• Proceedings of the Materials Research Society of Korea Conference
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• 2011.05a
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• pp.22.2-22.2
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• 2011

Reliability and degradation mechanism of conventional phosphor-converted white GaN-based light-emitting diodes (LEDs) were investigated. Under electro-thermal stress condition, the optical output degraded rapidly at the initial stress time accompanied by the change of chromatic properties. This could be attributed to the optical degradation of packaged materials, in particular, the browning of encapsulants and the darkening of reflective packages. At longer stress times, the optical output gradually decreased according to the degree of the reverse leakage currents, namely, the generation ofnonradiative recombination defects. This indicates that the optical degradation of white LEDs are dominated by the darkening of packaged materials and the generation of defects depending on the injection current and ambient temperatures. Using analyses of electroluminescence spectra, optical microscopy, electrical, optical, and thermal properties, optical degradations of white LEDs are discussed.

• Journal of Applied Reliability
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• v.17 no.3
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• pp.181-187
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• 2017

Purpose: The purpose of this study is to estimate the lifetime of commercial passenger vehicles shock absorber using degradation test and data. Method: The degradation factor of shock absorber was determined to be a damping force using FMEA. Degradation test was performed on damping force under real world usage condition and analysed the degradation data. Results: To estimate the lifetime of shock absorber, a degradation model was developed and a numerical example was provided. Conclusion: Evaluation of the lifetime of commercial and military vehicles shock absorber will be possible by using the proposed degradation analysis method.

• Journal of Applied Reliability
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• v.10 no.1
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• pp.25-38
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• 2010

In this paper, an accelerated degradation test procedure for an electronic appliance compressor is proposed. We investigate the amount of wear of the compressor and consider several factors as accelerating factors. Finally we select the operating pressure as a main accelerating factor. The test condition of accelerated degradation test is determined. The modified accelerating test reduces the test time in design phase by using the suggested accelerating factor.

• Proceedings of the Korean Reliability Society Conference
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• 2004.07a
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• pp.193-200
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• 2004

Determination of sample sizes and the inspection intervals for degradation tests is considered. The cases of degradation rate model and degradation path model are analyzed with some examples. Tightened critical value tests are also considered that are shown to be advantageous over non-tightened ones.

• Journal of the Institute of Electronics Engineers of Korea SD
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• v.42 no.12
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• pp.27-34
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• 2005

We have investigated the correlation between the active-layer uniformity and reliability of InGaN/GaN blue LEDs. According to initial characteristics, the devices are classified into two groups: group I devices of uniform light-emission and group II devices of non-uniform light-emission. The group II devices are more dependent on temperature and we have found two degradation mechanisms through reliability test. One is bulk degradation in which the degradation occurred over the entire chip and another one is edge degradation in which the degradation occurred from the edge of the chip. Bulk degradation caused by the nonradiative defects is found to be faster in group II devices while there is no difference in the rate of the much faster edge degradation, where darkening starts from the n-Ohmic contact edge. Therefore, more uniform active layer, more uniform current spreading, and the passivation of the dry-etched side-wall are essential for the high reliability of InGaN/GaN LEDs.

• Journal of Applied Reliability
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• v.18 no.1
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• pp.80-86
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• 2018

Purpose: This article provides a mathematical model for the accelerated degradation test when the performance degradation characteristic follows the lognormal distribution. Method: For developing test plans, the total number of test units and the test time are determined based on the minimization of the asymptotic variance of the q-th quantile of the lifetime distribution at the use condition. Results: The mathematical model for the accelerated degradation test is provided. Conclusion: Accelerated degradation test method is widely used to evaluate the product lifetime within a resonable amount of cost and time. In this article. a mathematical model for the accelerated degradation test method is newly developed for this purposes.

• Journal of Applied Reliability
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• v.9 no.2
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• pp.71-80
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• 2009

This paper discusses the service life prediction methods for CFRP bar for concrete reinforcement using accelerated degradation tests. The relationship between performance degradation and the rate of a failure-causing chemical reaction is assumed for the temperature accelerated degradation tests. Methods of obtaining acceleration factors and predicting service life of the CFRP bar using the degradation model are presented.

• Journal of Applied Reliability
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• v.18 no.1
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• pp.49-55
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• 2018

Purpose: To propose improved method for normalization, compare to the de facto international standard which is IESNA TM-21 or conventional normalization methods. Methods: Firstly, we analysed conventional methods and specified the problem of normalization method which is based on first measured data. Secondly, we proposed our approach which is based on the design specification. Lastly, we studied a real degradation data which is conducted for 15,000 hours. Conclusion: Proposed normalization method is better approach because it can reflect real data and design specification, and reduce distortion when analysing degradation data. Also, It is appliable to other long-life reliability items.

• Advances in concrete construction
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• v.9 no.5
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• pp.469-478
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• 2020

This study aims to establish a new methodological framework for the evaluation of the evolution of the reliability of plain concrete for pavement vs number of cycles under flexural fatigue loading. According to the framework, a new method calculating the reliability was proposed through probability simulation in order to describe a random accumulation of fatigue damage, which combines reliability theory, one-to-one probability density functions transformation technique, cumulative fatigue damage theory and Weibull distribution theory. Then the statistical analysis of flexural fatigue performance of cement concrete tested was carried out utilizing Weibull distribution. Ultimately, the reliability for the tested cement concrete was obtained by the proposed method. Results indicate that the stochastic evolution behavior of concrete materials under fatigue loading can be captured by the established framework. The flexural fatigue life data of concrete at different stress levels is well described utilizing the two-parameter Weibull distribution. The evolution of reliability for concrete materials tested in this study develops by three stages and may corresponds to develop stages of cracking. The proposed method may also be available for the analysis of degradation behaviors under non-fatigue conditions.

• Journal of the Korean Society of Industry Convergence
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• v.25 no.6_3
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• pp.1239-1246
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• 2022

This paper introduces the effect of grease on the degradation characteristics of bearings used as key components of packaging equipment and automation systems. Bearings parts are installed to fix and support the rotating body of the system, and performance degradation of the bearings has a great effect on the life of the system too. When bearings are used in various devices and systems, the grease is applied to reduce friction and improve fatigue life. Determining the type of lubricant (grease) is important because it has a great influence on the operating environment and lifespan and ensures long lifespan of systems and facilities. However, studies that simultaneously compared and analyzed the change in mechanical degradation characteristics and the comparison of chemical degradation characteristics according to grease types under actual operating conditions are insufficient. In this paper, three types of small harmonic drive, high-load reducer, and low-load reducer grease used in power transmission joint modules are experimentally selected and finally injected into ball bearings with a load (19,500N) to improve bearing durability. Degradation characteristics were tested by attaching to test equipment. At this time, after the durability test under the same load conditions, the mechanical degradation characteristics, that is temperature, vibration according to the three greases types. In addition, the chemical degradation characteristics of the corresponding grease was compared to present the results of mutual correlation analysis.