• International Journal of Reliability and Applications
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• v.1 no.1
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• pp.15-26
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• 2000

This paper considers an aperiodic preventive maintenance (PM) model for repairable systems, in which the time intervals between two consecutive preventive maintenances are unequal. To propose such an aperiodic PM model, we assume that each PM reduces the current hazard rate by a certain amount which depends on the number of PMs performed previously. If the system fails between PMs, the minimal repair is performed and the hazard rate remains unchanged after the repair. We give the exact expressions for the hazard rate function for the aperiodic PM model. Based on the proposed aperiodic PM model, we suggest the maximum likelihood method to estimate the parameters characterizing the model and apply the method to the case of Weibull distribution. Numerical examples for estimating the parameters are presented for the purpose of illustration.

• Journal of Applied Reliability
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• v.4 no.2
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• pp.105-119
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• 2004

Reliability program is essential to the development of space systems like launch vehicles and satellites, as they are non-repairable after launch and the failure of a launch vehicle resulted in catastrophic consequences for the mission. Foreign advanced space organizations have developed and implemented their own reliability management programs for launch vehicles from the conceptual design stage to the detail processes for the individual components, procedures and test reports. A study on the launch failures and the reliability analysis methods for one-shot devices contained in this paper will contribute to the reliability improvement for Korean launch vehicle and components.

• Journal of Applied Reliability
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• v.14 no.2
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• pp.122-128
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• 2014

In this paper, we consider the periodic preventive maintenance model for a repairable system following the expiration of extended warranty under replacement-repair warranty. Under the replacement-repair warranty, the failed system is replaced or minimally repaired by the manufacturer at no cost to the user. Also, under extended warranty, the failed system is minimally repaired by the manufacturer at no cost to the user during the original extended warranty period. As a criterion of the optimality, we utilize the expected cost rate per unit time during the life cycle from the user's perspective. And then we determine the optimal preventive maintenance period and the optimal preventive maintenance number by minimizing the expected cost rate per unit time. Finally, the optimal periodic preventive maintenance policy is given for Weibull distribution case.

• Journal of Applied Reliability
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• v.16 no.4
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• pp.313-322
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• 2016

Purpose: We introduce ways to employ Markov chain model to evaluate the effect of preventive maintenance process. While the preventive maintenance process decreases the failure rate of each subsystems, it increases the downtime of the system because the system can not work during the maintenance process. The goal of this paper is to introduce ways to analyze this trade-off. Methods: Markov chain models are employed. We derive the availability of the system consisting of N repairable subsystems by the methods under various maintenance policies. Results: To validate our methods, we apply our models to the real maintenance data reports of military truck. The error between the model and the data was about 1%. Conclusion: The models developed in this paper fit real data well. These techniques can be applied to calculate the availability under various preventive maintenance policies.

• Journal of the Korean Data and Information Science Society
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• v.18 no.2
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• pp.535-541
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• 2007

The Non-homogeneous Poisson process is probably the most popular model since it can model systems that are deteriorating or improving. The renewal process is a model that is often used to describe the random occurrence of events in time. But both these models are based on too restrictive assumptions on the effect of the repair action. The Modulated Power Law Process is a suitable model for describing the failure pattern of repairable systems when both renewal-type behavior and time trend are present. In this paper we propose maximum likelihood estimation of the next failure time after the system has experienced some failures, that is, Mean Time Between Failure for the MPLP model.

• Proceedings of the Korean Reliability Society Conference
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• 2000.11a
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• pp.335-342
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• 2000

Preventive maintenance(PM) is an action taken on a repairable system while it is still operating, which needs to be carried out in order to keep the system at the desired level of successful operation. The PM improves the reliability of the system by predicting the possible failures and thereby preventing such failures from its occurrence. In this paper, we develop the optimal preventive maintenance policies based on the aperiodic PM model. We investigate an aperiodic preventive maintenance policy and propose several optimal PM policies which minimize the expected cost over an infinite time span. Park, Jung and Yum(2000) determine the optimal period and the optimal number of PMs based on Canfield's(1986) periodic model. Our techniques to derive the optimal preventive maintenance policies based on our aperiodic PM model is similar to those in Park, Jung and Yum(2000), which can be considered as the special case of our results.

• Journal of Technologic Dentistry
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• v.22 no.1
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• pp.57-67
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• 2000

The 2 key bridge system has been developed to make crownless bridges without damaging sound teeth. Strong bridge work of single or multiple pontics is possible for replacing both anterior and posterior teeth. It is incresingly considerde to be unacceptable, by dentists as well as patients, to fully grind down healthy elements in order fit a conventional bridges. Because this technique uses a combination of hole and adhesive fitting, it has a number of adventages over etched bridges and conventional bridges: - In comparison with conventional bridges, hardly and healthy dental tissue is sacrificed - Due to the lack of crown edges there is no periodontal pressure, as is the case with conventional bridges - The treatment procedure is straight forward and involves less time than conventional bridges. - The treatment is largely resiverable and repairable

• Journal of Korean Institute of Industrial Engineers
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• v.32 no.2
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• pp.141-152
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• 2006

This paper presents the development of a decision model to examine the optimal repair action for a deteriorating system. In order to make a reasonable decision, it is necessary to perform an analysis of the uncertainties embedded in deterioration and to evaluate the repair actions based on the expected future cost. Focusing on the power law failure model, the uncertainties related to deterioration are analyzed based on the Bayesian approach. In addition, we develop a decision model for the optimal repair action by applying a repair cost function. A case study is given to illustrate a decision-making process by analyzing the loss incurred due to deterioration.

• The Korean Journal of Applied Statistics
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• v.15 no.1
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• pp.107-117
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• 2002

In this paper we present the optimal replacement policies following the expiration of combination warranty. We consider two types of combination warranty policies: renewing warranty and non-renewing warranty. The criterion used to determine the optimal replacement period is the expected cost rate per unit time from the user'perspective. The optimal maintenance period following the expiration of combination warranty is obtained. Some numerical examples are presented for illustrative purpose.

• Proceedings of the IEEK Conference
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• 2002.07c
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• pp.1559-1562
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• 2002

This paper describes a 0.13um ultra-high speed 1Mb CMOS SRAM macro with 1.7ns access time. It achieves ultra-high speed operation using two novel approaches. First, it uses process insensitive sense amplifier (Double-Equalized Sense Amplifier) which improves voltage offset by about 10 percent. Secondly, it uses new replica-based sense amplifier driver which improves bit- line evaluation time by about 10 percent compared to the conventional technique. The various memory macros can be generated automatically by using a compiler, word-bit size from 64kb to 1 Mb including repairable redundancy circuits.