Composite structure
Composites consist of two parts: the reinforcing phase and the binder, or matrix. In reinforced concrete, for example, the steel rods are the reinforcing phase; the concrete in which the rods are embedded are the binder or matrix.
In general, the reinforcing phase can exist in one of three forms: particles, fibers, or flat sheets. In the cutting wheels described above, for example, the reinforcing phase consists of tiny particles of cobalt metal in a binder of tungsten carbide. A plastic fishing rod is an example of a composite in which the reinforcing phase is a fiber. In this case, the fiber is made of threadlike strips of glass placed in an epoxy matrix. (Epoxy is a strong kind of plastic.) An example of a flat sheet reinforcing phase is plywood. Plywood is made by gluing together thin layers of wood so that the wood grain runs in different directions.
The binder or matrix in each of these cases is the material that supports and holds in place the reinforcing material. It is the tungsten carbide in the cutting wheel, the epoxy plastic in the fishing rod, or the glue used to hold the sheets of wood together.