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Composite materials (or composites for short) are engineered materials made from two or more constituent materials that remain separate and distinct on a macroscopic level while forming a single component.

There are two categories of constituent materials: matrix and reinforcement. At least one portion (fraction) of each type is required. The matrix material surrounds and supports the reinforcement materials by maintaining their relative positions. The reinforcements impart special physical (mechanical and electrical) properties to enhance the matrix properties. A synergism produces material properties unavailable from naturally occurring materials. Due to the wide variety of matrix and reinforcement materials available, the design potential is incredible.

Glass fibre roving strands

There are the so-called natural composites like bone and wood. Both of these are constructed by the processes of nature and beyond the scope of this text. Engineered composite materials must be formed to shape. This involves strategically placing the reinforcements while manipulating the matrix properties to achieve a melding event at or near the beginning of the component life cycle. A variety of methods are used according to the end item design requirements. These fabrication methods are commonly named moulding or casting processes, as appropriate, and both have numerous variations.

The principle factors impacting the methodology are the natures of the chosen matrix and reinforcement materials. Another important factor is the gross quantity of material to be produced. Large quantities can be used to justify high capital expenditures for rapid and automated manufacturing technology. Small production quantities are accommodated with lower capital expenditures but higher labour costs at a correspondingly slower rate.

Many commercially produced composites use a polymer matrix material often called a resin or resin solution. There are many different polymers available depending upon the starting raw ingredients. There are several broad categories, each with numerous variations. The most common categories are known as polyester, vinyl ester, epoxy, phenolic, polyimide, polyamide, and others. The reinforcement materials are often fibres but also commonly ground minerals. Fibres are often transformed into a textile material such as a felt, fabric, knit or stitched construction.

One component is often a strong fibre such as fibreglass, kevlar, or carbon fibre that gives the material its tensile strength, while another component (called a matrix) is often a resin such as polyester, or epoxy that binds the fibres together, transferring load from broken fibres to unbroken ones and between fibres that are not oriented along lines of tension. Also, unless the matrix chosen is especially flexible, it prevents the fibres from buckling in compression. Some composites use an aggregate instead of, or in addition to, fibres.

In terms of stress, any fibres serve to resist tension, the matrix serves to resist shear, and all materials present serve to resist compression, including any aggregate.Composite materials can be divided into two main categories normally referred to as short fibre reinforced materials and continuous fibre reinforced materials. Continuous reinforced materials will often constitute a layered or laminated structure.

Aramat a mixture of Kevlar and glass fibre