Research On Optical Fiber Cable Used In Crack Monitoring Of Concrete Bridges

The crack is a concentrated reflection of concrete bridge damage to a certain extent.The occurrence of cracks is random.Once a concrete bridge cracks,it will inevitably cause irreparable loss.Therefore,research on concrete cracks monitoring becomes more and more important.Based on the principle of distributed optical fiber sensor,this paper introduces the present and future development of distributed optical fiber sensor both at home and abroad.The basic principle and performance index of distributed optical fiber sensing system based on optical time domain reflectometry(OTDR)are expounded in detail.The influence of various performance index parameters on OTDR test results and the direction of parameter optimization are analyzed.At the same time,a theoretical study is made on the test curve of OTDR,and some typical test phenomena are analyzed theoretically.Distributed optical fiber sensing system uses optical fiber as the original sensor and transmission medium.Since the fiber is comparatively vulnerable,it is not only easy to break during the construction,but also tend to be destroyed by the external environment during the later maintenance.All these will seriously influence the life and accuracy of the fiber.In the concrete bridge with the demand of longer service life,optical fiber is not suitable to be used as the sensing element.Therefore,the optical fiber cable is used to replace the traditional optical fiber as the sensing element to monitor the concrete bridge cracks.Due to the structure of the cable,when a bridge crack occurs,the force generated by cracks in the cable will be continuously weakened while transmitting into the interior optical fiber of the cable.Therefore,in this paper,finite element software(ANSYS)is used to establish the finite element model of stress transfer in optical fiber cable.Finite element analysis(FEM)is used to analyze the stress transfer characteristics of optical fiber cable,so as to select the suitable optical fiber cable for concrete bridge crack monitoring.When a concrete bridge crack occurs,the cracks will make the fiber optic cable that is laid on the surface of the bridge bend,which will lead to reinforcement of the loss of backscattered optical power at the curved section.This change will be reflected in the OTDR test curve.By observing the OTDR test curve,the position where the loss occurs(that is,the location of the crack in the bridge)can be monitored,thus realizing the damage identification and damage location of concrete bridge.Through the theoretical analysis of the relationship between the bending loss and the radius of curvature caused by the cable when the crack occurs,the relationship between the bending radius and the loss is simulated by using MATLAB.In practical application,the laying method of optical fiber cable has a great influence on the monitoring accuracy of concrete bridges.In this paper,the laying of optical fiber cable has been studied in depth.According to different laying methods,the cracks in concrete bridges are simulated through experiments to measure the bending loss of the optical fiber inside the optical fiber cable.Through the analysis of experimental data,appropriate laying method of cable can be selected.Finally,the experimental data is consist with the MATLAB simulation results,which verifies that the optical fiber cable can replace the optical fiber as the sensor to realize the monitoring of concrete bridge cracks.