SHM with Long-gage Fiber Optic Sensors

Z.S. Wu, J. Zhang, Y.S. Tang, W. Hong, L. Huang

Southeast University, Nanjing, China

Ibaraki University, Hitachi, Japan

IBS Workshop, June 14, 2011

Content Background 1

Distributed sensing technique2

Sensor placement

Global parameter identification

Distributed long-gage FBG sensors

Utilizing distributed strain measurement for SHM

IBS Bridge3

Damage detection

1. Single Point Based Sensors

• Strain GaugeDamaged!

No damage! OK?!

Too Local!

Huge Limitation!

1. Single Point Based Sensors

2. Distributed long-gage FBG sensors

Connector

A packaged long-gage FBG sensor

125 250 500

1000

SMF

FBG

SMC

Composite package

Tube

Gauge length (sensing part)

Sheath Braid buffer layer

Connector

SMC: single-mode optical fiber cable SMF: single-mode optical fiber FBG: fiber Bragg grating

1-1 Cross section unit: μm

2. Distributed long-gage FBG sensors

Connector

A packaged long-gage FBG sensor

125 250 500

1000

SMF

FBG

SMC

Composite package

Tube

Gauge length (sensing part)

Sheath Braid buffer layer

Connector

SMC: single-mode optical fiber cable SMF: single-mode optical fiber FBG: fiber Bragg grating

1-1 Cross section unit: μm

2. Distributed long-gage FBG sensors

Connector

A packaged long-gage FBG sensor

125 250 500

1000

SMF

FBG

SMC

Composite package

Tube

Gauge length (sensing part)

Sheath Braid buffer layer

Connector

SMC: single-mode optical fiber cable SMF: single-mode optical fiber FBG: fiber Bragg grating

1-1 Cross section unit: μm

Distributed sensing technique provides both the local information and the global information of the structure!

2. Distributed long-gage FBG sensors

Distributed sensing system

Peripheral nervous system

Central nervous system

Brain

Network

Distributed sensors

Management and decision center

Modal parameters: too “global” Traditional Strain: too “ local ”

Human nervous system

How to realize a nervous system of structures 1)Very dense distribution of using smart point sensors –useful ?

2)Continuous or partially continuous wiring of using line Macro strain sensors including long –gauge sensors – natural !

Distributed sensing does not

means simple measurements!

R ep a ir in g

Stren gth en in g

D ifferen t A ction s

2. Packaged Long-gage FBG Sensors

Design of Long-gage FBG sensor

Long-gage FBG sensor and its mechanical property

Packaged with BFRP has no influence on strain sensitivity.

Long-gage FBG sensor specimen

0

100

200

300

400

500

600

700

800

0 50 100 150 200 250 300 350 400 450 500 550 600

bare FBG

packaged FBG1

packaged FBG2

packaged FBG3Wav

elen

gth

var

iati

on

(p

m)

Strain variation (µε)

Sε=1.2pm/με

Basic materials for packaging FBG sensor

(a) Carbon fiber tow (b) Epoxy resin (c) SMF and SMC

2. Distributed Strain Measurement for SHM

(a)

0 25 50 75 100 1251250

1

2

3

4

5

6

Magnit

ude o

f st

rain

FR

F (

/N

)

Frequency (Hz)

Mode 2

Mode 1

Mode 3

(a) Acceleration (b) Strain

(i) Global Information

(ii) Distribution of deformation from static strain distribution

Conjugated beam method

Deformation at the first joint and mid-span of the pth elementDistribution of deformation can be expressed by macro(long-gage)

strain distribution in an explicit formula!

MMS of a reference sensor, SR

MM

S o

f a target sen

sor,

Si

Best line of fit

Set of data at period t1

Feature = slope

Data set at period t2

Data set at period t3

Best line of fit

Increase in slope indicates damage within sensor Si between t2 and t3

No damage within sensor Si between t1 and t2

Interpretation

(iii) Damage Detection based on normalized modal macro-strain concept

Data set at period t3

3. Wayne Bridge: Sensor Layout

Totally 44 sensors were installed on the 3rd and 6th girders.

3. Wayne Bridge: Sensor Layout

Gage length Fixing endFixing end

ConnectorConnector

Fixing end

Gage lengthConnector Connector

Fiber sheath Plastic tube FBG Fixing end

(a)

(b)

3. Wayne Bridge Test Results: Global Information

Time history Time window 1

Time window 2 Time window 2

-5000

0

5000

10000

15000

20000

25000

2 2.2 2.4 2.6 2.8 3 3.2 3.4

F1 F2 F3

F4 F5 F6

F7 F8 F9

F10 F11 F12

F13 F14 F15

F16 F17

-1000

0

1000

2000

3000

4000

5000

6000

7000

2 2.2 2.4 2.6 2.8 3 3.2 3.4

F1 F2 F3

F4 F5 F6

F7 F8 F9

F10 F11 F12

F13 F14 F15

F16 F17

0.0E+00

2.0E-05

4.0E-05

6.0E-05

8.0E-05

1.0E-04

2 2.5 3 3.5

|fft|

Frequency (Hz)

2.82 Hz

2.81 Hz

2.81 Hz

Gird 3

Gird 6

3. Wayne Bridge Test Results: Global Information

Time history

Acceleration (Drexel University) Measured Strain

y = 1.0696x - 0.3627

R² = 0.9957

0102030405060708090

100110120130140150

0 15 30 45 60 75 90 105 120 135

MM

S o

f F

6

MMS of F4

y = 1.1408x - 0.7698R² = 0.9958

0102030405060708090

100110120130140150160

0 15 30 45 60 75 90 105 120 135

MM

S o

f F

7

MMS of F4

y = 1.1699x - 0.7834R² = 0.9943

0102030405060708090

100110120130140150160

0 15 30 45 60 75 90 105 120 135

MM

S o

f F

8

MMS of F4

y = 1.2116x - 0.7081R² = 0.9948

0

20

40

60

80

100

120

140

160

180

0 15 30 45 60 75 90 105 120 135

MM

S o

f F

9

MMS of F4

3. Wayne Bridge Test Results: Damage Detection

Increase in slope indicates damage

No damage if slope is stable

　Sensor

Variance Slope

F1 0.9915 0.7305

F2 0.9961 0.8438

F3 0.9968 0.9821

F4 　 1

F5 0.9959 1.0111

F6 0.9957 1.0694

F7 0.9958 1.1408

F8 0.9943 1.1698

F9 0.9948 1.2117

F10 0.9952 1.2439

F11 0.9956 1.2757

F12 0.9931 1.3182

F13 0.9916 1.3117

F14 0.985 1.2058

F15 0.9841 1.2119

F16 0.9829 1.2255

F17 0.9815 1.1661

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20 25 30 35

3. Wayne Bridge Test Results: Global Information

Fig. Magnitude relationship

( )t

( )t

M

X

-10

10

30

50

70

90

110

130

150

-50 0 50 100 150 200 250 300

Hei

ght (

cm)

MMS of Sensor (F9,W1,W10)

3. Wayne Bridge Test Results: Neutral Axis Determination

Neutral Axis Determination from dynamic strain measurement

-5000

0

5000

10000

15000

20000

25000

2 2.2 2.4 2.6 2.8 3 3.2 3.4

W1

W10

F9

Element 9 10 11 12 13 DREXEL Height 128 122 132 117 123 122

3. Wayne Bridge Test Results: Neutral Axis Determination

Static(Drexel Univ)

Dynamic

-10

10

30

50

70

90

110

130

150

-50 0 50 100 150 200 250 300

Hei

ght (

cm)

MMS of Sensor (F10,W2,W9)

-10

10

30

50

70

90

110

130

150

-50 0 50 100 150 200 250 300H

eigh

t (cm

)

MMS of Sensor (F11,W3,W8)

More interesting topics will be investigated by analyzing the measured distributed strains, e.g., comparing distributed strain time histories with traditional strain sensor outputs

Distributed long-gage FBG sensors can be used for both global and local information monitoring

Distributed strain measurement can be used for damage detection by utilizing developed damage index (like slopes, neural axis locations)

2

4. Conclusion

Thank you for your attention!