• Journal of Hydrospace Technology
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• 제2권1호
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• pp.41-54
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• 1996

Sculptured surface structures such as ship hulls are traditionally formed up to the required double curved shape by line heating method. The nature of the line heating process is a transient thermal process, followed by a thermo-elastic-plastic stress field. The permanant shape is dependent on many factors involved in the process, Among them are torch speed and path, supplied heat type and amount , and plate size. Thus, the work is essentially leaded by experts with lots of experiences. However, in order to effectively improve productivity through automation, each factor should be clearly examined how much it affects the final shape. This can not be done only by experiments, but can be achieved by a mechanics-based approach. In this paper, we propose a conceptual configuration for plate forming system, and then present simulations of the line heating process with numerical data in practices and suggest a computerized process of the line heating for practical applications. The modeling of heating torch, water cooling, and the plate to be formed is proposed for the finite element analysis after the mechanics of line heating is studied. Parametric studies are given and discussed for the effects of plate thickness, torch speed and initial curvature in forming a saddle typed surface.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2005년도 추계학술발표대회 개요집
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• pp.79-81
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• 2005

Bridges constructed recently are preferred to have long spans and simple structure details considering not only the function as bridge but scenic beauty, maintenance, construction term and life cycle cost, etc. Therefore, they require high performance steels like extra-thick plate steels and TMCP steels. A TMCP steel produced by themo-mechanical control process is now spot lighted due to the weldability for less carbon equivalent. It improved at strength and toughness in microstructure. Recently, the SM570-TMC steel which is a high strength TMCP steel whose tensile strength is 600MPa has been developed and applied to steel structures. But, for the application of this steel to steel structures, it is necessary to elucidate not only the material characteristics but also the mechanical characteristic of welded joints. In this study, the characteristics of residual stresses in welded joints of SM570-TMC steel were studied through the three-dimensional thermal elastic-plastic analyses on the basis of mechanical properties at high temperatures obtained from the elevated temperature tensile test.

• 한국정보통신학회:학술대회논문집
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• 한국해양정보통신학회 2004년도 SMICS 2004 International Symposium on Maritime and Communication Sciences
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• pp.70-73
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• 2004

The packaging of the integrated circuits requires knowledge of ceramics and metals to accommodate the fabrication of modules that are used to construct subsystems and entire systems from extremely small components. Composite ceramics (Al$_2$O$_3$-SiO$_2$) were tested for substrates. A stress analysis was conducted for a linear work-hardening metal cylinder embedded in an infinite ceramic matrix. The bond between the metal and ceramic was established at high temperature and stresses developed during cooling to room temperature. The calculations showed that the stresses depend on the mismatch in thermal expansion, the elastic properties, and the yield strength and work hardening rate of the metal. Experimental measurements of the surface stresses have also been made on a Cu/Al$_2$O$_3$-SiO$_2$ceramic system, using an indentation technique. A comparison revealed that the calculated stresses were appreciably larger than the measured surface stresses, indicating an important difference between the bulk and surface residual stresses. However, it was also shown that porosity in the metal could plastically expand and permit substantial dilatational relaxation of the residual stresses. Conversely it was noted that pore clusters were capable of initiating ductile rupture, by means of a plastic instability, in the presence of appreciable tri-axiality. The role of ceramics for packaging of microelectronics will continue to be extremely challenging.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2003년도 추계학술대회 논문집 Vol.16
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• pp.308-312
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• 2003

The packaging of the integrated circuits requires knowledge of ceramics and metals to accommodate the fabrication of modules that are used to construct subsystems and entire systems from extremely small components. Composite ceramics ($Al_2O_3-SiO_2$) were tested for substrates. A stress analysis was conducted for a linear work-hardening metal cylinder embedded in an infinite ceramic matrix. The bond between the metal and ceramic was established at high temperature and stresses developed during cooling to room temperature. The calculations showed that the stresses depend on the mismatch in thermal expansion, the elastic properties, and the yield strength and work hardening rate of the metal. Experimental measurements of the surface stresses have also been made on a $Cu/Al_2O_3-SiO_2$ ceramic system, using an indentation technique. A comparison revealed that the calculated stresses were appreciably larger than the measured surface stresses, indicating an important difference between the bulk and surface residual stresses. However, it was also shown that porosity in the metal could plastically expand and permit substantial dilatational relaxation of the residual stresses. Conversely it was noted that pore clusters were capable of initiating ductile rupture, by means of a plastic instability, in the presence of appreciable tri-axiality. The role of ceramics for packaging of microelectronics will continue to be extremely challenging.

• Steel and Composite Structures
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• 제10권3호
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• pp.207-222
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• 2010

Fire incident in buildings is common, so the fire safety design of the framed structure is imperative, especially for the unprotected or partly protected bare steel frames. However, software for structural fire analysis is not widely available. As a result, the performance-based structural fire design is urged on the basis of using user-friendly and conventional nonlinear computer analysis programs so that engineers do not need to acquire new structural analysis software for structural fire analysis and design. The tool is desired to have the capacity of simulating the different fire scenarios and associated detrimental effects efficiently, which includes second-order P-D and P-d effects and material yielding. Also the nonlinear behaviour of large-scale structure becomes complicated when under fire, and thus its simulation relies on an efficient and effective numerical analysis to cope with intricate nonlinear effects due to fire. To this end, the present fire study utilizes a second-order elastic/plastic analysis software NIDA to predict structural behaviour of bare steel framed structures at elevated temperatures. This fire study considers thermal expansion and material degradation due to heating. Degradation of material strength with increasing temperature is included by a set of temperature-stress-strain curves according to BS5950 Part 8 mainly, which implicitly allows for creep deformation. This finite element stiffness formulation of beam-column elements is derived from the fifth-order PEP element which facilitates the computer modeling by one member per element. The Newton-Raphson method is used in the nonlinear solution procedure in order to trace the nonlinear equilibrium path at specified elevated temperatures. Several numerical and experimental verifications of framed structures are presented and compared against solutions in literature. The proposed method permits engineers to adopt the performance-based structural fire analysis and design using typical second-order nonlinear structural analysis software.

• 대한조선학회논문집
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• 제39권2호
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• pp.69-80
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• 2002

선박의 곡외판은 냉간가공과 열간가공(선상가열) 과정을 거쳐 제작된다. 선상가열의 자동화 및 기계화에 대한 연구가 다양하게 수행되어 오고 있다. 특히 가열조건과 잔류 변형 사이의 관계규명은 자동화 및 기계화를 위한 주요 요소로 생각된다. 본 연구는 그러한 관계를 규명하기 위하여 일련의 체계적인 과정을 제시하고 관계식을 제안하였다. 선상가열에 의한 변형은 3차원 열탄소성 변형현상으로 정식화될 수 있으며, 열탄소성 변형현상을 유한요소해석을 이용하여 다양한 가열조건에 대한 수치해석을 수행하였다. 유한요소법의 유용성을 검증하기 위하여 실험을 수행하였고 그 유용성을 확인하였다 변형관계식에 사용된 입력변수는 차원해석을 통하여 선정하였다. 유한요소법을 이용하여 얻어진 결과를 활용하여 선상가열에 의한 변형관계식을 추정하기 위하여 다변수해석과 다차원 보관과정을 예시하였다. 일련의 과정을 통하여 본 연구는 선상가열의 가열조건과 잔류 변형 사이의 관계식을 선정하기 위한 방법을 제안하였다.

• 한국해양공학회지
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• 제22권6호
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• pp.75-82
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• 2008

Flame bending has been extensively used in the shipbuilding industry for hull plate forming In flame bending it is difficult to obtain the desired shape because the residual deformation dependson the complex temperature distribution and the thermal plastic strain. Mechanical bending such as reconfigurable press forming multi-point press forming or die-less forming has been found to improve the automation of hull plateforming because it can more accurately control the desired shape than line heating. Multi-point forming is a process in which external forces are used to form metal work-pieces. Therefore it can be a flexible and efficient forming technique. This paper presents an optimal approach to determining the press-stroke for multi-point press forming of curved shapes. An integrated configuration of Finite element analysis (FEA) and spring-back compensation algorithm is developed to calculate the strokes of the multi-point press. Not only spring-back is modeled by elastic plastic shell elements but also an iterative algorithm to compensate the spring-back is applied to adjust the amount of pressing stroke. An iterative displacement adjustment (IDA) method is applied by integration of the FEA procedure and the spring-back compensation work. Shape deviation between the desired surface and deform￡d plate is minimized by the IDA algorithm.

• 폴리머
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• 제25권1호
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• pp.56-62
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• 2001

Polyethylene terephthalate(PET) 필름 시편을 약 0.5~500 mm/min의 속도까지 단계적으로 연신시켰을 때, 플라스틱 변형이 일어나는 동안에 투명 및 불투명 밴드들이 존재하는 넥킹영역이 형성되었다. 상기 시편의 응력-변형 곡선을 살펴보면, 응력 진동이 뚜렷하게 발생하였음을 확인할 수 있었다. 한편, 열처리된 시편의 응력-변형 곡선을 살펴보면, 응력 진동이 발생하지 않았음을 확인할 수 있었다. 시편들의 미세구조는 광학 현미경을 통하여 동적으로 관찰하였고, 시편들의 열적 특성은 시차 주사 열량기를 사용하여 10 ${\circ}C$/min의 승온 속도에서 측정하였다. 또한, 시편들의 배향화 및 결정화 정도는 단색-핀홀법을 이용하여 측정하였고, 시편들의 탄성계수는 동적 기계 분석기를 통하여 -150~70 ${\circ}C$의 온도 범위에서 1 Hz의 주파수 대에서 측정하였다. PET 펠렛을 전기로에서 약 83${\circ}C$에서 30분 동안 열처리하여 투명한 PET 제품을 제작하였다.

• Nuclear Engineering and Technology
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• 제20권2호
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• pp.88-104
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• 1988

이차원의 유한요소법을 이용하여 axisymmetric R-$\theta$system으로 나누어서 정상과 부하추종 운전시에 핵연료 페렛트와 피복관의 열역학적 거동을 분석하기 위해서 FURA전산코드를 개발하였다. 온도분포와 내부압력을 정확히 계산하기 위해서 페렛트와 피복관의 변형과 핵분열의 기체방출을 전체 핵연료봉 길이로 고려하였다. 열역학적 평 형방정식을 얻기 위해서 Galerkin's Technique과 가상일의 원리를 사용하였고 역학적 해석을 위해서 탄성-소성, 크리프뿐만아니라 스엘링, 재배열, 고밀화 현상등을 고려하였다. 기하학적 모델에서는 4-결점 요소라 페레트 길이의 1/2만을 택하였다. 비선형식을 안정하게 해석하기 위해서 음해법을 도입하여 뉴튼-랩손 반복법을 적용하였다 이 코드의 검증은 해석해와 실험데이타로 비교하였다. 핵연료봉의 일반적인 거동은 axisymmetry system으로 계산하였고 균열된 페레트에 접촉하는 피복관의 거동은 R-$\theta$system을 사용하였다. 부하추종에 의한 피복관의 변형시효의 민감도는 출력율, 진동수, 진폭등으로 비교하였다.

• 한국강구조학회 논문집
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• 제16권5호통권72호
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• pp.519-528
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• 2004

대부분의 용접재는 용접 후 빠른 속도로 냉각하는 과정에서 재료조직의 변화, 즉 상변태(마르텐사이트변태)가 일어난다. 그리고 이러한 상변태에 의한 체적의 팽창은 특히 고강도강재의 용접시 용접부의 잔류응력완화에 기여한다. 따라서 고강도강을 적용한 강구조물 접합부의 잔류응력 특징을 파악하기 위해서는 고강도강 용접시 상변태가 잔류응력완화에 미치는 효과를 정량적으로 평가할 수 있는 수치적 모델을 확립해야 한다. 본 연구에서는 고강도강 용접시 상변태에 의한 체적의 팽창이 잔류응력 완화에 미치는 영향을 잔류응력 측정 실험을 통하여 조사하였고, 이를 바탕으로 상변태에 의한 잔류응력완화를 고려한 고강도강 및 이종강재 용접접합부의 잔류응력 특징을 3차원 열탄소성 해석기법 개발을 통하여 재현하였다.