Study On The Strain Sensing And Its Enlarging Technology Based On Fiber Grating

Fiber gratings have an extensive use in the fields of optical communication and optical sensing. As sensing cells, they can transduce physical quantities like strain, temperature, etc, having attractive merits such as being small and light, resistance to corrosion and immunity to electromagnetic interference, etc. Among fiber gratings, fiber Bragg grating(FBG) occupies the dominant position in the sensing field and now is studied focusing mainly on the demodulation technology and novel applications.This dissertation concerns on the FBG and its demodulation technology, the representative application in composite material engineering, and novel application in large-strain measuring. The main works are shown as follows:1.The thesis discusses the fiber grating's strain sensing ability and its application after the introduction of the normal strain measuring method. It also summarizes the large-strain measuring technology, especially the contemporary state-of-art of using fiber gratings. The project's contents and purpose are then determined.2.Detailed discussion is taken to the sensing model of FBG under uniform and nonuniform axial strain. The strain-transferring rule of FBG is analyzed numerically. The FBG demodulation methods are newly classified. A strain demodulation instrument based on the relative-matched method is made, which pays attention to its integration, digitization and automation, having a measuring range of 260ε, displaying resolution of 0.01εand repeatable accuracy of±4ε. As for the'two values'problem,'reference-signal-method'is proposed and achieved by LabVIEW.3.The thesis conducts typical researches in the composite material engineering using FBG sensing technology. Embedded FBGs are used to monitor the cure process of carbon fiber reinforced plastics, and the cure-induced strain's evolution and distribution rule through the thickness are studied. The composite beam specimens are employed in fatigue testing. The experimental results show the temperature increase is up to 2.3°C, keeping constantly of 1°C approximately, and the residual strains are 5ε～20ε. The responses of them under different loading frequencies and embedded thicknesses are studied emphatically. The optical low coherence reflectometry technology of measuring the FBG's axial nonuniform strain is studied, and is verified through double-cantilever-beam delamination experiments.4.Composed structure, cantilever-beam structure, basic-trapezoidal frame and changed-trapezoidal frame (folding-jib, multi folding-jib, arc, semi-circle and stepped frame) are proposed and used to sense large strain using FBGs. Preliminary analyses of basic-trapezoidal and changed-trapezoidal frame are taken through the finite element method analysis. Theoretical and experimental researches are conducted on the composed-structure and cantilever-beam-structure FBG sensors, the former with a strain sensitivity of 0.069 and a theoretical measurement range of 14.5%, the latter with a strain sensitivity of 0.00252 and an experimental measurement range of 50%.5.The thesis deduces the sensing model of the basic-trapezoidal-frame FBG strain sensor according to the'unit load method'. Installing ways of foot-inserting, horizontal-clamped and vertical-clamped are proposed, and the corresponding real FBG sensor specimens are home made. The experimental results show the agreement of the foot-inserting sensor with the model and the'five times'sensitivity-enhanced effect of the clamped than the model. All specimens have strain measuring range of more than 20%. The best sensing linearity is less than 0.9% with the hysterisis error less than 0.7% and the repeatability less than 0.4%, in the range of -10.8%~21.6%. FBG large-strain sensor based on folding-jib frame has been proved more excellent to sense large strain, with an experimental strain measurement range of 54.5%.The main creative points of the thesis are as follow:1.The'unit load method'is employed to deduce the model of the trapezoidal frame structure's sensing model, which is verified through FEM analysis and the optimal structure designing model of which is established also through FEM.2.Based on the sensing model, real trapezoidal-frame FBG large-strain sensor specimens are designed and made in ways of foot-inserting, horizontal-clamped and vertical-clamped, having the strain sensing range of more than 20%.3.When FBG sensing technology is used in the composite material engineering, creative experimental researches are taken on the fatigue performance of carbon-fiber-reinforced-plastic beam specimens, especially the response of the fatigue residual strain under the loading frequencies from 0.1Hz to 20Hz.4.The'reference-signal-method'is proposed to solve the'two values'problem in the FBG's relative-matched demodulation method, which is realized through LabVIEW programming.