Fiber Optic Strain Research Articles

Experimental study of anchor bolt stress evaluation with hybrid optical fiber monitoring

The evaluation of mechanical properties of bolts is crucial for ensuring the long-term stability and continuous maintenance of underground engineering. Current research prioritizes the development of highly adaptable bolt mechanical behavior monitoring methods. By utilizing bolts with embedded sensors, the working status and surrounding environment can be monitored without affecting the original support structure, enabling effective disaster prevention monitoring. In this study, we introduce an innovative hybrid monitoring system based on strain gauges, Fiber Bragg Gratings (FBG), and Brillouin Optical Time Domain Analysis (BOTDA). A specialized dynamometer with temperature compensation is employed to collect axial force data under step loading conditions. We compare the sensing characteristics of various methods and derive the strain transmission coefficients of fiber sensors at different distances. The results demonstrate that the FBG and BOTDA accurately reflect the magnitude and change patterns of the monitored parameters, highlighting the potential application of this novel system for bolt monitoring. This work proposes a unique fiber optic temperature compensator and strain transmission coefficient analysis method to promote the evaluation quality of anchor bolt stress behaviors, which would provide realistic significance to enhance the accuracy and reliability of bolt mechanical property evaluation in underground engineering.

Dynamic evolution law analysis of overburden separation and water flowing fracture under mining based on distributed optical fiber and information entropy theory

The water inrush from overburden separation is large and destructive, which is very easy to cause serious casualties and property losses. Therefore, a three-dimensional model of 150 cm × 150 cm × 60 cm (length × height × width) is established in the laboratory. And the dynamic development process of overburden separations and water flowing fractures in the process of coal mining in real time are analyzed based on distributed optical fiber and information entropy theory. The distributed optical fiber strain curve can well reflect the process and characteristics of overburden deformation and collapse. The strain curve of optical fiber shows a single peak or step shape at different mining stages. The separation position corresponds to the peak of the optical fiber strain curve or above the step. The change of strain value of optical fiber reflects the development, expansion and closure process of separation. The development height of the water flowing fractured zone corresponds to the lower part of the step of the strain curve. Strain information entropy realizes quantitative analysis of overburden system state. Strain information entropy can be divided into small amplitude rapid fluctuation period, sharp rapid adjustment period and stable slow fluctuation period. Water flowing fractures and separations are mainly produced during the period of the sharp rapid adjustment period. The height of the water flowing fractured zone and the height of the new separation increase step by step with the advance of the working face. The water flowing fractured zone finally develops to 86 cm of the model height. The highest position of the separation appears at the 112 cm height of the model. A new variable named growth rate of strain information entropy is defined, which can reflect the severity of the change of the overburden system state. When the key stratum breaks, the growth rate of strain information entropy increases significantly, which show that the key stratum plays a key role in controlling the overburden deformation process. In addition, the optical fiber strain curve can also reflect important information such as the change of advance support pressure, the distribution state of gangue in goaf and the breaking characteristics of key strata.

Comparison of methods for monitoring icing of high-voltage power lines and an overview of measuring equipment used to diagnose such lines

The low structural reliability of overhead lines, due to continuous climatic influence, becomes the cause of electrical network accidents and unreliable power supply. Ensuring the trouble-free operation of overhead lines is a complex task, the solution of which is of the utmost importance for the normal functioning of the entire infrastructure. In the autumn-winter period, difficult weather conditions almost completely exclude the quick completion of repair works. A fairly large share of accidents occurs during the formation of icy deposits on wires and lightning protection cables. Today, visual inspections of high-voltage power lines remain the main way of diagnosing icy deposits. In practice, it is necessary to equip overhead lines with ice load telemetry devices, which will allow monitoring the formation of ice deposits on wires in real time. The following diagnostic methods are used in modern ice monitoring devices: strainometric, fiber-optic, aerodynamic, locational, and instrumental-parametric. The tensiometric method of diagnosis is implemented by means of direct measurement of the icy load on the wire with subsequent comparison of the measured values with predetermined values of the threshold loads. To implement this diagnostic method, magnetoelastic force measuring sensors are used. Such a sensor is connected between the traverse of the U-shaped support and the upper end of the corresponding garland of insulators with a phase wire. Ice, wind and ice-wind loads are measured separately. This method has a limited scope of application, namely for diagnosing the formation of ice on the phase wires of intermediate spans of single-circuit lines with two-post U-shaped resistances. The fiber-optic method of strain gauge measurements is implemented in object control systems under difficult operating conditions, at nuclear power plants. The mechanical effort sensor (fiber-optic strain sensor) implements the fiber-optic method of strain measurement. Fiber-optic strain sensors are characterized by high accuracy due to resistance to interference and complexity of execution.

Distributed fiber optic strain sensing in axially loaded glued-in steel rods in glued-laminated timber

The rise in popularity of mass timber structures globally has increased the demand for high-performance timber connections. Glued-in rod connections which are advantageous compared to other dowel-type connections because of their high strength and stiffness, have been the subject of extensive research in the literature; However, the lack of an in-depth understanding of their behavior and a uniform set of guidelines for their design continues to limit the use of glued-in rods in practice. To develop an improved understanding of the behavior of glued-in rod connections this paper examines, for the first time, the use of distributed fiber optic sensors to measure the complete strain distribution along the length of a glued-in rod connection. Axial pullout tests were conducted on twenty-five glued-in rod connections in glued-laminated timber with varying embedment lengths and glue-line thicknesses. Results of the study demonstrate the ability of fiber optic sensors to capture the full strain distribution along the length of the steel rod and along the epoxy-adhesive interface, enabling the study of localized behavior that would be difficult to measure using conventional strain gauges. In addition, results demonstrate the ability of distributed fiber optic strain sensors to infer axial stress and force distributions along the length of a glued-in rod connection, which are compared with theoretical distributions from the Volkersen model, as well as measure bond-slip behavior.

Stretchable optical fiber strain sensor comprising zinc oxide and PDMS for human motion monitoring

Stretchable optical fiber strain sensor comprising zinc oxide and PDMS for human motion monitoring

Multimode optical fiber strain monitoring for smart infrastructures

The in-service monitoring of civil infrastructures is an important task required to achieve their smart operation. This task requires the installation of sensors to continuously check and control the structures’ status in order to fulfil the demanded operating conditions and avoid the occurrence of malfunctions. This work presents an investigation of the use of multimode optical fiber sensors in detecting strain and vibration of infrastructures. The multimode fiber sensor is composed of a single mode – multimode – single mode concatenated fiber structure. To explore its practical application for strain monitoring, the sensor is used to measure the strain on a pressurized pipe and its performance compared with the well-developed fiber Bragg grating sensor. The detuning of spectral features of the multimode sensor transmission with increasing pipe pressure is found to have a good linearity (R2 ∼ 0.95) and is more sensitive to strain compared to that of the fiber Bragg grating.

Detecting strain with a fiber optic cable on the seafloor offshore Mount Etna, Southern Italy

Oceans cover more than 70 percent of the Earth's surface making it difficult and costly to deploy modern seismological instruments here. The rapidly expanding global network of submarine telecom cables offers tremendous possibilities for seismological monitoring using laser light. Recent pioneer studies have demonstrated earthquake detection using lasers in onland and submarine fiber optic cables. However, permanent strain at the seafloor has never before been measured directly as it happens. With this aim, we deployed a dedicated 6-km-long fiber optic strain cable, offshore Catania Sicily, in 2000 m water depth, and connected it to a 29-km long electro-optical cable for science use. We report here that deformation of the cable equivalent to a total elongation of 2.5 cm was observed over a 21-month period (from Oct. 2020 to Jul. 2022). Brillouin laser reflectometry observations over the first 10 months indicate significant strain (+25 to +40 microstrain) at two locations where the cable crosses an active strike-slip fault on the seafloor, with most of the change occurring between 19 and 21 Nov. 2020. The cause of the strain could be fault slip or seabottom currents. During the following 11 months, the strain amplitude increased to +45 to +55 microstrain, affecting a longer portion of the cable up to 500 m to either side of the first fault crossing. A sandbag experiment performed on the distal portion of the cable (3.2–6.0 km) starting Sept. 2021 demonstrates how the fiber optic cable deforms in response to an applied load and how the deformation signal partially dissipates over time due to the elastic properties of the cable. These preliminary results are highly encouraging for the use of BOTDR (Brillouin Optical Time Domain Reflectometry) laser reflectometry as a technique to detect strain at the seafloor in near real time and to monitor the structural health of submarine cables.

Approaches to Determining the Residual Life of the Main Components of the Helicopter, Taking Into Account the Flight Profiles Performed

The purpose of research is development of an alternative approach to determining the residual life of the product, based on the conversion of the energy of deformed power elements of the airframe structure into the consumption of an equivalent product resource through a monitoring system based on fiber-optic strain sensors.Methods. The analysis of existing methods for assessing the technical condition of the main components of aircraft products and determining their residual life based on information about the technical condition, on the basis of which the synthesis of the algorithm for determining the residual life of the main components of the helicopter, taking into account the technical condition, was carried out.Results. In the course of the study, the existing methods of assessing the technical condition of the product were classified. The shortcomings in the existing concept of determining the residual resource are indicated, the main of which is the lack of the possibility of predicting the residual resource based on the technical condition of the object (operational history). An alternative approach to determining the residual life of the main components of the helicopter is proposed, with an explanation of the approximate methodology for assessing the technical condition of the main components and the product as a whole, the composition and description of the main stages of the implementation of this approach. The composition and the sensitive element in the control system of the current technical condition of the main components of the product are determined. A fiber-optic sensor with a Bragg grid (fiber-optic pressure sensor ВОДД-1.XXX ПИКВ.401143.003 ТУ) was selected as a sensitive element for the implementation of the monitoring system.Conclusion. The proposed approach to determining the residual life of the main components of the helicopter combines high reliability indicators, which is due to the fixation of the operation history of a particular helicopter, the simplicity of determining the residual life of the product and the absence of the need for special measuring instruments in operating companies.

Distributed optical strain sensing measurements down to cryogenic temperatures.

Rayleigh backscattering (RBS)-based distributed fiber sensors technology is becoming more and more crucial in various fields such as aerospace and defense, automotive, civil, and geotechnical. This technology is measuring the naturally occurring Rayleigh backscatter level in the optical fiber core; thus, any standard single-mode telecom optical fiber can be used. The application of distributed optical fiber strain sensing in the harsh environments of the European Organization for Nuclear Research required several mechanical tests to study the accuracy of strain sensing in cryogenic conditions. This study compares the performance of a RBS-based distributed optical fiber strain sensing down to cryogenic temperatures (4.2K) with previously validated instrumentations such as electrical strain gauges and fiber Bragg grating technologies.

Application of Physics-Informed Neural Networks for forward and inverse analysis of pile–soil interaction

The application of the Physics-Informed Neural Networks (PINNs) to forward and inverse analysis of pile–soil interaction problems is presented. The main challenge encountered in the Artificial Neural Network (ANN) modeling of pile–soil interaction is the presence of abrupt changes in material properties, which results in large discontinuities in the gradient of the displacement solution. Therefore, a domain-decomposition multi-network model is proposed to deal with the discontinuities in the strain fields at common boundaries of pile–soil regions and soil layers. The application of the model to the analysis and parametric study of single piles embedded in both homogeneous and layered formations is demonstrated under axisymmetric and plane strain conditions. The performance of the model in parameter identification (inverse analysis) of pile–soil interaction is particularly investigated. It is shown that by using PINNs, the localized data acquired along the pile length –possibly obtained via fiber optic strain sensing–can be successfully used for the inversion of soil parameters in layered formations.

Temporal changes in strain condition on all lateral surfaces during pretreatment before timber drying

Wood drying, such as boxed-heart square timber, often involves surface checks, which spoils the appearance of wood products and reduces their market value. Pretreatments are performed to avoid the surface checks, but detailed information about surface strain during timber drying remains unclear. In this study, surface strain detections were performed using two types of strain sensors (strain gauge and optical fiber strain sensor) to understand the surface dimensional changes during a series of pretreatments, consisting of steaming and high temperature and low humidity treatment. Simultaneous strain measurements based on the optical fiber sensor grasped the surface strain distributions in each lateral surface during timber drying; contraction behaviors were observed in the middle part of most surfaces in the early steaming stage, while one surface showed expansion. A remarkable expansion was detected in one surface during the high temperature and low humidity treatment, although most of the other surfaces showed gradual contraction behaviors. It was also discovered that the above detected behaviors were gradually reduced with the progress of each pretreatment.

Strain Monitoring of Overhead Transmission Line Based on Brillouin Scattering

The change in the optical fiber state and the status of transmission line often leads to the change of strain in its composite optical fiber. This paper attempts to use a commercial BOTDR (Brillouin optical time-domain reflectometry) to monitor the strain of optical fiber composited in transmission line. The Brillouin spectra of typical optical fiber used in the transmission line are measured by a commercial BOTDR at room temperature, the spectra are fitted and the key parameters are calculated, and then the strain along whole optical fiber is obtained by demodulation. The results show that BOTDR can effectively monitor the optical fiber strain, and the mean standard deviation of strain measured for multiple measurements under the same condition is only about 8 με, which can stastify the requirements of practical cases. The work of this paper lays a foundation for the strain measurement of transmission line.

Geomechanical Modeling of Fracture-Induced Vertical Strain Measured by Distributed Fiber-Optic Strain Sensing

Summary Distributed acoustic sensing (DAS) has recently gained importance in monitoring hydraulic fracturing treatments in the oil and gas industry. DAS data contain critical information about the fracture geometry as linearly relatable induced strain variations during the stimulation. The low-frequency components of the DAS (LF-DAS) data are known for their complexity as they exhibit various characteristic signals—caused by several mechanisms—that complicate their interpretation. LF-DAS data from horizontal monitoring wells (HMWs) have been used to detect fracture hits and characterize fracture geometry. However, the LF-DAS data from vertical monitoring wells (VMWs) have not been studied extensively as a means to infer fracture geometry. The major limitation of VMWs is the number of monitored stages, but the data contain more information about fracture height compared with LF-DAS measurements from HMWs. Hence, it is necessary to have a physical rock deformation model to simulate the strain rate responses in offset VMWs during fracture propagation to understand and interpret the various patterns that are observed in the field data sets. The objective of this study is to simulate strain rate signals in VMWs during hydraulic fracturing and to analyze the measurements to obtain information on the fracture geometry, especially the fracture height. The fracture boundary can be directly related to the strain rate signals. In this study, we propose a workflow to determine fracture height at different fiber-to-fracture (dff) distances for fracture heights ranging from 20 m to 300 m. We conduct a detailed sensitivity analysis to understand the impacts of the dff, the perforation location, the fracture passing time, and the well inclinations on the measured strain rate signals. The analysis helps interpret the various patterns observed in field data and the underlying mechanisms. Interpretation of field data from the Hydraulic Fracture Testing Site 2 (HFTS-2) using the results from our forward physical model provides valuable information on the fracture characteristics that can be captured by the physical model. The results of this study are expected to provide better interpretations of LF-DAS signals from VMWs.

Axial cyclic loading of piles in low-to-medium-density chalk

Comprehensive field investigations into the axial cyclic loading behaviour of open steel pipe piles driven and aged in low-to-medium-density chalk identify the conditions under which behaviour is stable, unstable or metastable. Post-cycling monotonic tests confirmed that stable cycling enhanced pile capacity marginally, while unstable cases suffered potentially large losses of shaft capacity. Metastable conditions led to intermediate outcomes. The patterns by which axial deflections grew under cyclic loading varied systematically with the normalised loading parameters and could be captured by simple fitting expressions. Cyclic stiffnesses also varied with loading conditions, with the highest operational shear stiffnesses falling far below the in situ seismic test values. The monotonic and cyclic axial responses of the test piles were controlled by the behaviour of, and conditions within, the reconsolidated, de-structured, chalk putty annuli formed around pile shafts during driving. Fibre-optic strain gauges identified progressive failure from the pile tip upwards. Large factors of safety were required for piles to survive repetitive loading under high-level, two-way conditions involving low mean loads, while low-amplitude one-way cycling had little impact. A simple ‘global’ prediction procedure employing interface shear and cyclic triaxial tests is shown to provide broadly representative predictions for field behaviour.

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.

Ultra-High Sensitivity and Temperature-Insensitive Optical Fiber Strain Sensor Based on Dual Air Cavities.

This study proposed an all-fiber Fabry-Perot interferometer (FPI) strain sensor with two miniature bubble cavities. The device was fabricated by writing two axial, mutually close short-line structures via femtosecond laser pulse illumination to induce a refractive index modified area in the core of a single-mode fiber (SMF). Subsequently, the gap between the two short lines was discharged with a fusion splicer, resulting in the formation of two adjacent bubbles simultaneously in a standard SMF. When measured directly, the strain sensitivity of dual air cavities is 2.4 pm/με, the same as that of a single bubble. The measurement range for a single bubble is 802.14 µε, while the measurement range for a double bubble is 1734.15 µε. Analysis of the envelope shows that the device possesses a strain sensitivity of up to 32.3 pm/με, which is 13.5 times higher than that of a single air cavity. Moreover, with a maximum temperature sensitivity of only 0.91 pm/°C, the temperature cross sensitivity could be neglected. As the device is based on the internal structure inside the optical fiber, its robustness could be guarantee. The device is simple to prepare, highly sensitive, and has wide application prospects in the field of strain measurement.

Monitoring shear deformation of sliding zone via fiber Bragg grating and particle image velocimetry

Monitoring shear deformation of sliding zones is of great significance for understanding the landslide evolution mechanism, in which fiber optic strain sensing has shown great potential. However, the correlation between strain measurements of quasi-distributed fiber Bragg grating (FBG) sensing arrays and shear displacements of surrounding soil remains elusive. In this study, a direct shear model test was conducted to simulate the shear deformation of sliding zones, in which the soil internal deformation was captured using FBG strain sensors and the soil surface deformation was measured by particle image velocimetry (PIV). The test results show that there were two main slip surfaces and two secondary ones, developing a spindle-shaped shear band in the soil. The formation of the shear band was successfully captured by FBG sensors. A sinusoidal model was proposed to describe the fiber optic cable deformation behavior. On this basis, the shear displacements and shear band widths were calculated by using strain measurements. This work provides important insight into the deduction of soil shear deformation using soil-embedded FBG strain sensors.

Experimental investigation of interfacial behavior of fiber optic cables embedded in frozen soil for in-situ deformation monitoring

Deformation monitoring of frozen soil is essential to prevent ground settlement problems and geological disasters in cold regions. Fiber optic sensing offers an effective tool for this purpose. However, the interfacial behavior of fiber optic strain sensing cables embedded in frozen soil remains elusive, leading to uncertainty in monitoring data analysis. This study presents an experimental investigation of cable-soil interfacial behaviors with different initial water contents and freezing durations. The results from the cable pullout test show that interfacial progressive failure conforms to the elastoplastic model under low water content conditions, while the softening model is applicable with higher water contents. Interfacial shear strengths and characteristic displacements are positively correlated with initial water contents and freezing durations. Freezing front migration and water redistribution during soil freezing were effectively characterized by shear strength profiles along the cable. These findings provide important references for implementing fiber optic-based deformation monitoring in frozen soil.

Distributed fibre optic inclinometer with cloud-based monitoring system

Geotechnical instrumentation and monitoring are important aspect of controlling subsurface ground movement, particularly in deep excavations and forewarning of geological disasters such as landslides, land subsidence, ground fissure, and surface collapse. A borehole inclinometer is an example of the established method to monitor such ground movements. For real-time data, a series of in-placed inclinometer probes are installed inside the borehole casing to provide the lateral displacement response over time. The limitation of the conventional system is the high cost and the practicality of inserting a high number of sensing probes into the casing for better measurement accuracy. Recent advancement in Distributed Optical Fibre Strain Sensing (DOFS) enables the incorporation of DOFS as a fibre optic (FO) inclinometer with lateral displacements to be plotted at centimetres spacing and is unaffected by the gauge lengths between the probes. This study enhances the FO inclinometer by developing a web-based application (SmartGeopipe©) to automate data acquisition which also includes complex data processing for the final pipe deflection profile visualisation. The FO inclinometer comprised of optical cables attached externally onto a remodified inclinometer casing and connected to a Brillouin Optical Time Domain Analysis (BOTDA) strain analyser. A standard installation procedure was established to safely secure the sensors during the casing installation process inside the borehole. SmartGeopipe© software is written using a predominantly monolithic Python / Cython backend and a multi-page frontend built with HTML, CSS, and JavaScript ES5 languages. All the computation work and data storing are done in a cloud server and SQL database, respectively. A case study of a station box excavation as part of the Mass Rapid Transit (MRT) tunnelling project was chosen to validate the performance of the FO inclinometer and the web application. Data from the FO inclinometer corroborated well with the manual measurements of the inclinometer probe. This ability supports the realization of an early warning system as an engineering control for hazard management in civil engineering infrastructure projects.

Wing dynamic shape-sensing from fiber-optic strain data using the Kalman state estimator

A novel method for shape sensing based on strain data and the use of the Kalman state estimator is presented and studied experimentally. Several recent studies proposed different methods in which strain data, measured in optical fibers, can be used to estimate the deformed shape of a flexible wing. Fiber optic sensors typically provide accurate, high resolution, strain data, yielding detailed prediction of a wing's deformed shape. However, in real life applications there could be moments in which the strain data is imperfect (for example, due to saturation or temperature effects), corrupted, or missing. The current study proposes to use a Kalman state estimator that weighs in strain-data and simulation output of an aeroelastic plant model to estimate the wing deformations. The method is demonstrated with a flexible wing model that is excited by prescribed control surfaces deflection in a wind-tunnel test. Strains over the front and rear wing spars are measured by Fiber Bragg Grating sensors, embedded in two optical fibers, and used to estimate the wing deformations. The latter are compared to wing deformations measured by a motion recovery camera system. Results show the advantages of the use of the Kalman state estimator when the strain data is corrupted. Additionally, with this approach, wing's modal velocities are estimated together with the wing deformations, resulting in smooth and accurate modal velocities that are readily usable by the vehicle's control system.