Mechanical Properties and
Property Testing
Chapter 6
Why?
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Use mechanical properties in the design
process
Properties are affected by
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Bonding type
Crystal Structure
Imperfections
Processing
Types of Mechanical Testing
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Slow application of stress
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Tensile testing
Allows dislocations to move
Rapid application of stress
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Impact testing
Ability of a material to absorb energy as it
fails
Types of Mechanical Testing
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Fracture Toughness
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Fatigue
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How does a material respond to cracks and
flaws
What happens when loads are cycled?
High Temperature Loads
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Creep
Strength of Materials
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You will study most of these tests in
more detail in “Strengths of Materials”
Tensile Testing
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We have a United brand tensile testing
machine
Often called an Instron – which is a
brand name
Slowly pulls on a sample
The sample deforms, then fails
We measure the load and the
deformation
Tensile test specimens
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Can be either flat or
round
Used for both
plastics and metals
– but not ceramics
Engineering Stress and Strain
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F
Engineering Stress
F is the applied force
A0 is the original crosssectional area of the
sample
Units are the same as
pressure
Engineering Strain
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Δl - Change in length of
the specimen
l0 – Original length of the
specimen
Dimensionless – though
often expressed as in/in
F

A0
l

l0
Results of the Tensile Test
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E – Modulus of Elasticity
y – Yield strength
based on 0.2% offset
Modulus of Resiliance
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Area under the elastic
portion of the curve
Tensile Strength
Failure stress
Ductility
Toughness
Tensile Strength
Engineering Stress, psi
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Fracture stress
Yield Strength
Toughness
Slope of the elastic portion of
the stress-strain
curve, E=σ/ε
Modulus of Resiliance
Slope = E
E=Modulus
of
Elasticity
Ductility
(Elasticity)
Engineering Strain, in/in
After John D. Russ – Materials Science – A
Multimedia Approach
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Yield Point
Some metals have
well defined yield
points, which are
easy to identify
Typical of Copper, Most
Low Carbon Steels and
Some Aluminum Alloys
Not all materials
have a well defined
yield point
Upper Yield Point
Lower Yield
Point
Typical of Copper, Most
Low Carbon Steels and
Some Aluminum Alloys
Typical of Aluminum and
Some Low Carbon Steels
Some low carbon steels and many aluminum alloys
exhibit a drop in stress once plastic deformation
starts. This occurs because dislocations that were
immobilized at point defects or dislocation tangles
are able to move again briefly.
Use
the average
value of
of lower
the lower
Toothed
shape typical
yieldyield
points
stress as the yield point, for these materials
Cast Iron and Polymers show a
gradual change from elastic to plastic
deformation – again, there is no
obvious yield point
Typical of Ductile Cast Iron
Typical of Polymers
The 0.2% offset yield allows us to
systematically select the yield point –
even when it isn’t obvious
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Draw a line starting at
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.002 strain (0.2% strain)
Parallel to the elastic portion of the curve
With the same slope as the modulus of
elasticity
The 0.2% offset yield is the point where
the stress strain curve and the offset
yield line intersect
Yield Point
Yield Point
Typical of Copper, Most
Low Carbon Steels and
Some Aluminum Alloys
Yield Point
0.002 strain
Typical of Ductile Cast Iron
Typical of Aluminum and
some Low Carbon Steels
Yield Point
0.002 strain
Typical of Polymers
Ductile Cast Iron
70000
60000
50000
40000
500
Stress N/mm2
Strain, psi
Copper Alloy
30000
20000
10000
0
400
300
200
100
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0
0.05
Strain in/in
0.15
0.2
Strain, mm/mm
Magnesium
PVC
300
14000
12000
10000
8000
Stress N/mm^2
Stress, psi
0.1
6000
4000
2000
0
250
200
150
100
50
0
0
0.02
0.04
Strain, in/in
0.06
0.08
0
0.02
0.04
0.06
Strain, mm/mm
0.08
0.1
Tensile Test
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Be sure to watch the movies on the CD
accompanying your textbook
The Stress-Strain Diagram Card is
especially good!!
Deformation happens when the atoms
slip.
Slip occurs on the close packed plane,
in the close packed direction.
Tensile Test
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Remember, we are not testing samples
that are a single crystal – there are
many grains, all aligned in different
directions.
However, within a single crystal
deformation occurs similarly to the
models shown on the next several slides
Force
Elastic
Deformation
Once the force
is released,
the material
returns to its
original shape
Force
In plastic
deformation
enough force
is applied that
slip eventually
occurs
Plastic
Deformation
Necking
True Stress/strain
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True stress
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A=actual cross
sectional area
True strain
F
t 
A
l 
 t  ln  
 l0 
Notice that true stress continues to increase with
increasing true strain
Ceramics
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Failure in ceramics usually occurs
because of flaws
A tensile test would give a wide range
of values, depending on the flaws in the
sample you chose
Three Point Compression or Bend Test
3 Point Bend Test
Typical graph showing 3 point bend
strength of a plastic material
http://www.lloyd-instruments.co.uk/testtypes/graphs/3point.jpg
Hardness Test
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Brinell
Rockwell
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our machine
(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used
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Hardness is a surface property
There are a number of processes that
can improve the surface hardness,
without affecting the bulk properties
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For example carburizing
Impact Test
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Charpy – our machine
Izod
Effect of Temperature
Effect of Structure
(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license.
Charpy Impact Test
Transition Temperature
(
c
)
2
0
0
3
B
r
o
o
k
s
/
C
o
l
e
,
a
d
i
Energy is not
absorbed,
causing failure
v
i
s
90
80
70
60
50
40
30
20
10
0
Energy is
readily
absorbed
East
West
North
1st
Qtr
3rd
Qtr
Affect of Crystal Structure
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FCC crystals absorb
more energy
Often they do not
exhibit a transition
temperature
(
c
)
2
0
0
3
B
r
o
o
k
s
/
C
o
l
e
,
a
d
i
Why?
v
i
s
90
80
70
60
50
40
30
20
10
0
East
West
North
1st
Qtr
3rd
Qtr
Slip is easy in FCC cells
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Even at low temperature slip is
comparatively easy in FCC cells
At low temperatures the slip systems in
BCC cells become inactive, making it
harder to absorb energy
Liberty Ships
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Liberty ships were cargo
ships built during WW II
They were mass produced
on an emergency basis
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2751 vessels built from
1941 to 1945
Largest number of ships
every produced to a single
design
Design life of 5 years
Average construction time
was 42 days
The Robert E Peary was
built in under 5 days
SS John W. Brown, one of two
surviving operational Liberty ships
http://en.wikipedia.org/wiki/File:SS_John_W_Brown.jpg
Rosy the Riveter
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The hulls were
welded instead of
riveted – a new
innovation
Riveted hulls were
labor intensive to
build – and there
was a lack of skilled
labor
J. Howard Miller's "We Can Do It!",
commonly mistaken to be Rosie the
Riveter
Brittle Fractures
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Early on 30% of the
ships suffered from
brittle fractures
Improved in later
designs to 5%
3 of the Liberty
ships broke in half!
SS John P. Gaines – which sank in the Aleutions – shown
is the back half of the ship – 10 lives were lost
http://www.mech.uwa.edu.au/DANotes/fracture/maritime/maritime.html
T2 Tanker
• Brittle fractures
were a problem
with other
designs as well
 Nearly 500 T2
tankers were
built during
WWII – with the
same problems
SS Schenectady
http://www.mech.uwa.edu.au/DANotes/fracture/maritime/mariti
me.html
In January 1943 the one-day old T2 tanker SS
Schenectady had just completed successful sea trials
and returned to harbour in calm cool weather when .
..
"Without warning and with a report which was heard
for at least a mile, the deck and sides of the vessel
fractured just aft of the bridge superstructure. The
fracture extended almost instantaneously to the turn
of the bilge port and starboard. The deck side shell,
longitudinal bulkhead and bottom girders fractured.
Only the bottom plating held. The vessel jack- knifed
and the center portion rose so that no water entered.
The bow and stern settled into the silt of the river
bottom."
The ship was successfully repaired.
The failures were attributed to:
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The all- welded construction which eliminated
crack- arresting plate boundaries which are
present in riveted joints
The presence of crack- like flaws in welded
joints performed by inexperienced operators
pressed into service by the exigencies of the
programme
The use of materials whose low resistance to
crack advance ( toughness ) was further
reduced by low temperatures.
http://www.mech.uwa.edu.au/DANotes/fracture/maritime/maritime.html
Constance Tipper
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Showed that the failures were
not due to faulty welds but to
the grade of steel, which when
subjected to low temperatures
became brittle
These ships were used in the
North Atlantic which was cold
They were regularly below the
ductile to brittle transition
temperature
Because the ship was welded
instead of riveted the cracks
could travel long distances
Welded hatch covers were a
common source of crack
initiation
http://www-g.eng.cam.ac.uk/125/1925-1950/tipper.html
Fracture Mechanics
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Stress Concentrators
Ductile materials are less subject to
failure from cracks or flaws
Fatigue
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Airplane wings vibrate
Cracks or flaws can grow
Eventually failure occurs
Endurance Limit
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Stress below which there is a 50%
probability that failure by fatigue will never
occur
Rotating Cantilever Beam Test
(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license.
Endurance Limit
(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license.
Aloha Airlines Flight 243
Creep
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Deformation over time
Silly Putty
(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license.
Importance of Mechanical
Testing
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If you weren’t all going to take
Strengths of Materials, this would be
the most important chapter in our
book!!
In order to use handbook data or
testing results effectively, you need to
understand the test limitations
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Consider adding information on SEM,
STM and AFM