• 대한기계학회논문집A
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• 제35권11호
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• pp.1461-1469
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• 2011

본 논문은 3 차원 유한요소해석 기법을 이용하여 토오크 시험을 위한 표준시험시편의 각도 및 동심 오열이 시편의 응력과 변형률 변화에 미치는 영향을 분석하였다. 해석 결과의 정량적 비교를 위해 각, 동심 및 복합 축 오열에 대한 평균 굽힘 변형률을 적용하였으며, 시편 형상에 따른 축 오열 영향을 확인하기 위해 환봉형 시편과 튜브형 시편에 대해 각각 유한요소해석을 실시하였다. 해석결과로부터 얻어진 변형률과 응력의 변화로 축 오열의 종류와 방향을 예측하는 일반적인 기준을 제시하였으며, 초기 항복조건을 적용하여 축 오열이 토오크 시편의 초기 항복 모멘트에 미치는 영향을 분석하였다.

• 대한기계학회논문집A
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• 제25권12호
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• pp.1995-2001
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• 2001

In this study, a small punch creep(SP-Creep) test using miniaturized specimen(10${\times}$10${\times}$0.5mm) is described to develop the new creep test method for high temperature structural materials. The SP-Creep test is applied to 2.25Cr-lMo(STBA24) steel which is widely used as boiler tube material. The test temperatures applied for the creep deformation of miniaturized specimens are between 550∼600$^{\circ}C$. The SP-Creep curves depend definitely on applied load and creep temperature, and show the three stages of creep behavior like in conventional uniaxial tensile creep curves. The load exponent of miniaturized specimen decreases with increasing test temperature, and its behavior is similar to stress exponent behavior of uniaxial creep test. The creep activation energy obtained from the relationship between SP-Creep rate and test temperature decreases as the applied load increases. A predicting equation or SP-Creep rate for 2.25Cr-lMo steel is suggested. and a good agreement between experimental and calculated data has been found.

• 한국지진공학회논문집
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• 제2권2호
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• pp.45-55
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• 1998

액상화 가능성을 예측하기위해 많은 방법들이 제안되어왔지만, 재료의 미세구조의 특성을 이용한 방법은 거의 없다. 본 연구에서는, 동하중을 받는 재료의 전체 응력-변형률의 특징에 대해 통합적 구성방식을 제공하는 교량상태개념(DSC)모델에 이론적 기초를 두며, 포화 사질토의 액상와 가능성을 평가하기 위한 새로운 방법을 제안한다. DSC이론으로부터, 미세구조의 변형시 재료내부 상태의 급격한 변화가 일어나는 시점의 교란도(한계교란도: Dc)를 초기 액상화 시기로 정의한다. 본 방법은 포화 Ottawa 모래를 이용한 진동 실삼축시험 결과로부토 얻은 자료를 이용하여 적합성이 검토되었다. 또한 본 연구로부터, 액상화와 초기 구속압사이의 관계가 Dc의 개념으로부터 밝혀졌다. 본 연구에서 제안한 새로운 액상화 평가방법은 액상화 거동을 예측할 수 있으며, 결론적으로 기존의 경험적 방법보다 향상된 방법이라고 사료된다.

• Applied Microscopy
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• 제30권4호
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• pp.337-345
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• 2000

사파이어 기판을 이용한 GaN 박막성장에서 완충층의 사용과 기판의 질화처리는 GaN 박막 내의 격자결함을 줄이는 가장 보편적인 방법이다. GaN박막의 초기 핵생성과 성장 거동을 향상시키기 위한 새로운 방법으로 사파이어 표면을 질소 활성화 이온빔으로 처리하는 방법이 시도되었다. 활성화 이온빔 처리의 결과 약 10nm두께의 비정질 $AlO_xN_y$ 층이 형성되었으며 GaN의 성장온도에서 부분적으로 결정화되어 계면 부위에 고립된 비정질 영역으로 존재하였다. 계면에 존재하는 비정질 층은 기판과 박막사이에서 발생하는 열응력을 효과적으로 감소시키는 역할이 가능하며 이를 확인하기 위하여 활성화 이온빔 처리에 의한 GaN박막 내의 격자변형량 차이를 비교하였다. GaN박막에서 얻어진 $[\bar{2}201]$ 정대축고차 Laue도형을 전산모사 도형과 비교하여 격자변형량을 측정하였다. 본 연구의 결과 활성화 이온빔 처리를 하지 않은 기판 위에 성장시킨 GaN박막의 격자변형량은 처리한 경우에 비해 6배 이상 높은 값을 가졌으며 따라서 활성화 이온빔 처리에 의해 GaN박막의 열응력은 크게 감소함을 확인하였다.

• Steel and Composite Structures
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• 제52권1호
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• pp.31-43
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• 2024

A very widely used analytical method (mathematical model), mentioned in Eurocode 3, to examine the connections' bending behavior is the component-based method that has certain weak points shown in the plastic behavior part of the moment-rotation curves. In the component method available in Eurocode 3, for simplicity, the effect of strain hardening is omitted, and the bending behavior of the connection is modeled with the help of a two-line diagram. To make the component method more efficient and reliable, this research proposed its advanced version, wherein the plastic part of the diagram was developed beyond the guidelines of the mentioned Regulation, implemented to connect the end plate, and verified with the moment-rotation curves found from the laboratory model and the finite element method in ABAQUS. The findings indicated that the advanced component method (the method developed in this research) could predict the plastic part of the moment-rotation curve as well as the conventional component-based method in Eurocode 3. The comparison between the laboratory model and the outputs of the conventional and advanced component methods, as well as the outputs of the finite elements approach using ABAQUS, revealed a different percentage in the ultimate moment for bolt-extended end-plate connections. Specifically, the difference percentages were -31.56%, 2.46%, and 9.84%, respectively. Another aim of this research was to determine the optimal dimensions of the end plate joint to reduce costs without letting the mechanical constraints related to the bending moment and the resulting initial stiffness, are not compromised as well as the safety and integrity of the connection. In this research, the thickness and dimensions of the end plate and the location and diameter of the bolts were the design variables, which were optimized using Particle Swarm Optimization (PSO), Snake Optimization (SO), and Teaching Learning-Based Optimization (TLBO) to minimization the connection cost of the end plate connection. According to the results, the TLBO method yielded better solutions than others, reducing the connection costs from 43.97 to 17.45€ (60.3%), which shows the method's proper efficiency.

• Smart Structures and Systems
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• 제3권3호
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• pp.281-298
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• 2007

The load carrying capacity of a bridge needs to be properly assessed to operate the bridge safely and maintain it efficiently. For the evaluation of load carrying capacity considering the current state of a bridge, static and quasi-static loading tests with weight-controlled heavy trucks have been conventionally utilized. In these tests, the deflection (or strain) of the structural members loaded by the controlled vehicles are measured and analyzed. Using the measured data, deflection (or strain) correction factor and impact correction factor are calculated. These correction factors are used in the enhancement of the load carrying capacity of a bridge, reflecting the real state of a bridge. However, full or partial control of the traffic during the tests and difficulties during the installment of displacement transducers or strain gauges may cause not only inconvenience to the traffic but also the increase of the logistics cost and time. To overcome these difficulties, an alternative method is proposed using an excited response part of full measured ambient acceleration data by ordinary traffic on a bridge without traffic control. Based on the modal properties extracted from the ambient vibration data, the initial finite element (FE) model of a bridge can be updated to represent the current real state of a bridge. Using the updated FE model, the deflection of a bridge akin to the real value can be easily obtained without measuring the real deflection. Impact factors are obtained from pseudo-deflection, which is obtained by double-integration of the acceleration data with removal of the linear components on the acceleration data. For validation, a series of tests were carried out on a steel plategirder bridge of an expressway in Korea in four different seasons, and the evaluated load carrying capacities of the bridge by the proposed method are compared with the result obtained by the conventional load test method.

• 한국철도학회논문집
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• 제10권2호
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• pp.186-194
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• 2007

This study was performed to compare the load-carrying capacities of the reinforced concrete structure between the carbon fiber adhesion reinforcement method and the external post-tensioning method and further estimate the effective stress of the reinforced material by analyzing the experimental reinforcing effect of each method and the behavior resulting from each method. As a result, it was found out that the effective stress of the carbon fiber reinforcement according to the carbon fiber adhesion reinforcement method had an unexpected value, and also, bearing of the stress was found to be far from sharing thereof. That is to say, while the carbon fiber was bearing the whole stress to some limits, it got to be momentarily ruptured as soon as it went beyond such limits. On the other hand, the external post-tensioning method has the advantage of inducing an initial effective stress by introducing a strain, and thus, it was found that behavior or bearing of the stress was also found to be a solid behavior of the steel wire. This method was also found to be more efficient and excellent than the carbon fiber adhesion reinforcement method in the reinforcing effect or securing the effective stress. Accordingly, we were to discuss the effective stress as comparatively examined, focusing on deriving of the more enhanced reinforcing effect on the basis of the experiment to which the field characteristic is added.

• Journal of Microbiology and Biotechnology
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• 제21권1호
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• pp.109-114
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• 2011

In this study, we investigated the feasibility of producing bioethanol from the hydrolysate of rape stem. Specifically, the most ideal yeast strain was screened, and the microaeration was performed by surface aeration on a liquid medium surface. Among the yeast strains examined, Pichia stipitis CBS 7126 displayed the best performance in bioethanol production during the surface-aerated fermentor culture. Pichia stipitis CBS 7126 produced maximally 9.56 g/l of bioethanol from the initial total reducing sugars (about 28 g/l). The bioethanol yield was 0.397 (by the DNS method). Furthermore, this controlled surface aeration method holds promise for use in the bioethanol production from the xylose-containing lignocellulosic hydrolysate of biomass.

• 대한기계학회논문집A
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• 제21권1호
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• pp.124-132
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• 1997

This paper describes an experimental and numerical study on growing ductile crack-tip behaviors. The hybrid experimental and numerical method by means of a computer image processign technique, was applied to the analysis of both base metal and weld metal CT specimens. In the weld metal specimen, the initial crack-tip was placed in front of fusion line, and the crack orientation was perpendicular to it. Finite element analysis of crack growth behaviors in both base and weld matal specimens made of A533B Class 1 steel were also performed to examine the effects of weldment on near crack-tip fields. a series of experimental studies on crack-tip behaviors have clearly shown the qualitative effects of material properties, especially a hardening exponent. The experimental and numerical results have also shown that weldment does not affect displacement and strain fields near a crack-tip while a stress field is influenced by the difference between yield stresses of both base and weld metals.

• 한국소음진동공학회논문집
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• 제15권3호
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• pp.341-345
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• 2005

Structural Dynamics Modification (SDM) is a very effective technique to improve structure's dynamic characteristics by adding or removing auxiliary structures, changing material properties and shape of structure. Among those of SDM technique, the method to change shape of structure has been mostly relied on engineer's experience and trial-and-error process which are very time consuming. In order to develop a systematic method to change structure shape, surface grooving technique is studied. In this work, the shape of base structure was modified to improve its dynamic characteristics such as natural frequencies via surface grooving technique. Grooving shape was formed by mergingthe neighboring small embossing elements after analyzing frequency increment sensitivities of all the neighboring emboss elements. For this process, Criterion Factor was introduced and the initial grooving was started from the element having highest strain energy and the grooving is expanded into neighboring element. The range of targeting grooving area to check its frequency variations restricted to their neighboring area to reduce the computation effort. This surface grooving technique was successfully applied to a hard disk drives (HDD) cover model to raise its natural frequency by giving some groove on its surface.