1

Session 10 – Fretting Fatigue Experiments

and Case Studyand Case Study

Prof. D. Nowell

University of Oxford, UK

UTMIS Autumn Course 2011 1

Types of fretting fatigue test• ‘Materials ranking’ tests

– e.g. bridge type pads

P

σA B

Q

Q

P

P

σ0

• Controlled geometry tests– e.g. Hertzian pads

• Component geometry tests

2

Materials ranking testsP

σA B

– Quick and easy to perform– Give qualitative information about materials performance

• But– Lack of control – slip and traction difficult to control

independently– Contact tractions difficult to estimate and susceptible to

alignment errors– Difficult to measure quantities needed for analysis– Analysis not straightforward (FE?)

Q

P

σ ωsin t

Fretting Pad

a a

R

Controlled Geometry tests (E.g. Hertz))

ωt

Q

P

σ ω0sin t

Specimen

-a a

Hertzian contact – repeatable and easy to analyseSalient variables can be controlled and measured

ωt

Salient variables can be controlled and measured (Q,P,σ, f, a)Experiments usually conducted in partial slipBut more difficult to performMay need specialised 2-actuator machine

3

Hertzian fretting tests

Implementation on single actuator machine

Two-actuator alternativesmall actuator

fretting Padsspecimen

hydraulic jaws

contact loads

fret

ting

mot

ion

hydraulic jaws

large actuatorThe Oxford ‘in-line’ rig

4

Tests on ‘realistic’ geometry

• Designed to simulate actual geometry and l diloading

• Useful for component lifing and model validation

• May be expensive and time-consuming to set upup

• Example – dovetail fretting

Fan Blade root failure– January 2001

5

Blade root failure – January 2001

Blade root failure – January 2001

6

Dovetail loads

Disk expansion

Biaxial Dovetail Fatigue Rig

7

Biaxial Rig

Fatigue Failures – biaxial rig

8

Dovetail fretting scar

Rig test results

9

Finite element analysisDifficult to resolve stress field with FEA

Design normally based on g yaverage pressure

Dovetail rig – ‘Fine’ FE modelLocal FE Model

2D

120,000 elements

800 along contact flank

10

Application to a practical problem

8

9

10ΔSIF Vs crack length

K-T limit

2

3

4

5

6

7

ΔK

I MPa

√m

μ=0.10μ=0.20

0 30

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.20

1

2

crack length (mm)

μ=0.30μ=0.40μ=0.50μ=0.65El-Hadad limit

Analysis shows importance of friction in the dovetail problem. Integrity of low-friction coating is important

Step-7

1200

14000.0060.10 0.15 0.20 0.30 0.40 0 50

Distribution of normal pressure in the contact flank

FrictionCoefficient

400

600

800

1000

norm

al tr

actio

n (M

Pa)

0.50 0.65

Step-8

-6 -4 -2 0 2 4 60

200

location in contact flank (mm)

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Distribution of shear traction in the contact flank

100

200

300

-300

-200

-100

0

shea

r tra

ctio

n (M

Pa)

0.0060.10 0.15 0.20 0.30 0 40

Step-8

-6 -4 -2 0 2 4 6-600

-500

-400

location in contact flank (mm)

0.40 0.50 0.65

Application of short crack arrest approach to the dovetail

problem• Short crack arrest approach applied to 22 dovetail

Test Model Coating Friction coefficient

Predicted result

Experimental result

1 T800 Uncoated 0.65 Broken Broken2 T800 Metco + Everlube 0.15 Unbroken Unbroken3 T800 Metco + PL237 0.12 Unbroken Broken4 T800 Metco + PL237 + Molykote 321R 0 1 Unbroken Unbroken22 dovetail

experiments at a range of loads

• 20 experimental results correctly predicted– One conservative

di ti

4 T800 Metco + PL237 + Molykote 321R 0.1 Unbroken Unbroken5 T800 Solgel 0.2 Unbroken Unbroken6 T800 Uncoated 0.65 Broken Broken7 T800 Uncoated 0.65 Broken Broken8 T800 Uncoated 0.65 Broken Broken9 T800 Uncoated 0.65 Broken Broken10 T800 Uncoated 0.65 Broken Broken11 T800 Uncoated 0.65 Broken Broken12 T900 Metco + PL237 0.12 Unbroken Unbroken13 T900 Metco + PL238 0.12 Unbroken Unbroken14 T900 Uncoated 0.65 Broken Unbroken15 T900 Uncoated 0.65 Broken Broken16 T900 Uncoated 0.65 Broken Broken17 T900 Uncoated 0.65 Broken Broken18 T900 Uncoated 0.65 Broken Broken

prediction– One unconservative

preduction

19 T900 Uncoated 0.65 Broken Broken20 T900 Uncoated 0.65 Broken Broken21 T900 Uncoated 0.65 Broken Broken22 T900 Uncoated 0.65 Broken Broken

Experiments include a range of different coatingsShot-peened residual stress field

12

Real geometry and simplified tests

Real geometry Simplified tests

3D view: realcomponent

2D Biaxial Test fixture

23

2D “slice”“In-line”F&R contact

Experimental and analytical studies may be carried Experimental and analytical studies may be carried out with an approximate geometrical model.out with an approximate geometrical model.

Dovetail fretting approximation

T

Di

M

V

P

Q

M

B

Flat and rounded

24Dovetail joint

Blade

DiscPQ

Dovetail

13

Q

P

Possible “in-line” test configurations

P

Q

Hertzian contactR

2b2a

y

B B x

25

R

x

y

2bB B

‘Flat and rounded’ approximation

PQ

Experimental work – In-line rig

A

PLoads

A

Tensile specimen

R

Q

Tensile specimen

σo

Bulk

t

BA

B

A B

Q

Bulk

t

B

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Fretting Pad

PQ

Fretting PadQQ

Bulk max

QQ

Bulkmin

Q=±Q max

P PP P

crack crack

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Shear actuator “In –line” fretting rig

1. Set of experiments carried out on the

fretting padsspecimen

P

f pOxford “in-line” fretting rig using flat and rounded contact pads. Specimens and pads Ti-6Al-4V

2. Tests designed in order to determine fretting performance for various loading conditions and geometries

27 Tension actuator

3. 7 series of tests carried out: 3 geometries, 3 different values of peak pressure and tangential/normal load ratio. Bulk load varied to obtain S-N curves

Test matrixInfluence of geometry

Geometry 2p /σ =0 72Influence of Q/P Series 4

Series 1

Geometry 1p0/σy=0.6Q/P=0.13

p0/σy=0.72Q/P=0.13

Geometry 1p0/σy=0.72Q/P=0.13

Geometry 1p0/σy=0.6Q/P=0.26

Influence of p0

Q Series 4

Series 5

Series 6

Increasing edge radius

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Geometry 3p0/σy=0.52Q/P=0.13

Geometry 3p0/σy=0.72Q/P=0.13

Series 5

Series 2

Series 3

15

Influence of geometry

100

120σ o_ampl

0.12

0.10

/σyNormalised

Increase R

40

60

80

Series 1, geometry 2Series 6, geometry 1Series 3, geometry 3Series 7 geometry 2

0.08

0.06

0.04

Increasing edge radius

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0

20

1.0E+04 1.0E+05 1.0E+06 1.0E+07N.cycles

Series 7, geometry 2Power-law best fit geometry 2Power-law best fit geometry 1Power-law best fit geometry 3

0.02

0.0

If peak pressure is held constant, increasing the edge radius reduces the threshold – a size effect

Influence of pressure (p0): Geometry 1

100

120σ o_amplNormalised

0.12

0.10

40

60

80

Series 4, geometry 1

0.08

0.06

0.04

Increase po

30

0

20

1.0E+04 1.0E+05 1.0E+06 1.0E+07N.cycles

Power-law best fit Series 4Series 6, geometry 1Power-law best fit Series 60.02

0.0

Increasing peak pressure (by increasing normal load) reduces threshold

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100

120σ o_amplNormalised

0.12

0.10

Influence of pressure (p0): Geometry 3

40

60

800.08

0.06

0.04

Increase po

31

0

20

1.0E+04 1.0E+05 1.0E+06 1.0E+07N.cycles

Series 2, geometry 3Power-law best fit Series 2Series 3, geometry 3Power-law best fit Series 30.02

0.0

Influence of tangential force, Q/P

100

120σ o_ampl

0.12

0.10

/σyNormalised

40

60

80

.

0.08

0.06

0.04

Increase Q/P

32

0

20

1.0E+04 1.0E+05 1.0E+06 1.0E+07N.cycles

Series 4, geometry 1Power-law best fit Series 4Series 5, geometry 1Power-law best fit Series 50.02

0.0

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Analysis of experiments• In the engine, the blade root

experiences a large no of HCF cycles• Therefore prediction of fretting fatigue

thresholds is important• Stress gradients are very steep –

traditional approaches may not work all that well

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that well• Therefore the short crack arrest method

is used– can a crack ‘escape’ from the stress concentration?

Short crack arrest approach for contacts: implementation

• Determine surface tractions using singular integral equation approach

• Use Muskhelishvili potentials to determine subsurface stress

• Calculate stress intensity factors using the distributed dislocation method (Nowell and Hills 1987) at the trailing

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dislocation method (Nowell and Hills, 1987), at the trailing edge of the contact, where cracks are found to nucleate.

• Plot SIFs on K-T diagram to establish bulk stress required to raise curve above threshold

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1

σ 0_ampl /σ fl

Validation – published results on Al alloyHertzian - geometry

Threshold predictionAl series1 (p0=157)Al series3 (p0=143)Al series4 (p0=143)Al series5 (p0=120)Al series6 (p0=120)Farris (1998)

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0.10.01 0.1 1 10 100

b/a 0 (p 0 /σ fl ) 1.3 (Q/fP) 1.18

Broken

Run out

Empirical scaling of x-axis may be used to collapse curves

1

Failurefl

amp

σσ _0

Predictions for current experiments on Ti 6-4Flat and rounded geometry, half-plane theory

0.1

p0=500, Q/fP=0.25,0.5,0.7,0.9,1

Non-FailureHighly dependent

on the loading conditions

f

36

0.010.1 1 10 100

Q/fP=0.5, p0=500,600,700,800,900

2

0

0⎟⎟⎠

⎞⎜⎜⎝

⎛−

fl

pa

abσ

Empirical scaling less successful here

19

1σ 0_ampl /σ fl

Point method prediction p0=665 [MPa] Q/fP=0 2p /σ =0 723

Comparison with experimental results (and with critical distance approach)

Point method prediction p0=665 [MPa], Q/fP=0.2Short crack prediction p0=665 [MPa], Q/fP=0.2Series 1: experimental dataSeries 3: experimental dataSeries 6: experimental data

test 6

test 9

tests 11-17, 65

test 27

test 28

66

test 60

test 55

test 54

test 63

p0/σy=0.723 ,p0/σy=0.723 ,

37

0.11 10 100(b-a)/a 0 (p 0 /σ fl ) 2

Run out 10 7Broken test 19

test 20

test 29

test 30

tests 67-68test 66

tests 59,61

test 58

test 57 test 64

tests 71,72

test 73

1

σ 0_ampl /σ fl

P i t th d di ti 0 478 [MP ] Q/fP 0 2/ 0 52

Comparison with experimental results (and with critical distance approach)

Point method prediction p0=478 [MPa], Q/fP=0.2Short crack prediction p0=478 [MPa], Q/fP=0.2Series 2: experimental data

test 23

test 26

test 24

p0/σy=0.52

p0/σy=0.52 ,

38

0.11 10 100(b-a)/a 0 (p 0 / σ fl ) 2

Run out 10 7Broken test 25

test 22

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Summary - Session 10• A range of experimental approaches are available, ranging from

materials ranking tests up to simulation of realistic geometry and loads

• A practical industrial problem was investigated using a geometrically representative test and using a ‘flat and rounded’ geometry to mimic dovetail traction distribution

• Fretting fatigue exhibits ‘notch-like’ behaviour – there is a size effect• Short crack arrest approach using fracture mechanics gives good

predictions of fretting thresholds, based on ΔK0 and σfl as independently measured material parametersindependently measured material parameters

• It is possible to incorporate residual stress fields such as those due to LSP or conventional shot peening

• Friction coefficient is an important parameter. It may be difficult to determine and can vary spatially and temporally

• The short crack approach is used for validation of current designs39