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Dive into the research topics where Yong Sang Cho is active.

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Featured researches published by Yong Sang Cho.


Key Engineering Materials | 2006

Evaluation of Hidden Corrosion in a Thin Plate Using a Non-Contact Guided Wave Technique

Ik-Keun Park; Tae Hyung Kim; Hyun Mook Kim; Yong Kwon Kim; Yong Sang Cho; Won Joon Song

In this paper, study on evaluation of thickness reduction in a thin plate with guided waves is presented. Ultrasonic guided wave techniques have been widely studied and successfully applied to various non-destructive tests with the advantage of long range inspection. In addition to application of guided waves to NDT, non-contact methods for ultrasonic wave generation and detection have become very useful and well combined with guided wave techniques due to their capability of ultrasonic wave generation and reception in surface of high temperature or on rough surface. An advanced non-contact technique for detection of thickness reduction simulating hidden corrosion in thin plates using guided waves is proposed. The proposed approach uses EMAT(Electro-Magnetic Acoustic Transducer) for the non-contact generation and detection of guided waves in aluminum plates. Interesting features of the dispersive behavior in selected wave modes are used to detect plate thinning. The experimental results show that the mode cutoff measurements provide a qualitative measurement of thinning detects and change in the mode group velocity can be used as quantitative parameter of thinning depth measurement.


Key Engineering Materials | 2006

Non-Contact Ultrasonic Inspection Technology of Fillet Weldments Monitoring

Ik-Keun Park; Hyun Mook Kim; Tae-Sung Park; Yong Kwon Kim; Yong Sang Cho; Won Joon Song

Its not easy to detect the defects in fillet weldment which is widely used in various building structures and power plants just with nondestructive inspection due to its complex geometrical shape and difficult access. But its easy to detect the cracks on the surface or just below the surface of fillet weldment heel part if surface SH-wave, among ultrasonic wave modes, is applied. The traditional ultrasound inspection using surface SH-wave is usually a contact method using piezoelectric transducer, so its not suitable for a field application because the reliability of inspection varies depending on field environments such as couplant, contact pressure and pre-process, etc. Therefore, the necessity for non-contact ultrasound inspection is increasing. This study proposes non-contact ultrasound inspection method using EMAT (electro-magnetic acoustic transducer), and presented non-contact ultrasound inspection method for fillet weldment through experimental verification.


Key Engineering Materials | 2006

Ultrasonic Guided-Wave Mode Identification in Pipe by Using Air-Coupled Transducer and Time-Frequency Analysis

Ik-Keun Park; Hyun Mook Kim; Yong Kwon Kim; Yong Sang Cho

For efficient NDE of pipes, essential components of power plant facilities, ultrasonic guided waves were generated and received applying an air-coupled transducer and comb one as non-contact technology. Mode generation and selection were predicted based on theoretical dispersive curve and the element space of a comb transducer. In addition, a receiving angle of the air-coupled transducer was determined to acquire the predicted modes by theoretical phase velocity of each mode. Theoretical dispersive curve was compared with the results of the time-frequency spectroscopes based on the wavelet transform and 2D-FFT to identify the characteristics of the received mode. The received modes show a good agreement with the predicted ones.


Key Engineering Materials | 2006

Long Range Ultrasonic Guided Wave Technique for Inspection of Pipes

Ik-Keun Park; Yong Kwon Kim; Hyun Mook Kim; Won Joon Song; Yong Sang Cho

Conventional non-destructive techniques for inspection of weld in pipelines require significant test time and high cost. In this paper, a study in the application of ultrasonic guided waves to long range inspection of the pipeline is presented. The characteristics and setup of a long range guided wave inspection system and experimental results in pipes of various diameters are introduced. The experimental results in mock-up pipes with cluster type defects show that the limit of detectable wall thickness reduction with this guided wave system is 2~3% in the pipe cross section area and the wall thickness reduction of 5% in cross section area can be detected when actual detection level is used. Therefore, the applicability of the ultrasonic guided wave technique to long range pipeline inspection for wall thickness reduction is verified.


Key Engineering Materials | 2006

A Non-Contact Guided Wave Technique for Defect Thinning Monitoring

Ik-Keun Park; Yong Kwon Kim; Tae Hyung Kim; Yong Sang Cho

This paper capitalizes on recent advances in the area of non-contact ultrasonic guided wave techniques. The present technique provides a decent method for nondestructive testing of defect thinning simulating a hidden corrosion or FAC(Flow Accelerated Corrosion) in a thin aluminum plate. The proposed approach is based on using EMAT(Electro-magnetic Acoustic Transducer) to generate guided waves and detect the wall thinning without any coupling. Interesting features in the dispersive behavior of selected guided modes are used for the detection of plate thinning. It is shown that mode cut-off measurement allows us to monitor a defect thinning level while a group velocity change can be used to quantify the thinning depth.


Key Engineering Materials | 2004

Implementation of Ultrasonic Immersion Technique for Babbitt Metal Debonding in Turbine Bearing

Gye Jo Jung; Yeon Shik Ahn; Yong Sang Cho; Sang Ki Park; Doo Song Gil; W.H. Lee

This study is aimed for the implementation of ultrasonic method to assess the reliability of turbine bearings. A modified ultrasonic immersion technique was carried out in both laboratory experiment and field application. From the laboratory results, we confirmed that the condition of interface layer between the babbitt and base metal be monitored by the C-Scan. The C-scan image by the ultrasonic immersion test can be used successfully to observe the condition of interface layer. The testing with a focused transducer provides a promising approach for estimating the extent of the damaged region and observing the interface layer effectively. The difference of the ultrasonic reflection ratio between the bonding and debonding area at the interface layer is one of the key parameters for assessing the extent of the damaged area; additionally, the reflection amplitude exhibits a favorable correlation with the overall damage level. The technique developed in this study was applied to the inspection of the turbine bearings at several power plants in Korea whereby the applicability in the field can be ascertained.


Materials Science Forum | 2008

The Study of Field Application of Digital Radiography Testing for Power Plant Weldments

Sang Ki Park; Doo Song Gil; Yeon Shik Ahn; Yong Sang Cho

Since Power Plant has tens of thousands welded parts in its structures, the quality on the welded parts are always a key factor for ensuring the reliability of the structures. Film based Radiography Test (hereinafter called “FRT”) is a generally accepted solution in inspecting the welded structure. However, the RT demands complex workflow, high cost, hazardous waste disposal, and etc…. In an effort to get over such shortcomings from FRT and present alternative inspection tools, this study was intended to evaluate a Digital Radiography Test (hereinafter called “DRT”). In this study, the evaluation were focused on if a DRT is applicable to the inspection of Welded structure in Power Plants, and what benefits can be generated in a use of DRT.


Key Engineering Materials | 2006

Application of Torsional Mode of Guided Waves to Long Range Pipe Inspection

Ik-Keun Park; Yong Kwon Kim; Won Joon Song; Yong Sang Cho

Conventional non-destructive techniques for inspection of weld in pipelines require significant test time and high cost. In order to overcome these drawbacks in conventional NDT techniques, various techniques using ultrasonic guided waves have been developed and applied to the pipeline inspection. Recently, a fast calculation technique for guided wave propagation using a semi-analytical finite element method (SAFEM), PIPE WAVE ver.1.0, has been developed by T. Takahiro et al [1]. In this paper, the calculation of torsional mode propagation in a pipe using PIPE WAVE ver. 1.0 is introduced as a preliminary study and the application of the torsional mode of ultrasonic guided waves to long range pipe inspection is presented.. The characteristics and setup of a long range guided wave inspection system and experimental results in pipes of various diameters are introduced. The experimental results in mock-up pipes with cluster type detects show that the limit of detectable wall thickness reduction with this guided wave system is 2~3% in the pipe cross section area and the wall thickness reduction of 5% in cross section area can be detected when actual detection level is used. Therefore, the applicability of the ultrasonic guided wave technique to long range pipeline inspection for wall thickness reduction is verified.


Key Engineering Materials | 2005

A Study on the Behavior of Ultrasonic Guided Wave Mode in a Pipe Using a Comb Transducer

Ik-Keun Park; Yong Kwon Kim; Youn Ho Cho; Won Joon Song; Yeon Shik Ahn; Yong Sang Cho; Gye Jo Jung

A preliminary study of the behavior of ultrasonic guided wave mode in a pipe using a comb transducer for maintenance inspection of power plant facilities has been verified experimentally. Guided wave mode identification is carried out in a pipe using time-frequency analysis methods such as wavelet transform (WT) and short time Fourier transform (STFT), compared with theoretically calculated group velocity dispersion curves for longitudinal and flexural mode. The results are in good agreement with analytical predictions and show the effectiveness of using the time-frequency analysis method to identify the individual guided wave modes. And, It was found out that longitudinal mode (0, 1) is affected by mode conversion less than the other modes. Therefore, L (0, 1) is selected as a optimal mode for evaluating location of the surface defect in a pipe.


Key Engineering Materials | 2004

A Study on the Characteristics of Surface Wave Induced by Phased Array Ut

Yong Sang Cho; Yeon Shik Ahn; Sang Ki Park; Jae-Hoon Kim

The aim of this study was to develop a new method for detecting surface crack on material or material structures, particularly for determining the length of the surface crack without moving the transducer. Modified phased array ultrasonic electronic scan technique was utilized to determine crack length and to detect surface crack as its depth and length changes. From previous laboratory study and actual field application, we confirmed that phased array UT provides a promising method for detection of cracks with two-dimensional visibility. Furthermore, it replicates the actual shape of the crack in power plant components which has complicated geometry. In this study, a developed technique was applied to several materials with differing surface cracks as well as variations in depth and length in order to verify the application ability and detection ability of surface flaw in the field. The Phased array UT surface wave response was analyzed to obtain this particular method of determining crack length without moving the transducer; additionally, information on the minimal length and depth of the surface crack for detection was also. Upon each case of the phased array UT parameter analysis, changes in the characteristic features of the sector scan results by frequency and electronic sweep angle range were noted. The results showed that the newly developed method for surface crack length determination is a very effective method whose accuracy and verified effectiveness is comparable to conventional crack length determining method.

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Ik-Keun Park

Seoul National University of Science and Technology

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Yong Kwon Kim

Seoul National University

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Yeon Shik Ahn

Electric Power Research Institute

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Sang Ki Park

Electric Power Research Institute

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Doo Song Gil

Electric Power Research Institute

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Gye Jo Jung

Electric Power Research Institute

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Jae-Hoon Kim

Chungnam National University

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Tae-Sung Park

Seoul National University of Science and Technology

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