The rise in the demand for new high performance materials

Composite Materials 1
An investigation into the mechanical properties of carbon fibre reinforced epoxy polymer
composite and prospective application in turbine fan blades
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Composite Materials 2
• Introduction to composites
The rise in the demand for new high performance materials to meet modern design
requirements or to eliminate the materials currently in use especially metal based ones has led
to the development of composite materials. The materials used for reinforcement in
composites such as carbon and glass are renewable which especially important considering
the transition towards the use of sustainable materials to curb climate change. The unification
of two or more different materials gives a material with different (usually better) properties
compared to those of the single ingredients. This new material is known as a composite
material. Unlike metallic alloys, each material in the composite material maintains its original
physical, mechanical and chemical properties (Barbero 2017). Composite materials have the
added advantage of improved stiffness, strength and low density in comparison with other
materials hence enabling a reduction in weight in the final material. The ability of a
composite material to provide superior strength is founded on the fact that desired materials
can be chosen for the reinforcement. The reinforcement of a material provides the required
strength and stiffness. Usually, the reinforcing material or materials are stronger and stiffer
than the matrix. Composites are assembled from two or more materials with fillers or fibres
for reinforcement and a compactable matrix which may be constructed from polymeric,
metallic or ceramic materials (Barbero 2017). The function of the matrix is to provide the
composite’s environmental tolerance, physical appearance and shape (Clyne & Hull 2019).
Most of the strength and stiffness is provided by the reinforcements which also carry the bulk
of the structural loads. Composite materials are widely used in engineering applications such
as aircraft manufacture, space vehicles and the construction industry due to their
extraordinary strength to weight ratio.

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Increasing operating efficiency in applications that utilize turbines such as wind
power generating stations is vital for the continued growth of the industry. The need for
increased efficiency necessitates the shift towards better materials such as carbon or glass
fibre-hybrid composites from glass fibres (Clyne & Hull, 2019). This change comes with
optimisation and design challenges in understanding the failure modes of these materials.
Currently, wind turbines still face scientific and technical difficulties in terms efficiency
which increases the cost of energy making wind less competitive with conventional energy
sources. A large percentage of composite materials are utilized by the turbine industry
(Barbero, 2017). According to Barbero (2017), turbine blades are the most important
components of a turbine. The performance and lifetime of a turbine is usually limited by the
turbines. Besides, the blades are regarded as the most expensive parts of the turbine according
to (Singh & Salem, 2011). The requirements for a high performance turbine include low
weight, strength and corrosion resistance. From the early 21st century, suggestions have been
put forward to manufacture all-carbon fibre composites for turbines. This would make the
blades significantly lighter and reduce the deflection of the blade tips. Besides, it would be
possible to have passive actuation with carbon fibre composites. Their aerodynamic
performance would be superior due to the increased bend-twist coupling as a result of their
greater anisotropy in comparison with glass fibre composites. However, the use of all-carbon
composites would greatly increase the cost of the blades making the use of these blades in
applications such as power generation unsuitable since competitiveness with traditional
power sources would be affected.
There are several types of composite materials that occur naturally. These include the
human bone, seaweeds and wood. Composite materials are reported to have been in use from
as early as 4000 BC (Jones 2018). These early materials mainly consisted of reinforced pitch
or bitumen.

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Composites can be grouped into three broad categories depending on the matrix material
which comprises the continuous phase. These include polymer matrix, metal matrix and
ceramic matrix composites (Yi 2017). Polymer matrix composites require relatively low
processing temperatures for fabrication hence they are easier to manufacture compared to the
other two types. Polymer matrix composites are usually made from synthetic fibres such as
carbon, glass, rayon and nylon embedded in a matrix of polymers. In this case, the polymer
makes up about 60 % of the composite material by volume (Tsai & Hahn 2018). The fibrous
reinforcing material may consist of short fibre segments or long continuous fibres. High
aspect ratio fibres are used for short fibre segments while continuous fibres are generally used
for structures designed for high performance. The figure shown below summarizes the
classification of composite materials.
Figure 1: Classification of composite fibres. Adapted from “Introduction to Composite
Materials Design, Third Edition” by Barbero, 2017
Composite materials can have specific stiffness and specific strength superior to standard
metal alloys (Campbell 2010). Specific stiffness is defined as the material’s modulus to
density ratio while specific strength is the strength to density ratio. Composite materials can
be tailored for specific applications depending on the arrangement of fibres in the matrix. For
instance, polymer concrete is being used widely in the building construction industry due to

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their ability to thrive in highly corrosive environments and their high strength to weight ratio
(Campbell 2010). As a result, maintenance costs are significantly reduced and the life cycle
prolonged.
Characteristics of composites
Despite the fact that there are different types of composite materials, there are universal
properties inherent in all composite materials.
i) High specific strength and modulus
The major advantages of composite materials are their superior specific strength and
modulus. Under the assumption of equal weight, these properties indicate the bearing
capacity and the stiffness of the material. These are very important properties in applications
such as aerospace component design (Campbell, 2010). The table below shows a comparison
of the mechanical properties of different materials. It is evident that composite materials
(especially fibre reinforced composites) have superior properties.
ii) High damage and fatigue resistance
Propagation of cracks in composite materials can be prevented by the interface between
the matrix and the reinforcing material. Consequently, the growth of a crack occurs gradually
and it may take a long time before complete failure of the structure (Wang, Zheng, & Zheng,
2011). Failure due to fatigue is usually initiated by the weakest links in fibres. Unlike the
damage in conventional materials, the failure in composite materials usually springs from a
combination of damages such as matrix cracking, debonding and fibre splitting. The large
number of fibres in a composite material makes the structure a statically indeterminate
mechanical system. If a small number of fibres fail, the load they bore is transferred to the
rest of the structure via the matrix. Therefore, the lifetime of composite materials is
significantly improved.

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iii) Excellent damping properties
The structure of a material as well as its shape determine the natural frequency of
vibration of the material. This frequency is also proportional to the specific modulus of the
material. Consequently, composite materials exhibit high natural frequencies due to their high
specific moduli (Barbero, 2017). Therefore, resonance rarely occurs in structures made of
composite materials. Besides, any vibrational energy is normally absorbed by the interface
between the reinforcing material and the matrix giving the structure high damping of
vibrations.
iv) Multi-functional performance
Composite materials have high resistance to heat and have good resistance to ablation.
For instance, the conductivity of fibreglass reinforced plastics is about 1 % that of metals.
The materials can also have high melting points, high specific heat vaporization heat. The
low conductivity of these materials makes them useful for use as electrical insulation
materials and as a dielectric in materials for high frequency applications. For example, it is
used in the construction of wave-transparent radomes for the protection of radar systems
(Clyne & Hull, 2019). Composite materials also have low friction coefficients and self-
lubricating characteristics. Besides, these materials have high resistance to chemical
corrosion as well as extraordinary optical and magnetic properties.
Classification of composites
Particulate composites
This is a composite whose reinforcement consists of particles (German, 2016). The
dimensions of the particles are roughly equal. Particulate fillers improve material temperature

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performance, lower friction improve the resistance to wear and reduce shrinkage. The
particles also contribute to load sharing with the matrix. However, their capabilities are
limited compared to fibres. Therefore, with the use of particles for reinforcement only
stiffness is improved considerably while strength is slightly increased (German, 2016).
Fibre composites
These composites consist of reinforcements whose lengths are greater than the cross-sectional
area (Piggott, 2016). These thin structures are called fibres. Fibre composites can be further
classified into single layer and multilayer composites which are further classified as shown in
the figure below.
Figure 2: Classification of Fibre Reinforced Composites. Adapted from “Introduction to
Composite Materials Design, Third Edition” by Barbero, 2017
The single layer composites may be reinforced with long or short fibres depending on
the overall dimension of the material. Reinforcing a material with long fibres produces a
continuous fibre reinforcement while the reinforcement of a material with short fibres
produces a discontinuous fibre reinforcement. Continuous fibre reinforcements have fibres
aligned in the same direction to enhance strength. The length of short fibres is chosen in such
a way to avoid entanglement and ensure that the fibres do not lose their fibrous properties.

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The behaviour of short and long fibre reinforced composites is distinct and well defined (Fu,
Lauke, & Mai, 2019).
Hybrid composites
These composites consist of two or more distinct material types (especially fibres)
incorporated in a single matrix (Jawaid, Nagarajan, Sukumaran, & Baets, 2018). Hybrids are
usually used to improve material properties while at the same time reducing the cost of
conventional composites (Jawaid, Nagarajan, Sukumaran, & Baets, 2018). Depending on the
method of incorporation of the component materials, hybrids are classified as intraply,
interplay, intimately mixed and sandwich type. As the name suggests, sandwich composites
consist of a material between layers of another material. An interplay material consists of
alternate layers of several materials stacked in a particular regular manner. Intimately mixed
hybrids is made up of several constituent materials are mixed in such a way that no
concentration of either type is greater than another. Intraply hybrids are made up of rows of
different constituent materials arranged regularly or randomly (Jawaid, Nagarajan,
Sukumaran, & Baets, 2018).
Laminate composites
To fabricate a laminate, several laminae are stacked in the matrix of interest in the
direction of thickness. To achieve reasonable bonding at least three layers are arranged
alternately between a polymer matrix and the reinforcement. An example of a laminate
composite consists of paper and plywood. Depending on the intended use of the composite,
the laminates can be oriented in a unidirectional or bi-directional manner. Laminated
materials normally utilize artificial fibres due to their superior mechanical, thermal and
physical properties.