Abstract

The increasing use of composite materials in modern structures of high performance in automotive and aircraft structures calls

for a deeper and more thorough knowledge of their properties. Composite materials are being used in an increasing number of

products as more manufacturers discover the benefits of these versatile materials.

Composites are an interesting class of materials for technological development, fulfilling the needs of numerous applications

where the properties of current monolithic alloys are not adequate. However, cost and manufacturing feasibility and/or

reliability have partially impaired the industrial application of these materials. In addition, how composite materials is desirable

to have high wear resistance coupled with high bulk tenacity, in order to allow the component to absorb impact loads because of

its enhanced mechanical and chemical properties, encompass many engineering problems. Following this concept, we will try to

focus upon the following:

● The definition and concept behind composites as well as the necessity to use composites.

● The comparison of the structural and mechanical characteristics of the composites with those of conventional used

materials.

● The study of processing of such materials and their viabilities with testing and some common as well as engineering

examples.

Introduction

Most of the products we see every day are made from monolithic materials. That means the individual components consist of a

single material (an unreinforced plastic), or a combination of materials that are combined in such a way that the individual

components are indistinguishable (a metal alloy).

Composite materials, on the other hand, consist of two or more materials combined in such a way that the individual materials

are easily distinguishable.

1.1What makes a material a composite?

“ A judicious combination of two or more materials that produces a synergistic effect. A material system composed of two or

more physically distinct phases whose combination produces aggregate and enhanced properties that are different from those of

its constituents.”

Composite materials for construction, engineering, and other similar applications are formed by combining two or more

materials in such a way that the constituents of the composite materials are still distinguishable, and not fully blended. One

example of a composite material is concrete, which uses cement as a binding material in combination with gravel as a

reinforcement. In many cases, concrete uses rebar as a second reinforcement, making it a three-phase composite, because of the

three elements involved.

Composite materials take advantage of the different strengths and abilities of different materials. In the case of mud and straw

bricks, for example, mud is an excellent binding material, but it cannot stand up to compression and force well. Straw, on the

other hand, is well able to withstand compression without crumbling or breaking, and so it serves to reinforce the binding action

of the mud. Humans have been creating composite materials to build stronger and lighter objects for thousands of years.

The majority of composite materials use two constituents: a binder or matrix and a reinforcement. The reinforcement is stronger

and stiffer, forming a sort of backbone, while the matrix keeps the reinforcement in a set place. The binder also protects the

reinforcement, which may be brittle or breakable, as in the case of the long glass fibers used in conjunction with plastics to

make fiberglass. Generally, composite materials have excellent compressibility combined with good tensile strength, making

them versatile in a wide range of situations.

Engineers building anything, from a patio to an airplane, look at the unique stresses that their construction will undergo.

Extreme changes in temperature, external forces, and water or chemical erosion are all accounted for in an assessment of needs.

When building an aircraft, for example, engineers need lightweight, strong material that can insulate and protect passengers

while surfacing the aircraft. An aircraft made of pure metal could fail catastrophically if a small crack appeared in the skin of the

airplane. On the other hand, aircraft integrating reinforced composite materials such as fiberglass, graphite, and other hybrids

will be stronger and less likely to break up at stress points in situations involving turbulence.

Many composites are made in layers or plies, with a woven fiber reinforcement sandwiched between layers of plastic or another

similar binder. These composite materials have the advantage of being very moldable, as in the hull of a fiberglass boat.

Composites have revolutionized a number of industries, especially the aviation industry, in which the development of higher

quality composites allows companies to build bigger and better aircraft.

1.2. Composition :

MATRIX + REINFORCEMENT = COMPOSITE

Composites are combinations of two materials in which one of the material is called the reinforcing phase, is in the form of

fibers, sheets, or particles, and is embedded in the other material called the matrix phase. Typically, reinforcing materials are

strong with low densities while the matrix is usually a ductile or tough material. If the composite is designed and fabricated

correctly, it combines the strength of the reinforcement with the toughness of the matrix to achieve a combination of desirable

properties not available in any single conventional material.

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.

1.3Fibers: Fibers are a special case of reinforcements. They are generally continuous and have diameters ranging from 120 to

7400 pin (3-200 pm). Fibers are typically linear elastic or elastic-perfectly plastic and are generally stronger and stiffer than the

same material in bulk form. The most commonly used fibers are boron, glass, carbon, and Kevlar.

1.4Matrix: The matrix is the binder material that supports, separates, and protects the fibers. It provides a path by which load

is both transferred to the fibers and redistributed among the fibers in the event of fiber breakage. The matrix typically has a

lower density, stiffness, and strength than the fibers. Matrices can be brittle, ductile, elastic, or plastic. They can have either

linear or nonlinear stress-strain behavior. In addition, the matrix material must be capable of being forced around the

reinforcement during some stage in the manufacture of the composite. Fibers must often be chemically treated to ensure proper

adhesion to the matrix. The most commonly used matrices are carbon, ceramic, glass, metal, and polymeric. Each has special

appeal and usefulness, as well as limitations.

1.5. Classification of Composites:

Composite materials are commonly classified at following two distinct levels:

• 1.5.1. The first level of classification is usually made with respect to the matrix constituent. The major composite classes

include Organic Matrix Composites (OMCs), Metal Matrix Composites (MMCs) and Ceramic Matrix Composites (CMCs). The

term organic matrix composite is generally assumed to include two classes of composites, namely Polymer Matrix Composites

(PMCs) and carbon matrix composites commonly referred to as carbon-carbon composites.

Fig. 1.1.Classification of composites on the basis of matrix

•1.5.2. The second level of classification refers to the reinforcement form - fibre reinforced composites, laminar composites and

particulate composites. Fibre Reinforced composites (FRP) can be further divided into those containing discontinuous or

continuous fibres.

Fig.1.2.Classification of composites on the basis of reinforcement

• 1.5.3. Fiber Reinforced Composites are composed of fibers embedded in matrix material. Such a composite is considered to be

a discontinuous fiber or short fiber composite if its properties vary with fiber length. On the other hand, when the length of the

fiber is such that any further increase in length does not further increase, the elastic modulus of the composite, the composite is

considered to be continuous fiber reinforced. Fibers are small in diameter and when pushed axially, they bend easily although

they have very good tensile properties. These fibers must be supported to keep individual fibers from bending and buckling.

•1.5.4. Laminar Composites are composed of layers of materials held together by matrix. Sandwich structures fall under this

category.

•1.5.5. Particulate Composites are composed of particles distributed or embedded in a matrix body. The particles may be flakes

or in powder form. Concrete and wood particle boards are examples of this category.

Fig.1.3.Enhancement of properties of glass

1.6.Concept behind composites:

Four fundamental concepts form the basis of the composite principle:

I. Heterogeneity : Non-uniformity of the chemical/physical structure. If the elastic properties are the same at every point

in the body, then it is said to be homogeneous.

II. Anisotropy : Direction dependence of the physical properties. At any point of such a body, the elastic properties are

different in different directions.

III. Symmetry : Tensorial nature of material properties.

IV. Hierarchy : Stacking of individual structural units.

1.7. Role and selection of fibers:

The role of the fiber reinforcement is to strengthen and stiffen the composites through prevention of matrix deformation by

mechanical restraint. . In continuous fiber reinforcement fiber reinforcement composites , the reinforcement is the principle

load bearing component. When fiber reinforcement is combined with metallic matrix of higher density the reinforcement also

serves as to reduce density of the composite, thus enhancing properties such as specific strength.

The points to be noted in selecting the reinforcements include compatibility with matrix material, thermal stability density,

melting temperature etc. the efficiency of discontinuously reinforced composites is dependent on tensile strength and density of

reinforcing phases. The compatibility , density, chemical and thermal stability of the reinforcement with matrix material is

important for material fabrication as well as end application. Also the role of the reinforcement depends upon its type in

structural composites. The metallic matrix serves to hold the reinforcing fiber together and transfer as well as distribute the load.

Discontinuous fiber reinforced composites display characteristic between those of continuous fiber and particulate reinforced

composites.

1.8. Role and selection of matrix.

Although it is undoubtedly true that the high strength of composites is largely due to the fiber reinforcement the importance of

matrix material cannot be underestimated as it provides support for the fibers and assists the fibers in carrying loads. It also

provides stability to the composite material. Matrix transfer stresses between the fibers. Provides barrier against an adverse

environment and protects the surface of the fiber from mechanical abrasion. Following factors considered for matrix selection:

The matrix must have mechanical strength with that of the reinforcement i.e. both should be compatible. Thus , if a high

strength fiber is used as the reinforcement , there is no point using a low strength matrix, which will not transmit stress

efficiently to the reinforcement.

1.9. Manufacturing of composites:

In order to select the most efficient manufacturing process, the manufacturing team considers several factors such as:

● User needs

● Performance requirements

● Size of the product

● Surface complexity

● Appearance

● Production rate

● Total production volume

● Economic targets/limitations

● Labor

● Materials

● Tooling/assembly equipment

Although the fabrication and application of composite materials by human being could be tracked back to ancient times

conceptually, the technologies for fabricating composite materials or advanced composites we refereed to nowadays are young

and still under development and improvement. Compared to conventional homogeneous materials (such as metals, plastics etc.),

the manufacturing techniques for composite materials are more demanding and more sophisticated. There are variety of

methods of practical interest are available one of them is mentioned below:

1.9.1 Filament winding method:

This process is primarily used for hollow, generally circular or oval sectioned components, such as pipes and tanks. Fiber tows

are passed through a resin bath before being wound onto a mandrel in a variety of orientations, controlled by the fiber feeding

mechanism, and rate of rotation of the mandrel.

Fig.1.5 Fiber winding method of production of composite

The relatively small demand, high initial investing on new manufacturing facilities, labor intensive and highly skilled workers

needed for manufacturing composite, there is a strong need to identify and to develop low-cost composite materials with high-

speed manufacturing technique. The other manufacturing techniques include:

1.9.2 Pultrusion: Pultrusion is a continuous molding process that combines fiber reinforcements and thermosetting resin.

Fig. 1.6 Pultrusion method of production of composite

1.9.3 Resin Transfer Molding (RTM):

Resin Transfer Molding or RTM as it is commonly referred to is a “Closed Mold Process” in which reinforcement material is placed between two matching mold surfaces – one being male and one being female.

Fig. 1.7 RTM method of production of composite

1.10. So why do we use composites?

Composites are able to meet diverse design requirements with significant weight savings as well as high strength-to-weight ratio

as compared to conventional materials. Some advantages of composite materials over conventional one are mentioned below :

• Tensile strength of composites is four to six times greater than that of steel or aluminium.

• Improved torsional stiffness and impact properties

• Higher fatigue endurance limit (up to 60% of the ultimate tensile strength).

• 30-45% lighter than aluminium structures designed to the same functional requirements

• Composites are less noisy while in operation and provide lower vibration transmission than metals

Composites are more versatile than metals and can be tailored to meet performance needs and complex design requirements

The greatest advantage of composite materials is strength and stiffness combined with lightness. By choosing an appropriate

combination of reinforcement and matrix material, manufacturers can produce properties that exactly fit the requirements for a

particular structure for a particular purpose.

Modern aviation, both military and civil, is a prime example. It would be much less efficient without composites. In fact, the

demands made by that industry for materials that are both light and strong has been the main force driving the development of

composites. It is common now to find wing and tail sections, propellers and rotor blades made from advanced composites, along

with much of the internal structure and fittings. The airframes of some smaller aircraft are made entirely from composites, as are

the wing, tail and body panels of large commercial aircraft.

The right composites also stand up well to heat and corrosion. This makes them ideal for use in products that are exposed to

extreme environments such as boats, chemical-handling equipment and spacecraft. In general, Another advantage of composite

materials is that they provide design flexibility. Composites can be molded into complex shapes – a great asset when producing

something like a surfboard or a boat hull.

Fig.1.4.Some fibers based and matrix based composites

The downside of composites is usually the cost. Although manufacturing processes are often more efficient when composites

are used, the raw materials are expensive. Composites will never totally replace traditional materials like steel, but in many

cases they are just what we need. And no doubt new uses will be found as the technology evolves. We haven’t yet seen all that

composites can do.

1.11 Composites as unique materials :

Composites are unique materials for following applications of practical interest:

• It is easier to achieve smooth aerodynamic profiles for drag reduction.

• Complex double –curvature parts with smooth surface finish can be made in one manufacture operation.

• High resistance to impact damage

• Like metals thermoplastics have indefinite shelf life.

• Composites are dimensionally stable as they have low coefficient of thermal expansion.

• The improved weatherability of composites in marine environment as well as their corrosion resistance and reduce the down

time for maintenance.

1.12 Applications of Composite materials:

1.12.1 Building Materials:

Composite products are used in a variety of residential and commercial construction applications. Entire homes can be framed

using plastic-laminated beams and trusses instead of traditional wood framing. By coating the beams with plastic, manufacturers

reduce the risk of rot or termite damage, which extends the life expectancy of the structure. Similar products are used to build

outdoor decks and porches.

Fiber-reinforced cement shingles create a maintenance-free roof that lasts for decades, while fiber-cement siding offers the look

of wood without the maintenance. Doors and flooring made from composite materials mimic the look of wood and often cost

much less. Fiber-reinforced panels, or FRP, are used on many bathroom and kitchen walls to create a durable and waterproof

surface that's easy to clean.

1.12.2 Aircraft:

According to the Australian Academy of Science, modern aircraft would be much less efficient if composite materials weren't

available. Composites like fiberglass-reinforced aluminum or carbon fiber create materials light enough to fly, but strong

enough to handle the pressure of high altitudes and frequent abuse.

Fig. 1.7 composites in aircraft

Items such as the wings and tail of an aircraft as well as the propellers and rotors are often made from composites. These

materials hold up under a wide range of temperatures and are highly resistant to rust and corrosion. In terms of aesthetics, many

composite materials used in aircraft construction are easy to shape and mold to fit design and engineering plans.

1.12.3 Sports:

A number of different sports products include composite-based components. Many modern baseball bats are either made

entirely from metal or wood composites or include a composite handle to resist breakage. Golf and tennis manufacturers also

rely on composites to improve strength and reduce weight in clubs and rackets. Other sporting equipment, including surfboards

and skis, contains composite materials that add flexibility while maintaining durability.

1.12.4Boating:

Many modern boats are made from composite materials such as fiberglass or thermoplastics. While traditional wooden boats

were subject to rot or warping over time, and metal boats were heavy and likely to rust, composites offer superior performance

and reduced maintenance. They are lightweight enough to stay afloat, yet strong enough to resist punctures and cracking. Most

composite boats are unlikely to corrode, even after frequent exposure to salt water and sea air. In addition to racing and pleasure

boats, composite materials are used on kayaks, canoes and jet skis.

1.12.5Ballistic Protection

Carbon-based composite fibers are often used to make bulletproof vests and other ballistic protection devices. These fibers,

including Kevlar, produce fairly lightweight body armor options that offer effective protection from bullets, flames and some

explosives.

Fig. 1.9 composites used in helmet and body guards

According to the Massachusetts Institute of Technology, Kevlar is five times stronger than steel and yet only half as dense as

fiberglass. Along with other composite fibers, Kevlar has saved thousands of lives and revolutionized protective gear for lawn

enforcement and military personnel.

1.13 Future scope of composites:

Armed with a wide gamut of advantages, composites have a key role to play in the growing market in India. Composites have

made an entry into diverse end-use segments and the developmental efforts for finding newer composites for existing & novel

applications is an area of top priority.

Repair technology is gaining more attention. Composites are showing a better service record than are metals, mainly due to their

better fatigue & corrosion resistance properties.

"The future is in composites" is the realization of many decades of high-technology progress toward different materials and

parts assembled and combined as monolithic units that would provide a combination of versatility, strength and other properties

beyond the possibilities of conventional materials like metal, wood or concrete.

Future business opportunities due to strong developmental activities focusing primarily on products & processes need to be

pursued in India. Towards such an objective, a multi-agency approach involving the industry, Government, academia, research

laboratory, certification/standardization and user agencies would be required for a quantum jump in composite technology in the

country.

1.14 Limitations of composites:

● Composites are heterogeneous: Properties in composites vary from point to point in the material. Most engineering

structural materials are homogeneous.

● Composites are highly anisotropic: The strength in composites vary as the direction along which we measure changes

(most engineering structural materials are isotropic). As a result, all other properties such as, stiffness, thermal

expansion, thermal and electrical conductivity and creep resistance are also anisotropic. The relationship between

stress and strain (force and deformation) is much more complicated than in isotropic materials.

● Composites materials are difficult to inspect with conventional ultrasonic, eddy current and visual NDI methods such

as radiography.

● High cost of raw materials and fabrication.

● Transverse properties may be weak.

● Matrix is weak , hence low toughness and it is difficult to attach.

1.15 Conclusion:

Due to their reduced weight, composite materials have an advantage over conventional metallic materials; although,

currently it is expensive to fabricate composites. Until techniques are introduced to reduce initial implementation costs and

address the issue of non-biodegradability of current composites, this relatively new material will not be able to completely

replace traditional metallic alloys. Who knows where technology will take us in future!!! only time is eligible candidate for

this question .

Acknowledgement :

We would like to acknowledge with appreciation the numerous and valuable comments, suggestions, constructive criticisms and proper guidance from the following faculty of the mechanical engineering department of our college:

[1] prof. vasant jog

[2] prof. anand khandekar .

[3 ] prof. arun jha .

References :

[1] http://www.tifac.org.in

[2] http://www.jjmechanic.com

[3] www.nptel.org