• Journal of Korean Institute of Industrial Engineers
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• v.36 no.1
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• pp.69-77
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• 2010

The failure rate model of External Environment Maintenance(EEM) for a system under severe environment is investigated. EEM, which is recently introduced concept, is a maintenance activity controlling external environment factors that potentially cause system failure such as cleaning equipment, controlling temperature (humidity) and removing dust inside of electronic appliances. EEM can not have any influence on the inherent failure rate of a system but reduce the severity of the external environment causing failure since it deals with only external environment factors. Therefore, we propose two failure rate models to express the improvement effect of EEM: The intensity reduction model and age reduction model. The intensity and age reduction models of EEM are developed assuming the quality of improvement effect is proportioned to an extra intensity or age respectively. The validation of proposed failure rate models is performed in order of data generation, parameter estimation and test for goodness-of-fit.

• Proceedings of the KIEE Conference
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• 2006.11a
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• pp.9-11
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• 2006

This paper suggests a revision method for historical fault data using Proportional Aging Reduction(PAR) to consider maintenance effect in time-varying failure rate. In order to product time-varying failure rate, the historical fault data are necessary. However, the maintenance record could be left out in historical data by spot operator's mistake. In this case, the failure rate is produced less than the average failure rate for increasing equipments' life-time by maintenance effect. Hence, it is necessary for new time-varying failure rate to extract maintenance effect from the existing fault data. In this paper, the revision method to reduce equipments' life-time, adversely using PAR among three techniques to consider maintenance effect.

• Smart Structures and Systems
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• v.22 no.2
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• pp.151-159
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• 2018

Maintenance activities are regarded as a key part of the repairable deteriorating system because they maintain the equipment in good condition. In practice, many maintenance policies are used in engineering fields to reduce unexpected failures and slow down the deterioration of the system. However, in traditional maintenance policies, maintenance activities have often been assumed to be performed at the same time interval, which may result in higher operational costs and more system failures. Thus, this study presents two non-periodic preventive maintenance (PM) policies for repairable deteriorating systems, employing the failure rate of the system as a conditional variable. In the proposed PM models, the failure rate of the system was restored via the failure rate reduction factors after imperfect PM activities. Operational costs were also considered, which increased along with the operating time of the system and the frequency of PM activities to reflect the deterioration process of the system. A numerical example was provided to illustrate the proposed PM policy. The results showed that PM activities performed at a low failure rate threshold slowed down the degradation of the system and thus extended the system lifetime. Moreover, when the operational cost was considered in the proposed maintenance scheme, the system replacement was more cost-effective than frequent PM activities in the severely degraded system.

• Proceedings of the Korea Society of Information Technology Applications Conference
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• 2005.11a
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• pp.143-146
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• 2005

The software reliability growth depends on the testing time because the failure rate varies whether it is long or not. On the other hand, it might be difficult to reduce failure rate for most of the cases are not available for debugging during operational phase, hence, there are some literatures to study that the failure rate is uniform throughout the operational time. The failure rate reduces and the reliability grows with time regardless of debugging. As a result, the products reliability varies with the time duration of these products in point of customer view. The reason of this is that it accumulates the products experience, studies the exact operational method, and then finds and takes action against the fault circumstances. I propose the simple model to represent this status in this paper.

• The Transactions of the Korean Institute of Electrical Engineers A
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• v.48 no.9
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• pp.1073-1080
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• 1999

Optimal routing of distribution networks can be attained by keeping the line power capacity limit to handle load requirements, acceptable voltage at customer loads, and the reliability indices such as SAIFI, SAIDI, CAIDI, and ASAI limits. This method is composed of optimal loss reduction and optimal reliability cost reduction. The former is solved relating to the conductor resistance of all alternative routes, and the latter is solved relating to the failure rate and duration of each alternative route. The routing considering optimal loss only and both optimal loss and optimal reliability cost are compared in this paper. The case studies with 10 and 24 bus distribution networks showed that reliability cost should be considered as well as loss reduction to achieve the optimal routing in the distribution networks.

• Journal of Trauma and Injury
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• v.25 no.1
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• pp.7-16
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• 2012

Purpose: Cervical dislocations with locked facets account for more than 50% of all cervical injuries. Thus, investigating a suitable management of cervical locked facets is important. This study examined factors of close reduction failure in traumatically locked facets of the subaxial cervical spine patients to determine suitable surgical management. Methods: We retrospectively analyzed of the case histories of 28 patients with unilateral/bilateral cervical locked facets from Nov. 2004 to Dec. 2010. Based on MRI evaluation of disc status at the injury level, we found unilateral dislocations in 9 cases, and bilateral dislocations in 19 cases, The patients were investigated for neurologic recovery, closed reduction rate, factors of the close reduction barrier, fusion rate and period, spinal alignment, and complications. Results: The closed reduction failed in 23(82%) patients. Disc herniation was an obstacle to closed reduction (p=0.015) and was more frequent in cases involving a unilateral dislocation (p=0.041). The pedicle or facet fracture was another factor, although some patients showed aggravation of neurologic symptoms, most patients had improved by the last follow up. The kyphotic angle were statistically significant (p=0.043). Sixs patient underwent anterior decompression/fusion, and 15 patients underwent circumferential fusion, and 7 patients underwent posterior fusion. All patients were fused at 3 months after surgery. The complications were 1 case of CSF leakage and 1 case of esphageal fistula, 1 case of infection. Conclusion: We recommend closed reduction be performed as soon as possible after injury to maximize the potential for neurological recovery. Patients fot whom closed reduction of the cervical locked facets have a higher incidence of anatomic obstacles to reduction, including facet fractures and disc herniation. Immediate direct open anterior reduction or circumferential fixation/fusion of locked cervical facets is recommended as a treatment of choice for traumatic locked cervical facet patients after closed reduction failure.

• Advances in pediatric surgery
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• v.19 no.2
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• pp.66-72
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• 2013

Intussusception is common cause of intestinal obstruction in children. Most of intussusceptions can be treated with non-operative reduction using air or barium. However, about 10% patients need operative treatment due to failure of reduction, peritonitis, and recurrence after reduction. We introduce our experience of laparoscopic surgery for intussusception. From April 2010 to March 2013, we reviewed 57 children who diagnosed intussusception. Twelve patients underwent an operation. The cause of operation was 7 of failure of air reduction and 5 of recurrence after air reduction. Median age was 21.5 months (range: 5.0~57.7 months) and 11 children (91.7%) underwent successful laparoscopic reduction. Median operating time was 50 minutes (range: 30~20 minutes) and median hospital days was 4.5 days (range: 3~8 days). One patient had a leading point as a heterotopic pancreas and underwent bowel resection through conversion. There was neither intra-operative nor postoperative complication. Laparoscopic reduction for intussusception can bring an excellent cosmetic effect with high success rate.

• International Journal of Reliability and Applications
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• v.10 no.1
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• pp.43-57
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• 2009

In this article, we study the reliability equivalence factor of a series system. The failure rates of the system components are functions of time t. we study two cases of non-constat failure rates (i) weibull distribution (ii) linear increasing failure rate distribution. There are two methods are used to improve the given system. Two types of reliability equivalence factors are discussed. Numerical examples are presented to interpret how one can utilize the obtained results.

• Journal of Information Technology Applications and Management
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• v.13 no.2
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• pp.17-28
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• 2006

The SRGM has been studied under the assumption that S/W reliability can grow as the fault causing failure is removed even during operational phase because the debugging is available. On the other hand, some papers insist on the uniform failure rate during operational phase because the debugging may not be available in case of universal software. The phenomenon, however, has been observed informally many times that the products S/W reliability grows as the time goes by even without any debugging in point of customer view. I propose the simple approaching method to model the S/W reliability phenomenon that the failure rate reduces as time goes on without modifying the existing reliability model in this paper.

• Proceedings of the Korean Reliability Society Conference
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• 2005.06a
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• pp.121-129
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• 2005

In this paper, we consider a periodic preventive maintenance(PM) policy in which each PM reduces the hazard rate of amount proportional to the failure intensity, which increases since the last PM and slows down the wear-out speed to that of new one. And the proportion of reduction in hazard rate decreases with the number of PMs. Our model is similar to $ARI_1$ proposed by Doyen and Gaudoin(2004) in the sense of reduction of hazard rate. Our model has totally different wear-out pattern of hazard rate after PM's, however, and the proportion of reduction depends on the number of PM's. Assuming that the system undergoes only minimal repairs at failures between PM's, the expected cost rate per unit time is obtained. The optimal number N of PM and the optimal period x, which minimize the expected cost rate per unit time are discussed. Explicit solutions for the optimal periodic PM are given for the Weibull distribution case.