DUCTILITY
When a metal can be permanently drawn, bent, or twisted into various
shapes without breaking, it has ductility.
This property is
essential in metals used to make wire or tubing. Ductile metals are
much preferred for aircraft because they are easily formed and resist
failure under loads.
For this reason, aluminum alloys are used for
cowl rings, fuselage, wing skin, and formed or extruded parts such as
ribs, spars, and bulkheads. Chrome-molybdenum steel is easily formed
into the desired shapes.
Although malleable and ductile are
frequently shown as synonymous, the two differ in meaning. Malleable
metals are generally shaped by compressive methods, and ductile
metals are shaped by expansive methods.
ELASTICITY
The characteristic that enables a metal to return to its original
shape when stresses are removed is elasticity.
This quality, or
property, is valuable because it is highly undesirable to have a part
permanently distorted after an applied load is removed.
However,
each metal has a point, its elastic limit, beyond which it cannot be
loaded without permanent distortion.
Members and parts used in
aircraft construction are designed so that the maximum loads to which
they are subjected never stress them beyond their elastic limits. An
example of a metal with a high elasticity limit is spring steel.
CONDUCTIVITY
The characteristic of a metal that enables it to transfer heat or
electricity is conductivity.
Heat conductivity in a metal is
especially important in welding because it governs the amount of heat
required for proper fusion.
To a certain extent, a metal's
conductivity determines the type of jig to be used to control
expansion and contraction during repairs.
Metals vary in how they
conduct heat and electricity; to eliminate radio interference,
careful thought should go into selecting metals to be bound together.
For
example,
copper
has
a
relatively
high
degree
of
heat
conductivity, and its low resistance makes it a good electrical
conductor.
Aluminum, on the other hand, is a good heat conductor,
but it is a poor electrical conductor.
DENSITY
The mass per unit volume of a substance is its density. In
aircraft work the preferred unit of volume is the cubic inch.
This
unit is convenient in determining the weight of a part
before manufacture. Give careful thought to the density of
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