Sin ttulo de diapositiva

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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,
EWEC09 Marseille, 18 March 2009
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Table of contents

• Why dealing with Fracture Mechanics?.
• Lets introduce the problem.
• Fracture Mechanics approach.
• VCCT approach.
• VCCT approach. In-house code.
• Application scenario 1.
• Application scenario 2.
• Conclusions Future work.
• Acknowledgements.

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Why dealing with Fracture Mechanics?

• 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?

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Why dealing with Fracture Mechanics?

• 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 107 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).

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Why dealing with Fracture Mechanics?

• 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.

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Why dealing with Fracture Mechanics?

• 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.

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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?.

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Fracture Mechanics approach

• 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.

Combinations mixed modes
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Fracture Mechanics approach

• 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
• GIc lt GIIc lt GIIIc Normalized tests.
• The crack grows under a pure mode deformation if
  
• G gt Gic with iI, II, III.
• For mixed modes, there are different approaches
  which try to deal with an equivalent G value.

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Fracture Mechanics approach

• 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.

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VCCT approach

• Numerical model definition.

• Adhesive paste is substituted by linear springs.
• The stiffness of each spring considers
• Bonded area.
• Elastic modulus of the adhesive (modified by
  Hookes laws).
• Thickness of the adhesive layer.

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VCCT approach

• Stable solution.
• As element size gets smaller, G reaches a stable
  value.

a reliable method for bonded components
design!!
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VCCT approach. In-house Code
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VCCT approach. In-house Code
In-house developed software. User interface.
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VCCT approach. In-house Code

• FMAC.
• STEP -1-
• FEM model definition. Rigid links for bonding
  areas .
• Adhesive elastic properties, critical energy
  release rate (GIc, GIIc, GIIIc) and thicknesses
  definition.
• Automatic definition of the modified model.
  NASTRAN analysis.
• STEP -2-
• Critical areas definition attending to stress
  criterion or other factors (manufacturing
  problems)
• Crack definitions.
• Automatic definition of the cracked model.
  NASTRAN analysis.
• STEP -3-
• GI, GII, GIII calculation by VCCT approach.
• Failure indexes definition.

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Application Scenario 1

• Lets imagine we must estimate the ultimate
  static load for a metallic component which is
  bonded to a composite panel
• Load direction 45º

Tests performed at CENER.

• How can we proceed?....Lets go step by step.

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Application Scenario 1

• STEP -1- Material Characterization (elastic
  properties).
• Steel
• Mechanical elastic properties are well known.
• Young modulus 210000MPa
• 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 0.088
• Adhesive (BETAMATE 7014/7065H)
• Universal traction tests.
• Elastic modulus 3.1MPa
• Poisson ratio 0.45

Tests performed at CENER.
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Application Scenario 1

• STEP -2- Gc testing for the bonding interfaces.
• Steel-adhesive interface
• ASTM D3433 standard.

Tests performed at CENER.
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Application Scenario 1

• STEP -2- Gc 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

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Application Scenario 1

• STEP -2- Gc testing for the bonding interfaces
  .
• Composite-adhesive interface
• ASTM D3433 standard.

Tests performed at CENER.
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Application Scenario 1

• STEP -2- Gc 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

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Application Scenario 1

• STEP -2- Gc 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 Gc, 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.

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Application Scenario 1

• STEP -3- Ultimate load estimation.
• The lowest value of Gc defines the de-bonding
  interface.
• A FEM model is defined considering real test
  scenario. Linear analyses are performed under
  different load magnitudes.

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Application Scenario 1

• STEP -4- Test Correlation.
• Two tests were performed.
• Problems with adhesive cure cycle for one
  component.
• So only one test result available for
  comparison.

Test failure load is 11400N, 21 higher than
predicted value (9428N)
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Application Scenario 2

• Lets compare VCCT approach and Cohesive
  elements technique against a 3 point bending test
  of an I-Beam
• Tests performed at WMC facilities. UPWIND
  project.

• Lets go step by step.

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Application Scenario 2

• STEP -1- Material Characterization.

UD Reinforcement (Flanges)
MD Reinforcement (Web)
Adhesive
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Application Scenario 2

• 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).

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Application Scenario 2

• 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).

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Application Scenario 2

• STEP -3- Failure prediction Correlation with
  test.

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 47.6KNjust a coincidence!!

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Conclusions Future work

• 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.

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Acknowledgements

• UPWIND WP3 partners.
• ALSTOM-ECOTECNIA wind power department.

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Thank you very much for your attention.
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