• International Journal of Reliability and Applications
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• v.17 no.1
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• pp.85-105
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• 2016

In this paper accelerated life testing is incorporated in quality control technique of acceptance sampling plan to induce early failures in high reliability products.Stress under accelerated condition can be applied in constant-stress, step-stress and progressive-stress or combination of such loadings. A ramp-stress results when stress is increased linearly (from zero) with time. In this paper optimum failure-censored ramp-stress accelerated life test sampling plan for log-logistic distribution has been formulated with cost considerations. The log-logistic distribution has been found appropriate for insulating materials. The optimal plans consist in finding optimum sample size, sample proportion allocated to each stress, and stress rate factor such that producer's and consumer's interests are safeguarded. Variance optimality criterion is used when expected cost per lot is not taken into consideration, and bilevel programming approach is used in cost optimization problems. The methods developed have been illustrated using some numerical examples, and sensitivity analyses carried out in the context of ramp-stress ALTSP based on variable SSP for proportion nonconforming.

• Journal of Korean Society for Quality Management
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• v.24 no.3
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• pp.37-49
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• 1996

For highly reliable devices it is often defined to "fail" when its performance degrades below a specified value. In this paper we consider a method for optimally designing accelerated degradation tests(ADTs) in which the performance over exposure time and stress has Weibull distribution. For the product whose performance has Weibull distribution, the optimum plan - low stress level and sample proportions allocated to each test condition - is obtained, which minimize the asymptotic variance of maximum likelihood estimator of a stated quantile at design stress. We also present compromise ADTs plan that can be used for the practical purpose.

• 한국신재생에너지학회:학술대회논문집
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• 2011.11a
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• pp.34.2-34.2
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• 2011

Wind guide can be installed on the top of buildings to collect wind. In this study, optimum shape of wind guide is developed, and proposed to combinate with the vertical wind turbine. Impact of parameters for wind guide is analyzed with several cases planned by Taguchi test plan. Front angle, rear angle, and roof angle are selected as key variables and changed into four different levels. By the experimental plan, totally, 64 cases are reduced to 16 cases of analysis. With optimum design of wind guide, the installed vertical axis wind turbines can be operated with maximum power output.

• International Journal of Reliability and Applications
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• v.18 no.2
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• pp.45-63
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• 2017

In this paper, we have formulated optimum Accelerated Life Test (ALT) plan for a parallel system with two independent components using masked data with ramp-stress loading scheme and Type-I censoring. Consider a system of two independent and non-identical components connected in parallel. Such a system fails whenever all of its components has failed. The exact component that causes the system to fail is often unknown due to cost and time constraint. For each parallel system at test, we observe its system's failure time and a set of component that includes the component actually causing the system to fail. The stress-life relationship is modelled using inverse power law, and cumulative exposure model is assumed to model the effect of changing stress. The optimal plan consists in finding out the optimum stress rate using D-optimality criterion. The method developed has been explained using a numerical example and sensitivity analysis carried out.

• International Journal of Reliability and Applications
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• v.15 no.2
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• pp.85-98
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• 2014

Accelerated life testing (ALT) is a well famous technique in life testing and reliability studies, this is particularly used to induce so high stress leading to failure of the highly reliable units quickly under stipulated duration of time. The step-stress ALT is one of the systematic experimental strategy of ALT applied to fail the units in steps. In this article we focus on two important issues (i) necessity of life tests at higher steps with relevant causes (ii) to develop a new optimum test plan for 3-step SSALT under the modified cumulative exposure model proposed by Khamis and Higgins (1998). It is assumed that the lifetime of test units follows Rayleigh distribution and its scale parameter at constant stress level is assumed to be a log-linear function of the stress. The maximum likelihood estimates of the parameters involved in the step-stress ALT model are obtained. A simulation study is performed for numerical investigation of the proposed new optimum plan 3-step, step-stress ALT. The necessity of the life test units at 3-step step-stress is also numerically examined in comparison to simple step-stress setup.

• Proceedings of the Korean Society of Marine Engineers Conference
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• 2006.06a
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• pp.189-192
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• 2006

For the safety and efficiency of voyage, the demand for decision support system in route planning has been increasing with the advance of information technology and the increase of oil price. According to the needs, the authors developed an optimum route assessment system. The system assists an navigator to make an optimum route plan with respect to sailing time and fuel consumption using weather forecast data. Also, the system provides the quantitative estimation for the various safety indexes including parametric roll and etc. Using these functions, a navigator is able to design the safe and efficient voyage plan. The effectiveness of system were verified by the operation during actual voyages and the simulation studies.

• International Journal of Reliability and Applications
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• v.13 no.1
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• pp.19-35
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• 2012

This paper presents an optimum design of step-stress partially accelerated life test (PALT) plan which allows the test condition to be changed from use to accelerated condition on the occurrence of fixed number of failures. Various life distribution models such as exponential, Weibull, log-logistic, Burr type-Xii, etc have been used in the literature to analyze the PALT data. The need of different life distribution models is necessitated as in the presence of a limited source of data as typically occurs with modern devices having high reliability, the use of correct life distribution model helps in preventing the choice of unnecessary and expensive planned replacements. Truncated distributions arise when sample selection is not possible in some sub-region of sample space. In this paper it is assumed that the lifetimes of the items follow Truncated Logistic distribution truncated at point zero since time to failure of an item cannot be negative. Optimum step-stress PALT plan that finds the optimal proportion of units failed at normal use condition is determined by using the D-optimality criterion. The method developed has been explained using a numerical example. Sensitivity analysis and comparative study have also been carried out.

• Journal of the Korean Society of Hazard Mitigation
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• v.9 no.3
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• pp.117-126
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• 2009

Various dimensions of watershed structural/non-structural planning can be applied in comprehensive flood mitigation plan in a river basin. Especially structural counterplans have very broad and diverse nature as flood control facilities. It is not easy to find the optimum alternative to maximize the ability of a basin to reduce flood risk using a combination of structural counterplans. In addition, there is no standard for evaluating the performance of structural counterplans and for selecting optimal combination of them. This study focused on how to select the best alternative of a comprehensive watershed structural plan from various flood defense alternative candidates. By introducing an analytic hierarchy process, we would like to show how we decide the best alternative using standard worksheets developed in this study for economics and policy evaluation, and Expert Choice 11.5, which calculates weights for evaluation items. Based on the results from this study, we would like to suggest the best practice of a standardized watershed plan for flood protection.

• Health Policy and Management
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• v.28 no.3
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• pp.233-239
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• 2018

A rapid decrease of total fertility rate to 1.08 in 2005 prompted the Korean government to plan and implement a '5-year plan for ageing society and population policy' starting from 2006. The 1st and 2nd 5-year plans had not shown any discernible impact on the fertility and the 3rd 5-year plan was launched in 2016. However, the fertility rate is going down further. The author reviewed the contents and assessment reports of the fertility promotion plan to suggest ideas for complementing the shortcomings of it. Author defined the major determinants of marriage and child birth as philosophy, politics, sense of value, social norm, culture, healthcare, and education. The plan was examined in view of these determinants. Transformation of Korea from an agricultural society to an industrialized society in a short period of time had brought about changes in most of the determinants of marriage and child birth; in particular philosophy and sense of value. These aspects were not put into consideration in the plan. Author suggested to launch a social education program for the general public to establish a sound philosophy of life, reform the sense of value on family, child birth and education, and cultivate the skill to draw a consensus through discussions on the social issues. A special program to promote marriage of women at the optimum age for child birth was proposed. The government should implement well balanced policy for economic development and labor. Multidisciplinary approach was recommended for these tasks.

• Journal of the Korean Data and Information Science Society
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• v.5 no.2
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• pp.87-94
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• 1994

In this paper, we consider optimum plan to determine stress change times under the three-step stress PALTs, assuming that each test units follows an exponential distribution. The tampered random variable(TRV) model for the three-step stress PALTs setup are introduced, and maximum likelihood estimators(MLEs) of the failure rate and the acceleration factors are obtained. The change times to minimize the generalized asymptotic variance(GAVR) of MLEs of the failure rate and the acceleration factors are proposed for the three-step stress PALTs.