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

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

Forsvarets forskningsinstitutt Fiberoptic sensors - 1 Quarts Core diameter 1-10 mm Cladding diameter mm 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

Forsvarets forskningsinstitutt 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

Forsvarets forskningsinstitutt Fiberoptic sensors - examples Biological/chemical sensors Electromagnetic sensors O 2 -sensors pH-sensors CO 2 -sensors Current/Voltage sensors Electric field sensors Voltage sensors Typical specs: Dynamic range:1A rms – 3.6kA rms (metering) 170kA rms (protection) Bandwith: 10Hz-6kHz

Forsvarets forskningsinstitutt Fiberoptic sensors - examples HydrophonesFiber optic gyroscope Measure rotation rate Typical performance Dynamic range: +/ deg/s ARW: 80 mdeg/ hr 1/2

Forsvarets forskningsinstitutt Fiberoptic Bragg Grating (FBG) Strain sensitivity: Linear response up to at least 3% elongation ( m  ) Temperature sensitivity: Desired resolution for structural health monitoring: 1-2 , 0.1ºC Necessary wavelength resolution: 1pm = 0.1GHz Desired measurement range: ±  Precision depends mainly on read-out technique Types –Strain sensors –Temperature sensors –Pressure sensors –Seismic sensors –Flow meters

Forsvarets forskningsinstitutt Scanning Fabry-Perot filter technique Analog differentiator Broadband source V(t) Scanning filter drive voltage detector amplifier t V 0 V 12 FBG peaks are passed to photodetector as filter scans through Bragg wavelength. Further processing Filter drive ~ 680 Hz 1 2 Derivative zero-crossing pinpoints time of Bragg peak, wavelength and strain calculated from time. Ramp voltage waveform converts time axis to wavelength.

Forsvarets forskningsinstitutt 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 

Forsvarets forskningsinstitutt Frequency analysis

Forsvarets forskningsinstitutt Structural Health Monitoring (SHM) 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

Forsvarets forskningsinstitutt Structural Health Monitoring (SHM) 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

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

Forsvarets forskningsinstitutt Structural Health Monitoring (SHM) 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

Forsvarets forskningsinstitutt Active operated guiding system CHESS II 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)

Forsvarets forskningsinstitutt CHESS II

Forsvarets forskningsinstitutt Active Operated Guiding System Fiber optic strain sensorsWave altimeterGPSMotion Reference Unit Data acquisition/ Signal processing Global loadsLocal loads Ship control/ information system Sea state Man-Machine Interface/Visualization

Forsvarets forskningsinstitutt Wave measurements 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

Forsvarets forskningsinstitutt Structural Health Monitoring (SHM) 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

Forsvarets forskningsinstitutt Vibration based damage detection 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

Forsvarets forskningsinstitutt Vibration based damage detection 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

Forsvarets forskningsinstitutt SaNDI – FE analysis Finite element model constructed to be able to simulate different damage scenarios 317 Hz456 Hz

Forsvarets forskningsinstitutt SaNDI - Experimental 4 panels under test –2 undamaged sandwich panel –1 panel with shear failure –1 panel with shear failure and debonding

Forsvarets forskningsinstitutt 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

Forsvarets forskningsinstitutt 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)

Forsvarets forskningsinstitutt A Statistical Pattern Recognition Paradigm for Structural Health Monitoring Statistical model building for damage detection –Using autoregressive models

Forsvarets forskningsinstitutt 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 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 Lasse Vines tel: fax: Gunnar Wang tel: fax: