High Temperature Irreversible Deformation Creep High Temperature Irreversible Deformation سيد ابراهيم موسوي ترشيزي دانشيار دانشگاه صنعت آب و برق (شهيد عباسپور) 26تیر ماه 91
Creep Time dependant permanent deformation At constant stress, strain continues to increase
Creep Stress Strain Time
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
Mechanisms of Creep
Creep Test Usually a tensile bar Dead load applied Strain is plotted with time Test usually ends with rupture (failure)
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
Creep Constitutive Models d = K2 n exp (-QC / RT) dt For T = constant d = K1 n
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
Stress Rupture Curve (Applied stress vs. time to rupture)
Threshold for Creep The Critical Temperature for Creep is 40% of the Melting Temperature. If T > 0.40 TM Creep Is Likely
Example Will Lead Creep at Room Temperature? TM = 327 C = 600 K TROOM = 23 C = 296 K
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
Creep in Ice How cold does your freezer need to be to avoid creep in the ice cubes?
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!
Creep of Ice Ice creeps at most terrestrial temperatures People have studied this for use in weather models & ship design Budd and Jacka, 1989).
How Do We Deal With Creep ? Reduce the effect of grain boundaries Use Single Crystals
Stress Rupture in Single Crystals CMSX-4 & CM 186 (yellow) are Single Crystals Advanced Materials & Processes April 1995
Single Crystal Turbine Blade Allison AE 2100 Turboprop engine First Stage vane doublet from CMSX-4 alloy Advanced Materials & Processes April 1995
How Do We Deal With Creep ? Reduce the effect of grain boundaries Use Single Crystals Change Materials
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