Unit 1 Introduction to Composite materials

Introduction

Modern technology required materials with unusual combination of properties that can’t

be met by conventional metal, alloys, ceramics, and polymeric materials. Composite is

the answer for structural materials that have low density, strong, stiff, abrasion, and

impact resistance and not easily corroded. In designing composite materials scientists and

engineers have indigenously combined various metals, ceramics and polymers to produce

a new generation of extra ordinary materials.

Definition

A composite material is defined as a structural material created by combining two or

more material having dissimilar characteristics. The constituent are combined at

macroscopic level and are not soluble in each other. One constituent is called Matrix

(Resin) phase and the other is called reinforcing (Fiber) phase. Reinforcing phase is

embedded in the matrix phase to give the desired characteristics.

Natural composite

Wood – lingix matrix reinforced with the cellulose fiber.

Bone – mineral matrix called apatite reinforced with collagen fibers.

Man made Composites

Mud walls of houses reinforced with bamboos

Glass fiber reinforced resin for helmet

Reinforced concrete

Automobile tyres

Need for developing composites

The main advantage of composite material is the combination of different

properties which are seldom found in conventional materials.

The unusual combination properties include high strength to weight ratio, higher

stiffness to weight ratio, improved fatigue resistance, improved corrosion

resistance, higher resistance to thermal expansion, excellent optical and magnetic

properties, wear resistance, good fracture toughness, acoustical insulation.

Present trend is to go for light weight construction for easy handling and reduced

space.

Classification of composites

1) Based on matrix

2) Based on Reinforcement

Reinforcements for the composites can be fibers, fabrics particles or whiskers. Fibers

are essentially characterized by one very long axis with other two axes either often

circular or near circular. Particles have no preferred orientation and so does their

shape. Whiskers have a preferred shape but are small both in diameter and length as

compared to fibers

Reinforcing material

ParticulateReinforced

FiberReinforced

Structural Reinforced

Functions of matrix

1) Binds the fiber together and acts as medium by which an externally applied stress

is transmitted and distributed to the fibers.

- matrix material should be ductile

- Elastic modulus of fiber should be much higher than that of matrix.

2) Matrix protects the individual fiber from surface damage as a result of mechanical

abrasion or chemical reaction with environment.

3) Matrix suppurates fibers and by the virtue of it relative softness and plasticity,

prevents the propagation of brittle cracks from fiber to fiber catastrophic failure.

In other word a matrix phase serves as a barrier to crack propagation.

Polymer Matrix Composites (PMC)

PMC’s consist of polymer resin as a matrix. They are used in greatest diversity of

composite applications as well as in largest quantity in light of there room temperature

properties, ease of fabrication and cost.

The matrix often determines the maximum service temperature, since it normally melts

softens and degrade as much as lower temperature then the fiber reinforcement.

Two main kinds of polymers are thermosets and thermoplastics. Thermosets have

qualities such as a well-bonded three-dimensional molecular structure after curing. They

decompose instead of melting on hardening. Merely changing the basic composition of

the resin is enough to alter the conditions suitably for curing and determine its other

characteristics. They can be retained in a partially cured condition too over prolonged

periods of time, rendering Thermosets very flexible. Thus, they are most suited as matrix

bases for advanced conditions fiber reinforced composites. Thermosets find wide ranging

applications in the chopped fiber composites form particularly when a premixed or

moulding compound with fibers of specific quality and aspect ratio happens to be starting

material as in epoxy, polymer and phenolic polyamide resins.

Thermoplastics have one- or two-dimensional molecular structure and they tend to at an

elevated temperature and show exaggerated melting point. Another advantage is that the

process of softening at elevated temperatures can reversed to regain its properties during

cooling, facilitating applications of conventional compress techniques to mould the

compounds.

Resins reinforced with thermoplastics now comprised an emerging group of composites.

The theme of most experiments in this area to improve the base properties of the resins

and extract the greatest functional advantages from them in new avenues, including

attempts to replace metals in die-casting processes. In crystalline thermoplastics, the

reinforcement affects the morphology to a considerable extent, prompting the

reinforcement to empower nucleation. Whenever crystalline or amorphous, these resins

possess the facility to alter their creep over an extensive range of temperature. But this

range includes the point at which the usage of resins is constrained, and the reinforcement

in such systems can increase the failure load as well as creep resistance.

Thermosets Thermoplastics

• Resin cost is low. • Resin cost is slightly higher.

• Thermosets exhibit moderate shrinkage. • Shrinkage of thermoplastics is

low

• Interlaminar fracture toughness is low. • Interlaminar fracture toughness

is high.

• Thermosets exhibit good resistance • Thermoplastics exhibit poor resistance

to fluids and solvents. to fluids and solvents.

• Composite mechanical properties are good. • Composite mechanical properties are good.

• Prepregability characteristics are excellent. • Prepregability characteristics are

poor.

• Prepreg shelf life and out time are poor. • Prepreg shelf life and out time are excellent.

Different types of thermosets and thermoplastic resins commonly in use are as follows:

Thermosets Thermoplastics

• Phenolics & Cyanate ester • Polypropylene

• Polyesters & Vinyl esters • Nylon (Polyamide)

• Polyimides • Poly-ether-imide (PEI)

• Epoxies • Poly-ether-sulphone (PES)

• Bismaleimide (BMI) • Poly-ether -ether-ketone (PEEK)

Metal Matrix Composite

Metal matrix composites, at present though generating a wide interest in research fraternity,

are not as widely in use as their plastic counterparts. High strength, fracture toughness and

stiffness are offered by metal matrices than those offered by their polymer counterparts. They

can withstand elevated temperature in corrosive environment than polymer composites. Most

metals and alloys could be used as matrices and they require reinforcement materials which

need to be stable over a range of temperature and non-reactive too. However the guiding

aspect for the choice depends essentially on the matrix material. Light metals form the matrix

for temperature application and the reinforcements in addition to the aforementioned reasons

are characterized by high moduli.

Most metals and alloys make good matrices. However, practically, the choices for low

temperature applications are not many. Only light metals are responsive, with their low

density proving an advantage. Titanium, Aluminum and magnesium are the popular matrix

metals currently in vogue, which are particularly useful for aircraft applications. If metallic

matrix materials have to offer high strength, they require high modulus reinforcements. The

strength-to-weight ratios of resulting composites can be higher than most alloys.

The melting point, physical and mechanical properties of the composite at various

temperatures determine the service temperature of composites. Most metals, ceramics and

compounds can be used with matrices of low melting point alloys. The choice of

reinforcements becomes more stunted with increase in the melting temperature of matrix

materials.

Ceramic Matrix Materials

Ceramics can be described as solid materials which exhibit very strong ionic bonding in

general and in few cases covalent bonding. High melting points, good corrosion resistance,

stability at elevated temperatures and high compressive strength, render ceramic-based

matrix materials a favorite for applications requiring a structural material that doesn’t give

way at temperatures above 1500ºC. Naturally, ceramic matrices are the obvious choice for

high temperature applications.

High modulus of elasticity and low tensile strain, which most ceramics posses, have

combined to cause the failure of attempts to add reinforcements to obtain strength

improvement. This is because at the stress levels at which ceramics rupture, there is

insufficient elongation of the matrix which keeps composite from transferring an effective

quantum of load to the reinforcement and the composite may fail unless the percentage of

fiber volume is high enough. A material is reinforcement to utilize the higher tensile strength

of the fiber, to produce an increase in load bearing capacity of the matrix. Addition of high-

strength fiber to a weaker ceramic has not always been successful and often the resultant

composite has proved to be weaker.

The use of reinforcement with high modulus of elasticity may take care of the problem to

some extent and presents pre-stressing of the fiber in the ceramic matrix is being increasingly

resorted to as an option.

When ceramics have a higher thermal expansion coefficient than reinforcement materials, the

resultant composite is unlikely to have a superior level of strength. In that case, the composite

will develop strength within ceramic at the time of cooling resulting in micro cracks

extending from fiber to fiber within the matrix. Micro cracking can result in a composite with

tensile strength lower than that of the matrix.

Reinforcement Types

Dispersoids –

Hard inert sub-micrometer size particles are disperse in to the metallic or non-metallic or

inter metallic matrix. More often particles of 0.01 micro-meter to 0.1 micro-meter are

uniformly dispersed in a value of concentration of 10 to 15%, which acts as obstacles for

dislocation movement. The presence of hard particle also increases the elastic limit causing

rapid hardening. The strength of composite depends up on the particle size, shape,

distribution and physical characteristics. In this composite material, matrix is the measure

load bearing constituent.

Particulate –

These types of composites consist of particles of one or more materials suspended in a matrix

of another material. Here size of particle varies from 1mm or more and volume concentration

varies from 20 to 40% volume. Because of slightly bigger size particle, they can’t interfere

with dislocation and exhibit strengthens effect by hydrostatically restraining the movement of

matrix close to it.

The elastic modulus of particulate composites follows the rule of mixture

Upper Limit Ec = Em Vm + Ep Vp

Lower Limit Ec= EmEp / (Em Vm + Ep Vp)

Ec, Ep, Em = Elastic modulus of composite, matrix and particulate

V = Volume fraction.

The rule of mixture equation predict that the elastic modulus should

Fiber Reinforcement

Fibers are grouped as whiskers, fibers and wires based diameter and character.

Whiskers – are vary thin single crystal, have extremely very large length to dia ratio (20 –

200). They have small size, high degree of crystalline perfection, high strength.

Ex – graphite, silicon, carbide, silicon nitride, Al oxide.

Fibers – Fiber materials are either polymer or ceramics which have small diameter.

Ex – polymer, glass, carbon, graphite, boron, silicon, quartz.

Wires – have relatively large diameter.

Ex – steel, tungsten, molybdenum.

* Discontinuous reinforcement -

Used in polymeric composite, referred as fillers of various shapes and short fibers up to

20 mm.

Ex – Metallic powders Al, Fe, Mg, Ti, Zr etc

Non metallic powder – Al oxides, calcium carbonate, silica, carbon etc

* Continuous fiber reinforcement -

Natural – Silk, Jute, Wool, Cotton

Organic – fibers from extended flexible polymers like polyethylene, polymer.

Glass fiber – Most inorganic fibers are based on amorphous 3D networks structure of

silica. They are isotropic in nature and can be drawn easily near their glass transition

temperature. Where they exhibit ‘Newtonian viscous flow’.

Depending on chemistry and property of fiber, they can be classified as

- E – glass – Electrical applications

- C – glass – Corrosion resistance application

- AR - Alkali resistance

- S – higher tensile and stiffness value

Structural Composites

a) Laminated

Multi layer composite consist several layers of fibrous composites bounded together by

organic adhesives. Each layer or lamina is a single layer composite and is very thin about

0.1 mm. when several such identical or different layers are bound together, form multi

layer composites. The constituent materials in each layer are called laminates. If multi

layer composite is made up of layers of different constituent materials. They are called

hybrid composites.

Ex – Reinforced plastic sheet clawed with copper, provided in the printed circuit which

gives better electrical conductivity.

Pure aluminum is bonded to high strength aluminum alloy protects ply from corrosion.

b) Sand witches

Sand witches consisting of thick low, density core, sand witched between two high

density facing materials. Sand witches can offer high strength and high bending stiffness.

Function of core – It suppurates the faces and resists deformation perpendicular to the

face plane.

It provides a certain degree of shear rigidity along planes which are perpendicular to the

face.

Core materials can be either sheets made of foam or honey comb structural made of

polystyrene foam, polyurethane foam, and polyvinyl chloride foam, plastic honey comb

carbon or Kevlar.

Face sheet is generally made of Al or FRP.

The challenge is making a structure as light as possible without sacrificing strength. This

requirement leads to stabilize thin surface to with stand tensile and compressive loads and

combination of face sheet and core to resist bending and torsion.

Prepegs

Is ready made type composite available in standard width of 76 to 1270 mm and

thickness varies from 0.1 to 3 mm.

A tape is stored at room temperature or in refrigerator. The resin content varies from 35

to 45%.

Prepegs are in 3 grades

- Continuous fiber embedded in resin matrix

- Discontinuous but aligned fibers embedded in resin matrix.

- Discontinuous and randomly distributed.

Rows of fibers such as glass, aramid, boron or carbon fibers are collected and passed

through collimator with tension rolls. The collimatric fibers pass through resin bath. In

heating chamber curing take place.

Stage A – Un cured stage, where in the resin is to flow easily.

Stage B – Middle stage –Semi viscous to allow process easily. In this stage both

heat and pressure applied.

Stage C – Prepegs become hard and partially cured.

After curing the prepegs pass through take up rolls back with releasing film. The

releasing film keeps prepegs from non-sticking.

Hybrid Composites

Hybrid is obtained by using two or more different types of fiber in a single matrix. A

verity of fiber combinations and matrix materials are used. There are number of ways in

which the two different fibers may be combined.

Fiber may be aligned and intimately mixed with one another.

Laminates may be constructed consisting of layers, each of which consists of single fiber

type, alternating one with another.

Ex – Commonly carbon and glass fiber are incorporated in to a polymeric resin. Carbon

fibers are strong and relatively stiff and provide low density reinforcement. Glass fibers

are inexpensive and lacks of stiffness of carbon. The glass-carbon hybrid is stronger and

tougher has a higher impact resistance and may be produced at lower cost. When hybrid

composites are stressed in tension, failure is usually doesn’t occur suddenly. The carbon

fibers are first to fail, at which the load is transferred to glass fiber. Upon the failure of

glass fiber the matrix phase must sustain the applied load.

Applications-

Light weight land, water, air transport structural components, sporting goods, light

weight orthopedic component.

Desirable characteristics of Fiber Reinforced Composites (Parameters or Factors)

1. Length of fiber

2. Diameter or size of fiber

3. Orientation of fiber

4. Volume fraction of fiber

5. Fiber properties

6. Matrix properties

7. Bonding and Interface strength

Length of fiber:

Usually the ends of fibers have lower load carrying ability and hence higher the ends

lower will be load carrying capacity of the composite. Longer the fiber, number of ends

will be lower and hence higher will be the load carrying capability. It has also been found

that for the same volume fraction of fibers, increasing the length of fibers is found to

increase the tensile strength of fibers.

Diameter of fibers

By reducing the diameter of fibers has following advantages:

The number of flaws is greatly reduced and strength is increased

Contact surface area increased. Smaller the size, more fibers will accommodated

and hence the contact surface is increased which in turn improves the efficiency

of load transfer from the matrix to the fiber.

Flexibility of fibers is greatly increased whereby the fibers can be bent, wound

and woven easily.

Volume fraction

Increasing the volume fraction or amount of fibers lead to increase in the specific

property of the composite. However, maximum volume fraction is restricted to 80% to

ensure that each fiber is surrounded by the matrix phase. It has been found that if the

volume fraction is below 28% the fibers do not effectively reinforce the matrix.

Orientation of fiber

1) Continuous aligned fiber under longitudinal load.

2) Continuous fiber under transverse load.

3) Discontinuous and aligned fiber.

4) Discontinuous and randomly oriented.

Fiber Properties

Fibre reinforcement not only comes in a variety of materials with different strengths and

stiffness, but also in a variety of forms, e.g. mats, straight rovings, woven fabrics. In

some forms the fibres are grossly kinked to conform to a weave pattern and this can

reduce the strength of the composite material. Unlike most conventional materials, the

strength and stiffness of the material can be varied by adjusting the fibre content.

Matrix Properties

The wide variety of polymers with different characteristics is further complicated by the

addition of fillers and Plasticizers which can significantly alter the composite properties. This

can result in published properties being of little value because the exact composition is not

stated or cannot be reproduced. Some matrices may exhibit poor bonding with the fibre

reinforcement and are thus unable to develop the full strength capacity of the fibres.

Bonding and Interface Strength

It is extremely important that the bonding between the reinforcing phase and matrix is

very good. The fibers should not pull out and get de-laminated or de-bonded. To facilitate

good wettability, coupling agents or coatings are used.

1.2 COMPOSITES

1.2.1 Why a composite?

Over the last thirty years composite materials, plastics and ceramics have been the

dominant emerging materials. The volume and number of applications of composite

materials have grown steadily, penetrating and conquering new markets relentlessly.

Modern composite materials constitute a significant proportion of the engineered

materials market ranging from everyday products to sophisticated niche applications.

While composites have already proven their worth as weight-saving materials, the current

challenge is to make them cost effective. The efforts to produce economically attractive

composite components have resulted in several innovative manufacturing techniques

currently being used in the composites industry. It is obvious, especially for composites,

that the improvement in manufacturing technology alone is not enough to overcome the

cost hurdle. It is essential that there be an integrated effort in design, material, process,

tooling, quality assurance, manufacturing, and even program management for composites

to become competitive with metals.

The composites industry has begun to recognize that the commercial applications of

composites promise to offer much larger business opportunities than the aerospace sector

due to the sheer size of transportation industry. Thus the shift of composite applications

from aircraft to other commercial uses has become prominent in recent years.

Increasingly enabled by the introduction of newer polymer resin matrix materials and

high performance reinforcement fibres of glass, carbon and aramid, the penetration of

these advanced materials has witnessed a steady expansion in uses and volume. The

increased volume has resulted in an expected reduction in costs. High performance FRP

can now be found in such diverse applications as composite armoring designed to resist

explosive impacts, fuel cylinders for natural gas vehicles, windmill blades, industrial

drive shafts, support beams of highway bridges and even paper making rollers. For

certain applications, the use of composites rather than metals has in fact resulted in

savings of both cost and weight. Some examples are cascades for engines, curved fairing

and fillets, replacements for welded metallic parts, cylinders, tubes, ducts, blade

containment bands etc.

Further, the need of composite for lighter construction materials and more seismic

resistant structures has placed high emphasis on the use of new and advanced materials

that not only decreases dead weight but also absorbs the shock & vibration through

tailored microstructures. Composites are now extensively being used for rehabilitation/

strengthening of pre-existing structures that have to be retrofitted to make them seismic

resistant, or to repair damage caused by seismic activity.

Unlike conventional materials (e.g., steel), the properties of the composite material can be

designed considering the structural aspects. The design of a structural component using

composites involves both material and structural design. Composite properties (e.g.

stiffness, thermal expansion etc.) can be varied continuously over a broad range of values

under the control of the designer. Careful selection of reinforcement type enables finished

product characteristics to be tailored to almost any specific engineering requirement.

Whilst the use of composites will be a clear choice in many instances, material selection

in others will depend on factors such as working lifetime requirements, number of items

to be produced (run length), complexity of product shape, possible savings in assembly

costs and on the experience & skills the designer in tapping the optimum potential of

composites. In some instances, best results may be achieved through the use of

composites in conjunction with traditional materials.

1.2.2 What is a composite?

A typical composite material is a system of materials composing of two or more materials

(mixed and bonded) on a macroscopic scale.

Generally, a composite material is composed of reinforcement (fibers, particles, flakes,

and/or fillers) embedded in a matrix (polymers, metals, or ceramics). The matrix holds

the reinforcement to form the desired shape while the reinforcement improves the overall

mechanical properties of the matrix. When designed properly, the new combined material

exhibits better strength than would each individual material. As defined by Jartiz, [7]

Composites are multifunctional material systems that provide characteristics not

obtainable from any discrete material. They are cohesive structures made by physically

combining two or more compatible materials, different in composition and characteristics

and sometimes in form. Kelly [8] very clearly stresses that the composites should not be

regarded simple as a combination of two materials. In the broader significance; the

combination has its own distinctive properties. In terms of strength or resistance to heat

or some other desirable quality, it is better than either of the components alone or

radically different from either of them.

Berghezan [9] defines as “The composites are compound materials which differ from

alloys by the fact that the individual components retain their characteristics but are so

incorporated into the composite as to take advantage only of their attributes and not of

their shortcomings”, in order to obtain an improved material

Van Suchetclan [10] explains composite materials as heterogeneous materials consisting

of two or more solid phases, which are in intimate contact with each other on a

microscopic scale. They can be also considered as homogeneous materials on a

microscopic scale in the sense that any portion of it will have the same physical property.

1.2.3 Characteristics of the Composites

Composites consist of one or more discontinuous phases embedded in a continuous

phase. The discontinuous phase is usually harder and stronger than the continuous phase

and is called the ‘reinforcement‘ or ‘reinforcing material’, whereas the continuous phase

is termed as the ‘ matrix’.

Properties of composites are strongly dependent on the properties of their constituent

materials, their distribution and the interaction among them. The composite properties

may be the volume fraction sum of the properties of the constituents or the constituents

may interact in a synergistic way resulting in improved or better properties. Apart from

the nature of the constituent materials, the geometry of the reinforcement (shape, size and

size distribution) influences the properties of the composite to a great extent. The

concentration distribution and orientation of the reinforcement also affect the properties.

The shape of the discontinuous phase (which may by spherical, cylindrical, or rectangular

cross-sanctioned prisms or platelets), the size and size distribution (which controls the

texture of the material) and volume fraction determine the interfacial area, which plays an

important role in determining the extent of the interaction between the reinforcement and

the matrix.

Concentration, usually measured as volume or weight fraction, determines the

contribution of a single constituent to the overall properties of the composites. It is not

only the single most important parameter influencing the properties of the composites,

but also an easily controllable manufacturing variable used to alter its properties.

The orientation of the reinforcement affects the isotropy of the system.

1.3 COMPONENTS OF A COMPOSITE MATERIAL

In its most basic form a composite material is one, which is composed of at least two

elements working together to produce material properties that are different to the

properties of those elements on their own. In practice, most composites consist of a bulk

material (the ‘matrix’), and a reinforcement of some kind, added primarily to increase the

strength and stiffness of the matrix.

1.3.1 Role of matrix in a composite

Many materials when they are in a fibrous form exhibit very good strength property but

to achieve these properties the fibres should be bonded by a suitable matrix.

The matrix isolates the fibres from one another in order to prevent abrasion and formation

of new surface flaws and acts as a bridge to hold the fibres in place. A good matrix

should possess ability to deform easily under applied load, transfer the load onto the

fibres and evenly distributive stress concentration.

A study of the nature of bonding forces in laminates [12] indicates that upon initial

loading there is a tendency for the adhesive bond between the reinforcement and the

matrix to be broken. The frictional forces between them account for the high strength

properties of the laminates.

1.3.2 Materials used as matrices in composites

In its most basic form a composite material is one, which is composed of at least two

elements working together to produce material properties that are different to the

properties of those elements on their own. In practice, most composites consist of a bulk

material (the matrix) and a reinforcement of some kind, added primarily to increase the

strength and stiffness of the matrix.

RULE OF MIXTURES

Rule of Mixtures is a method of approach to approximate estimation of composite

material properties, based on an assumption that a composite property is the volume

weighed average of the phases (matrix and dispersed phase) properties. According to

Rule of Mixtures properties of composite materials are estimated as follows:

Density

dc = dm*Vm + df*Vf

Where

dc,dm,df – densities of the composite, matrix and dispersed phase respectively;

Vm,Vf – volume fraction of the matrix and dispersed phase respectively.