EUROCODES Background and Applications

EN 1999-1-3 Fatigue of aluminium structures

Brussels, 18-20 February 2008 – Dissemination of information workshop

Fatigue of aluminium structures EN 1999-1-3 Eurocode 9: Design of aluminium structures

Prepared by: Univ. Prof. Dr.-Ing. Dr.-Ing. habil. Dimitris Kosteas Technische Universität München, Germany Presentation by: Johan Maljaars Eindhoven University of Technology, The Netherlands TNO, The Netherlands

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EUROCODES Background and Applications

EN 1999-1-3 Fatigue of aluminium structures

Brussels, 18-20 February 2008 – Dissemination of information workshop

Contents • Introduction • Fatigue design procedures – Safe life design (SN-curve) – Damage tolerant design (fracture mechanics) – Design assisted by testing

• Safety factors and reliability • Fatigue design and execution • Future developments – Harmonisation for imperfections / classification – Friction stir welding

• Conclusions

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EUROCODES Background and Applications

Introduction

Brussels, 18-20 February 2008 – Dissemination of information workshop

Fatigue Crack initiation and propagation due to repeated loading Examples of repeated loading: - Traffic loads - Wave loads - Wind loads

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EUROCODES Background and Applications

Introduction

Brussels, 18-20 February 2008 – Dissemination of information workshop

Fatigue and aluminium Important for aluminium structures, due to generally high ratio between life load and dead load Example of fatigue crack:

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Introduction

EUROCODES Background and Applications

Brussels, 18-20 February 2008 – Dissemination of information workshop

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Eurocodes Loads Eurocode 1 EN 1991

Design Eurocode 9 EN 1999 1-1: General structural rules 1-2: Fire design 1-3: Fatigue design 1-4: Cold-formed sheets 1-5: Shells

Execution / Inspection EN 1090-3 Execution Quality / Quality control Inspection / Supervision

EUROCODES Background and Applications

Introduction

Brussels, 18-20 February 2008 – Dissemination of information workshop

Background to EN 1999-1-3 • General format of code for steel structures, but with specific issues for aluminium

• Based on a comprehensive and reliable data bank – Small-size component tests (shown here) – Full-size component tests

• Refined statistical evaluation of test results

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EUROCODES Background and Applications

Fatigue design procedures

Brussels, 18-20 February 2008 – Dissemination of information workshop

Fatigue design procedures Three methods in EN 1999-1-3

• Safe life design – S-N curve approach • Damage tolerant design – Fracture mechanics approach • Design assisted by testing

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Safe life design

EUROCODES Background and Applications

Brussels, 18-20 February 2008 – Dissemination of information workshop

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S-N curves • S-N curve: Log-based relation between stress range and number

Stress range ∆σ

of cycles:

4

5

6

Number of cycles N Number of cycles N

EUROCODES Background and Applications

Safe life design

Brussels, 18-20 February 2008 – Dissemination of information workshop

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S-N curves for aluminium • Characteristic values depending on detail • 3 slopes in range N = 105 to 5.106: – m1 = 7 for base metal – m1 = 4.3 or 3.4 for welded details

• Influence of alloy on S-N curve: Base material – Same S-N curve for all 5xxx and 6xxx alloys listed EN1999-1-1 – Different (more favourable) curve for alloy 7020 Influence of alloy on S-N curve:

Welds – Same S-N curve for all alloys

m1 = 7 m1 = 4.3

m1 = 3.4

m2 = 7 m2 = 6.3

m2 = 5.4

Safe life design

EUROCODES Background and Applications

Brussels, 18-20 February 2008 – Dissemination of information workshop

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Mean stress effect • Taken into account by bonus factor ∆σC(R) = f ∆σC

Bonus factor f [-]

1.6

Base material 1.3 connections, σres = known 1

Normal case -1

-0.5

0 R [-]

0.5

1

Reff =

2σ res − Δσ 2σ res + Δσ

EUROCODES Background and Applications

Brussels, 18-20 February 2008 – Dissemination of information workshop

Safe life design 11

Types of details distinguished (1) • Parent metal

• Welded attachments

• Longitudinal welds

∆σc 70-100 18-36

28-63

• Butt welds

18-56

EUROCODES Background and Applications

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Safe life design 12

Types of details distinguished (2) • Fillet welds

∆σc

12-28

• Crossing welds in built-up beams

• Bolted joints

• Adhesive bonded joints

18-40

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EUROCODES Background and Applications

Safe life design

Brussels, 18-20 February 2008 – Dissemination of information workshop

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Variable amplitude load • Rainflow-counting method • Spectrum ∆σ

• Palmgren-Miner damage accumulation: ni D = Σi Ni

∆σ1

n1

∆σ2

n2

∆σ3

n3

∆σ4 n4

ni Number of cycles in spectrum with range Δσi (load) Ni Number of cycles according to S-N curve with range Δσi (resistance) D Accumulative damage (shall be ≤ 1.0)

n

Safe life design

EUROCODES Background and Applications

Brussels, 18-20 February 2008 – Dissemination of information workshop

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Geometric stress analysis • Local stress concentration factor (due to weld geometry) in classification of detail • Global stress concentration factor (due to overall joint geometry) depends on geometry: Simple geometry

Flat plate

Stress increase due to weld

Stress increase due to overall joint geometry

σnom Chord wall

Brace wall

σnom

Flat plate

Stress increase due to weld

Complex geometry

Safe life design

EUROCODES Background and Applications

Brussels, 18-20 February 2008 – Dissemination of information workshop

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Hot spot stress method Not applicable according to EN 1999-1-3, because: • Variable stress-strain relationship in HAZ • Finite element results depend on mesh density, type of modelling and type of elements

σnom t

σhs 0.4t

EUROCODES Background and Applications

Safe life design

Brussels, 18-20 February 2008 – Dissemination of information workshop

Reference detail procedure Method in EN 1993-1-1 to take into account the joint geometry: • Select similar detail (reference detail) and its S-N curve from EN 1999-1-2 • Identify type for stress of reference and assessed details • Establish same FEM on both reference and assessed details • Load both details with stress identified above • Determine hot spot stress ranges (HS) for both details • Assume for assessed detail the same slopes m1 and m2 as in reference detail • Estimate fatigue strength for assessed detail at 2 x 106 cycles:

σHS,ref ∆σC,assess = σ ∆σC,ref HS,assess

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EUROCODES Background and Applications

Safe life design

Brussels, 18-20 February 2008 – Dissemination of information workshop

Steel versus aluminium (1) Fatigue design values at 2.106 cycles for various details

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Safe life design

EUROCODES Background and Applications

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Steel versus aluminium (2) Different designs Steel bridge deck Welded

Aluminium bridge deck Extruded dekplaat

dwarsdrager trogstuiklas trog hoofdligger dwarsdrager kruising dwarsdragerlijf-trogbeen

EUROCODES Background and Applications

Damage tolerant design

Brussels, 18-20 February 2008 – Dissemination of information workshop

Damage tolerant design

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Damage tolerant design

EUROCODES Background and Applications

Brussels, 18-20 February 2008 – Dissemination of information workshop

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Explanation damage tolerant design (1) • Analytical background • Starts with an initial defect • Predicts crack growth Crack growth due to one stress cycle related to joint geometry: da / dN

ÅÆ ∆K

a

Damage tolerant design

EUROCODES Background and Applications

Brussels, 18-20 February 2008 – Dissemination of information workshop

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Explanation damage tolerant design (2) da / dN

ÅÆ ∆K

σ

• Central through-crack in infinitely large plate:

K=σ πa • Other geometries:

K = Yσ π a

σ

EUROCODES Background and Applications

Damage tolerant design

Brussels, 18-20 February 2008 – Dissemination of information workshop

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Determination of Y Y = geometry factor, to be determined with FEM

Model: TROG Def = .236E4

Y Z

Y Z

X

X

EUROCODES Background and Applications

Damage tolerant design

Brussels, 18-20 February 2008 – Dissemination of information workshop

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Crack propagation data Available for parent metal, HAZ and weld metal for (welded) aluminium alloys

da / dN

R=0.1

∆K

EUROCODES Background and Applications

Design by testing

Brussels, 18-20 February 2008 – Dissemination of information workshop

Design by testing

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EUROCODES Background and Applications

Design by testing

Brussels, 18-20 February 2008 – Dissemination of information workshop

Testing for fatigue design Required in case: • Applied load spectrum is not available • Geometry of structure is too complex for practical calculations • Different materials, dimensional details, or manufacturing methods than those given in detail category tables • Crack growth data are needed for damage tolerant design

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EUROCODES Background and Applications

Design by testing

Brussels, 18-20 February 2008 – Dissemination of information workshop

Acceptance For safe life design: TL =

Tm F

TL Tm F

= Design life (in cycles) = Mean life to failure determined by tests (in cycles) = Fatigue test factor, depending on number of tests

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EUROCODES Background and Applications

Design by testing

Brussels, 18-20 February 2008 – Dissemination of information workshop

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Fatigue test factor In agreement with EN 1990:

Fatigue test factor F [-]

4 3 independent 2 1

simultaneous

0 1

10 Sample size n [-]

100

EUROCODES Background and Applications

Safety factors and reliability

Brussels, 18-20 February 2008 – Dissemination of information workshop

Safety factors and reliability

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Safety factors and reliability

EUROCODES Background and Applications

Brussels, 18-20 February 2008 – Dissemination of information workshop

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Partial safety factors • Partial safety factor on loading γFf = between 1.0 and 1.5 • Partial safety factor for fatigue strength γMf = 1.0 (excluding adhesive bonded joints)

Log ∆σ

mR Rk

kRsR

Rk(1-1/γM)

Sk ms

S-N Curve standard

Sk(γF-1) kSsS

Log Ncrit

Characteristic value load

Log N

EUROCODES Background and Applications

Safety factors and reliability

Brussels, 18-20 February 2008 – Dissemination of information workshop

Reliability index β=

μR - μS √σR2 - σS2

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EUROCODES Background and Applications

Safety factors and reliability

Brussels, 18-20 February 2008 – Dissemination of information workshop

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Example of reliability EN 1999-1-3 By Prof. Kosteas

kR = 2.0 kS = 1.0 sr = 0.03-0.18 ss = 0.03-0.15

EUROCODES Background and Applications

Safety factors and reliability

Brussels, 18-20 February 2008 – Dissemination of information workshop

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Example of reliability EN 1999-1-3

EUROCODES Background and Applications

Safety factors and reliability

Brussels, 18-20 February 2008 – Dissemination of information workshop

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Example of reliability EN 1999-1-3 • γM γF = 1.0 Æ β ≈ 2.2 • γM γF > 1.35 Æ β ≈ 3.5 • Recommended value: β ≈ 3.8 • This issue needs further elaboration

EUROCODES Background and Applications

Fatigue design and execution

Brussels, 18-20 February 2008 – Dissemination of information workshop

Fatigue design and execution

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EUROCODES Background and Applications

Fatigue design and execution

Brussels, 18-20 February 2008 – Dissemination of information workshop

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Relation Eurocodes Consequence class CC (3)

EN 1990

Execution class EXC (4)

EN 1999-1-1

Execution requirements

EN 1090-3

Acceptance criteria for welds in EN ISO 10042

Fatigue design and execution

EUROCODES Background and Applications

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Execution classes Conseq. class Service category

CC1

Quasi static

CC2

Fatigue Quasi static

CC3

Fatigue

Quasi static

Fatigue

Nonwelded

1

1

2

3

3

3

Welded

1

2

2

3

3

4

EUROCODES Background and Applications

Further developments

Brussels, 18-20 February 2008 – Dissemination of information workshop

Further developments

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Further developments

EUROCODES Background and Applications

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Harmonisation of imperfections / classification

h

t

Further developments

EUROCODES Background and Applications

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Friction stir welding

400

∆σ 5083-O parent 5083-O fsw

100

190 % EC9-1-3 High quality

300 %

10

EC9-1-3 Normal quality 104

105

106

107

N

108

EUROCODES Background and Applications

Conclusions

Brussels, 18-20 February 2008 – Dissemination of information workshop

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Conclusions & recommendations • EN 1999-1-3 is powerful fatigue design tool. It enhances: – Safe life design (S-N curves) – Damage tolerant design (fracture mechanics) – Design by testing

• Evaluation and simplification of requirements for inspection and acceptance

• Further work – Friction stir welding – Damage tolerant design