Embedded Fiber Bragg Grating Sensors Research Articles

Development of Flexible Tooling for Deformation Sensing Applied to Composite Materials Fabrication Process

Among the existing techniques for composite materials manufacturing, Vacuum Assisted Resin Infusion (VARI) is a liquid composite molding (LCM) process where resin flows through a dry fiber preform to fully impregnate it. This method uses flexible film sealed onto a rigid mold to form the infusion cavity containing the fibers. As the fabric preform is impregnated, its thickness varies due to the changes in the applied compaction pressure. This thickness variation affects the resin flow and the final fiber volume fraction of the manufactured part. This study focuses on the initial steps of developing an integrated acquisition system for thickness variation monitoring during VARI. The conventional flexible tooling is to be replaced by a flexible membrane equipped with optical fiber Bragg grating (FBG) sensors. A prototype was developed by embedding FBG sensors in a silicon rubber material and initial measurements of a cylindrical profile curvature were performed. Preliminary results show satisfactory precision of the device, which opens a gap for a more precise and accurate thickness monitoring process during real part manufacturing.

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.

Application of FWM-Based OFC for DWDM Optical Communication System with Embedded FBG Sensor Network

Abstract Four-wave mixing optical frequency comb fibre-based setups (FWM-OFCs) have the potential to improve the combined dense wavelength division multiplexed passive optical network (DWDM-PON) and fibre Bragg grating (FBG) temperature sensors network providing easier application, broader technological opportunities for network development, and energy efficiency by substituting a power-demanding laser array. In this research, OFCs are generated for the purpose of combined network application of DWDM-PON and FBG optical sensors. The paper also investigates compatibility scenarios with OFCs in such systems. The mathematical simulation model has been developed and the performance of FWM-OFC based 8-channel 50 GHz spaced non-return-to-zero on-off keying (NRZ-OOK) modulated DWDM-PON transmission system, operating at 50 km single-mode fibre (SMF) with a bit rate of at least 10 Gbps embedded with 7 FBG optical temperature sensors, has been studied. As it is shown, FWM application results in OFC source that has fluctuations of the individual comb tones of less than 3 dB in power, and with an extinction ratio of about 33 dB for operation range of 192.9–193.25 THz, acting as a unified light source for all the data transmission channels. Embedded FBG optical sensors network causes negligible 0.3 dB power penalty.

Multitasking FBG sensors for condition monitoring with a wideband (DC-MHz) interrogation system.

We demonstrate diagnosis of several machine-condition failures using wide-frequency-band interrogation of fiber Bragg grating (FBG) sensors. In collaboration with Israel's national water company Mekorot Ltd., a scaled-down version of a semi-submerged pumping system was constructed. By monitoring broadband signals from DC to ultrasound (>M H z), at different points of the engine and the submersed pump, the system was able to diagnose incipient cavitation, faulty bearings, and submerged dynamic water-level measurements. In addition, a metal embedded FBG sensor was investigated, revealing the potential of using FBGs in applications where bonding is problematic such as bearing housing. These results prove that wideband data acquisition, together with advanced analytics, could open a variety of new applications in the fields of structural health and machine-condition monitoring.

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.

Health monitoring of carbon fiber-reinforced polymer composites in γ-radiation environment using embedded fiber Bragg grating sensors

Fiber-reinforced polymer (FRP) composites are considered suitable candidates for structural materials of spacecrafts due to their excellent properties of high strength, light weight, and corrosion resistance. An online health monitoring method for FRP composites must be applied to space structures. However, the application of existing health monitoring methods to space structures is limited due to the harsh space environment. Here, carbon fiber-reinforced polymer (CFRP) composites embedded with fiber Bragg grating (FBG) sensors were prepared to explore the feasibility of strain monitoring using embedded FBG sensors in γ-radiation environment. The analysis of the influence of radiation on the strain monitoring demonstrated that the embedded FBG can be successfully applied to the health monitoring of FRP composites in radiation environment.

Evaluation of transfer length of CFRP tendon in pretensioned concrete beam using embedded FBG sensors

This study aimed to determine the transfer length of pretensioned concrete beams strengthened with carbon fiber-reinforced polymer (CFRP) using a fiber Bragg grating (FBG) sensor. The transfer length is the distance along the tendon that is critical to ensuring effective prestressing force and structural capacity of the concrete beam. Typically, electric strain gauges are used to measure the transfer length, but they can produce measurement errors due to installation errors and data noise. In this study, an FBG sensor was embedded in a CFRP tendon instead of a strain gauge, enabling accurate measurements of tendon strain and transfer length.

Structural health monitoring of Cerasol® roof tiles with embedded FBG sensors

ABSTRACT There are currently little widely used structural health monitoring (SHM) techniques for structural adhesive joints. In this study, integrated fibre Bragg grating (FBG) sensors are used to examine the structural integrity and reliability of functional adhesive joints. There is an even greater demand for structural monitoring during operation or quality control of adhesive joints during production as more industries embrace high performance structural adhesives in fail-safe designs. Smartroof® photovoltaic (PV) roof tiles are used in this study to validate a real-time health monitoring of the adhesive connection between the PV glass plate and the polymer frame. A structural epoxy and a modified siloxane (MS) polymer are assessed. To assure measurement accuracy and to provide a better understanding of the strain sensing mechanism, the data is qualitatively compared with a finite element model. It is demonstrated that using embedded FBG sensors is accurate and adequate for in situ real-time structural health monitoring. Nevertheless, it remains difficult to ensure and manage the fibre’s position in the adhesive layer.

Experimental and numerical study of the additively manufactured carbon fibre reinforced polymers including fibre Bragg grating sensors

Fibre Bragg grating (FBG) sensors have been known as one of the smart localized and globalized Structural Health Monitoring (SHM) devices for various structural applications specifically those utilize composite materials. In recent decades the use of advanced composite materials green manufacturing methods has increased the requirement to provide effective means of performing experimental investigations to support analytical and numerical analyses. The main objective of this article is to analyse the performance of additively manufactured (AM) Carbon Fibre Reinforced Polymer (CFRP) sample with FBG sensors under the influence of environmental factors (temperature, relative humidity). The sample was manufactured using continuous carbon fibre applying Fused Deposition Modelling (FDM) technique. During the AM process one FBG sensor was embedded in the middle of the sample, while the second FBG sensor was attached to the finished sample surface. The environmental tests were conducted to investigate the durability of AM elements and influences of embedded FBG sensors on the composite specimen. Additionally, the behaviour of composite materials under environmental loading was modelled using the Finite Element Method (FEM) through an Abaqus software. It allows to achieve a more complex picture of embedded fibre optic influences on AM composite material durability.

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.

The cooperative deformation test of an embedded FBG sensor and strain correction curve verification

In order to accurately measure the internal structural strain of asphalt pavement using a fiber Bragg grating (FBG) sensor, the cooperative deformation characteristics of the FBG sensor and asphalt mixture were investigated. First, the cooperative deformation test and evaluation method were designed for the developed three FBG sensors, and the strain correction curve was drawn to modify the measurement results. Next, the sensitivity, stability, survival rate, and other factors were considered comprehensively to optimize the FBG sensor and make it suitable for the internal monitoring of asphalt roads. Finally, the accuracy of the average strain correction curve was verified. The main conclusions are as follows: (a) the strain data monitored by the embedded FBG sensor can reflect the deformation accumulation and elastic recovery of the material in asphalt roads; (b) the FBG sensors, with large or small correction coefficients, affect the measurement accuracy and stability of long-term monitoring; (c) the strain data fluctuated from 8.1 to 9.05% after temperature compensation; and (d) the average strain correction curve can modify the strain monitored by the FBG sensor. The measurement results are accurate, and the measurement error was about 3.01–9.14%.

Embedded Optical Fibre with Fibre Bragg Grating Influence on Additive Manufactured Polymeric Structure Durability.

Additive manufacturing (AM) polymers are applied in many branches of the industry due to the possibility of fast and accurate production of elements with various and complex shapes. Fibre Bragg grating sensors (FBG) are widely applied in structural health monitoring (SHM) systems. The main objective of this research is to perform analyses of the influence of embedded FBG sensors on AM polymer elements’ durability. Two polymers (M3 X and M3 Crystal) with different mechanical properties were analysed. The tests were performed on samples with FBG sensors embedded in (different alignment) and attached to the surfaces of the elements. Firstly, the samples were exposed to elevated or sub-zero temperatures under stable relative humidity levels. The strain in the samples was measured using fibre Bragg grating (FBG) sensors. The achieved results allow us to determine the relationships between strain and temperature for both materials and the differences in their mechanical response to the thermal loading. Then, the samples were subjected to a tensile test. A comparison of the tensile strength values was performed for the samples without and with embedded FBG sensors. The samples after the tensile tests were compared, showing differences in the mechanisms of failures related to the polymers and the thermal treatment influence on the material internal structure. Additionally, strain values measured by the FBG sensors were compared to the strain values achieved from the testing machine showing a good agreement (especially for M3 X) and indicating the differences in the materials’ mechanical properties. The achieved results allow us to conclude there is a lack of influence of embedded FBG sensors on the mechanical durability of AM polymers.

Model of the Temperature Influence on Additively Manufactured Carbon Fibre Reinforced Polymer Samples with Embedded Fibre Bragg Grating Sensors.

This study investigates the thermo-mechanical behaviour of additively manufactured Carbon Fiber Reinforced Polymer (CFRP) with embedded Fibre Bragg Grating (FBG) sensors with respect to their feasibility for utilising them under thermal loading. This was conducted through the Finite Element Method (FEM) inside an ABAQUS environment. Numerical simulation was complemented by several experimental investigations in order to verify the computational results achieved for the specimens exposed to thermal loading. FBG sensors, incorporated into the material by embedding technique, were employed to measure the strains of the samples subjected to elevated temperatures. It was shown that the strains given by numerical simulation were in good agreement with the experimental investigation except for a few errors due to the defects created within the layers during Additive Manufacturing (AM) process. It was concluded that the embedding FBG sensors were capable of identifying thermo-mechanical strain accurately for 3D-printed composite structures. Therefore, the findings of this article could be further developed for other types of material and loading conditions.

Development of FBG pressure sensors using FDM technique for monitoring sleeping postures

There are various methods used in 3D printing technology for its molding, prototyping, and fabrication applications, but fused deposition modeling (FDM) is most famous for its customized uses. In this research work, fiber Bragg grating (FBG) based pressure sensors were fabricated by embedding FBG sensors through the FDM process and Polylactic Acid (PLA) raw material was used during the fabrication of the pressure sensor. These FBG pressure sensors were placed into the smart bed for monitoring the sleeping postures. The loading and unloading tests were conducted for validating this new FBG pressure sensor. The Calibration tests showed that the maximum applied load on the pressure sensor was categorized by a measurement pressure range of ±58−352 N and the measurement sensitivity of two FBG sensors was around 0.00053 nm/N and 0.0012 nm/N. It is found that the most significant error of the FBG sensor is 13.3 % carrying out the vertical load of 0.35 K N. The measured wavelength changes exhibit a step-by-step increase and decrease during the sleeping postures which also agrees with the calibration data, and the measurement sensitivity of both FBG sensors remained approximately 0.00315 nm/N and 0.008 nm/N. The absolute errors of the sensor were less than 4% against the applied load during sleeping postures. FMD process is a new alternative method for its fast and straightforward embedment of FBG sensors fabrication for numerous applications. It is also initiated that FBG sensors can be adequately protected with molten PLA raw materials without losing their sensing performance and damage which has the advantage of easily washing smart sensors.

Small-diameter optical fibre sensor embedment for ambient temperature cure monitoring and residual strain evaluation of CFRP composite laminates produced by vacuum-assisted resin infusion

Out of autoclave (OoA) processing techniques, such as liquid composite moulding techniques (LCM) and, particularly, the vacuum-assisted resin infusion (VARI) technique, are being used, with increasing success, in replacement of prepreg/autoclave technologies to produce structural aircraft/aerospace polymer composite parts, due to its better cost effectiveness and competitiveness. This work aims to embed Fibre Bragg grating (FBG) sensors to monitor the VARI manufacturing of carbon fibre reinforced polymer (CFRP) composites and evaluate the associated phenomena: ambient curing and post curing reactions and resulting residual strains. The curing kinetics of the epoxy resin system alone was initially studied through isothermal differential scanning calorimetry (DSC) tests and applying the isoconversional Friedman method, and further studied by strain monitoring during ambient curing and post curing resorting to FBG sensors. The FBG sensors in the CFRP laminates were able to detect a subtle increase of strain as infusion of the CFRP started and to measure decreasing strain as resin filled in the dry fabric layers. Subtle strain decrease revealed forming crosslink bonds. Compressive strains measured by the FBG sensors during post curing show that further crosslink takes place. A comparison of resultant residual strains was made between specimens with embedded FBG sensors on small-diameter optical fibres (SDOF) and on large-diameter optical fibres (LDOF).

Embedded FBG Sensor Based Impact Identification of CFRP Using Ensemble Learning.

Impact brings great threat to the composite structures that are extensively used in an aircraft. Therefore, it is necessary to develop an accurate and reliable impact monitoring method. In this paper, fiber Bragg grating (FBG) sensors are embedded in unidirectional carbon fiber reinforced plastics (CFRPs) during the manufacturing process to monitor the strain that is related to the elastic modulus and the state of resin. After that, an advanced impact identification model is proposed. Support vector regression (SVR) and a back propagation (BP) neural network are combined appropriately in this stacking-based ensemble learning model. Then, the model is trained and tested through hundreds of impacts, and the corresponding strain responses are recorded by the embedded FBG sensors. Finally, the performances of different models are compared, and the influence of the time of arrival (ToA) on the neural network is also explored. The results show that compared with a single neural network, ensemble learning has a better capability in impact identification.

Prediction and detection of primary microcracks in carbon fiber reinforced polymer under load

Carbon fiber reinforced polymers (CFRP) are widely used in the manufacturing of critical parts and structures in the aerospace industry due to the combination of low density, high strength, and stiffness. Production of parts and structures with pre-predicted properties is a difficult task provided significant anisotropy of physical properties and complex microstructure. The solution to this problem has to be based on the correct mathematical model describing the behavior of parts and structures made of CFRP under operational loads. Moreover, experimental data on physical, mechanical, and thermophysical properties and their change depending on the number of loading cycles has to be implemented. In view of the above, prediction of the occurrence and development of microcracks in the material becomes significant. The aim of this article is to develop algorithms for prediction and detection of primary microcracks in CFRP under loading using fiber Bragg grating (FBG) sensors and multiscale mathematical modeling. The results of measuring the primary residual deformations in the plates made of CFRP at the manufacturing stage by embedded FBG sensors and testing its performance under loading was presented. Multiscale mathematical modeling of a numerical experiment performed to evaluate the occurrence of areas of primary microcracks in CFRP under loads. It is demonstrated that splitting of peaks of resonant wavelengths of embedded FBG sensors indicates the occurrence of primary damage and microcracks.

Damage Detection in Holed Carbon Fiber Composite Laminates Using Embedded Fiber Bragg Grating Sensors Based on Strain Information

Compared with metal materials, Carbon Fiber Reinforced Plastic (CFRP) is more excellent in performance, with high specific strength and high specific modulus. And among various CFRP structural elements, CFRP laminated plates have widespread applications. However, some indiscernible and hidden primary damage always appears on the CFRP laminated plate and will continuously evolve and expand after being loaded. This work starts with the analysis of the relationship between the changes of the strain distribution and damage expansion of CFRP laminated plates and proposes the damage recognition method of CFRP laminated plates based on strain information. Then, the CFRP laminate damage monitoring system based on Fiber Bragg Grating (FBG) sensor is established, and the CFRP laminate damage identification experiment based on FBG sensor is made to verify the accuracy of the CFRP laminate damage identification method. The results show that the maximum error between the load when the damage appears on the FBG sensor and that when the damage of CFRP laminated plates expands based on the simulated analysis does not exceed 16%. The accuracy of the damage recognition method of CFRP laminated plates is verified and the damage recognition of CFRP laminated plates and their expansion process is achieved.

Application of embedded fibre Bragg grating sensors for structural health monitoring of complex composite structures for marine applications

Fibre reinforced polymer (FRP) materials are recently widely applied in the marine industry. The high popularity of composites and safety requirements results in the development of Structural Health Monitoring (SHM) systems. One of the sensors types that can be permanently integrated with a composite structure and used as a part of SHM system are fibre Bragg grating (FBG) sensors. Among FBG sensors advantages small size, high corrosion resistance, and absence of electric current are especially important in monitoring of composite marine structures.The paper presents an application of embedded FBG sensors arrays for evaluation of complex composite structure — a fast patrol boat. The experimental investigations contain spectra analyses for thin laminate sample (skin), a composite sandwich panel and fast patrol boat hull (sandwich structure). The quality of sensors spectra (before/after embedding and loading) was determined using the Pearson correlation coefficient. The relationship between strain and temperature in exploitation range was determined for laminate and sandwich structures. A linear relationship was achieved for both elements. The relationship for the sandwich panel was then applied during strain analyses of the boat composite hull. The utility of designed FBG sensors array for the panel was also verified by sensors responses on impact excitation in chosen points on the structure. The achieved results were then used during designing FBG sensors array embedded into the hull. FBG sensors spectra deformations due to fast patrol boat hull manufacturing process and exploitation as well as strain curves determined for the boat during tests at dry dock and at sea trials are analysed.