Defense Intelligence Reference Document Materials For Advanced Aerospace Platforms

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leaving voids. This limitation makes it difficult to use many thermoplastic resins that
are otherwise attractive because they are recyclable and have much longer shelf
lives than thermosets (epoxies}. Another, more obvious limitation of RTM is
component size. This is in part because weaving of very large fiber preforms is
challenging and requires a very large weaving machine. It also requires a large
injection machine capable of multiple injection sites to ensure the complete
infiltration necessary to avoid formation of voids.
In practice, there are several significant challenges associated with manufacturing large
PMC structures today. Among these, perhaps the most significant are manufacturing
cost and the difficulty associated with making large, nonaxisymmetric components.
Large axisymmetric components that can be produced by winding tows of pre-preg are
clearly possible today, as demonstrated by the Boeing 787 fuselage whose section is
shown in Figure 2. For other shapes, because the load-bearing capability of PMCs
depends on the transfer of externally imposed loads to the strong, stiff fibers, joints
that intersect the principal load path become problematic. This is because the fibers
and, as a direct consequence1 the load path are discontinuous, requiring local section
size adjustments to offset this local structural inefficiency. Thus 1 achieving the most
structurally efficient use of PMCs requires monolithic structures with continuous fibers.
In large structures, this is at best a challenge. Even in the Boeing 787's PMC fuselage,
mechanically fastened joints are used to connect adjacent barrel sections. In the
fuselage, as in any cylindrical pressure vessel, the principal stresses are hoop stresses
stemming from pressurization during flight. However, the compromise in structural
efficiency is minimized by the circumferential orientation of the joints. Although these
bolted joints add weight, the overall structural efficiency of the PMC structure is still
better, albeit considerably more expensive, than a longitudinally and circumferentially
stiffened Al structure. The circumferential joints also create discontinuous longitudinal
crack paths that improve the structure's damage tolerance. Some of the added expense
of the PMC fuselage stems from the use of Ti alloy fasteners because of the galvanic
coupling issues that would accompany Al or steel fasteners.
For other applications, which are limited by different material properties, the PMC
system can be tailored to optimize structural performance. This is possible because
both the matrix and the fiber can be independently selected. Moreover, the fiber
\\architecture" (fiber orientation, weave geometry, and fiber volume fraction) can be
varied spatially to optimize load-bearing capability under complex stress states. For
example, again drawing on recent applications in commercial aircraft, the fan blades of
the large, high-bypass-ratio turbofan engine (GE90) produced by General Electric for
the Boeing 777 are made from PMCs. The limiting design consideration for these fan
blades is resistance to bird strikes. To optimize the PMCs' impact resistance, a medium-
modulus, high-tensile-strength carbon fiber was selected in combination with a
thermoplastic toughened epoxy matrix. Furthermore, the fiber architecture was set to
optimize the bending strength under the impact of a bird. The GE90 fan blades are
produced by hand layup and are quite costly to produce. In the 10-plus years that
these fan blades have been in service, not a single unscheduled engine removal related
to the PMC fan blades has occurred. The competitive fan blade technology for B-777-
class engines is hollow Ti, which is used by both Pratt & Whitney and Rolls Royce. By all
informal accounts, these blades are cost intensive. This example supports the unwritten
rule that the pathways leading to high-performance, high-value structures typically are
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