Blade Materials

Prof. Mike Kessler

Blade Materials

Mike Kessler

Motivation: Why composite materials?

Motivation – Structural CompositesPercentage of composite components in commercial aircraft*

*Source: “Going to Extremes” National Academies Research Council Report, 2005

Why PMCs?•Specific Strength and Stiffness•Part reduction•Multifunctional

20 % Wind Energy Scenario

• 300 GW of wind energy production by 2030• Keys for achieving 20% scenario Increasing capacity of wind

turbines Developing lightweight and low

cost turbine blades (Blade weight proportional to cube of length)

Fatigue• First MW scale wind turbine

– Smith-Putnam wind turbine, installed 1941 in Vermont

– 53 meter rotor with two massive steel blades

– Mass caused large bending stresses in blade root

– Fatigue failure after only a few hundred hours of intermittent operation.

– Fatigue failure is a critical design consideration for large wind turbines.

Material Requirements• High material stiffness is needed to

maintain optimal aerodynamic performance,

• Low density is needed to reduce gravitaty forces and improve efficiency,

• Long-fatigue life is needed to reduce material degradation – 20 year life = 108-109 cycles.

Material RequirementsMb=0.006

Mb=0.003

/2/1EM b

Merit index for beam deflection (minimize mass for a given deflection)

Absolute Stiffness (~10-20 Gpa)

Resistance against fatigue loads requires a high fracture toughness per unit density, eliminating ceramics and leaving candidate materials as wood and composites.

Blade Materials

Mike Kessler

Constituent Materials Used in Wind Turbine Blades

Materials For Turbine Blades• Fiber reinforced polymers (FRPs) are widely used for

bladesLightweightExcellent mechanical properties• Commonly used fiber reinforcements are glass

and carbonGlass Fiber vs. Carbon FiberGlass Fiber• Adequate Strength• High failure strain• High density• Low cost

Carbon Fiber• Superior mechanical properties• Low density• High cost (produced from PAN)

Material for Rotorblades• Fibers

– Glass– Carbon– Others

• Polymer Matrix• Composite Materials

• Composites: --Multiphase material w/significant proportions of ea. phase.• Matrix: --The continuous phase --Purpose is to: transfer stress to other phases protect phases from environment

• Dispersed phase: --Purpose: enhance matrix properties.

increase E, y, TS, creep resist. --For structural polymers these are typically fibers --Why are we using fibers?

For brittle materials, the fracture strength of a small part is usually greater than that of a large component (smaller volume=fewer flaws=fewer big flaws).

Terminology

Fibers

• Glass• Carbon• Others

woven fibers

cross section view

0.5mm

0.5mm

D. Hull and T.W. Clyne, An Introduction to Composite Materials, 2nd ed., Cambridge University Press, New York, 1996, Fig. 3.6, p. 47.

Fibers

• Most widely used for turbine blades

• Cheapest

• Best performance• Expensive

Composite properties from various fibers

Glass Fibers

• Most widely used for composites in turbine blades

• Diameters of 10–20 m• Produced by pulling from spinnerets• Coated with polymer sizing to

improve bonding between fibers and matrix

Carbon Fibers

• Graphite: crystallographic structure is stacked planes of hexagonal lattices

• Good mechanical properties in the hexagonal plane; weaker in the perpendicular direction

• Currently, fabrication starts with polyacrylonitrile (PAN) or natural tar– Both raw materials and processing methods

are expensive, so researchers are looking for other options

Lignin- A Natural Polymer

• Lignin, an aromatic biopolymer, is readily derived from plants and wood

• The cost of lignin is only $0.11/kg

• Available as a byproduct from wood pulping and ethanol fuel production

• Can decrease carbon fiber production costs by up to 49 %.

• Current applications for lignin use only 2% of total lignin produced

Carbon Fibers from Lignin• Production steps involve

Fiber spinningThermostabilizationCarbonization

• Current ChallengesPoor spinnability of ligninPresence of impuritiesChoice of polymer blending agentCompatibility between fibers and resins

Warren C.D. et.al. SAMPE Journal 2009 45, 24-36

Project Goals• Develop robust process for manufacturing

carbon fibers from lignin/polymer blend• Evaluate polymers for blending, including

polymers from natural sources• Optimize lignin/polymer blends to ensure

ease of processability and excellent mechanical properties

• Investigate surface functionalization strategies to facilitate compatibility with polymer resins used for composites

Technical Approach• Evaluate and pretreat high purity grade lignin• Spin fibers from lignin-copolymer blends using

unique fiber spinning facility• Characterize surface and

mechanical properties of carbon fibers made from lignin precursor• Perform fiber surface treatments (silanes and alternative sizing agents)

• Evaluate performance for a prototype coupon (Merit Index)

Polymer Matrices

• Thermosets– Unsaturated Polyesters– Vinyl Esters– Epoxies

• Thermoplastics

Properties of Polymer Matrices

• Low stiffness– 3–4 GPa for thermosets– 1–3 GPa for thermoplastics

• Good toughness– 5–8% failure strain for thermosets– 50–100% failure strain for thermoplastics

• Densities match fibers well– 1.1–1.3 g/cm3 for thermosets– 0.9–1.4 g/cm3 for thermoplastics– 1.77 g/cm3 for carbon fibers– 2.54 g/cm3 for glass-E fibers

Unsaturated Polyesters– Linear polyester with C=C

bonds in backbone that is crosslinked with comonomers such as styrene or methacrylates.

– Polymerized by free radical initiators

– Fiberglass composites– Large quantities

Epoxies

– Common Epoxy Resins

• Bisphenol A-epichlorohydrin (DGEBA)

• Epoxy-Novolac resins

Epoxide Group

•Cycloaliphatic epoxides

•Tetrafunctional epoxides

R CH

O

CH2

Epoxies (cont’d)

– Common Epoxy Hardners• Aliphatic amines

• Aromatic amines

•Acid anhydrides

H2N

HN

NH2

NH2H2N

DETA

M-Phenylenediamine (mPDA)

O

O

O

Hexahydrophthalic anhydride (HHPA)

Step Growth Gelation(a) Thermoset

cure starting with two part monomer.

(b) Proceeding by linear growth and branching.

(c) Continuing with formation of gell but incompletely cured.

(d) Ending with a Fully cured polymer network.

From Prime, B., 1997

Composite Materials

• Resin and fiber are combined to form composite material.

• Material properties depend strongly on 1. Properties of fiber2. Properties of polymer matrix3. Fiber architecture4. Volume fraction5. Processing route

Properties of Composite Materials• Stiffness

• Static strength• Fatigue properties• Damage Tolerance

Blade Materials

Mike Kessler

Characterizing Materials and Cure

Time-Temperature-Transformation (TTT) Diagrams

• TTT Diagrams– Useful tool for

illustrating gelation and vitrification

– Only meaningful if read horizontally (isothermal)

From Gillham, J. K., NATAS 200530

Characterization of Cure

• Differential Scanning Calorimetry (DSC) is a useful tool for measuring the extent and rate of cure.

• Degree of Conversion, α

• HR is found by running “dynamic scan” of a completely unreacted sample.

31

R

residualR

R H

HH

H

tH

)(H(t) is the enthalpy of the reaction up to time t

HR is the total heat of reaction

RH

dtdH

dt

d

Note that dH/dt is the ordinate of a DSC trace.

Hresidual is the residual heat of reaction of a partially cured sample.

DSC of Thermoset Cure

Hrxn

Hres

T

g

dH/dt(W, J/s)

Isothermal cure at 160°CWisanrakkit and Gillham,J.Appl.Poly.Sci 42, 2453 (1991)

Amine - epoxy

Data courtesy of J. GilhamSlide courtesy of B. Prime

Conversion – Time Curves

Conversion

()

Time (minutes)

Wisanrakkit and Gillham,J.Appl.Poly.Sci. 41, 2885 (1990)

Epoxy-Amine Cure DGEBA-PACM-20 (1:1)

Data courtesy of J. GilhamSlide courtesy of B. Prime

Tg – Time Curves

Wisanrakkit and Gillham,J.Appl.Poly.Sci. 42, 2453 (1991)

Epoxy-Amine Cure DGEBA-PACM-20 (1:1)Tg = 178°C

Data courtesy of J. GilhamSlide courtesy of B. Prime

Superposition of Tg vs. ln(time) data to form a

master curve

From Wisanrankkit and Gilham, 1990

Tref=140Vitrification point shown by arrows

Blade Materials

Mike Kessler

Wind Turbine Blade Design and Construction

Cross-section of Composite Blade

Manufacturing Processes

• Wet hand-lay-up– Most common when wind technology

was getting started

• Prepreg processes– Used by Vestas Wind Systems

• Resin-infusion processes– Usually vacuum-assisted– Most common today

Source: Hayman, B. & Wedel-Heinen, J. Materials challenges in present and future wind energy. MRS bulletin (2008).

Wet Hand-Lay-Up

• Oldest FRP production method, user earlier for boat building

• Composites layers are assembled by hand in open (one-sided) molds

• Randomly oriented fibers

• Cheap, but labor-intensive and poor control

Wet hand-lay-up of 27 m blade for three-bladed turbine at Tvind, Denmark. The turbine was built in the mid-1970s, but the blades were replaced in 1993. (Bröndsted 2005)

Filament Winding

• In simplest form, structure is rotated to wind oriented fibers around it

• More complicated winding geometries are necessary for turbine blades– Blades are not cylindrical– Fibers need to be oriented along long

axis of blade, but it’s not feasible to spin a blade end-over-end

Prepreg Technology

• Fibers are pre-impregnated with uncured resin• Prepregs are “tacky” sheets, which are stacked

into composite• Composites are cured by heating under vacuum

– 80 is common curing temperature for large wind turbine blades.

• Advantages– Easy to control– High fiber content, which gives good stiffness and

strength– Clean process, saving money on ventilation systems

Resin Infusion

• Basic idea: put fibers in sealed mold, inject liquid resin into mold, cure entire composite

• Biggest problem is incomplete wetting of fibers

• Solution: vacuum infusion technique, where vacuum is used to pull resin into fiber package

• Like prepreg, advantages are high fiber content and clean process

Blade Materials

Mike Kessler

Mechanical Properties and Damage Modes in Wind Turbine Blades, Laminated Composites, and Related Adhesive Joints

Common Production Defects

• Delaminations• Dry zones and voids• Poor curing• Wrinkles• Fiber reinforcement defects• Misalignment of fibers• Bonding defects (between layers in

sandwich structures, or between blocks of core material)

Delaminations

• Separation of plies in a laminar composite

• Reduces compressive strength by up to 34% (single delamination) or 64% (multiple laminations)

• Possible manufacturing causes are:– Contaminated reinforcing fibers– Insufficient wetting– Shrinkage during curing

Sandwich Debonds

• Lack of bonding between skin and core in a sandwich structure

• Critical defect because it compromises the advantages of the sandwich structure

• Possible manufacturing causes:– Voids in adhesive layer– Inadequate surface preparation– Inadequate curing

• Can often be detected by tapping a coin or light hammer on the surface

Geometric Imperfections

• Can happen throughout a structure– Wavy fibers within composite– Components that are not as flat or

straight as they should be– Joints that don’t fit perfectly

• Waviness in fibers reduces compressive strength

• Larger-scale geometric imperfections reduce fatigue life

Wrinkles

• One or many layers wrinkled outward

• Either in single-skin laminates or in face of sandwich structures

• Critical reduction in compressive strength for single-skin laminates

• Less of a problem in sandwich structures

References• Brøndsted, Povl, Hans Lilholt, and Aage Lystrup.

“Composite Materials for Wind Power Turbine Blades.” Annual Review of Materials Research 35, no. 1 (August 4, 2005): 505–538.

• Brøndsted, P, and JW Holmes. Wind rotor blade materials technology. European Sustainable Energy …, 2008.

• Hayman, B, and J Wedel-Heinen. “Materials challenges in present and future wind energy.” MRS bulletin (2008).

• Holmes, JW, and BF Sørensen. “Reliability of Wind Turbine Blades: An Overview of Materials Testing.” In proceedings Wind Power …, 2007.