composites ( as per mgu syllabus)

Department of Mechanical Engineering SSET

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Module 4 (second part)

Composites

Introduction to composites

A composite material is defined as a material which is composed of two or more materials which are

chemically and physically distinct phases in the final product material at a microscopic scale. The

two materials work together to give the composite unique properties. The materials which form

the composite are also called as constituents or constituent materials. The combination of

materials should result in significant property changes.

• The individual materials do not dissolve or merge completely in the composite, but

they act together as one.

• The properties of the composite material are superior to the properties of the

individual materials from which it is constructed.

• The biggest advantage of modern composites is that they are light as well as strong.

Many specific engineering applications can be met by modern composites. Composites also

provide design flexibility because many of them can be moulded into complex shapes.

In a composite, typically, there are two constituents. One of the constituent acts as a

reinforcement and other acts as a matrix. The constituents are also sometime referred as

phases.

A composite material consists of two phases:

Matrix phase or continuous phase: The base material surrounding reinforcement

material is (normally present in higher percentage) called a matrix. Common

matrixes are polymers, metals, or ceramics.

Fibre phase or re-inforcement phase or dispersive phase: The material which

reinforces the properties composite materials is called reinforcements. Generally fibre

is the load taking material in a composites.

Examples for composites

Natural composites

Wood (cellulose fibre plus + lignin matrix)

Bone ( calcium phosphate + collagen)

Synthetic composites

Concrete (cement matrix+sand and stone particle fibre)

Cemented carbide tool (WC and TiC

Tire (rubber matrix +carbon fibre)

Common fibres

Glass)

Carbon

Ceramics (boron,oxides, nitrides

and carbides)

Organic materials (polymers)

Metal

Common matrix

Metal

Ceramics

polymers


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Matrix phase

A matrix supports the fibers and bonds them together in the composite material. The matrix

transfers any applied loads to the fibers, keeps the fibers in their position and chosen

orientation, gives the composite environmental resistance, and determines the maximum

service temperature of a composite.

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

fibre reinforcement, the importance of matrix material cannot be underestimated as it provides

support for the fibres and assists the fibres in carrying the loads. It also provides stability to the

composite material.

Functions of a matrix material

1. The matrix material holds the fibres together. The matrix plays an important role

to keep the fibres at desired positions.

2. The matrix keeps the fibres separate from each other so that the mechanical

abrasion between them does not occur.

3. It transfers the load uniformly between fibers.

4. It provides protection to fibers from environmental effects.

5. It provides better finish to the final product.

6. The matrix material enhances some of the properties of the resulting material and

structural component (that fibre alone is not able to impart).

Properties of matrix

1. Reduced moisture absorption

2. Low shrinkage

3. Low co-efficient of thermal expansion

4. Excellent chemical resistance

5. Dimensional stability

6. Reasonable strength

Re-inforcement phase or Fibre phase or dispersed phase

The reinforcing phase is in the form of fibers, sheets, or particles which are embedded in the

matrix phase. Typically, reinforcing materials are strong with low densities while the matrix

is usually a ductile, or tough, material. Re-inforcement phase is the primary load carrying

element of the composite material. The classification of composites on the basis of fibre

phase is shown below


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Fibre phase are 3 types

1. Particle reinforced phase

2. Fibre reinforces phase

3. Structural reinforced phase

Functions of reinforcing agent (fibre)

1. These are the main load carrying constituents.

2. The reinforcing materials, in general, have significantly higher desired properties.

Hence, they contribute the desired properties to the composite.

3. It transfers the strength and stiffness to the matrix material.

Fibers occupy the most volume in a high performance composite and carry most of the

applied load. Fiber type, quantity and orientation have a major influence on the following

properties of the composite:

1. Specific Gravity

2. Tensile Strength & Modulus

3. Compressive Strength & Modulus

4. Fatigue Strength

5. Electrical & Thermal Conductivity's

6. Cost

Glass fibers are the earliest known fibers used to reinforce materials. Ceramic and metal

fibers (carbon) were subsequently found out and put to extensive use, to render composites

stiffer more resistant to heat.

Fibers are essentially characterized by high aspect ratio (length/diameter ratio). Particles have

no preferred orientation and so does their shape.

Fibres can be in the form rod, fibers, flakes and whiskers. Whiskers have a preferred shape

but are small both in diameter and length (single crystal) as compared to fibers.

Whiskers

• single crystals - very small diameter (~1 micron)

• virtually flaw free – so strong - expensive

• difficult to put in a matrix

Laminate:

Stacking of unidirectional or woven fabric layers at different fiber orientations are called

laminate. Effective properties vary with orientation, thickness and stacking sequence

Flake: Flake is a small, flat, thin piece or layer (or a chip) that is broken from a larger piece.

Since these are two dimensional in geometry, they impart almost equal strength in all

directions of their planes. For example, aluminum flakes are used in paints.


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Classification of composites

The first level of classification is usually made with respect to the matrix constituent.

1. Organic Matrix Composites (OMCs), (Polymer Matrix Composites (PMCs) namely

and carbon matrix composites)

2. Metal Matrix Composites (MMCs) and

3. Ceramic Matrix Composites (CMCs).

The second level of classification refers to the reinforcement form

1. Particle-reinforced (large-particle and dispersion-strengthened)

2. Fiber-reinforced (continuous (aligned) and short fibers (aligned or random)

3. Structural (laminates and sandwich panels)

Advantages or properties of the composite materials

Composites are a unique class of materials made from two or more distinct materials that

when combined are better (stronger, tougher) than each would be separately.

1. High Specific stiffness and specific strength:

The composites have high specific stiffness and strengths (Lighter and stronger).

2. Tailorable design: (Design Flexibility)

A component can be designed to have desired properties in specific directions.

(Choice of materials (fiber/matrix), volume fraction of fiber and matrix, layer

orientation, layers thickness etc makes tailorable design)

3. High Fatigue Life/strength:

The composites can with stand more number of fatigue cycles (than aluminium).

4. Good Dimensional Stability:

Composites retain their shape and size when they are hot or cool, wet or dry.

5. High Corrosion Resistance:

Polymer and ceramic matrix material used to make composites have high resistance to

corrosion from moisture, chemicals.

6. Thermal and electrical properties

Thermal Properties:

o Low thermal conductivity

o Low coefficient of thermal expansion

Electric Property:

o High dielectric strength

o Non-magnetic


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Disadvantages of Composites

Like all things in nature, composites materials have, their limitations as well. Some of the

important ones are:

1. Anisotropy property: have different property in various direction

2. Non‐ homogenous

3. Costly: Composite materials are in general expensive.

4. Difficult to fabricate: Further time taking, and expensive.

5. Sensitivity to temperature: Laminated composites are particularly sensitive to

temperature changes. They come in with residual thermal stresses, because they get

fabricated at high temperatures, and then cooled.

6. Moisture effects: Laminated composites are also sensitive to moisture, and their

performance varies significantly when exposed to moisture for long periods of time

7. Hidden defects are difficult to locate.

8. The composites, in general, are brittle in nature and hence easily damageable.

9. Matrix is subject to environmental degradation.

10. Parts may not be repairable or reusable

Application of composites

Composite materials have found applications in a wide range of industries.

1. Automotive industry: Lighter, stronger, wear resistance, rust ‐ free, aesthetics

o Car body

o Brake pads

o Drive shafts

o Hoods

o Spoilers

2. Aerospace: Lighter, stronger, temperature resistance, smart structures, wear resistance

Aircraft: Nose, doors, struts, fairings, ailerons, cowlings, outboard and inboard flaps,

stabilizers, elevators, rudders, fin tips, spoilers,

3. Rockets & missiles: Nose, body, pressure tanks, frame , fuel tanks , turbo motor

stators, - Satellites: frames, structural parts

4. Sports: Lighter, stronger, toughness, better aesthetics, higher damping properties

o Tennis

o Bicycles

o Badminton

o Boats

o Hockey

5. Transportation & Infrastructure: Lighter, stronger, toughness, damping

o Railway coaches

o Bridges

o Ships and boats

o Truck bodies and floors

In many applications, like in aircraft parts, there is a need for high strength-low weight

(specific strength). This can be achieved by composites consisting of a low-density (and soft)

matrix reinforced with stiff fibers.


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Fibre phase are 3 types

4. Particle reinforced phase

5. Fibre reinforces phase

6. Structural reinforced phase

Particle- reinforced composites:

A particle has no long dimension. Particle composites consist of particles of one of one

material dispersed in a matrix of a second material. Particle reinforced composites are much

easier and less costly than making fiber reinforced composites.

The reinforcement is in the form of particles which are of the order of a few microns are

generally added to increase the modulus and decrease the ductility of the matrix materials. In

this case, the load is shared by both particles and matrix materials. However, the load shared

by the particles is much larger than the matrix material. These reinforcing particles tend to

restrain movement of matrix phase of applied stress to particle which bear a friction of load,

the degree of reinforcement or improvement of behaviour depends on strong bonding at

matrix particle interface. Particle reinforced composites are the cheapest and most widely

used. The composite with reinforcement in particle form is also called as particulate

composite.

Particles re-inforced composites fall in two categories

Large-particle composites

Dispersion-strengthened composites

These classifications is not actually based on size of particle, but based mainly on bonding

mechanism between fibre and matrix. Usually we see classification on the basis size of

particles.

Large-particle composites

Large particle reinforcement, as the name suggests, involves larger particles but the particles

are small relative to the size of the structure and evenly distributed through it. The particle

diameter is typically on the order of a few microns. In this case, the particles carry a major

portion of the load. The particles are used to increase the modulus and decrease the ductility

of the matrix.

The most common large-particle composite is concrete, made of a cement matrix that bonds

particles of different size (gravel and sand.). Another example of particle reinforced

composites is an automobile tire which has carbon black particles in a matrix of poly-

isobutylene elastomeric polymer and ceramics particle (Tic, Sic) embedded in cobalt matrix

in the case of cutting tool.

• large-particle composites, restraining the movement of the matrix, if well bonded.

• Matrix – particle interaction (bonding) cannot be treated on atomic or molecular level.

• The particles in these composite are larger than in dispersion strengthened composites.

• Volume fraction of particles will be more than dispersed type composites.

• The volume fraction of the two phases influences the behaviour; mechanical

properties are enhanced with increasing particulate content.


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Dispersion-strengthened composites,

• Dispersion-strengthened means of strengthening materials where in very small

particles (usually less than 0.1 µm / 0.01-0.1μm)) of a hard yet inert phase are

uniformly dispersed within matrix phase (load bearing).

• The dispersed phase may be metallic or nonmetallic, oxide materials are often used.

• Bonding is at atomic level between particles and matrix

These particles act to help the matrix resist deformation. This makes the material harder and

stronger. The matrix bears the major portion of the applied load and the small particles hinder

dislocation motion, limiting plastic deformation. Matrix transfers some load to particles.

Here the strengthening occurs at atomic/molecular level i.e. mechanism of strengthening is

similar to that for precipitation hardening in metals where matrix bears the major portion of

an applied load. Volume concentration of fine particles is less than the large particles

composite.

Examples: thoria (ThO2) dispersed Ni-alloys matrix and aluminium with aluminium oxide

particles Al2O3). Metal may be strengthened and hardened by the uniform dispersion of

several volumes present of fine particles of a very hard and inert material. The high-

temperature strength of nickel alloys may be enhanced significantly by the addition of about

3 vol% of thoria (ThO2) as finely dispersed particles; this material is known as thoria-

dispersed nickel. Many metal-matrix composites would fall into the dispersion strengthened

composite category.

Advantages of particle reinforced composite materials

Low cost

High stiffness and strength (inorganic particles)

Wear resistance

Simpler manufacturing process

Fiber-reinforced composites

Most fiber-reinforced composites provide improved strength and other mechanical properties

and strength-to-weight ratio by incorporating strong, stiff but brittle fibers into a softer, more

ductile matrix. Fiberglass and carbon fiber composites are examples of fiber-reinforced

composites.

The strength depends on the fiber length and its orientation with respect to the stress

direction. The efficiency of load transfer between matrix and fiber depends on the interfacial

bond.


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The matrix material acts as a medium to transfer the load to the fibers, which carry most off

the applied load. The matrix also provides protection to fibers from external loads and

atmosphere.

Fiber Geometry

Some common geometries for fiber-reinforced composites:

Aligned

• The properties of aligned fiber-reinforced composite materials are highly anisotropic.

The longitudinal tensile strength will be high whereas the transverse tensile strength

can be much less than even the matrix tensile strength. It will depend on the

properties of the fibers and the matrix, the interfacial bond between them, and the

presence of voids. There are 2 different geometries for aligned fibers:

Continuous & Aligned

The fibers are longer than a critical length which is the minimum length necessary

such that the entire load is transmitted from the matrix to the fibers. If they are shorter

than this critical length, only some of the load is transmitted. Fiber lengths greater that

15 times the critical length are considered optimal. Aligned and continuous fibers give

the most effective strengthening for fiber composites.

Discontinuous & Aligned

• The fibers are shorter than the critical length. Hence discontinuous fibers are less

effective in strengthening the material, however, their composite modulus and tensile

strengths can approach 50-90% of their continuous and aligned counterparts. And they

are cheaper, faster and easier to fabricate into complicated shapes.

Random

This is also called discrete, (or chopped) fibers. The strength will not be as high as

with aligned fibers, however, the advantage is that the material will be istropic and

cheaper.

Woven

• The fibers are woven into a fabric which is layered with the matrix material to make a

laminated structure.

The objective of fiber-reinforced composites it to obtain a material with high specific strength

and high specific modulus. (i.e. high strength and high elastic modulus for its weight.)


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Factors influencing properties of composites

The mechanical properties of fiber-reinforced composites depend not only on the properties

of the fiber but also length of fibers, their orientation and volume fraction in addition to

direction of external load application etc. The efficiency of load transfer between matrix and

fiber depends on the interfacial bond also

Factors that affect the composite properties

There are various factors upon which the properties of the composite depend. Following are

the various factors:

1. Properties of the constituent materials. Apart from this, the properties of other phases

present, like additives, fillers and other reaction phases also affect the properties of

the composite.

2. Length of the fibre.

3. Orientation of the fibres (with respect to the loading direction).

4. Cross sectional shape of the fibre.

5. Distribution and arrangement of the fibres in the matrix material.

6. Proportions of the fibre and matrix material, that is, volume fractions of the

constituent materials.

The degree reinforcement or improvement of mechanical behavior depends on strong

bonding at the matrix- particle interface.

Influence of Fiber Length

Effect of fiber length: Some critical length (lc) is necessary for effective strengthening and

stiffening of the composite material, which is defined as:

m

f

c

dl

2

*

σf = ultimate tensile strength of fibre

τm= interface bonding shear strength (bonding between matrix and fibre)

lc= critical length

d=diameter of fibre

This critical length is the minimum length required for developing the full strength capacity

of the fiber. Critical length is depending on diameter of fibre, tensile strength of fibre and

bonding strength of matrix and fibre.

The longer the fiber, the more effectively the polymer is able to “grab on” and transfer stress

to the fiber. Generally speaking, continuous fiber composites have superior mechanical

properties. We want the fibre to carry as much load as possible. As fiber length l increases,

the fiber reinforcement becomes more effective. Load transfer between matrix and fibre is

depend on fibre length.

Fibers for which length is normally greater than 15 times cl are termed as continuous,

discontinuous or short fibers on the other hand. To achieve effective strengthening and

stiffening, the fibers must be larger than a critical length lc.

For a number of glass and carbon fiber–matrix combinations, this critical length is on the

order of 1 mm, which ranges between 20 and 150 times the fiber diameter.


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Load transfer from the matrix to the fibre causes the tensile stress in the fibre to rise to peak

in the middle. If the peak exceeds the fracture strength of the fibre, it breaks.

Influence of fiber orientation and concentration

The fibers is many composites are arranged in one direction; the fibers are aligned

unilaterally. The arrangement or orientation of the fibers relative to one another, the fiber

concentration, and the distribution all have a significant influence on the strength and other

properties of fiber-reinforced composites. With respect to orientation, two extremes are

possible: (1) a parallel alignment of the longitudinal axis of the fibers in a single direction,

and (2) a totally random alignment.

The long fiber will make the composite very strong in certain direction but not very strong in

the other direction. Hence, the material will be anisotropic. The values of their properties

depend on directions. The longitudinal tensile strength will be high whereas the transverse

tensile strength can be much less than even the matrix tensile strength. In random

arrangement, the strength will not be as high as with aligned fibers, however, the advantage is

that the material will be istropic and cheaper.


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Figure is a graphical way of representing the strength of a unilateral composite as a function

of the direction of fibre. As you can clearly see, the strength of a composite drops

dramatically when the stress is applied off the alignment of the fibers. The strength is

obtained by having the applied load transmitted from the matrix to the fibers.

The composite material is only strong and stiff in the direction of the fibers. So orientation,

length and volume of fibre in the composites influence the composite properties.

Unidirectional composites have predominant mechanical properties in one direction and are

said to be anisotropic, having mechanical and/or physical properties that vary with direction

relative to natural reference axes inherent in the material.

Short-fibre composites

In the case where l (length of fibre) is equal to lc, the tensile breaking stress in the middle of

the fibre can just be reached, and the fibre can therefore be broken, but the load-bearing

ability of the whole composite must be less than that of a continuous-fibre composite

containing an identical type of fibre. Only if a fibre is longer than the critical length, lc, can it

be broken by loading the composite and its full reinforcing potential realized.

Simplified illustration of the variation of tensile stress in short fibres as a function of fibre

length. σfis the fibre breaking stress and lc is the fibre critical length.

With very long fibre, composites can be loaded to the maximum fibre strength.

With short fibre length, composites can not be loaded to the maximum fibre strength.

With critical fibre length, maximum fibre strength at the centre of that fibre only.


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Syllabus for composites

Fundamentals of Composites: - particle reinforced composites – large particle composites -

fiber reinforced composites: influence of fiber length, orientation and concentration-fiber

phase – matrix phase.

Brief Notes on polymer, metal and ceramics matrix

(not in the syllabus but questions can be expected)

Concepts of Load Transfer in composites

The concept of load sharing between the matrix and the reinforcing constituent (fibre) is

central to an understanding of the mechanical behaviour of a composite. An external load

(force) applied to a composite is partly borne by the matrix and partly by the reinforcement.

Equating the externally imposed load to the sum of these two contributions, and dividing

through by the total sectional area, gives a basic and important equation of composite theory,

sometimes termed the "Rule of Averages".

Consider loading a composite parallel to the fibres. Since they are bonded together, both fibre

and matrix will stretch by the same amount in this direction, i.e. they will have equal strains.

This means that, since the fibres are stiffer (have a higher Young modulus, E), they will be

carrying a larger stress.

The fibers and the matrix experience the same strain

o Strain of composites= strain of fibre= strain of fibre

The load that the composite carries is the sum of the load on the fibers and the matrix:

o Pc = Pm + Pf

Substitute an expression for the load, P, using the stress (P = σA):


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σcAc = σmAm + σfAf

Now substitute an expression for the stress, using Young's modulus (σ = eE):

ec EcAc = emEmAm + efEfAf

And since ec = em = ef = e

we have: eEcAc = eEmAm + eEfAf

ie Ec = (Am/Ac) Em + (Af /Ac)Ef

If Vm & Vf are volume fractions of matrix and fibers respectively, we finally have :

Ec = VmEm + VfEf

So we see that for this case of isostrain conditions, the composite modulus, Ec, is simply the

weighted average of the moduli of the components.

Normally the matrix has a much lower modulus than the fiber so it strains more. Thus, for a

composite under tension, a shear stress appears in the matrix that pulls from the fiber. The

pull is uniform over the area of the fiber. This makes the force on the fiber be minimum at the

ends and maximum in the middle, like in the tug-of-war game.

Manufacturing or processing of composites

1. Open Mold Processes- laying resins and fibers onto forms

2. Closed Mold Processes-much the same as those used in plastic molding

3. Filament Winding- continuous filaments are dipped in liquid resin and wrapped

around a rotating mandrel, producing a rigid, hollow, cylindrical shape

4. Pultrusion Processes-similar to extrusion only adapted to include continuous fiber

reinforcement

Open mould /Hand Lay-Up:

The fibres are first put in place in the mould. The fibres can be in the form of woven, knitted,

stitched or bonded fabrics. Then the resin is impregnated. The impregnation of resin is done

by using rollers, brushes or a nip-roller type impregnator. The impregnation helps in forcing

the resin inside the fabric. The laminates fabricated by this process are then cured under

standard atmospheric conditions. The materials that can be used have, in general, no

restrictions. One can use combination of resins like epoxy, polyester, vinylester, phenolic and

any fibre material.


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Matrix types

The matrix materials used in composites can be broadly categorized as: Polymers, Metals,

Ceramics and Carbon and Graphite.

The metal matrix materials are: Aluminum, Copper and Titanium.

The ceramic materials are: Carbon, Silicon carbide, Silicon nitride.

Organic (polymer) matrix composites

Polymers make ideal materials as they can be processed easily, possess lightweight, and

desirable mechanical properties

The matrix is relatively soft and flexible

The reinforcement must have high strength and stiffness

Since the load must be transferred from matrix to reinforcement, the reinforcement-

matrix bond must be strong

There are two basic categories of polymer matrices:

1. Thermoplastics– soften upon heating and can be reshaped with heat and pressure.

Thermoset plastics–become cross linked during fabrication and does not soften upon

reheating

Roughly 95% of the composite market uses thermosetting plastics

Fibre-reinforced plastic (FRP) is a composite material made of a polymer matrix reinforced

with fibers. The fibres are usually glass, carbon, aramid, or basalt. Rarely, other fibres such as

paper or wood or asbestos have been used. The polymer is usually an epoxy, vinylester or

polyester thermosetting plastic still in use.

What are the thermoplastic matrix materials? What are their key features?

The following are the thermoplastic materials:

1. polypropylene,

2. polyvinyl chloride,

3. nylon,

4. polyurethane,

5. polyphenylene sulfide (PPS),

6. polysulpone.

http://en.wikipedia.org/wiki/Fiber

http://en.wikipedia.org/wiki/Aramid

http://en.wikipedia.org/wiki/Vinylester


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The key features of the thermoplastic matrix materials are:

1. higher toughness

2. low cost processing

3. The use temperature range is upto 225 0C.

What are the thermoset matrix materials? What are their key features?

The thermoset matrix materials are:

1. polyesters,

2. epoxies,

3. polyimides

Polyesters key features

1. Used extensively with glass fibers

2. Inexpensive

3. Light weight

4. Temperature range upto 100 0C.

5. Resistant to environmental exposures

What are the problems with the use of polymer matrix materials?

1. Limited temperature range.

2. Susceptible to environmental degradation due to moisture, radiation, oxygen (in space)

3. Low transverse strength.

4. High residual stress due to large mismatch in coefficients of thermal expansion

between fiber and matrix.

5. Polymer matrix cannot be used near or above the glass transition temperature.

Metal matrix composites

A metal matrix composite (MMC) is composite material with at least two constituent parts, one

being a metal necessarily, the other material may be a different metal or ceramic or organic

compound. Metal martices include aluminum, magnesium, copper, nickel, and intermetallic

compound alloys.

What are the common metals used as matrix materials? What are their advantages and

disadvantages?

The common metals used as matrix materials are aluminum, titanium and copper.

Advantages:

1. Higher transfer strength,

2. The attractive feature of the metal matrix composites is the higher temperature use.

3. High toughness (in contrast with brittle behavior of polymers and ceramics)

4. The absence of moisture

5. High thermal conductivity (copper and aluminum).

Dis-advantages:

1. Heavier

2. More susceptible to interface degradation at the fiber/matrix interface and

3. Corrosion is a major problem for the metals

The aluminum matrix composite can be used in the temperature range upward of 300ºC while

the titanium matrix composites can be used above 800 0C.


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Ceramics matrix composites

They consist of ceramic fibers embedded in a ceramic matrix, thus forming a ceramic fiber

reinforced ceramic (CFRC) material. The matrix and fibers can consist of any ceramic

material, whereby carbon and carbon fibers can also be considered a ceramic material.

What are the ceramic matrix materials? What are their advantages and disadvantages?

The carbon, silicon carbide and silicon nitride are ceramics and used as matrix materials.

Ceramic matrix material

The advantages of the ceramic matrix materials are:

1. The ceramic composites have very high temperature range of above 2000 0C .

2. High elastic modulus

3. Low density

The disadvantages of the ceramic matrix materials are:

1. The ceramics are very brittle in nature.

Carbon matrix

The advantages of the carbon matrix materials are:

1. High temperature at 2200 0C

2. Carbon/carbon bond is stronger at elevated temperature than room temperature.

The disadvantages of the carbon matrix materials are:

1. The fabrication is expensive

2. The multistage processing results in complexity and higher additional cost.

It should be noted that a composite with carbon fibres as reinforcement as well as matrix

material is known as carbon-carbon composite. The application of carbon-carbon composite

is seen in leading edge of the space shuttle where the high temperature resistance is required.

The carbon-carbon composites can resist the temperature upto 3000 0C.

The advantages of these composites are:

1. Very strong and light as compared to graphite fibre alone.

2. Low density

3. Excellent tensile and compressive strength

4. Low thermal conductivity

5. High fatigue resistance

6. High coefficient of friction

The disadvantages include:

1. Susceptible to oxidation at elevated temperatures

2. High material and production cost

3. Low shear strength

Figure depicts the range of use temperature for matrix material in composites. It should be

noted that for the structural applications the maximum use temperature is a critical parameter.

http://en.wikipedia.org/wiki/Fiber


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Advanced fibers:

An advanced fibre is defined as a fibre which has a high specific stiffness (that is, ratio of Young’s

modulus to the density of the material, ) and a high specific strength (that is the ratio of

ultimate strength to the density of the material,

The fibres made from following materials are the advanced fibres.

1. Carbon and/or Graphite

2. Glass fibers

3. Alumina

4. Aramid

5. Silicon carbide

6. Sapphire

It can be seen that the materials of the advanced fibres are lighter than the conventional

metals. These materials occupy higher position as compared to metals in the periodic table.

Thus, one can easily deduce that, in general, these materials have higher specific properties

(property per unit weight) than that of metals.

Carbon Fiber:

Sixth lightest element and carbon- carbon covalent bond is the strongest in nature.

Edison made carbon fiber from bamboo fibers made up of cellulose

The carbon content in carbon fibers is about 80-90 % and in Graphite fibers the

carbon content is in excess of 99%. Carbon fibre is produced at about 1300 0C while

the graphite fibre is produced in excess of 1900 0C.

Different fibers have different morphology, origin, size and shape

The size of individual filament ranges from 3 to 147 µm

Maximum use of temperature of the fibers ranges from 250 0C to 2000

0C.

Fiber properties vary with varying temperature.

Glass Fibre

Fibers of glass are produced by extruding molten glass, at a temperature around 1200 0C through holes in a spinneret with diameter of 1 or 2 mm and then drawing the

filaments to produce fibers having diameters usually between 5 to15µm.

The fibres have low modulus but significantly higher stiffness

Individual filaments are small in diameters, isotropic and very flexible as the diameter

is small.

The glass fibres come in variety of forms based on silica which is combined with

other elements to create speciality glass


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Miscellaneous notes

Fibre reinforced composites can be further divided into those containing discontinuous or

continuous fibres. Fibre Reinforced Composites are composed of fibres embedded in matrix

material. Such composite is considered to be a discontinuous fibre or short fibre composite if its

properties vary with fibre length. On the other hand, when the length of the fibre 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 fibre reinforced. Fibres are small in diameter and when

pushed axially, they bend easily although they have very good tensile properties. These fibres

must be supported to keep individual fibres from bending and buckling.

Fibre is an individual filament of the material with length to diameter ratio above 100 is

called. The fibrous form of the reinforcement is widely used.

Laminar Composites are composed of layers of materials held together by matrix. Sandwich

structures fall under this category.

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.

Alloys Vs Composite difference

Alloy is a homogenous and at least one material is metal in a alloy, but it is not

necessary to have metals in composites.