13 creep and generalized time dependent deformation notes

7/31/2019 13 Creep and Generalized Time Dependent Deformation Notes

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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

______dependant ________deformation

At constant stress, strain continues toincrease



Creep

Time

Stress

Strain



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 = K 2n exp (-QC / RT)

dt

For T = constant

d = K 1n

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



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 in Ice

How cold does your freezer need to be to avoid

creep in the ice cubes?



Creep of Ice

Ice creeps at most

terrestrial temperatures

People have studied this

for use in weathermodels & 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

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 ?

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

13 creep and generalized time dependent deformation notes

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