Distributed Fiber Optic Strain Research Articles

Distributed Strain Behavior of a Reinforced Concrete Bridge: Case Study

To effectively assess and manage aging infrastructure, sensing technologies that produce accurate and useful quantitative data are required. Distributed fiber optic strain measurement systems are one potential technology that could fulfill this requirement. This case study investigates the performance of a distributed fiber optic strain measurement technology with high accuracy and spatial resolution during a load test on a RC bridge. The measurements from the fiber optic system are compared with conventional strain gauges and linear transducers, and good agreement between the sensors was found. The fiber optic measurements are also used to examine the bridge support conditions as well as the load distribution between beams. The results show that assumptions made about the support conditions during design do not match the actual bridge behavior, although the load distribution between beams is as expected.

Structural health monitoring by using fiber-optic distributed strain sensors with high spatial resolution

In this paper, we review our researches on the topics of the structural health monitoring (SHM) with the fiber-optic distributed strain sensor. Highly-dense information on strains in a structure can be useful to identify some kind of existing damages or applied loads in implementation of SHM. The fiber-optic distributed sensors developed by the authors have been applied to the damage detection of a single-lap joint and load identification of a beam simply supported. We confirmed that the applicability of the distributed sensor to SHM could be improved as making the spatial resolution higher. In addition, we showed that the simulation technique considering both structural and optical effects seamlessly in strain measurement could be powerful tools to evaluate the performance of a sensing system and design it for SHM. Finally, the technique for simultaneous distributed strain and temperature measurement using the PANDA-fiber Bragg grating (FBG) is shown in this paper, because problems caused by the cross-sensitivity toward strain and temperature would be always inevitable in strain measurement for SHM.

Study of ballastless track structure monitoring by distributed optical fiber sensors on a real-scale mockup in laboratory

Ballastless track structure are presently seen as an attractive alternative to conventional ballast. A new model was designed and a real-scale mockup was built and tested in fatigue test at IFSTTAR laboratory. To verify numerical simulations, the railway structure was monitored by conventional strain gages. A distributed fiber-optic strain sensing based on Rayleigh backscatter was also tested in this experiment. Fiber-optic cables were used as long-gage sensors and they were embedded in the concrete slabs of the structure. Firstly, we verified that the sensors were enough mechanical and chemical resistant for withstanding the stresses and the caustic environment experienced during concrete pouring process. Secondly, we validated the strain profiles measurements by comparison with the values obtained by strain gages. A good agreement was found throughout the duration of the fatigue test of 10 million cycles. Moreover, some cracks were observed early during the fatigue test. It is important to be able to monitor them during the service of ballastless track line. We showed that cracks can be detected and localized by fiber-optic distributed strain measurements. Finally, this experiment demonstrates that fiber-optic distributed strain sensing technique based on Rayleigh backscatter is a promising sensing technique to monitor ballastless track structures and more generally, civil engineering structures.

A Method for Detecting Deformation of Pipeline Based on Distributed Fiber Optic Strain Sensor

A method for detecting pipeline’s deformation is presented in this paper, based on distributed fiber optic strain sensor. We simplify the pipeline and analysis it as a beam. According to beam’s geometrical relational expression, pipeline’s deflection could be calculated by using discrete strain with numerical method.

Performance Monitoring of a Secant-Piled Wall Using Distributed Fiber Optic Strain Sensing

An optical fiber strain-sensing technique, on the basis of Brillouin optical time domain reflectometry (BOTDR), was used to monitor the performance of a secant pile wall subjected to multiple props during construction of an adjacent basement in London. Details of the installation of sensors and data processing are described. Distributed strain profiles were obtained by deriving strain measurements from optical fibers installed on opposite sides of the pile to allow monitoring of both axial and lateral movements along the pile. Methods for analyzing the thermal strain and temperature compensation are also presented. Measurements obtained from the BOTDR were found to be in good agreement with inclinometer data from the adjacent piles. The relative merits of the two different techniques are discussed.

Measurement range enlargement in Brillouin optical correlation-domain reflectometry based on double-modulation scheme

We have demonstrated a double-modulation scheme to enlarge the measurement range of Brillouin optical correlation-domain reflectometry for fiber-optic distributed strain sensing. In this scheme, the frequency of the laser output is simultaneously modulated with two different frequencies. In the experiment, 53-cm resolution and 1.5-km measurement range were simultaneously obtained. Furthermore, 27-cm resolution and 1.5-km measurement range were also simultaneously achieved when a noise-floor compensation technique was employed.

Distributed Fiber Optic Strain Sensing

Distributed Fiber Optic Strain Sensing

Measurement range enlargement in Brillouin optical correlation-domain reflectometry based on temporal gating scheme

We newly develop a temporal gating scheme to enlarge the measurement range of Brillouin optical correlation-domain reflectometry (BOCDR) for a fiber-optic distributed strain measurement. In this scheme, the trade-off problem between the measurement range and the spatial resolution can be dissolved. 66-cm spatial resolution and 1-km measurement range were simultaneously obtained with 50-Hz sampling rate.

Fiber optic distributed temperature and strain sensing system based on Brillouin light scattering

We present an original method to improve the spatial resolution of a Brillouin distributed temperature and strain sensing system (DTSS). This method is shown to substantially improve the spatial resolution, while simultaneously strengthening the Brillouin backscattered light, which is based on the combination of an internal modulation of the laser source and an external modulator to generate two separate light pulses with different central wavelengths and pulse widths. Moreover, a novel Brillouin signal detection method, which we called isogenous heterodyne detection, is introduced, which is equivalent to a heterodyne detection scheme but is only with Rayleigh and Brillouin backscattered light without the need of an extra reference light. These new technical approaches have been incorporated into a fiber optic DTSS with 13 km single-mode fiber, which clearly successfully demonstrated all the advantages over conventional DTSS approaches in theory and the feasibility in experiment.

Application of a distributed fibre optic strain sensing system to monitoring changes in the state of an underground mine

As the first step in investigating the possibility of using a distributed fibre optic strain sensing system to detect changes in the state of an underground mine resulting from mining activities, we constructed an underground mine monitoring system based on a Brillouin optical time domain reflectometer (BOTDR) system in the El Teniente mine and conducted a field trial. This paper describes the structure of and mining processes in the mine, outlines the underground mine monitoring system that we developed and constructed, and presents some monitoring results. We consider from the results of the half-year field trial that the system could detect the deformation in a tunnel in the underground mine ventilation level due to a stress distribution imbalance produced surrounding the undercut face and the large-scale ore extraction area. The monitoring system using the BOTDR-based distributed fibre optic strain sensing system can be regarded as promising for underground mine monitoring.

Distributed Fiber Optic Strain Sensing with Embedded Small-Diameter Optical Fibers in CFRP Laminate

Although demand for composite structures rapidly increase due to the advantages in weight, there are few effective assessment techniques to enable the quality control and guarantee the durability. In particular, an invisible microscopic damage detection technology is highly required because damages such as transverse cracks, debondings, or delaminations can lead to the critical failure of the structures. Among many non-destructive evaluation (NDE) methods for composite structures, fiber optic sensors are especially attractive due to the high sensitivity, the lightweight, and the small size. In the current trend of the structural health monitoring technology, fiber Bragg gratings (FBG) sensors are frequently used as strain or temperature sensors, and Brillouin scattering sensors are also often used for a long distance distributed measurement. The Brillouin distributed sensors can measure strain over a distance of 10km while a spatial resolution was limited to 1m. Some novel sensing method is proposed to improve the spatial resolution. The pulse-prepump Brillouin optical time domain analysis (PPP-BOTDA) is one of the latest distributed sensing applications with a cm-order high spatial resolution. The PPP-BOTDA commercial product has the spatial resolution of 10cm, and can measure the strain with a precision of ±25og. This precision, however, can be achieved by using conventional single-mode optical fibers. In our research, small-diameter optical fibers with a cladding diameter of 40om were embedded in the CFRP laminate to avoid the deterioration of the CFRP mechanical properties. Thus, in order to verify the performance of PPP-BOTDA, the distributed strain measurement was conducted with the small-diameter optical fibers embedded in the CFRP laminate.

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.

Accurate high-resolution fiber-optic distributed strain measurements for structural health monitoring

The viability of fiber-optic frequency-domain Brillouin strain sensing for accurate high-resolution structural health monitoring, is demonstrated. High performances have been achieved by applying an iterative reconstruction algorithm which correctly takes into account the influence of the acoustic wave involved in Brillouin scattering. The quality of the reconstructions was confirmed by finite-element method (FEM) numerical simulations. A number of experiments with an aluminum beam under different load conditions are presented, demonstrating the potential benefits resulting from the use of SBS-based sensors in a wide range of structural health monitoring applications.

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%.

Application of correlation-based continuous-wave technique for fiber Brillouin sensing to measurement of strain distribution on a small size material

In this letter, we demonstrate a smart material using our technique utilizing correlation between pump-and-probe lightwaves for fiber optic distributed strain sensing with Brillouin scattering. This technique enables centimeter-order spatial resolution, which is 100 times higher than that in conventional pulsed-lightwave techniques. This letter describes verification of usefulness of the correlation-based continuous-wave technique for smart material applications. Strain distribution on the surface of a small ring is obtained with 1-cm spatial resolution.

Distributed strain sensing for structural monitoring applications

Strain sensors are a valuable tool for assessing the health of structures. The University of New Brunswick, in conjunction with ISIS Canada, is developing a distributed fibre optic strain sensor based on Brillouin scattering. This sensor can provide a virtually unlimited number of measurement points using a single optical fibre. A description of the operating principles of the system is given, along with a summary of laboratory test results. Strain measurement accuracy as high as approximately ±11 µε has been demonstrated at 1 m spatial resolution. Spatial resolutions as short as 100 mm can be used, although with decreased strain measurement accuracy. Future development of the technology will include an enhancement allowing both strain and temperature to be measured simultaneously.Key words: strain sensor, fibre optics, distributed sensing, structural monitoring, Brillouin scattering.

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.