• Advances in aircraft and spacecraft science
• /
• 제8권1호
• /
• pp.31-51
• /
• 2021

Selective laser melting (SLM), one of the most widely used powder bed fusion (PBF) additive manufacturing (AM) technology, enables the fabrication of customized metallic parts with complex geometry by layer-by-layer fashion. However, SLM inherently poses several problems such as the discontinuities in the molten track and the steep temperature gradient resulting in a high degree of residual stress. To avoid such defects, thisstudy proposes a temperature thread multiscale model of SLM for the evaluation of the process at different scales. In microscale melt pool analysis, the laser beam parameters were evaluated based on the predicted melt pool morphology to check for lack-of-fusion or keyhole defects. The analysis results at microscale were then used to build an equivalent body heat flux model to obtain the residual stress distribution and the part distortions at the macroscale (part level). To identify the source of uneven heat dissipation, a liquid lifetime contour at macroscale was investigated. The predicted distortion was also experimentally validated showing a good agreement with the experimental measurement.

• 한국치위생학회지
• /
• 제21권3호
• /
• pp.227-234
• /
• 2021

Objectives: The objective of this study was to investigate the relationship between smoking history and periodontal disease among the elderly in South Korea. Methods: The study subjects comprised 2,703 elderly people who underwent oral health examination as part of the 7th South Korea National Health and Nutrition Examination Survey (KNHANES; 2016-2018). Data were analyzed using frequency analysis, Rao-Scott chi-square test, t-test, and binary logistic regression analysis. Results: A complex sample logistic regression analysis showed that the odds for periodontal disease development were higher in past smokers (odds ratio [OR]=1.461; 95% CI=1.070-1.994) and current smokers (OR=1.601; 95% CI=1.011-2.536) than in lifetime non-smokers. Conclusions: Smokers must actively participate in smoking cessation programs and interventions starting from middle age.

• ETRI Journal
• /
• 제44권2호
• /
• pp.274-285
• /
• 2022

Retransmission in optical burst switching networks is a solution to recover data loss by retransmitting the dropped burst. The ingress node temporarily stores a copy of the complete burst and sends it each time it receives a retransmission request from the core node. Some retransmission schemes have been suggested, but uncontrolled retransmission often increases the network load, consumes more bandwidth, and consequently, increases the probability of contention. Controlled retransmission is therefore essential. This paper proposes a new controlled retransmission scheme for loss recovery, where the available bandwidth of wavelength channels and the burst lifetime are referred to as network conditions to determine whether to transmit a dropped burst. A retrial queue-based analysis model is also constructed to validate the proposed retransmission scheme. The simulation and analysis results show that the controlled retransmission scheme is more efficient than the previously suggested schemes regarding byte loss probability, successful retransmission rate, and network throughput.

• 대한기계학회논문집A
• /
• 제38권10호
• /
• pp.1185-1191
• /
• 2014

가스터빈 블레이드는 증기터빈 블레이드와 달리 냉각홀 및 냉각유로를 포함한 복잡한 형상으로 되어 있으며 복합화력의 운전특성에 따라 반복적이거나 지속적인 열-기계 하중 조건 하에서 운전된다. 따라서 블레이드는 운전시간에 따라 균일하지 못한 온도 분포나 응력 분포를 보이며, 이는 크리프나 열-기계피로 손상을 유발하며, 결국 가스터빈 블레이드의 수명을 단축시킨다. 결국 다양한 운전 조건에 따라 발생하는 응력을 정확하게 계산하는 것은 설비의 신뢰성을 보장하고 나아가 블레이드와 같은 고온 부품의 정확한 수명을 평가하는데 무엇보다 중요하다. 최근 들어 컴퓨터 기능이 좋아지고 상용 소프트웨어의 성능이 향상되어 실증 시험에 대한 대안으로 유동, 열 및 구조해석을 연결하는 전산해석이 많이 사용되고 있다. 본 논문에서는 가스터빈 실 운전조건을 고려하여 유동-열-구조 해석 기법을 연계하는 유체-구조 연성해석을 통해 블레이드 온도 및 응력분포를 계산하였다. 또한 해석 결과를 토대로 대표적인 손상기구인 크리프 및 열-기계 피로 손상 모델을 이용하여 블레이드의 수명을 평가하였다.

• 대한조선학회논문집
• /
• 제59권3호
• /
• pp.164-172
• /
• 2022

A HAZID is a brainstorming workshop to identify hazards in an early phase of a project. It should be flexible to capture all probable accidents allowing experienced participants to exploit their expertise and experiences. A bowtie analysis is a graphical representation of major accident hazards elaborating safety measures i.e. barriers. The result of these workshops should be documented in an organized manner to share as good as possible details of the discussion through the lifetime of the project. Currently results are documented using a three-step representation of an accident; causes, top event and consequences, which cannot capture correctly sequence of events leading to various accidents and roles of barrier between two events. Another problem is that barriers would be shown repeatedly leading to a misunderstanding that there are an enough number of safety measures. A new bowtie analysis method is proposed to describe an accident in multiple steps showing relations among causes or consequences. With causes and consequences shown in a format of a tree, the frequencies of having the top event (Fault tree analysis) and various consequences (Event tree analysis) are evaluated automatically based on the frequency of initiating causes and the probabilities of failure of barriers. It will provide a good description of the accident scenario and help the risk to be assessed transparently.

• 한국신뢰성학회지:신뢰성응용연구
• /
• 제14권3호
• /
• pp.191-196
• /
• 2014

The importance of railway safety has become increasingly significant domestically as well as internationally, as a series of high speed railway accidents and other major accidents have occurred recently. Especially for the domestic railway, the Korean Railway Safety Law has been revised recently, mandates all the domestic railway operation authorities to render the performance of RAMS and RCM. This study inspects and analyzes the current status of the sensing technology of the electric detection system to tell the status of railway facilities in the highway railway in a real time through a sensor. It also performs the reliability analysis of the electric detection system that is being progressed as a study assignment and suggests the system construction for the higher reliability.

• 대한기계학회:학술대회논문집
• /
• 대한기계학회 2007년도 춘계학술대회A
• /
• pp.172-174
• /
• 2007

A propeller shaft is the device which is used to transmit the power between two shafts in a vehicles, an industrial machinery, etc. The end of spline is worm due to the deflection of the propeller shaft, and a lifetime of it is reduced, because it for industrial machinery has the length of 2,500 mm, the weight of $300\;kg_{f},$ and the sliding distance of $\pm250\;mm.$ Accordingly in this study we analyzed the effect of the gap of spline on the deflection of a propeller shaft carrying out the finite element analysis, in order to determine the proper gap of spline to minimize the deflection of it. We adopt 10-kinds of gap of spline from 0.05 mm to 0.5 mm at interval of 0.05 mm as the design parameter for the finite element analysis and the centrifugal force as the load condition.

• International Journal of Aeronautical and Space Sciences
• /
• 제4권2호
• /
• pp.9-16
• /
• 2003

Piezoelectric materials are widely used to construct smart or adaptive structures. Although extensive efforts have been devoted to the analysis of piezoelectric materials in recent years, most researches have been conducted by assuming that the material properties are fixed and have no uncertainties. Intrinsically, material properties have a certain amount of scatter and such uncertainties can affect the performance of component. In this paper, the convex modeling is used to consider such uncertainties in calculating the crack extension force of piezoelectric material and the results are compared with the one obtained via the Monte Carlo simulation. Numerical results show that crack extension forces increase when uncertainties considered, which indicates that such uncertainties should not be ignored for reliable lifetime prediction. Also, the results obtained by the convex modeling and the Monte Carlo simulation show good agreement, which demonstrates the effectiveness of the convex modeling.

• 한국전기전자재료학회:학술대회논문집
• /
• 한국전기전자재료학회 2001년도 하계학술대회 논문집
• /
• pp.844-847
• /
• 2001

In this work, transient characteristics of the Non-Punch Through(NPT) Insulated Gate Bipolar Transistor(IGBT) has been studied. we has analyzed with lifetimes excess minority carrier injected into N-dirft, base region of IGBT's BJT part and accumulated charge of on-state which affected swiching characteristic. In this paper, excess minority carrier and charge distribution in active base region is expressed analytically. This analysis proposed optical trade-off between lifetimes and accumulated charge for decreasing switching losses because charge result in switching loss when device was tuned off.

• 전자공학회논문지A
• /
• 제31A권1호
• /
• pp.93-102
• /
• 1994

This paper, presents SECRET(SEC REliability Tool), which predicts reliability problems related to the hot-carrier and electromigration effects on the submicron MOSFETs and interconnections. To simulate DC and AC lifetime for hot-carrier damaged devices, we have developed an accurate substrate current model with the geometric sensitivity, which has been verified over the wide ranges of transistor geometries. A guideline can be provided to design hot-carrier resistant circuits by the analysis of HOREL(HOT-carrier RFsistant Logic) effect, and circuit degradation with respect to physical parameter degradation such as the threshold voltage and the mobility can also be expected. In SECRET, DC and AC MTTF values of metal lines are calculated based on lossy transmission line analysis, and parasitic resistances, inductances and capacitances of metal lines are accurately considered when they operate in the condition of high speed. Also, circuit-level reliability simulation can be applied to the determination of metal line width and-that of optimal capacitor size in substrate bias generation circuit. Experimental results obtained from the several real circuits show that SECERT is very useful to estimate and analyze reliability problems.