Deformation Monitoring For Key Parts Of Bridge Based On High-Toughness Optical Fiber Sensing Technology

Bridges are the key components of traffic.If the key parts of the bridge failed,it will prone to be largely deformed,seriously damaged and even collapsed under the action of earthquake,typhoon,mudslide,mountain torrent and other disaster loads.Whole-life cycle monitoring for the key parts of the bridge is of great significance to post-disaster emergency rescue,disaster relief,post-disaster resettlement,and post-disaster reconstruction.Deflection is an important monitoring index.It is an urgent problem to accurately monitor the deformation of key parts of bridge under the limitation of low monitoring range and insufficient toughness of the existing sensors.This paper developed a high-toughness optical fiber sensing technology based on the long gauge strain by toughness improvement of sensors,improved inversion algorithm,finite element simulation and experimental verification.And the high-precision deformation monitoring of key parts of bridges was realized.The main research contents and innovations of this paper are as follows:(1)The sensing performance and toughness of the long gauge fiber Bragg grating(FBG)sensor encapsulated by basalt fibers were studied.By arranging different types of strain sensors on the surface of BFRP and CFRP laminates for multi-cycle loading tests,the strain sensitivity coefficient,strain sensing repeatability,measurement accuracy,range and deformation ability were studied.The research results showed that the long gauge FBG sensor encapsulated by basalt fiber had good linearity and repeatability on the surface of the FRP laminate,the measured strain sensitivity coefficients were all around 1.2 pm/μ?,and the error was less than 1 μ?.The maximum strains monitored by the fiber-encapsulated FBG sensors on the surfaces of CFRP and BFRP lamiates were 11691 μ? and 14094 μ? respectively.And the sensoring ability was near 10000 μ?.Therefore,the long gauge FBG sensors encapsulated by basalt fibers owned superior deformation ability and strength,and it worked robustly under large deformation.(2)Optimization and error analysis were carried out for the algorithm that can identify the deformation of the structure by long gauge strain measured by the sensors.In order to be more suitable for the engineering application,the improved conjugate beam theory based on long gauge length strain was re-optimized and improved.Calculation of variable gauge length distribution,variable section structure,and different types of beam structures were realized by the reduced input parameters.Accuracy and effectiveness of the algorithm were analyzed from the aspects of sensor number,structure span,and measurement error.It was found that the calculation error of the deduction result was stably within 1%,and a high theoretical accuracy was gained when more than9 long gauge FBG sensors were arranged along the bottom of the beam.(3)The sensor layout scheme of key parts of the bridge was studied by the assistance of finite element simulation.Five kinds of sensor layout schemes covering key parts were designed according to the simulated damage distribution of the test beam.The influence of sensor number and the length of the gauge unit on the inversion accuracy in the mid-span area was studied,and the beam structure sensor layout suggestions were given: The gauge length of the sensors in the midspan area should be shorter to ensure the number and the key area should be covered in fully distributed manner.The length of the sensor gauge units in the two end areas can be longer to cover the non-critical area in a quasi-distributed manner.(4)The RC beam with a total length of 6 m which was strengthened with externally bonded prestressed CFRP laminate was selected to conduct damage test.At the same time,the hightoughness of the afore-mentioned sensors,the accuracy of the deduction algorithm and the reliability of the layout plan were verified.By comparing the load-strain curves of the fiberencapsulated FBG sensor and the strain gauge,it was found that the strain gauge fractured and out of work due to crack development.In contrast,the fiber-encapsulated FBG sensor had better hightoughness,and the crack width development ban be monitored while stable measurement of the full loading cycle can be achieved.By comparing the load-strain curve and displacement monitoring results of the FBG sensors arranged on the concrete surface and the CFRP laminate surface,it was found that the monitoring accuracy for both of them were within 10%,and the absolute error value does not exceed 4 mm.In the working stage with cracks,the absolute error value of displacement monitoring does not exceed 0.5mm,and the relative error does not exceed 3%.Therefore,the hightoughness optical fiber sensing technology is suitable in engineering application.