Note: Descriptions are shown in the official language in which they were submitted.
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MEANS FOR ATTENUATING SOUND ENERGY,
AND METHOD OF MANUFACTURE THEREOF
BACKGROUND OF THE INVENTION
Field of the Inventlon
This invention relates to means for attenuating sound
energy, more especially but not exclusively noise
attenuation panels (otherwise called acoustic panels),
and to methods of making such means.
D cription of the Prior Art
It has previously been proposed (see British Patent
Application No. 2056367A) to use for attenuation of
noise energy over a wide range of noise frequencies a
so called l'linear" acoustic panel having a layer
structure as follows:
i. an imperforate backing layer;
ii. an apertured layer, being a sheet which
has been so manufactured that it has a multitude of
through apertures, the apertures being of a
predetermined and ordered size and spacing thereby to
establish an open area characteristic of the layer;
iii. a cellular layer secured between the
imperforate and apertured layers, a first end of all of
the cells being covered by the imperforate layer and a
second opposed end being closed by the apertured layer,
a plurality of the apertures communicating with each of
the cells which thereby constitutes a Helmholtz resonant
cavity; and
iv. a porous layer adhered to the surface of
the apertured layer opposite the surface to which the
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cellular layer is adhered and providing th~ external
surface of the panel upon which air-borne noise i5
incident, the size of ~he pores of the porous layer
being substantially smaller than the apertures in the
apertured layer.
SUMMARY OF THE INVENTION
In such a panel, the honeycomb and the porous layer
together provide the required wide range of noise
attenuation and the apertured layer provides support for
the other layers and is a structural member by which the
panel is fastened to adjacent structure of the aircraft.
Performance of the panel is impaired by any substantial
change from a predetermined proportion of the surface
area of the apertured layer which is open as between the
porous layer and the cells of the cellular layer.
It is not easy to bond together the four layers of such
a panel in such a way ~hat, on the one hand, there is
minimal likelihood of delamination of the panel in use
and yet, on the other hand, a bonding step does not
occlude so many of the perforations and/or pores that
the characteristic open area of the apertured layer is
lost and the efficiency of attenuation of sound is
impaired. It is one aim of the present invention to
ameliorate these difficulties.
It is another aim to provide a panel which is less heavy
than equivalent prior art panels.
It is another object to provide a panel construction,
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which facilita-tes construction o-E panels oE complex shape.
The invention provides a facing sheet for an acous-tic panel, the
sheet comprising: i) an apertured layer, being a sheet which
defines a multi-tude of through apertures of a predetermined and
ordered size and spacing thereby to establish an open area charac-
teristic of the layer; and ii) a porous layer adhered to a surface
of the aperture~ layer, the size of the pores therein being sub-
s-tantially smaller than that of the apertures in the apertured
layer: characterised in that the apertured layer is formed from a
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carbon fibre/resin matrix composite material which is an open
weave material in which the apertures are constituted by the
hexagonal openings between three sets of carbon threads mutually
arranged at 60 to one another and in that the porous layer is of
stainless steel.
A number of advantages stem from such use of a composite material,
in place of the prior art material which is aluminium.
Firstly, the apertures can be formed while the resin matrix of the
composite is in a part-cured condition. This is likely to be a
far cheaper procedure than forming a like number of perforations
in a metal sheet.
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Secondly, the resin matrix of the composite coulrl itself
provide an adhesive medium by which the porous la~er
could be adhered to the aper-tured layer.
Thirdly, the fibrous component of the compo~ite material
could be arranged in a pattern, e.g. by weaviny which in
itself provides the requlred multitude of apertures, so
removing any necessity for a separate aperture-~orming
step.
Fourthly, a particular problem with electrolytic
corrosion is eliminated. One prior art panel has an
apertured layer of aluminium and a porous layer composed
of a stainless steel woven mesh or felt. Should these
two layers be in electrical contact, electrolytic
corrosion could occur in damp conditions. Prior art
fabrication methods may include special steps to
minimize the risk that in the finished assembly some
part of the mesh or felt is in contact with the
underlying apertured layer. A carbon ibre/epoxy resin
composite material has substantially the same electrode
potential as an overlying stainless steel porous layer,
so the li~elihood of electrolytic corrosion, and the
need to take special steps during assembly to prevent
contact between the porous layer and the apertured
layer, are both avoided.
It may be convenient to employ as a method of making a
noise attenuation means in accordance with the invention
a method characterised by the step of securing the
composite material layer to an adjacent one of said
layers by holding the adjacent layer in contact with the
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composite material layer at a -time when said resin is
other than fully-cured and curing the resin with said
two layers held in contact, whereby the resin secures
the two layers together. The resin will probably be a
so-called "controlled flow" resin, that is to say, a
resin with a viscosity which is high enough that the
resin remains jelly-like, duriny the early stages of its
cure, rather than fully fluid. rt may be convenient to
provide a "peel-ply" layer, as described below, to
absorb surplus resin.
In previously proposed panels, it is the apertured layex
which is used to secure the panel to the adjacent
supporting structure. In one embodiment of the present
invention, the imperforate backing layer is so used.
The present Applicants have noted that it is desirable
to secure in the apertured layer a proportion of open
aperture area, relative to the projected surface area of
the layer, of around 30~. Such a hish percentage of the
surface area open, and a requirement to keep the panel
as light as possible, detracts ~rom the suitability of
the apertured layer for mounting the panel to supporting
structure.
~5 BRIEF DESCRIPTION OF THE D~AWING
An embodiment of the present invention will now be
described, by way of example, with reference to the
accompanying drawings, in which:
Figure 1 is a section through an edge region of a noise
attenuating, acoustic panel, transverse to the plane of
the panel;
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Figure 2 is a plan view o~ part of the apertured la~er
of the panel of Figure l, drawn to a larger scale and
wi~h -the porous layer removed for clari~y bu-t with the
outline included of one of the hexagonal cells of -the
underlying cellular layer; and
Figure 3 is a section through a facing sheet of the
acoustic panel of Figures 1 and 2 during manufacture
thereof, with an associated forming tool and vacuum bag.
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DESCRIPTION OF A PREFERRED EMBODIMENT
With reference to Figure 1, the panel 10 comprises an
imperforate, metallic backing layer 11 of aluminium
alloy to which is secured by epoxy resin adhesive El a
cellular honeycomb layer 12 in which each cell has at
leask one drainage slot 18. To the other end of the
honeyco~b cells is secured by epoxy adhesive E2 an
apertured layer 13, described in more dekail below. To
the outside surface of the layer 13 is bonded a
stainless steel mesh 14 of fine wire. The apertured
layer 13 and mesh i4, taken together, are termed herein
the facing sheet of the panel 10.
The panel 10 provides part of the inward-facing surface
of a duct of a nose cowl for a turbo-fan aero engine,
the panel being one of several arcuate panels disposed
just upstream of the fan of the engine~ It is therefore
of extreme importance that the panel does not
deteriorate in use and, in particular, that no part of
it becomes detached from its supporting structure. This
structure comprises supporting flanges or stringers of
which only one stringer 15 is shown. By bonding means
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known to -those skilled in the ~rt, the panel 10 is
seeured -to the stringer, but it is to be noted that i~
is the backing sheet 11 o the panel, and not the
apertured layer 13, which is secured to a panel-mountiny
S flange 16 of the stringer 15. The gap 17 between the
external surface of the panel 10 and the surrounding
structure may be left open or could be sealed or closed
for example by use of a mastic.
Referring now to Fiyure 2, the apertured layer 13 is
formed from a woven material of three sets, 20, 21 and
22, of threads each composed of a multitude of carbon
fibres F. The three sets 20 t 21 and 22 are arranyed to
be at 60 to one another so as to provide a multitude of
apertures H of hexagonal shape. As can be seen from the
superimposed outline C of one of the honeycomb cells of
the cellular layer 12, there are a large number of
apertures H for each one of the open ends of the cells.
C.
Uncured resin R surrounds the fibres F in the threads of
the woven material but does not occlude the apertures H.
During manufacture of the panel 10, the resin R is cured
in an autoclave with the apertured layer 13 supported
and arranged in the shape required in the finished
panel, which may be a double curvature~
Providing las a so-called 'Ipeel-ply") a layer of woven
NYLON fabric on each side of the apertured layer 13
: 30 during the curing step will not only remove any "flash"
of adhesive from within the apertures but will also
apply the pattern of the weave to the surface of the
sheet so that adhesive, subsequently applied, will key
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to it. Preferably, however, -the outer layer 14 is held
pressed into contact with the apertured layer 13 duriny
the curing step, so that the resin serves to adhere
together the two layers of the Eacing sheet of the panel
5 10.
One specific manner of carrying such a method into
effect will now be described with reference to E'igure 3
of the drawings.
A former, herein referred to as a tool, 30 has a forming
surface 31 which has the shape of part of the panel 10
is cleaned, conveniently using methyl ethyl ketone, and
then receives two~coats 32 of a mould release agent,
such a Frekote 33 from Frekote Inc. of U.S.A., with a
period of air drying between coats. The agent is then
cured at 121C for 30 minutes.
An area 33 of 720/150 (this designation indicates the
number of wires per inch of fabric length, in warp and
weft respectively) stainless s-teel mesh (such as that
available from G. Bopp & Co. Ltd. of London N2, England
under the designation "Robusta weave") sufficient to
cover the working area of the tool 30 is subjected to a
vigorous vapour de-greasing treatment. It is then
draped, using great care to avoid the introduction o~
wrinkles into the mesh, on to the tool 30 and is secured
in position by tape (not shown~ applied to its
periphery. Where the tool 30 has a double curvature,
slitting of the mesh is acceptable provided it is in
positions laid down by design and production engineering
specirications. It may be desirable to provide a film
of a suitable adhesive around the periphery of the mesh
3~
and/or around any slit areas of the mesh in order to
improve adhesion between it and the fibre/resin
composite to be subsequently laid over the mesh.
The apertures P hetween the warp and weft strands of the
mesh 33 corlstitute the pores of this porous layer.
Next, a sufficient area 34 of an open--weave carbon fibre
fabric, such as is shown in F.igure 2, impregnated with a
co-curing resin system, is carefully laid over the mesh
33; The fabric carries as a "peel-ply" a layer of NYLON
fabric 34P on the surface of the carbon fibre fabric 34
opposite to the steel mesh 33.
Curing of the resin system is carried out under reduced
pressure in an au~oclave 29. The reduced pressure is
secured by overlaying the tool 30 and layer structure
33, 34 thereon with a NYLON vacuum bagging membrane 35
in the manner shown in Figure 3, and as now to be
described.
Around the periphery of the fibre fabric 34 is provided
a band 36 of a gas-permeable breather material, the band
having a width of around 2-5 cms. A thin release film
37 suitable for use at elevated temperature is applied
to the exposed surface of the uncured fibre fabric 34,
to overlap the band 36 by about l to l.S cms. O~erlying
the release film 37 and band 36 i5 a breather sheet 33
of the same material as forms the band 36. The vacuum
~mbrane 35 envelops the uncured workpiece and breather
overlying it. Around thQ periphery of the membrane 35,
where it meets the tool 30 is a sealing strip 35S ~f a
high temperature mastic sealant.
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Next, a test for leaks in the sealiny of the vacuum
membrane 35 and tool 30 is carried out by employing a
vacuum pump 40 to reduce the pressure within ~he bag
formed by -the membrane to 55 cms. o~ mercury~ as
indicated, on a gauge 41, sealing the vacuum line 42 at
~ cock 43 and monitoring any pressure rise within the
bag. A rise of not more than 12 cms. Hg in 5 minutes is
acceptable.
Next, with the air within the bag still under reduced
pressure, the autoclave cure is commenced. The ambient
pressure in the autoclave and external of the bag is 3.1
0.35 bar. When the pressure within the bag rises to
1.4 + 09 the bag is vented to atmosphere and,
thereafter, it is maintained during the remainder of the
curing txeatment at 0 + 0.o5 bar.
As to temperatures, these are controlled, by a
temperature controller 44 which receives temperature
data from a thermocouple 45 positioned adjacent to the
workpiece, to limit the rate of temperature rise in the
workpiece to a range of from 1 to 2.5C/min., up to a
curing temperature of 177 + 5C which is held for a
period of 120 + O mins. Thereafter the workpiece is
allowed to cool at a rate of 1.5C/min. to belvw 60C,
the autoclave pressure is released and the workpiece
removed from it~
As an alternative to slitting of the mesh 33, a complex
shape o double curvature could be provided by weaving
the mesh on a former of the required shape or by welding
~ogether at their peripheries a number of s~parate
pieces of the mesh.
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Other methods are envisaged. The aper-tures might be
formed after weaving but before cur1ng by moviny the
woven threads and res~raining them in their new
positions to provide the required apertures. The porous
layer might be attached to the apertured l~yer after the
latter has been cured to shape.
Referring to Figure 1, an unapertured (but nevertheless
usually woven) composite material for layer 13 might be
cured to shape and only afterwards might the apertwres
be provided, possibly by perforating the layer 13, the
outer layer 14 being attached subsequently. However,
some of the benefits of the invention would not be
gained by use of this procedure.
A sheet of the composite material could, while still
uncured or part-cured, be perforated when flat, and then
cured to shape in contact with the porous layer so as to
bond thereto.
The cured facing sheet is then adhered to one face of
the cellular layer 12, and an aluminium or fibre/resin
composite backing layer 11 is adhered to the other face,
both with an epoxy adhe~ive, as ~hown a~ E2 and El
respectively in Figure 1. Adherance of the facing sheet
is assisted ~y the pattern of the weave of the peel-ply
which remains on the surface of the cured resin o~ the
- facing sheetO The cellular layer 12 can be NOMEX, a
NYLON paper material, from CIBA-GEIGY (see hereinafterl.
The ce]l size is such that the largest circle which can
be inscribed within each cell has a diameter of 9 mm.
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The epox~ adhesives used were ob-tained Erom CIB~~GEIG~
Plastlcs and Additives Co. Limited of Cambridge,
England, and the triaxial weave, carbon fibre material
from Barber-Colman Co. of Rockford, Illinoi~, U.S.A.
Hexcel (U.K.) Limited of Liyhtwater, Surrey, England
impregnate the fabric with their F593 controlled flow
resin, and provide the impregnated fabric interleaved
between sheets of polythene. The porous woven layer i~.
from the British company, G. ~opp as aforesaid. As an
alternative to the Bopp material there can be used the
sintered s$ainless steel fibre mesh product sold as
' "BRUNS~T" by Brunswick Corporation of Dehand, Florida,
U~ S.A. Alternative woven carbon fibre materials are
available from Brochier Industrie of Villeurbanne,
France.
Non-metalllc porous layers are envisaged, e.g. woven
meshes of a polyester material, and would have the
advantage that they are of lower weight than an
equivalent metal mesh. Adhesives and resins need not be
of epoxy but could be, for example, a ph~nolic or
polyimide resin. Carbon fibres could be replaced by
other fibres provided they have sufficiently good
mechanical properties. Glass fibres and KE~JLAR are
believed to offer performarlce inferior to carbon fibre
for this application.
