Materials can be classified as either metals, ceramics, or polymers. The dominate aerospace structural materials since the mid-1920s have been metals. However, since the mid-1980s, high strength polymeric composite materials have started to replace metals in some aerospace structures. Since composites can be classified as a hybrid material with a reinforcement embedded in a matrix, they can consist of high strength fibers, or other types of reinforcements, in either polymer, metal, or ceramic matrices. The relationship among the three materials classes and composites is shown in Fig. B.1. This appendix provides a review of some of the basic aspects of these different material options.
The properties of materials are determined by both their composition and structure.1 At a very high level, the composition of metals, ceramics, and polymers is obviously as different as their properties. However, even within these material groups, small changes in composition can have a profound effect. For example, the addition of less than one percent of carbon to iron, combined with the proper heat treatment, can increase the strength of steel in the dramatic fashion shown in Fig. B.2. The further addition of minute amounts of hydrogen, in parts per million, can then ruin the properties, resulting in sudden brittle failures (hydrogen embrittlement). Even within a specific alloy group, such as aluminum, the number of compositions or alloys to choose from is very large with over 500 registered alloys.
The concept of structure is much more subtle but every bit as complicated, or even more complicated, as composition. While composition is somewhat fixed by the starting ingredients, a material's structure is influenced through a great number of steps during its processing, or manufacturing, into a finished component. While composition determines the types and amounts of atoms present, it is processing that determines the arrangements of the atoms and the number and types of defects in the atomic structure.
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