EP1157372B1 - Method for making a sound reducing panel with resistive layer having structural property and resulting panel - Google Patents

Description

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

The invention will be described in its application to the production of panels attenuation of the noise 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 likely applications in any other environment where necessary or desirable to use a panel type structure combining lightness, great mechanical resistance and acoustic properties.

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

The acoustically resistive layer has a dissipative role. When the wave sound passes through it, viscous effects occur which transform partially the acoustic energy in heat. The alveolar structure which 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 meet a number of conditions, the main ones being 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 of the impedance of resistive layers (septum and front side) so that they produce a maximum dissipation at frequencies of interest.

In addition, it is therefore essential to have acoustic homogeneity optimal both in terms of the resistive layers and the structure alveolar.

Furthermore, such a panel must, because of its environment, withstand severe conditions of use. In particular, he must not present any risk of delamination of the resistive layer even in the presence of strong depression and must be resistant to erosion or abrasion as well as 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 properties sufficient structural in particular to receive and transfer efforts aerodynamic, inertial and those related to the maintenance of the nacelle, towards the nacelle / engine structural connections.

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

Known acoustic attenuation panels, especially those used on turbine engine nacelles, respond with more or less happiness to 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 structure honeycomb closed by a rear reflector, mention may be made of those using a so-called non-linear processing 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 includes a honeycomb flanked on one side by a acoustic resistive layer consisting of a rigid and thin mesh of material composite and, on the other side, a reflector.

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

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

However, its drawbacks are also substantial. This layer resistive has a strong acoustic non-linearity which makes 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 provides a frequency window of efficiency limited, as well as low resistance to erosion.

According to another so-called linear processing method, also simple degree of freedom, illustrated for example by the document GB 2 130 963, the resistive layer is formed of two components, namely, a layer structural, honeycomb side, and a microporous layer on the surface.

The structural layer is formed of a fabric of carbon fibers with relatively wide meshes defining an opening rate of about 30% of the total surface of the layer.

The surface microporous layer is a fine mesh fabric of fibers mineral or synthetic or a metallic fabric, acting as a shock absorber acoustic.

The advantages of such a structure are the possibility of adjusting the acoustic resistance of the resistive layer by playing on both components of the latter, reducing acoustic non-linearity resulting in less dependence of the acoustic resistance on the sound level and the velocity of the tangential flow at the surface of the resistive layer. In addition, a more efficient frequency window is obtained. large in comparison with the previous technical solution.

On the other hand, such a structure has the major drawback of additional assembly penalizing in time and cost, due to the bi-component nature of the resistive layer. If the increased constraints of assembly of this structure are not well controlled, there are risks acoustic inhomogeneity, as well as delamination of the resistive layer.

Finally, there is also a risk of corrosion of the exposed layer. microporous imposing constraints on the choice of materials.

According to a third treatment technique known as double degree of freedom, the panel includes a resistive layer on the surface, two nests superimposed bees separated by a resistive layer, called a septum, usually microporous and a reflector.

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

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

Finally, by document EP 0.911.803 we know a panel acoustic attenuation formed of a sandwich comprising a structure honeycomb flanked on one side with a reflector and on the other side with a fabric metal itself covered with a perforated metal sheet.

Such an arrangement makes it possible to obtain panels whose exposed face to aerodynamic flows and which is defined by the tissue association metallic / metallic perforated sheet, has both good properties acoustic and good structural properties.

However, such panels can have drawbacks noticeable especially when they have an accentuated curvature, which is the case including fan channel inlet and outlet panels.

In fact, according to document EP 0.911.803, the metal sheet is first prepared then pierced before being put in place and shaped on the assembly, also carried out, 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 that can be accentuated, the shape of the pre-perforated sheet will cause local deformation of parts of the sheet and therefore of the holes located in these parts. These deformations are likely to significantly modify the area of the holes and therefore the rate of local porosity of the perforated sheet, thus causing inhomogeneity of the porosity of the sheet, detrimental to its effectiveness in terms of attenuation acoustic.

Furthermore, such shaping is difficult because the sheet is relatively rigid.

Finally, in general, all-metal type panels, this which is the case of the panel above, are likely to cause problems of corrosion.

The invention aims to overcome the various drawbacks of these techniques known by proposing a method of manufacturing attenuation panels acoustic of the honeycomb structure flanked, on the one hand, by a reflector and, on the other hand, an acoustically resistive layer with two components acoustic property and structural property respectively, allowing to obtain panels with complex shapes, in particular with evolving curvatures can be important and in particular of monobloc panels of form general annular with or without splint, such as those intended for entry and nacelle fan channel output, presenting both very good mechanical properties and optimal acoustic properties.

• des fils pré-imprégnés d'une résine thermoplastique ou thermodurcissable constituant une couche à propriété structurale. Ladite couche étant constituée par drapage, bobinage ou banderolage de sorte qu'elle présente un taux de surface ouverte de l'ordre de 30 % de la surface totale de la surface exposée,
• une couche à propriété acoustique par dessus la couche à propriété structurale, ladite couche à propriété acoustique étant constituée d'un tissu microporeux et d'épaisseur de l'ordre du dixième de celle de la couche à propriété structurale,
• la structure alvéolaire et le réflecteur avec éventuellement adjonction d'un adhésif entre composants,
• au moins une étape de cuisson en autoclave étant mise en oeuvre à la fin d'au moins une des étapes de mise en place ci-dessus.

To this end, the subject of the invention is a method of manufacturing an acoustic attenuation panel according to claim 1 comprising a honeycomb structure flanked, on the one hand, by a reflector and, on the other hand, 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 (M), of a shape suitable for the panel

• wires pre-impregnated with a thermoplastic or thermosetting resin constituting a layer with structural property. Said layer being formed by draping, winding or wrapping so that it has an open surface rate of the order of 30% of the total surface of the exposed surface,
• a layer with acoustic property over the layer with structural property, said layer with acoustic property consisting of a microporous fabric and with a thickness of the order of one tenth that of the layer with structural property,
• the honeycomb structure and the reflector with possibly 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 setting steps.

The method of the invention makes it possible to obtain an acoustic layer resistive with remarkable acoustic and structural properties, in particular the effectiveness of the acoustic attenuation due to the very good homogeneity of the porosity rate of said acoustically resistive layer, which can be defined precisely.

Indeed, the fact of using pre-impregnated wires shaped on a mold not only allows for complex shapes that can have pronounced curvatures, but above all allows a very good control of the porosity of the layer with structural property.

According to one embodiment, said layer is given to structural property the porosity required by the choice of wire spacing at weaving, in the case of a fabric, the flexibility of the latter allowing to fit the mold shapes without substantial deformation of the mesh of the tissue.

In the case of wound or wrapped wires, the adjustment of the spacing of the son allows to precisely adjust the porosity rate.

According to another embodiment, said layer is given to structural property the porosity required by drilling said layer after autoclave cooking.

Drilling is performed to precise diameters and in a workpiece shape and rigid, porosity control is perfectly ensured.

Advantageously and according to another mode of implementation and at for reinforcing purposes, the layer with structural properties consists of several plies of crossed threads, the plies being on either side of the layer to acoustic property.

The subject of the invention is also the panels obtained in accordance to the above process.

• la figure 1 est une vue en coupe et en éclaté schématique d'une structure de panneau obtenue conformément au procédé de l'invention ;
• la figure 2 est une vue en coupe similaire illustrant un autre mode de mise en oeuvre du procédé de l'invention ;
• la figure 3 est une vue partielle de dessus de la couche à propriété structurale du panneau de la figure 2 ;
• les figures 4a à 4e illustrent différentes étapes de réalisation d'un panneau du type de la figure 1,
• la figure 5 est une vue en coupe partielle illustrant un mode de collage de la couche acoustique bi-composant sur la structure alvéolaire, et
• la figure 6 est une vue en coupe partielle illustrant une variante du procédé illustré par la figure 2.

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 sectional view illustrating a variant of the method illustrated in FIG. 2.

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

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

Finally, over the honeycomb structure 2 is put in place a reflector conventional 3.

According to the invention, the layer with structural property 1a is formed from wires pre-impregnated with a thermoplastic resin or thermosetting suitable. By yarn is meant yarn, fiber, wicks under form of 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, glass, "Kevlar" or other mineral fibers or organic, natural or synthetic, dry or prepreg.

The honeycomb structure 2 is for example a paper of aramid fibers such than that known commercially as "NOMEX".

In the embodiment illustrated in FIG. 1, the layer to be structural property constituted by a fabric draped over the mold M, or by wires deposited by winding or wrapping, is produced, then polymerized by autoclave cooking.

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

This open surface rate is advantageously of the order of 30% of the exposed surface of layer 1a.

The perforations 4 produced for this purpose in layer 1 a have preferably a ratio of the diameter to the thickness of the layer 1 a greater than 1 to reduce the harmful effects of acoustic non-linearity.

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

After perforation of the holes 4, the layer 1a still being in place on the mold M, the acoustic property layer 1b is put in place, with 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 after layers 1b and 5 have been put in place, then a third polymerization by autoclave cooking is carried out after placement of the layers 2 and 3, a crosslinking adhesive being advantageously interposed between layers 2 and 3. Finally, the mold M is removed to release the panel finished.

The choice of adhesives 5, 6 and their installation methods, 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 the 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 structural property 1'a has the layer acoustically two-component resistive 1 'is made from wicks of deposited fibers along a weft of fabric, namely warl wicks 7 and wicks of frame 8, the mesh thus produced defining passage openings 9 (Figure 3) rectangular or square, constituting about 30% of the surface of the layer has it.

The fibers of the locks 7, 8 can be of the type indicated above, dry or prepreg. Wicks 7, 8 are deposited individually by winding, wrapping or manual deposit or not on a mold (no shown) similar to mold M of Figure 1. A polymerization is then performed.

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

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

It should be noted that the layer with structural property 1a can be consisting of several plies of pre-impregnated yarn fabrics or of several overlapping plies of wound or wrapped prepreg yarns.

The acoustically resistive layers (1, 1 ') of the panels according to the invention, although made up of two components, nevertheless have excellent mechanical qualities.

Indeed, the materials of the two components, structural and acoustic, are identical and compatible and lend themselves to good bonding and constitute after polymerization a single composite sheet with almost zero risk of delamination, very resistant to erosion, abrasion, impact and moreover easy to repair.

In addition, the resistive layers have, due to the precise control of their porosity rate during manufacturing, very good acoustic performance especially in terms of non-linearity, their impedance does not depend on the Mach number of grazing flow.

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

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%, we apply (figure 4a) the cross-linking adhesive layer 5, then the acoustic layer is placed 1 b (Figure 4b) and hot polymerization under pressure to assemble the two layers 1 a, 1 b.

Next, the crosslinking adhesive 6 is placed (FIG. 4c) on the honeycomb structure 2.

Finally (Figure 5d), we assemble all the elements of the panel by a new hot pressure polymerization step, adhesive 10 being also placed on the other side of the honeycomb in line with the feet of the cells for bonding 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 1 b, a very good adhesion between honeycomb 2 and layer 1 b.

Indeed, the adhesive 6 diffuses well in the porous mass of the layer 1 b and the junction between the end edge of the walls of the honeycomb cells 2 and the facing face of the layer 1b is established by constituting good connecting jumps to the right of the honeycomb cell bases defining cross-section bonds increasing in size as we get closer to the face of said layer 1 b.

It should also be noted that, in general, the invention allows to give the acoustic component (layer 1b) a very fine thickness, much lower than that of structural layer 1 a. For example, the layer 1a may have a thickness of one millimeter, while the thickness of the layer 1b can be reduced to 0.1 millimeter without degradation of its acoustic properties.

Figure 4e illustrates an alternative embodiment of the assembly of layers 1 a, 1 b and 2, in which the cross-linking adhesive 5 between the layers 1a and 1b is deleted. Due, in fact, to the small thickness and the large porosity of the acoustic layer 1b, it is possible to apply the adhesive 6 than on the receiving side of the honeycomb 2.

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

In this set, the only adhesive used (6) is applied only to the right of the feet of the honeycomb cells 2, which limits the obstruction of passage openings 4 through the structural layer 1 a only areas opposite said cell feet.

The technique illustrated in FIGS. 4a to 4e can be used with various variants of panel structure described above.

This technique makes it possible to easily design and produce acoustic attenuation panels with mechanical characteristics efficient and homogeneous, adapted to various environments, in particular those mentioned above such as the nacelles of turbo-engines.

FIG. 5 also illustrates an alternative embodiment of the holes 4 of the structural layer 1a during their perforation, according to which the the external outlet of said holes 4 is advantageously flared, by all appropriate means, as shown in 11, so as to improve the linearity acoustic.

FIG. 6 illustrates another variant embodiment of the method of the invention according to which the layer with structural property is reinforced. In this Indeed, the layer with structural property consists of several layers of crossed prepreg yarns arranged on either side of the property layer acoustic 1 "b.

On the left side of Figure 6, there is shown a first distribution of two plies of crossed wires, respectively a ply 13 of chain son, first deposited on a mold (not shown) similar to mold M of FIG. 1 and a web 14 of weft threads deposited over the layer 1 "b, ie after removal of the latter.

On the right side of Figure 6, a second is shown distribution of three layers, namely two layers crossed in a weft of weaving 15, deposited first on the mold and a third ply 16 of wires parallel to the wires of one of the layers of the weft 15, deposited on top the acoustic layer 1 "b.

All the components 13, 14, 15, 16, 1 "b thus form a 1 "acoustically resistive layer with both structural and acoustic.

This assembly is polymerized under pressure before installation of the other components 2, 3.

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

Claims (9)

1. A method of manufacturing an acoustic attenuation panel comprising an alveolar structure (2) flanked on the one hand by a reflector (3) and on the other hand by an acoustically resistive layer (1, 1', 1") with two components, respectively, with acoustic properties and structural properties, characterised in that it comprises placing on a mould (M), with a shape appropriate to the panel

   fibres preimpregnated with a thermoplastic or thermosetting resin forming a layer with structural properties (1a, 1'a, 13, 15), said layer being formed by stretch forming, winding or hoop casing so that it has an open surface ratio of around 30% of the exposed surface,

   a layer with acoustic properties on top of the layer with structural properties, said layer with acoustic properties being formed of a microporous fabric with a thickness of around one tenth of that of the structural layer,

   the alveolar structure and the reflector with possibly the addition of an adhesive between components,

   at least one step of curing in an autoclave being implemented at the end of at least one of the above placing steps.
2. A method according to claim 1, characterised in that the required porosity is conferred on said layer with structural properties (1'a) by the separation of the fibres (7, 8) at the time of weaving or when the fibres are wound or hoop cased.
3. A method according to claim 1, characterised in that the required porosity is conferred on said layer with structural properties (1a) by piercing said layer after curing in an autoclave, the layer with acoustic properties (1b) being put in place thereafter.
4. A method according to claims 1 and 2, characterised in that the layers with structural properties (1'a) and acoustic properties (1'b) are assembled with the possible interposing of a cross-linking adhesive (5) and subjected to curing in an autoclave, and then the whole is assembled on the structure with an alveolar core (2) and on the reflector (3), with the possible interposing of a cross-linking adhesive (6), and subjected to a further curing in an autoclave.
5. A method according to one of claims 1 to 4, characterised in that the layer with the structural properties is formed of several layers of crossed fibres.
6. A method according to claim 3, characterised in that the piercing holes (4) in the layer with structural properties (1a) have a diameter greater than the thickness of said layer and their external outlet (11) is splayed.
7. A method according to claim 5, characterised in that a layer of crossed fibres is put in place on top of the layer with acoustic properties.
8. A method according to one of claims 1 to 7, characterised in that the layer with acoustic properties (1b, 1'b, 1"b) is formed from a fabric of fibres chosen from the group comprising carbon, glass or "Kevlar" fibres, mineral or organic, natural or synthetic, dry or preimpregnated fibres.
9. A method according to claim 4, characterised in that the cross-linking adhesive (6) interposed between the structure with an alveolar core (2) and the layer with acoustic properties (1b) is placed in line with the bottoms of the cells of said structure with an alveolar core (2).

Images