Underestimated Low-Cycle Fatigue as a Contributor to Premature Failures

Ken T. Lee, MSAE, Blade Design Technical Lead

Amir Bachelani, Blade Structural Engineer

Cody Moore, Loads & Dynamics Engineer

Kyle K. Wetzel, Ph.d., CEO/CTO

Sandia Blade Reliability Workshop 2013Albuquerque, New Mexico

Outline

Wind Turbine Component Damage/Failure Low-Cycle Fatigue – Premature Failures Design Loads for Certification Fatigue Analysis for Certification Recommendations for Future Research

Copyright © 2013 Wetzel Engineering, Inc. All Rights Reserved.

WT Component Damage/FailureShare of main components of

total number of failuresDamage/failure occurrences

attributed to: electrical systemsrotor bladesgearbox

Structural in nature sustained early in the life of these components (sometimes within 3 or 6 months to a year) continuous and costly maintenance and repair

WT Component Damage/FailureWind Turbine

subassembly reliability:WT reliability & downtime

varies across subcomponents of WT

Failure frequencies of blades and gearboxes lower compared to other subcomponents

Downtime per failure for rotor blades and gearbox higher compared to other subcomponents

Pitch mechanism & electrical system shown to be significant sources.

Failure or fault-induced shutdowns

Common WT Component Damage/Failure

Common Types of Rotor Blade Damage/Failure:Crack propagation due to inherent

design or manufacturing defects

Blade damage due to extreme load buckling or blade-tower strike

Failure of adhesive bonds leading edge/trailing edge splitting

Blade failure at the blade root connection blade throw

Causes of WT Component Damage/Failure

Operating loads exceed design loads:Underestimated Design Loads (Ultimate, Low-cycle fatigue, or High-cycle fatigue)

Mistakes in applying standard methods

Shortcomings in standard methods

Malfunctioning control safety systems

Improper site assessment

Insufficient assessment of structural integrity:Mistakes in applying standard methods

Shortcomings in standard methods

Low-Cycle Fatigue (LCF)

Possible Contributor to Premature Wind Turbine Component Failures

Focus: LCF – Low-cycle fatigue characterized as:• High amplitude alternating stress

• Low cycle counts (approx. less than 104)

• Fatigue failure: propagation-dominated

LCF - OverviewComponents designed to 20 years of life (according to GL, IEC, DNV) Premature failure of WT subcomponents:Rotor Blades

Gearbox

Structural design & analysis of WT Components, Current Provision:Fatigue design focused on high-cycle fatigue (HCF) (i.e. S-N curve methods).

High amplitude loading events that occur at low cycle counts induce crack growth rates that far exceed that predicted by S-N curve analyses.

LCF - OverviewDesign Loads Simulations, Current Framework:Does not capture high-amplitude transient events (less than 104 – 504 cycles) Insufficient resolution of the “tails” of the loads spectra “Normal-operation” during

turbulent conditions

Under-reporting of loads induced by transient events, coherent inflow conditions, fault-induced shutdowns.

Blade Load Spectra as per Current Framework for Certification Design Loads:

Average Normalized Cumulative Blade Load Spectra with/without LCF:

Design Loads for Certification

Case Study on the Impact of Emergency Stops on Blade Fatigue Life

GL -Design Load Cases for Fatigue Analysis

•Power Production Load Case:DLC 1.1; NTM; Vin < V < Vout

• Idling or Parked Load Case:DLC 6.4; NTM; V<Vin and Vout < V < Vref

GL -Design Load Cases for Fatigue Analysis•Transient Load Cases, Faults during Operation:DLC 1.4; NWP; Grid-loss/E-stop – 20/year

DLC 1.8; NWP; Iced Conditions – 24 hours/year

DLC 2.1; NWP; Fault Occurrence – 24 hours/year

DLC 3.1; NWP; Start Up - 1000/year @ Vin, 50 @ Vr & Vout

DLC 4.1; Normal Shutdown - 1000/year @ Vin, 50 @ Vr & Vout

Possible Shortcomings in Current Guidelines• WT components could possibly be failing prematurely from fatigue.

• Current guidelines do not account for higher frequency rates of E-stops and/or Fault-Induced shutdown procedures

• E-stops due to grid loss could have a higher impact on fatigue life

• E-stops of more than 20/year, averaging 1 E-stop/day (360/year) Early life of WT operation, debugging of control and safety systems

Control system malfunction in response to abnormal inflow conditions

• Impact of an E-stop or a pitch control system fault during a gusting wind (with or without turbulent inflow) not considered.

Case Study of Impact of Increased Frequency of E-Stop Procedures

• Fatigue life of two turbines analyzed the impact of stricter provisions for Emergency Shutdown (E-stops) 20 cases/year of DLC 1.1 NTM (GL-

2010), grid loss timed at highest point of My

Increase cycle counts of DLC 1.4 (NWP) Estops from 20/year to 360/year (7200 in turbine lifetime)

1 case/year of DLC 1.5 EOG1 (GL-2010), grid loss timed at highest point of My

21 cases/year of grid loss during a gusting wind were added to the analysis

Results: 100 kW Turbine – 13m BladeVariable Speed-Pitch Regulated

Results: 2.0MWTurbine – 45m BladeVariable Speed-Pitch Regulated

Fatigue Analysis Methods for Certification

Current Fatigue Analyses GuidelinesFiber-Reinforced Composite Laminates:Characteristic S-N curve established for laminate. (GL 5.5.3.3.1)

Goodman diagram constructed using this curve. (GL 5.5.3.3.1)

Fatigue damage calculation using Miner’s rule (IEC 61400-1, 7.6.3.2)

95 % survival probability with a confidence level of 95 % used as basis for SN-curve (IEC 61400-1, 7.6.3.2)

Adhesives: 3 samples (being representative for the jointed components in geometry

and material) Minimum number of load cycles of N=106. (GL 5.5.6.10)

• HCF estimation is captured. LCF response of WT subcomponents/blade composites should also be considered in guidelines.

• Testing on pristine laminates. Manufacturing defects not tested. • Structural damage/failures due to:Manufacturing that is out of QCFailure to design for manufacturing quality that can be realistically

controlled.

• Current Safety Factors (SF’s) result in excessively conservative designs in areas not really the source of problems.

• Heavy reliance on SF’s still fails to address on-going problems industry has with blade reliability.

Current Fatigue Analyses Guidelines

Types of Damage & Defects

• Delamination

• Voids & Cracks in Laminates/Adhesive Bonds

• Wavy fiber plies, bridging of fibers

• Porosity, discontinuities in laminates

Crack Growth in Fiber-Reinforced Plastics

• Region I – Matrix Cracking

• Region II – Matrix-Fiber Interface Cracking

• Region III – Fiber Cracking

Crack Growth Modes in Adhesives

•Mode I – Opening Stresses (Peel Stress)•Mode II – Shear Stresses•Mixed Mode Loading of I and II

Low Cycle Fatigue (LCF) - Unidirectional Composites

• Normal S-N curve does not properly capture LCF.• A bi-linear S-N curve can be used as a basis to capture structural

response from LCF.

Crack Growth - Adhesives

Crack Growth - Adhesives

Crack Length Range, mm Best Fit Equation, mm/cycle0 – 20 0.99 .

20 – 40 2.77 .40 – 80 6.70 .

Future Research Directions & Recommendations

Design Fatigue Loads Estimation:Fatigue loads estimations methods should analyze: Increased probability of high amplitude transient events resulting from:

Fault-induced shutdown procedures

Control system fault or Emergency shutdown during a gusting wind and/or coherent inflow with wind directional changes

Rare occurrences of extreme oblique inflow

Increased cycle counts of high amplitude loading to capture extreme statistics where peak load will occur during the early stages in the operating life of WTG. Increase resolution of the tails of fatigue loads spectra.

Future Research Directions & RecommendationsStructural Design for Fatigue:

Structural influences of manufacturing defects, response to LCF

Crack growth modeling

Fracture mechanics

Improved S-N curve analyses:

Non-pristine laminates

Adhesive bonds

Sandwich cores

Anticipated manufacturing tolerances on structurally critical members

Analyses should define tolerances expected of the manufacturing QMS

Establish SF’s that rely less on testing laboratory coupons to establish material strength and properties, more on testing of components and subsystems that more closely reflect actual QC to establish.

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info@WetzelEngineering.com+1 785 856 0162 (office)

1310 Wakarusa Drive, Suite ALawrence, Kansas 66049

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