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© Fraunhofer
Improved fatigue design methods for offshore wind turbine rotor blades considering non-linear Goodman analysis combined with finite element analysis
Stefan Wessels
Michael Strobel
Dr. Arno van Wingerde
Isabel Koprek
Dr. Hans-Gerd Busmann
© Fraunhofer
Outline
Idea & Motivation
Used Fundamentals
Structural Model
Programming the fatigue Analysis based on NLGD´s
Validation
Fatigue Analysis on a rotor blade model
Conclusion & Outlook
© Fraunhofer
Idea & Motivation
Square – Cube – Law leads to increasing blade weights
Conservative approaches in blade design
Material properties of fiber composites can be exploited further
Goal:
Weight & Cost reduction in the early design stage
Track:
Damage calculation using FEM and Non-Linear-Goodman-Analysis
http://www.enercon.de
Sa(R3=-1)
Sm(R=1)
N=1R1
R2R4
UTSUCS
© Fraunhofer
Used Fundamentals
Non-Linear-Goodman-Diagram (NLGD)
Rain-Flow-Counting
Miner-Sum
Sa(R3=-1)
Sm(R=1)
N=1R1
R2R4
UTSUCS
N=x
Smi
Sai
-12
-1Time
elem
ents
tres
ses
orst
rain
s
1
i i
i
N
nD
© Fraunhofer
Structural Model
FEM-Rotor-Blade-Model
Model generated with FOCUS
ANSYS-Solver
8-noded SHELL99-Elements
4-noded SHELL181-Elements
Loads applied on arbitrary cross section
CARDS (JAVA - Open-Source Postprocessor)
z
M(z)
z1 z2 z3
M(z1)
M(z2)
M(z3)
© Fraunhofer
Principal procedure
-12
-1 time
loading
-12
-1
loading
time
-12
-1
loading
time
-12
-1
loading
time
Programming the fatigue Analysis based on NLGD´s
-12
-1Time
ele
me
nt
stre
sse
so
rst
rain
s
Am
pli
tud
e-C
las
s
Mean-Class
1 2 3 4 5
1
2 3
3 2
4 0.5 2
5 1
Sa
Sm
i i
i
N
nD
© Fraunhofer
Programming the fatigue Analysis based on NLGD´s
Programming the NLGD
Transforming S-N-curve data
Determination of maximum number of cycles out of NLGD
Using method to find a point in a triangle
Logarithmic interpolation
Sm
*P
Sa
*P
P1
P2
P3
P4
Sa(R3=-1)
Sm
N=1
R2R4 N=10
N=100
*P
UTSUCS
N100 101 102 103 104
87.56.55.5
4.5
71.50
Ms As -6.19 5.07 -5.80 4.75 -5.03 4.12 -4.26 3.48 -3.48 2.85
10R N
100 8 101 7.5 102 6.5 103 5.5 104 4.5
10R
UCS
1R
.....Sm
Sa
φ
© Fraunhofer
Validation
Results are compared with FOCUS
FOCUS beam model
Improved method 3D FEM-Model
Four points on an arbitrary cross section are investigated
Comparison of damage for UD-Laminate and +/- 45°-Laminate
Three different approaches1
2
3
4
UD-Laminat
+/-45°Laminat
+/-45°Laminat
Spar Caps
examined cross section
M(z)
zz0 z1 z2 ze z3
0
100000000
200000000
300000000
400000000
500000000
600000000
700000000
800000000
0 2000 4000 6000 8000 10000 12000
Mo
men
t in
[N
mm
]
Position am Radius in [mm]actual load progression (1): actual load
(2): non conservative (3): conservative
9650mm
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Validation
Comparison of the results
Big deviationsComparison of strain-time-series Factor k to adjust the damage calculation
0
0,05
0,1
0,15
0,2
0,25
0,3
0,35
1 2 3 4
da
ma
ge
D
Element Number
+/-45° Laminate
FOCUS (1): actual load (2): non conservative (3): conservative
0
0,005
0,01
0,015
0,02
0,025
0,03
0,035
1 2 3 4
da
ma
ge
D
Element Number
UD Laminate
FOCUS (1): actual load (2): non conservative (3): conservative
0
0,02
0,04
0,06
0,08
0,1
0,12
0,14
0,16
1 2 3 4
da
ma
ge
D
Element Number
+/-45° Laminate
FOCUS (1): actual load (2): non conservative (3): conservative
0
0,002
0,004
0,006
0,008
0,01
0,012
0,014
0,016
1 2 3 4
da
ma
ge
D
Element Number
UD Laminate
FOCUS (1): actual load (2): non conservative (3): conservative
© Fraunhofer
Fatigue Analysis on a rotor blade model
Transition zone of webs of particular interest
Analysis of:
UD-Laminate in spar caps
+/-45°-Laminate for outer shell
Adhesive bonding
webs
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Fatigue Analysis on a rotor blade model
Results of the damage calculation
Damage of UD-Lamiante in the spar caps
Increasing damage towards the tip due to:Deceasing thickness and cross section area
Constant load on the blade section
Change from two to one web
Damage of +/-45°-Lamiante Damage of adhesive bonding
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Conclusion & Outlook
The Improved fatigue design method helps:
Enhancing the material usage
Detecting critical areas in terms of fatigue
Increases the accuracy of the material prediction (using NLGD´s)
Issues to improve
Check other approaches for damage accumulation
The way of applying loads on 3D-FEM-Models
© Fraunhofer
Thank you for your attention!
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