Composites Essay

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This chapter introduces the general concepts and information pertaining to composites. The composition and properties of the various components that make up a composite will be investigated as well as the various types of composites that are commonly used. This will then be followed by a review of the testing techniques and apparatus that can be used in testing the mechanical properties of composites.

2.2. Composite types and background

There is no universally accepted definition for composites but the following explanation will give an indication as to what composites are. Composite materials are the combination of two or more materials in such a way that certain improved or desired properties are achieved [3]. The materials or elements that a composite is made of work together to produce a material property that is different to the properties of the original materials on their own [4]. This combination of properties is called the principle of combined action which states that better property combinations can be achieved by the judicious combination of two or more distinct materials [5]. Composites are mainly composed of two phases, namely the matrix and the dispersed phase, the matrix is also known as the bulk phase and it is continuous and surrounds the dispersed phase. The dispersed phase is a reinforcement which is added to increase the strength and stiffness of the matrix and it is usually in fibre form. Matrix materials have limited use in the manufacture of structures on their own since their mechanical properties are not very high when compared to most metals [4]. They do however have desirable properties such as the ability to be easily formed into complex shapes. The properties of a composite are a function of the pr...

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...t not deform at all under load and as such an elastomer matrix might not be suitable for this application.

Man-made polymers are generally called ‘resins’ and can be classified under two types; thermoplastic and thermoset, according to the effect of heat on their properties. Thermoplastic materials contain polymer molecules that are held together by weak van der Waals forces or hydrogen bonds [3]. Thermoplastics soften when heated and will eventually melt but they can be hardened again by cooling the material. This process of heating and cooling can be performed many times without having an effect on the material properties and this can be desirable for certain applications. Some types of thermoplastics include ABS, nylon and polypropylene and the main type of dispersed phase used in the creation of composites using thermoplastics is short fibres such as glass [4].

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