4.2.1: Properties

Several factors influence the selection of the structural materials for an aircraft. The most important one is the combination of strength and lightness. Other properties with different importance (sometimes critical) are stiffness, toughness, resistance to corrosion and fatigue, ease of fabrication, availability and consistency of supply, and cost (also very important). A brief description of some of the most important properties is given in the sequel:

Ductility: Ductility refers to a solid material’s ability to deform under tensile stress, withstanding large strains before fracture occurs. These large strains are accompanied by a visible change in cross-sectional dimensions and therefore give warning of impending failure.

Strength: The strength of a material is its ability to withstand an applied stress without failure. The applied stress may be tensile, compressive, or shear. Strength of materials is a subject which deals with loads, deformations, and the forces acting on a material. Looking at Figure 4.9, it is associated to \(\sigma_B\) (breaking stress); the greater \(\sigma_B\) is the more strengthless is the material.

Toughness: Toughness is the ability of a material to absorb energy and plastically deform without fracturing. Toughness requires a balance of strength and ductility. Strength indicates how much force the material can support, while toughness indicates how much energy a material can absorb before fracturing. Looking at Figure 4.9, it is associated to the difference between \(\sigma_B\) and \(\sigma_y\); the greater this difference is the more capacity the material ha to absorb impact energy by plastic deformation.

Brittleness: A brittle material exhibits little deformation before fracture, the strain normally being below 5%. Brittle materials therefore may fail suddenly without visible warning. Brittleness and toughness are antonyms.

Elasticity: A material is said to be elastic if deformations disappear completely on removal of the load. Looking at Figure 4.9, this property is associated to \(\sigma_y\) (elastic limit); the greater \(\sigma_y\) is the more elastic the material. Notice that, within the elastic zone, stress and strain are linearly related with the Young Modulus (E), i.e, \(\sigma = E \cdot \epsilon\).

Stiffness: Stiffness is the resistance of an elastic body to deformation by an applied force. Looking at Figure 4.9, this property is associated to \(\sigma_y\) (elastic limit); the lower \(\sigma_y\) is the more stiff the material. Elasticity and Stiffness are antonyms.

Plasticity: A material is perfectly plastic if no strain disappears after the removal of load. Ductile materials are elastoplastic and behave in an elastic manner until the elastic limit is reached after which they behave plastically. When the stress is relieved the elastic component of the strain is recovered but the plastic strain remains as a permanent set.

Fatigue: Mechanical fatigue occurs due to the application of a very large number of relatively small cyclic forces (always below the breaking stress \(\sigma_B\)) which results in material failure. For instance, every single flight of an aircraft can be considered as a cycle. In this manner, the aircraft can regularly withstand the nominal loads (always below the breaking stress \(\sigma_B\)), but after a large amount of cycles some parts of the structure might fail due to mechanical fatigue. For these reasons, aircraft may be tested for three times its life-cycle. In order to be able to withstand such testing, many aircraft components may be made stronger than is strictly necessary to meet the static strength requirements. The parts that might suffer from mechanical fatigue are termed fatigue-critical.

Corrosion: Corrosion is the gradual destruction of materials (usually metals) by chemical reaction with its environment. Roughly speaking, it has to do with the oxidation of the material and thus the loss of some of its properties. Corrosion resistance is an important factor to consider during material selection. Methods to prevent corrosion include: painting, which however incorporates an important amount of weight; anodizing, in which the aircraft is treated with a stable protective oxide layer; cladding, which basically consists of adding a layer of pure aluminum to the surface material (essentially, to attach a less noble material to a more noble material); and finally cadmium plating, which consists of covering the surface material with a more noble material (assuming the structure is made of a less noble material). These ideas are based on having two different materials with very different properties in terms of oxidation, so that if one suffers corrosion, the other does not.