Design And Application Of Low-temperature Sensitive Long-gauge FBG Strain Sensor

With the rapid development of economy and society and the objective demand of urbanization in China,many structures need to be demolished or rebuilt.Traditional demolition only depends on the experience of the construction personnel,the safety factor is very low,and the orientation of the collapsed structure cannot be accurately controlled,the randomness is relatively large.Therefore,it is necessary to conduct structural health monitoring in the demolition phase of the project.In response to the actual needs of concrete structure monitoring,this paper developed a low-temperature sensitive long-gauge fiber grating strain sensor and successfully applied it to the monitoring of the concrete silo demolition project,and obtained a good monitoring result.The structure of this paper is as follows:The first chapter introduced the status quo of the demolition field and the major casualties caused by the demolition work,indicating that it is of great significance to carry out structural health monitoring in the demolition project.Then it elaborated the research status of domestic and foreign scholars in the field of long-gauge sensors,summarized the shortcomings of existing long-gauge sensors,and finally introduced the main research contents of this paper.The second chapter was mainly about the design of long-gauge fiber Bragg grating strain sensor and the study of the sensing performance.Firstly,the determination principle of the length of the long-gauge sensor was derived in detail theoretically.The determination of the longest gauge length was mainly based on the load distribution and the instability of the sensor under pressure,and the determination of the shortest gauge length was mainly determined by the maximum particle size of the coarse aggregate.Then,the packaging method and working principle of the long-gauge fiber Bragg grating strain sensor were introduced in detail.Finally,the sensing performance of the sensor was studied by experiments from three aspects: uniaxial stretching,bending and cyclic loading.The third chapter introduced the design of clamped concrete long-gauge fiber grating strain sensor.The formula of low temperature sensing mechanism was derived theoretically,and the temperature decoupling characteristic of the sensor was realized by selecting the suitable negative thermal expansion material and adjusting the length of the outer sleeve and inner sleeve.Then,the applicability of the theory of low temperature sensing mechanism and the reliability of package were verified by experiments.Finally,it was installed on the Changtai Bridge to test the effect of low temperature sensitivity,and compared with the vibrational chord strain gauge at the same position.Through the analysis of the monitoring data for five months,it can be concluded that the low-temperature sensitive concrete long-gauge FBG strain sensor can effectively eliminate the measurement error caused by the alternating temperature field,showing a good low-temperature sensitive effect.The fourth chapter was about the application of long-gauge fiber Bragg grating strain sensor in the demolition of concrete silos.Firstly,the monitoring scheme was introduced.Then,the finite element analysis software ABAQUS was used to establish the silo model,and the dynamic and static parameters of the silo were simulated under different opening rates.The monitoring results showed that long-gauge fiber grating strain sensors were stable,which can quickly and accurately reflect the strain changes of the structure.The actual frequency changes during the demolition process were accurately measured,which was basically consistent with the results obtained by finite element simulation and successfully made early warning of the collapse of the structure.