Design
Philosophy
Imagine for a minute that composites do
not exist and then consider what one would need from a material
for any product, which has to work in an aggressive environment
such as the sea, for example. This material we seek needs
to be easily shaped, it has to be happy in a hot or cold salty
environment and ideally it needs little equipment to turn
it from a raw material into a product. It would also help
if the material is low in weight, relatively inexpensive and
can be tailored with regards to strength and stiffness. And
by the way, as we may be making a car or an expensive yacht,
we would like the material to be any colour and glossy.
It is easy now to see why composites (fibre reinforced polymers
or plastics) have become the mainstay material in the marine
designers’ drawer of materials.
However, a designer in the 1940’s would
now be extremely surprised to see the proliferation of fibres,
resins, sandwich core materials and manufacturing processes,
which abound in the composites industry. Since the first boat
was made in glass reinforced polyester resin, now some sixty
years ago, we have transgressed to high impact resistance
aramid fibres (Kevlar) to high strength and stiffness carbon
fibres using heat cured resin systems.
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History
Composite materials have been used
since man found that mud could be used to strengthen
straw in constructing useful items. Since that time
composite materials have played a part in technological
development from bridges to satellites.The first composite
material that was used in volume combining the properties
of a resin and a reinforcing fibre was in the 1950’s
with fibre glass commonly known as GRP or Glass Reinforced
Plastic. From that time the composites industry as we
know it was born.
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Not
that this progress has been without problems. Most people
know about blistering, no more so than the unfortunate boat
owner whose boat is looking like a bad attack of mini-mumps.
Also, because it is a material that requires little in the
way of equipment, (a bucket and a brush) it has been so
often used by the inexperienced, leading to poor products
and unfortunately, problems. This gave the material a bad
press on many occasions. But progress has been made and
composites are now used widely in almost every application.
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Advantages
of Composites
There
are a growing number of products that have developed
as a result of the properties of composites. One of
the critical factors is its strength to weight ratio.
Often the products that have a noticable increase in
performance are at the leading edge of product development
in their own field. Sporting activities where teams
and individuals are seeking an edge often provides the
context for development. In a similar way development
times and costs are not the most important issue when
developing safety equipment and products for the military.
Thes fields have all been a fertile area for composites
use.From Skis to Formula One cars Aircraft production
to artificial limbs anywhere where strengh, weight,
durability, and processability |
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Because
of the variation in strength and stiffness of the fibres,
an immediate advantage can be seen – it is possible to
‘engineer’ the required strength or stiffness and the
direction in which these properties are required.
So
let us take a look at the real benefits of composites
and basically how they work.
Fibre
reinforced polymers are what this says – a fibre of some
sort held within a resin matrix. The most common fibres
are glass, aramid (Kevlar) and carbon. The most common
resins are polyester, vinyl ester and epoxy. Phenolic
resins are also available and incidentally, the oldest
type. They have better fire resistance, but because they
are more difficult to use,
they
are not common within the marine industry.
The
fibres may be random or directional.
Glass
reinforced polyester is the cheapest and the most widely
used composite. The basic manufacturing process is simple
– a bucket of mixed resin (resin, accelerator and catalyst),
a brush to apply the resin and some fibre. The more sophisticated
manufacturing methods now include resin infusion, where
the resin is drawn into a closed mould under vacuum. The
mould already contains the fibre, in thicknesses and direction
to suit the load or stiffness requirement. Whole boat
hulls are now made by this method – one-shot manufacture.
At
the more expensive end, and the higher property end also,
we have carbon and aramid fibres. These are often in pre-impregnated
(pre-preg) form, that is the fibre has been coated with
a heat curing resin. To prevent curing before use, the
pre-preg is kept at low temperature.
It
is this huge range of fibres, resins, manufacturing processes
and supporting sandwich core materials, which give composites
the real advantage over other materials. It is also the
real advantage that is not always immediately appreciated
by those new to the material.
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Manufacturing Processes
Taking composite materials as a whole,
there are many different material options to choose from
in the areas of resins, fibres and cores, all with their
own unique set of properties such as strength, stiffness,
toughness, heat resistance, cost, production rate etc..
However, the end properties of a composite part produced
from these different materials is not only a function
of the individual properties of the resin matrix and fibre
(and in sandwich structures, the core as well), but is
also a function of the way in which the materials themselves
are designed into the part and also the way in which they
are processed.
Cost Issues
A misconceived disadvantage is the
apparent high cost of the higher strength and stiffness
carbon and aramid fibres. This tends to make designers
believe that the end product will be expensive when compared
to glass reinforced polyester or even metals. But, carbon
and aramid have much improved strength and stiffness over
glass fibre. Aramid is also very tough. Furthermore, when
the specific strength and stiffness (ie property divided
by material density) is compared to metals, the composites
are significantly better. More strength or stiffness per
kilogram of material. Also less weight means less material.
The higher material costs are then compensated.
If we now consider the fact that the
moulded surface will be very smooth and fair (as opposed
to look of welded aluminium alloy which requires filling
and fairing), the fact that the material will not degrade
in the salty environment and there will be no painting
required, we will equate costs to the more conventional
metal structure. Despite therefore the apparent high raw
material cost, we end up with a cost-effective product.It
is this philosophy of composite material selection that
is used by the experienced designers to create many of
the other well-designed and engineered products in composites.
However, a small word of warning – if the materials are
not fully understood and they are used by inexperienced
designers, errors can be dramatic. When they are used
correctly, composites can be shown to be the designers’
path to the optimum structure and composites are very
happy to be used in a wide variety of applications.
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