• 한국안전학회지
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• 제6권4호
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• pp.81-87
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• 1991

We propose the crack growth rate equation which will model fatigue crack growth rate behavior such that constant stress amplitude fatigue crack growth behavior can be predicted. Constant stress amplitude fatigue tests are conducted for four materials under three stress ratios of R＝0.2, R＝0.4 and R＝0.6. Materials which have different mechanical properties i.e. stainless steel, low carbon steel, medium carbon steel and aluminum alloy are used. Through constant stress amplitude fatigue test by using unloading elastic compliance method, it is confirmed that crack closure is a close relationship with fatigue crack propagation. We describe simply fatigue crack propagation behavior as a function of the effective stress intensity factor range ($\Delta$ $K_{eff}$＝U .$\Delta$K) for all three regions (threshold region, stable region). The fatigue crack growth rate equation is given by da / dN＝A($\Delta$ $K_{eff}$­$\Delta$ $K_{o}$ )$^{m}$ / ($\Delta$ $K_{eff}$­$\Delta$K) Where, A and m are material constants, and $\Delta$ $K_{o}$ is stress intensity factor range at low $\Delta$K region. $K_{cf}$ is critical fatigue stress intensity factor.actor.

• Journal of Mechanical Science and Technology
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• 제15권4호
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• pp.510-521
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• 2001

A nonlinear acoustic instability of subcritical liquid-oxygen droplet flames burning in gaseous hydrogen environment are investigated numerically. Emphases are focused on the effects of finite-rate kinetics by employing a detailed hydrogen-oxygen chemistry and of the phase change of liquid oxygen. Results show that if nonlinear harmonic pressure oscillations are imposed, larger flame responses occur during the period that the pressure passes its temporal minimum, at which point flames are closer to extinction condition. Consequently, the flame response function, normalized during one cycle of pressure oscillation, increases nonlinearly with the amplitude of pressure perturbation. This nonlinear response behavior can be explained as a possible mechanism to produce the threshold phenomena for acoustic instability, often observed during rocket-engine tests.

• 한국정보디스플레이학회:학술대회논문집
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• 한국정보디스플레이학회 2003년도 International Meeting on Information Display
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• pp.595-599
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• 2003

We have studied the phase transition to look for molecular structure by using several different techniques for new banana-shaped liquid crystals shown in Fig. 1. Based on the similarities to recently observed fluro-contaning materials (switching involves layer structure rearrangement, increasing threshold with increasing temperature) for HC sample (where x is H), we assume that the phase C has a triclinic symmetry corresponding to the double tilted $smC_G$ Phase. The observation that the polarization peak appears at lower field ($E_o{\sim}15V/{\mu}m$) than the amplitude of the threshold ($E_{th}$) can be explained assuming a field induced $SmC_G$ - SmCP (or SmAP) transition at $E_{th}$

• 대한기계학회논문집B
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• 제29권8호
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• pp.895-902
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• 2005

Lock-on characteristics of flow around a circular cylinder oscillating rotationally with a relatively high forcing frequency have been investigated experimentally. Dominant governing parameters are Reynolds number (Re), angular amplitude of oscillation (${\theta}_A$), and frequency ratio $F_R=f_f/f_n,\;where\;f_f$ is a forcing frequency and $f_n$ is a natural frequency of vortex shedding. Experiments were carried out under the conditions of $Re=4.14{\times}10^3,\;{\pi}/90{\leq}{\theta_A}{\leq}{\pi}/3,\;and\;F_R=1.0$. The effect of this active flow control technique on the lock-on flow characteristics of the cylinder wake was evaluated with wake velocity measurements and spectral analysis of hot-wire signals. The rotational oscillation modifies the flow structure of near wake significantly. The lock-on phenomenon always occurs at $F_R=1.0$, regardless of the angular amplitude ${\theta}_A$. In addition, when the angular amplitude is less than a certain value, the lock-on characteristics appear only at $F_R=1.0$,. The range of lock-on phenomena expands and vortex formation length is decreased, as the angular amplitude increases. The rotational oscillation create a small-scale vortex structure in the region just near the cylinder surface. At ${\theta}_A=60^{\circ}$, the drag coefficient was reduced about $43.7\%$ at maximum.

• 대한전자공학회논문지
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• 제5권2호
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• pp.8-14
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• 1968

트란지스타 단일안정 멀티바이부래타(monostable multi-vibrator)와 시리콘턴넬 다이오드 (T.D)로써 구성된 전류파 파고변별기를 설계하여 그 특성을 조사하였다. 피측정 전류액의 범위는 50㎂-5.23mA이며, 이 범위에 있어서 측정된 최대비직선도는 ±0.75% 이었다. 이 변별기의 전류액 분해능은 T.D를 통하여 흐르는 편의전류에 따라서 약간 달라지며 역방향 편의전류가 3 mA 일때, 만일 5%의 과잉파고를 주용한다면 그 분해시간은 2μS이다. 다음에 이 변별기의 임계치 안정도는 주로 T.D의 턴넬전류의 최대치 1 의 안정도에 의하여 좌우되며 환경온도의 변화범위가 0℃∼50℃일때는 최대비직선도 즉 ±0.75T 보다 더큰 임계치변화는 관측되지 않았다.

• 대한기계학회논문집
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• 제13권5호
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• pp.938-946
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• 1989

본 연구에서는 CT시험편을 이용하여 소규모 항복조건하에서 하한계(near threshold)영역과 안전균열성장(제II)영역의 균열진전거동을 파악하고 피로균열 진전속도와 음향방출거동, 파단면 특성을 각각 비교, 고찰하여 각 파라미터간의 상관 관계 및 유용성을 검토하고자 한다.

• Journal of the Korean Physical Society
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• 제73권10호
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• pp.1486-1494
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• 2018

Based on the semiclassical theory of chaotic scattering, Tanner et al. [J. Phys. B 40, F157 (2007)] proposed the fluctuation in the partial photoionization cross section of helium below the double-ionization threshold would show the same characteristics as in the total cross section, predicting that the Fourier spectrum of the fluctuation reveals peaks at the classical actions of closed triple collision orbits and the amplitude of the fluctuation decreases algebraically as the energy approaches the double-ionization threshold. In that paper, however, the predictions were not clearly confirmed due to the lack of experimental data with sufficient accuracy. So instead, we calculate the partial photoionization cross sections of collinear eZe helium for the energy range from the single-ionization threshold $I_{20}$ to $I_{32}$ in order to numerically confirm the predictions. Analysis of the fluctuation in the partial cross section shows that the predictions are indeed valid. Our findings mean that the fluctuation in the partial photoionization cross section can be described by classical triple collision orbits in the semiclassical limit. Thus it explains in a natural way the mirroring and mimicking structures observed in cross section signals for different ionization channels.

• 한국항행학회논문지
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• 제21권2호
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• pp.163-170
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• 2017

독립 협동형 감시시스템인 이차감시레이더(SSR)의 위치 정보의 해상도를 높이기 위해 다변측정감시시스템(MLAT)이 항공 감시시스템에 적용되고 있다. 다변측정감시시스템은 쌍곡선(hyperbola) 또는 쌍곡면(hyperboloid) 위치 측정법을 이용하는 시스템으로 4개 이상의 지상 수신기에서 TOA (time of arrival)값을 측정하고 이를 중앙처리장치에서 각 수신기 상호간의 수신시각차 (TDOA; time difference of arrival)를 이용하여 항공기의 위치를 계산한다. TDOA (time difference of arrival) 알고리즘을 이용하는 다변측정감시시스템의 위치 측정 해상도를 높이기 위해서는 먼저 항공기에 탑재된 트랜스폰더에서 송출된 신호가 수신기에 수신되는 시간인 TOA(time of arrival)를 정밀하게 측정하여야 한다. 본 논문에서는 다변측정감시시스템의 구성에 대해 간략히 설명하였으며 TOA를 측정하기 위한 방법 중 하나인 ATD (adaptive threshold detector)에 대하여 간략히 설명하였다. ATD는 동일 신호에 대한 수신기간 수신한 신호의 진폭 차에 의해 발생하는 펄스 위치 편차 문제를 해결하는 방식이다. 본 논문에서는 ATD의 종류 중 LAS (level adjuster System)과 CDS (constant-fraction discriminator system)에 대한 시뮬레이션 결과와 기본적인 임계값을 적용한 펄스 위치 계산 시뮬레이션 결과를 비교하여 ATD의 성능을 분석하였다.

• 대한전자공학회논문지TE
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• 제37권3호
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• pp.30-36
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• 2000

본 논문에서는 결함신호의 신호 대 잡음비를 높이기 위해 최소진복선택 알고리즘과 극 문턱치(PT : polarity threshold) 알고리즘을 결합한 최소극 문턱치(MPO : minimum polarity threshold) 알고리즘을 이용한 원전 고감쇠 용접부 초음파 검사방법을 제안하였다. 제안된 알고리즘의 유용성을 확인하기 위한 실험에서는 ASME Xl 코드에 준하는 원전의 실제 모재와 동일한 용접방법으로 제작된 대비 시험편과 4종류의 개선된 탐촉자를 이용하였다. 실험 결과, SE(safeend)와 CCSS(centrifugal casting stainless steel) 재료는 조직의 구조적 특성 때문에 산란신호가 발생하고, 용접 내부에서는 높은 감쇠 현상이 발생하여 정확한 결합검출 능력이 저하되며, 이러한 결함신호에 최소극 문턱치 알고리즘을 적용하면 신호 대 잡음비가 2.6배 개선됨을 확인하였다.

• 전기의세계
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• 제21권4호
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• pp.22-26
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• 1972

The vibration of power lines in rainy environments was researched using the concentric cylindrical gap to which D.C. voltage was applied. Consequently, it has been solidly conformed that the vibration of power lines begins at threshold voltage of corona, and that the amplitude of the vibration increases as the numbers of falling water drop per unit time increase. The vibration phenomen take place in straight line, elliptic or circular motion as voltage is applied the numbers of vibration nearay accord with the elastic vibrarion theory.