Optical Fiber Bragg Grating Sensors Research Articles

Remote Sensing of Lamb Waves Using Optical Fibres—An Investigation of Modal Composition

This paper reports on an experimental investigation into the differences between directly and remotely bonded optical fiber Bragg grating (FBG) sensors when measuring Lamb waves in an aluminum plate. A sensor array, comprising 16 individual short gauge-length FBG sensing elements, is used to decompose the wave into its constituent modes for the directly bonded case, where the array is attached directly to the plate and for the remotely bonded case where the same array is located in a suspended fiber 250 mm downstream from the fiber/plate contact region. The experimental results were consistent with model predictions and show that while the directly bonded fiber array can resolve the full multimodal Lamb wave spectrum in the plate, the remotely bonded sensor measures only a single nondispersive longitudinal acoustic mode predicted by the analytical solution for a cylindrical waveguide. The inability of remote sensing to distinguish between different modes of Lamb wave propagation represents a significant limitation with respect to the diagnostic utility of this sensing approach.

Impact localization for composite plate based on detrended fluctuation analysis and centroid localization algorithm using FBG sensors

Structure health monitoring of composite structures is essential for safety assurance and attracted more attention in recent years. In this paper, a low velocity impact (LVI) monitoring system based on detrended fluctuation analysis (DFA) and centroid localization algorithm is proposed. First, the distance from the LVI to neighboring FBG sensors can be estimated by analyzing the Hurst exponents calculated from response signal of LVI with different distance to monitoring FBG sensors by DFA. Then, the LVI location can be determined by region identification algorithm and centroid localization algorithm. To verify the performance of the proposed LVI monitoring system, a LVI localization system using optical fiber Bragg grating (FBG) sensor was constructed and validated on a CFRP plate with 240 mm*240 mm*3 mm experiment area. The experiment results showed that the average error of localization was 31.5 mm. The proposed localization method based on DFA and centroid localization algorithm demonstrates an effective means of LVI localization.

Process-parallel center deviation measurement of a BTA deep-hole drilling tool

The BTA deep-hole drilling process is used to manufacture holes with a high length-to-diameter ratio of up to 100. A major quality criteria is the difference between the drilled hole axis and the desired, ideal straight hole axis, called center deviation. A novel measuring system is being investigated in order to measure the center deviation simultaneously to the drilling process. The deviation is determined by calculating the bending line of the boring bar using a newly developed algorithm. The measuring system consists of four subsystems: Optical fiber Bragg grating (FBG) sensors, four eddy current sensors, a rotary encoder and a displacement encoder. These subsystems enable a spatial measurement visualization of the bending line and the center deviation at the head of the boring bar. A major challenge is the separation of the bending strain from other undesired strain fractions. This is done by using an arrangement of two FBG sensors and a digital band pass filter. Experimental investigations show sufficiently good agreement between the calculated and the actual center deviation. Furthermore, drilling experiments reveal that the measurement setup can be used for additional process monitoring by using frequency analysis. Therefore, the investigated system is able to visualize and observe the center deviation. The visualization can be used to derive adequate countermeasures in order to ensure a better process control for the machine operator. The objective of the investigation is to increase the process reliability and productivity by preventing scrap parts. Within this paper, the investigated measuring system, the calculation of the center deviation and first experimental results are described.

Fiber-Optic Sensor-Based Remote Acoustic Emission Measurement in a 1000 °C Environment.

Recently, the authors have proposed a remote acoustic emission (AE) measurement configuration using a sensitive fiber-optic Bragg grating (FBG) sensor. In the configuration, the FBG sensor was remotely bonded on a plate, and an optical fiber was used as the waveguide to propagate AE waves from the adhesive point to the sensor. The previous work (Yu et al., Smart Materials and Structures 25 (10), 105,033 (2016)) has clarified the sensing principle behind the special remote measurement system that enables accurate remote sensing of AE signals. Since the silica-glass optical fibers have a high heat-resistance exceeding 1000 °C, this work presents a preliminary high-temperature AE detection method by using the optical fiber-based ultrasonic waveguide to propagate the AE from a high-temperature environment to a room-temperature environment, in which the FBG sensor could function as the receiver of the guided wave. As a result, the novel measurement configuration successfully achieved highly sensitive and stable AE detection in an alumina plate at elevated temperatures in the 100 °C to 1000 °C range. Due to its good performance, this detection method will be potentially useful for the non-destructive testing that can be performed in high-temperature environments to evaluate the microscopic damage in heat-resistant materials.

An optical fiber Bragg grating and piezoelectric ceramic voltage sensor.

Voltage measurement is essential in many fields like power grids, telecommunications, metallurgy, railways, and oil production. A voltage-sensing unit, consisting of fiber Bragg gratings (FBGs) and piezoelectric ceramics, based on which an optical over-voltage sensor was proposed and fabricated in this paper. No demodulation devices like spectrometer or Fabry-Perot filter were needed to gain the voltage signal, and a relatively large sensing frequency range was acquired in this paper; thus, the cost of the sensing system is more acceptable in engineering application. The voltage to be measured was directly applied to the piezoelectric ceramic, and deformation of the ceramics and the grating would be caused because of the inverse piezoelectric effect. With a reference grating, the output light intensity change will be caused by the FBG center wavelength change; thus, the relationship between the applied voltage and the output light intensity was established. Validation of the sensor was accomplished in the frequency range from 50 Hz to 20 kHz and switching impulse waves with a test platform; good linearity of the input-output characteristic was achieved. A temperature validation test was completed, showing that the sensor maintains good temperature stability. Experimental results show that the optical over-voltage sensor can be used for voltage monitoring, and if applied with a voltage divider, the sensor can be used to measure high voltage.

Investigations on photo-mechanical and photo-thermo-mechanical strain variations in amorphous selenium using fiber Bragg grating sensor

Light-induced structural changes are investigated on 90nm thick amorphous-selenium (a-Se) thin film, coated on the cladding layer of an optical fiber Bragg Grating (FBG) sensor, upon illumination with 532nm laser of 10mW/mm2 power under different exposure times (ms, s, min, h). The changes induced are measured using the photo-induced strain produced. In the present experiments, the contributions of photo-mechanical and photo-thermo-mechanical effects have been well separated. Also, the photo-thermally induced crystallization effects on the magnitude of the photo-induced strain have also been investigated.

Preliminary Study on Integration of Fiber Optic Bragg Grating Sensors in Li-Ion Batteries and In Situ Strain and Temperature Monitoring of Battery Cells

Current commercial battery management systems (BMSs) do not provide adequate information in real time to mitigate issues of battery cells such as thermal runway. This paper explores and evaluates the integration of fiber optic Bragg grating (FBG) sensors inside lithium-ion battery (LiB) coin cells. Strain and internal and external temperatures were recorded using FBG sensors, and the battery cells were evaluated at a cycling C/20 rate. The preliminary results present scanning electron microscope (SEM) images of electrode degradation upon sensor integration and the systematic process of sensor integration to eliminate degradation in electrodes during cell charge/discharge cycles. Recommendation for successful FBG sensor integration is given, and the strain and temperature data is presented. The FBG sensor was placed on the inside of the coin cell between the electrodes and the separator layers towards the most electrochemically active area. On the outside, the temperature of the coin cell casing as well as the ambient temperature was recorded. Results show stable strain behavior within the cell and about 10 °C difference between the inside of the coin cell and the ambient environment over time during charging/discharging cycles. This study is intended to contribute to the safe integration of FBG sensors inside hermetically sealed batteries and to detection of real-time temperature and strain gradient inside a cell, ultimately improving reliability of current BMSs.

Development of a mechanical strain amplifying transducer with Bragg grating sensor for low-amplitude strain sensing

Vibration-based damage identification is a well-known method to support health monitoring of civil engineering structures. Damage in such structures can be identified by measuring changes of the natural frequencies, damping factors or modal displacements of the structure. However, this approach suffers from the low sensitivity of these natural frequencies and modal displacements to certain types of damage. Modal strains and curvatures can be more sensitive to local damage, but direct monitoring of these quantities with sufficient spatial resolution is not possible with current measurement techniques due to the very small strain levels (sub-microstrain) caused by ambient or operational excitation. To deal with this issue, we propose a novel mechanical transducer equipped with an optical fiber Bragg grating (FBG) sensor that enhances the sensitivity to strain with a factor larger than 30. The principle of operation of the transducer exploits a symmetric cantilever structure that enlarges the strain experienced by the FBG sensor compared to the strain applied to the transducer itself. We carried out dynamic and static tests to verify the ability of the strain-amplifying transducers to measure small-amplitude strain levels and to evidence the potential for carrying out FBG based modal strain measurements on concrete civil engineering structures.

In-situ simultaneous measurement of strain and temperature in automated fiber placement (AFP) using optical fiber Bragg grating (FBG) sensors

There has been a tremendous growth of utilizing automated fiber placement (AFP) to manufacture highly precise components and large structures like fuselage panels and wing skins for high-end applications in aircrafts and next generation of spacecrafts. Consequently, in-situ identification of potential defects and strain level within the laminates is critical to ensure the quality and integrity of the final product. In this study, optical fiber Bragg grating sensors (FBGs) have been implemented as an on-line monitoring technique for simultaneous measurement of strain and temperature in AFP. In addition, it is also shown that, the embedded FBG sensors can remain within the laminate for continuous health monitoring after manufacturing process toward the identification of crack induced acoustic emissions.

Long-term strain response of polymer optical fiber FBG sensors

We report on the viscoelastic response of PMMA microstructured polymer optical fibers (mPOFs) when exposed to long periods of strain and relaxation, with the strain period ranging from 0.5 min to 50 min. The behavior of the fibers was monitored by inscribing a fiber Bragg grating (FBG) in them and tracking the reflection peak. We demonstrate that the fiber, when relaxing from strains of up to 0.9%, has a two-phase recovery: initially linear (elastic driven) and subsequently nonlinear (viscoelastic driven) contraction. The linear (elastic) relaxation wavelength range depends both on the strain level and on the strain duration. For short strain durations, this wavelength range stays the same, but with increasing strain duration, it decreases, which will influence the operation range of mPOF and POF-based FBG sensors.

Monitoring of compressive behaviour of stiffened composite panels using embedded fibre optic and strain gauge sensors

PurposeCompression is critical loading condition for composite airframes. Compression behaviour of structures with or without damages is a weak point for composite fuselage panels. This is one of the reasons for need of continuous in-service health monitoring of composite structures. The purpose of this paper is to characterize the compression panel behaviour on the base of a developed and implemented structural health monitoring (SHM) system.Design/methodology/approachThe SHM system based on fibre optic Bragg grating (FOBG) sensors and standard resistance strain gauges (SGs) was placed onto/into (embedded or bonded) three stiffened carbon fibre reinforced polymer (CFRP) fuselage panels. The FOBG sensor system was used to monitor the structural integrity of the reference, impacted, and fatigued panels under compression loading. Both barely visible impact damage and visible impact damage were created to evaluate their influence on the panel behaviour. The functionality of the SHM system was verified through mechanical testing.FindingsExperimental data showed the presence of impact damages significantly changes the buckling modes development and deformation behaviour of the panels. Some differences between the optical and SG sensors during buckling were observed. The buckling waves and failure development were very well indicated during loading by all sensors located on the panel surface but not by the embedded sensors. Good agreement between the data from the SGs and FOBG sensors was achieved for all sensors placed on the stringers, which did not buckle. The good reliability of FOBG sensors during the fatigue and static testing up to panel failure was verified.Originality/valueThe paper gives information about different buckling behaviour of CFRP fuselage stiffened panels in compression. The paper gives detailed information about measured signals from different sensors based on their location on/in the panel structure for realistic loading scenario of composite aerostructures. The paper gives an integrated overview of sensors placement considering possibilities to predicate structure behaviour.

Improved KLT Algorithm for High-Precision Wavelength Tracking of Optical Fiber Bragg Grating Sensors

Fiber Bragg Gratings (FBGs) are among the most popular optical fiber sensors. FBGs are well suited for direct detection of temperature and strain and can be functionalized for pressure, humidity, and refractive index sensing. Commercial setups for FBG interrogation are based on white-light sources and spectrometer detectors, which are capable of decoding the spectrum of an FBG array. Low-cost spectrometers record the spectrum on a coarse wavelength grid (typically 78–156 pm), whereas wavelength shifts of 1 pm or lower are required by most of the applications. Several algorithms have been presented for detection of small wavelength shift, even with coarse wavelength sampling; most notably, the Karhunen-Loeve Transform (KLT) was demonstrated. In this paper, an improved algorithm based on KLT is proposed, which is capable of further expanding the performances. Simulations show that, reproducing a commercial spectrometer with 156 pm grid, the algorithm estimates wavelength shift with accuracy well below 1 pm. In typical signal-to-noise ratio (SNR) conditions, the root mean square error is 22–220 fm, while the accuracy is 0.22 pm, despite the coarse sampling. Results have been also validated through experimental characterization. The proposed method allows achieving exceptional accuracy in wavelength tracking, beating the picometer level resolution proposed in most commercial and research software, and, due to fast operation (>5 kHz), is compatible also with structural health monitoring and acoustics.

On the use of strain sensor technologies for strain modal analysis: Case studies in aeronautical applications.

This paper discusses the use of optical fiber Bragg grating (FBG) and piezo strain sensors for structural dynamic measurements. For certain industrial applications, there is an interest to use strain sensors rather than in combination with accelerometers for experimental modal analysis. Classical electrical strain gauges can be used hereto, but other types of strain sensors are an interesting alternative with some very specific advantages. This work gives an overview of two types of dynamic strain sensors, applied to two industrial applications (a helicopter main rotor blade and an F-16 aircraft), FBG sensors and dynamic piezo strain sensors, discussing their use and benefits. Moreover, the concept of strain modal analysis is introduced and it is shown how it can be beneficial to apply strain measurements to experimental modal analysis. Finally, experimental results for the two applications are shown, with an experimental modal analysis carried out on the helicopter main rotor blade using FBG sensors and a similar experiment is done with the aircraft but using piezo strain sensors instead.

Total Temperature Measurement of Fast Air Streams With Fiber-Optic Bragg Grating Sensors

The suitability of using fiber Bragg gratings (FBG) as sensor elements in total temperature probes for turbine compressor applications is shown. A series of five FBG-based probes was built and characterized experimentally in the harsh environment of an open-jet test bench at air stream velocities of up to Mach 0.8. The performances of the FBG-based probes were compared with that of a conventional thermocouple-based total temperature sensor. All fiber-optic probes showed good mechanical robustness and recovery factors in the range of 0.79–0.99. Employing fiber-optic sensor elements allowed a significant reduction of the diameters of the stagnation tubes of the transducers. Because of the smaller probe design and thinner signal transmission cables, less distortion of the air stream is expected with the fiber-optic sensors, when compared with the conventional instrumentation.

English

English

Measurement of sound pressure and temperature in tissue-mimicking material using an optical fiber Bragg grating sensor.

The experimental investigation of an optical fiber Bragg grating (FBG) sensor for biomedical application is described. The FBG sensor can be used to measure sound pressure and temperature rise simultaneously in biological tissues exposed to ultrasound. The theoretical maximum values that can be measured with the FBG sensor are 73.0MPa and 30°C. In this study, measurement of sound pressure up to 5MPa was performed at an ultrasound frequency of 2MHz. A maximum temperature change of 6°C was measured in a tissue-mimicking material. Values yielded by the FBG sensor agreed with those measured using a thermocouple and a hydrophone. Since this sensor is used to monitor the sound pressure and temperature simultaneously, it can also be used for industrial applications, such as ultrasonic cleaning of semiconductors under controlled temperatures.

The Role of Photo-Striction in Tailoring the Nano-Scale Phase Changes in Amorphous Selenium Thin Films

The photo-structural changes in a 2 μm thick amorphous selenium (a-Se) thin film, thermally evaporated on Si substrate, have been studied by micro-Raman spectrometer with 532 nm laser at different laser power densities (1 to 64 W/mm 2 ) and exposure times (15 – 60 s). In addition, the nanoscale photo-structural changes/photo-thermodynamic phase changes are examined on 90 nm a-Se film coated on SiO 2 cladding of the optical Fiber Bragg Grating (FBG) sensor on prolonged exposure to a low power (1 mW/mm 2 , 10 mW/mm 2 ) 532 nm laser, which is measured by means of photo-striction (light induced strain) in the material.

Estimation of the structural integrity of a runway repaired by compliant polymer concretes under thermomechanical loading conditions

When repair materials are applied for runway maintenance, possible material failures due to the differences in material properties and excessive shrinkage of the repair material needs to be investigated. Moreover, the repair part is subjected to various mechanical loadings, which should be taken into account for determining the best repair material. In this study, we investigate a real-time in-situ health monitoring system, which is composed of two Fiber Bragg Grating (FBG) optic sensors, to monitor the strain variations that are induced by environmental conditions and material shrinkage. The repair process with compliant polymer concrete (72:20:08(T)) was carried out at the Gimpo international airport (Korea) during the summer season. To consider the effect of mechanical loading on stress distribution in the materials, several dynamic analyses are also carried out by using the finite element analysis method. Based on the results, the feasibility and reliability of the health monitoring system using FBG optic sensors are verified.

Magnetic actuator based on giant magnetostrictive material Terfenol-D with strain and temperature monitoring using FBG optical sensor

We have designed a temperature and strain monitoring system for a magnetic actuator based on the giant magnetostrictive material Terfenol-D (Tb0.3 Dy0.7Fe1.92) with Fiber Bragg grating (FBG) sensors. Magneto-elastic properties of Terfenol-D depend on magnetization, stress pre-history, and temperature. In order to simultaneously monitor these effects, we have implemented a system based on a cylindrical Terfenol-D rod monitored with four FBGs that allows making the appropriate compensations on the strain measurement due to temperature drift. We have measured the magnetostriction in the axial and the transverse directions for the Terfenol-D rod with two perpendicular FBGs, and calculated the Poisson ratio. An additional mechanical system for strain amplification has been designed in order to increase sensitivity, by a factor of 4, in the strain measurement with the FBG sensor.

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.