• International Journal of Reliability and Applications
• /
• v.9 no.1
• /
• pp.95-112
• /
• 2008

This paper gives the reliability equivalence factors of a parallel system with n independent and non-identical components. It is assumed here that, the failure rates of the system's components are constants. We used three different methods to improve the system given. Two reliability characteristics (the mean time to failure and the reliability function) are used to perform the system improvement. For this purpose, the reliability functions and the mean times to failures of the original and improved systems are obtained. The results given in this paper generalize the results given in the literatures by setting n = 1, 2. An illustrative numerical example is presented to compare the different reliability factors obtained.

• Korean Journal of Computational Design and Engineering
• /
• v.20 no.1
• /
• pp.36-43
• /
• 2015

Recently, a number of evaluation studies on availability of plant were carried out. This study was conducted to verify of the reliability of a simulation with some variable such as configuration of process, failure probability density function and the number of iteration times for the natural gas liquefaction plant. The error rate of the KICT-RAM solution was evaluated as 0.03~1.79% compared with the result of the MAROS(commercial solution). And the error-rate change was observed in the range of 0.03~1.75 on the condition of the iteration times as 30, 100, 250. As a result the plant availability evaluation approach of KICT-RAM solution was verified as reasonable. However, the careful approach was required to use the solution because the error-rate increased according to iteration times change.

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

• Journal of Korean Society of Industrial and Systems Engineering
• /
• v.36 no.4
• /
• pp.100-108
• /
• 2013

In aerospace industry, MTBF (Mean Time Between Failure) and MFTBF (Mean Flight Time Between Failure) are generally used for reliability analysis. So far, especially to Korean military aircraft, MFTBF of avionic equipments is predicted by MIL-HDBK-217 and MIL-HDBK-338, however, the predicted MFTBF by military standard has a wide discrepancy to that of real-world operation, which leads to overstock and increase operation cost. This study analyzes operational data of avionic equipments. Operational MFTBF, which is calculated from operational data, is compared with predicted MFTBF calculated conventionally by military standard. In addition, failure rate trend is investigated to verify reliability growth in operational data, the investigation shows that failure rate curve from operational data has somewhat pattern with decreased failure rate and constant failure rate.

• Journal of Korean Neurosurgical Society
• /
• v.64 no.1
• /
• pp.60-68
• /
• 2021

Objective : Until recently, the transfemoral approach (TFA) was used as the primary method of arterial approach in acute ischemic stroke (AIS). However, TFA resulted in longer reperfusion times and worse outcomes in the mechanical thrombectomy (MT) of patients with complex aortic arches and significant carotid tortuosity. We found that the transradial approach (TRA) is a more favorable alternative approach for MT in such cases. Methods : We performed a retrospective review of our institutional database to identify 202 patients who underwent MT for AIS between February 2015 and December 2019. Patient characteristics, cause of TFA failure, procedure time, intra-procedural complications, and outcomes were recorded. Results : Eleven (5.4%) of 202 patients, who underwent MT for AIS, crossed over to TRA for recanalization, and eight (72%) of 11 achieved successful recanalization (≥modified Treatment in Cerebral Infarction 2b). The mean age (mean±standard deviation [median]) was 82.3±6.6 (76) years, and five of the 11 patients were male. The last seen normal to puncture time was 467.9±264.72 (264) minutes; baseline National Institutes of Health Stroke Scale score was 28.9±14.5 (16). Six (55%) of the 11 patients had right vertebrobasilar occlusions, and the remaining five (45%) had anterior circulation occlusive disease. The time from groin puncture to final recanalization time (overall procedural time) was 78.0±20.1 (62) minutes. The mean crossover time from TFA to TRA was 45.2±10.5 (41) minutes. The mean time from radial puncture to final recanalization was 33.8±10.5 (28) minutes. Distal thrombus migration events in previously unaffected territories occurred in 3/8 patients (37%). At 90 days, three patients (28%) had a favorable clinical outcome. Conclusion : Although rare, failure of TFA has been known to occur during MT for AIS. Our results demonstrate that TRA may be an alternative option for AIS intervention for select patients with subsequent timely revascularization. However, the incidence of distal thrombus migration was high, and the first puncture to reperfusion time was prolonged because of the time taken for the crossover to TRA after failure of TFA. This study provides some evidence that the TRA may be a viable alternative option to the TFA for MT of AIS.

• Journal of the Korean Institute of Gas
• /
• v.7 no.4 s.21
• /
• pp.20-23
• /
• 2003

The analysis of reliabilities of process facilities often uses models based on the Weibull distribution. The parameters in these models are functions of operating conditions, and determined by experiments. Using these values, we calculate the reliability, mean time to failure, and standard deviation. The conventional method assumes that the operating condition is constant, and thus treats the model parameters as constants. In this paper, a reliability function is proposed which is applicable when the scale parameter is a function of time, and an analysis method based on this is also presented. A case study on a cooling fan resulted in a big difference from the conventional method to which the average operating conditions were applied. The proposed method is also applicable to other process facilities, and expected to effectively take into account the effects of changes in the operating conditions on the reliabilities of the facilities.

• International Journal of Reliability and Applications
• /
• v.2 no.4
• /
• pp.241-251
• /
• 2001

In this paper, optimal bum-in time to minimize the total mean cost, which is the sum of manufacturing cost with burn-in and cumulative warranty-related cost, is obtained. When the products with cumulative pro-rata warranty have high failure rate in the early period (infant mortality period), a burn-in procedure is adopted to eliminate early product failures. After burn-in, the posterior product life distribution and the warranty-related cost are dependent on burn-in time; long burn-in period may reduce the warranty-related cost, but it increases the manufacturing cost. The paper provides a methodology to obtain total mean cost under burn-in and cumulative pro-rata warranty. Property of the optimal burn-in time is analyzed, and numerical examples and sensitivity analysis are studied.

• International Journal of Reliability and Applications
• /
• v.11 no.2
• /
• pp.89-106
• /
• 2010

This paper deals with the reliability and availability characteristics of three different series system configurations with warm standby components and a repairable service station. The failure time of the primary and warm standby are assumed to be exponentially distributed with parameters ${\lambda}$ and ${\alpha}$ respectively. The repair time distribution of each server is also exponentially distributed with parameter ${\mu}$. The breakdown time and the repair time of the service station are also assumed exponentially distributed with parameters ${\gamma}$ and ${\beta}$ respectively. We derive the reliability dependent on time, availability dependent on time, the mean time to failure, $MTTF_i$, and the steady-state availability $A_i$(${\infty}$) for three configurations and perform comparisons. Comparisons are made for specific values of distribution parameters and of the cost of the components. The three configurations are ranked based on: $MTTF_i$, $A_i$(${\infty}$), and $C_i/B_i$ where $B_i$ is either $MTTF_i$ or $A_i$(${\infty}$).

• Journal of the Korea Institute of Military Science and Technology
• /
• v.9 no.2 s.25
• /
• pp.51-59
• /
• 2006

The purpose of this paper is to present reliability analysis procedures for repairable systems and apply the procedures for assessing the reliabilities of two subsystems of a specific group of military equipment based on field failure data. The mean cumulative function, M(t), the average repair rate, ARR(t), and analytic test methods are used to determine whether a failure process follows a renewal or non-renewal process. For subsystem A, the failure process turns out to follow a homogeneous Poisson process, and subsequently, its mean time between failures, availability, and the necessary number of spares are estimated. For subsystem B, the corresponding M(t) plot shows an increasing trend, indicating that its failure process follows a non-renewal process. Therefore, its M(t) is modeled as a power function of t, and a preventive maintenance policy is proposed based on the annual mean repair cost.

• Journal of Power Electronics
• /
• v.16 no.4
• /
• pp.1426-1437
• /
• 2016

In different industrial and mission oriented applications, redundant or standby semiconductor systems can be implemented to improve the reliability of power electronics equipment. The proper structure for implementation can be one of the redundant or standby structures for series or parallel switches. This selection is determined according to the type and failure rate of the fault. In this paper, the reliability and the mean time to failure (MTTF) for each of the series and parallel configurations in two redundant and standby structures of semiconductor switches have been studied based on different failure rates. The Markov model is used for reliability and MTTF equation acquisitions. According to the different values for the reliability of the series and parallel structures during SC and OC faults, a comprehensive comparison between each of the series and parallel structures for different failure rates will be made. According to the type of fault and the structure of the switches, the reliability of the switches in the redundant structure is higher than that in the other structures. Furthermore, the performance of the proposed series and parallel structures of switches during SC and OC faults, results in an improvement in the reliability of the boost dc/dc converter. These studies aid in choosing a configuration to improve the reliability of power electronics equipment depending on the specifications of the implemented devices.