A composite material is formed from two (or more) different materials in order to obtain a new one with improved properties compared to those of the original materials.
In a composite material, one of the source materials will serve as a matrix while the other will serve as a reinforcement. The matrix forms the skeleton of the composite material and determines its shape. The reinforcing fibers are inserted into the matrix to modify its properties. Depending on their composition and the way in which they are incorporated, the reinforcing fibers modify the properties of the matrix in various ways, making it, for example, lighter or more resistant.
The reinforced concrete used to build this bridge is a composite material. The matrix consists of concrete in which steel rods have been inserted. The bridge can thus withstand tensile constraints by means of the steel rods; it can also resist compressive constraints due to the concrete.
Because composite materials have a wide variety of properties, they are being used in an increasing number of fields:
- Aeronautical sector: the fuselage (external structure) of an airplane for example;
- Sports sector: helmets and bicycle frames, surfboards, kayak hulls, tennis rackets, hockey sticks, etc.;
- Arts sector: for instance, violin bows;
- Mechanical sector: high performance brakes, certain engine parts, etc .;
- Military and police sector: bulletproof vests.
Unlike reinforced concrete, the name of composite materials usually comes from the type of reinforcement used. For example, when a windsurf board is made of a plastic matrix with glass fibers as reinforcement, it is called fiberglass.
Depending on the types of matrix and reinforcement used, a composite material has different properties.
| Part of composite material | Type of material used | Properties sought in the composite material |
| Matrix | Plastics | Durability, lightness, resilience, low-cost |
| Matrix | Metals | Ductility, thermal and electrical conductivity, rigidity |
| Matrix | Ceramics | Durability, heat resistance |
| Reinforcement | Fiberglass | Rigidity, corrosion resistance |
| Reinforcement | Aramid fibers (Kevlar) | Low density, resilience |
| Reinforcement | Carbon fibers | Rigidity, low density, electrical conductivity |
Composite materials undergo degradation when the matrix or the reinforcements undergo degradation as well. For example, a loss of adhesion between the matrix and the reinforcements will cause degradation of the composite material. The rate of degradation of the material will depend on the nature of the matrix, the type of reinforcement used, and the conditions to which the material is subjected.
The only way to protect composite materials is to ensure that the materials used in their design will withstand the conditions to which they will be subjected. Also, the manufacture of the composite material must ensure good cohesion between the matrix and the reinforcements.