material to be used because it affects the weight and balance of an
aircraft.
CONTRACTION AND EXPANSION
Heat applied to a metal causes it to expand or become larger, and
cooling metal causes it to contract or shrink.
Contraction and
expansion affect the design of welding jigs, castings, and tolerances
necessary for hot-rolled metal.
STRENGTH AND TOUGHNESS
When a metal can hold loads or withstand an applied force without
breaking, it has strength. This is a property that encompasses many
of the desirable qualities of metals. Strength with toughness is the
most important combination of properties a metal can possess. Metals
with this combination are used for vital structural members that may
become overloaded in service. Toughness describes the resistance of
a metal to tearing or shearing and permits the metal to be stretched
or otherwise deformed without breaking.
STRESS
This paragraph discusses the stresses associated with the qualities
or characteristics of metals.
Stress is a force placed upon a body
and is measured in terms of units of force per unit of area.
The
force is usually expressed in pounds and the unit of area in square
inches.
More simply, the expression is stated in pounds per square
inch (psi).
Stress can be in the form of compression, tension,
torsion, bending, shearing, or a combination of two or more of these.
All parts of an aircraft are subject to stresses.
The various
stresses acting on the aircraft parts while in flight have an
important bearing on the choice of metals used. The paragraphs that
follow describe the stresses. Refer to Figure 1-10a for examples.
Compression.
Compression is the decrease of volume of a
compressible
substance
because
of
pressure
being
applied.
Compressive strength is the resistance to applied pressure. Examples
of compression are the pressure applied to the fuel-air mixture in an
engine cylinder and the pressure applied to an airplane's landing
gear during landing.
Tension. Tension is the force or combination of forces that pulls
or stretches a material. The measurement of a material's resistance
to stretching or tearing is the material's tensile strength.
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