Reinforcements

Historically, reinforcements used in composites were composed of traditional textiles and fabrics, such as yarn. Modern reinforcements remain textile-like, and often are subject to application of a bonding agent or other treatment to improve the resin's adhesion to the fiber. A large variety of materials are capable of serving as reinforcement for FRPs, some like wood chips are natural, and others like nylon or glass-fibers are man-made.

Glass Fibers

The vast majority of reinforcing materials are man-made, and glass fibers are the most commonly used. Other materials used as reinforcement are carbon or graphite fibers, aramid, kevlar, nylon, or polyester. Even more specialized, usually employed for their high strength and ability to withstand high heat, are metals and metal oxides such as titanium. Ceramics and other high-technology materials, including nano-particle based compounds, are also increasingly finding application in composites.

Exposed Reinforcement Fibers

Reinforcements are utilized in several forms, they can be continuous fibers, or pieces of fiber called chopped strands. They can be woven, knitted, or braided to provide multidirectional support, or used for unidirectional support as is the case with rovings, or groupings of fibers or strands.

Glass fibers are grouped into two major categories, "E" glass and "S" glass. E-glass is the dominant type and are used broadly due to their non-conductivity, reduced susceptibility to moisture, and their high mechanical properties. S-glass uses stem from the need for higher strength, increased heat resistance, and the potential for specialized chemical resistance.

In addition to fiber reinforcements, more exotic materials are also used in composites in the form of cores or fillers. Inclusion of these components allow for increased strength and functionality while only marginally increasing the weight of the material. Cores are often employed in a sandwich-type design where the core material is enclosed between layers of so-called skin materials. After bonding the core to the skin with an adhesive, the resulting sandwich performs as one unit. Filler material use is increasing as these materials often result in reduced cost and improved performance for the composite material.

Wood Core	Honeycomb Core	Foam Core

The four main groups of cores are foam, syntactic foam, honeycomb structures, and wood. The most common foam materials used in composites are polyvinyl chlorides (PVCs) and urethanes. Urethanes find application in materials requiring floatational or insulating porperties. PVCs are more often used in larger structural applications, and can be manufactured to be rigid, similar to thermoset resins, or more malleable and capable of heat-induced shaping. Phenolic foams, similar to phenolic resins, are touted for their flame-resistant properties.

Fillers, in addition to their cost benefits, provide water resistance, weathering, surface smoothing, stiffness, dimensional stability, and temperature resistance. Calcium carbonate is a very common low-cost filler often derived from limestone or marble. Koalin, or mined clay, is another popular filler and, like calcium carbonate is available in a variety of particle sizes. Fillers offering improved flame/smoke performance are alumina trihydrate and calcium sulfate. Other commonly used fillers are: mica, feldspar, silica, talc, flake and milled glass, and microsphere products.

The figure above shows a cored laminate under a bending load. In this mode of loading it can be seen that the upper skin is put into compression, the lower skin into tension and the core into shear. It therefore follows that one of the most important properties of a core is its shear strength and stiffness.