Distributed Strain Sensor Research Articles

Health monitoring of flexural steel structures based on distributed fibre optic sensors

In our previous study, a distributed long-gauge fibre optic sensing system (with the ability to obtain effective average strain, or macro-strain, distributions) for practical adaptation in civil structural health monitoring was developed and verified. The present paper is devoted to proposing an integrated health monitoring scheme for elastic beam-like structures, where flexure dominates structural responses, based on static testing and measurements using the developed sensors. A series of experimental investigations on steel beams with different levels of damage are first carried out. The static strain measurements from distributed sensors are characterised and some concerns on the data processing and feature extraction are discussed. On the basis of the extracted features, structural health monitoring (SHM) investigations are deployed in three parts: damage identification with no requirement for a structural analytical model, parametric estimation based on finite element (FE) models, and evaluation of structural global behaviour. By comparing to traditional transducers such as linear variable displacement transducers (LVDTs) and foil ‘point’ strain gauges, the ability and ascendancy of the sensors developed here for SHM purposes are verified. A comprehensive health monitoring strategy for steel flexural structures based on the distributed strain sensors array is proposed finally.

A temperature-compensated high spatial resolution distributed strain sensor

We propose and demonstrate a scheme which utilizes the temperature dependence of spontaneous Raman scattering to provide temperature compensation for a high spatial resolution Brillouin frequency-based strain sensor.

Hybrid Interrogation System for Distributed Fiber Strain Sensors and Point Temperature Sensors Based on Pulse Correlation and FBGs

A hybrid interrogation scheme for distributed strain sensors and point temperature sensors is proposed and its feasibility is demonstrated. It exploits the advantages of distributed sensors and point sensors simultaneously. The proposed interrogation scheme uses a wavelength tunable pulse source and wavelength-dependent reflectors for region selective sensing. Experimental results confirm its linear response and a resolution of 0.02% of the total dynamic range, which in our experiment is over 2000 microstrains for sensing regions of around a 0.5-m length. The stability of the measurements is over 99.8%. Finally, the high input power from the signal source and the limited losses from the reflectors allow the usage of a large number of sensors in a fiber.

Application of structural health monitoring in civil infrastructure

The emerging sensor-based structural health monitoring (SHM) technology has a potential for cost-effective maintenance of aging civil infrastructure systems. The author proposes to integrate continuous and global monitoring using on-structure sensors with targeted local non-destructive evaluation (NDE). Significant technical challenges arise, however, from the lack of cost-effective sensors for monitoring spatially large structures, as well as reliable methods for interpreting sensor data into structural health conditions. This paper reviews recent efforts and advances made in addressing these challenges, with example sensor hardware and health monitoring software developed in the author's research center. The hardware includes a novel fiber optic accelerometer, a vision-based displacement sensor, a distributed strain sensor, and a microwave imaging NDE device. The health monitoring software includes a number of system identification methods such as the neural networks, extended Kalman filter, and nonlinear damping identificaiton based on structural dynamic response measurement. These methods have been experimentally validated through seismic shaking table tests of a realistic bridge model and tested in a number of instrumented bridges and buildings.

Distributed strain sensor based on dynamic grating in polarization-maintaining erbium-doped fiber

A novel distributed fiber-optic strain sensor based on dynamic grating in polarization-maintaining erbium-doped fiber (PM-EDF) is demonstrated experimentally. The dynamic grating is written with light beams polarized along one primary polarization axis of the PM-EDF, and read with a light beam polarized along the other primary polarization axis. When a strain is applied to the fiber, the detected Bragg reflection frequency of the dynamic grating shifts in proportion to the birefringence change, which is proportional to the strain. By employing the technique of synthesis of optical coherence function, the dynamic grating is localized in a region that can be shifted to sweep along the fiber to determine the location of the strain. In the experiment, a spatial resolution of 20 cm is demonstrated.

2504 分布型光ファイバセンサによる溶接部のひずみモニタリング(J10-1 計測・モニタリング・解析(1),J10 知的材料・構造システム)

Distributed strain sensor with particularly high spatial resolution is required for assessing integrity of welded joint by monitoring its strain distribution. A distributed strain measurement system based on optical frequency domain reflectometry (OFDR) can measure a strain at arbitrary position along a long gauge FBGs (fiber Bragg grating) with high spatial resolution less than 1mm. In this study, we evaluated the applicability of the system to strain monitoring of welded joints in static tensile tests. We could successfully measure the strain distribution along the weld line for 100mm, and could show the process of degradation of the specimen. In addition, we could observe that the strain distribution sharply fluctuates with acoustic emissions in the area where anomalous distortions in the strain distribution were observed under the relatively lower load. We could also measure strain distribution ranged over 8000με within the FBG with gauge length of 100mm. The measured strain was mare than -10000με at maximum. Thus, the system was proved to have excellent spatial resolution and the measurement range sufficient to monitor the strain distribution of the welded joints.

Guideline for the design of a fiber optic distributed temperature and strain sensor

Full analysis of a distributed temperature and strain sensor (DTSS), based on stimulated Brillouin amplification effect in an optical fiber, is given. Some new rules, e.g. optimized launch power, have been found and the optimized design guidelines for short and long distance DTSS are presented.

Structural Deflection Monitoring Using an Embedded ETDR Distributed Strain Sensor

Feasible application of the electrical time domain reflectometry (ETDR) technique for distributed strain measurement is investigated in this study. A prototype coaxial ETDR distributed strain sensor is embedded into a laboratory-scale reinforced concrete beam specimen, and three-point bending tests are conducted. The ETDR signal waveforms of the embedded sensor are used to determine the deflection profiles of the beam under various bending loads. The resulting deflection profiles are validated by their good agreements with the deflection measurements by LVDT displacement sensors at four discrete locations. It is shown that the ETDR distributed sensing technique has the capability to capture the variation of the load-induced axial strain along the sensing line. In addition, the ETDR distributed sensor could also clearly capture the location of localized high strain points.

Monitoring the stress of the post-tensioning cable using fiber optic distributed strain sensor

We applied a Brillouin-OTDR, which is a fiber optic distributed strain sensor, to measure the stress of four post-tensioning cables. The fiber optic sensor was bonded to one steel strand and three aramid fiber reinforced plastic (AFRP) cables, which were later installed in three precast beams. The monitoring test was performed both in the tension procedure and the load procedure. The experimental results showed that the fiber optic distributed sensor holds high accuracy, and the relative deviation of the measurement results between fiber optic sensor and strain gauge is under 2.7%. The stress distribution of the post-tensioning cable obtained under all loads can be used to evaluate the health state of the beam. Furthermore, the prestressing loss occurred in post-tensioning cable can be calculated. The initial prestressing loss of steel strand is 8.98% and the initial prestressing loss of AFRP cable is 6.32%. After 30 days, the prestressing loss of steel strand is 8.81% and the prestressing loss of AFRP cable is 9.61%.

Distributed Measurements with a Long Gauge FBG Sensor Using Optical Frequency Domain Reflectometry (1st Report, System Investigation Using Optical Simulation Model)

Optical fiber sensors are used in order to monitor strain distribution of a structure, because they have many advantages for structural health monitoring. By using Fiber Bragg Grating (FBG) sensors with a general interrogation system e.g. Wavelength Division Multiplexing (WDM) makes it possible to measure only average strain within the gauge length. In addition, it is necessary to allocate enough numbers of FBG sensors for monitoring stress concentration overall. Therefore, it is expected to apply a distributed strain sensor with the higher spatial resolution and the longer sensing length to accurate and effective monitoring of stress concentration. We present a new strain measurement system with a long gauge FBG sensor based on Optical Frequency Domain Reflectometry (OFDR), which enables us to measure fully distributed strain at special high spatial resolution. In this paper, we describe the principle and optical simulation model of the proposed measurement system and show results of numerical calculations.

Simulated annealing evolutionary algorithm for the fibre Bragg grating distributed strain sensor

A new method based on a simulated annealing evolutionary algorithm is developed to demodulate the strain profile along a fibre Bragg grating (FBG) distributed strain sensor. The method combines the merits of both simulated annealing and evolutionary programming. With just the intensity information of the reflected spectrum of the FBG, the strain profile along the FBG distributed senor can be reconstructed exactly, even in regions with significant gradients. Numerical simulations show good agreement between the original and the reconstructed strain profiles.

Structural Damage Detection Using an Embedded ETDR Distributed Strain Sensor

Feasible application of the Electrical Time Domain Reflectometry (ETDR) distributed strain sensing technique for structural damage detection was investigated in this study. A small-scale concrete beam specimen with an embedded ETDR distributed strain sensor was tested in three-point bending. The resulting ETDR waveforms of the sensor were studied in conjunction with the load–displacement curve of the concrete beam and the video images of the specimen recorded during the test. It was shown that the embedded ETDR sensor was capable of detecting weak points in the structure resulted from structural defects. The embedded sensor also had the capability to detect a load-induced damage at its early stage when no surface crack line was visible. On the occurrence of an apparent crack damage that passed through the sensing line, the embedded sensor could locate its position with a precision within 0.25′′. Moreover, the ETDR distributed sensor had the capability to detect multiple cracks simultaneously along a single sensing line.

Development of an electrical time domain reflectometry (ETDR) distributed strain sensor

In this paper, a detailed description for the ETDR distributed strain sensing mechanism of a coaxial cable was presented, and a signal calibration algorithm for interpreting ETDR signal waveforms was developed. In addition, a prototype coaxial ETDR distributed strain sensor with improved signal sensitivity was presented. The ETDR signal responses of the prototype sensor subjected to a concentrated lateral compression load and distributed axial tension load were experimentally tested. The test results showed that the prototype sensor has a substantially improved signal sensitivity over a commercial RG-174 cable of comparable size. It was also shown that the relation between the impedance change of the sensor and the applied axial tensile strain depends primarily on the mechanical stress–strain response of the sensor. From the test results, it was demonstrated that this relation could be empirically established with the aid of the calibration algorithm.

High spatial resolution microwave detection system for Brillouin-based distributed temperature and strain sensors

We present a microwave detection system for coherent detection of spontaneous Brillouin-based distributed temperature and strain measurements. The system was designed to overcome the existing bandwidth limitations of a previously used commercial spectrum analyser and provide a commercially practical solution to the detection of the Brillouin frequency shift and intensity for long range high spatial resolution measurements. The detection system bandwidth corresponds to a potential spatial resolution of ∼60 cm. The system was demonstrated as a temperature sensor over a range of 30 km, with a temperature resolution of 1.6 °C and spatial resolution of ∼2 m. It was also demonstrated as a combined temperature and strain sensor over a range of 6.3 km with ∼1.3 m spatial resolution; temperature and strain resolutions of 3 °C and 80 µε respectively were achieved.

Fabrication of self-apodized short-length fiber Bragg gratings.

A new technique for writing extremely short-length Bragg gratings in optical fibers is demonstrated. A physical model describes the diffraction effects on the spatial and wavelength spectra of the Bragg gratings. Selection of appropriate diffraction patterns and related parameters permits self-apodized Bragg gratings with a typical spatial length of several hundred micrometers and a bandwidth of several nanometers to be obtained. These gratings with well-defined spectra are suitable for use as miniature distributed strain sensors and other applications requiring small physical dimensions and broadband spectra.

Possibility of using a coaxial cable as a distributed strain sensor by timedomain reflectometry

In this paper, the possibility of using coaxial cables asdistributed strain sensors by time domain reflectometry is studied.Analytical analysis is conducted on the relation between the distributedaxial strain and the change of characteristic impedance as well as on therelation between the change of characteristic impedance and the voltage ofthe reflected wave. The analysis leads to the establishment of the quantitativerelation between the distributed axial strains of a coaxial cable and thechange in the voltage of the reflected wave. It is demonstrated that thevoltage of the reflected wave can be used to measure the magnitude anddetermine the location of the axial strains in a coaxial cable.Furthermore, a good linearity is obtained when the axial strains are small.The investigation highlights the potential of coaxial cables as distributedstrain sensors. Finally, the existing problems for such sensors arediscussed.

Health monitoring of large composite structures by using fiber optic distributed strain sensors. Experiences at America's Cup 2000.

Health monitoring technologies of a structure made of polymer matrix composites have lately become a subject of special interest. International America's Cup Class (IACC) yachts are specific boats whose most components made of carbon fiber reinforced plastic (CFRP) and that are regulated by the IACC rule. A Brillouin optical time domain reflectometer (BOTDR) is a fiber optic distributed sensor that can measure strain and temperature along an optical fiber. We equipped IACC yachts that were the Japanese entry in America's Cup 2000 with these sensors. Strain distributions of the yachts measured with BOTDR were used for structural health monitoring and assessments of the structural integrity were continued during races.

1 × N star coupler as a distributed fiber-optic strain sensor in a white-light interferometer

A novel technique of using a 1 x N star fiber optic coupler as a distributed strain sensor in a white-light interferometer to measure the distribution of strain is presented. The measuring principle and 1 x 4 star coupler with four fiber optic strain sensors are demonstrated. The experiment is performed with four sensors attached to a combination plastic specimen.

Continuous arbitrary strain profile measurements with fiber Bragg gratings

A new distributed strain sensing technique using a fiber optic Bragg grating has been developed and tested. This is the first `true' distributed strain sensor, to the authors' knowledge, with a high spatial resolution of about 1 mm. Since gratings can be made with a length of tens of centimeters, this new fiber optic measurement technique could have broad applications to smart materials and structures where monitoring of a continuous strain profile over a length of millimeters to tens of centimeters is needed. In this paper three different demodulation approaches are reviewed indicating a trade-off between a relatively simple measurement process for selected types of strain profile and a more complete measurement process that is suitable for any strain profile. Experimental results with different approaches are presented.

Simultaneous distributed measurement of strain and temperature from noise-initiated Brillouin scattering in optical fibers

The simultaneous determination of strain and temperature distributions from the measurement of noise-initiated Brillouin scattering (NIBS) power and frequency shift in optical fibers is discussed. Equations governing the growth of the NIBS signal are derived and from these, we calculate the dependence of the Brillouin power on temperature and strain. We study the potential problem given by the need to normalize the nonlinear Brillouin signal and present a new technique that solves this problem by mathematically combining the values of the Stokes and anti-Stokes powers to produce a linear effective power. Experimental results are presented that support this theory and allow the verification of the coefficients governing the dependence of the Brillouin power and frequency shift on temperature and strain. The signal-to-noise ratio of the sensor is discussed, and it is found that the noise associated with the field statistics plays a limiting role in the sensor performance and that an optimum value for the Brillouin gain factor can be determined. A simultaneous distributed temperature and strain sensor is demonstrated; preliminary results show a strain resolution of 100-/spl mu/m strain, a temperature resolution of 4/spl deg/C, and a spatial resolution of 40 m, over a sensing length of 1200 m.