The most widely used composite material in tactical aircraft
is a carbon fibre/epoxy mix.Carbon epoxy has eclipsed boron
based composites because it is much cheaper to produce,easier
to machine and drill, and can be formed into complex shapes
to produce structural members such as spars and ribs. Other
fibers typified by Kevlar also are being used in aircraft production.
Kevlar is less dense than carbon fibres but has inferior mechanical
properties. It is used in pressure vessels, for ballistic protection
and as lightweight fibreglass non-structural parts. Composites
have displaced conventional materials such as aluminum because
they have several advantages. They have lower density and
greater strength and stiffness than aluminum, therefore a smaller
lighter structure can carry the same load. Studies conducted by
Boeing indicate that a 38 per cent composite structural weight
can result in a 40 per cent reduction in empty weight, 39 per
cent reduction in wing area and a 33 per cent fuel saving for
the same mission profile when compared to an aircraft of
conventional metal structure. Another big advantage is that
composites are relatively insensitive to flaws. Fatigue testing
of composite structures demonstrated their high resistance to
cracking and that fractures generally do not propagate.
Composite materials are very stable and so are not subject
to corrosion as aremetallic structures. However, in the design
process, careful attention must be paid to composite-metal
interaction because through galvanic action some metals will
corrode when in contact with carbon fibre/resin laminate.
10.1.08
RTM RESIN TRANSFER MOLDING
Resin transfer molding (RTM) as a composites processing
technique is currently of great interest to the aerospace
industry for high performance, advanced composite parts due
primarily to its design flexibility and economics. For these
applications, high performance resin systems, such as epoxies
and bismaleimides, are required, but generally have high
viscosities at room temperature or are solids. To overcome
these processing difficulties, an epoxy resin and a bismaleimide
formulation have been developed, which have more desirable
processing conditions and wider processing envelopes for RTM
than conventional high performance resin systems.
technique is currently of great interest to the aerospace
industry for high performance, advanced composite parts due
primarily to its design flexibility and economics. For these
applications, high performance resin systems, such as epoxies
and bismaleimides, are required, but generally have high
viscosities at room temperature or are solids. To overcome
these processing difficulties, an epoxy resin and a bismaleimide
formulation have been developed, which have more desirable
processing conditions and wider processing envelopes for RTM
than conventional high performance resin systems.
9.1.08
THERMOPLASTIC COMPOSITE MATERIALS
Fibrereinforced thermoplastic composites produced by melting
resins and combining them with re-inforcing fibres under high
pressure in a mold, are another promising new area. Tests have
shown these composites to be highly resistant to damage,able
to be reshaped and quickly fabricated. Compared to carbon
epoxy, fibre-reinforced thermoplastics are equal in density,
equivalent in strength and part production may be less expensive.
The USAF and several other air arms, material suppliers and a
multitude of contractors are developing thermoplastic composites.
resins and combining them with re-inforcing fibres under high
pressure in a mold, are another promising new area. Tests have
shown these composites to be highly resistant to damage,able
to be reshaped and quickly fabricated. Compared to carbon
epoxy, fibre-reinforced thermoplastics are equal in density,
equivalent in strength and part production may be less expensive.
The USAF and several other air arms, material suppliers and a
multitude of contractors are developing thermoplastic composites.
CENTERS OF EXCELLENCE FOR COMPOSITES IN AEROSPACE
Centres of excellence for composites in
aerospace can be divided into:
aerospace can be divided into:
- Material suppliers.
- Part manufacturers.
- Aircraft manufacturers.
- National research centres.
- Universities.
been an extensive consolidation of the industry in recent years. Toray and Tenax /
Toho should be mentioned as carbon fibre manufacturers, whilst Hexcel and Cytec
Fiberite are the biggest producers of prepregs. However, several other companies
are well established with their own individual products.Independent part manufacturers include, for example, Fischer FACC (Austria),Composite Aquitaine (France), MAN(Germany), Fokker Special Products (The Netherlands), Sonaca (Belgium) and Gamesa (Spain). These organisations all run their own development projects and have specific expertise and fields of
application.The largest aircraft manufacturers with competence in composite development and
manufacturing are EADS (Airbus, Military Aircraft, Eurocopter, Astrium), Dassault (France), SAAB (Sweden), Alenia, Agusta Westland, Pilatus and Diamond Aircraft (Austria).
8.1.08
3 MANUFACTURING METHODS USING COMPOSITE MATERIALS
Filament winding, fiber placement, and tape laying are three known methods for applying unidirectional composite fibers to a rotating mandrel to form a continuous cylindrical skin.
In a filament winding process, the mandrel is typically suspended horizontally between end supports. The mandrel rotates about the horizontal axis as a fiber application instrument moves back and forth along the length of the mandrel, placing fiber onto the mandrel in a predetermined configuration. In most applications, the filament winding apparatus passes the fiber material through a resin "bath" just before the material touches the mandrel. This is called "wet winding." In other applications, the fiber has been preimpregnated with resin, eliminating the need for the resin bath. Following oven or autoclave curing of the resin, the mandrel can remain in place and become part of the wound component, or it can be removed.
The fiber placement process typically involves the automated placement of multiple "tows" (i.e., untwisted bundles of continuous filaments, such as carbon or graphite fibers, preimpregnated with a thermoset resin material such as epoxy) tape, or slit tape onto a rotating mandrel at high speed. A typical tow is between about 0.12'' and 0.25'' wide when flattened. Conventional fiber placement machines dispense multiple tows to a movable payoff head that collimates the tows (i.e., renders the tows parallel) and applies the tows to the rotating mandrel surface using one or more compaction rollers that compress the tows against the surface. In addition, such machines typically include means for dispensing, clamping, cutting and restarting individual tows during placement.
Tape laying is similar to the fiber placement process described above except that preimpregnated fiber tape, rather than individual tows, is laid down on a flat or contoured tool (e.g., a stationary or rotating mandrel) to form the part. One form of tape includes a paper backing that maintains the width and orientation of the fibers. The paper backing is removed during application. Slit tape is tape that has been slit after being produced in standard widths by the manufacturer. Slitting the tape results in narrower widths that allow enhanced stearability and tailoring during application to achieve producibility and design objectives. Slit tape can have widths varying from about 0.12 inch up to about 6 inches, and may or may not include backing paper. Another form of tape includes multiple individual fibers woven together with a cloth material. As used throughout this disclosure, unless otherwise indicated, the term "tape" refers to tape, tape with backing paper, slit tape, and other types of composite material in tape form for use in manufacturing composite structures. Tape laying is often used for parts with highly complex contours or angles because the tape allows relatively easy directional changes.
In a filament winding process, the mandrel is typically suspended horizontally between end supports. The mandrel rotates about the horizontal axis as a fiber application instrument moves back and forth along the length of the mandrel, placing fiber onto the mandrel in a predetermined configuration. In most applications, the filament winding apparatus passes the fiber material through a resin "bath" just before the material touches the mandrel. This is called "wet winding." In other applications, the fiber has been preimpregnated with resin, eliminating the need for the resin bath. Following oven or autoclave curing of the resin, the mandrel can remain in place and become part of the wound component, or it can be removed.
The fiber placement process typically involves the automated placement of multiple "tows" (i.e., untwisted bundles of continuous filaments, such as carbon or graphite fibers, preimpregnated with a thermoset resin material such as epoxy) tape, or slit tape onto a rotating mandrel at high speed. A typical tow is between about 0.12'' and 0.25'' wide when flattened. Conventional fiber placement machines dispense multiple tows to a movable payoff head that collimates the tows (i.e., renders the tows parallel) and applies the tows to the rotating mandrel surface using one or more compaction rollers that compress the tows against the surface. In addition, such machines typically include means for dispensing, clamping, cutting and restarting individual tows during placement.
Tape laying is similar to the fiber placement process described above except that preimpregnated fiber tape, rather than individual tows, is laid down on a flat or contoured tool (e.g., a stationary or rotating mandrel) to form the part. One form of tape includes a paper backing that maintains the width and orientation of the fibers. The paper backing is removed during application. Slit tape is tape that has been slit after being produced in standard widths by the manufacturer. Slitting the tape results in narrower widths that allow enhanced stearability and tailoring during application to achieve producibility and design objectives. Slit tape can have widths varying from about 0.12 inch up to about 6 inches, and may or may not include backing paper. Another form of tape includes multiple individual fibers woven together with a cloth material. As used throughout this disclosure, unless otherwise indicated, the term "tape" refers to tape, tape with backing paper, slit tape, and other types of composite material in tape form for use in manufacturing composite structures. Tape laying is often used for parts with highly complex contours or angles because the tape allows relatively easy directional changes.
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