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Module
2
Selection of Materials andShapes
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Lecture
1Physical and Mechanical Properties ofEngineering 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 an 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 and
their alloys, 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, metal matrix
composites or MMC, Concrete, Ceramic matrix composites, Engineering wood such as plywood,
oriented strand board, wood plastic composite 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.
Thephysical 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
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loading condition. The mechanical properties include elastic modulus, Poissons ratio, yield
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 2.1.1 depicts a
schematic representation of material family, which is utilized in selection of materials for a target
application.
Figure 2.1.1 Organized classification of materials and properties [1]
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)
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Melting point: Melting point is the temperature at which material changes its state fromsolid to liquid. (SI units: K)
Boiling point: Boiling point is the temperature at which material changes its state fromliquid 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.
Corrosion rate: Corrosion rate is measured in terms of corrosion penetration for givenperiod of time at specific surrounding condition. Corrosion rate is given by length of
penetration per unit time. (Units: mm/year)
Oxidation rate: Oxidation rate is measured in terms of amount of material consumedforming oxide or amount of oxide scale formed for given period of time at specific
surrounding temperature. Oxidation rate is given by amount of mass of material lost or
thickness of scale formed during oxidation per unit time. (Units: gms/min or m/min).
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 2.1.2 shows a
typical stress-strain curve of C-steel under uniaxial tensile loading. Point A indicates the
proportional limit. Stress strain behavior is linear only up to this point. Point B represents the
point at which the material starts yielding. Between point A and B, the stress strain behavior is
not linear, though it is in elastic region. Point C is referred to the upper yield point. The
material behavior after point D is highly nonlinear in nature. Point E is the maximum stress
that the material can withstand and the point F schematically indicates the point of rupture.
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Figure 2.1.2 Stress-strain curve for carbon-steel [3]
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 provide 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-2
Figure 2.1.3(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 Poissons ratio.
. The
elastic modulus is also referred to as the constant of proportionality between stress and strain
according to Hookes 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.
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(a) (b) (c)
Figure 2.1.3 Schematic presentation of (a) tensile, (b) shear and (c) hydrostatic compression [4]
The strength(SI units: Pa or N/m2
The hardness is another important mechanical property of engineering material and refers to the
resistance of a material against abrasion / scratching / indentation. The hardnessof 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, Mohrs 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 Brinell hardness (figure 2.1.5) test method
consists of indenting the test material with a 10 mm diameter hardened steel or carbide ball
) 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
proportional limitis referred to the stress just beyond the point where 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 strengthrefers 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. Different material behaves differently when subjected to load. Figure
2.1.4 indicates the different in stress strain behavior of typical cast iron, low carbon steel, and
aluminum alloy. Cast iron, being a brittle material generates steeper curve than low carbon steel
or aluminum alloy. There is no sign of yielding prior to failure, so the yield point has to be found
out graphically. The yield point strength in the case of low carbon steel and aluminum alloys can
be identified easily.
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subjected to a load of 3000 kg. For softer materials the load can be reduced to 1500 kg or 500 kg
to avoid excessive indentation. The full load is normally applied for 10 to 15 seconds in the case
of iron and steel and for at least 30 seconds in the case of other metals. The diameter of the
indentation left in the test material is measured with a low powered microscope. The Brinell
harness number is calculated by dividing the load applied by the surface area of the indentation.
The typicalBrinell hardnessvalues 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.
Figure 2.1.4 Comparison of behavior of different material
Another important mechanical property of engineering materials is the toughnessthat provides a
measure of a material to withstand shock and the extent of plastic deformation in the event of
rupture. Toughnessmay be considered as a combination of strength and plasticity. One way to
measure toughnessis 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. Figure 2.1.6 shows the schematic set-ups of Izod impact test and Charpy impact test.
In both the cases, impact loading is applied in notched specimen of predefined dimension.
Energy absorbed during the breakage of the specimen is the measure of the toughness. In a
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similar line, resilience of a material refers to the energy absorbed during elastic deformationand
is measured by the area under the elastic portion of the stress strain curve. Izod and charpy
tests are two important methods for evaluating toughness of a material.
Figure 2.1.5 Brinell Hardness Test [5]
Figure 2.1.6 Schematic set-up of (a) Izod Test and (b) Charpy Test [6]
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
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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 energythat 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
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 expansionprovides a measure of unit change in strain of a material for
unit change in temperature and is expressed in terms of K-1
E)-(1K T
in SI unit. The thermal strain in
material is considered to be isotropic in nature. The thermal shock resistanceprovides 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 ,where K is the thermal
conductivity, T
maximal tension the material can resist, the thermal expansion coefficient, E
the Youngs modulus and the Poissons 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 2.1.1 indicates the typical material properties
and common applications of some of the widely used metallic materials.
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Table 2.1.1 Common material properties of metallic materials [7]
Material
Properties
Iron Copper Aluminum C-Steel AA6061 Ti-6Al-4V
Type Pure Pure Pure Fe- Alloy Al-alloy Ti-Alloy
Density (kg.m-3 7870) 8930 2698 8000 2700 4420
Melting
Temperature (K)1808 1357 933 1750
Solidus = 855
Liquidus = 924
Solidus = 1877
Liquidus = 1933
Boiling
Temperature (K)3134 2835 2792 3300 3533
Youngs
Modulus(GPa)200 110 68 210 70-80 113.8
ShearModulus(GPa)
77.5 46 25 79.3 26 44
Bulk
Modulus(GPa)166 140 76 160 40.7
Poissons Ratio 0.291 0.343 0.36 0.27-0.3 0.33 0.342
Yield Strength
(MPa)50 33.3 250 275 880
Ultimate Tensile
Strength (MPa)210 90-180 410 310 950
Coefficient of
Thermal
Expansion X 10-6
(K-1
12.2
)
16.4 24 10.8 23.6 8.6
Thermal
Conductivity
(W.mm-1.K-1
76.2
)
400 210 35-55 180 6.7
Specific Heat
(J.kg-1.K-1440
)385 900 490 896 526.3
ApplicationHeat
Exchanger
Aerospace,
Construction,
Electrical
conductors
Utensils,
Naval
Construction,
Chemical
transport,
Aircraft fittings,
Pistons, Bike
frames
Aerospace,
Marine, Power
generation,
Offshore
Industries
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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 2.1.2 indicates the typical material properties and common applications of some
of the widely used ceramics.
Table 2.1.2 Material properties and applications of commonly used Ceramics [7]
Material
Properties
Alumina Silicon Carbide Silicon NitrideGlass
(Soda-lime glass)
Density (kg.m-3 3960) 3000 3290 2520
Melting
Temperature (K)2300 3000 2173 1313
Youngs
Modulus(GPa)370 410 310 72-74
Shear
Modulus(GPa)150 179 29.8
Bulk Modulus(GPa) 165 203
Poissons Ratio 0.22-0.27 0.14 0.27
Ultimate Tensile
Strength (MPa)300 250
Coefficient of
Thermal Expansion
X 10-6(K-1
5.4
)
2.77 3.3 8.5
Thermal
Conductivity
(W.mm-1.K-1
30
)
33-155 30
Specific Heat (J.kg-
1.K-1 850) 715 840
Application
Cutting
wheels,
polishing
clothes
High temperature
furnace, Heat
shield
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 2.1.3 indicates the typical material
properties and common applications of some of the widely used polymers.
Table 2.1.3 Material properties and applications of commonly used Polymers
Material
Properties
Polyvineyl
chloride (PVC)
Bakelite Silicone
Type thermoplastic elastomer Elastomer
Density (kg.m-3 1350) 1300968-1290
High density silicone-2800
Melting Temperature (K) 373-530 588
Youngs Modulus(MPa) 1-5
Yield Strength (MPa)10-60
(Flexible-rigid)
Ultimate Tensile Strength
(MPa)2.6 21-47 11
Coefficient of Thermal
Expansion X 10-6
(K-1
52)
8.1
Thermal Conductivity
(W.mm-1.K-10.14-0.28
)0.23 0.22
Specific Heat (J.kg-1.K-1 900) 1465
Application Plumbing Electrical
Insulators
Electrical appliances,
Structural application
(below 200C)
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Natural Materials
The most common examples of natural materials are wood, cotton, flax, wools, bamboo, jute
which primarily come from the plants or animals. Most of the natural materials are recyclable
and require considerable processing operations before use. Table 2.1.4 indicates the typical
material properties and common applications of some of the widely used natural materials.
Table 2.1.4 Material properties and applications of commonly used natural materials
Material
Properties
Oak Wood Wool Flax
Type
European
Oak
Density (kg.m-3 650) 22
Ignition Temperature (K) 523 873
Heat of Combution
(Kcal/g)4.9
Youngs Modulus (GPa) 9-13Longitudinal: 3.5
Transverse: 0.93
Shear Modulus(GPa)
Bulk Modulus(GPa)
Poissons Ratio
Yield Strength (MPa)
Ultimate Tensile Strength
(MPa)50-180 163
Coefficient of Thermal
Expansion (K-1
34-54)
Thermal Conductivity
(W.mm-1.K-1 0.3-0.35) 0.028
Specific Heat (J.kg-1.K-1 0.17)
ApplicationFurniture,
Packaging
Fabric, Thermal
insulator
Fabrication
of twine
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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 lengthof 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)
References
1. M F Ashby, Material Selection in Mechanical Design, Butterworth-Heinemann, 1999.2. G E Dieter, Mechanical Metallurgy, McGraw-Hill, 1961.3. http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-
ROORKEE/strength%20of%20materials/homepage.html,(28.05.2012).
4. http://www.grantadesign.com/education/datasheets/sciencenote.html,(28.05.2012).5. http://www.azom.com/article.aspx?ArticleID=2765,(28.05.2012).6. http://www.azom.com/article.aspx?ArticleID=2763,(28.05.2012).7. http://www.matweb.com,(28.05.2012).
http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-ROORKEE/strength%20of%20materials/homepage.htmlhttp://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-ROORKEE/strength%20of%20materials/homepage.htmlhttp://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-ROORKEE/strength%20of%20materials/homepage.htmlhttp://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-ROORKEE/strength%20of%20materials/homepage.htmlhttp://www.grantadesign.com/education/datasheets/sciencenote.htmlhttp://www.azom.com/article.aspx?ArticleID=2765http://www.azom.com/article.aspx?ArticleID=2763http://www.matweb.com/http://www.matweb.com/http://www.azom.com/article.aspx?ArticleID=2763http://www.azom.com/article.aspx?ArticleID=2765http://www.grantadesign.com/education/datasheets/sciencenote.htmlhttp://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-ROORKEE/strength%20of%20materials/homepage.htmlhttp://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-ROORKEE/strength%20of%20materials/homepage.html