Research On Structural Damage Monitoring Technology Based On Spectral Characteristics Of Fiber Bragg Grating

Much progress has been made in strucrural heath mornitoring using FBG sensor technology. It is one of effective structural health mornitoring techniques by measuring the shift of central wavelength. However, for subregion of some structure, such as repaired region of repaired structure, joint region of composite joint structure and so on, is subjected non-uniform strain. In such circumstances, the spectrum shape of FBG will be not the same as before, and we cannot identify damages directly by examine the shift of central wavelength. In this paper, spectral characteristics of fiber Bragg grating under non-uniform strain is researched, which is applied to typical damages monitoring for aerospace materials.The main works done in this dissertation include:Firstly, it was summarized on research background, present situation, significance of SHM and fiber Bragg grating sensing technology. Then, the researching contents were proposed.Secondly, starting form the transimission theory of FBG, strain sensing mechanism is derived strictly. On this basis, using Runge-Kutta method and transfer matrix method, the numerical simulation of reflection spectrum is studied when a non-uniform strain is subjected to FBG, then the advantages and disadvantages were analyzed. At last, Methods that have existed are optimized and experiment is designed to validate the veracity and feasibility.Thirdly, fiber Bragg grating sensor is used to monitor the fatigue crack propagation of aluminum alloy specimen with pre-crack under cyclic loading. The reflection spectra of FBG for different crack lengths have been calculated with the aid of a finite-element model of the strain field resulting from a crack, and compared with the results of the experiment.Fourthly, fiber Bragg grating sensor is used to monitor the propagation of crack beneath the repaired patch under cyclic loading condition. The reflection spectra of FBG for different crack lengths have been calculated with the aid of a finite-element model of the strain field resulting from a crack. Then, comparison between the theoretical result and the experimental result was done.Fifthly, taking carbon fiberπ-joint structure for research subject, a kind real-time online monitoring system using fiber Bragg grating sensor was studied. Firstly, based on the general FEM software, the 3-D finite element model of compositeπ-joint is established, the failure process and every layer failure load of compositeπ-joint were investigated by maximum stress criteria. Then, strain distributions belong the length of fiber Bragg grating were extracted, the reflection spectrum of fiber Bragg grating have been calculated according the strain distribution. Finally, to verify the numerical results, a test scheme was performed and the experimental spectrums of fiber Bragg grating were recorded. The experimental results indicate that the failure sequence and the corresponding loads were consistent with numerical predictions.Sixthly, a real-time monitoring system based on the fiber Bragg grating sensors is set up in this paper. Support vector machine is applied to detect the impact damage for the carbon fiber composite laminated plates, and compared with the results from traditional BP neural network.Lastly, summarize the contents and the achievements of this paper, then pointed out the further research direction.