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Characterization of impact damage in fibre reinforced composite plates using embedded FBG sensors

CompTest2011 5th International Conference on Composites Testing and Model Identification 14 Feb 2011 - 17 Feb 2011, Ecole Polytechnique Fédérale de Lausanne, Switzerland. Characterization of impact damage in fibre reinforced composite plates using embedded FBG sensors.

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Characterization of impact damage in fibre reinforced composite plates using embedded FBG sensors

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  1. CompTest2011 5th International Conference on Composites Testing and Model Identification 14 Feb 2011 - 17 Feb 2011, Ecole Polytechnique Fédérale de Lausanne, Switzerland Characterization of impact damage in fibre reinforced composite plates using embedded FBG sensors J. Frieden*, J. Cugnoni, J. Botsis, Th. Gmür Swiss National Science Foundation, grant N° 116715

  2. Objectives Primary objective of this work: • Impact localization and damage identification in CFRP plates with FBG sensors Methods: • Interpolation-based impact localization method using high rate FBG signals • Inverse numerical-experimental damage identification method based on eigenfrequency changes and homogenized damage model

  3. Objectives Primary objective of this work: • Impact localization and damage identification in CFRP plates with FBG sensors Today’s focus: • Influence of impact damage on the plate’s eigenfrequencies measured with FBG sensors • Experimental characterisation of impact damage • Finite element model of the plate with impact damage that reproduces the change of eigenfrequencies • Application

  4. Introduction: Materials and specimen • CFRP cross-ply plate with 28 UD plies • [0°2, 90°2, 0°2, 90°2, 0°2, 90°2, 0°2]s • Embedded FBG sensors Cross-section of plate Reference: Frieden J. et al, Composite Structures, 2010

  5. Sensitivity of eigenfrequencies to damage Impact 1.7J – 6.7J Damaged plate: Experimental modal analysis Intact plate: Experimental modal analysis Experiment carried out on 8 plates using different impact energies.

  6. Sensitivity of eigenfrequencies to damage • Relative frequency changes as a function of impact energy

  7. Experimental damage characterization High resolution X-ray computed tomography SkyScan, model 1076 Aluminium filter : 1 mm thickness X-ray source voltage : 100 kV X-ray source power : 10 W Exposure time : 1750 ms Damaged CFRP plate

  8. 2 mm Experimental damage characterization Cross-section (Cut through plate thickness) Impact energy : 5.1 J CT Resolution: 9 μm/pixel Distance between cross-section images: 9 μm Total of 10 000 images Convert to black & white images Impact Location Intralaminar cracks are rare and their occurrence is limited to a region located just beneath the impact point

  9. Experimental damage characterization Absorbed energy per unit of delamination area of 280 J/m2.

  10. Detailed 3D delamination model • FE model in Abaqus 6.8-2: • Numerical modal analysis • 20-nodes brick elements with reduced stiffness matrix integration • Mesh interfaces without node connection between plies • Element size : 2 mm x 2 mm

  11. Discrete delamination model • FE model in Abaqus 6.8-2: • Numerical modal analysis • 20-nodes brick elements with reduced stiffness matrix integration • Mesh interfaces without node connection between plies • Element size : 2 mm x 2 mm Eigenfrequency changes are mainly due to delamination type damage

  12. Homogenized damage model • Material properties: • Through-the-thickness homogenized material properties • Projected damage shape: • Rhombic area • Diagonal damage tensor D: • Affected: • Transverse shear moduli • Not affected: • Longitudinal, transverse and • through-the-thickness Young’s moduli • In-plane shear modulus • Poisson’s ratio

  13. Homogenized damage model • Material properties: • Through-the-thickness homogenized material properties • Projected damage shape: • Rhombic area • Diagonal damage tensor D: • Affected: • Transverse shear moduli Values of D13 and D23 identified through least square optimization: Minimize error between experimentally measured frequency change and numerically calculated frequency change • Not affected: • Longitudinal, transverse and • through-the-thickness Young’s moduli • In-plane shear modulus • Poisson’s ratio

  14. Homogenized damage model • Material properties: • Through-the-thickness homogenized material properties • Projected damage shape: • Rhombic area • Diagonal damage tensor D: • Affected: • Transverse shear moduli Values of D13 and D23 identified through least square optimization: Minimize error between experimentally measured frequency change and numerically calculated frequency change • Not affected: • Longitudinal, transverse and • through-the-thickness Young’s moduli • In-plane shear modulus • Poisson’s ratio

  15. Prediction of eigenfrequency change • Experimentally measured damage size • Using the previously determined values of D13 and D23

  16. Damage identification procedure • Values of D13 and D23 are fixed to 94 % • Parameters to identify: • Damage position • Damage surface • Damage aspect ratio • Reduce discrepancy between experimentally measured eigenfrequency changes and numerically calculated eigenfrequency changes • Iterative minimization algorithm: Levenberg-Maquardt

  17. Application example Impact energy: 3.4 J Predict the impact location Identify damage size and position

  18. Application • Reference measurements before impact: • Arrival time delays for interpolation-based localization method • Eigenfrequencies of intact plate

  19. Application: Reference data • Non-destructive hammer impacts • Grid of 3 x 3 reference points • Acquisition rate of FBG sensors : 1 GHz • Arrival time delays obtained by threshold method

  20. Application: Reference data • Non-destructive hammer excitation • Grid of 3 x 3 reference points • Acquisition rate of FBG sensors : 100 kHz • Eigenfrequencies obtained by modal curve fitting FRF

  21. Application: Impact • Impact with energy of 3.4 J • Acquisition rate of FBG sensors : 1 GHz

  22. Application: Impact • Impact with energy of 3.4 J • Acquisition rate of FBG sensors : 1 GHz

  23. Application: Identification of damage Experimental data

  24. Application: Identification of damage Experimental data • Parameters to identify: • Damage position • Damage surface • Damage aspect ratio • Initial guess for the damage identification: • Predicted impact location • Damage surface = 1 cm2 • Damage aspect ratio = 1

  25. Application: Identification of damage Identification results Predicted eigenfrequency changes compared to experimental eigenfrequency changes Convergence graph

  26. Conclusion • Embedded FBG sensors provide very accurate strain data for modal analysis and acoustic wave sensing. • The eigenfrequency changes can be mainly attributed to delamination type damage. • The simple homogenized damage model allows to reproduce the eigenfrequency changes. • The damage size can be identified by a numerical-experimental optimization method based on eigenfrequency changes.

  27. Thank you

  28. Introduction: Fast FBG interrogation

  29. FBG sensors for modal analysis 1st mode 3rd mode 2nd mode 4th mode

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