Detailed strain field analyses of fatigue cracks in friction stir - - PowerPoint PPT Presentation

6-7-2009

Challenge the future

Delft University of Technology

Detailed strain field analyses of fatigue cracks in friction stir welded joints

H.J.K. Lemmen, R.C. Alderliesten,

• R. Benedictus

ICAF 2009, Rotterdam Aerospace Structures and Materials, Faculty of Aerospace Engineering

2

Detailed strain field analyses of fatigue cracks in FS welded joints

σ∞ σ∞

Introduction

ICAF 2007: Fatigue crack initiation behaviour of friction stir

welded joints in aluminium alloy

Fracture Direction of crack growth Width of nugget Different material flows Hole

How affects an FS weld the fatigue crack growth behaviour? Width = 50mm

3

Detailed strain field analyses of fatigue cracks in FS welded joints

Contents

• Introduction
• Friction Stir (FS) welding
• Residual stress profiles
• Yield strength profiles
• Fatigue Crack Growth (FCG) test
• Loads & specimen configuration
• Digital Image Correlation (DIC)
• FCG test results
• FCG curves
• Results from DIC
• Conclusions
• Questions

4

Detailed strain field analyses of fatigue cracks in FS welded joints

Friction Stir Welding

Advancing Side (AS) Retreating Side (RS)

Tool rotation Downward welding force weld Welding direction

L LT

AS RS HAZ HAZ TMAZ Nugget

Process parameters: Alloys: AA2024-T3 AA7075-T6 AA6013-T4 Thickness: 2.5 mm 2.0 mm 1.8 mm Welding speed: 350 mm/min 300 mm/min 1000 mm/min Rotational speed: 550 rpm 280 rpm 1500 rpm Downward force: 19 kN 18 kN 14 kN

crown root

Pin Shoulder

FS welding performed by EADS in Munich

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Detailed strain field analyses of fatigue cracks in FS welded joints

• 100

100 200 300 400 500

• 20
• 15
• 10
• 5

5 10 15 20

distance from weld centre [mm] stress [MPa] RS AS Tool shoulder Tool Pin

Residual stress profiles in FS welds

AA2024-T3 AA6013-T4 AA7075-T6

Residual stresses measured by X-ray diffraction

6

Detailed strain field analyses of fatigue cracks in FS welded joints

• 100

100 200 300 400 500

• 20
• 15
• 10
• 5

5 10 15 20

distance from weld centre [mm] stress [MPa] RS AS Tool shoulder Tool Pin

Yield strength profiles of FS welds

AA2024-T3 AA7075-T6 AA6013-T4

results presented at AIAA/SDM conference 2008

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Detailed strain field analyses of fatigue cracks in FS welded joints

FS weld properties & FCG behaviour

yield strength: residual stress:

Direction of crack growth

FS weld Undisturbed plastic zone Fatigue crack

σ

y i e l d , . 2

Yield strength profile:

FS weld centre

σcrack tip = Kt * σ∞ Kt * σ∞ σresidual

Residual stress Profile:

σcrack tip

FCG Direction in FS weld

FCG retardation? ??

8

Detailed strain field analyses of fatigue cracks in FS welded joints

Fatigue crack growth test

Goal: to repeat and measure the behaviour observed in the FI specimen

AS RS

x y

2a = 5mm

90º

L LT

AS RS a = 2.5mm

x y

45º

L LT

d = 3mm AS RS

x y

0º

L LT

Three configurations of centre crack FCG test specimen: Effect of residual stress Combined effect & Change of FCG direction Effect of yield strength Constant amplitude loading; R = 0.1; σa = 60MPa

9

Detailed strain field analyses of fatigue cracks in FS welded joints

Digital image correlation

Digital camera X-Y-Z axes electronically controlled rack CCT specimen Grid of images Images after DIC analyses, with overlay of strain field

For examples of applications for this DIC tool have a look at the poster presentations of

• Mr. Wilson, Mr. Rodi, Mr. Khan, and Mr. Delgrange.

Strains are determined by analysing the images of a test DIC software has been developed by the author

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Detailed strain field analyses of fatigue cracks in FS welded joints

Centreline FS weld

Results DIC

Details: alloy = AA2024-T3; 90º configuration; y= 6.5mm;

Centreline FS weld Centreline FS weld

11

Detailed strain field analyses of fatigue cracks in FS welded joints

Plastic zone behaviour

a=6.5mm a=9mm a=11mm Crack path

Details: > AA2024-T3; > 90º conf.; > y= 6.5mm;

σ∞ σ∞

Small butterfly wings Large butterfly wings

150 250 350 450 550

• 15
• 10
• 5

5 10 15

distance from weld centre [mm] stress [MPa]

AA2024-T3 AA7075-T6 AA6013-T4

Y = 6.5 mm

Weld centre

12

Detailed strain field analyses of fatigue cracks in FS welded joints

FCG behaviour 90º conf.

0.E+00 1.E-03 2.E-03 3.E-03 4.E-03 5.E-03 5 10 15 20

a [mm] da/dN [mm/cycle]

base AA2024-T3 FS weld, 90º config, spec 1, y = 0.0mm FS weld, 90º config, spec 2, y = 3.0mm FS weld, 90º config, spec 3, y = 6.5mm FS weld, 90º config, spec 4, y = 6.5mm

0.0mm 3.0mm 6.5mm 6.5mm base

Details: > AA2024-T3; > 90º conf.;

σ∞ σ∞

FCG retardation FCG acceleration

13

Detailed strain field analyses of fatigue cracks in FS welded joints

Plastic zone behaviour

a=5.0mm a=8.0mm a=10.5mm

Details: > AA2024-T3; > 90º conf.;

σ∞ σ∞

Y= 0.0mm Y= 3.0mm base

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Detailed strain field analyses of fatigue cracks in FS welded joints

FCG behaviour 0º configuration

σ∞ σ∞

Details: > AA7075-T6; > 0º conf.;

0.E+00 1.E-03 2.E-03 3.E-03 4.E-03 5.E-03 5 10 15 20 25

a [mm] da/dN [mm/cycle]

base AA7075-T6-3, 1 saw cut FS welded AA7075-T6; 0º conf; spec 1 FS welded AA7075-T6; 0º conf; spec 2

Weld centre Tool edge Tool edge Weld centre Tool edge

retardation acceleration rapid acceleration Base material

15

Detailed strain field analyses of fatigue cracks in FS welded joints

σ∞ σ∞

Details: > AA2024-T3; > 45º conf.;

FCG behaviour 45º behaviour

16

Detailed strain field analyses of fatigue cracks in FS welded joints

FCG behaviour 45º behaviour

σ∞ σ∞

Details: > AA2024-T3; > 45º conf.;

a=5mm a=12mm a=20mm a=7.5mm W e l d i n g d i r e c t i

• n

Crack path

Weld centre Tool edge

17

Detailed strain field analyses of fatigue cracks in FS welded joints

FCG behaviour 45º behaviour

σ∞ σ∞

Details: > AA7075-T6; > 45º conf.;

1 2 3 4 5 6 7 8

Failure after several centimetres

18

Detailed strain field analyses of fatigue cracks in FS welded joints

FCG behaviour 45º behaviour

W e l d c e n t r e T

• l

e d g e W e l d i n g d i r e c t i

• n

Crack path

Material flows in Nugget σ∞ σ∞

Details: > AA7075-T6; > 45º conf.;

19

Detailed strain field analyses of fatigue cracks in FS welded joints

FCG behaviour 45º behaviour

σ∞ σ∞

Details: > AA7075-T6; > 45º conf.;

W e l d c e n t r e T

• l

e d g e W e l d i n g d i r e c t i

• n

Crack path

Material flows in Nugget

20

Detailed strain field analyses of fatigue cracks in FS welded joints

σ∞ σ∞

Introduction

ICAF 2007: Fatigue crack initiation behaviour of friction stir

welded joints in aluminium alloy

Fracture Direction of crack growth Width of nugget Different material flows Hole

How affects an FS weld the fatigue crack growth behaviour?

21

Detailed strain field analyses of fatigue cracks in FS welded joints

Conclusions

• The yield strength affects the FCG rate, depending on the
• rientation of the FS weld.
• The residual stresses affect the FCG rate, with the largest effect

found for the 90º configuration

• The micro structure in the nugget of the FS weld provides a path

with low fatigue resistance

• Digital image correlation has a large potential in future research in

fatigue.

22

Detailed strain field analyses of fatigue cracks in FS welded joints

Questions?

23

Detailed strain field analyses of fatigue cracks in FS welded joints

FCG behaviour 0º configuration

σ∞ σ∞

Details: > AA2024-T3; > 0º conf.;

a=5mm Weld centre Tool edge a=10mm Welding direction

Crack path

a=7.5mm a=15mm a=22mm Tool edge