1. High Temperature Irreversible Deformation
Creep
High Temperature Irreversible Deformation
سيد ابراهيم موسوي ترشيزي
دانشيار دانشگاه صنعت آب و برق (شهيد عباسپور)
26تیر ماه 91

2. Creep  Time dependant permanent deformation
 At constant stress, strain continues to increase

3. Creep
Stress
Strain
Time

4. 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

5. Mechanisms of Creep

6. Creep Test Usually a tensile bar Dead load applied
Strain is plotted with time
Test usually ends with rupture (failure)

7. Classical creep curve

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19. 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

20. Creep Constitutive Models
d = K2 n exp (-QC / RT) dt
For T = constant
d = K1 n

21. 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

22. Stress Rupture Curve (Applied stress vs. time to rupture)

23. Threshold for Creep
The Critical Temperature for Creep is 40% of the Melting Temperature.
If T > 0.40 TM  Creep Is Likely

24. Example Will Lead Creep at Room Temperature? TM = 327 C = 600 K
TROOM = 23 C = 296 K

25. Creep Example Will Lead Products Creep at Room Temperature?
TM = 327 C = 600 K
TROOM = 23 C = 296 K
100 x 296 / 600 =49.3 %  Will Creep

26. Creep in Ice
How cold does your freezer need to be to avoid creep in the ice cubes?

27. 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 = C! Brrrr!

28. Creep of Ice Ice creeps at most terrestrial temperatures
People have studied this for use in weather models & ship design
Budd and Jacka, 1989).

29. How Do We Deal With Creep ?
Reduce the effect of grain
boundaries
Use Single Crystals

30. Stress Rupture in Single Crystals
CMSX-4 & CM 186 (yellow) are Single Crystals
Advanced Materials & Processes April 1995

31. Single Crystal Turbine Blade
Allison AE 2100 Turboprop engine
First Stage vane doublet from CMSX-4 alloy
Advanced Materials & Processes April 1995

32. How Do We Deal With Creep ?
Reduce the effect of grain
boundaries
Use Single Crystals
Change Materials

33. How Do We Deal With Creep ?
مهم
How Do We Deal With Creep ?
Reduce the effect of grain
boundaries
Use Single Crystals
Change Materials
3. Change Operating Conditions