Fiber Optic Strain Sensors Research Articles

Femto-strain resolution fiber-optic sensing with an ultra-simple white-light round trip filtering method.

In the past decade, laser-driven resonant fiber-optic sensors (RFOSs) have been reported touching their ultimate resolution limit. The practicability of these high-performance sensors is, however, discounted because of high system complexity and dependence on narrow-linewidth lasers. In this paper, a novel, to the best of our knowledge, white-light-driven RFOS is established based on a round trip filtering (RTF) method. Via measuring the RTF loss of an add-drop fiber ring resonator (FRR) sensor, strain signal can be read out with an ultra-simple open-loop configuration. In the sensing experiment, even a resolution of several femto-strain around 1 kHz is demonstrated, representing the highest resolution level of RFOS to date. Thanks to the obvious superiority in both resolution, simplicity, and cost over traditional laser-driven RFOSs, the proposed white-light-driven RFOS is believed to be a milestone in the development of fiber-optic strain sensors.

Study on strain sensing property of fiber Bragg grating based on surface strain measurement of mechanical structure.

Strain monitoring is of great significance to identify the failure of key mechanical components and ensure the good operation of mechanical equipment. In terms of the common issue of the low sensitivity of fiber Bragg grating (FBG) strain sensor in strain measurement on the mechanical structure surface, this paper describes a flexible hinge strain sensor with the FBG as the sensitive element. A theoretical analysis of the sensitivity of the sensor is performed, a copper alloy material with a large elastic modulus is selected, the simulation analysis and optimization design for the structural parameters of the sensor are carried out by using ANSYS and Solidworks software, then the object of the sensor is developed according to the simulation results, and finally, the performance of the sensor is tested. The results show that the maximum deformation the sensor can measure is 0.5299 µε, the sensitivity of the sensor is ∼1.8870 pm/µε, which is approximately twice that of the bare FBG sensor, the linearity is greater than 99%, and the relative standard deviation of the repeatability is 0.9%. The results of the study provide a reference for developing the optical fiber strain sensor and further improving its sensitivity.

A strain reflection-based fiber optic sensor using thin core and standard single-mode fibers

We propose and demonstrate a fiber optic strain sensor based on a simple splice between a thin core fiber and a piece of conventional single-mode fiber. Mode dispersion generates an interference reflection spectrum sensitive to strain and temperature. The sensor offers a competitive strain sensitivity of 2.4 pm/μɛ, good linearity (R2=0.9969), and a cursory repeatability analysis yielded variation. The thermal studies indicate minimal cross-sensitivity (2.9 μɛ/°C). The sensor offers a simple implementation without any intricate manufacturing.

An Improved Strain Sensor Based on Long-Period Fiber Grating With a Local Ellipse-Core Structure

In this paper, we proposed a novel long-period fiber grating strain sensor based on local ellipse-core fiber (LEC-LPFG). The LEC-LPFG was fabricated by laser polishing and local heating methods, which could efficiently transform a single-mode fiber (SMF) into a periodic ellipse-core fiber. The effects associated with the fiber elliptical core shape were shown to significantly influence the strain sensitivity of the LEC-LPFG. Further, it was demonstrated that the LEC-LPFG has a high strain sensitivity of −20.2 pm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> and a temperature sensitivity of 68.6 pm/°C. Our work provides a new technology for straightforward, low-cost, and large-scale productions of premium fiber optical strain sensors which are desirable in many practical applications.

Nonnuclear Experimental Capabilities to Support Design, Development, and Demonstration of Microreactors

This work provides a summary of selected experimental capabilities being developed to support nonnuclear testing and demonstration of technology in support of microreactors under the U.S. Department of Energy’s (DOE’s) Microreactor Program. Major capabilities include the Single Primary Heat Extraction and Removal Emulator (SPHERE) and the Microreactor Agile Non-nuclear Experimental Test Bed (MAGNET). The SPHERE facility allows for controlled testing of the steady-state and transient heat rejection capabilities of a single heat pipe using electrical heaters that simulate nuclear heating. The facility is capable of monitoring axial temperature profiles along the heat pipe and surrounding test articles during startup, steady-state operation, and transients. Instrumentation includes noncontact infrared thermal imaging, surface thermocouples, spatially distributed fiber optic temperature and strain sensors, electrical power meters, and a water-cooled, gas-gap calorimeter for quantifying heat rejection from the heat pipe. The facility can be operated under both vacuum and inert-gas conditions. The MAGNET facility is a large-scale, 250-kW electrically heated microreactor test bed to enable nonnuclear experimental evaluation of a variety of microreactor concepts. It can be supplied to electrically heat a scaled section of a microreactor and further test the capabilities of heat rejection systems. The initial MAGNET experiments will support technology maturation and reduce uncertainty and risk associated with the design, operation, and deployment of monolithic heat pipe–based reactors. However, this test bed can broadly be applied to multiple microreactor concepts to evaluate a wide range of thermal-hydraulic and structural phenomena such as interface coupling with power conversion units and other collocated systems. MAGNET can evaluate integral thermomechanical effects during electrical heating of an array of heat pipes in a larger test article. Examples of initial testing will include thermal stresses in the monolith and the impact of debonding of a heat pipe from the core block and how that failure could impact surrounding heat pipes, i.e., understanding the potential for cascading failure. This work also discusses some modeling capabilities that can support experiment design, analysis, and interpretation, including the heat pipe code Sockeye and a comparison of thermal-structural simulations performed using ABAQUS and STAR-CCM+.

Fibre-optic measurement of strain and shape on a helicopter rotor blade during a ground run: 1. Measurement of strain

Optical fibre strain and shape measurement sensors were deployed on a 5-m long rotor blade during a full-speed (rotation rate 6.6 Hz) helicopter ground run, with real-time data wirelessly streamed from rotor hub-mounted sensor interrogators. In Part 1 of a 2-part paper series, the strain sensing capabilities of the two optical fibre-based sensing techniques, optical fibre Bragg grating (FBG) and fibre segment interferometry (FSI), are compared, while Part 2 (Kissinger et al 2022 Smart Mater. Struct. accepted) specifically investigates the blade shape measurement based on the FSI approach. In part 1, the rotor hub-mounted instrumentation is described, and data on the dynamics of the blade obtained from a sequence of controlled pilot inputs are analysed both in the time and spectral domains. It is shown that this can provide insights into the aeroelastic properties of the blade. Noise standard deviations of 0.2 n/ and 30 n/ for the FSI and FBG-based sensing approaches, respectively, were observed over a strain range of 3500 µϵ.

Fibre-optic measurement of strain and shape on a helicopter rotor blade during a ground run: 2. Measurement of shape

Optical fibre strain and shape measurement sensors were deployed on a rotor blade during a full-speed helicopter ground run, with real-time data wirelessly streamed from rotor hub-mounted sensor interrogators. In part 2 of a 2-part paper series, two-dimensional direct fibre-optic shape sensing (DFOSS), using fibre segment interferometry-based interrogation is investigated. The concept of blade shape change visualisation over one rotation period using rotation displacement surfaces is introduced and the usefulness of DFOSS data to gain additional insights by determining operational modal frequencies independently for both horizontal and vertical vibration directions of the blade is demonstrated.

Large Deformation Shape Sensing Using a Nonlinear Strain to Displacement Method

This paper presents a three-dimensional strain-to-displacement transformation method based on a nonlinear modal formulation, suitable for cases of large structural deformations. The method is first demonstrated numerically on a complex beam-like structure test case. A sensitivity study presents the effects of different solution parameters, such as the number of modes and sensors used and the measurement noise. The method is then applied to an experimental test case of the Pazy wing that undergoes very large deformations of 50% span. The wing is equipped with fiber optic strain sensors and a motion-tracking camera system that provides the reference displacement data. The modal data are extracted experimentally, and the scheme is validated using data from a structural loading test. The method is used in a wind tunnel test to map the wing’s deformations from measured strain data. Results are compared to those of a linear modal strain-to-displacement transformation method and to displacements computed by direct integration of the measured strains. The process is shown to yield superior results in cases of large deformations.

Mode-dependent dye absorption in phthalocyanine-doped acrylic optical fiber

Phthalocyanine has attractive features as a dopant for plastic optical fibers. Its planar molecular structure and red absorption band are suitable for the design of fiber-optic strain sensors that use the dynamic orientation of dye induced by the deformation of the host matrix. The guidelines to achieve a phthalocyanine-doped fiber with a ruled dispersing manner were investigated. A synthesized fiber using phthalocyanine decorated with tertiary butyl groups showed mode-dependent absorption, which suggested gradual dye distribution in the core radial direction.

Monitoring Flexure Behavior of Compacted Clay Beam Using High-Resolution Distributed Fiber Optic Strain Sensors

Abstract Understanding the mechanical characteristics of clayey soil is critical to estimate the stability of clay-related geotechnical engineering works. However, the distributed information of the internal deformation of clay is hard to capture through traditional methods. In this paper, a set of four-point bending tests were performed on a compacted clay beam instrumented with fiber optic strain sensors to investigate the flexure and cracking behaviors. The optical frequency domain reflectometry technique with high spatial resolution and precision was employed to establish a distributed strain sensing (DSS) system in this study. The results show that the DSS system exhibits a high accuracy in the collection of the clay deformation in terms of compression, tension, and cracking. The comparison of the internal strain profiles with the observation of particle image velocimetry analysis system verifies the feasibility of fiber optic sensing technology in the quantitative assessment of clay deformation. The performance of fiber optic sensors in the rebound deformation and creep deformation monitoring was preliminarily issued in the study. In addition, the progressive failure on the fiber–soil interface was analytically studied to evaluate the deformation compatibility between the fiber and surrounding clayed soil. The conclusions drawn in this paper can help to guide the state interpretation of clayey soil instrumented with fiber optic sensors.

Input torque measurements for wind turbine gearboxes using fiber-optic strain sensors

Abstract. Accurate knowledge of the input torque in wind turbine gearboxes is key to improving their reliability. Traditionally, rotor torque is measured using strain gauges bonded to the shaft. Transferring the resulting signal from the rotating shaft to a stationary data acquisition system while powering the sensing devices is complex and costly. The magnitude of the torques involved in wind turbine gearboxes and the high stiffness of the input shaft pose additional difficulties. This paper presents a new alternative method to measure the input torque in wind turbine gearboxes based on deformation measurements of the static first-stage ring gear. We have measured deformation using fiber-optic strain sensors based on fiber Bragg gratings because of their advantages compared to conventional electrical strain gauges. The present study was conducted on a Siemens Gamesa Renewable Energy gearbox with a rated power of 6 MW, in which a total of 54 fiber-optic strain sensors were installed on the outer surface of the first-stage ring gear. The gear mesh forces between the planets and the ring gear cause measurable deformations on the outer surface of the stationary ring gear. The measured strains exhibit a dynamic behavior. The strain values change depending on the relative position of the strain sensors to the planet gears, the instantaneous variations of the input torque, and the way load is shared between planets. A satisfactory correlation has been found between the strain signals measured on the static ring gear and torque. Two signal processing strategies are presented in this paper. The first procedure is based on the peak-to-peak strain values computed for the gear mesh events, and therefore torque can only be estimated when a gear mesh event is detected. The second signal processing procedure combines the strain signals from different sensors using a Coleman coordinate transformation and tracks the magnitude of the fifth harmonic component. With this second procedure, it is possible to estimate torque whenever strain data of all sensors are available, leading to an improved frequency resolution up to the sampling frequency used to acquire strain data. The method presented in this paper could make measuring gearbox torque more cost-effective, which would facilitate its adoption in serial wind turbines and enable novel data-driven control strategies, as well as a more accurate assessment of the consumed fatigue life of the gearboxes throughout their operation.

A neural network-informed self-aware deployable structure with application to phased array antennas

State of the art radio frequency (RF) arrays are growing larger in pursuit of increased signal-to-noise ratio. In support of this goal, elaborate forms of metrology are being developed to support the increased footprints. This work provides a unique solution to fulfill the metrology requirements of large-scale deployable RF antennas through the implementation of neural network demodulation of fiber optic strain sensors. The fiber optics are patterned with fiber Bragg gratings (FBGs) to encode strain on to back-reflected shifts in the wavelength of incident light. Experiments show the neural network can predict the deformation of a test structure within single millimeters for small amplitude motions. Therefore, the current technique meets the required precision needed for large scale deployable RF arrays operating at S-band or longer wavelengths.

Modeling of Supplemental Bar-Mounted Fiber Optic Strain Sensor for Structural Health Monitoring Applications

Abstract Fiber optic sensors have been increasingly utilized in structural health monitoring of large-scale civil structures. Bare fiber sensors are quite brittle, and therefore, their installation and embedment in reinforced concrete elements can be challenging, particularly when using uncommon materials as internal reinforcements in concrete. In the present study, a fiber optic strain sensor is preinstalled on a supplemental bar of adequate length and appropriate diameter. The sensor is attached to a glass fiber-reinforced polymer (GFRP) reinforcing bar in concrete flexural element. Performance under static-loading conditions has been evaluated, and the results have shown potential toward applying the technique to large-scale structures. Another objective of the present study is to develop a numerical model that represents the interaction between the concrete, the reinforcement steel, and the supplemental GFRP rebar, which has the sensor mounted on. The model is calibrated using experimental results. The model can be used to investigate varying parameters including material properties (e.g., compressive strength of concrete), geometrical data (e.g., the length of the supplemental rebar), and loading and boundary conditions, consequently eliminating the need to perform a large number of full-scale costly experiments. The developed model exhibited nearly identical behavior to the experiments after calibration. The study shows that the performance of the present sensing system is primarily affected by the relative sizes of the main and supplemental bars.

Calibration Technology of Optical Fiber Strain Sensor

Calibration Technology of Optical Fiber Strain Sensor

The Effect of Fault Architecture on Slip Behavior in Shale Revealed by Distributed Fiber Optic Strain Sensing

AbstractWe use Distributed Strain Sensing (DSS) through Brillouin scattering measurements to characterize the reactivation of a fault zone in shale (Opalinus clay), caused by the excavation of a gallery at ∼400 m depth in the Mont Terri Underground Laboratory (Switzerland). DSS fibers are cemented behind casing in six boreholes cross‐cutting the fault zone. We compare the DSS data with co‐located measurements of displacement from a chain potentiometer and a three‐dimensional displacement sensor (SIMFIP). DSS proves to be able to detect in‐ and off‐fault strain variations induced by the gallery excavated 30–50 m away. The total permanent displacement of the fault is ∼200 microns at rates up to 1.5 nm/s. DSS is sensitive to longitudinal and shear strain with measurements showing that fault shear is concentrated at the top and bottom interfaces of the fault zone with little deformation within the fault zone itself. Such a localized pattern of strain relates to the architecture of the fault that is characterized by a thick “layer” including an anastomosing network of polished surfaces where clay‐rich rock splits into progressively smaller flakes conferring the entire zone very little cohesion and friction (weak zone), and favoring slipping at the edges of the “layer”. Overall, DSS shows that slow slip may activate everywhere there is a weak fault within a shale series. Thus, our work demonstrates the importance of shear strain on faults caused by remote loading, highlighting the utility of DSS systems to detect and quantify these effects at large reservoir scales.

Long-Term Monitoring of a Tunnel in a Landslide Prone Area by Brillouin-Based Distributed Optical Fiber Sensors.

This paper shows the results of the monitoring of the deformations of a tunnel, carried out using a distributed optical fiber strain sensor based on stimulated Brillouin scattering. The artificial tunnel of the national railway crosses the accumulation zone of an active landslide, the Varco d’Izzo earthflow, in the southern Italian Apennines. Severely damaged by the landslide movements, the tunnel was demolished and rebuilt in 1992 as a reinforced concrete box flanked by two deep sheet pile walls. In order to detect the onset of potentially dangerous strains of the tunnel structure and follow their time trend, the internal deformations of the tunnel are also monitored by a distributed fiber-optic strain sensor since 2016. The results of the monitoring activity show that the deformation profiles are characterized by strain peaks in correspondence of the structural joints. Furthermore, the elongation of the fiber strands crossing the joints is consistent with the data derived by other measurement systems. Experiments revealed an increase in the time rate of the fiber deformation in the first and last part of the monitoring period when the inclinometers of the area also recorded an acceleration in the landslide movements.

In-situ calibration of in-service strain sensing system under passive excitation: methodology and impacts assessment

Aiming at the calibration of in-service strain sensing systems for structural health monitoring (SHM), a technical framework is proposed. A calibration method adapting to complex in-situ conditions is presented which defines the calibration prototype composed of measured variable, excitation source, reference system and sensing system. By analysing the operating procedure of the prototype, two measurement models are deduced for instantaneous and intermediate error analysis respectively. To discover the effects of the ambient temperature on the calibration process, the experiments are conducted using a temperature test chamber, and then the qualitative analysis and quantitative estimation are performed. The results show that: (a) With the variation of temperature, the responses of both resistance strain gauges (RSS) and fiber optic strain sensors (FOS) change significantly; (b) The relative strain deviations between the sensing system (SS) and the reference sensing system (RS) within −10 °C–50 °C fluctuate in a narrow range, for example 0.015–0.023 for RSS and 0.382–0.404 for FOS in this study, respectively; (c) The range of the deviations between SS and RS that is affected by temperature can be estimated with the proposed method, for example, 4.61 μϵ for RSS and 18.2 μϵ for FOS, respectively, with a confidence level of 95%.

Moving Load Identification with Long Gauge Fiber Optic Strain Sensing

Moving load identification has been researched with regard to the analysis of structural responses, taking into consideration that the structural responses would be affected by the axle parameters, which in its turn would complicate obtaining the values of moving vehicle loads. In this research, a method that identifies the loads of moving vehicles using the modified maximum strain value considering the long-gauge fiber optic strain responses is proposed. The method is based on the assumption that the modified maximum strain value caused only by the axle loads may be easily used to identify the load of moving vehicles by eliminating the influence of these axle parameters from the peak value, which is not limited to a specific type of bridges and can be applied in conditions, where there are multiple moving vehicles on the bridge. Numerical simulations demonstrate that the gross vehicle weights (GVWs) and axle weights are estimated with high accuracy under complex vehicle loads. The effectiveness of the proposed method was verified through field testing of a continuous girder bridge. The identified axle weights and gross vehicle weights are comparable with the static measurements obtained by the static weighing.

Long- and Short-Term Monitoring of a Dam in Response to Seasonal Changes and Ground Motion Loading: The Test Case of the Kurpsai Dam, Western Kyrgyz Republic

An experimental multi-parameter structural monitoring system has been installed on the Kurpsai dam, western Kyrgyz Republic. This system consists of equipment for seismic and strain measurements for making longer- (days, weeks, months) and shorter- (minutes, hours) term observations, dealing with, for example seasonal (longer) effects or the response of the dam to ground motion from noise or seismic events. Fibre-optic strain sensors allow the seasonal and daily opening and closing of the spaces between the dam’s segments to be tracked. For the seismic data, both amplitude (in terms of using differences in amplitudes in the Fourier spectra for mapping the modes of vibration of the dam) and their time–frequency distribution for a set of small to moderate seismic events are investigated and the corresponding phase variabilities (in terms of lagged coherency) are evaluated. Even for moderate levels of seismic-induced ground motion, some influence on the structural response can be detected, which then sees the dam quickly return to its original state. A seasonal component was identified in the strain measurements, while levels of noise arising from the operation of the dam's generators and associated water flow have been provisionally identified.

Evaluating in-service structural behaviour of an operational railway bridge using fibre optic sensing and structural model updating

Multi-girder prestressed concrete bridges are a very common structural form for bridges around the world. Their continued use is expected in the future given the advantages these structures have with respect to speed of construction, reduced maintenance and improved material efficiency. However, the in-service behaviour of these types of structures, such as their load distribution characteristics, support boundary conditions and the contribution of secondary elements, is still not well understood, and few studies have investigated these areas in depth. Gaining better insight into these aspects of bridge behaviour will enable more realistic structural assessment and thus facilitate more targeted maintenance and refurbishment of these bridges. This paper evaluates the in-service structural behaviour of an operational prestressed concrete railway bridge, which has been monitored using a dense network of fibre optic strain and temperature sensors since it was constructed in 2015. In particular, the normalised curvature profiles and the neutral axis positions across the composite solid slab bridge deck may be used to extract information about the bridge’s transverse load distribution behaviour, support boundary conditions and the stiffness contribution of secondary elements. Using this information, structural model validation and updating were performed with the goal of providing a more realistic assessment of structural behaviour and structural capacity. The proposed methodology and workflow in this study provide new insights into the real performance of multi-girder prestressed concrete bridges and thus can be used to improve their operation, maintenance, assessment and refurbishment.