Modelling of static and fatigue failure in wind turbine blades using a parametric blade model

A G Dutton, M Clarke1, P Bonnet2

Energy Research Unit (ERU)Rutherford Appleton Laboratory (RAL)

Science and Technology Facilities Council (STFC)(now at 1Oxford Brookes University and 2SAMTECH Iberica)

Presented at EWEC 2010, Warsaw, 23 April 2010

Background: SUPERGEN Wind

To undertake research to improve the cost-effective reliability & availability of existing and future

large scale wind turbine systems in the UK

Research Themes:– Baselining wind turbine

performance– Drive-train loads and monitoring– Structural loads and materials – Environmental issues

Background: Blade modelling

• Which are the best materials?

• What is the optimum lay-up?

• What is the best internal structure?

• What are the size limits for wind turbine blades?

• What additional stresses do smart control devices generate in a blade?

• How should NDT measurements be interpreted?

Picture credit: LM Glasfiber

Picture credit: EWEA

Parametric blade model:Design strategy

• Parametric processing tool for creation and running of the underlying FE model

• Suitable for sensitivity analyses, flexibility, documenting, re-usability

Python script front end for automation of the Abaqus FE package Modular program Realistic load application, including quasi-static aerodynamic loading Ultimate strength & fatigue analysis Developing dynamic implementation

Parametric blade model:Geometry definition

a b

c d

aerofoil shape

=> parameter sweeps: e.g.

tip d

efle

ctio

nor

max

str

ess

d - shear web offset (mm)

Parametric blade model:Geometry definition

Parametric blade model:Lay-up

Parametric blade model:Fully distrubuted aerodynamic load

Parametric blade model:Variable mesh density...

... at the push of a button

Parametric blade model

5 MW (61 m) blade model

• Basic lay-up information

• Target mass and stiffness distributions

• Limitations of lay-up information• Overall mass

• Discretisation of lay-up info

• Required spar-cap stress profile?

• Lay-up modification

• Materials variation

• Static load case (aerodynamic load distribution)

• Fatigue lifetime

5 MW (61 m) blade model:Spar-cap stress distribution (smoothed)

5 MW (61 m) blade model:Materials

Material property

Baseline UD material

High fatigue strength material

E1T (GPa) 39.0 56.3

E1C (GPa) 38.9 -

ν12 0.29 0.25

E2T (GPa) 14.1 9.0

E2C (GPa) 14.997 -

ν21 0.95036E-01 0.95036E-01

G12 (MPa) 4.24 4.24

Material property

Baseline UD material

High fatigue strength material

XT (MPa) 776.5 1757

XC (MPa) -521.8 -978

YT (MPa) 54 54

YC (MPa) -165 165

S (MPa) 56.1 135.4

Fatigue Baseline UD High fatigue strength

S-n curve at R=0.1

S0 = 1176

b = 9.74

S0 = 1250

b = 10.59bNS1

0max

5 MW (61 m) blade model:Static strength – skins and shear web

Choice of static failure criteria:•Tsai-Wu•Tsai-Hill•Other (user specified)

5 MW (61 m) blade model:Static strength – skins and shear web

Choice of static failure criteria:•Tsai-Wu•Tsai-Hill•Other (user specified)

5 MW (61 m) blade model:Static strength – bonding paste

Cohesive element model•Normal stress component•Shear stress component•Linear up to characteristic value•Material “softening”

5 MW (61 m) blade model:Fatigue strength estimation

• Complex loading• Stochastic / semi-deterministic (cyclic) loading

• Biaxial (triaxial) stress state

IEC 61400-1

• Fatigue characterisation• Predominantly uni-directional materials data

• Uncertainty in how best to combine different stress cycles

• R-ratio (minimum:maximum stress in a load cycle)

• Combine into constant life diagram…

5 MW (61 m) blade model:Fatigue strength estimation

Constant life diagram- Linear Goodman diagram

5 MW (61 m) blade model:Fatigue strength estimation

Constant life diagram- Multiple R-values diagram

5 MW (61 m) blade model:Fatigue strength estimation

Constant life diagram- Multiple R-values diagram

5 MW (61 m) blade model:Fatigue strength estimation

• Complex loading• Stochastic / semi-deterministic (cyclic) loading

• Biaxial (triaxial) stress state

IEC 61400-1

• Fatigue characterisation• Predominantly uni-directional materials data

• Uncertainty in how best to combine different stress cycles

• R-ratio (minimum:maximum stress in a load cycle)

• Combine into constant life diagram…

• …applies to a single material direction

• How to deal with complex stress states? Biaxial stress ratio

5 MW (61 m) blade model:Biaxial stress ratio

Biaxial stress ratio is the ratio between the two largest magnitude principal stress components

5 MW (61 m) blade model:Fatigue strength estimation

5 MW (61 m) blade model:Fatigue lifetime

Baseline glass fibre Uniaxial fatigue

743.9

1

9.1178

nS

Min: 1.3 x 109

593.10

1

2.1250

nS

High performance glass fibre

Min: 1.6 x 1010

Full scale blade testingThermoelastic stress analysis

Blade test: blade with defects

2211

pc

TTIsotropic materials:

Orthotropic materials: 22221111

pc

TT

Full scale blade testingThermoelastic stress analysis

Blade test: blade with defects

Blade model: normal blade

Blade model: blade with defects

Conclusions

• Flexible, parametric blade model for assessment of alternative materials

• Simple failure model in blade skin and developing damage model in bonding paste implemented

• Fatigue methodology under development

• Initial results also available for application to full-scale blade testing, control of smart blades and interpretation of condition monitoring data

• Future work planned on dynamic loading – operation in wakes from upstream turbines & “smart” blade devices

Acknowledgements

EPSRC grant no. EP/D034566/1

SUPERGEN Wind Energy Technologies Consortium

For further information please contact:

geoff.dutton@stfc.ac.uk