• Proceedings of the Korean Operations and Management Science Society Conference
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• 1997.10a
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• pp.193-196
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• 1997

In this paper we consider a Bayesian theoretic approach to periodic incomplete preventive maintenance with minimal repair at failure. We assume that the system failure rate is increasing as the frequency of PM increases and that the system is replaced at the K-th PM under this maintenance strategy. The optimal policies which minimize the expected cost rates are discussed. We seek the optimal periodic PM interval x and replacement time K under a Weibull failure intensity. Assuming suitable prior distribution for the Weibull parameters, we derive the posterior distribution incorporating failure data and obtain the updated optimal replacement strategies.

• Journal of Korean Society for Quality Management
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• v.30 no.3
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• pp.53-65
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• 2002

This paper studies an economic manufacturing quantity (EMQ) model with two types of failures and planned preventive maintenance of the production facility. One is a type I (major) failure which should be corrected by a failure maintenance and the other is a type H (minor) failure which can be minimally repaired without interrupting the production run. The objective is to determine the lot size and preventive replacement policy minimizing the long-run expected cost per unit time. We consider a control policy with a constant production lot size and preventive maintenance after completing n production runs. It is assumed that both preventive and failure maintenance times are random and the demand arriving during a stock-out period is lost. An expression for the expected cost per unit time is obtained in the general case. A special case is discussed and numerical results are provided.

• Journal of Korean Society for Quality Management
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• v.26 no.3
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• pp.71-81
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• 1998

In reliability analysis, the time difference between the expected next failure time and the current failure time or the Mean Time Between Failure(MTBF) is of significant interest. Until recently, in reliability growth studies, the reciprocal of the intensity function at current failure time has been used as being equal to MTBE($t_n$)at the n-th failure time $t_n$. That is MTBF($t_n$)=l/$\lambda (t_n)$. However, such a relationship is only true for Homogeneous Poisson Process(HPP). Tsokos(1995) obtained the upper bound and lower bound for the MTBF($t_n$) and proposed an estimator for the MTBF($t_n$) as the mean of the two bounds. In this paper, we provide the estimator for the MTBF($t_n$) which does not depend on the value of the shape parameter. The result of the Monte Carlo simulation shows that the proposed estimator has better efficiency than Tsokos's estimator.

• IE interfaces
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• v.25 no.4
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• pp.422-430
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• 2012

In this article, we introduced the estimation method by 'Safety Integrity Level'(SIL) for the criterion of safety assurance and performed a case study on a flame scanner. SIL requires probabilistic evaluation of each set of equipment used to reduce risk in a safety related system. FMEDA(Failure Modes, Effects and Diagnostic Analysis) method is widely used to evaluate the safety levels and provides information on the failure rates and failure mode distributions necessary to calculate a diagnostic coverage factor for a part or a component. Basically, two parameters resulting from FMEDA are used for SIL classification of the device : SFF(Safe Failure Fraction) and PFD(Probability of Failure on Demand). In this case study, it is concluded that the flame scanner is designed to fulfill the condition of SIL 3 in the aspect of SFF and PFD.

• Journal of the Korean Operations Research and Management Science Society
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• v.27 no.3
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• pp.115-133
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• 2002

In this paper, 15 South Korean telecommunications service failure cases were analyzed. Through the in-depth case study, 8 factors were found to be the major causes contributing to the telecommunications service failure. They were (1) ineffective marketing, (2) poor demand forecasting due to misjudgment of customer preference, (3) failure to satisfy technical specifications, (4) loss of cost advantage due to the price cut of competing services or new entry with lower price, (5) loss of utility advantage due to the increased utility of competing services or new entry with higher utility (6) decrease of market attractiveness due to change of customer preference, (7) impact of government policy, and (8) insufficient or low quality of contents. Additional analysis was done to derive managerial implications to the new telecommunications service development strategy. The findings from the paper will provide valuable Insight to the successful Implementation of new service development and service provisioning processes.

• Proceedings of the Korean Reliability Society Conference
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• 2011.06a
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• pp.21-26
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• 2011

Initial reliability prediction done by calculation would be more practical if support by evidence from customer usage profile and field failure data to improve the prediction. Thus, the consistency of the design and the product would be practically validated. In this paper, it will address rationale and method to decide on Acceleration Factor (AF) to be used in Accelerated Life Test (ALT) through usage profile and field failure. The case study of tractor transmission is used to demonstrate the method which data obtained from surveys done on farmers, field visits and field failure data from service center. By considering all the elements, it will determine more relevant AF which indicates the real use conditions of the component.

• Journal of Aerospace System Engineering
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• v.3 no.2
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• pp.31-38
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• 2009

COMS On-board FDIR(Failure Detection, Isolation and Recovery) functions are implemented on the on-board software to satisfy the autonomy and failure tolerance requirements. This paper presents concepts of COMS Failure Management with hierarchical layers and addresses the characteristics of the FDIR layer from low level to high level. It is aimed at giving the reader the understanding how the COMS FDIR was designed and how works. It first recalls what are the system level applicable requirements, which are based on the COMS mission requirements. Then it describes the philosophy and structure of the FDIR and subsequently breaks it down into the several FDIR layers. It could be used as an important and useful reference of the information to design and develop an automatic FDIR mechanism in the future.

• International Journal of Heart Failure
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• v.6 no.3
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• pp.107-116
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• 2024

Frailty is highly prevalent among patients with heart failure (HF) and independently predicts adverse outcomes. However, optimal frailty definitions, assessments, and management in HF remain unclear. Frailty is common in HF, affecting up to 80% of patients depending on population characteristics. Even pre-frailty doubles mortality risk versus robust patients. Frailty worsens HF prognosis through systemic inflammation, neurohormonal changes, sarcopenia, and micronutrient deficiency. Simple screening tools like gait speed and grip strength predict outcomes but lack HF-specificity. Comprehensive geriatric assessment is ideal but not always feasible. Exercise, nutrition, poly-pharmacy management, and multidisciplinary care models can help stablize frailty components and improve patient-centred outcomes. Frailty frequently coexists with and exacerbates HF. Routine frailty screening should guide supportive interventions to optimize physical, cognitive, and psychosocial health. Further research on HF-specific frailty assessment tools and interventions is warranted to reduce this dual burden.

• Korean Management Science Review
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• v.11 no.1
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• pp.187-196
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• 1994

Time-Space-Time(T-S-T) switching network is modeled as a graceful degrading system. Call blocking probability is defined as a measure of performance. Several performance related measures are suggested under the presence of failure. An optimization model is proposed, which determines optimal values of system parameters of the switching network.

• Journal of the Korean Operations Research and Management Science Society
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• v.16 no.1
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• pp.59-67
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• 1991

The hazard estimator is proposed for estimating system failure probability of a general network where all minimal cut sets are given. Theoretical variance of the hazard estimator is derived in a bridge system. It is demonstrated that variance of the hazard estimator is much smaller than that of the raw simulation estimator particularly for small arc failure probability.