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2014 Wind Turbine Blade Workshop- Westphal
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WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Rotor Blade Fatigue
Test Methods
Sandia Blade Workshops
Albuquerque, 27 August 2014
Tim Westphal
Knowledge Centre WMC
The Netherlands
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Wind turbine Materials & Constructions
● Full-scale structural testing
● Material research & testing
● Simulation and software
development
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Wind turbine Materials & Constructions
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Material research at WMC
● Rotor blade composites
● Focus on fatigue
● micromechanical modelling
● Influence of manufacturing
and laminate thickness
● Environmental influences
● Joints
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
● Aero-elasticity I: rotor pre-design
● Aero-elasticity II: turbine design +
Offshore
● Tidal stream turbines
● Structural blade design
FOCUS 6 integrated design tool
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Blade Testing
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
What is tested? (IEC-61400-23)
Minimum test programme acc. IEC 61400-23
● Mass, COG
● Modal / Natural frequency test
● Static tests
● Fatigue test(s)
● Post fatigue static tests
Validation of:
● Design assumptions
● Production
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Multiple-point Static Test
● Vertical: gravity in loading
direction
● Horizontal: easy access and
inspection
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Rotor Blade Fatigue Testing
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Fatigue Testing
● Fatigue loading equivalent to design
loading
● Commonly: Constant amplitude testing
for several million cycles
● Get correct fatigue strain distribution
● Spanwise Bending moment distribution
● Circumference Bi-axial loading
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Fatigue Test Methods
Single axial resonance Blade is tested twice, one test for flapwise loading and
one for edgewise loading
● Exciter with mass on blade
● Actuator to floor
Biaxial fatigue test Flapwise and edgewise loading in one single test, with
better off-axis loading
● Forced displacement
● Resonance for one or both axes
● same or different frequency for either axis
● Variable amplitude loading
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Single Axial Fatigue
● Advantages
● Simple hardware
● Simple tuning of bending
moment distribution
● High frequency
● Low energy consumption
● Distributed loading
● Disadvantages
● Separate flap and edge
test
● Not testing total
circumference
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Excitation methods
● Inertia based exciter ● Spinning eccentric mass or
● Linear actuator with mass
● Tuning of moments with masses on the blade
● Drawback – large masses on blade
● Ground based exciter
● Hydraulic actuator (with bell-crank, lever)
● Tuning of moments with masses and actuator
loading
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Single Axial Fatigue
Loading of circular section at 45° position
● Equal loading on both axes
● Strain factor ½2
● Cycles 2x
● S-N slope of 10
● logN = -10 logS
Underloading at 45°
● Factor 16 on life
● Factor 0.76 on strain
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Single Axial
Focus on spar caps
and leading/trailing
edge
Bi-axial
Better distribution over
circumference
Fatigue Loading Comparison
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Bi-axial Forced Displacement
Dual axis loads applied with actuators
Better loading of off axis panels
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Forced Displacement
Does not scale well to large blades!
● High energy consumption
● Test speed limited by hydraulic power
● Long test time
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Bi-axial test with resonance
Crank rod mechanism to drive flap
displacement in resonance
Mass
Mass
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Bi-axial Fatigue Testing
● Advantages
● Flat & edge loading combinations
● Short test time
● All R-values, weight compensation
possible
● Disadvantages
● Linear bending moment curve
● Higher shear load
● More hardware
● complex control
● Tuning of moment curve requires
additional hardware
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Bi-axial test setup development
Add crack rod mechanism in
edge direction
Mass
Mass
Mass
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Bi-axial test setup development
Bi-axial test of full blade at
flatwise frequency
Use coupled mass to reduce
edgewise frequency
Mass
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Full blade bi-axial test
● Distributed flapwise loading
● Tuning with masses and actuator loading
● Edgewise loading is combination of:
● Blade inertia
● Coupled masses
● actuator loading
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Bi-axial fatigue test with tip
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Full blade bi-axial test
● Advantages
● Flap and edge loading combinations
● Fast
● Low energy consumption
● distributed flapwise loading
● limited shear loads
● full blade tested
● Disadvantages
● More complex
● more time required for setup and tuning
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Bi-axial test tuning
● Bi-axial bending
moment envelope at
given section
● Minimize difference
between reference and
measured signal
● Optimization of
parameterized actuator
signals
Medge
Mflap
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Tuning of bi-axial testing
Tune moment envelope
at two sections
On-line optimisation
with ~8 parameters
Ma
ss
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Concluding Remarks
● Bi-axial tests can be performed on the full blade
● Limited shear loading
● Single axial tests focus on spar and leading and
trailing edge.
● Bi-axial tests give a more realistic distribution
of fatigue damage over the circumference of the
blade.
● Time savings possible for high number of
cycles
WMC
Knowledge
Centre
Wind turbine Materials and Constructions
Questions?
For more information please visit our
website: www.wmc.eu
Various movies available on
youtube.com/wmceu