Defense Intelligence Reference Document Materials For Advanced Aerospace Platforms

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not technology dependent but are cost intensive, no matter what technology is
employed to meet the requirements.
An additional characteristic of PMC structures-one related to the laminated
construction of components made by hand layup or automated tow placement
methods-is their susceptibility to fonnation of delaminations when impacted
perpendicular to the plane of the plies. Thfs is due to the mismatch in bending stiffness
between adjacent plies that have different unidirectional fiber orientations. This
mismatch causes shear stresses to develop that can exceed the shear strength of the
interlaminate bonds, causing small, embedded cracks to form. Under subsequent
in plane compression loading, the laminates bow because of the Poisson stresses and
separate because of the lack of an interlaminar bond to hold them together. In
significant compression loads, the laminates buckle, and this leads to structural failure.
This phenomenon, called compression after impact, is an insidious failure mode because
the delaminations are not externally detectable unless ultrasonic inspection methods
are used. Sources of such an impact include dropped tools, foreign objects (for
example, meteorites), and, perhaps most commonly, hail storms. Ultrasonic inspection,
if required, is expensive and time consuming. The latter concern in tum affects vehicle
availability and turnaround time. PMC parts made using RTM typically have reinforcing
fibers in the through-thickness direction, so concerns about compression after impact
are minimal.
The maximum temperature at which PMCs can be used is limited by PMCs' susceptibility
to oxygen degradation of the polymeric matrix. The maximum-use temperature for
prolonged exposure is determined by the thermal oxidative stabrlity (TOS) of a
particular resin. The TOS, like any chemical reaction, is determined by both time and
temperature. The glass transition temperature (T9) of the polymeric matrix also
imposes strength and dimensional stability limitations independent of the TOS limits.
This is particularly true for thermoplastics. However, the TOS limits usually impose
lower temperature limits than T9 if prolonged thennal exposure is contemplated. Three
distinct classes or groups of resins exist, each with a successively higher temperature
capability. These are conventional epoxies and most thermal plastics, bismaleimides
(BMis), and linear polyimides. The first class is limited by TOS to about 125° Celsius.
BMis can be used to about 175° Celsius. Linear polyimides, such as the in situ
polymerization of monomer reactants (PMR) group of thermosetting formulations, can
be used to about 300° Celsius. Many of the PMR resins contain the hazardous
compound methylenedianiline, which requires special care during use, including
protective clothing to limit personnel exposure (for example, during ply cutting and
hand layup). This requirement reduces productivity, adds cost, and creates a degree of
liability concern for the manufacturer of the PMC components. For RTM, the BMis and
PMR resins typically have higher viscosity and require commensurately higher injection
temperatures to reduce the viscosity to manageable levels for reasons discussed
earlier. New resins are constantly b.eing developed, including some with attractive
properties. However, many of the sources of these resins are startup companies that
exist on R&D funding, often in the form of LJ.S. government SBIR (Small Business
Innovative Research) projects. Such companies are good at innovation but often have
limited experience transitioning new products from the laboratory to large-scale
production. Furthermore, in the current economic climate, access to sufficient capital to
set up production-scale capacity can be a formidable problem for a small company.
Larger companies (for example, BASF, DuPont, GE Plastics) typically are not interested
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