Creep & Wear

High Temperature Irreversible Deformation

ObjectivesO Identify the primary mechanisms of

creep deformationO Determine the threshold

temperature for creep for an alloy of known melting temperature

O Use creep data to determine creep model parameters

O Read and apply a creep rupture curve

Creep Time dependant permanent deformation

At constant stress, strain continues to increase

Creep

Time

Stress

Strain

Mechanisms of CreepO High rates of diffusion permit

reshaping of crystals to relieve stressO Diffusion significant at both grain

boundaries and in the bulk O High energy and weak bonds allow

dislocations to “climb” around structures that pin them at lower temperature

Mechanisms of Creep

Creep TestO Usually a tensile bar

O Dead load applied

O Strain is plotted with time

O Test usually ends with rupture (failure)

Classical creep curve

Application of Classical Curve

O Steady state creep rate determined from straight section (secondary creep)

O Ahrennius model captures temperature dependence (diffusion!)

O 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 MethodO When rupture (catastrophic failure)

is the only concern (eg boiler tubes) a simpler method will do

O 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

ExampleWill Lead Creep at Room Temperature?

TM = 327 C = 600 KTROOM = 23 C = 296 K

Creep ExampleWill Lead Products Creep at Room Temperature?

TM = 327 C = 600 KTROOM = 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

O Ice creeps at most terrestrial temperatures

OPeople 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 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

How Do We Deal With Creep ?

1. Reduce the effect of grain

boundaries Use Single Crystals

2. Change Materials

3. Change Operating Conditions

WearO Three Mechanisms

1. Adhesive2. Abrasive3. Fretting

Adhesive WearOCompatible

MaterialsOSpecial coatingsOWelding of

AsperitiesOWelds break during

translation of surfaces

How Do you Prevent Adhesive Wear?

O Incompatible materials

OLubricateOPolish

Fundamentals of Machine Component Design, Juvinall & Marshak, Wiley

Abrasive Wear

Three body wear

Contamination orwear debris doesmuch of the damage

Why you use an oil filter.

Fretting Wear

O Parts in ContactO Small relative motionO Microwelding causes spot damage