Caiqian Yang

An Improved Conjugated Beam Method for Deformation Monitoring with a Distributed Sensitive Fiber Optic Sensor

Based on the distributed fiber optic sensing technique of pulse-prepump Brillouin Optical Time Domain Analysis, this paper proposes an improved conjugated beam method (ICBM) for distributed structural deformation monitoring. Because ICBM is suitable for the combined actions of loads, support settlements and temperature variations, it extends the application of the classical conjugated beam method. Other advantages include the linear and explicit relationship between structural deformation and strain and the fact that all calculation parameters can be easily determined free from random load or section stiffness distribution. Theoretical and experimental investigations are carried out to confirm that the accuracy of deformation monitoring with ICBM in one span of a continuous structure is related only to the accuracy of strain measurements for the same span. This characteristic is used to avoid the influence of strain measurement error accumulation from a series of strain measurement data on other spans. Such accumulation can result in uncontrolled deviation of monitoring deformation with the classical double integration method. In order to further increase the accuracy of deformation monitoring, a new distributed sensitive fiber optic sensor (DSFOS) is proposed to improve the accuracy of strain measurement with good designability according to detailed sensing requirements. The results of a uniaxial tensile experiment show that the strain measurements from DSFOSs agree well with the true values, and the measuring standard deviations appear to be lower than those obtained from UV coating fiber optic sensor. The flexural experimental results from reinforced concrete beams show that the obtained structural deflection data agree with the values observed from digital indicators. Relative to common fiber optic sensors, DSFOSs can obtain deformation and strain with higher accuracy to show applicability for daily monitoring.Based on the distributed fiber optic sensing technique of pulse-prepump Brillouin Optical Time Domain Analysis, this paper proposes an improved conjugated beam method (ICBM) for distributed structural deformation monitoring. Because ICBM is suitable for the combined actions of loads, support settlements and temperature variations, it extends the application of the classical conjugated beam method. Other advantages include the linear and explicit relationship between structural deformation and strain and the fact that all calculation parameters can be easily determined free from random load or section stiffness distribution. Theoretical and experimental investigations are carried out to confirm that the accuracy of deformation monitoring with ICBM in one span of a continuous structure is related only to the accuracy of strain measurements for the same span. This characteristic is used to avoid the influence of strain measurement error accumulation from a series of strain measurement data on other spans. Su...

Investigation on the damage identification of bridges using distributed long-gauge dynamic macrostrain response under ambient excitation

In this study, an output-only method for extraction of modal macrostrain and damage identification of bridges under ambient excitation is presented. It is theoretically proved that the modal macrostrain within the gauge length of a long-gauge macrostrain sensor is uniquely determined by the peak value of the power spectral density (PSD) of dynamic macrostrain response. Then damage occurred within the gauge length of a sensor can be identified by the ratio change between the peak value of PSD of this sensor response and that of the reference sensor response. The damage extent has also been verified to have corresponding relationship with the ratio change. Numerical simulation was carried out to confirm the feasibility of the proposed method. Analysis results of the numerical simulation reveal that the maximum error of the identified modal macrostrain relative to that of the modal analysis is 3%. Results of simulation show that the proposed PSD-based method can not only accurately localize the damage but also assess the damage extent. The influence of several typical excitations on real bridges was also investigated that further proves the robustness of the method. It is worth mentioning that only the first-order mode is necessary for the method to identify damage. Finally, the proposed method is employed for condition assessment of a real bridge located in New Jersey, wherein 17 long-gauge macrostrain sensors with the gauge length of 1 m were distributedly arranged along the critical region of the girder. Analysis results of the field measurements further verify that the PSD-based method can be utilized to assess the damage state of structures under ambient excitation.

A new type of smart basalt fiber-reinforced polymer bars as both reinforcements and sensors for civil engineering application

In this paper, a new type of smart basalt fiber-reinforced polymer (BFRP) bar is developed and their sensing performance is investigated by using the Brillouin scattering-based distributed fiber optic sensing technique. The industrial manufacturing process is first addressed, followed by an experimental study on the strain, temperature and fundamental mechanical properties of the BFRP bars. The results confirm the superior sensing properties, in particular the measuring accuracy, repeatability and linearity through comparing with bare optical fibers. Results on the mechanical properties show stable elastic modulus and high ultimate strength. Therefore, the smart BFRP bar has potential applications for long-term structural health monitoring (SHM) as embedded sensors as well as strengthening and upgrading structures. Moreover the coefficient of thermal expansion for smart BFRP bars is similar to the value for concrete.

Finite element model updating of flexural structures based on modal parameters extracted from dynamic distributed macro-strain responses

This article presents a novel strategy for finite element model updating of flexural structures. The method is based on modal parameters extracted from dynamic distributed macro-strain responses. The objective function that comprised low-order modal macro-strain and frequency was established first, while local bending stiffness, density, and boundary conditions of structures can be selected as the design variables. Both numerical simulation and experiment were conducted to verify the effectiveness of the proposed method. The long-gauge macro-strain sensors chosen in this article are first addressed, and the fundamental sensing properties of the macro-strain sensors confirm great dynamic measurement capacity. Simulation and experimental results show that both the local parameter (bending stiffness) and the global parameters (mass density and rotational stiffness of support) can be well identified. Notably, the updated finite element model can predict local response modal macro-strain and global response (frequency and displacement mode) because the local information-sensitive index and global information-sensitive index are included in the objective function. Therefore, the proposed finite element model updating strategy constitutes a new alternative in performance assessment of flexural structures.

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.

A model-free damage identification method for flexural structures using dynamic measurements from distributed long-gage macro-strain sensors

A damage identifying algorithm, named modal macro-strain vector, has been verified as efficient to locate local damages for flexural structures, with modal parameters directly extracted from the dynamic macro-strain measurements. However, the basic relation between the change in modal macro-strain vector and damage severity has not been established yet. In this article, a model-free damage identification method is proposed based on the modal macro-strain vector method to implement both damage location and quantification. In this method, one assumption is first proposed that the normalized modal macro-bending-moment is constant, as proved by the simulations. Then, damage quantification is performed with the combination of the spatial parameter of neutral axis and normalized modal macro-strain vector. In view of these, different finite element beam models are simulated with several damage cases for each specimen to verify the method. Simulation results show that the damage severity can be evaluated with a high accuracy with the proposed method. As a result, the efficiency of the method is verified through the experiments of a steel beam in the laboratory. Therefore, the proposed system is proved to be effective and useful for structural health monitoring.

A new optical fiber sensor with improved strain sensitivity based on distributed optical fiber sensing technique

In general, macro-strain is an effective index for health monitoring of civil infrastructures, which can reveal the unforeseen damage accumulation. However, it is difficult to acquire precise strain distribution with existing fully-distributed optical fiber sensing techniques. Based on the distributed optical fiber strain sensing technique of pulse-prepump Brillouin Optical Time Domain Analysis (PPP-BOTDA), a new optical fiber sensor with improved strain sensitivity (OFSISS) is proposed to enhance the precision of macro-strain measurements. The most advantage of the OFSISS sensor is that it can markedly reduce the measurement error of strain data with a proper designed magnified coefficient. The OFSISS has also good designability and durability according to detailed sensing requirements. Results of uniaxial tensile experiment show not only the high accuracy and precision of the OFSISS but also an important fact that the measured magnified coefficients of the manufactured OFSISSs with a recoating process agree well with the designed values. The bending experiment of using a steel beam illustrates that the linearity and reliability of macro-strain measurement from the OFSISS are good enough for the application in actual macro-strain monitoring and structural deformation monitoring.

In situ corrosion monitoring of PC structures with distributed hybrid carbon fiber reinforced polymer sensors

Firstly, the fabrication and sensing properties of hybrid carbon fiber reinforced polymer (HCFRP) composite sensors are addressed. In order to provide a distributed sensing manner, the HCFRP sensors were divided into multi-zones with electrodes, and each zone was regarded as a separate sensor. Secondly, their application is studied to monitor the steel corrosion of prestressed concrete (PC) beams. The HCFRP sensors with different gauge lengths were mounted on a PC tendon, steel bar and embedded in tensile and compressive sides of the PC beam. The experiment was carried out under an electric accelerated corrosion and a constant load of about 54 kN. The results reveal that the corrosion of the PC tendon can be monitored through measuring the electrical resistance (ER) change of the HCFRP sensors. For the sensors embedded in tensile side of the PC beam, their ER increases as the corrosion progresses, whereas for the sensors embedded in compressive side, their ER decreases with corrosion time. Moreover, the strains due to the corrosion can be obtained based on the ER change and calibration curves of HCFRP sensors. The strains measured with traditional strain gauges agree with the strains calculated from the ER changes of HCFRP sensors. The electrical behavior of the zones where the corrosion was performed is much different from those of the other zones. In these zones, either there exist jumps in ER, or the ER increases with a much larger rate than those of the other zones. Distributed corrosion monitoring for PC structures is thus demonstrated with the application of HCFRP sensors through a proper installation of multi-electrodes.

Structural health monitoring of PC structures with novel types of distributed sensors

In this paper, the structural health monitoring of a pre-stressed concrete (PC) structure based on two types of distributed sensing techniques is addressed. The sensing elements are Brillouin scattering-based fiber optic sensors (FOSs) and HCFRP (hybrid carbon fiber reinforced polymer) sensors composed of three types of carbon tows. Both types of sensors are characterized by a broad-based and distributed sensing function. The HCFRP sensors are bonded on PC tendon, steel reinforcing bar, and embedded in tensile and compressive concrete sides with epoxy resins and putties. The FOSs are embedded in the tensile and compressive concrete sides where the HCFRP sensors are embedded as well. The distributed sensors are arranged to detect and monitor the initiation and propagation of cracks, yielding of steel reinforcements and corrosion of PC tendons. The experimental investigations demonstrate that the initiation and location of cracks, yielding of steel reinforcements, corrosion of PC tendons and structural health of PC structures can be effectively detected and monitored with such kinds of distributed sensing systems.

Development of carbon fiber-based piezoresistive linear sensing technique

In this paper, the development of carbon fiber-based piezoresistive linear sensing technique and its application in civil engineering structures is studied and summarized. The sensing mechanism is based on the electrical conductivity and piezoresistivity of different types of carbon fibers. Firstly, the influences of values of signal currents and temperature on the sensing properties are studied to decide the suitable sensing current. Then, the linear temperature and strain sensing feasibility of different types of carbon fibers is addressed and discussed. Finally, the application of this kind of sensors is studied in monitoring the health of reinforced concrete (RC) and prestressed concrete (PC) structures. A good linearity of fractional change in electrical resistance (ER) (ΔR/R0)-strain and &DeltaR/R0-temperature is demonstrated. The &DeltaR/R0-strain and &DeltaR/R0-temperature curves of CFRP/HCFRP sensors can be well fitted with a line with a correlation coefficient larger than 0.978. All these reveal that carbon fibers reinforced polymer (CFRP) can be used as both piezoresistive linear strain and temperature sensors.