Fatigue of Wind Blade Laminates: Effects of Resin and Fabric Structure Details David Miller, Daniel D. Samborsky and John F. Mandell Montana State t University it MCARE 2012
Outline Overview of MSU Fatigue Program on Wind Blade Materials Recent Findings, Resin and Fabric Structure Interactions for Infused Laminates Comparison of Fatigue Trends for Various Wind Blade Component Materials Acknowledgements: Sandia National Laboratories/DOE (Joshua Paquette, Program Monitor). Thanks to our many industry collaborators
DOE/MSU Fatigue Database for Wind Blade Materials (Public, Sandia Website) Over 250 Materials 12,000+ test results Updates each March Excel based Trends analyzed in contractor reports (www.coe.montana.edu/composites/ / )
1. Blade Laminate Performance Purpose: Explore critical issues for basic blade laminates Characterize the static and fatigue resistance of blade composite laminates Current and potential fibers, fabrics, resins, fiber sizings, i processes, processing aids, laminate lay-ups, fiber contents, loading conditions, spectrum loading and design data Identify failure modes and mechanisms Ex: Cracking at fabric backing strands deleterious to lower cost polyester and vinyl ester resin laminates Identify potential ti materials with improved performance, lower cost, processing advantages, etc. Ex: pdcpd resin (tough, low viscosity); aligned strand laminates like Neptco RodPack
Glass Fabrics Standard Laminate Fatigue S-N Curves, MD Laminates Effect of R-Value Carbon vs Glass Carbon Prepreg
2. Complex Coupons with Material Transitions like Fabric Joints and Ply Drops Thickness Tapering
Purpose: Explore ply delamination issues related to structural details Mixed Mode Delamination Testing, Different Resins and Fabrics
Complex Structured Coupons with Ply Drops, Resin Infusion Purpose: Mini-substructure test. Simplified, less costly approach to substructure testing. Efficient comparisons of resins, fabrics, geometric details in structural context.. coupons represent more realistic internal (infused) blade structural detail areas than standard laminate tests
Damage Growth Curves Static Fatigue, R = 0.1 Simulation Damage Growth with Different Resins Correlates with Interlaminar G Ic,, G IIc
3. Blade adhesives Bulk adhesive strength, fatigue, fracture toughness, environmental effects Strength Based: standard joint geometry like lap shear; test includes crack initiation and propagation to failure. May include effects of typical flaws like porosity and poor surface prep. Fracture Mechanics Based: crack propagation resistance for relatively large cracks.
Adhesive Thickness Effects Fatigue at R = 0.1 and -1
Adhesive Flexural Mixed Mode Fracture Tests Typical load-deflection graph from an MMB test Mixed Mode Bending Apparatus
Mixed Mode Fracture for ADH-1
Typical Crack Path Transitions from Path B to C in MMB Specimens for ADH-1
4. Core Materials Purpose: To explore test methods for core materials which reflect critical core performance attributes for a wide range of emerging core materials and structures.
Static Failure Flexural Testing of Nextel Core Infused Laminate
5. Property Data for Analysis 3-D static properties of 100 mm thick glass/epoxy laminate Laminate Elastic Constants 1 Tensile Modulus E L (GPa) 44.6 Tensile Modulus E T (GPa) 17.0 Tensile Modulus E Z (GPa) 16.7 Compressive Modulus E L (GPa) 42.8 Compressive Modulus E T (GPa) 16.0 Compressive Modulus E Z (GPa) 14.2 Poisson Ratio ν LT 0.262 Poisson Ratio ν LZ 0.264 Poisson Ratio ν TL 0.079 Poisson Ratio ν TZ 0.350 Poisson Ratio ν ZL 0.090 Poisson Ratio ν ZT 0.353 Shear Modulus G LT (GPa) 3.49 Shear Modulus G LZ (GPa) 3.77 Shear Modulus G TL (GPa) 3.04 Shear Modulus G TZ (GPa) 3.46 Shear Modulus G ZL (GPa) 3.22 Shear Modulus G ZT (GPa) 3.50
Static Strength Properties in Three-Directions LAMINATE STRENGTH STRESS DIRECTION STRENGTH (MPa) ULTIMATE STRAIN PROPERTIES (%) Tension L 1240 3.00 Tension 1 T 43.9 028 0.28 Tension Z 31.3 0.21 Compression L 774 1.83 Compression T 179 1.16 Compression Z 185 1.44 Shear 2 LT 55.8 5.00 Shear 2 LZ 54.4 5.00 Shear TL 52.0 4.60 Shear 2 TZ 45.6 5.00 Shear ZL 33.9 1.10 Shear ZT 28.4 0.81 1 Transverse tension properties given for first cracking (knee) stress 2 Shear values given for 5% strain following ASTM D5379
Shear coupons and best fit stress-strain curves ()Sh (c) Shear Best tfitst Stress-Strain Curves
Recent Findings Effects of Fabric Construction and Resin Type on Fatigue Performance Poor tensile fatigue performance has been found for some lighter weight fabrics with all resins; and for most fabrics with vinyl esters and polyesters Consistent fatigue performance is found with some epoxies for a broad range of stitched unidirectional (UD) Fabrics Fatigue performance can decrease significantly ifi as fiber volume fraction (V f ) increases for many fabrics and resins
Data Representation Polyester (UP) vs Epoxy (EP) Million Cycle Strain Parameter; Power Law Fits: S = A N B ; Exponent B = 1/n S: Stress or Strain Linear-Log Plots, Multidirectional Laminates;TT: Database Laminate Designation; [±45/0/±45/0/±45]
PPG-Devold L1200/G50-E07 (MSU Fabric H, 1261 gsm) Front Back Aligned Strand
Unidirectional (0) 2 fabric H laminates; effect of removing 90 o backing strands. No effect with epoxy, significant improvement with polyester; failure along backing strands with UP, VE resins.
Poorly-performing fabric/resin combinations Resin cracks along transverse fabric backing strand take out primary uni-strands in current infusion fabric (polyester resin) Resin cracks along stitch line take out uni-strands in early hand lay-up triax fabric (tight stitching, polyester resin)
Aligned Strand (AS) vs UD Fabric H (0 2 ) Fatigue data, Three Resins (AS laminates fabricated by PPG/Reichhold by dry strand winding/infusion; same strands and resins as in the fabrics. Aligned strand laminates higher V f, stronger, significantly more fatigue resistant compared to UD fabrics)
Resin EP1/EP5 VE4 UP5 Fiber Volume Fraction, V f AS Laminates 0.64 0.66 0.68 Fabric Laminates 0.58 0.55 0.58 0 o V f, Fabric Laminates Fabric Efficiency: Fabric vs Aligned Strands (AS) 0.53 0.50 0.53 P F : Property for Fabric Laminates P AS : Property for AS Laminates 0 o Direction Fabric Efficiency, P F /P AS 0 o V Fabric efficiency: Translation of f 0.83 0.76 0.78 aligned strand (AS) structure Modulus, E 088 0.88 085 0.85 081 0.81 properties into UD fabric H (PPG- UTS 0.73 0.68 0.62 Devold L1200/G50-E07) laminate 10 6 cycle stress 0.64 0.37 0.40 properties for different resins 10 6 cycle strain 0.73 0.43 0.49 (PPG 2400 Tex rovings with Hybon 2026 sizing). P F /P AS Adjusted to AS V f [(P F /P AS )(AS V f /Fabric 0 o V f )] Modulus, E 1.06 1.1212 1.04 E and UTS translate efficiently UTS 10 6 cycle stress 10 6 cycle strain 0.88 0.77 088 0.88 0.89 0.49 049 0.49 0.79 0.51 063 0.63 for all resins; 10 6 Cycle Fatigue properties translate well for epoxy resin (EP1/EP5), but poorly for vinyl ester (VE) and polyester (UP)
Typical Infused Laminate Property Ratios Polyester (UP) to Epoxy (EP)* Property Ratio UP/EP Axial Tensile Modulus (UD) 1.0 Axial Tensile Modulus (MD) 10 1.0 Axial (UD) Static Tensile Strength 0.90-1.0 Axial (MD) Static Tensile Strength 0.90-1.0 Transverse (UD) Tensile Cracking Strain 042 0.42 Axial (AS) 10 6 Cycle Strain (R = 0.1) 0.66 Axial (UD) 10 6 Cycle Strain (R = 0.1) 0.51 Axial (MD) 10 6 Cycle Strain (R = 0.1) 0.65 Axial (Biax) 10 6 Cycle strain (R = 0.1) 0.91 Interlaminar G Ic (0-0 0 interface) 055 0.55 Interlaminar G IIc (0-0 interface) 0.48 Complex Coupon Ply Drop Delamination, 0.74 Threshold Fatigue Strain (R = -1) *V f = 0.5 to 0.6, UD Fabrics D and H, Biax Fabrics M and P AS: Aligned Strand UD: Unidirectional Fabric (0) 2 MD: Multidirectional Fabric (0/±45..) Biax: ±45 Fabric
Summary, Fabric/Resin Effects AS laminates give baseline for potential performance for particular resins and strands. Fabric efficiency relates the translation of AS properties p into typical UD fabric laminates, considering the reduced fiber content in the axial direction. Fabric efficiency is good for static strength and modulus. Fabric efficiency is good for high cycle fatigue for epoxy resin, but poor for polyester; vinyl esters range from poor to good. The cause of low fabric efficiency for some resins is cracking associated with transverse strands, and varies with fabric details. Multidirectional laminate efficiency may be similar to that of the UD fabric, but may be reduced in some cases by premature failure induced by biax fabric cracking.
Comparison of Fatigue Trends for Various Laminate Types and with Other Blade Materials and Structural Details Which laminates, materials, and material transitions will develop damage first as a function of service life and environment? How will the damage propagate? Consequences to structural performance? Compare fatigue exponents and strain capability in the context of detailed blade FEA and critical loading. Future work: defined failure mode substructure studies.
Tensile Fatigue Trends, Effects of Reinforcement Type and Lay-up (S = A N B ; exponent B = 1/n) Material Form Resin UTS, MPa A/ UTS B n 10 6 Cycle Strain, % UD Aligned Strand (AS) Laminates (PPG 2400 Tex, Hybon 2026 Finish) AS EP5 1369 1.149-0.072 13.9 1.20 AS VE4 1340 1.457-0.088 11.4 1.23 AS UP5 1382 1.558-0.123 813 8.13 079 0.79 UD Fabric H Laminates (contain PPG 2400 Tex/Hybon 2026 Strands) (0) 2 Fabric H EP1 995 1.265-0.088 11.4 0.88 (0) 2 Fabric H VE4 912 2.485-0.170 5.88 0.53 (0) 2 Fabric H UP5 884 1.940-0.173 5.78 0.39 MD Laminates, UD Fabric H and Biax Fabric T [(±45) 2 /(0) 2 ]s EP1 704 1.957-0.130 7.69 0.79 [(±45) 2 /(0) 2 ]s VE4 628 1.955-0.146 6.85 0.53 [(±45) 2 /(0) () 2 ]s UP5 663 1.736-0.151 6.62 0.42
Fatigue Trends, Other Laminate Types and Directions Material Resin UTS, A/UTS B n 10 6 Cycle MPa Form Strain, % Transverse Direction Fabric H UD Laminates (90) 6 Fabric H EP5 52.4 a 1.857-0.114 8.77 0.124 Biax Fabric M (±45/mat) Laminates (±45/m) 3 EP1 224 1.004-0.092 10.9 0.53 Fabric M (±45/m) 3 Fabric M VE1 239 1.000-0.090 11.1 0.44 (±45/m) 3 UP1 208 0.972-0.098098 10.2 041 0.41 Fabric M Triax Fabric W (±45/0)s EP1 585 2.20-0.143 6.99 0.70 Fabric W a First cracking stress
Fatigue Trends, Other Blade Details Material Resin Stre- A/ B n 10 6 Cycle Form ngth UTS Strain, % Delamination at thick ply drops 1 ply drop, Fabric D EP1 189 b 055 0.55 2 ply drop, Fabric D EP1 135 b -0.120 8.3 0.39 4 ply drop, Fabric D EP1 106 b -0.099 10.1 0.35 1 ply drop, Fabric D UP1 N/A 0.39 Thick Adhesive Lap Shear Joints Hexion Adhesive EP135G3/EKH1376G N/A 13.90 c 1.63-0.109 9.17 N/A 3M W1100 N/A 13.80 c 2.11-0.135 7.41 N/A Triax Skin/Core Sandwich Flexural Fatigue In Progress b Force (kn) at 30-mm delamination length c Apparent lap shear strength for 3.25 mm thick adhesive, 25 mm overlap length, 5 mm thick UD Fabric D/EP-1 adherends.
Summary: Comparison of Blade Materials for High Cycle Fatigue Performance Epoxy Resin EP1: Transverse and biax damage before UD laminate failure (damage progression) Ply drop delamination at similar strain to biax cracking Adhesive (ADH-1) fatigue exponent similar il to other materials UP and VE Resins: Similar failure strains for UD and biax components and single ply drop; very low transverse strain for UP Adhesive (ADH-1) fatigue exponent similar to biax laminate exponent, lower than UD exponent
Future Research Shift toward substructure oriented studies while maintaining database testing Major uncertainties lie in the performance of complex structure with realistic, multi-axial loading