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7/31/2019 13 Creep and Generalized Time Dependent Deformation Notes
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Creep & Wear
High Temperature Irreversible
Deformation
7/31/2019 13 Creep and Generalized Time Dependent Deformation Notes
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Objectives
Identify the primary mechanisms of creep
deformation
Determine the threshold temperature for creep
for an alloy of known melting temperature
Use creep data to determine creep model
parameters
Read and apply a creep rupture curve
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Creep
______dependant ________deformation
At constant stress, strain continues toincrease
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Creep
Time
Stress
Strain
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Mechanisms of Creep
High rates of diffusion permit reshaping of
crystals to relieve stress
Diffusion significant at both grain boundaries
and in the bulk
High energy and weak bonds allow dislocations
to “climb” around structures that pin them at
lower temperature
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Mechanisms of Creep
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Creep Test
Usually a tensile bar
Dead load applied
Strain is plotted with time
Test usually ends with rupture (failure)
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Classical creep curve
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Application of Classical Curve
Steady state creep rate determined from straight
section (secondary creep)
Ahrennius model captures temperature
dependence (diffusion!)
Several tests at different temperatures & stress
levels can be used to determine material model
parameters
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Creep Constitutive
Models
d = K 2n exp (-QC / RT)
dt
For T = constant
d = K 1n
dt
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Creep Rupture Method
When rupture (catastrophic failure) is the only
concern (eg boiler tubes) a simpler method will
do
Maps of time to rupture as a function of stress
and temperature are used to determine the
expected life
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Threshold for Creep
The Critical Temperature for Creep is 40%
of the Melting Temperature.
If T > 0.40 TM Creep Is Likely
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Example
Will Lead Creep at Room Temperature?
TM = 327 C = 600 K
TROOM = 23 C = 296 K
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Creep in Ice
How cold does your freezer need to be to avoid
creep in the ice cubes?
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Creep of Ice
Ice creeps at most
terrestrial temperatures
People have studied this
for use in weathermodels & ship design
Budd and Jacka, 1989).
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How Do We Deal With
Creep ?1. Reduce the effect of grain
boundaries
Use Single Crystals
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Stress Rupture in Single Crystals
CMSX-4 & CM 186 (yellow) are
Single Crystals
Advanced Materials & Processes
April 1995
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Single Crystal Turbine Blade
Allison AE 2100
Turboprop engine
First Stage vane doublet
from CMSX-4 alloy
Advanced Materials & Processes
April 1995
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How Do We Deal With Creep ?
1. Reduce the effect of grain
boundaries
Use Single Crystals
2. Change Materials
3. Change Operating Conditions
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Wear
Three Mechanisms
1. Adhesive
2. Abrasive
3. Fretting
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Adhesive Wear
Compatible Materials
Special coatings
Welding of Asperities
Welds break during
translation of surfaces
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How Do you Prevent Adhesive
Wear?
Incompatible materials
Lubricate
Polish
Fundamentals of Machine Component Design,
Juvinall & Marshak, Wiley
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Abrasive Wear
Three body wear
Contamination or wear debris does
much of the damage
Why you use an oil filter.
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Fretting Wear
Parts in Contact
Small relative motion
Microwelding causes spot damage