PROPERTIES OF METALS
This section is devoted primarily to the terms used in
describing various properties and characteristics of metals in general. Of
primary concern in aircraft maintenance are such general properties of metals
and their alloys as hardness, brittleness, malleability, ductility, elasticity,
toughness, density, fusibility, conductivity, and contraction and expansion.
You must know the definition of the terms included here because they form the
basis for further discussion of aircraft metals.
Hardness
Hardness refers to the ability of a metal to resist
abrasion, penetration, cutting action, or permanent distortion. Hardness may be
increased by working the metal and, in the case of steel and certain titanium
and aluminum alloys, by heat treatment and cold-working (discussed later).
Structural parts are often formed from metals in their soft state and then heat
treated to harden them so that the finished shape will be retained. Hardness
and strength are closely associated properties of all metals.
Brittleness
Brittleness is the property of a metal that allows little
bending or deformation without shattering. In other words, 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 brittle metals.
Malleability
A metal that can be hammered, rolled, or pressed into
various shapes without cracking or breaking or other detrimental effects is
said to be malleable. This property is necessary in sheet metal that is to be
worked into curved shapes such as cowlings, fairings, and wing tips. Copper is
one 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 that property that enables a metal to return
to its original 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. When metal is loaded
beyond its elastic limit and permanent distortion does result, it is referred
to as strained. In aircraft construction, members and parts are so designed
that the maximum loads to which they are subjected will never stress them
beyond their elastic limit.
NOTE: Stress is the internal resistance of any metal to
distortion.
Toughness
A material that possesses toughness will withstand tearing
or shearing and may be stretched or otherwise deformed without breaking.
Toughness is a desirable property in aircraft metals.
Density
Density is the weight of a unit volume of a material. In
aircraft work, the actual 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 and still maintain the proper weight and
balance of the aircraft.
Fusibility
Fusibility is defined as the ability of a metal to become
liquid by the application of heat. Metals are fused in welding. Steels fuse at
approximately 2,500°F, and aluminum alloys at approximately 1, 110°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 will be 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,
which is used to eliminate radio interference. Metals vary in their capacity to
conduct heat. Copper, for instance, has a relatively high rate of heat
conductivity and is a good electrical conductor.
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