Introduction
To fully appreciate the role and application
of composite materials to a structure, an understanding
is required of the component materials themselves and of
the ways in which they can be processed. This guide looks
at basic composite theory, properties of materials used,
various processing techniques commonly found and applications
of composite products.
In
its most basic form a composite material is one which is
composed of at least two elements working together to produce
material properties that are different to the properties
of those elements on their own. In practice, most composites
consist of a bulk material (the 'matrix'), and a reinforcement
of some kind, added primarily to increase the strength and
stiffness of the matrix. This reinforcement is usually in
fibre form.
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The
most common man-made composites can be divided into three
main groups:
Polymer Matrix Composites (PMC's) These are the
most common and will the main area of discussion in this
guide. Also known as FRP - Fibre Reinforced Polymers
(or Plastics) - these materials use a polymer-based resin
as the matrix, and a variety of fibres such as glass,
carbon and aramid as the reinforcement.
Metal Matrix Composites
(MMC's) - Increasingly found in the automotive industry,
these materials use a metal such as aluminium as the matrix,
and reinforce it with fibres such as silicon carbide.
Ceramic Matrix Composites (CMC's) - Used in very
high temperature environments, these materials use a ceramic
as the matrix and reinforce it with short fibres, or whiskers
such as those made from silicon carbide and boron nitride. |
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Resin systems such as epoxies and polyesters
have limited use for the manufacture of structures on their
own, since their mechanical properties are not very high
when compared to, for example, most metals. However, they
have desirable properties, most notably their ability to
be easily formed into complex shapes.
Materials
such as glass, aramid and boron have extremely high tensile
and compressive strength but in 'solid form' these properties
are not readily apparent. This is due to the fact that when
stressed, random surface flaws will cause each material
to crack and fail well below its theoretical 'breaking point'.
To overcome this problem, the material is produced in fibre
form, so that, although the same number of random flaws
will occur, they will be restricted to a small number of
fibres with the remainder exhibiting the material's theoretical
strength. Therefore a bundle of fibres will reflect more
accurately the optimum performance of the material. However,
fibres alone can only exhibit tensile properties along the
fibre's length, in the same way as fibres in a rope.
It
is when the resin systems are combined with reinforcing
fibres such as glass, carbon and aramid, that exceptional
properties can be obtained. The resin matrix spreads the
load applied to the composite between each of the individual
fibres and also protects the fibres from damage caused by
abrasion and impact. High strengths and stiffnesses, ease
of moulding complex shapes, high environmental resistance
all coupled with low densities, make the resultant composite
superior to metals for many applications.
Since
Polymer Matrix Composites combine a resin system and reinforcing
fibres, the properties of the resulting composite material
will combine something of the properties of the resin on
its own with that of the fibres on their own.
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Overall,
the properties of the composite are determined by:
·The
properties of the fibre
·The
properties of the resin
·The ratio of fibre to resin in the composite (Fibre
Volume Fraction)
·The geometry and orientation of the fibres in the
composite |
The first two will be dealt with
in more detail later. The ratio of the fibre to resin
derives largely from the manufacturing process used
to combine resin with fibre, as is described in the
section on manufacturing processes. However, it is also
influenced by the type of resin system used, and the
form in which the fibres are incorporated. In general,
since the mechanical properties of fibres are much higher
than those of resins, the higher the fibre volume fraction
the higher will be the mechanical properties of the
resultant composite. In practice there are limits to
this, since the fibres need to be fully coated in resin
to be effective, and there will be optimum packing of
the generally circular cross-section fibres. In addition,
the manufacturing process used to combine fibre with
resin leads to varying amounts of imperfections and
air inclusions. Typically, with a common hand lay-up
process as widely used in the boat-building industry,
a limit for Fibre Volume Fraction is approximately 30-40%.
With the higher quality, more sophisticated and precise
processes used in the aerospace industry, Fibre Volume
Fractions approaching 70% can be successfully obtained.
The
geometry of the fibres in a composite is also important
since fibres have their highest mechanical properties
along their lengths, rather than across their widths.
This leads to the highly anisotropic (having different
properties in different directions) properties of composites,
where, unlike metals, the mechanical properties of the
composite are likely to be very different when tested
in different directions. This means that it is very
important when considering the use of composites to
understand at the design stage, both the magnitude and
the direction of the applied loads. When correctly accounted
for, these anisotropic properties can be very advantageous
since it is only necessary to put material where loads
will be applied, and thus redundant material is avoided.
It
is also important to note that with metals the properties
of the materials are largely determined by the material
supplier, and the person who fabricates the materials
into a finished structure can do little to change those
'in-built' properties. However, a composite material
is formed at the same time as the structure is itself
being fabricated. This means that the person who is
making the structure is creating the properties of the
resultant composite material, and so the manufacturing
processes they use have an unusually critical part to
play in determining the performance of the resultant
structure.
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