Fiber Optic Strain Sensors Research Articles

APPLICATION OF A NEW MEASURING METHOD USING AN OPTICAL FIBER STRAIN SENSOR TO AN UNDERGROUND TUNNEL

The measuring methods using optical fiber strain sensors are being considered as structural monitoring technologies for concrete structures. B-OTDR method using optical fiber strain sensors can measure continuous strain along an optical fiber over 80km. Because of this advantage, it is expected to measure deformations in large-scale structures. But, this new measuring method, B-OTDR method, is just in development. There is very few case of applying this measuring method to actual structures. This paper describes a new measuring method using optical fiber strain sensors (B-OTDR method) applying to an actual concrete underground tunnel which was expected to deform by new shaft work near this tunnel. As a result, this method using optical fiber strain sensors can measure the deformation of the underground tunnel.

Wireless Instrumentation for Monitoring of Smart Structures

Smart structures needs lots of sensor installation to sense their status and also the external environmental change. Wireless technique can give a good solution to install sensors without heavy cables. So, in this work, a wireless device was developed to transmit static strain and elastic wave propagation of structures. The specification of this device was as follows: 2.4 GHz of transmitted frequency, 8 channels, 57,600 bps of the transmitted speed, and 10 mW of the transmitted power. In order to confirm the wireless device’s feasibility, a beam test was performed with five optical fiber strain sensors and two piezo-ceramic sensors with the wireless instrumentation.

Twin multiplexing strain sensing array based on a low-coherence fiber optic Mach–Zehnder interferometer

A twin multiplexed white light interferometric fiber optic strain sensor array was designed and demonstrated. In this sensing system, based on a low-coherence fiber optic Mach–Zehnder interferometer, an optical path matching technique is used to demodulate each sensor. The strain sensor array consists of 2 × N sensing elements linked by N 3 dB couplers. When one of the fiber sensors has been used to measure the strain, the variation caused by the temperature can be compensated by the other sensor. The multiplexing capacity of the sensing scheme has been analyzed. The results shown that it is depend on the light source power. The maximum number of the twin fiber sensor size is 4 for light source with 35 μW, and 7 for light source with 3 mW, respectively. The experimental result with a two-coupler sensor array was conducted and demonstrated which confirm this. It is useful for temperature compensation of distributed strain measurement in smart materials or structures health monitoring.

Low cost optical fibre based Fabry–Perot strain sensor production

The production of Fabry–Perot-based optical fibre sensors has long been an iterative and labour-intensive process. This paper demonstrates the production of Fabry–Perot-based optical fibre strain sensors using chemical etching techniques. Utilizing hydrofluoric acid and single-mode optical fibres, a preferential etching mechanism was observed around the core portion of the fibres. These etched fibre ends were then spliced together successfully to form enclosed Fabry–Perot cavities between 18 and 60 µm in length. These sensors have then been deployed for strain monitoring and have been subjected to strains of up to 1400 με on tensile test specimens. Etched Fabry–Perot cavity lengths were monitored using a white light interferometry system based on a CCD spectrometer and an 850 nm super luminescent diode. A linear and repeatable response to these strain tests has been shown with negligible sensitivity to temperature.

Distributed strain measurement for pile foundations

Recent advances in strain measurement using optical fibres provide new opportunities for monitoring the performance of piles. Brillouin optical time-domain reflectometry (BOTDR) is an innovative technique that allows measurement of full strain profiles using standard optical fibres. Distributed optical fibre strain sensors were installed at two sites, and the details of the installation methods are presented. A theoretical analysis that compares pile behaviour evaluated from localised strain measurement at discrete locations and from distributed strain measurements is conducted for the case of a vertically loaded pile in layered soil. Two problems are considered: a load test aiming to establish the load transfer function, and changes in pile–soil interface stress due to nearby tunnelling. Advantages of such a distributed reading for monitoring the performance of pile foundations in layered soils are demonstrated from the results of both cases. Cost analysis results show that the BOTDR measurement is a cost-effective pile monitoring method compared with discrete strain measurement methods such as vibrating-wire strain gauges and fibre Bragg gratings.

Assessment of corrosion‐induced damage and its effect on the structural behavior of RC beams containing supplementary cementitious materials

AbstractThis paper reports the results of an experimental program on the effect of steel loss and corrosion induced‐damage (cracking, delamination, spalling, etc.) on the post‐corrosion flexural response of reinforced concrete beams incorporating supplementary cementing materials (fly ash, slag and silica fume) used as partial replacement of ordinary Portland cement. Eight reinforced concrete beams measuring 2.5 m in length and 210 × 300 mm in cross‐section were cast, four of which were subjected to accelerated corrosion. Among all specimens incorporating supplementary cementing materials (SCM), the specimen containing fly ash showed the best performance in terms of the highest residual load carrying capacity at failure. Its best performance was also evident from the lowest measured steel loss, the least corrosion‐induced cracking and the lowest tendency for the concrete cover to delaminate as measured by a concrete‐embeddable fiber optic strain sensor (FOSS). Correlation between the amount of steel loss and the reductions in load carrying at failure was also established.

Demodulation algorithm for spatial-frequency-division-multiplexed fiber-optic Fizeau strain sensor networks

A demodulation algorithm for spatial-frequency-division-multiplexed fiber-optic Fizeau strain sensor networks with a large number of sensors is proposed to effectively reduce the cross talk between any two adjacent sensors and hence substantially enhance the multiplexing capability of the network, which is based on the Pisarenko algorithm. The cross talk between two fiber-optic Fizeau sensors is investigated experimentally. The experimental results show that a strain accuracy of better than +/-10 micro permittivity can be achieved even when the cavity length difference is approximately 100 microm. It was demonstrated that the multiplexing capability of the spatial-frequency-division-multiplexed fiber-optic Fizeau sensor network can be approximately 5 times greater than that of the conventional fast-Fourier-transform algorithm, and this can lead to achievement of a sensing network with a multiplexing capability of as many as 1000 Fizeau sensors.

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%.

Temperature-independent strain sensor made from tapered holey optical fiber

A large-mode-area holey fiber was tapered to a point in which the airholes collapsed, and its dependence on temperature and strain was studied. The transmission spectrum of such a fiber exhibits a series of peaks owing to the interference between the modes of the solid taper waist. We found that the interference peaks shifted to shorter wavelengths as the taper was elongated. However, the peaks were insensitive to temperature. The fabrication and advantages of our novel wavelength-encoded temperature-independent strain sensor compared with other optical fiber strain sensors are discussed.

Applications of fiber optic sensors for structural health monitoring

Large and complex structures are being built now-a-days and, they are required to be functional even under extreme loading and environmental conditions. In order to meet the safety and maintenance demands, there is a need to build sensors integrated structural system, which can sense and provide necessary information about the structural response to complex loading and environment. Sophisticated tools have been developed for the design and construction of civil engineering structures. However, very little has been accomplished in the area of monitoring and rehabilitation. The employment of appropriate sensor is therefore crucial, and efforts must be directed towards non-destructive testing techniques that remain functional throughout the life of the structure. Fiber optic sensors are emerging as a superior non-destructive tool for evaluating the health of civil engineering structures. Flexibility, small in size and corrosion resistance of optical fibers allow them to be directly embedded in concrete structures. The inherent advantages of fiber optic sensors over conventional sensors include high resolution, ability to work in difficult environment, immunity from electromagnetic interference, large band width of signal, low noise and high sensitivity. This paper brings out the potential and current status of technology of fiber optic sensors for civil engineering applications. The importance of employing fiber optic sensors for health monitoring of civil engineering structures has been highlighted. Details of laboratory studies carried out on fiber optic strain sensors to assess their suitability for civil engineering applications are also covered.

SFDM/CWDM of fiber-optic Fizeau strain sensors

In this letter, a novel method for multiplexing a large number of fiber-optic Fizeau sensors using coarse wavelength-division multiplexing (CWDM) and spatial frequency-division multiplexing (SFDM) is proposed and demonstrated. A CWDM device is used to multiplex signals from different sensor channels occupying different wavelength regions divided by the CWDM. More than 10 Fizeau sensors arranged in a single channel can be multiplexed by using SFDM due to the long cavity length of the Fizeau sensor. The experimental results show that a strain accuracy of /spl plusmn/5 /spl mu//spl epsiv/ is achieved. As up to 100 sensors can be multiplexed simultaneously, this approach can improve the multiplexing capacity of Fizeau strain sensors significantly.

Characterization of a FBG strain gage array embedded in composite structure

In recent researches, fiber Bragg grating (FBG) is proven the most promising fiber optic strain sensor upon its elaborate strain sensibility. Numerous efforts have been done in realizing practical FBG strain gage systems, either mounted on the surface, or integrated within the structure. Fiber reinforced composite materials are always regarded as the nominated candidate to embed FBG sensors upon the fact that, fiber reinforced filamentary topology enables the integration and effectively protects sensors from hostile attack, therefore a fiber optic smart structure is achievable. However, one must recognize that embedded FBG fibers are foreign entities to host structures, mechanical mismatches may eventually lead to alteration on sensors’ responsive behaviours, and measurand interpretation becomes complicated. This work reports a set of experiments on an orthogonal FBG array embedded in graphite/epoxy composite structures. Recurring to corresponding surface mounted resistance strain gages, the authors tend to provide a comprehensive investigation on strain-optic behaviours of embedded FBG sensors, and a quantitative discussion on inevitable deviations of the sensors when used as buried strain gages.

Novel Distributed Fiber-Optic Strain Sensor by Localizing Dynamic Grating in Polarization-Maintaining Erbium-Doped Fiber: Proposal and Theoretical Analysis

A novel scheme of distributed fiber-optic strain sensor by localizing a dynamic grating in a polarization-maintaining erbium-doped fiber is proposed. The dynamic grating, which is introduced by launching two counterpropagating coherent light beams into a pumped erbium-doped fiber per gain saturation, is examined. A theoretical model of the dynamic grating is given and used to perform simulations of the reflectivity of the dynamic grating localized by synthesizing the optical coherence function. Theoretical analysis and numerical simulation of the novel sensor scheme show that the Bragg frequency of the dynamic grating will shift according by 0.426 MHz/µε to strain in a typical polarization-maintaining erbium-doped fiber.

Spatial-frequency multiplexed fiber-optic Fizeau strain sensor system with optical amplification

A novel method for multiplexing fiber-optic Fizeau strain sensors with optical amplification is proposed and demonstrated. This method overcomes the two intrinsic disadvantages of fiber-optic Fabry–Perot (F–P) strain sensors, i.e. weak signal and difficult multiplexing. The amplified spontaneous emission (ASE) and optical amplification are used simultaneously to enhance the interferometric signal considerably. A Fizeau interferometer formed by two fiber ends with a quite different reflectivity is used to replace the F–P cavity in sensor head design. Such a Fizeau cavity can enlarge the cavity length by at least an order of magnitude and allows more than 10 sensors to be multiplexed simultaneously by using spatial-frequency multiplexing. The operating principle of the sensor system is discussed and an experiment is carried out to verify the concept of the method proposed. It is anticipated that such a sensor system could find important applications for health monitoring of large structures.

SMS-32: Development of Fiber Optic Strain and Vibration Sensors(SMS-V: SMART MATERIALS AND STRUCTURES, NDE)

SMS-32: Development of Fiber Optic Strain and Vibration Sensors(SMS-V: SMART MATERIALS AND STRUCTURES, NDE)

Temperature Compensation of a Fiber Optic Strain Sensor Based on Brillouin Scattering

Brillouin scattering-based fiber optic sensors are useful to measure strain or temperature in a distributed manner. Since the Brillouin frequency of an optical fiber depends on both the strain and temperature, it is very important to know whether the Brillouin frequency shift is caused by the strain change or temperature change. This article presents a temperature compensation technique of a Brillouin scattering-based fiber optic strain sensor. Both the changes of the Brillouin frequency and the Brillouin gain power is observed for the temperature compensation using a BOTDA sensor system. Experimental results showed that the temperature compensated strain values were highly consistent with actual strain values.

A TECHNIQUE FOR EMBEDDING EFPI FIBER OPTIC STRAIN SENSORS IN CONCRETE

SUMMARY EFPI fiber optic strain sensors possess good qualities for ap-plication to civil engineering structures. One of the majorand challenging tasks is the safe embedding of a fragile fiberoptic sensor inside concrete. From this work to develop asuitable technique of embedding fiber optic sensors in con-crete, it was observed that epoxy sheet encapsulation resultsin satisfactory performance compared to the use of acrylicsheets for encapsulation. The epoxy sheet encapsulation alsoshowed satisfactory performance at high strain ranges.

04/01002 Influence of domestic hot water parameters on the energy consumption of large buildings in Senegal: Ndoye, B. and Sarr, M. Energy Conversion and Management, 2003, 44, (16), 2635–2649

A fibre-optic measurement system to analyse the deformation of in-vessel components has successfully been developed, installed and commissioned at ASDEX Upgrade (AUG). This technology has thereby been qualified for in-vessel use at experimental fusion devices. AUG is equipped with an internal conductor for passive plasma stabilisation called the Passive Stabilisation Loop (PSL), on which the recently installed 16 internal coils (B-coils) are directly mounted. The PSL structure is highly prone to vibrations, and the risk of resonant oscillations in response to B-coil induced forces necessitated the development of the present diagnostic. The diagnostic system consists of 34 fibre-optic strain sensors incorporated in two glass fibres. It is completely insensitive to electromagnetic disturbances. The fibres are customised to avoid inconvenient excess fibre length in the vacuum vessel. They were tested for their neutron tolerance and vacuum compatibility prior to installation. The actual sensors are embedded in stainless steel carriers that were attached to the PSL, which is made of copper, by laser welding. Appropriate welding parameters were determined in view of the metallurgical dissimilarity. The weld quality was approved by tensile tests and microscopic investigations. Accurate in-vessel positioning of the sensors was assured using a 3D measurement system and coordinates from CAD. The data acquisition allows a sampling rate of 1 kHz. It was shown that the temporal and spatial resolutions of the system are sufficient to resolve the potentially dangerous bending eigenmodes of the PSL rings.

Error analysis of temperature-compensated white-light interferometric fiber-optic strain sensor

A software-based temperature-compensated fiber-optic white-light interferometric (WLI) strain sensor is presented. Measurement error of this sensor is studied theoretically and experimentally. The results quantitatively indicate how the CCD camera pixel resolution, interference fringe pattern center determination errors, strain sensing sensitivity, sensing fiber and compensating fiber lengths affect the measurement errors. Measurement accuracy limitation for the individual strain and temperature sensors with specified highly birefringent fibers and WLI decoding system is also investigated. The main sources of the central fringe position measurement error as well as solutions for further improvements are particularly discussed. Critical factors for sensor design and potential for civil and composite structure monitoring are highlighted by comparing sensitivities, fiber length limitation, power budget and dynamic ranges with those of other fiber interferometers. The analysis provides a convenient way for sensor design to satisfy applications with different accuracy requirements.

Strain monitoring of braided composites by using embedded fiber-optic strain sensors

Recently, fiber optic strain sensors have been applied to internal strain and damage monitoring of composites because of their small size, light weight and flexibility. Braided fiber reinforced plastics (FRP) are compatible with fiber optic sensors because optical fibers can be integrated directly and easily into fabrics. In the present paper, the strain monitoring of braided glass fiber reinforced plastics (GFRP) was conducted by using embedded fiber Bragg grating (FBG) and extrinsic Fabry–Perot interferometric (EFPI) sensors during the cure process, tensile tests and fatigue tests. From the experimental results of cure monitoring, it was found that both sensors can be used only for monitoring of thermal residual strain during cooling process. From the results of tensile tests, it was found that both sensors could measure strain correctly until damage initiation of braided GFRP. It also appeared that FBG sensors could monitor damage to FRP by observing the reflected spectral shape. From the fatigue tests, it appeared that the strain measured by embedded FBG sensors was affected by fatigue damage. Therefore, it is concluded that internal strain monitoring of braided FRP using fiber optic strain sensors is very useful for cure and health monitoring.