FR2527367A1 - DEVICE FOR ATTENUATING SOUND ENERGY AND MANUFACTURING METHOD THEREOF - Google Patents

Abstract

IN A "LINEAR" ACOUSTIC PANEL 10 A SIDING SHEET 13, 14 COVERS A CELLULAR HONEYCOMB LAYER 12 COVERING HIMSELF A NON-PERFORATED SUPPORT SHEET 11. THE SIDING SHEET INCLUDES A LAYERED LAYER 13 IN A FIBRESRESIN COMPOSITE MATERIAL, PREFERABLY HAVING AN OPEN TEXTURE TO PRESENT OPENINGS, AND A POROUS LAYER 14 APPLIED AGAINST THE FACE ON THE LAYERED LAYER, OF WHICH THE DOORS ARE MUCH MORE THE OPENINGS OF THIS LAYER. AN APPROPRIATE MANUFACTURE OF SUCH A SIDING SHEET CONSISTS OF CURING THE COMPOSITE MATERIAL SHAPED ON A TOOL, THE POROUS LAYER BEING IN CONTACT WITH THIS MATERIAL, SO THAT THE CURED RESIN OF THE COMPOSITE BONDES THE POROUS LAYER 14 ON THE LAYER ADJOURNED 13.

Description

The invention relates to means for at-

  to hold the sound energy and more especially, although not exclusively, to noise attenuation panels (also called acoustic panels), as well as to methods of manufacturing such means. It has already been proposed (see British patent application NO 2 056 367 A) to use for the attenuation of the noise energy over a wide range of noise frequencies an acoustic panel called "linear" having the multilayer structure following: i a non-perforated support layer ii an openwork layer, consisting of a sheet which has been manufactured so as to present a multitude

  through openings, whose size and space-

  ment are predetermined and ordered so as to establish an open surface characteristic of the layer; iii a cell layer fixed between the non-perforated and perforated layers, a first end of all the cells; being covered by the non-perforated layer while their second opposite end is closed by the perforated layer, and several of the openings communicating with each of the cells which, in this way each constitute a resonant Helmholtz cavity; and iv a porous layer bonded to the surface of the perforated layer which is opposite to the surface to which the cell layer is bonded and forming the external surface

  of the panel on which the transient noises strike

  carried by air, the size of the pores of the layer

  reuse being much lower than that of the openings of the

openwork layer.

  In such a panel, the cellular honeycomb layer and the porous layer together provide the desired wide range of noise attenuation and the perforated layer forms a support for the other layers and constitutes a structural element by which the panel can be fixed. to the adjacent structure of the aircraft The performance of the

  panel are deteriorated by any significant deviation from

  port has a predetermined proportion of the surface area of the openwork layer which is open as between the layer

  porous and cell layer cells.

  It is not easy to assemble the four layers of such a panel in such a way that, on the one hand, the risk of peeling of the sides of the panel in use is

  reduced to a minimum and that on the other hand, an assembly phase

  However, it does not close such a large number of perforations and / or pores that the surface opening characteristic of the perforated layer is altered and that the sound attenuation efficiency is reduced.

  invention is to reduce these difficulties.

  Another goal is to produce a panel which is

  lighter than equivalent panels of the an-

  later. Another object is to provide a panel construction and a method of assembling this construction which

  facilitate the production of panels of complex shapes.

  According to a first aspect, the present invention relates to a facing sheet for an acoustic panel, constituted by the perforated layer and the porous layer, and characterized in that the perforated layer is made of a material

  composite fiber / resin matrix.

  According to a second aspect, the invention relates to a method of manufacturing a facing sheet for an acoustic panel according to which: i) an openwork layer is prepared in a composite fiber / resinous matrix material which defines a multitude

  through openings having a dimension and a space-

  predetermined and ordered so as to introduce an open surface characteristic of the layer; ii) a porous layer is prepared, the pores of which have a dimension much smaller than that of the openings in the openwork layer; and iii) the openwork layer and the porous layer are bonded

  to each other, face to face, to form the barrier sheet

  is lying. A number of advantages arise from this use of a composite material instead of the material

  of the prior art which is aluminum.

  First, the openings can be formed while the resinous matrix of the composite is in the state

  partially hardened This is probably a process

  lasts much less costly than training the same name-

number of perforations in a sheet.

  Second, the resinous matrix of the composite can itself constitute an adhesive agent with the help of which

  the porous layer can be bonded to the perforated layer.

  Thirdly, the fibrous component of the composite material can be arranged in a pattern, for example in an intertwining, which in itself forms the desired multitude of openings, thus eliminating any need for a

  separate opening formation phase.

  Fourth, the particular problem of electrolytic corrosion is eliminated A prior art panel has an openwork aluminum layer and a porous layer composed of a stainless steel mesh or felt If these two layers come into electrical contact , electrolytic corrosion may occur in the presence of moisture The prior art manufacturing processes may include special phases to reduce the risk of contacting certain parts, the mesh or felt and the underlying perforated layer in the finished set A composite carbon fiber / epoxy resin has roughly the same electrode potential as the porous stainless steel layer above it, so the risk of electrolytic corrosion and the need for special measures during assembly to avoid any contact between the porous layer and the perforated layer are

thus deleted.

  It may be advantageous to use as a method for manufacturing a noise reduction device according to the invention a method which consists in fixing the layer of composite material to that of the layers which it

  is adjacent, keeping the adjacent layer in con-

  tact with the layer of composite material when the resin is in a state other than the fully

  hardened and harden the resin while both

  ches are kept in contact, so that the resin fixes the two layers to each other The resin will probably be what is called a resin "controlled flow / 'that is to say a resin having a viscosity

  which is high enough to keep the

  resistance of a gel during the first phases of its hardness

  cementing, instead of being entirely fluid It may be advantageous to provide a layer of a "peel-ply", as described below, to absorb

the excess resin.

  In the previously proposed panels, it is the perforated layer which is used to fix the panel to the adjacent support structure. In one embodiment of the present invention, it is the non-perforated support layer which is used for this. noticed that it is desirable to plan

  in the openwork layer a proportion of open area from

  vertices compared to the surface of the projected surface of the layer of approximately 30% This high percentage of the open surface and the need to keep the panel as light as possible is detrimental to the suitability of the perforated layer

  when mounting the panel on the porous structure.

  The characteristics and advantages of the invention

  tion will appear better during the description than will

  follow an embodiment shown in the accompanying drawings and given only by way of example In the drawings: FIG. 1 is a section through a singing region -

  an acoustic noise reduction panel, trans-

  versally to the plane of the panel; Fig 2 is a plan view of part of the perforated layer of the panel of Fig 1, drawn on a larger scale and without the porous layer for clarity of the drawing but with the outline of one of the hexagonal cells the underlying alveolar layer; and Fia 3 is a section of a facing sheet

  of the acoustic panel of Fig 1 and 2, during the manufacturing

  tion of this panel, with a forming tool and a bag

associated vacuum.

  As can be seen by referring to the

  Fig 1, the panel 10 comprises a layer of metallic support

  non-perforated metal, of aluminum alloy, to which

  is fixed by an epoxy resin adhesive, a layer

  honeycomb 12 which each cell has at least one flow slot 18 At the other end of the cells of the honeycomb is fixed, by an epoxy adhesive E 2, a perforated layer 13, which will be described with more details below To the external surface of the layer 13, is fixed a mesh 14 of stainless steel, with fine wires. The openwork layer 13 and the wire mesh 14, taken together, are designated here by the expression sheet.

facing the panel 10.

  The panel 10 forms part of the surface facing inward of a front cowling duct for a turbojet aviation engine, the panel being one of several curved panels arranged just upstream of the blower of the motor It is therefore of utmost importance that this panel does not deteriorate in use and, in particular, that none of its parts detaches from its support structure This structure includes flanges or support beams of which only 15 is shown The panel 10 is fixed to the heald by assembly means known to those skilled in the art, but it should be noted that it is the support sheet 11 of the panel and not the perforated layer 13 which is fixed to a wing 1 for fixing the panel 16 of the heald 15 The gap 17 between the external surface of the panel 10 and the surrounding structure can be left open but could be closed or sealed, for example using a sealant . As shown in Fig 2, the perforated layer 13 is formed of a woven material itself composed of three

  series of wires 20, 21 and 22, each composed of a multitu-

  of carbon fibers F The three series 20, 21 and 22 are arranged so as to form between them angles of 60 and to provide a multitude of openings H of hexagonal shape As it emerges from the superposition of the layout C of one of the honeycomb cells of the cell layer 12, there are a large number of 1 DH openings opposite the open end of each of the C cells. An uncured resin R surrounds the fibers F in the threads of the woven material but do not block the openings H During the manufacture of the panel 10, the resin R is hardened in an autoclave, the perforated layer

  13 then being supported and arranged with the necessary form

  for the finished panel, which may have a double curvature. By providing a layer of woven "nylon" fabric such as that which is called (skin-peelable), on each face of the perforated layer 13 during the curing phase, not only is any adhesive burr removed from the inside the openings but also applying the pattern of weaving on the surface of the sheet, so that the adhesive then applied is fixed mechanically on it However, the outer layer 14 is preferably kept compressed in contact with the perforated layer 13 for the curing phase so that the resin is used to bond the two layers of the sheet to each other

33 facing the panel 10.

  A particular mode of implementation of this method will now be described with reference to FIG. 3 of the drawings. A die 30, which will hereinafter be called a tool, and which has a molding surface 31, having

  the shape of part of the panel 10, is cleaned before

  mement using methyl ethyl ketone and then receives two coatings 32 of a release agent such as a Frekote 33 supplied by Frekote Inc, E U A, leaving a period of air drying between the coatings Then,

  this agent is hardened at 1210 ° C. for 30 minutes.

  We submit to an energetic treatment of degreasers-

  steam-wise, a surface 33 of a stainless steel lattice (such as that which can be obtained from the Company G Bopp & Co Lt D, of London N 2, Great Britain, under the designation of " Robusta wease ") 720 /, 150 (this designation indicates the number of threads per 25.4 mm length of fabric, warp and weft

  respectively) sufficient to cover the work surface

  vail of the tool 30 Then, one tends on the tool 30, taking great care to avoid forming gathers in the trellis, and one fixes it in position by means of ribbons

  adhesives (not shown) applied on its periphery.

  When the tool 30 has a double curvature, it is acceptable to split the wire mesh provided that this

  either in places which are established by the specifica-

  study and engineering engineering.

  it may be desirable to provide a film of suitable adhesive

  property around the periphery of the wire mesh and / or around any split surface of this mesh to improve the adhesion between it and the fiber / resin composite

  which must then be placed on the wire mesh.

  The openings P between the warp and weft threads of the mesh 33 constitute the pores of this porous layer. Then we carefully place it on the trellis

  metallic 33 sufficient surface 34 of a fabric

  carbon mesh with open mesh, such as that shown in Fig 2, which is impregnated with a system of resins which can be hardened together The fabric carries as a peelable layer, a layer of 34 P nylon fabric on the surface of the fabric carbon fiber 34 which is opposite to the wire mesh 33. The curing of the resin system is carried out under reduced pressure in an autoclave 29 The pressure

  reduced is obtained by covering the tool 30 and the structure

  re in layers 33, 34 placed on this tool with a nylon vacuum bag member 35 in the manner shown in the

  Fig 3 which will now be described.

  Along the periphery of the fiber fabric 34

  a strip 36 of permeable respirator material is provided

  gas-able, this strip having a width of approximately 2 to 5 cm is applied to the visible surface of the fiber fabric

  uncured 34 a thin film 37 of suitable release agent

  required to be used at high temperature, so as to cover the strip 36 for about 1 to 1.5 cm The release film 37 and the strip 36 are covered with a breathing sheet 38 made of the same material as that which forms the strip 36 The vacuum membrane 35 envelops the uncured part and the respirator which covers it Around the periphery of the membrane 35, at the place where this membrane meets the tool 30, a strip is provided

  35 S sealant in a high temperature sealant.

  Then, a leakage test is carried out in the sealing device of the vacuum membrane 35 and of the tool 30 using a vacuum pump 40 to reduce the pressure in the formed bag to a value of 55 cm of mercury. by the membrane 55, as indicated, on a pressure gauge 41, a vacuum line 42 is sealed

  by means of a tap 43 and the eventual elevation is monitored

  pressure trough inside the bag A rise

  less than or equal to 12 cm Hg in 5 minutes is ac-

ceptable.

  Then, while the air contained in the bag is still under reduced pressure, the curing begins

  in an autoclave The ambient pressure inside the

  toclave and outside the bag is 3.1 + 0.35 bars.

  When the pressure inside the bag rises to 1.4 + 0.69 bars, the bag is ventilated to the atmosphere and then

  maintained at O + 0.35 bar for the remainder of the processing time

  seriously. As regards the temperatures, they are regulated by a temperature regulator 44 which receives the

  temperature data of a thermocouple 45 placed near

  mite the room in order to limit the rate of temperature rise in the room to an interval of 1 to 2.50 C / min, up to a hardening temperature of 177 + 50 C, which is maintained for a period of 120 + 60 min Then, the part is allowed to cool at a speed of 1.50 C / min

  up to below 600 C, the pressure of the car is released

  clave and we extract the piece from the latter.

  Alternatively instead of splitting the metal mesh-

  lique 33, we could achieve a complex shape with double curvature by weaving the wire mesh on a matrix having the desired configuration, or by welding together at their periphery a number of separate pieces of mesh. Other processes are envisaged The openings

  can be formed after weaving but before hardening

  cementation, by moving the interlaced wires and retaining them in their new position so as to form the desired openings The porous layer can be fixed to the perforated layer after the latter has been cured

to its shape.

  In Fig 1, a non-perforated (but usually usually woven) composite material for the layer 13 can be hardened into shape and only after that provided with the openings, possibly by perforation of the layer

  13, the outer layer 14 being fixed later.

  However, some of the advantages of the invention would be

  lost through the use of this procedure.

  A sheet of the flat composite material for-

  rait while it is not yet hardened or partially hardened, to be perforated flat and then, hardened in shape,

  D in contact with the porous layer so as to be assembled

stuck to it.

  The hardened facing sheet is then bonded to one side of the cellular layer 12 and on the other side, a support layer-11 of aluminum or of fiber / resin composite is bonded, the two sheets or layers being bonded to

  using an epoxy adhesive as shown in E 2 and El res-

  pectively in Fig 1 The adhesion of the facing sheet is favored by the pattern of the weaving of the peelable layer which remains on the surface of the cured resin of the j D facing sheet The cellular layer 12 can be in

  "NOMEX", a nylon-paper material, from CIBA-GEIGY (see below)

  after) The size of the cells is such that the large circle which can be inscribed in each cell has a diameter

9 mm.

  The epoxy adhesives used are supplied by CIBA-

  GEIGY Plastics and Additives Co Limited of Cambridge,

  Great Britain, and the carbon fiber material

  tri-axial sage by Barber Colman Co of Rockford, EUA Hexcel (HK) Limited of Lightwater, Surrey, Great Britain impregnates the fabric with its flow-controlled resin F 593 and provides the impregnated fabric interposed between polyethylene sheets The layer porous woven material is supplied by the British company G Bopp as indicated above. In place of the Bopp material, the sintered stainless steel fiber mesh product sold under the designation "BRUNSMET" by Brunswick Corporation may be used.

  Dehand, Florida, E U A Other sources are also available.

  very carbon fiber materials woven from

  Industry Brochier, from Villeurbanne, France.

  Non-metallic porous layers, for example woven mesh of polyester material are envisaged, and these layers would have the advantage of having a lower weight than an equivalent metallic mesh. Adhesives and resins do not necessarily have to be epoxy but can also consisting of, for example, a phenolic or polymide resin Carbon fibers could be replaced by other fibers, provided that the latter have sufficiently good mechanical properties It is estimated that glass fibers and KEVLAR fibers provide performance lower than fiber

of carbon for this application.

  The electrode potentials of the porous layer (33) and of the perforated layer (34) are preferably

about the same.

2527367,

Claims (9)

REVENDICATI 0 NS 1, Facing sheet (33, 34) for an acoustic panel (10) comprising: i) an openwork layer (34), constituted by a sheet which defines a multitude of through openings (H) of dimension and d 'space predetermined and ordered so as to establish an open surface characteristic of the layer; and ii) a porous layer (33) fixed to a surface of the perforated layer, the dimensions of the pores (P) of this layer being significantly smaller than those of the openings (H) of the perforated layer; characterized in that the perforated layer (34) is formed from a composite fiber / resinous matrix material-   2 facing sheet according to claim 1,   characterized in that the composite material is a material   open weaving line in which the openings are formed by the spaces (H) between the threads (20, 21, 22) weaving.   3 facing sheet according to claim 2, characterized in that the woven material comprises three series (20, 21, 22) of threads arranged at 600 from each   others, to delimit hexagonal openings (H) gonal.   4 Facing sheet according to one of the claims   1 to 3, characterized in that the electrode potentials of the porous layer (33) and of the layer   openwork (34) are almost identical.   Facing sheet according to claim 4, characterized in that the fiber (F) of the composite material is made of carbon and that the porous layer (33) is made of stainless steel.   6 Method of manufacturing a barrier sheet   ment for an acoustic panel, characterized in that i) an openwork layer is prepared formed from a composite fiber / resinous matrix material which defines a multitude of through openings of a predetermined size and spacing and ordered so as to establish a surface characteristic or erection of the layer, iii) a porous layer is prepared whose pores   have dimensions much smaller than those of openings   openwork layer sizes; and iii) the perforated layer and the porous layer are bonded to each other, face against   face, to form the facing sheet.   7 Method according to claim 6, characterized in that one simultaneously performs the bonding of the perforated layer and the porous layer and the curing of the resin of the composite material, the resin acting as an adhesive between the porous layer and the openwork layer. 8 Method according to claim 6, characterized in that the perforated layer is shaped on a tool and that the shaped layer is hardened on the tool before joining by gluing the perforated layer and of the porous layer.   9 Method according to one of claims 6 and 8,   characterized in that the open surface is established before hardening the resin of the composite material.   Method according to one of claims 7 and 9,   characterized in that the establishment of the surface open is a weaving phase.   11 Method according to one of claims 7,9 and   , characterized in that the resin of the composite material hardens while at least one face of the perforated layer   is in contact with a peelable skin layer.   12 Method according to claim 11, characterized in that the face of the perforated layer is that which is remote from the porous layer of the cured facing sheet. 13 Sound energy attenuation device characterized in that it includes a facing sheet according to claim 1.   14 Sound energy attenuation device, characterized in that it is produced according to a process according to claim 6.   Sound energy attenuation device   according to one of claims 13 and 14, characterized in   what it constitutes a linear acoustic panel (10) whose support layer (11) is used to fix the   panel to an adjacent supporting structure.