the base metal. All other metals added to the alloy are
called alloying elements. Alloying elements, in either
small or large amounts, may result in a marked change
in the properties of the base metal. For example, pure
aluminum is relatively soft and weak. When small
amounts of other elements such as copper, manganese,
and magnesium are added, aluminum’s strength is
increased many times. An increase or a decrease in an
alloy’s strength and hardness may be achieved through
heat treatment of the alloy. Alloys are of great
importance to the aircraft industry. Alloys provide
materials with properties not possessed by a pure metal
alone.
Alloy steels that are of much greater strength than
those found in other fields of engineering have been
developed. These steels contain small percentages of
carbon, nickel, chromium, vanadium, and molybdenum.
High-tensile steels will stand stresses of 50 to 150 tons
per square inch without failing. Such steels are made
into tubes, rods, and wires.
Another type of steel that is used extensively is
stainless steel. This alloy resists corrosion and is
particularly valuable for use in or near salt water.
COMMON NONMETALLIC MATERIALS
In addition to metals, various types of plastic
materials are found in aircraft construction. Transparent
plastic is found in canopies, windshields, and other
transparent enclosures. Handle transparent plastic
surfaces with care, because this material is relatively
soft and scratches easily. At approximately 225°F,
transparent plastic becomes soft and very pliable.
Reinforced plastic is made for use in the con-
struction of radomes, wing tips, stabilizer tips, antenna
covers, and flight controls. Reinforced plastic has a high
strength-to-weight ratio and is resistant to mildew and
rot. Its ease of fabrication make it equally suitable for
other parts of the aircraft.
Reinforced plastic is a sandwich-type material. See
figure 1-22. It is made up of two outer facings and a
center layer. The facings are made up of several layers
of glass cloth, bonded together with a liquid resin. The
core material (center layer) consists of a honeycomb
structure made of glass cloth. Reinforced plastic is
fabricated into a variety of cell sizes.
High-performance aircraft require an extra high
strength-to-weight ratio material. Fabrication of
composite materials satisfies the special requirement.
This construction method uses several layers of bonding
Figure 1-22.—Reinforced plastic.
materials (graphite epoxy or boron epoxy). These
materials are mechanically fastened to conventional
substructures. Another type of composite construction
consists of thin graphite epoxy skins bonded to an
aluminum honeycomb core.
METALLIC MATERIALS
Learning Objective: Identify properties of
metallic materials used in aircraft con-
struction.
Metallurgists have been working for more than a
half century improving metals for aircraft construction.
Each metal has certain properties and characteristics that
make it desirable for a particular application, but it may
have other qualities that are undesirable. For example,
some metals are hard, others comparatively soft; some
are brittle, some lough; some can be formed and shaped
without fracture; and some are so heavy that weight
alone makes them unsuitable for aircraft use. The
metallurgist’s objectives are to improve the desirable
qualities and tone down or eliminate the undesirable
ones. This is done by alloying (combining) metals and
by various heat-treating processes.
You do not have to be a metallurgist to be a good
AM, but you should possess a knowledge and under-
standing of the uses, strengths, limitations, and other
characteristics of aircraft structural metals. Such
knowledge and understanding is vital to properly
construct and maintain any equipment, especially
airframes. In aircraft maintenance and repair, even a
slight deviation from design specifications or the
substitution of inferior materials may result in the loss
of both lives and equipment. The use of unsuitable
materials can readily erase the finest craftsmanship. The
selection of the specific material for a specific repair job
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