Composite materials are made of two or more constituent materials that retain their identities but act together to produce material properties that are unique compared to the individual components. Composites have a matrix phase that binds a dispersed reinforcing phase, which can be particles, short fibers, or continuous fibers. Composites are classified based on their matrix, such as polymer matrix composites, or based on their structure as fiber or particle reinforced composites. Natural composites like wood and bone exist in nature and use materials like cellulose fibers and calcium phosphate. Natural fiber composites are increasingly used in applications like automotive interiors to reduce weight and costs while supporting sustainability.

1. Composite:
Def: A composite material can be defined as a combination of two or more materials (having
significantly different physical or chemical properties) that results in better properties than those
of the individual components.
 The constituents retain their identities in the composite; that is, they do not dissolve or
otherwise merge completely into each other, although they act in concert.
Phases of composites: Composite material is a material composed of two or more distinct
phases (matrix phase and dispersed phase) and having bulk properties significantly different
form those of any of the constituents.
1) Matrix phase: The primary phase, having a continuous character, is called matrix. Matrix
is usually more ductile and less hard phase. It holds the dispersed phase and shares a load
with it.
2) Dispersed (reinforcing) phase:
The role of reinforcement in composite materials is primarily to add mechanical
properties to the material such as strength and
stiffness.
Reinforcements basically come in three
forms:
i. particulate
ii. discontinuous fiber
iii. continuous fiber
Reinforcements become fibers when one dimension becomes long compared to others.
Discontinuous reinforcements (chopped fibers, milled fibers) vary in length from a few
millimeters to a few centimeters.

2. Classification systems of composite materials:
There are two classification systems of composite materials. One of them is based on the matrix
material (metal, ceramic, polymer) and the second is based on the material structure:
Based on the matrix material:Composites are classified according to their matrix phase:
a) Polymer matrix composites (PMC's)
b) Ceramic matrix composites (CMC's)
c) Metal matrix composites (MMC's)
Based on the reinforcement :basically two type of composites exist based on the reinforcement
a) Fiber reinforced composites
b) Particle reinforced composites
Natural composites:
Def: Natural composites exist in both animals and plants. Natural fiber composites are
compositions of a polymer matrix embedded with high-strength natural fibers such as jute, oil
palm, PALF, sisal, kenaf, and flax.
One of the reasons for the growing interest is that natural fibers have a similar specific
modulus to glass fibers, and higher specific strength.
Other examples of natural composites are bones, teeth, plant leaves and bird feathers.

3. History of natural composites:
Natural composite materials exist since first ancient builder used straw to reinforce mud bricks.
In 12th century, Mongols made archery bow by using cattle tendons, horn, bamboo, silk, and
natural pine resin. In late 1800s, canoe builders glued together layers of kraft paper with shellac
to get paper laminates.
Though the concept was successful but performance of materials was not good. Between 1870
and 1890, a revolution took place with the development of first synthetic resin, called polyester
by American Cyanamid and DuPont. During same period, Owens-Illinois began weaving glass
fiber into a textile fabric on a commercial basis.
From 1934 and 1936, Ray Green, combined these two new products for making small boats and
this was the beginning of composite era. Composites industry began in late 1940s and expanded
rapidly through 1950-55s. Boats, car bodies, truck parts, aircraft components, underground
storage tanks, buildings are some of the products which were made at that time.
Examples of Natural Composite Materials:
Some of the examples of natural composite materials are given below:
 Wood:
Wood is an excellent example of natural composite in which cellulose fibers are held together
by lignin. Closely packed cellulose molecule provides higher density and strength in the
composites. Presence of cellulose is responsible for adequate strength in plants and trees.
 Mud Bricks:
The most primitive man made composite material is mud brick, made of straw and mud for
construction use. Mud has lower tensile strength and fail upon bending. When straw is embedded
into mud and allowed to dry, resulting mud brick resists both compressive and tensile forces,
making an excellent building material. Individual bricks of mud can be made and then laid by
using a mud mortar in a similar way to laying bricks or entire wall can be formed by continuous
molding process so that a wall is one big brick effectively.
 Bone and Teeth:
Bone is a naturally occurring composite material made of calcium phosphate (mineral)
embedded in collagen (protein) matrix, recent attempts to grow artificial hydroxylapatite bone

4. composite have proved successful. Bones contains short and soft collagen fibers i.e. inorganic
calcium carbonate fibres dispersed in a mineral matrix called apatite. These fibers grow and get
oriented in the direction of load. Tooth is a special type of bone consisting of flexible core and
hard enamel surface, outer enamel have high compressive strength. Tooth has piezoelectric
properties i.e. reinforcing cells are formed when pressure is applied.
 Concrete:
Concrete is a useful engineering material where aggregate (small stones or gravel) and sand are
bond together. Water is absorbed by the cement, and bonds other components together creating a
stone like material. Applications of concrete are in pavements, building structures, foundations,
motorways/roads, overpass, bridges, parking structures, brick/block walls and footings for gates,
fences and poles.
Applications of composites :
There are several industries such as automotive, construction, energy and aerospace, among
others which are being challenged by the society and governments to make products which are
more environmentally sound and reduce their dependence on fossil fuels .
 . Industrial Applications of Natural Fibre Composites in the Automotive Sector :
Natural fibre-based composites hold significant potential for automotive industry because the
demand for light weight and environmentally friendly materials is higher. Studies indicate that
natural fibre composites can contribute to a cost reduction of 20% and weight reduction of 30%
of an automotive part . Following those authors, light weight of components leads to lower fuel
consumption, good recycling possibilities, reduction in waste disposal and greenhouse emissions
which are some of main drivers for use of natural fibres. Natural fibre composites are mostly
being used for interior parts such as dashboards, door panels, parcel shelves, seat cushions,
backrests and cabin linings whereas the use of natural fibre composite parts for exterior
applications is very limited. As an example, the following Figure shows the production of door
from hemp fibre.

5. Figure: production of finished door by hemp fiber
 Applications in Other Sectors than Automotive
Despite most prominent applications of natural fibre composites are in
automotive sector there are additional fields of applications of natural fibre composites mainly:
textiles, medical, healthcare and pharmaceuticals, home and personal care, food and feed
additives, construction and furniture, packaging, pulp and paper, bioenergy and biofuels. The
lower weight and relatively lower cost of natural fibres are the main aspects referred to as the
reasons for the use of natural fibre composites in these applications.
Discussion and Conclusions:
Natural fibres polymeric composites are an effective way of improving the quality of parts
regarding the environment, economic and technical feasibility. Fibres such as flax, ramie, cotton
and hemp are reported to have costs that are superior to all the others but with no great advantage
in terms of mechanical properties. Therefore, selecting the most suitable natural fibre for a
particular application requires an integrated analysis followed by a decision-making process.
Despite all of these aforementioned issues, there are several markets and industries that have
interesting applications of natural fibres. The automotive industry is the most active and
knowledge-intensive sector which plays an important role in the development of non and semi-
structural parts. But there are other industries which gradually branched out to products segment
made of natural fibre composites as well, such as furniture, medical, sports, among others.

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