Any material that is heterogeneously made up out more than one base component. This rules out alloys, which are smooth mixes of different metals, but includes materials such as fiberglass, plywood, particle board, etc.

The vast majority of composite materials are made by combining thin fibers with high tensile strength with something goopy. Fiberglass is a combination of glass and resin, particle board is a combination of wood fragments and glue, etc. Materials produced in this was are ideally strong from the fibers, but flexible from the goop. Another advantage is that the materials are much lighter than comparable solid materials that have the same tensile properties.

Plywood is a different type of composite material, in that it is created by laminateing thin sheets of wood together in such a way that in each sheet, the grain runs a different way. This decreases the likelihood of the board to split, since there will always be a grain running in the pertainent direction.

Composite materials were designed the same way as most great engineering innovations: by blatantly ripping off Mother Nature. Wood, bone, and muscle are all good examples of natural 'fiber + goop' composites, and skin is a good example of the laminate approach.

The surrounding material in a composite is called the matrix. The reinforcing material that is surrounded is called the phase or the reinforcing phase.

Fiber Reinforced Plastics
  • Polyurethane is often reinforced with chopped glass fibers. The glass fibers increase the stiffness of the composite material and act as a cheap filler.
  • High strength nylon resins like Zytel by DuPont can be reinforced with chopped glass or glass fibers.
  • Epoxy is often reinforced with glass fibers or carbon fibers. Many aerospace composites are made from this combination.
  • Ribbons or mats of reinforcement fibers may be used as well.
  • The first fiberglass railroad car, a hopper car called the "Glasshopper", was the only car not destroyed in a catastrophic derailment.

Sometimes a coupling agent often must be used to make the reinforcement fibers stick to the plastic matrix. Silane is frequently used to make glass fibers stick to an epoxy matrix. When the reinforcement breaks free from the fibers due to stress, delamination occurs. This dramatically reduces the strength of the composite.

Metal Matrix Composites

Metals must be reinforced with other metals that have a higher melting temperature, or with ceramic materials. This is a still relatively new frontier in metallurgy.

  • Tank armors are often made from metal matrix composites. Carbon-based ceramics can not be used to reinforce steel armor because they will dissolve. For this reason, boron nitride is often the preferred reinforcement material.
  • Carbide Drill bits are made from tungsten carbide particles embedded in a cobalt matrix.
  • Aluminum diboride flakes can be used to dramatically increase the stiffness of aluminum.

Manufacturing Techniques

The most common manufacturing techniques for composites are
  • hand layup
  • filament winding
  • resin transfer molding
  • reaction injection molding with filler

As a Hobby

Some people make their own composite kayaks, canoes, model rockets, and even aircraft in their garages. A great deal of care must be taken to get good ventilation due to the often toxic fumes that come from curing epoxy and other plastic resins.

Composites on the Web

http://composite.about.com

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