5th International Conference on Composites Testing and Model Simulation Samuel Stutz Joël Cugnoni John Botsis 1 LMAF-STI, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne Switzerland

5th International Conference on Composites Testing and Model Simulation  Increasing use of composites  Layered structure  Delamination  Characterization of propagation Introduction Method Results & Discussion Conclusion 2

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 3 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 Introduction Method Results & Discussion Conclusion 4

5th International Conference on Composites Testing and Model Simulation  Prepregs from Gurit SP TM (SE 70) [0 20 ] Introduction Method Results & Discussion Conclusion 5 Sensor fibre Crack Crack initiator

5th International Conference on Composites Testing and Model Simulation Introduction Method Results & Discussion Conclusion 6 =2n 

5th International Conference on Composites Testing and Model Simulation Introduction Method Results & Discussion Conclusion 7

5th International Conference on Composites Testing and Model Simulation Introduction Method Results & Discussion Conclusion 8

5th International Conference on Composites Testing and Model Simulation Introduction Method Results & Discussion Conclusion 9

5th International Conference on Composites Testing and Model Simulation Introduction Method Results & Discussion Conclusion 10

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 11 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 12 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 13 Introduction Method Results & Discussion Conclusion

5th International Conference on Composites Testing and Model Simulation  Wavelength shift versus time 14 Introduction Method Results & Discussion Conclusion

5th International Conference on Composites Testing and Model Simulation  Wavelength shift versus time 15 Introduction Method Results & Discussion Conclusion  Strain versus crack length

5th International Conference on Composites Testing and Model Simulation  Wavelength shift versus time 16 Introduction Method Results & Discussion Conclusion  Strain versus crack length Using crack length versus time measurements to eliminate time

5th International Conference on Composites Testing and Model Simulation 17 Introduction Method Results & Discussion Conclusion

5th International Conference on Composites Testing and Model Simulation 18 Introduction Method Results & Discussion Conclusion Algorithms: Trust-region-reflective Levenberg-Marquardt

5th International Conference on Composites Testing and Model Simulation 19 Introduction Method Results & Discussion Conclusion

5th International Conference on Composites Testing and Model Simulation 20 Introduction Method Results & Discussion Conclusion Crack opening as a function of the distance from the crack tip (from simulation)  (z*) and the identified bridging traction distribution  (z*) were combined to obtain the bridging law  (  )

5th International Conference on Composites Testing and Model Simulation 21 Introduction Method Results & Discussion Conclusion Cohesive element properties: Thickness : 20  m Damage initiation : 20 MPa Cohesive stiffness : 9000GPa/mm G(a) = G i + G b (a)

5th International Conference on Composites Testing and Model Simulation  Experimental load- displacement curve  Two initial crack lengths, 30 and 60 mm 22 Introduction Method Results & Discussion Conclusion

5th International Conference on Composites Testing and Model Simulation  Simulated load - displacement curve  No bridging in the cohesive law 23 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/m 2 24 Introduction Method Results & Discussion Conclusion

5th International Conference on Composites Testing and Model Simulation 25 Introduction Method Results & Discussion Conclusion Fibre end

5th International Conference on Composites Testing and Model Simulation 26 Introduction Method Results & Discussion Conclusion Using crack length versus time measurements to eliminate time

5th International Conference on Composites Testing and Model Simulation 27 Introduction Method Results & Discussion Conclusion  Identified energy release rate (cohesive elements) G II =1070 J/m 2  Identified crack initiation:  max = 38.7 MPa  Identified friction between the loading pins and the sample:  =0.35  There was no sensitivity to friction between the fracture surfaces (  =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 28 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 29 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 parameters o Bridging tractions in mode I o The energy release rate in mode II  The experimental load displacement curves were entirely reproduced 30 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 _ Introduction Method Results & Discussion Conclusion

5th International Conference on Composites Testing and Model Simulation 32 Introduction Method Results & Discussion Conclusion

5th International Conference on Composites Testing and Model Simulation 33 Introduction Method Results & Discussion Conclusion