Most materials used in structural applications are either polymers, metals, or ceramics, and in many present applications these materials perform satisfactorily in their unmodified or unreinforced form. When the thermal stability and strength of the material are not critical, low-cost polymeric materials such as acrylates, epoxies, and polycarbonates can perform acceptably. Likewise, metals such as aluminum, steel, copper, or tungsten are adequate for lightweight structural components, tooling, electrical conductors, and lamp filaments, respectively. Because cost is the controlling factor, the present performance of many ceramic structural products such as window glass, structural bricks, and cement blocks is considered satisfactory. However, in many applications where performance is the controlling factor (i.e., aerospace, transportation, underwater vessels), advanced structural materials are needed that are stronger, stiffer, lighter weight, and more resistant to hostile environments. Unreinforced, the polymer, metal, and ceramic materials available today cannot meet many of these requirements. This is especially true if the structural component must be exposed to extremely high temperatures for extended periods of time. The graph shown in Figure 1.1 provides the approximate temperature limits for the use of various structural materials.

Figure 1.1 Maximum-use temperatures of various structural materials.¹