• 대한금속재료학회지
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• 제56권11호
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• pp.779-786
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• 2018

There have been previous attempts to observe the occurrence of dynamic ferritic transformation at temperatures even above $Ae_3$ in a low-carbon steel, and not only in steels, but recently also in titanium alloys. In this study, a new approach is proposed that involves treating true stress-true strain curves in uniaxial compression tests at various temperatures, and different strain rates in 0.1C-6Ni steel, which is a model alloy used to decelerate the kinetics of ferrite transformation from austenite. The initial flow stress up to peak stress was used to analyze the change in dynamic softening phenomena, such as dynamic recovery, dynamic recrystallization, and dynamic transformation. It is worth mentioning that for predicting the occurrence of dynamic transformation, flow stress before reaching peak stress is much more sensitive to the change in the dynamic softening rate due to dynamic transformation, compared to peak stress. It was found that the occurrence of dynamic ferritic transformation could be successfully obtained even at temperatures above $Ae_3$ once the deformation condition was satisfied. This deformation condition is a function of both the strain rate and the deformation temperature, which can be described as the Zener - Hollomon parameter. In addition, the driving force of dynamic ferritic transformation might be much less than that of the dynamic recrystallization of austenite at a given deformation condition. By applying this technique, it is possible to predict the occurrence of dynamic transformation more sensitively compared with the previous analysis method using peak stress during deformation.

• 한국농공학회지
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• 제44권1호
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• pp.103-115
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• 2002

During this study, various stress path tests in previous isotropic and anisotropic (compression and tension) stress histories are performed on weathered granite soil sampled at Iksan, Jeonbuk. Yielding points are determined from various stress-strain curves(stress ratio-shear strain, volumetric strain, normalized energy and dissipated total energy curves). The shape and characteristics of isotropic and anisotropic yielding curves are examined. The main results are summarized as follows . 1) Yielding curries defined from stress ratio - normarized energy and dissipated total energy curves show almost perfect ellipse. 2) Directions of plastic strain incremental vector are not perpendicular to yielding curve. 3) Normarized energy and dissipated total energy spread with similar tendency with respect to yielding currie in stress space.

• Structural Engineering and Mechanics
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• 제20권3호
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• pp.335-346
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• 2005

The T-stress of cracks in elastic sheets is solved by using the fractal finite element method (FFEM). The FFEM, which had been developed to determine the stress intensity factors of cracks, is re-applied to evaluate the T-stress which is one of the important fracture parameters. The FFEM combines an exterior finite element model with a localized inner model near the crack tip. The mesh geometry of the latter is self-similar in radial layers around the tip. The higher order Williams series is used to condense the large numbers of nodal displacements at the inner model near the crack tip to a small set of unknown coefficients. Numerical examples revealed that the present approach is simple and accurate for calculating the T-stresses and the stress intensity factors. Some errors of the T-stress solutions shown in the previous literature are identified and the new solutions for the T-stress calculations are presented.

• 대한기계학회논문집
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• 제18권7호
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• pp.1705-1712
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• 1994

Hilbert problems are derived to evaluate thermal stress intensity factors for a partially insulated crack subjected to vertically uniform heat flow in infinite bonded dissimilar materials. In case of fully insulated crack surface, the present solutions of thermal stress intensity factors are reduced into the same as the previous results. For the homogeneous material, mode II thermal stress intensity factor only exists. However, in the bonded dissimilar materials, both mode I and II thermal stress intensity factors are obtained. Specially, in this case, mode II thermal stress intensity factor is dominent. Also, thermal stress intensity factors are strongly influenced by the material properties. Thermal stress intensity factors decrease when the degree of insulation decreases.

• International Journal of Reliability and Applications
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• 제18권2호
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• pp.21-44
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• 2017

Accelerated life tests (ALTs) are frequently used in manufacturing industries to evaluate the reliability of products within a reasonable amount of time and cost. Test units are subjected to elevated stresses which yield quick failures. Most of the previous works on designing ALT plans are focused on tests that involve a single stress. Many times more than one stress factor influence the product's functioning. This paper deals with the design of optimum modified ramp-stress ALT plan for Burr type XII distribution with Type-I censoring under two stress factors, viz., voltage and switching rate each at two levels- low and high. It is assumed that usage time to failure is power law function of switching rate, and voltage increases linearly with time according to modified ramp-stress scheme. The cumulative exposure model is used to incorporate the effect of changing stresses. The optimum plan is devised using D-optimality criterion wherein the ${\log}_{10}$ of the determinant of Fisher information matrix is maximized. The method developed has been explained using a numerical example and sensitivity carried out.

• Geomechanics and Engineering
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• 제25권4호
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• pp.341-345
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• 2021

1D sand compression response to ko-loading experiences volume contraction from low to high effective stress regimes. Previous study suggested compressibility model with physically correct asymptotic void ratios at low and high stress levels and examined only for both remolded clays and natural clays. This study extends the validity of Enhanced Terzaghi model for different sand types complied from 1D compression data. The model involved with four parameters can adequately fit 1D sand compression data for a wide stress range. The low stress obtained from fitting parameters helps to identify the initial fabric conditions. In addition, strong correlation between compressibility and the void ratio at low stress facilitates determination of self-consistent fitting parameters. The computed tangent constrained modulus can capture monotonic stiffening effect induced by an increase in effective stress. The magnitude of tangent stiffness during large strain test should not be associated with small strain stiffness values. The use of a single continuous function to capture 1D stress-strain sand response to ko-loading can improve numerical efficiency and systematically quantify the yield stress instead of ad hoc methods.

• 한국콘텐츠학회논문지
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• 제13권11호
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• pp.740-753
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• 2013

본 연구의 목적은 보다 효과적인 실습교육체계 개선방안을 모색하기 위해 사회복지실습지도 현황과 학생들의 실습스트레스 및 대처를 파악하고, 실습지도에 따른 스트레스의 차이를 살펴보는 것이다. 이를 위해 2012년에 사회복지현장실습을 경험하고 수업을 수강한 서울 및 경기북부 4개 대학교 학생들을 대상으로 설문조사를 실시하였다. 202개 설문분석 결과, 실습생은 전문지식과 기술 부족, 실습환경, 실습업무부담 영역의 순으로 높은 스트레스를 보고하였고, 스트레스 대처전략에 있어서는 문제해결중심, 사회적지지 추구, 회피중심 전략 순으로 활용하였다. 실습경험에 따른 실습스트레스의 차이에 있어서는 현장체험 유무, 봉사시간, 실습사전교육 참여 유무에 따라, 그리고 슈퍼비전의 정기성과 빈도, 전공 만족도, 실습 및 슈퍼비전 만족도에 따라 차이를 보였다. 이상의 연구결과를 토대로 실습교육체계 개선방안을 제시하였다.

• Perspectives in Nursing Science
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• 제11권2호
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• pp.87-93
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• 2014

Purpose: The use of salivary biomarkers for stress research is increasing based on the convenience of collection, affordability and scientific merit. This short review provides an overview of the state of the science of salivary biomarkers utilized in research related to stress. Methods: An integrative review was conducted. Results: The trend of utilizing salivary biomarkers in stress research was reviewed, specifically, focusing on the use of endocrine and inflammatory biomarkers incorporated in previous stress research. Then, a review of sampling procedures for salivary biomarkers and the analytic methods is provided. Finally, a discussion on the strengths and areas for improvement in the use of salivary biomarkers in stress research is included. Conclusion: Salivary biomarkers as an alternative to blood biomarkers are increasingly being recognized as a legitimate source for analyzing the stress response in humans.

• 한국신뢰성학회지:신뢰성응용연구
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• 제17권2호
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• pp.112-121
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• 2017

Purpose: The most previous works on designing accelerated life tests (ALTs) are focused on the application of a single stress. Because of the difficulty to obtain the sufficient information in a reasonable duration using single stress only, there is needed in practice to use multiple-stress ALTs frequently. This paper presents new practical plans with two stresses for Weibull distribution. Methods: The four-level practical plans based on rectangle test region are proposed and compared with the corresponding three-level statistically optimal plans. Sensitivity analyses for assumed design parameters and life-stress relationship are conducted. Results: A procedure to choose practical ALT plans is illustrated with a numerical example and guidelines for planning two-stress ALTs are provided. Conclusion: The proposed two-stress ALT plans on practical constraints to assess a quantile of Weibull lifetime distribution at the use condition are efficient and robust.

• 아동학회지
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• 제26권4호
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• pp.55-70
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• 2005

The Teachers' Job Stress Scale(D'Arienzo, 1981) and the General Self-Efficacy Scale(Kim & Cho, 1996) were used to study 170 child care teachers. Data were analyzed by one-way ANOVA, Pearson's partial correlation, stepwise multiple regression(forward), path analysis, Internal consistency(Cronbach's $\alpha$), and factor analysis. Results showed that job stress was low for teachers of the national child care center, university graduates, higher salaried, and married teachers. Self-efficacy was high among teachers with more than 2 years experience, married status, and previous experience. Correlation between teacher's job stress and self-efficacy was -.19. Self-efficacy and demographic variables for job stress explained 8~22% of the variability. Self-efficacy intermediated between job stress and demographic variables. Monthly salary directly affected job stress.