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Fiber optic Bragg gratings as sensors
and FFI’s activity in Structural Health
Monitoring
Lasse Vines
Gunnar Wang
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Outline
 Introduction to fiberoptic sensors
 Fiber Bragg Gratings (FBG) as sensors
 Structural health monitoring at FFI
 Vibration based damage detection
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Fiberoptic sensors - 1
Extrinsic fiberoptic sensors
• Quarts
• Core diameter 1-10 mm
• Cladding diameter 80-250mm
• Difference in index of
refraction: ca. 4%
– 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
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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
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Fiberoptic sensors - examples
Biological/chemical
sensors
Electromagnetic
sensors
Typical specs:
Dynamic range:1Arms – 3.6kArms (metering)
170kArms (protection)
Bandwith: 10Hz-6kHz
•O2-sensors
•Current/Voltage sensors
•pH-sensors
•Electric field sensors
•CO2-sensors
•Voltage sensors
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Fiberoptic sensors - examples
Hydrophones
Fiber optic gyroscope
Measure rotation rate
Typical performance
Dynamic range: +/- 1000 deg/s
ARW: 80 mdeg/ hr1/2
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Fiberoptic Bragg Grating (FBG)

 1.2 pm
me
e
 Strain sensitivity:
 Linear response up to at least 3% elongation
(30 000 me)

pm
 Temperature sensitivity: T  12 K
 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
b  2neff 
Types
–Strain sensors
–Temperature sensors
–Pressure sensors
–Seismic sensors
–Flow meters
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Scanning Fabry-Perot filter technique
1
2
Broadband
source
V(t)
Ramp voltage waveform
converts time axis to
wavelength.
Filter
~ 680 Hz
drive
Scanning filter
drive voltage
detector
FBG peaks are passed to
photodetector as filter scans
through Bragg wavelength.
amplifier
Analog
differentiator
Further
processing
0V
V
t
1  2
Derivative zero-crossing
pinpoints time of Bragg
peak, wavelength and strain
calculated from time.
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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
 e1   k1,1 k1, 2
e   k
k 2, 2
 2   2,1
  
 
   

  
  
  
e14  k14,1 k14, 2
k1,3
k 2,3
k1, 4
k 2, 4


k14,3
k14, 4
k1,5 
k 2,5   M sag 


  M hb 
 M 
   to 
  M sk 
 M 
  st 
k14,5 
M = k -1 e
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Frequency analysis
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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
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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
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Verification of design
CHESS I (Composite Hull Embedded Sensor System)
Cooperation project between US
Naval Research Lab, Optical
Sciences Div and FFI, 19962000
 A strain monitoring system
was installed onboard KNM
Skjold to verify ship design
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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
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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)
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CHESS II
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Active Operated Guiding System
Fiber optic strain sensors
Motion Reference Unit
GPS
Wave altimeter
Data acquisition/ Signal processing
Global loads
Local loads
Sea state
Man-Machine Interface/Visualization
Ship control/
information system
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Wave measurements
 Measure wave height at bow
 The boat move compared to
earth
 What are the wave profile
along the ship
Laplace wave equation
2
2




 2  2  2  0
 z
For linear monochromatic waves
one can find the relationship
~
rest
xwavesurfac

C

(
R
eˆz ' )  vteˆx  Heˆz
e
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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
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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
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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
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THAL
Inspec
of San
in Nav
SaNDI – FE analysis
Finite element model
constructed to be able to
simulate different damage
scenarios
317 Hz
456 Hz
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SaNDI - Experimental
4 panels under test
– 2 undamaged sandwich
panel
– 1 panel with shear failure
– 1 panel with shear failure
and debonding
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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
THALE
Inspecti
of Sandw
in Nava
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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

t
S (t )  Ae  sin(2 ft   )
(gives   13ms for the first
order resonance, 317Hz)
THALE
Inspecti
of Sandw
in Nava
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A Statistical Pattern Recognition
Paradigm for Structural Health
Monitoring
 Statistical model building for
damage detection
– Using autoregressive
models
n
e (t )  x(t )    i x(t  i )
i 1
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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
Lasse Vines
lasse.vines@ffi.no
tel: +47 6380 7416
fax: +47 6380 7212
Gunnar Wang
gunnar.wang@ffi.no
tel: +47 6380 7372
fax: +47 6380 7212