4 High Temp Fatigue

Huseyin Sehitoglu

Department of Mechanical and Industrial Engineering1

University of Illinois at Urbana-Champaign

Overview of High Temperature and Thermo-mechanical

Fatigue (TMF)

Overview of High Temperature and Thermo-mechanical

Fatigue (TMF)

Huseyin SehitogluMechanical and Industrial Engineering,University of Illinois, Urbana, Il. 61801

Tel : 217 333 4112 Fax: 217 244 6534e-mail: [email protected]

Huseyin Sehitoglu



Talk OutlineTalk Outline

Examples of High Temperature Problems Basic Terminology at High Temperatures Introduction to Constraint : Plasticity and ratchetting,

Out of Phase and In phase TMF Experimental Techniques at High Temperatures Fatigue Lives of Selected Materials under IF and TMF Mechanics- Stress-strain Models Life Models-Fatigue-Oxidation and Fatigue-Creep

Modeling Future Directions

Huseyin Sehitoglu



Railroad Wheels undergoing Friction Braking Brake Rotors Pistons, Valves and Cylinder Heads of Spark-

ignition and Diesel Engines Turbine Blades and Turbine Disks Pressure Vessel and Piping

Examples of Components Experiencing High

Temperatures

Examples of Components Experiencing High

Temperatures

Huseyin Sehitoglu



Mechanical Strain

-2x10 -3 -3 -3-10 10

.

.

.

..

.

.

..

......

.

..

.. ....

.

2x10 -3-3x10 -3

-100

-200

-300

Stre

ss (M

Pa)

100

200

60 min, 615 C

50 min, 589 C

55 min, 603 C45 min, 572 C

40 min, 552 C 35 min, 527 C

30 min, 497 C25 min, 459 C

20 min, 438 C 15 min, 362 C

10 min, 295 C

5 min, 210 C

65 min, 462 C

70 min, 401 C75 min, 354 C80 min, 314 C

85 min, 279 C

90 min, 249 C95 min, 223 C

100 min, 200 C110 min, 162 C

120 min, 133 C

122.5 min, 78 C125 min, 24 C

Railroad Wheels under Friction Braking

Railroad Wheels under Friction Braking

Huseyin Sehitoglu



Schematic of a Railroad Wheel,Strain-Temperature-Stress Changes on the B1 location under brake shoe heating (laboratory simulation based

on strain temperature measurements on wheels)

-400

-200

0

200

400

Stre

ss (

MPa

), T

empe

ratu

re (

C)

6005004003002001000

Time (minutes)

Laboratory Simulation of Stress at B1 Location in Railroad Wheel

Stress

Temperature

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cold

hot-200

-100

0

100

200

stre

ss (

MP

a)

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3

mechanical strain (%)

cycle 1 cycle 20 cycle 36

Cast IronOP TMFMinimum Temperature = 150 CMaximum Temperature = 500 CCycle Time = 4 mintuesDem = 0.6%Nf = 37 cycles

Brake Rotor Cracking

Huseyin Sehitoglu



Design-Manufacturing-Life Prediction Methodology for Cylinder Heads and BlocksDesign-Manufacturing-Life Prediction Methodology for Cylinder Heads and Blocks

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Percentage of Vehicles with Aluminum Engine Blocks and Heads(*)

(*) Delphi VIII Study, 1996

20%10%5%Light trucks

50%30%13%Passenger cars

Blocks

60%40%20%Light trucks

95%85%78%Passenger cars

Heads

200520001994

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Cylinder Heads (FEM and Fatigue Life Contours)

Inlet Valve

Exhaust Valve

Spark Plug

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Turbine Blades

TF

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Turbine Blades( Thermo-mechanical fatigue failure)Turbine Blades( Thermo-mechanical fatigue failure)

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Turbine Blades (strain-temperature variation)

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Basic Terminology at High Temperatures

What is a high temperature problem? Deformation under Constant or Variable Stress at homologous temperatures above 0.35 ( T/Tm >0.35 where Tmis melting temperature).

Stress Relaxation: Decrease in Stress at Constant Strain

Creep: Increase in Strain at Constant Stress

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High temperaturefatigue testing or modeling

TMFexternal stresses

TFinternal stresses

HCFDein 0

LCFDein > 0

Isothermal fatigue Non-isothermal fatigue

Isothermal vs. Thermo-mechanical fatigue

Huseyin Sehitoglu



Disk Specimen under TF loading (Simovich)

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Limitations in our Understanding of High

Temperature Material Behavior

Limitations in our Understanding of High

Temperature Material Behavior

Experiments on TMF are missing (difficult, expensive).Microstructural damage mechanisms are not well understood.

Stress-strain (constitutive) models havenot been established

Proposed failure models have severe drawbacks.

Huseyin Sehitoglu



ControlTower

LABWIEV

MACINTOSH IICi

LOAD CELL

Loading History

High TemperatureExtensometer Induction

Coil

TemperatureController

ACTUATOR

GPIB

InductionHeater

RF

C/L

THERMOCOUPLEC/L

Strain, load,position control

Test Frame

Experimental Techniques at High Temperatures

Experimental Techniques at High Temperatures

Huseyin Sehitoglu



Total Constraint Total Constraint

T

Tot BD A

O

C

E

a (T-To)

so

so-

Mechanical Strain

Stre

ss

T

e net = 0

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The compatibility equation

enet = eth+emech = a (T-T o ) + emech

When the net strain is zero and all of the thermal strain is converted to

mechanical strain. Then,

emech = - a (T-T o )

Total Constraint (ctd.)

Total Constraint (ctd.)

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Two-Bar Model(ctd.)Two-Bar Model(ctd.)

A , l 1 1

A , l 22

P

DT

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Simple RelationsSimple Relations

Equilibrium : A1 s1 + A2 s2 = P Compatability : l1 e1 = l2 e2 Strain :

e1 = e1 e + e 1 in + e1 th

e2 = e2 ee 1 th = a ( T- T0 )

e 1 in = inelastic (plastic) straine1 e = elastic strain

Huseyin Sehitoglu



The Concepts of Total, Partial, Over and Notch Constraint

The Concepts of Total, Partial, Over and Notch Constraint

e1=-s1E2C

, C = A2.l1A1.l2

C ; Total ConstraintC finite ; Partial Constraint

A , l 1 1

A , l 22

P

DT

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The Stress-strain Response under Total and Partial Constraint

The Stress-strain Response under Total and Partial Constraint

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The Stress-strain Response under Total and Partial Constraint (ctd.)

The Stress-strain Response under Total and Partial Constraint (ctd.)

Huseyin Sehitoglu



Out-of-Phase TMF Response

Mechanical Strain

Tmax

Str

ess

Tmin

In-Phase TMF Response

Mechanical Strain

Str

ess

T

minT

max

Stress-strain Behavior under Out-of-Phase versus In-Phase Stress-strain Behavior under Out-of-Phase versus In-Phase

Huseyin Sehitoglu



Mechanical StrainTmax

Stre

ss

Tmin

Ds

smin

smax

Dem

AB

C

Some DefinitionsSome Definitions

Inelastic St rain range:

Dein @ Dem - sBEB

+ sCEC

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Comparison of TMF IP and TMF OP Tests on 1010 Steel (Jaskes Data)

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TMF experiments of Coffin

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Thermal Block Histories on Steels under Total Constraint

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Metallurgical Studies : (a) Damage Mechanisms (Crack nucleation from slip bands,

precipitates, porosities, surface and internal oxidation, grain boundary cavitation)(b) Alloy Development

Mechanistic Studies : (a) Constitutive Modeling ( phenomenological:non-unified

and unified models for stress-strain prediction)(b) Life Prediction Modeling (Crack nucleation (stress,

strain, time), Crack Growth (Mean stress, crack length)

Classification of IF and TMF StudiesClassification of IF and TMF Studies

Huseyin Sehitoglu



Engineering Application : (a) Material Selection(b) Early Design(c) Residual Life Assesment

Classification of IF and TMF Studies (ctd.)Classification of IF and TMF Studies (ctd.)

Huseyin Sehitoglu



1015

1010

105

100

10-5

ein/A

0.012 3 4 5 6 7

0.12 3 4 5 6 7

12 3 4 5 6 7

10s/Ko

Al319- Solutionized Small SDAS

Power Law Creep

Plasticityn2=7.96

n1=3.12

Constitutive Modeling-Experimentally Determined Flow Rule

Constitutive Modeling-Experimentally Determined Flow Rule

Huseyin Sehitoglu



TMF OP 100-300C 1.0%

100C

300C

200C

-200

-100

0

100

200

Str

ess

(MP

a)

-6x10 -3 -4 -2 0 2 4 6Strain

STRESS1-2fea Stress1-2 STRESS300fea Stress300

Huseyin Sehitoglu



Hysteresis loops for the tests performed at 5x10-3 s-1

-240

-180

-120

-60

0

60

120

180

240

Str

ess

(MP

a)

-6x10-3

-4 -3 -2 -1 0 1 2 3 4 5 6Strain

experimental TNET

20oC

250oC

130oC

300oC

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Drag stress recoveryHyteresis loops at 20C for the material pre-exposed at 300C

-300

-200

-100

0

100

200

300

Str

ess

(MP

a)

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6Strain (%)

0 h

1 h10 h

100 h

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Coarsening of the PrecipitatesCoarsening of the Precipitates

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300

200

100

0

-100

-200

Stre

ss (M

Pa)

-0.4 -0.2 0.0 0.2 0.4Mechanical strain, (%)

TMF OP DT=100-300C, De=0.6%, 5x10-5s-1

Al 319-T7B Small SDAS

Experiment Simulation

Cycle 1

Cycle 350

TMF OP Stress-Strain PredictionTMF OP Stress-Strain Prediction

Huseyin Sehitoglu



Constitutive Modeling:

Mechanistic Studies

Requirements for a good model: Incorporate strain rate, temperature and mean

stress effect on stress-strain response Incorporate temperature-strain induced

changes on materials stress-strain response

Huseyin Sehitoglu



Constitutive Modeling: Non-unified Plasticity (stress-strain) Models:

Plastic strains (time-independent) and creep strains are added.

Unified Creep-Plasticity Models: Plastic strain and creep srain is combined as inelastic strain.

Mechanistic Studies

Huseyin Sehitoglu



Requirements for a good model: Incorporate stress,strain, thermal expansion,

mean stress, stress state effect on life Predict the effect of temperature, strain rate,

metallurgical changes on life.

Life Prediction ModelingLife Prediction Modeling

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Coffins ApproachCoffins Approach

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Coffins Approach (Frequency Modified Life)

Coffins Approach (Frequency Modified Life)

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Coffins ApproachCoffins Approach

Advantages:

(1) Simple to use; accounts for frequency effects

Disadvantages;

(1) Not sensitive to location of hold time within the cycle (tension or compression).

(2) Does not account for creep damage effects

(3) TMF life prediction not explicitly handled.

(4) No stress-strain model

Advantages:

(1) Simple to use; accounts for frequency effects

Disadvantages;

(1) Not sensitive to location of hold time within the cycle (tension or compression).

(2) Does not account for creep damage effects

(3) TMF life prediction not explicitly handled.

(4) No stress-strain model

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Strain Range Partitioning Method(SRP)

DeDe

De

DeDe

De

pp

cp

cppc

pc

cc

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SRP Data on Two Class of Steels(Manson et al.)

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SRP ApproachSRP Approach

Advantages:

(1) Accounts for location of hold time within a cycle

Disadvantages;

(1) Life curves are often too close, expensive to generate all these curves

(2) Does not account for oxidation/environment effects

(3) TMF Life prediction not explicitly handled.

Advantages:

(1) Accounts for location of hold time within a cycle

Disadvantages;

(1) Life curves are often too close, expensive to generate all these curves

(2) Does not account for oxidation/environment effects

(3) TMF Life prediction not explicitly handled.

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Damage per cycle is sum of the dominant mechanisms Dfat, Dox , Dcreep.

The terms in the damage equations should be physically based, specifically, they should be linked to specific experiments, stress-strain behavior and microstructural observations.

Development of a Mechanism Based Failure Model (Sehitoglu et al.)

Development of a Mechanism Based Failure Model (Sehitoglu et al.)

Huseyin Sehitoglu



Neu, Sehitoglu, Boismier, Kadioglu, 1987-

1Nf

ox = hcr do

BFox Kpeff

-1b 2 Demechox 2b +1

e (1-a' /b)

This equation accounts for the strain range at the oxide tip hence the oxide-metal properties the shapeof the oxide are included.

depends on the temperature strain history

and the temperature- time variation in the cycle.

Fatigue - Oxidation Models (ctd.)

Fatigue - Oxidation Models (ctd.)

F ox Kpeff

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Combined Damage Model Predictions

Combined Damage Model Predictions

10-4

10-3

10-2

10-1

Mec

hani

cal S

trai

n R

ange

101

102

103

104

105

106

107

108

Nf

WAP319-T7B Small SDAS All Fatigue Results

RT 40Hz 250?C 0.5Hz

250?C 5 x10-5s-1

300?C 5 x10-5s-1

TMF OP 100-300?C TMF IP 100-300?C

300?C Dox=0

300?C Dox

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Combined Damage Model Predictions (1070 Steel)


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Combined Damage ModelCombined Damage Model

Advantages:

(1) Accounts for TMF loading.

(2) Damage due to oxidation and creep are included.

Disadvantages:

(1) Requires some time to understand how it all works.

Advantages:

(1) Accounts for TMF loading.

(2) Damage due to oxidation and creep are included.

Disadvantages:

(1) Requires some time to understand how it all works.

Huseyin Sehitoglu



Modified Strain-Life Relation

- initial pore size fatigue strength coefficient fatigue strength exponent fatigue ductility coefficient fatigue ductility exponent

C'b

e f'

c

Fatigue Damage Equation

a0

Demech2

= C'a02-b2b (2N f

fat )-1b +e f

' (2N ffat )c

Huseyin Sehitoglu



- empirical constants- activation energy

- universal gas constant- hydrostatic stress- effective stress

- initial pore size

Cc,mc

ss H

HDR

Creep Damage Equation

a0

Dcr = Cc(mc - 1)a0mc -1

s Hs H

s n+1 exp -

DHRT

dt

0

tc

m c

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TMF IP versus TMF OP Comparison- Al 319

2

3

4

5

6

7

8

90.01

2

3

4

5

101

2 3 4 5 6 7 8 9

102

2 3 4 5 6 7 8 9

103

2 3 4 5 6 7 8 9

104

Nf

WAP EAP PredictionTMF-IPTMF-OP a0 = 70 mm

300 mm

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Initial Voids and after TMF IP

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Future DirectionsFuture Directions

A simple model is developed to predict lifefor a given mechanical strain range, maximumtemperature, and material.

Given a strain and temperature field in a component,the model can predict the most critical location wherecrack nucleation will occur.

Huseyin Sehitoglu



Given an elastic strain, temperature history from FEM, the model is able to predict the stresses and plastic strains assuming the mechanical strain is equal to the elastic strain from FEM. This is known as the strain invariance method.

To predict component behavior the model accounts for the initial defect size.

Future Directions (ctd.)Future Directions (ctd.)

4 High Temp Fatigue

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