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Fracture Mechanics and New Techniques and Criteria for the Design of Structural Components for Wind Turbines. Daniel Trias, Raquel Rojo, Iñaki Nuin, Esteban Belmonte Analysis and Design of Aerogenerators – Wind Department. Index. INTRODUCTION Failure of composites: a matter of scale

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Fracture Mechanics and New Techniques and Criteria for the Design of Structural Components for Wind Turbines

Daniel Trias, Raquel Rojo, Iñaki Nuin, Esteban Belmonte

Analysis and Design of Aerogenerators – Wind Department


Index
Index Design of Structural Components for Wind Turbines

  • INTRODUCTION

    • Failure of composites: a matter of scale

    • Failure criteria for fibre-reinforced composites

  • FRACTURE MECHANICS FOR ADHESIVE/DELAMINATION ASSESSEMENT: VCCT

    • Stresses in a single lap joint (Illustrative example)

    • VCCT Implementation in a commercial FE code

    • Application example

  • FRACTURE MECHANICS IN FAILURE CRITERIA: LaRC criteria

    • (Short) Description

    • Application example (only on article)


INTRODUCTION Design of Structural Components for Wind Turbines


Failure in composites a matter of scale
Failure in composites: a matter of scale Design of Structural Components for Wind Turbines

Failure depends on phenomena (matrix and fibre cracking, debonding, kinking …) which take place at a scale of about 10um and which are nearly-brittle


Failure in composites a matter of scale1

60 m Design of Structural Components for Wind Turbines

Failure in composites: a matter of scale

5.000.000 : 1 scale relation with microscale (fibre diametre)

46 m

2.29 m

Liberty

Blade

Yao Ming


Failure criteria for fibre reinforced composites
Failure criteria for fibre reinforced composites Design of Structural Components for Wind Turbines

  • MACROSCOPICAL CRITERIA

    • Empirically obtained from global behaviour of laminae

    • Generally symmetrical

    • “Black box”

    • Ply level or laminate level

    • Tsai-Hill, Tsai-Wu, etc.

  • PHENOMENOLOGICAL CRITERIA

    • Bridge micro and macro behaviour by analyzing specific phenomena

    • Ply level

    • Hashin, Hashin-Roten, Puck, etc.

    • Puck: Analyzes fracture plane successfully spread since WWFE

    • Puck: Physically meaningless parameters

  • MICROSCOPICAL CRITERIA

    • Failure of single constituents: fibre, matrix

    • May be used in multi-scale analysis

    • Computationally unaffordable for large structures

Refine some failure criteria

Adhesive joints/ Delamination assessment:

- VCCT

- Decohesive elements

  • FRACTURE MECHANICS

    • Theory 1900s. Application in Computational Mechanics 1970s

    • Introduce the effect of defects in brittle behaviour, analyze kinking.

    • NASA: LaRC Criteria. Physically based parameters




Stresses in a single lap joint
Stresses in a single lap joint VCCT

Single lap joint


Stresses in a single lap joint1
Stresses in a single lap joint VCCT

Single lap joint

LARGE stress gradients!

Shear stresses

(Induced) Peel stresses


Adhesive implementation in fe model stress based approach
Adhesive implementation in FE model: stress-based approach VCCT

Single slab joint

(FE model)

2 nodes with same coordinates joined with a MPC/rigid link

Adhesive

Elastic spring element

2 nodes with same coordinates joined with a MPC/rigid link


Stress dependence on mesh size

- VCCT

+

Peeling Stress peak

Mesh-size

+

-

Stress dependence on mesh size


Fracture mechanics approach

Combinations: mixed modes VCCT

Mode I

Mode III

Mode II

Fracture Mechanics approach

  • Based on crack propagation analysis:

  • Specially well-suited for cracked materials and brittle behaviour

  • Provides concepts and tools which allow the analysis of microscale phenomena and their application to component-scale situations.

  • Energy based analysis: stable solution for stress singularities


Fracture mechanics approach1
Fracture Mechanics approach VCCT

  • GIc, GIIc, GIIIc are material properties. Usually:

    • GIc < GIIc < GIIIc

  • Critical values of G are needed for each mode. Tests with a standard:

    • Mode I : DCB test (ASTM, DIN, ISO)

    • Mode II: ENF test (DIN)

    • Mixed mode I/II: MMB test (ASTM)

    • Mode III: some proposals

  • Failure criteria (Loss of adhesion / delamination)

    • GI > GIc ?

    • GII > GIIc ?

    • GIII > GIIIc?

  • We need to compute GI, GII, GIII numerically: Virtual Crack Closure Technique (VCCT)

    • Basic assumption: the energy needed to open a crack some Δa length is the same energy needed to close it some Δa length


Fracture mechanics approach vcct
Fracture Mechanics approach : VCCT VCCT

Debonded region

Bonded region

Crack tip

Adhesive

G>Gc? : Would a potential crack propagate?


Crack tip local coordinate system

Modified formulae: 3D non-regular meshes VCCT

Need to find information on neighbor nodes and elements

Crack tip: Local coordinate system

Crack tip

Non-straight crack tip: Local coordinate system to be defined at each node of the crack tip

Bonded region

yL

Debonded region

xL

i

k

x*


Implementation with a commercial fe code
Implementation with a commercial FE code VCCT

Modification of adhesive model:

r.link  spring + r.link

Model with defined adhesive zone (r.link)

Model with non-rigid adhesive zone

FE commercial software (Nastran, Marc)

FE SOLUTION

External code (MATLAB)

Stress solution

USER INTERACTION

Initiation criteria

Definition of critical zones to crack initiation

Computation of G (VCCT)



Application to a turbine blade 2
Application to a Turbine Blade (2) VCCT

Initiation criteria (stress)  Detect zones where crack may appear


Application to a turbine blade 21
Application to a Turbine Blade (2) VCCT

Crack “creation”: Adhesive is removed from those nodes showing larger value of the stress-based criteria

Need to solve again!


Application to a turbine blade 3
Application to a Turbine Blade (3) VCCT

GI, GII, GIII computed through VCCT formula, considering crack local coordinate system

Check adhesive failure criteria based on energy release rate

Nearly the same methodology may be used for delamination



Improvements achieved with larc
Improvements achieved with LaRC VCCT

  • Fracture Mechanics employed for tensile matrix failure. In situ effects (dependence on ply thickness) are considered

  • Fibre kinking computed through Fracture Mechanics

  • Drawbacks:

    • Iteration required for the computation of fracture plane angles

    • Not (yet) spread in industry


Application to a component
Application to a component VCCT

σ11>0 and σ22>0


Application to a component1
Application to a component VCCT

σ11<0 and σ22<0


Final remarks and conclusions
Final Remarks and conclusions VCCT

  • Fracture Mechanics can be used successfully even in commercial finite element codes for adhesive assessment.

  • VCCT can be used for both adhesive and delamination assessment.

  • Fracture Mechanics has been used (NASA) to improve some failure criteria:

    • Biaxial Compression

    • Fibre Kinking

  • Future work:

    • Compare with models with analytical solution (almost done!)

    • Compare with tests on a substructure

    • Fatigue model


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