hardness: a measure of strength

resilience: a measure of energy stored in an elastic manner; that is, the strain is restored when the stress is relieved

toughness (fracture toughness): a measure of resistance to crack propagation

creep resistance: a slow deformation with time under load; strain can increase without applying much stress

wearability: a measure of surface degradation mainly under exposure (e.g., corrosion)

fatigue quality: set up with alternate cycling of applied load; a good material dissipates vibration energy for a number of cycles

ability to hold strength at elevated temperature

The limit load, ultimate load, and factor of safety (FS) associated with material are described in Section 5.5.2. Limit load is up to the point where there is no permanent deformation under load. Certifying agencies stipulate strict control on aircraft structural integrity. For unpredictability (e.g., under a gust load or material defect), an FS is incorporated to accept an ultimate load when some deformation is allowed but is still below the ultimate strength. For metals, the FS = 1.5; that is, a 50% increase from the limit load is allowed. The properties of composite materials have reduced values of the stress level to allow for damage tolerance and environmental issues and to maintain an FS of 1.5 (see Section 5.6). The manufacturing process also determines the allowable stress level. These considerations can penalize part of the weight-saving associated with using lighter materials.

The strength and other properties vary among materials. Table 15.4 lists important materials used in the aircraft industry (only typical values are given). Wood has many variations and is not used much anymore.

With an increase in temperature, material properties degrade. Special alloys of steel and titanium retain better strength at elevated temperatures. Components experiencing a hot temperature have titanium and stainless-steel alloys that are available in many variations. In the quest to find still-better materials, nickel, beryllium, magnesium, and lithium alloys have been produced. The more exotic the nature of an alloy, the more costly it is. Typically, an aluminum–lithium alloy is three to four times more expensive than duralumin (in 2005). Aluminum alloys are still the dominant material used in the aircraft industry. The variety of aluminum alloys indicates a wide range available for specific uses. Various types of aluminum alloys are designated (i.e., classified) with a numbering system, as shown in Table 15.5.

15.5.2 Material Selection

Material selection for any engineering product depends on its function, shape, manufacturability (i.e., process), and cost. For aircraft applications, there is the additional consideration of weight. Within the material classes, there are subclasses of alloys: composites that offer appropriate properties to suit a product – a large variety is available with ever-increasing newer types. In the conceptual design phase, engineers must screen and rank the types of materials that suit the requirements, listing the limitations and constraints involved and whether a change to another type is

required. Military aircraft designers can use case-specific materials that have never been tried.

To select an appropriate material for a component, the following factors must be considered:

Table 15.5. Aluminum alloys (other types do not have numerical designations)

Note:

Both surfaces of the aluminum-alloy sheet are clad with copper. These have high electrical and thermal conductivity, corrosion resistance, good formability, and are not heat treatable.