composite materials forwind turbine blades -...

Composite materialsfor wind turbine blades:

issues and challenges

Francesco Aymerich

Department of Mechanical, Chemical and Materials Engineering University of Cagliari, Italy

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Francesco AymerichDepartment of Mechanical, Chemical and Materials Engineering

University of Cagliari, Italy

SYSWIND Summer school – July 2012 – University of Patras

Outline of presentation

• Use of composites in wind turbine blades

• Manufacturing processes

• Mechanisms of damage and failure in composite materials

• Strength analysis and damage tolerance approach


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• Composites for delamination resistance

• Carbon fibres in wind turbine blades

• Summary

Wind turbine bladesWind turbine blades are complex structures whose design involves the two basic aspects of • Selection of the aerodynamic shape• Structural configuration and materials selection (to ensure that

the defined shape is maintained for the expected life)


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Modern blades - consist of different kinds of materials (typically composite materials in

monolithic or sandwich configuration)- use various connections solutions between different substructures- include many material or geometric transitions

Growth of blade mass with blade lengthThe growth rate of blade mass with length has been reducing in the past decades

Key drivers for reduction:• Improved manufacturing processes• Introduction of new materials • More efficient use of materials and improved structural configurations


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[Lekou, 2010]

Main loads on bladesThe main loads on the blades are generated by wind and by gravity.

Wind loads mainly induce both flapwise and edgewise bending. These loads have both a static and a dynamic component (variations in wind speed and natural wind shear) that induce fatigue on the blade material.

Gravity loads mainly induce edgewise bending, when the blade is horizontal.The rotation of the blades cause alternating edge-wise bending and thus fatigue of the material.


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Cross-section of a bladeThe cross-section of a blade consists essentially of :

- Outer shells (ensure the stability of the aerodynamic shape) - Internal structural support of the outer shells (longitudinal beam

or webs)OUTER SHELL


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OUTER SHELL

LEADINGEDGE

TRAILINGEDGE

INTERNAL STRUCTURAL

SUPPORT

Cross-section concepts: main sparThe two aeroshells are bonded to a load-carrying spar-beam (box-beam) The main spar and the wing shells are manufactured separately and then joined in a separate bonding process.

FLANGES : THICKMONOLITHIC COMPOSITE(0° fibres)SHELL: SANDWICH

(0°/±45° fibres)


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MAINSPAR

ADHESIVEBONDING

ADHESIVE BONDING

WEBS: SANDWICH(±45° fibres)

Cross-section concepts: Internal stiffenersThe two aeroshells are bonded to two or more internal webs (stiffeners).The wing shells are manufactured with relatively thick monolithic composite laminates (spar-caps).

ADHESIVE BONDINGTHICK MONOLITHICCOMPOSITE


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INTERNAL STIFFENERSADHESIVE BONDING

Cross-section concepts: Integral stiffenersThe entire blade structure, including internal webs/stiffeners, is manufactured in one single process (no secondary bonding).

THICK MONOLITHICCOMPOSITE


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INTEGRAL STIFFENER

Key structural design requirements

• the blades must be strong to resist the extreme (ultimate) loads;

• the blades must resist the time-varying (fatigue) loads through the entire life of the blade;

• the blades must be stiff to prevent collision with the tower under extreme loads. Local stiffness must be also sufficient to prevent extreme loads. Local stiffness must be also sufficient to prevent instability of components under compression (to avoid local or global buckling)

• the blade construction needs to be as light as possible to minimize the cost of generated power

• the blades should be stiff and light to avoid resonance


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Materials requirementsThe structural design requirements translate to the followingmaterials requirements in terms of material properties:

• High material strength is needed to withstand the extreme loads

• High fatigue strength is needed to resist varying loads and reduce material degradation during servicereduce material degradation during service

• High material stiffness is needed to maintain aerodynamic shape of the blade, to prevent collision with the tower, and to prevent local instability (buckling) under compressive loads

• Low density is needed to reduce gravity forces and to minimize the cost of power


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Why composite materials on blades?

ρALm = 2/1

MATERIAL INDEX


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ρALm =

3LEIkFS ==

δ

( )

= 2/1

32/112

EL

kLSm ρMass of the beam

Stiffness of the beam

To minimize mass for a givenstiffness S we have to maximize ρ

2/1E

Why composite materials?Minimize mass for assigned stiffness

Line with constant E1/2/ρ values


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Ashby plot

Why composite materials?Minimize mass for assigned strength

Line with constant σf2/3/ρ values


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Ashby plot

Composite materialsA composite material consists of two or more materials combined to obtain properties different from those of the individual materials.

- Reinforcing fibres (to add strength and stiffness )- Matrix (holds and protects fibres, and distributes the load)

Polymer Matrix Composite (PMC) materials are typically used in wind turbine blades.


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Polymer MatrixThermoset materials are obtained from a chemical reaction between the resin and the hardener to form a hard infusible product.

Polyester : easy to process (does not require post curing), inexpensiveVinyl ester : cost and strength intermediate between polyester and epoxyEpoxy : best mechanical properties, less shrinkage, expensive

Advantages Disadvantages

Easy to process Long curing times – Limited toughnessExotherm during curing (thick components)


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High toughness - Recyclability High process temperature and pressure

Thermoplastic materials soften and melt with heating, then hardening again with cooling. The softening process can be repeated without any significant degradation of the material properties.

PP or L-PET: used in film or fibre form and consolidated by heating and vacuum Reactive thermoplastics (APA-6): suitable for liquid moulding (similar to thermosets)

Strength and stiffness of polymer resins


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[Gurit, 2012]

Degradation of resin from water ingressThe absorption of water affects the resin and the resin/fibre interface. leading to gradual reduction of mechanical properties.

Strengthretained: 85%


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Short-beam shear strength test on a glassfibre laminate

[Gurit, 2012]

Strengthretained: 65%

Reinforcing fibresTypical reinforcement used in composite materials are stiff, strong and lightweight fibres such as

Glass fibres (good specific strength, low specific stiffness, relatively inexpensive)Carbon fibres (high specific strength and stiffness, expensive)Aramid fibres (hygroscopic, low compression strength, few data on fatigue)


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Properties of reinforcing fibresMaterial Tensile

strength(MPa)

Young’s modulus E

(GPa)

Density (g/cm3)

Specificstiffness

E/ρ

HS Carbon 3500 160-270 1.8 90 - 150IM Carbon 5300 270-330 1.8 150 - 180UHM Carbon 2000 >440 2.0 > 220E-Glass 2400 69 2.5 28


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E-Glass 2400 69 2.5 28S-Glass 3450 86 2.5 34Aramid LM 3600 60 1.45 41Aramid UHM 3400 180 1.45 125Aluminium 7020 400 70 2.7 26Mild Steel 450 210 7.8 26HS Steel 1250 200 7.8 25

Comparative fibre costComparison of fibre cost for unidirectional fabrics (300 g/m2)


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Approximative cost of E- glass = ~ 2.5 Euro/m2 (~ 3.1 USD/m2)[Gurit, 2012]

Final properties of a composite The final properties of the composite are mainly determined by• Properties of fibres • Properties of matrix• Percentage of fibres (fibre volume fraction, typically ranging from 35% to 65%)• Orientation and geometry of fibrous reinforcement

For example, stiffness properties are strongly dependent on fibre orientation and fibre fraction


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FIBRE ORIENTATION FIBRE FRACTION

Tensile and compressive properties of unidirectional composites


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Reinforcement architecture

Unidirectional fibres

Common geometries of the fibrous reinforcement include

Continuous or choppedstrand mat (CSM)

Unidirectional fibres


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Fabric type reinforcement Woven fabrics

Stitch bonded (Non crimp) fabrics

Woven fabrics are obtained by interlacing yarns of fibres with different orientations (usually 0° (warp) , 90°(weft), and ±45°)

Plain weave(1 warp yarnover 1 weft yarn)

Twill weave(1 warp yarnunder 3 weft yarns)

Woven fabrics

Woven fabrics


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Woven fabrics (vs unidirectional layers)ADVANTAGES DISADVANTAGES

Higher stability for fibre placement Lower fibre fraction

Laminates have higher resistance tocrack propagation

Lower in-plane properties (crimped fibres and stress concentrations)More difficult to infuse with resin

Non crimp fabrics (NCFs) are obtained by stitching together unidirectional yarns with different orientations, using non-structural threads.

Non Crimp Fabrics (NCF)

NCF fabric

Woven fabric

-45°

90°+45°


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Non crimp fabrics (vs woven fabrics)ADVANTAGES DISADVANTAGES

Higher fibre fraction Stitching may induce fibre fracture

Higher stiffness/strength (straight fibres)No stress concentration due to fibrewavinessEasier lay-up (fewer layers)

Basic structural configurations used in blades

Monolithic laminatesconsist of different layers (plies) of multidirectional fabrics or unidirectional fibres


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Sandwich compositesconsist of of a low density core between thin faces (skins) of composite material.

Polymeric (PVC, PET, PMI) foams with density in the range 40-200 kg/m3

Sandwich compositesThe insertion of a core increases the thickness of the structure (and thus flexural stiffness and strength) without increasing its weight.

The core carrythe shear load

The skins carry the tensile and the compressive loads


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Weight 1 ~1 ~1Bending stiffness 1 ~12 ~48Bending strength 1 ~6 ~12

Manufacturing techniquesfor composite blades

•Wet hand layup (laminating technique)•Filament winding•Resin infusion•Prepregs

Potential for automation


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WET HAND LAYUP

FILAMENT WINDING

VACUUM INFUSION

PREPREG MATERIALSAutomation

Minimization of cycle times and costReduction of defectsImproved structural performance

Wet hand layup (laminating technique)Dry fibre material (mats, fabrics or unidirectional tapes) are laid in various layers into the mould of the component.The layers are then impregnated with resin and cured at room or higher temperature (70° to 100° C for epoxy).


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[Brøndsted et al., 2005]


• Production of complex shapes.• Fibres can be oriented along preferred

directions

• The process is labor intensive and time-consuming (hand–made)

• Large amount of voids and defects• Low fibre fraction

Filament windingThe fibres are passed through a resin bath and are then wound onto a rotating mandrel. The process is primarily used for cylindrical componentsbut can be adapted for blade manufacturing


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[Bussolari, 1983]

Advantages Disadvantages• The process can be carried out in an automatic way.

• Different mandrels must be used to gradually build the airfoil.

• Fibres cannot be easily oriented along the axis of the blade (0° direction).

Resin Infusion Techniques Dry fibres (mats, fabrics or unidirectional tapes) are placed in a mould and encapsulated in a vacuum bag. Liquid resin is then pulled through the reinforcement by vacuum and allowed to cure at room or higher temperature.

[Grande, 2008]


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[Ref]

Advantages Disadvantages• Large components can be made in a

single step• Clean and safe process• Good final material quality• Potential for automation

• Relatively complex process (especially for large components)

• Low viscosity resins should be used (resulting in lower mechanical properties)

Blade Infusion- The two airfoils and the webs or spar are usually manufactured separately and subsequently bonded tocomplete the blade.- In some technologies however the full blade is infused in a single step.


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[Hogg, 2010]

Resin Infusion Techniques Key issues for final quality of resin infusion are:• Improvement of fibre impregnation (to avoid regions with dry fibres)• Reduction of voids

Possible ways to tackle these issues are

Selection of appropriate fibre coating/sizing to improve wettability


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Use of low viscosity resins (at room or moderate temperature) to improve wettability and reduce the process time for large components

Use of fibre fabrics with special architecture (or special resin distribution meshes) to facilitate flow of the resin

Improvement of resin flow (optimal placement of inlet and outlet lines for resin by simulation of the flow and data from sensors)

Prepreg technologyPrepreg tapes consist of fibre fabrics pre-impregnated with a resin that is not fully cured.The prepregs are laid up onto the mould surface, vacuum bagged and then heated. The pressure required to consolidate the stacked layers of prepregs is achieved by vacuum. Process temperatures range between 70°C and 120°C.


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[Ref]

Advantages Disadvantages• High fibre ratio and low void content• Consistent material properties• Easy control of fibre alignment• Large components can be made in a

single step• Clean and safe process

• High cost for prepreg material• Tooling must whitstand process

temperature• It is difficult to correctly cure thick

laminates (temperature not uniform through-the-thickness)

Automation of blade manufacturingManufacturing of turbine blades consists of a combination of manual, labour-intensive operations

• Fabrics pattern cutting• Lay-up• Vacuum bagging• Infusion• Demoulding• Secondary bonding


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• Secondary bonding

[Black, 2009]

To reduce labour and manufacturing time, and improve quality the trend is toward automation

Automated Cutting - Bagging Automated Tape Layup (ATL) Automated Bonding

MECHANISMS OF FAILURE IN COMPOSITE MATERIALSCOMPOSITE MATERIALS


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Mechanisms of failure in composite materials

Failure in composites can be examined at different scales(fibre/matrix/interface level; ply level; laminate level, etc.).

Strength analyses of composite structures carried out at the laminate level may often lead to unsafe predictions.Most adopted criteria have been developed to estimate failure at the ply level.

First-ply-failure (FPF) is often used as a criterion for laminate strength, but


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First-ply-failure (FPF) is often used as a criterion for laminate strength, but this is often a very conservative approach and there may be a large distance between the load for FPF and the collapse load of the laminate (LPF – Last Ply Failure).

Final collapse of a laminate is the result of the accumulation of different damage modes, which can induce significant degradation of the material properties during life.

Idealized stress-strain curvefor a [0/+45/-45/90]s laminate


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Main failure modes in composite materials

COMPOSITE LAMINATES• Fibre failure• Matrix failure• Fibre-matrix debonding• Inter-laminar failure (delamination)• Buckling instability


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• Buckling instability

COMPOSITE SANDWICHES• Core failure/crushing• Core/facesheet debonding

Fibre Failure

TENSIONFibres have brittle fracture.Failure occurs by unstable growth of a cluster of adjacent broken fibres.

Cluster offibre breaks


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COMPRESSIONFibre failure initiates by instability (buckling)followed by kinking

Matrix failure

TENSION loading involves failure of the matrix perpendicular to the tensile load direction

Matrix failure is controlled by tensile or compressive stresses perpendicular to the fibre direction and by shear stresses.


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COMPRESSION loading involves failure of the matrix along inclined planes

Out-of-plane SHEAR loading involves failure of the matrix along a 45° plane

Examples of criteria for ply failure

1=−ultx

x

σσ

Fibre failure in tension

Fibre failure in compression

1=+ultx

x

σσ

1=−ulty

y

σ

σ

Matrix failure in tension

Matrix failure in compression

Matrix failure in shear

1=+ulty

y

σ

σ

1=xy

τ

τ

Maximum stress criteria


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More complex failure criteria include the interaction between the various stress components.

ultxyτ

1222

=

+

+

−

++++

ultxy

xy

ulty

y

ultx

y

ultx

x

ultx

x

τ

τ

σσ

σσ

σσ

σσ

Tsai-Hill criterion

Delamination (Interlaminar failure)Delamination is the separation between adjacent plies due to normal (through-thickness z-direction) or shear stresses at the interface.It is one of the most common failure processes in laminates, because of the low through-thickness strength of laminates.

DELAMINATIONDELAMINATION


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Delaminations are typically induced in composite laminates during service- by out-of-plane loads (impacts) or - by in-plane loads in the presence of strain concentrations such as at discontinuities (ply drops, wrinkles, material or geometric transitions) or existing defects.

Delamination (Interlaminar failure)Delamination may propagate -under static or cyclic loads- with three different propagation modes


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Growth of delamination may be modelled with a fracture mechanics approach, assuming that the crack propagates when the energy available (strain energy release rate) reaches the fracture energy of the material. Specific FE approaches include the VCC technique or the use of interface elements implementing a cohesive law of fracture.

BucklingBuckling is a mode of collapse occurring under compression, which ischaracterized by the appearance, at a critical applied load, of out-of-planebending deflections (corresponding to new equilibrium configurations).


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Buckling can significantly reduce the compressive strength and stiffness of composite structures and can lead to the development of other failure modes (i.e. fibre failure)

Buckling of delaminated compositesDifferent buckling modes can be induced in delaminated compositestructures depending on the thickness of the laminate and on the size anddepth location of the delamination.

Local buckling


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• Buckling load is reduced in the presence of delamination.• Bending of plies due to buckling results in higher stresses and may promote

the growth of delamination

Mixed bucklingGlobal buckling

Failure modes in sandwich structuresSandwich structures show typical damage modes (in addition todamage in the composite skins).

Failure shearin the core

Face buckling


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Core crushing or laminate fracture due to local loads

Global or shear buckling Face/core debonding

+ buckling

Typical manufacturing defects

•Voids and dry zones•Delaminations•Bonding defects•Foreign inclusions•Fibre waviness

Aerospace quality

•Fibre waviness•Wrinkles


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Dry regions and fibre waviness

0° direction

PREPREG

INFUSION

INFUSION

Typical failure modes in blades- Main spar -


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The most critical failure modes involve interface failure

[Sørensen et al., 2005]

Typical failure modes in blades- Wing shell -


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[Sørensen et al., 2005]

The most critical failure modes involve interface failure

Main structural design constraintsfor wind turbine blades

- Ultimate strength (controlled by material strength)

- Fatigue strength (controlled by material strength)

- Global stiffness (controlled by material stiffness)


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- Global stiffness (controlled by material stiffness)

- Natural frequencies (controlled by material stiffness and mass)

- Global and local buckling (controlled by material strength and stiffness)

ALL AFFECTED BY DAMAGE/DEFECTS

Basic steps of strength analysis on a bladeDesign loads• Fatigue loads (wind variation, wind shear, blade rotation)• Ultimate loads (extreme wind, turbulence, emergency braking)

By aeroelastic calculations, loads are quantified in terms of• Flapwise bending moments• Edgewise bending moments• Flapwise shear forces


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• Flapwise shear forces• Edgewise shear forces• Axial force (secondary importance)• Torsional moment (secondary importance)

Stresses and strains are calculated in blade sections and appropriate failure criterions are used to estimate strength or life.

Fatigue of composite bladesWind turbine blades must withstand the time-varying loads through the entire life of the blade.

Major cyclic loads are induced by gravity (edgewise bending) and by varying winds (flapwise bending)

The expected fatigue life of a wind turbine is of the order of 20-25 years (more than 108 cycles)20-25 years (more than 108 cycles)


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Fatigue testing of blade materials• Fatigue strength data of representative laminates are typically

generated by cyclic tests carried out at constant stress amplitude.• The cyclic loading is described by the maximum applied stress and by

the R ratio (ratio of minimum applied stress to maximum applied stress).• Specimens are tested at different maximum stresses and with selected

values of R ratios.• The fatigue behaviour is described by S-N curves at specific R-ratios.• The fatigue behaviour is described by S-N curves at specific R-ratios.


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Constant Life Diagrams (CLD)• Constant Life Diagrams (CLD or Goodman diagrams) describe the

influence of R-ratio on the fatigue life. CLD are constructed from S-N curve obtained at different R ratios.

- Continuous lines connect points of equal fatigue life (constant life lines)- Radial lines indicate stress states with a given R ratio


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Constant Life Diagrams (CLD)• Constant Life Diagrams are constructed with fatigue data obtained at

different R ratios.

• R ratios typically characterized are- R = 0.1 (tension-tension fatigue)- R = -1 (symmetric tension-compression fatigue) - R = 10 (compression-compression fatigue)

CLDs built with insufficient fatigue data may result inhighly non-conservative fatigue life predictionshighly non-conservative fatigue life predictions


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R = -1

R=0.5R=0.1R=-0.35R=-1R = -2.5R=10

R =

-1

R =

-1

Fatigue analysis with CLD

• Representative load histories are transformed into stress (or strain) histories at a given blade location;

• A counting procedure (i.e. rainflow) is applied to transform variable amplitude signals to a collection of blocks of constant amplitude cycles. Each block is identified by a mean stress value and a stress range.


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Number of cycles Mean Stress (Strain) Stress (Strain) Amplitude

N1 S1 A1

N2 S2 A2

… … …… … …Nn Sn An

Fatigue analysis with CLD• Fatigue life is estimated by a damage summation rule (i.e. Palmgren-Miner)

( )( )∑ < 1j

appliedcharact

jappliedactual

SNSNγ

From CLD

• Partial safety factors for materials are applied to account for variability and uncertainties in material strength


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and uncertainties in material strength

DNV-OS-J102 (2006)

• type of material•manufacturing method•post curing•ply drops•size effect•humidity• temperature•UV radiation•……

Static failure criterions are used to estimate strength at specific locations.Allowable design stresses/strain for the material are reduced by safety factors for materials.

Ultimate strength (static) analysis


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DNV-OS-J102 (2006)

Ultimate strength and fatigue life estimationMain limitations of conventional analyses

The presence of defects is only accounted for by limiting safety factors (empirically determined). Large safety margins are needed if the effect of damage on residual strength is not well understood.

The change of properties (stiffness degradation, residual strength) associated to damage evolution is not predicted or taken into account


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associated to damage evolution is not predicted or taken into account in the estimation of strength, life or performance.

Extensive testing is required to develop accurate CLDs. The approach is conservative only when a sufficient set of fatigue data is available. (i.e. at least three R=0.1, -1, 10).

Fatigue data are only valid for the tested lay-ups. A change in lay-up requires new characterization of the material

Improving accuracy of strength/life estimationComposites have unique damage modes and exhibit different levels of sensitivity to the various damage mechanisms.

To address limitations or conventional design tools, more reliable methods for strength/life assessment of composite structures should thus be adopted to:


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• Account for the main failure and damage mechanisms• Simulate the growth and interaction of single failure modes• Predict the degradation of properties due to damage• Predict the residual strength in the presence of damage

Basic concepts of a DAMAGE TOLERANCE APPROACH

Damage tolerance approachA Damage Tolerance approach aims at estimating the ability of the structure to sustain design loads and to perform its function in the presence of a specified amount of damage/defects

Specific procedures and tools (i.e. fracture mechanics, cohesive laws) may be used to predict damage progression during service (under both static and cyclic loads) and to simulate the effect of damage on blade performance (strength, stiffness, etc.)

The introduction of damage tolerance concepts would allow to • optimize the use of materials • minimize effort (cost, time) for material testing• reduce safety factors values• improve the blade reliability


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damage on blade performance (strength, stiffness, etc.)

Damage progression approachesPossible approaches incorporating damage progression:

• Strength degradation models : residual strength of the composite is estimated as a function of the number of cycles. Failure occurs when the residual strength is lower than the maximum applied stress.

• Stiffness degradation models: stiffness degradation is estimated as a function of the number of cycles by means of appropriate laws. Failure function of the number of cycles by means of appropriate laws. Failure occurs when the residual stiffness decreases to a predefined stiffness level.

• Physical damage models: based on the simulation real damage and fracture mechanisms developing in individual plies of the composite. Only characterization at the ply level is required; models are potentially applicable to composites with any stacking sequence.


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Possible basic steps of a physically-based progressive damage approach

- Mechanical characterization of the basic unidirectional ply in terms of static strength, fatigue life (damage initiation and propagation) and damage.

- Stress/strain analyses carried out cycle by cycle- Ply failure is predicted using appropriate multiaxial failure criteria- Properties of the laminate are estimated as a function of existing damage.- Final failure is defined from performance requirements (stiffness) or predicted at the last ply failure.at the last ply failure.

Main challenges in building physically-based models:• Development of fracture mechanics criteria for propagation of the different

damage mechanisms• Development of simple models for associating damage pattern (e.g. crack

density, delamination) to residual mechanical properties of composite.


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Example of damage tolerance analysisTensile behaviour of a [03/903] composite plate in the presence of a hole


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Static damageFatigue damage

Matrix cracks in 90° pliesSplitting in 0° plies

Delamination

The growth of axial splitting and delaminations with static or cyclic loads can be simulated by a FE model adopting a fracture mechanics based technique (Virtual Crack Closure + Paris law).

Example of damage tolerance analysis

The stress distribution at the hole is affected by the entity of damage


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The residual strength of the damaged plate is predicted as the load inducing failure of 0°plies

Example of damage tolerance analysis

Prediction of change in residual strength with fatigue cycling

Damage reduces stress concentration at the hole


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N = 104 cycles N = 105 cycles

COMPOSITES FORDELAMINATION AND DELAMINATION AND

FRACTURE RESISTANCE


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Delamination Delamination and face/core debonding are two of the most critical failure modes in composite blades.The presence of delamination and debonding strongly affects the compression resistance and the stiffness of the blade.

In laminated composites, fibres provide no reinforcement along the thickness direction and crack growth resistance is


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along the thickness direction and crack growth resistance is largely controlled by matrix properties.

Improving the delamination resistance (interlaminar fracture toughness) of composites is thus a very effective way to improve structural reliability and efficiency of composite blades

Delamination Effective routes for improvement of interlaminar toughness include two approaches

STRUCTURAL APPROACH• 3D reinforcement (Stitching - Z-pinning)• Interlayers (Interleaving - Veils)


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MATERIAL APPROACH• Thermoplastic or toughened resins

(disadvantages: expensive; process require high temperature; toughening additives increase viscosity and make resins difficult to infuse at low temperatures)Toughening by Nano-modification of the matrix is an option being explored

Stitching / Z-pinningStitching consists in sewing a structural thread (kevlar, polyethylene) through the plies to produce a preform with a 3D fibre structure.

Stitching can be performed on both dry fabric and uncured prepreg tape.

Z-pinning consists in inserting thin and stiff pins (steel, carbon fibres)


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Z-pinning consists in inserting thin and stiff pins (steel, carbon fibres)through the plies, typically by an ultrasonic process.Z-pinning is typically performed on uncured prepreg tapes but can be adapted for use

on dry fabrics.

Stitching / Z-pinningStitching /Z pinning are very efficient techniques to improveinterlaminar properties of compositesStitching delays the growth of delaminations by bridging the delaminated interface and reducing the interlaminar stresses at the crack tip.

Main disadvantages


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These combined effects may induce a reduction of in-plane static and fatigue strength of stitched/Z-pinned composites.

Main disadvantagesLocalized fibre fracturePresence of a weak (resin rich) area around the stitch/pin In-plane and out-of-plane fibre waviness produced by the processEnvironmental effects (increased moisture absorption)

Improvement of interlaminar fracture properties by Z-pinning

Mode II fracture test (ENF)Mode I fracture test (DCB)


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more then10 times increase in fracture toughness

[Partridge et al., 2005]

Improvement of impact resistance by stitching

5 J impact on a [03/903]s carbon/epoxy laminate

UNSTITCHED


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STITCHED(5 mm x 5 mm stitching pattern)

more than 30% reduction in damage area

Improvement of impact resistance by stitching


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7 J impact on a stitched [03/903]s carbon/epoxy laminate

Bridgingaction of stitches

Improvement of compression after impact(CAI) strength by stitching


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CAI strength of stitched [0/90]3s carbon/epoxy laminates

Impact damage up to 30% increase incompression after impact strength

Buckling of delaminated plies under compression (CAI)


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UNSTITCHED STITCHED

Buckling and delamination growth are restrained by the bridging action of stitches

Buckling followed by delamination growth

Delamination growth

Influence of stitching on static/fatigue strengthStatic/Fatigue strength of composite materials is usually reduced by stitching in fibre dominated laminates (failure controlled by fibre damage)


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S-N curves of [±45/0/90]s carbon/epoxy laminates under cyclic tension

Fatigue strength of composite materials may be improved by stitchingin matrix-dominated laminates (failure is controlled by delamination)

In unstitched [+45/-45]s laminates, fatigue damage sequence involves:

Influence of stitching on static/fatigue strength

1. Matrix cracking 70%fatigue life


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2. Localization of extensive matrix cracking in specific regions

3. Growth of delamination from highly cracked regions

UNSTITCHED

85%fatigue life

At failure

• Improves fatigue life (up to 3 times increase)

Influence of stitching on static/fatigue strengthStitching in [+45/-45]s laminates (failure is controlled by delamination)


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• Improves resistance to stiffness degradation

Delamination initiation

Fatigue strength of composite materials is improved by stitchingIn [±30/90]s laminates ((failure is controlled by delamination)

S-N curves of [±30/90]s carbon/epoxy laminates

Delaminationarrested bystitches

Influence of stitching on static/fatigue strength


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UNSTITCHED STITCHED

stitches

more than 20 times increase in fatigue life

Failure often occur at joints and connections

TRAILING EDGE

LEADINGEDGE


83

WEBS / SPAR CAP

Improvement of T-joints strength by Z-pinning

Up to 2 times strength increase


84[Koh et al., 2012]

Improvement of lap-joints strength by stitching[03/903]s carbon epoxy laminate


85

Up to 5 times increase in fatigue life

Stitching in sandwich composites

Stitching can be performed in composite sandwich structures to reduce skin/core debonding (and thus increasing buckling load)


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up to 2 times increase in buckling load

[Sharma, 2004]Buckling load

Improvement of delamination resistanceby interleaving or veils

• tough resin films between the layers of the composite (problems for infusion)• veils between layers (veil is a low weight layer made of randomly oriented

fibres

Delamination resistance may be improved in laminates by the insertion of

VEIL


87

Fibres from the veil migrate to the upper and lower fabric plies and provide a bridging effect between delaminated layers

Delamination resistance may be improved by using tough resins

(high fracture resistance and large deformation at failure)

Examples of TOUGH RESINS •Rubber modified thermosets (difficult to infuse)•Thermoplastic resins (difficult to infuse and process)•Nano-reinforced resins


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Main general issueHigher resin’s fracture toughness does not necessarily translate tohigher composite’s fracture toughness(large plastic deformation of resin is constrained by the presence of fibres, poor fibre/matrix adhesion, residual stresses)

Impact resistance of an advanced thermoplastic prepregAPC2 – Carbon/PEEK (Poly-ether-ether-ketone)

Impact damage in [±45/0/90]2s carbon fibre laminates

8 J impact


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Carbon/epoxy

Carbon/PEEK

PEEK is a thermoplastic resin with a processing temperature of about 340°C, requiring autoclave or press moulding, and a quick cooling rate.

Alternative thermoplastics resins and processing routesIntertwined or commingled fabrics: reinforcing fibres are mixed with fibres spun from thermoplastic polymers (PET, PP). Impregnation is achieved by application of sufficient heat and pressure.

Liquid moulding: monomers are heated, activated with a catalyst and injected into the fabrics as a low viscosity resin; the polymerisation takes place in situ and the basic process steps are similar to those of thermosets (vacuum infusion).

Main issues with thermoplastic resins


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Main issues with thermoplastic resins

High processing temperatures(160°C to 250°C)

More expensive toolingHigher energy inputsLarge thermal expansion

Faster heating and cooling ratesare required

Special tooling(Brittle matrix with low cooling rates)

High sensitivity to moisturebefore processing Reduced mechanical properties

Nanomodification of epoxy resinDispersion of nanoclay particles (1 nm thick platelets) in epoxy resins improves both static and fatigue fracture properties.

Fracture properties of nanomodified epoxy resin


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Fracture toughnessParis curve

[Quaresimin et al., 2012]

Use of nanomodified epoxy resin in laminatesGlass/epoxy laminates infused with nano-clay modified epoxy resins have worse static and fatigue fracture properties than conventional laminates.

Fracture properties of nanomodified laminates


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Fracture energyParis curve

[Quaresimin et al., 2012]

USE OF CARBON FIBRESIN WIND TURBINE BLADES


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Potentials for use of carbon fibres in blades

Carbon fibres have been considered and used in recent years as a way to reduce weight and increase stiffness in large blades.

Main issues related to the use of carbon fibres

Cost (about 5-10 times more expensive than glass fibres)

Compressive performance of carbon fibres is sensitive to alignment


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Compressive performance of carbon fibres is sensitive to alignment (prepreg better than wet process).

Fatigue performance of carbon fibres is strongly degraded by stress concentrations (ply drops; carbon-glass interfaces, voids, wrinkles, delaminations)

Infusion of thick carbon fabrics is difficult because of the lower permeability than fibreglass fabrics (this suggests use of prepregs)

Potentials for use of carbon fibres in blades

Parametric analyses show potential for significant structural improvements both for complete and hybrid/selective use of carbon fibres

§ 30% to 40% reduction in mass § 20% reduction in tip deflection for complete replacement of glass with carbon in main spar


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for complete replacement of glass with carbon in main spar

§ 50% reduction in root moment§ 10% reduction in tip deflection For selective replacement of glass with carbon in the outer span of the blade

[Griffin et al., 2003]

Carbon/Glass transition and ply-dropsAn issue for the use of carbon fibres is the design of the transition between carbon and glass fibres and of ply-drops.

Critical aspects includeLarge difference between stiffness and strain-to-failure of carbon and glass layers (higher stress in carbon; load is transferred by matrix shear and high matrix strength is required)


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•Stress concentrations at ply drops/transitions may induce- strong decrease of fatigue strength in carbon structures (high shear

stress in matrix)- delaminations at relatively low strains under fatigue

Compression strength of carbon plies is strongly reduced by fibre waviness

Design recommendations for transitions/ply drops

§ Thin plies are better than thick plies in the presence of ply drops for delamination resistance.

§ Thicker laminates are better than thin laminates, for the same number of ply

[Griffin et al., 2003]STAGGERED PLY DROPS


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§ Thicker laminates are better than thin laminates, for the same number of ply drops, in resisting delamination.

§ No more than one ply drop at the same location (staggered pattern).§ Internal ply drops are more resistant to delamination than external ply drops

Z-spiking

Z-spiked region: fibre tows are driven into adjacent layer

Carbon fibres for bend-twist coupled blades

Bend-twist coupling is a form of aeroelastic tailoring in which a flapwise bending load induces a twist of the blade section.

A change in wind velocity (which induces a change in bending

Carbon layers are especially effective in achieving bend-twist coupling because of their high orthotropic ratio


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A change in wind velocity (which induces a change in bending moment and thus in twist) results in a change in the angle of attack.The change in the angle of attack provides opportunities for load mitigation.

This passive approach for load mitigation is especially attractive because of its simplicity and economy (lighter blade, increased reliability, less maintenance, etc.).

Bending-extension coupling of off-axis compositesThe bend-twist coupling may be achieved by exploiting the orthotropic elastic properties of laminated composite materials (off-axis loading)

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Blade with on-axis laminates

Extension of the laminates does not induce shear forces → the blade does not twist with bending

The symmetry axis of the composite is aligned with the blade axis.


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[de Goeij, 1999]

Bend-twist blade with off-axis laminates

Extension of the laminates induces shear forces and generates a torsion → the blade twists with bending

The symmetry axis of the composite is not aligned with the blade axis


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[de Goeij, 1999]

Bend-twist configurations

Fully (whole span) or partially (outer span)

Bend-twist coupling in shell, spar cap, or both


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Parametric analyses suggest partially coupled blades combining off-axis orientation of fibres both in skin and spar cap as an efficient design for fatigue load mitigation and mass reduction.

partially (outer span) coupling.

[Bottasso 2012]

Bend-twist configurations

Main issues associated to coupled designLarge off-axis angle for fibres in the spar caps would reduce significantly the bending stiffness of the spar (thicker spar caps needed)


103

The presence of off-axis fibres results in increasing importance of transverse matrix cracks (stiffness degradation; delamination initiation; reduced fatigue strength)Lack of fatigue data on off-axis lay-ups

Trade-off in use of carbon fibres : introduction of bend-twist coupling does notallow to fully exploit high strength/stiffness of fibres (off-axis orientation)

SummaryThe development of innovative composite blades involves a strict interaction between main design drivers related to • Materials• Manufacturing process• Structural design

Key issues and research trends include:MaterialsImprovement of material propertiesBetter understanding of material behaviour (reduction of safety factors)Prediction of the effect of defects (damage tolerance approach)Optimized use of materials


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Manufacturing processReduction of defectsReduction in cycle/production timesReduction of labour intensive steps (automation)

Optimized use of materials

Structural designBetter design of structural details (ply drops, transitions, bonding)Better understanding of size effectInnovative structural configurations

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Wind Turbines pp. 259-266, 1983.WC de Goeij, MJL van Tooren, A Beukers. Implementation of bending-torsion coupling in the design of a wind-turbine rotor-blade.

Applied Energy 63:191-207, 1999.DNV - Design And Manufacture of Wind Turbine Blades - Offshore And Onshore Wind Turbines, DNV-OS-J102, 2006.JA Grande. Wind power blades energize composites manufacturing, Plastics Technology (www.ptonline.com), October 2008.DA Griffin, TD Ashwill. Alternative composite materials for megawatt-scale wind turbine blades: design considerations and

recommended testing, J Solar Energy Eng. 125: 515-521, 2003.Gurit Guide to Composites, www.gurit.com, 2012.B Hayman, J Wedel-Heinen, P Brøndsted. Materials Challenges in present and future wind energy. MRS Bulletin, 33:343-353,

2008.P Hogg. Wind Turbine Blade Materials. SUPERGEN Wind Phase 1 Final Assembly, Loughborough, March 2010TM Koh, S Feih, AP Mouritz. Strengthening mechanics of thin and thick composite T-joints reinforced with z-pins, Composites: Part


TM Koh, S Feih, AP Mouritz. Strengthening mechanics of thin and thick composite T-joints reinforced with z-pins, Composites: Part A 43: 1308–1318, 2012

DJ Lekou. Scaling Limits & Costs Regarding WT Blades. UpWind Deliverable 3.4.3, 2010.IK Partridge, DDR Cartie´. Delamination resistant laminates by Z-Fiber pinning: Part I manufacture and fracture performance.

Composites: Part A 36: 55–64, 2005.M Quaresimin, M Salviato, M Zappalorto. Fracture and interlaminar properties of clay-modified epoxies and their glass reinforced

laminates. Engineering Fracture Mechanics 81: 0-93, 2012.SC Sharma, M Krishna, N. Narasimhamurthy. Buckling response of stitched polyurethane foam composite sandwich structures,

Journal of reinforced plastics and composites, 23(12): 1267-1277, 2004BF Sørensen, K Branner et al. Improved design of large wind turbine blades of fibre composites (phase 2) – summary report, Risø-

R-1526(EN), Risø National Laboratory, Denmark, 2005.R Stewart. Wind turbine blade production – new products keep pace as scale increases. Reinforced Plastics, Jan/Feb 2012: 19-25.OT Thomsen. Sandwich materials for wind turbine blades - present and future. Journal of Sandwich Structures and Materials 11:7-

26, 2009

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