1.Reinforcements
Reinforcements are important constituents of a composite material and give
all the necessary stiffness and strength to the composite. These are thin rod
like structures. The most common reinforcements are glass, carbon, aramid
and boron fibers. Typical fiber diameters range from 5m
m (0.0002 in.) to 20m m (0.0008 in.). The diameter of a glass fiber is in the
range of 5 to 25mma carbon fiber is 5 to 8mm, an aramid fiber is 12.5
mm, and a boron fiber is100mm. Because of this thin diameter, the fiber is
flexible and easily conforms to various shapes. In general, fibers are made
into strands for weaving or winding operations. For delivery purposes,
fibers are wound around a bobbin and collectively called a “roving.” An
untwisted bundle of carbon fibers is called “tow.” In composites, the
strength and stiffness are provided by the fibers. The matrix gives rigidity to
the structure and transfers the load to fibers.
Fibers for composite materials can come in many forms, from continuous
fibers to discontinuous fibers, long fibers to short fibers, organic fibers to
inorganic fibers. The most widely used fiber materials in fiber-reinforced
plastics (FRP) are glass, carbon, aramid, and boron. Glass is found in
abundance and glass fibers are the cheapest among all other types of fibers.
There are three major types of glass fibers: E-glass, S-glass, and S2-glass.
The properties of these fibers are given in Table 1. The cost of E-glass is
around $1.00/lb, S-glass is around $8.00/lb, and S-2 glass is $5.00/lb.
Carbon fibers range from low to high modulus and low to high strength.
Cost of carbon fibers.
Aramid fibers cost approximately $15.00 to $20.00/lb. Some of the
common types of reinforcements include:
• Continuous carbon tow, glass roving, aramid yarn
• Discontinuous chopped fibers
• Woven fabric
• Multidirectional fabric (stitch bonded for three-dimensional
properties)
• Stapled
• Woven or knitted three-dimensional preforms
Continuous fibers are used with most thermoset and thermoplastic resin
systems. Chopped fibers are used for making injection molding and
compression molding compounds.
Chopped fibers are made by cutting the continuous fibers. Woven fabrics
are used for making prepregs as well as for making laminates for a variety
of applications (e.g. ,boating, marine, and sporting).
1.1 Glass Fiber
The properties of fibers depend on how the fibers are manufactured. The
raw materials used for making E-glass fibers are silica sand, limestone,
fluorspar, boric acid, and clay. Silica accounts for more than 50% of the
total ingredients.
By varying the amounts of raw materials and the processing parameters,
other glass types are produced. The raw materials are mixed thoroughly and
melted in a furnace at 2,500 to 3,000°F. The melt flows into one or more
bushings containing hundreds of small orifices. The glass filaments are
formed as the molten glass passes through these orifices and successively
goes through a quench area where water and/or air quickly cool the
filaments below the glass transition temperature. The filaments are then
pulled over a roller at a speed around 50 miles per hour. The roller coats
them with sizing. The amount of sizing used ranges from 0.25 to 6% of the
original fiber weight. All the filaments are then pulled into a single strand
and wound onto a tube.
Sizing is applied to the filaments to serve several purposes; it promotes
easy fiber wetting and processing, provides better resin and fiber bonding,
and protects fibers from breakage during handling and processing. The
sizing formulation depends on the type of application; for example, sizing
used for epoxy would be different than that used for polyester.
Due to their low cost, high tensile strengnth ,high impact resistance and
good chemical resistance. Glass fibers are used extensively in commercial
composite applications. However their properties cannot match those of
carbon fibers for high performance composite applications. They possess
low modulus and high fatigue properties compered to carbon fibers. The
maximum use temperature used for glass fibers ranges from 930oF for Eglass up to 1920 oF for quartiz.
1.2 Carbon Fiber
Carbon and graphite fibers are produced using PAN-based or pitch-based
Precursors. The precursor undergoes a series of operations. In the first step,
the precursors are oxidized by exposing them to extremely high
temperatures.
Later, they go through carbonization and graphitization processes. During
these processes, precursors go through chemical changes that yield high
stiffness- to-weight and stength-to-weight properties. The successive
surface treatment and sizing process improves its resin compatibility and
handle ability.
Pitch-based carbon fibers are produced in the same way as PAN-based
fibers but pitch is more difficult to spin and the resultant fiber is more
difficult to handle. Pitch itself costs pennies a kilogram, but processing and
purifying it to the fiber form are very expensive. Generally, pitch-based
fibers are more expensive than PAN-based fibers.
The cost of carbon fibers depends on the strength and stiffness properties
as well as on the tow size (number of filaments in a fiber bundle). Fibers
with high stiffness and strength properties cost more.
Carbon and graphite has wide range of properties; however, they generally
exhibit superior tensile and compressive strength possess high moduli, have
explant fatigue characteristics and do not corrode. graphite fibers are 1)
subjected to heat treatments above 3000oF.2) possess 3D ordering of their
atoms3)have carbon content greater than 99% 4) have elastic moduli(E)
greater than 50msi. Carbon fibers has lower carbon content (93-95) %.
The strength of carbon and graphite fibers depend on the type of precursor
used. The processing conditions during manufacturing such
as fiber tension and temperatures and the presence of flows and defects.
carbon fibers combined with polymer more closely than any other material.
Carbon fibers are elastic to failure at normal temperatures, creep resistant
and don’t susceptible to failure chemically inert, except in strong oxidizing
environment or in content with molten metals and have excellent damping
characteristics.
Some disadvantages of carbon fibers are:
1. They are brittle and have low impact resistance.
2. They have low strain to failure.
3. Their compressive strength is less than their tensile strength.
4. they are relatively expansive compered to glass.
1.3 Aramid Fiber
Aramid fibers provide the highest tensile strength-to-weight ratio among
Reinforcing fibers. They provide good impact strength. Like carbon fibers,
they provide a negative coefficient of thermal expansion. The disadvantage
of aramid fibers is that they are difficult to cut and machine. The
advantages of organic fibers are that:
1. they have stiffness and strength intermediate between that of glass and
carbon.
2. they are actually thermoplastics that have a glass transition
temperature(Tg) higher than their degradation temperature.
3. they have highly oriented molecular chains in the fiber direction that are
held together by strong covalent bonds resulting in high longitudinal tensile
strength. However, the chains in the transvers direction are held together by
hydrogen bonds resulting in low transvers strength.
4. Aramid fibers have combination of good tensile and modulus.
5. they have light weight and exillent toughness and impact resistance.
These aromatic polyamides are part of the nylon family.
Figure 3 represent stress- strain diagram of different fibers of composite
materials.