APPLICATIONS OF APPLICATIONS OF ENERGY CONCEPTS FOR ENERGY CONCEPTS FOR FATIGUE ANALYSIS OF FATIGUE ANALYSIS OF AIRPORT PAVEMENTSAIRPORT PAVEMENTS

APPLICATIONS OF APPLICATIONS OF ENERGY CONCEPTS FOR ENERGY CONCEPTS FOR FATIGUE ANALYSIS OF FATIGUE ANALYSIS OF AIRPORT PAVEMENTSAIRPORT PAVEMENTS

FAA Fatigue Project BriefingOctober 7th, 2004 Urbana, IL

Samuel H. Carpenter, ProfessorShihui Shen, Graduate Research Assistant

University of Illinois at Urbana-Champaign

PRESENTATIONPRESENTATIONPRESENTATIONPRESENTATION

Introduction

Objectives to the New Approach

Problems in Traditional Fatigue Analysis Approach

Energy Concepts & RDEC Approach

Findings & Results

Implications for Airport Pavements

Conclusions & Future Work

Introduction

Objectives to the New Approach

Problems in Traditional Fatigue Analysis Approach

Energy Concepts & RDEC Approach

Findings & Results

Implications for Airport Pavements

Conclusions & Future Work

INTRODUCTION : A NEW INTRODUCTION : A NEW APPROACHAPPROACH

RDEC APPROACHRDEC APPROACH

INTRODUCTION : A NEW INTRODUCTION : A NEW APPROACHAPPROACH

RDEC APPROACHRDEC APPROACH Fatigue is a Damage Phenomenon

Described by Energy Principles

All Fatigue Behavior can be Described by a Single Parameter

Ratio of Dissipated Energy Change (RDEC) : Percent of Load Cycle Input Energy Producing Damage

Plateau Value (PV): the Value When Material’s Ratio of Dissipated Energy Change is Constant During the Fatigue Test

Fatigue is a Damage Phenomenon Described by Energy Principles

All Fatigue Behavior can be Described by a Single Parameter

Ratio of Dissipated Energy Change (RDEC) : Percent of Load Cycle Input Energy Producing Damage

Plateau Value (PV): the Value When Material’s Ratio of Dissipated Energy Change is Constant During the Fatigue Test

OBJECTIVES OF THE OBJECTIVES OF THE NEW APPROACHNEW APPROACHOBJECTIVES OF THE OBJECTIVES OF THE NEW APPROACHNEW APPROACH Overcome the problems of traditional

fatigue analysis approach; Examine energy concepts and the

ratio of dissipated energy change (RDEC) approach;

Apply RDEC approach to airport pavement, and develop design considerations suitable for airfield conditions:

Heavy aircraft loads; Very thick pavements; Low load frequency.

Overcome the problems of traditional fatigue analysis approach;

Examine energy concepts and the ratio of dissipated energy change (RDEC) approach;

Apply RDEC approach to airport pavement, and develop design considerations suitable for airfield conditions:

Heavy aircraft loads; Very thick pavements; Low load frequency.

PROBLEMS IN PROBLEMS IN TRADITIONAL FATIGUE TRADITIONAL FATIGUE ANALYSIS APPROACHANALYSIS APPROACH

PROBLEMS IN PROBLEMS IN TRADITIONAL FATIGUE TRADITIONAL FATIGUE ANALYSIS APPROACHANALYSIS APPROACH

Cannot explain fatigue behavior under heavy loads on pavements of varying thicknesses;

Non-unique strain-Nf relationship. Such relationship is not fundamentally material based.

A distinctly different fatigue behavior appears when airport pavement is built very thick (low strain/damage levels);

Low load frequency in thick pavement amplifies the healing effect which cannot be observed and described by traditional approach.

Cannot explain fatigue behavior under heavy loads on pavements of varying thicknesses;

Non-unique strain-Nf relationship. Such relationship is not fundamentally material based.

A distinctly different fatigue behavior appears when airport pavement is built very thick (low strain/damage levels);

Low load frequency in thick pavement amplifies the healing effect which cannot be observed and described by traditional approach.

TRADITIONAL FATIGUE TRADITIONAL FATIGUE PLOTPLOTTRADITIONAL FATIGUE TRADITIONAL FATIGUE PLOTPLOT

10

100

1000

10000

1 100000 1E+10 1E+15 1E+20 1E+25 1E+30 1E+35 1E+40

Nf @ 50% stiffness reduction, log

Str

ain

(mic

rost

rain

), lo

g

3N701N1051N105P3N905N1055N90P1N80D1N80DP8N70P1B41-75-76-77-78-48-79-711-717-721-4post

Low Strain/Damage Range

Normal Strain/Damage Range

Nf=1.1E+7

Fatigue Under Heavy LoadFatigue Under Heavy Load

Fatigue In Very Thick PavementFatigue In Very Thick Pavement

ENERGY CONCEPTS: ENERGY CONCEPTS: DISSIPATED ENERGYDISSIPATED ENERGYENERGY CONCEPTS: ENERGY CONCEPTS: DISSIPATED ENERGYDISSIPATED ENERGY

• Ratio of Dissipated Energy Change (RDEC)• Ratio of Dissipated Energy Change (RDEC)

STRAIN

STR

ES

SINITIAL LOAD CYCLE

SECOND LOAD CYCLE

DIFFERENT DISSIPATED ENERGY BETWEEN FIRST AND SECOND LOAD CYCLE

)( iiii SinW

RATIO OF DISIPATED RATIO OF DISIPATED ENERGY CHANGE ENERGY CHANGE CALULATIONCALULATION

RATIO OF DISIPATED RATIO OF DISIPATED ENERGY CHANGE ENERGY CHANGE CALULATIONCALULATION

a

baa DE

DEDE

DE

DERDEC

TYPICAL RDEC PLOT TYPICAL RDEC PLOT WITH THREE WITH THREE BEHAVIOR ZONESBEHAVIOR ZONES

TYPICAL RDEC PLOT TYPICAL RDEC PLOT WITH THREE WITH THREE BEHAVIOR ZONESBEHAVIOR ZONES

IIIIII

IIIIII

Plateau ValuePlateau Value

Rati

o o

f D

issi

pate

d E

nerg

y

Rati

o o

f D

issi

pate

d E

nerg

y

Ch

an

ge,

Log

Ch

an

ge,

Log

Load Repetitions, Load Repetitions, LogLog

CURRENT FINDINGS IN CURRENT FINDINGS IN RDEC APPROACHRDEC APPROACHCURRENT FINDINGS IN CURRENT FINDINGS IN RDEC APPROACHRDEC APPROACH

Unique relationship between PV and Nf

Different mixtures, load levels, loading modes and testing conditions

PV is a comprehensive energy based parameter.

A lower (higher) PV is always associated with a lower (higher) damage, producing a longer (shorter) fatigue life

Unique relationship between PV and Nf

Different mixtures, load levels, loading modes and testing conditions

PV is a comprehensive energy based parameter.

A lower (higher) PV is always associated with a lower (higher) damage, producing a longer (shorter) fatigue life

RDEC APPROACHRDEC APPROACHRDEC APPROACHRDEC APPROACH

PV vs. Nf @ 50% stiffness reduction

y = 0.5746x-1.1121

R2 = 0.9769

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

loading cycles @ 50% stiffness reduction

Pla

teau

Val

ue

Various mixture types and testing conditions under heavy aircraft loads

RDEC RDEC APPROACHAPPROACHRDEC RDEC APPROACHAPPROACH

Normal PV: y = 0.5662x-1.1106

R2 = 0.9885

Low PV: y = 0.2871x-1.0793

R2 = 0.9984

1.E-35

1.E-30

1.E-25

1.E-20

1.E-15

1.E-10

1.E-05

1.E+00

1.E+00 1.E+04 1.E+08 1.E+12 1.E+16 1.E+20 1.E+24 1.E+28 1.E+32 1.E+36 1.E+40

loading cycles @ 50% stiffness reduction, log

Pla

teau

Val

ue,

log

Normal PVLow PV

8.57E-9

1.10E+7

Including the strain conditions (low strain/damage level) in very thick airport pavements

IMPLICATIONS FOR IMPLICATIONS FOR AIRPORT PAVEMENTSAIRPORT PAVEMENTSIMPLICATIONS FOR IMPLICATIONS FOR AIRPORT PAVEMENTSAIRPORT PAVEMENTS A fatigue endurance limit exists,

and is an important consideration for the design/performance of thick pavements (low strain/damage)

Healing effect is significant in pavements with low load occurrence.

A fatigue endurance limit exists, and is an important consideration for the design/performance of thick pavements (low strain/damage)

Healing effect is significant in pavements with low load occurrence.

UNIQUE ENERGY LEVEL UNIQUE ENERGY LEVEL AT WHICH NO FATIGUE AT WHICH NO FATIGUE DAMAGE EXISTSDAMAGE EXISTS

UNIQUE ENERGY LEVEL UNIQUE ENERGY LEVEL AT WHICH NO FATIGUE AT WHICH NO FATIGUE DAMAGE EXISTSDAMAGE EXISTS

PV vs. Nf @ 50% stiffness reduction

y = 0.5746x-1.1121

R2 = 0.9769y = 0.0659x-1.026

R2 = 0.9983

1.E-41

1.E-38

1.E-35

1.E-321.E-29

1.E-26

1.E-23

1.E-20

1.E-17

1.E-14

1.E-111.E-08

1.E-05

1.E-02

1.E+01

1.E+00 1.E+04 1.E+08 1.E+12 1.E+16 1.E+20 1.E+24 1.E+28 1.E+32 1.E+36 1.E+40

loading cycles @ 50% stiffness reduction

Pla

teau

Val

ue

Normal PV

Low PVPV=8.57E-PV=8.57E-99

Nf=1.10ENf=1.10E+7+7

FATIGUE ENDURANCE FATIGUE ENDURANCE LIMITLIMITFATIGUE ENDURANCE FATIGUE ENDURANCE LIMITLIMIT

Crucial for Design and Performance of Thick Pavements

Limit to HMA Thickness Unique fatigue curves

Independent of Traffic Level Significant element for structural

design

Minimizes Effect of Overloads

Crucial for Design and Performance of Thick Pavements

Limit to HMA Thickness Unique fatigue curves

Independent of Traffic Level Significant element for structural

design

Minimizes Effect of Overloads

1.E-27

1.E-24

1.E-21

1.E-18

1.E-15

1.E-12

1.E-09

1.E-27 1.E-25 1.E-23 1.E-21 1.E-19 1.E-17 1.E-15 1.E-13 1.E-11 1.E-09

PV from extended testing

PV

fro

m s

ho

rten

ed t

esti

ng

500,000

1 million

2million

3 million

5 million

Line of Equality

Compare PV projected from shortened testing with extended testingCompare PV projected from shortened testing with extended testing

USE OF PV TO USE OF PV TO SHORTEN FATIGUE SHORTEN FATIGUE TESTING FOR THICK TESTING FOR THICK PAVEMENTSPAVEMENTS

USE OF PV TO USE OF PV TO SHORTEN FATIGUE SHORTEN FATIGUE TESTING FOR THICK TESTING FOR THICK PAVEMENTSPAVEMENTS

USE OF PV TO USE OF PV TO SHORTEN FATIGUE SHORTEN FATIGUE TESTING FOR THICK TESTING FOR THICK PAVEMENTSPAVEMENTS

USE OF PV TO USE OF PV TO SHORTEN FATIGUE SHORTEN FATIGUE TESTING FOR THICK TESTING FOR THICK PAVEMENTSPAVEMENTS Plateau Value period can be reached much earlier than Nf @ 50% stiffness reduction point;

Reasonable projection can be obtained through greatly shortened low strain/damage testing.

500,000 Load repetitions

Plateau Value period can be reached much earlier than Nf @ 50% stiffness reduction point;

Reasonable projection can be obtained through greatly shortened low strain/damage testing.

500,000 Load repetitions

HEALINGHEALINGHEALINGHEALING Accepted Description

Between loads the damage is reversed as the asphalt-aggregate interface reattaches, removing micro-cracks

Actual Occurrence A continuous physical-chemical reaction

that occurs even during continuous loadings at low strain levels

Accepted Description Between loads the damage is reversed as

the asphalt-aggregate interface reattaches, removing micro-cracks

Actual Occurrence A continuous physical-chemical reaction

that occurs even during continuous loadings at low strain levels

HEALING IN AIRPORT HEALING IN AIRPORT PAVEMENTSPAVEMENTSHEALING IN AIRPORT HEALING IN AIRPORT PAVEMENTSPAVEMENTS

A Material Property Constant The HMA has the potential to recover

a relative amount of damage

When Load Damage Falls Below Healing Potential, Damage Accumulation is Minimal or Non-Existent

Fatigue Endurance Limit exists

Field Fatigue Life is Increased Over Lab Testing

A Material Property Constant The HMA has the potential to recover

a relative amount of damage

When Load Damage Falls Below Healing Potential, Damage Accumulation is Minimal or Non-Existent

Fatigue Endurance Limit exists

Field Fatigue Life is Increased Over Lab Testing

CURRENT HEALING CURRENT HEALING STUDYSTUDYCURRENT HEALING CURRENT HEALING STUDYSTUDY

y = -0.0282x2 - 0.0011x - 5.0937

R2 = 0.9786

-7

-6.5

-6

-5.5

-5

0 1 2 3 4 5 6 7

rest period (sec.)

log

(PV

)

PV – An energy level related to fatigue life, Nf

CONCLUSIONSCONCLUSIONSCONCLUSIONSCONCLUSIONS

Ratio of Dissipated Energy Change (RDEC) provides a unique way to study fatigue behavior of HMA;

Plateau Value, PV, is a function of material properties and pavement response;

PV-Nf relationship is unique for mixture type, loading mode, and all testing conditions;

Ratio of Dissipated Energy Change (RDEC) provides a unique way to study fatigue behavior of HMA;

Plateau Value, PV, is a function of material properties and pavement response;

PV-Nf relationship is unique for mixture type, loading mode, and all testing conditions;

CONCLUSIONSCONCLUSIONSCONCLUSIONSCONCLUSIONS PV shows a unique threshold for the

fatigue endurance limit (PVL). . Current results shows such PVL is around 8.57E-9;

PV-Nf uniqueness can be used to predict long fatigue life without running test to failure;

Healing can be observed with the PV. Represents an energy level of damage,

which decreases with an increase in rest periods (healing effect).

PV shows a unique threshold for the fatigue endurance limit (PVL). . Current results shows such PVL is around 8.57E-9;

PV-Nf uniqueness can be used to predict long fatigue life without running test to failure;

Healing can be observed with the PV. Represents an energy level of damage,

which decreases with an increase in rest periods (healing effect).

FUTURE WORKFUTURE WORK FUTURE WORKFUTURE WORK

Substantiate the relationship between PV and rest periods:

Relate PV with healing using energy concepts

Healing Index

Healing Rate

Asphalt Type Influence

Substantiate the relationship between PV and rest periods:

Relate PV with healing using energy concepts

Healing Index

Healing Rate

Asphalt Type Influence

FUTURE WORKFUTURE WORK FUTURE WORKFUTURE WORK

Integrate the energy based healing effects, rest periods, into an improved rational design procedure suitable for airfield conditions:

Heavy aircraft load

Thick pavement layers

Low loading frequency and extended rest periods

Integrate the energy based healing effects, rest periods, into an improved rational design procedure suitable for airfield conditions:

Heavy aircraft load

Thick pavement layers

Low loading frequency and extended rest periods

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