Fiber optic Bragg gratings as sensors and FFI’s activity in Structural Health Monitoring

1. Fiber optic Bragg gratings as sensors and FFI’s activity in Structural Health Monitoring Lasse Vines Gunnar Wang

2. Outline • Introduction to fiberoptic sensors • Fiber Bragg Gratings (FBG) as sensors • Structural health monitoring at FFI • Vibration based damage detection

3. Quarts Core diameter 1-10 mm Cladding diameter 80-250mm Difference in index of refraction: ca. 4% Extrinsic fiberoptic sensors sensing takes place in a region outside the fiber Encoder plates/disks Reflection/Transmission Gratings Fluorescence Intrinsic fiberoptic sensors sensing takes place within the fiber itself Microbend Distributed sensors (Rayleigh,Raman,Mode coupling etc.) Blackbody sensors Interferometric Fiberoptic sensors - 1

4. Fiberoptic sensors - 2 Advantages intrinsic sensors • Immune to electromagnetic interference • Can be used in harsh environment (water, oil, etc.) • Passive • Small size and weight • High Sensitivity and large dynamic range • Multiplexing possibilities • The accuracy are dependent of the readout technique • Can operate in high temperatures

5. Fiberoptic sensors - examples Electromagnetic sensors Biological/chemical sensors Typical specs: Dynamic range:1Arms – 3.6kArms (metering) 170kArms (protection) Bandwith: 10Hz-6kHz • O2-sensors • pH-sensors • CO2-sensors • Current/Voltage sensors • Electric field sensors • Voltage sensors

6. Fiberoptic sensors - examples Hydrophones Fiber optic gyroscope Measure rotation rate Typical performance Dynamic range: +/- 1000 deg/s ARW: 80 mdeg/ hr1/2

7. Fiberoptic Bragg Grating (FBG) • Strain sensitivity: • Linear response up to at least 3% elongation (30 000 me) • Temperature sensitivity: • Desired resolution for structural health monitoring: 1-2me, 0.1ºC • Necessary wavelength resolution: 1pm = 0.1GHz • Desired measurement range: ±1000 - 10 000me • Precision depends mainly on read-out technique • Types • Strain sensors • Temperature sensors • Pressure sensors • Seismic sensors • Flow meters

8. l l 1 2 Scanning Fabry-Perot filter technique 1 2 Broadband source V(t) Filter drive Ramp voltage waveform converts time axis to wavelength. ~ 680 Hz Scanning filter drive voltage detector FBG peaks are passed to photodetector as filter scans through Bragg wavelength. amplifier Analog differentiator 0 V Derivative zero-crossing pinpoints time of Bragg peak, wavelength and strain calculated from time. V Further processing t

9. Fiberoptic SHM technology FBG are used as strain sensors to calculate global moments • Sagging/hogging – vertical bending • Horizontal bending moment • Longitudinal compression force • Torsion- twisting moment • Vertical shear force • Splitting moment and local loads at exposed locations Why FBG sensors in SHM? • High sensitivity • Multiplexing • Expected long lifetime M = k -1

10. Frequency analysis

11. Structural health monitoring is a question of verification of constructional design (both short and long term) Verification of design Active operated guiding system Minimizing load to prolong lifetime of object Operate close to capacity when necessary (military) Damage detection Condition based maintenance Acoustic signature for Naval ships Structural Health Monitoring (SHM)

12. Structural health monitoring is a question of verification of constructional design (both short and long term) Verification of design Active operated guiding system Minimizing load to prolong lifetime of object Operate close to capacity when necessary (military) Damage detection Condition based maintenance Acoustic signature for Naval ships Structural Health Monitoring (SHM)

13. Cooperation project between US Naval Research Lab, Optical Sciences Div and FFI, 1996-2000 A strain monitoring system was installed onboard KNM Skjold to verify ship design Verification of design CHESS I (Composite Hull Embedded Sensor System)

14. Structural health monitoring is a question of verification of constructional design (both short and long term) Verification of design Active operated guiding system Minimizing load to prolong lifetime of object Operate close to capacity when necessary (military) Damage detection Condition based maintenance Acoustic signature for Naval ships Structural Health Monitoring (SHM)

15. Cooperation between FFI, FiReCo (ship design) and Norwegian Navy 1999 – 2002 Objectives Development of operational system Installation and trials on Norwegian Navy Mine Counter Measure Vessel Extensive sea trials Determine operational limits and reduce damages Industrialization (necessary for future installation on Norwegian naval ships) Active operated guiding systemCHESS II

16. CHESS II

17. Active Operated Guiding System Fiber optic strain sensors Motion Reference Unit GPS Wave altimeter Ship control/ information system Data acquisition/ Signal processing Global loads Local loads Sea state Man-Machine Interface/Visualization

18. Measure wave height at bow The boat move compared to earth What are the wave profile along the ship Laplace wave equation For linear monochromatic waves one can find the relationship Wave measurements

19. Structural health monitoring is a question of verification of constructional design (both short and long term) Verification of design Active operated guiding system Minimizing load to prolong lifetime of object Operate close to capacity when necessary (military) Damage detection Condition based maintenance Acoustic signature for Naval ships Structural Health Monitoring (SHM)

20. Participants: Finland, Sweden, Denmark, United Kingdom, Norway Objectives: Develop NDI-methods Improve knowledge about behavior and growth of damages Establish acceptance criteria of damages Develop and verify methods for repair FFI tasks: Detection of dynamic properties of sandwich constructions FE Analysis Analysis of undamaged and damaged panels Fiber optic monitoring Experimental investigation of undamaged and damaged panels Shearography Develop an instrument for field measurements on Naval ships Vibration based damage detection

21. Damage Changes to the material and/or geometric properties of a structural or mechanical system, including changes to the boundary conditions and system connectivity, that adversely affect current or future performance of that system. Vibration response of structures are influenced by global properties, and is therefore a possible feature for damage detection Common excitation techniques Random Chaotic Frequency sweep Transient excitation Accelerometer not ideal in SHM Other sensor types are investigated,e.g. Strain sensors Vibration based damage detection

22. Finite element model constructed to be able to simulate different damage scenarios SaNDI – FE analysis 317 Hz 456 Hz

23. 4 panels under test 2 undamaged sandwich panel 1 panel with shear failure 1 panel with shear failure and debonding SaNDI - Experimental

24. Experimental analysis - 1 • Excitation using a vibration exciter • Frequency sweep gives resonance frequencies and profile of frequency response • Stationary excitation gives amplitude and phase relation of sensors on test panel

25. Experimental analysis - 2 • Transient (shock) excitation • gives resonance frequencies and decay time of the system without perturbation • ordinary signal processing gives low accuracy  modeled based signal processing Assuming signal of form (gives for the first order resonance, 317Hz)

26. A Statistical Pattern Recognition Paradigm for Structural Health Monitoring • Statistical model building for damage detection • Using autoregressive models

27. Litterature Udd E(1992): Fiber Optic Sensors: an introduction for Engineers and scientists, Wiley Interscience Kashyap R (1999): Fiber Bragg Gratings, Academic Press Pran K, Havsgård G B, Sagvolden G, Farsund Ø, Wang G (2002): Wavelength multiplexed fibre Bragg grating system for high-strain health monitoring applications, Measurement Science and Technology, vol. 13, pp 471-476 Sagvolden G, Pran K, Farsund Ø, Vines L, Torkildsen H E, Wang G (2002): Fiber Optic System for Ship Hull Monitoring, Proceedings of the first European Workshop on Structural Health Monitoring Point of contact