Fiber Bragg Grating Strain Sensors Research Articles

Semi-automated methodology for damage assessment of a scaled wind turbine tripod using enhanced empirical mode decomposition and statistical analysis

One of the reasons structural health monitoring (SHM) techniques are preferred over the traditional visual inspection is that the assessment of the condition is not dependent on the expertise and experience of the person performing the inspection. In reality it is seen that the performance of the SHM techniques is still dependent on the inputs and the preferences of the engineer and as a result, on the experience and the skill of the engineer. The paper presents a semi-automated methodology for the damage detection and localization in a scaled model of an offshore support structure using fibre Bragg grating strain sensors for two damage scenarios namely deterioration of the support condition (simulated by introducing change in the attachment of a leg of a tripod) and a circumferential fatigue crack (simulated by unbolting the flange).The method consists of the offline setup step where the human inputs are provided. The system is trained based on the inputs of system performance, the factors such as thresholds for damage detection, the signal processing, etc. are determined. Once the system is trained, it is then deployed for automated, online damage detection. For the damage identification, a two-step damage method is employed which makes use of the ensemble empirical mode decomposition (EEMD) technique. In the first step, if the damage metric is above a certain threshold, damage is said to be detected. Once the damage has been detected, the EEMD based metric is again used to calculate the damage localization index (DLI) which is used for the localization of the damage. The DLI is used to overcome biases due to higher relative amplitude of vibration of some rosettes. Application of the methodology for the different damage scenarios on several different excitation conditions indicates that the developed methodology is robust and may be used for damage identification in tripod structures.

一种玻璃纤维封装的光纤布拉格光栅应变传感器

一种玻璃纤维封装的光纤布拉格光栅应变传感器

A Fiber Bragg Grating Anchor Rod Force Sensor for Accurate Anchoring Force Measuring

This paper presents a novel anchor rod force sensor based on fiber Bragg grating (FBG) for accurate anchoring force measurement. A special FBG strain sensor with enhanced sensitivity has been designed and embedded in the centre line of an elastic structural body, which forms the main body of the anchor rod force sensor. The mechanical structure design and measuring principle of the proposed anchor rod force sensor have been presented detailedly. Then a sensor prototype has been manufactured and fully tested. The experimental results show that the sensor provides a sensitivity of 11.58 pm/kN with a fitting linear correlation coefficient of 0.9996 in a measurement range of 50 kN, a repeatability error of 3.26%, and a hysteresis error of 3.87%. Furthermore, good resistance to bending effects of anchor rod, good creep resistance (fluctuant value is within 2 pm), and good temperature compensation capacity (maximum value is 11 pm) have also been verified. These results promise the sensor better applicability in the health monitoring of large civil engineering structures.

Experimental and numerical study on formation of interface separation and interfacial dielectric strength of GIL insulator

As the transmission capacity of gas insulated transmission line (GIL) increases, the synergistic improvement in dielectric and mechanical strengths of GIL insulators is urgently needed for the development of advanced power systems. Mechanical defects in power equipment are prone to induce discharges during high voltage operations, which need to be mitigated. In this paper, the strains on the GIL insulator surface are measured by fiber Bragg grating (FBG) strain sensors during hydrostatic tests. An abnormal change of strain is detected near the interface between center conductor and the spacer. The numerical results of stress distributions of GIL insulator during manufacturing and high-voltage operation are calculated by the finite element method. The results indicate that the mechanical stresses are concentrated at the interface, which lead to the development of conductor-spacer interface separation. In addition, based on the hydrostatic test and the FEM calculations, the dielectric interfacial strength, including the electric field distribution, and flashover are investigated by a combination of numerical and experimental studies. The results indicate that the air gaps generated by the separated interface distort the electric field at the tip of the gap, which is likely to cause discharges and reduce the gas-solid interfacial flashover strength of the insulator, placing concerns of insulation failure and even operating failures of GIL. This work sheds light on the importance of interfacial structure of power equipment and will guide the design and the manufacture of GIL insulator.

OPTYRE-Real Time Estimation of Rolling Resistance for Intelligent Tyres.

The study of the rolling tyre is a problem framed in the general context of nonlinear elasticity. The dynamics of the related phenomena is still an open topic, even though few examples and models of tyres can be found in the technical literature. The interest in the dissipation effects associated with the rolling motion is justified by their importance in fuel-saving and in the context of an eco-friendly design. However, a general lack of knowledge characterizes the phenomenon, since not even direct experience on the rolling tyre can reveal the insights of the correlated different dissipation effects, as the friction between the rubber and the road, the contact kinematics and dynamics, the tyre hysteretic behaviour and the grip. A new technology, based on fibre Bragg grating strain sensors and conceived within the OPTYRE project, is illustrated for the specific investigation of the tyre dissipation related phenomena. The remarkable power of this wireless optical system stands in the chance of directly accessing the behaviour of the inner tyre in terms of stresses when a real-condition-rolling is experimentally observed. The ad hoc developed tyre model has allowed the identification of the instant grip conditions, of the area of the contact patch and allows the estimation of the instant dissipated power, which is the focus of this paper.

Shape reconstruction of woven fabrics using fiber bragg grating strain sensors

In this paper we develop a methodology that uses in-plane strain measurements to determine out-of-plane deflection for woven fabrics using an optical fiber based sensor network. A multiplexed fiber Bragg grating (FBG) network is used to collect strain at discrete locations in the fabric. To simplify the problem a circularly shaped two-dimensional woven fabric material under a spherical indenter load is studied. A finite element (FE) model of the fabric behavior, derived from benchmark testing, was used to help develop reconstruction algorithms, some of which account for slipping of the fabric at the clamped boundaries. Due to the large deflections, complex material behavior of the fabric and slipping of the fabric at the outer boundaries, a modified empirical approximation approach was found to be the optimal choice for the deflection reconstruction. Experiments are performed to evaluate one of the algorithms on strain data from FBG sensors for two test cases: bonded to and woven into the fabric. Despite the complex strain on the FBGs bonded to the fabric, the empirical approach well predicts the out-of-plane deflection, except in the region under the indentor, where the fabric deformation was different than that modeled in the FE simulations. This result is promising for structural applications were direct observations of the out-of-plane deflections are not possible. To increase the maximum deflection of the fabric that could be measured, weaving of the FBGs into the fabric is also attempted. This method was less successful, due to the large amount of relative slipping between the optical fiber and the fabric, drastically reducing the strain measured by the FBGs.

Research on Applicability of Fiber Bragg Grating Sensor for Ship Structure Monitoring System

A complete application research of Fiber Bragg Grating sensing technology in ship structure stress monitoring and evaluation system is studied in this paper. The working principles of Fiber Bragg Grating strain sensor in monitoring system are briefly introduced. And a kind of new differential FBG strain sensor for ship structure stress monitoring is applied and verified by a series of calibration and force measurement test systematically. Through finite element calculation and test towards ship structure, some defects of monitoring sensor used in bending deformation are found. Then, a three-dimensional strain sensor arrangement for stress monitoring of hull structure is designed and developed. The satisfactory test results are obtained by this sensor arrangement. Finally, the accuracy and reliability of hull structure monitoring system is improved further through this research results.

Structural Health Monitoring of Gantry Girder using Fibre Bragg Grating Sensors

Most of the industries have an in-build overhead crane that helps in lifting and transportation of heavy loads from one location of work place to another. Gantry girders are provided to support the loads transmitted through the moving wheels of the overhead cranes. The paper presents the structural health monitoring (SHM) practices that can be implemented for damage detection of gantry girder using Fiber Bragg Grating (FBG) sensors that are positioned on the key parts of the crane. The sensor is designed for monitoring the stress and strain of the girder. The alternating stress was determined based on distributed fiber bragg grating strain sensors. The analysis of the signals based on finite element (FE) models helps to determine the safety condition of the crane.

A Comparison of Strain-Based Methods for the Evaluation of the Relative Displacement of Beam-Like Structures

This research presents an evaluation and comparison of the various strain to displacement transformation methods for a beam-like structure. Displacements can provide useful information for the monitoring and assessment of structural performance, health, and safety. The displacement of a structure is correlated with the curvature of a structure, so any unusual behavior of the structure that alters the curvature will also affect the displacement of the structure. Additionally, monitoring the displacement of a structure is useful for evaluating service limits as excessive displacement are uncomfortable to users and can cause damage to surrounding structures. Direct displacement monitoring of a real-life structure can be challenging, especially for long-term measurements. Because of this, the focus of this research is on in-direct displacement monitoring based on strain sensors. Several methods to determine displacements from strain measurements have been presented in the literature; this paper provides a quantitative comparison of selected methods using both a static and dynamic analysis, providing the errors of the methods. The methods were applied in a small-scale laboratory test to a beam with fiber Bragg-grating strain sensors, with both static and dynamic loading cases. The experimental displacement results are compared to displacement results from LVDT displacement sensors. Finally, the methods are applied to an existing structure equipped with long-gauge fiber Bragg-grating strain sensors, an in-service highway overpass subjected to vehicle loading. The displacements for the overpass were obtained and compared with the service requirements.

Dynamic Consolidation Measurements in a Well Field Using Fiber Bragg Grating Sensors.

Currently available groundwater flow prediction tools and methods are limited by insufficient spatial resolution of subsurface data and the unknown local heterogeneity. In this field study, fiber Bragg grating (FBG) sensors were installed in an extraction well field to investigate its potential to measure groundwater flow velocity. Reference in-situ pore pressure and temperature measurements were used to identify possible sources of FBG responses. FBG strain sensors were able to detect soil consolidation caused by groundwater extraction from 250 m distance. The results show that FBG responses were influenced by interface friction between soil and FBG packaging. FBG packaging slipped in soil and the effect was more pronounced during higher groundwater flow around a nearby well. These FBG fibers could be applied for indirect flow monitoring that does not require any tracer and provide real-time and long-term data during regular operation of extraction wells.

Laboratory Simulation of Flow through Rough-Walled Microfractures under High Hydraulic Gradient

Laboratory experiments on fluid flow through fracture are important in solving the fluid-in-rush problems that happen during the tunnel excavation. In order to study the mechanism of fluid flow through a rough-walled microfracture, fluid flow experiments were carried out and the fiber Bragg grating (FBG) strain sensors were applied to monitor the deformation of the microfracture surface during the seepage process. Considering the difficulty of collection of undisturbed rock samples from the deep locations, a methodology to simulate fluid flow through a fractured rock mass using analog materials containing a single fracture was developed. This method is easy to simulate the fluid flow through a fracture of certain aperture. Experimental data showed that Forchheimer equation could provide an excellent description of the nonlinear relationship between hydraulic gradient and flow velocity, and the variations of Forchheimer coefficients with joint roughness coefficient (JRC) were studied. It was found that the deformation of the microfracture surface subjected to seepage could be accurately captured by the quasi-distributed FBG strain sensors. The test results also demonstrated that the surface strain is significantly affected by hydraulic pressure.

Shallow bore-hole three-axial fiber Bragg grating strain sensor for Etna volcano monitoring.

We present the realization, installation, and first results of a three-axial Fiber Bragg Grating (FBG) strain sensor prototype. This sensor has been developed in the framework of the Mediterranean supersite volcanoes (http://www.med-suv.eu, 2013) project and, in particular, with the aim at contributing to the study and monitoring of Etna volcano. The FBG sensor was installed in the facilities of the Serra La Nave Astrophysical Observatory (Catania, Italy) about 7 km south-west from the summit craters, at an elevation of about 1740 m. The three-axial device showed a dynamic range of some hundreds of microstrains with microstrain resolution (submicrostrain concerning the vertical component). That is a good trade-off among performances, cost, and power consumption. The sensor structure and its read-out system are innovative in their assembly and offers practical advantages in comparison with traditional strain meters. As a demonstration of the performances of our device, the data of about 28 months of operation are presented together with the records of some local, regional, and teleseismic events. The sensor along the vertical axis showed to be the best performing one, having a power spectral density of about -90 dB re. 1ε2/Hz around one day period.

A new Fiber Bragg Grating sensor based circumferential strain sensor fabricated using 3D printing method

A new Fiber Bragg Grating (FBG) circumferential strain sensor was fabricated using fused deposition modeling (FDM) method and used to measure the circumferential strain of Rubber cylinders in a uniaxial compression test. Advantages of this FBG based sensor include small size, flexible, low cost, immunity to electromagnetic interference (EMI) and quick prototyping. Calibration test data indicate that the wavelength of FBG strain sensors is linearly proportional to the change of occurred elongation. Measurement sensitivity and resolution of the FBG sensor are 0.0218 nm/mm and 114 μm, respectively. The maximum wavelength difference was 0.0124 nm with a maximum measurement error of 1.59%. The designed new FBG based sensor has shown excellent performance and can be used for both vertical pressure measurements and circumferential strain monitoring.

Highly Sensitive FBG Strain Sensor With Enhanced Measurement Range Based On Higher Order FWM

To meet the simultaneous requirement of high sensitivity and high measurement range is an uphill task in the development of fiber Bragg grating (FBG) strain sensor. A unique scheme of highly sensitive FBG strain sensor is proposed based on nonlinear four wave mixing (FWM) with a high measuring range. In this paper, both the edges of FBG reflection spectrum are used without increasing the number of sources. The FWM products generated are tuned to the reflection edges of the FBG. The signals reflected from the edges of the FBG are then utilized in the subsequent FWM process to enhance the sensitivity of the system. The proposed scheme can provide a strain measurement range of 2310 με with strain sensitivity of 91.21 dBm/nm.

High Precision, Small Size and Flexible FBG Strain Sensor for Slope Model Monitoring

In this paper, a soft fiber Bragg grating (FBG) strain sensor was constructed of a rubber strip, FBGs and steel plates, which exhibits the advantages of high precision and a small size. A series of FBGs was uniformly pasted on a flexible rubber strip which can monitor the slope deformation by measuring the bending deformation of the rubber strip. Most notably, this sensor can be used to monitor horizontal displacement in the subsurface of the slope model. The relationships among the bending angle of the rubber strip, the strain of the rubber strip, and the subsurface deformation of the slope model were established. In addition, the subsurface deformation of the slope model can be obtained by the FBG strain sensor monitoring. Since a rigid-flexible structure was formed by uniformly pasting a series of steel plates on the other side of the rubber strip, the sensitivity of the FBG strain sensor was improved to be 1.5425 nm/°. The measurement results verify that the FBG strain sensor shows good performance, and the model test results demonstrate that the FBG strain sensor can be used for monitoring the subsurface deformation of the slope model.

Deflection Monitoring of Bi-Dimensional Structures by Fiber Bragg Gratings Strain Sensors

In this paper, we report about the deflection estimation of bi-dimensional structure by using fiber Bragg gratings (FBGs) as strain sensors embedded in the structure and an algorithm based on the classical beam theory with small deformation assumption. The proposed technique allows a real-time indirect structural monitoring solving the existing difficulties in measuring the deflection directly and can be applied to small as well large structure, and it is crucial in high-energy physics, where particle detectors have to measure the position of the incoming radiation with a resolution of few tens of micrometers. The obtained results have been compared with mechanical comparator direct deflection measurements showing good agreement, showing a resolution of few tens of micronmeters over a surface of the order of about 1 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The dynamic range of deflection monitoring is dependent on the panel dimension: in the hypothesis of 1-m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> panel, the deflection limit is around 15 cm since FBG can measure very high strain (greater than 10000 μm/m).

Effect of vehicle speed and overload on dynamic response of semi-rigid base asphalt pavement

Premature deterioration in semi-rigid base asphalt pavement is still considered as one of the main distress related to Chinese highway pavements. There are several reasons for this premature damage and these include poor design and construction, a sudden increase in traffic loading and the dynamic loading of overloaded heavy truck. This paper investigated the effect of vehicle speed and traffic overload on the dynamic response as well as on service life of a semi-rigid base asphalt pavement with typical functional requirements. The study involved simulating the critical stress–strain response at the bottom of asphalt layer under different load weight level and vehicle speed. A large scale three-dimensional viscoelastic finite element model of transient dynamic analysis of the pavement was developed. The Sinotruck HOWO-A7 6×4 and HOWO-A7 8×4 were selected as a representative heavy truck and the associated contact stress effect of each isolated wheel load on the pavement structure was incorporated in the implicit dynamic analysis by a developed DLOAD subroutine. Field dynamic strain measurement from Fiber Bragg Grating strain sensors at the Rizhan-LanKao G1511 highway test section was successfully used to validate the developed model. The results of this study indicate that under heavy moving truck load the response time of the longitudinal, vertical and shearing stress–strain at the bottom of the asphalt layer has both a tensile and compressive component, while the response time history of the transversal stress–strain has only a tensile component. The result also indicated that a decrease in the vehicle speed not only induce a significant increase in the load duration on the structure but also amplified the shock effect induced by the wheel load. The damage caused by the overload is greatly influenced by the axle weight extra extent. The fatigue analysis results revealed that, factors such as overloading or low-speed traffic could result in adverse effect on the service life of pavement. However, as expected the tensile strain at the bottom of the asphalt layer of the proposed semi-rigid base asphalt pavement structure is less than the critical strain of a conventional pavement structure with an ordinary asphalt mix even under critical conditions such as overloading or low-speed traffic.

Strain Monitoring of a Composite Drag Strut in Aircraft Landing Gear by Fiber Bragg Grating Sensors.

This work reports on the use of Fiber Bragg Grating (FBG) sensors integrated with innovative composite items of aircraft landing gear for strain/stress monitoring. Recently, the introduction of innovative structures in aeronautical applications is appealing with two main goals: (i) to decrease the weight and cost of current items; and (ii) to increase the mechanical resistance, if possible. However, the introduction of novel structures in the aeronautical field demands experimentation and certification regarding their mechanical resistance. In this work, we successfully investigate the possibility to use Fiber Bragg Grating sensors for the structural health monitoring of innovative composite items for the landing gear. Several FBG strain sensors have been integrated in different locations of the composite item including region with high bending radius. To optimize the localization of the FBG sensors, load condition was studied by Finite Element Method (FEM) numerical analysis. Several experimental tests have been done in range 0–70 kN by means of a hydraulic press. Obtained results are in very good agreement with the numerical ones and demonstrate the great potentialities of FBG sensor technology to be employed for remote and real-time load measurements on aircraft landing gears and to act as early warning systems.

Dynamic Response of Interior Ballistic Process and Rail Stress in Electromagnetic Rail Launcher

The dynamic response of interior ballistic process in an electromagnetic (EM) rail launcher (EMRL) directly affect the stress concentration of the rails and the launch performance of the projectile. By simplifying the rail as a Bernoulli–Euler beam fixed on the elastic support under the action of moving loads and acquiring the equivalent stiffness of the elastic support through a static EM-structural coupled model, the critical velocity of the EMRL is captured. Also, the hybrid finite-element/boundary-element method is used to establish a coupled EM-structural-motion model to derive the dynamic contact force between the armature and the rail as well as the stress distribution of the rails. Finally, the measuring data of fiber Bragg grating strain sensors fixed on the back of the rails are used to verify the stress concentration of the rails and analyze the stability of the projectile in the bore. For the typical 30 mm $\times30$ mm rectangular caliber launcher whose critical velocity is less than muzzle velocity, analysis and test results show that the EM extrusion force of the C-type armature exerted on the rail reaches the peak value at the beginning of the flat-top stage and gradually decreases as time goes on; the stress waves aroused by armature passing are easily resonant with the reflected stress waves in the rail in the high-speed stage of the armature, in which the stresses of rails are the severest, and the stress concentration level is about 2.44 times that in the initial low-speed stage; when the armature operates in the high-speed stage, there will be a vibration phenomenon which will lead to the asymmetry of loads on the upper and lower rails. These results have guiding significance to design the EMRL.

A High Sensitivity FBG Strain Sensor Based on Flexible Hinge.

For the purpose of improving the sensitivity of the fiber Bragg grating (FBG)-based strain sensor. A novel FBG-based strain sensor with high sensibility was designed by means of a flexible hinge bridge displacement magnification structure. This sensor can be used to accurately measure the strain of a mechanical structure surface. In this paper, the strain sensitization amplification factor of the sensor was calculated by using the flexible matrix method and the strain energy theory. The magnification had been verified by using simulation analysis and experimental results, and the error between theoretical calculation and simulation analysis was less than 7%. The result shows that the strain sensitivity of the sensor is 10.84 pm/με, which is about 10 times to that of the bare FBG sensor. This sensor is sensitive to micro-strain, so it can be well applied to health monitoring of a mechanical system.