Of primary concern in aircraft maintenance are such general properties of metals and their alloys as hardness, strength, malleability, ductility, elasticity, toughness, density, brittleness, fusibility, conductivity contraction and expansion, and so forth. These terms are explained to establish a basis for further discussion of structural metals.
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| Properties of Metals |
Hardness
Hardness refers to the ability of a material to resist abrasion, penetration, cutting action, or permanent distortion. Hardness may be increased by cold working the metal and, in the case of steel and certain aluminum alloys, by heat treatment. Structural parts are often formed from metals in their soft state and are then heat treated to harden them so that the finished shape is retained. Hardness and strength are closely associated properties of metals.
Strength
One of the most important properties of a material is strength. Strength is the ability of a material to resist deformation. Strength is also the ability of a material to resist stress without breaking. The type of load or stress on the material affects the strength it exhibits.
Tensile Strength
When a piece of sheet metal is pulled from each end, the resultant force is called tension. The ability to withstand tension is called tensile strength, and is measured in pounds per square inch. Since the strength values for several metals are rated to several thousand psi, the letter "K" is often used to represent the last three zeros of the psi rating. Therefore, a tensile strength of 70,000 psi is written as 70 KSI.
Yield Strength
The ability of a metal to resist deformation is called its yield strength. For example, when a tensile load is applied to a material, the material resists any deformation until its yield point is reached. However, once the yield point is reached the metal stretches without an increase in the applied stress. Furthermore, as the metal stretches, the molecular structure changes enough to increase the metal's strength and, therefore, resist further deformation. This continues until the ultimate load is reached, at which time the material breaks.
Shear Strength
Shear strength describes a metal's ability to resist opposing forces. A rivet holding two or more sheets of metal together resisting the force of the sheets trying to slide apart is an example of a shear load. When the rivets installed in a joint have more strength than the metal in the surrounding joint, the joint is said to be loaded in shear.
Bearing Strength
Bearing strength is the ability of a joint to withstand any form of crushing or excessive compressive distortion. Material under a compression load usually fails by buckling or bending. The force at which something buckles while being compressed varies with an object's length, cross-sectional area, and shape.
Density
Density is the weight of a unit volume of a material. In aircraft work, the specified weight of a material per cubic inch is preferred since this figure can be used in determining the weight of a part before actual manufacture. Density is an important consideration when choosing a material to be used in the design of a part to maintain the proper weight and balance of the aircraft.
Malleability
A metal that can be hammered, rolled, or pressed into various shapes without cracking, breaking, or leaving some other detrimental effect, is said to be malleable. This property is necessary in sheet metal that is worked into curved shapes, such as cowlings, fairings, or wingtips. Copper is an example of a malleable metal.
Ductility
Ductility is the property of a metal that permits it to be permanently drawn, bent, or twisted into various shapes without breaking. This property is essential for metals used in making wire and tubing. Ductile metals are greatly preferred for aircraft use because of their ease of forming and resistance to failure under shock loads. For this reason, aluminum alloys are used for cowl rings, fuselage and wing skin, and formed or extruded parts, such as ribs, spars, and bulkheads. Chrome molybdenum steel is also easily formed into desired shapes. Ductility is similar to malleability.
Elasticity
Elasticity is a property that enables a metal to return to its original size and shape when the force that causes the change of shape is removed. This property is extremely valuable, because it would be highly undesirable to have a part permanently distorted after an applied load was removed. Each metal has a point known as the elastic limit, beyond which it cannot be loaded without causing permanent distortion. In aircraft construction, members and parts are so designed that the maximum loads to which they are subjected do not stress them beyond their elastic limits. This desirable property is present in spring steel.
Toughness
A material that possesses toughness withstands tearing or shearing and may be stretched or otherwise deformed without breaking. Toughness is a desirable property in aircraft metals.
Brittleness
Brittleness is the property of a metal that allows little bending or deformation without shattering. A brittle metal is apt to break or crack without change of shape. Because structural metals are often subjected to shock loads, brittleness is not a very desirable property. Cast iron, cast aluminum, and very hard steel are examples of brittle metals.
Fusibility
Fusibility is the ability of a metal to become liquid by the application of heat. Metals are fused in welding. Steels fuse around 2,600 °F and aluminum alloys at approximately 1,100 °F.
Conductivity
Conductivity is the property that enables a metal to carry heat or electricity. The heat conductivity of a metal is especially important in welding, because it governs the amount of heat that is required for proper fusion. Conductivity of the metal, to a certain extent, determines the type of jig to be used to control expansion and contraction. In aircraft, electrical conductivity must also be considered in conjunction with bonding to eliminate radio interference.
Thermal Expansion
Thermal expansion refers to contraction and expansion that are reactions produced in metals as the result of heating or cooling. Heat applied to a metal causes it to expand or become larger. Cooling and heating affect the design of welding jigs, castings, and tolerances necessary for hot rolled material.