A typical composite material is a system of materials composing of two or more materials (mixed and bonded) on a macroscopic scale. For example, concrete is made up of cement, sand, stones, and water. If the composition occurs on a microscopic scale (molecular level), the new material is then called an alloy for metals or a polymer for plastics.

Generally, a composite material is composed of reinforcement (fibers, particles, flakes, and/or fillers) embedded in a matrix (polymers, metals, or seramics). The matrix holds the reinforcement to form the desired shape while the reinforcement improves the overall mechanical properties of the matrix. When designed properly, the new combined material exhibits better strength than would each individual material.

Based on the form of reinforcement, common composite materials can be classified as follows:

1. Fibers as the reinforcement (Fibrous Composites):
  a. Random fiber (short fiber) reinforced composites

  b. Continuous fiber (long fiber) reinforced composites

2. Particles as the reinforcement (Particulate composites):

3. Flat flakes as the reinforcement (Flake composites):

4. Fillers as the reinforcement (Filler composites):

Different materials are suitable for different applications. When composites are selected over traditional materials such as metal alloys or woods, it is usually because of one or more of the following advantages:

• Cost:
  • Prototypes
  • Mass production
  • Part consolidation
  • Maintenance
  • Long term durability
  • Production time
  • Maturity of technology
• Weight:
  • Light weight
  • Weight distribution
• Strength and Stiffness:
  • High strength-to-weight ratio
  • Directional strength and/or stiffness
• Dimension:
  • Large parts
  • Special geometry
• Surface Properties:
  • Corrosion resistance
  • Weather resistance
  • Tailored surface finish
• Thermal Properties:
  • Low thermal conductivity
  • Low coefficient of thermal expansion
• Electric Property:
  • High dielectric strength
  • Non-magnetic
  • Radar transparency

Note that there is no one-material-fits-all solution in the engineering world. Also, the above factors may not always be positive in all applications. An engineer has to weigh all the factors and make the best decision in selecting the most suitable material(s) for the project at hand.