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EWEC09 Marseille, 18 March 2009 Fracture mechanics techniques for the design of structural components with adhesive joints for wind turbines. Authors:

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Presentation on theme: "EWEC09 Marseille, 18 March 2009 Fracture mechanics techniques for the design of structural components with adhesive joints for wind turbines. Authors:"— Presentation transcript:

1 EWEC09 Marseille, 18 March 2009 Fracture mechanics techniques for the design of structural components with adhesive joints for wind turbines. Authors: Iñaki Nuin, Carlos Amézqueta, Daniel Trias, Javier Estarriaga, Marcos del Río, Ana Belén Fariñas,

2 Why dealing with Fracture Mechanics?. Let’s introduce the problem. Fracture Mechanics approach. VCCT approach. VCCT approach. In-house code. Application scenario 1. Application scenario 2. Conclusions – Future work. Acknowledgements. Table of contents

3 Years ago, CENER was involved in a 40 meter length glass fiber epoxy blade. Guidelines reading.  Design scenarios: Static and fatigue.  For static loads:  Fiber Failure: Common theories. (MAX.STRAIN / TSAI-WU…)  Matrix Failure: General agreement. (PUCK / LARC03-04…)  ¿How to deal with bonding lines?.  For fatigue loads:  Detailed S-N approach for glass and carbon epoxy / polyester composites. (GL guidelines)  ¿How to deal with bonding lines? Why dealing with Fracture Mechanics?

4 GL guideline:  Static:  7 MPa: Limit defined for the characteristic shear stresses.  It covers stress concentration factors up to a factor of 3.0.  Fatigue:  1 MPa: Limit defined for the equivalent constant-range spectrum for 10 7 load cycles.  It covers stress concentration factors up to a factor of 3.0. NOTES: The adhesive must be approved by GL. The bonding lines must not include discontinuities (fatigue). Why dealing with Fracture Mechanics?

5 DNV guideline:  Difficult to match real local stresses with numerical analyses.  Due to simplifications.  Due to FEM-meshing effects.  It is necessary to combine analytical with testing approach.  Purpose:  Update the predicted resistance of the joint with the results from the tests.  Gain knowledge. Why dealing with Fracture Mechanics?

6 Testing and field experience:  Adhesive failure may happen…  Comment from a Blade manufacturer:  The most difficult part of the manufacturing process is trying to bond the two shells together.  Trailing edge defects can grow to full blade failure.  Bonding problem is the biggest issue.

7 Let's introduce the problem Stress approach.  Local stress levels dependent on the mesh size.  As element size gets smaller, local stress gets higher.  No reliable method for bonded components design.  If we refine the mesh…..¿when do we stop?.

8 History:  Theoretical concepts developed at the beginning of 20th century.  First real applications for the industry in the eighties.  From 1995 till today it is commonly used. Concept:  Specially well-suited for brittle behaviour.  Provides concepts which fill the gap between micro-scale and real component dimensions.  Energy based analysis: Stable solution for local effects.  Based on crack propagation analysis. Fracture Mechanics approach Combinations mixed modes

9 Energy release rate (G): Elastic energy released when the defect grows one unit of area. The critical value for G is a material property. It is common that:  G Ic < G IIc < G IIIc : Normalized tests. The crack grows under a pure mode deformation if:  G > G ic with i=I, II, III. For mixed modes, there are different approaches which try to deal with an equivalent G value. Fracture Mechanics approach

10 How can we measure it?  FCEM: Finite crack extension method. (two analyses)  Based on Griffith balance.  CCT: Crack closure technique. (two analyses)  Energy necessary for the crack to grow = External work needed for the crack to close.  VCCT: Virtual crack closure technique. (one analysis)  Based on the auto-similarity concept. Fracture Mechanics approach

11 Numerical model definition. VCCT approach  Adhesive paste is substituted by linear springs.  The stiffness of each spring considers:  Bonded area.  Elastic modulus of the adhesive (modified by Hooke’s laws).  Thickness of the adhesive layer.

12 Stable solution.  As element size gets smaller, G reaches a stable value. VCCT approach ¡¡…a reliable method for bonded components design!!

13 NASTRAN model FEM model with rigid links at the adhesive area (rbe2) Modified model (including adhesive behaviour) Pre-process (PATRAN) Stresses Crack initiation criterium Post-process (PATRAN) Critical areas definition (crack initiation) FEM model modification: Equivalent stiffness Adhesive properties Bonding paste thickness Bonding area. (MATLAB) NASTRAN model (MATLAB) G calculation (VCCT) RESULTS (Crack stability) VCCT approach. In-house Code

14 In-house developed software. User interface. VCCT approach. In-house Code

15 FMAC.  STEP FEM model definition. Rigid links for bonding areas. 2.Adhesive elastic properties, critical energy release rate (G Ic, G IIc, G IIIc ) and thicknesses definition. 3.Automatic definition of the modified model. NASTRAN analysis.  STEP Critical areas definition attending to stress criterion or other factors (manufacturing problems…) 4.Crack definitions. 5.Automatic definition of the cracked model. NASTRAN analysis.  STEP G I, G II, G III calculation by VCCT approach. 8.Failure indexes definition. VCCT approach. In-house Code

16 Let’s imagine we must estimate the ultimate static load for a metallic component which is bonded to a composite panel:  Load direction; 45º Application Scenario 1 How can we proceed?....Let’s go step by step. Tests performed at CENER.

17 STEP -1-: Material Characterization (elastic properties).  Steel:  Mechanical elastic properties are well known. Young modulus: MPa Poisson ratio: 0.3  Composite panel:  3 point bending test to obtain the flexural modulus.  Biaxial strain gauge to define Poisson ratio. Flexural modulus: 7972MPa Poisson ratio:  Adhesive (BETAMATE 7014/7065H)  Universal traction tests. Elastic modulus: 3.1MPa Poisson ratio: 0.45 Application Scenario 1 Tests performed at CENER.

18 STEP -2-: G c testing for the bonding interfaces.  Steel-adhesive interface:  ASTM D3433 standard. Application Scenario 1 Tests performed at CENER.

19 STEP -2-: G c testing for the bonding interfaces.  Steel-adhesive interface:  Huge dispersion for Maximum load results (4787N – 5411N).  Different values of G depending on the standard approach:  Considering the DCB specimen FEM model and FCEM, CCT & VCCT approaches: Application Scenario 1

20 STEP -2-: G c testing for the bonding interfaces.  Composite-adhesive interface:  ASTM D3433 standard. Application Scenario 1 Tests performed at CENER.

21 STEP -2-: G c testing for the bonding interfaces.  Composite-adhesive interface:  Huge dispersion for Maximum load results (276.9N – 466.7N).  Different values of G depending on the standard approach:  Considering the DCB specimen FEM model and FCEM, CCT & VCCT approaches: Application Scenario 1

22 STEP -2-: G c testing for the bonding interfaces.  Depending on the standard, the values of G are quite scattered:  ASTM D3433 and “Classical Beam Theory” approaches do not consider adhesive paste stiffness. Rigid adhesives (epoxy). Small thickness of the bonding layer.  “Orthotropic Theory” and “Modified Classical Beam Theory” take into account shear in plane effects of the adherents.  “Adhesive Theory” considers the adhesive layer stiffness.  FCEM, CCT and VCCT theories are based on FEM models. As a consequence the values for G c, are supposed to consider all these global effects.  When designing a real bonded component, it is necessary to compare the values of G in between analogous approaches. Application Scenario 1

23 STEP -3-: Ultimate load estimation.  The lowest value of G c defines the de-bonding interface.  A FEM model is defined considering real test scenario. Linear analyses are performed under different load magnitudes. Application Scenario 1

24 STEP -4-: Test Correlation.  Two tests were performed.  Problems with adhesive cure cycle for one component.  So… only one test result available for comparison. Application Scenario 1 Test failure load is 11400N, 21% higher than predicted value (9428N)

25 Let’s compare VCCT approach and Cohesive elements technique against a 3 point bending test of an I-Beam:  Tests performed at WMC facilities. UPWIND project. Application Scenario 2 …Let’s go step by step.

26 STEP -1-: Material Characterization. Application Scenario 2 UD Reinforcement (Flanges)MD Reinforcement (Web)Adhesive

27 STEP -2-: FEM models definition.  MSC.MARC.  Linear material behaviour.  Large displacements assumption.  Cohesive elements to simulate the adhesive interface with glass fiber laminates (UD &MD).  3D laminate properties (out of plane characterization). Application Scenario 2

28 STEP -2-: FEM models definition.  MSC.NASTRAN.  Linear material behaviour.  Small displacements assumption.  VCCT technique defined via in house developed software (FMAC).  3D orthotropic properties (calculated from laminate properties). Application Scenario 2

29 STEP -3-: Failure prediction – Correlation with test. Application Scenario 2 MSC.MARC (Cohesive Elements)MSC.NASTRAN (VCCT)  Critical local points for both models are located at the same area.  MSC.MARC: First bonding failure under 40.6KN load.  MSC.NASTRAN: First bonding failure under 48.1KN load.  Test Failure 47.6KN……just a coincidence!!

30 Conclusions.  Fracture mechanics approach is confirmed as a reliable method when designing bonded components.  VCCT approach predicts the possibility of one defect to start growing… nothing about how it grows (cohesive elements).  Nevertheless, due to bonding process complexity and uncertainties, it is difficult to estimate accurately bonded joints capacity.  Ignorance factors must be considered. Future work.  In-house code development:  Spring model development (coupled behaviour).  Non-linear behaviour implementation.  Validation test plans:  ENF specimen tests performance.  Mixed mode tests performance.  Subcomponent tests. Conclusions – Future work

31 Acknowledgements UPWIND WP3 partners. ALSTOM-ECOTECNIA wind power department.

32 Thank you very much for your attention.

33


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