A composite is created by combining different materials to create a new one. A rudimentary example would be mixing mud and straw and forming it into a brick shape to make adobe bricks. It takes two materials which, by themselves, wouldn't usually be used for the same purpose as they are when combined into a composite material for building. In construction trades, concrete would be a slightly more complex composite of stone mixed with cement. If you add re-bar (strong steel rods), it becomes a three-phase composite that adds both strength and flexibility. In engineering, an engineer may design something that is under certain stresses that require the uses of a material that is a composite (either because conventional material can't meet the stress demands or could be too heavy for the purpose it has been designed).
Composites Are Versatile
Make a quantum leap from cement to the aerospace industry. Many jets and airplanes are made of composite materials stronger and lighter than the materials they were made from. The new Boeing 787 Dreamliner, for example, will use 50 percent composite materials, dropping its overall weight by 12 percent. That is significant because of added strength and the lower weight allows the plane to use less fuel. One of the best known examples of a composite in the flying world is the B-2 or Stealth Bomber. The body is engineered to deflect radar away from detection and the body and parts of the wings are covered in a radar absorbing composite material, making it virtually undetectable to radar.
Many composites are made of scraps or byproducts of materials that would not have another, or limited, use. Scrap wood and dust from a sawmill is one good example. The mills sell the sawdust and wood scraps to companies that mix the dust and small bits of lumber with a binding agent to produce compressed wood. Compressed wood is used primarily in two ways. Without veneer, it is used to build parts of furniture that are not visible in the finished product. It is usually used for support of pieces to add structural integrity. Compressed wood with veneer can be used on parts of furniture that are outwardly visible. The finished product is less expensive than furniture made entirely from whole pieces of lumber. There are other examples of scraps, from sawdust to food, that at one time would have been thrown away but have found uses as a composite material.
Composites Can Break Without Breaking
In other words, some composites consist of various bonded strands of material that would allow one or more of the strands or even bundles of strands to fail without undermining the overall structural integrity of the material. Advanced composite materials such as carbon fiber reinforced plastics (CFRP) are being applied to many aircraft structures to improve performance and save weight. The composite is comprised of strands (strings) of bundled fibers and then mixed, heated and compressed into what might be a wing on an aircraft or its body or nose. If for whatever reason, wind-shear for example, some of the strands snap from the stress, the remaining strands remain intact. Same for entire bundles, which are reinforced with yet other bundles. It would take a catastrophic force to cause all the strands in all the bundles to fail. CFRP is designed intentionally for just such events.
Around the Home
You know that aluminum siding that has faded to a pale yellow since you first bought your house in 1963 (when aluminum siding became popular and really was made of just aluminum)? There is now a composite that mixes polyurethane foam with the aluminum. Any color can be added and it won't fade. It offers the benefit of greater insulation for the home because polyurethane is mixed with air, forming air pockets that help serve as insulation.
Green and Going Greener
One of the latest uses for recycled materials has been the advent of recycled wood-plastic composite for building products like decking, door and window frames, fencing and exterior moldings. Manufacturers claim the new wood-plastic is more durable than preservative-treated lumber, and recover saw dust and waste plastics that include high-density polyethylene (PVC).
- Radar-absorbing Stealth Bomber made with composite material; photo credit, Sneaky at flickr.com