Exploring effect of medium coupling on ultrasonic lamb waves in engineering structures and synthesised soft tissue-bone phantoms

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Exploring effect of medium coupling on ultrasonic lamb waves in engineering structures and synthesised soft tissue-bone phantoms

 

Author: Chen, Jiangang
Title: Exploring effect of medium coupling on ultrasonic lamb waves in engineering structures and synthesised soft tissue-bone phantoms
Degree: Ph.D.
Year: 2011
Subject: Hong Kong Polytechnic University -- Dissertations
Lamb waves
Department: Dept. of Mechanical Engineering
Pages: xxii, 199 p. : ill. ; 30 cm.
InnoPac Record: http://library.polyu.edu.hk/record=b2425056
URI: http://theses.lib.polyu.edu.hk/handle/200/6039
Abstract: Lamb waves, the modality of elastic waves propagating in thin plate/shell-like structures, have been the core of intensive researches towards developing various nondestructive evaluation (NDE) techniques for engineering structures and quantitative ultrasound (QUS) techniques for human bone assessment, by taking advantage of their superb capability of sensing material deterioration and geometric changes in the medium in which they are travelling. However, a number of engineering structures work in liquid environments, as typified by boat/submarine hulls, offshore platforms and petroleum pipelines; and on the other hand human bones are covered with soft tissues. These surrounding media such as fluid and soft tissues have very distinct elastic properties from those of engineering structures and bone structures principally accommodating wave propagation, leading to perceivable coupling effect and modulating wave propagation characteristics, to different extents for different modes. Without appropriate rectification of such a coupling effect, accuracy of NDE and QUS techniques can be impaired considerably. Therefore to calibrate and compensate for the coupling effects arising from surrounding media remains of significance but challenge. In this thesis work, the above-addressed coupling effect has been investigated and calibrated quantitatively. Calibration results have been used to compensate for medium coupling effects in engineering NDE and medical QUS. Investigations were divided into the following five parts: Firstly, analytical description of propagation characteristics of Lamb waves in a two-layer medium (a solid plate coupled with a layer of fluid) was briefed. The dispersion properties of different wave modes in this coupled medium were observed to be markedly different from those of their counterparts in a free solid plate. This observation entailed the necessity of quantitative interrogation and calibration of coupling effect of coupled medium on wave propagation. Secondly, the coupling effect of a fluid medium (pure water) on Lamb waves in engineering structures (typified by aluminium plates) was investigated through three-dimensional (3D) finite element (FE) simulation and experimental validation. With quantitative knowledge of the coupling effect from the surrounding fluid media, traditional Lamb-wave-based NDE approaches were rectified when used to evaluate a through-thickness hole and chemical corrosion in submerged aluminium plates, with the assistance of a probability-based diagnostic imaging technique. The identification results articulated the necessity of rectification and compensation for the medium coupling effect when applying Lamb-wave-based damage identification to structures with coupled media.
Thirdly, expanding the above study to a bio-medical application, the coupling effect arising from a layer of soft tissues on Lamb waves in mimicked human long bone was canvassed. A series of soft tissue-bone phantoms was fabricated, each consisting of a soft phase (mimicking coupled soft tissues) and hard phase (imitating bone). Ultrasonic tests were carried out on these phantoms in which the effect of soft tissues of different thicknesses and elastic properties on Lamb wave propagation was examined in a range reflecting normal to pathological conditions of soft tissues of human beings. Study conducted has established quantitative relationships between the manifestation of the captured ultrasound signal (e.g., changes in propagation velocity and signal magnitude) and diversity of coupled soft tissues. Fourthly, bearing in mind that real human long bones are tube-like structures, the coupling effect of soft tissues on propagation of cylindrical Lamb waves (the form of Lamb waves in tube-like structures) was evaluated using cylindrical bone phantoms made of acrylic tubes coupled with the soft phase layer. Such a way of modelling is deemed to offer closer similarity to real bone-soft tissue system. The propagation velocity and signal intensity of two fundamental cylindrical Lamb wave modes were interrogated and calibrated when the coupled soft medium had different thicknesses and elastic moduli. The results indicated more prominent coupling effect on the concerned waves in tubes than that in plate models. Monitoring of the healing processes of bone fracture is an important application of QUS-based bone assessment in the clinic. However during the practical application, the coupling effect from soft tissues is often ignored. Without differentiating the influence due to degradation of bone from that contributed by coupled soft tissues, the accuracy in determination of healing progress of bone fracture may be comprised. In this part, the fundamental Lamb modes in fractured bone phantoms at different healing stages were examined via 3D FE simulation and experimental validation. With the previously obtained quantitative calibration of the coupling effect of soft tissues on concerned wave modes, prediction of a particular healing stage of a mimicked fractured bone was conducted. The results indicated that the estimate of the healing progress of bone fracture based on propagation of Lamb waves might be inaccurate without compensation for the coupling effect of soft tissues. In summary, the coupling effects arising from surrounding fluid/soft tissues on propagation characteristics of Lamb waves were examined systematically in this thesis work via numerical simulation and experimental validation. The results revealed that there was prominent influence of the coupling medium on wave propagation, to different extents for different wave modes, indicating that calibration and rectification of such coupling effects are an essential factor that should be taken into account in both NDE applications and clinical QUS-based bone evaluation.

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