MATERIAL FORMS AND MANUFACTURINGComposite materials come in a wide variety of material forms. The fiber
itself may be used in continuous form or as a chopped fiber. Chopped
glass fibers are used typically to reinforce various polymers, with concomitant
lower strength and stiffness relative to continuous fiber composites.
Chopped fibers in conjunction with automated fabrication techniques
have been utilized to fabricate automotive body parts at high
production rates.
Continuous-fiber materials are available in a number of different
forms, with the specific form utilized depending on the manufacturing
process. Thus it is useful to consider both the material and the manufacturing
process at the same time. The fibers themselves have a small
diameter, with sizes of 5 to 7 mm (0.0002 to 0.0003 in) typical for
carbon fibers. A large number of fibers, from 2,000 to 12,000, are
gathered in the manufacturing process to form a tow (also called a roving
or yarn). The filament winding process utilizes these tows directly. The
tows may be further processed by prepregging, the process of coating the
individual fibers with the matrix material. This process is widely used
with thermosetting polymeric resins. The resin is partially cured, and
the resulting ‘‘ply’’ is placed on a paper backing. The prepregged material
is available in continuous rolls in widths of 75 to 1,000 mm
(3 to 40 in). These rolls must be kept refrigerated until they are utilized
and the assembled product is cured. Note that the ply consists of a
number of fibers through its thickness, and that the fibers are aligned
and continuous. Typical volume fractions of fiber are on the order of 60
percent. The material forms discussed here are used in a variety of
specific manufacturing techniques. Some of the more popular techniques
are described briefly below.
Filament Winding The process consists of winding the fiber around
a mandrel to form the structure. Usually the mandrel rotates while fiber
placement is synchronized to proceed in a longitudinal direction. The
matrix may be added to the fiber by passing the fiber tow through a
matrix bath at the time of placement, a process called wet winding; or the
tows may be prepregged prior to winding. Filament winding is widely
used to make glass fiber pipe, rocket motor cases, and similar products.
Filament winding is a highly automated process, with typical low manufacturing
costs. Obviously, it lends itself most readily to axisymmetric
shapes, but a number of specialized techniques are being considered for
nonaxisymmetric shapes.
Prepreg Layup This common procedure involves laying together
individual plies of prepregged composite into the final laminated structure.
A mold may be used to control the part geometry. The plies are laid
down in the desired pattern, and then they are wrapped with several
additional materials used in the curing process. The objective is to remove
volatiles and excess air to facilitate consolidation of the laminate.
To this end, the laminate is covered with a peel ply, for removal of the
other curing materials, and a breather ply, which is often a fiberglass net.
Optionally, a bleeder may be used to absorb excess resin, although the
net resin process omits this step. Finally, the assembly is covered with a
vacuum bag and is sealed at the edges. A vacuum is drawn, and after
inspection heat is applied. If an autoclave is used, pressure on the order
of 0.1 to 0.7 MPa (20 to 100 lb/in2) is applied to ensure final consolidation.
Autoclave processing ensures good lamination but requires a
somewhat expensive piece of machinery. Note that the individual plies
are relatively thin [on the order of 0.13 mm (0.005 in)], so that a large
number of plies will be required for thick parts. The lamination process
is often performed by hand, although automated tape-laying machines
are available. Although unidirectional plies have been described here,
cloth layers can also be used. The bends in the individual fibers that
occur while using cloth layers carry a performance penalty, but manufacturing
considerations such as drapeability may make cloth layers
desirable.
Automated Tape and Tow Placement Automated machinery is
used for tape layup and fiber (tow) placement. These machines can be
large enough to construct wing panels or other large structures. Thermosetting
matrices have been used extensively, and developments using
thermoplastic matrices are underway.
Textile Forms The individual tows may be combined in a variety of
textile processes such as braiding and weaving. Preforms made in these
ways then can be impregnated with resin, often called resin transfer
molding (RTM). These textile processes can be designed to place fibers
in the through-the-thickness direction, to impart higher strength in this
direction and to eliminate the possibility of delamination. There is considerable
development underway in using braided and/or stitched preforms
with RTM because of the potential for automation and high production
rates.
DESIGN AND ANALYSIS
The fundamental way in which fiber composites, and in particular continuous-
fiber composites, differ from conventional engineering materials
such as metals is that their properties are highly directional. Stiffness
and strength in the fiber direction may be higher than in the direction
transverse to the fibers by factors of 20 and 50, respectively. Thus a
basic principle is to align the fibers in directions where stiffness and
strength are needed. A well-developed theoretical basis called classical
lamination theory is available to predict stiffness and to calculate stresses
within the layers of a laminate. This theory is often applied to filamentwound
structures and to textile preform structures, if allowances are
made for the undulations in the fiber path. The basic assumption is that
fiber composites are orthotropic materials. Thin composite layers require
four independent elasticity constants to characterize the stiffness;
those are the fiber direction modulus, transverse direction modulus,
in-plane shear modulus, and one of the two Poisson ratios, with the
other related through symmetry of the orthotropic stress-strain matrix.
The material constants are routinely provided by material suppliers.
Lamination theory is then used to predict the overall stiffness of the
laminate and to calculate stresses within the individual layers under
mechanical and thermal loads.
Transverse properties are much lower than those in the fiber direction,
so that fibers must usually be oriented in more than one direction,
even if only to take care of secondary loads. For example, a laminate
may consist of fibers in an axial direction combined with fibers oriented
at 660° to this direction, commonly designated as an [0m/660n]s laminate,
where m and n refer to the number of plies in the axial and 660°
directions and s stands for symmetry with respect to the midplane of the
laminate. Although not all laminates are designed to be symmetric,
residual stresses will cause flat panels to curve when cooled from elevated-
temperature processing unless they are symmetric. Other popular
laminates have fibers oriented at 0°, 645°, and 90°. If the relative
amounts of fibers are equal in the [0/660] or the [0/645/90] directions,
the laminate has in-plane stiffness properties equal in all directions and
is thus termed quasi-isotropic.
Note that the stiffness of composite laminates is less than that of the
fibers themselves for two reasons. First, the individual plies contain
fibers and often a much less stiff matrix. With high-stiffness fibers and
polymeric matrices, the contribution of the matrix can be neglected, so
that the stiffness in the fiber direction is essentially the fiber volume
fraction VF times the fiber stiffness. Fiber volume fractions are often on
the order of 60 percent. Second, laminates contain fibers oriented in
more than one direction, so that the stiffness in any one direction is less
than it would be in the fiber direction of a unidirectional laminate. As an
example, the in-plane stiffness of a quasi-isotropic carbon fiber/polymeric
matrix laminate is about 45 percent of unidirectional plies, which
in turn is approximately 60 percent of the fiber modulus for a 60 percent
fiber volume fraction material.
Strength Properties Fiber composites typically have excellent
strength-to-weight properties; see Table 6.13.1. Like stiffness properties,
strength properties are reduced by dilution by the weaker matrix,
and because not all the fibers can be oriented in one direction. The
overall reduction in apparent strength because of these two factors is
similar to that for stiffness, although this is a very rough guide. As an
example, the strength of a quasi-isotropic carbon/epoxy laminate under
uniaxial tensile load has been reported to be about 35 to 40 percent of
that provided by the same material in a unidirectional form.
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