• Communications for Statistical Applications and Methods
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• 제23권4호
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• pp.269-285
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• 2016

In this paper, a simple step-stress accelerated life test (ALT) under progressive type-II censoring is considered. Progressive type-II censoring and accelerated life testing are provided to decrease the lifetime of testing and lower test expenses. The cumulative exposure model is assumed when the lifetime of test units follows an extension of the exponential distribution. Maximum likelihood estimates (MLEs) and Bayes estimates (BEs) of the model parameters are also obtained. In addition, a real dataset is analyzed to illustrate the proposed procedures. Approximate, bootstrap and credible confidence intervals (CIs) of the estimators are then derived. Finally, the accuracy of the MLEs and BEs for the model parameters is investigated through simulation studies.

• 소성∙가공
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• 제11권7호
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• pp.581-588
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• 2002

Among the processes to produce micro lens, the process using press molding is a new technology to simplify the process, but it contains many unknown variables. The press-molding process proposed in this paper was simplified into two step process, the first step is the pressing to design the preform for glass element, the second step is the annealing to reduce the residual stress. It is important to estimate the amount of shrinkage of glass gob and the residual stress during process. It Is difficult to evaluate the process variables as mentioned above through the experiment. The influences due to process variables was evaluated by using FEM parametric analysis. The results in this paper can be applicable to produce micro lens.

• Journal of the Korean Statistical Society
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• 제18권2호
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• pp.125-134
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• 1989

This paper presents optimum simple step-stress accelerated life test plans for the case where the test process is observed periodically at intervals of the same length. Two types of failure data, periodically observed complete data and periodically observed censored data, are considered. An exponential life distribution with a mean that is a log-linear function of stress, and a cumulative exposure model for the effect of changing stress are assumed. For each type of data, the optimum test plan which minimizes the asymptotic variance of the maximum likelihood estimator of the mean life at a design stress is obtained and its behaviors are studied.

• 한국간호교육학회지
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• 제10권1호
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• pp.54-63
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• 2004

The purpose of this study is to identify the factors that influence stress experienced by nursing students and to provide a perceived causal structure model among these variables. The ultimate goal of this study is to develop efficient guidance to clinical nursing education in this population. This study intends to apply perceived causal structure: network analysis method which was developed by Kelly(1983), and has been applied in nursing research. This method is selected to show dynamic relationship of stressor using network method. Data was collected from convenient sample of 186 junior college nursing students who had the clinical practice experience during 10 weeks. Data collection and analysis was conducted in 2 steps from December, 9, 2002 to February, 8, 2003. Step 1.: Data was collected using literature review(10 articles) to identify the causes of stress. Nine causes of stress were extracted. Step 2.: As perceived casual structure network study, data was collected using questionnaires which included 9 extracted cause and stress. The questionnaire contained a 10 X 10 grid table with 10 causes and effects printed. In network analysis, 'Yes' was scored as 1, 'No' was scored as 0, and the mean(maximum 1, minimum 0) was calculated. Construction of the network under inductive eliminative analysis which stopped the construction of the network when the consensual agreement level dropped near 50% was proceeded by adding causes in order of the mean rating level. In this study, construction of the final network was stopped by consensual agreement level of 52% of the total subjects. The results are summarized as follows : Step 1: Investigation of the causes of stress ; The extracted causes of stress from quality data was identified 9 categories ; negative nurse, lack of clinical practice opportunity, ambiguous role, negative patient, lack of nursing knowledge and skill, difficult of personal relations, inefficient clinical practice guidance, gap of theory and practice, lack of support. Step 2 : Construction of the perceived causal structure model ; 1) The most central cause of stress is ambiguous role in the systems of causation. 2) The distal cause of stress is inefficient clinical practice guidance 3) The causes that have a number of outgoing link are negative nurse, ambiguous role. 4) The causes that have a number of incoming link are ambiguous role, gap of theory- practice, lack of clinical practice opportunity, lack of nursing knowledge- skill. 5) There is a mutual relationship between stress and difficult of personal relations, stress and ambiguous role, ambiguous role and negative nurse, ambiguous role and lack of clinical practice opportunity, ambiguous role and lack of nursing knowledge-skill, lack of nursing knowledge-skill and gap of theory- practice. In conclusion, the network suggests that the first centre cause is related on ambiguous role and the second on negative nurse, inefficient clinical practice guidance in the systems of causation

• 한국공간구조학회논문집
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• 제11권2호
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• pp.111-118
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• 2011

단층 래티스 돔과 같은 스페이스 프레임 구조물은 대공간을 확보하기 위하여 강접합으로 설계함으로써 경제성과 심미성 등의 장점을 확보할 수 있다. 그러나 실제 구조물의 접합상태는 완전한 강접합도, 완전한 핀접합도 아닌 접합부 상세에 따라 중간적인 특성을 나타내는 반강접합이 일반적이다. 본 연구에서는 저자 등에 의해 제안된 Step-Up Erection 공법을 적용하여 단층 라멜라 돔의 접합부 강성에 따른 시공 중의 좌굴 특성을 해석적으로 규명하였다. 얻어진 결과는 시공 Step이 증가할수록 그리고 접합부 휨강성이 핀접합에 가까울수록 좌굴내력은 감소하였으며, 좌굴모드는 시공 Step이 증가할수록, 접합부 휨강성이 강접합에 가까울수록 돔의 정점 부근에서 뜀좌굴 현상에 의한 절점좌굴이 발생하였다. 또한, 시공 Step이 낮을 경우 저면 경계부의 원주방향 부재에서 큰 인장응력 분포를 나타내었으며, 시공 Step이 증가할수록 정점 중앙부에서 큰 압축응력 분포를 보였다. 완성돔을 제외한 시공 중 Step에서는 Step이 증가할수록, 접합부 휨강성이 핀접합에 가까울수록 저면부 경선방향 부재에서 큰 인장응력 분포를 나타내었다.

• Structural Engineering and Mechanics
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• 제19권6호
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• pp.639-652
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• 2005

In this paper, a full-scale K-joint specimen was tested to failure under cyclic combined axial and in-plane bending loads. In the fatigue test, the crack developments were monitored step by step using the alternating current potential drop (ACPD) technique. Using Paris' law, stress intensity factor, which is a fracture parameter to be frequently used by many designers to predict the integrity and residual life of tubular joints, can be obtained from experimental test results of the crack growth rate. Furthermore, a scheme of automatic mesh generation for a cracked K-joint is introduced, and numerical analysis of stress intensity factor for the K-joint specimen has then been carried out. In the finite element analysis, J-integral method is used to estimate the stress intensity factors along the crack front. The numerical stress intensity factor results have been validated through comparing them with the experimental results. The comparison shows that the proposed numerical model can produce reasonably accurate stress intensity factor values. The effects of different crack shapes on the stress intensity factors have also been investigated, and it has been found that semi-ellipse is suitable and accurate to be adopted in numerical analysis for the stress intensity factor. Therefore, the proposed model in this paper is reliable to be used for estimating the stress intensity factor values of cracked tubular K-joints for design purposes.

• 한국해양공학회지
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• 제35권4호
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• pp.257-265
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• 2021

Many studies have been performed to predict a reliable and accurate stress-range distribution and fatigue damage regarding the Gaussian wide-band stress response due to multi-peak waves and multiple dynamic loads. So far, most of the approximation models provide slightly inaccurate results in comparison with the rain-flow counting method as an exact solution. A step-by-step study was carried out to develop new approximate spectral moments that are close to the rain-flow counting moment, which can be used for the development of a fatigue damage model. Using the special parameters and bandwidth parameters, four kinds of parameter-based combinations were constructed and estimated using the R-squared values from regression analysis. Based on the results, four candidate empirical formulas were determined and compared with the rain-flow counting moment, probability density function, and root mean square (RMS) value for relative distance. The new approximate spectral moments were finally decided through comparison studies of eight response spectra. The new spectral moments presented in this study could play an important role in improving the accuracy of fatigue damage model development. The present study shows that the new approximate moment is a very important variable for the enhancement of Gaussian wide-band fatigue damage assessment.

• 한국초전도ㆍ저온공학회논문지
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• 제5권1호
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• pp.71-75
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• 2003

The conduit fer superconducting cable is welded and plastically deformed during the jacketing process to make the CICC (Cable-in-Conduit-Conductors) fer a fusion magnet. The jacketing process of KSTAR (Korea Superconducting Tokamak Advanced Research) conductors is composed of several sequential steps such as rounding, welding, sizing, and square-rolling. Since the welded zone in Incoloy 908 conduit is brittle and easy to have flaws, there may be a possibility of stress corrosion cracking during the heat treatment of coil when both the induced tensile residual stress and the concentration of oxygen in the furnace are sufficiently high. The steps of the jacketing process were simulated using the finite element method of the commercial ABAQUS code, and the stress distribution in the conduit in each step was calculated, respectively. Furthermore, the variations of residual welding stresses through the steps of the jacketing process were calculated and analyzed to anticipate the possibility of the stress corrosion cracking in the conduit. The concentrated high tensile residual welding stresses along the welding bead decrease by the plastic deformation of the following sizing step. The distribution in residual stresses in the conductor for magnet coil is mainly governed by the last step of square-rolling.

• 한국환경과학회지
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• 제14권4호
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• pp.389-395
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• 2005

The changes of O-J-I-P transients were investigated using leaves of four subtropical plant species (Crinum asiaticum var. japonicum, Osmanthus insularis, Chloranthus glaber and Asplenium antiquum) under the natural conditions in winter, in order to select the stress indicators for diagnosing physiological states of plants under low temperature. In the O-J-I-P transients of these species, the fluorescence intensity was found to be higher in O-step and lower in P-step in winter than in summer. Particularly, the fluorescence intensity of the P-step in Crinum asiaticum var. japonicum was lower than those of other three plant species, indicating that Crinum asiaticum var. japonicum is the most sensitive to low temperature. Of the chlorophyll fluorescence parameters derived from O-J-I-P transients of four subtropical plants, Fm, Fv/Fo, ABS/CS, TRo/CS, $\phi_{po}$ and $\phi_{po}/(1-\phi_{po})$ decreased significantly with the increase of Fo, Sm, N, EToICS, ETo/RC and $\psi_o/(1-\psi_o)$ depending on temperature drop in winter. Therefore, these parameters could be used as indicators for estimating low temperature stress and diagnosing physiological states of plants under the natural conditions in winter.

• Journal of the Korean Data and Information Science Society
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• 제7권2호
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• pp.295-299
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• 1996

In this paper, optimal change times are determined for fully three-step stress accelerated life tests, which minimize the asymptotic variance for maximum likelihood estimator of logarithm of the failure rate at the usual condition and exponential distribution is given for life time data.