CHAPTER 5

COMPOSITE MATERIALS

Composite materials (or composites for short) are engineered materials made from two
or more constituent materials with significantly different physical or chemical properties
which remain separate and distinct on a macroscopic level within the finished structure.
Composites are made up of individual materials referred to as constituent
materials. There are two categories of constituent materials: matrix and reinforcement. At
least one portion of each type is required. The matrix material surrounds and supports the
reinforcement materials by maintaining their relative positions. The reinforcements
impart their special mechanical and physical properties to enhance the matrix properties.
A synergism produces material properties unavailable from the individual constituent
materials, while the wide variety of matrix and strengthening materials allows the
designer of the product or structure to choose an optimum combination.
Engineered composite materials must be formed to shape. The matrix material can
be introduced to the reinforcement before or after the reinforcement material is placed
into the mould cavity or onto the mould surface. The matrix material experiences a
melding event, after which the part shape is essentially set. Depending upon the nature of
the matrix material, this melding event can occur in various ways such as chemical
polymerization or solidification from the melted state.

Figure 5.1 Plywood is a commonly encountered composite material

Most commercially produced composites use a polymer matrix material often

called a resin solution. There are many different polymers available depending upon the
starting raw ingredients. There are several broad categories, each with numerous
variations. The most common are known as polyester, vinyl ester, epoxy,
phenolic, polyamide, polypropylene and others. The reinforcement materials are often
fibers but also commonly ground minerals. The various methods have been developed to
reduce the resin content of the final product, or the fibre content is increased. As a rule of
thumb, lay up results in a product containing 60% resin and 40% fibre, whereas vacuum
infusion gives a final product with 40% resin and 60% fibre content. The strength of the
product is greatly dependent on this ratio.

5.1 FIBERS AND MATRIX MATERIALS

Composite materials are created and applied to take advantage of the high strength and/or
stiffness of fibers. Fibers are combined with a matrix material in order to create a useful
structure. The matrix

1. Binds the fibers together

2. Transfers loads to the fibers

3. Protects fibers from damage

Specific matrix materials are usually selected according to the manufacturing processes to
be used.

Figure 5.2.Matrix & Fibre

5.1.1 Fiber Materials

Filaments of reinforcing material usually are circular in cross-section. Diameters range

from less than 0.0025 mm to about 0.13 mm, depending on material. Filaments provide
greatest opportunity for strength enhancement of composites. The filament form of most
materials is significantly stronger than the bulk form. As diameter is reduced, the material
becomes oriented in the fiber axis direction and probability of defects in the structure
decreases significantly

Continuous vs. Discontinuous Fibers

• Continuous fibers – They are very long; in theory, they offer a continuous path by
which a load can be carried by the composite part.

• Discontinuous fibers (chopped sections of continuous fibers) - short lengths (L/D

= roughly 100). Important type of discontinuous fiber are whiskers - hair-like
single crystals with diameters down to about 0.001 mm (0.00004 in.) with very
high strength

Fiber Orientation – Three Cases

• One-dimensional reinforcement, in which maximum strength and stiffness are

obtained in the direction of the fiber.

• Planar reinforcement, in some cases in the form of a two-dimensional woven

fabric.

• Random or three-dimensional in which the composite material tends to possess

isotropic properties.

Figure 5.3 Fiber Orientations in Composite Materials

(a) One-dimensional, continuous fibers; (b) planar, continuous fibers in the form of a
woven fabric; and (c) random, discontinuous fibers

For many materials, much higher strength can be achieved in fiber form than in bulk

form. Several of these materials are suitable for use in fiber composites are:
 1. Glass
2. Polymer
3. Carbon
4. Boron

5.1.2 Matrix Materials

Although some fiber composite structures have been produced with thermoplastic matrix,
most to date have utilized thermosets. Many lower cost composite structures have used
polyester resins. For higher performance requirements, such as in aerospace applications,
epoxies are most commonly used. Epoxies have reasonably high modulus and strength.
Epoxies exhibit low shrinkage on cure (good for adhesives). Epoxy service temperature to
125-175ºC.

Several of these materials suitable for use in composites are:

1. Thermoplastics
2. Polyster
3. Vinyl Ester
4. Epoxy

5.2 MECHANICS OF COMPOSITE MATERIALS

The basic element of a unidirectional composite is a thin sheet (ply). Material axes are
defined as follows:
• Longitudinal direction (1) – parallel to fibers
• Transverse direction (2) – perpendicular to fibers in plane
• Normal direction (3) – out of plane
 Figure 5.4 Unidirectional Composite Material Coordinates

Modeling Unidirectional Composite Behavior

• Fibers are assumed to be all the same (perhaps with circular cross section) and
uniformly distributed throughout the matrix.

• The quantity of fibers present is expressed as the fiber volume fraction, Vf.

• The matrix volume fraction is given by Vm = 1 - Vf.

• Composite density: rc = rfVf + rmVm.

Composite Modulus using Rule of Mixtures:

Longitudinal Modulus: E1 = V f Ef + V m Em

1 Vf V m
Transverse Modulus: = +
E2 E f Em

1 V V
Shear Modulus: = f+ m
G G f Gm

Composite laminates

Composite laminates is a useful structure will require that fibers be oriented in more than
one direction. A common approach to creating such a structure is by stacking layers, or
plies, to form a laminate.

Figure 5.5 Ply orientations are defined using the angle θ (1-2 axis relative to the x-y axis)
 High stiffness and strength usually require a high proportion of fibres in the
composite. This is achieved by aligning a set of long fibres in a thin sheet (a lamina or
ply). However, such material is highly anisotropic, generally being weak and compliant
(having a low stiffness) in the transverse direction. Commonly, high strength and stiffness
are required in various directions within a plane. The solution is to stack and weld
together a number of sheets, each having the fibres oriented in different directions. Such a
stack is termed a laminate. An example is shown in the diagram 5.6. The concept of a
laminate, and a pictorial illustration of the way that the stiffness becomes more isotropic
as a single ply is made into a cross-ply laminate

Laminate Stacking Sequence

0o 0o
90 o  30 o
45 o  30 o
45 o 0o
90 o  30 o
0o  30 o

[0/90/45]S [(0/-30/30)2]

 45 o 0o
 45 o 90 o
0o 0o
0o
90 o
90 o
45 o
90 o
90 o
0o
0o 0o
 45 o 90 o
 45 o 0o

[±45/02/90]S [(0/90)2/45]S
 Figure 5.6 Different laminate stacking sequences of composite material

Figure 5.7 Effect of fiber orientation on the tensile strength of E-glass fiber-reinforced
epoxy composites.

5.3 WIND TURBINE APPLICATION

Wind turbine blades are subjected to static and dynamic lift, drag and inertial loads over a
wide range of temperatures and other severe environmental
conditions (e.g., UV, rain, hail, bird strikes) during a typical 20 year service life.

Blades must be possess

• Low weight and rotational inertia

 • High rigidity
• Resistance to fatigue and wear

Because of the unique requirements for large‐scale wind turbine blades, advanced compos
itesare the materials of choice.

Composite fibers used as reinforcements in blade construction:

Traditional E‐glass fiber (70‐75% by weight) bonded with epoxy or unsaturated polyester
resin (most common).

Carbon fiber bonded with same resin (less common, although provides high stiffness and
less weight for longer turbine blade)

Composite resins used in blade construction:

Epoxy was preferred matrix as blades grew longer since it offers better mechanical
performance particularly tensile and flexural strength
particularly tensile and flexural strength

Polyester is easier to process (needs no post‐curing) and is less expensive

Vinyl ester (limited use, but growing)

Core materials used primarily for large area unsupported stability in lead/trailing
edge panels and shear webs.

Balsa (low cost, good shear properties, higher weight)

Foam cores (PVC, Urethane, PET)

Engineered core materials (Webcore TYCOR, NexCore)