distortion. Hardness may be increased by working the
distortion does result, it is referred to as strained. In
metal and, in the case of steel and certain titanium and
aircraft construction, members and parts are so
aluminum alloys, by heat treatment and cold-working
designed that the maximum loads to which they are
(discussed later). Structural parts are often formed from
subjected will never stress them beyond their elastic
metals in their soft state and then heat treated to harden
limit.
them so that the finished shape will be retained.
NOTE: Stress is the internal resistance of any
Hardness and strength are closely associated properties
metal to distortion.
of all metals.
Toughness
Brittleness
A material that possesses toughness will withstand
Brittleness is the property of a metal that allows
tearing or shearing and may be stretched or otherwise
little bending or deformation without shattering. In
deformed without breaking. Toughness is a desirable
other words, a brittle metal is apt to break or crack
property in aircraft metals.
without change of shape. Because structural metals are
often subjected to shock loads, brittleness is not a very
Density
desirable property. Cast iron, cast aluminum, and very
hard steel are brittle metals.
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
A metal that can be hammered, rolled, or pressed
manufacture. Density is an important consideration
into various shapes without cracking or breaking or
when choosing a material to be used in the design of a
other detrimental effects is said to be malleable. This
part and still maintain the proper weight and balance of
property is necessary in sheet metal that is to be worked
the aircraft.
into curved shapes such as cowlings, fairings, and wing
tips. Copper is one example of a malleable metal.
Fusibility
Ductility
Fusibility is defined as the ability of a metal to
become liquid by the application of heat. Metals are
Ductility is the property of a metal that permits it to
fused in welding. Steels fuse at approximately 2,500F,
be permanently drawn, bent, or twisted into various
and aluminum alloys at approximately 1,110F.
shapes without breaking. This property is essential for
metals used in making wire and tubing. Ductile metals
Conductivity
are greatly preferred for aircraft use because of their
ease of forming and resistance to failure under shock
Conductivity is the property that enables a metal to
loads. For this reason, aluminum alloys are used for
carry heat or electricity. The heat conductivity of a
cowl rings, fuselage and wing skin, and formed or
metal is especially important in welding, because it
extruded parts, such as ribs, spars, and bulkheads.
governs the amount of heat that will be required for
Chrome-molybdenum steel is also easily formed into
proper fusion. Conductivity of the metal, to a certain
desired shapes. Ductility is similar to malleability.
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
Elasticity is that property that enables a metal to
interference. Metals vary in their capacity to conduct
return to its original shape when the force that causes
heat. Copper, for instance, has a relatively high rate of
the change of shape is removed. This property is
heat conductivity and is a good electrical conductor.
extremely valuable, because it would be highly
undesirable to have a part permanently distorted after
Contraction and Expansion
an applied load was removed. Each metal has a point
known as the elastic limit, beyond which it cannot be
Contraction and expansion are reactions produced
loaded without causing permanent distortion. When
in metals as the result of heating or cooling. A high
metal is loaded beyond its elastic limit and permanent
degree of heat applied to a metal will cause it to expand
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