The industry must constantly revise its practices to ensure continued system reliability and safety and structural integrity. Advanced materials such as composites are used extensively in military aircraft, helicopters, and business planes. In large civilian transport aircraft, the introduction of such materials is much slower (Table 2.5); they appear primarily in secondary structures. Broader use of composite materials will require large changes in design and in manufacturing plants.

Advances in turbine airfoil materials are illustrated in Figure 2.1. The volume of materials consumed by the industry is not large (e.g., about 80,000 tons/year for large commercial aircraft), but the value is extraordinary. The cost of a commercial airframe is approximately the value of its weight in silver. The cost of a spacecraft approximates the value of its weight in gold. Because of these economic factors, substantial costs can be tolerated for materials that possess the desired combination of properties.

Some principal determinants in the selection of materials for the aerospace industry are life cycle cost, strength-to-weight ratio, fatigue life, fracture toughness, survivability, and reliability. Additional considerations for spacecraft include high specific stiffness and strength, a low coefficient of thermal expansion, and durability in a space environment.

The payoff for successful materials development can be large. In a shuttlelike orbiter, for example, replacement of conventional aluminum airframes with currently unobtainable aluminum/silicon carbide or magnesium-graphite composites would yield a severalfold increase in pay load capability. Similarly, a major reduction in airframe weight could lead to an increase in fuel efficiency that would make an aircraft attractive to commercial airlines. Lifetime sales for a successful new generation of commercial transport planes could be expected to amount to about $45 billion.

Conventional materials (e.g., metals, alloys, ceramics, and polymer composites) are approaching developmental limits in terms of properties for aerospace applications. This limit is based on fatigue (or service life) criteria