| This dissertation focuses on the application of PZT-based ultrasonic guided waves time reversal method (TRM) which is used in pipeline structure health monitoring. Compared with the traditional pipeline conventional ultrasonic nondestructive testing process, the developed TRM can solve such problems which traditional method can’t solved as the defocusing effect at the focal point, the aberrations formation, and image distortion and blurring. TRM of ultrasonic guided waves for structural health monitoring (SHM) can focus or reconstruct complex signals. The more sensors work, the more obvious the focusing effect would be; the higher the degree of the signal complexity, the greater the contrast of the signal focusing. Fluid-filled and buried in soil pipelines are theoretically and experimentally researched for the first time in China and reported in this dissertation. A fast and accurate testing system is developed which has important practical significance for accurate determination of the pipeline structure integrity and pipeline monitoring. The main research works are given as follows.Firstly, the time reversal focusing equations for ultrasonic guided waves are derived and experimentally validated according to the basic principle of the time reversal process. An experimental research is performed to validate the time reversal focusing process for signals excited and propagated in an aluminum plate placing a single or multiple piezoelectric transducers, and the time reversal focusing of the narrowband ultrasonic guided wave signal is realized which lay the foundation for the follow-up experiments. The results show that the focusing signal is divided into two parts. One part is the main peak signal at zero moment which is the focusing of the multiple monitoring signal modes and is also the focusing of various monitoring signals, resulting in a significant increase in the amplitude. The other part is the side lobe peaks with the various appearing moment. The side lobe peaks will not superimpose among them or even sometimes offset each other, and meanwhile the main peak signal will be significantly prominent.Secondly, the excitation frequency rules for ultrasonic guided wave modes are studied to determine a reasonable choice in the pipeline transmission, which can minimize the frequency dispersion and simplify the complexity of the analysis on pipeline ultrasonic echo signals. The guided wave mode and the excitation frequency are determined by the characteristic and reference displacement of dispersion curves and the pipeline guided wave modal dispersion curves. The cutoff frequency is calculated from dispersion curves to determine the excitation frequency range of blind, and the period of the excitation signal is determined in accordance with the width of the frequency spectrum of the signal and the excitation pulse number. The narrowband excitation signals, which increase the signal strength and the propagation distance of guided waves, can be made by the plurality of single tone superimposed signals modulated by HANNING window. The results show that the signal of a high number of cycles is usually of the narrow band and conducive to the frequency control, more number of cycles, and long signal duration in time, and the waveforms are easily superimposed. Generally, a compromise method is selected.Thirdly, numerical simulations are performed respectively for the cases of a non-destructive pipeline, circumferential crack pipes, axial crack pipe and the oblique crack pipes from different angles, according to the pulse-echo method. L(0,2) mode and T(0,1) mode are given as the visualization results of the guided waves propagation, and the guided wave propagation in the health or damaged tubes is analyzed respectively. A variety of damage signals are processed and compared in time reversal method. The simulation results of the cracked pipelines show that guided waves would not only reflect and transmit but also convert mode at the defective position, and the converting mode are usually multi-bending. As the energy of the emitted guided waves is considerable, the amplitude of the flaw echo dealt with TRM is obviously greater than that obtained by the direct method; therefore, the capacity of small defect detection is enhanced The mode conversion phenomenon caused by defects and the access point rate for time reversal signals are the main factors determining the time reversal focusing efficiency and enhancing the defect detection capability.Fourthly, the experimental research on the defect detecting in the pipeline by using PZT-based ultrasonic guided waves is performed. The guided waves with L(0,2) and T(0,1) modes are excited and used in the tube to monitor the presence of a circumferential crack, axial cracks and inclined crack detection respectively, to determine the defect location and signal mode form sensitive to defects. The experimental results show that the method for separately emitting signals by TRM and then superimposing the signals can enhance the monitoring capacity by using the PZT-based guided waves, and this process is named as approximate time reversal detection. After processing the signals in time reversal method, the signals’reflectivity to all of circumferential cracks, axial cracks, or diagonal crack defects increase more greatly than the method of direct guided wave detect dose, indicating that the TRM method is conducive to detecting small defects. Meanwhile, many other modal wave packets appear and interfere with the signal discrimination. Under the same condition of pipe parameters, the more serious modal conversion phenomenon, the greater extent to improve the detection capabilities of the original guided wave defect detection. This is consistent with the numerical simulation analysis results.Fifthly, the dispersion equations for the liquid filled and buried pipe are derived, and the different modal dispersion and attenuation characteristics in the structure are calculated. With the increase of the frequency, the modal attenuation value also increases correspondingly, and the guide wavelength becomes shorter which is more sensitive to defects. Since the attenuation increases with the frequency increasing, the detection distance will also be shortened. Therefore, it is advisable to choose the proper frequency and moderate attenuation longitudinal mode state for detecting different kinds of defects in such pipelines according to the practical engineering requirement of the pipeline motoring. An experimental system is setup for the analysis on the propagation of the guided wave state in the liquid-filled underground pipeline. The experimental research for the liquid-filled underground pipeline is performed and the time reversal method is applied in the structural health monitoring, and the theoretical results are validated by the proposed experiment. |
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