Defense Intelligence Reference Document Materials For Advanced Aerospace Platforms

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construction to be considered too risky. Today, the combination of higher toughness
alloys and FSW opens up the possibility of greater design flexibility resulting in lighter
large structures with equal or greater reliability than earlier ones.
In sum, metallic, nonreusable (at least nominally so) launch vehicles made from
advanced Al alloys and fabricated through FSW constitute an incremental but significant
improvement over earlier versions.
In recent years, PMCs have matured significantly. For many components that are not
exposed to elevated temperatures, PMCs provide a degree of design flexibility not
readily available in metals. Consequently, PMC materials have begun to supplant Al
alloys in the construction of commercial subsonic aircraft. The use of PMCs in the
empennage of the Boeing 777 was one of the first examples of Al alloys being
displaced. Subsequently, the new Boeing 787 has more structure made from
composites than from metallic materials. Once PMCs are introduced into a structure in
significant quantities, a constraint related to galvanic incompatibility between the PMC
structure and any adjoining Al alloys also is introduced. When a PMC structure is in
direct contact with an Al alloy structure, catastrophic corrosion of the Al alloy
components can occur. In the Boeing 787, the remedy for this concern is the use of
titanium (Ti) alloys in areas where there is direct contact between the metallic and the
PMC structures. This is directly analogous to the plastic bushing a plumber puts in the
joint between copper and iron piping. Notwithstanding this constraint, the specific
strength and stiffness of PMC structures make a compelling argument for their
application in high-performance structures, such as launch vehicles.
Composite structures can be manufactured using one of three methods: hand layup of
pre-preg, automated tow placement, and resin transfer molding.
• The most rudimentary of these, but also the most flexible, is hand layup of pre-preg.
This method uses sheets of material that contain both the fiber and the polymeric
matrix (called pre-preg). The polymeric matrix can be either a thermoset (for
example, epoxy) or a thermoplastic. Individual plies are cut from the pre-preg
typically using a numerfcally controlled laser or mechanical cutting device and are
laid up to form the desired shape. Areas that have heavier loads contain more plies
locally, and the plies are cut in an orientation with respect to the fiber direction in
the pre-preg to achieve the desired strength relative to the principal load path.
These plies are carefully placed according to a drawing (blueprint), making hand
layup a labor-intensive process and, therefore, making parts made using this
method expensive. During ply placement, it is critical that no ply wrinkles are
introduced, as these create severe reductions in the local load-bearing capability of
the final component. Once all the plies are in their proper places, the article is
placed in a vacuum-tight bag that is evacuated and placed in an autoclave for curing
of the epoxy matrix or fusing of the thermoplastic. A disadvantage of a pre-preg
whose matrix is a thermoset is limited shelf life. In practice, this is managed to a
degree by storing the pre-preg in a freezer to slow the rate of chemical reaction that
sets the epoxy. However, this does not completely halt the reaction, causing these
materials to have a shelf life beyond which they are not easily manipulated during
layup and do not develop full strength after curing in the autoclave. An additional
issue is out time-the time the pre-preg can be out of the freezer during layup
before the reaction proceeds at an accelerated rate and reaches a point at which the
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