Fatigue of Wind Blade Laminates:Fatigue of Wind Blade Laminates: Effects of Resin and Fabric Structure

DetailsDetails

David Miller, Daniel D. Samborsky and John F. Mandell

M t St t U i itMontana State University

MCARE 2012

Outline• Overview of MSU Fatigue Program on

Wind Blade MaterialsWind Blade Materials • Recent Findings, Resin and Fabric

Structure Interactions for InfusedStructure Interactions for Infused Laminates

• Comparison of Fatigue Trends for Various• Comparison of Fatigue Trends for Various Wind Blade Component Materials

Acknowledgements: Sandia National Laboratories/DOE (Joshua Paquette, Program Monitor).

Thanks to our many industry collaboratorsThanks to our many industry collaborators

• DOE/MSU Fatigue Database for Wind Blade Materials(Public, Sandia Website)

– Over 250 Materials

12 000+ test results– 12,000+ test results

– Updates each MarchUpdates each March

– Excel based

– Trends analyzed in contractor reports ( t d / it / )(www.coe.montana.edu/composites/ )

1. Blade Laminate Performance• Purpose: Explore critical issues for basic

blade laminatesblade laminates – Characterize the static and fatigue resistance of blade

composite laminatesC d i l fib f b i i fib i i• Current and potential fibers, fabrics, resins, fiber sizings, processes, processing aids, laminate lay-ups, fiber contents, loading conditions, spectrum loading and design data

Identify failure modes and mechanisms– Identify failure modes and mechanisms• Ex: Cracking at fabric backing strands deleterious to lower

cost polyester and vinyl ester resin laminatesId tif t ti l t i l ith i d– Identify potential materials with improved performance, lower cost, processing advantages, etc.

• Ex: pDCPD resin (tough, low viscosity); aligned strand l i t lik N t R dP klaminates like Neptco RodPack

Glass FabricsStandard Laminate Fatigue

Glass Fabrics

S-N Curves, MD LaminatesEffect of R-ValueCarbon vs GlassCarbon vs Glass

Carbon Prepregp g

2. Complex Coupons with MaterialT iti lik F b i J i t d Pl DTransitions 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

related to structural details

Complex Structured Coupons with Ply Drops Resin InfusionDrops, Resin Infusion

Purpose: Mini-substructure test. Simplified, less costly p , yapproach to substructure testing. Efficient comparisons of resins, fabrics, geometric details in structural context..coupons represent more realistic internal (infused) blade structural detail areas thanstructural detail areas than standard laminate tests

Damage Growth CurvesStatic Fatigue, R = 0.1

Damage Growth with Different Resins Correlates with Interlaminar GIc, GIIc

Simulation

Ic, IIc

3. Blade adhesives

• Bulk adhesive strength, fatigue, g , g ,fracture toughness, environmental effects

• Strength Based: standard joint geometry g j g ylike lap shear; test includes crack initiation and propagation to failure. May include ff f feffects of typical flaws like porosity and

poor surface prep.F M h i B d k• Fracture Mechanics Based: crack propagation resistance for relatively large crackscracks.

Fatigue at R = 0.1 and -1

Adhesive Thickness Effects

Adhesive Flexural Mixed Mode Fracture Tests

Mixed Mode Bending ApparatusTypical load-deflection graph from an MMB test

Mixed Mode Fracture for ADH-1

Typical Crack Path Transitions from Path B to C in MMB Specimens for ADH-1Path 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.

Flexural Testing of NextelCore Infused LaminateCore Infused Laminate

Static Failure

5. Property Data for Analysis

• 3-D static properties of 100 mm thick glass/epoxy

Laminate Elastic Constants1

Tensile Modulus EL (GPa) 44.6g p ylaminate

L ( )

Tensile Modulus ET (GPa) 17.0

Tensile Modulus EZ (GPa) 16.7

Compressive Modulus EL (GPa) 42.8

C i M d l E (GP ) 16 0Compressive Modulus ET (GPa) 16.0

Compressive Modulus EZ (GPa) 14.2

Poisson Ratio νLT 0.262

Poisson Ratio νLZ 0.264

Poisson Ratio νTL 0.079

Poisson Ratio νTZ 0.350

Poisson Ratio νZL 0.090

Poisson Ratio ν 0 353Poisson Ratio νZT 0.353

Shear Modulus GLT (GPa) 3.49

Shear Modulus GLZ (GPa) 3.77

Shear Modulus GTL (GPa) 3.04

Shear Modulus GTZ (GPa) 3.46

Shear Modulus GZL (GPa) 3.22

Shear Modulus GZT (GPa) 3.50

LAMINATE STRENGTH

STRESS DIRECTION

STRENGTH(MPa)

ULTIMATE STRAIN

Static Strength Properties in Three-Directions

STRENGTH PROPERTIES

DIRECTION (MPa) STRAIN(%)

Tension L 1240 3.00T i 1 T 43 9 0 28Tension1 T 43.9 0.28Tension Z 31.3 0.21Compression L 774 1.83Compression T 179 1.16Compression Z 185 1.44Shear2 LT 55.8 5.00Shear LT 55.8 5.00Shear2 LZ 54.4 5.00Shear TL 52.0 4.60

2Shear2 TZ 45.6 5.00Shear ZL 33.9 1.10Shear ZT 28.4 0.81

1Transverse tension properties given for first cracking (knee) stress2Shear values given for 5% strain following ASTM D5379

( ) Sh B t Fit St St i CShear coupons and best fit stress-strain curves

(c) Shear Best Fit Stress-Strain Curves

Recent FindingsEffects of Fabric Construction andEffects 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 andresins; and for most fabrics with vinyl esters and polyesters

• Consistent fatigue performance is found with g psome epoxies for a broad range of stitched unidirectional (UD) FabricsF ti f d i ifi tl• Fatigue performance can decrease significantly as fiber volume fraction (Vf) increases for many fabrics and resinsfabrics and resins

Data Representation Polyester (UP) vsEpoxy (EP)

Million CycleMillion Cycle Strain Parameter;

Power Law Fits:S = A NB; 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)

B kFront Back

Aligned Strand

Unidirectional (0)2 fabric H laminates; effect of removing 90o backing strands. No effect with epoxy significantNo effect with epoxy, significant improvement with polyester; failure along backing strands with UP, VE resins.resins.

Poorly-performing fabric/resincombinations

Resin cracks along transversefabric backing strand take out primary uni-strands in currentinfusion fabric (polyester resin)

Resin cracks along stitch linetake out uni-strands in earlyyhand lay-up triax fabric (tight stitching, polyester resin)

Aligned Strand (AS) vs UD Fabric H (02) 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 highersame strands and resins as in the fabrics. Aligned strand laminates higherVf, stronger, significantly more fatigue resistant compared to UD fabrics)

Fabric Efficiency: Fabric vs Aligned

Resin EP1/EP5 VE4 UP5

Fiber Volume Fraction, Vf gStrands (AS)

AS Laminates 0.64 0.66 0.68

Fabric Laminates 0.58 0.55 0.58

0o Vf , Fabric L i t

0.53 0.50 0.53 PF: Property for Fabric Laminates

Fabric efficiency: Translation of aligned strand (AS) structure

Laminates

0o Direction Fabric Efficiency, PF/PAS

0o Vf 0.83 0.76 0.78

M d l E 0 88 0 85 0 81

PAS: Property for AS Laminates

g ( )properties into UD fabric H (PPG-Devold L1200/G50-E07) laminate properties for different resins (PPG 2400 Tex rovings with

Modulus, E 0.88 0.85 0.81

UTS 0.73 0.68 0.62

106 cycle stress 0.64 0.37 0.40 6 (PPG 2400 Tex rovings with

Hybon 2026 sizing).

E and UTS translate efficiently

106 cycle strain 0.73 0.43 0.49

PF/PAS Adjusted to AS Vf [(PF/PAS) (AS Vf/Fabric 0o Vf)]

Modulus E 1 06 1 12 1 04 for all resins; 106 Cycle Fatigue properties translate well for epoxy resin (EP1/EP5), but poorly for vinyl ester (VE) and

Modulus, E 1.06 1.12 1.04

UTS 0.88 0.89 0.79

106 cycle stress 0.77 0.49 0.51

106 l t i 0 88 0 49 0 63 poorly for vinyl ester (VE) and polyester (UP)

106 cycle strain 0.88 0.49 0.63

Typical Infused Laminate Property Ratios Polyester (UP) to Epoxy (EP)*

Property Ratio UP/EPAxial Tensile Modulus (UD) 1.0Axial Tensile Modulus (MD) 1 0 AS Ali d St dAxial Tensile Modulus (MD) 1.0Axial (UD) Static Tensile Strength 0.90-1.0Axial (MD) Static Tensile Strength 0.90-1.0T (UD) T il C ki St i 0 42

AS: Aligned StrandUD: Unidirectional

Fabric (0)2MD: MultidirectionalTransverse (UD) Tensile Cracking Strain 0.42

Axial (AS) 106 Cycle Strain (R = 0.1) 0.66Axial (UD) 106 Cycle Strain (R = 0.1) 0.51

MD: MultidirectionalFabric (0/±45..)

Biax: ±45 Fabric

Axial (MD) 106 Cycle Strain (R = 0.1) 0.65Axial (Biax) 106 Cycle strain (R = 0.1) 0.91Interlaminar GI (0-0 interface) 0 55Interlaminar GIc (0 0 interface) 0.55Interlaminar GIIc (0-0 interface) 0.48Complex Coupon Ply Drop Delamination, Threshold Fatigue Strain (R = -1)

0.74Threshold Fatigue Strain (R = -1)

*Vf = 0.5 to 0.6, UD Fabrics D and H, Biax Fabrics M and P

Summary, Fabric/Resin Effects• AS laminates give baseline for potential performance for

particular resins and strands.• Fabric efficiency relates the translation of AS properties into y p p

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 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• 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 L i t T d ith Oth Bl dLaminate 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 loadingdetailed blade FEA and critical loading.Future work: defined failure mode substructure studies.

Tensile Fatigue Trends, Effects of Reinforcement Type and Lay-up (S = A NB; exponent B = 1/n)

MaterialForm

Res-in

UTS, MPa

A/UTS

B n 106 CycleStrain,

%

UD Ali d St d (AS) L i t (PPG 2400 T H b 2026 Fi i h)UD Aligned Strand (AS) Laminates (PPG 2400 Tex, Hybon 2026 Finish)

AS EP5 1369 1.149 -0.072 13.9 1.20AS VE4 1340 1.457 -0.088 11.4 1.23AS UP5 1382 1 558 0 123 8 13 0 79AS UP5 1382 1.558 -0.123 8.13 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[( )2 ( )2]

Material Resin UTS, A/UTS B n 106 Cycle

Fatigue Trends, Other Laminate Types and Directions

MaterialForm

Resin ,MPa

10 CycleStrain,

%

Transverse Direction Fabric H UD Laminates

(90)6 Fabric H EP5 52.4a 1.857 -0.114 8.77 0.124

Biax Fabric M (±45/mat) Laminates

(±45/m)3b i

EP1 224 1.004 -0.092 10.9 0.53Fabric M

(±45/m)3Fabric M

VE1 239 1.000 -0.090 11.1 0.44

(±45/m) UP1 208 0 972 -0 098 10 2 0 41(±45/m)3Fabric M

UP1 208 0.972 0.098 10.2 0.41

Triax Fabric W

(±45/0)s EP1 585 2.20 -0.143 6.99 0.70( 45/0)s Fabric W

EP1 0.70

aFirst cracking stress

Material Resin Stre- A/ B n 106 Cycle

Fatigue Trends, Other Blade Details

Form ngth UTS Strain, %

Delamination at thick ply drops

1 l d F b i D EP1 189b 0 551 ply drop, Fabric D EP1 189b 0.55

2 ply drop, Fabric D EP1 135b -0.120 8.3 0.39

4 ply drop, Fabric D EP1 106b -0.099 10.1 0.35

1 ply drop, Fabric D UP1 N/A 0.39

Thick Adhesive Lap Shear Joints Hexion Adhesive

EP135G3/EKH1376GN/A 13.90c 1.63 -0.109 9.17 N/A

EP135G3/EKH1376G

3M W1100 N/A 13.80c 2.11 -0.135 7.41 N/A

Triax Skin/Core Sandwich Flexural FatigueIn ProgressIn Progress

bForce (kN) at 30-mm delamination lengthcApparent lap shear strength for 3.25 mm thick adhesive, 25 mmoverlap length 5 mm thick UD Fabric D/EP-1 adherendsoverlap length, 5 mm thick UD Fabric D/EP 1 adherends.

Summary: Comparison of Blade Materialsfor High Cycle Fatigue Performanceg y g

• Epoxy Resin EP1: – Transverse and biax damage before UD laminate g

failure (damage progression)– Ply drop delamination at similar strain to biax cracking

Adh i (ADH 1) f i i il h– Adhesive (ADH-1) fatigue exponent similar to other materials

• UP and VE Resins:• UP and VE Resins:– Similar failure strains for UD and biax components

and single ply drop; very low transverse strain for UPg y y– Adhesive (ADH-1) fatigue exponent similar to biax

laminate exponent, lower than UD exponent

Future Research• Shift toward substructure oriented studies

while maintaining database testingwhile maintaining database testing• Major uncertainties lie

in the performance ofin the performance of complex structure with realistic multi axialrealistic, multi-axial loading