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Composites

Introduction

A composite materials is created by combining two or more materials that have very different properties.   The different materials work together to give the composite superior properties compared to the individual properties of the original materials.  Within the composite the different materials are identifiable, they do not dissolve or blend into each other.

The three main types of composite materials are

  • Particle filled composite ....Particles have approximately the same dimensions in all directions in a matrix. An example of this type of composite is concrete
  • Discontinuous fibre- reinforced...fibres have limited length/diameter ratio in a matrix.   An example of this type of composite is Glass fibre
  • Continuous fibre-reinforced ... Continuous fibres are constructed by winding or by using prepared layers.... An example of this form of composite is the carbon fibre reinforced materials used in sports equipment.

Most engineering materials metals,plastics etc are homogeneous in that the properties are not a function of position in the solid.    They have isotropic characteristics (the same in all directions).   Composites can be isotropic but, importantly, they can be engineered to have very selective directional properties.  Therefore a composite material can be engineered to have very high strengths to meet specific directional load requirements.  Composites are often orthotropic.  An orthotropic material has properties which are different in three mutually perpendicular directions at a point, with three mutuallly perpendicular planes of symmetry

A piece of wood is a composite, with long fibres of cellulose (a very complex form of starch) held together by a much weaker substance called lignin.   Cellulose is also found in cotton and linen, but it is the binding power of the lignin that makes a piece of timber much stronger than a bundle of cotton fibres.

Other composite material uses include reinforced concrete, car tyres, fibreglass, mud (adobe) bricks, wattle and daub.

Plastic Composites

The addition of high strength fibers to a polymer matrix can significantly improve its mechanical properties such as ultimate tensile strength and flexural modulus.

The table below illustrates typical benefits of composite plastics

PlasticUltimate Tensile Strength
       Unfilled      30% Glass Fibres
MPaMPa
Epoxy70150
Phenolic6090
Polyester 60140
ABS4090

Filling materials can include glass fibres, carbon fibres and aramid fibres, which improve the mechanical properties.   Graphite, PTFE, or molybdenum disulfide are also added to plastics to enhance the lubrication properties.


The benefits of composite plastics compared to unfilled plastics are to offset by the increased costs and the increased difficulty in processing.

Glass fibre reinforced polyester (GRP)composites have virtually completely replaced wood for the construction of small marine craft.   Continuous carbon fibres have used extensively in high performance structures.   The monocoque structure of a Formula 1 racing car is normally a carbon fibre composite.  The material combines low structural weight, with high strength enabling a safe structure with minimum weight penalty allowing high performance.

Composites may also be used for anything from a shuttle in a loom to large civil airliners. A number of applications for composites are listed below:


  • Windsurf Mast - Carbon Fibre
  • Golf Club Shaft - Carbon Fibre
  • Tennis, Badmington, Squash Rackets - Carbon Fibre
  • Fishing Rod - Carbon Fibre
  • Sailing Craft Hulls - GRP
  • High perfomance car shells - GRP /Carbon Fibre
  • Bearings - Glass Fibre filled PTFE


Links Providing information on Composites
  1. Composites.. Macrogallaria- A very useful set of notes.
  2. Science/Technology of Composites.. Australian Academy of Science
  3. Composites basics.. MDA Composites- Very detailed series of articles on polymer composites
  4. Engineering Composites Mechanical Properties.. Useful source of relevant information
  5. Substech.com.. A site provide lots of information on material properties including composites

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Last Updated 12/03/2010