PHYSICS 231 Lecture 21: Some material science

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PHYSICS 231Lecture 21 Some material science
liquid
solid
gas

• Remco Zegers
• Walk-in hour Thursday 1130-1330 am
• Helproom

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States of matter
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Phase transformations
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solids
ordered
crystalline
amorphous
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The deformation of solids

• Stress Tells something about the force causing
  the
• deformation
• Strain Measure of the degree of deformation
• For small stress, strain and stress are linearly
  correlated.
• Strain ConstantStress
• Constant elastic modulus
• The elastic modulus depends on
• Material that is deformed
• Type of deformation (a different modulus is
  defined for
• different types of deformations)

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The Youngs modulus
Beyond the elastic limit an object is permanently
deformed (it does not return to its original
shape if the stress is removed).
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example
An architect wants to design a 5m high circular
pillar with a radius of 0.5 m that holds a bronze
statue that weighs 1.0E04 kg. He chooses
concrete for the material of the pillar
(Y1.0E10 Pa). How much does the pillar
compress?
5m
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Ultimate strength
Ultimate strength maximum force per unit area a
material can withstand before it breaks or
fractures.
Different for compression and tension.
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example
A builder is stacking 1 m3 cubic concrete blocks.
Each blocks weighs 5E03 kg. The ultimate
strength of concrete for compression is 2E07 Pa.
How many blocks can he stack before the lowest
block is crushed?
The force on the low end of the lowest block is
FNmblockg. N total number of
blocks mblockmass of one block g9.81
m/s2 Ultimate strength 2E07F/A Nmblockg/1
N408 blocks.
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The Shear Modulus
?x
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Moving earth crust
100 m
30 m
A tectonic plate in the lower crust (100 m deep)
of the earth is shifted during an earthquake by
30m. What is the shear stress involved, if the
upper layer of the earth does not move?
(S1.5E10 Pa)
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Bulk Modulus
Compressibility 1/(Bulk modulus)
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example
What force per unit area needs to be applied to
compress 1 m3 water by 1? (B0.21E10 Pa)
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Some typical elastic moduli
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Density
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Pressure
PressureF/A (N/m2Pa)
Same Force, different pressure
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example
A nail is driven into a piece of wood with a
force of 700N. What is the pressure on the wood
if Anail1 mm2? A person (weighing 700 N) is
lying on a bed of such nails (his body covers
1000 nails). What is the pressure exerted by each
of the nails?
PnailF/Anail700N/1E-06m27E08
Pa PpersonF/(1000Anail)700/1E-03 7E05 Pa
(about 7 times the atmospheric pressure).