The Application Of Fiber Bragg Grating Technology In Structure Health Monitoring

The field of structural health monitoring (SHM) is concerned with accurately and reliably assessing the integrity of a given structure in order to reduce ownership costs, to increase operational lifetime, and to improve safety. In civil SHM systems, fiber optic sensors (FOSs) are promising sensing alternatives. They exhibit several advantages such as, flexibility, embeddability, multiplexity and EMI immunity, as compared with traditional sensors. Among the optical fiber sensors based on varied principles, fiber Bragg grating (FBG) sensors are the most promising candidates to effectively replace conventional strain gages for long-term monitoring applications in harsh circumstances.The main points of this dissertation are to present the design and performance of FBG sensors, including strain, temperature and displacement sensors, apply the FBG sensors to the model dynamic characteristics tests under simulated earthquake and introduce the applications of FBG sensors in structural health monitoring. The layout of the paper is as follows:The chapter 1 is the introduction of fiber optic sensor (FOS) technology, the research and application of FOS in structural health monitoring and the present status of fiber Bragg grating sensor technology. This chapter presents the sensing principle of fiber Bragg grating, summarizes the different type of fiber Bragg grating, lists the important parameters of fiber Bragg grating and introduces some kinds of sensors based in fiber Bragg gating such as the sensors of strain, temperature, displacement, pressure, accelerometer and shear force. In addition, this chapter illustrates the main research content of dissertation.The design and performance of FBG sensors, including strain, temperature and displacement sensors are presented in the chapter 2. The two kinds of FBG strain sensor, which is packaged by strainless steel tube and twocside-gripper respectively, are designed to measure the strain variation of structure. The strain trandferring characteristics of sensors are studied in detail and a packaging method of FBG strain sensor is put forward to realize the enhanced and reduced strain sensitivity. The FBG temperature sensors with the enhanced and non-enhanced sensitivity are invented to measure temperature variation. The calibration results provide the temperature characteristics of sensor and prove the reproducibility of packaging method. In addition, the indirect measuring sensor, which are pull-rod type FBG displacement sensor and FBG micro-displacement sensor, are developed to measure displacement with both large and small scale.The chapter 3 introduces the FBG and electronic sensor synchronous sampling system which includes a Si425 FBG demodulation device, a strain-to-voltage conversion unit and a PXI-1044 system. This synchronous sampling system in which the voltage sampling unit is embedded in the software of Si425 device, realizes the function of FBG and electronic sensor synchronous sampling and saving. The system is applied in the the vibration experiment of mansion model. The durability and reliability of the system are investigated in a series tests. In addition, these tests prove the performance of the short gage FBG strain sensor.The chapter 4 presents the applications of FBG sesnors in vibration experiment of submarine pipeline model, the roller compacted artificial concrete dams model and earthquake destructive test of the whole concrete dam model. The results the strain measured by FBG sensor agrees well with the electric resistance strain sensor and the theoretical result. The results prove this FBG strain sensor has the advantages of small size, high precision and flexible use, and demonstrates promising potentials.The chapter 5 introduces several engineering applications in which the FBG sensors served as key sensing element. A distributed FBG temperature sensor system was applied in temperature monitoring of ground heat pump system. The ultimate purposes of sensing system are to measure the water temperature fluctuation along the pipe at the places where the sensors were installed during the operation of the ground source heat pump system, and evaluate the reliability of FBG sensor system for a long period. The FBG sensors including strain and temperature sensor, also were applied to the health monitoring of the oil production offshore platforms. Strain responses induced by the impacts of ocean wave and a ship of hundred-ton's weight are monitored on-site successfully. In the monitoring, FBG sensors exhibit excellent performance and higher tolerance to harsh environment in the long-term real-time health monitoring of ocean offshore platform. Morever, a load monitoring system of ocean offshore platform was successfully developed to monitor the whole load variation of platform.In the chapter 6, an application of FBG sensor in medical engineering was conducted to mornitor the movement of tendon and ligament in different postures and locomotion.