• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2005년도 춘계 학술발표회 논문집
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• pp.392-399
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• 2005

Shear modulus (or rigidity) of dam material is an important parameter which can be directly associated with the deformation of dam. Seepage or leakage of water can cause the defects or cracks of dam body. The existence of cracks and rigidity of dam body are decisive information for the estimation of dam safety. Rigidity of material is mainly determined from S-wave velocity and the defects of dam body can be detected by seismic reflection survey. Therefore, seismic reflection survey will be a desirable method which can give a solution about dam safety problem. Among various physical properties of dam body, S-wave velocity is the most important information but it is not easy to get the information. In this study, diverse measuring techniques of S-wave reflection survey were attempted to get the information about S-wave velocity of dam body. Ultimately, S-wave velocity could be estimated by the analysis of SH reflection events which can be easily observed in shot gather data obtained from SH measuring technique. Meanwhile, P-wave reflection survey was also performed at the same profile. P-beam radiation technique which can reduce the surface waves and reinforce the P-wave reflection events was applied for giving a help to analyse P-wave velocity. In the end, P-and S-wave velocity, Vs/Vp, Poisson's ratio distribution of the vertical section under the profile could be acquired.

• 한국공작기계학회:학술대회논문집
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• 한국공작기계학회 2001년도 추계학술대회(한국공작기계학회)
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• pp.319-323
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• 2001

In significant number of energy-related facilities for like thermal power plant or petro-chemical industry, CrMo steels are widely used energy conversion industries. However, these materials undergo precipitation of carbides or intermetallic compounds into grain boundary and change of internal microstructure such as coarsening of precipitation, decrease of solute elements and impurity segregation under more severe service conditions, which results in deterioration of inherent superior material characteristics. In this study, it was verified experimentally the feasibility of the aging degradation evaluation for degraded 2.25Cr-lMo steel specimens prepared by isothermal aging heat treatment at 63$0^{\circ}C$ by high frequency longitudinal ultrasonic and surface SH wave investigating the change of attenuation coefficient analyzed by spectral analysis. Attenuation coefficient had a tendency to increase as degradation proceeded.

• 비파괴검사학회지
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• 제22권2호
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• pp.149-154
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• 2002

실제 사용 온도 보다 높은 $630^{\circ}C$에서 열처리 시간을 변수로 하여 최대 1000 시간 동안 등온 열처리를 통해 2.25Cr-1Mo 강의 경년열화재를 제작하고 열화에 따른 미세조직과 기계적 특성 변화에 대한 초음파 비파괴평가 법으로 표면 특성평가에 우수한 표면 SH파를 적용하였다. 열화에 따른 초음파의 감쇠와 전파시간 및 웨이블렛을 이용한 음속과 감쇠계수의 주파수 의존성평가를 수행한 결과 열화가 진행됨에 따라서 탄화물은 점차 조대화되고 경도는 감소하였다. 감쇠계수는 열화시간이 증가함에 따라서 증가하였고 전파시간 역시 열화의 초기단계에 급격히 증가했다. 초음파의 감쇠와 경도와는 좋은 상관관계를 나타내어 감쇠계수는 경년열화 평가에 중요한 파라미터임을 알 수 있었다.

• Geomechanics and Engineering
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• 제16권2호
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• pp.169-176
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• 2018

An attempt has been made here to study the propagation of SH-type surface waves in an elastic medium, which is initially stressed and heterogeneous and has a point source inside the medium. The upper portion of the composite medium is a sandy layer. It is situated on an initially stressed heterogeneous half-space, whose density, rigidity and internal friction are function of depth. The analysis has been carried out by using Fourier transform and Green's function approach. The phase velocity has been investigated for several particular situations. It has been shown that the results of the study agree with those the case of Love wave propagation in a homogeneous medium in the absence of the sandy layer, when the initial stress is absent. In order to illustrate the validity of the analysis presented here, the derived analytical expression has been computed numerically, by considering an illustrative example and the variances of the concerned physical variables have been presented graphically. It is observed that the velocity of shear wave is amply influenced by the initial stress and heterogeneity parameters and the presence of the sandy layer. The study has an important bearing on investigations of different problems in the earth's interior and also in seismological studies.

• 한국지반공학회논문집
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• 제20권1호
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• pp.61-73
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• 2004

레일레이파와 러브파는 탄성파 중에서 표면파에 속하는 것으로, 매우 중요한 표면파로 인식되고 있다. 러브파의 파동 전파시 매질내에 발생되는 변형은 SH파와 같은 전단형태이므로, 러브파는 P-파에 의해 교란되지 않는 특성이 있다. 이러한 특성으로 인하여 러브파가 레일레이파나 다른 체적파보다 더 유용한 탄성파로 인식되고 있다. 본 연구에서는 표면파를 활용하는 대표적 실험기법인 SASW 기법에서 러브파를 도입하고자 하는 목적으루 이론적, 수치해석적, 실험적 방법으로 러브파의 위상속도 분산 특성을 연구하였다. 특히, 러브파의 수평성분과 레일레이파의 수직 수평 성분의 특성을 연구하기 위하여, 반무한 지반, 2층 구조의 지반에 대하여 2차원,3차원 유한요소 해석을 수행하였다. 또한, 수치해석으로 획득한 러브파와 레일레이파에 대한 결과를 확인하기 위하여 일반 지반에서 현장실험을 수행하였다. 이와 같은 수치해석과 현장실험을 통하여 러브파의 위상속도 분산특성이 SASW 기법의 정확성과 신뢰성을 더욱 향상시키기 위한 추가 자료로 매우 유용할 것이라는 것을 확인하였다.

• 한국지진공학회:학술대회논문집
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• 한국지진공학회 1997년도 춘계 학술발표회 논문집 Proceedings of EESK Conference-Fall 1997
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• pp.128-135
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• 1997

The indirect boundary integral equation is formulated to analyze the behavior of a cavity in a multilayered half-plane subjected to earthquake waves. This formulation uses the fundamental solutions that are numerically calculated by the generalized transmission and reflection coefficient method. The free surface of the cavity without external excitation influences the behavior of the half-plane. Consequently this analysis adds the consideration of scattering-field into the analysis and the total motion field of the cavity is decomposed into the free-field and scattering-field motions. The free-field motion is obtained from the modification of the transmission and reflection coefficient method. The scattering-field motion is calculated is calculated by the indirect boundary value problem which has the ficticious boundaries and sources. In this study, P wave, SV wave, SH wave, and Rayleigh wave are analyzed respectively.

• Earthquakes and Structures
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• 제5권2호
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• pp.161-205
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• 2013

We study seismically induced, anti-plane strain wave motion in a non-homogeneous geological region containing tunnels. Two different scenarios are considered: (a) The first models two tunnels in a finite geological region embedded within a laterally inhomogeneous, layered geological profile containing a seismic source. For this case, labelled as the first boundary-value problem (BVP 1), an efficient hybrid technique comprising the finite difference method (FDM) and the boundary element method (BEM) is developed and applied. Since the later method is based on the frequency-dependent fundamental solution of elastodynamics, the hybrid technique is defined in the frequency domain. Then, an inverse fast Fourier transformation (FFT) is used to recover time histories; (b) The second models a finite region with two tunnels, is embedded in a homogeneous half-plane, and is subjected to incident, time-harmonic SH-waves. This case, labelled as the second boundary-value problem (BVP 2), considers complex soil properties such as anisotropy, continuous inhomogeneity and poroelasticity. The computational approach is now the BEM alone, since solution of the surrounding half plane by the FDM is unnecessary. In sum, the hybrid FDM-BEM technique is able to quantify dependence of the signals that develop at the free surface to the following key parameters: seismic source properties and heterogeneous structure of the wave path (the FDM component) and near-surface geological deposits containing discontinuities in the form of tunnels (the BEM component). Finally, the hybrid technique is used for evaluating the seismic wave field that develops within a key geological cross-section of the Metro construction project in Thessaloniki, Greece, which includes the important Roman-era historical monument of Rotunda dating from the 3rd century A.D.

• 센서학회지
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• 제17권4호
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• pp.289-294
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• 2008

The purpose of this study is to materialize the viscosity sensor by using the SH-SAW sensor of which the center frequency is operated at higher than 50 MHz. In order to measure the viscosity, SAW sensor of which the center frequency is operated at 100 MHz is developed. By using the developed sensor, phase shift, delay time, insertion loss, and frequency variation are measured at different viscosity. The result shows that the phase shift difference between the viscosity variations is such that the difference between the distilled water and the 100 % glycerol solution is approximately $45^{\circ}$, the change of the insertion loss is approximately 9 dB, and the difference of frequency variation is approximately 5.9 MHz. Therefore, it is shown that viscosity of unknown solution can be measured with the surface acoustic wave sensor.

• 지구물리와물리탐사
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• 제21권1호
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• pp.33-40
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• 2018

이 연구는 한국, 중국, 일본을 포함하는 동아시아 지역의 지각과 최상부맨틀의 SV파 및 SH파 속도구조와 지진파의 속도 방사 이방성(radial anisotropy)을 알아보기 위해 수행하였다. IRIS (Incorporate Research Institutions for Seismology)에서 동아시아에 설치한 광대역 지진관측소에 기록된 지진자료를 사용하여 레일리파와 러브파를 추출하였으며, MFT (Multiple Filter Technique)을 사용하여 각 성분에 기록된 주기 3 ~ 200초 범위의 레일리파와 러브파의 군속도 분산자료를 획득하였다. 수직성분에서 62466개의 레일리파의 분산곡선 측정값을, 접선(transverse)성분에서 54141개의 러브파의 분산곡선 측정값을 얻을 수 있었다. 얻어진 분산자료를 역산하여 속도모델을 구하였고, 역산된 모델을 통해 깊이 100 km 까지의 SV파 및 SH파 속도 구조를 구하였다. SV파와 SH파 속도구조의 경우, 동일하게 깊이 30 km 까지 동해에서 강한 고속도 이상이 나타나며, 30 km 이상의 깊이에서는 중국 남서쪽의 티벳 고원에서 강한 저속도 이상이 나타난다. SH파 속도구조의 경우, 30 km 이상의 깊이에서 동해에서 상대적으로 더 강한 저속도 이상이 나타난다. 그 결과, 지진파 이방성은 평균적으로 동해지역에서 음의 이방성을, 중국 내륙지역에서 양의 이방성이 관측된다.

• Nuclear Engineering and Technology
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• 제50권6호
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• pp.890-898
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• 2018

Nondestructive inspection (NDI) is an integral part of structural integrity analyses of dry storage casks that house spent nuclear fuel. One significant concern for the structural integrity is stress corrosion cracking in the heat-affected zone of welds in the stainless steel canister that confines the spent fuel. In situ NDI methodology for detection of stress corrosion cracking is investigated, where the inspection uses a delivery robot because of the presence of the harsh environment and geometric constrains inside the cask protecting the canister. Shear horizontal (SH) guided waves that are sensitive to cracks oriented either perpendicular or parallel to the wave vector are used to locate welds and to detect cracks. SH waves are excited and received by electromagnetic acoustic transducers (EMATs) using noncontact ultrasonic transduction and pulse-echo mode. A laboratory-scale canister mock-up is fabricated and inspected using the proposed methodology to evaluate the ability of EMATs to excite and receive SH waves and to locate welds. The EMAT's capability to detect notches from various distances is evaluated on a plate containing 25%-through-thickness surface-breaking notches. Based on the results of the distances at which notch reflections are detectable, NDI coverage for spent nuclear fuel storage canisters is determined.