EWEC09Marseille, 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,

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

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?

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).

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.

Why dealing with Fracture Mechanics?

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.

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?.

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

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 < GIIc < GIIIc : Normalized tests.

The crack grows under a pure mode deformation if: G > Gic with i=I, II, III.

For mixed modes, there are different approaches which try to deal with an equivalent G value.

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.

Fracture Mechanics approach

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.

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

VCCT approach

¡¡…a reliable method for bonded components design!!

NASTRAN model

FEM model with rigid links at the adhesive area (rbe2)

Modified model (including adhesive

behaviour)

Pre

-pro

cess

(PA

TR

AN

)

Stresses

Crack initiation criterium

Post-

pro

cess

(PA

TR

AN

)

Critical areas definition (crack initiation)

FEM model modification:

Equivalent stiffness

• Adhesive properties

• Bonding paste thickness

• Bonding area.

(MA

TLA

B)

NASTRAN model

(MATLAB)

G calculation (VCCT)

RESULTS(Crack stability)

VCCT approach. In-house Code

In-house developed software. User interface.

VCCT approach. In-house Code

FMAC. STEP -1-1. FEM model definition. Rigid links for bonding areas .2. Adhesive elastic properties, critical energy release rate (GIc, GIIc, GIIIc)

and thicknesses definition.3. Automatic definition of the modified model. NASTRAN analysis.

STEP -2-3. 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 -3-7. GI, GII, GIII calculation by VCCT approach.

8. Failure indexes definition.

VCCT approach. In-house Code

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.

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

Application Scenario 1

Tests performed at CENER.

STEP -2-: Gc testing for the bonding interfaces. Steel-adhesive interface:

ASTM D3433 standard.

Application Scenario 1

Tests performed at CENER.

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:

Application Scenario 1

STEP -2-: Gc testing for the bonding interfaces . Composite-adhesive interface:

ASTM D3433 standard.

Application Scenario 1

Tests performed at CENER.

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:

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.

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.

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.

Application Scenario 1

Test failure load is 11400N, 21% higher than predicted value (9428N)

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.

STEP -1-: Material Characterization.

Application Scenario 2

UD Reinforcement (Flanges) MD Reinforcement (Web) Adhesive

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

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

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!!

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

Acknowledgements

UPWIND WP3 partners. ALSTOM-ECOTECNIA wind power department.

Thank you very much for your attention.