Propagation Characteristics Of Piezoelectric Ultrasonic Guided Waves In Layered Pipeline Structure

Pipeline transportation industry began to rapidly develop from the middle of the nineteenth century. Because of development of oil and gas resources and the increasing demand for energy market, it is becoming one of the five means of highway, railway, aviation, water and pipeline transportantion industries, playing important role in the economic construction and national defense industry. In practical engineering, in order to protect the pipeline structure, a certain thickness of the insulation and protection layer is usually arranged on the pipe outside, and the layered pipeline is directly placed in the underground or underwater. However, with the increase of the use time, the damage in the layered pipeline with a pipe insulation and protection may also appear, affecting the normal production and resulting in a waste of resources, economic losses, even causing huge losses to people's life and property safety. It is very necessary for the damage detection of layered pipe structure. However, because of the limitation of conventional detection methods, a new non-destructive inspection method based on ultrasonic guided wave examination by using PZT-based waves is gradually paid attention and developed. The characteristic analysis of pipeline structure dispersion curves and the wave prpogation is the key scientific issues to be solved in the field through systematic research.This dissertation focuses on researches of ultrasonic guided wave frequency dispersion curve characteristics of non-defective monolayer stell pipe and layered pipeline, respectively, through the theoretical analysis, experimental verification and numerical simulation, and the main contents are conluded as following:Firstly, the ultrasonic guided wave dispersion equation of the hollow pipeline structure and the hollow layered pipeline structure are derived with the help of the previous research results, and the Matlab software is used to solve the guided wave dispersion equation. The two kinds of group velocity and phase velocity dispersion curves for pipeline structures with the given material and geometrical parameters are plotted out. The dissertation also analyzes three guided waves modes:the longitudinal mode (L mode), the torsional mode (T mode) and flexural mode (F mode) for the two kinds of pipeline structures, the results show that the change of the dispersion curves for T mode and F mode are complex, and it is unsuitable for using as excitation signal. Meanwhile, two kinds of pipeline L (0,2) mode dispersion curves are analyzed which will provide a theoretical basis for the selection of signals during the damage detection of the pipeline.Secondly, two kinds of non-defective pipeline structure verification tests are performed. It is found that the superimposed sinusoidal signal which is modulated by the Hanning window is suitable for the excitation signal through analysis of the excitation signal before the experiment. Piezoelectric ceramic patches are used as both the signal emitting and receiving devices, respectively, to generate L(0,2) mode and L(0,1) mode guided waves for damage detection of the single steel pipeline and layered one. And the dispersion curves are plotted out according to experimental data, and the experimental dispersion curves are compared with the theoretical one and they matches well, therefore, the accuracy of dispersion curve drawing method is verified.Finally, ultrasonic guided wave dispersion curve characteristic in the single pipeline are studied. Taking the L(0,2) mode of the single pipeline ultrasonic guided wave dispersion curves as an example, numerical analysis for both the single pipeline (monolayer steel pipeline) and other material pipe is performed, and the fitting empirical formula for calculating the specific frequency and weak dispersion range is set up. Based on the analysis, the principle of selecting the excitation signal in the monolayer steel pipeline for damage detection by using ultrasonic guide wave detection technology is established. The results show that the specific frequency of the ultrasonic guided wave dispersion curve for a single layer pipeline with different materials is closely associated with the ration its modulus of elasticity and density. The specific frequency is becoming greater with the increase of the ratio.