Mechanical Properties - Universiti Teknologi Malaysianhayati/Chapter_5_mechanical_properties.pdf ·...
Transcript of Mechanical Properties - Universiti Teknologi Malaysianhayati/Chapter_5_mechanical_properties.pdf ·...
Mechanical Properties
5
Learning Objectives
4-2
Introduce the basic concepts associated with
mechanical properties of materials.
Stress and Strain (engineering and true stress, engineering and true strain)
Stress – Strain Curve
Elastic and Plastic deformation in metals
Types of Fracture: ductile and brittle
Common mechanical properties testing: hardness and
impact testing
Technological Importance
4-3
The materials used in sports equipment must be lightweight, stiff, tough, and impact resistant. (Courtesy of Getty Images.)
Aircraft, such as the one shown here, makes use of aluminum alloys and carbon-fiber-reinforced composites
Technological Importance
4-4
Terminology for Mechanical Properties
4-5
Stress - Force or load per unit area of cross-section over
which the force or load is acting.
Strain - Elongation change in dimension per unit length.
Young’s modulus - The slope of the linear part of the
stress-strain curve in the elastic region, same as
modulus of elasticity.
Shear modulus (G) - The slope of the linear part of the
shear stress-shear strain curve.
Viscosity ( ) - Measure of resistance to flow, defined as
the ratio of shear stress to shear strain rate (units Poise
or Pa-s).
Thixotropic behavior - Materials that show shear
thinning and also an apparent viscosity that at a
constant rate of shear decreases with time.
4-6
(a) Tensile, compressive, shear and bending stresses. (b) Illustration showing how Young’s modulus is defined for elastic material. (c) For nonlinear materials, we use the slope of a tangent as a variable quantity that replaces the Young’s modulus constant
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Stress – Strain
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Engineering strain = oo
oi
l
l
l
ll
Ao : original cross sectional area li : instantaneous length lo : original length Strain has no unit
Engineering stress = oA
F
Tension and Compression The above definitions allow us to compare
test results for specimens of different cross
Sectional areas
Stress - Strain
4-8
Pure shear stress =
oA
F
Pure shear strain = tan
Shear
Stre
ss –
Stra
in T
est (T
ensile
Test)
4-9
(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein
under license.
4-1
0
(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Te
nsile
stre
ss-s
train
cu
rve
s fo
r diffe
ren
t ma
teria
ls
4-11
Stress – Strain Test (Tensile Test)
4-12
(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
The stress-strain curve for an aluminum alloy from Table 6-1
Properties Obtained from Tensile Test
4-13
Elastic limit
Tensile strength, Necking
Hooke’s law
Poisson’s ratio
Modulus of resilience (Er)
Tensile toughness
Ductility
Elastic Deformation
4-14
Elastic Deformation is a non-permanent deformation where the material completely recovers to its original state upon release of the applied stress.
Hooke’s Law: = E
stress
Modulus of elasticity (Young’s modulus)
strain
E = slope in the linear region
For metals, typically E ~ 45 – 400 GPa
Measure of material’s resistance to elastic deformation (stiffness).
Elastic Deformation
4-15
F
bonds
stretch
return to
initial
1. Initial 2. Small load 3. Unload
Elastic means reversible! From Callister 6e resource CD.
Plastic Deformation
4-16
1. Initial 2. Small load 3. Unload
Plastic means permanent!
F
linear elastic
linear elastic
plasticFrom Callister 6e resource CD.
Yield Strength (y)
4-17
Is the strength required to produce a very slight yet specified amount of plastic deformation.
What is the specified amount of strain?
Strain offset method
0.002
1. Start at 0.002 strain (for most metals).
2. Draw a line parallel to the linear region.
3. y = where the dotted line crosses the
stress-strain curve. y
Elastic region
P P = proportional limit (beginning of deviation
from linear behavior.
Mixed elastic-plastic behavior
For materials with nonlinear elastic region: y is
defined as stress required to produce specific
amount of strain (e.g. ~0.005 for most metals).
Yield Strength (y)
4-18
(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
(a) Determining the 0.2% offset yield strength in gray cast ion, and (b)
upper and lower yield point behavior in a low-carbon steel
Yield Strengths of Different Materials
4-19
Room T values
y(ceramics)
>>y(metals)
>> y(polymers)
Based on data in Table B4,
Callister 6e.
a = annealed
hr = hot rolled
ag = aged
cd = cold drawn
cw = cold worked
qt = quenched & tempered
From Callister 6e resource CD.
Ultimate Tensile Strength (UTS)
4-20
Stress at the maximum of stress-strain curve.
0.002
y
Elastic region
P
TS M
F P = proportional limit y = yield strength
TS = tensile strength
M = max. stress
F = fracture point
Note: For most engineering materials, strength should be specified by
yield strength (not tensile strength). Why?
Necking
4-21
y P
TS M
F
• Necking starts at max stress (A decreases) • Past max stress, decreases • The material becomes weak (true or false?)
Necking
4-22
Engineering Stress = oA
F
Original cross
sectional area
True Stress = i
TA
F
Ai = instantaneous area
Tensile Test
4-23
(a) Original and final shape of a standard tensile-test specimen. (b)
Outline of a tensile-test sequence showing stages in the elongation of
the specimen.
Ductility
4-24
• Is measure of degree of plastic deformation that has been sustained at fracture.
• Ductile materials can undergo significant plastic deformation before fracture. • Brittle materials can tolerate only very small plastic deformation.
Types of Fracture
4-25
Cup & Cone in Al Brittle in mild steel
Highly ductile
Moderately ductile
Brittle
How to Measure Ductility?
4-26
% elongation = %100
o
of
l
ll
% reduction in area = %100
o
fo
A
AA
Ao and lo are initial.
Af and lf are at fracture.
Mechanical Properties of Materials
4-27
METALS (WROUGHT) E (GPa) Y (MPa) UTS (MPa) ELONGATION(%) in 50 mm
POISSONÕSRATIO (
Aluminum and its alloysCopper and its alloysLead and its alloysMagnesium and its alloysMolybdenum and its alloysNickel and its alloysSteelsStainless steelsTitanium and its alloysTungsten and its alloys
69-79105-150
1441-45
330-360180-214190-200190-20080-130
350-400
35-55076-1100
14130-30580-2070105-1200205-1725240-480344-1380550-690
90-600140-1310
20-55240-38090-2340345-1450415-1750480-760415-1450620-760
45-565-350-921-540-3060-565-260-2025-7
0
0.31-0.340.33-0.35
0.430.29-0.35
0.320.31
0.28-0.330.28-0.300.31-0.34
0.27
NONMETALLIC MATERIALS
CeramicsDiamondGlass and porcelainRubbersThermoplasticsThermoplastics, reinforcedThermosetsBoron fiberCarbon f ibersGlass f ibers (S, E)Kevlar fibers (29, 49, 129)Spectra f ibers (900, 1000)
70-1000820-1050
70-800.01-0.11.4-3.42-50
3.5-17380
275-41573-8570-11373-100
------------
140-2600-
140-
7-8020-12035-1703500
2000-53003500-46003000-34002400-2800
0-0-
1000-510-1
00
1-25
3-43
0.2-
0.240.5
0.32-0.40-
0.34-----
Note: In the upper table the lowest values for E, Y, and UTS and the highest values for elongation are for thepure metals. Multiply GPa by 145,000 to obtain psi, and MPa by 145 to obtain psi. For example, 100 GPa =14,500 ksi, and 100 MPa = 14,500 psi.
Hardness Testing
4-28
Hardness is defined as the resistance of a material to plastic penetration of its surface. That is the ability of a material to resist indentation, abrasion and wear
There are three main types of tests used to determine hardness:
Scratch tests, dynamic and static hardness tests
For engineering purposes, only the static hardness tests are used.
Hardness Testing
4-29
Higher values of hardness means:
Resistance to plastic deformation or cracking in
compression. Better wear properties.
Hardness test is: Simple (just press indent into material’s surface) Cheap (no need to machine the specimen, no big
equipment needed)
Hardness Testing
4-30
Apply known force (kg) Measure size of indenter
Smaller indents mean higher hardness
Indentation will depend not only on the material being tested but also
indenter composition and geometry.
Types of Hardness Tests
4-31
General characteristics of hardness testing methods. The Knoop test is known as a
microhardness test because of the light load and small impressions. Source: After H. W.
Hayden, W. G. Moffatt, and V. Wulff.
Types of Hardness Tests
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Impact Testing: Toughness
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• Toughness (notch toughness) is the ability of a
material to absorb energy
1. Material Toughness (slow absorption) - Not a readily observable property - Defined by the area under the stress-strain curve 2. Impact Toughness (rapid absorption) - Ability to absorb energy of an impact without
fracturing • Toughness, ductility and brittleness are related
Impact Testing: Toughness
4-34
Impact test - Measures the ability of a material to
absorb the sudden application of a load without
breaking.
Impact energy - The energy required to fracture a
standard specimen when the load is applied suddenly.
Impact toughness - Energy absorbed by a material,
usually notched, during fracture, under the conditions of
impact test.
Fracture toughness - The resistance of a material to
failure in the presence of a flaw.
Properties Obtained from the Impact Test
4-35
Ductile to brittle transition temperature (DBTT) - The
temperature below which a material behaves in a
brittle manner in an impact test.
Notch sensitivity - Measures the effect of a notch,
scratch, or other imperfection on a material’s
properties, such as toughness or fatigue life.
Impact Testing: DBTT
4-36
The Charpy V-notch properties for a BCC
carbon steel and a FCC stainless steel. The FCC crystal structure typically leads top higher absorbed energies and no transition temperature
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DBTT
Brittle Ductile
Rough surface
Smooth surface
Impact Testing
4-37
- At low temperature, where the material is brittle and
not strong, little energy is required to fracture the material.
- At high temperature, where the material is more ductile
and stronger, greater energy is required to fracture the
material
-The transition temperature is the boundary between brittle
and ductile behavior.
The transition temperature is an extremely important
parameter in selection of construction material.
Example of DBT failure
4-38
4-39
Charpy
Test
Izod Test
Fatigue Testing
4-40
Fatigue is the lowering of strength or failure of a material due
to repetitive or cyclic stress which may be above or below
the yield strength.
Aloha Airlines Flight 243, April 1988
Eschede train crash, Germany 1988
Fatigue Testing
4-41
Time Co
mp
res
sio
n Te
ns
ion
Time
max
min
Co
mp
res
sio
n Te
ns
ion
0
Time
max
min
Co
mp
res
sio
n T
en
sio
n
0
a r
m
Irregular or Random stress cycle
Reversed stress cycle Repeated stress cycle
Fatigue Testing
4-42
Fatigue Testing
4-43
(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a
trademark used herein under license.
Beach marks
Fatigue Testing
4-44