Composite Materials for Wind Turbine Bladescam.bue.edu.eg/Presentations/15 May 2011...

Composite Materials for Wind Turbine Blades

Povl Brøndsted

Materials Research Division

Risø National Laboratory for Sustainable Energy

Technical University of Denmark

Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark

ApplicationWindmills – Wind turbines

Entertaining

Challenging

Larger

and Larger

2011-05-152 MatWind, Keynote

Small


5M Wind Power Turbine, Brunsbüttel, Germany, 61.5 m blades

(Courtesy of LM Glasfiber A/S)2011-05-153 MatWind, Keynote


Growth

2011-05-15MatWind, Keynote4


The Wind Turbine Blade

LM 61.5 m (17.7 tons)

y = 0.0005x2.6589

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70

Blade Length (m)

Bla

de W

eig

ht

(metr

ic t

on)

Trendline blades < 40 m



Blade construction - an aerodynamic shell and a load-carrying beam



Blade construction



Load Type



Blade construction - an aerodynamic shell and a load-carrying beam


Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark 2011-05-15MatWind, Keynote10


Adhesive Joints



Material Selection for Blades

• Optimise against Stiffness

• Optimise against fatigue

• Optimise agaist Weight

• Life time 20 Years => >100.000.000 load cycles



Selection Tool - Stiffness



Mechanical performanceof composites

Fibre content

Fibre orientation

Fibre length

Porosity

Fibre properties

Matrix properties

Fibre/matrixinterfaceproperties

Fibre packingability

Composite Parameters



Typical properties of fibres and composites

Type Stiffness

Ef

GPa

Tensile

Strength

f MPa

Density,

f

g/cm3

Vol.

Fraction

Vf

Orientati

on

Stiffness

Ec

GPa

Tensile

Strength

c MPa

Density,

c

g/cm3

Merit

Ec1/2

/c

Glass-E 72 3500 2.54 0.5 0º 38.0 1800 1.87 3.3

0.3 Random 9.3 420 1.60 1.9

Carbon 350 4000 1.77 0.5 0º 176.0 2050 1.49 8.9

0.3 Random 37.0 470 1.37 4.4

Aramid 120 3600 1.45 0.5 0º 61.0 1850 1.33 5.9

0.3 Random 14.1 430 1.27 2.9

Polyethylene 117 2600 0.97 0.5 0º 60.0 1350 1.09 7.1

0.3 Random 13.8 330 1.13 3.3

Cellulose 80 1000 1.50 0.5 0º 41.0 550 1.35 4.7

0.3 Random 10.1 170 1.29 2.5

CompositesFibres



Matrix:

• Thermosetting polymers

• Thermoplastic polymers

• Metals

• Ceramics

Fibres:

• Glass fibres

• Carbon fibres

• Aramid fibres

• Cellulose fibres

• and more ….

+ =

Composite materials

Spider silk fibres

Composite Materials



Composite Architectures


Fibre-orientation

unidirectional

weaves/patterns

random orientation

Boundary fibre/matrix:

interface

interphase (zone)


Manufacturing

• Hand layup

• Vacuum Assisted Resin Transfer Moulduíng



Tvind Møllen



Root End


Composite Length Scales

26

MatWind, Keynote


Properties


• Measurements of Mechanical Properties on Different Scales

• Full scale structures

• Wind turbines

• Components

• Blades

• Subcomponents

• construction details from blades, adhesive joints

• Materials performance

• Standards, recommendations, experimental models

• Microstructures

• Single fibre tests, ESEM tests


Blade test - quality control

LM 61.5 m



Blade tested to failure – static loading (well above design load)



Complicated failure - many failure modes



Fracture modes

Adhesive joint failure

Delamination

(+/-45 )

Adhesive joint failure

Cracks in gelcoat(chanal cracks)

Splitting alongfibres

Skin/adhesivedebonding



Fracture modes - example: a wind turbine blade

Sandwichdebonding

Laminate

Foam

Delamination

Splitcracks




Sandwichdebonding

Splitcracks

Delamination Compressionfailure




Delamination

Delamination

Multipledelaminations

Split cracks insurface layer

Splitting

Splitting

Buckling-drivendelamination

Compression failure



Multiscale modelling


z

Combine a coarse 3D model with a fine model of each damage mode


Subcomponent Test – Girder Section



Mechanical testing – Test Coupons

Measurement of mechanical properties used in

• qualification of materials

• constitutive models based on material structures and micromechanical behaviour

• design, reliability and lifetime estimation

• models describing elastic-plastic behaviour and for use in solid mechanics modelling



Material Qualification

• Characterisation of Laminates

• Characterisation of Core material and structure

• Characterisation of interface between skin and core

• Characterisation of sandwich

The materials are to be tested in static loading and in fatigue loading



Laminates

The following static mechanical tests are suggested

• Static tensile testing according to ISO 527/4, ISO 527/5 or ASTM D3039. At least 5 test coupons are to be tested.

• Static compression testing according to ISO 14126 or ASTM D3410. At least 5 test coupons are to be tested. In order to obtain the best results it is advised to use combined shear and end loading fixtures.

• V-notch shear testing according to ASTM D 5379. At least 5 specimens are to be tested

• Interlaminar shear strength measurements according to ASTM D 2344. At least 10 test coupons are to be tested.



Laminates

The following fatigue mechanical tests are suggested• Tension-tension fatigue testing according to ISO 13003. At least 10 test coupons are to be tested.

Range of Number of Cycles: 104 - 107.

• Compression-compression fatigue testing partly according to ISO 13003. No specific standard is available. An antibuckling device must be used in order to avoid global buckling, or short gauge length specimens as in the static compression tests should be used. In order to obtain the best results it is advised to use combined shear and end loading fixtures. At least 10 test coupons are to be tested. Range of Number of Cycles: 104 - 107.

• Tension-Compression Fatigue testing partly according to ISO 13003. No specific standard is available. An antibuckling device might be used in order to avoid global buckling, or short gauge length specimens as in the static compression tests should be used. At least 10 test coupons are to be tested. Range of Number of Cycles: 104 - 107.



Tensile Tests



Compression test



Test specimen with back-to-back mounted extensometers



Shear test


Fatigue Test - Universal Testing Machine


S-N curves

Stage A

Stage B

Stage C

Log S

Log N

In polymer matrix fibre composites the damage modes and the microstructural damage mechanisms do change basically between the different stages.

In constant amplitude fatigue tests stage B can be analytically expressed in a power law:

N S Cm



Fatigue model

The number of cycles, NL, to a damage level definedby a stiffness degradation criteria, EL/E, can becalculated from

NL E

nE

E

KC

L

0

11

Assuming n = m:

The constant C in the fatigue power law will depend on the change in stiffness in the material.



Fatigue experiments.Materials

The material is a 4 layers 90:10 fabric with chopped strand mat on both sides, again a nominal symmetric lay-up. The average fibre volume fraction and the porosity content were measured on 3 panels for this material: Vf =38 % and Vp = 7 %.

Mechanical data

Test type Modulus

E0 (GPa)

Max

Stress

(MPa)

Max

Strain

(%)

Longitudinal

tension

26.0

± 1.1

451

± 110

2.0

± 0.6

Transverse

tension

11.1

± 0.9

48

± 2

2.0

± 0.3

K-value m value R-value

CA fatigue test 1.28 1015 10.55 0.1



Constant load amplitude fatigue curve for glass/polyester

based on stiffness degradation



0%

20%

40%

60%

80%

100%

120%

0 40000 80000 120000 160000

Cycles

% o

f m

axim

um

load

Level 1: 50000

cycles

Level 4: 1000

cyclesLevel 3: 20000


cycles

Level 5: 20000


cycles

Fatigue experiments

Block loading sequence



Fatigue modelling

Block loading stiffness reduction curve

0.900

0.925

0.950

0.975

1.000

1.025

0.0.E+00 5.0.E+06 1.0.E+07 1.5.E+07 2.0.E+07

Rela

tive S

tiff

ness

Number of Cycles

190

240 230

180

220

210

200

260 250

Max. stress levels in MPa

0

50

100

150

200

250

300

350

1.0.E+04 1.0.E+05 1.0.E+06 1.0.E+07 1.0.E+08S

tress L

evel (M

Pa)

Number of Cycles



Fatigue following either a regular or a random pattern as multi-level load histories can be simulated in

Fatigue under varying stress amplitudes.

Step tests

Repeated block tests

Stochastic tests

It is the goal to be able to compare variable amplitude fatigue behaviour to constant amplitude fatigue test results to predict the varying amplitude behaviour based on constant amplitude fatigue test results

BUT IS IT POSSIBLE? 2011-05-1558 MatWind, Keynote


Palmgren-Miner’s rule

Cumulative damage in fatigue can be expressed in a one parameter sum:

Mn

N

i

ii

j

1

Suggested “rule”:

Failure takes place when M = 1.



Equivalent stress parameter

Using N=S ni in N•Sm=C

n S Ci eq

m

Seq is the equivalent stress or strain parameter

S C neqm

im

1 1

Palmgren-Miner’s sum: Mn

N

n

C S Cn Si

i

i

i

m i i

m

1

1 1

C S neq

m

i

S

n S

n Meq

i i

m

i

mm

11

1



Equivalent stress parameter

For any load history an equivalent load value can be calculated.

The lifetime of a specimen subjected to a varying fatigue load history is equal to the lifetime of the same specimen subjected to a constant fatigue load history at the equivalent level

In order to calculate the equivalent load Palmgren-Miner’s sum must be known.

Why does Palmgren-Miner’s rule M=1 not work?

Because the results depend on M1/m and not on M. For example allowing a scatter of 10% in equivalent stress based on experimental results means that

m = 3, 0.7 < M > 1.3 - metals

m =10, 0.3 < M > 2.6 - GFRP

m =20, 0.1 < M > 6.7 - CFRP 2011-05-1561 MatWind, Keynote


Fatigue experiments

Modelled S-N curve and experimental results from block load

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

1000 10000 100000 1000000 10000000

Cycles

No

rmali

sed

str

ess

Eq. values

Max. values

97.5% stiffness

lineConstant Load

line



0

10

20

30

40

50

60

70

0 2000 4000 6000 8000 10000 12000 14000 16000# Cycles

Load L

evel

Peaks

Troughs

Ten

sio

nC

om

pre

ssio

n

Fatigue experiments

WisperX loading sequence



Fatigue experiments

Modelled S-N curve and experimental results from WisperX load history

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

1000 10000 100000 1000000 10000000 100000000

Cycles

No

rmali

sed

str

ess

Eq. values

Max. values

97.5% stiffness line

Constant Load line



Damage model

Damage: w = D = 1 -E/E1

E/E1

Stage 1Stage 2

Stage 3

N

In stage 2:

E

EA N B

1

d

dNA

EE( )

1



Assumption: The rate of change in stiffness, is only a function of the stress level .

dE

E

dNK

E

n1

0

Damage model

dE

EK

EdN

E

EK

EN

EE

nN

n

11

00

1 0

1

1


Fracture mechanics approach to fatigue & fracture

critIc aCK

aCK

K (MPa√m)

da/d

N

Kth KIc

Stage II

Sub-critical crack growth

a0

K

Static:

Dynamic:

K

Fast fracture

Nocrackgrowth

IF K > KIc ………. Fast fracture

IF Keff < Kth …. No crack growth

IF Kmax > KIc …. ...Fast fracture

IF Keff < Kth &Kmax < KIc ………. Sub-critical crack growth

Static Dynamic


Fracture mechanics concepts

M

M

L

Large-scale bridging



Fatigue Life-Time Prediction

• As long as the damage modes and the microstructural damage mechanisms do not change basically we have found that:

Fatigue design curves (nominally equivalent to traditional S-N-curves) can be constructed based on measurements of the stiffness reduction in a material under constant amplitude fatigue loading .

Fatigue lifetime for materials subjected to varying load amplitudes can be predicted using an equivalent stress or strain parameter weighted by the value of Palmgren-Miner’s sum. However, this value can only be determined experimentally. A prediction can be made based on constant amplitude results if it can be assumed that M1/m equals 1.

Alternatively fatigue lifetime for materials subjected to varying load amplitudes can be predicted using the stiffness (or damage) model. The stiffness reduction can be calculated on a cycle to cycle basis using the constant amplitude fatigue curve.

This approach is based on the assumption that the sequential effect is small (the damage level of the next cycle does not depend on the preceding load history).



Interface testing• Interlaminar fracture toughness tests in 4 different crack opening modes. No specific standard is

available. Double Cantilever Beam test specimens are manufactures by cutting a crack starter notch in a manufactured test beam. The tests are performed in Mode I, Mixed Mode 1:2, Mixed Mode 2:1, and in Mode II. 5 specimens are required for each test mode. Test fixture to be used is special mixed mode test fixture developed by Risø DTU.



Loading Principle



Interface strength - single fibre tests



Saturated specimen: glass fibre embedded in a polymer matrix

Counting number of fibre breaks:a) Optical observationb) Acoustic emission

Fragmentation test



Single fibre fragmentation set-up

Specimen’s geometry



a) Fragment l=lcb) Fragment l>lc

c) Fragment l<lc

Single fibre fragmentation test



Polarised-light micrograph showing fibre/matrix debonding

debonding yielding



Weak interface: debonding at the fibre/matrix interface

Strong interface: crack propagates into the matrix



Epoxy specimen tested at 80% of the fibre’s tensile strengtha. 1 cycle. b. b. 1000 cycles. c. c. 5000 cycles d. d. 10000 cycles

Single Fibre Fragmentation Test - FATIGUE



0

10

20

30

40

50

60

70

80

1 10 100 1000 10000 100000 1000000

Cycles

Deb

on

din

g L

en

gth

(1E

-6m

)

80% - d1

80% - d2

80% - d3

80% - d4

60% - d2

60% - d3

60% - d4

Fatigue of epoxy at 60% and 80% of the fibre’s tensile strength

2

Single Fibre Fragmentation Test – FATIGUE



Concluding remarks

Material mechanics and the continuum damage mechanics approach is used to

account for the basic behaviour of the materials and to characterize the damage

evolution in long fiber laminated composites.

Fracture mechanics and cohesive laws are used for analysing construction details and

joints

The experimentally determined behaviour are based on both component and full

scale tests

Larger structures necessitates analyses and tests of scaled components.

The relationships between component behaviour and full scale can be analysed using

models and mechanical laws based on the fundamentals in materials mechanics



Challenges

Celluloses fibres• wood, bamboo, sisal, coco, flax, hemp, jute, straw

Sandwich and core• Layered constructions, core materials

• Holllow components

• Energy absorbing

Interfaces/interphases• Chemestry, physics and mechanics in fibre/matrix interfases

• Relation to manufacturing technology

• Controlling of interfasedesign of composite properties

Sustainability/Recycling• Life cycle analyses

• Craddle to Craddle

• Environmantal friendly resources

• CO2 balance



Wind Rotor Blade



Material Needs


Composite Materials for Wind Turbine Bladescam.bue.edu.eg/Presentations/15 May 2011...

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