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 technique 2 Sensor placement Global parameter identification Distributed long-gage FBG sensors Utilizing distributed strain measurement for SHM IBS Bridge 3 Damage detection

1. Single Point Based Sensors

Strain Gauge Damaged! No damage! OK?! Too Local! Huge Limitation! 1. Single Point Based Sensors

2. Distributed long-gage FBG sensors

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

2. Distributed long-gage FBG sensors 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!

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 Wavelength variation (pm) Strain variation (µε) S ε =1.2pm/με

2. Distributed Strain Measurement for SHM (a) 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 element Distribution of deformation can be expressed by macro(long-gage) strain distribution in an explicit formula!

MMS of a reference sensor, S R MMS of a target sensor,Si Best line of fit Set of data at period t 1 Feature = slope Data set at period t 2 Data set at period t 3 Best line of fit Increase in slope indicates damage within sensor Si between t 2 and t 3 No damage within sensor Si between t 1 and t 2 Interpretation (iii) Damage Detection based on normalized modal macro-strain concept Data set at period t 3

3. Wayne Bridge: Sensor Layout Totally 44 sensors were installed on the 3 rd and 6 th girders.

3. Wayne Bridge: Sensor Layout Gage length Fixing end Connector Fixing end Gage length Connector Fiber sheath Plastic tube FBG Fixing end (a) (b)

3. Wayne Bridge Test Results: Global Information Time historyTime window 1 Time window 2

2.82 Hz 2.81 Hz Gird 3 Gird 6 3. Wayne Bridge Test Results: Global Information Time history Acceleration (Drexel University) Measured Strain

3. Wayne Bridge Test Results: Damage Detection Increase in slope indicates damage No damage if slope is stable

Sensor VarianceSlope F F F F4 1 F F F F F F F F F F F F F Wayne Bridge Test Results: Global Information Fig. Magnitude relationship

M X 3. Wayne Bridge Test Results: Neutral Axis Determination Neutral Axis Determination from dynamic strain measurement

3. Wayne Bridge Test Results: Neutral Axis Determination Static (Drexel Univ) Dynamic

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!