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

t.westphal@wmc.eu

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Wind turbine Materials and Constructions

Wind turbine Materials & Constructions

● Full-scale structural testing

● Material research & testing

● Simulation and software

development

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Wind turbine Materials and Constructions

Wind turbine Materials & Constructions

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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

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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

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Blade Testing

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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

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Multiple-point Static Test

● Vertical: gravity in loading

direction

● Horizontal: easy access and

inspection

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Wind turbine Materials and Constructions

Rotor Blade Fatigue Testing

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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

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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

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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

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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

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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

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Wind turbine Materials and Constructions

Single Axial

Focus on spar caps

and leading/trailing

edge

Bi-axial

Better distribution over

circumference

Fatigue Loading Comparison

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Wind turbine Materials and Constructions

Bi-axial Forced Displacement

Dual axis loads applied with actuators

Better loading of off axis panels

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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

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Wind turbine Materials and Constructions

Bi-axial test with resonance

Crank rod mechanism to drive flap

displacement in resonance

Mass

Mass

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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

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Wind turbine Materials and Constructions

Bi-axial test setup development

Add crack rod mechanism in

edge direction

Mass

Mass

Mass

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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

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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

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Bi-axial fatigue test with tip

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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

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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

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Tuning of bi-axial testing

Tune moment envelope

at two sections

On-line optimisation

with ~8 parameters

Ma

ss

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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

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Questions?

For more information please visit our

website: www.wmc.eu

Various movies available on

youtube.com/wmceu