EP1157372A1

EP1157372A1 - Method for making a sound reducing panel with resistive layer having structural property and resulting panel

Info

Publication number: EP1157372A1
Application number: EP00993660A
Authority: EP
Inventors: Robert Andre; Alain Porte; Hervé Batard
Original assignee: Airbus Operations SAS
Current assignee: Airbus Operations SAS
Priority date: 1999-12-24
Filing date: 2000-12-21
Publication date: 2001-11-28
Anticipated expiration: 2020-12-21
Also published as: ES2217038T3; FR2803078B1; DE60008861T2; FR2803078A1; US20020157764A1; DE60008861D1; EP1157372B1; CA2365100A1; WO2001048734A1; CA2365100C

Abstract

The invention concerns a method for making a sound reducing panel comprising an honeycomb core structure (2) flanked, on one side, by a reflector (3), and, on the other by an acoustically resistive layer (1') with two components having acoustic and structural property respectively. The invention is characterised in that it consists in: fixing on a mould having a shape suited for the panel to obtain a layer with structural property (1'a) consisting of yarns (7, 8) pre-impregnated with a thermoplastic or thermoset resin, by lay-up, winding or wrapping, such that said layer exhibit an open surface proportion of the order of 30 % of the whole exposed surface, and in fixing on the structural property layer a layer with acoustic property (1'b) consisting of a macroporous fabric having a thickness of the order of one-tenth of that of the structural property layer, then in fixing the open-cell structure (2) and the reflector (3) optionally with the addition of an adhesive (5, 6) between the components, at least a step of autoclave curing being carried out at the end of at least one of said fixing steps. The invention is particularly useful for making sound reducing panels for aircraft turbo engines.

Description

METHOD FOR MANUFACTURING A RESISTANT LAYER ACOUSTIC MITIGATION PANEL WITH STRUCTURAL PROPERTY AND

PANEL OBTAINED

The present invention relates to an acoustic attenuation panel more particularly intended for the at least partial absorption of sound energy from gas flows at high speed.

The invention will be described in its application to the production of noise attenuation panels generated in particular by aircraft turbine engines, in certain locations of the nacelle, for example at the entrance and at the exit of the fan channel, but it is understood that the invention is capable of applications in any other environment where it is necessary or desirable to use a structure of the panel type combining lightness, high mechanical strength and acoustic properties.

The panel according to the invention is of the well known type consisting of a sandwich comprising a honeycomb structure of the honeycomb type flanked, aerodynamic vein side, an acoustically resistive layer and, on the opposite side, a rear reflector. The honeycomb structure may be simple, that is to say with a single resonator or with a monolayer honeycomb core, or else multiple, that is to say with superimposed resonators or with honeycomb core formed by several superimposed layers separated or not by septa.

The acoustically resistive layer has a dissipative role. When the sound wave passes through it, viscous effects occur which partially transform acoustic energy into heat. The alveolar structure behind the resistive layer traps this sound wave thanks to the cells which behave like waveguides perpendicular to the surface of said layer, the wave reflecting on the rear reflector of the panel.

To obtain good acoustic attenuation, it is necessary to combine a certain number of conditions, the main ones being a good adequacy of the height of the cells of the alveolar structure at the frequencies of the sound wave that we want to process and the adaptation the impedance of the resistive layers (septum and front face) so that they produce maximum dissipation at the frequencies of interest.

In addition, it is therefore essential to have optimal acoustic homogeneity both at the level of the resistive layers and at that of the honeycomb structure.

Furthermore, such a panel must, because of its environment, withstand severe conditions of use. In particular, it must not present any risk of delamination of the resistive layer even in the presence of a high vacuum and must be resistant to erosion or abrasion as well as to corrosion, have good electrical conductivity and be able to absorb the energy of a mechanical impact.

Such a panel must also and of course have sufficient structural properties to in particular receive and transfer the aerodynamic, inertial forces and those related to the maintenance of the nacelle, towards the structural nacelle / engine connections.

The surface condition of the resistive layer must finally meet the aerodynamic requirements of the environment.

Known acoustic attenuation panels, in particular those used on nacelles of turbine engines, more or less successfully meet all of the above requirements.

Among these panels, all made on the same principle of a resonant structure comprising a resistive front layer and a honeycomb structure closed by a rear reflector, mention may be made of those implementing a so-called non-linear treatment with a single degree of freedom and illustrated for example by European patent EP 0 038 746 issued in the name of the Applicant. Such a panel comprises a honeycomb flanked, on one side, by an acoustic resistive layer constituted by a rigid and thin lattice of composite material and, on the other side, by a reflector.

Such a structure has the advantage of good control of the percentage of open area of the resistive layer because said lattice is formed of orthogonal wicks of fibers, for example carbon, delimiting between them openings the size of which can be adjusted during process of impregnating the fibers with the aid of a thermosetting resin and then hardening of the resin, the fabric being subjected to shaping under pressure and at temperature in order to obtain said rigid and thin lattice.

The resistive layer thus obtained also has good structural strength and finally has the advantage of being a monolayer component.

However, its drawbacks are also substantial. This resistive layer has a high acoustic non-linearity which means that its surface impedance varies significantly with the acoustic level.

In addition, for this type of layer, the grazing flow will produce a phenomenon of narrowing of the air passage sections in the holes. The acoustic resistance of this layer will also depend on the speed of this grazing flow.

In addition, the resistive layer offers a frequency window of limited efficiency, as well as a low resistance to erosion.

According to another so-called linear processing method, also with a single degree of freedom, illustrated for example by document GB 2 1 30 963, the resistive layer is formed of two components, namely, a structural layer, on the honeycomb side, and a microporous layer on the surface.

The structural layer is formed from a fabric of relatively wide mesh carbon fibers defining an opening rate of approximately 30% of the total surface of the layer. The surface microporous layer is a fine mesh fabric of mineral or synthetic fibers or a metallic fabric, acting as an acoustic damper. The advantages of such a structure are the possibility of adjusting the acoustic resistance of the resistive layer by acting on the two components of the latter, the reduction of the acoustic non-linearity resulting in a less dependence of the acoustic resistance on -vis the acoustic level and the speed of the tangential flow at the surface of the resistive layer. In addition, a wider frequency window of efficiency is obtained in comparison with the previous technical solution.

By cons, such a structure has the major drawback of an additional assembly penalizing in time and cost, due to the two-component nature of the resistive layer. If the increased assembly constraints of this structure are not well controlled, there are risks of acoustic non-uniformity, as well as delamination of the resistive layer.

Finally, there is also a risk of corrosion of the microporous exposed layer imposing constraints on the choice of materials. According to a third treatment technique called a double degree of freedom, the panel comprises a resistive layer on the surface, two superposed honeycombs separated by a resistive layer, called a septum, generally microporous and a reflector.

The advantages of this structure are the obtaining of a very important frequency window of effectiveness, the possibility of adjustment of the acoustic resistance by playing on the two resistive layers, the weak or moderate acoustic non-linearity.

On the other hand, the installation of two superimposed honeycomb structures separated by a resistive layer makes the manufacturing process longer and costly and introduces the risks of acoustic inhomogeneities caused by possible misalignments of the honeycombs, combined with the effects of glue, as well as transverse sound propagation in areas of misalignment.

Finally, from document EP 0.91 1.803, an acoustic attenuation panel is known, formed of a sandwich comprising a honeycomb structure flanked, on one side, by a reflector and, on the other side, by a fabric. metal itself covered with a perforated metal sheet. Such an arrangement makes it possible to obtain panels whose face exposed to aerodynamic flows and which is defined by the combination of metallic fabric / metallic perforated sheet, has both good acoustic properties and good structural properties. However, such panels can have significant drawbacks, in particular when they have an accentuated curvature, which is the case in particular for the fan channel inlet and outlet panels.

In fact, according to document EP 0.91 1.803, the metal sheet is first prepared and then pierced before being put in place and shaped on the assembly, produced in addition, of the honeycomb structure, the reflector and the metallic fabric.

Due to the shape of the panel which is not of revolution and which can have convexities or concavities which can be accentuated, the shaping of the pre-perforated sheet will cause local deformations of parts of the sheet and therefore holes located in these parts. These deformations are capable of appreciably modifying the area of the holes and therefore the local porosity rate of the perforated sheet, thus causing inhomogeneity of the porosity of the sheet, detrimental to its effectiveness in terms of acoustic attenuation. Furthermore, such shaping is difficult because the sheet is relatively rigid.

Finally, in general, all-metal type panels, which is the case of the above panel, are likely to pose corrosion problems. The invention aims to overcome the various drawbacks of these known techniques by proposing a method of manufacturing an acoustic attenuation panel of the type with a honeycomb structure flanked, on the one hand, by a reflector and, on the other hand, by an acoustically resistive layer with two components respectively with acoustic property and with structural property, making it possible to obtain panels with complex shape in particular with evolving curvatures which can be significant and in particular monoblock panels of generally annular shape with or without splint, such as those intended at the entrances and nacelle fan channel outlet, having both very good mechanical properties and optimal acoustic properties.

To this end, the subject of the invention is a method of manufacturing an acoustic attenuation panel comprising a honeycomb structure flanked, on the one hand, by a reflector and, on the other hand, by an acoustically resistive layer with two components respectively with acoustic property and with structural property, characterized in that it consists: in placing on a mold of a shape suitable for the panel to obtain a layer with structural property consisting of wires pre-impregnated with a thermoplastic resin or thermosetting, by draping, coiling or wrapping, so that said layer has an open surface rate of the order of 30% of the total surface of the exposed surface, to be placed over the layer with structural property a layer with an acoustic property made up of a microporous fabric and with a thickness of the order of a tenth of that of the layer with a structural property, then putting in place lveolar and the reflector with optionally the addition of an adhesive between components, - at least one autoclave cooking step being implemented at the end of at least one of the above implementation steps. The method of the invention makes it possible to obtain an acoustically resistive layer with remarkable acoustic and structural properties, in particular the efficiency of the acoustic attenuation due to the very good homogeneity of the porosity rate of said acoustically resistive layer, which can be defined precisely.

In fact, the fact of using pre-impregnated wires shaped on a mold not only makes it possible to produce complex shapes which may have pronounced curvatures, but above all allows very good control of the porosity of the layer with structural property. According to one embodiment, said layer with structural property is given the porosity required by the choice of the spacing of the threads for weaving, in the case of a fabric, the flexibility of the latter allowing it to match the shapes. of the mold without substantial deformation of the mesh of the fabric.

In the case of wound wires or streamers, adjusting the spacing of the wires makes it possible to precisely adjust the porosity rate.

According to another mode of implementation, said layer with structural property is given the porosity required by piercing said layer after baking in an autoclave.

The drilling is carried out to precise diameters and in a shaped and rigid part, the control of the porosity is perfectly ensured.

Advantageously and according to another mode of implementation and for reinforcement purposes, the layer with structural property consists of several layers of crossed wires, the layers being on either side of the layer with acoustic property.

The invention also relates to the panels obtained in accordance with the above process.

Other characteristics and advantages will emerge from the description which follows of various embodiments of the invention, description given by way of example only and with reference to the appended drawings in which:

- Figure 1 is a schematic sectional and exploded view of a panel structure obtained according to the method of the invention; - Figure 2 is a similar sectional view illustrating another embodiment of the method of the invention;

- Figure 3 is a partial top view of the structural property layer of the panel of Figure 2;

FIGS. 4a to 4e illustrate different stages in the production of a panel of the type of FIG. 1,

FIG. 5 is a partial sectional view illustrating a method of bonding the two-component acoustic layer to the cellular structure, and FIG. 6 is a partial section view illustrating a variant of the method illustrated in FIG. 2.

More precisely, the panel is in one piece, annular without a splice or with a single splice and is produced using a mold symbolized in M in FIG. 1, of shapes and dimensions appropriate to those of the panel to be obtained. and on which the successive layers of the panel will be draped, wound or wrapped.

The first of these layers is a layer with a structural property 1 a, on which a layer with acoustic property 1 b will then be put in place, the assembly 1 a-1 b forming the two components of a layer 1 known as acoustically resistive, on which will be placed a honeycomb structure 2, simple as illustrated or multiple as mentioned above.

Finally, on top of the honeycomb structure 2 is placed a conventional reflector 3. According to the invention, the layer with structural property 1a is formed from wires pre-impregnated with an appropriate thermoplastic or thermosetting resin. By yarn is meant yarn, fiber, wicks in the form of a ribbon of square or rectangular section, of carbon, glass, "Kevlar", or other mineral or organic, natural or synthetic fibers. The acoustic property layer 1b is formed from a very fine fabric of carbon fibers, glass, “Kevlar” or other mineral or organic, natural or synthetic fibers, dry or prepreg.

The honeycomb structure 2 is for example a paper of aramid fibers such as that commercially called "NOMEX". In the embodiment illustrated in FIG. 1, the layer with structural property consisting of a fabric draped over the mold M, or by threads deposited by winding or wrapping, is produced, then polymerized by autoclave cooking.

This gives a composite sheet, rigid, smooth and shaped, which is then drilled according to the desired open surface rate.

This open surface rate is advantageously of the order of 30% of the exposed surface of the layer 1a. The perforations 4 produced for this purpose in the layer 1 a preferably have a ratio of the diameter to the thickness of the layer 1 a greater than 1 to reduce the harmful effects of the acoustic non-linearity.

The perforations 4 are produced by various mechanical means, by laser or by electroerosion.

After perforation of the holes 4, the layer 1 a still being in place on the mold M, the layer having an acoustic property 1 b is put in place, with the possible interposition of an adhesive layer 5, then the honeycomb structure 2 is put in place with possible interposition of a second adhesive layer 6 and finally the reflector 3.

A second polymerization by autoclave cooking can be carried out after the layers 1b and 5 have been put in place, then a third polymerization by autoclave cooking is carried out after the layers 2 and 3 have been put in place, a crosslinking adhesive being advantageously interposed between the layers 2 and 3. Finally, the mold M is removed to release the finished panel.

The choice of adhesives 5, 6 and their methods of implementation, as well as the choice of fabric for layer 1b and the polymerization methods are determined so as to obtain an open surface rate after bonding in layer 1b , corresponding to the desired rate, that is to say giving the resistive layer 1 the required non-linearity factor.

The embodiment of FIG. 2 is similar to that of FIG. 1, except that the layer with a structural property Ta of the two-component acoustically resistive layer 1 ′ is formed from strands of fibers deposited in a fabric weft, namely warp wicks 7 and weft wicks 8, the mesh thus produced defining passage openings 9 (FIG. 3) rectangular or square, constituting approximately 30% of the surface of the layer a.

The fibers of the locks 7, 8 can be of the type indicated above, dry or prepreg. The wicks 7, 8 are deposited individually by winding, wrapping or manual deposit or not on a mold (not shown) similar to the mold M of Figure 1. A polymerization is then carried out.

The spacing between wicks 7, 8 and the polymerization conditions are defined so as to give the layer 1a the required non-linearity factor.

In the examples illustrated by FIGS. 1 and 2, the thickness of the layer with structural property 1a, l 'is of the order of 10 times the thickness of the layer with acoustic property 1b, Tb.

It should be noted that the layer with a structural property 1 a may consist of several plies of fabrics of prepreg yarns or of several layers of superimposed coiled prepreg yarns or streamers.

The acoustically resistive layers (1, 1 ') of the panels according to the invention, although made up of two components, nevertheless have excellent mechanical qualities. In fact, the materials of the two components, structural and acoustic, are identical and compatible and lend themselves to good bonding and, after polymerization, form a single composite sheet with almost zero risk of delamination, very resistant to erosion and abrasion. , shock and moreover easy to repair. In addition, the resistive layers have, due to the precise control of their porosity rate during manufacture, a very good acoustic performance in particular in terms of non-linearity, their impedance not depending on the Mach number of the grazing flow. .

The panels according to the invention are also simple and easy to produce.

Figures 4a to 4d illustrate an embodiment of a panel of the type of Figure 1, on a mold (not shown) similar to the mold M.

After constitution and shaping of the structural component 1 a, with the desired open surface rate, for example 30%, the layer of crosslinking adhesive 5 is applied (FIG. 4a), then the acoustic layer is placed

1 b (FIG. 4b) and hot polymerization under pressure to assemble the two layers 1 a, 1 b. Then, the crosslinking adhesive 6 is put in place (FIG. 4c) on the alveolar structure 2.

Finally (FIG. 5d), all the elements of the panel are assembled by a new step of polymerization under hot pressure, an adhesive 1 0 also being placed on the other face of the honeycomb at the level of the feet of the cells to the bonding of the rear reflective layer 3 which is itself mono or multi-layer and whose structure is conventional.

Due to the high porosity of the acoustic layer 1b, very good adhesion is obtained between honeycomb 2 and layer 1b. Indeed, the adhesive 6 diffuses well in the porous mass of the layer 1b and the junction between the end edge of the walls of the cells of the honeycomb 2 and the opposite face of the layer 1b is established by constituting good bridging connections at the level of the honeycomb cell bases defining cross-sectional links which increase as one approaches the face of said layer 1b.

It should also be noted that, in general, the invention makes it possible to give the acoustic component (layer 1 b) a very fine thickness, much less than that of the structural layer 1 a. By way of example, the layer 1 a could have a thickness of one millimeter, while the thickness of the layer 1 b could be reduced to 0.1 millimeters without degrading its acoustic properties.

FIG. 4e illustrates an alternative embodiment of the assembly of layers 1 a, 1 b and 2, in which the crosslinking adhesive 5 between the layers

1 a and 1 b is deleted. Because of the small thickness and the large porosity of the acoustic layer 1b, it is possible to apply the adhesive 6 only to the receiving face of the honeycomb 2.

The adhesive 6, as illustrated in FIG. 5, migrates during the polymerization throughout the thickness of the porous layer 1 b and comes into contact with the opposite face of the external structural layer 1 a. The assembly 1 a, 1 b, 2 is thus securely fixed.

In this assembly, the only adhesive used (6) is deposited only at the level of the feet of the cells of the honeycomb 2, which limits the obstruction of the passage openings 4 through the structural layer 1a to the only zones facing said cell feet.

The technique illustrated in FIGS. 4a to 4e can be used with the various variants of panel structure described above. This technique makes it possible to easily design and produce acoustic attenuation panels with efficient and homogeneous mechanical characteristics, adapted to various environments, in particular those mentioned above such as the nacelles of turbo-engines.

In FIG. 5, an alternative embodiment of the holes 4 of the structural layer 1 a has also been illustrated during their perforation, according to which the external outlet of said holes 4 is advantageously flared, by any appropriate means, as shown in 1 1, so as to improve the acoustic linearity.

FIG. 6 illustrates another alternative embodiment of the method of the invention according to which the layer with structural property is reinforced. For this purpose, the layer with structural property consists of several layers of crossed prepreg son placed on either side of the layer with acoustic property 1 "b.

On the left-hand side of FIG. 6, there is shown a first distribution of two plies of crossed threads, respectively a ply 13 of warp threads, deposited first on a mold (not shown) similar to the mold M of FIG. 1 and a ply 1 4 of weft threads deposited over the layer 1 "b, that is to say after the latter has been removed.

On the right-hand side of FIG. 6, a second distribution of three plies is shown, namely two plies crossed in a weaving weft 1 5, deposited first on the mold and a third ply 1 6 of threads parallel to the threads of the 'one of the layers of the weft 1 5, deposited over the layer with acoustic property 1 "b.

All of the components 1 3, 1 4, 1 5, 1 6, 1 "b thus form an acoustically resistive layer 1" with properties that are both structural and acoustic. This assembly is polymerized under pressure before installation of the other components 2, 3.

The spacing of the son of the plies 13, 14, 15, 16 deposited by winding or wrapping determines the rate of porosity of the layer 1 ".

Claims

1. Method for manufacturing an acoustic attenuation panel comprising a honeycomb structure (2) flanked, on the one hand, by a reflector (3) and, on the other hand, by an acoustically resistive layer (1, 1 M ") with two components respectively with acoustic property and with structural property, characterized in that it consists: in placing on a mold (M) of a shape suitable for the panel to obtain a layer with structural property (1 a, a, 1 3, 1 5) consisting of wires pre-impregnated with a thermoplastic or thermosetting resin, by draping, winding or wrapping, so that said layer has an open surface rate of the order of 30% of the total surface of the exposed surface, to put on top of the layer with structural property a layer with acoustic property (1 b, Vb, 1 "b) made of a microporous fabric and with a thickness of the order of a tenth of that of the structural property layer, then lay ace the honeycomb structure (2) and the reflector (3) with optionally the addition of an adhesive (5, 6, 1 0) between components, - at least one autoclave baking step being carried out at the end of minus one of the above installation steps.

2. Method according to claim 1, characterized in that said layer with structural property (l 'a) is given the porosity required by the spacing of the threads (7, 8) during weaving or during winding or wrapping sons.

3. Panel according to claim 1, characterized in that said layer with structural property (1 a) is given the porosity required by piercing said layer after baking in an autoclave, the layer with acoustic property (1 b) being put then in place.

4. Method according to claims 1 and 2, characterized in that the layers with structural property (Va) and acoustic property (Vb) are assembled with possible interposition of a crosslinking adhesive (5) and subjected to autoclaving , then the assembly is assembled to the honeycomb core structure (2) and to the reflector (3), with possible interposition of a crosslinking adhesive (6), and subjected to a new autoclave cooking.

5. Method according to one of claims 1 to 4, characterized in that the layer with structural property consists of several plies (1 3 to 1 6) of crossed threads, the plies being on either side of the layer with acoustic property (1 "b).

6. Method according to claim 3, characterized in that the holes (4) for drilling the layer with structural property (1 a) have a diameter greater than the thickness of said layer and their external outlet (1 1) is flared .

7. Panel produced in accordance with any one of claims 1 to 6.