• Korean Journal of Materials Research
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• v.5 no.6
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• pp.633-638
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• 1995

Two types of $Ta_2O_{5}$, films, prepared by thermal oxidation and PECVD, on P-type(100) Si wafers were studied to examine the relationship between electrical properties and stresses of the films. For the thermally oxidized films, Ta films were depositied on the Si wafers by dc magnetron sputtering followed by thermal oxidation as functions of oxidation temperature and time. The PECVD films were deposited on the Si wafers as a fuction of RF power density. The relationship between the electrical properties and film stresses were studied. In the case of thermally oxidized $Ta_2O_{5}$ film, the electrical properties and film stress were not found to be dependent on each other, while PECVD $Ta_2O_{5}$ films showed that the electrical properties were depended on the film stress.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.3
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• pp.405-412
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• 2013

This paper reports the steady-state stresses within the heat affected zone (HAZ) of a welded straight pipe subject to creep. The creep constants and exponent are varied systematically to see the effect of various mismatches in creep properties on the steady-state creep stresses, via detailed two-dimensional finite element (FE) creep analyses. The weldments consist of the base metal and weld metal with the HAZ, which are characterized using the idealized power creep laws with the same creep exponent. The internal pressure and axial loading are considered to see the effect of the loading mode. To quantify the creep stresses, a creep mismatch factor is introduced as a function of the creep constants and exponent. It is concluded that the ratio of the section-averaged stresses for a mismatched case to those for an evenmatched case are linearly dependent on the mismatch factor. The results are compared with the FE results, including the Type IV region, as well as the R5 procedure.

• Journal of Korean Society of Steel Construction
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• v.19 no.3
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• pp.291-301
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• 2007

The overlay model is a certain kinds of numerical analysis method to present the material non-lineariy which is represented the baushinger effect and the strain hardening. This model simulates the complex behavior of material by controlling the properties of the layers which like the hardening ratio, the section area and the yield stress. In this paper, the constitutive equation and plastic flow rule of each layer which are laid in the plane stress field are obtained by using the thermodynamics. Two numerical examples were tested for the validity of proposed method in uniaxial stress and plane stress field with comparable experimental results. The only parameter for the test is the yield stress distribution of each layers.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.2
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• pp.129-138
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• 2010

Technical ceramics, due to their unique physical properties, are excellent candidate materials for engineering applications involving extreme thermal and chemical environments. When ceramics are rapidly cooled, they receive thermal shock. The thermal shock parameter is defined as the critical temperature difference. The critical temperature difference for ceramic parts is influenced by its size, the convective heat transfer coefficient, etc. The thermal shock for a component is analyzed by using the transient thermal stress. If the transient thermal stress exceeds the modulus of rupture (MOR), cracking by thermal shock is initiated. The critical temperature difference for water is less than the critical temperature difference for air. The three-way catalyst substrate used in this study has an adequate performance against thermal shock because its radial and axial temperature differences existed below the critical temperature differences.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.25 no.6
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• pp.505-512
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• 2012

In this paper, an efficient yet accurate method for the thermal stress analysis using a first order shear deformation theory(FSDT) is presented. The main objective herein is to systematically modify transverse shear strain energy through the mixed variational theorem(MVT). In the mixed formulation, independent transverse shear stresses are taken from the efficient higher-order zigzag plate theory, and the in-plane displacements are assumed to be those of the FSDT. Moreover, a smooth parabolic distribution through the thickness is assumed in the transverse normal displacement field in order to consider a transverse normal deformation. The resulting strain energy expression is referred to as an enhanced first order shear deformation theory, which is obtained via the mixed variational theorem with transverse normal deformation effect(EFSDTM_TN). The EFSDTM_TN has the same computational advantage as the FSDT_TN(FSDT with transverse normal deformation effect) does, which allows us to improve the through-the-thickness distributions of displacements and stresses via the recovery procedure. The thermal stresses obtained by the present theory are compared with those of the FSDT_TN and three-dimensional elasticity.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 1996.05a
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• pp.265-272
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• 1996

액츄에이터에 사용되는 터어보 펌프의 터어빈을 구동시키기 위한 가스발생기용 고체연료 추진기관의 뒷마개부에 대하여 열/구조 해석을 수행하였다. 가스발생기는 장시간 연소모타로써 뒷마개에는 단열이 되어 있지 않은 배출튜브가 나사로 체결되어 고온, 고압의 연소가스에 의해 뒷마개 구조물에 작용하는 열 하중이 상당히 클 것으로 판단되므로, 최적설계를 위하여 뒷마개부의 열 및 구조해석을 수행하여 열하중의 영향을 예측하고 경량화를 위한 설계자료를 얻고자 하였다. 본 논문에서는 해석결과만을 언급하였으며 차후에 수행될 지상시험시에 해석치와 실험치를 비교한 후 좀 더 정확히 모델링을 하여 열/구조 해석 결과를 뒷마개부의 최적설계에 활용하고자 한다. 해석 결과 열하중이 연소관과의 조립부에는 거의 영향을 주지 않았으나, 열과 압력하중이 동시에 작용할 경우에 뒷마개 배출튜브의 조립부 근접한 곳에서 항복응력을 넘는 응력이 발생하여 정확한 구조 해석을 위해서는 탄소성해석을 수행하여야 할 것으로 판단된다.

• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.2
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• pp.286-294
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• 1988

The thermal stress intensity factors (TSIF's) for the cusp cracks such as hypocycloid crack, symmetric airfoil crack and symmetric lip crack are determined by using Bogdanoff's complex variable approaches in plane thermoplasticity. The results are expressed in terms of the periodic functions of the direction of uniform heat flow. The TSIF's are shown to be sensitive to both the direction of uniform heat flow and be thermal boundary conditions. It is also shown that Fourence's solutions for an insulated circular hole and Sih's solutions for an insulated Griffith crack are derived from the results of the stress and displacement fields for the hypocycloid crack and the TSIF's for the various cusp cracks, respectively.

• Polymer(Korea)
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• v.27 no.3
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• pp.189-194
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• 2003

Interfacial evaluation, damage sensing and cure monitoring of single carbon fiber/thermo setting composite with different curing processes were investigated using electro-micromechanical test. After curing, the residual stress was monitored by measurement of electrical resistance and then compared to various curing processes. In thermal curing case, matrix tensile strength, modulus and interfacial shear strength were higher than those of ultraviolet curing case. The shrinkage measured during thermal curing occurred significantly by matrix shrinkage and residual stress due to the difference in thermal expansion coefficient. The apparent modulus measured in the thermal curing indicated that mechanical and interfacial properties were highly improved. The reaching time to the same stress of thermal curing was faster than that of UV curing case.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.39 no.1
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• pp.11-17
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• 2015

High-density polyethylene (HDPE) piping, which has recently been applied to safety class III piping in nuclear power plants, can be butt-joined through the thermal fusion process, which heats two fused surfaces and then subject to axial pressure. The thermal fusion process generates bead shapes on the butt fusion. The stress concentrations caused by the bead shapes may reduce the fatigue lifetime. Thus, investigating the effect of the thermal butt fusion beads on fatigue behavior is necessary. This study examined the fatigue behavior of thermal butt fusion via a tensile fatigue test under stress-controlled conditions using finite element elastic stress analysis. Based on the results, the presence of thermal butt fusion beads was confirmed to reduce the fatigue lifetime in the low-cycle fatigue region while having a negligible effect in the medium- and high-cycle fatigue regions.

• Journal of Welding and Joining
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• v.13 no.4
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• pp.7-13
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• 1995

마이크로솔더링에 의한 전자기기는, 사회기능의 중추가 되는 컴퓨터, 통신 기기, 항공기 인공위성 등의 제어계를 구성하므로, 그 접속부에 대한 높은 신뢰성의 요구는 그 무엇보다 중요하다. 전자기기에 있어서의 솔더 접속부는 집과 기판의 전기 적.기계적 접속의 역할을 하고 있으며, 따라서 개개의 접속부의 파단은 전체의 불량 으로 연결된다. 실제 전자콤포넌트와 그 시스템의 단선 등의 사고에 있어서 자주 발생 하는 사고중의 하나가 솔더접속부의 단선에 의한 것이며, 그 단선중에서도 가장 보편 적이며 또한 대단히 심각한 문제로서 주목을 받고 있는 것이 솔더접속부의 열피로파단 이다. 전자기기를 사용할 때, 스위치의 on-off에 의한 power cycle과 환경의 온도변화 에 기인하는 반복열 사이클은 솔더접속부의 피로를 일으키게 되고, 결국에는 사용중에 파단을 초래하게 된다. 이러한 온도변화의 범위는 약 -55.deg. - 150.deg.C로 예상할 수 있으며, 여기서 최고온도인 150.deg.C는 Pb-Sn 공정합금의 경우 0.9Tm.p.이상의 고온에 해당한다. 이 피로는 등온적으로 또는 열사이클중에 발생하기도 한다. 솔더접 속부의 열피로수명은 대부분의 공업재료에서 나타나는 저사이클피로거동과 유사하게 발생하며, 솔더 접속부에 인가되는 열변형/응력(thermal strain/stress)의 크기에 크게 의존하는 것으로 알려져 있다. 솔더는 서로 다른 열팽창계수를 갖는 칩과 회로 기관의 두종류의 재료를 접속하기 때문에, 상기한 바와 같은 반복열사이클에 의하여 발생하는 열변형/응력이 접속부의 피로.파단을 야기시킨다. 이러한 솔더접속부에 대한 주기적인 응력/변형의 인가는 접속부에 내.외적으로 현저한 변화를 야기시키게 되고, 열피로로 연결되며 결국에는 시스템의 전기적 단선을 초래하게 된다. 또한 열피로파단 현상는 변형/응력의 크기 뿐 만아니라 솔더합금자체의 야금학적인 물성에도 크게 의존 하며, 내적.외적인 열변화에 의한 야금학적인 특성변화도 크게 영향을 미친다. 솔더 접속부의 신뢰성에 대한 연구는, 그 중요성에 비추어 볼 때, 지금까지 수많은 연구가 행하여져 왔다. 그러나 신뢰성과 관련된 열피로파단현상에 대한 야금학적인 면에서의 연구는 비교적 적은 편이다. 따라서 본 해설에서는 전자기기의 마이크로 솔더접속부 에서 발생하는 열피로파단현상에 대한 야금학적인 면에 중점을 두어 서술하고자 한다.