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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
3. Mechanical Properties of Materials
1. Stress-Strain Relationships
2. Hardness
3. Effect of Temperature on Properties
4. Fluid Properties
5. Viscoelastic Behavior of Polymers
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ME 351
Manufacturing Technology I
Lecture Objective
Mechanical Properties of Materials
Stress Strain Relat ion ship s
Tensile Properties
Compressive Properties
Shear Properties
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Mechanical Properties in Design and
Manufacturing
Mechanical properties determine a material’s behavior when subjected to mechanical stresses
Properties include elastic modulus, ductility,
hardness, and various measures of strength
Dilemma: mechanical properties desirable to the
designer, such as high strength, usually make
manufacturing more difficult
The manufacturing engineer should appreciatethe design viewpoint
And the designer should be aware of the
manufacturing viewpoint
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
3. Mechanical Properties of Materials
1. Stress-Strain Relationships
2. Hardness
3. Effect of Temperature on Properties
4. Fluid Properties
5. Viscoelastic Behavior of Polymers
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Stress-Strain Relationships
Three types of static stresses to which materialscan be subjected:
1. Tensile - tend to stretch the material
2. Compressive - tend to squeeze it
3. Shear - tend to cause adjacent portions of
material to slide against each other
Stress-strain curve - basic relationship thatdescribes mechanical properties for all three
types
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Typical Engineering Stress-Strain Plot
Figure 3.3 Typical engineering stress-strain plot in a
tensile test of a metal.
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Tensile Test
Most common test forstudying stress-strain
relationship, especially
metals
In the test, a force pullsthe material, elongating
it and reducing its
diameter
Figure 3.1 Tensile test: (a)
tensile force applied in (1)
and (2) resulting elongation
of material
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Tensile Test Specimen
ASTM (AmericanSociety for Testing
and Materials)
specifies
preparation of testspecimen
Figure 3.1 Tensile test:
(b) typical test specimen
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Tensile Test Setup
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Tensile Test Sequence
Figure 3.2 Typical progress of a tensile test: (1) beginning oftest, no load; (2) uniform elongation and reduction of
cross-sectional area; (3) continued elongation, maximum
load reached; (4) necking begins, load begins to decrease;
and (5) fracture. If pieces are put back together as in (6),
final length can be measured.
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Engineering Strain
Defined at any point in the test as
where e = engineering strain; L = length at any
point during elongation; and L o = original gage
length
o
o
L
LLe
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Engineering Stress
Defined as force divided by original area:
oe
A
F
where e = engineering stress, F = applied
force, and A o = original area of test specimen
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Two Regions of Stress-Strain Curve
The two regions indicate two distinct forms ofbehavior:
1. Elastic region – prior to yielding of the
material
2. Plastic region – after yielding of the material
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Elastic Region in Stress-Strain Curve
Relationship between stress and strain is linear
Material returns to its original length when stress
is removed
Hooke's Law: e = E e
where E = modulus of elasticity
E is a measure of the inherent stiffness of amaterial
Its value differs for different materials
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Yield Point in Stress-Strain Curve
As stress increases, a point in the linearrelationship is finally reached when the
material begins to yield
Yield point Y can be identified by the
change in slope at the upper end of the
linear region
Y = a strength property
Other names for yield point = yield strength,yield stress, and elastic limit
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Plastic Region in Stress-Strain Curve
Yield point marks the beginning of plasticdeformation
The stress-strain relationship is no longer
guided by Hooke's Law
As load is increased beyond Y , elongation
proceeds at a much faster rate than before,
causing the slope of the curve to change
dramatically
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Tensile Strength in Stress-Strain Curve
Elongation is accompanied by a uniformreduction in cross-sectional area, consistent
with maintaining constant volume
Finally, the applied load F reaches a maximum
value, and engineering stress at this point is
called the tensile strength TS (a.k.a. ultimate
tensile strength)
TS =o A
F max
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Ductility in Tensile Test
Ability of a material to plastically strain withoutfracture
Ductility measure = elongation EL
where EL = elongation; L f = specimen length atfracture; and L o = original specimen length
L f is measured as the distance between gagemarks after two pieces of specimen are put backtogether
o
of
LLLEL
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
True Stress
Stress value obtained by dividing theinstantaneous area into applied load
where = true stress; F = force; and A = actual
(instantaneous) area resisting the load
A
F
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
True Strain
Provides a more realistic assessment of"instantaneous" elongation per unit length
o
L
L L
L
L
dL
o
ln
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
True Stress-Strain Curve
Figure 3.4 - True stress-strain curve for the previousengineering stress-strain plot in Figure 3.3
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Strain Hardening in Stress-Strain Curve
Note that true stress increases continuously inthe plastic region until necking
In the engineering stress-strain curve, the
significance of this was lost because stress
was based on an incorrect area value
It means that the metal is becoming stronger
as strain increases
This is the property called strain hardening
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Flow Curve
Because it is a straight line in a log-log plot, therelationship between true stress and true strain
in the plastic region is
where K = strength coefficient; and n = strain
hardening exponent
nK
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
True Stress-Strain in Log-Log Plot
Figure 3.5 True stress-strain curve plotted on log-logscale.
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Categories of Stress-Strain Relationship
Perfectly elastic
Elastic and perfectly plastic
Elastic and strain hardening
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Perfectly Elastic
Behavior is definedcompletely by
modulus of
elasticity E
Fractures ratherthan yielding to
plastic flow
Brittle materials:
ceramics, manycast irons, and
thermosetting
polymers
Figure 3.6 Three categoriesof stress-strain relationship:
(a) perfectly elastic.
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Elastic and Perfectly Plastic
Stiffness defined by E
Once Y reached,
deforms plastically at
same stress level
Flow curve: K = Y, n =
0
Metals behave like this
when heated tosufficiently high
temperatures (above
recrystallization)
Figure 3.6 Three categories
of stress-strain relationship:
(b) elastic and perfectlyplastic.
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Elastic and Strain Hardening
Hooke's Law in elasticregion, yields at Y
Flow curve: K > Y , n > 0
Most ductile metalsbehave this way when
cold worked
Figure 3.6 Three categories
of stress-strain relationship:(c) elastic and strain
hardening.
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Compression Test
Applies a load thatsqueezes the ends of a
cylindrical specimen
between two platens
Figure 3.7 Compression test:
(a) compression force applied
to test piece in (1) and (2)
resulting change in height.
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Compression Test Setup
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Engineering Stress in Compression
As the specimen is compressed, its height isreduced and cross-sectional area is
increased
e = -
where A o = original area of the specimen
o A
F
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Engineering Strain in Compression
Engineering strain is defined
Since height is reduced during compression, value
of e is negative (the negative sign is usually
ignored when expressing compression strain)
o
o
h
hhe
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Stress-Strain Curve in Compression
Shape of plasticregion is different
from tensile test
because cross
section increases
Calculated value of
engineering stress is
higher
Figure 3.8 Typical
engineering stress-strain
curve for a compression
test.
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Barreling effect in a compression test
Barreling effect in a
compression test:
(1) start of test
(2) after considerable
compression has
occurred
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Testing of Brittle Materials
Hard brittle materials (e.g., ceramics) possesselasticity but little or no plasticity
Often tested by a bending test (also called
flexure test )
Specimen of rectangular cross-section is
positioned between two supports, and a load
is applied at its center
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Bending Test
Figure 3.10 Bending of a rectangular cross-section results inboth tensile and compressive stresses in the material: (1)
initial loading; (2) highly stressed and strained specimen;
and (3) bent part
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Testing of Brittle Materials
Brittle materials do not flex
They deform elastically until fracture
Failure occurs because tensile strength of
outer fibers of specimen are exceeded
Failure type: cleavage - common with
ceramics and metals at low temperatures,
in which separation rather than slip occurs
along certain crystallographic planes
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Transverse Rupture Strength
The strength value derived from the bending test:
2
51
bt
FLTRS
.
where TRS = transverse rupture strength; F =
applied load at fracture; L = length of specimen
between supports; and b and t are dimensions of
cross-section
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Shear Properties
Application of stresses in opposite directions oneither side of a thin element
Figure 3.11 Shear (a) stress and (b) strain.
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Shear Stress and Strain
Shear stress defined as
where F = applied force; and A = area overwhich deflection occurs.
Shear strain defined as
where = deflection element; and b = distance
over which deflection occurs
A
F
b
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Torsion Test
Figure 3.12 Torsion test setup
t R
T 22
L
R
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Torsion Stress-Strain Curve
Figure 3.13 Typical shear stress-strain curve from a torsion test
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Shear Elastic Stress-Strain Relationship
In the elastic region, the relationship is defined as
G
where G = shear modulus, or shear modulus ofelasticity
For most materials, G 0.4E , where E = elastic
modulus
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Shear Plastic Stress-Strain Relationship
Relationship similar to flow curve for a tensiletest
Shear stress at fracture = shear strength S
Shear strength can be estimated fromtensile strength: S 0.7(TS)
Since cross-sectional area of test specimen
in torsion test does not change as in tensile
and compression, engineering stress-straincurve for shear true stress-strain curve
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That’s All for Today
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
ME 351
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ME 351
Manufacturing Technology I
Lecture Objective
Mechanical Properties of Materials
Hardness
Tensile Properties
Compressive Properties
Shear Properties
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
3. Mechanical Properties of Materials
1. Stress-Strain Relationships
2. Hardness
3. Effect of Temperature on Properties
4. Fluid Properties
5. Viscoelastic Behavior of Polymers
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Hardness
Resistance to permanent indentation
Good hardness generally means material is
resistant to scratching and wear
Most tooling used in manufacturing must behard for scratch and wear resistance
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Hardness Tests
Commonly used for assessing materialproperties because they are quick and
convenient
Variety of testing methods are appropriate due
to differences in hardness among differentmaterials
Most well-known hardness tests are Brinell and
Rockwell
Other test methods are also available, such as
Vickers, Knoop, Scleroscope, and durometer
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Widely used for
testing metals and
nonmetals of low
to medium
hardness
A hard ball is
pressed into
specimen surface
with a load of 500,
1500, or 3000 kg
Figure 3.14 Hardness testing methods: (a) Brinell
Brinell Hardness Test
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Brinell Hardness Number
Load divided into indentation area = Brinell
Hardness Number (BHN)
)(
22
2
i bbb DDDD
F HB
where HB = Brinell Hardness Number (BHN),
F = indentation load, kg; D b = diameter of ball,
mm, and D i = diameter of indentation, mm
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Rockwell Hardness Test
Another widely used test
A cone shaped indenter is pressed into
specimen using a minor load of 10 kg, thus
seating indenter in material
Then, a major load of 150 kg is applied,
causing indenter to penetrate beyond its
initial position
Additional penetration distance d is convertedinto a Rockwell hardness reading by the
testing machine
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Rockwell Hardness Test
Figure 3.14 Hardness testing methods: (b) Rockwell:
(1) initial minor load and (2) major load.
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Common Rockwell Hardness Scales
Rockwellscale
Hardnesssymbol
Indenter
Load(kg)
Typicalmaterials tested
A HRA Cone 60 Carbides,
ceramics
B HRB 1.6 mm
ball
100 Nonferrous
metals
C
HRC
Cone
150
Ferrous metals,
tool steels
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Vickers Hardness Test
Vickers hardness test uses pyramid shapedindenter made of “Diamond”.
Can be used for all metals – thus this test
has widest scale among hardness tests
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Vickers Hardness Test
where F = applied load, kg, and D = the
diagonal of impression made by indenter, mm
2
854.1
D
F HV
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Knoop Hardness Test
Uses pyramid shaped
diamond indenter, likeVickers test uses.
Length to width ratio is
about 7:1
Applied loads are
generally lighter than in
Vickers test
Suitable for measuringsmall and thin specimen or
hard materials that might
fracture under pointed
heavy loads
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Knoop Hardness Test
where F = load, kg, and D = the long diagonal
of impression, mm
2
2.14
D
F HK
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
3. Mechanical Properties of Materials
1. Stress-Strain Relationships
2. Hardness
3. Effect of Temperature on Properties
4. Fluid Properties
5. Viscoelastic Behavior of Polymers
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufactur ing 3/e
Effect of Temperature on Properties
Temperature has significant effect on nearly allproperties of a material.
Designer and manufacturer should be aware
of it
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Effect of Temperature on Properties
Figure 3.15 General effect of temperature on strength and ductility
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Hot Hardness
Ability of a materialto retain hardness
at elevated
temperatures
Figure 3.16 Hot
hardness - typicalhardness as a function
of temperature for
several materials.
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Recrystallization in Metals
Most metals strain harden at room temperatureaccording to the flow curve (n > 0)
But if heated to sufficiently high temperature
and deformed, strain hardening does not occur
Instead, new grains are formed that are free
of strain
The metal behaves as a perfectly plastic
material; that is, n = 0
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Recrystallization Temperature
Formation of new strain-free grains is calledrecrystallization
Recrystallization temperature of a given metal
= about one-half its melting point (0.5 T m) as
measured on an absolute temperature scale
Recrystallization takes time - the
recrystallization temperature is specified as the
temperature at which new grains are formed in
about one hour
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Recrystallization and Manufacturing
Recrystallization can be exploited inmanufacturing
Heating a metal to its recrystallization
temperature prior to deformation allows a
greater amount of straining, and lower forcesand power are required to perform the
process
Forming metals at temperatures above
recrystallization temperature is called hotworking
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3. Mechanical Properties of Materials
1. Stress-Strain Relationships
2. Hardness
3. Effect of Temperature on Properties
4. Fluid Properties
5. Viscoelastic Behavior of Polymers
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Fluid Properties and Manufacturing
Fluids flow - They take the shape of the
container that holds them
Many manufacturing processes areaccomplished on materials converted from solidto liquid by heating
Called solidification processes
Examples:
Metals are cast in molten state
Glass is formed in a heated and fluid state
Polymers are almost always shaped as fluids
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Viscosity in Fluids
Viscosity is the resistance to flow that ischaracteristic of a given fluid
Flow is a defining characteristic of fluids, but the
tendency to flow varies for different fluids
Viscosity is a measure of the internal friction
when velocity gradients are present in the fluid
The more viscous the fluid, the higher the
internal friction and the greater the resistanceto flow
Reciprocal of viscosity is fluidity - the ease
with which a fluid flows
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Viscosity
Viscosity can be defined using two parallel platesseparated by a distance d and a fluid fills thespace between the two plates
Figure 3.17 Fluid flow between two parallel plates, onestationary and the other moving at velocity v
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Shear Rate
Shear stress is related to shear rate, defined asthe change in velocity dv relative to dy
where = shear rate, 1/s; dv = change in velocity,
m/s; and dy = change in distance y , m
Shear rate = velocity gradient perpendicular to flowdirection
dy
dv
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Shear Viscosity
Shear viscosity is the fluid property that defines therelationship between F/A and dv /dy ; that is,
or
where = a constant of proportionality called
the coefficient of viscosity , Pa-s For Newtonian fluids, viscosity is a constant
For non-Newtonian fluids, it is not
dy
dv
A
F
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Coefficient of Viscosity
Rearranging, coefficient of viscosity can beexpressed:
Viscosity of a fluid is the ratio of shear stressto shear rate during flow
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Viscosity of Polymers and Flow Rate
Viscosity of a thermoplastic polymer melt is notconstant
It is affected by flow rate
Its behavior is non-Newtonian
A fluid that exhibits this decreasing viscosity
with increasing shear rate is called
pseudoplastic
This complicates analysis of polymer shapingprocesses such as injection molding
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Newtonian versus Pseudoplastic Fluids
Figure 3.18 Viscous behaviors of Newtonian and pseudoplastic
fluids. Polymer melts exhibit pseudoplastic behavior. For
comparison, the behavior of a plastic solid material is shown
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Viscoelastic Behavior
Material property that determines the strain thatthe material experiences when subjected to
combinations of stress and temperature over
time
Combination of viscosity and elasticity
In a viscoelastic solid, the relationship between
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stress and strain is time dependent :
where f(t) can be conceptualized as a
modulus of elasticity that depends on time
)()( t f t
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Elastic versus Viscoelastic Behavior
Figure 3.19 (a) perfectly elastic response of material to stress applied overtime; and (b) response of a viscoelastic material under same conditions.The material in (b) takes a strain that is a function of time andtemperature
Viscoelastic modulus as a function of temperature
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p
for a thermoplastic polymer
Viscoelastic Behavior of Polymers:
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Shape Memory
A problem in extrusion of polymers is die swell ,
in which the profile of extruded material grows in
size, reflecting its tendency to return to its
previously larger cross section in the extruder
barrel immediately before being squeezed
through the smaller die opening
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That’s All for Today