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Composite materials for construction, engineering, and other similar applications are formed by combining two or more materials in such a way that the constituents are still distinguishable and not fully blended. One example is concrete, which uses cement as a binding material in combination with aggregate, like gravel, as a reinforcement. In many cases, concrete uses rebar as a second reinforcement, making it a three-phase composite, because of the three elements involved.
This type of material takes advantage of the different strengths and abilities of its different elements. In the case of mud and straw bricks, for example, mud is an excellent binding material, but it cannot stand up to compression and force well. Straw, on the other hand, is well able to withstand compression without crumbling or breaking, and so it serves to reinforce the binding action of the mud. Humans have been creating composite materials to build stronger and lighter objects for thousands of years.
The majority of composite materials use two constituents: a binder or matrix and a reinforcement. The reinforcement is stronger and stiffer, forming a sort of backbone, while the matrix keeps the reinforcement in a set place. The binder also protects the reinforcement, which may be brittle or breakable, as in the case of the long glass fibers used in conjunction with plastics to make fiberglass. Generally, composites have excellent compressibility combined with good tensile strength, making them versatile in a wide range of situations.
Engineers building anything, from a patio to an airplane, look at the unique stresses that their construction will undergo. Extreme changes in temperature, external forces, and water or chemical erosion are all accounted for in an assessment of needs. When building an aircraft, for example, engineers need lightweight, strong material that can insulate and protect passengers while surfacing the aircraft. An aircraft made of pure metal could fail catastrophically if a small crack appeared in the skin of the airplane. On the other hand, one integrating reinforced materials such as fiberglass, graphite, and other hybrids will be stronger and less likely to break up at stress points in situations involving turbulence.
Many composites are made in layers or plies, with a woven fiber reinforcement sandwiched between layers of plastic or another similar binder. These materials have the advantage of being very moldable, as in the hull of a fiberglass boat. Composites have revolutionized a number of industries, especially the aviation industry, in which the development of higher quality materials allows companies to build bigger and better aircraft.