Creep _ Wear by gjmpzlaezgx

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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

   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

CMSX-4 & CM 186 (yellow) are   April 1995
Single Crystals
Allison AE 2100
Turboprop engine

First Stage vane doublet
from CMSX-4 alloy

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
2.   Abrasive
3.   Fretting
   Compatible Materials
   Special coatings
   Welding of Asperities
   Welds break during
translation of surfaces
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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