A composite combines two or more materials so that the result has a property neither has alone: a strong, stiff reinforcement carries the load and a matrix holds it in shape, shares load between pieces and protects it. Reinforced concrete is the classic case, with concrete carrying compression and steel carrying tension. Composites are usually strongest along the reinforcement and weak across it, and they fail at the interface if the bond between the two is poor.
Part of Materials Science
Composite materials are engineered combinations of a reinforcing phase, such as fibres, particles or layers, embedded in a continuous matrix of a different material.
Each Part Does One Job
The Advanced Materials article states the principle: concrete resists compression but cracks under tension, steel resists tension but buckles in compression, and together they handle both, with the concrete surrounding and protecting the steel while the steel carries the tension. The general pattern is:
- Reinforcement supplies stiffness and strength. It is usually strong but may be brittle or thin, like glass fibre.
- Matrix supplies shape, transfers load into the reinforcement through the interface, and protects it from damage and the environment. It is usually weaker and cheaper.
Much older building practice is composite too: Adobe is mud reinforced with straw, plywood and bent wood are layered, and Pattern Welding layers hard and tough steels.
A Short Calculation Shows The Gain
Along the fibres, the stiffness of a composite is roughly the sum of each part’s stiffness weighted by its volume share: E(composite) = Vf × E(fibre) + (1 − Vf) × E(matrix). This is the rule of mixtures.
Worked example, using illustrative values: let glass fibre be 70 GPa and a polyester matrix 3 GPa, with 30% fibre by volume (Vf = 0.30). Then E = 0.30 × 70 + 0.70 × 3 = 21 + 2.1 = 23.1 GPa. The composite is about 7.7 times as stiff as the matrix alone (23.1 / 3), but much less than the fibre, since 70% of the volume is soft matrix. Across the fibres, though, the matrix governs and the stiffness is far lower. These material values are typical textbook figures, not corpus numbers, and only the arithmetic is the point.
Four Things Decide Whether A Composite Works
- Bond at the interface. The matrix must wet and stick to the reinforcement. A poor bond leaves the fibre to pull out.
- Orientation. Align fibres with the load. Random mat is weaker in any one direction but equal in all.
- Fraction. More reinforcement means more strength until the matrix cannot fill the gaps. Dry spots and air voids are weak points.
- Match of expansion. Parts that expand differently stress the interface on heating. Steel and concrete are fortunately close.
The Family
| Type | Reinforcement | Article |
|---|---|---|
| Fibre-reinforced | Glass or plant fibre in resin | Fiber-Reinforced Composites |
| Laminated | Layers of sheet material | Laminated Composites |
| Concrete | Steel in cement | Concrete Composites |
| Particle | Hard grains in metal | Cermet |
Common Mistakes
- Expecting equal strength in all directions — composites are directional.
- Assuming the matrix just fills space — it carries load across the interface.
- Using a resin the reinforcement cannot bond to — the strength is never realised.
What’s Next
- Learn next: Fiber-Reinforced Composites — the composite a settlement can make soonest
- Combine with: Mechanical Properties — the measurements behind the comparison