constant cross-sectional area by using continuous
strand fiber roving. For improving transverse
Laminate
90/0/90
strength, mats or woven roving are added at or
near the outer surface. The ratio of continuous
roving to mat determines the mechanical proper-
ties. The use of polyester or vinyl ester resins is
Figure 5. Schematic representation of a fiber
common in the pultrusion process. The process
composite laminate.
involves pulling the continuous strand roving and
mats from one end of the line into a liquid resin
bath where strands are thoroughly wetted. Then
Manufacturing processes
the fiber resin stream is pulled first through a
FRP composites are created by combining pri-
series of preformers and then through a long, pre-
marily two materials, one serving as the reinforce-
heated die, in which final shaping, compacting,
ment and the other as the matrix. Because com-
and curing take place.
posites present such immense opportunities for
In a filament winding process, a band of resin-
tailoring the material to the specific requirements
impregnated roving (monofilaments) is wrapped
of structures, the processes available are multiple
around a rotating mandrel and cured, to produce
and varied. They include molding, compression
axisymmetric hollow components.
molding, pultrusion, filament winding, resin
SCRIMP, which is similar in concept to resin
transfer molding, and the recently developed
transfer molding (RMT), is common in the plas-
Seeman Composite Resin Infusion Molding Pro-
tics industry. SCRIMP requires only one tool side
cess (SCRIMP). The key ingredient in selecting a
and a simple vacuum bag. Before the resin is
successful production process is the cost effective-
infused, all air voids are first eliminated and the
ness, which depends largely on high rates of pro-
resin is infused in a controlled fashion. The resin
duction and uniformity of quality.
wets out the reinforcing fibers and eliminates
The early manufacturing methods included the
any voids. The major advantage of this process is
slow and labor-intensive hand-layup technique,
that the manufacture of very large parts in a high-
which slowly gave way to the methods that can
quality fashion can be accomplished without large
support mass production rates. With the introduc-
investments.
tion of automation, well-controlled resin curing
techniques, new fiber forms, and high-resolution
Durability of FRP composites
quality control tools, the manufacturing technol-
The durability of FRP composites has been re-
ogy of FRP composites is advancing at a rapid
searched in depth by many authors (Monaghan
pace.
and Brinson 1994, Hahn and Kim 1978, Dewimille
The bag molding process, which is common in
and Burnsell 1983), and their results have been
the aerospace industry, requires a "prepeg,"
briefly reviewed recently (Dutta 1995a, 1995b).
which contains fibers impregnated with resin in
The mechanisms that control the durability
a partially cured state. The prepeg plies are laid
include chemical or physical changes of the poly-
up in the desired fiber orientation angle, as well
mer matrix, the loss of adhesion or debonding at
as in the desired sequence. The entire assembly
the fiber/matrix interface, and the reduction of
is then cured in a preheated autoclave. While the
fiber strength and modulus. Environment plays
bag molding process is relatively simple, it can-
a crucial role in changing the properties of poly-
not handle parts of complex geometry. For parts
mer matrix composites. By environment we mean
of complex geometry, the compression molding
both the ambient environment and the loading
process is more suited. It involves transferring
environment, because both can affect the durabil-
sheet molding compounds (SMC) into finished
ity of the composites. Considering the ambient
products in relatively short periods of time. In this
environment, we find that both matrix and fibers
process, precut amounts of SMC plies are placed
may be affected by moisture, temperature, sun-
as a charge on to the bottom half of a preheated
light (UV radiation), ozone, and the presence of
mold cavity, and the top half of the mold is then
degrading chemicals like salts or alkalis. Repeated
lowered on the charge, applying pressure on it
excursions to very high and low temperatures
and squeezing out the excess resin. With the
(freeze-thaw cycling) may also introduce some
applied heat, the charge assembly is cured.
changes. Under a mechanical loading environ-
Pultrusion is a continuous molding process. It
ment, as in steel, repeated loading may introduce
produces long, straight structural members of
5
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