Combining these different materials obtains new ones with chemical-physical properties that cannot be found individually in the substances they are made of.
Composites are distinguished from metal or metal alloys in that the materials making up the combination are different in composition or shape. Each constituent maintains its own identity and characteristics in the end compound, without merging fully in the other.
Reinforced concrete is an example of a composite structure in which cement and steel maintain their identity and characteristics. The steel rods support the stress load whereas the concrete supports compression.
Composite materials are mainly used for their outstanding qualities regarding strength, rigidity, lightness and resistance to fatigue, corrosion and shocks.
Advanced composite materials comprise a highly resistant fiber inserted into an epoxy matrix.
Graphite and epoxy resin structures weigh 20% less than aluminium.
A reduction in weight for parts involved is the most obvious advantage over traditional materials (e.g. metals). Another advantage of composite materials is their good resistance to cyclic loading (fatigue resistance).
Most modern composite materials combine a thermosetting resin matrix reinforced with fiber as well as strengthening cells such as rigid foam and honeycomb structures.
The most commonly used reinforcement is glass, carbon and aramid fibers available in several types (continuous, fragmented, multi-axial or braided fibers).
Careful choice of the type of reinforcement makes it possible to fine tune the characteristics of strength and resistance for the end structure to meet all the requirements for the finished product.
The most commonly used thermosetting resin matrices include polyesters, epoxy, vinyl esters and phenols. The choice of type of resin will vary the characteristics of working temperature, resistance to chemicals and weather conditions, the electrical conductivity and fireproofing required for the end product. All items produced with traditional materials can be made of composite materials. Whereas the use of composites is almost obligatory for certain types of applications, the selection of the material to use generally depends on the working life expected for the end product, the complexity of its shape, economical assembly costs, and experience in the use of composites. In spite of the fact that production processes have also evolved constantly, manual layering or lamination is still the most common method in use.
Carbon fiber is a filiform graphite structure made up of millions of very fine carbon filaments. It is the main reinforcement used to make most of the evolved composites where the fibers are “composed” or joined together with a matrix, usually resin. The function of the resin matrix is to keep the resistant fibers in place (so that they keep the same orientation when absorbing stress), protect them and maintain the shape of the composite product. To make composite structures carbon fibers are first woven and put together to organize sheets of carbon which are placed in position and immersed in or backed onto the resin matrix. The main characteristics of carbon fiber are high mechanical strength, low density, heat insulation capacity, resistance to temperature variations and chemicals, as well as good fireproofing properties.
Carbon fiber materials often have high anisotropy, i.e. the mechanical characteristics are directionally dependent. This makes it possible to distribute the fibers according to the main stress or the behaviour expected of the piece to be created (bending, radial, twisting, etc. resistance).
For a long time structural composites were usually made with glass fibers. Although these have good resistance and low density, they have a relatively low elastic modulus. For this reason, about 25 years ago experiments began to convert organic composites into carbon and graphite fibers and fabrics. The extremely good mechanical properties of carbon fiber derive from the special crystal structure of graphite. The better the crystal structure, the better the performance of the material.