Structural Monitoring with Fiber Optic Technology

Abstract. With micro-strain resolution and the capability to sample at rates of 100 Hz and higher, fiber optic (FO) strain sensors offer exciting new possibilities for in-situ landslide monitoring. Here we describe a new FO monitoring system based on long-gauge fiber Bragg grating sensors installed at the Randa Rockslide Laboratory in southern Switzerland. The new FO monitoring system can detect sub-micrometer scale deformations in both triggered-dynamic and continuous measurements. Two types of sensors have been installed: (1) fully embedded borehole sensors and (2) surface extensometers. Dynamic measurements are triggered by sensor deformation and recorded at 100 Hz, while continuous data are logged every 5 min. Deformation time series for all sensors show displacements consistent with previous monitoring. Accelerated shortening following installation of the borehole sensors is likely related to long-term shrinkage of the grout. A number of transient signals have been observed, which in some cases were large enough to trigger rapid sampling. The combination of short- and long-term observation offers new insight into the deformation process. Accelerated surface crack opening in spring is shown to have a diurnal trend, which we attribute to the effect of snowmelt seeping into the crack void space and freezing at night to generate pressure on the crack walls. Controlled-source tests investigated the sensor response to dynamic inputs, which compared an independent measure of ground motion against the strain measured across a surface crack. Low frequency signals were comparable but the FO record suffered from aliasing, where undersampling of higher frequency signals generated spectral peaks not related to ground motion.

This is an ambitious book aimed at introducing the relatively new concepts and possibilities of optical fibre sensors for structural monitoring to the uninitiated who have an engineering or general physics background.Measures draws the reader into the volume with a description of smart structures - the structural monitoring equivalent of artificial nervous systems - before a series of back-to-basics tutorials on optical theory and photonic technology.The emphasis on smart structures early in the book is a worthy attention-grabber since it elevates the subject of structural health monitoring above just another set of techniques for making engineering measurements. The promise is to `revolutionize engineering design philosophy' by creating `intelligence within otherwise inanimate structures'.In the latter two thirds of the book, the author steps through the main issues of structural monitoring using fibre optic sensors. Intensity-based, interferometric, polarimetric and spectral sensors (including the ubiquitous Bragg grating) are compared and contrasted. The hot topic of strain versus temperature discrimination in fibre sensors earns a whole chapter and several useful techniques for overcoming this cross-sensitivity are portrayed.Installation of sensors is also discussed with reference to retro-fit and co-manufacturing (embedding) approaches. Examples of concrete constructions such as bridges (a frequent theme in the book) and fibre-reinforced plastics such as glass-fibre and carbon composite materials are considered.A chapter on `short-gauge' sensors and applications deals in some depth with the Bragg grating as a strain sensor. The methods of multiplexing and interrogating these devices are explored with many examples from both Measures' own research and the work of other groups worldwide. The Beddington Trail bridge trial in Calgary, one of the first such installations of Bragg gratings, followed by the more ambitious Confederation Bridge, also in Canada, provide concrete examples of the technology's application.The material is marred somewhat by the inferior reproduction of some of the photographs, especially those showing field installations of the optical sensors.Other applications are not neglected. A description of trials aboard a Norwegian Naval vessel with composite hull monitored by Bragg gratings is also given.Interferometric sensors in similar applications trials are also covered in chapters on short and long gauge length devices.Distributed strain and temperature sensing techniques using Fourier transform, low coherence and stimulated backscattering are covered in the penultimate chapter, which draws together distributed measurementat a small physical scale in the form of intra-Bragg grating strain profile measurements (on the scale of millimetres)and measurements over kilometres using stimulated Brillouin scattering.In this reviewer's opinion the book dwells on strain monitoring in civil engineering structures at the expense of a broader scope, which could have included, for example, the detection of impacts or the acoustic emissions from crack propagation and other forms of structural damage.Nevertheless, this volume is an impressive collection of background and examples of real applications in heavyweight engineering. It adds significantly to the claim that fibre optic sensors have at last arrived.Peter Foote

Streicker Bridge is a pedestrian bridge on the Princeton University campus. It consists of a main span and four curved continuous girders (legs). The main span and the southeast leg of the bridge are equipped with fiber optic strain and temperature sensors, allowing the bridge to also function as an on-campus laboratory for the Structural Health Monitoring research group. Parallel sensors were embedded at critical cross-sections in the deck prior to the pouring of concrete. The deck of the southeast leg experienced early age cracking within a few days of concrete pouring, which was detected by the strain sensors. Post-tensioning was then performed and it is assumed that it closed off the cracks. Evaluation of post-tensioning forces is complex due to the existence of the cracks, and this paper researches a procedure to estimate the post-tensioning forces at cracked and uncracked locations. The obtained post-tensioning forces were compared to design forces and conclusions regarding the status of post-tensioning were made. This is important as it gives information on the actual health condition and performance of the structure. It also provides information on the safety of the structure. The objective of this paper is to present a methodology for the evaluation of the post-tensioning force along the deck based on strain measurements. The monitoring system, results, data analysis method, and conclusions regarding the bridge health condition and performance are presented in this paper.

Polymer modified concrete repair materials (PMCRMs) offer significant protection to concrete reinforcement, if the PMCRM is compatible with the concrete substrate, and the instructions given by the manufacturer are followed. However, if the PMCRM is not compatible with the concrete substrate, or the manufacturer’s instructions are not followed, volume changes within concrete repair materials (CRMs) caused by shrinkage may result in the repair system failing by means of delamination or cracking. This paper demonstrates the use of an optical fibre strain and temperature sensor to provide valuable information regarding the movement and internal temperature changes which occur within concrete repairs, which may help extend the service life and provide a better understanding of PMCRMs. A strain measuring (fibre Bragg grating) FBG sensor was implemented to measure strain within a polymer modified concrete repair patch (PMCRP). The optical fibre monitoring system consisted of an internal temperature measuring FBG sensor and a strain measuring FBG sensor, which monitored the temperature and movement changes within a PMCRP. The device allowed for easy installation of the strain measuring FBG, and allowed for the optical fibre sensor to be reused, thus it was possible to measure both the internal temperature and the movement of the PMCRP soon after placement of the repair material.

In the present study, real-time measurement of internal and residual strains in GFRP laminates with an optical fiber strain sensor was performed in an autoclave molding. An Extrinsic Fabry-Perot Interferometric (EFPI) type optical fiber strain sensor was embedded into prepreg sheets before the molding. The ion viscosity and the temperature of the epoxy resin matrix were also measured simultaneously with the internal strain measurement. The results of the experiments showed that the internal and residual strain of GFRP laminates during the molding could be monitored by the optical fiber sensor embedded in the parallel direction to the reinforcing fibers. It was also found that the circumstances around the embedded optical fiber strain sensor should be taken into account for the interpretation of the measured internal and residual strain.

Optical fiber sensors have been extensively adapted as structural health monitoring devices. Due to the existence of the adhesive layer, a portion of the strain is absorbed by the adhesive. As a result, the structural strain sensed by the optical fiber is underestimated and required to be corrected. An analytical solution is presented through which it is possible to establish the relationship between the strains in the host structure and the surface bonded optical fiber sensor. Experimental measurements based on the Mach–Zehnder interferometric technique were performed to validate the theoretical prediction and reveal the differential strains between the optical fiber strain sensor and test specimen. Parametric studies show that the percentage of the strain in the test specimen actually transferred to the optical fiber is dependent on the bonding length of the optical fiber and the adhesive. The strain transfer is increasing from 56% to 82% as the bonding length increases from 5 cm to 12 cm with the epoxy adhesive. The general trend of the strain transfer obtained from both experimental tests and theoretical predictions shows that the longer the bonding length and the stiffer the adhesive, the more strain is transferred to the optical fiber.

Wet deck slamming experiments with a FRP sandwich panel using a network of 16 fibre optic Bragg grating strain sensors

This paper presents the findings of a project sponsored by the BP Technology Innovation Board with the objective of developing a technology for continuously monitoring casing and completion mechanical deformation in intelligent wells. The innovation idea: " a permanent strain-sensing system in intelligent wells to continuously monitor the axial, lateral and cross-sectional deformation of casing and completion, with the desire to have the combined features of gyroscope, multifinger caliper, radioactive logs, and conventional mechanical deformation monitoring systems" was proposed to and funded for research by the BP Technology Innovation Board in 2000. The preliminary development evaluation was completed in late 2002. Production from soft reservoirs can suffer significant compaction resulting in casing and completion damage. This often leads to well failures with the consequent production loss and costs of replacement or repair. Current reservoir compaction and wellbore deformation monitoring technologies suffer from several drawbacks. A new technology is needed that combines characteristics of long distance profile sensing, 2 to 3 feet spatial resolution, the ability to measure large deformation (at least 2% strain), and multiple deformation modes. Based on a review of available sensing technologies, Sensornet's DTSS (simultaneous Distributed Temperature and Strain Sensing) technology18,19,20 was selected for the evaluation as it appeared to meet some special sensing requirements of this application. A large scale mechanical test (15 feet long, 7 inch OD, P110 casing deployed with both fiber optic sensors and conventional strain gauge rosettes) was designed and executed with the aim of evaluating:the capability and technical limit of the DTSS technology for large and sophisticated strain profile sensing;the effectiveness of load transfer; andthe reliability of the designed downhole sensing system. The experiments produced some very interesting results:It was for the first time, to our knowledge, that pure mechanical compressional strain was directly measured (without the need for pre-tensioning the fiber) by a fiber sensor using Brillouin scattering;For the first time temperature and sophisticated deformation modes were simultaneously and independently measured in a single fiber;The DTSS system measured strain up to the breaking point of the casing testing system (2.83% stretch), which is the largest strain ever published in the literature measured by fiber sensor. The experiments also helped to outline further work needed for adopting this technology for downhole applications. Introduction Production from soft reservoirs, especially those that are over pressured and thick, can cause significant amounts of reservoir compaction, leading to surface subsidence, overburden deformation, casing damage (offset, bending, crushing/ovalization, and restricted entry, see Figure 1), and well failures (lost access to the well due to severe casing damage such as severing of casing in the overburden, casing and completion buckling/collapse in the producing intervals resulting in completion failure such as loss of sand control integrity, and intolerable annular pressure build up due to loss of seal integrity, see Figure 2)1–6.

Optical fiber strain and temperature sensor based on an in-line Mach–Zehnder interferometer using thin-core fiber

A novel fiber optic distributed temperature and strain sensor for building applications

A review of our recent work on ultrahigh resolution optical fiber sensors in the quasi-static region is presented, and their applications in crustal deformation measurement are introduced. Geophysical research such as studies on earthquake and volcano requires monitoring the earth’s crustal deformation continuously with a strain resolution on the order of nano-strains (nɛ) in static to low frequency region. Optical fiber sensors are very attractive due to their unique advantages such as low cost, small size, and easy deployment. However, the resolution of conventional optical fiber strain sensors is far from satisfactory in the quasi-static domain. In this paper, several types of recently developed fiber-optic sensors with ultrahigh resolution in the quasi-static domain are introduced, including a fiber Bragg grating (FBG) sensor interrogated with a narrow linewidth tunable laser, an FBG based fiber Fabry-Perot interferometer (FFPI) sensor by using a phase modulation technique, and an FFPI sensor with a sideband interrogation technique. Quantificational analyses and field experimental results demonstrated that the FBG sensor can provide nano-order strain resolution. The sub-nano strain resolution was also achieved by the FFPI sensors in laboratory. Above achievements provide the basis to develop powerful fiber-optic tools for geophysical research on crustal deformation monitoring.

Intensity-modulated optical fibre strain sensor for continuous measurements of below-the-surface food deformation during drying

A novel strain sensor based on polarisation-maintaining optical fibre and FMCW laser diode technology is described. The sensor consists of a l00m length of fibre, a frequency-modulated laser diode light source and a PIN photodiode detector. The principle of operation relies on that facts that the optical frequency of the laser light is modulated by approximately 1GHz during a current modulated cycle of 1ms, and that the fibre has a twist inserted to induce coupling between the eigenmodes HEx 11 or HEy 11 due to circular birefringence. By selectively launching the modulated laser light into one of the eigenmodes and retaining the other for detection of coupled modes, the length of fibre from the launch end to the twist was rendered insensitive to changes in environmental conditions such as temperature. The system was shown to have a sensitivity of approximately 2πrad per 1000 microstrain with a resolution of 2 microstrain within a dynamic range of 5000 microstrain.

The field of communications has been revolutionized by the advent of optical fiber. Optical fiber now connects most of the world carrying a vast amount of information through a very limited physical medium. To the materials engineer, optical fiber technology has stimulated interest in a new type of micro-sensor application, where the size, weight, and the ability to integrate the sensor into a material structure play a major role. Areas of interest currently include materials characterization, cure monitoring, and structural health monitoring. The fiber Bragg grating (FBG) is such a sensor technology, creating an optical strain gauge within the core of an optical fiber through the use of a wavelength specific filter. As the FBG experiences induced strain along its major axis, its light signal indicates the amount of strain with great accuracy and sensitivity. In FBG applications, where loading may occur in all directions, complex changes take place in the FBG signal. As these changes impact the usefulness of the FBG as a strain sensor, this thesis endeavors to study and demonstrate the effect of transverse load on the FBG structure. A review of the basics of optical fiber fabrication, light transmission, and optical fiber sensor technology is presented first, followed by a detailed discussion of the FBG's manufacturing and operation. The crux of the thesis is presented with the results of experiments conducted to characterize the FBG signal in both unloaded and transversely loaded configurations. Polarization and wavelength based experiments and analyses are conducted to detect changes in the FBG signal. The results of the study are used to propose cause and effect relationships between transverse loading, its effect on the material properties of the FBG, and changes in the FBG signal.

This article explores the prospects of using spatially distributed optical fiber temperature sensors based on Rayleigh optical frequency domain reflectometry (OFDR) technology in the continuous casting of molten steel. The measurement capability of the optical fiber sensors in a simulated steelmaking environment was demonstrated using a mock refractory-lined tundish, which was fabricated in-house. Single-mode optical fibers, contained in protective stainless-steel tubing, were embedded in the refractory lining of the mock tundish. The instrumented tundish was preheated up to a temperature of 960 °C (recorded at the surface of the working lining) before the molten steel pour. A low-alloy steel (AISI 4140 grade) was melted in a 200 lb (90.7 kg) coreless induction furnace and was poured into the instrumented preheated tundish. The embedded optical fiber sensors were used to measure the temperature distribution in the castable lining during the preheating process of the tundish and during its contact with molten steel. Temperatures were metered with a spatial resolution of 0.65 mm along the embedded optical fibers. The measurements were recorded with an update rate of 1 Hz. The maximum temperature recorded by the optical fiber sensors in the castable lining after the steel pour was <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$591^{\circ }\text{C}$ </tex-math></inline-formula> . The spatially continuous optical fiber sensors provided useful information on thermal gradients in the castable lining. Real-time monitoring of spatially distributed thermal profiles in the refractory lining can improve superheat control, reduce energy losses, detect refractory wear, and improve the quality of the cast.