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