Corrosive Properties
or become larger. Cooling hot metal will shrink or
contract it. Contraction and expansion affect the design
Corrosion is the eating away or pitting of the
of welding jigs, castings, and tolerances necessary for
surface or the internal structure of metals. Because of
hot-rolled material.
the thin sections and the safety factors used in aircraft
design and construction, it would be dangerous to select
QUALITIES OF METALS
a material subject to severe corrosion if it were not
The selection of proper materials is a primary
possible to reduce or eliminate the hazard. Corrosion
consideration in the development of an airframe and in
can be reduced or prevented by using better grades of
the proper maintenance and repair of aircraft. Keeping
base metals; by coating the surfaces with a thin coating
in mind the general properties of metals, it is now
of paint, tin, chromium, or cadmium; or by an
possible to consider the specific requirements that
electrochemical process called "anodizing." Corrosion
metals must meet to be suitable for aircraft purposes.
control is discussed at length in Aviation Maintenance
Ratings Fundamentals, and it is not covered in detail in
Strength, weight, and reliability determine the
this course.
requirements to be met by any material used in airframe
construction and repair. Airframes must be strong and
Working Properties
as light in weight as possible. There are very definite
limits to which increases in strength can be
Another significant factor to consider in the
accompanied by increase in weight. An aircraft so
selection of metals for aircraft maintenance and repair
heavy that it could not support more than a few hundred
is the ability of material to be formed, bent, or
pounds of additional weight would be of little use. All
machined to required shapes. The hardening of metals
metals, in addition to having a good strength/weight
by cold-working or forming is called work hardening.
ratio, must be thoroughly reliable, thus minimizing the
If a piece of metal is formed (shaped or bent) while
possibility of dangerous and unexpected failures. In
cold, it is said to be cold-worked. Practically all the
addition to these general properties, the material
work you do on metal is cold-work. While this is
selected for definite application must possess specific
convenient, it causes the metal to become harder and
qualities suitable for the purpose. These specific
more brittle.
qualities are strength, weight, corrosive properties,
If the metal is cold-worked too much (that is, if it is
working properties, joining properties, and shock and
bent back and forth or hammered at the same place too
fatigue properties.
often), it will crack or break. Usually, the more
Strength
malleable and ductile a metal is, the more cold-working
it can withstand.
The material must possess the strength required by
the demands of dimensions, weight, and use. There are
Joining Properties
five basic stresses that metals may be required to
Joining metals structurally by welding, brazing, or
withstand. These are tension, compression, shear,
soldering, or by such mechanical means as riveting or
bending, and torsion.
bolting, is a tremendous help in design and fabrication.
Weight
When all other properties are equal, material that can be
welded has the advantage.
The relationship between the strength of a material
and its weight per cubic inch, expressed as a ratio, is
Shock and Fatigue Properties
known as the strength/weight ratio. This ratio forms the
Aircraft metals are subject to both shock and
basis of comparing the desirability of various materials
fatigue (vibration) stresses. Fatigue occurs in materials
for use in airframe construction and repair. Neither
that are exposed to frequent reversals of loading or
strength nor weight alone can be used as a means of true
repeatedly applied loads, if the fatigue limit is reached
comparison. In some applications, such as the skin of
or exceeded. Repeated vibration or bending will
monocoque structures, thickness is more important
ultimately cause a minute crack to occur at the weakest
than strength; and in this instance, the material with the
point. As vibration or bending continues, the crack
lightest weight for a given thickness or gauge is best.
lengthens until complete failure of the part occurs. This
Thickness or bulk is necessary to prevent buckling or
is termed "shock and fatigue failure." Resistance to this
damage caused by careless handling.
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