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Module
2
Selection of Materials and
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Lecture
1
Physical and Mechanical Properties of
Engineering Materials
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Instructional objectives
At the of this lecture, the student should be able to appreciate
(a) general classification of engineering materials, and
(b) physical and mechanical properties of engineering materials
Engineering Materials
Materials play and important role in the construction and manufacturing of various parts and
components. An appropriate selection of a material for a given application adds to economy,
working and life of the final part and component.
Classification of Engineering Materials
Engineering materials can be broadly classified as metals such as iron, copper, aluminum, etc.,
and non-metals such as ceramics (e.g. alumina and silica carbide), polymers (e.g. polyvinyle
chloride or PVC), natural materials (e.g. wood, cotton, flax, etc.), composites (e.g. carbon fibre
reinforced polymer or CFRP, glass fibre reinforced polymer or GFRP, etc.) and foams.
Properties of Engineering Materials
Material property is the identity of material, which describes its state (physical, chemical) and
behavior under different conditions. The material properties can be broadly categorized as
physical, chemical, mechanical and thermal.
The physical properties define the physical state of material and are independent of its chemical
nature. The physical properties of engineering materials include appearance, texture, mass,
density, Melting point, boiling point, viscosity, etc. The chemical properties describe the
reactivity of a material and are always mentioned in terms of the rate at which the material
changes its chemical identity, e.g. corrosion rate, oxidation rate, etc. The mechanical properties
describe the resistance against deformation, in particular, under static and dynamic mechanical
loading condition. The mechanical properties include elastic modulus, Poisson’s ratio, yield
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strength and ultimate tensile strength, hardness and toughness, etc. The thermal properties
describe material behavior under thermal loading and include thermal conductivity, specific heat,
thermal diffusivity, coefficient of thermal expansion, etc. For a given application or service, an
engineering material is selected based on a set of appropriate material properties, often referred
to as attributes, that would be requisite to sustain various expected loads. Figure 1 depicts a
schematic representation of material family, which is utilized in selection of materials for a target
application.
Figure 1 Organized classification of materials and properties
Physical Properties
Physical properties describe the state of material, which is observable or measurable. Color,
texture, density, melting point, boiling point, etc. are some of the commonly known physical
properties.

Color: Represents reflective properties of substance

Density: Amount of mass contained by unit volume of material. The higher the density
the heavier is the substance. (SI unit: kg/m3)

Melting point: Melting point is the temperature at which material changes its state from
solid to liquid. (SI units: K)
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
Boiling point: Boiling point is the temperature at which material changes its state from
liquid to gaseous. (SI units: K)
Chemical Properties
Chemical properties are the measure of reactivity of a material in the presence of another
substance or environment which imposes change in the material composition. These properties
are always mentioned in term of the rate of change in its composition. Corrosion rate, oxidation
rate, etc. are some of the chemical properties of material.
Mechanical Properties
Mechanical properties describe the behavior of material in terms of deformation and resistance to
deformation under specific mechanical loading condition. These properties are significant as they
describe the load bearing capacity of structure. Elastic modulus, strength, hardness, toughness,
ductility, malleability are some of the common mechanical properties of engineering materials.
Every material shows a unique behavior when it is subjected to loading. Figure 2 shows a typical
stress-strain curve of C-steel under uniaxial tensile loading.
Figure 2 Stress-strain curve for carbon-steel
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Stresses computed on the basis of the original area of the specimen are often referred to as the
conventional or nominal stresses. Alternately, the stresses computed on the basis of the actual
area of the specimen gives the so called true stress. Within the elastic limit, the material returns
to its original dimension on removal of the load. The elastic modulus is referred to the slope of
the stress-strain behavior in the elastic region and its SI unit is conceived as N.m-3. The elastic
modulus is also referred to as the constant of proportionality between stress and strain according
to Hooke’s Law. Beyond the elastic limit, the materials retains a permanent, irreversible strain
(or deformation) even after the load is removed. The modulus of rigidity of a material is defined
as the ratio of shear stress to shear strain within the elastic limit. The bulk modulus is referred to
the ratio of pressure and volumetric strain within the elastic limit.
(a)
(b)
(c)
Figure 3 Schematic presentation of (a) tensile, (b) shear and (c) hydrostatic compression
Figure 3(a) to (c) schematically shows the uniaxial tensile, shear and hydrostatic compression on
a typical block of material. When a sample of material is stretched in one direction it tends to get
thinner in the other two directions. The Poisson's ratio becomes important to highlight this
characteristic of engineering material and is defined as the ratio between the transverse strain
(normal to the applied load) and the relative extension strain, or the axial strain (in the direction
of the applied load). For an engineering material, the elastic modulus (E), bulk modulus (K), and
the shear modulus (G) are related as: G = E/2(1+) and K = E/3(1-2), where  refers to the
Poisson’s ratio.
The strength (SI units: Pa or N/m2) is the property that enables an engineering material to resist
deformation under load. It is also defined as the ability of material to withstand an applied load
without failure. Based on the typical stress-strain behavior of an engineering material, a few
reference points are considered as important characteristics of the material. For example, the
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proportional limit is referred to the stress at which the stress / strain behavior of a material first
becomes non-linear. The yield strength refers to the stress required to cause permanent plastic
deformation. The ultimate tensile strength refers to the maximum stress value on the engineering
stress-strain curve and is often considered as the maximum load-bearing strength of a material.
The rupture strength refers to the stress at which a material ruptures typically under bending.
The hardness is another important mechanical property of engineering material and refers to the
resistance of a material against abrasion / scratching / indentation. The hardness of a material is
always specified in terms of the particular test that is used to measure the same. For a measure of
resistance against indentation, Vickers, Brinell, Rockwell, Knoop hardness tests are common.
Alternately, for a measure of resistance against scratch, Mohr’s hardness test is followed. The
basic principle used in these testing involves the pressing of a hard material against the candidate
material, whose hardness is to be measured. The typical Brinell hardness values of some of the
commonly used engineering materials are as follows: aluminum – 15, copper – 35, mild steel –
120, austenitic stainless steel – 250, hardened tool steel – 650, and so on.
Another important mechanical property of engineering materials is the toughness that provides a
measure of a material to withstand shock and the extent of plastic deformation in the event of
rupture. Toughness may be considered as a combination of strength and plasticity. One way to
measure toughness is by calculating the area under the stress strain curve from a tensile test. The
toughness is expressed in Joule to indicate the amount of energy absorbed in the event of failure
or rupture. In a similar line, resilience of a material refers to the energy absorbed during elastic
deformation and is measured by the area under the elastic portion of the stress – strain curve.
Thermal Properties
The thermal properties of an engineering material primarily refer to the characteristic behaviors
of the material under thermal load. For example, thermal conductivity is a measure of the ability
of material to conduct heat and is expressed as W.K-1.m-1 in SI unit. The specific heat refers to
the measure of energy that is required to change the temperature for a unit mass and is expressed
as J.kg-1.K-1. The product of density and specific heat is often referred to the heat capacity of a
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unit mass of material. The thermal diffusivity refers to the ratio of thermal conductivity and heat
capacity of a material and provides a measure the rate of heat conduction. The thermal diffusivity
is expressed in terms of m2.s-1.
When a material is subjected to both thermal and mechanical loading, two more characteristics
of materials - coefficient of thermal expansion and thermal shock resistance - become significant.
The coefficient of thermal expansion provides a measure of unit change in strain of a material for
unit change in temperature and is expressed in terms of K-1 in SI unit. The thermal strain in
material is considered to be isotropic in nature. The thermal shock resistance provides a measure
to which a material can withstand an impact load which is either thermal or thermo-mechanical
in nature. The thermal shock resistance is expressed as [KT(1-  )/(E)] where K is the thermal
conductivity, σT maximal tension the material can resist, α the thermal expansion coefficient, E
the Young’s modulus and ν the Poisson’s ratio.
Getting Familiar with Different Materials
Metals
Metals have free valance electrons which are responsible for their good thermal and electrical
conductivity. Metals readily loose their electrons to form positive ions. The metallic bond is held
by electrostatic force between delocalized electrons and positive ions. Metals are primarily used
in the form of alloys which depict a combination of two or more materials, in which at least one
is metal. The iron based alloys are characterized as ferrous alloys. For example, steel is an alloy
of iron, carbon and other alloying elements, brass is an alloy of copper and zinc, bronze is an
alloy of copper and tin, and so on. Metals and alloys are typically characterized by an excellent
blend of mechanical and thermal properties. Table 1 indicates the typical material properties and
common applications of some of the widely used metallic materials.
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Table 1
Common material properties of metallic materials
Material
Iron
Copper
Aluminum
C-Steel
AA6061
Ti-6Al-4V
Pure
Pure
Pure
Fe- Alloy
Al-alloy
Ti-Alloy
Density (kg.m )
7850
8930
2700
8000
2700
4420
Melting
Temperature (K)
1811
1357
933
1750
860
1922
Boiling
Temperature (K)
3134
2835
2792
3300
Young’s
Modulus(GPa)
210
120
70
210
70-80
113.8
Shear
Modulus(GPa)
82
48
26
79.3
26
44
Bulk
Modulus(GPa)
170
140
76
160
Poisson’s Ratio
0.29
0.34
0.35
0.27-0.3
0.33
0.33
250
275
880
90-180
410
310
1000
17
23
10.8
23.4
8.6
Properties
Type
-3
Yield Strength
(N.mm-2)
322
Ultimate Tensile
Strength (N.mm-2)
Coefficient of
Thermal
Expansion X 10-6
(K-1)
Thermal
Conductivity
(W.mm-1.K-1)
80
400
237
35-55
180
7.2
Specific Heat
(J.kg-1.K-1)
450
385
897
490
896
560
Aircraft
fittings,
Pistons,
Bike
frames
Aerospace,
Marine,
Power
generation,
Offshore
Industries
Application
Utensils,
Aerospace,
Naval
Heat
Construction,
Construction,
Exchanger
Electrical
Chemical
conductors
transport,
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Ceramics and Glasses
Ceramics are non-metallic in nature and refer to the carbide, boride, nitride and oxides of
Aluminum, silica, zirconium, etc. However, the ceramics possess excellent resistance to thermal
and chemical corrosion and wear resistant. Ceramics are also good thermal and electrical
insulators. Table 2 indicates the typical material properties and common applications of some of
the widely used ceramics.
Table 2 Material properties and applications of commonly used Ceramics
Material
Alumina
Silicon Carbide
Silicon Nitride
Glass
(Soda-lime glass)
Density (kg.m-3)
3950
3000
3290
2520
Melting
Temperature (K)
2300
3000
2173
1313
Young’s
Modulus(GPa)
340
410
310
72-74
Shear
Modulus(GPa)
124
179
Bulk
Modulus(GPa)
165
203
Poisson’s Ratio
0.22-0.27
0.14
Ultimate Tensile
Strength (N.mm-2)
260
250
Coefficient of
Thermal Expansion
X 10-6 (K-1)
5.4
2.77
3.3
Thermal
Conductivity
(W.mm-1.K-1)
15-40
33-155
30
Specific Heat
(J.kg-1.K-1)
930
715
Application
Cutting
wheels,
polishing
clothes
High
temperature
furnace, Heat
shield
Properties
29.8
0.27
8.5
840
Balls and roller of
bearing, Cutting
tools, Engine
valves, Turbine
blades
Windows, food
Preparation
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Polymer
Polymer is a chain of molecules connected by covalent (sharing of electrons) chemical bond.
Three types of polymers are most common: (a) thermoplastics which can be reworked on
heating, (b) thermosets which cannot be worked with after curing is over, and (c) elastomers,
which typically provide very high elastic deformation. The polymers cannot withstand high
temperature due to their low transition temperature Table 3 indicates the typical material
properties and common applications of some of the widely used polymers.
Table 3 Material properties and applications of commonly used Polymers
Material
Polyvineyl
chloride (PVC)
Bakelite
Silicone
thermoplastic
elastomer
elastomer
1350
1300
968-1290
High density silicone-2800
Properties
Type
Density (kg.m-3)
Melting Temperature (K)
373-530
588
Young’s Modulus(MPa)
1-5
Yield Strength (N.mm-2)
10-60
(Flexible-rigid)
Ultimate Tensile Strength
(N.mm-2)
2.6
Coefficient of Thermal
Expansion X 10-6 (K-1)
52
Thermal Conductivity
(W.mm-1.K-1)
0.14-0.28
0.23
900
1465
Plumbing
Electrical
Insulators
Specific Heat (J.kg-1.K-1)
Application
21-47
11
8.1
0.22
Electrical appliances,
Structural application
(below 200°C)
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Natural Materials
The most common examples of natural materials are wood, cotton, flax, wools, which primarily
come from the plants or animals. Most of the natural materials are recyclable and require
considerable processing operations before use.
Table 4 Material properties and applications of commonly used natural materials
Material
Oak Wood
Wool
Flax
Properties
Type
Density (kg.m-3)
European
Oak
650
Melting Temperature (K)
Boiling Temperature (K)
Young’s Modulus (GPa)
9-13
Shear Modulus(GPa)
Bulk Modulus(GPa)
Poisson’s Ratio
Yield Strength (N.mm-2)
Ultimate Tensile Strength
(N.mm-2)
50-180
Coefficient of Thermal
Expansion (K-1)
34-54
Thermal Conductivity
(W.mm-1.K-1)
0.3-0.35
Specific Heat (J.kg-1.K-1)
Application
0.028
0.17
Furniture,
Packaging
Fabric,
Thermal
insulator
Fabrication
of twine
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Composite
Composite material is formed by combining one or more different materials. Unlike alloy
system each constituent is distinguishable and remain their properties. Composite materials
consists of matrix material with reinforcement to enhance its strength. Few common examples of
composites are FRP(Glass/carbon fiber reinforced polymers), Metal matrix composites. Using
composites one can combine attractive qualities of other materials and engineer properties to
demand. On the other side they are expensive and difficult to fabricate and join. Table 5 indicates
common properties and applications of composites.
Table 5 Material properties and applications of commonly used composites
Material
Properties
Density (kg.m-3)
Carbon fiber
reinforced
polymer
matrix
Aluminum
matrix
Titanium
matrix
1800
2650
3860
210
300
Young’s Modulus(GPa)
Poisson’s Ratio
Alumina
matrix
Cermet
85
100
350
500
1750
385
500
Aerospace
Turbines
High
temperature
Mechanical
Application
Cutting
tools,
Polishing
materials
0.295
Yield Strength (N.mm-2)
Ultimate Tensile Strength
(N.mm-2)
Application
7000
1500
Mechanical
Aerospace,
components,
Sporting
Protection
equipments,
screen,
Electronic
Sporting
packaging
equipments
Foams
Foam is a substance formed by trapping many gaseous bubbles in liquid or solid. Solid foams are
very important class of structure due to its light weight. The foams can be metallic (eg. Titanium
foam), ceramic (alumina foam) or based on polymer (polyurethane foam). The metallic foams
are commonly used for medical implants. The ceramic foams are used typically as insulators
while the polymer based foams are primarily used for packaging and acoustic insulators.
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Exercise
Choose the correct answer.
1. Hydrostatic stress results in
(a) linear strain
(b) shear strain
(c) both linear and shear strain
(d) None
2. Toughness of a material is equal to the area under ____ part of the stress-strain curve.
(a) Elastic
(b) Plastic
(c) Both elastic and plastic
(d) None
3. During a tensile loading, the length of a steel rod is changed by 2 mm. If the original length
of the rod has been 20 mm, what is the amount of strain induced
(a) 0.1
(b) 2
(c) 0.9
(d) 0.22
4. ____ is an example of a chemical property.
(a) Density
(b) Mass
(c) Acidity
(d) Diffusivity
Answers:
1. (d)
2. (c)
3. (a)
4. (c)
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