Distributed Fiber Optic Sensors Research Articles

Structural damage identification of subseabed shield tunnels based on distributed fiber optic sensors and information fusion

Subseabed shield tunnels are constantly subjected to erosion induced by seepage and corrosive ions and electrochemical effects of diffuse currents from subway traction locomotives during service, resulting in damage to the outer wall of the segment. However, the outer wall cannot be directly observed, making damage identification a challenge. In this research, an approach based on distributed fiber optic sensors (DFOSs) and information fusion is proposed to solve this issue. To monitor strain, DFOSs were installed along the circumference of the segment inner wall. The strain curvature was adopted to solve the adverse influence of the spatial resolution (SR) averaging effect in DFOSs. Using the strain and strain curvature as information sources, the strain damage probability index (SDPI) and strain curvature damage probability index (SCDPI) were developed. Based on Dempster-Shafer (D-S) evidence theory, the SDPI and SCDPI were further utilized to build basic probability assignment (BPA) functions and perform information fusion for damage identification. To validate the feasibility of this approach, model experiments were conducted. The analysis of the test data indicated that information fusion can suppress the independent features of the two information sources while emphasizing their common features. The SR averaging effect, which can cause data distortion and diminish or even mask the damage anomaly feature in strain, and multiple extrema in the strain curvature that can interfere with damage localization are simultaneously eliminated. The damage identification accuracy is improved, and the damage localization error for the segment outer wall can reach 0.79 cm.

Axial pullout behaviour of fusible PVC pipes and fusion weld joints assessed using distributed fibre optic sensors

Pipes are subjected to axial pulling forces that lead to shear stresses at the soil–pipe interface and axial strain distributions from various circumstances, including pulled-in-place pipe installation and permanent ground deformations. This study investigates the axial pullout response of fusible polyvinyl chloride pipes in dense sand. Three tests were conducted using a large-scale soil chamber to assess the development of axial resistance and the impact of non-continuous pulling and fusion weld joints. The measured maximum load response was significantly underpredicted by guidelines typically used in practice. Additionally, non-continuous pulling and changes to the pull rate had negligible impacts on the load–displacement response. Pipes were instrumented with distributed fibre optic sensors (DFOS) to capture the nonlinear development of strains along these flexible pipes and the mobilization of soil friction along the soil–pipe interface. Measurements have provided insight into the propagation of axial strains along the pipe, the additional longitudinal restraint provided by fusion weld joints, and their impact on the axial force distribution along pipes. The “locked-in” strains due to residual soil–pipe friction remaining after the axial pulling force is removed have also been measured using DFOS.

A data driven methodology for upscaling remaining useful life predictions: From single- to multi-stiffened composite panels

In this paper we execute a complex test campaign to develop a novel methodology for the Remaining Useful Life (RUL) estimation of complex multi-stiffened composite aeronautical panels utilizing Machine Learning models trained with Structural Health Monitoring (SHM) data from hierarchically simpler elements, i.e., single-stiffened panels. Distributed Fiber Optical sensors (DFOS) are employed to monitor the panels’ behavior undergoing variable amplitude compression-compression fatigue after multiple impacts. A data processing methodology is first applied to the DFOS data, to both alleviate the effect of the variable loading conditions on the monitored strain and ease the computational burden. In this upscaling endeavor, an advanced strain-based Health Indicator (HI) based on Genetic algorithms, created and validated on the single-stiffened panel data, is utilized as the prognostic feature for the RUL estimations of the multi-stiffened panels. The HI displays favorable characteristics in terms of monotonicity and prognosability which are highly desirable for more accurate RUL estimations. For the prognostic task, standard machine learning models are trained using the historical degradation data of the single-stiffened panels and a similarity analysis is performed to enhance the accuracy when predicting the RUL of the multi-stiffened panels. Despite the increased structural complexity of the multi-stiffened panels, we demonstrate that the RUL is able to be predicted with reasonable accuracy. The present work paves the road for upscaling and applying prognostic methodologies to more complex structures beyond simple coupons or generic elements.

Customizing nanoparticle characteristics in Ba-rich nanoparticle-doped optical fibers to tune Rayleigh scattering

Rayleigh scattering enhanced nanoparticle-doped silica-based optical fibers have recently emerged for distributed sensing applications. To date, Ca-, Sr- and Mg-based nanoparticles have been demonstrated to be very promising for it. In this work, we add in situ growth Ba-rich nanoparticles to alkaline earth-based compositions suitable to fabricate tunable Rayleigh scattering enhanced nanoparticle-doped optical fibers. Preform fabrication conditions allow largely customizing the features of the nucleated nanoparticles in the preform core. Fiber drawing process further impact on their size and composition due to a possible reaction with the silica-based glass and allows large customization, which is translated into an optimized trade-off between Rayleigh scattering enhancement of 35.4–44.9 dB, regarding a SMF-28, and two-way optical losses, 1.2–6.9 dB/m. Sensing lengths from 6.5 to 28.7 m become possible. The possibility of easily tailoring Ba-rich nanoparticle characteristics strengthen the potential of in situ grown alkaline earth nanoparticles for future distributed optical fiber sensors.

A Model-Assisted Probability of Detection Framework for Optical Fiber Sensors

Optical fiber sensors (OFSs) represent an efficient sensing solution in various structural health monitoring (SHM) applications. However, a well-defined methodology is still missing to quantify their damage detection performance, preventing their certification and full deployment in SHM. In a recent study, the authors proposed an experimental methodology to qualify distributed OFSs using the concept of probability of detection (POD). Nevertheless, POD curves require considerable testing, which is often not feasible. This study takes a step forward, presenting a model-assisted POD (MAPOD) approach for the first time applied to distributed OFSs (DOFSs). The new MAPOD framework applied to DOFSs is validated through previous experimental results, considering the mode I delamination monitoring of a double-cantilever beam (DCB) specimen under quasi-static loading conditions. The results show how strain transfer, loading conditions, human factors, interrogator resolution, and noise can alter the damage detection capabilities of DOFSs. This MAPOD approach represents a tool to study the effects of varying environmental and operational conditions on SHM systems based on DOFSs and for the design optimization of the monitoring system.

Young’s Modulus-Independent Determination of Fibre Parameters for Rayleigh-Based Optical Frequency Domain Reflectometry from Cryogenic Temperatures up to 353 K

The magnetic spectrometer AMS-100, which includes a superconducting coil, is designed to measure cosmic rays and detect cosmic antimatter in space. This extreme environment requires a suitable sensing solution to monitor critical changes in the structure such as the beginning of a quench in the superconducting coil. Rayleigh-scattering-based distributed optical fibre sensors (DOFS) fulfil the high requirements for these extreme conditions but require precise calibration of the temperature and strain coefficients of the optical fibre. Therefore, the fibre-dependent strain and temperature coefficients KT and Kϵ for the temperature range from 77 K to 353 K were investigated in this study. The fibre was integrated into an aluminium tensile test sample with well-calibrated strain gauges to determine the fibre’s Kϵ independently of its Young’s modulus. Simulations were used to validate that the strain caused by changes in temperature or mechanical conditions was the same in the optical fibre as in the aluminium test sample. The results indicated a linear temperature dependence of Kϵ and a non-linear temperature dependence of KT. With the parameters presented in this work, it was possible to accurately determine the strain or temperature of an aluminium structure over the entire temperature range from 77 K to 353 K using the DOFS.

Coherent OTDR with large dynamic range based on double-sideband linear frequency modulation pulse.

Rayleigh scattering-based distributed optical fiber sensors with long sensing distance and large dynamic range are highly desired for application scenarios such as vehicle tracking, structure health monitoring, and geological survey. To enlarge the dynamic range, we propose a coherent optical time domain reflectometry (COTDR) based on double-sideband linear frequency modulation (LFM) pulse. By utilizing I/Q demodulation, both the positive and negative frequency band of the Rayleigh backscattering (RBS) signal can be properly demodulated. Consequently, the dynamic range is doubled without increasing the bandwidth of signal generator, photodetector (PD), and oscilloscope. In the experiment, the chirped pulse with 10 μs pulse width and 498 MHz frequency sweeping range is launched into the sensing fiber. Single-shot strain measurement is achieved over 5 km single-mode fiber with a spatial resolution of 2.5 m and a strain sensitivity of 7.5 pε/H z. A vibration signal with 3.09 με peak-to-peak amplitude (corresponding to 461 MHz frequency shift) is successfully measured with the double-sideband spectrum, which cannot be properly recovered with the single-sideband spectrum.

Degradation Analysis of Single-Mode and Multimode Fibers in a Full-Scale Wellbore and Its Impact on DAS and DTS Measurements

Distributed fiber optic sensors (DFOS) are viewed as a promising technology for efficient, cost-effective, and non-intrusive real-time monitoring for a variety of applications. The chemical passivity, non-magnetic interference behavior, and ability to acquire continuous measurements at high-spatial resolution have made DFOS very attractive for in-situ measurement of temperature, strain, vibration, etc. The accuracy of the measured parameter by the DFOS is highly dependent on the strength and quality of the optical signal returning from the point of interest. The signal degradation in DFOS results from optical losses due to attenuation, absorption, scattering, bending, dispersion, and coupling. This study analyzes the degradation observed in both the single-mode and multimode fibers installed in a 1574 mdeep wellbore for acquiring distributed acoustic sensing (DAS) and distributed temperature sensing (DTS) data, respectively. The degradation was unexpected as it was observed within three years of DFOS installation in an experimental, non-producing well that contains non-corrosive fluids and operates in a low-temperature (<54.4 °C) and low-pressure (<3500 psi or 24131 kPa) environment. A root-cause analysis was performed to investigate the cause of this degradation using time-lapse optical loss measurements acquired at different wavelengths using an optical time-domain reflectometer. Degradation of the multimode fiber was also examined by the time-lapse analysis of the Raman backscatter that is used for DTS measurement. Based on the investigation, microbending was found to be the main cause of the observed degradation. Its impact on the quality of the DAS and DTS measurements was also analyzed both qualitatively and quantitively. Recommendations for minimizing degradation are presented.

Transient Strain Monitoring of Weldments Using Distributed Fiber Optic System

The primary objective of this study was to evaluate the capability of a distributed fiber optic sensor to capture in situ dynamic transient strain formation during and post-weldment on the surface of a steel plate. The study involved a vertical manual weld of a bead on a plate on a 300 mm × 300 mm × 6.35 mm A36 steel plate (European equivalent S235J2; Chinese equivalent Q235B) clamped at the corners. A fiber optic distributed sensor was used to measure the surface total and thermal strains on the welded side of the plate adjacent to the weld path. Experimental results show a complex behavior of strain changes during the welding process and the residual strain formation post-welding. This study aims to document the use of distributed fiber optic sensing techniques in welding applications. Validations of the experimental data were performed using VrWeld, a commercial software framework for computational weld mechanics, and an iPhone FLIR One Pro. thermographic camera. The experimental results demonstrated that although distributed fiber optic sensing based on Rayleigh backscattering is an appropriate and useful technique for total strain measurements, the manufacturing and the materials used for the thermal sensors are critical in obtaining optimal results. Finally, this study highlights the challenges encountered in synchronizing large experimental data sets captured with different instruments with computational welding mechanic (CWM) models.

Comparative analysis of anchor cables in pullout tests using distributed fiber optic sensors

The interaction mechanism between grouted anchor cables and the surrounding rock mass or soil are complicated and difficult to investigate using conventional monitoring technologies. This study focuses on the in-situ pullout behavior of grouted anchor cables using Brillouin Optical Time-Domain Analysis technique. A series of pullout tests were conducted on anchor cables with five different anchor lengths and three grouting methods. Distributed fiber optic sensors were used to measure the strain distribution of cable bolts from tip to top. The in-situ pullout test results show that both ultimate pullout resistance and ultimate displacement at the ultimate pullout resistance increase with anchor length. The bond strength suffers a slight variation with an increase in anchor length. Both ultimate pullout resistance and the ultimate displacement for Type-B grouting (grouting with reaming) and Type-C grouting (secondary grouting) are larger than those for Type-A grouting (one-time grouting). The shear bond strength of Type-C grouting is greater than that of Type-A grouting. Results from Gaussian functions analysis show that it is feasible to use the function to characterize the axial stress of anchor cables. Furthermore, the overall mobilized percentage η was evaluated. The anchor cables with smaller anchor lengths will result in greater η. The η for the anchor cables of 6 m reaches 92% at the last load, whereas the anchor cables of 18 m were mobilized by only 29% at the last load. The grouting method has a negligible effect on the η, increasing from 22% to 29% in process of the pullout test.

Unified inverse isogeometric analysis and distributed fiber optic strain sensing for monitoring structure deformation and stress

Inverse analysis is an efficient and reliable approach to the estimation of structural state such as deformation and stress distribution from measured real-world data and numerical algorithms. In this work, a new method of inverse analysis is proposed which integrates Isogeometric Analysis (IGA) and distributed fiber optic sensing (DFOS) using a framework based on non-uniform rational B-spline (NURBS) functions. This formulation expands on the existing inverse Finite Element Methods (iFEM) by developing a new approach to use the exact geometry of strain-sensing optical fibers. The constraint equations of theoretical and measured strains are derived using a discrete least-squares functional and then coupled with isogeometric finite element equations to provide regularization for ill-posed inverse problems. The explicit strain fiber model not only separates the structure mesh and strain sensing locations but also can adapt to the sensing parameters such as spatial resolution of DFOS instrument, which is not yet possible with the current models. The proposed method is validated with four numerical examples using plate, hemisphere and cylindrical geometries under prescribed force or displacement boundary conditions. Furthermore, the sensitivity of the inverse solution to the sampling interval and measurement noise is studied. The unified inverse IGA and DFOS is tested under simulated measurement noise up to 30 micro-strains with strain sampling intervals ranging from 10 mm to 1 m, and validated with the comparisons of reference and inverse solutions.

Damage Analysis of Segmental Dry Joint Full-Scale Prestressed Cap Beam Based on Distributed Optical Fiber Sensing.

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.

Study on the bond performance of glass fiber-reinforced polymer bars considering the relative position between longitudinal bars and stirrups

Fiber-reinforced polymer (FRP) reinforcements are increasingly recognized as an effective solution to the corrosion problems faced by reinforced concrete structures, and the bond between FRP bars and concrete is an essential basis for the application. Previous research has shown that stirrup restraints on the concrete can significantly enhance the bond performance of middle-arranged longitudinal FRP bars. However, in practical applications, longitudinal bars are typically placed at the corners or sides of stirrups to form reinforcement cages, and there has been limited research on the effects of stirrups on the bond performance of FRP bars under these conditions. To address this gap, this study attempts to further reveal the stirrup effects by varying the relative positions between the longitudinal bars and stirrups. Based on forty-four axial pull-out tests on glass fiber-reinforced polymer (GFRP) bars equipped with Distributed Optical Fiber Sensors (DOFS), the overall and local bond performances of three GFRP bars (sand-coated, helical wound, single helical wrapped) considering four relative positions between the GFRP bars and stirrups (no stirrups, side, middle, corner) were investigated. The test results indicated that the bond strength of FRP bars increased significantly, up to 71.5%, for the relative position of the middle case, but could be reduced by up to 17.5% for the side and corner positions compared with the no-stirrup situation. DOFS proved to be an effective method to obtain the continuous strain distribution and reveal the interfacial bond mechanism. Based on DOFS results, a bond stress-slip model considering the effect of relative positions was developed. Using this model, the development lengths of GFRP bars were proposed and compared with the ACI, CSA, and JSCE codes.

Fatigue Repair of Cracked Steel Plates Using Small-Patch Ultrahigh-Modulus CFRP Governed by Bond Failure

This study evaluates the effectiveness of externally bonded small-patch ultrahigh-modulus (UHM) carbon fiber–reinforced polymer (CFRP) plates of 460 GPa modulus in extending fatigue life and slowing crack propagation in steel plates with simulated fatigue cracks. The study investigates a range of very short bond lengths for applications where bonding space may be limited or accessibility restricted, together with the effect of single- versus double-sided application, on fatigue performance. It also compares short UHM CFRP plates with similar length normal modulus (NM) (165 GPa) CFRP plates with regard to extending fatigue life and reducing stress concentration at the crack tip. Distributed fiber-optic sensors (DFOS) and digital-image correlation (DIC) are used to capture the full strain field and track crack growth. Results showed that fatigue life was increased by up to 2.0 and 2.34 times for single- and double-sided UHM CFRP repairs. As bond length increased from 25 to 100 mm, fatigue life increased from 1.36 to 2.0. UHM CFRP more effectively reduced stress concentration at the crack tip, by 60% compared with 37% for NM CFRP, indicating that it has the potential for superior fatigue life gains, relative to NM CFRP if the necessary bond length to prevent or delay debonding is provided. As debonding failure governed in this study, NM CFRP plates achieved comparable fatigue life extension. DIC tracking of crack growth matched the traditional “beach marking” technique with less than 6% difference.

Calculation the opening of neighboring surface cracks in concrete structure based on OFDR technology

Under the influence of temperature change, construction quality, and external force, concrete structures are prone to generate cracks which poses a significant threaten to the safety of the structure. With the advantages of distributed monitoring over traditional methods, the distributed fiber optic sensing (DFOS) technology has become a widely accepted method in concrete cracks detection and monitoring. Both laboratory tests and field monitoring indicate that the crack location can be determined by the strain peak and the width of cracks can be derived by integrating the strain exerted on the optical cable. Nevertheless, when the neighboring crack spacing distance (CSD) is sufficiently small, the strain induced by cracks will overlap and two strain peaks will be partially weighted, forming a saddle-shaped double strain peak. Thus, how to calculate the crack opening displacement (COD) becomes a big challenge. To tackle this issue, an analytical method to calculate the opening of neighboring cracks based on the shear strain transfer mechanism is proposed. The effectiveness of this method was evaluated through a crack simulation experiment and further validated by a concrete beam loading test. The strain variations induced by neighboring cracks were recorded by optical frequency domain reflectometry (OFDR) technology and the width of neighboring cracks was calculated using the proposed method. By comparing the calculation results with measured values, the accuracy of the proposed method was verified, which provided a feasible way to calculate the COD of neighboring cracks with strain overlap.

Fiber optic monitoring of an anti-slide pile in a retrogressive landslide

Anti-slide piles are one of the most important reinforcement structures against landslides, and evaluating the working conditions is of great significance for landslide mitigation. The widely adopted analytical methods of pile internal forces include cantilever beam method and elastic foundation beam method. However, due to many assumptions involved in calculation, the analytical models cannot be fully applicable to complex site situations, e.g. landslides with multi-sliding surfaces and pile-soil interface separation as discussed herein. In view of this, the combination of distributed fiber optic sensing (DFOS) and strain-internal force conversion methods was proposed to evaluate the working conditions of an anti-sliding pile in a typical retrogressive landslide in the Three Gorges reservoir area, China. Brillouin optical time domain reflectometry (BOTDR) was utilized to monitor the strain distribution along the pile. Next, by analyzing the relative deformation between the pile and its adjacent inclinometer, the pile-soil interface separation was profiled. Finally, the internal forces of the anti-slide pile were derived based on the strain-internal force conversion method. According to the ratio of calculated internal forces to the design values, the working conditions of the anti-slide pile could be evaluated. The results demonstrated that the proposed method could reveal the deformation pattern of the anti-slide pile system, and can quantitatively evaluate its working conditions.

Numerical investigation of fibre-optic sensing for sinkhole detection

This paper forms part of the SINEW (sinkhole early warning) project and continues the work conducted by Möller and co-workers in 2022, where 1g experiments demonstrated the feasibility of using distributed fibre-optic sensing (DFOS) for sinkhole early warning. Their experimental campaign highlighted an order of magnitude difference in the strain between the soil and the cable that remains unexplained and weakens confidence in the technology and/or the experimental method. This paper uses three-dimensional finite-element analyses to examine further this discrepancy and the soil–cable interface. The results support the experimental findings and demonstrate that the DFOS signature strain profile is induced by the horizontal movement of the ground, and enhanced when sufficient coupling at the soil–cable interface is achieved. This result holds when modelling is scaled to realistic confining pressure, and its significance is twofold. First, this needs to be accounted for in the DFOS laying technique. Second, particles of cohesionless soils undergo relatively high horizontal displacement away from the centre of the sinkhole, and this means that DFOS cables are able to detect subsidence away from the centre of the sinkhole. The paper illustrates this result and the signature strain profile expected in this case.

Experimental Study on Deformation Behavior of Shield Tunnel Subjected to Riverbed Scour Based on DOFS

Exploring the deformation behavior of shield tunnels is significant to design and safety assessment for shield tunnel structures. This paper carries out a reduced scale model test of shield tunnel subjected to riverbed scour for the first time. A new loading device system is designed to simulate the initial stress field of stratum and effect of riverbed scour. Distributed optical fiber sensing (DOFS) technology is adopted as measuring method to achieve the surrounding soil deformation and structural deformation, ovality, and radius of curvature of shield tunnel. The test results show that uneven rebound deformation of stratum and shield tunnel structure occurs in both transverse and longitudinal directions under the action of riverbed scour. Variations in ovality differences exist along the longitudinal direction of the shield tunnel, indicating that the transverse and longitudinal deformation of the shield tunnel affects each other. The radius of curvature of the longitudinal deformation of the shield tunnel at the vault is much smaller than that of the bottom. The results demonstrate the broad application value of DOFS technology in shield tunnel structure monitoring and reveal the deformation mechanism of shield tunnels subjected to riverbed scour, which can provide important reference values for the structure design and safety monitoring of shield tunnels.

Performance Analysis of a Distributed Optical Fiber Vibration Sensor Based on a Sagnac Interferometer

In this work, measuring capabilities of a Sagnac Interferometer-based Distributed Optical Fiber Sensor (DOFS) system are demonstrated by comparing its results to a conventional amplitude-based Distributed Acoustic Sensing (DAS) system. Through a set of three different experiments, it was demonstrated that both sensing systems can detect and locate dynamical deformation along the fiber, in addition to determining its frequency components. The coherent amplitude-based DAS presented a lower background noise. In contrast, even though the Sagnac interferometry-based system presented low-frequency noise in the measurements, it is less susceptible to harmonic distortion for higher amplitude deformations since its sensing principles rely on the measurement of the accumulated phase rather than the amplitude of the backscattered signal purely. Accordingly, the Sagnac interferometry-based system arises as a promising and rather accessible topology for a DOFS implementation.

Automatic interpretation of strain distributions measured from distributed fiber optic sensors for crack monitoring

Distributed fiber optic sensors have exhibited superior capabilities in monitoring cracks in engineering structures through measuring detailed strain distributions. However, manually interpreting the measurements from long distributed sensors deployed in large-scale structures is time-consuming, labor-intensive, and subject to human errors. This paper proposes to automate the identification, localization, quantification, and visualization of cracks through intelligent interpretation of strain distributions measured from distributed fiber optic sensors based on machine learning. Based on these intelligent capabilities, a live digital twin model based on building information modeling is developed to visualize cracks. The digital twin model is updatable with real-time measurements from strain distributions from distributed fiber optic sensors. The proposed approach is evaluated via laboratory testing of a concrete beam. The results show that the proposed approach achieves high accuracy in interpretation of sensor data for crack monitoring. This research advances the capabilities of structural health monitoring using distributed fiber optic sensors.