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2 Fiber-Reinforced Composites

A composite is a combination of two or more materials (reinforcing elements, fillers, and matrix binder) with different form or composition which, when combined into a material system, exhibit properties which are a combination of its individual components. The system constituents retain their distinct identities, meaning they do not dissolve or merge completely into each other, but act in concert to provide an overall function. The matrix can be a ceramic, metal, or polymer. Fillers may be mineral or metallic powders. Reinforcing can be particles, fibers, rods, or bars. For example, reinforced concrete is a composite consisting of steel reinforcement, sand and gravel fillers, and a portland cement matrix.

Fiber-reinforced composites or fiber-reinforced polymers (FRP) consist primarily of a typical reinforcement of glass, carbon or aramid fibers, and a polymer matrix. Fillers to modify the physical, mechanical, thermal, electrical, and other properties or to lower the cost or density, may or may not be included. The polymer matrix may be a thermoplastic, a thermoset, or an elastomer. A thermoplastic polymer, polyethylene, polyvinyl chloride, or polystyrene for example, is one which becomes pliable or plastic when heated and then becomes hard again when cooled. A thermoset polymer changes into a crosslinked, substantially infusible material when cured by heat or chemical reaction. Epoxy, polyester, and polyurethane are examples of thermosets. An elastomer is a rubber-like polymer which recovers its original shape and size after removal of a deforming force.

The key component of an FRP is the fibrous reinforcement; it is the primary load bearing component. The matrix serves as the mechanism by which loads are transferred within the member from one fiber to another. Each type of fiber has certain advantages and disadvantages; reinforcement is selected on the basis of its physical, mechanical, and thermal properties.

Modern glass fibers were first developed in the 1930s for military purposes. Soon after, its primary commercial use was for the reinforcement of plastics. E-glass is the standard because of its electrical and mechanical properties. This fiber has a tensile strength nearly double that of steel and has modified versions that resist strong acids. An interesting characteristic of glass fibers is that they are elastic - elongating until failure without yielding. After the load is released the fiber returns to its original length.

Carbon fibers are the most widely used variety of reinforcement having a very wide range of physical properties. Their strength can vary from that of steel to about four times that. What separates carbon fiber reinforced polymer (CFRP) from the rest is its low weight. Its performance based on stiffness to density is very high. It also has very good fatigue and damping characteristics. Manufactured carbon fibers can vary from the weakest of all fibers to among the strongest. Likewise, their price also varies from inexpensive for the weaker fibers to expensive for the strongest fibers. The most commonly produced versions of CFRP are the intermediate strength fibers. They have tensile strengths stronger than glass and somewhat weaker than aramids.

Like carbon, aramid fibers are lightweight, have high tensile strengths, and good damping and wear resistance. They also have excellent fiber toughness. A popular version of the aramid fiber is marketed under the trademark Kevlar. However, its drawbacks are low resistance to acid attack and high cost.

As the manufacture of FRP composites improve and their mechanical properties are better understood, they are being used in a wider variety of applications. Because of the ultra-conservative nature of the civil engineering community and the relatively short history of FRP composites use, fiber-reinforced composites are just beginning to be considered as a civil engineering material alternative to steel and reinforced concrete. Although many factors, including material form, will significantly influence any design, some general differences between metals and composites may make the latter appear to be the more attractive choice. Differences between composites and metals are as follows:

FRP composites consist primarily of fiber reinforcement and a polymer. Fibers that are typically used for civil and structural engineering applications are E-glass, carbon, and aramid; polymers are either polyester, vinyl ester, or epoxy. A major reason these polymers are used is because they cure by chemical reaction at ambient temperature. FRP composites may take several forms. The fibers can be in a woven or stitched fabric, or unidirectional sheet, tow or yarn. The composite may be a prepreg (fabric with uncured polymer infusion at the factory), preform (extruded, cast, or shaped at the factory), laminate plate, rod/cable, or a hybrid of these. Various methods exist for applying composites to a structural member. They include hand lay-up, filament winding, vacuum resin transfer molding, and any compaction process. When preforms or laminate plates are used for repair or upgrade, matrix binders or adhesives made of polyester, vinyl ester, epoxy, or polyurethane are used to bond them to the structural members. Depending upon the composite specifications, additives, fillers, or coatings may also be incorporated in the composite to provide UV and/or fire resistance and special moisture or chemical resistance.

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