Kyung-Cho Kim

Evaluation of material degradation using nonlinear acoustic effect

A method to measure the nonlinear effect of ultrasonic waves is suggested as a new approach for the effective evaluation of material degradation. In sonic wave propagation, nonlinear effect can be interpreted as the generation of higher order harmonic waves. The generation mechanism of higher order harmonic components due to nonlinear effect is firstly explained using nonlinear elasticity. Next, we attempt to measure how much higher order harmonic component was generated in the degraded material. For this purpose, a measurement system mainly based on a high power nonlinear ultrasonic signal analysis system that adopts heterodyne signal detection technique was constructed. From the results, we confirm that the nonlinear acoustic effect may be a useful technique for the evaluation of material degradation.

Ultrasonic Imaging of Welded Metals Using Simplified Ultrasonic Computerized Tomography.

In this paper, as a new measurement method to estimate the infinitesimal change of material condition, the simplified ultrasonic computerized tomography (CT) system, which uses the information of three directions, i.e., the 90°, +45° and -45° directions about the inspection plane, is proposed. The use of the simplified ultrasonic CT system has two merits: First, the measurement time is very short compared with that of the conventional CT method. Second, it can detect sensitively infinitesimal defects in vertical or oblique directions about the inspection plane because the obtained image is not a C scan image but a CT image calculated from the three directions. Due to these merits, this method can be considered to be a very effective method for the evaluation of material condition. In order to determine the applicability of actual NDT, several kinds of welded specimens are investigated. The results showed that the CT images obtained were very similar to actual defects of specimens. Also, the same CT images were obtained by computer simulation and the obtained CT images by computer simulation were almost the same as the experimental images.

The development of simplified ultrasonic CT system and its application to the evaluation of weld metal

In this paper, as a new measurement method to estimate the change of material condition, the simplified ultrasonic CT system, which uses the information of three directions, that is, 90°, + 45° and −45° about inspection plane is proposed. Use of simplified CT system has two merits: Firstly, the measurement time is very short compared with general CT. Secondly, it can detect sensitively small defect in vertical or slant direction about inspection plane because the obtained image is CT image calculated from three directions. From these merits, this method can be considered as an effective method to evaluate material conditions. The basic performance of the proposed method was confirmed through several specimens with several simple defects. In order to confirm the applicability of actual NDT, several kinds of welded specimens are investigated. The result showed that the CT image obtained had good agreement with actual defect of specimens.

Degradation Assessment of Aluminum Alloy 6061-T6 Using Ultrasonic Attenuation Measurements

Abstract Ultrasonic methods are widely used to degradation assesment. Remaining-life cycle of metal can be estimated by ultrasonic parameters because ultrasonic velocity and attenuation are affected by change of material properties with accumulated fatigue in the metal. Therefore, in this study, we will estimate overall change of material properties by 2D C-scan image. Fatigued aluminum alloy 6061-T6 samples from 0 to 85% were prepared for evaluating fatigue life cycle. Also, degraded image of materials using attenuation is proposed to estimate degree of material degradation for determining degraded area of fatigued samples. Finally, we will predicts process pf degradation with measured attenuation of fatigued aluminum alloy 6061-T6 samples. Keywords: Al6061-T6, Attenuation, Ultrasonic, Degradation, Fatigue[접수일: 2012. 3. 14, 수정일: (1차: 2012. 12. 4, 2차: 2013. 2. 4) 게재확정일: 2013. 2. 7] *성균관대학교 기계공학부, **한국원자력안전기술원 ✝Corresponding Author: School of Mechanical Engineering, Sungkyunkwan University, Suwon 440-746, Korea (E-mail: sjsong@skku.edu)

Korean Regulatory Experience on PWSCC in Korean NPP

The degradation of alloy 600 and its weld material (alloy 82/182) has been reported in many nuclear power plants. In Korea, the crack induced by PWSCC was discovered in the drain nozzle of Yongkwang units 3 & 4 in 2006∼2008 and SG plug weld of Yongkwang unit 3 in 2007. In July 2007, during visual inspections of SG tube plugs at Yonggwang unit 4, boric acid deposits were observed around five Alloy 600 welded plugs. The root cause of the cracking in alloy 600 plugs was revealed to be due to the fact that the cracks were mainly caused by residual stress induced from the welding, expanding and tight-fitting. Younggwang unit 3 found the white small deposits on the drain nozzle on the 10th RFO in 2007. The root cause of the cracking in drain nozzle was revealed to be due to the initiation of a crack on the inside surface of drain nozzle and propagated to through wall cracks in the axial and circumferential direction. Younggwang unit 3 found the white widespread deposits on the upper head of a reactor vessel on the 12th RFO in 2010. Utility is trying to reveal the root cause of the cracking in the vent line of the reactor head according the KINS requirement. In this article, Korean regulatory experiences for PWSCC are introduced. After these PWSCC experiences, all SG tubes welded by Alloy 600 were replaced and all SG drain and instrumentation nozzles with Alloy 600 have been replaced into Alloy 690 material.© 2010 ASME

Evaluation of nonlinear acoustic effect using bispectum analysis and its NDE application

It has recently got known that the nonlinear acoustic effect is sensitive to the infinitesimal deformation, dislocation, or degradation of material. Also how to measure the nonlinear acoustic effects is an important point in the actual NDE application. In this paper, as a new signal processing method to estimate the nonlinear acoustic effects (nonlinear parameter β), the bispectrum method is proposed. Use of bispectrum has two merits: In first, the gaussian additive noise is eliminated in the bispectrum domain. In second, it can detect two frequency components that have special phase relation. From these merits, this method can be considered as a very effective method for the measurement of nonlinear parameter β, which requires detecting pure harmonics in phase without noise interference. The performance of the proposed method was confirmed by through computer simulations, which showed that the bispectrum method could obtain the value of β near to the true value in the case of low S/N ratio signal. Also, ...