Post on 12-Jan-2017
Creep Creep
High Temperature High Temperature Irreversible DeformationIrreversible Deformation
ObjectivesObjectives Identify the primary mechanisms of Identify the primary mechanisms of
creep deformationcreep deformation Determine the threshold temperature Determine the threshold temperature
for creep for an alloy of known for creep for an alloy of known melting temperaturemelting temperature
Use creep data to determine creep Use creep data to determine creep model parametersmodel parameters
Read and apply a creep rupture curveRead and apply a creep rupture curve
CreepCreep Time dependant permanent Time dependant permanent deformationdeformation
At constant stress, strain At constant stress, strain continues to increasecontinues to increase
CreepCreep
Time
Stress
Strain
Mechanisms of CreepMechanisms of Creep High rates of diffusion permit High rates of diffusion permit
reshaping of crystals to relieve stressreshaping of crystals to relieve stress Diffusion significant at both grain Diffusion significant at both grain
boundaries and in the bulk boundaries and in the bulk High energy and weak bonds allow High energy and weak bonds allow
dislocations to “climb” around dislocations to “climb” around structures that pin them at lower structures that pin them at lower temperaturetemperature
Mechanisms of CreepMechanisms of Creep
Creep TestCreep Test Usually a tensile bar Usually a tensile bar
Dead load appliedDead load applied
Strain is plotted with timeStrain is plotted with time
Test usually ends with rupture Test usually ends with rupture (failure)(failure)
Classical creep curveClassical creep curve
Application of Classical Application of Classical CurveCurve
Steady state creep rate determined Steady state creep rate determined from straight section (secondary from straight section (secondary creep)creep)
Ahrennius model captures Ahrennius model captures temperature dependence (diffusion!)temperature dependence (diffusion!)
Several tests at different Several tests at different temperatures & stress levels can be temperatures & stress levels can be used to determine material model used to determine material model parametersparameters
Creep Constitutive Creep Constitutive ModelsModels
dd = K = K2 2 nn exp exp (-Q(-QC C / RT) / RT) dtdt
For T = constantFor T = constant
dd = K = K1 1 nn dtdt
Creep Rupture MethodCreep Rupture Method When rupture (catastrophic failure) When rupture (catastrophic failure)
is the only concern (eg boiler tubes) is the only concern (eg boiler tubes) a simpler method will doa simpler method will do
Maps of time to rupture as a Maps of time to rupture as a function of stress and temperature function of stress and temperature are used to determine the expected are used to determine the expected lifelife
Stress Rupture CurveStress Rupture Curve(Applied stress vs. time to (Applied stress vs. time to
rupture)rupture)
Threshold for CreepThreshold for Creep
The Critical Temperature for The Critical Temperature for Creep is 40% of the Melting Creep is 40% of the Melting Temperature.Temperature.
If T > 0.40 TIf T > 0.40 TMM Creep Is Likely Creep Is Likely
ExampleExampleWill Lead Creep at Room Will Lead Creep at Room Temperature?Temperature?
TTMM = 327 = 327 C = 600 C = 600 KKTTROOMROOM = 23 = 23 C = 296 C = 296 KK
Creep ExampleCreep ExampleWill Lead Products Creep at Room Will Lead Products Creep at Room Temperature?Temperature?
TTMM = 327 = 327 C = 600 C = 600 KKTTROOMROOM = 23 = 23 C = 296 C = 296 KK
100 x 296 / 600 =49.3 % 100 x 296 / 600 =49.3 % Will Will CreepCreep
Creep in IceCreep in IceHow cold does your freezer need to be How cold does your freezer need to be
to avoid creep in the ice cubes?to avoid creep in the ice cubes?
Creep in IceCreep in IceHow cold does your freezer need to be How cold does your freezer need to be
to avoid creep in the ice cubes?to avoid creep in the ice cubes?
0C = 273 K0C = 273 K
0.4 * 273 = 109.2K = -163.8 C! 0.4 * 273 = 109.2K = -163.8 C! Brrrr!Brrrr!
Creep of IceCreep of Ice Ice creeps at most Ice creeps at most
terrestrial terrestrial temperaturestemperatures
People have People have studied this for use studied this for use in weather models in weather models & ship design& ship design
Budd and Jacka, 1989).
How Do We Deal With How Do We Deal With CreepCreep ??
1.1. Reduce the temperature of Reduce the temperature of the surrounding below the surrounding below 40% of the critical 40% of the critical temperature.temperature.
How Do We Deal With Creep ?
* Reduce the effect of grain boundaries
Use Single Crystals
* Change Materials* Change Operating Conditions
THE END