• 대한전기학회:학술대회논문집
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• 대한전기학회 1987년도 전기.전자공학 학술대회 논문집(I)
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• pp.469-472
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• 1987

The hot-carrier induced degradation in very short-channel MOSFET was studied systematically. Under the traditional DC stress conditions, the threshold voltage shift (${\Delta}Vt$) and the transconductance degradation (${\Delta}Gm$/(Gmo-${\Delta}Gm$)) were confirmed to depend exponentially on the stress time and the dependency between the two parameters was proved to be linear. And the degradation due to the DC stress across gate and drain was studied. As the AC dynamic process is more realistic in actual device operation, the effects of dynamic stresses were studied.

• Structural Engineering and Mechanics
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• 제71권1호
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• pp.37-43
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• 2019

This paper studies application of modified couple stress theory and first order shear deformation theory to magneto-electro-mechanical vibration analysis of three-layered size-dependent curved beam. The curved beam is resting on Pasternak's foundation and is subjected to mechanical, magnetic and electrical loads. Size dependency is accounted by employing a small scale parameter based on modified couple stress theory. The magneto-electro-mechanical preloads are accounted in governing equations to obtain natural frequencies in terms of initial magneto-electro-mechanical loads. The analytical approach is applied to investigate the effect of some important parameters such as opening angle, initial electric and magnetic potentials, small scale parameter, and some geometric dimensionless parameters and direct and shear parameters of elastic foundation on the magneto-electro-elastic vibration responses.

• 대한기계학회논문집A
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• 제32권8호
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• pp.642-653
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• 2008

In this paper, we propose a 3-D finite element (FE) analysis model with combined physical behavior and kinematical impact factors for evaluation of residual stress in multi-impact shot peening. The FE model considers both physical behavior of material and characteristics of kinematical impact. The physical parameters include elastic-plastic FE modeling of shot ball, material damping coefficient, dynamic friction coefficient. The kinematical parameters include impact velocity and diameter of shot ball. Multi-impact FE model consists of 3-D symmetry-cell. We can describe a certain repeated area of peened specimen under equibiaxial residual stress by the cell. With the cell model, we investigate the FE peening coverage, dependency on the impact sequence, effect of repeated cycle. The proposed FE model provides converged and unique solution of surface stress, maximum compressive residual stress and deformation depth at four impact positions. Further, in contrast to the rigid and elastic shots, plastically deformable shot produces residual stresses closer to experimental solutions by X-ray diffraction. Consequently, it is confirmed that the FE model with peening factors and plastic shot is valid for multi-shot peening analyses.

• 한국응급구조학회지
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• 제17권3호
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• pp.87-100
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• 2013

purpose: This study aims to understand the relationship between stress in college life, vulnerability to depression, and smartphone overuse among undergraduate students majoring in public health. Methods: The subjects of the study were 320 students who were selected through convenience sampling from the department of emergency medical technology, physical therapy, and public health and administration attending G University in Gwangju. The study was carried out from October 22 to 26, 2012. For final analysis, 266 copies were used. Statistical analysis was done using SPSS WIN 12.0. Results: Excessive use of smartphones had a negative effect on stress in college life and vulnerability to depression. The stress of college life had an effect on students' sense of value, their friends of the same sex, and job. Depressive vulnerability had an effect on a students' dependency. Conclusion: For the prevention of smartphone overuse, programs to prevent college life stress and vulnerability to depression should be developed. Counselling programs for the groups should be made available to the students.

• 소성∙가공
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• 제24권1호
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• pp.52-61
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• 2015

Evaluation of the elevated temperature flow stress for thermoplastic fiber metal laminates(TFMLs) sheet, comprised of two aluminum sheets in the exterior layers and a self-reinforced polypropylene(SRPP) in the interior layer, was conducted. The flow stress as a function of temperature should be evaluated prior to the actual forming of these materials. The flow stress can be obtained experimentally by uniaxial tensile tests or analytically by deriving a flow stress model. However, the flow stress curve of TFMLs cannot be predicted properly by existing flow stress models because the deformation with temperature of these types of materials is different from that of a generic pure metallic material. Therefore, the flow stress model, which includes the effect of the temperature, should be carefully identified. In the current study, the flow stress of TFMLs were first predicted by using existing flow stress models such as Hollomon, Ludwik, and Johnson-Cook models. It is noted that these existing models could not effectively predict the flow stress. Flow stress models such as the modified Hollomon and modified Ludwik model were proposed with respect to temperatures of $23^{\circ}C$, $60^{\circ}C$, $90^{\circ}C$, $120^{\circ}C$. Then the stress-strain curves, which were predicted using the proposed flow stress models, were compared to the stress-strain curves obtained from experiments. It is confirmed that the proposed flow stress models can predict properly the temperature dependent flow stress of TFMLs.

• 터널과지하공간
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• 제22권6호
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• pp.462-470
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• 2012

일반화된 Hoek-Brown 암반파괴조건식을 Mohr-Coulomb 파괴조건에 기초한 암반구조물 해석법에 적용시키기 위해서는 등가 마찰각과 등가 점착력을 계산하는 과정이 필요하다. Balmer(1952)이론에 기초한 기존의 접선 순간마찰각과 순간점착력 계산식은 최소주응력 ${\sigma}_3$의 함수로 표시되므로 등가 강도정수의 정수압 의존성 및 응력경로 의존성을 이해하는 데 적합지 않다. 이 연구에서는 응력불변량을 이용하여 일반화된 Hoek-Brown식의 접선 순간마찰각과 순간점착력 계산하는 방법을 제시하여 기존의 방법이 갖는 단점을 극복하였다. 제시된 방법을 이용한 예제 해석을 통해 접선 순간마찰각과 순간점착력의 정수압 의존특성 및 파괴곡면의 팔면체 단면에서 Lode각의 의존성을 고찰하였다. 접선 순간마찰각은 삼축신장 응력조건에서 가장 크며, 접선 순간점착력은 삼축압축 응력조건에서 가장 큰 것으로 나타났다. 접선 순간마찰각과 순간점착력의 정수압 및 Lode각 의존성은 GSI 값이 큰 양호한 암반에서 상대적으로 큰 것으로 나타났다.

• 대한기계학회논문집A
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• 제33권10호
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• pp.1158-1162
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• 2009

Fatigue limit of the copper film coated by Sn was estimated using Goodman diagram and Gerber diagram. To obtain the high cycle fatigue life curve, S-N curve, of the film, the high cycle fatigue test was carried out by applying the constant amplitude load to the film specimen with three different stress ratio of 0.05, 0.3 and 0.5 and the frequency of 40 Hz at room temperature in air. The free-standing film specimen 15.26${\mu}m$ thick was fabricated by etching process. The fatigue limits and S-N curves at the respective stress ratios were determined from the experimental works. It was shown that the S-N curves were dependent on the stress ratio and the fatigue limit was increased with decreasing the stress ratio. The dependency of the fatigue behavior was presented in empirical relationship. Using these relationships, the fatigue limit was predicted.

• 소성∙가공
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• 제27권4호
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• pp.227-235
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• 2018

The flow stresses have been identified prior to a numerical simulation for predicting a deformation of materials using the experimental or analytical analysis. Recently, the flow stress models considering the temperature effect have been developed to reduce the number of experiments. Artificial neural network can provide a simple procedure for solving a problem from the analytical models. The objective of this paper is the prediction of flow stress on the fiber metal laminate using the artificial neural network. First, the training data were obtained by conducting the uniaxial tensile tests at the various temperature conditions. After, the artificial neural network has been trained by Levenberg-Marquardt method. The numerical results of the trained model were compared with the analytical models predicted at the previous study. It is noted that the artificial neural network can predict flow stress effectively as compared with the previously-proposed analytical models.

• 터널과지하공간
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• 제5권3호
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• pp.251-265
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• 1995

One of the most important matters in stress measurement by hydraulic fracturing technique is the determination of the breakdown pressure, reopening pressure, and shut-in pressure, since these values are the basic input data for the calculation of the in-situ stress. The control of the fracture propagation is also important when the hydraulic fracturing technique is applied to the development of groundwater system, geothermal energy, oil, and natural gas. In this study, a laboratory scale hydraulic fracturing device was built and a series of model tests were conducted with cube blocks of Machon gabbro. A new method called 'flatjack method' was adopted to determine shut-in pressure. The initial stress calculated from the shut-in pressure measured by flatjack method showed much higher accuracy than the stress determined by the conventional method. The dependency of the direction of fracture propagation on the state of the initial stresses was measured by introducin g artificial slots in the borehole made by water jet system. Numerical modeling by BEM was also performed to simulate the fracture propagation process. Both results form numerical and laboratory tests showed good agreement. From this study which provides the extensive results on the determination of shut-in pressure and the control of fracture propagation which are the critical issue in the recent hydraulic fracturing, it is conclued that in-situ stress measurement and the control of fracture propagation could be achived more accurately.

• Corrosion Science and Technology
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• 제2권2호
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• pp.82-87
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• 2003

Stress corrosion cracking of Alloy 600 and Alloy 690 has been studied with a C-ring specimen in 1%, 10% and 40% NaOH at $315^{\circ}C$. SCC test was performed at 200 mV above corrosion potential. Initial stress on the apex of C-ring specimen was varied from 300 MPa to 565 MPa. Materials were heat treated at various temperatures. SCC resistance of Ni-$_\chi$Cr-10Fe alloy increased as the Cr content of the alloy increased if the density of an intergranular carbide were comparable. SCC resistance of Alloy 600 increased in caustic solution as the product of coverage of an intergranular carbide in grain boundary, intergranular carbide thickness and Cr concentration at grain boundary increased. Low temperature mill annealed Alloy 600 with small grain size and without intergranular carbide was most susceptible to SCC. TT Alloy 690 was most resistant to SCC due to the high value of the product of coverage of an intergranular carbide in grain boundary, intergranular carbide thickness and Cr concentration at grain boundary. Dependency of SCC rate on stress and NaOH concentration was obtained.