16.6 Composites

Modern structural composites are a blend of two or more materials. One component is generally made of reinforcing fibers, either polymeric or ceramic. The other component is generally made up of a resinous binder or matrix that is polymeric in nature, at least for polymeric composites. The fibers are strong and stiff relative to the matrix. Composites are generally orthotropic (having different properties in two different directions) materials. When the fiber and matrix are joined to form a composite, they both retain their individual identities and both directly influence the composite's final properties. The resulting composite is composed of layers (laminates) of the fibers and matrix stacked to achieve the desired properties in one or more directions. The reinforcing fiber can be either continuous or discontinuous in length. The common commercially available reinforcing fibers are:

• Glass

• Polyester

• Graphite

• Aramide

• Polyethylene

• Boron

• Silicon carbide

• Silicon nitride, silica, alumina, and alumina silica

Glass fiber composites are the most common type of composite. However, graphite, aramid, and other reinforcements are finding applications in demanding aerospace functions and in premium sporting equipment such as fishing rods, tennis rackets, and golf clubs.

The resin matrix can be either thermosetting or thermoplastic. Thermosetting resins such as epoxy, polyimide, polyester, phenolic, etc. are used in applications where physical properties and environmental resistance are important. Polyester and epoxy composites make the bulk of the thermoset composite market. Of these two, polyesters dominate by far. Reinforced with glass fiber, these are known as "fiberglass" reinforced plastics (FRP).