5th International Conference on Composites Testing and Model Simulation

Experimental and numerical characterization of modes I

& II delamination in unidirectional composites

Samuel StutzJoël Cugnoni John Botsis

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LMAF-STI, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne Switzerland

E-mail: samuel.stutz@a3.epfl.ch

5th International Conference on Composites Testing and Model Simulation

Increasing use of composites Layered structure Delamination Characterization of propagation

Introduction

Introduction Method Results & Discussion Conclusion

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5th International Conference on Composites Testing and Model Simulation

Approach: Measuring strain with optical

fibre sensors during crack propagation

Identify relevant properties with a parametric FE-model

Comparing the numerical and experimental load-displacement curve

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Introduction

Introduction Method Results & Discussion Conclusion

5th International Conference on Composites Testing and Model Simulation

Method◦ Manufacturing◦ Multiplexed Fiber Bragg grating (FBG) sensors◦ Distributed strain measurements

Results & Discussion◦ Mode I (DCB) tests◦ Mode II (4ENF) tests

Conclusion

Outline

Introduction Method Results & Discussion Conclusion

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5th International Conference on Composites Testing and Model Simulation

Prepregs from Gurit SPTM (SE 70) [020]

Manufacturing

Introduction Method Results & Discussion Conclusion

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Sensor fibre

Crack

Crack initiator

5th International Conference on Composites Testing and Model Simulation

Optical fibre sensor (FBG)

Introduction Method Results & Discussion Conclusion

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l=2nL

5th International Conference on Composites Testing and Model Simulation

Optical fibre sensor (FBG)

Introduction Method Results & Discussion Conclusion

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5th International Conference on Composites Testing and Model Simulation

Optical fibre sensor (FBG)

Introduction Method Results & Discussion Conclusion

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5th International Conference on Composites Testing and Model Simulation

Multiplexed sensor array

Introduction Method Results & Discussion Conclusion

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5th International Conference on Composites Testing and Model Simulation

Multiplexed sensor array

Introduction Method Results & Discussion Conclusion

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5th International Conference on Composites Testing and Model Simulation

No strain at the FBG positions

Strain ahead of the crack tip reaches the FBGs

All FBGs are in the bridging zone

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Strain sensors during test

Introduction Method Results & Discussion Conclusion

5th International Conference on Composites Testing and Model Simulation

No strain at the FBG positions

Strain ahead of the crack tip reaches the FBGs

All FBGs are in the bridging zone

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Strain sensors during test

Introduction Method Results & Discussion Conclusion

5th International Conference on Composites Testing and Model Simulation

No strain at the FBG positions

Strain ahead of the crack tip reaches the FBGs

All FBGs are in the bridging zone

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Strain sensors during test

Introduction Method Results & Discussion Conclusion

5th International Conference on Composites Testing and Model Simulation

Wavelength shift versus time

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Strain measurements mode I

Introduction Method Results & Discussion Conclusion

5th International Conference on Composites Testing and Model Simulation

Wavelength shift versus time

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Strain measurements mode I

Introduction Method Results & Discussion Conclusion

Strain versus crack length

5th International Conference on Composites Testing and Model Simulation

Wavelength shift versus time

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Strain measurements mode I

Introduction Method Results & Discussion Conclusion

Strain versus crack length

Using crack length versus time measurements to eliminate time

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Identification of bridging tractions

Introduction Method Results & Discussion Conclusion

5th International Conference on Composites Testing and Model Simulation

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Identification of bridging tractions

Introduction Method Results & Discussion Conclusion

Algorithms:

Trust-region-reflective

Levenberg-Marquardt

5th International Conference on Composites Testing and Model Simulation

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Identification of bridging tractions

Introduction Method Results & Discussion Conclusion

5th International Conference on Composites Testing and Model Simulation

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Cohesive zone element model

Introduction Method Results & Discussion Conclusion

Crack opening as a function of the distance from the crack tip (from simulation)d(z*)

and the identified bridging traction distributions(z*)

were combined to obtain the bridging laws(d)

5th International Conference on Composites Testing and Model Simulation

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Cohesive zone element model

Introduction Method Results & Discussion Conclusion

Cohesive element properties:Thickness : 20 mmDamage initiation : 20 MPaCohesive stiffness : 9000GPa/mm

G(a) = Gi + Gb(a)

5th International Conference on Composites Testing and Model Simulation

Experimental load-displacement curve

Two initial crack lengths, 30 and 60 mm

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Load-displacement curve

Introduction Method Results & Discussion Conclusion

5th International Conference on Composites Testing and Model Simulation

Simulated load - displacement curve

No bridging in the cohesive law

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Load-displacement curve

Introduction Method Results & Discussion Conclusion

5th International Conference on Composites Testing and Model Simulation

Simulated load - displacement curve

With bridging in the cohesive law

Energy release rate of the bridging fibres: 350 J/m2

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Load-displacement curve

Introduction Method Results & Discussion Conclusion

5th International Conference on Composites Testing and Model Simulation

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Mode II test (4ENF)

Introduction Method Results & Discussion Conclusion

Fibre end

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Strain measurements mode II

Introduction Method Results & Discussion Conclusion

Using crack length versus time measurements to eliminate time

5th International Conference on Composites Testing and Model Simulation

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Identification of GII and friction

Introduction Method Results & Discussion Conclusion

Identified energy release rate (cohesive elements) GII =1070 J/m2

Identified crack initiation: smax = 38.7 MPa

Identified friction between the loading pins and the sample: m =0.35

There was no sensitivity to friction between the fracture surfaces (m=0.1 – 0.4)

5th International Conference on Composites Testing and Model Simulation

Experimental load-displacement curves

The different slopes are due to different initial crack lengths

Some unstable crack propagation

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Load-displacement curve

Introduction Method Results & Discussion Conclusion

5th International Conference on Composites Testing and Model Simulation

Results from the numerical simulation

Cohesive elements with the identified properties

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Load-displacement curve

Introduction Method Results & Discussion Conclusion

5th International Conference on Composites Testing and Model Simulation

The multiplexed FBG sensor array proved to be an excellent embedded sensor to measure non-homogeneous strain in mode I and mode II delamination

The measured strain distribution was successfully used for identification of material parameterso Bridging tractions in mode I o The energy release rate in mode II

The experimental load displacement curves were entirely reproduced

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Conclusions

Introduction Method Results & Discussion Conclusion

5th International Conference on Composites Testing and Model Simulation

The authors acknowledge the financial support from the Swiss National Science Foundation (SNF) under Grant 200020_124397.

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Acknowledgements

Introduction Method Results & Discussion Conclusion

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Different friction coefficients between loading pin and composite

Introduction Method Results & Discussion Conclusion

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Different friction coefficients between fracture surfaces

Introduction Method Results & Discussion Conclusion