Composite Materials Emt311 Lecture Notes

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Composite Materials EMT311 Lecture Notes
materials technology and engineering (Harare Institute of Technology)
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COMPOSITE MATERIALS: SYNTHESIS, PROPERTIES & APPLICATIONS
EMT311 LECTURE NOTES
B.TECH (HONOURS) MATERIALS TECHNOLOGY ENGINEERING

SCHOOL OF ENGINEERING AND TECHNOLOGY
HARARE INSTITUTE OF TECHNOLOGY
LECTURER: ENGINEER AARON RUNGANI [BSC. HEN (UZ), MIP (AU)]
Contacts: rungani@yahoo.co.uk, +263778817943

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COMPOSITE MATERIALS: SYNTHESIS, PROPERTIES & APPLICATIONS (EMT311)
CONTENTS PAGE
1.0 INTRODUCTION ............................................................................................................................. 3

1.1 Advantages and Disadvantages of Composite Materials ................................................................. 6 Page | 1
2.0 Applications of Composite Materials ................................................................................................. 9
3.0 CLASSIFICATION OF COMPOSITE MATERIALS ...................................................................... 12
3.1 Classification based on type of matrix .......................................................................................... 12
3.2 Classification based on type of reinforcement form ...................................................................... 14
3.3 ORGANIC METAL COMPOSITES: Polymer Matrix Composites (PMC) ............................... 15
3.4 Metal Matrix Composites (MMC) ............................................................................................ 17
3.5 Ceramic Matrix Composites (CMC) ......................................................................................... 17
4.0 CLASSIFICATION OF REINFORCEMENTS ................................................................................ 19
4.1 Reinforcements Definition ........................................................................................................... 21
4.2 Particle Reinforced Composites ................................................................................................... 21
4.3. Cermets/ Ceramal ....................................................................................................................... 22
5.0 Fiber-Reinforced Polymer (FRP) Composites .................................................................................. 23
5.1 Whisker Reinforced Composites .................................................................................................. 24
5.2 Flakes Reinforced Composites ..................................................................................................... 25
5.3 Filled Composites ........................................................................................................................ 26
5.4 Microspheres Composites ............................................................................................................ 27
5.5 Role and Selection of Reinforcements .......................................................................................... 28
5.6 Role and Selection of Matrix ........................................................................................................ 28
5.7 Functions of the Matrix ................................................................................................................ 29
5.8 Desired Functions of Matrix......................................................................................................... 30
6.0 STRUCTURAL COMPOSITES ...................................................................................................... 31
7.0 MECHANICAL PROPERTIES OF COMPOSITES ........................................................................ 33
7.1 Idealization of microstructure of fibrous composites..................................................................... 33
7.2 Mechanical properties stiffness and strength................................................................................. 33
7.3 Isotropic, Anisotropic, and Orthotropic Materials ......................................................................... 40
7.4 Laminates .................................................................................................................................... 40
7.5 Fundamental property relationships .............................................................................................. 41
8.0 MANUFACTURING METHODS FOR COMPOSITES .................................................................. 43
8.1 SPRAY LAY-UP METHOD ....................................................................................................... 43
8.2 WET/ HAND LAY-UP ................................................................................................................ 44
Compiled by: Engineer A. Rungani rungani@yahoo.co.uk +263778817943 2020 Files

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8.3 VACUUM BAGGING ................................................................................................................ 46

8.4 FILAMENT WINDING............................................................................................................... 47
8.5 PULTRUSION ........................................................................................................................... 49
8.6 RESIN TRANSFER MOULDING ............................................................................................... 50
8.7 VACUUM INFUSION PROCESS ............................................................................................... 52 Page | 2
8.8 RESIN FILM INFUSION ............................................................................................................ 53
9.0 MECHANICAL TESTING OF COMPOSITES ............................................................................... 55
9.1 Objectives of Mechanical Testing ................................................................................................ 55
9.2 Tensile Testing ............................................................................................................................ 56
9.3 Measurement of Modulus............................................................................................................. 57
9.4 Compression Testing ................................................................................................................... 58

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DISCLAIMER
This document does not claim any originality and cannot be used as a substitute for prescribed Page | 3
textbooks. The information presented here is merely a collection by the author for their respective
teaching assignments as an additional tool for the teaching-learning process. Various sources as
indicated in the references of the document as well as freely available material from internet were
consulted for preparing this document. The ownership of the information lies with the respective
author or institutions. Further, this document is not intended for use in commercial purposes and
the Department is not accountable for any issues, legal or otherwise, arising out of use of this
document. The Department members make no representations or warranties with respect to the
accuracy or completeness of the contents of this document and specifically disclaim any implied
warranties of merchantability or fitness for a particular purpose.

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1.0 INTRODUCTION
Composites are engineered or naturally occurring materials made from the physical amalgamation
of two or more constituent materials with significantly different physical or chemical properties
that remain separate and distinct within the finished structure. A composite material is a material
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that consists of one or more discontinuous components (particles/fibers/reinforcement) that are
placed in a continuous medium (matrix). Composite materials, or shortened to composites, are
microscopic or macroscopic combinations of two or more distinct engineered materials (those with
different physical and/or chemical properties) with a recognizable interface between them in the
finished product. One of the materials is present in the matrix phase, and another one could be in
particle or fiber form. Composites are broadly categorized into two types: structural composites
with outstanding mechanical properties and functional composites with various outstanding
physical, chemical or electrochemical properties. For structural applications, the definition can be
restricted to include those materials that consist of a reinforcing phase such as fibers or particles
supported by a binder or matrix phase. Some features of composites include:
1) The distribution of materials in the composite is controlled by mechanical means.

2) The term composite is usually reserved for materials in which distinct phases are
separated on a scale larger than atomic, and in which the composite’s mechanical
properties are significantly altered from those of the constituent components.
3) The composite can be regarded as a combination of two or more materials that are used
in combination to rectify a weakness in one material by a strength in another.
4) A recently developed concept of composites is that the composite should not only be a
combination of two materials, but the combination should have its own distinctive
properties. In terms of strength, heat resistance, or some other desired characteristic,
the composite must be better than either component alone.
Composites were developed because no single, homogeneous structural material could be found
that had all of the desired characteristics for a given application. The main advantages of composite
materials are their high strength and stiffness, combined with low density, when compared with
bulk materials, allowing for a weight reduction in the finished part. The reinforcing phase provides
the strength and stiffness. In most cases, the reinforcement is harder, stronger, and stiffer than the
matrix. The reinforcement is usually a fiber or a particulate. Particulate composites have
dimensions that are approximately equal in all directions. They may be spherical, platelets, or any

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other regular or irregular geometry. Particulate composites tend to be much weaker and less stiff
than continuous fiber composites, but they are usually much less expensive. Particulate reinforced
composites usually contain less reinforcement (up to 40 to 50 volume percent) due to processing
difficulties and brittleness. The continuous phase is the matrix, which is a polymer, metal, or
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ceramic. Polymers have low strength and stiffness, metals have intermediate strength and stiffness
but high ductility, and ceramics have high strength and stiffness but are brittle. The matrix
(continuous phase) performs several critical functions, including maintaining the fibers in the
proper orientation and spacing and protecting them from abrasion and the environment. In polymer
and metal matrix composites that form a strong bond between the fiber and the matrix, the matrix
transmits loads from the matrix to the fibers through shear loading at the interface. In ceramic
matrix composites, the objective is often to increase the toughness rather than the strength and
stiffness; therefore, a low interfacial strength bond is desirable. The type and quantity of the
reinforcement determine the final properties. There is a practical limit of about 70 volume percent
reinforcement that can be added to form a composite. At higher percentages, there is too little
matrix to support the fibers effectively. Fiber-reinforced composites were first developed to
replace aluminum alloys, which provide high strength and fairly high stiffness at low weight but
are subject to corrosion and fatigue. The discontinuous filler phase in a composite is usually stiffer
or stronger than the binder phase. There must be a substantial volume fraction of the reinforcing
phase (about10%) present to provide reinforcement. Natural composites include wood and bone.
Wood is a composite of cellulose and lignin. Cellulose fibers are strong in tension and are flexible.
Lignin cements these fibers together to make them stiff. Bone is a composite of strong but soft
collagen (a protein) and hard but brittle apatite (a mineral).
1.1 Advantages and Disadvantages of Composite Materials

The advantages of composites are many, including lighter weight, the ability to tailor the layup for
optimum strength and stiffness, improved fatigue life, corrosion resistance, and, with good design
practice, reduced assembly costs due to fewer detail parts and fasteners. The specific strength
(strength/density) and specific modulus (modulus/density) of high strength fibers (especially
carbon) are higher than those of other comparable aerospace metallic alloys. This translates into
greater weight savings resulting in improved performance, greater payloads, longer range, and fuel
savings. Corrosion of aluminum alloys is a major cost and a constant maintenance problem for

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both commercial and military aircraft. The corrosion resistance of composites can result in major
savings in supportability costs. Carbon fiber composites cause galvanic corrosion of aluminum if
the fibers are placed in direct contact with the metal surface, but bonding a glass fabric electrical
insulation layer on all interfaces that contact aluminum eliminates this problem. As long as
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reasonable strain levels are used during design, fatigue of carbon fiber composites should not be a
problem. Assembly costs can account for as much as 50 percent of the cost of an airframe.
Composites offer the opportunity to significantly reduce the amount of assembly labor and the
number of required fasteners. Detail parts can be combined into a single cured assembly either
during initial cure or by secondary adhesive bonding.
Disadvantages of composites includes: high raw material costs, usually high fabrication and
assembly costs, adverse effects of both temperature and moisture, poor strength in the out- of plane
direction where the matrix carries the primary load (they should not be used where load paths are
complex, such as with lugs and fittings), susceptibility to impact damage and de-laminations or
ply separations, and greater difficulty in repairing them compared to metallic structures. The major
cost driver in fabrication for a composite part using conventional hand lay-up is the cost of laying
up or collating the plies. This cost is generally 40 to 60 percent of the fabrication cost, depending
on part complexity. Assembly cost is another major cost driver, accounting for about 50 percent
of the total part cost. As previously stated, one of the potential advantages of composites is the
ability to cure or bond a number of detail parts together to reduce assembly costs and the number
of required fasteners. Temperature has an effect on composite mechanical properties. Typically,
matrix-dominated mechanical properties decrease with increasing temperature. Fiber-dominated
properties are somewhat affected by cold temperatures, but the effects are not as severe as those
of elevated temperature on the matrix-dominated properties. Design parameters for carbon/epoxy
are cold dry tension and hot-wet compression. An important design factor in the selection of a
matrix resin for elevated-temperature applications is the cured glass transition temperature. The
cured glass transition temperature (Tg) of a polymeric material is the temperature at which it
changes from a rigid, glassy solid into a softer, semi-flexible material. At this point, the polymer
structure is still intact but the crosslinks are no longer locked in position. Therefore, the Tg
determines the upper use temperature for a composite or an adhesive and is the temperature above
which the material will exhibit significantly reduced mechanical properties. The amount of

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absorbed moisture depends on the matrix material and the relative humidity. Elevated temperatures
increase the rate of moisture absorption. Absorbed moisture reduces the matrix-dominated
mechanical properties and causes the matrix to swell, which relieves locked-in thermal strains from
elevated-temperature curing. These strains can be large, and large panels fixed at their edges can
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buckle due to strains caused by swelling. During freeze-thaw cycles, absorbed moisture expands
during freezing, which can crack the matrix, and it can turn into steam during thermal spikes. When
the internal steam pressure exceeds the flatwise-tensile (through-the-thickness) strength of the
composite, the laminate will delaminate. Composites are susceptible to de-laminations (ply
separations) during fabrication, during assembly, and in service. During fabrication, foreign
materials such as pre-preg backing paper can be inadvertently left in the lay-up. During assembly,
improper part handling or incorrectly installed fasteners can cause de-laminations. In service, low-
velocity impact damage from dropped tools or forklifts running into aircraft can cause damage.
The damage may appear as only a small indentation on the surface but it can propagate through
the laminates, forming a complex network of de-laminations and matrix cracks. Depending on the
size of the de-lamination, it can reduce the static and fatigue strength and the compression buckling
strength. If it is large enough, it can grow under fatigue loading. Typically, damage tolerance is a
resin-dominated property.

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2.0 APPLICATIONS OF COMPOSITE MATERIALS

Applications include aerospace (advanced spacecraft and aircraft components), transportation
(automobiles components), construction (structural applications), marine goods, sporting goods,
biomedical materials and more recently infrastructure, with construction and transportation being
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the largest. In general, high-performance but more costly continuous-carbon-fiber composites are
used where high strength and stiffness along with lightweight are required, and much lower-cost
fiberglass composites are used in less demanding applications where weight is not as critical. In
military aircraft, low weight is “king” for performance and payload reasons, and composites often
approach 20 to 40 percent of the airframe weight. For decades, helicopters have incorporated glass
fiber-reinforced rotor blades for improved fatigue resistance, and in recent years helicopter
airframes have been built largely of carbon-fiber composites. Military aircraft applications, the
first to use high performance continuous-carbon-fiber composites, drove the development of much
of the technology now being used by other industries. Both small and large commercial aircraft
rely on composites to decrease weight and increase fuel performance, the most striking example
being the 50 percent composite airframe for the new Boeing 787. All future Airbus and Boeing
aircraft will use large amounts of high-performance composites. Composites are also used
extensively in both weight-critical reusable and expendable launch vehicles and satellite structures.
Weight savings due to the use of composite materials in aerospace applications generally range
from 15 to 25 percent. The major automakers are increasingly turning to composites to help them
meet performance and weight requirements, thus improving fuel efficiency. Cost is a major driver
for commercial transportation, and composites offer lower weight and lower maintenance costs.
Typical materials are fiberglass/polyurethane made by liquid or compression molding and
fiberglass/ polyester made by compression molding. Recreational vehicles have long used glass
fibers, mostly for their durability and weight savings over metal. The product form is typically
fiberglass sheet molding compound made by compression molding. For high-performance
Formula 1 racing cars, where cost is not an impediment, most of the chassis, including the
monocoque (a structure design in which the frame and body are built as a single integrated
structure), suspension, wings, and engine cover, is made from carbon fiber composites. Corrosion
is a major headache and expense for the marine industry. Composites help minimize these
problems, primarily because they do not corrode like metals or rot like wood. Hulls of boats
ranging from small fishing boats to large racing yachts are routinely made of glass fibers and

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polyester or vinyl ester resins. Masts are frequently fabricated from carbon fiber composites.
Fiberglass filament-wound SCUBA tanks are another example of composites improving the
marine industry. Lighter tanks can hold more air yet require less maintenance than their metallic
counterparts require. Jet skis and boat trailers often contain glass composites to help minimize
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weight and reduce corrosion. More recently, the topside structures of many naval ships have been
fabricated from composites. Using composites to improve the infrastructure of our roads and
bridges is a relatively new, exciting application. Many of the world’s roads and bridges are badly
corroded and in need of continual maintenance or replacement. In the United States alone, it is
estimated that more than 250,000 structures, such as bridges and parking garages, need repair,
retrofit, or replacement. Composites offer much longer life with less maintenance due to their
corrosion resistance. Typical processes/materials include wet lay-up repairs and corrosion-
resistant fiberglass pultruded products. In construction, pultruded fiberglass rebar is used to
strengthen concrete, and glass fibers are used in some shingling materials. With the number of
mature tall trees dwindling, the use of composites for electrical towers and light poles is greatly
increasing. Typically, these are pultruded or filament-wound glass. Wind power is the world’s
fastest-growing energy source. The blades for large wind turbines are normally made of
composites to improve electrical energy generation efficiency. These blades can be as long as 37
m and weigh up to 5200 kg. In 2007, nearly 50,000 blades for 17,000 turbines were delivered,
representing roughly 180 million kg of composites. The predominant material is continuous glass
fibers manufactured by either lay-up or resin infusion. Tennis racquets have been made of glass
for years, and many golf club shafts are made of carbon. Processes include compression molding
for tennis racquets and tape wrapping or filament winding for golf shafts. Lighter, stronger skis
and surfboards also are possible using composites. Another example of a composite application
that takes a beating yet keeps on performing is a snowboard, which typically involves the use of a
sandwich construction (composite skins with a honeycomb core) for maximum specific stiffness.
Although metal and ceramic matrix composites are normally very expensive, they have found uses
in specialized applications. Frequently, they are used high temperature applications. However, the
much higher temperatures and pressures required for the fabrication of metal and ceramic matrix
composites lead to very high costs, which severely limits their application. In conclusion, advanced
composites are a diversified and growing industry due to their distinct advantages over competing
metallics including lighter weight, higher performance, and corrosion resistance. They are used in

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aerospace, automotive, marine, sporting goods, and, more recently, infrastructure applications. The
major disadvantage of composites is their high cost. However, the proper selection of materials
(fiber and matrix), product forms, and processes can have a major impact on the cost of the finished
part.
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Figure 2.1: applications of composite materials

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3.0 CLASSIFICATION OF COMPOSITE MATERIALS

Apart from being broadly categorized into structural and functional, composites are also classified
according to their content, i.e., base material and filler: (1) the matrix and (2) the type of
reinforcement form. The base material, which binds or holds the filler material in structures, is
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termed as a matrix or a binder material, while filler material is present in the form of sheets,
fragments, particles, fibers, or whiskers of natural or synthetic material.
3.1 Classification based on type of matrix

This classification gives three main categories, which include:
 Organic Matrix Composites (OMC)
i. Polymer Matrix Composites (PMC)
ii. Carbon Matrix Composites (CMC) (these are carbon-carbon composites i.e. carbon
fiber in a graphite matrix)
 Metal Matrix Composites (MMC)
 Ceramic Matrix Composites (CMC)
Polymer matrix composites: Based on type of polymer resin used, composite materials can be
classified into thermoplastic and thermoset composites.
Thermoplastic composites- this is a type of composite with a thermoplastic resin like

polyester, HDPE etc. They are lesser used as high-tech materials due to their high viscosity
which cause problems during their penetration into the reinforcement.
Thermoset composites- in these composites thermoset polymers like epoxy,
unsaturated polyester and vinyl-ester are used as resin. They are the most used composite
materials in automotive, naval, aeronautical, and aerospace applications.

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Metal Matrix Composites
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Ceramic Matrix Composites

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3.2 Classification based on type of reinforcement form
This classification gives three main categories, which include:

 Fiber Reinforced Composites (FRC), continuous or discontinuous.
i. Considered to be a discontinuous fiber or short fiber composite if its properties vary Page | 14
with fiber length.
ii. On the other hand, when the length of the fiber is such that any further increase in
length does not further increase, the elastic modulus of the composite, the
composite is considered to be continuous fiber reinforced.
iii. Fiber are small in diameter and when pushed axially, they bend easily although they
have very good tensile properties. These fibers must be supported to keep individual
fiber from bending and buckling.
 Laminar Composites (LC); layers of materials held together by matrix (Sandwich
structures).
 Particulate Composites (PC); particles distributed or embedded in a matrix body. The
particles may be flakes or in powder form (e.g. Concrete and wood particle).
Figure 3.2: summary of types of matrices and reinforcement materials
In the following section, we shall look into all these main classes in detail.

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3.3 ORGANIC METAL COMPOSITES: Polymer Matrix Composites (PMC)
Polymers make ideal materials as they can be processed easily, possess lightweight, and desirable
mechanical properties. Two main kinds of polymers are thermosets and thermoplastics.
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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, rendering
thermosets very flexible. Thus, they are most suited as matrix bases for advanced fiber
reinforced composites.
 Thermosets find wide ranging applications in the chopped fiber composites form,
particularly, when a premixed compound with fibers happens to be the starting material as
in epoxy, polymer and phenolic polyamide resins.
• Thermoset resins are – epoxy, polyester, phenolic polyamide resins.
Epoxy Resins
 Widely used in filament-wound composites and electrical circuit boards.
 Reasonably stable to chemical attacks and are excellent adherents having slow shrinkage
during curing and no emission of volatile gases.
 These advantages make epoxies expensive.
 Cannot be used above 140oC (limiting their applications).
Polyester Resins
 Easily accessible, cheap and used widely.
 Stored at room temperature for long periods and the mere addition of a catalyst can cure
the matrix material within a short time.
 Cured polyester is usually rigid or flexible and transparent.
 Used in automobile and structural applications.

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 Withstand the variations of environment and stable against chemicals and can be used up
to about 75oC or higher.
 Compatibility with few glass fibers and can be used with variety of reinforced plastic.
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Aromatic Polyamides
 Most sought after as the matrices of advanced fiber composites for structural applications
demanding long duration exposure for continuous service at around 200- 250oC.
Thermoplastics- include: polyethylene, polystyrene, polyamides, nylons, and polypropylene.

 Have one- or two-dimensional molecular structure and they tend to show an exaggerated
melting point at an elevated temperature.
 Soften at elevated temperatures can be reversed to regain its properties during cooling,
facilitating applications of conventional techniques to mold the compounds.
 Resins comprise an emerging group of composites and the main goal is to improve the base
properties of the resins, which would extract the greatest functional advantages from them.
 Whether crystalline or amorphous, these resins possess the facility to alter their creep over
an extensive range of temperature.
 Reinforcement in such systems can increase the failure load as well as creep resistance.
Moreover, addition of filler raises the heat resistance.
Advantages of thermoplastics include:

 There are no chemical reactions involved, which often result in the release of gases or heat.
 Manufacturing is limited by the time required for heating, shaping and cooling the
structures.
Thermoplastics resins are sold as molding compounds. Fiber reinforcement is apt for these resins.
Since the fibers are randomly dispersed, the reinforcement will be almost isotropic. However,
when subjected to moulding processes, they can be aligned directionally. They tend to lose their
strength at elevated temperatures. However, their redeeming qualities like rigidity, toughness and
ability to avoid creep, place thermoplastics in the important composite materials bracket. They are
used in automotive control panels, electronic products encasement etc.

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3.4 Metal Matrix Composites (MMC)

 Though generating a wide interest in research, are not as widely in use as plastic.
 High strength, fracture toughness and stiffness are offered by metal matrices when
compared to their polymer counterparts.
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 Withstand elevated temperature in corrosive environment than polymer composites.
 Most metals and alloys - used as matrices. Hence, require reinforcement materials – stable
over a range of temperatures and non-reactive too.
 Guiding aspect for the choice depends on matrix material.
 Light metals (low strength) form the matrix while the reinforcements have high moduli.
 If metal matrix has high strength, they require even higher modulus reinforcements.
 Hence, light metals (Al, Ti, and Mg) are the popular matrix metals with their low density.
e.g. carbide in a metal matrix.
 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. As the melting points of matrix materials become high, the choice of reinforcements
becomes small.
3.5 Ceramic Matrix Composites (CMC)

 Ceramics- solid materials which exhibit strong ionic bonding (in some cases covalent
bonding)
 High melting points, good corrosion resistance, stability at elevated temperatures and high
compressive strength - ceramic matrix materials used above 1500ºC (high temperature
applications). e.g. cermet, concrete.
 Most ceramic possess high modulus of elasticity and low tensile strain and hence addition
of reinforcements to improve their strength have proved futile. 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. However,
addition of any high-strength fiber (as reinforcing material) to a weaker ceramic has not
always been successful and often the resultant composite has proved to be weaker.

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 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 stress within ceramic at the time of cooling resulting in microcracks
extending from fiber to fiber within the matrix. Microcracking can result in a composite
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with lower tensile strength than that of the matrix.
 Motivation to develop CMCs - to overcome the problems associated with the conventional
technical ceramics like alumina, silicon carbide, aluminium nitride, silicon nitride or
zirconia –they fracture easily under mechanical or thermo-mechanical loads because of
cracks initiated by small defects or scratches. The crack resistance is – like in glass – very
low.
 Multi-strand fibres has drastically increased the crack resistance/ fracture toughness,
elongation and thermal shock resistance, and resulted in several new applications.
 Carbon (C), silicon carbide (SiC), alumina (Al2O3) and mullite (Al2O3–SiO2) fibres are
most widely used with the same matrix materials i.e. C, SiC, alumina and mullite.
 CMC names include a combination of type of fibre/type of matrix. For example, C/C stands
for carbon-fibre-reinforced carbon (carbon/ carbon), or C/SiC for carbon-fibre-reinforced
silicon carbide (commercially available CMCs are C/C, C/SiC, SiC/SiC and Al2O3/Al2O3).
 They differ from conventional ceramics in the following properties:
 Elongation to rupture up to 1%
 Strongly increased fracture toughness
 Extreme thermal shock resistance
 Improved dynamical load capability
 Anisotropic properties following the orientation of fibers

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4.0 CLASSIFICATION OF REINFORCEMENTS
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Figure 4.1: classification of reinforcements
Figure 4.2: classification of reinforcements

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Types of Reinforcements
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Figure 4.3: illustrations of reinforcement arrangement
Figure 4.4: orientation of reinforcements

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4.1 Reinforcements Definition
 A strong, inert, woven and nonwoven fibrous material incorporated into the matrix to
improve its mechanical and physical properties e.g. asbestos, boron, carbon, metal or glass
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or ceramic fibers, graphite, jute, sisal, whiskers, macerated fabrics, and synthetic fibers.
 Reinforcement and filler difference: reinforcement markedly improves tensile and flexural
strength, whereas filler usually does not to be effective, reinforcement must form a strong
adhesive bond with resin.
 Role of the reinforcement: increase the mechanical properties of the neat resin system.
Different fibers have different properties - affect the properties of the composite in different
ways. However, individual fibers or fiber bundles can only be used on their own in a few
processes such as filament winding. For most other applications, the fibers need to be
arranged into some form of sheet, known as a fabric, to make handling possible.
 Different ways for assembling fibers into sheets and the variety of fiber orientations - Lead
to different types of fabrics, each of which has its own lead characteristics.
 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 in both diameter and length as compared to
fibers.
4.2 Particle Reinforced Composites
 Microstructures of metal and ceramics composites, which show particles of one phase
scattered in the matrix, are known as particle-reinforced composites.
 Square, triangular, irregular and round shapes of reinforcement are known, but the particle
dimensions are observed to be more or less equal.
 The size and volume concentration of the dispersant distinguishes particle-reinforced
composites from dispersion-strengthened composites.
 The dispersed size in particulate composites is of the order of a few microns and volume
concentration is greater than 28%.

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 Particle reinforced composite - particle-matrix interaction do not occur at atomic or

molecular level. Particles harder and stiffer than matrix – hence, tend to bear the load.
Degree of reinforcement depends on strong matrix particle bonding. e.g. Concrete.
 Dispersion strengthened composites – particles smaller (diameter 0.01 - 0.1 x 10-6 m). Page | 22
Particle matrix interactions that lead to strengthening occur on atomic level. The matrix
bears the major portion of load. While dispersed particles hinder motion of dislocations.
Thus, plastic deformation is restricted such that yield and tensile strength as well as
hardness improve. e.g. thoria dispersed nickel.
 The composite’s strength depends on - diameter of the particles, the inter-particle spacing,
and the volume fraction of the reinforcement.
4.3. Cermets/ Ceramal (Cermet – ‘ceramic’ and ‘metal’ composite)

Designed to have the optimal properties of both ceramic (high temperature resistance and
hardness) and metal (the ability to undergo plastic deformation). The metal (Ni, Mo,
Co etc.) is used as a binder for an oxide, boride, carbide, or alumina. Cermets are usually less than
20% metal by volume – used in:
 Manufacture of resistors (potentiometers), capacitors etc. - experience high temperatures.
 Spacecraft shielding (as they resist the high velocity impacts of micrometeoroids and
orbital debris better than Al and other metals.
 Vacuum tube coatings of solar hot water systems.
 Material for fillings and prostheses.
 Machining of cutting tools. Titanium nitride (TiN), titanium carbonitride (TiCN), titanium
carbide (TiC)
Cermets are usually produced by:
 Using power metallurgy techniques (ceramic and metal powder mixed and sintered).
 Impregnation of a porous ceramic structure with a metallic matrix binder.
 Coating in powder form, which is sprayed through a gas flame and fused to a base material.

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5.0 FIBER-REINFORCED POLYMER (FRP) COMPOSITES
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Figure 5.1: fiber pre-form arrangements
Fibers - important class of reinforcements, as they effectively transfer strength to the matrix
influencing and enhancing composite properties as desired. Glass fibers (earliest known
reinforcing fibers). Ceramic and metal fibers used subsequently to make composites stiffer and
more heat resistant. The performance of a fiber composite is judged by:
 The length, shape, orientation, and composition of the fibers and the mechanical properties
of the matrix.
 Orientation of the fiber in the matrix (strength greatest along the longitudinal directional &
slightest shift in the angle of loading drastically reduce the strength)
Since unidirectional loading is found in very few structures, a mix of fiber orientations is given to
withstand load from different angles (particularly more fibers in the direction where load is
expected to be the heaviest). Monolayer tapes consisting of continuous or discontinuous fibers can
be oriented unidirectional stacked into plies containing layers of filaments also oriented in the
same direction. Properties of angle-plied composites, which are not quasi-isotropic, may vary with
the number of plies and their orientations.

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Orientation of short fibers - random orientations by sprinkling on to given plane, addition of matrix
in liquid or solid state before or after the fiber deposition. Experience has shown that continuous
fibers (or filaments) exhibit better orientation, although it does not reflect in their performance.
These are found in mass production of filaments in different ways like winding, twisting, weaving
Page | 24
and knitting, which exhibit the characteristics of a fabric.
Organic and inorganic fibers are used to reinforce composites. Almost all organic fibers have low
density, flexibility, and elasticity. Inorganic fibers (glass fibers, silicon carbide fibers, high silica
and quartz fibers, aluminina fibers, metal fibers and wires, graphite fibers, boron fibers, aramid
fibers and multiphase fibers) are of high modulus, high thermal stability and possess greater
rigidity than organic fibers.
5.1 Whisker Reinforced Composites

Whiskers are Single crystals grown with nearly zero defects (usually discontinuous and short fibers
of different cross-sections made from materials like graphite, silicon carbide, copper, iron etc.).
Typical lengths are range from 3 - 55 nm. Whiskers differ from particles in that, whiskers have a
definite length to width ratio (> 1) with extraordinary strengths up to 7000 MPa. Whiskers
(laboratory produced) Metal-whisker combination, strengthening the system at high temperatures,
has been demonstrated at the laboratory level. Since whiskers are fine, small sized materials and
not easy to handle, it becomes a hindrance in composite fabrication. Early research has shown that
whisker strength varies inversely with effective diameter. When whiskers were embedded in
matrices, whiskers of diameter up to 2 - 10μm yielded fairly good composites. Ceramic whiskers
have high moduli, useful strengths and low densities. Specific strength and specific modulus are
very high and this makes them suitable for low weight structure composites. They also resist
temperature, mechanical damage and oxidation more than metallic whiskers (which are denser
than ceramic whiskers). However, they are not commercially viable because they are damaged
while handling.

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Figure 5.2: whiskers
5.2 Flakes Reinforced Composites

Flakes are often used in place of fibers as they can be densely packed. Metal flakes that are in
close contact with each other in polymer matrices can conduct electricity or heat, while mica flakes
and glass can resist both. Flakes are not expensive to produce and usually cost less than fibers.
However, limitations include control of size, shape of flakes and defects in the end product. Glass
flakes tend to have notches or cracks around the edges, which weaken the final product. (also
resistant to be lined up parallel to each other in a matrix, causing uneven strength). They are usually
set in matrices, or held together by a matrix with a glue-type binder.
Advantages of flakes over fibers –
 Parallel flakes filled composites provide uniform mechanical properties in the same plane
as the flakes.
 While angle plying is difficult in continuous fibers which need to approach isotropic
properties, it is not so in flakes.
 Flake composites have a higher theoretical modulus of elasticity than fiber reinforced
composites.
 They are relatively cheaper to produce and be handled in small quantities.

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Figure 5.2: flakes arrangements
5.3 Filled Composites

Addition of filler materials to plastic matrices to replace a portion of the matrix, which would
enhance or change the properties of the composites. The fillers also enhance strength and reduce
weight in some cases. Fillers may be the main ingredient or an additional one in a composite. The
filler particles may be irregular structures, or have precise geometrical shapes like polyhedrons,
short fibers or spheres. They also occasionally impart colour or opacity to the composite, which
they fill. As inert additives, fillers can change almost any basic resin characteristic in all directions
required, to surpass many limitations of basic resins. The final composite properties can be affected
by the shape, surface treatment, blend of particle types, size of the particle in the filler material
and the size distribution. Filled plastics tend to behave like two different constituents. They do not
alloy and accept the bonding (they desist from interacting chemically with each other). Although
the matrix forms the bulk of the composite, the filler material is also used in such great quantities
relatively that it becomes the rudimentary constituent. The benefits of fillers - increase stiffness,
thermal resistance, stability, strength and abrasion resistance, porosity and a favorable coefficient
of thermal expansion.
Disadvantages of fillers are that methods of fabrication are very limited and the curing of some
resins is greatly inhibited, shorten the life span of some resins, and weaken a few composites.

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Fillers produced from powders are also considered as particulate composite. In a porous or spongy
composite, metal impregnates are used to improve strength or tolerance of the matrix. Metal
casting, graphite, powder metallurgy parts and ceramics belong to this class of filled composites.
In the honeycomb structure, sheet materials in the hexagonal shapes are impregnated with resin or
Page | 27
foam and are used as a core material in sandwich composites.
5.4 Microspheres Composites

Microspheres - useful fillers due to specific gravity, stable particle size, strength and controlled
density modify products without compromising profits or physical properties. Solid glass
microspheres are most suitable for plastics. Microspheres coated with a binding agent - bonds
between sphere’s surface and resin. This increases the bonding strength and basically removes
absorption of contaminants/ moisture (reduce attraction between particles). Solid Microspheres
have relatively low density, and therefore, influence the commercial value and weight of the
finished product. Hollow microspheres are essentially silicate based, made at controlled specific
gravity. They are larger than solid glass spheres, used in polymers and have wider range of particle
sizes. Commercially, silicate-based hollow microspheres with different compositions using
organic compounds are also available. Due to this modification, they are less sensitive to moisture
(reduce attraction between particles) - vital in highly filled polymer composites as viscosity
increase constraints limit filler loading. They were earlier used in thermosetting resin. Now,
several new strong spheres are available and they are at least 5 times stronger than hollow
microspheres in static crush strength and 4 times long lasting in shear. Recently, ceramic alumino-
silicate microspheres have been used in thermoplastic systems. Greater strength and higher density
of this system in relation to siliceous microspheres and their abrasion resistance make them
suitable for high-pressure applications. Hollow microspheres have a lower specific gravity than
the pure resin. They find wide applications in aerospace and automotive industries where weight
reduction for energy conservation is one of the main considerations. Microspheres, whether solid
or hollow, due to their spherical shape behave like minute ball bearing, and hence, they give better
flow properties. They also distribute stress uniformly throughout resin matrices. In spherical
particles, the ratio of surface area to volume is minimal (smallest). In resin-rich surfaces of
reinforced systems, the microspheres, which are free of orientation and sharp edges, are capable
of producing smooth surfaces.

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5.5 Role and Selection of Reinforcements

 Compatibility with matrix material, thermal stability, density, melting temperature etc.
 Efficiency of discontinuously reinforced composites - dependent on tensile strength and
density of reinforcing phases.
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 Difference between the coefficients of thermal expansion of the matrix and reinforcement
-composites used in thermal cycling application.
 The manufacturing process selected and the reinforcement affects the crystal structure.
 In particulate/whisker reinforced composites, the matrix (major load bearing constituent).
Role of the reinforcement - strengthen and stiffen the composite through prevention of matrix
deformation by mechanical restraint. This restraint is generally a function of the ratio of
interparticle spacing to particle diameter.
 In continuous fiber reinforced composites, the reinforcement (principal load-bearing
constituent). The matrix serves to hold the reinforcing fibers together and transfer as well
as distribute the load.
 Discontinuous fiber reinforced composites - properties between continuous fiber and
particulate reinforced composites.
 Addition of reinforcement increases the strength, stiffness and temperature capability.
 Reduce density of composite if the matrix material has high density.
5.6 Role and Selection of Matrix

 The matrix provides support for the fibers and assists them in carrying the loads. It also
provides stability to the composite material. Resin matrix system acts as a binding agent in
a structural component in which the fibers are embedded.
 When too much resin is used, the part is classified as resin rich. On the other hand, if there
is too little resin, the part is called resin starved.
 A resin rich part is more susceptible to cracking due to lack of fibre support, whereas a
resin starved part is weaker because of void areas and the fact that fibres are not held
together and they are not well supported.
 In case of MMCs, thermodynamically stable dispersoids (particles) are essential for high
temperature applications.

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 Done using an alloy-matrix (alloy particles in a metal matrix) system in which solid state
diffusivity, interfacial energies and elemental solubility are minimized, in turn reducing
interfacial reactions.
 Al and Mg alloys matrices widely used due to low density and high thermal conductivity. Page | 29
 Additionally, composites with low alloying additions to the matrix result in attractive
combinations of ductility, toughness and strength.
 Alloying elements (usually used as grain refiners) may form coarse inter-metallic
compounds during consolidation, thus, reducing the tensile properties of the composite.
 The choice of a matrix is dictated by - continuously or discontinuously reinforced fibers.
 Continuous fibers - transfer of load to the reinforcing fibers (hence composite strength will
be governed by the fiber strength.
 The role of matrix - provide efficient transfer of load to the fibers and blunt cracks in the
event that fiber failure occurs (matrix used for toughness than strength). Hence, lower
strength, ductile, and tough matrix may be utilized in continuous reinforced composites.
 Discontinuous reinforced composites - the matrix may govern composite strength.
 Then, the choice of matrix will be influenced by consideration of the required composite
strength and higher strength matrix may be required.
 Choice of the matrix also include - potential reinforcement/matrix reactions (during
processing or in service - degraded composite), thermal stresses (thermal mismatch
between reinforcements and matrix), and matrix fatigue behavior under cyclic conditions.
 Thermal mismatch - a large melting temperature difference may result in matrix creep
while the reinforcements remain elastic, even at temperatures approaching the matrix
melting point.
 However, creep in both the matrix and reinforcement must be considered when there is a
small melting point difference in the composite.
5.7 Functions of the Matrix

 Holds the fibers together.
 Protects the fibers from environment.
 Distributes the loads evenly between fibers so that all fibers are subjected to the same
amount of strain.

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 Enhances transverse properties of a laminate.

 Improves impact and fracture resistance of a component.
 Avoid propagation of crack growth through the fibers by providing alternate failure path
along the interface between the fibers and the matrix. Page | 30
 Carry inter-laminar shear (design consideration for structures under bending loads); in-
plane shear strength (under torsion loads)
 Minor role - in the tensile load-carrying capacity of a composite structure.
 Provide lateral support against the possibility of fiber buckling under compression loading.
 Finally, the processing ability and defects in a composite material depend strongly on the
physical and thermal characteristics, such as viscosity, melting point, and curing
temperature of the matrix.
5.8 Desired Functions of Matrix

 Reduced moisture absorption.
 Low shrinkage.
 Low coefficient of thermal expansion.
 Good flow characteristics so that it penetrates the fiber bundles completely and eliminates
voids during the compacting/curing process.
 Reasonable strength, modulus and elongation (elongation should be greater than fiber).
 Must be elastic to transfer load to fibers.
 Strength at elevated temperature (depending on application).
 Low temperature capability (depending on application).
 Excellent chemical resistance (depending on application).
 Should be easily fabricated into the final composite shape.
 Dimensional stability (maintains its shape).

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6.0 STRUCTURAL COMPOSITES

Composite materials and homogeneous materials form structural composites. Properties depends
on the geometry of the structural elements. There are two types: laminated composites and
sandwich structures.
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Laminated composites: involves piling of layers or laminar of unidirectional composite material.
Laminar composite examples include: continuous and aligned fiber reinforced plastics with
matrices such as epoxy, polyester, PE, PA, PET, etc. In order to get different mechanical
properties, layers of materials with different properties are piled, or a different way of piling layers
on top of each other.
Figure 6.1: sandwich composites

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Figure 6.2: honeycomb sandwich

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7.0 MECHANICAL PROPERTIES OF COMPOSITES
7.1 Idealization of microstructure of fibrous composites

As mentioned earlier, the micromechanics is a study at fiber and matrix level. Thus, the geometry
of arrangement of the fibers and matrix in a composite is an essential requirement to develop a Page | 33
model for the study. Some of the methods do not use the geometry of arrangement. Most of the
methods developed for micromechanical analysis assume that:
1. The fibers and matrix are perfectly bonded and there is no slip between them.
2. The fibers are continuous and parallel.
3. The fibers are assumed circular in cross section with a uniform diameter along its length.
4. The space between the fibers is uniform throughout the composite.
5. The elastic, thermal and hygral properties of fiber and matrix are known and uniform.
6. The fibers and matrix obey Hooke’s law.
7. The fibers and the matrix are only two phases in the composite.
8. There are no voids in the composite.
7.2 Mechanical properties stiffness and strength

Volume fractions-Consider a composite consisting of fiber and matrix. Take the following symbol
notations:
𝑣𝑐,𝑓,𝑚 = Volume of composite, fiber, and matrix respectively
𝜎𝑐,𝑓,𝑚 = Density of composite, fiber, and matrix respectively
Now define the fiber volume fraction 𝑉𝑓 and matrix volume fraction 𝑉𝑚 as:
𝑣𝑓 𝑣𝑚
𝑉𝑓 = , 𝑉𝑚 =
𝑣𝑐 𝑣𝑐
Note that the sum of volume fractions is:

𝑉𝑓 + 𝑉𝑚 = 1 , 𝑣𝑓 + 𝑣𝑚 = 𝑣𝑐
Mass fractions- Consider a composite consisting of fiber and matrix and take the following symbol
notation:
𝑤𝑐,𝑓,𝑚 = mass of composite, fiber and matrix respectively. The mass fraction (weight fraction) of
the fibers (𝑊𝑓 ) and the matrix (𝑊𝑚 ) are defined as:

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𝑤𝑓 𝑤𝑚
𝑊𝑓 = and 𝑊𝑚 =
𝑤𝑐 𝑤𝑐
Note that the sum of mass fractions is:

𝑊𝑓 + 𝑊𝑚 = 1 , 𝑤𝑓 + 𝑤𝑚 = 𝑤𝑐 Page | 34
From the definition of the density (r) of a single material,

𝑤𝑐 = 𝑟𝑐 𝑣𝑐 , 𝑤𝑓 = 𝑟𝑓 𝑣𝑓 and 𝑤𝑚 = 𝑟𝑚 𝑣𝑚
Substituting above equations, the mass fractions and volume fractions are related in terms of the
fiber and matrix volume fractions as:
𝜌𝑓 𝜌𝑚
𝑊𝑓 = 𝑉 and 𝑊𝑚 = 𝑉𝑚
𝜌𝑐 𝑓 𝜌𝑐
In terms of individual constituent properties, the mass fractions and volume fractions are related
by:
𝜌𝑓
𝜌𝑚 1
𝑊𝑓 = 𝜌𝑓 𝑉𝑓 and 𝑊𝑚 = 𝜌𝑓 𝑉𝑚
𝑉 +𝑉𝑚 (1−𝑉𝑚 )+𝑉𝑚
𝜌𝑚 𝑓 𝜌𝑚
One should always state the basis of calculating the fiber content of a composite. It is given in
terms of mass or volume. Based on above Equation, it is evident that volume and mass fractions
are not equal and that the mismatch between the mass and volume fractions increases as the ratio
between the density of fiber and matrix differs from one.
Density- The derivation of the density of the composite in terms of volume fractions is found as
follows. The mass of composite 𝑤𝑐 is the sum of the mass of the fibers 𝑤𝑓 and the mass of the
matrix 𝑤𝑚 as: 𝑤𝑓 + 𝑤𝑚 = 𝑤𝑐
𝑣𝑓 𝑣𝑚
𝜌𝑐 𝑣𝑐 = 𝜌𝑓 𝑣𝑓 + 𝜌𝑚 𝑣𝑚 Thus: 𝜌𝑐 = 𝜌𝑓 + 𝜌𝑚
𝑣𝑐 𝑣𝑐
Using the definitions of fiber and matrix volume fractions from Equation
𝜌𝑐 = 𝜌𝑓 𝑉𝑓 + 𝜌𝑚 𝑉𝑚
Now, consider that the volume of a composite 𝑣𝑐 is the sum of the volumes of the fiber 𝑣𝑓 and
matrix 𝑣𝑚 : 𝑣𝑓 + 𝑣𝑚 = 𝑣𝑐
The density of the composite in terms of mass fractions can be found as:

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𝟏 𝑾𝒇 𝑾𝒎
= +
𝝆𝒄 𝝆𝒇 𝝆𝒎
The following section gives a series of slides to introduce the equations for analysis of mechanical
Page | 35
properties of composites.

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7.3 Isotropic, Anisotropic, and Orthotropic Materials

Materials can be classified as either isotropic or anisotropic. Isotropic materials have the same
material properties in all directions, and normal loads create only normal strains. By comparison,
anisotropic materials have different material properties in all directions at a point in the body.
Page | 40
There are no material planes of symmetry, and normal loads create both normal strains and shear
strains. A material is isotropic if the properties are independent of direction within the material.
For example, consider the element of an isotropic material. If the material is loaded along its 0°,
45°, and 90° directions, the modulus of elasticity (E) is the same in each direction (E0° = E45° =
E90°). However, if the material is anisotropic, it has properties that vary with direction within the
material. The moduli are different in each direction (E0° ≠ E45° ≠ E90°). While the modulus of
elasticity is used here, the same dependence on direction can occur for other material properties,
such as ultimate strength, Poisson’s ratio, and thermal expansion coefficient.
Bulk materials, such as metals and polymers are normally treated as isotropic materials, while
composites are treated as anisotropic. However, even bulk materials such as metals can become
anisotropic for example, if they are highly cold worked to produce grain alignment in a certain
direction. Consider the unidirectional fiber-reinforced composite ply (also known as a lamina). If
the composite is loaded parallel to the fibers, the modulus of elasticity E approaches that of the
fibers. If the composite is loaded perpendicular to the fibers, the modulus E is much lower,
approaching that of the relatively less stiff matrix. When the modulus varies with direction within
the material, the material is anisotropic. Composites are a subclass of anisotropic materials that are
classified as orthotropic. Orthotropic materials have properties that are different in three mutually
perpendicular directions. They have three mutually perpendicular axes of symmetry, and a load
applied parallel to these axes produces only normal strains. However, loads that are not applied
parallel to these axes produce both normal and shear strains. Therefore, orthotropic mechanical
properties are a function of orientation.
7.4 Laminates
When there is a single ply or a lay-up in which all of the layers or plies are stacked in the same
orientation, the lay-up is called a lamina. When the plies are stacked at various angles, the lay-up
is called a laminate. Continuous-fiber composites are normally laminated materials in which the
individual layers, plies, or laminae are oriented in directions that will enhance the strength in the

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primary load direction. Unidirectional laminae are extremely strong and stiff in the 0° direction.
However, they are very weak in the 90° direction because the load must be carried by the much
weaker polymeric matrix. While a high-strength, fiber can have a tensile strength of 3500 MPa or
more, a typical polymeric matrix normally has a tensile strength of only 35 to 70 MPa. The
Page | 41
longitudinal tension and compression loads are carried by the fibers, while the matrix distributes
the loads between the fibers in tension, stabilizes the fibers and prevents them from buckling in
compression. The matrix is also the primary load carrier for inter-laminar shear (i.e., shear between
the layers) and transverse (90°) tension. Because the fiber orientation directly affects mechanical
properties, it seems logical to orient as many of the layers as possible in the main load-carrying
direction. While this approach may work for some structures, it is usually necessary to balance the
load-carrying capability in a number of different directions, such as the 0°, +45°, -45°, and 90°
directions. A balanced laminate having equal numbers of plies in the 0°, +45°, –45°, and 90°
degrees directions is called a quasi-isotropic laminate, because it carries equal loads in all four
directions.
7.5 Fundamental property relationships

When a unidirectional continuous-fiber lamina or laminate is loaded in a direction parallel to its
fibers, the longitudinal modulus E can be estimated from its constituent properties by using what
is known as the rule of mixtures:
𝐸 = 𝐸𝑓 𝑉𝑓 + 𝐸𝑚 𝑉𝑚
Where 𝐸𝑓 is the fiber modulus,𝑉𝑓 is the fiber volume percentage, 𝐸𝑚 is the matrix modulus, and
𝑉𝑚 is the matrix volume percentage. The longitudinal tensile strength 𝜎 also can be estimated by
the rule of mixtures:
𝜎 = 𝜎𝑓 𝑉𝑓 + 𝜎𝑚 𝑉𝑚
Where 𝜎𝑓 and 𝜎𝑚 are the ultimate fiber and matrix strengths, respectively. When loads are parallel
to the fibers (0°), the ply is much stronger and stiffer than when loads are transverse (90°) to the
fiber direction. There is a dramatic decrease in strength and stiffness resulting from only a few
degrees of misalignment off the 0°.
When the lamina is loaded in the transverse (90°), the fibers and the matrix function in series, with
both carrying the same load. The transverse modulus of elasticity E is given as:

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1 𝑉𝑓 𝑉𝑚
= +
𝐸 𝐸𝑓 𝐸𝑚
Physical properties, such as density (𝜌), can also be expressed using rule of mixture relations:
𝜌 = 𝜌𝑓 𝑉𝑓 + 𝜌𝑚 𝑉𝑚
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While these micromechanics equations are useful for a first estimation of lamina properties when
no data are available, they generally do not yield sufficiently accurate values for design purposes.
For design purposes, basic lamina and laminate properties should be determined using actual
mechanical property testing.

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8.0 MANUFACTURING METHODS FOR COMPOSITES
8.1 SPRAY LAY-UP METHOD
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Description
Fiber is chopped in a hand-held gun and fed into a spray of catalyzed resin directed at the mould.
The deposited materials are left to cure under standard atmospheric conditions. Spray-up technique
is no different from hand lay-up. However, it uses a handgun that sprays resin and chopped fibers
on a mold. Simultaneously, a roller is used to fuse these fibers into the matrix material. It is an
open mold type of technique, where chopped fibers provide good conformability and quiet faster
than hand lay-up.
Material Options
 Resins: Primarily polyester
 Fibers: Glass roving only

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 Cores: None. These have to be incorporated separately

Typical Applications
Simple enclosures, lightly loaded structural panels, e.g. caravan bodies, truck fairings, bathtubs,
shower trays, some small dinghies.
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Advantages
 Widely used for many years.
 Low cost way of quickly depositing fiber and resin.
 Low cost tooling. Disadvantages:
 Laminates tend to be very resin-rich and therefore excessively heavy.
 Only short fibers are incorporated which severely limits the mechanical properties of the
laminate.
 Resins need to be low in viscosity to be sprayable. This generally compromises their
mechanical/thermal properties.
 The high styrene contents of spray lay-up resins generally mean that they have the potential
to be more harmful and their lower viscosity means that they have an increased tendency
to penetrate clothing.
 Limiting airborne styrene concentrations to legislated levels is becoming increasingly
difficult.
8.2 WET/ HAND LAY-UP
Description
Resins are impregnated by hand into fibers, which are in the form of woven, knitted, stitched or
bonded fabrics. This is usually accomplished by rollers or brushes, with an increasing use of nip-
roller type impregnators for forcing resin into the fabrics by means of rotating rollers and a bath
of resin. Laminates are left to cure under standard atmospheric conditions.

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Materials Options
 Resins: Any, e.g. epoxy, polyester, vinylester, phenolic
 Fibers: Any, although heavy aramid fabrics can be hard to wet-out by hand.
 Cores: Any.
Standard wind-turbine blades, production boats, architectural mouldings
Advantages
 Widely used for many years.
 Simple principles to teach.
 Low cost tooling, if room-temperature cure resins are used.
 Wide choice of suppliers and material types.
 Higher fiber contents and longer fibers than with spray lay-up.
Disadvantages
 Resin mixing, laminate resin contents, and laminate quality are very dependent on the skills
of laminators. Low resin content laminates cannot usually be achieved without the
incorporation of excessive quantities of voids.
 Health and safety considerations of resins. The lower molecular weights of hand lay-up
resins generally mean that they have the potential to be more harmful than higher molecular

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weight products. The lower viscosity of the resins also means that they have an increased
tendency to penetrate clothing.
 Limiting airborne styrene concentrations to legislated levels from polyesters and
vinylesters is becoming increasingly hard without expensive extraction systems.
Page | 46
 Resins need to be low in viscosity to be workable by hand. This generally compromises
their mechanical/ thermal properties due to the need for high diluent/styrene levels.
8.3 VACUUM BAGGING

This is basically an extension of the wet lay-up process described above where pressure is applied
to the laminate once laid-up in order to improve its consolidation. This is achieved by sealing a
plastic film over the wet laid-up laminate and onto the tool. A vacuum pump extracts the air under
the bag and thus up to one atmosphere of pressure can be applied to the laminate to consolidate it.
Materials Options
 Resins: Primarily epoxy and phenolic. Polyesters and vinylesters may have problems due
to excessive extraction of styrene from the resin by the vacuum pump.
 Fibers: The consolidation pressures mean that a variety of heavy fabrics can be wet-out.
 Cores: Any.
Large, one-off cruising boats, racecar components, core-bonding in production boats.

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Advantages
 Higher fiber content laminates can usually be achieved than with standard wet lay-up
techniques.
 Lower void contents are achieved than with wet lay-up. Page | 47
 Better fiber wet-out due to pressure and resin flow throughout structural fibers, with excess
into bagging materials.
 Health and safety: The vacuum bag reduces the amount of volatiles emitted during cure.
Disadvantages
 The extra process adds cost both in labour and in disposable bagging materials.
 The operators require a higher level of skill.
 Mixing and control of resin content still largely determined by operator skill.
8.4 FILAMENT WINDING
This process is primarily used for hollow, generally circular or oval sectioned components, such
as pipes and tanks. Fiber tows are passed through a resin bath before being wound onto a mandrel

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in a variety of orientations, controlled by the fiber feeding mechanism, and rate of rotation of the
mandrel.
Materials Options
 Resins: Any, e.g. epoxy, polyester, vinylester, phenolic
Page | 48
 Fibers: Any. The fibers are used straight from a creel and not woven or stitched into a fabric
form
 Cores: Any, although components are usually single skin Typical Applications: Chemical
storage tanks and pipelines, gas cylinders, fire-fighters breathing tanks
Advantages
 This can be a very fast and therefore economic method of laying material down.
 Resin content can be controlled by metering the resin onto each fiber tow through nips or
dies.
 Fiber cost is minimized since there is no secondary process to convert fiber into fabric prior
to use.
 Structural properties of laminates can be very good since straight fibers can be laid in a
complex pattern to match the applied loads.
Disadvantages
 The process is limited to convex shaped components.
 Fiber cannot easily be laid exactly along the length of a component.
 Mandrel costs for large components can be high.
 The external surface of the component is unmoulded, and therefore cosmetically
unattractive.
 Low viscosity resins usually need to be used with their attendant lower mechanical and
health and safety properties.

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8.5 PULTRUSION
Page | 49
Fibers are pulled from a creel through a resin bath and then on through a heated die. The die
completes the impregnation of the fiber, controls the resin content and cures the material into its
final shape as it passes through the die. This cured profile is then automatically cut to length.
Fabrics may also be introduced into the die to provide fiber direction other than at 0°. Although
pultrusion is a continuous process, producing a profile of constant cross-section, a variant known
as 'pulforming' allows for some variation to be introduced into the cross-section. The process pulls
the materials through the die for impregnation, and then clamps them in a mould for curing. This
makes the process non-continuous, but accommodating of small changes in cross-section.
Material Options
 Resins: Generally epoxy, polyester, vinylester and phenolic
 Fibers: Any
 Cores: Not generally used
Beams and girders used in roof structures, bridges, ladders, frameworks

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Advantages
 This can be a very fast, and therefore economic, way of impregnating and curing materials.
 Resin content can be accurately controlled.
 Fiber cost is minimized since the majority is taken from a creel. Page | 50
 Structural properties of laminates can be very good since the profiles have very straight
fibers and high fiber volume fractions can be obtained.
 Resin impregnation area can be enclosed thus limiting volatile emissions.
Disadvantages
 Limited to constant or near constant cross-section components.
 Heated die costs can be high.
8.6 RESIN TRANSFER MOULDING
Fabrics are laid up as a dry stack of materials. These fabrics are sometimes pre-pressed to the
mould shape, and held together by a binder. These 'preforms' are then more easily laid into the
mould tool. A second mould tool is then clamped over the first, and resin is injected into the cavity.
Vacuum can also be applied to the mould cavity to assist resin in being drawn into the fabrics. This

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is known as Vacuum Assisted Resin Injection (VARI). Once all the fabric is wet out, the resin
inlets are closed, and the laminate is allowed to cure. Both injection and cure can take place at
either ambient or elevated temperature.
Page | 51
Material Options
 Resins: Generally epoxy, polyester, vinylester and phenolic, although high temperature
resins such as bismaleimides can be used at elevated process temperatures.
 Fibers: Any. Stitched materials work well in this process since the gaps allow rapid resin
transport. Some specially developed fabrics can assist with resin flow
 Cores: Not honeycombs, since cells would fill with resin, and pressures involved can crush
some foams
Small complex aircraft and automotive components, train seats.
Advantages
 High fiber volume laminates can be obtained with very low void contents.
 Good health and safety, and environmental control due to enclosure of resin.
 Possible labour reductions.
 Both sides of the component have a moulded surface.
Disadvantages
 Matched tooling is expensive and heavy in order to withstand pressures.
 Generally limited to smaller components.
 Unimpregnated areas can occur resulting in very expensive scrap parts.

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8.7 VACUUM INFUSION PROCESS
Page | 52
Fabrics are laid up as a dry stack of materials as in RTM. The fiber stack is then covered with peel
ply and a knitted type of non-structural fabric. The whole dry stack is then vacuum bagged, and
once bag leaks have been eliminated, resin is allowed to flow into the laminate. The resin
distribution over the whole laminate is aided by resin flowing easily through the non-structural
fabric, and wetting the fabric out from above.
Materials Options
 Resins: Generally epoxy, polyester and vinylester.
 Fibers: Any conventional fabrics. Stitched materials work well in this process since the
gaps allow rapid resin transport.
 Cores: Any except honeycombs.
Semi-production small yachts, train and truck body panels
Advantages
 As RTM above, except only one side of the component has a moulded finish.
 Much lower tooling cost due to one half of the tool being a vacuum bag, and less strength
being required in the main tool.

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 Large components can be fabricated.

 Standard wet lay-up tools may be able to be modified for this process.
 Cored structures can be produced in one operation.
Page | 53
Disadvantages
 Relatively complex process to perform well.
 Resins must be very low in viscosity, thus comprising mechanical properties.
 Unimpregnated areas can occur resulting in very expensive scrap parts.
8.8 RESIN FILM INFUSION
Dry fabrics are laid up interleaved with layers of semi-solid resin film supplied on a release paper.
The lay-up is vacuum bagged to remove air through the dry fabrics, and then heated to allow the
resin to first melt and flow into the air-free fabrics, and then after a certain time, to cure.
Materials Options
 Resins: Generally epoxy only.
 Fibers: Any

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 Cores: Most, although PVC foam needs special procedures due to the elevated
temperatures involved in the process
Aircraft radomes and submarine sonar domes.
Page | 54
Main Advantages
 High fiber volumes can be accurately achieved with low void contents.
 Good health and safety and a clean lay-up, like prepreg.
 High resin mechanical properties due to solid state of initial polymer material and elevated
temperature cure.
 Potentially lower cost than prepreg, with most of the advantages.
 Less likelihood of dry areas than SCRIMP process due to resin traveling through fabric
thickness only.
Disadvantages
 Not widely proven outside the aerospace industry.
 An oven and vacuum bagging system is required to cure the component as for prepreg,
although the autoclave systems used by the aerospace industry are not always required.
 Tooling needs to be able to withstand the process temperatures of the resin film (which if
using similar resin to those in low-temperature curing prepregs, is typically 60-100°C).
 Core materials need to be able to withstand the process temperatures and pressures.

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9.0 MECHANICAL TESTING OF COMPOSITES
9.1 Objectives of Mechanical Testing

The development of the mechanical testing of the materials depends upon other scientific factors.
These factors help in better understanding and facilitate the progress in evaluating the various Page | 55
processes. These processes include:
1. Quality control of a process
2. Quality assurance for the material developed and structure fabricated from thereof
3. Better material selection
4. Comparisons between available materials
5. Can be used as indicators in materials development programmes
6. Design analysis
7. Predictions of performance under conditions other than test conditions
8. Starting points in the formulation of new theories
It should be noted that these processes are dependent upon each other. However, if they are
considered individually then the data required can be different for the evaluation. For example,
some tests are carried out as multipurpose tests using various processes. A conventional tensile
test carried out under fixed conditions may serve quality control function whereas one carried out
varying factors like temperature, strain rate, humidity etc. may provide information on load bearing
capacity of the material. The properties evaluated for materials like composite is very sensitive to
various internal structure factors. However, these factors depend mainly upon the fabrication
process or other factors. The internal structure factors that affect the properties are, in general, at
atomic or molecular level. These factors mostly affect the matrix and fiber-matrix interface
structure. The mechanical properties of the fibrous composite depend on several factors of the
composition. These factors are listed below again for the sake of completeness.
1. Properties of the fiber

2. Surface character of the fiber
3. Properties of the matrix material
4. Properties of any other phase
5. Volume fraction of the second phase (and of any other phase)

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6. Spatial distribution and alignment of the second phase (including fabric weave)
7. Nature of the interfaces
Another important factor is processing of the composites. There are many parameters that control
Page | 56
the processing of composites that access the quality of adhesion between fiber and matrix, physical
integrity and the overall quality of the final structure. In case of composite the spatial distribution
and alignment of fibers are the most dominating factor, which causes the variation of properties.
The spatial distribution and alignment of the fibers can change during the same fabrication process.
Thus, for a given fabrication process the property evaluated from the composite material may show
a large variation.
9.2 Tensile Testing

The well-known purpose of the tensile testing is to measure the ultimate tensile strength and
modulus of the composite. However, one can measure the axial Poison’s ratio with additional
instrumentations. The standard specimen used for tensile testing of continuous fiber composites is
a flat, straight-sided coupon. A flat coupons in ASTM standard D 3039/D 3039M-93 for and have
been shown.
The specimen, as mentioned above is flat rectangular coupon. The tabs are recommended for
gripping the specimen. It protects the specimen from load being directly applied to the specimen
causing the damage. Thus, the load is applied to the specimen through the grips. Further, it protects
the outer fibers of the materials. The tabs can be fabricated from a variety of materials, including
fiberglass, copper, aluminum or the material and laminate being tested. When the tabs of composite
material are used then according to ASTM specifications the inner plies of the tabs should match
with the outer plies of the composite. This avoids the unwanted shear stresses at the interface of
the specimen and tabs. However, the recent versions of the ASTM standards allow the use of tabs
with reinforcement. Further, end-tabs can also facilitate accurate alignment of the specimen in the
test machine, provided that they are symmetrical and properly positioned on the specimen. The
tabs are pasted to the specimen firmly with adhesive.
1. The axial modulus
2. In-plane and through thickness Poisson’s ratio
3. Tensile ultimate stress

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4. Tensile ultimate strain

5. Any nonlinear, inelastic response
In general, the tensile tests are done on coupons with laminae/laminate for corresponding axial
properties and coupons with laminae/laminate for corresponding transverse properties. The off
Page | 57
axis laminae specimen also provides data on coefficient of mutual influence and the in-plane shear
response.
9.3 Measurement of Modulus

It should be noted that due to progressive damage the stiffness of the lamina or laminae/laminate
changes causing the stress strain curve to be non-linear. The measurement of modulus in a tensile
testing from a non-linear loading curve can be done by three methods. In the first method the

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modulus is taken as a tangent to the initial part of the curve. In the second method a tangent is
constructed at a specified strain level. For example, in the Figure the modulus is measured at 0.25%
strain or 0.0025 strain (Point B). In the third method, a secant is constructed between two points.
For example in Figure a secant is constructed between points A and B. Typically, the strain values
Page | 58
at these points are 0.0005 and 0.0025. In ASTM standards the secant is called as chord. The
modulus measured by these methods is known as ‘initial tangent modulus’, ‘B% modulus’ and
‘A%-B% secant (chord) modulus’, respectively.
9.4 Compression Testing

Most of the structural members include the compression members. Such members can be loaded
directly in compression or under a combination of flexural and compression loading. The axial
stiffness of such members depends upon the cross-sectional area. Thus, it is proportional to the

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weight of the structure. One can alter the stiffness by changing the geometry of the cross section
within limits. However, some of the composites have low compressive strength and this fact limits
the full potential application of these composites. The compression testing of the composites is
very challenging due to various reasons. The application of compressive load on the cross section
Page | 59
can be done in three ways: directly apply the compressive load on the ends of a specimen, loading
the edges in shear and mixed shear and direct loading. These three ways of imposing the loads for
compression testing are shown. During compression loading the buckling of the specimen should
be avoided. This demands a special requirement on the holding of the specimen for loading
purpose. Further, it demands for special geometry of the specimen. These specimens are smaller
as compared to the tensile testing specimens. A compression test specimen according to ASTM
D695 (modified) standard is shown. The compression testing of composites is a vast topic.
Additional reading on this topic from other literature is suggested to readers.

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Page | 60

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REFERENCES
1. D. Hull and T.W. Clyne, (1996), Introduction to Composite Materials, Cambridge
University Press
2. Frank L Matthews and R D Rawlings, (2006), Composite Materials: Engineering and
Page | 61
Science, Taylor and Francis.
3. Stuart M Lee, J. Ian Gray, Miltz, (1989), Reference Book for Composites Technology,
CRC press
4. K.K. Chawla, (1998), Composite Materials, Springer-Verlag, New York
5. B.T. Astrom, (1997), Manufacturing of Polymer Composites, Chapman & Hall
6. Composite materials by J. N. Reddy
7. Introduction to Physical Metallurgy; McGraw Hill Education (India) Private Limited;
Sidney H. Avner
8. Material Science & Engineering; Prentice- Hall of India Private Limited; V. Raghavan
9. Material Science & Engineering; William D. Callister
10. The Science & Engineering of Materials; Cengage Learning, Donald. R. Askeland &
Pradeep Phule
11. Modern Ceramic Engineering; David W. Richerson
12. Modern Physical Metallurgy & Materials Engineering; R.E. Smallman & R.J. Bishop
13. Physical Metallurgy & Advanced Materials; R.E. Smallman & A.H.W. Ngan
14. Physical Metallurgy Principles; Cengage Learning; Reza Abbhaschian etal
15. Engineering Physical Metallurgy; foreign Language publishing house, Moscow; Y.
Lakhtin

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Composite Materials EMT311 Lecture Notes

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COMPOSITE MATERIALS: SYNTHESIS, PROPERTIES & APPLICATIONS

EMT311 LECTURE NOTES

B.TECH (HONOURS) MATERIALS TECHNOLOGY ENGINEERING

HARARE INSTITUTE OF TECHNOLOGY

LECTURER: ENGINEER AARON RUNGANI [BSC. HEN (UZ), MIP (AU)]

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COMPOSITE MATERIALS: SYNTHESIS, PROPERTIES & APPLICATIONS (EMT311)

1.0 INTRODUCTION ............................................................................................................................. 3

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8.3 VACUUM BAGGING ................................................................................................................ 46

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1) The distribution of materials in the composite is controlled by mechanical means.

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1.1 Advantages and Disadvantages of Composite Materials

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2.0 APPLICATIONS OF COMPOSITE MATERIALS

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Figure 2.1: applications of composite materials

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3.0 CLASSIFICATION OF COMPOSITE MATERIALS

3.1 Classification based on type of matrix

Thermoplastic composites- this is a type of composite with a thermoplastic resin like

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