Analysis of the coupling effect on ultrasonic lamb wave propagation in the coupled medium

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Analysis of the coupling effect on ultrasonic lamb wave propagation in the coupled medium

 

Author: Liu, Mingran
Title: Analysis of the coupling effect on ultrasonic lamb wave propagation in the coupled medium
Degree: M.Sc.
Year: 2013
Subject: Lamb waves.
Ultrasonic waves.
Hong Kong Polytechnic University -- Dissertations
Department: Faculty of Engineering
Pages: xiv, 80 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2576819
URI: http://theses.lib.polyu.edu.hk/handle/200/7321
Abstract: Nondestructive evaluation (NDE) techniques have been widely used to avoid further failure in a structure over a long period of time. Quantitative ultrasound (QUS) is applied to assist in the diagnosis of certain bone-related diseases. Lamb waves, extensively used as guided waves in many NDE applications and clinical QUS, have been attracting great attention because of their properties such as low attenuation when propagated in a structure, fast propagation velocity and high sensitivity to the change of the propagation media. Although the application of Lamb waves in NDE applications and clinical QUS has been confirmed and validated, some aspects which may influence their results such as the coupling effect should also be of concern. The coupling effect on the propagation of Lamb waves in coupled medium influences the precision and accuracy of many studies and applications such as QUS-based bone assessment. In this thesis, the existing studies on coupling effect are reviewed first. The coupling effect has been of concern for a long time, and to this day the results of existing studies have achieved compensation for the coupling effect in engineering NDE and medical QUS. However, when the coupling effect of coupled media has been investigated, transducers were commonly used for transmitting and receiving the Lamb waves and the integratedm analysis of the influences of the transducerspositions was not focused on or described. It is necessary to investigate whether the variation of the transducerspositions between the coupled layers have influences on the propagation of ultrasonic Lamb wave signals.
Allowing for the preliminary review, this study consisted of the following two primary sections. Firstly, Lamb waves were investigated. The propagation characteristics of Lamb waves in a free solid plate and in coupled medium are both described since they are influenced by the coupling effect during propagation through the medium. Secondly, an experimental system was designed and built which was prepared for experimental validation of the theoretical investigation into the influences of the different transducerspositions on wave propagation. In the experiment of this thesis, the main target was to investigate the influence of the coupled layer of a proper thickness on wave propagation. Whether there are differences caused to the wave propagation between the transducers located atop the coupled layer and those at the interface of the coupled media was investigated under the specific and practical situation. Therefore, five sets of the different positions of the transducers were designed and investigated. The transmitter and the receiver were located at the interface of the coupled media or atop the coupled layer, respectively. By changing the positions of transducers, the experiments were performed and the resulting signals captured. By calibrating and analyzing the five sets of experimental results, the main conclusion was drawn and is described below. With the energy of Lamb waves obviously dispersing and attenuating, the influences of the various positions of the transducers (atop the free plate, atop the coupled medium and at the interface of the coupled medium) on the velocity of ultrasonic waves could be ignored when the wavelength of Lamb waves was in order of the thickness of the propagation media.

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