blade materials
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Blade Materials. Prof. Mike Kessler. Blade Materials. Motivation: Why composite materials?. Mike Kessler. Motivation – Structural Composites. Percentage of composite components in commercial aircraft*. Why PMCs? Specific Strength and Stiffness Part reduction Multifunctional. - PowerPoint PPT PresentationTRANSCRIPT
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.