Blade Materials

1. Blade Materials Prof. Mike Kessler

2. Blade Materials Motivation: Why composite materials? Mike Kessler

3. Motivation – Structural Composites Percentage of composite components in commercial aircraft* • Why PMCs? • Specific Strength and Stiffness • Part reduction • Multifunctional *Source: “Going to Extremes” National Academies Research Council Report, 2005

4. 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)

5. 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.

6. 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.

7. Material Requirements Mb=0.003 Mb=0.006 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.

8. Blade Materials Constituent Materials Used in Wind Turbine Blades Mike Kessler

9. Materials For Turbine Blades • Fiber reinforced polymers (FRPs) are widely used for blades • Lightweight • Excellent mechanical properties • Commonly used fiber reinforcements are glass and carbon Glass Fiber vs. Carbon Fiber • Glass Fiber • Adequate Strength • High failure strain • High density • Low cost • Carbon Fiber • Superior mechanical properties • Low density • High cost (produced from PAN)

10. Material for Rotorblades • Fibers • Glass • Carbon • Others • Polymer Matrix • Composite Materials

11. Terminology • 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, sy, 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).

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

13. Fibers • Most widely used for turbine blades • Cheapest • Best performance • Expensive

14. Composite properties from various fibers

15. 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

16. 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

17. 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

18. Carbon Fibers from Lignin • Production steps involve • Fiber spinning • Thermostabilization • Carbonization • Current Challenges • Poor spinnability of lignin • Presence of impurities • Choice of polymer blending agent • Compatibility between fibers and resins Warren C.D. et.al. SAMPE Journal 2009 45, 24-36

19. 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

20. 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)

21. Polymer Matrices • Thermosets • Unsaturated Polyesters • Vinyl Esters • Epoxies • Thermoplastics

22. 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

23. 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

24. Epoxies • Common Epoxy Resins • Bisphenol A-epichlorohydrin (DGEBA) • Epoxy-Novolac resins Epoxide Group • Cycloaliphatic epoxides • Tetrafunctional epoxides

25. Epoxies (cont’d) • Common Epoxy Hardners • Aliphatic amines • Aromatic amines • Acid anhydrides DETA Hexahydrophthalic anhydride (HHPA) M-Phenylenediamine (mPDA)

26. Step Growth Gelation • Thermoset cure starting with two part monomer. • Proceeding by linear growth and branching. • Continuing with formation of gell but incompletely cured. • Ending with a Fully cured polymer network. From Prime, B., 1997

27. Composite Materials • Resin and fiber are combined to form composite material. • Material properties depend strongly on • Properties of fiber • Properties of polymer matrix • Fiber architecture • Volume fraction • Processing route

28. Properties of Composite Materials • Stiffness • Static strength • Fatigue properties • Damage Tolerance

29. Blade Materials Characterizing Materials and Cure Mike Kessler

30. 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 2005

31. Characterization of Cure • Differential Scanning Calorimetry (DSC) is a useful tool for measuring the extent and rate of cure. • Degree of Conversion, α • HRis found by running “dynamic scan” of a completely unreacted sample. H(t) is the enthalpy of the reaction up to time t HR is the total heat of reaction Hresidual is the residual heat of reaction of a partially cured sample. Note that dH/dt is the ordinate of a DSC trace.

32. dH/dt (W, J/s) DHres Tg DHrxn Isothermal cure at 160°C Wisanrakkit and Gillham, J.Appl.Poly.Sci 42, 2453 (1991) DSC of Thermoset Cure Amine - epoxy Data courtesy of J. Gilham Slide courtesy of B. Prime

33. Conversion – Time Curves Data courtesy of J. Gilham Slide courtesy of B. Prime

34. Tg – Time Curves Data courtesy of J. Gilham Slide courtesy of B. Prime

35. Superposition of Tg vs. ln(time) data to form a master curve Tref=140 Vitrification point shown by arrows From Wisanrankkit and Gilham, 1990

36. Blade Materials Wind Turbine Blade Design and Construction Mike Kessler

37. Cross-section of Composite Blade

38. 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).

39. 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)

40. 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

41. 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

42. 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

43. Blade Materials Mechanical Properties and Damage Modes in Wind Turbine Blades, Laminated Composites, and Related Adhesive Joints Mike Kessler

44. 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)

45. 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

46. 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

47. 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

48. 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

49. References • Brøndsted, Povl, Hans Lilholt, and AageLystrup. “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.