Packaged Fiber Bragg Grating Sensors Research Articles

Modeling analysis and experimental study on epoxy packaged FBG sensor for cryogenic temperature measurement

The Fiber Bragg grating (FBG) is a kind of temperature-sensitive optical device that has irreplaceable advantages for cryogenic measurement. In this work, a model-driven FBG temperature sensor packaging method was proposed to improve the sensor’s sensitivity and linearity and predict the sensor's response time in cryogenic. The numerical model explores the relationship between the sensor’s size and performance. The packaging method can prepare high-precision samples of multiple sizes and provide experimental support for this study. The accuracy of the model is verified by comparing the calculated results with the results of a warming-up experiment and a liquid nitrogen experiment. The results show an improvement in sensitivity and linearity, and the model can also be used to predict the characteristics of FBG coated with epoxy resins at cryogenic temperature. The sensor with a thickness of 0.2 mm has a sensitivity and linearity of 26.6 pm/K and 0.997, respectively, in the range of 80 K-270 K.

Long-term performance of packaged fiber Bragg grating sensors for strain monitoring inside creep medium

ABSTRACT To investigate the long-term performance of the packaged fiber Bragg grating (FBG) sensors embedded in civil infrastructure for strain monitoring, in this paper, the influence of host matrix’s creep effect on the behavior of the FBG sensors was systematically studied through theoretical, numerical, and experimental analysis. A theoretical strain transfer analysis between the optic fiber, packaging layer, and host matrix to consider the creep effect of the host matrix was performed accordingly for long-term strain monitoring. Parametric studies were carried out using numerical analysis for FBG sensors packaged with glass fiber reinforced plastic (GFRP), also known as FBG-GFRP sensors in concrete, as an example. The results show that embedded in a creep medium, an acceptable long-term performance of packaged FBG sensors requires the packaging layer to have a minimum length and maximum thickness. Laboratory long-term creep tests using epoxy resin and concrete as host matrix for FBG-GFRP sensors also clearly demonstrated that the influence of creep effect cannot be ignored for strain measurements if the host matrix has a creep potential and the developed correction model performed well to reduce measurement errors of such sensors in creep medium.

Dynamic Performance Detection of CFRP Composite Pipes based on Quasi-Distributed Optical Fiber Sensing Techniques

For the high strength, corrosion resistance, and good stability, carbon fiber–reinforced polymer (CFRP) composites have been made into pipes to transfer gasses and oils in subsea environment. Structural performance of CFRP composite pipes is particularly important to sustain the regular operation of the delivery system. To obtain the in-field behavior of the CFRP composite pipes, quasi-distributed optical fiber sensing techniques are developed based on the multiple configuration of fiber Bragg grating (FBG) sensing elements. Theoretical investigation on the dynamic response of the pipes is performed. Experiments on cantilever CFRP pipes with surface-attached FBGs in series and packaged FBG sensors have been conducted to check the feasibility and effectiveness of the proposed sensing technique. Results validate the good measurement performance of the proposed sensors and the accuracy of the vibration analysis. The study can be adopted to instruct the establishment of the structural health monitoring system of CFRP composite pipes and assess the safety operation state of the pipe systems.

Reliable packaging of optical fiber Bragg grating sensors for carbon fiber composite wind turbine blades

A sensing module composed of a carbon fiber reinforced polymer (CFRP) packaging and an embedded fiber Bragg grating (FBG) sensor is proposed for strain monitoring of wind turbine blades. The sensing unit is designed and manufactured with the aim of maximizing the strain transfer from a CFRP wind blade to the FBG sensor. The performance of the packaged FBG sensors is experimentally investigated through exhaustive aging and fatigue tests. Then, a series of CFRP-packaged FBG sensors is used to monitor a full-scale 56.85 m-long wind blade during flapwise and edgewise fatigue tests over a few million dynamic strain cycles. Results demonstrate that the independent sensing modules provide efficient strain transfer, verifying a longer lifetime and more reliable measurements compared to conventional electrical strain gauges.

Performance improvement of FBG sensors based on the pre-stressed package technique

Packaging technology is a key factor in fiber Bragg grating (FBG) sensing, which can greatly affect the sensing performance of FBG sensors. In this paper, a pre-stressed packaging method for FBG sensors is proposed. The method enables the pre-stress applied on the FBG to be continuously and accurately adjusted when the FBG sensor is packaged, thereby ensuring the stability of the pre-stress. In order to implement the method, an instrument is designed and fabricated as well. The proposed method is demonstrated through some experiments, experimental researches indicate that using the proposed method, the spectrums of the fourteen packaged FBG sensors have no chirping nor broadening the spectral 3 dB bandwidth. The average temperature-dependent sensitivity coefficient of fourteen sensors is 2.03 × 10−5/℃, the relative error is 1.7%, and the average linearity is better than 0.999. So, the proposed pre-stressed packaging method has advantages of great stability, good repeatability, easy operation and low cost.

Surface-Mounted Bare and Packaged Fiber Bragg Grating Sensors for Measuring Rock Strain in Uniaxial Testing.

The paper explores the possibility of using high-resolution fiber Bragg grating (FBG) sensing technology for on-specimen strain measurement in the laboratory. The approach provides a means to assess the surface deformation of the specimen, both the axial and radial, through a chain of FBG sensor (C-FBG), in a basic setup of a uniaxial compression test. The method is cost-effective, straightforward and can be commercialized. Two C-FBG; one was applied directly to the sample (FBGBare), and the other was packaged (FBGPack) for ease of application. The approach measures the local strain with high-resolution and accuracy levels that match up to the existing local strain measuring sensors. The approach enables the evaluation of small-strain properties of the specimen intelligently. The finite element model analysis deployed has proven the adaptability of the technique for measuring material deformation. The adhesive thickness and packaging technique have been shown to influence the sensitivity of the FBG sensors. Owing to the relative ease and low-cost of instrumentation, the suggested method has a great potential to be routinely applied for elemental testing in the laboratory.

Design of storage and management device of packaged FBG sensors based on RFID

In the application of fiber Bragg grating(FBG) sensor in structural condition monitoring, the reasonable layout scheme is designed according to its central wavelength. However, the paper label with FBG central wavelength information may be lost when FBG is used, therefore it is difficult to store and manage FBG sensors. This paper designs a storage device and its management system of packaged independent FBG sensor based on RFID technology. The oblique grid structure can store the FBG sensors with the same central wavelength. The RFID reader module is arranged on the top of the box-type device with multi-layer oblique grid. The packaged FBG sensors are stored, read and classified with the help of special software.

Reliability Analysis of Fiber Bragg Grating Sensors packaged with Carbon Fiber Reinforced Plastics based on FMEA

The performance of packaged Fiber Bragg grating (FBG) sensors with carbon fiber reinforced plastic (CFRP) may degraded even functional failure with the working time increases. The reliability of this kind of packaged FBG sensors has been analyzed by using the FMEA method in this paper. Considering the working environment of FBG sensor, such as alternating loads and ageing, the related failure modes are discussed. The risk index of each failure mode is calculated. The relationship between different failure modes is analyzed, and as well as the weak links of the packaging design and process of the FBG sensor. The results are helpful to improve the availability and the reliability of packaged FBG sensor by CFRP.

Design of a management system of packaged FBG sensor based on RFID for wind turbine blade

With the application of fiber bragg grating (FBG) sensors in the condition monitoring of wind turbine blades, the layout management of FBG sensors with different central wavelengths and manufacturing processes has attracted much attention. A management system of FBG sensor based on RFID technology is proposed. The management system includes three parts: packaging process of FBG sensor with RFID chip, RFID chip stored information and the FBG sensors status monitoring system, the FBG sensor installation on full scale field wind turbine blade and its maintenance. The proposed management system introduces RFID technology into the blade monitoring using FBG sensor. It provides effective digital management method for the manufacturing, the layout, the installation and the maintenance process of the FBG sensor on the wind turbine blade.

The Study of the Directional Sensitivity of Fiber Bragg Gratings for Acoustic Emission Measurements

Structural health monitoring (SHM) of engineering structures plays a crucial role in ensuring their safety and integrity. Acoustic emissions (AE) can be one of the several ways to monitor the structural health. Fiber Bragg grating (FBG)-based AE measurement technique is emerging as a solution to assess cracks in structures remotely and has advantages of being less bulky and having low noise, high bandwidth and ease of implementation. This paper presents a study on the impact of AE signals on FBGs attached to a substrate, focusing on the directional sensitivity using different configurations. For ease of installation, the FBG sensors are metallically packaged similar to lead zirconium titanate (PZT) sensors in which they can withstand temperatures of up to 250 °C. The directional AE sensitivity of the packaged FBG sensor device is studied and compared with that of a surface attached FBG. Experimentally, the AE signals are generated by a metal ball drop impact on aluminum plate and are measured using an FBG AE interrogation system. A numerical simulation of the design was carried out using ANSYS explicit dynamics and the AE wave propagation in structure was analyzed and experimentally verified.

Selection and Characterization of Packaged FBG Sensors for Offshore Applications.

With the development in the exploitation of maritime resources, the structural health monitoring (SHM) of offshore structures becomes necessary. This study focuses on addressing the practical issues of application of fiber Bragg grating (FBG) sensors for the SHM of offshore structures, in particular an FPSO (floating, production, storage, and offloading unit) vessel. Due to the harsh marine environment and tough working conditions, the FBG sensors must have sufficient protection and good repeatability for long-term monitoring. Thorough research has been conducted to identify the most suitable, commercially available protection packaging for FBG sensors for offshore applications. Further, the performance of the selected FBG sensor packaging is tested under conditions of strong sunlight, heavy rain, and salty water in order to emulate the marine environment. Moreover, the installation method of the packaged FBG sensors is equally important, as it ensures the repeatability and durability of the sensors for their long-term performance. It is shown that the packaged FBG sensors can be installed using resin-based epoxy to maintain the repeatability of the sensor over the long-term. Further, the packaged FBG sensors are installed and tested on a simple FPSO model. The experimental results under full load and ballast draft conditions show that the proposed FBG sensors are competent for the SHM of offshore structures.

Early corrosion monitoring of reinforcing steel bars by using long-gauge carbon fiber sensors

Corrosion of main steel reinforcements is one of the foremost causes of early deterioration and shortening of service life of reinforced concrete structures. Early detection of corrosion can aid in reducing the maintenance cost and extending the service life of structures. This research proposes an approach for early corrosion monitoring of steel reinforcements in concrete structures using strain measurements of long-gauge packaged carbon fiber line (PCFL) sensors. An accelerated corrosion method utilizing a salt solution and constant current was used to achieve the required corrosion levels. The effects of the salt solution, electrical current, and change in temperature during the corrosion process on the signals of the proposed packaged sensors were studied. A PCFL with temperature compensation and a protective cover was installed on a steel bar and tested under accelerated corrosion and a constant tensile load. Monitored measurements of the PCFL sensors during the corrosion progress were compared with a laboratory-scale model and measurements of packaged fiber Bragg grating (PFBG) sensors. The results confirmed that the proposed packaging could effectively protect the carbon fiber sensors from electric effects, long-term salt attack, and temperature variation during the corrosion process. The experimental results of both the PCFL and the PFBG sensors were in good agreement with the corrosion values calculated theoretically from Faraday’s law with a relative weight loss ratio error of less than 0.8. Therefore, the use of long-gauge PCFL sensors can be considered as a promising approach for monitoring the corrosion progress of steel reinforcements of large-scale structures.

Application of a Packaged Fiber Bragg Grating Sensor to Outdoor Optical Fiber Cabinets for Environmental Monitoring

Outdoor optical fiber cabinets are generally installed along roadsides where environmental conditions, such as sun exposure, rain, and dust, can cause damage to the cabinets. Furthermore, high temperatures and unexpected impacts that cause cabinet deformation may damage the network equipment inside the cabinets, affecting the quality of communications. This paper developed an optical fiber sensing system for real-time monitoring of the temperature variation and external forces exerted on outdoor cabinets using fiber Bragg gratings (FBGs). This FBG sensing system was established in the network management center, and was linked to the outdoor cabinets at the remote end through optical fibers in order to monitor the impacts and temperature changes of the cabinets. Experimental results revealed that exerting an external impact force of 297 N on an outdoor cabinet severely dented the impacted cabinet surface. When the monitoring system detects external impacts exceeding 297 N or cabinet temperatures greater than 65 °C, the network management center can then immediately send maintenance personnel to check for any network impairments.

Aluminum-thin-film packaged fiber Bragg grating probes for monitoring the maximum tensile strain of composite materials.

In this paper, new fiber Bragg grating (FBG) sensor probes are designed to intermittently detect the maximum tensile strain of composite materials, so as to evaluate the structural health status. This probe is fabricated by two thin Al films bonded to an FBG optical fiber and two supporting brackets, which are fixed on the surface of composite materials. The residual strain of the Al packaged FBG sensor probe is induced by the strain of composite materials. This residual strain can indicate the maximum strain of composite materials. Two types of sensor probes are prepared-one is an FBG with 18 μm thick Al films, and the other is an FBG with 36 μm thick Al films-to compare the thickness effect on the detection sensitivity. These sensor probes are bonded on the surfaces of carbon fiber reinforced plastics composite specimens. In order to determine the strain sensitivity between the residual strain of the FBG sensor probe and the maximum strain of the composite specimen, tensile tests are performed by universal testing machine, under the loading-unloading test condition. The strain sensitivities of the probes, which have the Al thicknesses of 18 and 36 μm, are determined as 0.13 and 0.23, respectively.

Performance characterization of fiber Bragg grating thermal response in space vacuum thermal environment

We investigated the fiber Bragg grating (FBG) thermal response in space vacuum thermal environment. The FBGs were packaged with 6061-T6 aluminum. The liquid nitrogen immersion experiment results show that its wavelength shift standard deviation is 0.76 pm for 217 h. The combination effect of vacuum and cryogenic temperature was studied by thermal cycling process in space environment simulator. The FBG sensors show accuracy better than 2% full scale, and the hysteresis errors are below 1%. It proves that these metal packaged FBG sensors can survive and meet the requirement of space measurement.

Application of Packaging Technique in Fiber Bragg Grating Temperature Sensor for Measuring Localized and Nonuniform Temperature Distribution

The development of Fiber Bragg Grating (FBG) sensing technique has improved significantly especially in the sensor head design and real-time data acquisition technique. This paper presents the development of a simple and cost effective packaging technique that further enhances the performances of the FBG sensor. The packaged FBG sensor overcomes the nonuniform heat distribution of a bare FBG that causes eccentric response of FBG spectrum. Therefore, the packaged FBG sensor could be operated for a localized area with high temperature differential. The packaging also compensates the unwanted strain effect from the surrounding which makes temperature measurement become more accurate. The experimental works have been successfully carried out to demonstrate the system operation and the packaging functionalities. The temperature sensitivity coefficient of the bare FBG sensor measured in experiment is 10.05 pm/°C, while the packaged fiber sensor is 10.09 pm/°C, which are expected from the design.

Measurement of Temperature Field for the Spindle of Machine Tool Based on Optical Fiber Bragg Grating Sensors

The change of spindle temperature field is an important factor which influences machining precision. Many methods of spindle temperature field measurement have been proposed. However, most of the methods are based on the electric temperature sensors. There exist some defects (e.g., anti-interference, multiplexing, and stability capacity are poor). To increase the temperature sensitivity and reduce strain sensitivity of the bare Fiber Bragg Grating (FBG) sensor, a cassette packaged FBG sensor is proposed to measure spindle temperature field. The temperature characteristics of the packaged FBG sensor are studied by comparative experiment with traditional thermal resistor sensor. The experimental results show that the packaged FBG sensor has the same capacity of temperature measurement with the thermal resistor sensor but with more remarkable antiinterference. In the further measurement experiment of the temperature field, a spindle nonuniform temperature field is acquired by the calibrated FBG sensors. It indicates that the packaged FBG sensor can be used to measure the temperature field for the spindle of machine tool.

Effects of Bonding Layer Characteristics on Strain Transmission and Bond Fatigue Performance

Fiber Bragg Grating (FBG) sensor arrays can be used to monitor the mechanical behavior of large composite structures. However, brittle FBG sensors, especially multiplexed FBG sensors can be easily damaged when they are installed in flexible structures. As a protection for brittle FBG sensors, epoxy packaged FBG sensors have been suggested. Packaged FBG sensors are usually installed using epoxy adhesives onto the composite specimen since they have high mechanical, heat-resistance and insulation properties. However, the adhesive could decrease strain transmission from the bonding layer and suffer from aging problem. In this paper, strain transmission and bond fatigue performance of the bonding layer depending on the elastic modulus of adhesive, elastic modulus of packaging material and bonding layer thickness are discussed. Static tests were performed to evaluate the effects of bonding layer thickness and elastic modulus ratio of adhesive to the packaging material on strain transmission. Fatigue tests were performed to evaluate the bond fatigue performance depending on the elastic modulus of the adhesive and bonding layer thickness. It was observed that thin bonding layer with high elastic modulus gave high strain transmission, however, it suffered from debonding problem. The elastic modulus ratio and the bonding layer thickness resulting in high strain transmission while maintaining a long bond fatigue life were found in this study.

Feasibility of Fiber Bragg Grating and Long-Period Fiber Grating Sensors under Different Environmental Conditions

This paper presents the feasibility of utilizing fiber Bragg grating (FBG) and long-period fiber grating (LPFG) sensors for nondestructive evaluation (NDE) of infrastructures using Portland cement concretes and asphalt mixtures for temperature, strain, and liquid-level monitoring. The use of hybrid FBG and LPFG sensors is aimed at utilizing the advantages of two kinds of fiber grating to implement NDE for monitoring strains or displacements, temperatures, and water-levels of infrastructures such as bridges, pavements, or reservoirs for under different environmental conditions. Temperature fluctuation and stability tests were examined using FBG and LPFG sensors bonded on the surface of asphalt and concrete specimens. Random walk coefficient (RWC) and bias stability (BS) were used for the first time to indicate the stability performance of fiber grating sensors. The random walk coefficients of temperature variations between FBG (or LPFG) sensor and a thermocouple were found in the range of −0.7499 °C/ to −1.3548 °C/ . In addition, the bias stability for temperature variations, during the fluctuation and stability tests with FBG (or LPFG) sensors were within the range of 0.01 °C/h with a 15–18 h time cluster to 0.09 °C/h with a 3–4 h time cluster. This shows that the performance of FBG or LPFG sensors is comparable with that of conventional high-resolution thermocouple sensors under rugged conditions. The strain measurement for infrastructure materials was conducted using a packaged FBG sensor bonded on the surface of an asphalt specimen under indirect tensile loading conditions. A finite element modeling (FEM) was applied to compare experimental results of indirect tensile FBG strain measurements. For a comparative analysis between experiment and simulation, the FEM numerical results agreed with those from FBG strain measurements. The results of the liquid-level sensing tests show the LPFG-based sensor could discriminate five stationary liquid-levels and exhibits at least 1,050-mm liquid-level measurement capacity. Thus, the hybrid FBG and LPFG sensors reported here could benefit the NDE development and applications for infrastructure health monitoring such as strain, temperature and liquid-level measurements.

Packaging methods of fiber-Bragg grating sensors in civil structure applications

Fiber-Bragg grating (FBG) sensors made on bare fibers are easily damaged when handled improperly during and after fabrication. As a protection from such damage, a novel technique for protecting and packaging FBG sensors has been developed and is presented in this paper. To characterize the strain transmission efficiency of the packaged FBG sensors, an analytical finite-element method is used, and the results are compared with the experiments. It is observed that the thickness and Young's moduli of glues have little influence on the strain transmission, especially when the thickness of the glue is less than the diameter of an optical fiber. However, recoating and steel-tube packaging will markedly affect the strain transmission rate. The strain transmission rates decrease with the increase in thickness of the packaging material. Also, the aging problem of the polymide or acrylate coating and epoxy glue must be considered, since the service life of most structures is usually designed for more than 50 years. The metallic recoated FBG sensor developed in this research uses different approaches, such as low-temperature solder welding, which shows no aging problem, to install the sensors in the structures. Based on the simulated and experimental results, the nickel recoating method is shown to have good strain transmission efficiency compared with other packaging methods.