Chunfeng Wan

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.

Bridge Assessment and Health Monitoring with Distributed Long-Gauge FBG Sensors

Most sensors for structural testing and health monitoring are “point” sensors which strongly limit the ability to correct damage detection and structural assessment. In this paper, long-gauge FBG sensor which can sense the whole area within the gauge length is introduced. Bridge assessment and health monitoring with the microstrain distribution acquired by the distributed long-gauge FBG sensor are also studied. Experiments were conducted and application to a real prestressed box bridge was also implemented. Static and dynamic testing results show that distributed long-gage FBG sensing technique can obtain not only the global information such as bridge deflection and natural frequency, but also the local parameters such as strain and modal macrostrain to detect damage of the bridge. It shows that structural assessment and health monitoring based on the proposed technique have great potential in maintenance of civil engineering infrastructures.

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.

Identification of modal macro-strain vector based on distributed long-gage FBG sensors under ambient vibration

Recent reports show that modal macro-strain vector (MMSV) obtained by using distributed long-gage FBG sensors is an effective indicator for damage detection. However, in previous researches, MMSV was always obtained under impulsive load such as hammer impact. In structural health monitoring of real large-scale structures, however, it is often very difficult to apply such impulsive load. This paper therefore introduces a new method to abstract MMSV under ambient excitation. Theoretical deduction reveals that MMSV can be uniquely determined by auto-spectrum of dynamic macro-strain responses under ambient excitation. Both numerical simulation and experiment were conducted to verify the proposed methods. Simulation results showed that that the identified frequencies and MMSV vectors under random excitation are in good agreement with those obtained from theoretical analysis, while experimental results showed the identified frequencies and MMSV agreed well with those obtained using point impulsive excitation.

Experimental Study on Force Sensitivity of the Conductivity of Carbon Nanotubes-Modified Epoxy Resins

The addition of a conductive material into polymer improves its mechanical properties, electrical properties and thermal conductivity and bestows it with good self-sensing and self-adjusting properties. In this study, carbon nanotubes-modified epoxy resins (CNTs-EP) were successfully prepared with good dispersion through the combined methods of three roller rolling, ultrasonic processing and adding surfactant. Tests were conducted to evaluate the resistivity of unloaded modified epoxy resins with different mixing amounts of carbon nanotubes (CNTs), to determine the conductive percolation threshold. On the basis of the test results, a series of monotonic and cyclic uniaxial tensile tests were then conducted to investigate the force sensitivity of the conductivity of epoxy resins modified with different mixing amounts of CNTs. The relationship between the stress and the resistivity under various mixing amounts was studied, indicating that the resistance response could play a good warning role on the damage of the modified polymer material.

Parameter identification for structural health monitoring based on Monte Carlo method and likelihood estimate

Structural parameters are the most important factors reflecting structural performance and conditions. As a result, their identification becomes the most essential aspect of the structural assessment and damage identification for the structural health monitoring. In this article, a structural parameter identification method based on Monte Carlo method and likelihood estimate is proposed. With which, parameters such as stiffness and damping are identified and studied. Identification effects subjected to three different conditions with no noise, with Gaussian noise, and with non-Gaussian noise are studied and compared. Considering the existence of damage, damage identification is also realized by the identification of the structural parameters. Both simulations and experiments are conducted to verify the proposed method. Results show that structural parameters, as well as the damages, can be well identified. Moreover, the proposed method is much robust to the noises. The proposed method may be prospective for the application of real structural health monitoring.

DAMAGE DETECTION TO URBAN GIRDER BRIDGE USING DISTRIBUTED STRAIN RESPONSE

Detection of damage to an urban highway bridge is establishing a prominent role in modern engineering. In this paper, damage localization index called the relative strain energy (RSE) based on the distributed strain response was introduced, and it is employed to validate the effectiveness in locating damage elements by using a finite element (FE) model of an urban girder bridge under various degrees of damage assumption in the bridges girder. The numerical analysis results show that the RSE index could locate damage position accurately in early damage of the structure, and they also have an excellent noise pollution resistance performance. Two random excitations were introduced into the analysis of the damage location in before and after damage occurrence, respectively. All of the results are presented in the form of tables and graphs.