demands familiarity with the most common properties
of various metals.
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
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 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.
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 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 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.
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 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
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.