Em . 06 . Tensile Properties of Materials

8/3/2019 Em . 06 . Tensile Properties of Materials

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P. C. SEE 2009 FKPPT UMP

BFM1113 ENGINEERING

MATERIALS

Faculty of Manufacturing Engineering and

Technology Management


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Lecture 06Tensile Properties of MaterialsMany materials, when in service, are subjected toforces or loads; examples include the aluminum alloy

from which an airplane wing is constructed and thesteel in an automobile axle. In such situations it isnecessary to know the characteristics of the materialand to design the member from which it is made sothat any resulting deformation will not be excessiveand fracture will not occur.

The mechanical behavior of material reflects therelationship between its response or deformation to anapplied load or force. Important mechanical propertiesare strength, hardness, ductility and stiffness.

The mechanical properties of materials are ascertainedby performing carefully designed laboratoryexperiments that replicate as nearly as possible theservice conditions.

Callister and Rethwisch (2008)


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Where are we now??

The structure ofmetals

The properties ofmaterials

Metal alloys

Polymer materialsCeramic materialsCompositematerials


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Learning Outcome (LO6)Tension

Compression

Torsion

Bending

Hardness

Fatigue

Creep

Impact

Failure and Fracture of Materials InManufacturing and Service

Residual Stresses

Work, Heat and Temperature


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Learning Outcome (LO6)At the end of this session you should be able to

Understand the purpose of learning the mechanical properties ofof materials

Understand the tensile properties of materials

Construct and analyze the stress-strain curves

Understand the effect of external parameters on the tensilebehaviour of materials

Adapted from Wikipedia


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LO6 - Part 1

Understand the purpose of learning themechanical properties of of materials


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Mechanical Properties of Materials

Manufacturing operations

Parts and components are formed intovarious shapes

By applying forces to the workpiece

Through various tools and dies

Why study mechanical properties?

Design and development

a.) Determine stresses and stressdistribution within members that are

subjected to well defined loads Materials characterization

a.) To predict materials performancethrough stress analysis

b.) To understand mechanism of fractures

and ways to prevent it

Parts and components are formed into various

shapes in manufacturing operations


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In a finite element analysis, the real structure is represented by a finite number of interconnected

elements. The behaviour of the finite elements under an applied load represents the overall

behaviour of the real structure. See http://bit.ly/18USR9 for more information.


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Temperature

CTE Substrate >CTE PCB > CTE Si Die

High

Temperature

Time


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Warpage contours of FC-PBGA package documented at (a) 150C, (b) 100C and (c)

room temperature, where the contour interval is 5.3 mm per fringe order. A 3-D

warpage map at room temperature obtained by digital image processing is shown in(d). See http://bit.ly/z6Fw0 for more information.


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Mechanical Properties of Materialsx, y = (0, 0)

Strain distribution

in critical solder ball

Von Mises stressdistribution

in critical solder ball

Location of critical solder ball


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An oil tanker that fractured in a brittle manner by crack propagation around its girth


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LO6 - Part 2

Understand the tensile properties ofmaterials


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Tensile Properties of Materials

Tensile test determines thefollowing mechanical propertiesof materials

Strength

Ductilit

Toughness Elastic modulus, and

Strain hardening ability

Instron 5560 Universal Materials Tensile

Testing Machine (see http://bit.ly/e7VD7 for

more information)


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A typical stress-strain curve obtained from a tension test, showing various features


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Proportional limit

The point up to which the stress and strain arelinearly related

Ultimate stress

The largest stress in the stress strain curve

Rupture stressThe stress at the point of rupture

Elastic region

The region of the stress-strain curve in which thematerial returns to the undeformed state when

applied forces are removed

A closer view on tensile test using Instron

http://bit.ly/18UibN


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Plastic region

The region in which the material deforms

permanently

Yield point

The point demarcating the elastic from thelastic re ion

Yield stress

The stress at yield point

Plastic strain

The permanent strain when stresses are zero

Off-set yield stress

Stresses that would produce a plastic straincorresponding to the specified off-set strain

Tensile test of an Al-Mg-Si alloy. This is a ductile

fracture type, as seen by the local necking and the

cup and cone fracture surfaces


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Tensile Properties of MaterialsDuctile material

A material that can undergo large plastic

deformation before fracture

Brittle material

A material that exhibits little or no plasticdeformation at failure

Picture showing the failure of brittle material.

See http://bit.ly/3AoWon for more

information

Hardness

Resistance to indentation

Strain hardening

The raising of the yield point with increasing

strain (see beyond proportional limit)

Necking

The sudden decrease in the area of cross-sectionafter ultimate stress


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(a) A standard tensile-test specimen before and after pulling, showing original and final gage lengths. (b) Atensile-test sequence showing different stages in the elongation of the specimen


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Typical engineering stress-strain behavior to fracture point F. The tensile strength TS is indicated at point M.

The circular insets represent the geometry of the deformed specimen at various points along the curve.


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Engineering stress

Specimen elongates when theload is first applied

Known as linear elastic

Engineering stress (or nominalstress) is defined as the ratio ofthe applied load, P, to theoriginal cross-sectional areaAo,of the specimen

Engineering stress,

0A

P=

Simulation of a Tensile Test With Necking Localization.

See http://bit.ly/MCGPm for more information


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Engineering strain

Also known as nominal strain

Tensile strain calculated bytaking into account the linear

-

sample.

Engineering strain,

where l is the instantaneouslength of the specimen

Picture showing a sample before and after tensile

test. See http://bit.ly/93FEI for more information

0

0 )(

l

lle

=


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Yield strength

As the load is increased --> plasticdeformation

Characterized by yield stress, y

Yield point --> strain offset of 0.002,or 0.2% elongation

Area decrease permanently anduniformly

During unloading, the curve followsa path parallel to the original elasticslope

Schematic illustration of the loading and unloading of

a tensile-test specimen. Note that, during unloading,

the curve follows a path parallel to the original elastic

slope.


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Ultimate tensile strength

Max. engineering stress --> tensilestrength/Ultimate Tensile Strength(UTS)

Loaded beyond its ultimate tensilestrength --> begins to neck (neckdown)

Engineering stress drops further, finally--> fracture at necked region

Engineering stress at fracture -->breaking/fracture stress

A typical stress-strain curve obtained from a

tension test, showing various features


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Tensile Properties of MaterialsModulus of Elasticity, E

Ratio of stress to strain in elastic region

Also known as Youngs Modulus

Modulus of elasticit

=

Measure the slope of the elastic portion--> stiffness of material

e

A typical stress-strain curve obtained from a

tension test, showing various features


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Tensile Properties of MaterialsThe Poissons effect

A positive (tensile) strain contributes anegative (compressive) strain in the otherdirection

This is called Poisson effect

Poissons ratio,

The Poissons effect

allongitudin

lateral

=


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Tensile Properties of MaterialsTrue stress and true strain

Engineering stress --> based onoriginal cross sectional areaAo ofthe specimen

True stress --> ratio of the load, P,

to the actual (instantaneous)cross-sectional area,A, of thespecimen

True stress,

True strain is calculated as

True strain,A comparison of typical tensile engineering stress-strain and true

stress-strain behaviours. Necking begins at point M on the

engineering curve, which corresponds to M on the true curve.

The corrected true stress-strain curve takes into account the

complex stress state within the neck region.

A

P=

0

lnl

le =


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Tensile Properties of MaterialsToughness

Resistance to fracture of a materialwhen stressed

Area under the true stress-strain curve

--> energy per volume dissipate bymater a ur ng e ormat on

Also known as the specific energy

Total area up to fracture --> toughness

Depends on the height and width of

the curve, while on the other hand,

Strength depends on height

Ductility depends on width Toughness of material is equal to the area under thestress-strain curve up to fracture


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LO6 - Part 3

Determine the ductility of materials


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Ductility of MaterialsDefinition and concepts

The extend to which materials canbe plastically deformed withoutfracture

Also --> materials abilit todeform under tensile stress

For deformation undercompressive stress --> malleability

Important in metalworking

Schematic appearance of round metal bars after tensile

testing. (a) Brittle fracture. (b) Ductile fracture. (c) Completely

ductile fracture. See http://bit.ly/16zyEg for more

information.


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Ductility of MaterialsDuctility measurement

Two common measurement

Total elongation

0llf ,

lo and lfare original and final(fracture) length measured in test

Reduction of area

Reduction of area,

whereA0 andAf are the originaland final (fracture) cross-sectional

area

Tensile test of a nodular cast iron with very

low ductility (http://bit.ly/16zyEg).

0l

=

1000

xA

AARA

f

f

=


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Ductility of Materials

Approximate relationship between elongation and tensile reduction of area for

various groups of metals


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Ductility of MaterialsBrittle materials

No yield point and no strain hardening

Ultimate strength same with breakingstrength

Brittle materials do not show plastic

deformation but fail within elasticregion (linear stress-strain curve)

Characteristic --> broken parts can bereassembled as original shape (no

necking is observed)

Stress Strain Curve for Brittle materials. Point 1 indicates

the ultimate strength and point 2 indicates the yield

strength. See http://bit.ly/pEbS2 for more information.


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LO6 Part 4

Construct and analyze the stress-strain curves


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Stress-Strain CurveProcedure

Divide load data byA0, and theelongation by lo

Calculate data for true stress-straincurve in lastic re ion usin thefollowing equation

True stress,

K= strength coefficient

n = strain-hardening coefficient

(a) Load-elongation curve in tension testing of a

stainless steel specimen. (b) Engineering stress-strain

curve. (c) True stress-strain curve. (d) True stress-strain

curve based on the corrected curve in (c) plotted on alog-log paper.

nK =


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Stress-Strain Curve


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Stress-Strain Curve

True stress-strain curves in tension at room temperature for various metals. The curves start at a

finite level of stress: The slope associated to the elastic regions are too steep to be shown in

this figure, thus each curves starts at the yield stress of the material.


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Stress-Strain CurveStrain at necking in tension test

Necking onset corresponds toultimate strength of material

Specimen cannot support the loadanymore

Cross-sectional area reduction ratehigher than the strain hardening rate

True strain at the onset of neckingequals to strain hardeningcoefficient, n

Higher n --> longer uniform strainbefore necking (recall strainhardening)

Stress vs. Strain curve typical of structural

steel. 1.) Ultimate Strength. 2.) Yield Strength.

3.) Rupture. 4.) Strain hardening region.

5.) Necking region. Point A: Engineering

stress. Point B: True stress


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LO6 Part 5

Understand the effect of externalparameters on the tensile behaviour of

materials


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Temperature Effects on Tensile Behavior

Higher temperature generally

Raises the ductility and toughness Lowers the yield stress and the

modulus of elasticity

In most metals, the strain-

hardening exponent, n decreaseswith increasing temperature

Typical effects of temperature on stress-strain curves. Note

that temperature affects the modulus of elasticity, the yield

stress, the ultimate tensile strength, and the toughness of

materials.


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Rate of Deformation Effects on Tensile Behavior

Deformation rate --> the speed atwhich tension test is being carried

out

A function of specimen length

Increasing the strain rate increasesthe strength of material (strain-

rate hardening)

Slope of graph (see figure) isknown as strain-rate sensitivityexponent, m

Stress,

C is known as strength coefficient The effect of strain rate on the ultimate tensile strength foraluminium. Note that, as the temperature increases, the

slopes of the curves increase; thus, strength becomes more

and more sensitive to strain rate as temperature increases.

mC=


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Materials stretch further at higher mvalue --> delays necking

Higher strain rate increases strength,hence reduces necking and allowsfurther deformation

Su er lasticit --> hi h ductilitcaused by high strain-rate sensitivity at

higher temperature

Observation at higher temperature

Higher strain-rate sensitivity

Lower strength compared to 300C

The effect of strain rate on the ultimate tensile strength for

aluminium. Note that, as the temperature increases, the

slopes of the curves increase; thus, strength becomes more

and more sensitive to strain rate as temperature increases.


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Superplasticity

Ability to undergo large, uniform

elongation prior to necking and fracture

Elongation: range from 100% to 2000%

Cause: hi h strain rate sensitivit atincreased temperature

A balance between dislocationmultiplication and annihilation

Dislocation density does not increaseduring deformation

Used to manufacture complex structuralcomponents Superplasticity is the ability of certain materials to

undergo extreme elongation at the proper

temperature and at a controlled strain rate. Under the

certain conditions these materials can be stretched to

several times their original length.


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Hydrostatic Pressure Effects on Tensile Behavior

Effect of hydrostatic pressure

Increases the strain at fracture andductility

Happens in both ductile andbrittle materials

Explanation --> suppression ofmicro-void development

Used in manufacturing process i.e.,hydrostatic extrusion

Brittle material can be extrudedbecause the hydrostatic pressureincreases its ductility

The appearance of the fractured tensile bars tested

under applied pressure


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Radiation Effects on Tensile Behavior

Important in nuclear applications

Typical changes at high energyradiation:

Increases yield stress, tensilestrength and hardness

ecreases uc y an

toughness

Plastic materials --> same effect

Nuclear power for the Astute will be provided by

the Rolls-Royce PWR 2 pressurised water reactor.


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Summary Manufacturing processes involve shaping materials by plastic

deformation.

Hence the yield strength, ultimate tensile strength, modulus ofelasticity, ductility, hardness, and the energy required for plasticdeformation are important factors.

The tensile test is the most commonly used test to determine suchmechanical properties.

Temperature, rate-of-deformation, hydrostatic pressure andradiation affects tensile behavior of materials


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Whats Next?


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an you


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ReferenceCallister, W. D., and Rethwisch, D. G. (2008) Fundamentals of Materials Science

and Engineering, John Wiley & Sons.

Kalpakjian, S., and Schmid, S. (2006) Manufacturing Engineering and

Technology, Pearson Education.


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Em . 06 . Tensile Properties of Materials

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