• Journal of the Korean Data and Information Science Society
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• v.20 no.6
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• pp.1169-1175
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• 2009

In this paper, the inferences of data obtained from periodic inspection and type I censoring for the step-stress accelerated life test are studied. The exponential distribution with a failure rate function that a log-linear function of stress and the tampered failure rate model are considered. The maximum likelihood estimators of the model parameters are estimated and also the optimal stress change time which minimize the asymptotic variance of maximum likelihood estimators of parameters is determined. A numerical example will be given to illustrate the proposed inferential procedures and the sensitivity of the asymptotic variance of the estimated mean by the guessed parameters is investigated.

• Journal of Korean Institute of Industrial Engineers
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• v.17 no.1
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• pp.95-108
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• 1991

For the case where the lifetime at a constant stress level has exponential distribution, optimal accelerated life test plans are developed under the assumptions of intermittent inspection and Type I censoring. In a optimal plan, the low and high stress levels, the proportion of test units allocated and the inspection times at each stress are determined such that the asymptotic variance of the maximum likelihood estimator of logarithmic transformed mean at the use condition is minimized. In addition to the optimal plan in which numerical technique to solve the set of nonlinear equations must be employed to determine inspection times at each stress level, we also propose another plans which employ equally-spaced or equal probability inspection schemes at two overstress levels of corresponding optimal one. For both optimal and proposed plans, computational results indicate that the asymptotic variance of the estimated mean at the use stress is insensitive to number of inspections at overstress levels for the range of parameter values considered.

• Journal of Korean Society for Quality Management
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• v.24 no.4
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• pp.44-58
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• 1996

This paper presents optimal accelerated life test plans with different censoring times for exponential, Weibull, and lognormal lifetime distributions, respectively. For an optimal plan, low stress level, proportion of test units allocated and censoring time at each stress are determined such that the asymptotic variance of the maximum likelihood estimator of a certain quantile at use condition is minimized. The proposed plans are compared with the corresponding optimal plans with a common censoring time over range of parameter values. Computational results indicate that those plans are statistically optimal ones in terms of accuracy of estimator when total censoring times of two plans are equal.

• Journal of Korean Institute of Industrial Engineers
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• v.23 no.1
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• pp.177-196
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• 1997

In this paper, statistically optimal accelerated life test(ALT) plans considering statistical efficiency only and new compromise ALT plans to sacrifice some statistical efficiency in return for improved overall properties including estimobility probability and robustness for the model assumptions are developed under the assumptions of constant stress, intermittent inspection, Type I censoring and lognormal failure distribution which has been one of the popular choices of failure distributions in the extensive engineering applications of ALT. Computational experiments are conducted to compare with four ALT plans including two proposed ones under continuous and intermittent inspections over a range of parameter values in terms of asymptotic variance, sensitivities for guessed input values, and proportion of estimable samples, etc. The small and moderate sample properties for the proposed ALT plans designed under asymptotic criterion are also investigated by Monte Carlo simulation.

• Journal of the Korean Society for Aviation and Aeronautics
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• v.27 no.3
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• pp.15-23
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• 2019

This paper contains analyses and development processes of the requirements to detect the possible intermittent failure in an old avionics backplane. Interconnections for signal transmission between electronic components, such as Pin-to-PCB, FPCB-to-FPCB, pin-to-FPCB, and pint-to-wire, were selected as the main cause of intermittent failure by analyzing target equipment and documents. The possibility of detecting intermittent failures occurring in the target equipment is verified by physics-of-failure analyses. In order to verify the occurrence of intermittent failures and their detectability, latching continuity circuit testers were manufactured and accelerated life tests were performed by applying temperature and vibration cycle in consideration of flight conditions. Through the above process, the detection requirements for the major intermittent failure in the target avionics backplane was developed.

• Proceedings of the KSR Conference
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• 2003.10c
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• pp.606-610
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• 2003

The 200 class insulation system which adopted to traction motor have excellent dielectric strength but weaken to thermal stress therefore deterioration phenomena analysis according to thermal stress is necessary. Accelerated thermal aging tests have been used to determine thermal reliability of stator coils used as traction motor in electric multiple unit. The conventional aging test is carried on according to IEC 60034-18-31 and IEEE Std. 275-1992. Variation in insulation resistance, P.I, capacitance, dielectric loss($tan{\delta}$) and partial discharge are measured during the aging cycle. Sample coils for traction motor were tested by accelerated aging test which composed of heat, vibration and moisture. Reliability and expected life were evaluated on the insulation system for traction motor.

• KEPCO Journal on Electric Power and Energy
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• v.8 no.2
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• pp.73-77
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• 2022

Recently, as the number of years of operation has increased for more than 30 years, interest in evaluating the remaining life of major power facilities such as transformers and ultra-high voltage cables is increasing. In particular, the risk of failure is increasing because the underground transmission XLPE cable has been built since 1980 and has been operating in excess of 30 years of design life or close proximity. Therefore, it is necessary to develop an algorithm to evaluate the residual life of the XLPE cable considering the load to determine the risk of failure. Since load data is large amount of data, it is necessary to make the variable load information equivalent to the time unit first in order to calculate the remaining life of the system quickly. In overseas literature, transformers are reported to be standardized for variable load equivalent conversion formulas, but they have not been reported for ultra-high voltage cables. Therefore, in this paper, whether the equivalent conversion formula of a transformer can be applied to XLPE cables was reviewed through accelerated degradation tests under equivalent and variable temperature conditions, and considerations were studied when evaluating the remaining operating life of XLPE cables based on the experimental results.

• Journal of the Korean Mathematical Society
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• v.31 no.4
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• pp.709-715
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• 1994

The aim of of the study is a powerful test for the discrimination and therefore an optimal desin for that purpose. This problem is studied by Chernoff ([5]) and used in Chernoff ([6]) for accelerated life tests using the exponential distribution for life times. The approach used here is similar to that suggested by Lauter ([10]) and used in Chaloner ([3]) and Chaloner and Larntz ([4]) where it is motivated using Bayesian arguments. The approach taken in this paper the loss function $L(\cdot)$ evaluating a test procedure and a design d.

• Journal of the Korean Operations Research and Management Science Society
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• v.19 no.1
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• pp.145-152
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• 1994

This paper deals with an economic design of acelerated life test under constant stresses where failure times are exponentially distributed. In this case the optimization criterion is the information amount per unit cost. Fisher's information matrix of exponential distribution's parameters and expected cost considering fixed and variable costs are obtained. The decision variable is the censoring time in the model. In the 2-level constant stress case, it is proved that the optimal solution exists and is unique under some condition. Numerical examples are also included.

• Fire Science and Engineering
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• v.33 no.2
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• pp.114-123
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• 2019

The paper relates to a study on the changes in performance of insulation sheath resulting from accelerated degradation of IV and HIV insulated wire. To assume insulation degradation of IV and HIV insulated wire, accelerated life tests using Arrhenius equation were conducted among accelerated life test models, and experimental samples of 0 year, 10 years, 20 years, 30 years, and 40 years in equivalent life were produced. Whereas the maximum tensile load were increased as accelerated degradation of IV and HIV insulated wire progressed, elongation percentage, rupture time, and flexibility of insulated wires were found to be gradually reduced. According to the additional surface analysis results for the insulated wires per equivalent life using a scanning electron microscope, mechanical properties of the insulator were observed to be reduced as insulation degradation resulting from aging progressed since phenomena such as formation of crystalline structures and perforation, etc. occurred on the sample surface with progression of accelerated degradation. Consequently, institutional replacement of insulated wires and preparation of repair times considering performance degradation of the insulator installed inside buildings are considered necessary in order to prevent in advance the risks of electrical fire resulting from degradation in insulation performance.