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strength-to-weight ratio and offer high fuel efficiency over the lifetime of the aircraft. There has been a steady
increase in the use of composites in both military and commercial transport aircraft. Until recently, the use of composite
materials in commercial transports was limited to non-load or low-low carrying structural parts. However, with
the modern technology, composites are increasingly used for primary, load-carrying structural parts. There has
been a three-fold increase in the structural weight of composite parts used in Boeing 777 aircraft compared to the
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previous generation airplanes. In Boeing 777, the vertical and horizontal tail sections as well as major wing sections
were made with toughened carbon fiber-reinforced epoxy composite [7]. Given the lightweight, low cost, and performance
advantages of advanced composite materials, their use in future aircraft will continue to grow. Boeing estimates
that the average structural weight fraction of polymer composites in their commercial airplanes will increase
from about 7 percent currently to about 20 percent over the next 15 years [7-8]. Already, smaller business aircraft
and helicopters are now being produced with entire aircraft structure made from composite materials [9].
Polymer matrix composites are engineered materials comprised of continuous, high-strength fibers impregnated
with a polymer matrix to form a reinforced layer (ply), which is subsequently bonded together with other layers under
heat and pressure to form an laminate. The resin acts to hold the fibers together and protect them and to transfer the
load to the fibers in the fabricated composite part. The strength and stiffness of the laminate are determined by the
orientation of the fibers with respect to the loading direction and their volume fraction in the composite. For a typical
polymer matrix composite, fibers comprise about 55-60 volume percent of the laminate with the polymer resin being
the remainder.
Resins
There are two main classes of polymer matrix composites depending upon the type of resin used, thermosets and
thermoplastics. Thermoset resins are the predominant type in use today with epoxies and phenolics by far being the
most dominant resins in commercial aircraft applications because they are relatively tough, easy to process, and
require moderate forming temperature. Composite panel materials used in aircraft cabin interiors are required to
comply with strict heat release rate regulations. Epoxies are highly flammable and thus cannot be used in composites
for large surface area, interior panels such as partitions, stowage bins, galley walls, and ceilings. Phenolics are
currently the thermoset resin of choice for aircraft interiors because of their low heat release rate. Thermoplastic
resins have found limited use as matrix resins in aircraft interior and structural composites because they require high
forming temperatures in manufacturing. Unlike the thermosets, the thermoplastics can usually be reheated and
reformed into another shape, if required. Thermally stable engineering thermoplastics, such as polyetheretherketone
and polysulfone, are used as resin tougheners in commercial and military applications [8]. The development of highperformance
thermoplastic resin systems is an area of evolving research that holds great promise for future applications
of polymer matrix composites.
Carbon/Graphite Fibers
Continuous carbon fibers are the most commonly used reinforcement materials because of their high strength-toweight
ratio. Fibers are used alone or in combination with other fibers (hybrids) in the form of continuous fiber
fabrics, tapes, and tows or as discontinuous chopped strands. Common raw materials, also known as precursors,
for carbon and graphite fibers are polyacrylonitrile (PAN), rayon, and petroleum pitch. The synthesis process involves
controlled pyrolysis at 1000-2000 C for carbon fibers while graphite fibers require pyrolysis temperatures of
2000-3000 C and contain 93-95 and 99 percent carbon atoms, respectively [8]. Typical carbon fibers used as
reinforcement material in composites are 6-8 μm in diameter.
HAZARDS FROM INHALATION EXPOSURE
The assessment of fiber toxicity is complex process that generally requires substantial epidemiological data along
with long-term exposure studies for the health effects on humans. This is because it a long time for the biological
processes to manifest following the exposure. The gestation period for physiological changes in humans may be as
long as 20-30 years depending upon the severity and duration of the exposure.
There are two major routes to exposure from fibers - dermal and inhalation. Dermatitis results from mechanical or
chemical irritation or sensitization of the skin. This condition is a typical response to surface abrasion and puncture
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