What's a Composite?
The use of composites in all products - from sporting goods to bridges to satellites - is increasing. Outside of the profession, though, many people would be hard pressed to identify a composite. This article presents a simple definition of composite materials. Although it is primarily written for people new to the materials, composite professionals may also find a few things of interest.
Most of the products we see every day are made from monolithic materials. That means the individual components consist of a single material (an unreinforced plastic), or a combination of materials that are combined in such a way that the individual components are indistinguishable (a metal alloy).
Composite materials, on the other hand, consist of two or more materials combined in such a way that the individual materials are easily distinguishable. A common example of a composite is concrete. It consists of a binder (cement) and a reinforcement (gravel). Adding another reinforcement (rebar) transforms concrete into a three-phase composite.
The individual materials that make up composites are called constituents. Most composites have two constituent materials: a binder or matrix, and a reinforcement. The reinforcement is usually much stronger and stiffer than the matrix, and gives the composite its good properties. The matrix holds the reinforcements in an orderly pattern. Because the reinforcements are usually discontinuous, the matrix also helps to transfer load among the reinforcements.
Reinforcements basically come in three forms: particulate, discontinuous fiber, and continuous fiber. A particle has roughly equal dimensions in all directions, though it doesn't have to be spherical. Gravel, microballoons, and resin powder are examples of particulate reinforcements. Reinforcements become fibers when one dimension becomes long compared to others. Discontinuous reinforcements (chopped fibers, milled fibers, or whiskers) vary in length from a few millimeters to a few centimeters. Most fibers are only a few microns in diameter, so it doesn't take much length to make the transition from particle to fiber.
With either particles or short fibers, the matrix must transfer the load at very short intervals. Thus, the composite properties cannot come close to the reinforcement properties. With continous fibers, however, there are few if any breaks in the reinforcements. Composite properties are much higher, and continuous fibers are therefore used in most high performance components, be they aerospace structures or sporting goods.
Matrix materials are usually some type of plastic, and these composites are often called reinforced plastics. There are other types of matrices, such as metal or ceramic, but plastics are by far the most common. There are also many types of plastics, but a discussion of them is beyond the scope of this week's column. Suffice it to say for now that the two most common plastic matrices are epoxy resins and polyester resins.
Composite materials are available as plies or lamina. A single ply consists of fibers oriented in a single direction (unidirectional) or in two directions (bidirectional; for example a woven fabric). There are other forms, but these are the most important for this discussion.
Composite properties are best in the direction of the fibers. Perpendicular, or transverse, to the fibers, the matrix properties dominate because load must be transfered by the matrix every fiber diameter. Because most structures are not loaded in a single direction, even though one direction may dominate, it is necessary to orient fibers in multiple directions. This is accomplished by stacking multiple plies together. Such a stack is called a laminate.
The most efficient composites have most of their fibers oriented in the primary load direction, and just enough fibers oriented in the other directions to carry secondary loads and hold the structure together. Efficiency means both low weight and low cost, because any fibers which don't carry much load could probably be removed.
For further introductory reading, see the What's a Composite? subject page.