COMPOSITES

Composite materials are composed of two or more distinct phases, usually a reinforcing fiber within a binder or matrix. The concept of increasing strength and toughness by adding fibers to binders is very old; mud bricks containing straw or animal hair are among the first examples. However, it is only relatively recently that composites as we currently recognize them have become widely used. Today, fiber-reinforced composites are at the heart of the materials revolution.

Composites have advantages of design flexibility, high stiffness and strength, low weight and cost savings. One unique attribute of these materials is that the properties can be selected by the design engineer to suit the component and application. For example, some proportion of the fibers can be oriented in the direction of principal load on the component, while others can be configured to carry shear and off-axis loads. Cost and weight savings can often be achieved by replacing several parts made from conventional materials with a single composite component.

Testing composites requires the greatest possible attention to test conditions, grips, fixtures and specimen preparation if meaningful results are to be obtained. Although this type of testing is a relatively new technology, the level of standardization is high, and a number of specialized tests are used to determine parameters such as inter-laminar shear strength, compressive strength after impact, and peel strength.

Although the most common composites are based on polymer matrices, the use of fiber-reinforcement in metals and ceramics is being actively developed for a host of applications in the aerospace, power generation and biomedical industries.