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Introduction
A composite material is a non uniform solid consisting of two or more different materials
that are mechanically bonded together. Each of the various components retains its identity
in the composite and maintains its characteristic structure and properties. Generally, the
structure of a composite consists of two phases, matrix and reinforcement. The matrix is a
continuous phase and the reinforcement is a discontinuous one. The duty of reinforcements
is attaining strength of the composite and the matrix has the responsibility of bonding of the
reinforcements. There are recognizable interface between the materials of matrix and
reinforcements. The composite materials, however, generally possess combination of
properties such as stiffness, strength, weight, high temperature performance, corrosion
resistance, hardness and conductivity which are not possible with the individual
components. Indeed, composites are produced when two or more materials or phases are
used together to give a combination of properties that cannot be achieved otherwise.
Composite materials especially the fiber reinforced polyester (FRP) kind highlight how
different materials can work in synergy. Analysis of these properties shows that they
depend on (1) the properties of the individual components; (2) the relative amount of
different phases; (3) the orientation of various components; the degree of bonding between
the matrix and the reinforcements and (4) the size, shape and distribution of the
discontinuous phase. The material involves can be organics, metals or ceramics. Therefore, a
wide range of freedom exists, and composite materials can often be designed to meet a
desired set of engineering properties and characteristics [1].
There are many types of composite materials and several methods of classifying them. One
method is based on the matrix materials which include polymers, metals and ceramics. The
other method is based on the reinforcement phase which has the shape of fiber, particulate
and whisker. Whiskers are like fibers but their length is shorter. The bonding between the
particles, fibers or whiskers and the matrix is also very important. In structural composites,
polymeric molecules known as coupling agent are used. These molecules form bonds with the dispersed phase and become integrated into the continuous matrix phase as well. The
most popular type of composite material is the fiber-reinforced polyester composites, in
which continuous thin fibers of one material such as glass, carbon or natural fibers are
embedded in a polyester matrix. They are also called glass fiber reinforced polyester
(GFRP), carbon fiber reinforced polyester (CFRP) and natural fiber reinforced polyester
(NFRP). The objective is usually to enhance strength, stiffness, fatigue, resistance, or
strength to weight ratio by incorporating strong and stiff fibers in a softer, more ductile
matrix. The microstructure of a selected GFRP composite is shown in figure 1.
The usages of fiber reinforced polyesters are in airplanes, electronics components,
automotives, rail ways and wagon systems and sporting equipments. Beside their desired
mechanical properties, their resistance to corrosion is also a tempting factor to use these
composite in different areas. Although they are sensitive to UV light, heat and moisture
environments, good maintenance could increase their life time. In this chapter different
phases of FRPs, the mechanical relationships between different components of FRPs, the
mechanism of degradation and aging of FRPs and application of them is discussing
2. Different phases of FRP composites
2.1. Fiber reinforcements
The fiber is an important constituent in FRP composites. A great deal of research and
development has been done with the fibers on the effects in the types, volume fraction,
architecture, and orientations. The fiber generally occupies 30% - 70% of the matrix volume
in the composites. The fibers can be chopped, woven, stitched, and braided. They are
usually treated with sizing such as starch, gelatin, oil or wax to improve the bond as well as
binders to improve the handling. The most common types of fibers used in advanced
composites for structural applications are the fiberglass, aramid, and carbon. The fiberglass
is the least expensive and carbon being the most expensive. The cost of aramid fibers is about the same as the lower grades of the carbon fiber. Other high-strength high-modulus
fibers such as boron are at the present time considered to be economically prohibitive.
2.2. Glass fibers
The glass fibers are divided into three main classes E-glass, S-glass and C-glass. The E-glass
is designated for electrical use and the S-glass for high strength. The C-glass is for high
corrosion resistance, and it is uncommon for civil engineering application. Of the three
fibers, the E-glass is the most common reinforcement material used in civil and industrial
structures. It is produced from lime-alumina-borosilicate which can be easily obtained from
abundance of raw materials like sand. The fibers are drawn into very fine filaments with
diameters ranging from 2 to 13 X 10 -6 m. The glass fiber strength and modulus can degrade
with increasing temperature. Although the glass material creeps under a sustained load, it
can be designed to perform satisfactorily. The fiber itself is regarded as an isotropic material
and has a lower thermal expansion coefficient than that of steel [3].
There are also the other fiber glasses which are used for FRP reinforcement as well as;
- A-glass, soda lime silicate glasses used where the strength, durability, and good
electrical resistivity of E-glass are not required.
- D-glass, borosilicate glasses with a low dielectric constant for electrical applications.
- ECR-glass, calcium alumino silicate glasses with a maximum alkali content of 2 wt.%
used where strength, electrical resistivity, and acid corrosion resistance are desired.
- AR-glass, alkali resistant glasses composed of alkali zirconium silicates used in cement
substrates and concrete.
- R-glass, calcium alumino silicate glasses used for reinforcement where added strength
and acid corrosion resistance are required.
- S-2-glass, magnesium alumino silicate glasses used for textile substrates or
reinforcement in composite structural applications which require high strength,
modulus, and stability under extreme temperature and corrosive environments.
Table 1 and 2 show the chemical and mechanical properties of different glass fibers
respectively.
2.1.2. Aramid fibers
These are synthetic organic fibers consisting of aromatic polyamides. The aramid fibers have
excellent fatigue and creep resistance. Although there are several commercial grades of
aramid fibers available, the three most common ones used in structural applications are
Kevlar 29, Kevlar 49 and Kevlar 149. The Young's Modulus curve for Kevlar 29 is linear to a
value of 83 GPa but then becomes slightly concave upward to a value of 100 GPa at rupture;
whereas, for Kevlar 49 the curve is linear to a value of 124 GPa at rupture (see Table 3). As
an anisotropic material it's transverse and shear modulus are an order of magnitude less
than those in the longitudinal direction. The fibers can have difficulty achieving a chemical
or mechanical bond with the resin