Research On Characteristics Of Polymer-matrix Carbon Fiber Smart Stripes And Strain Monitoring

The dissertation came from the project of "The mechanism of compound sensitive surface and its imaging method based on stress monitoring of nuclear containment vessel"(NO.50878169) supported by NSFC (National Natural Science Foundation of China), aimed at condition monitoring of structures under active service. Comparing experiments were conducted to study the piezoresitivities of Polymer-matrix Carbon Fiber Smart Stripes (CFSS) in unidirectional strains and in plane strains respectively. Then the plane strain-states of a target structure were sensed by using the piezoresitivity of CFSS and strains monitoring at spots were reached. In order to realize structure health monitoring of the whole field, a kind of carbon fiber smart layer of ERT system based on neural network was developed.The main research contents and contributions are listed as follows:1. The piezoresistivities of CFSS respectively under unidirectional tension and unidirectional strains (that is the direction along the axis of the carbon fiber) were comparatively researched. Results show that the change of the measured resistance generated by the former is bigger when the axial strains of same values occur. Comparing the two different load conditions, the same axial strains cause the same geometric dimensional change, which is the main factor of resistance change. While in the load condition of unidirectional tension, transverse strains occur for Poisson effect exists. Though transverse strain is much smaller compared with axial strain, results show it still causes resistance change by changing the overlapping degree of adjacent carbon fibers.2. Comparing experiments were conducted under unidirectional strains along the axial and the transverse of carbon fibers respectively to study the piezoresistivity of CFSS. Results manifest the relative resistance changes caused by transverse strain is larger than that caused by axial strain with the same value strains, which means transverse strain can also be a main factor of causing the longitudinal resistance change by causing the change of contact resistance between adjacent carbon fibers. 3. CFSS distributed in two ways were stuck on a bending plate to detect strains based on the good piezoresistivity of CFSS. Monotonous tests were carried out to calibrate the surface piezoresistivity of the CFSS and cyclic tests were conducted to measure the strains of the structure using the constitutive relation of CFSS. Results demonstrate the experimental values by CFSS are fairly good with those by gage factors, therefore it is feasible to sense the strains by CFSS.4. A set of carbon fiber smart layer based on GA(Genetic Algorithm) in Electrical Resistance Tomography (ERT) was programmed. ERT system includes forward problem and inverse problem, and the forward problem was solved by using finite element method. The software Matlab was used to continually invoke the calculation of ANSYS, which was used as input vectors inGA(Genetic Algorithm). GA(Genetic Algorithm) was used to simulate ERT inverse problem for obtaining the resistivity distribution of the smart layer, which could reflect the real strains of the structure. Results indicate that it’s viable to apply the ERT inversion system developed in this paper into strain monitoring in complex strain states.5. From maxwell’s equations and variational principle, field theory has carried on the resistivity converted to vector field research, establish the resistivity response and structural strain field vector scalar field between the physical and mathematical model. Further theory under the complex stress function is studied by complex scalar field sensitive layer resistivity inversion method of solving strain vector field.