Embedded Fiber Bragg Grating Sensors Research Articles

Localized Temperature Variations in Laser-Irradiated Composites with Embedded Fiber Bragg Grating Sensors.

Fiber Bragg grating (FBG) temperature sensors are embedded in composites to detect localized temperature gradients resulting from high energy infrared laser radiation. The goal is to detect the presence of radiation on a composite structure as rapidly as possible and to identify its location, much the same way human skin senses heat. A secondary goal is to determine how a network of sensors can be optimized to detect thermal damage in laser-irradiated composite materials or structures. Initial tests are conducted on polymer matrix composites reinforced with either carbon or glass fiber with a single optical fiber embedded into each specimen. As many as three sensors in each optical fiber measure the temporal and spatial thermal response of the composite to high energy radiation incident on the surface. Additional tests use a 2 × 2 × 3 array of 12 sensors embedded in a carbon fiber/epoxy composite to simultaneously measure temperature variations at locations on the composite surface and through the thickness. Results indicate that FBGs can be used to rapidly detect temperature gradients in a composite and their location, even for a direct strike of laser radiation on a sensor, when high temperatures can cause a non-uniform thermal response and FBG decay.

Effective seismic strengthening and monitoring of a masonry vault by using Glass Fiber Reinforced Cementitious Matrix with embedded Fiber Bragg Grating sensors

Abstract The research proposes the combined use of a Glass Fiber Reinforced Cement Matrix (GFRCM) composite with an integrated fiber optic sensing system for innovative seismic retrofitting of masonry vaults. The need of eco-compatibility of bonding material with masonry support implies the use of Hydraulic Lime Mortar (HLM) as bonding matrix that, in contrast, is characterized by lower adhesion capacity respect to polymeric resins and not well-known carrying-load properties. Hence, monitoring of the operating features of the GFRCM reinforcement has been pursued with an advanced fiber optic sensing system realized through Fiber Bragg Grating (FBG) sensors. The use of FBG sensors is justified by a large number of advantages such as small sensor dimensions, low weight as well as high static and dynamic resolution of measured values, distributed sensing feature allowing to detect anomalies in load transfer between reinforcement and substrate. Specifically, the proposed retrofitting and monitoring technique is designed and applied to an old masonry pavilion vault. A nonlinear finite element model of the reinforced structure is developed in order to quantify the effectiveness of the GFRCM strengthening layer and to derive the convenient position of the optic fibers for a correct monitoring of the reinforced vault. An experimental campaign is carried out in order to verify the proper behavior of the proposed strengthening and sensing strategy.

Impact Localization Method for Composite Plate Based on Low Sampling Rate Embedded Fiber Bragg Grating Sensors

Fiber Bragg Grating (FBG) sensors have been increasingly used in the field of Structural Health Monitoring (SHM) in recent years. In this paper, we proposed an impact localization algorithm based on the Empirical Mode Decomposition (EMD) and Particle Swarm Optimization-Support Vector Machine (PSO-SVM) to achieve better localization accuracy for the FBG-embedded plate. In our method, EMD is used to extract the features of FBG signals, and PSO-SVM is then applied to automatically train a classification model for the impact localization. Meanwhile, an impact monitoring system for the FBG-embedded composites has been established to actually validate our algorithm. Moreover, the relationship between the localization accuracy and the distance from impact to the nearest sensor has also been studied. Results suggest that the localization accuracy keeps increasing and is satisfactory, ranging from 93.89% to 97.14%, on our experimental conditions with the decrease of the distance. This article reports an effective and easy-implementing method for FBG signal processing on SHM systems of the composites.

An Investigation of the Use of Embedded FBG Sensors to Measure Temperature and Strain Inside a Concrete Beam During the Curing Period and Strain Measurements under Operational Loading

This paper covers detailed investigation on the use of a FBG sensor for temperature and strain measurement in a reinforced concrete beam within first 28 days after pouring and structural response after 28 days curing period. A Fiber Bragg grating (FBG) sensor and a few thermo-couples (TC) have been embedded in the concrete beam at the casting stage. After curing the beam was laterally loaded on a three-point bending arrangement until a crack formed on the beam and subsequently heated to 200 0C with a 5kN lateral load at mid-span. A detailed analysis was performed on the data obtained from FBG sensor, thermo-couples and surface mounted electrical strain gauges (SG). Further, the cracked surface was continuously monitored with an Infrared Red (IR) camera during the heating to establish the temperature profile of the cracked surface of the concrete beam. A detailed Finite Element Analysis (FEA) of the concrete beam (structural and thermal) was performed using ABAQUS and compared with the experimental results.

Strain Transfer Analysis of Embedded Fiber Bragg Grating Strain Sensor

Abstract Due to the difference in the physical and mechanical properties between the optical fiber, protective layer, adhesive layer, and the host material, the strains measured by a fiber Bragg grating (FBG) sensor may not be the actual strains of the host material, which impedes the reliable applications of FBG sensors. To overcome this problem, in this paper, the strain transfer formula was derived by using elastic analysis, the shear-lag method, and several reasonable assumptions taking into account temperature variations and nonaxial stresses, which was an improvement to an existing study (Li, H. N., Zhou, G. D., Ren, L., and Li, D. S., “Strain Transfer Analysis of Embedded Fiber Bragg Grating Sensor Under Nonaxial Stress,” Optic. Eng., Vol. 46, No. 5, 2007, 054402). The analytical results were validated by simulating the behavior of a bare optical fiber through finite element analysis. The data enabled the identification of the parameters that influence the strain transfer from the host material to the embedded FBG sensors and the impact of the temperature variation and sensor alignment angle on the measurement accuracy. This work provided additional knowledge for the improvement of the existing strain transfer theory of FBG sensors in order to achieve a more accurate strain measurement with this sensing technology.

Measurement of Elastic Properties of Epoxy Molding Compound by Single Cylindrical Configuration with Embedded Fiber Bragg Grating Sensor

We propose a novel experimental method, based on a fiber Bragg grating (FBG) sensor, to measure the elastic properties of epoxy molding compound (EMC) from a single specimen configuration. The FBG sensor is embedded in the center of a cylindrical EMC specimen, and deforms together with the EMC. The Bragg wavelength (BW) shifts are documented during compressive and hydrostatic loadings. Young’s modulus and bulk modulus are determined from the BW shifts using the relationships between the elastic constants and the BW shift. Two major developments to accommodate the unique requirements of EMC testing include: (1) a large mechanical pressure to be applied during curing; and (2) a very high gas pressure required for hydrostatic testing. The shear modulus and Poisson’s ratio are calculated from the two measured constants to provide a complete set of elastic properties of EMC.

Monitoring Poisson’s Ratio Degradation of FRP Composites under Fatigue Loading Using Biaxially Embedded FBG Sensors

The significance of strain measurement is obvious for the analysis of Fiber-Reinforced Polymer (FRP) composites. Conventional strain measurement methods are sufficient for static testing in general. Nevertheless, if the requirements exceed the capabilities of these conventional methods, more sophisticated techniques are necessary to obtain strain data. Fiber Bragg Grating (FBG) sensors have many advantages for strain measurement over conventional ones. Thus, the present paper suggests a novel method for biaxial strain measurement using embedded FBG sensors during the fatigue testing of FRP composites. Poisson’s ratio and its reduction were monitored for each cyclic loading by using embedded FBG sensors for a given specimen and correlated with the fatigue stages determined based on the variations of the applied fatigue loading and temperature due to the autogenous heating to predict an oncoming failure of the continuous fiber-reinforced epoxy matrix composite specimens under fatigue loading. The results show that FBG sensor technology has a remarkable potential for monitoring the evolution of Poisson’s ratio on a cycle-by-cycle basis, which can reliably be used towards tracking the fatigue stages of composite for structural health monitoring purposes.

Fiber Bragg gratings-based sensing for real-time needle tracking during MR-guided brachytherapy.

The development of MR-guided high dose rate (HDR) brachytherapy is under investigation due to the excellent tumor and organs at risk visualization of MRI. However, MR-based localization of needles (including catheters or tubes) has inherently a low update rate and the required image interpretation can be hampered by signal voids arising from blood vessels or calcifications limiting the precision of the needle guidance and reconstruction. In this paper, a new needle tracking prototype is investigated using fiber Bragg gratings (FBG)-based sensing: this prototype involves a MR-compatible stylet composed of three optic fibers with nine sets of embedded FBG sensors each. This stylet can be inserted into brachytherapy needles and allows a fast measurement of the needle deflection. This study aims to assess the potential of FBG-based sensing for real-time needle (including catheter or tube) tracking during MR-guided intervention. First, the MR compatibility of FBG-based sensing and its accuracy was evaluated. Different known needle deflections were measured using FBG-based sensing during simultaneous MR-imaging. Then, a needle tracking procedure using FBG-based sensing was proposed. This procedure involved a MR-based calibration of the FBG-based system performed prior to the interventional procedure. The needle tracking system was assessed in an experiment with a moving phantom during MR imaging. The FBG-based system was quantified by comparing the gold-standard shapes, the shape manually segmented on MRI and the FBG-based measurements. The evaluation of the MR compatibility of FBG-based sensing and its accuracy shows that the needle deflection could be measured with an accuracy of 0.27 mm on average. Besides, the FBG-based measurements were comparable to the uncertainty of MR-based measurements estimated at half the voxel size in the MR image. Finally, the mean(standard deviation) Euclidean distance between MR- and FBG-based needle position measurements was equal to 0.79 mm(0.37 mm). The update rate and latency of the FBG-based needle position measurement were 100 and 300 ms, respectively. The FBG-based needle tracking procedure proposed in this paper is able to determine the position of the complete needle, under MR-imaging, with better accuracy and precision, higher update rate, and lower latency compared to current MR-based needle localization methods. This system would be eligible for MR-guided brachytherapy, in particular, for an improved needle guidance and reconstruction.

The performance of embedded fiber Bragg grating sensors for monitoring failure modes of foam cored sandwich structures under flexural loads

In this study, failure modes of foam core sandwich composites are investigated by using embedded Fiber Bragg Grating sensors. Sandwich specimens with Fiber Bragg Grating sensors, embedded inside the face sheet, are manufactured using vacuum infusion process and then subjected to a static and a cyclic loading under the three-point bending mode. Different failure modes are monitored utilizing the wavelength shift and the spectrum of Fiber Bragg Grating sensors. It is shown that the responses of the Fiber Bragg Grating sensor differ depending on damage modes thereby making structural health monitoring of sandwich structures possible.

Application of Embedded Fiber Bragg Grating (FBG) Sensors in Monitoring Health to 3D Printing Structures

The 3D printing technology is known as a core technology in the third industrial revolution. The 3D printing structures are more popular and used as key components of products, so that monitoring health of 3D printing structures is particularly important. Moreover, the fiber Bragg grating (FBG) sensing technology is a new type of sensing technology, and FBG sensors have its unique advantages so that they have great application prospects in monitoring health of structures. FBG sensors were embedded inside a 3D printing structure, which strain variation is monitored by the FBG sensors during the loading process in this paper. Comparison was carried out between the measured and theoretical results to investigate thoroughly validity and reliability of the embedded FBG sensors for monitoring health of the 3D printing structure. Test results show that the embedded FBG sensors may be utilized to effectively monitor the strain variation of the 3D printing structure during the loading process; the measured and theoretical results show a good agreement and the straightness may be up to above 0.9998, so that the reliability is very good. Our results can also be guidance for monitoring health of 3D print structures.

Strain Characterization of HighTemperature Superconductor Tapes Using Embedded Fibre Bragg Grating Sensor in Mid-rip and Parallel Configuration

The aim of this work was to check the feasibility of embedding fibre Bragg grating (FBG)-based sensors to study the behaviour of the second-generation yttrium barium copper oxide (YBCO)-based high-temperature superconductor (HTS) tapes when subject to torsion. The variation in strain at multiple points was found out by embedding FBG sensors at strategic points on the tape. Torsional tests were carried out with tapes with different configurations of sensors. Configurations included embedding the FBG sensors on the mid-rib of the tape and stacking them parallel as an array. The newly fabricated tape was subject to torsion at both room temperature and liquid nitrogen temperature, and the variation in strain was studied. Also, certain parameters like the degree of freedom of one end of the HTS tape under twist were varied. The results obtained for the free-ended tape and the fixed-ended tape showed significant differences in the type of strain experienced by the tape. When the tape was given freedom to move axially during twisting, the strain variation was uniform and high repeatability was noticed. This reflects on the importance of the condition of the ends of the tape during torsion. In this paper, the design and technology requirements to adapt the wavelength division multiplexing (WDM) FBG sensor concept that could be installed in parallel configuration for coated conductors is presented. Initial experiments, which demonstrate the torsional stress distribution at room temperature and at 77 K are reported.

Cyclic hysteresis of AZ31B extrusion under load‐control tests using embedded sensor technology

AbstractThe fatigue behaviour of AZ31B extrusion magnesium alloy under load‐control cyclic test conditions is estimated using a combination of simulation and experimental results. The strain measurement of this asymmetric material is found experimentally using a Fibre Bragg Grating (FBG) sensor during rotating bending tests. Then, to analyse applied stresses in the sample – particularly in the plastic deformation range – the Variable Material Property (VMP) method is employed. Using this simulation method, the hysteresis loops of two critical top and bottom elements of the sample's cross section under different bending moments are obtained. Finally, the strain of the sample during rotating bending, as measured by the embedded FBG sensor, is related to the stresses obtained from the modeling using a mapping function. The hystereses obtained from this combination of the modeling and experimental results are compared with the results of a companion strain‐control pull–push test in which the input strain history was that of measured by the FBG sensor. Observations verify that the stresses of the combined VMP‐FBG hysteresis loops have good compatibility with the stress responses obtained through the experiment. The hybrid model introduced in this work can be employed to capture cyclic hysteresis, and hence estimate the fatigue life, under load‐controlled rotating bending tests.

WE‐AB‐BRA‐10: Assessment of Fiber Bragg Grating (FBG)‐Based Sensing for Real‐Time Needle Tracking During MR‐Guided Brachytherapy

Purpose:This study assesses the potential of Fiber Bragg Grating (FBG)‐based sensing for real‐time needle (including catheter or tube) tracking during MR‐guided HDR brachytherapy.Methods:The proposed FBG‐based sensing tracking approach involves a MR‐compatible stylet composed of three optic fibers with nine sets of embedded FBG sensors each. When the stylet is inserted inside the lumen of the needle, the FBG sensing system can measure the needle's deflection. For localization of the needle in physical space, the position and orientation of the stylet base are mandatory. For this purpose, we propose to fix the stylet base and determine its position and orientation using a MR‐based calibration as follows. First, the deflection of a needle inserted in a phantom in two different configurations is measured during simultaneous MR‐imaging. Then, after segmentation of the needle shapes on the MR‐images, the position and orientation of the stylet base is determined using a rigid registration of the needle shapes on both MR and FBG‐based measurements. The calibration method was assessed by measuring the deflection of a needle in a prostate phantom in five different configurations using FBG‐based sensing during simultaneous MR‐imaging. Any two needle shapes were employed for the calibration step and the proposed FGB‐tracking approach was subsequently evaluated on the other three needles configurations. The tracking accuracy was evaluated by computing the Euclidian distance between the 3D FBG vs. MR‐based measurements.Results:Over all needle shapes tested, the average(standard deviation) Euclidian distance between the FBG and MR‐based measurements was 0.79mm(0.37mm). The update rate and latency of the FBG‐based measurements were 100ms and 300ms respectively.Conclusion:The proposed FBG‐based protocol can measure the needle position with an accuracy, precision, update rate and latency eligible for accurate needle steering during MR‐guided HDR brachytherapy.M. Borot de Battisti is funded by Philips Medical Systems Nederland B.V.; M. Moerland is principal investigator on a contract funded by Philips Medical Systems Nederland B.V.; G. Hautvast and D. Binnekamp are fulltime employees of Philips Medical Systems Nederland B.V.

Advances in Fiber Optic Sensors Technology Development for Temperature and Strain Measurements in Superconducting Magnets and Devices

The luminosity upgrade of the Large Hadron Collider (HL-LHC) requires the development of a new generation of superconducting magnets based on Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn technology. To monitor the magnet thermo-mechanical behavior during its service life, from the coil fabrication to the magnet operation, reliable sensing systems need to be implemented. In the framework of the FP7 European Project EUCARD, Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn racetrack coils are developed as test beds for the fabrication validation, the cable characterization, and the instrumentation development. Fiber optic sensors (FOS) based on fiber Bragg grating (FBG) technology have been embedded in the coils of the short model coil (SMC) magnet. The FBG sensitivity to both temperature and strain required the development of a solution able to separate the mechanical and temperature effects. This paper presents the feasibility study of the implementation of embedded FBG sensors for the temperature and strain monitoring of the 11-T-type conductor. We aim to monitor and register these effects during the coil fabrication and cool down in a standalone configuration.

Monitoring Poisson’s ratio of glass fiber reinforced composites as damage index using biaxial Fiber Bragg Grating sensors

Damage accumulation in Glass Fiber Reinforced Polymer (GFRP) composites is monitored based on Poisson’s ratio measurements for three different fiber stacking sequences subjected to both quasi-static and quasi-static cyclic tensile loadings. The sensor systems utilized include a dual-extensometer, a biaxial strain gage and a novel embedded-biaxial Fiber Bragg Grating (FBG) sensor. These sensors are used concurrently to measure biaxial strain whereby the evolution of Poisson’s ratio as a function of the applied axial strain is evaluated. It is observed that each sensor system indicates a non-constant Poisson’s ratio, which is a sign of damage accumulation under the applied tensile loading. As the number of off-axis plies increases, transverse strain indicates a notable deviation from linearity due to the formation of transverse cracking, thereby leading to a larger reduction in Poisson’s ratio as a function of applied axial strain. Here, it is demonstrated that biaxially embedded FBG sensors are reliable to monitor the evolution of Poisson’s ratio, unlike biaxial strain gages which record strain values that can be significantly influenced by the cracks formed on the surface of the specimen.

Method for independent strain and temperature measurement in polymeric tensile test specimen using embedded FBG sensors

A novel method to obtain independent strain and temperature measurements using embedded Fibre Bragg Grating (FBG) in polymeric tensile test specimens is presented in this paper. The FBG strain and temperature cross-sensitivity was decoupled using two single mode FBG sensors, which were embedded in the specimen material with a certain angle between them. It is demonstrated that, during temperature variation, both FBG sensors show the same signal response. However, for any applied load the signal response is different, which is caused by the different levels of strain acting in each sensor. Equations to calculate independently the strain and temperature are presented in the article, together with a measurement resolution study.This multi-parameter measurement method was applied to an epoxy tensile specimen, tested in a unidirectional tensile test machine with a temperature controlled cabinet. A full calibration procedure (temperature and strain) was performed to this material-sensor pair, where a calibration error < 1% was achieved. This was followed by a strain-temperature test case, where multiple two loading/strain stages of ε = 0.30% and ε = 0.50% were applied during a continuous variation of temperature, from 40 °C to –10 °C.The consistency of the expected theoretical results with the calibration procedure and the experimental validation shows that this proposed method is applicable to measure accurate strain and temperature in polymers during static or fatigue tensile testing. Two different calibration protocols are presented and analysed.

Dynamic Strain Measurements on Automotive and Aeronautic Composite Components by Means of Embedded Fiber Bragg Grating Sensors

The measurement of the internal deformations occurring in real-life composite components is a very challenging task, especially for those components that are rather difficult to access. Optical fiber sensors can overcome such a problem, since they can be embedded in the composite materials and serve as in situ sensors. In this article, embedded optical fiber Bragg grating (FBG) sensors are used to analyze the vibration characteristics of two real-life composite components. The first component is a carbon fiber-reinforced polymer automotive control arm; the second is a glass fiber-reinforced polymer aeronautic hinge arm. The modal parameters of both components were estimated by processing the FBG signals with two interrogation techniques: the maximum detection and fast phase correlation algorithms were employed for the demodulation of the FBG signals; the Peak-Picking and PolyMax techniques were instead used for the parameter estimation. To validate the FBG outcomes, reference measurements were performed by means of a laser Doppler vibrometer. The analysis of the results showed that the FBG sensing capabilities were enhanced when the recently-introduced fast phase correlation algorithm was combined with the state-of-the-art PolyMax estimator curve fitting method. In this case, the FBGs provided the most accurate results, i.e., it was possible to fully characterize the vibration behavior of both composite components. When using more traditional interrogation algorithms (maximum detection) and modal parameter estimation techniques (Peak-Picking), some of the modes were not successfully identified.

Structural Stability Monitoring of a Physical Model Test on an Underground Cavern Group during Deep Excavations Using FBG Sensors.

Fiber Bragg Grating (FBG) sensors are comprehensively recognized as a structural stability monitoring device for all kinds of geo-materials by either embedding into or bonding onto the structural entities. The physical model in geotechnical engineering, which could accurately simulate the construction processes and the effects on the stability of underground caverns on the basis of satisfying the similarity principles, is an actual physical entity. Using a physical model test of underground caverns in Shuangjiangkou Hydropower Station, FBG sensors were used to determine how to model the small displacements of some key monitoring points in the large-scale physical model during excavation. In the process of building the test specimen, it is most successful to embed FBG sensors in the physical model through making an opening and adding some quick-set silicon. The experimental results show that the FBG sensor has higher measuring accuracy than other conventional sensors like electrical resistance strain gages and extensometers. The experimental results are also in good agreement with the numerical simulation results. In conclusion, FBG sensors could effectively measure small displacements of monitoring points in the whole process of the physical model test. The experimental results reveal the deformation and failure characteristics of the surrounding rock mass and make some guidance for the in situ engineering construction.

Instrumentation of integrally stiffened composite panel with fiber Bragg grating sensors for vibration measurements

We evaluate the performance of fiber Bragg grating (FBG) sensors for the measurement of dynamic strains in complex composite structures. The particular structure used in this study is an integrally stiffened composite panel for which the stiffeners and skin are fabricated in a single layup and cure process. Surface-mounted FBG sensors are bonded to the panels after curing, whereas embedded FBG sensors are successfully incorporated during the fabrication process. A finite element model was also constructed of the stiffened panel. The panels were subjected to repeated impacts and the post-impact vibration response of the panel was measured through the FBG sensor responses. Little change to the global response of the panel was observed after the repeated impacts, through the dynamic response of the surface-mounted FBGs. Pulsed phase thermography and micro-computer-tomography imaging of the panel confirmed that the damage was localized near the impact locations, producing negligible changes to the global response of the panel. All of the embedded FBG sensors survived the fabrication and multiple impacts; however, as these were embedded close to the neutral axis of the panel, they were not very sensitive to the vibration modes. Excitation of the panel near the first natural frequency did produce a measurable response in the FBG sensors, confirming their functionality.

An experimental–numerical investigation of hydrothermal response in adhesively bonded composite structures

Water absorption and thermal response of adhesive composite joints were investigated by measurements and numerical simulations. Water diffusivity, saturation, swelling, and thermal expansion of the constituent materials and the joint were obtained from gravimetric experiments and strain measurements using embedded fiber Bragg grating (FBG) sensors. The mechanical response of these materials at different temperatures and water content was characterized by dynamic mechanical analysis. Thermal loading and water absorption in joint specimens were detected by monitoring the FBG wavelength shift caused by thermal expansion or water swelling. The measured parameters were used in finite element models to simulate the response of the embedded sensor. The good correlation of experimental data and simulations confirmed that the change in FBG wavelength could be accurately related to the thermal load or water absorption process. The suitability of the embedded FBG sensors for monitoring of water uptake in adhesive composite joints was demonstrated.