In an advanced society like ours we all depend on composite materials in some aspect of our lives. Fibreglass, developed in the late 1940s, was the first modern composite and is still the most common. It makes up about 65 per cent of all the composites produced today and is used for boat hulls, surfboards, sporting goods, swimming pool linings, building panels and car bodies.
Most composites are made up of just two materials. One material (the matrix or binder) surrounds and binds together a group of threads or remains of a much stronger material (the reinforcementIn fibreglass, the reinforcement is provided by fine threads or fibres of glass, often woven into a sort of cloth, and the matrix is a plastic.
The threads of glass in fibreglass are very strong under tension but they are also brittle and will snap if bent sharply. The matrix not only holds the fibres together, it also protects them from damage by sharing any stress among them. The matrix is soft enough to be shaped with tools, and can be softened to allow repairs to be made. It is also quite light, which is an advantage in many applications.
Over recent decades many new composites have been developed, some with very valuable properties. By carefully choosing the reinforcement, the matrix, and the manufacturing process that brings them together, engineers can modify the properties to meet specific requirements. They can, for example, make the composite sheet very strong in one direction by aligning the fibres that way, but weaker in another direction where strength is not so important. They can also select properties such as resistance to heat, chemicals, and weathering by choosing an appropriate matrix material.
Making an object from a composite material usually involves some form of mould. The reinforcing material is first placed in the mould and then semi-liquid matrix material is sprayed or pumped in to form the object. Pressure may be applied to force out any air bubbles, and the mould is then heated to make the matrix set solid.
The moulding process is often done by hand, but automatic processing by machines is becoming more common. One of the new methods is called pultrusion (a term derived from the words 'pull' and 'extrusion'). This process is ideal for manufacturing products that are straight and have a constant cross section, such as bridge beams.
In many thin structures with complex shapes, such as curved panels, the composite structure is built up by applying sheets of woven fibre reinforcement, saturated with the plastic matrix material, over an appropriately shaped base mould. When the panel has been built to an appropriate thickness, the matrix material is then cured.
The greatest advantage of composite materials is strength and firmness combined with lightness. By choosing an appropriate combination of reinforcement and matrix material, manufacturers can produce properties that exactly fit the requirements for a particular structure for a particular purpose.
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