Research On Application Of Distributed Optical Fiber Sensor In Structural Strain And Cracking Monitoring

Distributed optical fiber sensor based on brilliouin scattering(BOTDA/R) has been imported into China for nearly ten years, it has been adopted in several projects by other researchers; meanwhile some remarkable achievements are reported in literature. However, the embedding method of distributed optical fiber sensor plays as a key technology while applying BOTDA in on-site project, because several factors such as fiber rupture, obvious bending would induce the failure of sensing route. Speically for the large-scale foundation engineering under constructing, the optical fiber embedded on the surface of structure would be destroyed by other cross activities. Embedding the optical fiber into concrete not only provides a safe circumstance but also record the inner strain field. BOTDA is a kind of distributed sensing technology; therefore reasonable sensing net design would achieve full-scale monitoring of structure. On the other hand, structural local performance like cracking is another significant factor affects the safety, serviceability, durability. Cracking monitoring based on BOTDA would play as an important complementarity for structural health monitoring.The main research contents in this thesis are as following:1. A new optical fiber sensor embedded method based on air-blowing and vacuum grouting is introduced which can lay the long distance distributed optical fiber (LEOFS) into the structure in a short time. The monitoring sensor nets are firstly designed to meet the objectives of structural health monitoring. Then tubes are fixed to formwork or steel bars before concrete casting. Finally air-blowing and vacuum grouting techniques are used for laying the distributed optical fibers and fixing them inside the tubes to form the LEOFS. However, special techniques are required when applying air-blowing technique to install distributed optical fiber sensors in civil engineering. The resistance like loss of air pressure and unevenness between tubes caused by the connectors will increase the difficulty of air-blowing. Therefore simulation experiment is carried out to estimate the maximum length for the LEOFS’s installation.2. Accuracy of strain measurement is an important issue to ensure the efficiency of structural diagnosing and evaluation. Test accuracy not only depends on the performance of equipment, but also influences by strain transferring capability of distributed optical fiber sensor which used to receive structural strain signal. The performance of strain transferring is mainly affected by the characteristics of optical fiber, physical properties of binder and configuration of sensor. The influence of above parameters to strain measuring sensitivity is studied by theoretical analysis, and their quantitative relation is obtained by calibration experiments. The results of theoretical and experimental analysis can be used for guiding optimization design of distributed optical fiber sensor.3. Based on tunnel health monitoring project, the application of LEOFS has be put into practice. The long distance distributed optical fibers are embedded into concrete of secondary lining by air-blowing and vacuum grouting. In order to achieve life-cycle health monitoring, the initial stress field and time-varying stress field are established respectively of secondary lining established by using traditional sensors and finite element analysis, and by monitoring strain of distributed optical fibers. The long-term true stress field which used to diagnose secondary lining is calculated by adding initial stress field to time-varying stress field. At last, a structural health monitoring software based on Labview and optical switch is developed to deal with monitored data.4. Security detection has always been the most complicated but significant task in the preservation of cultural relics. To ensure the structural safety of a canoe with thousands of year’s history during its excavation, a whole-process monitoring method based on the technique of distributed optical fiber sensor is presented. Considering the situation of the project, the layout of optical fiber sensor is determined. By analyzing the monitoring data, the influences have been evaluated in the process of reinforcement and reversal of the canoe. Monitoring results show that the distributed optical fiber sensing technology can effectively capture the changes of structural response in each process. And the analysis results show that the hull is still in safety under the cultural relic’s protection. The effectiveness of this technique is also proved by practical application in construction.5. Due to the change of structural function, one teaching building had to be retrofitted to a large-space office building. The underpinning technology had been adopted to transform this brick-concrete structure to a frame structure. In order to ensure its safety operation, fiber Bragg grating technology (FBG) and distributed optical fiber sensor (BOTDA) had been used to monitor structural stress and strain during the whole period of construction. The test results shows that the designed underpin system performances well and can provide sufficient bearing capacity for this new retrofitted structure.6. Cracking monitoring provides an important way to evaluate structural safety. Cracking monitoring method based on distributed optical fiber sensor can effectively avoid undetected phenomenon existed in point-wise test method, and it can run automatically. A theoretical model of cracking monitoring based on obliquely intersected fibers and Brillouin Optical Time Domain Analysis (BOTDA) is introduced, and the numerical equation among fiber strain, crack width and crack angle is established by the calibration experiment. The experiments are also carried out to study the performance of different kinds of cracking sensors with different gauge length and pretension. The cracking simulation test proved that this new method can effectively record the width and the angle of cracks during the whole cracking process. Finally, the feasibility of corrosion monitoring based on BOTDA is considered by accelerated corrosion test.7. In this theis, several on-site projects and in-door experiments based on BOTDA are presented. The summarization of this research would figure out some key technology while applying BOTDA.