Fatigue Crack Propagation and Stable Tearing in Friction-Stir-Welded Aluminum Sheet

Presented by: Eui I. LimThe Boeing Company

At: ASIP 2006San Antonio, TexasNovember 28, 2006

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Collaborative Effort Univ of So Carolina

Raphael MuzzoliniJean-Christophe Ehrstrom, PhD

Nathalie FuzierAnthony Reynolds, PhD

Kenneth K. ChanEui I. Lim

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Initiation

• Seemingly Consistent Crack Turnings– FCGR test on 7xxx-T7 Sheet– 3 Samples each, from 3 Lots

FSW sample Base metal

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Motivation

• Understand Phenomenon– Why did the Cracks Turned?– Can it be Captured, and

Controlled?• Via Experiments

– FCGR and stable tearing– C(t) and M(t) coupons– Cracks to FSW @ 90º & 45º

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Outline

1. Material and weld properties

2. Fatigue tests– Geometry– Crack path– da/dN curves– Residual stress measurements

3. Stable tearing tests– Crack path– Properties

4. Fractography

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Material and Welding Description• Aluminum 7xxx-T7 Sheet , t = 5mm.• Post Weld Aging heat treat to stabilize properties of the weld.

Profile of the Tool :R

etre

atin

g si

de

Adva

ncin

g sid

e

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Weld Properties

5.95477 (69)Weld (T-L)13.98.1552 (80)Base metal (T-L)

Elongation at rupture (%)

Elongation at UTS (%)

UTSMPa (ksi)

Weld UTS : 86% of base material

engineering strain (%)0 2 4 6 8 10 12 14 16

engi

neer

ing

stre

ss (M

Pa)

0

100

200

300

400

500

600

(ksi

)

0

20

40

60

80

welded sample

base material

TS

L

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Hardness Profile

Distance from the centerline of the weld (mm)-30 -20 -10 0 10 20 30 40

Mic

roha

rdne

ss(H

V)

100

120

140

160

180

200

(inches)

-1.0 -0.5 0.0 0.5 1.0 1.5

Advancing side Retreating side

ST

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Outline

1. Material and weld properties

2. Fatigue tests– Geometry– Crack path– da/dN curves– Residual stress measurements

3. Stable tearing tests– Crack path– Properties

4. Fractography

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C(T) Samples

Dimensions are in inches.

LT 90º orientation 45º orientation

L

T

dθ

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Crack Path 90º OrientationFatigue cycling at constant ΔK = 15 MPa*m1/2

Propagation along straight line, slight deviation angle.

R=0.1

Angle 13.7º 15º < 5º

Angle 7º < 5º < 5º

TL

R=0.5

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Crack Path 45º OrientationCurved crack path.

L T

R=0.1

32º (local max) 38º (local max) 10º

17º (local max) 21º (local max) 11.5º

R=0.5

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Fatigue Results

R=0.5 ΔK=15MPa*m1/2L

T

Distance from the centerline of the weld (mm)

-20 -10 0 10 20 30 40 50 60

da/d

N(m

m/c

ycle

)

0.0001

0.001

0.01

(inches)

-0.5 0.0 0.5 1.0 1.5 2.0

(inch

/cyc

le)

1e-5

1e-4

weld

parent

Cracking Direction

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Fatigue Results

R=0.1 ΔK=15MPa*m1/2

LT

weld

parent

Cracking Direction

Distance from the centerline of the weld (mm)

-30 -20 -10 0 10 20 30 40 50 60

da/d

N(m

m/c

ycle

)

1e-7

1e-6

1e-5

1e-4

1e-3

(inches)

-1 0 1 2

(inch

/cyc

le)

1e-8

1e-7

1e-6

1e-5

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Residual Stress Evaluation

Forward determination of the stress intensity factor due to residual stresses :

By “Cut Compliance” technique - Measure Δε at each additional cut

dad

aZEaK resI

ε)(')( =

where Z(a): a geometrical function

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Kresidual

True Kmax applied at the crack tip, Kmaxtotal, is a summation of Kresidualand Kmax generated by loading.

LT

(inch)

Slot cutting direction

K re

s (M

Pa*m

1/2 )

(ksi

*inch

1/2 )

Distance from the centerline of the weld (mm)

-40 -30 -20 -10 0 10 20 30 40 50-20

-15

-10

-5

0

5

10

15-1 0 1

-15

-10

-5

0

5

10

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Residual Stress Profile Calculated from K

Advancing side Retreating side

Distance from the centerline of the weld (mm)-40 -30 -20 -10 0 10 20 30 40 50 60

Res

idua

l stre

ss (

MP

a)

-100

-50

0

50

100

150

(inch)-1 0 1 2

(ksi

)

-10

0

10

20

LT

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Outline

1. Material and weld properties

2. Fatigue tests– Geometry– Crack path– da/dN curves– Residual stress measurements

3. Stable tearing tests– Crack path– Properties

4. Fractography

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Stable Tearing - Parent Material

Rolling Direction appears to be the Preferred Path for Crack Extension.

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Stable Tearing - Welded Samples

Crack turns so it extends along the L direction. Combined effect of the microstructure and the welding?

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Stable Tearing Data

0

2

4

6

8

10

12

0 5 10 15 20 25 30 35delta a (mm)

load

(kN

)

-0.3

0.2

0.7

1.2

1.7

2.2

2.7-0.1 0.1 0.3 0.5 0.7 0.9 1.1 1.3

(inch)

(kip

)

base metal

classic weld (12 ipm)

slow weld (4 ipm)

Series2

weld

LT

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Stable tearing data

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Outline

1. Material and weld properties

2. Fatigue tests– Geometry– Crack path– da/dN curves– Residual stress measurements

3. Stable tearing tests– Crack path– Properties

4. Fractography

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Fractography – Fatigue R=0.1

SEM pictures of Fracture Surface

ST

100 μm

10 μm

Inside the nugget

Appears mostly inter-granular

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Inter-granular Fracture

Surface rougher than for R=0.1

Fractography – Fatigue R=0.5

ST

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Stable Tearing – Nugget

ST

R A

SEM pictures

100 μm

20 μm 5 μm

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Summary

• Weld characterization:– With appropriate welding parameters, weld UTS would be 86%

of the base material.

• Crack path:– For 90º oriented fatigue samples, propagation along a straight

line, with sometimes a slight deviation angle observed.

– For 45º oriented fatigue sample, curved crack path.

– In base metal stable tearing tests, the rolling direction is often a preferred path direction for the crack.

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Summary

• Fatigue crack growth rate : – Compared to parent material, reduced crack growth rate is

observed in the HAZ. This reduction is higher for lower R.

– Residual stress seem to have a predominant effect in this variation. Closure and microstructure might also be involved.

• Fractography:– In fatigue, fracture is mostly inter-granular. The surface is

rougher for higher R.

– In stable tearing, the fracture is also clearly inter-granular.

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Future Work

• Investigate Residual Stress Affect due to FSW

• Acquire Crack Growth Properties Along the Weld, HAZ from both the Advancing Side and the Retreating Side of the Weld

• Acquire Crack Growth Properties Across the Weld, HAZ from both the Advancing Side and the Retreating Side of the Weld

• Repeat Testing and Data Acquisitions for other Materials Commonly used in FSW