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This article presents a comprehensive study on the impact of irradiation optical fiber cores with a femtosecond-pulsed laser, operating at a wavelength of 1030 nm, on the signal amplitude in Rayleigh scattering-based optical frequency domain reflectometry (OFDR). The experimental study involves two fibers with significantly different levels of germanium doping: the standard single-mode fiber (SMF-28) and the ultra-high numerical aperture fiber (UHNA7). The research findings reveal distinct characteristics of reflected and scattered light amplitudes as a function of pulse energy. Although different amplitude changes are observed for the examined fibers, both can yield an enhancement of amplitude. The paper further investigates the effect of fiber Bragg grating inscription on the overall amplitude of reflected light. The insights gained from this study could be beneficial for controlling the enhancement of light scattering amplitude in fibers with low or high levels of germanium doping. [ABSTRACT FROM AUTHOR]

This article presents a comprehensive study on the impact of irradiation optical fiber cores with a femtosecond-pulsed laser, operating at a wavelength of 1030 nm, on the signal amplitude in Rayleigh scattering-based optical frequency domain reflectometry (OFDR). The experimental study involves two fibers with significantly different levels of germanium doping: the standard single-mode fiber (SMF-28) and the ultra-high numerical aperture fiber (UHNA7). The research findings reveal distinct characteristics of reflected and scattered light amplitudes as a function of pulse energy. Although different amplitude changes are observed for the examined fibers, both can yield an enhancement of amplitude. The paper further investigates the effect of fiber Bragg grating inscription on the overall amplitude of reflected light. The insights gained from this study could be beneficial for controlling the enhancement of light scattering amplitude in fibers with low or high levels of germanium doping.

Despite the promising application of Distributed Optical Fiber Sensors (DOFS) in monitoring damage in composite structures, their implementation outside academia is still unsatisfactory due to the lack of a systematic approach to assessing their damage detection performance. The existing tool developed for non-destructive evaluation, Probability of Detection (POD) curves, needs to be adapted for structural health monitoring applications to account for spatial and temporal dependency. Damage detection performance with DOFS is deeply related to the inherent variability sources of the system, the strain transfer properties of the optical fiber, and the loading conditions, which determine the damage-induced strain on the structure. This paper establishes a systematic approach based on the Length at Detection (LaD) method to qualify DOFS for damage detection in composites under different scenarios. Specifically, this study considers two DOFS with different strain transfer properties for monitoring delamination in carbon fiber reinforced polymers double-cantilever beam specimens under mode I quasi-static and fatigue loading. The POD curves derived from the LaD method confirm that this methodology can quantify the change in the detection performance due to the DOFS type and the loading conditions. The study also proposes a practical solution to compare POD curves obtained with different sample sizes, by introducing the concept of virtual specimens to simulate the lower confidence bound convergence. [ABSTRACT FROM AUTHOR]

Distributed fiber optic sensors (DFOS) can detect structural cracks and structural deformation with high accuracy and wide measurement range. This study monitors the segmental prestressed bent cap, assembled with a large key dry joint, based on optical fiber technology, and it allows the comparison of its damaging process with that of a monolithic cast in place counterpart. The obtained results, comprising cross-section strain distributions, longitudinal strain profiles, neutral axis location, crack pattern, and the damage process, show that the DFOS technology can be successfully used to analyze the complex working stress state of the segmental beam with shear key joints, both in the elastic range and at the ultimate load, and to successfully identify the changing characteristics of the stress state of the segmental capping beam model when elastic beam theory no longer applies. The DFOS data confirm that the shear key joint, as the weak point of the segmental cap beam, results in the high stress concentration area, and the damage rate is higher than that of the cast-in-place beam. The accurate monitoring by the DFOS allows for the realization that the damage occurs at the premature formation of a concentrated compression zone on the upper part of the shear key. [ABSTRACT FROM AUTHOR]

In this paper, we propose and demonstrate a damage detection technique based on the automatic classification of the Lamb wave signals acquired on a metallic plate. In the reported experiments, Lamb waves are excited in an aluminum plate through a piezoelectric transducer glued onto the monitored structure. The response of the monitored structure is detected through a high-resolution phase-sensitive optical time-domain reflectometer (ϕ-OTDR). The presence and location of a small perturbation, induced by placing a lumped mass of 5 g on the plate, are determined by processing the optical fiber sensor data through support vector machine (SVM) classifiers trained with experimental data. The results show that the proposed method takes full advantage of the multipoint sensing nature of the ϕ-OTDR technology, resulting in accurate damage detection and localization. [ABSTRACT FROM AUTHOR]

Distributed acoustic sensors (DASs) are able to monitor in real-time acoustic excitation and mechanical vibration of the external environment for many kilometers along the length of the optical fiber. In the oil industry, the technology allows the monitoring of well structural health, identifying flow profile, acquiring seismic data, updating reservoir information, and feeding geological models. The spatial resolution of DAS is a limiting factor on the quality of information gathered. In this article, we propose and analyze a novel method of measuring local dynamic strain of structures using DAS. The method uses serpentine configurations to measure the frequency spectrum of the in-phase strain signals at discrete regions of the structure, enabling DAS to determine, among other vibration parameters, the strain frequency response function (SFRF). An experimental study of the sensor application with different arrangements in a small structure has been performed. The structure in the experiment consists of a free–free aluminum beam of approximately 2.7 m. Experiment results show agreement with numerical results developed by the finite element method (FEM) and using reference sensors, demonstrating a significant improvement of the DAS SFRF accuracy using the serpentine configuration and up to 12.9 dB increase in peak-to-noise ratio (PNR) for low excitation forces. The recovered response enabled the reconstruction of mode shapes. [ABSTRACT FROM AUTHOR]

Owing to their high sensitivity with respect to external measurands, Rayleigh-based distributed optical fiber sensors (DOFS) find their way into applications in many industrial and academic sectors. To further transcend the limit of these sensors in terms of sensing range, low spatial resolution, speed and accuracy of the measurement, improving the signal-to-noise ratio (SNR) of the system plays a pivotal role. Out of the several existing techniques for enhancing the SNR, one such method, solely dedicated to the Rayleigh-based sensors, is through intrinsically increasing the backreflected signal of the fibers. The enhanced backreflected signal provided by such fibers, generally known as reflection-enhanced fibers (REF), is rigorously analyzed in the present work. It is inferred from the analysis that the enhanced signal is essentially accompanied by enhanced signal-dependent noises, which can adversely affect their performance. For instance, when a performance comparison is carried out between the REF and a standard single-mode fiber (SMF) under identical experimental conditions, due to their different intrinsic backreflected signal levels, the two fibers experience dissimilar noise regimes leading to an erroneous estimation of the performance of the former. This necessitates the optimization of the interrogation system and the experimental parameters while employing such fibers for specific sensing applications. Additionally, a distributed temperature measurement is presented by taking advantage of the enhanced SNR of a 100 m long REF exhibiting a sub-mK temperature uncertainty of 0.5 mK at metric spatial resolution yielding a sixfold improvement compared to the 3 mK uncertainty of an SMF.

Short-time Fourier transform (STFT) algorithm with Lorentzian fitting is popular in distributed Brillouin optical fiber sensors to acquire Brillouin frequency. However, STFT often suffers from the tradeoff between Brillouin frequency measurement resolution and computation complexity. To lower the complexity of Brillouin frequency analysis and speedup measurement, a novel method based on instantaneous frequency (IF) analysis is proposed. The instantaneous Brillouin frequency can be obtained through the instantaneous phase change between two adjacent samples in the time domain. The requirement on the spectral resolution is, therefore, greatly relaxed, significantly reducing computation complexity and lowering measurement time cost. A Brillouin optical time-domain reflectometry (BOTDR) scheme is demonstrated with a spatial resolution of 1.2 m over a 5-km sensing fiber. For an average count of 4096 over measurements, experiments show that the IF algorithm can reduce computation complexity by 122 times at a standard deviation (STD) of 0.5065 MHz for BFS estimation which is comparable to the STD obtained through STFT in conjunction with Lorentzian fitting. [ABSTRACT FROM AUTHOR]

To improve the understanding of failure mechanisms and behaviour of hard rock tunnel linings, local load conditions were experimentally simulated and monitored using a comprehensive set of sensors and imaging techniques. The data includes measurements from distributed optical fiber sensors (DOFS), high-resolution cameras, load cells, pressure cells and LVDTs. Two types of loads were examined: rock block load and bond loss combined with a distributed load over the area of lost bond. The experiments replicated these conditions and were conducted in a laboratory setting where the shotcrete and substrate rock were substituted by cast fiber reinforced concrete (FRC) and cast concrete, respectively. To facilitate the loads, concrete cones were cast into the substrate concrete and pushed through the FRC top layer with a hydraulic jack to mimic rock block loads. To simulate the bond loss and the associated distributed load, lifting bags were installed and inflated between the FRC layer and substrate cast concrete. All specimens were monitored using DOFS embedded in two perpendicular directions and in two layers in the top FRC layer. In addition, the hydraulic jack was instrumented with LVDTs and load cells to measure displacement and load, and the pressure in the lifting bags was monitored using a pressure cell. Two cameras continuously photographed the top surface of the FRC layer, which had been painted with a speckle pattern, during the testing and the pictures can be used for digital image correlation (DIC). Lastly, each specimen was scanned with a 3D scanner prior to and after testing of the specimen., (© 2024 The Author(s).)

In this study, corrosion-induced mortar/concrete cracking is experimentally investigated with optical frequency domain reflectometry (OFDR) distributed optical fiber sensors, and numerical simulation is performed with finite-element method (FEM). Mortar/concrete cylindrical specimens are instrumented with OFDR distributed optical fiber sensors and then subjected to accelerated corrosion tests till surface cracking. A total of 12 specimens are prepared and tested in consideration with three steel rebar sizes and four cover thicknesses. A localized corrosion model is proposed and incorporated into FEM to simulate mortar/concrete cover cracking and the evolution of OFDR fiber sensor strain with an increase of corrosion time. Experimental results show that strain peaks appear along the length of fiber sensors due to either localized corrosion of steel rebar or mortar/concrete cover cracking, and the initiation time of corrosion-induced mortar/concrete cracking is identified to be the time when the fiber sensor strain peak starts to increase rapidly. The cracking strain of mortar/concrete obtained based on OFDR fiber sensors is affected by environmental factors such as moisture level. Empirical equations are proposed to predict the critical penetration depth of steel rebar corrosion-induced mortar and concrete cracking, and the limitations for actual engineering applications are discussed. [ABSTRACT FROM AUTHOR]

This paper presents a study on the suitability and accuracy of detecting structural cracks in brick masonry by exploiting the breakage of ordinary silica optical fibers bonded to its surface with an epoxy adhesive. The deformations and cracking of the masonry specimen, and the behavior of pilot optical signals transmitted through the fibers upon loading of the test specimen were observed. For the first time, reliable detection of structural cracks with a given minimum value was achieved, despite the random nature of the ultimate strength of the optical fibers. This was achieved using arrays of several optical fibers placed on the structural element. The detection of such cracks allows the degree of structural danger of buildings affected by earthquake or other destructive phenomena to be determined. The implementation of this technique is simple and cost effective. For this reason, it may have a broad application in permanent damage-detection systems in buildings in seismic zones. It may also find application in automatic systems for the detection of structural damage to the load-bearing elements of land vehicles, aircraft, and ships. [ABSTRACT FROM AUTHOR]

Since the optical signal is attenuated when it propagates forward, the uncertainty of the measurement at the end of the optical fiber is increased, which restricts the performance of the Raman Scattering Distributed Temperature Sensing System (RDTS). Therefore, it is essential to study the signal denoising method of the system. Existing signal denoising methods, such as wavelet transform, median filtering, singular value decomposition, 1DDCNN, etc., still have room for improvement in parameter adjustment, computational complexity, and denoising effect. Thus, we have proposed a novel deep-learning model called the down-sampling double network model (DSDN) for RDTS signal denoising with high performance based on down-sampling and convolutional neural network (CNN). The DSDN model includes a down-sampling part, a one-dimensional full convolution part, and a ResNet part. It uses synthetic data as a training dataset. We have designed model performance evaluation experiments based on synthetic data and real RDTS data under different noise intensities. The root mean square error (RMSE) of the RDTS signal after DSDN noise reduction has been reduced to 0.189 °C, which is better than 2.117 °C of wavelet, 2.077 °C of median filter, 0.968 °C of 1DDCNN model. The mean absolute error (MAE) has been reduced to 0.165 °C, which is better than 1.716 °C of the wavelet transform, 1.402 °C of median filter, 0.564 °C of 1DDCNN. And smooth has been reduced to 0.074, which is better than wavelet's 1.298, median filter's 1.171, and 1DDCNN's 0.149. The results show that the DSDN model proposed in this article has excellent signal denoising performance and good robustness on different noise intensities and can be applied to RDTS systems. [ABSTRACT FROM AUTHOR]

Pitting corrosion of steel bars can greatly impair mechanical properties of steel bars, so detecting pitting corrosion timely and accurately is crucial for assessing the health of structures. However, most current corrosion detection methods have some difficulties measuring pitting corrosion level of steel bars effectively. Considering that corrosion pits can cause strain redistribution of steel bar in the tensile state, this study proposes a method to quantitatively determine the pitting corrosion level through monitoring the steel strain distribution. Distributed optical fiber sensors (DOFS) based on Rayleigh scattering and capable of achieving the spatial resolution in millimeter scale were adopted to monitor the strain distributions of notched HRB400E steel bars subjected to tensile stress, and finite element (FE) method was employed to analyze the strain distributions of steel bars with different pit geometries. Experimental results showed that the bare fiber sensors (PI-FS) measured steel strain more accurately than jacketed fiber sensors. From strain distributions monitored by PI-FS, the pit location and pit length can be directly determined. Further, FE results revealed that the strain distribution along the fiber sensor position was significantly affected by the fiber sensor position relative to the pit mouth. Finally, the procedures to determine the pitting corrosion level from DOFS data and FE results were presented by mathematically comparing the similarity of strain curves given by the fiber sensor on a pitting-corroded bar and FE analyses of steel bars with different corrosion levels. In the end, the issues related to the practical applicability of the proposed method were discussed and limitations of this study were addressed. Although with limitations, this study provided a novel and promising way to quantitatively monitor the pitting corrosion level based on the strain distribution of steel bars. • New method to monitor pitting corrosion level by DOFS was proposed. • Position of fiber sensors relative to the pit mouth governed the strain distribution according to FEM. • Pitting corrosion quantification was achieved by matching strain distributions from DOFS and FEM. [ABSTRACT FROM AUTHOR]

A pattern recognition method for intrusion signals is introduced, leveraging distributed optical fiber sensors and convolutional neural networks. To enhance the model's generalization capabilities, two strategies are employed. Firstly, the sensor structure is refined, incorporating the strengths of both the Sagnac and Michelson interferometers to create a hybrid design with a wide frequency response. Comparative experiments confirm that this exceptional frequency response significantly boosts the model's generalization power. Secondly, to address the limitations of distributed fiber optic sensors, a multimodal approach is proposed, integrating video frame difference and fiber optic pattern recognition. The trained convolutional neural network model exhibits low computational overhead, robust generalization, and practical applicability. When applied to a dataset collected in diverse scenarios not included in the model training, the model achieved a CNR of 0% and a FPR of only 0.26% for the intrusion behavior of climbing fences. [ABSTRACT FROM AUTHOR]

Distributed fiber optic sensors (DFOS) can detect structural cracks and structural deformation with high accuracy and wide measurement range. This study monitors the segmental prestressed bent cap, assembled with a large key dry joint, based on optical fiber technology, and it allows the comparison of its damaging process with that of a monolithic cast in place counterpart. The obtained results, comprising cross-section strain distributions, longitudinal strain profiles, neutral axis location, crack pattern, and the damage process, show that the DFOS technology can be successfully used to analyze the complex working stress state of the segmental beam with shear key joints, both in the elastic range and at the ultimate load, and to successfully identify the changing characteristics of the stress state of the segmental capping beam model when elastic beam theory no longer applies. The DFOS data confirm that the shear key joint, as the weak point of the segmental cap beam, results in the high stress concentration area, and the damage rate is higher than that of the cast-in-place beam. The accurate monitoring by the DFOS allows for the realization that the damage occurs at the premature formation of a concentrated compression zone on the upper part of the shear key.

We live in an environment of ever-growing demand for transport networks, which also have ageing infrastructure. However, it is not feasible to replace all the infrastructural assets that have surpassed their service lives. The commonly established alternative is increasing their durability by means of Structural Health Monitoring (SHM)-based maintenance and serviceability. Amongst the multitude of approaches to SHM, the Digital Twin model is gaining increasing attention. This model is a digital reconstruction (the Digital Twin) of a real-life asset (the Physical Twin) that, in contrast to other digital models, is frequently and automatically updated using data sampled by a sensor network deployed on the latter. This tool can provide infrastructure managers with functionalities to monitor and optimize their asset stock and to make informed and data-based decisions, in the context of day-to-day operative conditions and after extreme events. These data not only include sensor data, but also include regularly revalidated structural reliability indices formulated on the grounds of the frequently updated Digital Twin model. The technology can be even pushed as far as performing structural behavioral predictions and automatically compensating for them. The present exploratory review covers the key Digital Twin aspects—its usefulness, modus operandi, application, etc.—and proves the suitability of Distributed Sensing as its network sensor component. [ABSTRACT FROM AUTHOR]

Glass fiber/epoxy (GF/E) composites are being extensively scaled-up as anti-corrosion materials in chemical industries because of the relatively low-cost, and are easy to manufacture. Nonetheless, Glass fiber/epoxy composites are not entirely immune to corrosion when exposed to dilute acidic solutions due to the reduction in mechanical strength caused by swelling, which adversely affects the structural integrity. Mathematical correlations quantifying the stress and mechanical property reduction with the time of exposure are a tool to design the service life. The composite structure has been monitored throughout its service life in the present study, by instrumenting the GF/E composite with distributed optical fiber sensors (DOFS). The instrumented composites along with test specimens are aged in aqueous 10 % sulphuric acid environment at room temperature to measure the mechanical properties and weight gain due to the transport of the acidic medium. The strain evolution with the time of exposure is recorded by processing the signals received from the DOFS. The experimental gravimetric data are fitted into the Langmuir model, and the model parameters such as diffusivity (D), β and γ are evaluated. Finally, mathematical correlations developed by quantifying the stress and reduction in mechanical properties with exposure time can be used to determine the mechanical health of GF/E composite structure exposed to sulphuric acid solutions. The changes in the microstructure of GF/E composites is also investigated using field emission-scanning electron microscope (FE-SEM). The technique of instrumenting the GF/E composites by embedding it in the composite structure during the fabrication process is also developed. [ABSTRACT FROM AUTHOR]

We have designed and fabricated a single-mode Raman gain fiber (RGF) and use it to experimentally demonstrate a Raman distributed temperature sensor (RDTS) for high-precision temperature measurement with low power consumption. We have performed simulation to analyze the impact of fluorine-doped inner cladding on the control of the acoustic field and suppression of stimulated Brillouin scattering (SBS), showing that the SBS threshold can be improved by 37dB at 6wt% fluorine-doping concentration. Taking advantage of high Raman gain, a maximum signal-to-noise ratio (SNR) improvement of 6dB has been realized by using RGF under the same pump power as by using single-mode fiber (SMF), and the temperature uncertainty at the fiber end is enhanced from 0.9°C to 0.5°C. To achieve comparable sensing accuracy as that of SMF-based RDTS, our RGF-based RDTS only consumes half of the power, showing good energy efficiency. The magnitude of SNR and temperature uncertainty enhancement is limited by the relatively large fiber attenuation, but it can be further improved through optimization of the fiber design with high fluorine-doping concentration and graded index structure. [ABSTRACT FROM AUTHOR]

In this paper, we propose and demonstrate a damage detection technique based on the automatic classification of the Lamb wave signals acquired on a metallic plate. In the reported experiments, Lamb waves are excited in an aluminum plate through a piezoelectric transducer glued onto the monitored structure. The response of the monitored structure is detected through a high-resolution phase-sensitive optical time-domain reflectometer (ϕ-OTDR). The presence and location of a small perturbation, induced by placing a lumped mass of 5 g on the plate, are determined by processing the optical fiber sensor data through support vector machine (SVM) classifiers trained with experimental data. The results show that the proposed method takes full advantage of the multipoint sensing nature of the ϕ-OTDR technology, resulting in accurate damage detection and localization.

This paper presents a study on the suitability and accuracy of detecting structural cracks in brick masonry by exploiting the breakage of ordinary silica optical fibers bonded to its surface with an epoxy adhesive. The deformations and cracking of the masonry specimen, and the behavior of pilot optical signals transmitted through the fibers upon loading of the test specimen were observed. For the first time, reliable detection of structural cracks with a given minimum value was achieved, despite the random nature of the ultimate strength of the optical fibers. This was achieved using arrays of several optical fibers placed on the structural element. The detection of such cracks allows the degree of structural danger of buildings affected by earthquake or other destructive phenomena to be determined. The implementation of this technique is simple and cost effective. For this reason, it may have a broad application in permanent damage-detection systems in buildings in seismic zones. It may also find application in automatic systems for the detection of structural damage to the load-bearing elements of land vehicles, aircraft, and ships.

The present study investigates the environmental durability of a distributed optical fiber sensing (DOFS) cable on the market, commonly used for distributed strain measurements in reinforced concrete structures. An extensive experimental program was conducted on different types of specimens (including samples of bare DOFS cable and plain concrete specimens instrumented with this DOFS cable) that were exposed to accelerated and natural ageing (NA) conditions for different periods of up to 18 months. The instrumentation of both concrete specimens consisted of DOFS cables embedded at the center of the specimens and bonded at the concrete surface, as these two configurations are commonly deployed in the field. In these configurations, the alkalinity of the surrounding cement medium and the outdoor conditions are the main factors potentially affecting the characteristics of the DOFS component materials and the integrity of the various interfaces, and hence impacting the strain transfer process between the host structure and the core optical fiber (OF). Therefore, immersion in an alkaline solution at an elevated temperature or freeze/thaw (F/T) and immersion/drying (I/D) cycles were chosen as accelerated ageing conditions, depending on the considered configuration. Mechanical characterizations by tensile and pull-out tests were then carried out on the exposed specimens to assess the evolution of the mechanical properties of individual component materials as well as the evolution of bond properties at various interfaces (internal interfaces of the DOFS cable, and interface between the cable and the host structure) during ageing. Complementary physico-chemical characterizations were also performed to better understand the underlying degradation processes. The experimental results highlight that immersion in the alkaline solution induced a significant and rapid decrease in the bond properties at internal interfaces of the DOFS cable and at the cable/concrete interface (in the case of the embedded cable configuration), which was assigned to chemical degradation at the surface of the cable coating in contact with the solution (hydrolysis and thermal degradation of the EVA copolymer component). Meanwhile, F/T and I/D cycles showed more limited effects on the mechanical properties of the component materials and interfaces in the case of the bonded cable configuration. A comparison with the same specimens exposed to outdoor NA suggested that the chosen accelerated ageing conditions may not be totally representative of actual service conditions, but provided indications for improving the ageing protocols in future research. In the last part, an analysis of the distributed strain profiles collected during pull-out tests on instrumented concrete specimens clearly illustrated the consequences of ageing processes on the strain response of the DOFS cable.

Steel piles are used in foundation structures to support offshore wind power system, but they suffer from serious non-uniform corrosion in the marine environment, which can threaten the stability and safety of the overall wind tower infrastructure. Thus, real-time and in-situ monitoring of steel pile corrosion is of great significance to provide timely warning and guidance for maintenance. This study deployed a distributed optical fiber sensor beneath the simulated anti-corrosion wrapping system to monitor non-uniform corrosion of steel pile in the four marine corrosion zones: atmospheric zone, water level fluctuation zone, immersion zone, and sub-mud zone. The sensor was applied to measure corrosion products induced strain variation and analyze non-uniform corrosion influenced by different environmental factors. Two analytical models considering build-ups and run-offs of corrosion products were established to quantify non-uniform corrosion induced mass loss at any location on the surface of steel pile. Functions of volume expansion and reduction coefficients of corroded steel pile were proposed, and sensitivity of mass loss measurement with the distributed optical fiber sensor was investigated. Finally, visualization through color cloud map was applied to illustrate the severity of non-uniform corrosion on the pile surface. • Distributed optical fiber sensors are applied to monitor and quantify non-uniform corrosion of steel pile in the marine corrosion zones. • Two non-uniform corrosion induced mass loss models were established for steel pile in the marine environment. • Non-uniform distribution of mass loss of corroded steel pile across the marine corrosion zones is visualized through color cloud map. [ABSTRACT FROM AUTHOR]

In the field of structural deformation and strain measurement for civil structural health monitoring, Brillouin optical time-domain analyzer (BOTDA) system with sub-meter high spatial resolution is required. Differential pulse-width pair Brillouin optical time-domain analyzer (DPP-BOTDA) is an excellent method to achieve the goal without the need to modify the system structure. But the signal-to-noise ratio (SNR) of DPP-BOTDA is poor due to the differential operation, which limits its practical applications. Here, we propose to use Block-Matching and 3D filtering (BM3D) algorithm to improve the SNR of DPP-BOTDA with less degradation of measurement accuracy and spatial resolution. The influence of each BM3D parameter on the denoising performance is fully analyzed in detail for the first time which helps the optimization of the algorithm. Three pulse pairs of 50/47 ns, 50/45 ns and 50/43 ns are used for the demonstration with a sensing range of 9.9 km. With the optimized BM3D, the experimental spatial resolution only degrades by 0.08 m, 0.05 m and 0.04 m after denoising when the SNR improvement is 9 dB, respectively. And the temperature uncertainty for the three cases is improved to be 0.79 °C, 0.36 °C and 0.24 °C, respectively. We believe that this research will improve the efficiency of the parameter optimization when using the BM3D method for noise reduction and contribute to the practical application of the DPP-BOTDA system. [ABSTRACT FROM AUTHOR]

[Display omitted] • Conducting three-edge bearing tests of prestressed concrete cylinder pipes. • Collecting the full strain distribution of a pipe until failure. • Providing empirical assumptions to fit test results after cracking in concrete. • Iterative solution of strains considering elastoplastic constitutive relations. • Achieving an improved understanding of design assumptions and parameters. A prestressed concrete cylinder pipe (PCCP) with quality defects or being subjected to overloads can crack in the inner concrete core, thereby the internal water acting directly on the steel cylinder and further weakening the bearing capacity of the pipe. This study aims to explore calculation methods of when and after a PCCP cracks through the comparative analysis between the test results measured from the three-edge bearing test (TEBT) and the calculation results through the improved procedures based on ANSI/AWWA C304. To this end, the full strain distribution around the circumference of the first pipe is measured until failure by distributed optical fiber sensors. The thrust and moment coefficients of loads under the TEBT are derived, and strains are solved considering the stress–strain relationships of the constituent materials. The first pipe cracks in the inner core at the crown and invert as the load reaches 280 kN/m, but the second pipe with the same parameters does not crack when the load arrives at 360 kN/m. The measured strains reveal that the inner side at the crown and invert and the outer side at the springline are subjected to tension. Before appearing the cracks in the inner concrete core, the calculated results are in good agreement with experimentally observed data. After cracking of the inner concrete core, both the vertical displacement at the crown and the fluctuation amplitude of test strains begin to accelerate. The results calculated by considering two empirical assumptions can reflect the trend of the test results. The proposed comparative analysis approach provides the cracking test load that meets the serviceability limit-state design criterion for quality inspection and helps designers a better understanding of design assumptions and parameters. [ABSTRACT FROM AUTHOR]

Conventional Brillouin optical time-domain analyzer (BOTDA) requires the process of frequency scanning, which is very time-consuming. The data acquisition speed of the conventional BOTDA is too slow to realize dynamic sensing. The use of the optical frequency comb-based BOTDA can avoid the frequency scanning and speed up the measurement. But, to achieve dynamic measurement, it is also necessary to reduce the time for signal averaging to further improve the measurement speed. However, few averaging times lead to poor signal-to-noise ratio (SNR), which will limit the sensing performance of the system. Block-matching and 3D filtering (BM3D) algorithm is proposed to achieve the denoising of sensing signals in the sweep-free BOTDA using intensity-modulated direct-detection optical orthogonal frequency division multiplexing. The impact of the BM3D parameters on the denoising performance is analyzed, which is used to realize parameter optimization. To demonstrate the denoising performance of BM3D under different noise levels, white Gaussian noise with different standard deviation (STD) is added to the original sensing signal. When the STD of the added noise is 0.07 mV, the SNR after denoising using optimized BM3D is improved by 9.7 dB for the Brillouin gain spectrum (BGS) and 10.6 dB for the Brillouin phase spectrum (BPS), respectively, and the frequency uncertainty is reduced to be below 1 MHz. For a wide range of noise levels, BM3D has shown excellent denoising performance, and the average SNR after denoising is improved to be 16.8 and 15.6 dB for BGS and BPS, respectively, and the corresponding frequency uncertainty is reduced to be below 1.2 MHz. [ABSTRACT FROM AUTHOR]

The present study investigates the environmental durability of a distributed optical fiber sensing (DOFS) cable on the market, commonly used for distributed strain measurements in reinforced concrete structures. An extensive experimental program was conducted on different types of specimens (including samples of bare DOFS cable and plain concrete specimens instrumented with this DOFS cable) that were exposed to accelerated and natural ageing (NA) conditions for different periods of up to 18 months. The instrumentation of both concrete specimens consisted of DOFS cables embedded at the center of the specimens and bonded at the concrete surface, as these two configurations are commonly deployed in the field. In these configurations, the alkalinity of the surrounding cement medium and the outdoor conditions are the main factors potentially affecting the characteristics of the DOFS component materials and the integrity of the various interfaces, and hence impacting the strain transfer process between the host structure and the core optical fiber (OF). Therefore, immersion in an alkaline solution at an elevated temperature or freeze/thaw (F/T) and immersion/drying (I/D) cycles were chosen as accelerated ageing conditions, depending on the considered configuration. Mechanical characterizations by tensile and pull-out tests were then carried out on the exposed specimens to assess the evolution of the mechanical properties of individual component materials as well as the evolution of bond properties at various interfaces (internal interfaces of the DOFS cable, and interface between the cable and the host structure) during ageing. Complementary physico-chemical characterizations were also performed to better understand the underlying degradation processes. The experimental results highlight that immersion in the alkaline solution induced a significant and rapid decrease in the bond properties at internal interfaces of the DOFS cable and at the cable/concrete interface (in the case of the embedded cable configuration), which was assigned to chemical degradation at the surface of the cable coating in contact with the solution (hydrolysis and thermal degradation of the EVA copolymer component). Meanwhile, F/T and I/D cycles showed more limited effects on the mechanical properties of the component materials and interfaces in the case of the bonded cable configuration. A comparison with the same specimens exposed to outdoor NA suggested that the chosen accelerated ageing conditions may not be totally representative of actual service conditions, but provided indications for improving the ageing protocols in future research. In the last part, an analysis of the distributed strain profiles collected during pull-out tests on instrumented concrete specimens clearly illustrated the consequences of ageing processes on the strain response of the DOFS cable. [ABSTRACT FROM AUTHOR]

The present study investigates the environmental durability of a distributed optical fiber sensing (DOFS) cable on the market, commonly used for distributed strain measurements in reinforced concrete structures. An extensive experimental program was conducted on different types of specimens (including samples of bare DOFS cable and plain concrete specimens instrumented with this DOFS cable) that were exposed to accelerated and natural ageing (NA) conditions for different periods of up to 18 months. The instrumentation of both concrete specimens consisted of DOFS cables embedded at the center of the specimens and bonded at the concrete surface, as these two configurations are commonly deployed in the field. In these configurations, the alkalinity of the surrounding cement medium and the outdoor conditions are the main factors potentially affecting the characteristics of the DOFS component materials and the integrity of the various interfaces, and hence impacting the strain transfer process between the host structure and the core optical fiber (OF). Therefore, immersion in an alkaline solution at an elevated temperature or freeze/thaw (F/T) and immersion/drying (I/D) cycles were chosen as accelerated ageing conditions, depending on the considered configuration. Mechanical characterizations by tensile and pull-out tests were then carried out on the exposed specimens to assess the evolution of the mechanical properties of individual component materials as well as the evolution of bond properties at various interfaces (internal interfaces of the DOFS cable, and interface between the cable and the host structure) during ageing. Complementary physico-chemical characterizations were also performed to better understand the underlying degradation processes. The experimental results highlight that immersion in the alkaline solution induced a significant and rapid decrease in the bond properties at internal interfaces of the DOFS cable and at the cable/concrete interface (in the case of the embedded cable configuration), which was assigned to chemical degradation at the surface of the cable coating in contact with the solution (hydrolysis and thermal degradation of the EVA copolymer component). Meanwhile, F/T and I/D cycles showed more limited effects on the mechanical properties of the component materials and interfaces in the case of the bonded cable configuration. A comparison with the same specimens exposed to outdoor NA suggested that the chosen accelerated ageing conditions may not be totally representative of actual service conditions, but provided indications for improving the ageing protocols in future research. In the last part, an analysis of the distributed strain profiles collected during pull-out tests on instrumented concrete specimens clearly illustrated the consequences of ageing processes on the strain response of the DOFS cable.