Long-gauge Fiber Bragg Grating Research Articles

Parametric study of a rapid bridge assessment method using distributed macro-strain influence envelope line

Abstract Dynamic strain-based methods are promising for bridge status assessment; however, the influences of vehicle parameters variations on their capabilities for damage identification are usually indeterminate. In this paper, parametric study for highway bridge rapid assessment method is conducted under moving vehicle loads, which is based on the distributed macro-strain envelope line. First, the relationship between the average bending stiffness and macro-strain history is studied according to the theory of the strain influence line. To determine the influence of different vehicle parameters (velocity, weight and axle numbers) on the method, a 1:10 scale model of a bridge-vehicle system is established for the parametric experimental study. A type of long-gauge Fiber Bragg Grating (FBG) sensor is used in the experiment. The empirical mode decomposition method (EMD) is applied to extract the static macro-strain component, and the maximum values of the static macro-strain histories are accumulated as the long-gauge strain envelope line. The experimental results show that the proposed method not only effectively identifies the location of damage but also damage extent and is not affected by the parameters of the vehicle. This method has engineering significance in bearing capacity evaluation and post-bridge structure reinforcement design.

Distributed Deformation Monitoring for a Single-Cell Box Girder Based on Distributed Long-Gage Fiber Bragg Grating Sensors.

Distributed deformation based on fiber Bragg grating sensors or other kinds of strain sensors can be used to monitor bridges during operation. However, most research on distributed deformation monitoring has focused on solid rectangular beams rather than box girders—a kind of typical hollow beam widely employed in actual bridges. The deformation of a single-cell box girder contains bending deflection and also two additional deformations respectively caused by shear lag and shearing action. This paper revises the improved conjugated beam method (ICBM) based on the long-gage fiber Bragg grating (LFBG) sensors to satisfy the requirements for monitoring the two additional deformations in a single-cell box girder. This paper also proposes a suitable LFBG sensor placement in a box girder to overcome the influence of strain fluctuation on the flange caused by the shear lag effect. Results from numerical simulations show that the theoretical monitoring errors of the revised ICBM are typically 0.3–1.5%, and the maximum error is 2.4%. A loading experiment for a single-cell box gilder monitored by LFBG sensors shows that most of the practical monitoring errors are 6–8% and the maximum error is 11%.

Development of a Bridge Weigh-in-Motion System Based on Long-Gauge Fiber Bragg Grating Sensors

Monitoring the weight of vehicles is essential for policymakers to maintain the condition of bridges. In the group of methods for weighing vehicles, bridge weigh-in-motion (B-WIM) systems are among the most prominent and have several advantages, such as convenience and no interruption to traffic. However, they still have one drawback in that, for most systems, a vehicle’s velocity and wheelbase need to be given in advance. Currently, this problem is usually solved by adding an axle detector, which leads to higher costs. Under such circumstances, this paper presents an alternative B-WIM system to measure a vehicle’s velocity, wheelbase, and axial and gross weight merely based on a single set of long-gauge fiber Bragg grating (FBG) sensors without additional devices. To test the effectiveness of this method under various conditions, a vehicle-bridge interaction (VBI) system simulation was performed based on the result of an indoor experiment. Also, an in situ test on an expressway bridge was performed to initially verify its feasibility. The results showed that this method can achieve its function with good accuracy under various conditions.

An Indirect Method for Monitoring Dynamic Deflection of Beam-Like Structures Based on Strain Responses

An indirect method for monitoring dynamic deflection of beam-like structures using strain responses measured by long-gauge fiber Bragg grating (FBG) sensors is proposed in this paper. Firstly, a theoretical derivation shows that structural deflection is in direct and linear relationship to long-gauge strain. Meanwhile, the method is suitable for structures with different boundary conditions and irrelevant to external loads. Secondly, the influence of boundary conditions, load type and sensor gauge length on the method is investigated by numerical simulation. Finally, an experiment of a simply supported beam subjected to dynamic loads was designed to verify the method. Experimental results show that both deflection time-history of arbitrary points of structures and deflection distribution along structures at a certain time can be obtained with high-precision. Therefore, the method presented can be a new alternative for the deflection evaluation and maintenance of engineering structures.

Damage detection of rotating wind turbine blades using local flexibility method and long-gauge fiber Bragg grating sensors

Wind turbines are a cost-effective alternative energy source; however, their blades are susceptible to damage. Therefore, damage detection of wind turbine blades is of great importance for condition monitoring of wind turbines. Many vibration-based structural damage detection techniques have been proposed in the last two decades. The local flexibility method, which can determine local stiffness variations of beam-like structures by using measured modal parameters, is one of the most promising vibration-based approaches. The local flexibility method does not require a finite element model of the structure. A few structural modal parameters identified from the ambient vibration signals both before and after damage are required for this method. In this study, we propose a damage detection approach for rotating wind turbine blades using the local flexibility method based on the dynamic macro-strain signals measured by long-gauge fiber Bragg grating (FBG)-based sensors. A small wind turbine structure was constructed and excited using a shaking table to generate vibration signals. The structure was designed to have natural frequencies as close as possible to those of a typical 1.5 MW wind turbine in real scale. The optical fiber signal of the rotating blades was transmitted to the data acquisition system through a rotary joint fixed inside the hollow shaft of the wind turbine. Reversible damage was simulated by aluminum plates attached to some sections of the wind turbine blades. The damaged locations of the rotating blades were successfully detected using the proposed approach, with the extent of damage somewhat over-estimated. Nevertheless, although the specimen of wind turbine blades cannot represent a real one, the results still manifest that FBG-based macro-strain measurement has potential to be employed to obtain the modal parameters of the rotating wind turbines and then locations of wind turbine segments with a change of rigidity can be estimated effectively by utilizing these identified parameters.

Normalized Curvature Ratio for Damage Detection in Beam-Like Structures

Fiber optic sensors (FOS) offer numerous advantages for structural health monitoring. In addition to being durable, lightweight, and capable of multiplexing, they offer the ability to monitor strain in both static and dynamic mode. FOS also allow for instrumentation of large areas of a structure with long-gauge sensors which helps enable global monitoring of the structure. Drawing upon these benefits, the Normalized Curvature Ratio (NCR), a curvature based damage detection method, has been developed. This method utilizes a series of long-gauge fiber Bragg grating (FBG) strain sensors for damage detection of a structure through dynamic strain measurements and curvature analysis. The main assumption is that the ratios between cross-sectional curvature amplitudes under free vibration remain unchanged given the state of the structure is unchanged. The theoretical development of this method is presented along with an analytical study of a simply supported beam with two damage cases: a loss of flexural stiffness in the span and a change in rotational stiffness of the support. Validation of the method is then performed through two implementations. First, through a small-scale laboratory test with a simply supported aluminum beam subjected to a change in the rotational stiffness of the support. Second, the method is applied to an existing in-service highway overpass with over 5 years of data collection of dynamic strain events. The advantages and limitations of the method are identified and discussed. This research shows encouraging results and the potential for the NCR to be used as a simplistic metric for damage detection.

Deflection monitoring for a box girder based on a modified conjugate beam method

After several years of operation, a box girder bridge would commonly experience excessive deflection, which endangers the bridge’s life span as well as the safety of vehicles travelling on it. In order to avoid potential risks, it is essential to constantly monitor the defection of box girders. However, currently, the direct deflection monitoring methods are limited by the complicated environments beneath the bridges, such as rivers or other traffic lanes, which severely impede the layouts of the sensors. The other indirect deflection monitoring methods mostly do not thoroughly consider the inherent shear lag effect and shear deformation in the box girder, resulting in a rather large error. Under these circumstances, a deflection monitoring method suiting box girders is proposed in this article, based on the conjugate beam method and distributed long-gauge fibre Bragg grating (FBG) sensor. A lab experiment was conducted to verify the reliability and feasibility of this method under practical application. Further, the serviceability under different span-depth ratios and web thicknesses was examined through a finite element model.

Structural health monitoring of maglev guideway PC girders with distributed long-gauge FBG sensors

Summary Health monitoring for maglev guideway PC girders is critical to ensure the safe operation of a high-speed maglev train. A long-gauge fiber Bragg grating (FBG) sensor has been recently developed to achieve the high-precision distributed macro-strain measurements and the capability of dynamic testing. This paper presents an integrated strategy for health monitoring of maglev PC girders equipped with the developed FBG sensors. Using the quasi-static measurements under the maglev train load that is uniformly distributed over the span, the deflection and the flexural stiffness of the PC girder can be evaluated. Using the dynamic measurements of the free vibration after the quick passing of the maglev train, the modal parameters of the girder is determined, based on which the flexural stiffness can be calculated in a different way. Using the long-term quasi-static measurements with no maglev train passing by, the deflection can be estimated due to the other effects such as temperature variation, concrete creep, or prestress loss. If assuming that the prestress loss is the only cause of the long-term deflection, the prestress loss can be determined according to its quantitative relation with the strain variation. Through analytical investigations and field testing on a typical maglev PC girder, the ability of the FBG sensors to evaluate the deflection, stiffness, modal parameters and prestress loss is validated.

Flexibility identification and deflection prediction of a three-span concrete box girder bridge using impacting test data

Abstract Deflection is an important parameter for condition assessment of in-service bridges. However, accurately measuring bridge deflections especially that of long span bridges is still a challenging problem. In this article, field tests including multiple reference impact test (MRIT) and static test were performed on a three-span post-tensioned concrete box girder bridge to study structural deflections. Firstly, a new impacting device, which can generate high-magnitude and wide-frequency-band forces, has been developed and employed to execute the MRIT on the bridge. The output data from MRIT was utilized to estimate structural Frequency Response Function (FRF) and to identify deep-level structural parameters including modal scaling factor and structural flexibility, from which structural deflections under arbitrary loads were predicted. Field test results have shown that the developed impacting device can produce FRF with much better reciprocity than the FRF estimated from traditional sledge hammer, which demonstrates the superiority of the developed impacting device for bridge excitation and deflection prediction. Secondly, static test was also conducted on the bridge. The developed long gauge Fiber Bragg Grating (LG-FBG) strain sensors were mounted on the bottom of the box girder bridge. The measured long-gauge strains have been adopted to calculate structural deflections through a modified conjugate beam method which has high precision and there is no error accumulation from other span. Finally, data processing strategy was carried out according to the procedure of structural flexibility identification from MRIT data, and the identified flexibility matrix has been used to predict structural deflections by multiplying a static force vector corresponding to the tested truck load. The well agreement between the predicted deflections and the calculated deflections indicates that the proposed methods can be utilized to obtain structural deflections.

Dynamic Method of Neutral Axis Position Determination and Damage Identification with Distributed Long-Gauge FBG Sensors.

The neutral axis position (NAP) is a key parameter of a flexural member for structure design and safety evaluation. The accuracy of NAP measurement based on traditional methods does not satisfy the demands of structural performance assessment especially under live traffic loads. In this paper, a new method to determine NAP is developed by using modal macro-strain (MMS). In the proposed method, macro-strain is first measured with long-gauge Fiber Bragg Grating (FBG) sensors; then the MMS is generated from the measured macro-strain with Fourier transform; and finally the neutral axis position coefficient (NAPC) is determined from the MMS and the neutral axis depth is calculated with NAPC. To verify the effectiveness of the proposed method, some experiments on FE models, steel beam and reinforced concrete (RC) beam were conducted. From the results, the plane section was first verified with MMS of the first bending mode. Then the results confirmed the high accuracy and stability for assessing NAP. The results also proved that the NAPC was a good indicator of local damage. In summary, with the proposed method, accurate assessment of flexural structures can be facilitated.

基于长标距光纤布拉格光栅阵列动态响应的参数识别

基于长标距光纤布拉格光栅阵列动态响应的参数识别

Distributed strain measurement based on long-gauge FBG and delayed transmission/reflection ratiometric reflectometry for dynamic structural deformation monitoring.

In this paper, we propose a delayed transmission/reflection ratiometric reflectometry (DTR(3)) scheme using a long-gauge fiber Bragg grating (FBG), which can be used for dynamic structural deformation monitoring of structures of between a few to tens of meters in length, such as airplane wings and helicopter blades. FBG sensors used for multipoint sensing generally employ wavelength division multiplexing techniques utilizing several Bragg central wavelengths; by contrast, the DTR(3) interrogator uses a continuous pulse array based on a pseudorandom number code and a long-gauge FBG utilizing a single Bragg wavelength and composed of simple hardware devices. The DTR(3) scheme can detect distributed strain at a 50 cm spatial resolution using a long-gauge FBG with a 100 Hz sampling rate. We evaluated the strain sensing characteristics of the long-gauge FBG when attached to a 2.5 m aluminum bar and a 5.5 m helicopter blade model, determining these structure natural frequencies in free vibration tests and their distributed strain characteristics in static tests.

基于长标距FBG传感器宏应变技术的损伤识别研究

基于长标距FBG传感器宏应变技术的损伤识别研究

Damage Detection Strategy Using Strain-Mode Residual Trends for Long-Span Bridges

In this paper, a damage detection strategy based on distributed strain responses measured by long-gauge fiber Bragg grating (FBG) sensors is investigated. First, the distributed strain modes of a system with multiple degrees of freedom (DOFs) are derived via the frequency response function (FRF) of distributed strain response. Then the distributed strain mode is used to identify different degrees of damage incurred in a long-span, cable-stayed bridge under ambient excitation. The robustness of the distributed strain mode for detecting damage is analyzed, and it is found that its robustness is affected by ambient excitation, mode normalization, degree of damage, and noise pollution. To improve the robustness of the distributed strain-mode method, a damage detection strategy based on distributed strain-mode residual trends is developed using statistical tendency analysis and confidence probability. Finally, a numerical analysis is performed on the bridge structure under different operational conditions. The analytical results show that the proposed strategy can reduce the effects of loading variability and environmental diversity on the identified strain mode and locate damage accurately with good damage severity assessment and a high level of robustness.

Structural health monitoring of a steel stringer bridge with area sensing

The Federal Highway Administration Long-term Bridge Performance Programme initiated an International Bridge Study by selecting a steel stringer bridge as a benchmark structure for structural health monitoring. As a part of this programme, the authors studied the application of the Long-Gauge Fibre Bragg Grating (LG-FBG) sensors on this bridge. This paper aims at illustrating the LG-FBG-related state-of-the-art technologies by taking the bridge as the test bed. (1) The concept of the LG-FBG sensor for area sensing is presented. Most fibre optic sensors measure point strains for local monitoring. In contrast, the developed LG-FBG area sensor has a long gauge (e.g. 1–2 m), and it can be connected to each other to make a sensor array for distributed strain measuring; (2) spectral analyses of the macro-strain time histories are performed to identify structural frequencies, and the results are compared with those estimated from acceleration measurements; (3) the neutral axis position of the girder of the investigated bridge is estimated from the recorded macro-strain time histories, and the results are compared with those from static truck tests and (4) a modal macro-strain-based damage index is applied for damage detection of the steel stringer bridge.

Condition assessment of reinforced concrete beams using dynamic data measured with distributed long-gage macro-strain sensors

A new condition assessment strategy of reinforced concrete (RC) beams is proposed in this paper. This strategy is based on frequency analysis of the dynamic data measured with distributed long-gage macro-stain sensors. After extracting modal macro-strain, the reference-based damage index is theoretically deducted in which the variations of modal flexural rigidity and modal neutral axis height are considered. The reference-free damage index is also presented for comparison. Both finite element simulation and experiment investigations were carried out to verify the proposed method. The manufacturing procedure of long-gage fiber Bragg grating (FBG) sensor chosen in the experiment is firstly presented, followed by an experimental study on the essential sensing properties of the long-gage macro-strain sensors and the results verify the excellent sensing properties, in particular the measurement accuracy and dynamic measuring capacity. Modal analysis results of a concrete beam show that the damage appearing in the beam can be well identified by the damage index while the vibration testing results of a RC beam show that the proposed method can not only capture small crack initiation but its propagation. It can be concluded that distributed long-gage dynamic macro-strain sensing technique has great potential for the condition assessment of RC structures subjected to dynamic loading.

Dynamic fiber Bragg gratings based health monitoring system of composite aerospace structures

The main purpose of the current work is to develop a new system for structural health monitoring of composite aerospace structures based on real-time dynamic measurements, in order to identify the structural state condition. Long-gauge Fibre Bragg Grating (FBG) optical sensors were used for monitoring the dynamic response of the composite structure. The algorithm that was developed for structural damage detection utilizes the collected dynamic response data, analyzes them in various ways and through an artificial neural network identifies the damage state and its location. Damage was simulated by slightly varying locally the mass of the structure (by adding a known mass) at different zones of the structure. Lumped masses in different locations upon the structure alter the eigen-frequencies in a way similar to actual damage. The structural dynamic behaviour has been numerically simulated and experimentally verified by means of modal testing on two different composite aerospace structures. Advanced digital signal processing techniques, e.g. the wavelet transform (WT), were used for the analysis of the dynamic response for feature extraction. WT's capability of separating the different frequency components in the time domain without loosing frequency information makes it a versatile tool for demanding signal processing applications. The use of WT is also suggested by the no-stationary nature of dynamic response signals and the opportunity of evaluating the temporal evolution of their frequency contents. Feature extraction is the first step of the procedure. The extracted features are effective indices of damage size and location. The classification step comprises of a feed-forward back propagation network, whose output determines the simulated damage location. Finally, dedicated training and validation activities were carried out by means of numerical simulations and experimental procedures. Experimental validation was performed initially on a flat stiffened panel, representing a section of a typical aeronautical structure, manufactured and tested in the lab and, as a second step, on a scaled up space oriented structure, which is a composite honeycomb plate, used as a deployment base for antenna arrays. An integrated FBG sensor network, based on the advantage of multiplexing, was mounted on both structures and different excitation positions and boundary conditions were used. The analysis of operational dynamic responses was employed to identify both the damage and its position. The system that was designed and tested initially on the thin composite panel, was successfully validated on the larger honeycomb structure. Numerical simulation of both structures was used as a support tool at all the steps of the work providing among others the location of the optical sensors used. The proposed work will be the base for the whole system qualification and validation on an antenna reflector in future work.

IDENTIFICATION OF DAMAGE IN REINFORCED CONCRETE COLUMNS UNDER PROGRESSIVE SEISMIC EXCITATION STAGES

Prompt and accurate detection of realistic damage in constructed facilities is critical for effective condition assessment and structural health monitoring. This paper reports the experimental investigations of eccentric reinforced concrete columns mounted onto a shaking table and subject to progressively increasing seismic excitations. The investigation was aimed at studying the changes in the dynamic parameters in order to assess the structural conditions of the concrete columns after each post-seismic stage. The dynamic response of the structure was measured using accelerometers, traditional foil-strain gauges, and long-gauge fiber Bragg grating (FBG) sensors. The post-seismic conditions of the columns were evaluated via vibration-based damage identification methods. Results from this study demonstrate the applicability of specially packaged surface-mounted long-gauge FBG sensors for detecting the initiation and the progression of cracks due to reverse dynamic loads. The concept of modal macrostrain analysis was also introduced to identify and localize mild damage due to the applied seismic excitations of increasing intensities. The performance of the sensors for structural identification is also discussed.

Sensitivity Enhancement of Long-gage FBG Sensors for Macro-strain Measurements

In our recent study, a structural health monitoring strategy based on distributed fiber optic sensing techniques has been proposed to utilize the strain responses throughout the full or some partial areas of structures to detect the arbitrary and unforeseen damage. However, to perform this strategy more effectively, there is an increasing demand for improving the ability of such sensors to measure small structural responses, especially for the cases such as damage identification based on ambient vibration tests, fatigue crack monitoring of steel structures and crack detection of reinforced concrete structures. This work puts forward a novel packaging design for long-gage fiber Bragg grating (FBG) sensors to enhance the measurement sensitivity of strain responses. The basic idea is to utilize two materials of different stiffness to package the in-tube optical fiber and impose the deformation within the gage length largely on the short-gage sensing part of FBG. Proof-tests indicate that the measurement sensitivity of the innovative FBG sensor can be artificially enhanced by adjusting the two recoating materials and their respective lengths. A series of tests are carried out to verify the ability of the improved sensors to detect small crack and measure slight vibration.

Structural health monitoring using fiber optic distributed sensors for vacuum-assisted resintransfer molding

In this study we implemented manufacturing process and strain monitoring of a compositestructure by optical fiber sensors for vacuum-assisted resin transfer molding (VaRTM).Optical fibers with fiber Bragg gratings were embedded into a glass fiber reinforced plasticspecimen made by VaRTM and the applicability of structural health monitoring with fiberBragg grating (FBG) sensors based on optical frequency domain reflectometry(OFDR) was investigated. In this study, long-gage FBGs which are 10 timeslonger than ordinary FBGs (which are about 10 mm long) were employed fordistributed sensing. We can easily map the strain or temperature profile alonggratings by OFDR and the spatial resolution of this sensing technique is about1 mm. The resin flow process in VaRTM could be monitored by measuring thedifference in temperature between the resin and preform. Then, the shrinkageof resin could be also monitored during the curing process. The specimen wasthen subjected to a bending load in a three-point bending test and the straindistributions along the FBGs were measured. From these results we could show theapplicability of distributed sensors to quality assurance of a composite structure madeby VaRTM and assessment of the structural integrity of in-service compositestructures.