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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.
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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.
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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.