• Journal of Korean Society of Industrial and Systems Engineering
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• v.45 no.3
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• pp.78-89
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• 2022

The purpose of this study was to propose useful suggestion by analyzing preventive replacement policy under which there are minor and major failure. Here, major failure is defined as the failure of system which causes the system to stop working, however, the minor failure is defined as the situation in which the system is working but there exists inconvenience for the user to experience the degradation of performance. For this purpose, we formulated an expected cost rate as a function of periodic replacement time and the number of system update cycles. Then, using the probability and differentiation theory, we analyzed the cost rate function to find the optimal points for periodic replacement time and the number of system update cycles. Also, we present a numerical example to show how to apply our model to the real and practical situation in which even under the minor failure, the user of system is not willing to replace or repair the system immediately, instead he/she is willing to defer the repair or replacement until the periodic or preventive replacement time. Optimal preventive replacement timing using two variables, which are periodic replacement time and the number of system update cycles, is provided and the effects of those variables on the cost are analyzed.

• Journal of the military operations research society of Korea
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• v.18 no.2
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• pp.114-125
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• 1992

A policy of periodic replacement with minimal repair at failure is considered for a complex system. Under such a policy the system is replaced at periodic times. iT(i=1,2, $\ldots$), while minimal repair is performed at any intervening system failures. The cost of the j-th minimal repair to the component which fails at age t is g(C(t). $c_j$ (t)), where C(t) is the age-dependent random part, $c_j$(t) is the deterministic part which depends on the age and the number of the minimal repair to the component, and g is a positive nondecreasing continuous function. The cost of replacement is expensive when the number of failures occurring in (0. T) is greater than a threshold level. The problem of determining the optimal replacement period, $T^{\ast}$, which minimizes the total expected cost per unit time over an infinite time horizon is considered. Various special cases are considered.

• Journal of Korean Institute of Industrial Engineers
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• v.21 no.4
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• pp.507-517
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• 1995

The failure rate of an item depends on operational environment. When an item has a chance failure period and a wearout failure period in sequel, the severity of operational environment causes the increase in the slop of wearout failure rate or the increase in the magnitude of chance failure rate. For such a change of operational environment, this paper concerns the change of optimal preventive replacement time. Two preventive replacement policies, age replacement policy and periodic replacement policy with minimal repair, are considered. Investigated properties are: (a) in age replacement policy, optimal preventive replacement time increases as the chance failure rate increases and optimal preventive replacement time decreases as the slope of wearout failure rate increases, and (b) in periodic replacement policy with minimal repair, optimal preventive replacement time increases as the slope of wearout failure rate increases; however, the change of chance failure rate does not alter the optimal preventive replacement time.

• Journal of Korean Institute of Industrial Engineers
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• v.11 no.1
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• pp.3-10
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• 1985

Periodic replacement policies are proposed for a system whose repair cost, when it fails, can be estimated by inspection. The system is replaced when it reaches age T (Policy A), or when it fails for the first time after age T (Policy B). If it fails before reaching age T, the repair cost is estimated and minimal repair is then undertaken if the estimated cost is less than a predetermined limit L; otherwise, the system is replaced. The expected cost rate functions are obtained, their behaviors are examined, and ways of obtaining optimal T and L are explored.

• Journal of Korean Society for Quality Management
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• v.25 no.3
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• pp.86-95
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• 1997

This paper considers a repairable system, which is maintained preventively at periodic times and is minimally repaired at each failure. Most preventive maintenance policies for such repairable systems assume that the cost of minimal repair is constant regardless of its age at failure. However, it is more practical to consider the situations where the cost of minimal repair is dependent not only on its age at failue, but also on the number of preventive maintenance carried out prior to its failure. We consider the preventive maintenance carried out prior to its failure. We consider the preventive maintenance policy with age-dependent minimal repair cost. The optimal policies which minimize the expected cost rate over an infinite time span are discussed. We obtain the optimal period and number of preventive maintenance prior to replacement of the system.

• Journal of the Korea Institute of Military Science and Technology
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• v.4 no.1
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• pp.170-176
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• 2001

An optimization problem to obtain the optimal replacement interval considering the salvage values is studied. The system is minimally repaired at failure and is replaced by new one at age T(periodic replacement policy with minimal repair of Barlow and Hunter〔2〕). Our model assumes that the time horizon associated with the number of replacements is random The total expected cost considering the salvage values with random time horizon is obtained and the optimal replacement interval minimizing the cost is found by numerical methods. Comparisons between non-considered salvage values and this case are made by a numerical example.

• Journal of Applied Reliability
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• v.18 no.2
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• pp.104-113
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• 2018

Purpose: This paper proposes the non-periodic preventive maintenance policy based on the level of cumulative hazard intensity. We aim to construct a cost-effectiveness on the proposed model with relaxing the constraint on reliability. Methods: We use the level of cumulative hazard intensity as a condition variable, instead of reliability. Such a level of cumulative hazard intensity can derive the reliability which decreases as the frequency of preventive maintenance action increases. We also model the imperfect preventive maintenance action using the proportional age setback model. Conclusion: We provide a numerical example to illustrate the proposed model. We also analyze how the parameters of our model affect the optimal preventive maintenance policy. The results show that as long as high reliability is guaranteed, the inefficient preventive maintenance action is performed reducing the system operation time. Moreover, the optimal value of the proposed model is sensitive to changes in preventive maintenance cost and replacement cost.

• Journal of the korean academy of Pediatric Dentistry
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• v.23 no.3
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• pp.697-705
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• 1996

Ankyloglossia, or tongue-tie, is a congenital condition which occurs as a result of fusion between the tongue and the floor of the mouth. Ankyloglossia often results in malocclusion with an anterior "open bite" deformity, early prognathism, swallowing problem, speech disorder, and periodontal problem. Generally lingual frenectomy is used for treatment of ankyloglossia, but incomplete operation and simple frenectomy may produce a scar contracture resulting in a more deformed ankyloglossia than was present initially. The Z-plasty is used for the correction of scar contractures and the replacement of missing tissue and this procedure is ideally suited for the treatment of an ankylosed frenum. Most authors advise postponement of any decision for surgical correction of tongue-tie until the age of 4 years, unless the child is having much difficulty with sucking or swallowing. We treated 4 patients with ankyloglossia using Z-plasty technique. As a result, we found out that it was effective for correction of movement limitation of tongue, prevention of relapse. Further, periodic check ups are needed for evaluation of relapse, improvement of speech, and other functions of the tongue.

• Journal of Korean society of Dental Hygiene
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• v.16 no.6
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• pp.849-861
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• 2016

Objectives: The purpose of the study was to investigate the relationships among snack habits, oral health practice, and oral health status in preschool children. Methods: The subjects were 209 preschool children and their mothers in Jeonbuk from March 24 to May 30, 2014. The study instruments comprised snack habits, oral health practice, and oral health status. Data were analyzed for independent t-test, one way ANOVA and hierarchical multiple regression using SPSS Win 21.0 version. Results: Higher dft index was shown in the older age (p=0.033), lower mother's education (p<0.001), lower mother's daily toothbrushing (p<0.001), check-up after tooth-brushing (p<0.001), tooth-brushing practice before sleeping (p<0.001), tooth-brushing practice after snack (p<0.001), regular dental check-up (p<0.001), fluoride and sealant for prevention caries (p<0.001), limitation of sugar snack intake (p<0.007), periodic replacement toothbrush (p=0.022). The cause of higher dft index included soda (p<0.001), yogurt (p<0.001), snack (p=0.002), bread and cake (p=0.002) and caramel and candy (p<0.001). Fruit (p<0.001), vegetable (p<0.001) and milk (p=0.004) decreased dft index. Factors affecting oral health status were tooth-brushing practice before sleeping, tooth-brushing practice after snack, regular dental check-up, fluoride and sealant for prevention caries, and intake of soda, yogurt, caramel, candy, and fruit. The explanation power of the final model was 67.6%. Conclusions: It is necessary to develop the oral health education program for the preschool children and mothers to enhance the best oral health condition.

• Journal of Chest Surgery
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• v.33 no.7
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• pp.547-551
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• 2000

Background; The purpose of this study was to analyze the early and midterm results of Cabrol technique to assist in making future decisions on a more adequate technique for repairing aortic root diseases. Material and Method; From August 1993 to July 1999, we performed Cabrol technique in 18 patients ; 12 annuloaortic ectasia, 6 Stanford type A aortic dissection. Male and female ratio was 11;7, mean age was 46.9$\pm$12.3 years and mean follow up period was 22.5$\pm$21.5 months. We analysed the factors influencing postoperative complications and early mortality. The factors were old age(>60 years), high NYHA(>III), preoporative concomitant disease, urgency of operation, concomitant procedure, long pump preoperative concomitant disease, urgency of operation, concomitant procedure, long pump time(>200 minute), and hospital stay time (>30 days). Result; Operative mortality was 11.1%, late mortality was 11.1%, and overall mortality was 22.2%. The causes of operative death were a heart failure and an arrhythmia. The causes of late death were an acute myocardial infarction and an unknown etiology. Postoperative complications were bleeding, wound infection, toxic hepatitis, acute renal failure, and cerebral infarction. The factors influencing postoperative complications were hihg MYHA Fc(>III) (p=0.044), concomitant disease (p=0.044), long pump time(>200 minute)(p=0.015), and concomitant procedure(p=0.004). There were no significant factors influencing early mortality. Conclusion; The lower postoperative bleeding rate and no complication related to tension of anastomosis after Cabrol technique warrant its consideration in patients requiring aortic root replacement, especially without feasible mobilization of coronary arteries. However, to confirm the graft thrombosis, a more detailed study including periodic angiography will be required.