Download - Creep and wear
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Creep & Wear
High Temperature Irreversible Deformation
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ObjectivesO Identify the primary mechanisms of
creep deformationO Determine the threshold
temperature for creep for an alloy of known melting temperature
O Use creep data to determine creep model parameters
O Read and apply a creep rupture curve
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Creep Time dependant permanent deformation
At constant stress, strain continues to increase
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Creep
Time
Stress
Strain
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Mechanisms of CreepO High rates of diffusion permit
reshaping of crystals to relieve stressO Diffusion significant at both grain
boundaries and in the bulk O 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 TestO Usually a tensile bar
O Dead load applied
O Strain is plotted with time
O Test usually ends with rupture (failure)
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Classical creep curve
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Application of Classical Curve
O Steady state creep rate determined from straight section (secondary creep)
O Ahrennius model captures temperature dependence (diffusion!)
O Several tests at different temperatures & stress levels can be used to determine material model parameters
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Creep Constitutive Models
d = K2 n exp (-QC / RT) dt
For T = constant
d = K1 n dt
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Creep Rupture MethodO When rupture (catastrophic failure)
is the only concern (eg boiler tubes) a simpler method will do
O Maps of time to rupture as a function of stress and temperature are used to determine the expected life
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Stress Rupture Curve(Applied stress vs. time to
rupture)
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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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ExampleWill Lead Creep at Room Temperature?
TM = 327 C = 600 KTROOM = 23 C = 296 K
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Creep ExampleWill Lead Products Creep at Room Temperature?
TM = 327 C = 600 KTROOM = 23 C = 296 K
100 x 296 / 600 =49.3 % Will Creep
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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 in Ice
How cold does your freezer need to be to avoid creep in the ice cubes?
0C = 273 K
0.4 * 273 = 109.2K = -163.8 C! Brrrr!
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Creep of Ice
O Ice creeps at most terrestrial temperatures
OPeople have studied this for use in weather models & 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
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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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WearO Three Mechanisms
1. Adhesive2. Abrasive3. Fretting
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Adhesive WearOCompatible
MaterialsOSpecial coatingsOWelding of
AsperitiesOWelds break during
translation of surfaces
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How Do you Prevent Adhesive Wear?
O Incompatible materials
OLubricateOPolish
Fundamentals of Machine Component Design, Juvinall & Marshak, Wiley
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Abrasive Wear
Three body wear
Contamination orwear debris doesmuch of the damage
Why you use an oil filter.
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Fretting Wear
O Parts in ContactO Small relative motionO Microwelding causes spot damage