monitoring of acoustic emissions using a fiber bragg

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

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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

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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

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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

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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

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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!