Embedded Fiber Bragg Grating Research Articles

Multi-Directional Strain Measurement in Fiber-Reinforced Plastic Based on Birefringence of Embedded Fiber Bragg Grating.

Embedded fiber Bragg gratings are increasingly applied for in-situ strain measurement in fiber-reinforced plastics, integral to high-end aerospace equipment. Existing research primarily focuses on in-plane strain measurement, limited by the fact that fiber Bragg gratings are mainly sensitive to axial strain. However, out-of-plane strain measurement is equally important for comprehending structural deformation. The birefringence of fiber Bragg gratings shows promise for addressing this problem; yet, the strain transfer relationship between composites and optical fibers, along with the decoupling method for multi-directional strains, remains inadequately explored. This study introduces an innovative method for multi-directional strain measurement in fiber-reinforced plastics using the birefringence of a single-fiber Bragg grating. The strain transfer relationship between composites and embedded optical fibers was derived based on Kollar's analytical model, leading to the development of a multi-directional strain decoupling methodology. This method was experimentally validated on carbon fiber/polyetherimide laminates under thermo-mechanical loading. Its reliability was confirmed by comparing experimental results and finite element simulations. These findings significantly broaden the application scenarios of fiber Bragg gratings, advancing the in-situ measurement technology crucial for the next generation of high-end aerospace equipment.

Fatigue and residual strain monitoring for thermoplastic composite using embedded FBG inscribed in highly‑birefringent side-hole elliptical core optical fiber

Monitoring of thermoplastic carbon reinforced composites is critical for the safe operation and optimization of composite structures. Strain measurement during their manufacturing and operation is often achieved with embedded Fiber Bragg Gratings (FBG). However, conventional FBGs can only measure axial strain. To measure multiaxial strain in thermoplastic composites, FBGs inscribed in highly-birefringent side-hole elliptical core optical fiber (SH2 FBG) are proposed. SH2 FBG were used to monitor composite produced by automated tape laying process, first − to measure residual in-plane and out-of-plane strain during the manufacturing stage, second − in plane transverse strain during cyclic loading. It was demonstrated that the sensors can be used to measure multiaxial residual strain, observe changes in material stiffness, and calculate the area of the mechanical hysteresis loop. As a result − it was shown that embedding SH2 FBG can be an effective and robust method for monitoring the carbon fiber reinforced thermoplastic composite.

Ultrasonic measurements with embedded Fiber Bragg Gratings in polyurethane resins

In geophysics, Distributed Acoustic Sensing (DAS) optical fibers are increasingly being exploited to probe the subsurface using seismic waves. However, the integration of the deformation undergone by the fiber over a gauge length (typically a few meters) and a limited sampling frequency prohibit the use of this technology for ultrasonic applications. On the other hand, fiber Bragg gratings (FBGs) offer a sufficiently small sensitive element (500 μm to several cm) to measure the dynamic deformation field at ultrasonic frequencies. Recently, it has been shown that FBGs bonded to a surface or embedded in a structure can be used to measure the dynamic deformation field caused by the propagation of an ultrasonic wave. However, when inserting an optical fiber into a material, the effects of the structure on the fiber mechanical and optical properties need to be taken into account. For FBGs embedded in a reacting material, such as a polymer or concrete, the polymerization stage can affect the FBG spectrum leading to a variation of the Bragg wavelength, a decrease of the grating amplitude and, in the worst case, a modification of the FBG signature (lobe broadening, introduction of harmonics, etc.) due to the material’s shrinkage inducing strain on the fiber. In order to prepare for large-scale on-site measurements with embedded FBGs, it is necessary to carry out preliminary laboratory tests to address the instrumentation issues associated with inserting optical fibers into a curing material. For this purpose, several FBGs with different characteristics (insensitive to microbending, acrylate/polymide coating, multicore, etc.) have been integrated into different polyurethane resins to compare their response to insertion, heat transfer and the measurement of dynamic deformations at ultrasonic frequencies. In this contribution, the reduce scale models produced in polyurethane resins, and the results related to fiber insertion and ultrasound measurements, are presented in detail.

Optimization and characterization of a 3D-printed wearable strain sensor for respiration and heartbeat measurements

Recently, the healthcare scenario recognized the advantages of embedding fiber Bragg gratings (FBGs) into 3D-printed structures for biomedical purposes. Material extrusion is considered a next-generation method to develop sensors based on FBG with exciting characteristics (e.g., low cost, lightweight, and fast prototyping with customized designs). The aim of this study is to combine the FBG and material extrusion benefits for proposing a novel strain sensor for respiration and heartbeat measurements from the detection of chest wall deformations. A finite-element analysis guided the sensor design to maximize the strain localization in the sensor area. The bonding at the fiber - encapsulation material interface was discussed by analyzing the spectral response and microstructural imaging. The sensor response was investigated by applying mechanical and thermal inputs. Finally, the skin-mountable system was used in a pilot test for estimating respiratory and heart rates, showing promising results in human physiological monitoring.

Embedded Fiber Bragg Grating (FBG) Sensors Fabricated by Ultrasonic Additive Manufacturing for High-Frequency Dynamic Strain Measurements

Embedded Fiber Bragg Grating (FBG) Sensors Fabricated by Ultrasonic Additive Manufacturing for High-Frequency Dynamic Strain Measurements

Finger tapping test evaluated by embedded fiber Bragg gratings

Finger tapping test evaluated by embedded fiber Bragg gratings

Embedding Fiber Bragg Grating Sensors in Carbon Composite Structures for Accurate Strain Measurement

Fiber Bragg Grating (FBG) sensors written by femtosecond laser pulses in polyamide-coated low bending loss optical fibers are succesfully embedded in carbon composite structures, following laminating and light resin moulding processes which optimize the size of each ply to address aesthetic, drapability and structural requirements of the final components. The sensors are interrogated by a tunable laser operating at around 1.55 μm and their response to temperature and strain variations is characterized in a thermally controlled chamber and by bending tests using suspended calibrated loads and a laser scanning system. Experimental results are in good agreement with simulations, confirming that the embedding process effectively overcomes potential issues related to FBG spectral distortion, birefringence and losses. In particular the effects of the composite material non homogeneity and FBG birefringence are investigated to evaluate their impact on the monitoring capabilities. A bi-material mechanical beam model is proposed to characterize the orthotropic laminates, pointing out better accuracy in estimating the applied load with respect to the classical homogeneous beam model. A comparative analysis, performed on different instrumented carbon composite samples and supported by theory, points out the repeatability of the FBG sensors embedding process and the effectiveness of the technology for real time accurate strain measurement. Based on such measurements damages and/or changes in local stiffness can be effectively detected, allowing for structural health monitoring of composite structures for applications in specific industrial fields like automotive and aerospace.

Embedded Fiber Bragg Grating Sensors for Monitoring Temperature and Thermo-Elastic Deformations in a Carbon Fiber Optical Bench.

A composite optical bench made up of Carbon Fiber Reinforced Polymer (CFRP) skin and aluminum honeycomb has been developed for the Tunable Magnetograph instrument (TuMag) for the SUNRISE III mission within the NASA Long Duration Balloon Program. This optical bench has been designed to meet lightweight and low sensitivity to thermal gradient requirements, resulting in a low Coefficient of Thermal Expansion (CTE). In addition to the flight model, a breadboard model identical to the flight one has been manufactured, including embedded fiber Bragg temperature and strain sensors. The aim of this is to explore if the use of distributed fiber Bragg gratings (FBGs) can provide valuable information for strain and temperature mapping of an optical instrument on board a space mission during its operation as well as its on-ground testing. Furthermore, surface-mounted strain FBG sensors and thermocouples have been installed in the optical bench for intercomparison purposes. This paper presents the results obtained from a thermal vacuum test consisting of three thermal cycles with stabilization steps at 100 °C, 60 °C, 20 °C and -20 °C. Experimental results provide information about how FBG embedded temperature sensors can provide a proper and quick response to the temperature changes of the optical bench and that embedded FBG strain sensors are able to measure micro-deformation induced in a close-to-zero CTE optical bench.

Experimental evaluation of the use of embedded fiber Bragg gratings to measure steady and unsteady flow-induced marine propeller blade deformation

This paper presents an experimental study that investigates the deformation of a composite propeller blade subjected to a uniform inflow. An array of fiber Bragg grating sensors were embedded inside the blade, allowing for real-time monitoring of strain. Measurements were carried out in the towing tank at Centrale Nantes, France. An original experimental setup was developed to support and fully submerge the blade in the hydrodynamic facility, which reduced the effect of the free surface during testing. The blade, with a root chord of 0.3 m and a span of 0.5 m, was instrumented with embedded fiber Bragg grating (FBG) sensors on both the pressure and suction sides to measure the deformation field. The FBG measurements were dynamically synchronized with a six degree-of-freedom hydrodynamic balance to measure the loads and moments on the blade. Visualizations of the boundary layer regime were observed to determine the flow detachment regions at the surface. The carriage velocities utilized were between 1 and 4m/s and the angle of attack of the blade, based on the zero-lift angle of attack, ranged between 0 to 12∘. After determining the natural frequencies and modes of the blade using laser doppler vibrometry, the forces and deformations were investigated under hydrodynamic flow for both averaged and unsteady data. For averaged results, a good correlation is observed between the lift force and deformations, which demonstrates the accuracy of the measurement system. The fluid–structure interaction dynamic is analyzed using the frequency spectra of drag, lift, pitching moment, and deformations. The interaction between Strouhal frequencies of vortex shedding resulting from flow detachments at the blade surface is identified with the flow visualization, and the modal response of the blade is demonstrated.

Three-dimensional force-tactile sensors based on embedded fiber Bragg gratings in anisotropic materials.

Three-dimensional force-tactile sensors have attracted much attention for their great potential in the applications of human-computer interaction and bionic intelligent robotics. Herein, a flexible haptic sensor based on dual fiber Bragg gratings (FBGs) embedded in a bionic anisotropic material is proposed for the detection of 3D forces. To achieve the discrimination of normal and tangential force angles and magnitudes, FBGs were orthogonally embedded in a flexible silicone cylinder for force determination. Fe3O4 nanoparticles were used as a modifying agent to induce anisotropic elasticity of the silicone structure to improve the angle detection resolution. The results show that the flexible tactile sensor can detect the angle and magnitude of the 3D force.

Structural Health Monitoring of Composite Pipelines Utilizing Fiber Optic Sensors and an AI-Based Algorithm-A Comprehensive Numerical Study.

In this paper, a structural health monitoring (SHM) system is proposed to provide automatic early warning for detecting damage and its location in composite pipelines at an early stage. The study considers a basalt fiber reinforced polymer (BFRP) pipeline with an embedded Fiber Bragg grating (FBG) sensory system and first discusses the shortcomings and challenges with incorporating FBG sensors for accurate detection of damage information in pipelines. The novelty and the main focus of this study is, however, a proposed approach that relies on designing an integrated sensing-diagnostic SHM system that has the capability to detect damage in composite pipelines at an early stage via implementation of an artificial intelligence (AI)-based algorithm combining deep learning and other efficient machine learning methods using an Enhanced Convolutional Neural Network (ECNN) without retraining the model. The proposed architecture replaces the softmax layer by a k-Nearest Neighbor (k-NN) algorithm for inference. Finite element models are developed and calibrated by the results of pipe measurements under damage tests. The models are then used to assess the patterns of the strain distributions of the pipeline under internal pressure loading and under pressure changes due to bursts, and to find the relationship of strains at different locations axially and circumferentially. A prediction algorithm for pipe damage mechanisms using distributed strain patterns is also developed. The ECNN is designed and trained to identify the condition of pipe deterioration so the initiation of damage can be detected. The strain results from the current method and the available experimental results in the literature show excellent agreement. The average error between the ECNN data and FBG sensor data is 0.093%, thus confirming the reliability and accuracy of the proposed method. The proposed ECNN achieves high performance with 93.33% accuracy (P%), 91.18% regression rate (R%) and a 90.54% F1-score (F%).

Damage Monitoring of Composite Adhesive Joint Integrity Using Conductivity and Fiber Bragg Grating

Adhesive joints possess a number of advantages over traditional joining methods and are widely used in composite structures. Conventional non-destructive examination techniques do not readily reveal joint degradation before the formation of explicit defects. Embedded fiber Bragg grating (FBG) sensors and the resistance of carbon nanotube (CNT)-doped conductive joints have been proposed to monitor the structural integrity of adhesive joints. Both techniques will be employed and compared in the current work to monitor damage development in adhesive joints under tensile and cyclic fatigue loading. Most of the previous works took measurements under an applied load, which by itself will affect the monitoring signals without the presence of any damage. Moreover, most FBG works primarily relied on the peak shifting phenomenon for sensing. Degradation of adhesive and inter-facial defects will lead to non-uniform strain that may chirp the FBG spectrum, causing complications in the peak shifting measurement. In view of the above shortfalls, measurements are made at some low and fixed loads to preclude any unwanted effect due to the applied load. The whole FBG spectrum, instead of a single peak, will be used, and a quantitative parameter to describe spectrum changes is proposed for monitoring purposes. The extent of damage is revealed by a fluorescent penetrant and correlated with the monitoring signals. With these refined techniques, we hope to shed some light on the relative merits and limitations of the two techniques.

Three-Dimensional-Printed Sensing Samples Embedding Fiber Bragg Gratings: Metrological Evaluation of Different Sample Materials and Fiber Coatings

Embedding optical fibers with fiber Bragg grating (FBG) sensors in 3D-printed samples can effectively facilitate the systematic use of smart materials in many fields such as civil, bio-medical and soft robotics applications. The aim of the study is to analyze different combinations of filament materials and FBG coatings, and to assess their metrological characteristics. Eighteen samples are fabricated and tested under different mechanical and thermal conditions. The repeated tests allow to perform an evaluation of the measurement repeatability for each sample, along with an analysis of the samples sensitivity. The filaments employed are acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), and thermoplastic polyurethane (TPU). The fiber coatings are acrylate, Ormocer® and polyimide. The fiber coating has no significative influence on the performance of the sensors. The tests for temperature sensitivity highlight a good performance of ABS (116 pm/°C) and TPU samples (32 pm/°C) up to 60 °C, whereas the fabricated PLA samples (139 pm/°C for polyimide, 55 pm/°C for acrylate, 14 pm/°C for Ormocer®) cannot be used above 40 °C. The tests for strain sensitivity in axial elongation show an average sensitivity of 3.049 nm/mm for ABS, 1.991 nm/mm for PLA, 3.726 nm/mm for TPU. The bending tests show that all specimens materials have different sensitivity to elongation (2.994 nm/mm for ABS, 0.668 nm/mm for PLA, 0.149 nm/mm for TPU). Only for acrylate in PLA samples an effective difference for bending sensitivity resulted (1.241 nm/mm for the acrylate coating vs 2.366 nm/mm for the other coatings).

A Fiber-Optic Sensor-Embedded and Machine Learning Assisted Smart Helmet for Multi-Variable Blunt Force Impact Sensing in Real Time

Early on-site diagnosis of mild traumatic brain injury (mTBI) will provide the best guidance for clinical practice. However, existing methods and sensors cannot provide sufficiently detailed physical information related to the blunt force impact. In the present work, a smart helmet with a single embedded fiber Bragg grating (FBG) sensor is developed, which can monitor complex blunt force impact events in real time under both wired and wireless modes. The transient oscillatory signal "fingerprint" can specifically reflect the impact-caused physical deformation of the local helmet structure. By combination with machine learning algorithms, the unknown transient impact can be recognized quickly and accurately in terms of impact magnitude, direction, and latitude. Optimization of the training dataset was also validated, and the boosted ML models, such as the S-SVM+ and S-IBK+, are able to predict accurately with complex databases. Thus, the ML-FBG smart helmet system developed by this work may become a crucial intervention alternative during a traumatic brain injury event.

Residual Strain Monitoring and Dynamic Characteristics of Hybrid Hollow Square Tube with Metal–FRP–Metal Sandwich Walls

In order to design and fabricate a novel hollow square tube (HST) for ram structure in machine tools, the hybrid HST with sandwich walls based on steel skins and unidirectional carbon fiber-reinforced polymer (CFRP) composite core is proposed. A detailed co-cured fabrication method with embedded fiber Bragg grating (FBG) sensors for residual strains/stresses determination in a hybrid metal–composite structure is presented. Results reveal that the hybrid HST has undergone complex residual strain history, and the strain rate is about 10 times the cooling rate. The tensile strains in the dwell stage transform into compressive strains in the cooling stage due to the mismatch of the coefficients of thermal expansion of the steel plates and the CFRP composite. A comparison of the residual strains in the cooling phase obtained by FBG sensors with those obtained by theoretical calculation is carried out. Furthermore, the dynamic characteristics of the hybrid HST and the steel HST are tested. The results showed that the damping of the hybrid HST is 586% higher than that of the steel HST, while the hybrid HST has a lower first natural frequency (4.6% reduction) and mass (15.9% weight reduction). The influence of co-cure temperature and cooling rate on the size and state of the residual strains is analyzed, which might be helpful to guide the manufacturing of sandwich structures in machine tools. This novel hybrid HST may be used for online health monitoring and safety evaluation to build intelligent machine tools structures.

Verification of embedding conditions for FBG sensor into textile product for the development of wearable healthcare sensor.

To develop wearable healthcare sensors that use fiber Bragg grating (FBG) sensors, a stretch textile product with an embedded FBG sensor is required. The FBG sensor, which is an optical fiber, was embedded into a textile product following a wavy pattern by using a warp knitting machine. When an optical fiber is embedded in a textile product, the effect of the cycle length of wavy pattern and the number of cycles on the optical loss is verified. The shorter the cycle length of the wavy pattern of the optical fiber, and more increase in the number of cycles, the longer the textile product in which the optical fiber is embedded can expand and contract. However, when the cycle length of the wave pattern is 30 mm (shortest), large in optical loss, the pulse wave signal cannot be measured. If the cycle length of the wavy pattern is 50 mm or more, small in optical loss, the pulse wave signal is measured. Compared with a straight pattern embedding FBG sensor, the amplitude value of the pulse wave signal measured with a cycle length of 50 mm is large, therefore the sensor sensitivity in this state is greater. This result is consistent with the measurement sensitivity depending on the angle of installation with respect to the direction of the artery. With a cycle length of wavy pattern of 50 mm and 4 cycles, a stretch textile product with an embedded FBG sensor can be fabricated. Pulse wave signals are measured with this textile product, and the development of wearable healthcare sensors is expected.

Characterization of cure residual strain development and associated structural distortion in carbon fiber polymer matrix composites

A comprehensive experimental methodology has been developed to measure the in situ cure residual strain (CRS) generation in carbon fiber reinforced polymer (CFRP) laminates using embedded fiber Bragg grating (FBG) sensors. In order to delineate the effect of anisotropy, heterogeneity, laminate thickness and geometry, in situ CRS is measured in pristine epoxy, uni-directional (UD)-CFRPs, multi-directional (MD)-CFRPs and L-shaped MD-CFRP laminates. In this study, emphasis is laid on quantifying the variation in CRS magnitudes as a function of laminates lay-up sequence and thickness. Calibration of FBG sensors are carried out by following well established procedures. Structural distortion due to CRS is correlated using embedded FBG sensor and measured using the coordinate measurement machine (CMM). The presented results show a uniform CRS development across the thickness in MD-CFRP laminates as compared to L-shaped MD-CFRP laminates.

A High-Precision Extensometer System for Ground Displacement Measurement Using Fiber Bragg Grating

The design and performance of an innovative high-precision extensometer system, fabricated inexpensively using 3D printing technology, are discussed in this paper. In the development of the extensometer, an embedded Fiber Bragg Grating (FBG) strain sensor was 3D printed using a thermoplastic polyurethane (TPU) filament, which was used as the primary sensing element of the extensometer system, taking advantage of its excellent flexibility and high sensitivity to variations in localized strain. In the performance assessment carried out, the results obtained during the experimental test and validation have demonstrated that it could be used very effectively to measure strain variations, with an average wavelength responsivity of 0.0158 nm/cm (for displacement) and very high linearity (up to 99&#x0025;). Furthermore, the protection integrated into the sensor systems design makes it well-suited for in-the-field applications, such as monitoring ground displacements which can lead to dangerous slippages of sloped earthworks. In addition, a field testing of the extensometer under simulated conditions has shown that a Fiber Bragg Grating (FBG)-based approach could be applied effectively to the measurement of strain, offering a wavelength responsivity of 0.0012 nm/<inline-formula> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> (for strain-sensitive FBGs) under both dry and wet soil conditions. Moreover, taking advantage of the high (&#x007E;99&#x0025;) linearity, the extensometer is a reliable instrument for use in different underground conditions, creating an easy-to-use ground movement monitoring system which then enables an excellent representation of the displacement profile of the earth to be made.

Fiber bragg grating sensors to monitor strain response of epoxy resin during curing and cryogenic temperature

The epoxy resin is one of the critical components of large-scale superconducting magnets which find wide applications in magnetic confinement fusion, high energy accelerator and magnetic resonance imaging. In general, magnetic confinement fusion, high energy accelerator and magnetic resonance will be used at cryogenic temperatures. An approach with respect to real-time monitor of epoxy curing and cryogenic process is practically significant for magnetic reliability, maintainability as well as future mass production. Fiber Bragg Grating (FBG) sensors are excellent candidates of this measurement for their anti-electromagnetic interference characteristics and high sensitivity. The aim of this work is to use the embedded Fiber Bragg Gratings (FBG) to obtain real-time strain response of curing epoxy, and the strain change of the epoxy at cryogenic temperature from 4.2K to 300K. An FBG pre-tensioning device was designed to keep the FBG embedded in the epoxy in a stretched state during the epoxy curing process. Moreover, strain gauge sensors were also embedded to monitor the curing process at the same time. There are subtle differences in strain values obtained by the two sensors, which illustrate higher sensitivity of FBGs. This work verifies the reliability of FBG monitoring the strain response of epoxy during curing process as well as the cryogenic service.

Strain Sensor Based on Embedded Fiber Bragg Grating in Thermoplastic Polyurethane Using the 3D Printing Technology for Improved Sensitivity

A new and easy-to-fabricate strain sensor has been developed, based on fiber Bragg grating (FBG) technology embedded into a thermoplastic polyurethane filament using a 3-dimensional (3D) printer. Taking advantage of the flexibility and elastic properties of the thermoplastic polyurethane material, the embedding of the FBG provides durable protection with enhanced flexibility and sensitivity, as compared to the use of a bare FBG. Results of an evaluation of its performance have shown that the FBG sensors embedded in this way can be applied effectively in the measurement of strain, with an average wavelength responsivity of 0.013 5 nm/cm of displacement for tensile strain and −0.014 2 nm/cm for compressive strain, both showing a linearity value of up to 99%. Furthermore, such an embedded FBG-based strain sensor has a sensitivity of ∼1.74 times greater than that of a bare FBG used for strain measurement and is well protected and suitable for in-the-field use. It is also observed that the thermoplastic polyurethane based (TPU-based) FBG strain sensor carries a sensitivity value of ∼2.05 times higher than that of the polylactic acid based (PLA-based) FBG strain sensor proving that TPU material can be made as the material of choice as a “sensing” pad for the FBG.