Experimental And Numerical Analysis Of Skin/Stiffener ... · 2D models were developed in Ansys for...

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Experimental And Numerical Analysis Of Skin/Stiffener

Debonding Under Bending

Iker Urresti

6th International Conference on Composites Testing and Model Identification

I. Urresti1, A. Barrio1, J. Renart2 and L. Zubillaga1

1Department of Mechanical Engineering, [email protected],

http://www.ikerlan.es

2AMADE, Polytechnic School, University of [email protected], http://www.udg.edu/

24/04/2013

mailto:[email protected]

http://www.ikerlan.es/

mailto:[email protected]

http://www.udg.edu/

OUTLINE

OUTLINE

1 INTRODUCTION 1.1 IK-4 IKERLAN

1.2 MOTIVATION

1.3 OBJECTIVE OF THE PRESENT WORK

2 MULTI-LEVEL APPROACH• 2.1 MULTI-LEVEL APPROACH SUMMARY

• 2.2 FRACTURE MECHANICS MODELS AND VALIDATION (2D, 3D)

• 2.3 EXPERIMENTAL CHARACTERIZATION OF ADHESIVE

• 2.4 NON-SPECIFIC 3 POINT BENDING : MODELS AND VALIDATION

• 2.5 SKIN-STIFFENER DEBONDING UNDER BENDING: MODELS AND VALIDATION

• 3 DISCUSSION OF THE RESULTS

• 4 CONCLUSIONS

• 5 PERSPECTIVES AND APPLICATIONS


1. INTRODUCTION

1.1 IK4-IKERLAN

Private not-for-profit Technology Centre

Staff of more than 300 qualified researchers and engineers

IK-Ikerlan is part of “IK-4 Research alliance” and also the key technological R&D actor within “Grupo Mondragon” (7th largest business group in Spain)

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Works for a wide variety of domains: transportation (railway and vertical), automation, industrial, medical, home appliances, wind-turbines, etc.

Main industrial partners:


1. INTRODUCTION

1.2 MOTIVATION

More and more structures with adhesive joints are being designed in several industries: aeronautical, automotive, wind turbines...

Adhesive joints offer the potential to reduce weight and cost

Prediction of debonding is of vital importance in order to correctly dimension the joints

• Analytical models difficult to apply for complex loading scenarios

• Cohesive Zone Models (CZM) possibility to predict debonding using FEM

EXPLORATION USING A COMMERCIAL FEM CODE (ANSYS)

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Ch.V. Katsiropoulos, et. al,” Fracture toughnessand shear behavior of composite bonded jointsbased on a novel aerospace adhesive”, Composites Part B: Engineering

Lars Chr. Terndrup Overgaard, “Structural Response Analyses of Vestas V52 Wind Turbine Blade”


1. INTRODUCTION

1.3 OBJECTIVE OF THE PRESENT WORK

To develop the FEM models and experimental test capabilities to predict the onset (and growth) of adhesive debonding

• Develop and validate the methodology step by step

• Apply methodology to a structural component (skin-stiffener panel)

To test the capabilities of cohesive element formulations that are already implemented in commercial Finite Element software (Ansys).

Acquire knowledge in order to apply the methodology to industrial problems


CORRELATION

2. MULTI-LEVEL APPROACH

2.1 MULTI-LEVEL APPROACH SUMMARY Based on the work developed by Bertolini in [1]

Idea is to build the methodology front bottom to top:

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[1] Julien Bertolini, et al. , “Multi-level experimental and numerical analysis of composite stiffener debonding. Part 1: Non-specific specimen level”, Composite Structures, Volume 90, Issue 4, October 2009, Pages 381-391

FRACTURE MECHANICS 2D MODELS

DCB, ENF, MMB tests

FRACTURE MECHANICS 3D MODELS

DCB, ENF, MMB tests

NON-SPECIFIC COMPONENTS

3 point bending test

STRUCTURAL COMPONENTS

7 point bending



2.2 FRACTURE MECHANICS MODELS AND VALIDATION (2D)

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[2] Turon, A.; Camanho, P.P.; Costa, J.; Renart, J. "Accurate simulation of delamination growth under mixed-mode loading using cohesiveelements: Definition of interlaminar strengths and elastic stiffness ." Composite Structures 92 (2010): 1857-1864.[3] G. Alfano and M.A. Crisfield. “Finite Element Interface Models for the Delamination Anaylsis of Laminated Composites: Mechanical and Computational Issues”. International Journal for Numerical Methods in Engineering. Vol. 50. pp. 1701-1736. 2001.

DCB: Mode I opening(Double Cantilever Beam)

ENF: Mode II shear(End Notch Fracture)

MMB: Mixed Mode(Mixed Mode Bending)


2D models were developed in Ansys for fracture mechanics test models

Input data, material properties, test conditions from Turon et al. [2]

PLANE182, CONTA171, TARGE169 elements are used in Ansys for the 2D model, 0.5mm element size, implicit time integration

Bilinear Cohesive Zone Model (CZM) implemented in Ansys [3]

Results validated against analytical LEFM and Abaqus model (next slide)

Propagation criteria in Ansys


2.2 FRACTURE MECHANICS MODELS AND VALIDATION (2D) Results validated against analytical crack growth predictions and Abaqus model [4]

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DCB(Double Cantilever Beam)

ENF(End Notch Fracture)

MMB(Mixed Mode Bending)

[4] Sarrado, C.; Turon, A.; Renart, J.; Urresti, I. "Assessment of energy dissipation during mixed-mode delamination growth using cohesive zone models." Composites Part A-Applied Science and Manufacturing (2012): 2128-2136.



2.2 FRACTURE MECHANICS MODELS AND VALIDATION (3D) 2D fracture mechanics models were extended to 3D

Solid-Shell elements (SOLSH190) were employed

Results were validated against the 2D model previously validated in [6]

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DCB (3D model)(Double Cantilever Beam)

ENF (3D model)(End Notch Fracture)

MMB (3D model)(Mixed Mode Bending)



2.3 EXPERIMENTAL CHARACTERIZATION OF ADHESIVE FM-300 film adhesive GIc measured using a DCB test

configuration.

Unidirectional AS4/8552 Carbon/Epoxy

Test Standards were employed [5,6,7]

Stable crack growth with cohesive failure was obtained in 5 out of 10 specimens.

Using the CBT (Corrected Beam Theory) method (ISO 25217): • GIc = 1169 J/m2

Good agreement with other authors: Bertolini: GIc = 1131 J/m2 ,

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[5] ASTM D5528 - 01(2007)e3 Standard Test Method for Mode I Interlaminar Fracture Toughness of Unidirectional Fiber-ReinforcedPolymer Matrix Composites

[6] ASTM D3433 - 99(2012) Standard Test Method for Fracture Strength in Cleavage of Adhesives in Bonded Metal Joints

[7] ISO 25217:2009, Adhesives -- Determination of the mode 1 adhesive fracture energy of structural adhesive joints using double cantilever beam and tapered double cantilever beam specimens


GIIc = 2866 J/m2


2.4 NON-SPECIFIC 3 POINT BENDING : MODELS AND VALIDATION (1/3)

Objective to test cohesive elements in a real component with simplified BC-s and loading. 2 test configurations: flat stiffener and T stiffener (sections of the 7 point bending stiffened panel) Skin and stiffener are AS4/8552 Carbon/Epoxy (Skin layup: (90,45,0,-45,-45,0,45,90)s )

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FEM model description

• Solid/Shell element types (SOLSH190)

• CZM properties:

GIc = 1169 J/m2, max=30 MPa, GIIc = 2866 J/m2, max=60 Mpa

• Frictional contact between skin and cylindrical support (to allow for rotation and contact position change)

• Mesh size of 0.5 mm

• Misalignment delamination only on one side

Experimental Setup

• 3 point bending standard tooling + Zwick standard test machine

• Force sensor is located on top of central cylinder

• Displacement from machine head, laser and inductive sensors


ADHESIVE FAILURE INSTEAD OF COHESIVE FAILURE



Flat Stiffener: Correlation FEM vs Experimental tests


1st sidedebondingload margin

ADHESIVE FAILURE INSTEAD OF COHESIVE FAILURE



T Stiffener: Correlation FEM vs Experimental tests


2nd sidedebondingload margin

1st sidedebondingload margin


2.5 SKIN-STIFFENER DEBONDING UNDER BENDING: MODELS AND VALIDATION

7 point bending test: 5 supports + 2 loading points

AS4/8552 Carbon/Epoxy panels

• Skin layup: (90,45,0,-45,-45,0,45,90)s

• Stiffener layup: (90,45,0,0,-45,0,0)s

Several configurations were simulated and tested

• 2 stiffener types: Flat stiffener and T stiffener

• 2 test configurations: Symmetric and Anti-Symmetric


20

0

50

300

Flat Stiffener and symmetric configuration is selected for this presentation



FEM model description

• Solid/Shell element types (SOLSH190)

• Cohesive zone model properties between skin and stiffener.

• Refined mesh (1mm mesh size) at debondingcritical zone and support contacts

• Supports modeled and frictional contact with composite panel. Support rotation is allowed

• Displacement is applied to both top supports

• Large displacements and stress stiffening

• Sensitivity analysis is performed on:

• Mesh size (1, 3, 5 mm)

• Artificial Damping

• CZM Contact Stiffness

• Fracture Toughness

• One side delamination vs 2 sidedelamination



2.5 SKIN-STIFFENER DEBONDING UNDER BENDING: MODELS AND VALIDATION EXPERIMENTAL SETUP

• 7 point bending tool (own design) with positioning system for the test specimen

• Top and bottom sides are positioned using the columns

• Modular tool to allow for several test configurations

• A force sensor is located at each support.

• Oscillating support are employed to allow for specimen rotation

• Standard static test machine

• Displacement is measured with machine head, laser and inductive sensor.

• A data acquisition system (National Instruments) was employed to record all the channels at the sametime (force at each support and displacement)



2.5 SKIN-STIFFENER DEBONDING UNDER BENDING: MODELS AND VALIDATION Flat Stiffener: Correlation FEM vs Experimental tests


C-SCAN

PSRPS-1 PSRPS-2

ADHESIVE FAILURE INSTEAD OF COHESIVE

FAILURE

Experimental debondingload margin



Mesh size sensitivity (1, 3, 5 mm)

• No clear trend in onset force is found when mesh is refined.

• Differences with experimental values cannot be explained by mesh size

• Mesh size lower than 1mm could not be simulated due to computational limitations

• 2D DCB shows meshconvergence for 0.5mm meshsize


1mm mesh size 3mm mesh size 5mm mesh size

Debonding

onset

Last converged substep

Experimental debonding load margin



Sensitivity analysis of Artificial Damping (3e-4, 5e-4, 8e-4)

• With values between 3e-4 and 5e-4 debonding occurs almost at thesame load level

• No clear trend found in artificial damping the value 5e-4 is selected which helps convergence during debonding



Art. Damp. 8e-4

CONVERGENCE

PROBLEMS



Sensitivity analysis of Contact Stiffness (1e2, 1e4, 1e6, 1e7 N/mm3)

• High contact stiffness values have convergence difficulties (1e7 N/mm3)

• Values between 1e4 N/mm3and 1e6 N/mm3 have debonding onset on the same load level and have also similar stiffness

• Values lower than 1e3 N/mm3

show different behavior and do not even show debonding


Kcont=1e2 N/mm3 NO DEBONDING

Kcont=1e7 N/mm3

CONVERGENCE

PROBLEMS




Sensitivity analysis Fracture Toughness

• At the initiation point mode mixity is close to 0.5 GIIc and Mixed mode law important (Ansys has a simple one)

• If lower fracture toughness values are used the obtained debonding onset forces can be matched



ONGOING WORK

Mode II fracture Toughness

measurement

FM-300 Bonded: GIc = 1169 J/m2, GIIc = 2866 J/m2

Co-cured: GIc = 262 J/m2, GIIc = 1363 J/m2



Comparison: Delamination on 2 sides vs delamination on 1 side


• If debonding is allowed on one side only, debonding initiation load does not change significantly


3. DISCUSSION OF THE RESULTS

3 DISCUSSION OF THE RESULTS

FEM models (3 point bending and 7 point bending) clearly over estimates delamination onset force.

Difference in debonding force is not justified by important numerical parameters

• Mesh size convergence must be further studied

Possible explanations for the differences• Initial crack when measuring GIc but NO initial crack in 3 point bending and 7 point bending

• Gic measured for 0º-0º bonded interface, debonding direction in 7 point bending is in the 90º

• Mode II fracture toughness might be smaller for our adhesive

• Adhesive failure vs cohesive failure:

Adhesive failure in all bending tests, GIc measured for cohesive failure of the adhesive


DCB specimen with cohesive failure Bending tests with adhesive failure

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4. CONCLUSIONS

4 CONCLUSIONS

A complete metholodology was built starting from LEFM models to stiffened panel size in order to predict debonding failure in real components

2D and 3D FEM fracture mechanics models were validated

Adhesive failure was obtained in all bending tests (3 point and 7 point)

Good correlation is obtained for the debonding shape comparing FEM model and C-Scan measurement

With the actual models we have debonding force over estimation:

• Work in progress to determine GIIc of the adhesive


5. PERSPECTIVES AND APPLICATIONS

5 PERSPECTIVES AND APPLICATIONS

Mode II fracture toughness measurement in ENF test using Digital Image Correlation

Implementation of more advanced Mixed Mode dalamination laws (Benzeggagh-Kenane)

FEM models with explicit time integration for easier convergence

Application for some other industries: elevators, trains, wind turbine blades


DIC images for ENF test performed in collaboration with GOM

Image courtesy of RISO

6. AKNOWLEDGEMENTS

6 AKNOWLEDGEMENTS

AMADE

(for the C-scan)

Ansys Iberia

Basque Government


http://www.udg.es/

P.º J.M. Arizmendiarrieta, 2

20500 Arrasate-Mondragón (Gipuzkoa)

Tel.: 943 71 24 00

Fax: 943 79 69 44

www.ikerlan.es

Eskerrik asko

Muchas gracias

Thank you

Merci beaucoup

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http://www.ikerlan.es/

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