Center for Quality Engineering and Failure Prevention
Monitoring of Acoustic Emissions Using a Fiber Bragg Grating Dynamic
Strain Sensing System
Department of Mechanical Engineering / Center for Quality Engineering and Failure Prevention
Northwestern University Evanston, IL 60208
Brad Regez, Yan-Jin Zhu, Yinian Zhu, Oluwaseyi Balogun, and Sridhar Krishnaswamy
AEWG, Denver, Colorado, May 18, 2011. Acknowledgement: ONR, NSF, CCITT
Center for Quality Engineering and Failure Prevention
Goal: Develop a network of high-frequency strain sensors with the following requirements:
• Always ready to detect and locate impact and other transient signals; • Adaptive to detect dynamic strains (ultrasound-induced) in the presence of large quasi-static strains (structural deformation-induced) or thermal drift; • Multiplexable for large sensor arrays.
Technology: Optical Fiber Bragg Grating (FBG) Sensors and Multiplexed Two-Wave Mixing (MTWM) in adaptive photorefractive crystals.
Dynamic Strain Monitoring System - requirements -
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Talk Outline
• Fiber Bragg-Grating sensors as dynamic strain sensors • Current methods of demodulation
• Two Wave Mixing demodulator system • frequency response • sensitivity • cross-talk
•Applications: •acoustic emission
Center for Quality Engineering and Failure Prevention
Talk Outline
• Fiber Bragg-Grating sensors as dynamic strain sensors • Current methods of demodulation
• Two Wave Mixing demodulator system • frequency response • sensitivity • cross-talk
•Applications: •acoustic emission
Center for Quality Engineering and Failure Prevention
Fiber Bragg Grating Sensors
Strain or temperature signal is spectrally encoded in the reflected / transmitted light from an FBG sensor.
1µε 1.2pm 1oC 13pm
Ref: A.Othonos, and K.Kalli, “Fiber Bragg Gratings,” Artech House, Boston. (1999)
• Bragg-gratings are refractive-index gratings in the optical fiber. • They are very easy to fabricate. • They are local sensors • Several sensors can be readily multiplexed. • Useful as temperature, strain sensors. • Can be used for dynamic strain sensing
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Demodulation Scheme
Readiness Adaptivity Multiplexability
CCD Spectrometer / AWG
Always No Demodulator for each sensor
Tunable Filter Intermittent – Scanned
Feedback Filter for each sensor
Tunable Source Intermittent – Scanned
Feedback yes
Interferometric Always Feedback Demodulator for each sensor
MTWM system Always Self Single demodulator
Current Approaches for Spectral Shift Demodulation
Center for Quality Engineering and Failure Prevention
Talk Outline
• Fiber Bragg-Grating sensors as dynamic strain sensors • Current methods of demodulation
• Two Wave Mixing demodulator system • frequency response • sensitivity • cross-talk
•Applications: •acoustic emission
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Pump
Signal
Transmitted Signal +
Diffracted Pump
In a nutshell: • PRC’s act as “novelty filters” • Output only “sees” sudden variations in the input • what is new is dictated by the PRC response time
Principle: • Pump + Signal create a refractive index grating in the PRC • Pump and signal beams diffract off the index grating • Diffracted pump is a replication of the quasistatic signal beam • Dynamic changes in the signal beam are not tracked by the PRC • The transmitted signal beam effectively interferes with the diffracted pump beam and only dynamic changes in the signal beam are observed.
Ref: Yi Qiao, Yi Zhou, and Sridhar Krishnaswamy, (July 2006), “Adaptive two-wave mixing wavelength demodulation of Fiber Bragg Grating dynamic strain sensors”, Applied Optics, vol. 45, No. 21, pp 5132-5142.
Two Wave Mixing Interferometry in Photorefractive Crystals
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TWM Interferometer for Spectral Demodulation
• Bragg-sensor signal at λB split into two legs with optical path difference OPD ‘d’ • The two-beams are mixed in a PRC to create a grating. • Path-mismatch causes a phase difference between the two legs of:
• Quasistatic drift in λB compensated for by creation of new grating in PRC • Dynamic changes in λB cause instantaneous phase shift at the PRC output.
2
2( ) ( )B nB
dt tπ λ ϕλ
∆Φ = − ∆ +
Fiber Coupler Fiber Bragg Grating Sensor Broadband Laser Source
Optical Fiber Amplifier PRC
Fiber Coupler
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Multiplexability
• A single TWM spectral demodulator can be used to demodulate multiple FBG sensors simultaneously by wavelength multiplexing.
• The different channels are separated after the PRC by band-drop filters.
Circulator
ASEBroadband Optical Amplifier
1 by 2Coupler
Collimator
Collimator
PRC
DC field 6kV/cm
InP:Fe
λ/2
λ/2
Freespace to
fibercoupler
Photodetector
Photodetector
1548nm
1552nm
Band drop filters
Photodetector
Photodetector
1560nm
1556nm
1536nm 1540nm 1544nm 1548nm
Optical Analyzer
bandsplittertransmission band
1537-1543nm
FBG reflection0.1nm
1540nm 1548nm
bandsplitterreflection band
<1537nm & >1543nm
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4.0 4.5 5.0 5.5 6.0 6.5Time(ms)
20 kHz
2 kHz
5 kHz
10 kHz
Signal
Amplit
ude(a.u
.)
Ch4
Ch3
Ch2
Ch1
-0.04-0.020.000.020.04
-0.04-0.020.000.020.04
-0.04-0.020.000.020.04
-0.04-0.020.000.020.04
0 5000 10000 15000 20000 25000
0.00.20.40.60.81.0
Frequency (Hz)
ch
1
0.00.20.40.60.81.0
ch
2
0.00.20.40.60.81.0
ch
3
0.00.20.40.60.81.0
ch
4
Time response Frequency response
Multiplexed 4-channel TWM spectral demodulator - crosstalk
• 20 kHz, 10kHz, 5 kHz and 2 kHz 5με strains were applied onto the four FBG sensors respectively.
• No detectable cross-talk
Center for Quality Engineering and Failure Prevention
Talk Outline
• Fiber Bragg-Grating sensors as dynamic strain sensors • Current methods of demodulation
• Two Wave Mixing demodulator system • frequency response • sensitivity • cross-talk
•Applications: •acoustic emission
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TWM System NWU Prototype
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Pencil Lead Break AE Data FBG vs. PZT
450 500 550 600 650 700 750 800 850 900 950
-1.0-0.8-0.6-0.4-0.20.00.20.40.60.81.01.21.4
FBG Response PZT Response
Relat
ive A
mpl
itude
Time (µs)
Source to Receiver Distance: 2 cm
450 500 550 600 650 700 750 800 850 900 950-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
FBG Response PZT Response
Relat
ive A
mpl
itude
Time (µs)
Source to Receiver Distance: 3 cm
450 500 550 600 650 700 750 800 850-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
FBG Response PZT Response
Rela
tive
Ampl
itude
Time (µs)
Source to Receiver Distance: 4 cm
450 500 550 600 650 700 750 800 850 900 950-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
FBG Response PZT Response
Relat
ive A
mpl
itude
Time (µs)
Source to Receiver Distance: 5 cm
Pencil Break
Experimental Setup
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TWM Spectral Demodulation - Pencil Lead Break AE Monitoring -
Fmax = 210kHz
Time (µs)
Freq
uenc
y (k
Hz)
900 1000 1100 1200 1300 1400 1500
-10
-5
0
5
10
15
20
Rela
tive
Ampl
itude
Time (µs)
Time – Frequency Transform Experimental Waveform
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• Tyfo SEH-51 Composite – Tyfo S Epoxy – Uni-directional Glass/Aramid
custom weave [0/90]
• Composite Gross Laminate Properties: – σu =575 MPa, ε=2.2%, E=26.1 GPa – Laminate Thickness: 1.3mm
Collaborative work between NU & Harbin Institute of Technology (Li Hui)
Glass Fiber Composite Coupon
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Continuous AE – Matrix Damage
487.72 487.74 487.76 487.78 487.80.17
0.18
0.19
0.2
0.21
Time (s)
Am
plitu
de (v
)
487.744 487.746 487.748 487.75
0.18
0.185
0.19
0.195
0.2
0.205
Time (s)
Am
plitu
de (
v)
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Burst AE – Fiber Damage
511.38 511.39 511.4 511.41 511.42
0.16
0.17
0.18
0.19
0.2
0.21
0.22
0.23
Time (s)Am
plitud
e (v)
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FBG Sensors Disadvantages / Advantages Over Piezoelectric Based Sensors
DISADVANTAGES • Systems are more expensive than piezoelectric based systems • FBG sensors are less sensitive than piezoelectric sensors
ADVANTAGES • Smaller footprint • Immune to electromagnetic signals or noise • Minimum signal loss since cables are replaced by a fiber optic • Exhibit long term stability • Can be mounted underwater in needed • Can be embedded within the structure • Can be used in high temperature applications.
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Conclusions TWM Wavelength demodulation demonstrated:
1. Always On: Wavelength demodulation induced by transient events is demonstrated.
2. Adaptive: The TWM wavelength demodulator is demonstrated to have adaptivity to quasistatic drift (both strains and temperature).
3. Frequency response: High pass – no upper limit from TWM.
4. Multiplexability: Little detectable cross-talk.
Center for Quality Engineering and Failure Prevention
Monitoring of Acoustic Emissions Using a Fiber Bragg Grating Dynamic
Strain Sensing System
Department of Mechanical Engineering / Center for Quality Engineering and Failure Prevention
Northwestern University Evanston, IL 60208
Brad Regez, Yan-Jin Zhu, Yinian Zhu, Oluwaseyi Balogun, and Sridhar Krishnaswamy
AEWG, Denver, Colorado, May 18, 2011. Acknowledgement: ONR, NSF, CCITT
Thank You!