Research On Distributed Fiber Sensing Technology And Its Application To Civil Structural Health Monitoring

Health monitoring on civil structures is related to project quality and operation safety. Therefore, it is of striking economic and social benefits and has become the trend in large civil projects all over the world. In recent years, our country has witnessed the fast development in steel-concrete composite structures, especially in concrete-filled steel tube (CFST) structures, which are broadly applied in bridges, buildings, and underground works and so on. However, interfacial damages (disengaging) in CFST have seriously weakened structure quality and safety, thus they attract much attention in engineering field. Since the occurrence and development of disengaging-cracking is invisible and unpredictable, effective detection of it is not yet developed by now both at home and abroad. Drilling hole on tube wall and ultrasonic detection are two conventional detecting methods. The former one may cause damage on the tube wall, while the latter results in a qualitative evaluation by experiences rather than by quantitative detection; it is a kind of point-type detection, which means lots of practical operational work but limited range detected; it can only be applied during the construction period. As the focus of the frontier researches in intelligent material-structure field and on health monitoring of structures, optic-fiber sensing has attracted much attention with its unique features, and it has been applied to civil works (especially the bridges), mostly to the monitoring of strain and temperature etc. by the Fiber Bragg Grating. Nevertheless, there is little report about the research on distributed fiber sensing for monitoring of interfacial damages in large CFST structures. This paper is devoted to distributed fiber sensing for health monitoring of structures. The combination of theoretical analyses, numerical analyses, model and emulation tests, and practical application has been applied to study distributed fiber disengaging-crack sensing technology and monitoring system. As a practical case, its application in the Wu Gorge Yangtze River Bridge Project has been realized successfully. (1) For the first time, the core technology is presented, which includes the non-orthogonal sensing pattern of the direct microbending transduction mechanism between mechanical and optic effects as well as the combination and optimization of optic fibers, which have established the distributed fiber sensing technology and system for interfacial damage detection. The optic fiber is laid in non-orthogonal broken line against the crack. In case of disengaging (or cracking), microbending will be induced on fiber by composite force of pulling-shearing and thus leads to high power loss of guided light wave, which realizes direct modulation of the mechanical parameter(disengaging deformation) and optical parameter(optic loss) without any need of deformer. For optimum composite sensing fiber, different types of fibers are combined to attain high sensibility, reliability and large dynamic range. The monitoring system developed is able to realize large-scale, continuous, long-term and quantificational monitoring on damages. (2) For the first time, micromechanical theoretical model containing bi-surface for fiber-concrete complex and its nonlinear numerical analyses are presented. Mechanical model of the complex is established, and its basic equation and solution conditions are given. In addition, mechanical parameters on the interface are determined by tests, and the effects of friction on contact surface and that of rigid protecting structures on micromechanical field around fiber are taken into accounts. The nonlinear finite element algorithm, which is realized by augmented Lagrangian method, is given. Several analysis cases, such as fiber bearing pulling, fiber bearing shearing-pulling, and fiber with the protecting structure bearing shearing-pulling forces, are computed. Special micro-behaviors and relative rules of fiber and interface in the microbending area are obtained by numerical analyses: bi-surface separating, interface slippage (mainly on coating-concrete interface), extruded deformation of coating, stress concentration and breaking. It is concluded that coating improves dynamic range but greatly attenuates the transmission of stress and strain; that when distance between the fiber and the protecting structure is less, sensing accuracy will be influenced. By the criteria of ultimate strain, dynamic ranges of fiber of type I and II are computed, and the results fit that of tests well. This proves the correctness of the theoretical methods and providestheoretical means for mechanical analysis on optic fiber embedded in concrete. (3) New concept of mechanical-optic constitutive relation is presented. A digital mechanics-optics coupling test system is established to realize automatic gathering of data in disengaging sensing model test; large-scale prestrained CFST model experiments are carried out for disengaging sensing. Mechanical-optic constitutive relation and technical performance characteristics (disengaging resolution: 0.02mm; dynamic range: 4.8mm) of fiber sensing are acquired, based on which the algorithm for disengaging quantification is defined and special software for optic fiber monitoring system is prepared, which forms up the integral software-hardware system. (4) The contrast between fiber refinement and civil construction is remarkable, causing embedding technology of fiber a key problem for its practical application. For the first time, this paper has initiated a research using large-scale emulation tests with engineering machinery employed, and thus developed the embedment-protecting technique that was proved feasible in practical works. With the experiences from the practical application, effective measures to improve fiber survival probability are obtained. (5) The paper gives a detailed practical project case using obtained research results on health monitoring for Wu Gorge Yangtze River Bridge Project. A fiber sensing net is installed on 12 key monitoring areas and their disengaging situations are monitored. Large-scale disengaging is detected, and its values, locations, ranges and develop processes are determined. The results coincide with that of the ultrasonic detection qualitatively and in particular confirmed by "drilling hole" inspection. Monitoring results provide evidence for evaluation of project quality. Based on these data, treatment is performed to improve safety of bridge. It is for the first time that distributed fiber monitoring is successfully applied to bridge health monitoring both at home and abroad, and the first time that applied for damage detection in civil works too. Therefore, the works follow the technical development way of combination of multi-discipline, as well as the link of hi-tech fiber sensing and traditional civil engineering field, and progresses are made in aspects such as distributed sensing type based on non-orthogonal pattern of fibers and sensing fiber optimization and composition, the micromechanical model and theory on nonlinear analyses of fiber embedded in concrete, embedment-protecting technology and practical application and so on. Tangible benefits are attained on a key project in the Three Gorge Reservoir Area, and an integrated progress of "theory-test-project application" is fulfilled. It proves preliminarily the result of the study is scientific, advanced and applicable. It has developed the health monitoring technology in engineeringstructures and thus shows great academic value and practical importance.