Surface-bonded Fiber Bragg Grating Research Articles

Proof-of-concept study of bolt connection status monitoring using fiber Bragg grating curvature sensor

For connection status monitoring of bolted joint groups, a novel method with fiber Bragg grating (FBG)-based curvature sensor is proposed. An FBG curvature sensor with an inextensible elastic matrix and an offset grating is adopted and clamped between a fixed support and the monitored bolt in a plane bending status. Based on the shape deformation of the FBG curvature sensor induced by the bolt loosening, the bending curvature change of the elastic matrix is measured by the FBG. By detecting the wavelength shift of the FBG induced by the matrix curvature variation, the relationship between the bolt loosening angle and the wavelength shift of FBG can be obtained, and hence the bolt connection status can be determined by the wavelength shift of FBG. Details of the proposed method are presented, and a surface-bonded FBG curvature sensor is designed, fabricated, and experimentally studied to verify the proposed method, and the parameters on sensing performance are also investigated. Experimental results show that the proposed method can monitor bolt loosening angle with high sensitivity and linear output by adjusting the parameters of the FBG curvature sensor. The looseness angle resolution of the proposed sensor can reach 0.0767°. This paper presents the first attempt to monitor bolt loosening angle using an FBG curvature sensor. With the advantages of high sensitivity and resolution, strong applicability, convenience and reusability for the maintenance of bolts, and ease of forming quasi-distributed sensor networks, the proposed method is able to provide a general solution for the on-line monitoring of bolt connection status for large-span distributed bolted-joint groups.

Strain-Sensing Mechanism and Axial Stress Response Characterization of Bolt Based on Fiber Bragg Grating Sensing

The anchoring quality of bolts is related to roadway safety and the surrounding rock stability. Due to the limitations of conventional monitoring methods in capturing strain, there still exists a gap in the real-time perception of the mechanical properties of bolts at the micro-scale. This paper proposes a new approach to detecting bolts’ anchoring qualities based on the fiber Bragg grating sensing principle. Moreover, it studies the strain transmission mechanism between the surface-bonded fiber Bragg grating and the bolt. A fiber-optic monitoring test platform of anchor bolt anchoring quality is built. The full-length anchor bolt’s strain evolution law and axial force distribution characteristics are studied during the pull-out test. The study results have shown that the theoretical value of the fiber strain transfer coefficient can be used to calculate the strain of the bolt. The bolt pull-out test verified the accuracy of using the fiber Bragg grating bolt axial force characterization equation to estimate the bolt stress. On the other hand, the correlation between the bolt axial force and the fiber Bragg grating monitoring value follows an exponential pattern. This study provides an important basis for improving the understanding of a bolt anchoring mechanism and the stability control of a roadway’s surrounding rock.

A Sensing Mechanism and the Application of a Surface-Bonded FBG Dynamometry Bolt

In the present paper, a new type of surface-bonded fiber Bragg grating (FBG) dynamometry bolt is designed. It is assumed that the adhesive layer is a linear viscoelastic material and its creep mechanical behavior is expressed by the standard linear solid model. The shear strain transfer model of the surface-bonded FBG sensor is established. Additionally, the instantaneous and quasistatic strain transfer functions of the surface-bonded FBG sensor are obtained. The functions are validated by a uniaxial tensile test and a long-term constant-load tensile test. The test results show that the strain measured by the FBG sensor has a proportional relationship with the strain measured by the resistance strain gauge. Furthermore, under the fixed load for a long period of time, the strain of the FBG sensor has a tendency to drift and the strain reduction rate is about 40.5%. Finally, the field application is carried out in a mining area. It has been proved that the ground pressure online monitoring system based on the FBG sensing technology can successfully monitor the stress of the rock bolt.

表面粘贴布拉格光纤光栅传感器的应变传递耦合机理研究

表面粘贴布拉格光纤光栅传感器的应变传递耦合机理研究

Method for $In-Situ$ Strain Transfer Calibration of Surface Bonded Fiber Bragg Gratings

This paper reports on a method for strain calibration of fiber optic sensors which have been adhesively bonded to a structural member. Generally, the coefficient of strain transfer (kST) can be used to calibrate the strain sensitivity of the surface bonded FBG. Here we propose an alternative method to determine kST which is based solely on observing the temperature-dependent behaviour of the adhesively mounted FBG sensor. In this paper we prove the validity of our method by comparing kST determined via a conventional strain (mechanical) calibration and via our alternative temperature (thermal) calibration. A set of 18 surface bonded FBGs with coefficients ranging from 0.5 to 1 are investigated. The average deviation of kST determined via a temperature calibration versus a mechanical strain calibration was ~5%, with a maximum deviation of 13% obtained from the set of bonded sensors measured in this work. This led to an average relative accuracy of <; 6%, when the strain sensors were calibrated using our temperature-based calibration method.

The Analysis of FBG Central Wavelength Variation with Crack Propagation Based on a Self-Adaptive Multi-Peak Detection Algorithm.

We propose a peak seeking algorithm to extract the damage characteristic-variation of central wavelength to monitor the crack damage status in aluminum alloy plates using surface bonded fiber Bragg grating (FBG) sensors. The FBG sensors are sensitive to the uniform and non-uniform strain distribution along their longitudinal direction, and the effect appears in the power spectrum of the reflected light from the gauge section. In this paper, we propose a fast-self-adaptive multi-peak seeking algorithm to detect the central wavelength shifting of the FBG reflection spectrum with the crack propagation. The proposed peak searching algorithm results point to a significant improvement compared to other conventional methods. Then the central wavelength shifting is applied to explain the crack propagation behavior of the aluminum plates under quasi-static tensile test conditions. The different damages feature changing intervals which are associated with the crack position and the FBGs location, demonstrating that central wavelength shifting performs as an indicator to detect structural crack damage.

Strain transfer mechanism of grating ends fiber Bragg grating for structural health monitoring

The grating ends bonding fiber Bragg grating (FBG) sensor has been widely used in sensor packages such as substrate type and clamp type for health monitoring of large structures. However, owing to the shear deformation of the adhesive layer of FBG, the strain measured by FBG is often different from the strain of actual matrix, which causes strain measurement errors. This investigation aims at improving the measurement accuracy of strain for the grating ends surface-bonded FBG. To fulfill this objective, a strain transfer equation of the grating ends bonding FBG is derived, and a theoretical model of the average strain transfer from the matrix to the optical fiber is developed. Moreover, parameters that influence the average strain transfer rate from the matrix to the optical fiber are analyzed. A selection scheme of bonding parameters by numerical simulation is provided, which is significantly advantageous over that of the grating bonding FBG. The theoretical equation is verified by finite element method (FEM). Compared with the existing model, the proposed model has higher measurement accuracy. Experimental tests are performed to validate the effectiveness of the proposed model on the equal-intensity cantilever beam, whose surface is attached to the bare FBG with grating ends bonding and strain gauge by using epoxy glue. The results show that there is a great agreement between the outcome of the bare FBG and that of the strain gauge, and the corrected strain is closer to the true strain. The proposed model provides a theoretical basis for the design of the grating ends surface-bonded FBG strain sensor for health monitoring of large structures.

Strain transferring mechanism analysis of the surface-bonded FBG sensor.

The mechanical analysis model of surface-bonded fiber Bragg grating (FBG) shear strain transfer is established in consideration of the linear viscoelastic effect of the adhesive layer, which is simplified as a standard linear solid model. And the strain transfer relation is acquired and validated by anchor bar uniaxial tension experiments. There is good correspondence between calculated values and experimental results. Finally, the influences of shear modulus, length and width of the adhesive layer, and thickness of the interlayer on the instantaneous and quasistatic strain transfer are analyzed by numerical simulation.

Measurement accuracy of FBG used as a surface-bonded strain sensor installed by adhesive

Material and dimensional properties of surface-bonded fiber Bragg gratings (FBGs) can distort strain measurement, thereby lowering the measurement accuracy. To accurately assess measurement precision and correct obtained strain, a new model, considering reinforcement effects on adhesive and measured object, is proposed in this study, which is verified to be accurate enough by the numerical method. Meanwhile, a theoretical strain correction factor is obtained, which is demonstrated to be significantly sensitive to recoating material and bonding length, as suggested by numerical and experimental results. It is also concluded that a short grating length as well as a thin but large-area (preferably covering the whole FBG) adhesive can enhance the correction precision.

Optimization design of the tuning method for FBG spectroscopy based on the numerical analysis of all-fiber Raman temperature lidar

Abstract All fiber Raman temperature lidar for space borne platform has been proposed for profiling of the temperature with high accuracy. Fiber Bragg grating (FBG) is proposed as the spectroscopic system of Raman lidar because of good wavelength selectivity, high spectral resolution and high out-of-band rejection rate. Two sets of FBGs at visible wavelength 532 nm as Raman spectroscopy system are designed for extracting the rotational Raman spectra of atmospheric molecules, which intensities depend on the atmospheric temperature. The optimization design of the tuning method of an all-fiber rotational Raman spectroscopy system is analyzed and tested for estimating the potential temperature inversion error caused by the instability of FBG. The cantilever structure with temperature control device is designed to realize the tuning and stabilization of the central wavelengths of FBGs. According to numerical calculation of FBG and finite element analysis of the cantilever structure, the center wavelength offset of FBG is 11.03 nm/°C with the temperature change in the spectroscopy system. By experimental observation, the center wavelength offset of surface-bonded FBG is 9.80 nm/°C with temperature changing when subjected to certain strain for the high quantum number channel, while 10.01 nm/°C for the low quantum number channel. The tunable wavelength range of FBG is from 528.707 nm to 529.014 nm for the high quantum number channel and from 530.226 nm to 530.547 nm for the low quantum number channel. The temperature control accuracy of the FBG spectroscopy system is up to 0.03 °C, the corresponding potential atmospheric temperature inversion error is 0.04 K based on the numerical analysis of all-fiber Raman temperature lidar. The fine tuning and stabilization of the FBG wavelength realize the elaborate spectroscope of Raman lidar system. The conclusion is of great significance for the application of FBG spectroscopy system for space-borne platform Raman lidar.

Experimental Study on Directionality of Ultrasonic Wave Coupling Using Surface-Bonded Fiber Bragg Grating Sensors

Recent studies demonstrated the potential of increasing the Lamb wave detection sensitivity of fiber Bragg grating (FBG) sensors by bonding the optical fiber away from the grating location, instead of the conventional method of bonding the FBG directly. The FBG located at a remote location further along the optical fiber collects the guided traveling wave in the optical fiber generated from the Lamb wave signal. This remote bonding method could potentially be extended to a series of multiplexed FBGs. However, previous experiments also detected coupling to guided traveling waves in both directions in the optical fiber, which could have significant effects on multiplexed signals. In this paper, we measure the coupled signal amplitudes in both forward and backward directions, when ultrasonic waves couple from a thin plate to an optical fiber and from an optical fiber to a thin plate. The forward- and backward-induced modes are measured in both the optical fiber and the plate. The same experiment is then performed for the case when ultrasonic signal is coupled from the optical fiber to the plate. In addition, two different types of bonding, cyanoacrylate adhesive and frictional bond, are explored to investigate how the signal conversion depends on the bonding method. The results demonstrate that the coupling of ultrasonic waves from a thin structure to an optical fiber and from an optical fiber to the structure is complex. The coupling does not only occur in the direction of the wave propagation, but can be coupled into both forward and backward modes, depending on the bonding configuration used.

Effects of the bonding length on the reflected spectra and strain measurement of the surface bonded fiber Bragg grating sensor

Surface bonded fiber Bragg grating (FBG) sensors have the limitation that signal characteristics are affected by the bonding layer. Previously conducted studies with regard to this limitation used different optical fiber sensors and load types during the experiments, and didn’t consider the effects on the reflected spectra. Therefore, in this study, the effects of the bonding length on the reflected spectra and strain measurement of the surface bonded FBG sensor were experimentally investigated. From the experimentation results, sufficiently bonded FBG sensor showed stablereflected spectra and enough strain transfer rate. However, insufficiently bonded FBG sensor showed distorted reflected spectra with high multiple peak ratio due to internal strain gradients within the grating length of the FBG sensor. In addition, distorted reflected spectra made it difficult to calculate adequate peak and decreased the strain transfer rates. Therefore, it was found that effective bonding length need to be determined to get a stable reflected spectra and enough strain transfer rate from the surface bonded FBG sensors.

Comparison of Metal-Packaged and Adhesive-Packaged Fiber Bragg Grating Sensors

A feasible metallic packaging technique of fiber Bragg grating (FBG) sensors is developed for measurement of strain, and it can be simply achieved via one-step ultrasonic welding. The strain transfer coefficient of the FBG sensor is theoretically evaluated by a proposed model aiming at determining the proper packaging parameters of a surface-bonded FBG sensor. According to analytical results, the metallic packaging shows higher strain transfer coefficient compared with traditional adhesive packaging under the same conditions. Comparisons of the experimental results with the numerical predictions for the strain sensitivity show a satisfactory agreement. Experimental strain tests are performed on an elaborate uniform strength beam for both tensile and compressive strain, and the strain sensitivity of $\sim 1.26$ and 1.34 pm/ $\mu \upvarepsilon $ is obtained for the tensile and compressive situation, respectively. The results of comparison demonstrate that the metal-packaged FBG sensor has a certain advantage in respect of strain measurement in specific sensing field, such as long-term, high-precision, and harsh-environment structural health monitoring.

Design, fabrication, and testing of fiber Bragg grating sensors for cryogenic long-range displacement measurement

It is essential to measure the shrinkage/expansion and positioning/aligning of magnets and to control valve displacement which plays a vital role in experiments like the Karlsruhe tritium neutrino experiment beam tube and Cryo pumps. Hence, a displacement sensor which, over a long working range, can be operated under extreme environmental conditions needs to be developed. Fiber Bragg gratings (FBG) have been considered to be excellent sensor elements useful for a variety of applications. This paper will discuss a long range displacement sensors based on fiber Bragg gratings for cryogenic temperature applications. The cryo pump inlet valve control requirements have been taken as example specifications for sensor design. To achieve the development goal, a proper signal transducer and sensor package were designed. A study of the strain transmission of surface-bonded FBG was conducted. The influence of bonding thickness and bonding length was reported. The design, fabrication, and performance were tested at low temperature of around 77K. The sensor performance was found to be satisfactory at both room temperature and 77K and linearly for long-range displacement of 550mm with 14pm/mm sensitivity and 0.142mm accuracy.

Effects of bonding layer on the available strain measuring range of fiber Bragg gratings

Stress-induced birefringence can lead to distortion in the reflection spectra of fiber Bragg grating (FBG) sensors, thereby resulting in the loss of accuracy and stability of strain measurements. The bonding layer is a direct factor in producing stress birefringence within FBGs. To assess the impacts quantitatively, a theoretical model that links the bonding layer and the reflection spectrum was established. At the same time, the finite element method, based on the theoretical model, was used to study the relationships between characteristics of the bonding layer and reflection spectrum in detail. The analytical results indicate that high elastic modulus and mismatched Poisson's ratio of bonding layer decrease the available strain measuring range of FBGs remarkably, and that unreasonable geometric parameters of the bonding layer should be avoided. In addition, a validation experiment was conducted and experimental results proved the prediction of the theoretical analysis. It can be concluded from the results that the bonding layer is the major limiting factor for the application of surface-bonded FBG sensors in large strain measurements. The bonding materials and bonding processes used in producing FBG sensors deserve serious consideration.

表面粘贴式光纤布拉格光栅传感器层状结构对测量应变的影响

When a surface-bonded Fiber Bragg Grating(FBG)sensor is used to measure the strain of a host material,it will effect the strain distribution of the host material.Therefore,this paper explores the relationship between optical fiber strain and host material strain.A theoretical model of strain transfer of the host material to the optical fiber was presented to modify the measured strain,and the interaction between FBG sensor and host material was considered.Finally,the theoretical predictions proposed in this paper were verified by Finite Element Analysis(FEA)and practical experiments.The results show that the error between FEA and theoretical solution is controlled within 5%,and thatbetween theoretical solution and experimental data is controlled within 8%.These mean that the strain transfer mode satisfies the accuracy requirement of surface-bonded FBG sensors.Moreover,the effects of geometrical and material parameters on the average strain transfer rate and the strain transfer rate were analyzed.The obtained results indicate that the average strain transfer rate and the strain transfer rate increase with the Young's modulus of the host material.However,they decrease with increasing the top thickness and bottom thickness of an adhesive layer.

An Investigation of Optical-Mechanical Transfer of Surface-Bonded Fiber Bragg Grating Sensors for Aerostat Envelope Strain Monitoring

An Investigation of Optical-Mechanical Transfer of Surface-Bonded Fiber Bragg Grating Sensors for Aerostat Envelope Strain Monitoring

Strain transfer of surface-bonded fiber Bragg grating sensors for airship envelope structural health monitoring

This paper deals with an improved bonding approach of surface-bonded fiber Bragg grating (FBG) sensors for airship envelope structural health monitoring (SHM) under the strain transfer theory. A theoretical formula is derived from the proposed model to predict the strain transfer relationship between the airship envelope and fiber core. Then theoretical predictions are validated by numerical analysis using the finite element method (FEM). Finally, on the basis of the theoretical approach and numerical validation, parameters that influence the strain transfer rate from the airship envelope to fiber core and the ratio of effective sensing length are analyzed, and some meaningful conclusions are provided.

Analysis of strain transfer of six-layer surface-bonded fiber Bragg gratings

A theoretical analysis of strain transfer of six-layer surface-bonded fiber Bragg gratings (FBGs) subjected to uniform axial stress is presented. The proposed six-layer structure consists of optical fiber, protective coating, adhesive layer, substrate layer, outer adhesive layer, and host material, which is different from the four-layer case of common acknowledgement. A theoretical formula of strain transfer rate from host material to optical fiber is established to provide an accurate theoretical prediction. On the basis of the theoretical analysis, influence parameters of the middle layers that affect the average strain transfer rate of the six-layer surface-bonded FBG are discussed. After the parametric study, a selection scheme of sensor parameters for numerical validation, which makes the average strain transfer rate approach unity, is determined. Good agreement is observed between numerical results and theoretical predictions. In the end, the six-layer model is extended to the general situation of multiple substrate layers, which lays a theoretical groundwork for the research and design of surface-bonded FBGs with substrate layers in the future.