Module 2 Lecture 1 Final

8/13/2019 Module 2 Lecture 1 Final

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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

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Module 2 Lecture 1 Final

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