Composites (composite materials): An introduction from Explain that Stuff!

What is a composite?

Composites are made by combining two or more natural or artificial materials to maximize their useful properties and minimize their weaknesses. One of the oldest and best-known composites, glass-fiber reinforced plastic (GRP), combines glass fibers (which are strong but brittle) with plastic (which is flexible) to make a composite material that is tough but not brittle. Composites are typically used in place of metals because they are equally strong but much lighter.

Most composites consist of fibers of one material tightly bound into another material called a matrix. The matrix binds the fibers together somewhat like an adhesive and makes them more resistant to external damage, whereas the fibers make the matrix stronger and stiffer and help it resist cracks and fractures. Fibers and matrix are usually (but not always) made from different types of materials. The fibers are typically glass, carbon, silicon carbide, or asbestos, while the matrix is usually plastic, metal, or a ceramic material (though materials such as concrete may also be used).

A brown nanocomposite powder being poured from a test tube in a laboratory

These three types of matrixes produce three common types of composites:

Polymer matrix composites (PMCs), of which GRP is the best-known example, use ceramic fibers in a plastic matrix.
Metal-matrix composites (MMCs) typically use silicon carbide fibers embedded in a matrix made from an alloy of aluminum and magnesium, but other matrix materials such as titanium, copper, and iron are increasingly being used. Typical applications of MMCs include bicycles, golf clubs, and missile guidance systems; an MMC made from silicon-carbide fibers in a titanium matrix is currently being developed for use as the skin (fuselage material) of the US National Aerospace Plane.
Ceramic-matrix composites (CMCs) are the third major type and examples include silicon carbide fibers fixed in a matrix made from a borosilicate glass. The ceramic matrix makes them particularly suitable for use in lightweight, high-temperature components, such as parts for airplane jet engines.

Photo: A typical PMC (polymer matrix composite). This brown powder, N-CAS (nanocomposite absorbent solvent), can remove poisonous arsenic from drinking water. It's made by embedding nanoparticles of metal oxide, which absorb the arsenic, in a polymer matrix. Picture courtesy of Idaho National Laboratory and US Department of Energy.

Making composites

Objects made from glass-reinforced plastic, such as row boats, are frequently made by hand by cutting layers of composite from a continuous reel and sticking them into a mold with resin. The reel of composite used for this process is typically manufactured by drawing continuous lengths of the fiber material through a tank containing the resin that will act as the matrix. The coated fibers are then pressed onto a backing tape and formed into long sheets or continuous rolls called laminates. The tape is a composite whose fibers all run in the same direction. This makes it anisotropic, which means its physical properties vary in different directions. For example, it is stronger and stiffer in one direction than in others.

That may be a good thing if the material needs to be particularly strong along one axis but, for something like a boat hull or the fuselage or an airplane, the composite needs to be equally strong in every direction. The problem can be solved in one of two ways. First, when the hull or fuselage is being manufactured, the laminates can be pasted in so that each successive layer has its fibers pointing in a different direction. Alternatively, another kind of laminate can be used in which the fibers have been chopped up before they are stuck to the backing tape. Although this type of laminate is weaker in any one direction, it has the advantage of being equally strong in all directions.

A B2 stealth bomber photographed from above

The world of composites

Photo: The B2 stealth bomber uses clever design and composite materials to evade radar detection. Picture courtesy of US Air Force.

The strength and lightness of composites has made them particularly attractive for transportation. From the $42 million B-2 "Stealth" Bomber to low-cost, home-build glider kits, composites have made airplanes lighter, more economical, and more affordable and solved problems such as cracking and metal fatigue. Composites have also made possible new craft called tiltrotors—airplanes with a swiveling propeller at the end of each wing that can hover or take off vertically like a helicopter. Made from traditional materials such as aluminum, craft of this sort would have been simply too heavy to carry their cargo. Space rockets and satellites are also benefiting from composites, and in some unusual ways. Instead of having fuel tanks that must be jettisoned part way through a mission, the next generation of spacecraft may have tanks made from composites that can themselves be burned up as fuel.

Smart cars are made from composite materials

Composites are not just useful in making things fly. Cars of the future must be safer, more economical, and more environmentally friendly, and composites could help achieve all three. Although composites such as GRP have been used in the manufacture of automobile parts since the 1950s, most cars are still made from steel. Engineers believe carefully designed composites could cut the weight of a typical steel car by as much as 40 percent, increasing fuel economy by as much as a quarter, yet maintaining body strength and crash-resistance. High-temperature ceramic-matrix composites are also making possible cleaner-burning, more fuel-efficient engines for both cars and trucks.

Photo: Smart cars are lightweight, composite cars. A steel safety shell holds together a variety of different parts and panels mostly made of plastics, including polypropylene (PP), polyvinyl butyral (PVB), polycarbonate (PC), and polyethylene terephthalate (PET). As on most cars, the "rubber" tires are actually composites made from rubber and numerous other materials, such as silica.

The strength and lightness of composites has made them equally popular in the design of sports equipment. Tennis racquets once made from wood, then aluminum, are now typically made from graphite or graphite-based composites, and improved composites based on fiberglass, Kevlar (aramid fibers), titanium, and ceramics are now being developed. The composite fibers used in tennis racquets are angled specifically to reduce bending and twisting and to improve stability. Similar materials are used in other types of sports equipment. The latest composite hockey sticks made from aerospace-grade carbon fibers in a nylon polymer matrix are twice as tough and six times stronger than sticks made from ordinary composites. The same material is used to make wheels for mountain bicycles, but more advanced Kevlar composites are used to make the light, super-strong, solid wheels used in Olympic-style cycles.

Composites are so versatile that they are now being used even to build large-scale structures. A 460-ft (140-m) bridge that carries four lanes of traffic through San Diego, California, has recently been constructed from composites and is estimated to be one fifth as light as an equivalent metal bridge. (For more on the advantages of composite bridges, see this article on building more durable bridges from the Federal Highway Administration.) NASA scientists and industry engineers are currently developing composites that could be used in place of metals to construct offshore oil platforms and the pipelines that carry their oil to shore. Once developed, the technology is expected to yield extra-durable pipes that could be used for everyday applications such as sewage disposal.

Composites in action

Graphite tennis racquet

Photo: Most modern tennis racquets are made from carbon-based (graphite) composites.

Traditional wooden tennis racquets were heavy, had a tendency to break, and often warped in extreme heat. Today, graphite composites have made racquets lighter and stronger and eliminated warping. More advanced materials such as titanium may offer greater strength and reduced weight in future.

Cement drill

Composites are increasingly used in place of metals in machine tools. Apart from being lighter and stronger, they can offer better performance than metals at high temperatures and do not develop potentially dangerous weaknesses such as fractures and fatigue.

Laminates

Laminates made from composites such as GRP have their fibers aligned in a single direction. To ensure something like a boat hull is equally strong in all directions, the laminates must be built up in layers with their fibers oriented in alternate ways. Read more in our main article about laminates.

Composites