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Composite materials
(or composites for short) are engineered materials made from two or more
constituent materials that remain separate and distinct on a macroscopic level
while forming a single component.
There are two categories of
constituent materials: matrix and reinforcement. At least one portion (fraction)
of each type is required. The matrix material surrounds and supports the
reinforcement materials by maintaining their relative positions. The
reinforcements impart special physical (mechanical and electrical) properties to
enhance the matrix properties. A synergism produces material properties
unavailable from naturally occurring materials. Due to the wide variety of
matrix and reinforcement materials available, the design potential is
incredible.
Glass fibre roving strands
There are the so-called natural
composites like bone and wood. Both of these are constructed by the processes of
nature and beyond the scope of this text. Engineered composite materials must be
formed to shape. This involves strategically placing the reinforcements while
manipulating the matrix properties to achieve a melding event at or near the
beginning of the component life cycle. A variety of methods are used according
to the end item design requirements. These fabrication methods are commonly
named moulding or casting processes, as appropriate, and both have numerous
variations.
The principle factors impacting
the methodology are the natures of the chosen matrix and reinforcement
materials. Another important factor is the gross quantity of material to be
produced. Large quantities can be used to justify high capital expenditures for
rapid and automated manufacturing technology. Small production quantities are
accommodated with lower capital expenditures but higher labour costs at a
correspondingly slower rate.
Many commercially produced
composites use a polymer matrix material often called a resin or resin solution.
There are many different polymers available depending upon the starting raw
ingredients. There are several broad categories, each with numerous variations.
The most common categories are known as polyester, vinyl ester, epoxy, phenolic,
polyimide, polyamide, and others. The reinforcement materials are often fibres
but also commonly ground minerals. Fibres are often transformed into a textile
material such as a felt, fabric, knit or stitched construction.
One component is often a strong
fibre such as fibreglass, kevlar, or carbon fibre that gives the
material its tensile strength, while another component (called a matrix)
is often a resin such as polyester, or epoxy that binds the fibres together,
transferring load from broken fibres to unbroken ones and between fibres that
are not oriented along lines of tension. Also, unless the matrix chosen is
especially flexible, it prevents the fibres from buckling in compression. Some
composites use an aggregate instead of, or in addition to, fibres.
In terms of stress, any fibres
serve to resist tension, the matrix serves to resist shear, and all materials
present serve to resist compression, including any aggregate.Composite materials can be divided
into two main categories normally referred to as short fibre reinforced
materials and continuous fibre reinforced materials. Continuous reinforced
materials will often constitute a layered or laminated structure.
Aramat
- a mixture of Kevlar
and glass fibre
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