Fiber reinforcements dominate composites. The fiber industry is divided between natural fibers -- those from plant, animal, or mineral sources -- and synthetic fibers. Many synthetic fibers have been developed specifically to replace natural fibers because synthetics usually behave more predictably and are more uniform in size. Often, synthetic fibers are less costly than their natural counterparts. In the garment industry, for example, the acrylic and rayon fibers were developed to replace more costly natural wool and silk.

For engineering purposes, metal, ceramic, glass, and organically derived synthetic fibers are more significant. Nylon, for example, is used for belting, nets, hose, rope, parachutes, webbing, ballistic cloths, and as reinforcements in tires.

Metal and ceramic fibers are used in high-strength, high-temperature, lightweight composite materials for aerospace applications. Fibers improve the strength-to-weight ratio of base materials such as titanium and aluminum. Anisotropic properties can be designed into a part made from a fiber composite by selectively aligning the fiber/base layup.

Fibers of stainless steel or aluminum provide conductivity in plastic components for static-electricity dissipation or, with higher loadings, shielding from electromagnetic interference (EMI). Shielding is particularly important for housings of computers, copiers, and other business machines. Because only small levels of stainless-steel fibers are needed, base-resin properties remain relatively unchanged, and the composite has good colorability.

Among the strongest materials are metal fibers formed by controlled solidification and cold drawing. Some nonmetallic fibers such as aluminum oxide and silicon carbide are nearly as strong as metal fibers but have a higher modulus of elasticity. Fibers in a metal matrix combine the strength of the fiber with ductility or other characteristics of the matrix. Many combinations of properties are possible -- for example, tungsten fibers in a copper matrix add the strength of the fiber to the conductivity of the matrix. Aluminum oxide and silicon carbide are among several fibers added to aluminum to produce high strength-to-weight ratio composites.

One application using an alumina/silica ceramic-fiber reinforcement in an aluminum matrix is that of diesel pistons. The reinforcement, placed either in the combustion-bowl or ring-groove area, is a preform of Fiberfrax (from Carborundum Co.) ceramic fiber. The pistons are produced by the squeeze-cast process.

Glass fibers, the most widely used reinforcement for plastic and rubber products, are also the finest (smallest diameter) of all fibers, typically 1 to 4 microns in diameter. Because glass fibers have a large surface area in proportion to volume (a -in.-diameter bead of glass is stretched into over 97 miles of fiber) surface conditions of the fiber have a strong influence on its strength and behavior.

Most glass-reinforced products are made with E-glass (electrical glass), which has good electrical and mechanical properties and high heat resistance. E-glass is available as chopped fiber, milled fiber, continuous roving, woven roving, woven fabric, and reinforcing mat. Tensile strength is 500,000 psi modulus is 10.5 million, and elongation can be as high as 4.8%.

Applications are in many industries, ranging from tub/shower units and boat hulls to tanks, ducts, and automotive exterior panels. Fabrication of components, using both thermoplastic and thermoset matrix resins is done by all conventional molding processes.

For higher performance than provided by E-glass, S-glass offers 30% higher tensile strength and 18% higher modulus. S-glass is used in such applications as aircraft flooring, helicopter blades, and filament-wound pressure containers.

Carbon fibers< offer the widest range of stiffness of any material -- from about 5 million to as high as 100 million psi. Most commonly used of these materials, however, are those fibers in the midrange, having moduli in the 30 to 40 million-psi range because they have the most useful balance of properties.

Used alone or as a part of a hybrid reinforcement with glass or aramid, carbon adds considerable strength and stiffness to engineering resins. In chopped form, molding procedures of carbon-reinforced composites is essentially the same as those used for glass-reinforced compounds. In tape form, often with an epoxy resin, the fibers are usually laid up as laminates, with the continuous fibers at various angles to one another.

In between chopped and continuous fibers are the recently introduced long-fiber-reinforced composites, which are available with either carbon or glass-fiber reinforcement. In these compounds (ICI Advanced Materials and Polymer Composites Inc.) the carbon fibers averaging about 0.5 in. long (same length as the pellets) provide strength values between those of the chopped and continuous-fiber-reinforced composites.

Because of their light weight and high strength and stiffness, carbon-reinforced composites are used in aircraft components. Their high-temperature properties qualify them for applications such as pump packings, bearings, and brake components. Sports equipment of "graphite" materials include skis, racquets, golf club shafts, and lightweight bicycle parts.

Aramid fibers (aromatic polyamides) are characterized by excellent environmental and thermal stability, static and dynamic fatigue resistance, and impact resistance. These fibers have the highest specific tensile strength (strength/density ratio) of any commercially available continuous-filament yarn. Aramid-reinforced thermoplastic composites have excellent wear resistance and near-isotropic properties -- characteristics not available with glass or carbon-reinforced composites.

Aramid fiber, tradenamed Kevlar (Du Pont), is available in several grades and property levels for specific applications. The grade designated simply as Kevlar is made specifically to reinforce tires, hoses and belting, such as V-belts and conveyor belts.

Kevlar 29 is similar to the basic Kevlar in properties but is designated specifically for use in ropes and cables, protective apparel, and as the substrate for coated fabrics. In short fiber or pulp form, Kevlar 29 can substitute for asbestos in friction products or gaskets. Fabrics of Kevlar 29 can be made into bullet-resistant vests. Clothing made from Kevlar 29 can be as heat resistant as that made from asbestos and also be extremely cut resistant.

Kevlar 49 has half the elongation (2.5%) and twice the modulus (18 10 (to the 6th power) psi) of Kevlar 29. Applications are principally in reinforcing plastic compounds used in lightweight aircraft boat hulls and sports equipment. Composites containing Kevlar are also used as interior panels and secondary structural parts, such as fairings and doors on commercial aircraft.

Kevlar 149 is a highly crystalline aramid that has a modulus of elasticity 40% greater than that of Kevlar 49 and a specific modulus nearly equal to that of high-tenacity graphite fibers. It is used to reinforce composites for aircraft components.

Nomex aramid fiber (also a Du Pont product) is characterized by excellent high-temperature durability with low shrinkage. It will self-extinguish and does not melt, retaining a high percentage of its initial strength at elevated temperatures. It is available as continuous filament yarn, staple, and tow. Nomex is used in military and civilian protective apparel, dry gas filtration, rubber reinforcement, and industrial fabrics. Nomex aramid fibers are also available as a paper for use in high-temperature electrical insulation and in resilient, corrosionproof honeycomb core for aerospace and other transportation applications.

Thermoplastic fibers are also used to reinforce composite materials. Two such families are Compet and Spectra fibers, both products of Allied-Signal Corp. Thermoplastic fibers are particularly effective where high-shear processing would degrade conventional glass-fiber reinforcement, thereby reducing performance of the composite.

Compet fibers of nylon and polyester provide excellent impact resistance, surface appearance, and abrasion and corrosion resistance. They were developed to provide a degree of toughness and impact strength in brittle thermoset resins. Two polyester grades provide regular and reduced shrinkage characteristics, and a nylon grade is particularly resistant to alkalis. Tensile strength of the grades ranges from 120,000 to 150,000 psi. Compet fibers are often used in hybrid reinforcement systems, along with a stronger, higher modulus fiber.

Spectra, a lightweight, high-strength, extended-chain polyethylene fiber, is claimed to be 10 times stronger than steel and 75% stronger than any other organic fiber available. Two grades of Spectra are available. One is a 1,200-denier fiber designed for high strength under intermittent loading conditions -- sports equipment, ballistic fabrics, and medical products. The other is a 650-denier fiber for high strength under continuous load -- sailcloth, high-tension ropes, and cables that must withstand flex-fatigue conditions. Tensile strength of Spectra fibers ranges from 370,000 to 430,000 psi.