Composite materials: combining for new properties
Design & Technology · Key Stage 3 · The School
Two materials, one new set of properties
A COMPOSITE is made from two distinct materials, combined so the result performs better than either alone. Usually one material gives strength, often as fibres or strands. The other, called the MATRIX, holds those fibres in place and passes force between them.
Fibreglass, worked through
FIBREGLASS mixes glass fibres, strong but brittle on their own, with a plastic resin matrix, flexible but not very strong on its own. Together, the glass fibres carry the load. The resin holds them in shape, and stops a crack in one fibre spreading through the whole piece. Neither material alone does the job as well as the pair does together.
Plywood is a composite too
PLYWOOD glues layers of thin wood together. Each layer's grain runs at right angles to the layer below it. Solid wood is strong along the grain, but weak across it. Plywood's alternating layers give it real strength in every direction, which solid wood of the same thickness simply cannot match.
A composite's strength is not the same in every direction — it depends on which way the reinforcing fibres run. A sheet of fibreglass with all its fibres running one way is very strong along that direction and much weaker across it, exactly like solid wood along versus across the grain. This is why plywood alternates its layers at right angles: each layer covers the direction the one below it is weak in. Designers choosing or specifying a composite must think about which direction the actual load in the finished product will run, and orient the fibres or layers to match — getting this wrong can leave a composite structurally weaker than a much simpler, single material would have been.