Research On Propagation Of Guided Waves In Complex Structures And Its Application For Damage Detection

Engineering structures are widely used in various fields of productionand life. In recent years; accidents about engineering structures emergeendlessly. Therefore, more and more eyes focus on their safety andreliability.However, Structural health monitoring technology can achieve thefunction of the real time, online and active surveillance which have brought aflurry of research. It’s known that based on the application of ultrasonicguided wave structural health monitoring technology mainly confined to theplate, tube and other simple structure, but less for the use of guidedwave-based damage detection of complex structure in engineering practice.Hence Research on propagation of guided waves in complex structures and itsapplication for damage has important practical significance.This article bases on the basic theory of guided waves, using the finiteelement simulation and experimental methods to research the propagationmechanism of guided wave in the complex structure (thick beams, pressurevessels and high-speed rail axle) and analyze the damage on the guided wavepropagation. On this basis, It carries on the research of the real damage(fatigue cracks) and the true condition state (solid-liquid coupling state), andmeanwhile introducesthe based damage identification methods. The paper firstly elaborates the basic theory of guided waves includingelastic waves in an infinite medium, the plane of the elastic half-spaceharmonic guided waves in infinite sheet and hollow cylindrical structureguided wave. Secondly, dispersion equation is deduced in free board andhollow cylindrical structure, and mean while guided wave dispersioncharacteristics analysis software is developed. In processing, the finiteelement simulation is introduced to do the numerical modeling of drivers,sensors and damage. Experimental platform is set up to introduce theexperiment signal processing methods.The paper describes the standardization process of excitation frequencyselection in guided wave-based structural health monitoring technology,totally considering a variety of factors on the impact of guided waveexcitation center frequency selection, such as selecting the appropriate waveguide mode, reducing the effects of dispersion, increasing the signalamplitude of the wave guide, improving the signal time domain resolution,restraining the local symmetry effects and considering the solid-liquidcoupling, etc. Based on the above analysis, the parameters of the exciteexcitation is optimizedBy introduction of the slot and fatigue crack damage in the thick beamstructure, research finds that the asymmetric type injury will cause the guidedwave mode conversion (S0mode and A0mode convert to each other) and thatthe symmetrical type injury does not cause the wave guidemode conversion.When injury angle is unchanged, the amplitude of the injury reflected wavesignal will amplify with the increase of damage and the arrival time also will lag. In addition, it takes the guided wave methods to do damage identificationfor the true fatigue crack and then to compare the experimental results withthe slot injury results.The paper strengthens the ability to research the guided wavepropagation mechanism of the pressure vessel and use shape similarprinciples to analyze the dispersion properties of the guided wave propagationin the pressure vessel. Finally, it verifies the correctness of the principles bycomparing the theoretical group velocity, the group velocity and theexperimental group velocity of finite element simulation. And paper alsoanalyzes the circumferential propagation and the axial propagation (includingtorsional mode, portrait mode and bending mode) in the pressure vessel.Furthermore, overall considering the mode excitation, the repeatability, thedispersion characteristics, the propagation velocity, the sensitivity of thedamage, and suppression of the guided wave energy leakage in the case ofsolid-liquid coupling, L (0,2) mode is selected as the final choice to dostructural damage monitoring.The finite element model of the pressure vesselis set up to do the numerical simulation of elastic guided wave propagation inthe structure as well as interaction between guided waves and structural holes.The contrast of experimental signal and finite element simulation signalachieves a good consistency between experimental results and numericalsimulation demerit, and verifies the correctness of the finite element model atlast.According to the guided wave propagation path in the pressure vesseland the use of the Tof, combining with the finite element simulation curve to analyze of the wave packet components of experimental signal as well as theformation reasons of the wave packet, the experimental results has betterconsistency with the theoretical analysis which proves the correctness of thetheoretical analysis and the feasibility of the guided wave-based structuralhealth monitoring technology applications in the nondestructive testing ofpressure vesselsFor variable cross-section thick-walled structure (high-speed rail axle),the conventional single illuminator and multiple receivers mode is no longerapplicable. Due to high-speed rail axle is axially symmetric structure, whenthe structure exists injury, this symmetrical environment will be destroyed andthe symmetric mode of the wave guide will convert the asymmetric mode.The excitation introduction of symmetrical arrangement drive in the structurewill suppresses the non-symmetric mode in the wave guide, and then onlyexcites a symmetrical longitudinal mode.When the symmetrical longitudinalmode acts with the injury, the bending mode forms. And this mode will causethe particle vibration along the circumferential direction. Then it is available to judge whether there is damage in the structure by obtaining the particlecircumferential vibration of the corresponding area in the structure. Thismethod is not the limit of axle wall thickness and variable cross-section,andgreatly improve the feasibility of the method in practical engineeringapplications, in addition, the method is very sensitive to slight injury and isrightly suitable to monitor initial crack in structure.