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Comparative Study on the Elongation Measurement of a Soil Nail Using Optical Lower Coherence Interferometry Method and FBG Method |
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Authors
Cheng-Yu Hong1, Jian-Hua Yin1, Wei Jin2, Chao Wang2, Wan-Huan Zhou1, Hong-Hu Zhu1
1 Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China
2 Department of Electrical Engineering, The Hong Kong Polytechnic University Hong Kong, China
Advances in Structural Engineering, Issue Volume 13, Number 2, Pages 309-320
DOI: 10.1260/1369-4332.13.2.309
Abstract
Optical fiber sensor has gained popularity for structural monitoring in recent decades. Studies on the monitoring for geotechnical structures with fully-distributed fiber sensors, however, are quite limited. In this study, the fully-distributed Lower Coherence Interferometry (LCI) sensors and quasi-distributed Fiber Bragg Grating (FBG) sensors are applied for measuring the strain distribution of a soil nail as a slender structure during pullout test. First, this paper introduces an innovative calibration method for the LCI system. Second, the paper describes the procedures of fiber instrumentation on soil nail. Third, the measured results by LCI technology and FBG technology are compared to study the strain distribution and friction resistance of soil nail during pullout test. Comparative study from test results indicates the characteristics of strain distribution and resistance contribution for different parts of soil nail. Through this study, the advantages and limitations of two sensing technologies are examined.
Keywords
optical fiber sensor, pullout, soil nail, strain, elongation
Fulltext link: http://multi-science.metapress.com/content/41l2520746gg11h8/?p=546ee3d6660b4f0b853f19bef1950388&pi=7 |
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Stimulated Brillouin Scattering (SBS) |
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Extrinsic Fabry-Perot Interferometer (EFPI) |
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The construction of the Extrinsic Fabry-Perot Interferometer (EFPI) is first described by Murphy et al. The Fabry-Perot (FP) cavity is formed by a single mode fiber and a multimode fiber sealed inside a hollow core tube.
The single mode fiber acts as the inputoutput fiber whereas the multimode fiber is used solely as a reflector. The hollow core tube acts as a guide tube in addition to protecting the cavity from external elements. Light enters the single mode fiber and it is partially reflected from the first glass-air interface as R1. The transmitted light travels through the cavity, is reflected from the second air-glass interface and enters the single mode fiber again as R2. These reflections then interfere in the single mode iber. The output depends on the difference in the optical path lengths traveled by the two interfering waves. The effect of subsequent reflections inside the cavity are negligible. |
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Monitoring Internal Displacements of a Model Dam Using FBG Sensing Bars |
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Hong-Hu Zhu 1, Jian-Hua Yin 1,2, Lin Zhang 2,3, Wei Jin 4 and Jian-Hua Dong 2,3
1. Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hong Kong, China
2. State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University, Chengdu, Sichuan, China
3. School of Water Resources and Hydropower Engineering, Sichuan University, Chengdu, Sichuan, China
4. Department of Electrical Engineering, The Hong Kong Polytechnic University, Hong Kong, China
Advances in Structural Engineering, Volume 13, Number 2 / April 2010, Pages 249-262, Online Date Tuesday, June 08, 2010
DOI 10.1260/1369-4332.13.2.249
ABSTRACT: Conventional instrumentation has difficulty to measure displacements inside laboratory-scale physical models. In this paper, an innovative fiber Bragg grating (FBG) sensing bar has been developed to perform high-accuracy and real-time monitoring of internal displacements. According to Euler-Bernoulli beam theory, the strain distribution measured by the quasi-distributed FBG sensors adhered on the bar surface can be used to compute the displacement profile along the sensing bar. The effectiveness of the FBG sensing bar has been verified by calibration tests. In the physical model of the Wudu gravity dam, two FBG sensing bars developed by the authors were installed, together with linear variable displacement transformers (LVDTs) and strain rosettes. The FBG sensing bars successfully captured the variation of internal displacement profiles in the model dam. The monitoring results during the overloading test were presented, based on which the deformation mechanism of the dam-foundation system is explained in details. The displacements measured by the FBG sensing bars are validated by their good agreements with those from LVDTs.
KEYWORDS: optical fiber sensing technology, fiber Bragg grating (FBG), internal displacement, dam, physical model test
Fulltext link: http://multi-science.metapress.com/content/22u762vj0212412r/?p=9f93759384d344b98d004c72cb3f09bd&pi=3 |
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Brillouin Optical Time Domain Analysis (BOTDA) |
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Fiber Bragg Grating (FBG) |
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Fiber Bragg Grating (FBG) is one of the most popular optical fiber sensing technologies. Bragg grating is written into a segment of Ge-doped single-mode fiber in which a periodic modulation of the core refractive index is formed by exposure to a spatial pattern of ultraviolet (UV) light.
According to Bragg’s law, when a broadband source of light has been injected into the fiber, FBG reflects a narrow spectral part of light at certain wavelength, which is called the Bragg wavelength and dependent on the grating period and the refractive index of fiber. |
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