제 4 학술분과 : 실리콘 카바이드보강 알루미늄합금의 미시구조와 결함이 재료물성에 미치는 영향

정현조 한국복합재료학회 1992 한국복합재료학회 학술발표회 논문초록집 Vol.- No.2

비선형 고체의 응력자유 경계로부터 반사된 위상배열 집속 빔의 2차 고조파 거동

정현조 한국비파괴검사학회 2019 한국비파괴검사학회지 Vol.39 No.1

응력자유 경계면을 갖는 고체에서의 비선형 고조파 발생에서 응력자유 경계는 2차 고조파 발생을 파괴적으로 변화시키므로 펄스-에코법은 실제로 사용될 수 없었다. 집속 빔은 종종 펄스-에코 모드에서 유체의 비선형 및 생체 역학적 이미징에 사용되었다. 본 논문에서는 선형 위상배열 탐촉자의 집속 빔을 이용하여 펄스-에코 모드에서 응력자유 경계면을 갖는 비선형 고체에서 2차 고조파의 생성을 크게 개선할 수 있음을 보였다. 응력자유 및 강체 경계에 집속된 후에 반사되는 기본파 및 2차 고조파의 빔 음장을 계산하였고 그 속성을 수신 평균 음장을 통하여 관찰하였다. 경계면으로부터 반사된 후에 얻게 되는 두 개의 2차 고조파 성분, 즉 최초 전파 과정에서 발생한 후 반사된 2차 고조파와 반사된 기본파의 역방향 전파 과정에서 새롭게 생성된 2차 고조파, 사이의 위상차를 분석하였으며, 시뮬레이션 결과에서 관찰되는 2차 고조파의 생성을 이 두 성분의 위상차를 이용하여 설명하였다. In nonlinear acoustic harmonic generation in solids with stress-free boundaries, such a boundary is known to destructively change the second harmonic generation, and the pulse-echo method was not practically applicable. Focused beams have often been used for fluid nonlinearity and biomechanical imaging in pulse-echo test setups. This paper considers the focused beam fields of linear phased array transducers to ensure that pulse-echo harmonic generation can be applied to solids with stress-free boundaries. The fundamental and second harmonic beam fields that are focused at and reflected from the stress-free and rigid boundaries are calculated and their properties are investigated in terms of the received average fields. The phase difference between the two second harmonic components after reflection from the boundary, i.e., the reflected and newly generated second harmonics, is emphasized. The phase difference is used to explain the improved and accumulated second harmonic observed in the simulation results.

콤트랜스듀서에 의해 발생한 레일리 표면파의 비선형 음장 해석

정현조,장수증 한국비파괴검사학회 2020 한국비파괴검사학회지 Vol.40 No.1

Comb transducers can be considered to effectively generate nonlinear Rayleigh surface waves, including second harmonics. In this paper, we calculate the propagation and reception fields for the fundamental and second harmonics of Rayleigh surface waves generated by the finite-amplitude excitation of a comb transducer composed of single and multiple line sources. The sound fields of the comb transducer with multiple line sources are formulated by extending an existing single-line source model, and the received sound fields are calculated for three different types of receivers, namely the point, single line, and multiple line reception. In addition, nonlinear parameters of the surface waves, which are calculated from the amplitudes of the fundamental and second harmonics, are defined, and accompanying diffraction and attenuation correction equations are obtained and the results are presented. The analysis presented in this paper can be used for the quantitative measurement of nonlinear parameters of materials using comb transducers. 제2고조파를 포함한 레일리 표면파의 비선형 파동을 효과적으로 발생시키기 위하여 콤트랜스듀서가 고려될 수 있다. 본 논문에서는 단일 선원 및 다중 선원으로 구성된 콤트랜스듀서의 유한진폭 방사에 의해 생성된 레일리 표면파의 기본파 및 제2고조파에 대한 전파 음장과 수신 음장을 해석적 방법을 사용하여 계산하고 그 결과를 수록하였다. 다중 선원 콤트랜스듀서의 음장 계산은 이전에 개발된 단일 선원 음장 모델을 확장 적용하였으며, 수신 음장의 경우 점 수신, 단일 선 수신, 다중 선 수신으로 구분하여 그 결과를 구하고 비교하였다. 또한 기본파와 제2고조파의 진폭으로 계산되는 표면파의 비선형 파라미터를 정의하였으며, 이에 수반되는 회절 및 감쇠 보정 식을 구하고 그 결과를 제시하였다. 본 논문의 해석 결과는 콤트랜스듀서를 이용한 재료의 비선형 파라미터의 정량적 측정에 적용될 수 있다.

하절기 기화냉각장치 설치에 따른 가스터빈 성능변화

정현조,유호선,문승재,Chung, Hyeon-Jo,Yoo, HoSeon,Moon, Seung-Jae 한국플랜트학회 2017 플랜트 저널 Vol.13 No.1

This study analyzed the change of gas turbine performance with air temperature decrease by the evaporation cooling system in summer season. Gas turbine performance was tested on the condition that ambient temperature is $29{\pm}1^{\circ}C$. As a result, Air temperature at the compressor inlet was decreased by $4.12^{\circ}C$ after the installation of evaporation cooling system. Decreased air temperature followed by increased air density affected gas turbine performance, Which increased compressor pressure ratio by 0.27, improved compressor efficiency of 0.29 %p, improved gas turbine enthalpy drop efficiency of 0.31 %p, improved the gas turbine efficiency by 0.44 %p, improved electric power output by 4,489 kW. On the other side, the influence of the humidity increase and flow resistance increase was negligible. 본 연구는 하절기 가스터빈 압축기 입구에 설치된 기화냉각장치로 인하여 저하된 공기온도가 가스터빈 성능에 어떠한 변화를 미치는지에 대하여 해석하였다. 연구 대상은 인천화력본부 가스터빈 3호기이며, 기종은 Siemens SGT6-4000F(V 84.3A)이다. 기화냉각장치 설치 전후를 비교하기 위하여 특설계측기를 설치하였으며, 2015년 8월 3일에서 14일까지 11시부터 15시 사이에 대기온도 $29{\pm}1^{\circ}C$ 조건에서 설치 전후 각각 5회씩 총 10회에 걸쳐 정밀성능시험을 시행하고 각각의 평균값을 적용하였다. 시험 결과, 가스터빈 압축기 입구 공기온도 $4.12^{\circ}C$ 저하로 공기 밀도가 증가되어 압축기 압력비는 0.27 증가, 압축기 효율은 0.29 %p 상승, 가스터빈 엔탈피강하 효율은 0.31 %p 증가하였고, 가스터빈 효율은 0.44%p, 출력은 4,489 kW가 증대되는 효과를 얻을 수 있었으며, 습도 및 유동저항 증가에 따른 영향은 미미하였다.

잡음 신호가 제거된 원형 봉 웨이브가이드에 관한 연구

정현조 한국기계기술학회 2014 한국기계기술학회지 Vol.16 No.2

Bar type waveguides are frequently employed as a buffer rod between ultrasonic senor and hard-to-access object in many measurement applications. Difficulty in pulse echo measurements using a buffer rod is caused by spurious echoes due to reflection, mode conversion and interference within the buffer rod of finite size. These spurious echoes degrade the signal to noise ratio (SNR) because of their possible interference with desired target signals. A tapered buffer rod or rods with non-symmetrical cross sections were used before to reduce these unwanted spurious echoes. In this paper, we propose signal processing techniques to remove the spurious echo signals. Experiments were performed in order to examine the refocusing and recovering the initial input waveform in the long range propagation of ultrasonic waves in cylindrical waveguides. Two different time reversal processes (regular TR and reciprocal TR or inverse filtering) are tested and the experimental results were compared

Time Reversal Focusing and Imaging of Point-Like Defects in Specimens with Nonplanar Surface Geometry

정현조,이현기,배성민,이정식 한국비파괴검사학회 2010 한국비파괴검사학회지 Vol.30 No.6

Nonplanar surface geometries of components are frequently encountered in real ultrasonic inspection situations. Use of rigid array transducers can lead to beam defocusing and reduction of defect image quality due to the mismatch between the planar array and the changing surface. When a flexible array is used to fit the complex surface profile, the locations of array elements should be known to compute the delay time necessary for adaptive beam focusing. An alternative method is to employ the time reversal focusing technique that does not require a prior knowledge about the properties and structures of the specimen and the transducer. In this paper, a time reversal method is applied to simulate beam focusing of flexible arrays and imaging of point-like defects contained in specimens with nonplanar surface geometry. Quantitative comparisons are made for the performance of a number of array techniques in terms of the ability to focus and image three point-like reflectors positioned at regular intervals. The sinusoidal profile array studied here exhibits almost the same image quality as the flat, reference case.

Time Reversal Reconstruction of Ultrasonic Waves in Anisotropic Media

정현조 한국비파괴검사학회 2008 한국비파괴검사학회지 Vol.28 No.1

Time reversal (TR) of body waves in fluids and isotropic solids has been used in many applications including ultrasonic NDE. However, the study of the TR method for anisotropic materials is not well established. In this paper, the full reconstruction of the input signal is investigated for anisotropic media using an analytical formulation, called a modular Gaussian beam (MGB) model. The time reversal operation of this model in the frequency domain is done by taking the complex conjugate of the Gaussian amplitude and phase received at the TR mirror position. A narrowband reference signal having a particular frequency and number of cycles is then multiplied and the whole signal is inverse Fourier transformed. The original input signal is seen to be fully restored by the TR process of MGB model and this model can be more generalized to simulate the spatial and temporal focusing effects due to TR process in anisotropic materials.

Time Reversal Beam Focusing of Ultrasonic Array Transducer on a Defect in a Two Layer Medium

정현조,이정식,배성민 한국비파괴검사학회 2009 한국비파괴검사학회지 Vol.29 No.3

The ability of time reversal techniques to focus ultrasonic beams on the source location is important in many aspects of ultrasonic nondestructive evaluation. In this paper, we investigate the time reversal beam focusing of ultrasonic array sensors on a defect in layered media. Numerical modeling is performed using the commercially available software which employs a time domain finite difference method. Two different time reversal approaches are considered – the through transmission and the pulse-echo. Linear array sensors composed of N elements of line sources are used for signal reception/excitation, time reversal, and reemission in time reversal processes associated with the scattering source of a side-drilled hole located in the second layer of two layer structure. The simulation results demonstrate the time reversal focusing even with multiple reflections from the interface of layered structure. We examine the focusing resolution that is related to the propagation distance, the size of array sensor and the wavelength.

Imaging of a Defect in Thin Plates Using the Time Reversal of Single Mode Lamb Wave: Simulation

정현조,이정식,배성민,이현기 한국비파괴검사학회 2010 한국비파괴검사학회지 Vol.30 No.3

This paper presents an analytical investigation for a baseline-free imaging of a defect in plate-like structures using the time-reversal of Lamb waves. We first consider the flexural wave (A0 mode) propagation in a plate containing a defect, and reception and time reversal process of the output signal at the receiver. The received output signal is then composed of two parts: a directly propagated wave and a scattered wave from the defect. The time reversal of these waves recovers the original input signal, and produces two additional side bands that contain the time-of-flight information on the defect location. One of the side band signals is then extracted as a pure defect signal. A defect localization image is then constructed from a beamforming technique based on the time-frequency analysis of the side band signal for each transducer pair in a network of sensors. The simulation results show that the proposed scheme enables the accurate, baseline-free detection of a defect, so that experimental studies are needed to verify the proposed method and to be applied to real structure.

방향분포를 가진 다상 금속복합재료의 탄성 및 전도해석에 관한 연구

정현조 대한기계학회 1995 대한기계학회논문집 Vol.19 No.9

The effective elasticity and conduction of composite materials containing arbitrarily oriented multiple phases has been analyzed using the concept of orientation-dependent average fields and concentration factors. The analysis provided closed form expressions for the effective stiffnesses and conductivities. Under the prescribed boundary conditions, the concentration factors were evaluated by the equivalent inclusion principle, through which the interaction between various phases is approximated by the Mori-Tanaka mean-field approximation. SiC particulate(SiC$_{p}$) reinforce aluminum(Al) matrix composites were fabricated and their elastic constants and electrical conductivities were measured together with a careful study of their microstructure. The measured properties showed a systematic anisotropy and this behavior could be attributed to the preferred orientation of SiC$_{p}$. The theoretical model developed was applied to the computation of the anisotropic properties of these composites. Both two-phase and three-phase composites were considered based on the microstructural information. The SiC$_{p}$ was modeled as an ellipsoid with planar random orientation distribution in the extruded Al/SiC$_{p}$ composites. The effect of extraneous phase such as intermetallic compounds was also investigated.tigated.