Coaxial Cable Sensing System For Distributed Large Strain Monitoring Of Structures

Local large strain often occurs together with structural failure,such as buckling deformation of lifeline engineering(such as oil and gas pipeline,etc.)under the action of landslide,ground fracture and karst collapse,as well as interface cracking,slipping and stripping of heterogeneous materials or composite structures.Local large strain monitoring can not only control the safety of the structure in real time,but also scientifically understand and grasp the damage and collapse mechanisms of the structure.However,traditional strain sensing technologies,such as optical fiber sensors,resistance strain gauges,etc.,have a small measurement range owing to their limited deformability,which is difficult to meet needs of structural large strain monitoring.At present,there is a continuing need for developing new sensors to address the challenges of the entire process of strain monitoring.In view of this,this dissertation focuses on the development and application of structural large strain sensor.Due to high ductility coaxial cable(strain up to 15%)and optical fiber adhere to the same electromagnetic theory,many optical sensing techniques can also be implemented on coaxial cables,thus developing coaxial cable-based large strain sensor.Inspired by this,the sensing mechanism of Fabry-Perot interferometer(FPI)in optical fiber sensors has been transplanted to the coaxial cable,and has developed coaxial cable Fabry-Perot interferometer(CC-FPI)large strain sensor.On the basis of the relative studies on CC-FPI sensor,its distributed strain sensing mechanism,design and fabrication method,performance test,related testing techniques,and the applications of CC-FPI sensor in large strain measurement are systematically studied in this thesis.The main contents are as follows:Firstly,the sensing mechanism of CC-FPI sensor has been studied by the electromagnetic wave interference theory,and the signal processing method for distributed strain sensing of CCFPI sensor has also been proposed.Through simulation,the characteristics of interference spectrum,spatial resolution,and the impact of manufacturing parameters on the signal quality and sensing performance have been discussed.Secondly,three fabrication methods of CC-FPI sensor have been developed;the distributed strain measurement platform of CC-FPI sensor is built based on the virtual instrument technology and vector network analyzer.On this basis,the strain and temperature sensing performances of CC-FPI sensors with different fabrication methods,and CC-FPI sensors fabricated by different types of coaxial cables were tested and analyzed.Thirdly,based on the advantage of CC-FPI sensor that any two reflection points can reconstruct multi-scale measurement gauge length,an adaptive gauge length approach for damage measurement using CC-FPI sensor has been developed.It includes the damage characterization with adaptive gauge length measurements,and gauge length reconstruction algorithm of CC-FPI sensor.Furthermore,an experiment with damages on a cantilever beam was carried out to verify that the adaptive gauge length measurement methods using CC-FPI sensor can characterize the structural damage accurately.Fourthly,considering that the CCFPI sensor and the optical fiber sensor are complementary in strain measurement,the former has a high-measuring range with low accuracy,whereas the latter has a high accuracy but a small-measuring range,an opticalelectrical co-sensing tape(OECST)has been developed.The OEC ST is capable of simultaneously providing accurate lower-range strain measurements and the higher-range rough strain measurements on one single tape,it can be used to monitor structures whose deformations exhibit discontinuous characteristics.The conceptual and geometric design,fabrication method,sensing mechanism,and sensing performance test have been discussed.The effectiveness and the reliability of OECST for discontinuous deformation monitoring have been verified through a shield tunnel segment model test.Finally,structures with large strain monitoring needs have been selected as research objects,and three kinds of engineered CC-FPI large strain sensing probes have been developed,including CC-FPI smart steel strand,CC-FPI sensing probe for RC beam and CC-FPI sensing probe for buried pipe crossing fault.In research and development process of these CC-FPI large strain sensing probes,the sensor selection,design,manufacturing and packaging technology,and the structural characteristics were discussed.The sensing probes have been respectively used to monitor the strain of steel strand,the strain of RC beam covering its entire process of failure,and the strain of buried pipe crossing fault covering its entire process of deformation.Moreover,the applicability and the effectiveness of CC-FPI large strain sensing probes have been verified by a series of model tests.