The big idea: A composite consists of two or more materials combined to enhance their properties.
One is the matrix — continuous, gives shape and surface. The other is the reinforcement — carries the load. Each supplies what the other lacks.
Each constituent tested alone and failing, then combined. That is the definition doing work.
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Glass-reinforced plastic, step by step
Resin alone
Rigid, mouldable into any shape, gives a surface — and cracks under very little load. It has form and no strength.
Glass fibre alone
Very high tensile strength for its weight — far stronger than the resin. But a bundle of loose fibres has no stiffness in compression, no surface and no shape.
Cured together
The resin transfers load into the fibres and stops them buckling; the fibres stop the resin cracking. The composite has the strength of one and the form of the other.
Better than either
That is what "enhance their properties" means — the composite outperforms both constituents, not just the weaker one.
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| Composite | Matrix | Reinforcement | Used for |
|---|---|---|---|
| GRP | Polyester resin | Glass fibre | Boat hulls, shower trays |
| Carbon-fibre composite | Epoxy resin | Carbon fibre | Bike frames, aircraft parts |
| Plywood | Adhesive | Cross-bonded veneers | Stiff in both directions |
| Reinforced concrete | Concrete | Steel bars | Strong in tension as well |
What it costs: Composites buy an exceptional strength-to-weight ratio and properties tuned by fibre direction.
They cost more per kilogram, are labour-intensive, fail suddenly rather than bending first, and are very hard to recycle — the constituents cannot easily be separated again.
How this is tested — explaining why combining materials creates a more suitable material, using an example. It comes up two ways:
Paper 1 — multiple choice
- Identify the matrix or reinforcement in a named composite.
- Pick the property a composite gains over its constituents.
Paper 2 — analysing a product
- Explain why a composite suits a named product.
- Explain a disadvantage of using a composite.
The trap: Saying a composite is "stronger". Say which property is enhanced, which constituent supplies it, and what the other contributes.
Explain why a racing bicycle frame is made from carbon-fibre composite rather than from steel or from resin alone.
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