Composite Material

Composite Material, substance that is made up of a combination of two or more different materials. A composite material can provide superior and unique mechanical and physical properties because it combines the most desirable properties of its constituents while suppressing their least desirable properties. For example, a glass-fibre reinforced plastic combines the high strength of thin glass fibres with the ductility and chemical resistance of plastic; the brittleness that the glass fibres have when isolated is not a characteristic of the composite. The opportunity to develop superior products for the aerospace and motor vehicle industries, and recreational applications, has sustained the interest in advanced composites. Currently composites are being considered on a broader basis—for applications that include civil engineering structures such as bridges and motorway pillar reinforcement; and for biomedical products, such as prosthetic devices.

Composite materials usually consist of synthetic fibres embedded within a matrix, a material that surrounds and is tightly bound to the fibres. The most widely used type of composite material is polymer matrix composites (PMCs). PMCs consist of fibres made of a ceramic material such as carbon or glass embedded in a plastic matrix. Typically, the fibres make up about 60 per cent of a polymer matrix composite by volume. Metal matrices or ceramic matrices can be substituted for the plastic matrix to provide more specialized composite systems called metal matrix composites (MMCs) and ceramic matrix composites (CMCs), respectively.

The fibrous reinforcing constituent of composites may consist of thin continuous fibres or relatively short fibre segments. When using short fibre segments, however, fibres with a high aspect ratio (length-to-diameter ratio) are used. Continuous-fibre composites are generally required for high performance structural applications. The specific strength (strength-to-density ratio) and specific stiffness (elastic modulus-to-density ratio) of continuous carbon fibre PMCs, for example, can be vastly superior to conventional metal alloys. Composites can also have other attractive properties, such as high thermal or electrical conductivity, and a low coefficient of thermal expansion. Also, depending on how the fibres are oriented or interwoven within the matrix, composites can be fabricated that have structural properties specifically tailored for a particular structural use.

Although composite materials have certain advantages over conventional materials, composites also have some disadvantages. For example, PMCs and other composite materials tend to be highly anisotropic—that is, their strength, stiffness, and other engineering properties are different depending on the orientation of the composite material. For example, if a PMC is fabricated so that all the fibres are lined up parallel to one another, then the PMC will be very stiff in the direction parallel to the fibres, but not stiff in the perpendicular direction. These anisotropic properties pose a significant challenge for the designer who uses composite materials in structures that place multidirectional forces on the structural members. Also, forming strong connections between separate composite material components is difficult.

The broader use of advanced composites is inhibited by high manufacturing costs. Currently, fabricating composite materials is a labour-intensive process. However, as improved manufacturing techniques are developed, it will become possible to produce composite materials at higher volumes and at a lower cost than is now possible, accelerating the wider exploitation of these materials.

See also Condensed-Matter Physics.