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CHAPTER5

PROPERTIES OF MATERIALS –PART 1

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OUTLINE

3.1 Mechanical Properties

3.1.1 Definition

3.1.2 Factors Affecting Mechanical Properties

3.1.3 Kinds of Mechanical Properties

3.1.4 Stress and Strain

3.1.5 Elastic Deformation

3.1.6 Plastic Deformation & Plasticity

3.1.7 Strength

3.1.8 Brittleness, Toughness, Resilience & Ductility

3.1.9 Fatigue

3.1.10 Creep and Shrinkage Design and Safety Factors

3.2 Electrical Properties

3.3 Optical Properties

3.4 Magnetic Properties

3.5 Thermal Properties

3.6 Corrosion Properties

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3.1 MECHANICAL PROPERTIES3.1.1 DEFINITION

Properties or deformationobserved when a material is subjected

� to an applied external force (F = ma)

� to a mechanical force of stretching, compressing, bending, strikingare called the mechanical properties.

e.g. Mechanical properties of airplane

wing made of aluminum alloy

Mechanical properties of a bridge made of steel.

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3.1.2 FACTORS AFFECTING THE

MECHANICAL PROPERTIES

� Nature of the applied load, e.g. Tensile, compressive, shear

� Magnitude of the applied force

� The duration (application time): may be less than a second, may extend over a period of many years.

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3.1.3 KINDS OF MECHANICAL

PROPERTIES

Elasticity the ability of a material to deform under load and return to its original size and shape when the

load is removed.

Stiffness

the slope of the linear segment of stress – strain curve is Elastic Modulus or Young’s Modulus.

The value of the Modulus is the measure of STIFFNESS, material’s resistance to elastic

deformation (MPa)

Plasticity the property of a material to deform permanently under the application of a load.

Yield Strength the stress level at which the plastic deformation begins. (MPa)

Tensile Strength the stress at the maximum on the engineering stress-strain curve.the ability of a material to

withstand tensile loads without rupture when the material is in tension (MPa)

Compressive Strength the ability of a material to withstand compressive (squeezing) loads without being crushed

when the material is in compression. (MPa)

Fracture Strength corresponds to the stress at fracture (MPa)

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3.1.3 KINDS OF MECHANICAL

PROPERTIES

Toughness the ability of a material to withstand shatter. A material which easily shatters is brittle. The ability of a

material to absorb energy (J/m3)

Resilience The capacity of material to absorb energy when it is deformed elastically and then, upon unloading, to

have this energy recovered (J/m3)

Ductility the ability of a material to stretch under the application of tensile load and retain the deformed shape on the

removal of the load. Measure of ability to deform plastically without fracture (no units or m/m)

Brittleness brittle materials approximately have a fracture strain of less than about 5%.

Malleability the property of a material to deform permanently under the application of a compressive load. A material

which is forged to its final shape is required to be malleable

Fatigue

Strength the property of a material to withstand continuously varying and alternating loads

Hardness the property of a material to withstand indentation and surface abrasion by another hard object. It is an

indication of the wear resistance of a material.

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3.1.4 STRESS & STRAIN

Tension Compression Shear Torsion

Reference: Callister, Material Science and Eng., 5th Ed., p114

Types of force(load) applied on the object

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3.1.4.1 ENGINEERING STRESS (σ):

(Gerilme)

� Stress is defined as force F applied over the original cross-sectional area Ao.

� For a tensile test the stress is given by,

� Stress, (MPa or psi)

� Where, � F = applied tensile force (N or lbs) � A0= original cross-sectional area of the test specimen (m2 or in2)

� Units for Engineering Stress: � US customary: pounds per square inch (psi)� SI: N m-2 = Pascal (Pa)� 1psi = 6.89 x 10 3 Pa

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3.1.4.1 ENGINEERING STRESS (σ):

(Gerilme)

� Example: A 1.25 cm diameter bar is subjected to a load of 2500 kg. Calculate the engineering stress on the bar in megapascal (MPa)

� Sol’n:

� F= ma = 2500 x 9.81 = 24 500 N

� Ao = π r 2 = π ( 0.0125 2 / 4 )

� σ = Ft / Ao = 2 x 10 8 Pa = 200 MPa

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3.1.4.2 ENGINEERING STRAIN:

(Şekil Değiştirme)

� When an unaxial tensile force is applied to a rod, it causes the rod to be elongated in the direction of the force.

� Engineering strain is the ratio of the change in the length of the sample in the direction of the force divided by the originallength.

� ε = ( l – lo ) / lo = ∆ l / lo� Where, � ∆l = l - lo is the change in length � l0 = original length of the specimen� In engineering practice it is common to convert engineering

strain into percent strain or percent elongation� % engineering strain = engineering strain x 100 % = %

elongation� Unit of engineering strain: � Inch / inch or m/m which is dimensionless

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3.1.4.2 ENGINEERING STRAIN:

(Şekil Değiştirme)

L

Engineeringstress

Engineering(normal) strain

==

= =A

F

δε

σ

A

F

L

A

F

δε

σ

=

==2

2

LL

A

F

δδε

σ

==

=

2

2

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3.1.4.3 STRESS – STRAIN TESTING

� Tension tests: they are common, since they are easier to perform for most structural materials, steel etc.

� Compression tests: are used, when a material’s under large and permanent strains is desired, or when the material is brittle in tension, concrete

� Shear and torsion tests: Torsion test are performed on cylindrical solid shafts or tubes, machine axles and drive shafts

Typical tensile Specimen

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3.1.4.3 STRESS – STRAIN TESTING

Hydraulic

Wedge

Grips

SpecimenExtensometer

Schematic representation of the apparatus used to conduct tensile stress - strain tests

Typical tensile test machine

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3.1.4.4 YOUNG'S MODULUS (E)

� During Elastic Deformation: Stress / Strain = a constant

� σ / ε= E =Modulus of elasticity (Young’s Modulus) (ElastisiteModülü) (MPa)

� Modulus of Elasticity gives an idea about material’s resistance to elastic deformation.

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STIFFNESS:Material’s resistance to Elastic Deformation.

Atomic Origin of Stiffness

Strongly Bonded

Weakly Bonded

Net In

tera

tom

ic F

orc

e

Interatomic Distance

E ∝dF

dr

ro

The value of the Modulus of Elasticity is the measure of STIFFNESS

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Metal Alloy Modulus of Elasticity,

E ( GPa)

Aluminum

Brass

Copper

Magnesium

Nickel

Steel

Titanium

Tungsten

69

97

110

45

207

207

107

407

3.1.4.4 YOUNG'S MODULUS (E)

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3.1.4.4 YOUNG'S MODULUS (E)

Engineering Strain, ε = ∆L/Lo)0.002

Engineering

Stres

s, σ

= F

/Ao

Total Elongation

E

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3.1.5 ELASTIC DEFORMATION

� Elasticity, or elastic deformation is defined as ability of returning to an initial state or form after deformation.

� In most engineering materials, however, there will also exist a time-dependent elastic strain component. That is, elastic deformation will continue after the stress application, and upon load release some finite time is required for complete recovery. This time-dependent elastic behavior is known as ANELASTICITY, and it is due to time-dependent microscopic and atomistic processes that are attendant to the deformation.

� For metals the inelastic component is normally small and is often neglected. However, for some polymeric materials its magnitude is significant; in this case it is termed VISCOELASTIC BEHAVĐOR.

A simplified view of a metal bar's structure

P

The same metal bar, this time with an applied load.

After the load is released, the bar returns to its original shape. This is called elastic deformation.

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3.1.5 ELASTIC DEFORMATION

� EXAMPLE:� A piece of copper originally 305 mm (12 in.) long is

pulled in tension with a stress of 276 MPa (40,000 psi). If the deformation is entirely elastic, what will be the resultant elongation?

� Sol’n:

� Since the deformation is elastic, strain is linearly dependent on stress the magnitude of E for copper is 110 GPa

� ε= (l – lo ) / lo = ∆ l / lo� ∆l = (276 MPa) (305 mm)/ 110 x 103 MPa = 0.77 mm

= Eεεεεσ

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3.1.6 PLASTIC DEFORMATION & PLASTICITY

� For most metallic materials, elastic deformation exists only to strains of about 0.005. As the material is deformed beyond this point, the stress is not proportional to strain. And permanent, nonrecoverabledeformations, PLASTIC DEFORMATION, occurs.

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3.1.6 PLASTIC DEFORMATION & PLASTICITY

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3.1.7 STRENGTH3.1.7.1 YIELD STRENGTH ( -Y ) ( MPa or psi )

� Stress at which noticeable plastic deformation has occurred.

� The magnitude of the yield strength for a metal is a measure of its resistance to plastic deformation.

� A straight line is drawn parallel to the elastic deformation part of the curve from the engineering strain value of 0.002. The stress corresponding to the intersection point of these two lines is YIELD STRENGTH.

� Yield strengths may range from 35 MPa for a low strength Al to over 1400 MPa for high strength steels.

� Comparison of Yield Strength :� σy (ceramics) >> σ y (metals) >> σ y (polymers)

>> σ y (composites)

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3.1.7.2 TENSĐLE STRENGTH (TS) ( MPa or psi )

� The stress at the maximum on the engineering stress-strain curve.

� This corresponds to the maximum stress that can be resisted by a structure in tension. It is the maximum stress without fracture.

Examples:� metals: occurs when noticeable “necking” starts� ceramics: occurs when crack propagation starts� polymers: occurs when polymer backbones are all

aligned and about to break.

� Tensile Strengths may vary from 50 MPa to 3000 MPa

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3.1.7.3 COMPRESSIVE (CRUSHING) STRENGTH

� It is important in ceramics used in structures such as buildings or refractory bricks. The compressive strength of a ceramic is usually much greater than their tensile strength.

� Tensile, compressive and bending testing for materials

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3.1.7.3 COMPRESSIVE (CRUSHING) STRENGTH

Comparison of Stress -Strain

Curves for Metals,

Ceramics,Polymers

and Elastomers

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3.1.7.3 COMPRESSIVE (CRUSHING) STRENGTH

The Relationship between Elastic Modulus and Melting Temperature

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3.1.8 BRITTLENESS, TOUGHNESS, RESILIENCE & DUCTILITY

3.1.8.1 BRITTLENESS

� A material that experiences very little or no plastic deformation upon fracture is termed brittle.

Ductile vs Brittle Materials

• Only Ductile materials will exhibit necking.

• Ductile if EL%>8% (approximately)

• Brittle if EL% < 5% (approximately)

X

XXA

B C

XDBrittle Ductile

A & B C & D

Engin

eer

ing S

tres

s

Engineering Strain

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3.1.8.1 BRITTLENESS

Brittle Fracture Surfaces

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3.1.8.2 TOUGHNESS

� A measure of the ability of a material to absorb energy without fracture.

� (J/m3 or N. m/m3= MPa)� It is a measure of the ability of a material to absorb

energy up to fracture.� Energy needed to break a unit volume of material.� Area under stress-strain curve� For a material to be tough, it must display both

strength and ductility.� Often ductile materials are tougher than brittle ones.� Examples:

� smaller toughness (ceramics), � larger toughness(metals, PMCs)� smaller toughness unreinforced ( polymers)

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3.1.8.2 TOUGHNESS

Toughness, Ut

Engineering Strain, e = ∆L/Lo)

Engin

eer

ing S

tres

s, S

=P/A

o

X

Ut = Sdeo

e f

∫

≈(Sy + Su )

2

EL%

100

SuSy

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3.1.8.2 TOUGHNESS

� Toughness is really a measure of the energy a sample can absorb before it breaks.

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3.1.8.3 RESILIENCE

� A measure of the ability of a material to absorb energy without plastic or permanent deformation. (J/m3 or N. m/m3= MPa)

X

Resilience, Ur

Engineering Strain, e = ∆L/Lo)

Engin

eering S

tres

s, S

=P/A

o

Ur = Sdeo

ey

∫

≈Sy ey

2

=Sy

2

2E

SuSy

E

ey

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3.1.8.4 DUCTILITY (% EL)

� Ductility is another important mechanical property.

� It is a measure of the degree of plastic deformation that has been sustained at fracture.

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3.1.8.4 DUCTILITY (% EL)

Stress-Strain diagrams for typical (a) brittle and (b) ductile

materials

Stress- Strain Curves for Brittle

and Ductile Materials

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3.1.8.4 DUCTILITY (% EL)

Ductile Materials

Brittle Materials

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3.1.8.4 DUCTILITY (% EL)

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STRESS – STRAIN CURVES

Stress- Strain Curves for Different Materials

CURVE EXAMPLEA. Stiff but Weak: CERAMICB. Stiff and Strong: CERAMICC. Stiff and Strong: METALC'. Moderately Stiff and Strong: METALD. Flexible and Moderately Strong: POLYMERE. Flexible and Weak: POLYMER

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3.1.9 FATIGUE

� If placed under too large of a stress, metals will mechanically fail, or fracture. This can also result over time from many small stresses. The most common reason (about 80%) for metal failure is fatigue.

� The most common reason (about 80%) for metal failure is fatigue.

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FATIGUE MECHANISM

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FATIGUE MECHANISM

This front brake assembly broke off under braking and severely injured the cyclist. Poor maintenance had allowed the brake bolt to loosen and allow the assembly to

"chatter" when braking imposing cyclic loads instead of steady stress on the fastening bolt.

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MECHANICAL PROPERTIES

Typical Mechanical Properties

Material Yield Stress(MPa)

UltimateStress (MPa)

DuctilityEL%

Elastic Modulus(MPa)

Poisson’sRatio

1040 Steel 350 520 30 207000 0.30

1080 Steel 380 615 25 207000 0.302024 Al Alloy 100 200 18 72000 0.33

316 Stainless Steel 210 550 60 195000 0.30

70/30 Brass 75 300 70 110000 0.35

6-4 Ti Alloy 942 1000 14 107000 0.36AZ80 Mg Alloy 285 340 11 45000 0.29

Metals in annealed (soft) condition