Propagation Characteristics Of Piezoelectric Ultrasonic Guided Waves In Liquid-filled Layered Pipeline Structure

The pipeline structure has unique advantages in many respects on delivering liquid,gas,grout and so on.At present,the pipeline is widely used in oil,natural gas,urban water supply,heating,gas supply,sewage and other projects.In practical engineering,in order to strengthen the durability,the pipeline structure can usually set the protective layer in different ways,producing layered pipe structures.One way is setting the protective layer in the outer part of the pipeline structure,such as the external insulation layer and the protective layer;the other way is placing it in the internal structure,to prevent the transport of material from corrosion of the pipeline structure.The safety factor of the pipeline structure will decrease with the change of conditions such as the material being corroded,the internal and external protective layer aging and falling off,the pressure of the fluid gas or the residual stress in the tube,so that the application period of the composite pipe is greatly reduced.In recent years,domestic and foreign pipeline leakage accidents occur somewhere,not only causing serious pollution to the surrounding environment and immeasurable economic loss,but also affecting the normal production and taking a serious threat to the safety of people’s lives and property.Therefore,it is necessary to carry out proper damage identification and health monitoring for the layered liquid-filled pipeline structure.Since conventional detection method has the weakness of comprehensively monitoring a long distance and large range of pipeline structure,a novel nondestructive testing method which uses ultrasonic guided wave for damage detection based on piezoelectric ceramics is developed,and has been widely concerned.It is the most important scientific issue in this field to research ultrasonic guided wave dispersion curves and the propagation law of the pipeline structure,which is need to be resolved through the systematic study.In this dissertation,we combine theoretical analysis,experimental validation,and numerical simulation,to research the damage detection of healthy layered pipeline,the layered liquid-filled pipe,the layered pipeline with cracks,and the layered liquid-filled pipe with cracks.The purpose is to establish a set of health monitoring system for the damage assessment of layered pipeline structure with piezoelectric ultrasonic guided wave.The main content includes:First,the frequency dispersion equation of a layered pipeline in theory is established and numerically solved.The frequency dispersion equations of the ultrasonic guided wave in the structure of the layered hollow pipe and the layered liquid filled pipe are derived,respectively.The Matlab software is used to solve the dispersion equation of guided wave,then the group velocity and phase velocity dispersion curves of the required size and material is plotted.Then,the multiple modes of the propagation of guided waves in two kinds of pipes are analyzed,and the frequency dispersion curves of L(0,2)mode in the two pipelines are also plotted.The differences of frequency dispersion curves of two cases are compared,and the effect of water on frequency dispersion curve is analyzed.The principle of selecting the ultrasonic guided wave modes and excitation frequency which is suitable for the propagation of the layered pipeline and the layered liquid filled tube is proposed.The study results will provide a theoretical basis for the selection of the guided wave signal in the pipeline inspection.Secondly,the characteristics of the frequency dispersion curve of the ultrasonic guided wave in the layered pipeline are mainly studied.A large amount of analysis and research on single pipe and multiple tubes pipe show that the L(0,2)mode of ultrasonic guided wave dispersion curves in a laminar pipe are suitable for being used as the analysis mode.The empirical formula of the characteristic frequency and weak dispersion region of the layered pipeline is fitted.Then,the selection principle of excitation signal for damage detection in plastic composite pipe is proposed.It is found that the frequency dispersion group velocities of different material layered pipes are changed,but the initial frequency difference is not obvious.The initial frequency is associated with pipe size,but material parameters have little effect on it,and the general trend of the frequency dispersion curve is the same.Finally,according to the pulse echo method,a validation experiment was carried out for two kinds of pipes,which are the structure of the layered hollow pipe and the layered liquid filled pipeline and the crack.Before testing,through the analysis of the excitation signal,we find that the sinusoidal superimposed signal modulated by Harming window is suitable for the damage detection.The piezoelectric ceramic pieces are used as the excitation and receiving device of the signal to generate guided wave signal with L(0,1)mode,and a validation experiment was carried out on two kinds of pipes,which are the structure of the layered hollow pipe and the layered liquid filled pipeline and the crack.The measured experimental data is used to draw the frequency dispersion curve.The experimental and theoretical dispersion curves are compared and they match well which validates the correctness of the proposed frequency dispersion curve.The propagation of guided waves in these four kinds of pipes is analyzed and compared,and the relationship between the damage degree and the reflection coefficient is also analyzed.The results show that the greater the damage degree is,the greater the damage reflection coefficient is,and the two variables have a approximate linear relationship.