Research On Cable Tension Monitoring Technology Based On Pre-implanting FBG Strain Sensors Into Anchorage Zone

Cable-stayed bridges are the most widely used bridge structure as it has manyadvantages, such as large spanability, linear appearance, high economic benefit. As themain force bearing and transmission component, the cable tension is easily influencedby the stress distribution of the bridges and their health status. As the key index toreflect the running state of the cable, based on cable tension real-time monitoring, wecan not only get the stress distribution of the bridge, but also help grasp the recessionand failure situation of the cable itself.At present, there are two ways to measure cable tension: external sensor methodsand internal sensor methods. External sensor methods, such as the frequency method,the pressure ring method and magnetic method, have its principle limits, furtherresearch is needed to improve their measurement accuracy and dynamic performance.Internal sensor methods, such as the smart bars method and optic-fiber cable tensionmeter method, still have some limitations in collaborative variant, large strainmeasurement and low survival rate. Due to many advantages:high accuracy, gooddynamic performance, full integration between the sensors and cables, without addingexternal measurement components, et al, internal sensor methods represent the futuretrend in the field of bridge health monitoring.Relying on the support of National Science and Technology Support Program(2007BAE15B04)“Research and industrialization of smart cables for large spanbridges”，based on the mechanical analysis of cable structures, a novel cable tensionmonitoring method based on pre-planting FBG strain sensor into anchorage zone isproposed. The main research is shown in the following aspects:The three-dimensional finite element analytical model of cable anchoring area isestablished, and the stress distribution regularities of cable anchoring area is tentativeexplored. On the basis of finite element analysis, an analysis unit of steel wire andencapsulated materials is selected, the axial stress of steel wire in anchoring area istheoretical analyzed by utilizing elastic mechanics equations. Stress distribution formulaof steel wire when anchoring depth varies is deduced, and the axial stress distribution ofsteel wire in anchoring area is obtained by utilizing living example calculations, whichprovides basic foundation and support to cable tension monitoring program。By analyzing the strain distribution trend of cable anchoring zone, combining the optimum measurement interval of FBG, the implantation depth of FBG is determined.By analyzing the interface mechanical properties of anchoring area inside, the geometriccenter of anchoring material surrounded by steel wires is selected as implantationlocation of FBG. By analyzing the construction and performance features of FBG, cableanchor structural component characteristics and anchor production process, theimplantation scheme is determined. Finally the scheme of cable tension measurementbased on strip FBG sensors pre-implantation in anchoring area was proposed, and thefeasibility is verified by scale model experiments.The overall non-uniform gratings are simplified cascading multiple uniformsub-gratings, FBG reflected spectrum equation is derived under inhomogeneous straineffect. Three typical non-exponential decays at the depth of10cm,15cm,20cm aresimplified to linear inhomogeneous strain, and are loaded to FBG, the effects on theoutput spectrum of FBGs are simulated. The simulation results shows: FBG spectrumvaries degrees of broadening, fission and shift phenomenon at three typical depths,Central wavelength peaks are reduced to83.6%,93.3%and98.32%respectively;reflected spectrum of FBGs at the depth of10cm and15cm has emerged competition,peak wavelength difference between the two competition sidelobes are0.176nm,0.189nm respectively. Simulation results show that the conventional FBG strainsensor is not suitable for anchor axial strain measurement.According to the stress distribution pattern, structural characteristics, sensortechnology and anchor production technology, the implanting FBG strain sensors areproposed to meet five key performance requirements. The structure of FBG strainsensors packaged in thin-walled capillaries is proposed, and thin-walled capillaries canhelp isolate the transferred strain from packaging materials to FBG, and homogenize thenon uniform strain. The sensor samples are developed, and the mechanical and thermaltests are carried out, the test results show that the sensors are of good strain sensingperformance and thermal stability. Performance expectations are achieved.The test object on prototype cable is analyzed, and the test program is designed.In the cable production process, four FBG strain sensors are pre-implanted into twoanchors, the process validation tests and mechanical tensile tests are carried out. theprocess validation test results show: the process of implanting sensors only takes46minutes, has less impact on the existing cable stranding process; sensor assemblytechnology is effective, no epoxy outside. Mechanical tensile tests results show: duringthe tensile process, FBG spectrum occurs no broadening, fission sign, which can meet all the requirements of the prototype cable tests; The real strain distribution trends areobtained by fitting measured axial strains, which provide an accurate basis to determinethe location of FBG sensors; The relationship between FBG outputs and cable tensionare of good consistency, and are of good reproducibility and linearity, which proves thatthe tension measurement method is entirely feasible.