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Industrial effluent is largely being used for irrigation of crops. Hence, monitoring its pollution level is essential to check its suitability for the purpose. We propose a simple and highly sensitive Fiber Bragg grating (FBG) sensor to determine the concentration of Chloride (Cl) and Lead (Pb) ions present in the treated waste water effluent of sugar factories. It is based on the principle of shift in the Bragg wavelength of an etched FBG with change in the ambient refractive index. The etched FBG was calibrated by known concentration of solutions containing the element under study and then was used to quantify the element in the waste effluent. Proper reagents were used for selective determination of the element. The concentrations of Cl and Pb ions in the effluent sample determined by our designed FBG sensor are 95.73 ppm and 0.008 ppm respectively which are in good agreement with the results obtained by APHA (American Public health association) standard techniques employed for wastewater analysis. Sensitivities of Cl and Pb sensors are calculated as 0.76 pm/ppm and 38.6 pm/ppm respectively. Sensitivity of the sensor for Pb ions being about 50 times greater than that for Cl ions, suggests a higher sensitivity of sensor in lower range of concentration in ppm. Limit of detection of our sensor for Cl and Pb are 1.2 ppm and 0.003 ppm respectively which are lower than that of APHA techniques, suggesting the superiority of our sensors. Simple working, miniaturization, cost-effectiveness and ability of online measurement are the most important features of FBG chemical sensors that give them an edge over existing techniques for continuous effluent monitoring. [ABSTRACT FROM AUTHOR]

Relative Humidity (RH) sensors based on Fiber Bragg Grating (FBG) technology have been fabricated by polymer functionalization, using hygroscopic material, namely agar. The sensor is specifically intended to monitor moisture in stones of cultural heritage buildings and monuments, in particular to be positioned under hydrophobic coatings used for protecting stones from absorption of water and dissolved chemicals, thus providing early warning of coating deterioration by RH monitoring. The performance of the prototype was first tested exposing the sensors in controlled environment and then assessed for the application simulating “in field” operating conditions, with sensors installed on marble and travertine tiles. Results show that the proposed sensor is able to monitor the presence of moisture inside the stones. [ABSTRACT FROM AUTHOR]

To apply Fiber Bragg Grating (FBG) sensors in the hole structural health monitoring of aircraft, this study investigates a temperature compensation algorithm based on Fourier curve-fitting, combined with an analysis of FBG layout. The results identify that the wavelengths have a linear relationship with stress and temperature and the linear correlation coefficients are all greater than 0.9. In the experiment, the temperature range was 17 °C–100 °C (step is 10 °C) and the tensile force changed from 0 N to 10 KN (step is 1 KN). With the Fourier curve-fitting of the wavelength variation, the temperature compensation can be actualized, and the average error of temperature is 0.17 °C. By integrating the algorithm in the self-developed FBG interrogation system, the real-time monitoring of stress is achieved with the average error of stress is 3.9 μϵ. [ABSTRACT FROM AUTHOR]

Once a geothermal site has been selected for exploration, a full understanding of the formation temperature distribution on the surface and in the underground reservoir is always needed to assess the geothermal capability and productivity. Commercially available electronic measuring instruments cannot overcome the highest downhole temperatures obtained in the world’s deepest and hottest geothermal reservoirs. In this study, fiber optic Bragg grating (FBG) measurement technology is utilized applied in an attempt to replace more expensive electronic sensors and to obtain more accurate downhole pressure and temperature data. This has been our target since 2013. For this purpose, a portable pressure and temperature (P/T) optic fiber wire-line instrument prototype was developed, and it can measure up to 1500 m in depth. The prototype was named SINOTECH-OPTIC-P/T in 2015. It has been tested in various well types, including a hot spring well and a pioneering geothermal exploration well (JY-01). The primary results have proved to be satisfactory. This paper summarizes the temperature and pressure measurement results obtained in JY-01 well at Sanshing geothermal site in I-lan County, Taiwan. The maximum measured well depth was 1200 m, and data measured includes a downhole temperature profile with the maximum temperature of around 75 °C. [ABSTRACT FROM AUTHOR]

Glass-fiber reinforced polymer (GFRP) composite materials have an undisputed dominance over conventional metallic materials. However, susceptibility to barely visible or invisible internal damage due to impact has increased the demand for these composite materials in robust real-time structural health monitoring (SHM) system since they are capable of localizing the source of impact. Thus, in this paper, an in situ FBG sensor was embedded in a GFRP beam, providing an online real-time monitoring system and with the knowledge of cross-correlation linear source location (CC-LSL) algorithm, the impact location was capable of being determined in a split second. The consistency of cross-correlation function in providing repeatable results for all trials estimated a consistent time difference for all the impact points. The CC-LSL algorithm also revealed that the highest percentage of error was only 4.21% away from the actual hit. In the meantime, FBGs also showed good results as a dynamic strain measuring device in capturing frequency response at certain orientations compared to the AE sensor. [ABSTRACT FROM AUTHOR]

A combined stress-vibration sensor was developed to measure stress and vibration simultaneously based on fiber Bragg grating (FBG) technology. The sensor is composed of two FBGs and a stainless steel plate with a special design. The two FBGs sense vibration and stress and the sensor can realize temperature compensation by itself. The stainless steel plate can significantly increase sensitivity of vibration measurement. Theoretical analysis and Finite Element Method (FEM) were used to analyze the sensor’s working mechanism. As demonstrated with analysis, the obtained sensor has working range of 0–6000 Hz for vibration sensing and 0–100 MPa for stress sensing, respectively. The corresponding sensitivity for vibration is 0.46 pm/g and the resulted stress sensitivity is 5.94 pm/MPa, while the nonlinearity error for vibration and stress measurement is 0.77% and 1.02%, respectively. Compared to general FBGs, the vibration sensitivity of this sensor is 26.2 times higher. Therefore, the developed sensor can be used to concurrently detect vibration and stress. As this sensor has height of 1 mm and weight of 1.15 g, it is beneficial for minimization and integration. [ABSTRACT FROM AUTHOR]

[Display omitted] • Optical fiber sensors have better corrosion, electromagnetic interference resistance and signal-to-noise ratio. • Optical fiber sensors can measure the strain, temperature and pressure information of pavement in real time. • The applications of optical fiber sensors in pavement are more and more extensive. Optical fiber sensor (OFS) is a commonly used tool to monitor the serviceability and integrity of pavements. The earliest application of optical fiber sensors in the road can be traced back to the early 21st century, and development of this technique has been extensively studied after that. In view of the increasing demand for pavement nondestructive testing and the rapid development of optical fiber sensor technology, this work summarizes the optical fiber sensors commonly used in road engineering and the classification methodologies. After that, the basic principle of fiber Bragg grating (FBG) sensor and the application of this sensor in the pavement were discussed. Finally, the principle and application of distributed optical fiber sensors (DOFS) in the pavement were included as well. Overall, this review found that FBG sensors can accurately measure and monitor road strain, temperature, pressure and WIM measurement by employing analytical-based methods. Distributed optical fiber sensors can measure pavement crack width, identify the void under the concrete slab, and obtain vehicle load (load size, load moving speed, axle type) and other data. Currently, it has been proved that the installation of the installation of fiber optic sensors on the road surface is an effective method to monitor the integrity of the road structure under traffic, but further tests in real traffic environment to further improve the survival rate of fiber optic sensors and evaluate the accuracy of the measured results are still needed. [ABSTRACT FROM AUTHOR]

Fiber Optic Sensors (FOS) offer numerous advantages for structural health monitoring. In addition to being durable, lightweight, and capable of multiplexing, they offer the ability to monitor strain in both static and dynamic mode. FOS also allow for instrumentation of large areas of a structure with long-gage sensors which helps enable global monitoring of the structure. Drawing upon these benefits, the Normalized Curvature Ratio (NCR), a curvature based damage detection method, has been developed. This method utilizes a series of long-gage Fiber Bragg Grating (FBG) strain sensors for damage detection of a structure through dynamic strain measurements and curvature analysis. The main assumption is that the ratios between cross-sectional curvature amplitudes under free vibration remain unchanged given the state of the structure is unchanged. The theoretical development of this method is presented along with an analytical study of a simply supported beam with two damage cases: a loss of flexural stiffness in the span and a change in rotational stiffness of the support. Validation of the method is then performed through two implementations. First, through a small-scale laboratory test with a simply supported aluminum beam subjected to a change in the rotational stiffness of the support. Second, the method is applied to an existing in-service highway overpass with over 5 years of data collection of dynamic strain events. The advantages and limitations of the method are identified and discussed. This research shows encouraging results and the potential for the NCR to be used as a simplistic metric for damage detection.

This study investigated the reflected spectral deformation mechanism of fiber Bragg grating (FBG) sensors with crack propagation. This analysis was performed based on the simulated FBG response by applying modified-transfer matrix modeling (TMM) with the strain states, which were extracted by the finite element method (FEM) analysis. Experimental data were obtained from FBG sensors bonded in an aluminum alloy structure and subjected to multiple crack lengths, and the strain values were obtained by digital image correlation (DIC) technology. Based on the simulations and the experimental full spectral response, we compared the performance of two damage features: The full width at half maximum (FWHM) and the spectral difference. In addition, results showed that the two features were insensitive to experimental noise and were highly sensitive to the complex strain field caused by crack propagation. Moreover, the damage features changes in the crack propagation process also provided a way for crack position measurement. Ultimately, the 10 mm grating lengths sensors showed better performance to the crack detection with longer sensitivity distance. According to the research in this paper, the crack position was quantitatively determined by evaluating different damage features of the reflected spectrum.

The characterization of Fiber Bragg Grating (FBG) sensors on a high-scattering fiber, having the core doped with MgO nanoparticles for polarization-dependent temperature sensing is reported. The fiber has a scattering level 37.2 dB higher than a single-mode fiber. FBGs have been inscribed by mean of a near-infrared femtosecond laser and a phase mask, with Bragg wavelength around 1552 nm. The characterization shows a thermal sensitivity of 11.45 pm/°C. A polarization-selective thermal behavior has been obtained, with sensitivity of 11.53 pm/°C for the perpendicular polarization (S) and 11.08 pm/°C for the parallel polarization (P), thus having 4.0% different sensitivity between the two polarizations. The results show the inscription of high-reflectivity FBGs onto a fiber core doped with nanoparticles, with the possibility of having reflectors into a fiber with tailored Rayleigh scattering properties.

We propose a peak seeking algorithm to extract the damage characteristic-variation of central wavelength to monitor the crack damage status in aluminum alloy plates using surface bonded fiber Bragg grating (FBG) sensors. The FBG sensors are sensitive to the uniform and non-uniform strain distribution along their longitudinal direction, and the effect appears in the power spectrum of the reflected light from the gauge section. In this paper, we propose a fast-self-adaptive multi-peak seeking algorithm to detect the central wavelength shifting of the FBG reflection spectrum with the crack propagation. The proposed peak searching algorithm results point to a significant improvement compared to other conventional methods. Then the central wavelength shifting is applied to explain the crack propagation behavior of the aluminum plates under quasi-static tensile test conditions. The different damages feature changing intervals which are associated with the crack position and the FBGs location, demonstrating that central wavelength shifting performs as an indicator to detect structural crack damage.

As the applications of fiber Bragg gratings (FBGs) continue to grow and become more advanced, it becomes necessary to understand their behavior when exposed to high temperatures in unique situations. In these experiments, uniform 1530-nm fiber Bragg gratings and Type K Cr-Al thermocouples were embedded in three-ply carbon fiber composites. A 100 W high energy laser (HEL) heated the composites to high temperatures over timespans less than one second, and FBG spectral data and thermocouple temperature data were collected during each HEL heating test. The data from three high energy laser tests that represent different levels of damage to the FBG are analyzed to explore the spectral response and thermal decay of embedded FBG sensors when exposed to high temperatures over short timespans. Results are compared to a previously proposed power-law model describing the decay of FBGs in bare fiber when held at constant temperatures over much longer timespans.

In many applications in the industry, securely attaching fiber optic sensors to metallic structures is important for optimum monitoring, overcoming the limitations of glues and adhesives which are known to degrade under certain circumstances. To avoid that problem, creating a metallic bond to attach the sensors securely to the metal surface is important. Commercial fiber optics with metal coatings can be used but it is important not to damage the sensor itself which is written in the thin optical fiber. In this work, an alternative laser cladding technology has been studied for embedding metal-coated fiber optics into which fiber Bragg grating (FBG) sensors have been written. A three-step strategy was selected for embedding the metal coating fibers to create the best conditions to allow high-quality measurements. This has been seen to allow good control of the embedding process to be achieved and to minimize the thermal and mechanical stress generated. The research undetaken has shown that it is possible to embed Cu- and Ni-coated fiber optics containing sensors to over 300 μm with low losses, of between 0-1.5 dB (or 0-30%) and yet still enable satisfactory strain and temperature measurement results to be obtained. The research has shown that both Ni- and Cu-coated FBG-based fiber optic sensors could be embedded successfully and shown to give good mechanical and thermal response to similar nonembedded sensors and excellent cross-comparison with the conventional gauge used for calibration. The results are, therefore, particularly encouraging for the use of sensors of this type when incorporated to create metallic "smart structures" achieving durability of the sensors through the use of this innovative technique. [ABSTRACT FROM AUTHOR]

In this paper, a fiber Bragg grating (FBG)-based stress monitoring system instrumented on an orthotropic steel deck arch bridge is demonstrated. The FBG sensors are installed at two types of critical fatigue-prone welded joints to measure the strain and temperature signals. A total of 64 FBG sensors are deployed around the rib-to-deck and rib-to-diagram areas at the mid-span and quarter-span of the investigated orthotropic steel bridge. The local stress behaviors caused by the highway loading and temperature effect during the construction and operation periods are presented with the aid of a wavelet multi-resolution analysis approach. In addition, the multi-modal characteristic of the rainflow counted stress spectrum is modeled by the method of finite mixture distribution together with a genetic algorithm (GA)-based parameter estimation approach. The optimal probability distribution of the stress spectrum is determined by use of Bayesian information criterion (BIC). Furthermore, the hot spot stress of the welded joint is calculated by an extrapolation method recommended in the specification of International Institute of Welding (IIW). The stochastic characteristic of stress concentration factor (SCF) of the concerned welded joint is addressed. The proposed FBG-based stress monitoring system and probabilistic stress evaluation methods can provide an effective tool for structural monitoring and condition assessment of orthotropic steel bridges. [ABSTRACT FROM AUTHOR]

The aim of this work was to develop and validate an experimental methodology suitable for analysing on-site the behaviour of fibre-reinforced wooden structures. The proposed measurement method is based on the application of fibre Bragg grating (FBG) strain sensors. An analysis of adhesive behaviour was performed preliminarily, which provided indications for choosing the type of adhesive and for the fibre bonding length in accordance with the volume of measurement. The first series of tests was carried out on wood samples to verify the coupling between the measuring sensor and the wood support when the latter is subject to mechanical stresses. The second investigation was done on site to test the behaviour of a historical wood floor before and after reinforcement by means of a series of tests performed using optical fibres with the Bragg grating. The optical fibre system measurements were compared to those obtained using a laser vibrometer, a measurement system of proven stability and precision. The comparison makes it possible to confirm the validity of the results and the reliability of the system for the monitoring of historic wooden structures. [ABSTRACT FROM AUTHOR]

The actual tensile force in the pre-stressed (PS) tendons of a pre-stressed concrete (PSC) girder is an important factor for evaluating the performance of PSC girder bridges. To measure the tensile force in a PS tendon, an artificial neural network (ANN)-based tensile force measurement method is proposed in this study, incorporating a fibre Bragg grating (FBG) sensor and an elasto-magnetic (EM) sensor. The FBG sensor measures the strain change in the whole of a single tendon while the EM sensor measures the local permeability changes in all tendons. An FBG-encapsulated tendon is fabricated by installing an FBG sensor onto a perforated tendon and EM sensors are fabricated by embedding the EM sensor into the girder. An experimental study is performed to verify the capability of the sensors using a material testing system (MTS) and a down-scaled girder model. The FBG sensor measures the change of strain due to the tension variation, while the EM sensor measures the magnetic flux change. The ANN is used to improve the accuracy of estimation. The measured strain and permeability are used to train the ANN to estimate the tensile force in a PS tendon. To verify the capability of the trained ANN, the long-term tensile force is estimated using the ANN, the result is compared with that from a conventional regression model and the reference tensile force is measured by a load cell. The results show that the proposed method can monitor the pre-stressing force in the PS tendon of a PSC girder with high accuracy. [ABSTRACT FROM AUTHOR]

Chemical shrinkage and gelation are critical issues in curing process of composite structures since they have significant influence on generation of distortion or residual stress. In this paper, a new in-situ method to capture the gel point and to measure the effective transverse chemical shrinkage was presented. A tailed FBG (Fiber Bragg Grate) set consists of several FBG sensors with various tail lengths was embedded perpendicular to the fiber direction to monitor the curing process. Gel point was determined with the aid of profile turning point of the FBG set and the results matched well with DMA test. The effective transverse chemical shrinkage which occurs between gelation and vitrification was obtained and compared with the one determined by TMA or bi-material strip test. Both results were used to predict the spring-in of C-specimen manufactured on CFRP tube. It turns out that accurate prediction of spring-in can be obtained with the effective transverse chemical shrinkage determined by tailed FBG set. The in-situ monitoring method with tailed FBG set is an alternative and promising technology to capture the gel point and to reveal the mechanism of curing distortion. [ABSTRACT FROM AUTHOR]

FBG sensors offer many advantages, such as a lack of sensitivity to electromagnetic waves, small size, high durability, and high sensitivity. However, their maximum strain measurement range is lower than the yield strain range (about 1.0%) of steel strands when embedded in steel strands. This study proposes a new FBG sensing technique in which an FBG sensor is recoated with polyimide and protected by a polyimide tube in an effort to enhance the maximum strain measurement range of FBG sensors embedded in strands. The validation test results showed that the proposed FBG sensing technique has a maximum strain measurement range of 1.73% on average, which is 1.73 times higher than the yield strain of the strands. It was confirmed that recoating the FBG sensor with polyimide and protecting the FBG sensor using a polyimide tube could effectively enhance the maximum strain measurement range of FBG sensors embedded in strands. [ABSTRACT FROM AUTHOR]

In order to monitor the crack tip propagation of aluminum alloy, this study investigates the variation of the spectrum characteristics of a fiber Bragg grating (FBG), combined with an analysis of the spectrum simulation. The results identify the location of the subordinate peak as significantly associated with the strain distribution along the grating, corresponding to the different plastic zones ahead of the crack tip with various crack lengths. FBG sensors could observe monotonic and cyclic plastic zones ahead of the crack tip, with the quadratic strain distribution along the grating at the crack tip-FBG distance of 1.2 and 0.7 mm, respectively. FBG sensors could examine the process zones ahead of the crack tip with the cubic strain distribution along the grating at the crack tip-FBG distance of 0.5 mm. The spectrum oscillation occurs as the crack approaches the FBG where the highly heterogeneous strain is distributed. Another idea is to use a finite element method (FEM), together with a T-matrix method, to analyze the reflection intensity spectra of FBG sensors for various crack sizes. The described crack propagation detection system may apply in structural health monitoring. [ABSTRACT FROM AUTHOR]

Fibre Bragg grating (FBG) based acoustic emission (AE) measurement technique is utilized to monitor ballast breakage activities during uniaxial compressive loading. AE activity is recorded continuously using an FBG attached to the ballast sample and using a high speed FBG acoustic interrogation system. Crack induced AE events, amplitude, spectrograph of AE frequencies and strain induced by the cracks are obtained using the fibre optic system. This information can be crucial in analyzing ballast breakage, one of the key parameters that determine the permanent settlement of railway track. The successful implementation of the method underpins the potential of using fibre optic acoustic emission method to study ballast breakage with further investigation into the frequency analysis and modelling the spectrum of AE at microscopic level can shed light into a deeper understanding of railway track settlements. [ABSTRACT FROM AUTHOR]

The stability of gas storage cavern complex constructed in bedded salt rock formation might be influenced by multiple factors during the operation process. In order to assess these effects, three-dimensional rheological model tests were conducted in this study. Physical model with four ellipsoid caverns were constructed with scaled in-situ stresses applied through independent sets of hydraulic jacks and internal pressure applied by digital-controlled latex balloons. Influences of different factors on the deformation properties of surrounding rock were systematically evaluated, including the gas recovery and injection rate, the loss of pressure, the extremely low and high gas pressure, and the pillar width and pressure difference between adjacent caverns. Test results revealed that the deformation of the cavern wall was accelerated when the gas extraction/injection rate exceeds 0.65 MPa/d. Loss of internal pressure dramatically promoted the deformation. Tertiary creep stage arose when the internal pressure was too low (3 MPa) or too high (22 MPa). Internal pressure difference and pillar width worked together to affect the deformation properties. Tests results obtained in this study provided physical evidence for the safety analysis of cavern group as well as the optimization of design and operation scheme. These results can also serve as benchmark for the validation of numerical models. [ABSTRACT FROM AUTHOR]

Condition monitoring of critical and expensive carbon fiber reinforced polymer (CFRP) composite infrastructures is extremely essential task for uninterrupted operation and the durability of structures. Therefore, accurate and robust monitoring techniques and correlated damage identification algorithms need to be developed to characterize the health status of the CFRP structures. However, the effectiveness of existing methods is highly dependent on the structural properties of the CFRP plates. To identify the random damages caused by static or dynamic forces, the concept to establish the real-time strain fields of the structures based on measured signals is proposed. A moving surface spline interpolation algorithm based on Green's function has been developed to map the real-time strain fields of the CFRP plate using strain measured by surface-attached FBG sensors. A few static and impact loadings have been applied to a sample CFRP plate and the strain-field prediction by the proposed algorithm has been evaluated by a finite element model. Time and frequency domain analysis has also been conducted to recognize the dynamic response of the CFRP plates. Results indicate that the proposed algorithm can establish real-time strain field predictions with high precision, which can be used to recognize the static and dynamic responses of CFRP plate. Most importantly, the method is independent on the structural properties of the plate and the sensor layout, which is particularly significant for constructing the smart health monitoring system of CFRP. [ABSTRACT FROM AUTHOR]

• Experimental tests address the impact of corrosion degree on the fatigue and sensing performance of corroded smart steel wire. • The fatigue life of smart steel wire decreases exponentially when the weight loss rate increases. • The sensing performance of the smart steel wire remains unchanged when the number of fatigue cycles increases. • The proposed sensing model exhibits excellent accuracy, with an error of less than 3%. • The proposed models can effectively estimate the long-term behavior of smart steel wire. Smart steel wire (SSW), with the advantage of both high-strength and sensitive force-sensing, is promising in wide engineering applications. However, the coupling effect of corrosion and fatigue may cause measurement misalignment or even fracture of SSW during its service life. To study the fatigue and sensing performance of SSW under corrosion, this paper presented the sensing performance of SSW according to the Fiber Bragg Grating (FBG) sensing principle and the fatigue life estimation method of SSW based on the Basquin Equation. Then, the effects of corrosion on the fatigue and sensing performance of SSW were experimentally investigated in terms of the weight loss rate (WLR). Finally, the fatigue life and sensing models for corroded SSW were established and verified using the experimental data. The proposed models show excellent accuracy for the estimation of the long-term performance of SSW and can be used to guide the engineering practice of smart cables. [ABSTRACT FROM AUTHOR]

Dielectric barrier discharge plasma actuators (DBD-PA) and fiber Bragg grating (FBG) sensors have been investigated for active control of flow separation around a NACA0024 airfoil. Tangential jets were produced in the vicinity of the DBD-PA slightly aft of the leading edge of the airfoil. The flow separation control ability was evaluated at low Reynolds number in an open-circuit wind tunnel. Phase- and time-averaged velocity distributions around the airfoil were measured using Particle Image Velocimetry, and the flow separation control ability of the DBD-PA was assessed at 8°, 12° and 16° angle of attack. An FBG sensor was attached to a chord-wise cantilever near the trailing edge of the airfoil on the pressure surface. This was used to measure strain fluctuations in the cantilever root due to flow-induced oscillations. The feasibility of this system to detect flow separation was studied, where the standard deviations of strain fluctuations significantly increased when the flow was separated. This was utilized in an open-loop control system to detect flow separation by FBG then apply active control with the DBD-PA to reattach the flow.

A novel fiber Bragg grating (FBG)-based passive sensor architecture, which can be used to protect the fiber cut and monitor the multiple sensors simultaneously, is proposed and experimentally demonstrated. Here, we employ a wavelength-tunable erbium-doped fiber (EDF) laser scheme with 25 km cavity length acting as the detecting light source in central office (CO). Each FBG sensor, serving as a feedback element, is used in proposed sensor architecture. By tuning the tunable bandpass filter (TBF) placing inside cavity to match the corresponding Bragg wavelength of FBG over the amplification bandwidth, we can retrieve the related wavelength lasing for the FBG sensing and monitoring simultaneously. Moreover, the survivability and capacity of the passive FBG sensor architecture can be also enhanced.

With the development in the exploitation of maritime resources, the structural health monitoring (SHM) of offshore structures becomes necessary. This study focuses on addressing the practical issues of application of fiber Bragg grating (FBG) sensors for the SHM of offshore structures, in particular an FPSO (floating, production, storage, and offloading unit) vessel. Due to the harsh marine environment and tough working conditions, the FBG sensors must have sufficient protection and good repeatability for long-term monitoring. Thorough research has been conducted to identify the most suitable, commercially available protection packaging for FBG sensors for offshore applications. Further, the performance of the selected FBG sensor packaging is tested under conditions of strong sunlight, heavy rain, and salty water in order to emulate the marine environment. Moreover, the installation method of the packaged FBG sensors is equally important, as it ensures the repeatability and durability of the sensors for their long-term performance. It is shown that the packaged FBG sensors can be installed using resin-based epoxy to maintain the repeatability of the sensor over the long-term. Further, the packaged FBG sensors are installed and tested on a simple FPSO model. The experimental results under full load and ballast draft conditions show that the proposed FBG sensors are competent for the SHM of offshore structures.

Structural health monitoring has been used to confirm the durability evaluation and safety of a structure in which long-term use is required. Because the transmission loss of the optical fiber sensor is extremely small, it can measure the present status of structures from far away. Generally the FBG (Fiber Bragg Grating) optical sensing systems are often used to get reliable indices from structures. The shipping industry recently, there has been a growing interest in fuel efficiency improvements and energy regulation in international shipping. Therefore, during sailing, to measure the exact power of the main shaft of the ship has become important. We proposed the monitoring system using space optical transmitting of the FBG optical fiber to get information from a rotational shaft. And we evaluated the performance of the space optical transmission of the FBG optical fiber sensing system for rotational shaft in the previous study. Consequently, we reported that it can be effectively measured and space optical transmission metric is effective for performance prediction. In this paper, the strain and the temperature on the rotating torsion shaft, is measured by the FBG optical fiber sensor and the space optical transmission. We confirm reliability of the information that is retrieved from the rotating shaft subjected to torsion in the experiment. The conditions for reliable data from the rotating shaft under loading is obtained on the basis of the experimental results of the proposed systems.

Structural health monitoring is intended as a mechanism to ensure the operation and to evaluate the durability of the structure whose use is expected during a long time. It reduces life-cycle costs with ensuring the safety and the reliability of the structural systems. In addition, the optical fiber sensing systems are used for structural health monitoring because the high accuracy measurement and the transmission line, can be combined by the optical fibers. On the other hand, accurate measurement of horsepower for a rotary shaft in ships is needed for higher fuel efficiency and the energy regulation in international shipping. The FBG (Fiber Bragg Grating) optical fibers sensing are applied to measure several strains at different location which are induced by the torsion for the rotary shaft rotating in 100-400 rpm. The measured data of the strains have to be transmitted to the outside of the rotary shaft to use the FBG optical fiber sensing. The optical space transmission system makes it possible to transmit the data of the strains on the surface of the rotary shaft to outside the measurement system. In this study, we propose to use FBG optical fiber sensor to the shaft horsepower meter of main rotary shaft in ship. The optical transmission efficiency of the FBG measurement system is studied by a static and rotational experiment. Based on the experimental results, we verify the effectiveness of the optical space transmission performance of the system, and of the metrics for its improving performance.

The hydraulic characteristics of pavement, such as pore water pressure, hydrodynamic pressure, and permeability, have a great influence on the functional performance and durability of pavement. The main objective of this study is to design a device for measuring the hydrodynamic pressure of pavement surface and analyze its characteristics with different vehicle speeds. In order to investigate the influence of vehicle speeds on the hydrodynamic pressure of pavement surface, a device equipped with five fiber Bragg grating (FBG) sensors was designed and utilized to measure the surface hydraulic characteristics at four vehicle speeds in the field. The calibration of FBG sensor was conducted by the interrogator (SmartScan Aero) under given pressures from 0 MPa to 1 MPa with an interval of 0.1 MPa. Then, the device was embedded in the pavement and compacted by an auto wheel, which traveled over the device at different speeds. Furthermore, the experimental data was collected by the interrogator during the process of a moving tire touching the surface of the device. The experimental results showed that the correlation coefficient between the given pressure and the center wavelength change of the FBG sensor was 0.99, which means the FBG sensor was accurate and reliable. The hydrodynamic pressure of the pavement surface increased with the increase in vehicle speed from 40 km/h to 100 km/h. Moreover, the directional anisotropy of the hydraulic pressure was found to be dependent on vehicle speed. [ABSTRACT FROM AUTHOR]

Fibre Bragg grating (FBG) sensors have been increasingly adopted to detect the dynamic strain of structures. When the sensor is attached on the surface, adhesive material is employed to assist the installation, which leads to indirect contact of sensing fibre and the monitored structure. To correct the strain transfer error induced by the shear lag effect and improve the measurement accuracy of FBG sensors under dynamic response, strain transfer mechanism of a three-layered testing model constituted of sensing fibre, adhesive layer and host material has been studied in this paper. Laboratory test on steel beam attached with FBG sensor under fatigue load has been projected to investigate the feasibility of the derived strain transfer formula, and numerical simulation by MATLAB has been used as a supporting tool to offer the reference dynamic strain. Based on the analysis, sensitive parameters that affect the strain transfer coefficient have been discussed to instruct the application design of FBG sensors. Results indicate that strain transfer coefficient under dynamic response is much lower than that in static state, and error modification is particularly significant; in the dynamic testing model, bonded length, shear modulus and thickness of adhesive layer are more sensitive, which should be precisely selected in practical engineering to guarantee the effective strain measurement. [ABSTRACT FROM AUTHOR]

This paper presents a novel framework for probabilistic crack size quantification using fiber Bragg grating (FBG) sensors. The key idea is to use a high-order extended finite element method (XFEM) together with a transfer (T)-matrix method to analyze the reflection intensity spectra of FBG sensors, for various crack sizes. Compared with the standard FEM, the XFEM offers two superior capabilities: (i) a more accurate representation of fields in the vicinity of the crack tip singularity and (ii) alleviation of the need for costly re-meshing as the crack size changes. Apart from the classical four-term asymptotic enrichment functions in XFEM, we also propose to incorporate higher-order functions, aiming to further improve the accuracy of strain fields upon which the reflection intensity spectra are based. The wavelength of the reflection intensity spectra is extracted as a damage sensitive quantity, and a baseline model with five parameters is established to quantify its correlation with the crack size. In order to test the feasibility of the predictive model, we design FBG sensor-based experiments to detect fatigue crack growth in structures. Furthermore, a Bayesian method is proposed to update the parameters of the baseline model using only a few available experimental data points (wavelength versus crack size) measured by one of the FBG sensors and an optical microscope, respectively. Given the remaining data points of wavelengths, even measured by FBG sensors at different positions, the updated model is shown to give crack size predictions that match well with the experimental observations. [ABSTRACT FROM AUTHOR]

Monitoring of roof activity is a primary measure adopted in the prevention of roof collapse accidents and functions to optimize and support the design of roadways in underground coalmines. However, traditional monitoring measures, such as using mechanical extensometers or electronic gauges, either require arduous underground labor or cannot function properly in the harsh underground environment. Therefore, in this paper, in order to break through this technological barrier, a novel monitoring system for roof safety control in underground coal mining, using fiber Bragg grating (FBG) material as a perceived element and transmission medium, has been developed. Compared with traditional monitoring equipment, the developed, novel monitoring system has the advantages of providing accurate, reliable, and continuous online monitoring of roof activities in underground coal mining. This is expected to further enable the prevention of catastrophic roof collapse accidents. The system has been successfully implemented at a deep hazardous roadway in Zhuji Coal Mine, China. Monitoring results from the study site have demonstrated the advantages of FBG-based sensors over traditional monitoring approaches. The dynamic impacts of progressive face advance on roof displacement and stress have been accurately captured by the novel roadway roof activity and safety monitoring system, which provided essential references for roadway support and design of the mine. [ABSTRACT FROM AUTHOR]

A method for detection and localization of a damage in a model of an offshore support structure is addressed in this paper. The damage has been simulated as a dismantled flange located in one of the upper braces of a tripod model. The strain measurements have been obtained from a network of Fibre Bragg Grating (FBG) sensors bonded to the model. The root mean square deviation (RMSD) estimator applied to a chosen part of averaged and smoothed frequency spectrum of the strain data has been considered here as a damage indicator. A capability of the method has been checked for the freely hanging model as well as for the tripod fixed to a table. Experimental results have been supported by numerical modal analysis using Abaqus software. The results show that additional peaks have appeared in some frequency regions revealing vibration modes which are associated with damage in the structure. Applying the RMSD estimator to the regions where new peak has emerged it is possible to detect the damage by comparing the same part of an averaged frequency spectrum of the same rosette before and after the damage or by comparing the spectra of two rosettes for model with the damage without referencing to the healthy state. The good localization performance of the RMSD estimator for some new frequency peaks can be justified by analysing the corresponding localized numerical maximal in-plane principal strain modes. The determination of damaged structural member is understood as the correct damage localization. [ABSTRACT FROM AUTHOR]

This study is to achieve a new interrogation system for fiber Bragg grating (FBG) sensors, wavelength demodulation is commonly performed using a Fabry-Pérot (FP) scanner. However, for the thermal effects to a FP scanner, using a single A-thermal FBG as the reference to extract the correct sequence index and position of FBG sensors is introduced. In addition, a third-order polynomial curve fitting function is used to account for the nonlinear relationship between the FP driving voltage and the FP transmission wavelength. To enhance the accuracy of the FP scanning system, an additional interrogation structure based on four A-thermal reference FBGs is also proposed and the proposed interrogation system can accurately extract the Bragg wavelength down to 7.2 pm. Also, the performance of a FP scanning system with a high-end or low-end FP scanner is discussed. [ABSTRACT FROM AUTHOR]