Composites have two distinct advantages over conventional
materials: light weight in the form of high specific properties, and excellent
corrosion resistance. The specific properties (stiffness or strength divided by
specific gravity) of composites are much greater than other engineering
materials. These benefits are self-evident with even glass-reinforced materials
having properties equivalent to, or greater than, steel.
One note of caution, however, is that the high specific
properties are only in the direction of reinforcement and stiffness and strength
in other directions can be markedly lower. Here lies one of the complications,
and one of the greatest challenges, with composites - anisotropy and
heterogeneity. This arises when the properties of a material vary with direction
and although it is possible to put fibres precisely where they are needed, i.e.
in the direction of applied loading, this may be difficult in some cases.
Indeed, there may be insufficient knowledge regarding the loads in the first
instance. Design with composites therefore requires much more property data and
is inherently more complex than design with more traditional materials.
Corrosion resistance is another outstanding attribute,
particularly those with a polymeric matrix. The table indicates the range of
media where the materials have been used successfully Scott
Bader 1986. To be successful, special attention must be given to the
details of the design to preclude ingress of fluids into the composite laminate
where it may have a deleterious effect on the interlace between matrix and
reinforcement and on the properties of the reinforcement itself.
Chemical Resistance of GRP composites
|
Acids |
Sulphuric
Hydrochloric
Nitric*
Phosphoric |
|
Alkalis |
Sodium Hydroxide
Potassium Hydroxide |
|
Aqueous salts |
Ammonium, calcium, sodium, potassium salts |
|
Oxidising agents |
Chlorine
Sodium hypochloride |
|
Organics |
Alchohols, some acids, glycols, Aviation and diesel
fuels |
Tabulated information is indicative only. Reference must
be made
to technical data sheets pertaining to individual resin systems
Flexibility of manufacture is also a unique characteristic.
Large, complex structures can be fabricated in one piece minimising tooling
costs and obviating the need for joints and fastenings. The fact that during
manufacture the material itself is being made at the same time as the component
gives rise to tremendous scope for innovative design without the constraints of
conventional metal-forming or machining processes. Other significant advantages
may include:
- Thermal properties. It is possible to
design components with very low, even zero, thermal expansivity, a key factor
in controlling dimensional tolerances. Also thermal conductivities are
generally low which may be of significance in certain applications.
- Fatigue. Certain composites, particularly
those with carbon reinforcement have excellent fatigue characteristics - a
clear benefit in many areas.
- Wear resistance. Again, carbon fibre
materials behave well due to the formation of a graphitic layer as wear
begins.
- Electrical properties. GRP is an
excellent electric insulator. Electromagnetic characteristics can be
especially valuable for a certain range of structures.
- Finish. Because of the flexibility
available during manufacture finishes such as pigmentation and textures can be
easily incorporated.
In terms of disadvantages, there are two that require
particular attention: Lack of ductility and
inspectability. By and large whenever a ductile material is filled
with reinforcement, be it in particulate or fibre form, a loss in toughness will
result. This leads to a stress/strain curve, which is essentially linear to
failure. Composites, therefore, are less forgiving materials than, say, metals,
where stress concentrations and peak loadings can often be accommodated by
redistribution as a result of local yielding. Great care must be taken with
composites to ensure that all loadings are accounted for in the initial design.
Coping with this can be made more difficult by the effects of anisotropy.
The difficulty with inspectability is not so much associated
with the technique, although the heterogeneity of many composites can cause
difficulties, but the scale of the required task. Metals can be procured in part
finished form, e.g. plate, rod or tube, and supplied with a certificate of
conformity which provides evidence of quality. Subsequent component inspection,
over and above that required to check manufacturing tolerance, etc., can be
limited to welds and joints. Composite components on the other hand require
inspection over the whole surface, which can lead to a different scale of
problem altogether. Whilst this issue is by no means insurmountable, quality
assurance and inspection procedures must feature in the initial feasibility
assessment. It should be emphasised that composites are not a panacea to all
problems. An advantage for one application may be a disadvantage for another and
often a compromise may be the most appropriate course of action. The point which
can be made, however, is that the added flexibility offered by composites,
together with the inherent advantages of light weight, corrosion resistance,
etc., they possess over other materials, ensures that they must feature in the
first rank of materials' options available to the engineer.
-
'Maximum
working temperatures for crystic polyester resins', Scott Bader
