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Creep _ Wear

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									Creep & Wear

High Temperature Irreversible
       Deformation
                  Objectives
   Identify the primary mechanisms of creep
    deformation
   Determine the threshold temperature for creep
    for an alloy of known melting temperature
   Use creep data to determine creep model
    parameters
   Read and apply a creep rupture curve
                 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
    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
dt
          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 ?
1.    Reduce the effect of grain
      boundaries
        Use Single Crystals
  Stress Rupture in Single Crystals




                               Advanced Materials & Processes
CMSX-4 & CM 186 (yellow) are   April 1995
Single Crystals
    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 ?

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

1. Reduce the effect of grain
   boundaries
   Use Single Crystals
2. Change Materials
3. Change Operating Conditions
                    Wear
    Three Mechanisms
    1.   Adhesive
    2.   Abrasive
    3.   Fretting
                     Adhesive Wear
   Compatible Materials
   Special coatings
   Welding of Asperities
   Welds break during
    translation of surfaces
      How Do you Prevent Adhesive
                Wear?

   Incompatible materials
   Lubricate
   Polish




                             Fundamentals of Machine Component Design,
                             Juvinall & Marshak, Wiley
               Abrasive Wear
Three body wear

Contamination or
wear debris does
much of the damage

Why you use an oil filter.
               Fretting Wear



   Parts in Contact
   Small relative motion
   Microwelding causes spot damage

								
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