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Samuel Stutz Joël Cugnoni John Botsis

Experimental and numerical characterization of modes I & II delamination in unidirectional composites. Samuel Stutz Joël Cugnoni John Botsis. LMAF-STI, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne Switzerland E-mail: samuel.stutz@a3.epfl.ch. Introduction.

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Samuel Stutz Joël Cugnoni John Botsis

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  1. Experimental and numericalcharacterization of modes I & II delamination in unidirectional composites Samuel Stutz Joël Cugnoni John Botsis LMAF-STI, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne Switzerland E-mail: samuel.stutz@a3.epfl.ch

  2. Introduction • Increasing use of composites • Layered structure • Delamination • Characterization of propagation IntroductionMethodResults & Discussion Conclusion

  3. Introduction 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 IntroductionMethodResults & Discussion Conclusion

  4. Outline • Method • Manufacturing • Multiplexed Fiber Bragg grating (FBG) sensors • Distributed strain measurements • Results & Discussion • Mode I (DCB) tests • Mode II (4ENF) tests • Conclusion IntroductionMethodResults & Discussion Conclusion

  5. Manufacturing • PrepregsfromGurit SPTM (SE 70) [020] Crack initiator Sensor fibre Crack IntroductionMethodResults & Discussion Conclusion

  6. Optical fibre sensor (FBG) l=2nL IntroductionMethodResults & Discussion Conclusion

  7. Optical fibre sensor (FBG) IntroductionMethodResults & Discussion Conclusion

  8. Optical fibre sensor (FBG) IntroductionMethodResults & Discussion Conclusion

  9. Multiplexedsensorarray IntroductionMethodResults & Discussion Conclusion

  10. Multiplexedsensorarray IntroductionMethodResults & Discussion Conclusion

  11. Strainsensorsduring test • No strainat the FBG positions • Strainahead of the crack tip reaches the FBGs • All FBGs are in the bridging zone IntroductionMethodResults & Discussion Conclusion

  12. Strainsensorsduring test • No strainat the FBG positions • Strainahead of the crack tip reaches the FBGs • All FBGs are in the bridging zone IntroductionMethodResults & Discussion Conclusion

  13. Strainsensorsduring test • No strain at the FBG positions • Strain ahead of the crack tip reaches the FBGs • All FBGs are in the bridging zone IntroductionMethodResults & Discussion Conclusion

  14. Strainmeasurements mode I • Wavelength shift versus time IntroductionMethodResults & Discussion Conclusion

  15. Strainmeasurements mode I • Wavelength shift versus time • Strain versus crack length IntroductionMethodResults & Discussion Conclusion

  16. Strainmeasurements mode I • Wavelength shift versus time • Strain versus crack length Using crack length versus time measurements to eliminate time IntroductionMethodResults & Discussion Conclusion

  17. Identification of bridging tractions IntroductionMethodResults & Discussion Conclusion

  18. Identification of bridging tractions Algorithms: Trust-region-reflective Levenberg-Marquardt IntroductionMethodResults & Discussion Conclusion

  19. Identification of bridging tractions IntroductionMethodResults & Discussion Conclusion

  20. Cohesive zone element model Crack opening as a function of the distance from the crack tip (from simulation) d(z*) and the identified bridging traction distribution s(z*) were combined to obtain the bridging law s(d) IntroductionMethodResults & Discussion Conclusion

  21. Cohesive zone element model Cohesive element properties: Thickness : 20 mm Damage initiation : 20 MPa Cohesive stiffness : 9000GPa/mm G(a) = Gi + Gb(a) IntroductionMethodResults & Discussion Conclusion

  22. Load-displacementcurve • Experimental load-displacement curve • Two initial crack lengths, 30 and 60 mm IntroductionMethodResults & Discussion Conclusion

  23. Load-displacementcurve • Simulated load - displacement curve • No bridging in the cohesive law IntroductionMethodResults & Discussion Conclusion

  24. Load-displacementcurve • Simulated load - displacement curve • With bridging in the cohesive law • Energy release rate of the bridging fibres: 350 J/m2 IntroductionMethodResults & Discussion Conclusion

  25. Mode II test (4ENF) Fibre end IntroductionMethodResults & Discussion Conclusion

  26. Strainmeasurements mode II Using crack length versus time measurements to eliminate time IntroductionMethodResults & Discussion Conclusion

  27. Identification of GII and friction • 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) IntroductionMethodResults & Discussion Conclusion

  28. Load-displacementcurve • Experimental load-displacement curves • The different slopes are due to different initial crack lengths • Some unstable crack propagation IntroductionMethodResults & Discussion Conclusion

  29. Load-displacementcurve • Results from the numerical simulation • Cohesive elements with the identified properties IntroductionMethodResults & Discussion Conclusion

  30. Conclusions • 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 parameters • Bridging tractions in mode I • The energy release rate in mode II • The experimental load displacement curves were entirely reproduced IntroductionMethodResults & DiscussionConclusion

  31. Acknowledgements The authors acknowledge the financial support from the Swiss National Science Foundation (SNF) under Grant 200020_124397. IntroductionMethodResults & DiscussionConclusion

  32. Different friction coefficients between loading pin and composite IntroductionMethodResults & DiscussionConclusion

  33. Different friction coefficients between fracture surfaces IntroductionMethodResults & DiscussionConclusion

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