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It’s easy to get confused about composites. Wet lay-ups, vacuum
bagging, auto-claves, pre-preg, pultrusions – even the language seems like it
comes from a different planet! But in this series, courtesy of SP Composites,
we’ll give you a complete grounding in the technology.
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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. Today, the most common man-made composites can be divided into
three main groups:
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Polymer Matrix Composites
(PMC) – These are the most common and will be discussed here. 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
(Kevlar) as the reinforcement.
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Ceramic Matrix Composites
(CMC) - 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.
Polymer Matrix
Composites
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 PMC 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
something of the fibres on their own.
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. 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 an 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 FVF is approximately 30-40%. With the higher quality, more
sophisticated and precise processes used in the aerospace industry, FVF’s
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 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. (But clearly if these properties are NOT accounted for, you’ve got big
problems! – Ed)
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 almost nothing 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.
Loading
There are four main direct loads that any material in a structure
has to withstand: tension, compression, shear and flexure.
Tension - this diagram shows a tensile load applied to a
composite. The response of a composite to tensile loads is very dependent on the
tensile stiffness and strength properties of the reinforcement fibres, since
these are far higher than the resin system on its own.
Compression - this diagram shows a composite under a
compressive load. Here, the adhesive and stiffness properties of the resin
system are crucial, as it is the role of the resin to maintain the fibres as
straight columns and to prevent them from buckling.
Shear - this diagram shows a composite experiencing a shear
load. This load is trying to slide adjacent layers of fibres over each other.
Under shear loads the resin plays the major role, transferring the stresses
across the composite. For the composite to perform well under shear loads, the
resin element must not only exhibit good mechanical properties but must also
have high adhesion to the reinforcement fibre. The interlaminar shear strength
(ILSS) of a composite is often used to indicate this property in a multilayer
composite (‘laminate’).
Flexure - flexural loads are really a combination of
tensile, compression and shear loads. When loaded as shown, the upper face is
put into compression, the lower face into tension and the central portion of the
laminate experiences shear.
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Comparison with Other
Structural Materials
Due to the factors described above,
there is a very large range of mechanical properties that can be achieved with
composite materials. Even when considering one fibre type on its own, the
composite properties can vary by a factor of 10 with the range of fibre
contents and orientations that are commonly achieved. The comparisons that
follow therefore show a range of mechanical properties for the composite
materials. The lowest properties for each material are associated with simple
manufacturing processes and material forms (e.g. spray lay-up glass fibre), and
the higher properties are associated with higher technology manufacture (e.g.
autoclave moulding of unidirectional glass fibre prepreg), such as would be
found in the aerospace industry.
For the other materials
shown, a range of strength and stiffness (modulus) figures is also given to
indicate the spread of properties associated with different alloys, for example.
Tensile Strength of
Common Structural Materials
Tensile Modulus (ie Stiffness) of
Common Structural Materials
The above figures clearly show the
range of properties that different composite materials can display. These
properties can best be summed up as high strengths and stiffnesses combined with
low densities. It is these properties that give rise to the characteristic high
strength and stiffness to weight ratios that make composite structures ideal for
so many applications. This is particularly true of applications which involve
movement, such as cars, trains and aircraft, since lighter structures in such
applications play a significant part in making these applications more
efficient. The strength and stiffness to weight ratio of composite materials can
best be illustrated by the following graphs that plot ‘specific’ properties.
These are simply the result of dividing the mechanical properties of a material
by its density (ie mass per volume). Generally, the properties at the higher end
of the ranges illustrated in the previous graphs are produced from the highest
density variant of the material. The spread of specific properties shown in the
following graphs takes this into account.
Densities of Common
Structural Materials
Specific Tensile
Modulus Specific Tensile Modulus of Common Structural Materials
Specific Tensile
Strength Specific Tensile Strength of Common Structural Materials
Next week: the
different resins
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Stress
versus strain – these concepts are very important for a good understanding of
some of the above graphs. See Making Things, Part 6
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