fatigue of aluminium structures en 1999-1-3 -...
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Brussels, 18-20 February 2008 – Dissemination of information workshop 1
EUROCODESBackground and Applications EN 1999-1-3 Fatigue of aluminium structures
Fatigue of aluminium structuresEN 1999-1-3Eurocode 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
Brussels, 18-20 February 2008 – Dissemination of information workshop 2
EUROCODESBackground and Applications EN 1999-1-3 Fatigue of aluminium structures
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
Brussels, 18-20 February 2008 – Dissemination of information workshop 3
EUROCODESBackground and Applications Introduction
FatigueCrack initiation and propagation due to repeated loading
Examples of repeated loading:- Traffic loads- Wave loads- Wind loads
Brussels, 18-20 February 2008 – Dissemination of information workshop 4
EUROCODESBackground and Applications Introduction
Fatigue and aluminiumImportant for aluminium structures, due to generally high ratio
between life load and dead load
Example of fatigue crack:
Brussels, 18-20 February 2008 – Dissemination of information workshop 5
EUROCODESBackground and Applications Introduction
EurocodesLoads Design Execution / Inspection
Eurocode 1EN 1991
1-1: General structural rules
1-2: Fire design
1-5: Shells
1-4: Cold-formed sheets
1-3: Fatigue design
Eurocode 9EN 1999
EN 1090-3ExecutionQuality /Quality controlInspection /Supervision
Brussels, 18-20 February 2008 – Dissemination of information workshop 6
EUROCODESBackground and Applications Introduction
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
Brussels, 18-20 February 2008 – Dissemination of information workshop 7
EUROCODESBackground and Applications Fatigue design procedures
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
Brussels, 18-20 February 2008 – Dissemination of information workshop 8
EUROCODESBackground and Applications Safe life design
S-N curves• S-N curve: Log-based relation between stress range and number
of cycles:
Number of cycles N
Number of cycles N
4 5 6
Stre
ss ra
nge
∆σ
Brussels, 18-20 February 2008 – Dissemination of information workshop 9
EUROCODESBackground and Applications Safe life design
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 and6xxx alloys listed EN1999-1-1
– Different (more favourable) curvefor alloy 7020 Influence of alloy on S-N curve:
Welds– Same S-N curve for all alloys
m1 = 7
m2 = 7m1 = 4.3
m2 = 6.3m1 = 3.4
m2 = 5.4
Brussels, 18-20 February 2008 – Dissemination of information workshop 10
EUROCODESBackground and Applications Safe life design
Mean stress effect• Taken into account by bonus factor
∆σC(R) = f ∆σC
σσσσ
Δ+Δ−
=res
reseffR
22
-1 -0.5 0 0.5 1R [-]
1
1.3
1.6
Bon
us fa
ctor
f [-]
connections,σres = known
Base material
Normal case
Brussels, 18-20 February 2008 – Dissemination of information workshop 11
EUROCODESBackground and Applications Safe life design
Types of details distinguished (1)• Parent metal
• Welded attachments
• Longitudinal welds
• Butt welds
∆σc
70-100
18-36
28-63
18-56
Brussels, 18-20 February 2008 – Dissemination of information workshop 12
EUROCODESBackground and Applications Safe life design
Types of details distinguished (2)• Fillet welds
• Crossing weldsin built-up beams
• Bolted joints
• Adhesive bondedjoints
∆σc
12-28
18-40
56
Brussels, 18-20 February 2008 – Dissemination of information workshop 13
EUROCODESBackground and Applications Safe life design
Variable amplitude load• Rainflow-counting method
• Spectrum
• Palmgren-Miner damageaccumulation:
D = Σ
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)
ni
Nii
∆σ
n
∆σ1∆σ2∆σ3
∆σ4
n1 n2 n3 n4
Brussels, 18-20 February 2008 – Dissemination of information workshop 14
EUROCODESBackground and Applications Safe life design
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
pla
te
Flat plate
σnom
Stress increase due to weld
Complex geometry
Bra
ce w
all
Chord wall
σnom
Stress increase due to weld
Stress increase due to overall joint geometry
Brussels, 18-20 February 2008 – Dissemination of information workshop 15
EUROCODESBackground and Applications Safe life design
Hot spot stress methodNot 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 0.4t
σhs
Brussels, 18-20 February 2008 – Dissemination of information workshop 16
EUROCODESBackground and Applications Safe life design
Reference detail procedureMethod 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:
∆σC,assess = ∆σC,refσHS,ref
σHS,assess
Brussels, 18-20 February 2008 – Dissemination of information workshop 17
EUROCODESBackground and Applications Safe life design
Steel versus aluminium (1)Fatigue design values at 2.106 cycles for various details
Brussels, 18-20 February 2008 – Dissemination of information workshop 18
EUROCODESBackground and Applications Safe life design
Steel versus aluminium (2)Different designs
Steel bridge deck Aluminium bridge deckWelded Extruded
dwarsdrager
dwarsdrager
hoofdliggertrog
dekplaat
trogstuiklas
kruising dwarsdragerlijf-trogbeen
Brussels, 18-20 February 2008 – Dissemination of information workshop 19
EUROCODESBackground and Applications Damage tolerant design
Damage tolerant design
Brussels, 18-20 February 2008 – Dissemination of information workshop 20
EUROCODESBackground and Applications Damage tolerant design
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 ∆Ka
Brussels, 18-20 February 2008 – Dissemination of information workshop 21
EUROCODESBackground and Applications Damage tolerant design
Explanation damage tolerant design (2)
da / dN ∆K
• Central through-crackin infinitely large plate:
• Other geometries:
a π σ =K
a π σY =K
σ
σ
Brussels, 18-20 February 2008 – Dissemination of information workshop 22
EUROCODESBackground and Applications Damage tolerant design
Determination of YY = geometry factor, to be determined with FEM
X
Y
Z
X
Y
Z
Model: TROG Def = .236E4
Brussels, 18-20 February 2008 – Dissemination of information workshop 23
EUROCODESBackground and Applications Damage tolerant design
Crack propagation dataAvailable for parent metal, HAZ and weld metal for (welded) aluminium alloys
∆K
R=0.1da / dN
Brussels, 18-20 February 2008 – Dissemination of information workshop 24
EUROCODESBackground and Applications Design by testing
Design by testing
Brussels, 18-20 February 2008 – Dissemination of information workshop 25
EUROCODESBackground and Applications Design by testing
Testing for fatigue designRequired 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
Brussels, 18-20 February 2008 – Dissemination of information workshop 26
EUROCODESBackground and Applications Design by testing
AcceptanceFor safe life design:
TL =
TL = Design life (in cycles)Tm = Mean life to failure determined by tests (in cycles)F = Fatigue test factor, depending on number of tests
TmF
Brussels, 18-20 February 2008 – Dissemination of information workshop 27
EUROCODESBackground and Applications Design by testing
Fatigue test factorIn agreement with EN 1990:
1 10 100Sample size n [-]
0
1
2
3
4
Fatig
ue te
st fa
ctor
F [-
]
simultaneous
independent
Brussels, 18-20 February 2008 – Dissemination of information workshop 28
EUROCODESBackground and Applications Safety factors and reliability
Safety factors and reliability
Brussels, 18-20 February 2008 – Dissemination of information workshop 29
EUROCODESBackground and Applications Safety factors and reliability
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)
mR
ms
Log N
Log ∆σ
Log Ncrit
Rk
S-N Curve standard
kRsR
Sk Characteristic value loadkSsS
Rk(1-1/γM)Sk(γF-1)
Brussels, 18-20 February 2008 – Dissemination of information workshop 30
EUROCODESBackground and Applications Safety factors and reliability
Reliability index
β = μR - μS
√σR2 - σS
2
Brussels, 18-20 February 2008 – Dissemination of information workshop 31
EUROCODESBackground and Applications Safety factors and reliability
Example of reliability EN 1999-1-3By Prof. Kosteas
kR = 2.0kS = 1.0
sr = 0.03-0.18ss = 0.03-0.15
Brussels, 18-20 February 2008 – Dissemination of information workshop 32
EUROCODESBackground and Applications Safety factors and reliability
Example of reliability EN 1999-1-3
Brussels, 18-20 February 2008 – Dissemination of information workshop 33
EUROCODESBackground and Applications Safety factors and reliability
Example of reliability EN 1999-1-3• γM γF = 1.0 β ≈ 2.2• γM γF > 1.35 β ≈ 3.5
• Recommended value: β ≈ 3.8
• This issue needs further elaboration
Brussels, 18-20 February 2008 – Dissemination of information workshop 34
EUROCODESBackground and Applications Fatigue design and execution
Fatigue design and execution
Brussels, 18-20 February 2008 – Dissemination of information workshop 35
EUROCODESBackground and Applications Fatigue design and execution
Relation EurocodesConsequence 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
Brussels, 18-20 February 2008 – Dissemination of information workshop 36
EUROCODESBackground and Applications Fatigue design and execution
Execution classes
Conseq. class
CC1 CC2 CC3
Service category
Quasi static
Fatigue Quasi static
Fatigue Quasi static
Fatigue
Non-welded
1 1 2 3 3 3
Welded 1 2 2 3 3 4
Brussels, 18-20 February 2008 – Dissemination of information workshop 37
EUROCODESBackground and Applications Further developments
Further developments
Brussels, 18-20 February 2008 – Dissemination of information workshop 38
EUROCODESBackground and Applications Further developments
Harmonisation of imperfections / classification
h
t
Brussels, 18-20 February 2008 – Dissemination of information workshop 39
EUROCODESBackground and Applications Further developments
Friction stir welding
5083-O parent5083-O fsw
EC9-1-3 High qualityEC9-1-3 Normal quality
10
100
400
104 105 106 107 108
190 %
300 %
N
∆σ
Brussels, 18-20 February 2008 – Dissemination of information workshop 40
EUROCODESBackground and Applications Conclusions
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 andacceptance
• Further work– Friction stir welding– Damage tolerant design