creates a fibrous and porous network, which makes it possible to achieve acoustic / phonic attenuation but also to improve the mechanical performance of the panel and damage tolerance, for example by increasing the stiffness of the sandwich structure of the panel. The panel according to the invention is therefore self-stiffened.

[0112] Furthermore, the addition of the non-woven strip on the upper skin makes it possible to have an ad-hoc surface which improves the aerodynamic performance of the turbomachine, the presence of the strip preventing the appearance of the “telegraphing” phenomenon at the level of the upper skin and / or the core.

[0113] The invention also makes it possible to achieve acoustic optimization in addition to the size of the cell used. Indeed, the fibrous network of the non-woven strip creates an intrinsic porosity. This is directly associated with the volume / mass fraction of fibers (local) which will change the density and therefore the local porosity and, as a result, an associated level of sound / acoustic attenuation. Consequently, it is possible to optimize or achieve different acoustic performances depending on the area of ​​the room by varying this local porosity / density.

[0114] The invention also allows for total lightening of the structure while maintaining equivalent mechanical performance (particularly in bending) to that of panels of the prior art, for example and in a non-limiting manner, a self-stiffened panel with an aluminum honeycomb structure. Indeed, the non-woven strip obtained from carbon fiber scraps is as efficient as the solutions current state-of-the-art materials (glass fibers in particular), for a lower thickness (or density) (no superposition of acoustic panels).

[0115] The invention also allows for a shorter manufacturing time compared to standard solutions using Nida-type sandwich acoustic structures. This reduction in manufacturing time is achieved through the use of thermoplastic materials in the composition of the non-woven strip and which are perfectly compatible with the implementation of compression / stamping / rapid thermoforming type processes.

[0116] The invention can also allow a gain in cost, in mass. Indeed, if the consolidation of the non-woven strip is carried out using thermoplastic and / or miscible materials between them, it is not necessary to use an adhesive at the interface of the strip and the upper skin. This saves on cost. The use of the adhesive can be eliminated as well as the manufacturing operations associated with the use of the adhesive (elimination of the adhesive film and associated secondary operations).

[0117] Finally, the invention allows a reduction in the ecological footprint relating to the manufacture of acoustic panels because it uses recycled materials.

Claims

Claims

1. Acoustic panel (212i.2) for a casing (200) of an aircraft turbomachine (100), comprising a sandwich structure, said structure comprising a cellular material (300) called the “core” interposed between a first layer (302) of composite material called the lower “skin” on which a lower surface of the core rests and a second layer (304) called the upper “skin” of composite material comprising perforations (PF304) and arranged on an upper face of the cellular material (300) and opposite its lower surface, characterized in that the panel (212i_2) further comprises at least one non-woven strip (400) containing a fibrous and porous network which comprises at least carbon fiber scraps (402) having a length less than or equal to 160 mm, and a binder ensuring the cohesion of the fibers between them and the holding of the strip (400), said strip (400) covering at least a portion of the upper skin (304).

2. Panel according to claim 1, in which the binder is thermoplastic or thermosetting.

3. An acoustic panel according to claim 1 or 2, wherein the strip (400) further comprises thermoplastic fibers.

4. Acoustic panel according to one of claims 1 to 3, in which the non-woven strip (400) is arranged on an external face (Sext) of the upper skin (304).

5. Acoustic panel according to the preceding claim, wherein the strip (400i) further comprises protruding portions (404) housed in the perforations (PF304).

6. Acoustic panel according to the preceding claim, in which it further comprises another non-woven strip (4002) arranged on an internal face (Sint) of the upper skin (304) and opposite its external face (S ext), the internal strip (4002) being connected to the external strip (400i) via the projecting portions (404) of said external strip (400i).

7. Acoustic panel according to the preceding claim, in which the strips arranged respectively on the internal face (4002) and the external face (4000) of the upper skin (304) have identical or different volume / mass fractions of fibers.

8. Acoustic panel according to one of the preceding claims, in which the binder is a thermoplastic and for example PE, PP or a PE-PP copolymer.

9. Housing (200) comprising at least one panel (212i.2) according to one of claims 1 to 8.

10. Turbomachine (100) comprising at least one casing (200) according to the preceding claim.

11. Method of manufacturing (500) an acoustic panel (212i.2) according to one of claims 1 to 8, the method (500) comprising at least: a. a step of depositing a non-woven strip (400) on a portion of the upper skin (304); b. a compression of the assembly formed by the sandwich structure and the non-woven strip (400).

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