FATIGUE TESTING

Dr Ignacio Artamendi

Aggregate Industries Technical & Development Department

IAT National Training Day 24 September 2008

2Fatigue of asphalt mixtures

Fatigue cracking occurs when an asphalt layer is subjected to repeated loading under the passing traffic

Alligator pattern typical in fatigue cracking

3Severity levels

Low Moderate High

4Mechanism

compression compressiontension

Development of tensile strains at the bottom of the asphalt layer of sufficient magnitude to initiate cracking that eventually propagates up to the surface (bottom-up cracking).

5Test configurations

Trapezoidal

4-point bending

Tension/compression Indirect tension

3-point bending

Prismatic

2-point bending

6Testing modes

Controlled strain: the strain (deformation) is kept constant by decreasing the stress (load) during the test.

Applicable to thin flexible pavements.

Controlled stress: the stress (load) is maintained constant and the strain (deformation) increases during the test.

Applicable to thick pavement construction.

7Controlled strain test

0

50

100

150

200

250

300

0 100000 200000 300000 400000 500000 600000

N (cycles)

S

t

r

a

i

n

(

m

/

m

) strain amplitude

strain mean

0.00.20.40.60.81.01.21.41.61.82.0

0 100000 200000 300000 400000 500000 600000

N (cycles)

S

t

r

e

s

s

(

M

P

a

)

stress amplitude

stress mean

StressStrain

0 0

Sinusoidal loading

8Controlled stress test

0.0

0.3

0.5

0.8

1.0

1.3

1.5

1.8

2.0

0 10000 20000 30000 40000 50000 60000

N (cycles)

S

t

r

e

s

s

(

M

P

a

)

stress amplitude

stress mean

0

50

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450

0 10000 20000 30000 40000 50000 60000

N (cycles)

S

t

r

a

i

n

(

m

/

m

)

strain amplitude

strain mean

StressStrain

0 0

Sinusoidal loading

9Definition of failure

11 m

fN C =

where:Nf = number of load applications to failure

= tensile strainC1 , m = material regression coefficients

Failure in constant strain 50 % reduction in initial stiffness

Failure in constant stress 10 % reduction in initial stiffness or when the sample fractures

Arbitrary definitions of failure

10

Fatigue data: stiffness evolution

Phase I: rapid reduction of stiffness due to internal heating of the sample.

Phase II: approximate linear reduction in the stiffness (crack initiation stage)

Phase III: rapid drop in stiffness attributed to coalescence of micro-cracks to form a sharp crack (crack propagation stage).

0

2000

4000

6000

8000

10000

0 10000 20000 30000 40000 50000 60000

N (cycles)

E

*

(

M

P

a

)

Phase II Phase IIIP

h

a

s

e

I

11

Dissipated energy

-800

-600

-400

-200

0

200

400

600

800

-150 -100 -50 0 50 100 150

Strain (microstrain)

S

t

r

e

s

s

(

k

P

a

)

95% E

70% E

Dissipated energy (area within the hysteresis loop) is lost in the material in the form of mechanical work, heat generation or damage.

StressStrain

0 0

sini i i iw =

12

EN 12697-24 Resistance to fatigue

Two point bending test on trapezoidal specimens

Two point bending test on prismatic shaped specimens

Three point bending test on prismatic shaped specimens

Four-point bending test on prismatic shaped specimens

Indirect tensile test on cylindrical shaped specimens

13

Two-point bending trapezoidal test

14

Two-point bending trapezoidal test

Standard European fatigue test (EN 12697-24).

Specified test in France for material design (Level 4).

Large database (France).

Test is performed at low-intermediate temperature (10 0C).

Controlled strain (deflection) and stress (load) modes of testing but strain control specified.

Specimens are difficult to to fabricate.

Large number of specimens required (18).

Duration 30 days

15

Four-point bending (4PB) test

16

Four-point bending (4PB) test

Standard European fatigue test (EN 12697-24).

Preferred test in most European countries and USA (AASHTO TP9).

Good repeatability and reproducibility has been reported.

Specimens are easy to fabricate.

Test is performed at low temperatures (10 0C) where cracking occurs.

Controlled strain (deflection) and stress (load) modes of testing.

2nd Workshop on 4PB, 24-25 September, Portugal.

17

ITFT

18

ITFT

The ITFT is the most commonly used fatigue test in the UK.

The ITFT as per BS DD ABF is not a European Standard test. (The Indirect Tensile Test is , however, included in EN 12697-24).

Test is performed at 20 0C (No fatigue cracking at this temperature!).

Controlled stress test only.

No good control of the applied load (stress).

Load capacity of the equipment to test stiff materials such as EME2 is limited.

19

ITFT specimens after testing

Fatigue cracking Vertical permanent deformation

Indentation of loading strips

Failure modes for ITFT ( high stiffness materials and/or low temperatures)

20

ITFT load control

0

1

2

3

4

5

6

7

8

9

10

0 20000 40000 60000 80000

No of load applications

V

e

r

t

i

c

a

l

d

e

f

o

r

m

a

t

i

o

n

(

m

m

)

0

400

800

1200

1600

2000

Tensile stress (kPa)

Vertical deformation (mm)Tensile stress (kPa)

Poor control of the load (stress) at high stresses (>1000 kPa) for high stiffness materials and/or low temperatures.

Temperature = 10 0C Stiffness = 13500 MPa Target stress = 1000 kPa

21

European work on fatigue (RILEM)

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

1.E+09

10 100 1000

0(m/m)

N

f

ENTPE (T/C) KTH (4PB) LCPC (2PB) CRR(SS)CRR(BS) UofL (4PB) IBDiM (4PB) DWW (4PB)DWW (3PB) VTI (ITT) CONSULPAV (4PB)

Indirect tension test ITT

Same material

11 laboratories

T = 10 0C

22

European work on fatigue (RILEM)

20

40

60

80

100

120

140

160

180

200

ENTP

E (T/C

)

KTH (

4PB)

LCPC

(2PB

)

CRR(S

S)

CRR(B

S)

CONS

ULPA

V (4P

B)

IBDiM

(4PB)

DWW

(4PB)

DWW

(3PB)

VTI (I

TT)

UofL

(4PB)

6

(

m

/

m

)

Indirect tension test ITT

23

Healing

3000

3500

4000

4500

5000

5500

6000

6500

7000

7500

8000

0 10000 20000 30000 40000 50000 60000

Number of cycles

S

t

i

f

f

n

e

s

s

(

M

P

a

)

During a rest period the material recovers its properties (stiffness)

24

Pavement design

Horizontal tensile strain in cement bound layers

Bituminous layer

Cementitious or unbound granular layer

Subgrade

Horizontal tensile strain

Vertical compressive stress and strain

Moving wheel

Maximum tensile strain (m )

Analytical design

11 m

fN C =

Fatigue relationship

N (million standard axles)

Transfer function

LR1132

Log N= -9.38 - 4.16logm (DBM)Log N= -9.78 - 4.32logm (HRA)

25

Summary

Fatigue cracking major distress mode in asphalt pavements.

Different equipment available (2PB, 3PB, 4PB, ITT).

Different testing modes (strain / stress).

Different failure criteria

Fatigue testing is specified in EN 12697-24.

Variability between test methods.

ITT and UKs ITFT not recommended.

Data can be used to rank materials and in pavement design.

26

Thank you

Any questions?

FATIGUE TESTINGFatigue of asphalt mixturesSeverity levelsMechanismTest configurationsTesting modesControlled strain testControlled stress testDefinition of failureFatigue data: stiffness evolutionDissipated energyEN 12697-24 Resistance to fatigueTwo-point bending trapezoidal testTwo-point bending trapezoidal testFour-point bending (4PB) testFour-point bending (4PB) testITFT ITFT ITFT specimens after testingITFT load controlEuropean work on fatigue (RILEM)European work on fatigue (RILEM)HealingPavement designSummarySlide Number 26