Functional Properties And Mechanism Of Polymer-matrix Carbon Fiber Smart Layer

The dissertation was supported by the project of National Science Foundation "The mechanism of compound sensitive surface and imaging method of stress monitoring for nuclear safety shells" (No.50878169).Aiming at the long-term health monitoring for large-scale structures, the research on the functional properties and its mechanism of the polymer-matrix carbon fiber smart layer was carried out. The study includes one-dimensional load sensitivity, two-dimensional load sensitivity, mechanism of load sensitivity, temperature sensitivity and its mechanism, and the related applicatons.The following conclusions and innovation can be obtained:Firstly, by constructing of unidirectional strain field, the responses of polymer-matrix carbon fiber smart layer to the longitudinal or the transverse unidirectional strain were obtained. The result indicated that each of the above strain can induce the resistance increase of the smart layer, and the piezoresistive coefficients were revealed by the further analyses. Based on the piezoresistive coefficients, the two-dimensional load sensitivity constitutive model was built up. The constitutive mode was modified by the result of off-axis experiment.Secondly, the laminated construction of polymer-matrix carbon fiber smart layer was built up. Two kind of contact interface, the overlapped contact interface and the crossed contact interface were researched. The high gage factors and favorable stabilities of the two contact interfaces were discovered, which revealed the mechanism of load sensitivity for polymer-matrix laminated carbon fiber smart layer. The electrical model was built up for the overlapped contact interface, and the mechanism of its sensitivity was discussed.Thirdly, the temperature sensitivity of epoxy matrix carbon fiber smart layer was revealed. The epoxy matrix carbon fiber smart layer exhibits NTC effect in-10℃～25℃and exhibits PTC effect in 25℃～50℃. Further experiment demonstrates its sensing properties in the temperature scale in which the NTC effect or PTC effect exhibits individually. The linear NTC effect for the single carbon fiber in the epoxy matrix and the non-linear PTC effect for the contact interface of carbon fiber in the epoxy matrix were revealed. Base on the two effects, the mechanism of the temperature sensitivity for the epoxy matrix carbon fiber smart layer was explained.The fourth, the damage sensitivity of the smart layer was revealed by the monitoring for a beam with prefabricated defect. Based on its damage sensitivities, the smart layer was effectively applied in the health monitoring for a concrete beam under three-point bending.Finally, the temperature effect on the deformation detection was clarified. The temperature change influences the deformation detection in two aspects:on one hand, the temperature sensitivity directly causes the resistance change; on the other hand, the temperature increase causes the gage factor increase. In additional, the temperature compensation was realized by the differential voltage method in the experiment.