Under load (i. e., stress), all materials deform (i. e., strain) – some more than others – but they can recover their original shape when the load is removed, provided that the application is within a specific limit. Beyond this load level, materials do not recover to their original shape. See , , and  for details on stress and strain.
Stress is the applied force per unit area of a material. It is termed as tensile or compressive stress when the force is acting normal to an area and shear stress when it is acting tangentially. The associated deformation per unit length or area is the normal or shear strain, respectively. How a material is prepared affects the
Wing to body fairing
T. E. flap linkage fairing
(a) Aluminum alloy
Figure 15.12. Material stress-strain relationship
characteristics of the stress-strain relationship. The nature of alloys, crystal formation, heat treatment, and cooling affects a materials characteristics.
A typical stress-strain characteristic of an aluminum alloy is shown in Figure 15.12. The figure shows that initially, the stress-strain relationship behaves linearly according to Hooke’s Law, which represents the elastic property of a material. Within the elastic limit, the material strength (i. e., how much stress it can bear) and stiffness (i. e., how much deformation occurs) are the two main properties considered by designers in choosing materials; of course, the cost, weight, and other properties are also factors to consider. The maximum point within which linearity holds is the yield point. Past the yield point, permanent deformation occurs: The material behaves like plastic and the slope is no longer linear. The highest point in the stress-strain graph is known as the ultimate strength, beyond which the component continues to deform and results in a rupture that is a catastrophic failure. The linear portion gives the following:
stress/strain = constant = Young’s Modulus (the slope of the graph)
Sometimes raw material is supplied with a small amount of prestretching (i. e., strain hardening) and a permanent deformation set in which the yield point is higher. Typically, some aluminum sheet metals are supplied with 0.2% built-in prestretched strain (see Figure 15.12a). However, with prestretching, the ultimate strength is unchanged. Figure 15.12b compares various types of typical aircraft materials. A steeper slope indicates higher stiffness, which often has a higher elastic limit. Brittle materials rupture abruptly with minimal strain buildup; ductile materials exhibit significant strain buildup before rupturing, thereby warning of an imminent failure. Rubber-like materials do not have a linear stress-strain relationship. The pertinent properties associated with materials follow (some are shown in Figure 15.12): 
• 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.