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Monday, September 15, 2014
8:50 AM
Mechanical Properties of Metals
Tensile
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Elastic, and plastic, Video https://www.youtube.com/watch?v=n7LXYyohmgg
Plastic deformation of single crystal; video
Plastic deformation of polycrystal: video
Graph of Force/elongation would depend on geometry (Length and cross
sectional area). To eliminate this dependency, we define stress and strain.
stress
Unit:
N/m2 or Pa
Lb(f)/in2 = PSI
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strain
No unit, but
cm/cm or in/in,..
From: http://www.engineeringarchives.com/les_mom_stressstraindiagram.html
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Note the definitions of Yield strength, Ultimate Tensile strength
If Sample is "unloaded" in the elastic region, it will return to its
original length, L0.
If sample is unloaded in the plastic region, it will only recover
Maximum dL corresponding to the elastic region
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Permanent
deformation
Process of unloading during plastic deformation,
establishes a new yield strength point, higher than
previous one.
Plastic deformation is permanent slip of a plane ovr
another plane in a specific direction
Deformation of
single crystal
Movies: slip in poly-crystals
More on slip later!
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What happens if loading/unloading repeatedly?
Yield Strength increases each time sample is unloaded and
loaded again. As the result, sample becomes more brittle.
This is the basic reason for cold working.
Stress strain test video (watch it yourself) : https://www.youtube.com/watch?v=D8U4G5kcpcM
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Samples are intentionally machined with smaller diameter in the middle to
ensure necking (and eventual failure) AT THE CENTER.
True stress,
Engineering stress,
True Strain,
Engineering strain,
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Up to necking
After necking
n and K are experimental values
Types of Y.S
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Example: A 400 kg load is suspended by a wire(d=p1Cm,L=
12m) made from pure Cu.
wire is stretched to 12.08m while the load is on. Is the wire
deformed permanently?
.
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Use 69 Mpa for yield strength and o.5 cm for radius
This answer could be wrong!
Correct approach would be to compare strain, not stress.
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what is the final length when land is removed?
A steel rod (T.S =380 Mpa, Y.S =180 Mpa and E=
207 Gpa) is being used in construction of suspension
bridge. Each rod can experience 50,000N load.
What is the minimum diameter of rods so the they
would never stretch plastically?
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Previously we
saw for the
force between
two atoms:
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Effect of T: Later we will disuse D. B. T Temperature
Some definitions:
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Some definitions:
Ductility =
% elongation =
% reduction in area =
Toughness=
Resilience =
Modulus of resilience =
Modulus of elasticity =
Poisson's ratio = measure of homogeneity
Area under the area in the
inelastic region is Called
Modulus of resilience.
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Poisson's ratio
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Poisson's ratio
A sample with Y.S.=180 MPa and 20 cm diameter is used for a part that should
never be plastically deformed. Using M=1.2, calculate working stress and
maximum load.
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A coper wire (YS=69MPa and E=110GPa) is subjected to 400
tensile load. As a result, its length increases from 2 to 2.02 m.
Is the wire permanently deformed?
If Do=2mm, what is its final diameter when load is re
moved?
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LINKS:
Pres-stressed concrete:
https://www.youtube.com/watch?v=2QQsaPYssVU
Long Movie: https://www.youtube.com/watch?v=LsoU4SeqhNk
How it is made, concrete: https://www.youtube.com/watch?v=0ALY_VmxaOk
Documentary about concrete: https://www.youtube.com/watch?v=e6dxe4wpm5M
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