EP0698159A1 - Acoustic attenuating liner and method of making same - Google Patents

Acoustic attenuating liner and method of making same

Info

Publication number
EP0698159A1
EP0698159A1 EP94916684A EP94916684A EP0698159A1 EP 0698159 A1 EP0698159 A1 EP 0698159A1 EP 94916684 A EP94916684 A EP 94916684A EP 94916684 A EP94916684 A EP 94916684A EP 0698159 A1 EP0698159 A1 EP 0698159A1
Authority
EP
European Patent Office
Prior art keywords
perforated sheet
backsheet
core
adhesive
epoxy
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP94916684A
Other languages
German (de)
French (fr)
Inventor
Charles A. Parente
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Grumman Corp
Original Assignee
Grumman Aerospace Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Grumman Aerospace Corp filed Critical Grumman Aerospace Corp
Publication of EP0698159A1 publication Critical patent/EP0698159A1/en
Pending legal-status Critical Current

Links

Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B3/00—Layered products comprising a layer with external or internal discontinuities or unevennesses, or a layer of non-planar shape; Layered products comprising a layer having particular features of form
    • B32B3/10—Layered products comprising a layer with external or internal discontinuities or unevennesses, or a layer of non-planar shape; Layered products comprising a layer having particular features of form characterised by a discontinuous layer, i.e. formed of separate pieces of material
    • B32B3/12—Layered products comprising a layer with external or internal discontinuities or unevennesses, or a layer of non-planar shape; Layered products comprising a layer having particular features of form characterised by a discontinuous layer, i.e. formed of separate pieces of material characterised by a layer of regularly- arranged cells, e.g. a honeycomb structure
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/20—Layered products comprising a layer of metal comprising aluminium or copper
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B3/00—Layered products comprising a layer with external or internal discontinuities or unevennesses, or a layer of non-planar shape; Layered products comprising a layer having particular features of form
    • B32B3/26—Layered products comprising a layer with external or internal discontinuities or unevennesses, or a layer of non-planar shape; Layered products comprising a layer having particular features of form characterised by a particular shape of the outline of the cross-section of a continuous layer; characterised by a layer with cavities or internal voids ; characterised by an apertured layer
    • B32B3/266—Layered products comprising a layer with external or internal discontinuities or unevennesses, or a layer of non-planar shape; Layered products comprising a layer having particular features of form characterised by a particular shape of the outline of the cross-section of a continuous layer; characterised by a layer with cavities or internal voids ; characterised by an apertured layer characterised by an apertured layer, the apertures going through the whole thickness of the layer, e.g. expanded metal, perforated layer, slit layer regular cells B32B3/12
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B7/00—Layered products characterised by the relation between layers; Layered products characterised by the relative orientation of features between layers, or by the relative values of a measurable parameter between layers, i.e. products comprising layers having different physical, chemical or physicochemical properties; Layered products characterised by the interconnection of layers
    • B32B7/04—Interconnection of layers
    • B32B7/12—Interconnection of layers using interposed adhesives or interposed materials with bonding properties
    • E—FIXED CONSTRUCTIONS
    • E04—BUILDING
    • E04C—STRUCTURAL ELEMENTS; BUILDING MATERIALS
    • E04C2/00—Building elements of relatively thin form for the construction of parts of buildings, e.g. sheet materials, slabs, or panels
    • E04C2/30—Building elements of relatively thin form for the construction of parts of buildings, e.g. sheet materials, slabs, or panels characterised by the shape or structure
    • E04C2/34—Building elements of relatively thin form for the construction of parts of buildings, e.g. sheet materials, slabs, or panels characterised by the shape or structure composed of two or more spaced sheet-like parts
    • E04C2/36—Building elements of relatively thin form for the construction of parts of buildings, e.g. sheet materials, slabs, or panels characterised by the shape or structure composed of two or more spaced sheet-like parts spaced apart by transversely-placed strip material, e.g. honeycomb panels
    • E04C2/365—Building elements of relatively thin form for the construction of parts of buildings, e.g. sheet materials, slabs, or panels characterised by the shape or structure composed of two or more spaced sheet-like parts spaced apart by transversely-placed strip material, e.g. honeycomb panels by honeycomb structures
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
    • G10K11/00—Methods or devices for transmitting, conducting or directing sound in general; Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
    • G10K11/16—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
    • G10K11/172—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using resonance effects
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2305/00—Condition, form or state of the layers or laminate
    • B32B2305/02—Cellular or porous
    • B32B2305/024—Honeycomb
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2307/00—Properties of the layers or laminate
    • B32B2307/10—Properties of the layers or laminate having particular acoustical properties
    • B32B2307/102—Insulating
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2307/00—Properties of the layers or laminate
    • B32B2307/70—Other properties
    • B32B2307/714—Inert, i.e. inert to chemical degradation, corrosion
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2311/00—Metals, their alloys or their compounds
    • B32B2311/24—Aluminium

Definitions

  • the present invention relates to an acoustic attenuating liner. More particularly, the invention relates to an acoustic attenuating liner and a method of making the same.
  • the engines of older aircraft have acoustic liners in the engine inlet duct which are used for attenuating specific noise spectra. These liners reduce the engine noise to an extent which permits the aircraft to meet government specified noise regulations.
  • acoustic liners in the engine inlet duct which are used for attenuating specific noise spectra. These liners reduce the engine noise to an extent which permits the aircraft to meet government specified noise regulations.
  • airports which have implemented, or are attempting to implement, lower noise level regulations for the communities around them.
  • the principal object of the invention is to provide an acoustic attenuating liner which will fit within a specific depth and will provide a lower frequency acoustic attenuation.
  • An object of the invention is to provide an acoustic attenuating liner which will fit within a specific depth and will provide acoustic attenuation of more than one frequency.
  • Another object of the invention is to provide an acoustic attenuating liner which will fit within a specific depth and will provide acoustic attenuation of two different frequencies.
  • Still another object of the invention is to provide an acoustic attenuating liner of simple structure which functions efficiently, effectively and reliably to provide lower frequency reduction of the noise of a jet engine.
  • Yet another object of the invention is to provide an acoustic attenuating liner which is lightweight, durable and corrosion-resistant and will provide a lower frequency acoustic attenuation.
  • Another object of the invention is to provide an acoustic attenuating liner which is suitable for use with all types of aircraft and engines, including jet engines, combustion engines, turbines and APU systems, and provides a lower frequency acoustic attenuation in an area of predetermined dimensions whereby said acoustic attenuating liner may be installed as a replacement for existing acoustic attenuating liners.
  • an acoustic attenuating liner comprises a backsheet.
  • a honeycomb core on the backsheet is bonded thereto, wherein at least part of the core is at an angle of other than 90° with the backsheet.
  • a perforated sheet is provided in the honeycomb core and adhesive between the perforated sheet and the core bonds the perforated sheet to the core.
  • part of the core is at an angle of 90° with the backsheet, and part of the core is at an angle other than 90° with the backsheet.
  • a corrosion-insulated perforated sheet is provided on the honeycomb core.
  • An adhesive is provided between the perforated sheet and the core for bonding the perforated sheet to the core.
  • a mesh woven to a determined weave pattern from a corrosion- resistant metal is placed on the perforated sheet.
  • the perforated sheet is reticulated.
  • An additional adhesive is provided between the mesh and the perforated sheet for bonding the mesh to the perforated sheet.
  • the additional adhesive has predetermined characteristics including a minimum viscosity of 1000 poises during curing and a predetermined thickness.
  • the backsheet comprises aluminum
  • the perforated sheet comprises anodized aluminum
  • the mesh comprises corrosion-resistant stainless steel wire
  • the honeycomb core comprises aluminum in a modification of the invention.
  • the honeycomb core comprises non-metallic material and the perforated sheet comprises a graphite epoxy open weave woven to a determined open area with the weave cross-section flattened to provide a bonding surface and smooth aerodynamics in another modification of the invention. Furthermore, part of the core is at an angle of 90° with the backsheet and part of the core is at an angle of other than 90° with the backsheet.
  • a method of making an acoustic attenuating liner comprises the steps of weaving a perforated sheet, forming a honeycomb core, bonding the perforated sheet to the core, forming a backsheet and bonding at least part of the core to the backsheet at an angle of other than 90° with the backsheet.
  • part of the core is bonded to the backsheet at an angle of 90° with the backsheet and part of the core is bonded to the backsheet at an angle of other than 90° with the backsheet.
  • a method of making an acoustic attenuating liner comprises the steps of weaving a perforated sheet of composite material, impregnating the perforated sheet of composite material with a resin, curing the perforated sheet of composite material with heat and pressure, forming a honeycomb core of composite material, reticulating the perforated sheet with an epoxy reticulating adhesive comprising a precatalyzed epoxy adhesive in film form which may be applied to the perforated sheet by reticulative process and curing with heat and pressure to bond the perforated sheet of composite material to the core, forming a backsheet of composite material, bonding at least part of the core to the backsheet at an angle of other than 90° with the backsheet with an epoxy supported film adhesive and curing with heat and pressure, weaving a mesh of corrosion-resistant material having acoustic properties which meet desired acoustic resistance values, spraying the perforated sheet with an epoxy adhesive to a thickness of substantially 1.0 to 1.5 mils, staging the epoxy adhesive on
  • the mesh is woven in a reverse plain Dutch weave pattern
  • the perforated sheet comprises a graphite epoxy open weave woven to a determined open area with the weave having a low profile so as to improve the bonding footprint and provide good aerodynamics
  • the backsheet comprises a graphite epoxy structural laminate.
  • the perforated sheet, the epoxy reticulating adhesive, the bonding of the mesh to the perforated sheet and the bonding of the core to the backsheet are cured at substantially 10 to 45 psi pressure at a temperature of substantially 330 to 350°F. This sequence results in a face sheet with a low profile which is smooth and does not induce aerodynamic drag in the engine which would reduce engine performance.
  • the epoxy reticulating adhesive is a precatalyzed epoxy adhesive in film form which may be applied by reticulative process, such as, for example, Hysol EA9649 or EA9689, which consist of epoxy resin of trifunctional aromatic glycidyl ether and epoxidized novolac and a curative of 4,4'-diaminodiphenyl sulfone.
  • reticulative process such as, for example, Hysol EA9649 or EA9689, which consist of epoxy resin of trifunctional aromatic glycidyl ether and epoxidized novolac and a curative of 4,4'-diaminodiphenyl sulfone.
  • the perforated sheet is sprayed with Minnesota Mining and Manufacturing EC3710 which consists of epoxy resin of a mixture of diglycidy ethers of Bisphenol "A”, epoxy novolacs and multifunctional epoxy resins and a curative of 4,4'- diaminodiphenylsulfone/dicyandiamide.
  • the core is bonded to the backsheet with a film adhesive which may be a precatalyzed epoxy adhesive in film form which may be applied by reticulative process, such as, for example, Hysol EA9649.
  • a film adhesive which may be a precatalyzed epoxy adhesive in film form which may be applied by reticulative process, such as, for example, Hysol EA9649.
  • a method of making an acoustic attenuating liner for an engine cowling comprises the steps of weaving graphite to form a perforated sheet of composite material, prepregging the graphite with an epoxy resin, curing the prepregged graphite into a desired configuration at a temperature of substantially 340°F and a pressure of substantially 45 psi, applying the epoxy reticulating adhesive to the surface of an open weave of the perforated sheet in a manner whereby the holes are left open, placing a honeycomb core of non-metallic material on the surface of the open weave having the adhesive, placing a graphite reinforced composite backsheet on the opposite side of the honeycomb, bonding the backsheet, core and perforated sheet and curing at a temperature of substantially 340°F and a pressure of substantially 45 psi, at least part of the core being bonded to the backsheet at an angle of other than 90° with the backsheet, spraying the open weave with an epoxy adhesive on its opposite surface to a thickness of substantially
  • FIG. 1 is a cross-sectional view of an embodiment of an acoustic attenuating liner of the prior art
  • FIG. 2 is a cross-sectional view of a first embodiment of the acoustic attenuating liner of the invention
  • FIG. 3 is a cross-sectional view of a modification of the embodiment of FIG. 2 of the acoustic attenuating liner of the invention
  • FIG. 4 is a cross-sectional view of a second embodiment of the acoustic attenuating liner of the invention.
  • FIG. 5 is a cross-sectional view of a modification of the embodiment of FIG. 4 of the acoustic attenuating liner of the invention.
  • FIG. 6 is a cross-sectional view of a third embodiment of the acoustic attenuating liner of the invention.
  • FIG. 7 is a cross-sectional view of a modification of the embodiment of FIG. 6 of the acoustic attenuating liner of the invention.
  • the embodiment of the acoustic attenuating liner of the prior art comprises a backsheet 1 of solid aluminum.
  • a corrosion-resistant aluminum honeycomb core 2 such as, for example, the PAA (Reg. Trademark) core of American Cyanamid, is placed on the backsheet 1, which is preferably aluminum, and bonded thereto.
  • the core 2 is further protected against corrosion by dip priming in corrosion-inhibiting adhesive primer, such as, for example, Hysol EA9205 primer, which consists of a trifunctional aromatic glycidyl ether and epoxidized novolac epoxy resin cured with a 4,4 , -diaminodiphenylsulfone/dicyandiamide filled with strontium chromate, all dissolved or suspended in methyl ethyl ketone solvent to provide a sprayable solution.
  • a corrosion- insulated perforated sheet 3 is provided on the honeycomb core 2. the perforated sheet 3 is corrosion-insulated by anodizing the aluminum sheet coupled with the use of a corrosion-inhibiting adhesive primer, such as, for example, Hysol EA9205 primer.
  • the perforated sheet 3 has a per cent open area ranging from 27 to 35% of its surface.
  • the selected opening percentage is uniform over the entire surface area of the perforated sheet 3.
  • Adhesive 4 of a type capable of reticulation is placed between the perforated sheet 3 and the honeycomb core 2 for bonding said perforated sheet to said core.
  • This adhesive is reticulated on the perforated skin to eliminate adhesive blockage of the perforated holes. The only blockage that results is at the intersection of the perforate.
  • Standard reticulation methods result in reticulation of the honeycomb core and the inherent blockage of many holes which will result in decreased acoustic performance of the line.
  • This "reverse reticulation” is accomplished by using a reticulation system designed to move the perforated skin across an air knife at a predetermined rate and air temperature and flow rate. The air knife is designed to fit the contour of the part such that the reticulation adhesive forms uniformly around the holes. The reticulation is performed on the perforated sheet 3 in order to prevent blockage of the holes.
  • the honeycomb core 5 is at an angle of other than 90° with the backsheet 1 whereas in the prior art device, the core 2 is at an angle of 90° with said backsheet.
  • the angular positioning of the core 5 permits said core to attain desired goals, since it provides longer core cells in the same dimension of space between the backsheet 1 and the perforated sheet 3.
  • the angled honeycomb core 5 of the invention decreases the honeycomb resonance frequency.
  • a one inch deep core having a resonant frequency of 2,600 Hz would be reduced to 1,800 Hz when the core is 1.4 inches deep. This angle may be adjusted to meet requirements.
  • a modification of the embodiments of FIGS. 2, 4 and 6, shown in FIGS. 3, 5 and 7, provides part of the core 2 at an angle of 90° with the backsheet 1 and part of the core 5 at an angle of other than 90° with said backsheet. This permits two different frequencies to be attenuated. Different parts of the honeycomb core may be provided at different angles with the backsheet 1 thereby permitting different frequencies of sound to be attenuated.
  • the acoustic attenuating liner of the invention has the same backsheet 1, honeycomb core 5 at an angle of other than 90° with said backsheet and perforated sheet 3 as the first embodiment, shown in FIGS. 2 and 3.
  • the perforated sheet 3 of the second embodiment is preferably corrosion-insulted by anodizing the aluminum sheet in sulfuric acid, followed by priming with a corrosion-inhibiting adhesive primer, such as, for example, Hysol EA9205 primer.
  • a corrosion-inhibiting adhesive primer such as, for example, Hysol EA9205 primer.
  • Another key to improving the acoustic performance is reducing the thickness of the perforated sheet and maintaining the hole size and spacing.
  • a thin skin with a large open area of 27 to 35%, along with the proper wire mesh, will result in a liner which has a low mass reactance factor and low non-linearity factor.
  • a mesh 6 woven from corrosion-resistant metal is placed on the perforated sheet 3.
  • the mesh 6 preferably comprises a stainless steel which has been woven from an alloy which has been drawn in fine wires prior to this and to a weave pattern which has been shown to yield uniformly repeatable acoustic properties, low non-linearity factors and low mass reactance factors.
  • Low non-linearity factors result in liners that are not sensitive to air flow. Grazing flow over the liner surface modifies the liner's orifice discharge coefficient.
  • the particular woven mesh pattern can be altered during the weaving process by removing or adding wires in the warp direction.
  • the mesh 6 may comprise any suitable known corrosion-resistant wire, a preferred wire alloy is that known as 254SMO produced by Avesta of Sweden and is woven in a reverse plain Dutch weave pattern to meet specific flow resistance values.
  • the drawing of thin strands of wire from the alloy 254SMO and weaving this wire into a particular weave for the face skin has a two-fold purpose. The first is for corrosion resistance and the second is for improved acoustic performance.
  • a tightly woven mesh having 700 to 755 strands in the warp direction and 130 and 155 strands in the fill direction is selected because it exhibits excellent flow resistance properties. These properties include low non-linearity factors which aid in yielding good acoustic properties of the bond ent.
  • the acoustic properties may be adjusted by removing or adding strands in the warp direction.
  • an additional adhesive 7 is placed between the mesh 6 and the perforated sheet 3 to bond said mesh to said perforated sheet.
  • the additional adhesive 7 is selected especially, because it has predetermined characteristics, including a minimum viscosity of 1000 poises during subsequent curing.
  • the additional adhesive is preferably a precatalyzed epoxy adhesive in solution form, suitable for application by spray technique, such as, for example, that manufactured by the Minnesota Mining and Manufacturing Company and known as EC3710.
  • the adhesive spray thickness must fall within a designed thickness in order to meet the acoustic requirements, not become blocked, and also have sufficient peel strength to prevent delamination in service.
  • the adhesive spray is applied by robot to a set pattern which will apply a uniform coating in addition to an adhesive of predetermined thickness.
  • the adhesive 7 When it is used to join metals, the adhesive 7 is substantially 0.6 to 0.9 mil in thickness and when it is used to join graphite composites, the adhesive 7 is substantially 1.0 to 1.5 mils in thickness.
  • the additional adhesive is sprayed on by a robot, so that a calculated amount of adhesive thickness is applied to the perforated surface which will provide the correct amount of peel strength and no blockage of the woven wire mesh.
  • part of the core 2 is at an angle of 90° with the backsheet 1 and part of the core 5 is at an angle of other than 90° with said backsheet.
  • the acoustic attenuating liner of the invention comprises a backsheet 8 of solid composite material of any suitable type, preferably reinforced graphite.
  • a honeycomb core 9 of non-metallic material, such as, for example, composite material, such as, for example, NomexTM is bonded to the backsheet 8 with adhesive 10 of any suitable known type.
  • the honeycomb core 9 is at an angle of other than 90° with the backsheet 8.
  • the key to this part of the structure is the reticulating adhesive.
  • the normal acoustic liners have the reticulating adhesive applied to the honeycomb core, which results in blockage of many holes on the perforated skin during the bonding. This blockage results in reduced acoustic performance.
  • a method has been devised whereby the reticulating adhesive is applied on the perforated skin. This is accomplished by utilizing a reticulating system which controls the temperature, airflow rate and sweep time of the perforate over the air knife.
  • the resulting skin has no blocked holes, so that when it is combined with the other components results in an improved liner.
  • a perforated sheet 11 of graphite yarn woven in an open weave and impregnated with epoxy is bonded to the honeycomb core 9 with adhesive 12 of a reticulating type and applied to the open surface by a reticulation process.
  • the open weave material is woven to provide an open area of consistent percentage. This material is woven such that the tows of the graphite weave leave a low profile. This is accomplished by dividing the number of graphite strands into smaller bundles during the weaving process.
  • a low profile perforated skin results which accomplishes two important tasks. The first is that the low profile provides a wider footprint for bonding the wire mesh and provides greater peel strength.
  • the second is that the low profile of the cured perforate is an improved aerodynamic surface which is required for both engine and acoustic performance.
  • the per cent open area may be increased or decreased during the weaving thereof to account for changes in acoustic attenuation requirements.
  • the stainless steel wire mesh 6 of the second embodiment of the invention is bonded to the perforated sheet 11 by the additional adhesive 7 of the second embodiment of the invention.
  • the additional adhesive 7 is controlled to provide good peel strength and low blockage for good acoustic performance.
  • part of the core 13 is at an angle of 90° with the backsheet 1 and part of the core 9 is at an angle of other than 90° with said backsheet.
  • the acoustic attenuating liner of the invention may be made for a jet engine inlet or bypass cowling.
  • graphite is woven to form a perforated sheet.
  • the graphite is then prepregged with epoxy resin and cured into a desired configuration, preferably of a portion of an engine cowling.
  • Adhesive is reticulated onto the surface of an open weave, such that the open area remains the same, and a honeycomb core of non-metallic material is placed on the surface of the open weave having the adhesive.
  • a solid graphite backsheet is placed on the opposite side of the honeycomb core from the open weave. The assembly of the backsheet, the honeycomb core and the perforated sheet is cured in a one-shot autoclave operation.
  • the open weave is sprayed with an additional adhesive, which is the same as the additional adhesive of the second and third embodiments of the invention, on its opposite surface.
  • the assembly is staged in an oven at substantially 210°F for substantially one hour.
  • a stainless steel mesh which is the same as the mesh of the second and third embodiments of the invention, is added to the additional adhesive on the opposite surface of the open weave.
  • the additional adhesive is cured and the mesh is bonded to the previously bonded backsheet, honeycomb core and perforated sheet and all are bonded at a pressure of substantially 45 psi.

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  • Engineering & Computer Science (AREA)
  • Architecture (AREA)
  • Civil Engineering (AREA)
  • Structural Engineering (AREA)
  • Physics & Mathematics (AREA)
  • Acoustics & Sound (AREA)
  • Multimedia (AREA)
  • Laminated Bodies (AREA)

Abstract

An acoustic attenuating liner (figs. 6, 7) has a non-metallic honeycomb core (9) bonded on a backsheet (8). At least part of the honeycomb core (9) is at an angle of other than (90) with the backsheet (8). A corrosion-insulated perforated sheet (11) is bonded to the honeycomb core (9) by adhesive (12) between the perforated sheet and the core. A mesh woven (6) from corrosion-resistant metal is bonded to the perforated sheet (11) by additional adhesive (7) between the mesh (6) and the perforated sheet (11) for bonding the mesh (6) to the perforated sheet (11).

Description

ACOUSTIC ATTENUATING LINER AND METHOD OF MAKING SAME
The present invention relates to an acoustic attenuating liner. More particularly, the invention relates to an acoustic attenuating liner and a method of making the same.
The engines of older aircraft have acoustic liners in the engine inlet duct which are used for attenuating specific noise spectra. These liners reduce the engine noise to an extent which permits the aircraft to meet government specified noise regulations. However, there are many airports which have implemented, or are attempting to implement, lower noise level regulations for the communities around them.
Redesign of the acoustic treatment of an engine nacelle is costly. Thus, if a lower frequency attenuation is required, for example, the entire structure must be modified. Efforts are made to remove the existing acoustic treatment and to substitute therefor an improved treatment in the same physical location. However, since a lower frequency attenuation requires material having a deeper core, this is impossible. It is thus of critical importance to design an acoustic treatment for older engines that can provide a lower frequency attenuation to fit within a specific depth. This is also useful for the acoustic attenuating lines of new engines.
The principal object of the invention is to provide an acoustic attenuating liner which will fit within a specific depth and will provide a lower frequency acoustic attenuation. An object of the invention is to provide an acoustic attenuating liner which will fit within a specific depth and will provide acoustic attenuation of more than one frequency.
Another object of the invention is to provide an acoustic attenuating liner which will fit within a specific depth and will provide acoustic attenuation of two different frequencies.
Still another object of the invention is to provide an acoustic attenuating liner of simple structure which functions efficiently, effectively and reliably to provide lower frequency reduction of the noise of a jet engine.
Yet another object of the invention is to provide an acoustic attenuating liner which is lightweight, durable and corrosion-resistant and will provide a lower frequency acoustic attenuation.
Another object of the invention is to provide an acoustic attenuating liner which is suitable for use with all types of aircraft and engines, including jet engines, combustion engines, turbines and APU systems, and provides a lower frequency acoustic attenuation in an area of predetermined dimensions whereby said acoustic attenuating liner may be installed as a replacement for existing acoustic attenuating liners.
In accordance with the invention, an acoustic attenuating liner comprises a backsheet. A honeycomb core on the backsheet is bonded thereto, wherein at least part of the core is at an angle of other than 90° with the backsheet. A perforated sheet is provided in the honeycomb core and adhesive between the perforated sheet and the core bonds the perforated sheet to the core.
In a modification of the invention, part of the core is at an angle of 90° with the backsheet, and part of the core is at an angle other than 90° with the backsheet.
A corrosion-insulated perforated sheet is provided on the honeycomb core. An adhesive is provided between the perforated sheet and the core for bonding the perforated sheet to the core. A mesh woven to a determined weave pattern from a corrosion- resistant metal is placed on the perforated sheet. The perforated sheet is reticulated. An additional adhesive is provided between the mesh and the perforated sheet for bonding the mesh to the perforated sheet. The additional adhesive has predetermined characteristics including a minimum viscosity of 1000 poises during curing and a predetermined thickness.
The backsheet comprises aluminum, the perforated sheet comprises anodized aluminum, the mesh comprises corrosion-resistant stainless steel wire, and the honeycomb core comprises aluminum in a modification of the invention.
The honeycomb core comprises non-metallic material and the perforated sheet comprises a graphite epoxy open weave woven to a determined open area with the weave cross-section flattened to provide a bonding surface and smooth aerodynamics in another modification of the invention. Furthermore, part of the core is at an angle of 90° with the backsheet and part of the core is at an angle of other than 90° with the backsheet.
In accordance with the invention, a method of making an acoustic attenuating liner comprises the steps of weaving a perforated sheet, forming a honeycomb core, bonding the perforated sheet to the core, forming a backsheet and bonding at least part of the core to the backsheet at an angle of other than 90° with the backsheet.
In the modification, part of the core is bonded to the backsheet at an angle of 90° with the backsheet and part of the core is bonded to the backsheet at an angle of other than 90° with the backsheet.
In accordance with the invention, a method of making an acoustic attenuating liner comprises the steps of weaving a perforated sheet of composite material, impregnating the perforated sheet of composite material with a resin, curing the perforated sheet of composite material with heat and pressure, forming a honeycomb core of composite material, reticulating the perforated sheet with an epoxy reticulating adhesive comprising a precatalyzed epoxy adhesive in film form which may be applied to the perforated sheet by reticulative process and curing with heat and pressure to bond the perforated sheet of composite material to the core, forming a backsheet of composite material, bonding at least part of the core to the backsheet at an angle of other than 90° with the backsheet with an epoxy supported film adhesive and curing with heat and pressure, weaving a mesh of corrosion-resistant material having acoustic properties which meet desired acoustic resistance values, spraying the perforated sheet with an epoxy adhesive to a thickness of substantially 1.0 to 1.5 mils, staging the epoxy adhesive on the perforated sheet to ensure a minimum viscosity of 1000 poises during subsequent curing and bonding the mesh to the perforated sheet and curing with heat and pressure.
The mesh is woven in a reverse plain Dutch weave pattern, the perforated sheet comprises a graphite epoxy open weave woven to a determined open area with the weave having a low profile so as to improve the bonding footprint and provide good aerodynamics and the backsheet comprises a graphite epoxy structural laminate.
The perforated sheet, the epoxy reticulating adhesive, the bonding of the mesh to the perforated sheet and the bonding of the core to the backsheet are cured at substantially 10 to 45 psi pressure at a temperature of substantially 330 to 350°F. This sequence results in a face sheet with a low profile which is smooth and does not induce aerodynamic drag in the engine which would reduce engine performance. The epoxy reticulating adhesive is a precatalyzed epoxy adhesive in film form which may be applied by reticulative process, such as, for example, Hysol EA9649 or EA9689, which consist of epoxy resin of trifunctional aromatic glycidyl ether and epoxidized novolac and a curative of 4,4'-diaminodiphenyl sulfone.
The perforated sheet is sprayed with Minnesota Mining and Manufacturing EC3710 which consists of epoxy resin of a mixture of diglycidy ethers of Bisphenol "A", epoxy novolacs and multifunctional epoxy resins and a curative of 4,4'- diaminodiphenylsulfone/dicyandiamide.
The core is bonded to the backsheet with a film adhesive which may be a precatalyzed epoxy adhesive in film form which may be applied by reticulative process, such as, for example, Hysol EA9649.
In accordance with the invention, a method of making an acoustic attenuating liner for an engine cowling comprises the steps of weaving graphite to form a perforated sheet of composite material, prepregging the graphite with an epoxy resin, curing the prepregged graphite into a desired configuration at a temperature of substantially 340°F and a pressure of substantially 45 psi, applying the epoxy reticulating adhesive to the surface of an open weave of the perforated sheet in a manner whereby the holes are left open, placing a honeycomb core of non-metallic material on the surface of the open weave having the adhesive, placing a graphite reinforced composite backsheet on the opposite side of the honeycomb, bonding the backsheet, core and perforated sheet and curing at a temperature of substantially 340°F and a pressure of substantially 45 psi, at least part of the core being bonded to the backsheet at an angle of other than 90° with the backsheet, spraying the open weave with an epoxy adhesive on its opposite surface to a thickness of substantially 1.0 to 1.5 mils whereby the peel strength is adequate and the mesh remains unblocked, staging the perforated sheet in an oven, adding a stainless steel mesh to the epoxy adhesive on the opposite surface of the open weave, and bonding the mesh to the perforated sheet and curing at a temperature of substantially 340°F and a pressure of substantially 45 psi.
In order that the invention may be readily carried into effect, it will now be described with reference to the accompanying drawings, wherein:
FIG. 1 is a cross-sectional view of an embodiment of an acoustic attenuating liner of the prior art;
FIG. 2 is a cross-sectional view of a first embodiment of the acoustic attenuating liner of the invention;
FIG. 3 is a cross-sectional view of a modification of the embodiment of FIG. 2 of the acoustic attenuating liner of the invention;
FIG. 4 is a cross-sectional view of a second embodiment of the acoustic attenuating liner of the invention;
FIG. 5 is a cross-sectional view of a modification of the embodiment of FIG. 4 of the acoustic attenuating liner of the invention;
FIG. 6 is a cross-sectional view of a third embodiment of the acoustic attenuating liner of the invention; and FIG. 7 is a cross-sectional view of a modification of the embodiment of FIG. 6 of the acoustic attenuating liner of the invention.
The embodiment of the acoustic attenuating liner of the prior art, as shown in FIG. 1, comprises a backsheet 1 of solid aluminum. A corrosion-resistant aluminum honeycomb core 2, such as, for example, the PAA (Reg. Trademark) core of American Cyanamid, is placed on the backsheet 1, which is preferably aluminum, and bonded thereto. The core 2 is further protected against corrosion by dip priming in corrosion-inhibiting adhesive primer, such as, for example, Hysol EA9205 primer, which consists of a trifunctional aromatic glycidyl ether and epoxidized novolac epoxy resin cured with a 4,4,-diaminodiphenylsulfone/dicyandiamide filled with strontium chromate, all dissolved or suspended in methyl ethyl ketone solvent to provide a sprayable solution. A corrosion- insulated perforated sheet 3 is provided on the honeycomb core 2. the perforated sheet 3 is corrosion-insulated by anodizing the aluminum sheet coupled with the use of a corrosion-inhibiting adhesive primer, such as, for example, Hysol EA9205 primer.
The perforated sheet 3 has a per cent open area ranging from 27 to 35% of its surface. The selected opening percentage is uniform over the entire surface area of the perforated sheet 3.
Adhesive 4 of a type capable of reticulation is placed between the perforated sheet 3 and the honeycomb core 2 for bonding said perforated sheet to said core. This adhesive is reticulated on the perforated skin to eliminate adhesive blockage of the perforated holes. The only blockage that results is at the intersection of the perforate. Standard reticulation methods result in reticulation of the honeycomb core and the inherent blockage of many holes which will result in decreased acoustic performance of the line. This "reverse reticulation" is accomplished by using a reticulation system designed to move the perforated skin across an air knife at a predetermined rate and air temperature and flow rate. The air knife is designed to fit the contour of the part such that the reticulation adhesive forms uniformly around the holes. The reticulation is performed on the perforated sheet 3 in order to prevent blockage of the holes.
The acoustic attenuating liner of FIG. 1 and other embodiments and a method of making same are described in United States patent No. 5,151,311.
In accordance with the invention, as shown in FIGS. 2, 4 and 6, the honeycomb core 5 is at an angle of other than 90° with the backsheet 1 whereas in the prior art device, the core 2 is at an angle of 90° with said backsheet. The angular positioning of the core 5 permits said core to attain desired goals, since it provides longer core cells in the same dimension of space between the backsheet 1 and the perforated sheet 3. The angled honeycomb core 5 of the invention decreases the honeycomb resonance frequency. Thus, when the honeycomb core 5 is at an angle of 45° with the backsheet 1, for example, a one inch deep core having a resonant frequency of 2,600 Hz would be reduced to 1,800 Hz when the core is 1.4 inches deep. This angle may be adjusted to meet requirements. A modification of the embodiments of FIGS. 2, 4 and 6, shown in FIGS. 3, 5 and 7, provides part of the core 2 at an angle of 90° with the backsheet 1 and part of the core 5 at an angle of other than 90° with said backsheet. This permits two different frequencies to be attenuated. Different parts of the honeycomb core may be provided at different angles with the backsheet 1 thereby permitting different frequencies of sound to be attenuated.
In the second embodiment of FIGS. 4 and 5, the acoustic attenuating liner of the invention has the same backsheet 1, honeycomb core 5 at an angle of other than 90° with said backsheet and perforated sheet 3 as the first embodiment, shown in FIGS. 2 and 3. The perforated sheet 3 of the second embodiment is preferably corrosion-insulted by anodizing the aluminum sheet in sulfuric acid, followed by priming with a corrosion-inhibiting adhesive primer, such as, for example, Hysol EA9205 primer. Another key to improving the acoustic performance is reducing the thickness of the perforated sheet and maintaining the hole size and spacing. A thin skin with a large open area of 27 to 35%, along with the proper wire mesh, will result in a liner which has a low mass reactance factor and low non-linearity factor. In addition, in the second embodiment, a mesh 6 woven from corrosion-resistant metal is placed on the perforated sheet 3. The mesh 6 preferably comprises a stainless steel which has been woven from an alloy which has been drawn in fine wires prior to this and to a weave pattern which has been shown to yield uniformly repeatable acoustic properties, low non-linearity factors and low mass reactance factors. Low non-linearity factors result in liners that are not sensitive to air flow. Grazing flow over the liner surface modifies the liner's orifice discharge coefficient. Low mass reactance factors of liners has been demonstrated to improve the acoustic properties. The particular woven mesh pattern can be altered during the weaving process by removing or adding wires in the warp direction. Although the mesh 6 may comprise any suitable known corrosion-resistant wire, a preferred wire alloy is that known as 254SMO produced by Avesta of Sweden and is woven in a reverse plain Dutch weave pattern to meet specific flow resistance values. The drawing of thin strands of wire from the alloy 254SMO and weaving this wire into a particular weave for the face skin has a two-fold purpose. The first is for corrosion resistance and the second is for improved acoustic performance. A tightly woven mesh having 700 to 755 strands in the warp direction and 130 and 155 strands in the fill direction is selected because it exhibits excellent flow resistance properties. These properties include low non-linearity factors which aid in yielding good acoustic properties of the bond ent. The acoustic properties may be adjusted by removing or adding strands in the warp direction.
In the second embodiment of the invention, an additional adhesive 7 is placed between the mesh 6 and the perforated sheet 3 to bond said mesh to said perforated sheet. The additional adhesive 7 is selected especially, because it has predetermined characteristics, including a minimum viscosity of 1000 poises during subsequent curing. More particularly, the additional adhesive is preferably a precatalyzed epoxy adhesive in solution form, suitable for application by spray technique, such as, for example, that manufactured by the Minnesota Mining and Manufacturing Company and known as EC3710. The adhesive spray thickness must fall within a designed thickness in order to meet the acoustic requirements, not become blocked, and also have sufficient peel strength to prevent delamination in service. The adhesive spray is applied by robot to a set pattern which will apply a uniform coating in addition to an adhesive of predetermined thickness. When it is used to join metals, the adhesive 7 is substantially 0.6 to 0.9 mil in thickness and when it is used to join graphite composites, the adhesive 7 is substantially 1.0 to 1.5 mils in thickness. The additional adhesive is sprayed on by a robot, so that a calculated amount of adhesive thickness is applied to the perforated surface which will provide the correct amount of peel strength and no blockage of the woven wire mesh.
In the modification of FIG. 5, part of the core 2 is at an angle of 90° with the backsheet 1 and part of the core 5 is at an angle of other than 90° with said backsheet.
In the third embodiment of FIGS. 6 and 7, the acoustic attenuating liner of the invention comprises a backsheet 8 of solid composite material of any suitable type, preferably reinforced graphite. A honeycomb core 9 of non-metallic material, such as, for example, composite material, such as, for example, Nomex™ is bonded to the backsheet 8 with adhesive 10 of any suitable known type. The honeycomb core 9 is at an angle of other than 90° with the backsheet 8. The key to this part of the structure is the reticulating adhesive. The normal acoustic liners have the reticulating adhesive applied to the honeycomb core, which results in blockage of many holes on the perforated skin during the bonding. This blockage results in reduced acoustic performance. Instead, a method has been devised whereby the reticulating adhesive is applied on the perforated skin. This is accomplished by utilizing a reticulating system which controls the temperature, airflow rate and sweep time of the perforate over the air knife. The resulting skin has no blocked holes, so that when it is combined with the other components results in an improved liner.
In the third embodiment of FIGS. 6 and 7 of the invention, a perforated sheet 11 of graphite yarn woven in an open weave and impregnated with epoxy is bonded to the honeycomb core 9 with adhesive 12 of a reticulating type and applied to the open surface by a reticulation process. The open weave material is woven to provide an open area of consistent percentage. This material is woven such that the tows of the graphite weave leave a low profile. This is accomplished by dividing the number of graphite strands into smaller bundles during the weaving process. A low profile perforated skin results which accomplishes two important tasks. The first is that the low profile provides a wider footprint for bonding the wire mesh and provides greater peel strength. The second is that the low profile of the cured perforate is an improved aerodynamic surface which is required for both engine and acoustic performance. The per cent open area may be increased or decreased during the weaving thereof to account for changes in acoustic attenuation requirements. The stainless steel wire mesh 6 of the second embodiment of the invention is bonded to the perforated sheet 11 by the additional adhesive 7 of the second embodiment of the invention. The additional adhesive 7 is controlled to provide good peel strength and low blockage for good acoustic performance. In the modification of FIG. 7, part of the core 13 is at an angle of 90° with the backsheet 1 and part of the core 9 is at an angle of other than 90° with said backsheet.
The acoustic attenuating liner of the invention may be made for a jet engine inlet or bypass cowling. In the method of making an acoustic attenuating liner of the invention for an engine cowling, graphite is woven to form a perforated sheet. The graphite is then prepregged with epoxy resin and cured into a desired configuration, preferably of a portion of an engine cowling. Adhesive is reticulated onto the surface of an open weave, such that the open area remains the same, and a honeycomb core of non-metallic material is placed on the surface of the open weave having the adhesive. A solid graphite backsheet is placed on the opposite side of the honeycomb core from the open weave. The assembly of the backsheet, the honeycomb core and the perforated sheet is cured in a one-shot autoclave operation.
The open weave is sprayed with an additional adhesive, which is the same as the additional adhesive of the second and third embodiments of the invention, on its opposite surface. In the second and third embodiments of the invention, the assembly is staged in an oven at substantially 210°F for substantially one hour.
A stainless steel mesh, which is the same as the mesh of the second and third embodiments of the invention, is added to the additional adhesive on the opposite surface of the open weave. The additional adhesive is cured and the mesh is bonded to the previously bonded backsheet, honeycomb core and perforated sheet and all are bonded at a pressure of substantially 45 psi.
Although shown and described in what are believed to be the most practical and preferred embodiments, it is apparent that departures from the specific method and design described and shown will suggest themselves to those skilled in the art and may be made without departing from the spirit and scope of the invention. I, therefore, do not wish to restrict myself to the particular construction described and illustrated, but desire to avail myself of all modifications that may fall within the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:
1. An acoustic attenuating liner comprising
a backsheet;
a honeycomb core on said backsheet and bonded thereto, at least part of said core being at an angle of other than 90° with said backsheet;
a perforated sheet on said honeycomb core; and
adhesive between said perforated sheet and said core for bonding said perforated sheet to said core.
2. An acoustic attenuating liner as claimed in claim 1, wherein said backsheet comprises aluminum and said perforated sheet comprises anodized aluminum.
3. An acoustic attenuating liner as claimed in claim 1, wherein said perforated sheet has a per cent open area ranging from 27 to 35% of its surface and a maximum thickness of .025 inch for good mass reactance.
4. An acoustic attenuating liner as claimed in claim 1, wherein said honeycomb core is corrosion-resistant and said perforated sheet is corrosion-insulated.
5. An acoustic attenuating liner as claimed in claim 1, wherein said honeycomb core comprises aluminum.
6. An acoustic attenuating liner as claimed in claim 1, wherein said honeycomb core comprises non-metallic material.
7. An acoustic attenuating liner as claimed in claim 1, wherein said perforated sheet comprises a graphite epoxy weave woven to a determined pattern which provides low non-linearity factors and a weave pattern which permits resistance changes for tuning the final resistance of said liner.
8. An acoustic attenuating liner as claimed in claim 7, wherein said perforated sheet comprises a graphite epoxy. open weave woven to a determined open area with the weave cross-section flattened to provide a bonding surface and smooth aerodynamics.
9. An acoustic attenuating liner as claimed in claim 1, wherein part of said core is at an angle of 90° with said backsheet and part of said core is at an angle of other than 90° with said backsheet.
10. An acoustic attenuating liner comprising
a backsheet;
a corrosion-resistant honeycomb core on said backsheet and bonded thereto, at least part of said core being at an angle of other than 90° with said backsheet;
a corrosion-insulated perforated sheet on said honeycomb core; adhesive between said perforated sheet and said core for bonding said perforated sheet to said core;
a mesh woven to a determined weave pattern from corrosion-resistant metal on said perforated sheet, said perforated sheet being reticulated; and
additional adhesive between said mesh and said perforated sheet for bonding said mesh to said perforated sheet, said additional adhesive having predetermined characteristics including a minimum viscosity of 1000 poises during curing and a predetermined thickness.
11. An acoustic attenuating liner as claimed in claim 10, wherein said backsheet comprises aluminum, said perforated sheet comprises anodized aluminum and said perforated sheet has a per cent open area ranging from 27 to 35% of its surface and a maximum thickness of .025 inch for good mass reactance.
12. An acoustic attenuating liner as claimed in claim 10, wherein said mesh comprises stainless steel wire, said honeycomb core comprises aluminum and said perforated sheet comprises a graphite epoxy weave woven to a determined pattern which provides low non-linearity factors and a weave pattern which permits resistance changes for tuning the final resistance of said liner.
13. An acoustic attenuating liner as claimed in claim 10, wherein said honeycomb core comprises non-metallic material.
14. An acoustic attenuating liner as claimed in claim 10, wherein part of said core is at an angle of 90° with said backsheet and part of said core is at an angle of other than 90° with said backsheet.
15. A method of making an acoustic attenuating liner, said method comprising the steps of
weaving a perforated sheet;
forming a honeycomb core;
bonding said perforated sheet to said core;
forming a backsheet; and
bonding at least part of said core to said backsheet at an angle of other than 90° with said backsheet.
16. A method as claimed in claim 15, wherein part of said core is bonded to said backsheet at an angle of 90° with said backsheet and part of said core is bonded to said backsheet at an angle of other than 90° with said backsheet.
17. A method of making an acoustic attenuating liner, said method comprising the steps of
weaving a perforated sheet of composite material; impregnating said perforated sheet of composite material with a resin;
curing said perforated sheet of composite material with heat and pressure;
forming a honeycomb core of composite material;
reticulating said perforated sheet with an epoxy reticulating adhesive comprising a precatalyzed epoxy adhesive in film form applicable to said perforated sheet by reticulative process and curing with heat and pressure;
bonding said perforated sheet of composite material to said core;
forming a backsheet of composite material;
bonding at least part of said core to said backsheet at an angle of other than 90° with said backsheet with an epoxy supported film adhesive and curing with heat and pressure;
weaving a mesh of corrosion-resistant material having acoustic properties which meet desired acoustic resistance values;
spraying said perforated sheet with an epoxy adhesive to a thickness of substantially 1.0 to 1.5 mils; staging said epoxy adhesive on said perforated sheet to ensure a minimum viscosity of 1000 poises during subsequent curing; and
bonding saidmesh to said perforated sheet and curing with heat and pressure.
18. A method as claimed in claim 17, wherein said mesh is woven in a reverse plain Dutch weave pattern, said perforated sheet comprises a graphite epoxy weave woven to a determined open area with the weave having a low profile so as to improve the bonding footprint and provide good aerodynamics, said perforated sheet comprises a graphite epoxy open weave and said backsheet comprises graphite epoxy structural laminate.
19. A method as claimed in claim 17, wherein said perforated sheet, said epoxy reticulating adhesive, the bonding of said mesh to said perforated sheet and the bonding of said core to said backsheet are cured at substantially 10 to 45 psi pressure at a temperature of substantially 330 to 350°F and said core is bonded to said backsheet with a film adhesive comprising a precatalyzed epoxy adhesive in film form applicable by reticulative process.
20. A method of making an acoustic attenuating liner for an engine cowling, said method comprising the steps of
weaving graphite to form a perforated sheet of composite material; prepregging said graphite with an epoxy resin;
curing the prepregged graphite into a desired configuration at a temperature of substantially 340°F and a pressure of substantially 45 psi;
applying an epoxy reticulating adhesive comprising a precatalyzed epoxy adhesive in film form applicable by reticulative process to the surface of an open weave of said perforated sheet in a manner whereby the holes are left open;
placing a honeycomb core of non-metallic material on the surface of said open weave having said adhesive;
placing a graphite reinforced composite backsheet on the opposite side of said honeycomb;
bonding said backsheet, core and perforated sheet and curing at a temperature of substantially 340°F and a pressure of substantially 45 psi, at least part of said core being bonded to said backsheet at an angle of other than 90° with said backsheet;
spraying said open weave with an epoxy adhesive on its opposite surface to a thickness of substantially 1.0 to 1.5 mils whereby the peel strength is adequate and the mesh remains unblocked;
staging said perforated sheet in an oven; adding a stainless steel mesh to said epoxy adhesive on said opposite surface of said open weave; and
bonding saidmesh to said perforated sheet and curing at a temperature of substantially 340°F and a pressure of substantially 45 psi.
EP94916684A 1993-05-06 1994-05-05 Acoustic attenuating liner and method of making same Pending EP0698159A1 (en)

Applications Claiming Priority (3)

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US5886793A 1993-05-06 1993-05-06
US58867 1993-05-06
PCT/US1994/005062 WO1994026994A1 (en) 1993-05-06 1994-05-05 Acoustic attenuating liner and method of making same

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US6176964B1 (en) * 1997-10-20 2001-01-23 Vought Aircraft Industries, Inc. Method of fabricating an acoustic liner
DE10238460B3 (en) * 2002-08-22 2004-03-11 Airbus Deutschland Gmbh Lightweight structure made of thin sheet metal layers
ATE365120T1 (en) * 2004-06-25 2007-07-15 Ford Global Tech Llc COVERING ELEMENT FOR A VEHICLE
US10196979B2 (en) 2014-12-02 2019-02-05 United Technologies Corporation Acoustic panel repair methods and apparatus
US11719160B2 (en) 2020-02-03 2023-08-08 Rohr, Inc. Acoustic liner and method of forming same
CN113878886B (en) * 2021-06-17 2022-08-09 中国航空工业集团公司济南特种结构研究所 Composite material silencing nacelle sound lining maintenance method

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