Interaction of target molecules with the evanescent wave of light guided in optical fibers is among the most promising sensing schemes for building up smart chemical sensor technologies. If the technique of optical time domain reflectometry (OTDR) is combined with silicone-clad quartz glass fibers distributed chemical sensing is possible. Hydrocarbon (HC) detection and location is done by automated identification of the position of the corresponding step drop (light loss) in the backscatter signal induced by local refractive index increase in the silicone cladding due to a penetrating HC compound. A commercially available mini-OTDR was adapted to sensing fibers of up to nearly 2-kilometer length and location of typical HC fuels could be demonstrated. The instrument is applicable for fuel leakage monitoring in large technical installations such as tanks or pipelines with spatial resolution down to 1 m. A similar technique using measurements in the Vis spectral range is being developed for health monitoring of large structures, e.g., for early detection of corrosion caused by water ingress and pH changes in reinforced concrete. Here, a pH indicator dye and a phase transfer reagent are immobilized in the originally hydrophobic fiber cladding, leading to a pH induced absorption increase and a step drop signal in the backscatter curve. The configuration of the distributed sensing cables, the instrumental setups, and examples for HC and pH sensing are presented.

The strain and temperature dependence of the Brillouin loss spectra using Brillouin frequencies in a photonic crystal fiber with a partially Ge-doped core are studied for the first time. The spectrum of the photonic crystal fiber displays a multi-peak structure due to waveguide and antiwaveguide types of sound propagation. Our experiments demonstrate that the Brillouin frequency shift v_B depends on strain and temperature of the photonic crystal fiber. For two main peaks that are attributed to the scattering from longitudinal acoustic waves in the Ge-doped center region and solid pure silica region of the core of the photonic crystal fiber, the strain coefficients are 0.048 and 09.055 MHz/με and temperature coefficients are 0.96 and 1.25 MHz/°C at wavelength of 1320 nm. Combining the measurements of the strain dependence of Brillouin frequency shift with measurements of the temperature dependence of Brillouin frequency shift, we demonstrate a simultaneosly distributed strain and temperature sensor with a spatial resolution of 15 cm, a temperature resolution of 1.3°C, and a strain resolution of 15με along a single photonic crystal fiber.

The monitoring of temperature profiles over long distance by means of optical fibers represents a highly efficient way to perform leakage detection along pipelines, in dams, dykes, or tanks... Different techniques have been developed taking advantages of the fiber geometry and of optical time domain analysis for the localization of the information. Among fiber optics distributed temperature sensing techniques, Brillouin-based systems have demonstrated to have the best potential for applications over distances up to several tens of kilometers. The key features and performances are reviewed in the present article and a 55km pipeline equipped with a fiber optics leakage detection system is presented as a case study.

This paper describes the basic principle of an interferometric angle and length sensor and its application. The sensor is the main part of a compact calibration system that can be used to calibrate and test one-dimensional length measurement systems. It is built by integrating the sensor into a positioning stage and setting up a DSP-based hardware and software signal processing system. A PC-based frontend using Matlab allows for easy setup of user-specific scripts for numerous calibration tasks. The sensor's and system's capabilities have been demonstrated using a high-resolution autocollimator, an AFM and an inductive tactile probe as calibration samples. Calibrations are possible with step sizes ranging from 20pm up to ±6μm and several waveforms for dynamic calibrations at frequencies currently up to 2 kHz.

Long-guage interferometric SOFO sensors have been in use for the last 10 years for the monitoring of civil, geotechnical, oil & gas and other structures. The sensors are based on an all-fiber Mechelson interferometer with one arm mechanically coupled to the structure and one used as reference. The original demodulation system is based on low-coherence interferometry. The path-matching readout unit features excellent long-term stability and preceision, but is not suitable for dynamic measurements because the scanning mirror requires about 7 seconds to acquire a complete interferogram. In order to allow the use of the same sensors for the measurement of dynamic events, a new demodulator was developed. It is based on the coherent demodulation of a coherence-colloapsed laser source and is able to demodulate 8 SOFO sensors at frequencies of up to 1 kHz with resolutions of 0.01 microns. This system is particularly adapted for the analysis of dynamic structural deformations produced by traffic, wind, seismic and impacts. The use of long-gauge sensors allows a direct measurement of the dynamic strains and the calculation of modal strains and curvatures. This paper presents the optical setup of the SOFO dynamic demodulation system and discusses its performances through the presentation of laboratory and field tests.

The Compressed Natural Gas (CNG) used as a carburant in automotive industry offers low cost and notably less pollution. Full composite tank used to store the CNG onboard features low weight and extended lifespan. However, the safety issues and maintenance fees remain a challenge for its use in ordinary cars. The structural health monitoring of tanks with accent to damage detection can significantly increase the safety and decrease the maintenance fees. Structural health monitoring and damage detection of composite tanks impose important challenges to the monitoring strategy and monitoring system to be used. The issues of non-intrusive installation of sensors, their topologies and network, and particularly analysis and interpretation of resulting data are very complex. The long-gage interferometric sensors of SOFO type, for direct embedding in the full composite tank during production are developed. The sensor consists of single mode optical fiber embedded into the very thin composite tape. Such packaging offers to optical fiber excellent protection during handling and embedding and makes sensor non-intrusive to the tank material. Appropriate topologies of the sensors are combined in single sensor network used to monitor strain state and damage. The results of monitoring are analyzed at several levels, and the damage is detected using algorithms combining the global deformation and changes in both the tank stiffness and sensors cross-correlation. The monitoring strategy, sensors used in full composite tank monitoring, installation issues and the results of the structural health monitoring performed in laboratory are presented in details in this paper.

In a quest for fiber optic sensors that could monitor soil moisture, Blue Road Research implemented fiber Bragg grating sensors in such a way that they could detect humidity, soil moisture evaporation rates, and pressure changes from soil weight. These were then used to monitor soil in controlled flood tests to determine the moisture levels in a soil test bed. Fiber optic sensors seem well suited for humidity and soil moisture monitoring since they can easily be multiplexed with many sensors on one fiber line, and they have distinct longevity advantages that enable their use in applications involving wet environments, remote locations or long distances, electromagnetic interference, flammability, or other harsh environmental conditions that may degrade ordinary electronic sensors or their measurements. This paper describes the workings of a highly accurate optical humidity sensor that can be multiplexed on a fiber optic strand to monitor humidity that may lead to corrosion, soil moisture levels and changes, weather conditions, etc. as well as means to record such data.

A fiber optic-based irreversible moisture sensor that can be used to determine if a location has ever been wet during a given monitoring period is presented. The irreversible sensor offers a simple means to detect moisture when more complex dedicated continuous monitoring is not feasible and/or where traditional moisture sensors are not applicable, such as in confided spaces. The irreversible moisture sensor has three response states that indicate if an area is actively wet, has been wet but is currently dry, or has never been wet. The irreversible moisture sensor is based on long period grating fiber optic technology. The functionality and survivability of the sensor to exposure in typical atmospheric environments has been evaluated. Testing consisted of long-term exposure to low and high humidity levels (<10 to 97 % relative humidity), and exposure to freezing and elevated temperatures (-10 and 60 °C). The sensors were monitored periodically to examine their functionality. The sensors exhibited rapid irreversible response to water even after extended exposure to the simulated service conditions, and thus are suitable for long-term service in typical atmospheric environments. Potential applications for this sensor include remote, confined or difficult to access locations, aggressive environments where electrical-based sensors perform inadequately, or where approval for fielding instrumentation for continuous monitoring maybe prohibitive, such as on aircraft.

An engineering solution for development of smart composite materials is proposed. It is based on the implanting of piezoelectric sensors into the composite material during its manufacturing. Sweeping of sensor's input alternating voltage frequency enables tracking of sensors resonance. The resonance spectrum changes due to the changes of sensor's boundary conditions during cure of the composite material. Those changes are related to the degree of composite cure. Moreover, such design of composite constructions with implanted sensors enables monitoring of structural defects during explotation of those constructions. Numerical as well as experimental investigations validated the applicability of the described solidification monitoring methodology.

New methods for structural health monitoring are being assessed, especially in high-performance, extreme environment, safety-critical applications. One such application is for composite cryogenic fuel tanks. The work presented here attempts to characterize and investigate the feasibility of using imbedded piezoelectric sensors to detect cracks and delaminations under cryogenic and ambient conditions. A variety of damage detection methods and different sensors are employed in the different composite plate samples to aid in determining an optimal algorithm, sensor placement strategy, and type of imbedded sensor to use. Variations of frequency, impedance measurements, and pulse echoing techniques of the sensors are employed and compared. Statistical and analytic techniques are then used to determine which method is most desirable for a specific type of damage. These results are furthermore compared with previous work using externally mounted sensors. Results and optimized methods from this work can then be incorporated into a larger composite structure to validate and assess its structural health. This could prove to be important in the development and qualification of any 2nd generation reusable launch vehicle using composites as a structural element.

Health monitoring techniques that utilize structural materials with the ability to diagnose their own condition, so-called self-diagnosis materials, have been under development. The authors have developed two types of electrically conductive fiber reinforced composite to diagnose cracks in concrete structures: a high sensitivity detection sensor and maximum strain memory sensor. Three points bending tests on pre-notched reinforced concrete beam under the cyclic loading is presented using these two self-diagnosis materials, with attention towards the relationship between crack width of the concrete beam and electric resistance. Moreover, effects of volume fraction of carbon particle on memorizing maximum strain are investigated. It has been proved that both self-diagnosis materials are highly effective to detect the cracks in the concrete. And present strain can be obtained by the proposed fiber reinforced plastic composites. Although volume fraction of carbon particle has significant influence on the characteristics of memorizing maximum strain, maximum strain of the concrete structures can be memorized using the appropriate self-diagnosis materials.

Monitoring the structural condition of road and airport pavement is an extremely critical task to ensure the safety and efficiency of teh transportation. The topic is relevant to both civil and military transportation infrastructure. The presence of damage in pavement, including surface cracking, depressions, swells, and wear, is inevitable due to the sever environmental and service loads that these structures must be subject to. Existing NDE techniques aimed at assessing the structural condition of pavement include Falling Weight Deflectometer, Ground Penetrating Radar, and acoustic methods based on surface waves. This paper presents improvements to the traditional surface-wave method for the detection of surface-breaking cracks in pavement. The advances include 1) the modeling of the problem as dipsersive waves propagating in a multilayer system, 2) the inclusion of post-processing algorithms based on the Wavelet Transform to improve the sensitivity and accuracy of the inspection, and 3) the use of non-contact, air-coupled acoustic detectors to enhance the mobility of the inspection unit. The crack detection procedure consists of first generating a dispersive wave with an impulse hammer, and then measuring the changes in velocity, amplitude and/or frequency content as the wave travels across the flaw with the aid of the Continuous Wavelet Transform. Multilayer wave propagation modeling provides a better understanding of the experimental results by predicting how the various frequencies interact with cracks of different depths. The results of field tests will be presented for both rigid (concrete-based) and flexible (bitumen-based) pavement with surface cracks.

Magnetoelastic materials used as sensors are obtaining an increasing interest in the last few years. The magnetoelastic wave resonance frequency is the sensitive parameter generally measured to obtain key information on the magnetoelastic material; nevertheless, it was shown that also the amplitude of the magnetoelastic wave, stimulated at a frequency close to the sensor core resonance frequency (pseudo-resonance), is very sensitive to measure magnetic fields. By basing on that characteristic, both parameters, "pseudo-resonance" amplitude and resonance frequency, could be used as the "sensitive parameter" in no-contact vibration sensors or stress, flux and magnetic field sensors. In a previous paper the authors proposed a signal condition technique for magnetoelastic elements, and used it for no-contact vibration and displacement measurements. The measurements, even though had a good linearity and low distortion, comparable to that of traditional devices (LVDT or accelerometer), had a too poor repeatability, due to a time-varying behavior of the sensor magnetoelastic core. This paper proposes an On-Line self-Calibration technique for magnetoelastic sensors, OLC, that solves the repeatability problem by improving the sensor accuracy of about one order of magnitude. No external reference sensor is used, but just an easy to obtain reference stimulus. A tuning procedure is then shown, able to put the sensor into a repeatable, time-invariant status, by acting on a static polarization magnetic field. Finally, the proposed technique is applied for no-contact vibration measurements. Results are presented, comparing them with the case of no OLC applied.

The coupling phenomenon between stress and magnetic induction, known as magnetostriction, has been successfully applied to generate and measure elastic waves. Most applications of this phenomenon thus far, however, are rather limited to cylindrical ferromagnetic waveguides. The main objective of this work is to develop a new patch-type, orientation-adjustable magnetostrictive transducer that is applicable for non-cylindrical, non-ferromagnetic waveguides. The existing patch-type transducer consisting of a ferromagnetic patch and a racetrack coil is useful to generate elastic waves only in one specific direction once the patch is bonded to a test specimen. However, the proposed transducer can transmit and receive elastic waves in any direction only with one patch at a given location. The proposed magnetostrictive transducer consists of a circular nickel patch, a figure-of-eight coil, and a couple of bias permanent magnets. Because of the unique configuration of the transducer, the propagating direction of the generated waves can be freely controlled since the set of bias magnets and the coil is not bonded to the magnetostrictive patch. In this work, the characteristics of the proposed transducer were investigated experimentally

In order to maintain sustainability of infrastructures in which have significant influence to social lifelines, an economical and rational maintenance management system is absolutely needed. Maintenance management of infrastructures and reduction of their life cycle cost (LCC), become serious problems those have to be solved urgently. In this paper, Structural health monitoring based maintenance (SHMBM) aimed to reduce structure’s life cycle cost is proposed. Theoretical background of SHMBM and contribution of monitoring information in LCC reduction are reviewed. As the tools to retrieve the structural deformational properties information, the intelligent sensory principles and their sensory system are introduced such as GPS based MMS monitoring technology, peak memory sensory technology, elasto-magnetic sensory technology and fiber optical sensory technology. Their reliabilities were verified by some verification test and field application test. Some examples of SHMBM application in the monitoring of structural properties are also described as well.

We report the development of nanostructured strain sensors formed by electrostatic self-assembly (ESA) processing. The sensors may be used to measure strains from 1 microstrain to more than 100% strain, over gauge lengths ranging from approximately 1 millimeter to tens of centimeters.

Detecting and locating cracks in structural components and joints that have high feature densities is a challenging problem in the field of Structural Health Monitoring. There have been advances in piezoelectric sensors, actuators, wave propagation, MEMS, and optical fiber sensors. However, few sensor-signal processing techniques have been applied to the monitoring of joints and complex structural geometries. This is in part because maintaining and analyzing a large amount of data obtained from a large number of sensors that may be needed to monitor joints for cracks is difficult. Reliable low cost assessment of the health of structures is crucial to maintain operational availability and productivity, reduce maintenance cost, and prevent catastrophic failure of large structures such as wind turbines, aircraft, and civil infrastructure. Recently, there have also been advances in development of simple passive techniques for health monitoring including a technique based on mimicking the biological neural system using electronic logic circuits. This technique aids in reducing the required number of data acquisition channels by a factor of ten or more and is able to predict the location of a crack within a rectangular grid or within an arbitrarily arranged network of continuous sensors or neurons. The current paper shows results obtained by implementing this method on an aluminum plate and joint. The plates were tested using simulated acoustic emissions and also loading via an MTS machine. The testing indicates that the neural system can monitor complex joints and detect acoustic emissions due to propagating cracks. High sensitivity of the neural system is needed, and further sensor development and testing on different types of joints is required. Also indicated is that sensor geometry, sensor location, signal filtering, and logic parameters of the neural system will be specific to the particular type of joint (material, thickness, geometry) being monitored. Also, a novel piezoresistive carbon nanotube nerve crack sensor is presented that can become a neuron and respond to local crack growth.

Commercially used fiber optic measurement methods have different performances because of different physical princi-ples and different sensor designs. Environmental influences, type of application and operational conditions can cause perturbations in the sensor system, depending on the chosen measurement principle. Such perturbations lead to faulty measurements, and thus, the structural behavior can be misinterpreted. In order to ensure reliable measurements, not only the characteristics of the sensor system must be known, but also its ability for the specific use must be proven. This paper points out essential criteria for optimum choice of fiber optic sensor systems and demonstrates necessary steps to evaluate the long-term stability and reliability of a sensor system. The paper summarizes the conditions required for evaluation of a sensor system, and briefly touches on the correct use of vocabulary of general terms in fiber sensor measurement technique to describe the real performance unambiguously. On the way towards drafting guidelines and standards for an appropriate use of fiber sensors, firstly survey of important issues is presented. Two details of these are explained with the example of FBG strain sensors. Finally, when fiber sensors are to be installed on-site, the procedure of validation is presented to assure that the particular requirements for a specific intended use of fiber sensors are fulfilled.

Optical fiber Bragg gratings are unique among strain sensors due to their potential to measure strain distributions over gage lengths of a few centimeters with a spatial resolution of a few nanometers. The application of these sensors requires modeling of the grating output spectrum due to an applied axial strain profile. The most computationally efficient method for this calculation is the transfer-matrix model (T-matrix) derived originally for chirped gratings. This approach models a grating with varying properties as a series of smaller grating segments with constant parameters. Huang and colleagues first applied the T-matrix approach to model the inverse problem of a grating subjected to non-uniform strain by varying the period of each segment. The current work shows that, in the presence of strain gradients, this approach does not converge to the numerical solution of the grating coupled mode equations in the limit of a large number of segments. A modified T-matrix representation is then derived for the sensor problem and is shown to approach the coupled mode solutions for a large number of segments. Finally, the application of the modified T-matrix model to Bragg grating sensors is outlined, including inversion of the grating spectrum via a genetic algorithm.

We discuss the current status of fiber Bragg grating (FBG) sensor metrology. High-accuracy wavelength measurements are critical for FBG strain sensors, because wavelength measurement uncertainties even as small as 1 pm lead to an uncertainty of nearly 1 microstrain. We administered an FBG wavelength measurements round robin in which twelve participants measured the spectral reflectance of four FBGs; we found that the measured peak wavelengths of a single FBG varied by as much as 35 pm. We have determined that this variation results from two factors: the uncertainty of the measurement systems and the wavelength instability of the athermally packaged FBGs. We found that the wavelengths of athermally packaged gratings will drift with time and can undergo large jumps and that it is difficult to achieve stability better than 4 pm/year. We discuss the measurement uncertainties for several common wavelength measurement systems, including tunable laser, optical spectrum analyzer (OSA), and interferometric measurements. We show that when using an OSA, as many of the round-robin participants did, it is difficult to achieve a measurement uncertainty better than 10 pm, and if the OSA is not accurately calibrated to a known wavelength reference; then wavelength measurement uncertainty can be as large as 1 nm. Wavelength references that are based on molecular absorption lines are ideal for calibrating OSAs, but they are not optimal for calibrating FBG interrogation units where a reference based on reflection lines rather than absorption lines is often preferred. We discuss other possible candidates for wavelength references for FBG sensor interrogation units.

As the strain sensing element of structural health monitoring, the study and application of the fibre-optic Bragg grating (FBG) has been widely accepted. The accuracy of the FBG sensor is highly dependent on the physical and mechanical properties among the bare optical fibre, protective coating, adhesive layer and host material, namely, the strain interface transferring characteristics among layers. This paper takes the embedded FBG sensor as an object of study. Firstly, the general expression of multiplayer interface strain transferring mechanism is derived. Secondly, based on the defined average strain, the error modified equation of FBG sensor is obtained. Finally, in the light of embedded tube-packaged FBG and FRP-OFBG strain sensors, which developed by ourself, the corresponding strain transferring error, laws were studied, and the corresponding error modification coefficients were given too, which were validated by experiments. The achievements provide theories for the development and application of the embedded FBG sensors.

Optical fiber sensors have the capability of sensing environmental factors, including strain and temperature. In particular, optical fiber Bragg gratings have been used to create multi-parameter sensors capable of measuring axial and transverse strain in addition to temperature. These measurements are made by writing gratings into the optical fibers at two separated wavelengths, 1300 and 1550 nm, and monitoring the polarization sensitive output from the sensor. While there is an analytical model for determining the strain on a homogeneous cylinder under transverse load, these fiber optic sensors are not homogeneous as they consist of distinct regions within the fiber: core, cladding, and stress rods. We measure the strain on a multi-parameter fiber Bragg grating written at 1550nm under transverse load at 0, 15, 30, 45, 60, 75, and 90° and compare these values with an analytical solution accounting for internal stresses and transverse load.

The research presented in this paper demonstrates the feasibility of detecting deformations of wing-like structures (cantilever plates) using a distributed, multiplexed fiber Bragg grating (FBG) sensor system. Overall, the work accomplished during the research effort demonstrates the ability of a multiplexed network of FBG sensors to measure strain across a structure and have those strains determine the deformation or shape of the structure. A neural network approach and a structural mechanics approach were examined to determine the deformation of the cantilever plate based on the recorded strain readings from the FBG sensors. This paper presents analytical and experimental results for both approaches. The experimental setup consisted of a cantilever plate with 36 FBG sensors mounted in longitudinal, transverse, and off-axis orientations on the plate’s surface. The neural network method performed best when trained with experimentally measured strain and deflection data. The structural mechanics approach demonstrated more accurate comparisons to the ground truth shape, taken with a laser range finder, than the neural network approach. In all experimental cases using the structural mechanics approach, the root-mean-square error between the ground-truth measurements and the FBG sensor based measurements were less than 0.52 inches (approximately 10% of maximum deflection). In approximately 30% of the test cases, the root-mean-square error was less than 0.2 inches.

An overall strategy in infrastructure health monitoring systems is given through the concept of Infrastructure Optics. The focus is to design and build optical devices and systems, primarily fiber optic communication technology, for health monitoring of infrastructure. Recent developments in the use of Optical Frequency Domain Reflectometry (OFDR) to demodulate Fiber Bragg Grating Arrays (FBGA) have shown promise in its use in infrastructure health monitoring systems. However, the number of papers on the simulation and characteristics of FBGA using OFDR demodulation for health monitoring purposes is not great. In this paper, a FBGA is simulated using OFDR demodulation technique to extract strain information from a simulated cantilever beam host. The structure is first simulated using a Finite Element Model (FEM) to determine displacement and strain response, and the results are used as inputs to the FBGA. An OFDR program then demodulates the array to extract the strain response of the cantilever beam. The characteristics of OFDR and FBGA system is analyzed and compared to actual FEM results.

It is desirable to make dynamic strain measurements in multiple directions at a single point. In particular this technology could be useful in monitoring multidimensional deflections of ship hulls measuring both axial and transverse strain fields. Conventional strain gages are unable to deliver this type of performance due to the size requirements and lack of available technology. Fiber grating sensors have been shown to deliver accurate strain measurements in two orthogonal directions, but have been limited by the speed at which the signal can be demodulated. As it is desirable to make multi-directional dynamic measurements, a system is required that can deliver a high speed demodulation of the signal from a dual-axis fiber grating sensor. Using available optical components a system was developed to read out orthogonal directions of strain at a point at up to 100 kHz. The resolution in one direction is +/- 40 microstrain and in the other +/- 180 microstrain. The system was initially tested over 4000 microstrain in tension along the higher sensitivity axis.

We study a new sensing system for health monitoring of aircraft structures made of composite materials. This sensing system is composed of fiber Bragg grating (FBG) sensors and a piezoelectric transducer (PZT). The FBG sensors receive elastic wave generated by the PZT. For high-frequency vibration monitoring, or acoustic emission (AE) detection, we have developed high-speed optical wavelength interrogator for FBG sensors. In this FBG interrogator, we used an optical filter which converts wavelength shift of the reflected light from the FBG into the output optical power change. This system is suitable for high-speed wavelength detection because there is no mechanical moving part. We studied two types of optical filter. One is a Mach-Zender interferometer and the other is an arrayed waveguide grating (AWG). Both of them were fabricated using silica-based planar lightwave circuit (PLC) technology. The optical filters based on the PLC technology have the advantage in integration of optical components. By combining the FBG interrogator using the AWG optical filter with the PZT actuator, we succeeded to detect elastic wave propagating in the CFRP laminated plate. As a result, we found that this FBG/PZT hybrid sensing system is very promising for detection of internal defects in composite materials.

Fiber Bragg grating sensors are one of many fiber optic sensor technologies that are currently being used in structural health monitoring systems. The sensors operate by detecting shif in the wavelength of relfected maxima due to applied strain. This paper studies a new fiber Bragg interrogation method that utilizes a swept wavelength laser in combination with wavelength references. These include a gas cell, which is used as the long term wavelength standard and an etalon used for accurate interpolation of peak wavelengths. An etalon is essentially a filter that has a periodic response over a broad wavelength range. Since its wavelength response spacing is smaller than the gas cell, it can be used to determine the intermediate wavelengths between two gas cell absorption lines. Peak location is a key element of this interogation method and several detection algorithms are investigated. It was determined that polynomial peak fitting is the most computationally efficient method and yields a resolution of better than 0.5 pm with signal to noise ratios of 30:1 or better. With higher signal to noise ratios, polynomial peak fitting can yield a resolution of better than 0.25 pm and a resolution of bettern than 0.25 pm. Using a tunable laser, a HCN gas cell and an etalon with maxima every 140 pm, static load tests will demonstrate that a resolution of 1 pm and an accuracy of less than 5pm can be achieved. Also, this accuracy will be maintained over a long period of time as it is based on absorption lines in the gas cell. The results of this study demonstrate that absolute accurate strain measurements can be obtained with the use of wavelength references in conjunction with a suitable peak location algorithm.

We report the use of a fiber-optic distributed sensing system to monitor temperature at a multitude of discrete points on an industrial motor undergoing qualification after a rewinding. This technique involves using optical frequency domain reflectometry to demodulate the reflected signal from multiplexed Bragg gratings that have been photoetched in the core of an optical fiber. In this work, high-resolution optical sensing fiber was applied along the stator windings and end-windings of the motor to assess their suitability for long-term temperature monitoring. Performance tests were conducted at different heat loads representing different electrical conditions. Results indicate excellent agreement with collocated Resistance Temperature Devices (RTDs) and demonstrate significant potential for mitigating costly motor failure due to insulation breakdown resulting from highly localized hotspots.

The consolidation of thermoplastic composites produces internal residual strains due to the differences between the coefficients of thermal expansion of the component materials. In the case of AS4/PPS (carbon fibre-polyphenylene sulphide), where the melting/solidification temperature is 280°C, there exists a 255°C range wherein the various constituents will contract/expand to different degrees. A fibre Bragg grating (FBG) sensor may be embedded into this laminate with the goal of characterizing the residual strains; however, these strains may be non-uniform in the longitudinal and transverse directions, and may also vary depending on the laminate architecture. Non-uniform axial strains typically broaden and split the FBG sensor's spectral response, making it difficult to measure the strain distribution. Also, load-induced birefringence caused by the consolidation process will complicate the interpretation of the spectral response. This research is directed at understanding the residual strain state in FBG sensors due to the fabrication process. It is the aim of this study to experimentally investigate the residual strains in long and short gauge length FBG sensors embedded in the 0° plies of AS4/PPS unidirectional and cross-ply laminates (200 x 50 x 3.6 mm). Long gauge length sensors are monitored throughout the fabrication process, to provide insight into the development of the residual strains.

Impact location monitoring is one of the major concerns of the smart health monitoring. For this application, multipoint ultrasonic sensors are to be employed. In this study, a multiplexed FBG sensor system with wide dynamic range was proposed and stabilization controlling system was also developed for the maintenance of maximum sensitivity of sensors. For the intensity demodulation system of FBG sensors, Fabry-Perot tunable filter (FP-TF) with 23.8 nm FSR (free spectral range) was used, which behaves as two separate filters between 1530 ~ 1560 nm range. Two FBG sensors were attached on the bottom side of the graphite/epoxy composite beam specimen, and low velocity impact tests were performed to detect the one-dimensional impact locations. Impact locations were calculated by the arrival time differences of the impact longitudinal waves acquired by the two FBGs. As a result, multiplexed in-line FBG sensors could detect the moment of impact precisely and found the impact locations with the average error of 1.32 mm.

Life cycle monitoring of civil infrastructure such as bridges and buildings is critical to the long-term operational cost and safety of aging structures. The widespread use of Structural Health Monitoring (SHM) systems is limited due to unavailability of specialized data acquisition equipment, high cost of generic equipment, and absence of fully automatic decision support systems. The goals of the presented project include: first, design of a Wireless Intelligent Sensor and Actuator Network (WISAN) and creation of an inexpensive set of instrumentation for the tasks of structural health monitoring; second, development of a SHM method, which is suitable for autonomous structural health monitoring. The design of the wireless sensor network is aimed at applications of structural health monitoring, addressing the issues of achieving a low cost per sensor, higher reliability, sources of energy for the network nodes, energy-efficient distribution of the computational load, security and coexistence in the ISM radio bands. The practical applicability of the sensor network is increased through utilization of computational intelligence and support of signal generation capabilities. The automated SHM method is based on the method of modal strain energy, though other SHM methods will be supported as well. The automation tasks include automation of the modal identification through ambient vibrations, classification of the acquired mode shapes, and automatic evaluation of the structural health.

Various structural materials have been used in construction projects using stones, concretes, and steels materials. Among of these projects, concretes may use widely because concretes have high compressive strength, and comparatively easy maintenance and management. Reinforced concrete Buildings will be deteriorated as time passed. These problems will be accelerated by propagation of cracks. In order to manage such cracks, time, efforts and expense are required. In this study, leakages of fluorescence and adhesive material were investigated using glass sensors that were embedded in a model beam and column. In addition, currents in glass pipe sensor were observed to find leakage of liquid in glass pipes. Progressive cracks were generated by fracture of glass pipe sensor. In this investigation, a reinforcement clothing system was wrapped for a glass pipe sensor. The glass pipe sensor that can make control and reinforce cracks simultaneously.

Structural fatigue life prediction, especially for the structures in service, is still a world-wide big problem, which is the safety evaluation focus of structural health monitoring system. Generally speaking, by now we have no choice but to rely on the strain-time course to give statistical prediction of structural fatigue life. In this paper, a new kind of structural fatigue life prediction smart sensor, named smart fatigue life gauge, is developed. And its effect of resistance accumulation and statistical expression are studied. Then, the solution of fatigue damage life prediction based on smart fatigue life gauge is given. Results show that the smart fatigue life gauge is prominent in the field of structural health monitoring system.

The morphological characteristics such as surface area, thickness, and roughness of the Polypyrrole (PPy) on metal electrodes of electrochemical sensors affect sensitivity and selectivity of electrochemical sensors because the surface morphology of PPy affects reduction and oxidation of the electrodes. The PPy morphology was investigated by changing several polymerization parameters such as polymerization time, electrolytes, and metal electrodes. Indium tin oxide (ITO) or Au was used as a seed metal electrode on which PPy was electropolymerized. The electropolymerization was performed in an electrolyte, either sodium dodecyl sulfate (NaDS) or sodium dodecyl benzene sulfonate (NaDBS). A homogeneous PPy morphology was obtained on ITO electrode when NaDBS solution was used as an electrolyte, but when NaDS solution was used for polymerizing on ITO electrode, the electropolymerized PPy showed uniform surface morphology and film thickness up to 1 min polymerization time while further polymerization resulted in non-uniform networks like a spider web. This increases the detection surface area with identical metal electrodes of electrochemical sensors. When Au was used, neither NaDBS nor NaDS solution resulted in any morphology change of the PPy film regardless of the polymerization time. The effect of the PPy morphology on the electrochemical sensors will also be presented.

In this paper we describe and compare two methods for detecting Lamb waves used for damage detection. One is a polarimetric sensor that integrates the pressure effects of the acoustic wave along its length, whilst the other is the Bragg grating that measures linear strain and is in effect a point sensor. Both sensors are highly directional, which gives benefits in damage signal detection and source location. We demonstrate both the detection and location of damage and discuss the advantages and disadvantages of the two sensors.

This paper describes an all optical non-contact technique to determine material properties in a mechanical structure. Using optical generation of ultrasound and interferometric optical detection to determine the dispersion curves for acoustic (Lamb) waves, readily available numerical algorithms have produced values of Young's Modulus and Poisson ratio with confidence limits of a few percent. The approach may be applied to inspection regions with dimension in the range microns to centimetres.