Composite materials have been used throughout history to achieve combinations of properties that could not be achieved with individual materials. Concrete is a composite of cement, sand, and gravel. Poured concrete is usually reinforced with steel rods. Other examples of composites include steel-belted tires; asphalt blended with gravel for roads; plywood with alternating directions of fibers, carbon, or glass fiber-reinforced polyester; or epoxy used for furniture, boats, and sporting goods. Composite materials offer combinations of properties otherwise unavailable. The reinforcing material may be in the form of particles, fibers, or sheet laminates.

Fiber-reinforced Composites

Fiber composites may also be classified according to the nature of the matrix and the fiber. Examples of a number of possibilities are listed in Table 17.1.

Different geometric arrangements of the fibers are possible. In two-dimensional products, the fibers may be unidirectionally aligned at 90° to one another in a woven fabric or randomly oriented (Figure 17.1). The fibers may be very long or chopped into short segments for easy fabrication. In thick sections it is possible to randomly orient short fibers in three dimensions. Fiber reinforcement is used to impart stiffness (increased modulus) or strength to the matrix. Fiber reinforcement also increases toughness.