• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.30 no.9
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• pp.551-557
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• 2017

It is extremely important to improve methodologies for the lifetime assessment of porcelain insulators. While there has been a considerable amount of work regarding the phenomena of lifetime distributions, most of the studies assume that aging distributions follow the Weibull distribution. However, the true underlying distribution is unknown, giving rise to unrealistic inferences, such as parameter estimations. In this article, we review several distributions that are commonly used in reliability and survival analysis, such as the exponential, Weibull, log-normal, and gamma distributions. Some properties, including the characteristics of failure rates of these distributions, are presented. We use a Bayesian approach for model selection and parameter estimation procedures. A well-known measure, called the Bayes factor, is used to find the most plausible model among several contending models. The posterior mean can be used as a parameter estimate for unknown parameters, once a model with the highest posterior probability is selected. Extensive simulation studies are performed to demonstrate our methodologies.

• Communications for Statistical Applications and Methods
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• v.24 no.4
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• pp.325-337
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• 2017

In this paper, we proposed a new long-term lifetime distribution with four parameters inserted in a risk competitive scenario with decreasing, increasing and unimodal hazard rate functions, namely the Weibull-Poisson long-term distribution. This new distribution arises from a scenario of competitive latent risk, in which the lifetime associated to the particular risk is not observable, and where only the minimum lifetime value among all risks is noticed in a long-term context. However, it can also be used in any other situation as long as it fits the data well. The Weibull-Poisson long-term distribution is presented as a particular case for the new exponential-Poisson long-term distribution and Weibull long-term distribution. The properties of the proposed distribution were discussed, including its probability density, survival and hazard functions and explicit algebraic formulas for its order statistics. Assuming censored data, we considered the maximum likelihood approach for parameter estimation. For different parameter settings, sample sizes, and censoring percentages various simulation studies were performed to study the mean square error of the maximum likelihood estimative, and compare the performance of the model proposed with the particular cases. The selection criteria Akaike information criterion, Bayesian information criterion, and likelihood ratio test were used for the model selection. The relevance of the approach was illustrated on two real datasets of where the new model was compared with its particular cases observing its potential and competitiveness.

• Transactions of the Korean Society of Pressure Vessels and Piping
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• v.11 no.2
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• pp.45-50
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• 2015

In many cases, the field fracture mechanism of the thermoplastic pipe is considered as either brittle or environmental fractures. Thus the estimation of the lifetime by modeling slow crack growth considering such fracture mechanisms is required. In comparison of the some conventional and empirical equations to explain the slow crack growth rate such as the Paris' law, the crack layer theory can be used to simulate the crack and process zone growth behaviors precisely, so the lifetime of thermoplastic pipe can also be accurately estimated. In this study, the modified crack layer theory for the stress corrosion cracking (SCC) of high density polyethylene is introduced with detailed algorithm. The oxidation induction time of the HDPE is also considered for the reduction of specific fracture energy during exposed to chemical environments. Furthermore, the parametric study for an important SCC parameter is conducted to understand the slow crack growth behavior of SCC.

• Journal of Applied Reliability
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• v.13 no.2
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• pp.109-116
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• 2013

The solder is any of various fusible alloys, usually tin and lead, used to join metallic parts that provide the contact between the chip package and the printed circuit board. Solder plays an important role of electrical signals to communicate between the two components. In this study, two kinds of Ag-free solder as sample is made to conduct the thermal shock test and the high humidity temperature test. Low resistance is measured to estimate crack size of solder, using daisy chain. The low speed shear test is also performed to analyze strength of solder. The appropriate degradation model is estimated using the result data. Depending on the composition of solder, lifetime estimation is conducted by adopted degradation model. The lifetime estimated two kinds of Ag-free solder is compared with expected lifetime of Sn-Ag-Cu solder. The result is that both Ag-free composition are more reliable than Sn-Ag-Cu solder.

• Safety and Health at Work
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• v.12 no.2
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• pp.174-183
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• 2021

Background: Toluene diisocyanate (TDI) is a highly reactive chemical that causes sensitization and has also been associated with increased lung cancer. A risk assessment was conducted based on occupational epidemiologic estimates for several health outcomes. Methods: Exposure and outcome details were extracted from published studies and a NIOSH Health Hazard Evaluation for new onset asthma, pulmonary function measurements, symptom prevalence, and mortality from lung cancer and respiratory disease. Summary exposure-response estimates were calculated taking into account relative precision and possible survivor selection effects. Attributable incidence of sensitization was estimated as were annual proportional losses of pulmonary function. Excess lifetime risks and benchmark doses were calculated. Results: Respiratory outcomes exhibited strong survivor bias. Asthma/sensitization exposure response decreased with increasing facility-average TDI air concentration as did TDI-associated pulmonary impairment. In a mortality cohort where mean employment duration was less than 1 year, survivor bias pre-empted estimation of lung cancer and respiratory disease exposure response. Conclusion: Controlling for survivor bias and assuming a linear dose-response with facility-average TDI concentrations, excess lifetime risks exceeding one per thousand occurred at about 2 ppt TDI for sensitization and respiratory impairment. Under alternate assumptions regarding stationary and cumulative effects, one per thousand excess risks were estimated at TDI concentrations of 10 - 30 ppt. The unexplained reported excess mortality from lung cancer and other lung diseases, if attributable to TDI or associated emissions, could represent a lifetime risk comparable to that of sensitization.

• ETRI Journal
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• v.30 no.1
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• pp.47-58
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• 2008

In sensor networks, analyzing power consumption before actual deployment is crucial for maximizing service lifetime. This paper proposes an instruction-level power estimator (IPEN) for sensor networks. IPEN is an accurate and fine grain power estimation tool, using an instruction-level simulator. It is independent of the operating system, so many different kinds of sensor node software can be simulated for estimation. We have developed the power model of a Micaz-compatible mote. The power consumption of the ATmega128L microcontroller is modeled with the base energy cost and the instruction overheads. The CC2420 communication component and other peripherals are modeled according to their operation states. The energy consumption estimation module profiles peripheral accesses and function calls while an application is running. IPEN has shown excellent power estimation accuracy, with less than 5% estimation error compared to real sensor network implementation. With IPEN's high precision instruction-level energy prediction, users can accurately estimate a sensor network's energy consumption and achieve fine-grained optimization of their software.

• The Korean Journal of Applied Statistics
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• v.12 no.2
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• pp.463-478
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• 1999

가속수명시험은 제품의 사용조건 보다 높은 스트레스 수준에서 시험하여 제품의 신뢰수명을 추정하는 것으로써 스트레스 수준을 일정하게 유지하는 일정형 시험이 일반적이다. 그러나 경우에 따라서는 시험절차의 편리와 시험기간의 단축을 위해서 스트레스를 시간에 따라 선형적으로 증가시키는 램프(ramp)형 시험을 사용하기도 한다. 이 논문에서는 일정스트레 s에서 제품의 수명이 모수 $\Theta$(s), $\beta$인 와이블분포를 따르고 수명과 스트레스의 관계가 역거듭제곱모형인 경우에 스트레스를 사용조건에서 가하고, 스트레스 수준의 최대 한계가 주어져 있는 램프시험 하에서 시험제품이 갖는 수명분포를 유도하고, 정시관측중단시험의 경우에 대해서 수명분포의 최우추정량과 추정량의 점근분포를 구하며, 최우추정치를 구하는 알고리즘을 제안한다.

• Journal of the Korean Operations Research and Management Science Society
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• v.22 no.4
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• pp.151-165
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• 1997

We propose a method for estimating the probability of perfect PM from successive failure times of a repairable system. The system under study is maintained preventively at periodic times, and it undergoes minimal repair at failure. We consider Brown-Proschan imperfect PM model in which the system is restored to a condition as good as new with probability P and is otherwise restored to its condition just prior to failure. We discuss the identifiability problem when the PM modes are not recorded. The expectation-maximization principle is employed to handle the incomplete data problem. We assume that the lifetime distribution belongs to a parametric family with increasing failure rate. For the two parameter Weibull lifetime distribution, we propose a specific algorithm for finding the maximum lifelihood estimates of the reliability parameters : the probability of perfect PM (P), as well as the distribution parameters. The estimation method will provide useful results for maintaining real systems.

• Communications for Statistical Applications and Methods
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• v.23 no.4
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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.

• Journal of Applied Reliability
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• v.16 no.4
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• pp.280-286
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• 2016

Purpose: Recently, Jiang defines the tradeoff B life to minimize a sum of life lost by preventive maintenance (PM) and corrective maintenance (CM) contribution parts and sets up an optimal replacement age of age replacement policy as this tradeoff life. In this paper, Jiang's model only considering the known lifetime distribution is extended by assigning different weights to two parts of PM and CM in order to reflect the practical maintenance situations in application. Methods: The new age replacement model is formulated and the meaning of a weight factor is expressed with the implied cost of failure under asymptotic expected cost model and also discussed with one-cycle expected cost criterion. Results: The proposed model is applied to Weibull and lognormal lifetime distributions and optimum PM replacement ages are derived with corresponding implied cost of failure. Conclusion: The new age replacement policy to escape the estimation of cost of failure in classical asymptotic expected cost criterion based on the renewal process is provided.