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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
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
5M Wind Power Turbine, Brunsbüttel, Germany, 61.5 m blades
(Courtesy of LM Glasfiber A/S)2011-05-153 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Growth
2011-05-15MatWind, Keynote4
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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
2011-05-155 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Blade construction - an aerodynamic shell and a load-carrying beam
2011-05-156 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Blade construction
2011-05-15MatWind, Keynote7
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Load Type
2011-05-15MatWind, Keynote8
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Blade construction - an aerodynamic shell and a load-carrying beam
2011-05-159 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark 2011-05-15MatWind, Keynote10
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Adhesive Joints
2011-05-15MatWind, Keynote11
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Material Selection for Blades
• Optimise against Stiffness
• Optimise against fatigue
• Optimise agaist Weight
• Life time 20 Years => >100.000.000 load cycles
2011-05-15MatWind, Keynote12
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Selection Tool - Stiffness
2011-05-15MatWind, Keynote13
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Mechanical performanceof composites
Fibre content
Fibre orientation
Fibre length
Porosity
Fibre properties
Matrix properties
Fibre/matrixinterfaceproperties
Fibre packingability
Composite Parameters
2011-05-1514 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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
2011-05-1515 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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
2011-05-1516 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Composite Architectures
2011-05-15MatWind, Keynote17
Fibre-orientation
unidirectional
weaves/patterns
random orientation
Boundary fibre/matrix:
interface
interphase (zone)
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Manufacturing
• Hand layup
• Vacuum Assisted Resin Transfer Moulduíng
2011-05-15MatWind, Keynote18
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark 2011-05-15MatWind, Keynote19
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Tvind Møllen
2011-05-15MatWind, Keynote20
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Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark 2011-05-15MatWind, Keynote22
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark 2011-05-15MatWind, Keynote23
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark 2011-05-15MatWind, Keynote24
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Root End
2011-05-15MatWind, Keynote25
Composite Length Scales
26
MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Properties
2011-05-15MatWind, Keynote27
• 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
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark 2011-05-15MatWind, Keynote28
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Blade test - quality control
LM 61.5 m
2011-05-1529 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Blade tested to failure – static loading (well above design load)
2011-05-1530 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Complicated failure - many failure modes
2011-05-1531 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Fracture modes
Adhesive joint failure
Delamination
(+/-45 )
Adhesive joint failure
Cracks in gelcoat(chanal cracks)
Splitting alongfibres
Skin/adhesivedebonding
2011-05-1532 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Fracture modes - example: a wind turbine blade
Sandwichdebonding
Laminate
Foam
Delamination
Splitcracks
2011-05-1533 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Fracture modes - example: a wind turbine blade
Sandwichdebonding
Splitcracks
Delamination Compressionfailure
2011-05-1534 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Fracture modes - example: a wind turbine blade
Delamination
Delamination
Multipledelaminations
Split cracks insurface layer
Splitting
Splitting
Buckling-drivendelamination
Compression failure
2011-05-1535 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Multiscale modelling
2011-05-15MatWind, Keynote36
z
Combine a coarse 3D model with a fine model of each damage mode
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Subcomponent Test – Girder Section
2011-05-15MatWind, Keynote37
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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
2011-05-1538 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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
2011-05-1540 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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.
2011-05-15MatWind, Keynote41
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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.
2011-05-1542 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Tensile Tests
2011-05-15MatWind, Keynote43
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Compression test
2011-05-1544 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Test specimen with back-to-back mounted extensometers
2011-05-1545 MatWind, Keynote
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Shear test
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Fatigue Test - Universal Testing Machine
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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
2011-05-1549 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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.
2011-05-1553 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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
2011-05-1554 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Constant load amplitude fatigue curve for glass/polyester
based on stiffness degradation
2011-05-1555 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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
cyclesLevel 2: 30000
cycles
Level 5: 20000
cyclesLevel 6: 30000
cycles
Fatigue experiments
Block loading sequence
2011-05-1556 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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
2011-05-1557 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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.
2011-05-1559 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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
2011-05-1560 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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
2011-05-1562 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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
2011-05-1563 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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
2011-05-1564 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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
2011-05-1566 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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
2011-05-1567 MatWind, Keynote
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
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Fracture mechanics concepts
M
M
L
Large-scale bridging
2011-05-1569 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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).
2011-05-15MatWind, Keynote70
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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.
2011-05-1571 MatWind, Keynote
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Loading Principle
2011-05-1572 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Interface strength - single fibre tests
2011-05-15MatWind, Keynote73
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Saturated specimen: glass fibre embedded in a polymer matrix
Counting number of fibre breaks:a) Optical observationb) Acoustic emission
Fragmentation test
2011-05-1574 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Single fibre fragmentation set-up
Specimen’s geometry
2011-05-1575 MatWind, Keynote
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a) Fragment l=lcb) Fragment l>lc
c) Fragment l<lc
Single fibre fragmentation test
2011-05-1576 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Polarised-light micrograph showing fibre/matrix debonding
debonding yielding
2011-05-1577 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Weak interface: debonding at the fibre/matrix interface
Strong interface: crack propagates into the matrix
2011-05-1578 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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
2011-05-1579 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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
2011-05-1580 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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
2011-05-1581 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
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
2011-05-1582 MatWind, Keynote
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Wind Rotor Blade
2011-05-1583 MatWind, Keynote
Risø DTU, Technical University of DenmarkRisø DTU, Technical University of Denmark
Material Needs
2011-05-15MatWind, Keynote84
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