composite materials forwind turbine blades -...

Composite materialsfor wind turbine blades

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

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

• 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)

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

[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.

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

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)

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

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

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

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

Why composite materials on blades?

ρALm = 2/1

MATERIAL INDEX

ρALm =

3LEIkFS ==

32/112

kLSm ρMass of the beam

Stiffness of the beam

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

Why composite materials?Minimize mass for assigned stiffness

Line with constant E1/2/ρ values

Ashby plot

Why composite materials?Minimize mass for assigned strength

Line with constant σf2/3/ρ values

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.

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)

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

[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%

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)

Properties of reinforcing fibresMaterial Tensile

strength(MPa)

Young’s modulus E

Density (g/cm3)

Specificstiffness

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

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)

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

FIBRE ORIENTATION FIBRE FRACTION

Tensile and compressive properties of unidirectional composites

Reinforcement architecture

Unidirectional fibres

Common geometries of the fibrous reinforcement include

Continuous or choppedstrand mat (CSM)

Unidirectional fibres

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 (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

90°+45°

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 of multidirectional fabrics or unidirectional fibres

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

Weight 1 ~1 ~1

Bending stiffness 1 ~12 ~48

Bending strength 1 ~6 ~12

Manufacturing techniquesfor composite blades

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

Potential for automation

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).

[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

[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]

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.

[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

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.

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)

Typical manufacturing defects

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

Aerospace quality

•Fibre waviness•Wrinkles

Dry regions and fibre waviness

0° direction

PREPREG

INFUSION

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

• 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

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

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

Main failure modes in composite materials

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

• 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

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.

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

Fibre failure in tension

Fibre failure in compression

1=+ultx

1=−ulty

Matrix failure in tension

Matrix failure in compression

Matrix failure in shear

1=+ulty

Maximum stress criteria

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

ultxyτ

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

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

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).

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

• 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

Core crushing or laminate fracture due to local loads

Global or shear buckling Face/core debonding

+ buckling

Typical failure modes in blades- Main spar -

The most critical failure modes involve interface failure

[Sørensen et al., 2005]

Typical failure modes in blades- Wing shell -

[Sørensen et al., 2005]

The most critical failure modes involve interface failure

USE OF CARBON FIBRESIN WIND TURBINE BLADES

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

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

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 include

Large 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)

•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

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

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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SHEAR-EXTENSION COUPLING

BEND-TWIST COUPLING

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BEND-EXTENSION COUPLING

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.

[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

[de Goeij, 1999]

Bend-twist configurations

Fully (whole span) or partially (outer span)

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

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)

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)

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