Composite materials are defined as those that contain two or more materials that have been bonded together. Wood is an example of a natural composite material. It is made from lignin—a natural resin—which is reinforced with cellulose fibers. For thousands of years, straw has been used to reinforce mud bricks, forming a two-phase composite, and more recently, reinforced concrete has been developed. Concrete contains a cement binder with a gravel reinforcement. By adding another reinforcing material (steel rebar), concrete becomes a three-phase composite. Metal matrix and metal or ceramic composites have now also been developed.
Perhaps the most familiar class of composite materials are polymer composites. These are a class of reinforced plastics in which fibers are used to reinforce a particular polymer matrix. Phenolic laminates made from phenol formaldehyde resin and paper were developed around 1912, finding a use as an electrical insulating material. Glass is the most common fiber used to reinforce a polymer matrix, forming fiber-glass or glass-reinforced plastic. Other more expensive and higher strength materials such as carbon and, more recently, aramid fibers, are used in advanced applications such as aircraft components. Polyester, vinyl ester and epoxy resins are the most commonly used thermoset resins for the formation of the polymer matrix. PEEK (polyether ether ketone) may also be selected as the resin for applications where cost is less of a problem, such as in aerospace applications.
Other resins, including phenolic, silicone, and polyurethane may be used for particular functions. The designer can alter the chemical resistance properties, service temperature capabilities, weather resistance, electrical properties, resistance to fire and adhesive properties by choosing the right type of resin. Fibers for reinforcing can be obtained in a variety of different formats. They can be woven or multiaxial, continuous or chopped (to give a random form). Alternatively they can be variations or combinations of all these types. By selecting the right orientation of fiber lay-up, it is possible to ''design-in'' a variety of properties connected with the physical strength of the ultimate composite. Composite materials are often what we call layered composites—they are made up of layers of fibers, supplied as plies or lamina. A single ply comprises fibers oriented in a single direction (unidirectional) or in two directions (bidirectional); for example, in a woven fabric. Random fiber layers are also used. These are supplied as ''prepregs,'' meaning that they are prepared before being molded by using a thermoplastic binder that is applied to the reinforcement and then heating the reinforcement. This softens the binder, which can then be formed into the desired shape in a separate operation. A specialist form of composite production involves filament winding on a mandrel prior to molding, producing high-strength rods.
Polymers fall into two classes—thermoplastics and thermosetting plastics, or thermosets. Thermoplastics include familiar plastics such as acrylic (polymethyl methacrylate), polyethylene, acrylic, polypropylene, and polystyrene. They can be heated and formed, then remelted and reformed into a new shape. Composite materials are normally made with thermoset resins. These start as liquid polymers. Following the cross-linking process, which they undergo on heating, the liquid polymers are changed during the molding process into an irreversible solid form. Composite materials therefore have superior properties to thermoplastics as they have better heat and chemical resistance, enhanced physical properties, and more structural resilience.
The development of composite materials that possess a range of advantageous properties have inspired a range of new uses in areas ranging from global uses, such as in aerospace, transport (both by road and sea), and building applications, as well as domestic products—high-tech sporting equipment, electrical goods and office equipment. Composites have various advantages including high strength, light weight, and some temperature resistance. Their high strength has enabled the design of composite materials that meet the taxing needs of a particular function, for example an aircraft nose cone, which needs high impact strength. A variety of resins and reinforcements (from random to woven) can be used to meet the exact physical and mechanical properties needed for a particular structure.
The light weight of composite together with their high strength properties can be designed to meet very demanding specifications, for example in Formula One racing cars or in aerospace uses. Composites can be used to produce the highest strength-to-weight ratio structures known, as for example when aramid fibers are embedded in an epoxy matrix. Due to these high strength-to-weight ratios, these composites contribute enormous weight savings to aerospace structures—a vital consideration when more weight means more fuel use. Other advantages of these composite materials are their high resistance to corrosion and fatigue. More complicated composite materials known as composite hybrids have been developed. These are formed by adding another material such as glass or aramid fiber to the original carbon fiber and epoxy matrix. These extra materials improve mechanical properties such as impact resistance and fracture toughness. Glass-reinforced plastic can gain improved stiffness by adding carbon and epoxy to the matrix. While most modern engineering composites are made from a thermosetting resin matrix reinforced with fibers, some advanced thermoplastic resins may also be used. Other composites, in particular phenolic composites, have filler reinforcements, which may be mineral or fibrous, or a mixture of both. Foams and honeycombs can also be utilized as cellular reinforcements to bestow stiffness together with a very light weight. An example of such a polymer composite was used in the rotor blades of the EH101 Westland helicopter. The only disadvantage of this type of reinforcement, apart from its high cost, is that if the surface is damaged and the honeycomb becomes wet, it may lose its mechanical properties. Composite plastic materials are made using a range of processes. The original process was a hand lay-up process, which can be slow, time-consuming and expensive. New manufacturing methods include pultrusion, vacuum infusion, resin transfer molding (RTM), sheet molding compound (SMC), low-temperature curing prepregs and low-pressure molding compounds. These methods are being used in high tech areas such as aerospace, and RTM in particular looks as though it is developing into a very good value for money process. Choosing composites for certain applications is straightforward in some cases, but in others, their selection will rely on parameters such as service life, production run, complexity of mold, cost savings in assemblage, and on the experience and skills of the designer in tapping into the ultimate potential of composites. Sometimes it is best to use composites together with more traditional materials.
The development of polymer composites has been pushed forward by the needs of the aerospace industry but has revolutionized the design of furniture, allowing the design of organic forms, as well as the design of boats, and sporting equipment where, as for example in the case of tennis rackets and racing cars, their performance has radically exceeded expectations.
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