Composites are defined as materials which consist of not less than two different component materials, neither of which are well suited for construction purposes on their own, but which in combination result in a very strong and rigid material. Composites have been known and used in buildings for thousands of years. Straw mixed with clay for building clay huts in the Stone Age is one example. Steel-reinforced concrete is an example from modern times. In combinations of this type, tensile force is absorbed by the steel reinforcement, while the concrete absorbs the compressive load.
Plastics reinforced with various forms of fibre make up a significant portion of the composites that are used by modern society. Fibre-reinforced plastics can be divided roughly into two groups: synthetic materials reinforced with short fibres, and synthetic materials reinforced with long (continual) fibres. Composites that are reinforced with short fibres are used primarily for injection moulding or extruded plastic products. Composites reinforced with long or continual fibres are often used in large structures such as ships, pressure tanks, aircrafts, and wind turbine wings. In fibre-reinforced plastic materials, the properties of the fibres are used to resist tensile and compressive loads, while the plastic – the matrix material – transfers shear.
When using composite materials instead of traditional materials such as steel or aluminium for example, there are normally significant reductions in weight due, in part, to the specific properties of the individual components and low dead weight, and partly because it is possible to produce composites for specific purposes. Because it is a combination of materials, a composite product can be combined and designed with a view to specific load-bearing capacities, while providing a number of advantages in relation to traditional materials, such as resistance to chemicals, as well as electrical and thermal insulating properties.
During recent decades, composite materials have steadily gained ground in nearly all sectors. The rise in use of composites can be explained by better and more comprehensive knowledge of the fundamental properties of composites and their long service life. This has enabled more specific uses and has reduced security factors to realistic levels.