Biomedical subjects

Hak-Joon Kim

Publications and source records attributed to Hak-Joon Kim.

5 recordsLinked to original sources

PubMed2006

An ultrasonic measurement model using a multi-Gaussian beam model for a rectangular transducer.

To date, ultrasonic measurement models have primarily treated systems where circular transducers are used. Recently, however, a highly efficient ultrasonic beam model for a rectangular transducer has also become available where the transducer is represented as a superposition of a relatively few Gaussian beams. Thus, using the multi-Gaussian beams, we developed ultrasonic measurement models for systems where a rectangular transducer is employed. In this paper, we describe the developed models including the beam model, the efficiency factor for a rectangular transducer and far-field scattering models for some standard scatterers. Furthermore, the accuracy of the proposed model is verified by the comparison of the model-based predictions to the experimental measurements.

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PubMed2006

Characterization of TiN coating layers using ultrasonic backward radiation.

Since ceramic layers coated on machinery components inevitably experience the changes in their properties it is necessary to evaluate the characteristics of ceramic coating layers nondestructively for the reliable use of coated components and the remaining life prediction. To address such a need, in the present study, the ultrasonic backward radiation technique is applied to examine the very thin TiN ceramic layers coated on AISI 1045 steel or austenitic 304 steel substrate. Specifically, the ultrasonic backward radiation profiles have been measured with variations in specimen preparation conditions such as coating layer thickness and sliding loading. In the experiments performed in the current study, the peak angle and the peak amplitude of ultrasonic backward radiation profile varied sensitively according to two specimen preparation conditions. In fact, this result demonstrates a high possibility of the ultrasonic backward radiation as an effective tool for the nondestructive characterization of the TiN ceramic coating layers even in such a thin regime.

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PubMed2006

Evaluation of CVD diamond coating layer using leaky Rayleigh wave.

In the present study, the possibility of using leaky Rayleigh waves as a nondestructive tool for the evaluation of CVD diamond coating layer is explored experimentally. For this purpose, a set of CVD diamond coated specimens are prepared and the leaky Rayleigh waves are measured in an immersion, pulse-echo setup. For the proper analysis of the acquired signals we propose a novel signal analysis approach, namely the "time trace angular scan (TTAS)" image. Then, the proposed approach together with the backward radiation profiles are applied for the analysis of signals acquired in the initial experiments. The TTAS image shows the entire information on both time-of-arrival and angle of incidence of the signals for the proper "time-angle windowing." Then, the backward radiation profile of the windowed signals provides adequate parameters from which nondestructive evaluation of the coated specimens is carried out.

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PubMed2006

Generation of the basis sets for multi-Gaussian ultrasonic beam models--an overview.

By using a small number of Gaussian basis functions, one can synthesize the wave fields radiated from planar and focused piston transducers in the form of a superposition of Gaussian beams. Since Gaussian beams can be transmitted through complex geometries and media, such multi-Gaussian beam models have become powerful simulation tools. In previous studies the basis function expansion coefficients of multi-Gaussian beam models have been obtained by both spatial domain and k-space domain methods. Here, we will give an overview of these two methods and relate their expansion coefficients. We will demonstrate that the expansion coefficients that have been optimized for circular piston transducers can also be used to generate improved field simulations for rectangular probes. It will also be shown that because Gaussian beams are only approximate (paraxial) solutions to the wave equation, a multi-Gaussian beam model is ultimately limited in the accuracy it can obtain in the very near field.

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PubMed2004

Comparison of modeling approaches to ultrasonic testing at near critical angles.

Modeling of ultrasonic testing has been paid a great attention in nondestructive evaluation community recently since it can provide thorough understanding of underlying physics of ultrasonic testing. As a result, there have been developed various modeling approaches up to now. Especially, many practical models have been developed based on either the multi-Gaussian beam or the Rayleigh-Sommerfeld integral. This paper discusses the modeling of ultrasonic testing with oblique incidence at the near critical angles using these two approaches. The theoretical models that can predict the reflection signals from side drilled cylindrical holes in solid specimen immersed in water are developed. Then, the theoretical predictions for the oblique incidence at the near critical angles are compared to the experiments for the investigation of model behavior.

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