laminates. These laminates can number from 2 to
ideal for use in radomes. Its high strength/weight ratio,
greater than 50, and are generally bonded to a
resistance to mildew and rot, and ease of fabrication
substructure such as aluminum or nonmetallic
make it equally suited for other parts of the aircraft.
honeycomb. The much stiffer fibers of graphite, boron,
The manufacture of reinforced plastic laminates
and Kevlar epoxies have given composite materials
involves the use of liquid resins reinforced with a filler
structural properties superior to the metal alloys they
material. The resin, when properly treated with certain
have replaced.
agents known as catalysts, or hardeners, changes to an
infusible solid.
The use of composites is not new. Fiber glass, for
example, has been used for some time in various
The reinforcement materials are impregnated with
aircraft components. However, the term advanced
the resin while the latter is still in the liquid (uncured)
composites applies to graphite, boron, and Kevlar,
state. Layers or plies of cloth are stacked up and heated
which have fibers of superior strength and stiffness.
under pressure in a mold to produce the finished, cured
The use of these advanced composite materials does
shape. Another technique, called "filament winding,"
represent a new application for naval aircraft.
consists of winding a continuous glass filament or tape,
impregnated with uncured resin, over a rotating male
Composite materials are replacing and
form. Cure is accomplished in a manner similar to the
supplementing metallic materials in various aircraft
woven cloth reinforced laminates. Glass fiber
structural components. The first materials were used
reinforced honeycomb consists of a relatively thick,
with laminated fiber glass radomes and helicopter rotor
central layer called the "core" and two outer laminates
blades. In recent years, the replacement of metallic
called "facings." (See figure 2-25.)
materials with more advanced composite materials has
rapidly accelerated. This has become particularly
The core material commonly used in radome
evident with the advent of the F/A-18, AV-8B, SH-60B,
construction consists of a honeycomb structure made of
and CH-53E aircraft; and it is anticipated that
glass cloth impregnated either with a polyester or
composite materials will continue to comprise much of
epoxy or a combination of nylon and phenolic resin.
the structure in future aircraft. As a result, there is a
The material is normally fabricated in blocks that are
growing requirement to train you in the use of advanced
later cut on a band saw to slices of the exact thickness
composite materials.
desired, or it may be originally fabricated to the proper
thickness.
There are numerous combinations of composite
materials being studied in laboratories and a number of
The facings are made up of several layers of glass
types currently used in the production of aircraft
cloth, impregnated and bonded together with resin.
components. Examples of composite materials are as
Each layer of cloth is placed in position and
follows: graphite/epoxy, Kevlar/epoxy, boron
impregnated with resin before another layer is added.
polyamide, graphite polyamide, boron-coated boron
Thicker cloths are normally used for the body of the
aluminum, coated boron titanium, boron graphite
facings, with one or more layers of finer weave cloth on
epoxy hybrid, and boron/epoxy. The trend is toward
the surface.
minimum use of boron/epoxy because of the cost when
The resins are thick, syrupy liquids of the so-called
compared to current generation of graphite/epoxy
contact-pressure type (requiring little or no pressure
composites.
during cure), sometimes referred to as contact resins.
Composites are attractive structural materials
They are usually thermosetting polyester or epoxy
because they provide a high strength/weight ratio and
resins. Cure can be affected by adding a catalyst and
offer design flexibility. In contrast to traditional
heating, or they can be cured at different temperatures
materials of construction, the properties of these
by adjusting the amount and type of catalysts.
materials can be adjusted to more efficiently match the
requirements of specific applications. However, these
Types of Composite Material
materials are highly susceptible to impact damage, and
the extent of the damage is difficult to determine
Composites are materials consisting of a
visually. Nondestructive inspection (NDI) is required to
combination of high-strength stiff fibers embedded in a
analyze the extent of damage and the effectiveness of
common matrix (binder) material; for example,
repairs. In addition, repair differs from traditional
graphite fibers and epoxy resin. Composite structures
metallic repair techniques.
are made of a number of fiber and epoxy resin
2-42
