• Journal of Korean Society for Quality Management
• /
• v.27 no.3
• /
• pp.79-93
• /
• 1999

The common accelerated life test(ALT) consists of test methods applying a constant stress, higher than the use condition stress, to items. There we, however, situations for which a progressive stress ALT, in which the stress on a test item is continuously increased with time, Is more convenient to perform testing and simpler in analyzing data than a constant stress ALT. When a product under constant stress s follows a Weibull distribution with parameters $\theta$(5) and $\beta$, maximum likelihood(ML) estimators of parameters involved in the model are obtained and their asymptotic distributions are derived under stress bounded ramp tests in which the stress is increased linearly from use condition stress to the stress upper bound. The optimum test plans are also found which minimize the asymptotic variance of the ML estimator of the log mean life at design constant stress. For selected values of the design parameters, tables useful for finding optimal test plans are given. The effect of the pre-estimates of design parameters is studied.

• Proceedings of the Korean Operations and Management Science Society Conference
• /
• 1996.04a
• /
• pp.262-265
• /
• 1996

An ALT(accelerated life testing) method is to test a product in over-stressed conditions, and then the test result is extrapolated to an usual (normal stressed) condition. It is the major disadvantage of ALT method that the more extrapolation to an usual condition applies the bigger error is indispensable. Therefore a reliability model combining field failure data and laboratory test data is required in pratice. We propose several methods of estimating the failure rate of a product life which is assumed to follow an exponential distribution. Structural similarity and technological advances are also cosidered. Finally, We derive the acceleratio factor which can be used to predict the failure rate for a new product.

• Journal of the Korea Concrete Institute
• /
• v.12 no.3
• /
• pp.21-30
• /
• 2000

Recently, reinforced concrete structures exposed to severe enviroment are increased in metropolitan area. The acid rain and CO2 penetrated towad rebar, thus rebar corrosion occurred. The corrosion of rebar in concrete is, as in most corrosion processes, an electrochemical nature. The corrosion may severely affect on durability and service life of such a concrete structures. This study was performed for the purpose of acquiring data about corrosion condition and considering a countermeasure to prevent rebar from corroding due to carbonation of concrete. An accelerated car bonation testing procedure was applied to measure the evolution of carbonation and rebar corrosion with time for various water-binder ratios and cement types.

• International Journal of Concrete Structures and Materials
• /
• v.9 no.3
• /
• pp.295-306
• /
• 2015

Simulating natural leaching process for cementitious materials is essential to perform long-term safety assessments of repositories for nuclear waste. However, the current test methods in literature are time consuming, limited to crushed material and often produce small size samples which are not suitable for further testing. This paper presents the results from the study of the physical (gas permeability as well as chloride diffusion coefficient) and mechanical properties (tensile and compressive strength and elastic modulus) of solid cementitious specimens which have been depleted in calcium by the use of a newly developed method for accelerated calcium leaching of solid specimens of flexible size. The results show that up to 4 times increase in capillary water absorption, 10 times higher gas permeability and at least 3 times higher chloride diffusion rate, is expected due to complete leaching of the Portlandite. This coincides with a 70 % decrease in mechanical strength and more than 40 % decrease in elastic modulus.

• Journal of Electrical Engineering and Technology
• /
• v.4 no.1
• /
• pp.123-130
• /
• 2009

The partial discharge (PD) measurement is a very effective method to assess the winding insulation condition of high-voltage machines, since most of the insulation failure processes are directly or indirectly caused by PD. On-line PD measurements, which can detect insulation defects of winding in the early stages on rotating machines in operation, have been accepted as the most important technique. The epoxy mica capacitive coupler is currently and extensively used for on-line detection of PD pulses of high-voltage rotating machines. To evaluate the feasibility of developing a capacitive coupler that is easier to manufacture at a lower cost compared to epoxy mica couplers, a 100pF capacitive coupler made of ceramic material is designed, fabricated and tested for on-line PD measurements of 23 kV electrical machines. A series of electrical tests and accelerated aging tests are performed on the ceramic coupler to evaluate the performance requirements, long-term reliability and thermal stability for in field application. The test results show that the newly developed ceramic coupler provides equal and improved performance at a lower cost compared to epoxy mica couplers, and estimated voltage life is anticipated to surpass 100 years.

• KIEE International Transaction on Electrical Machinery and Energy Conversion Systems
• /
• v.5B no.2
• /
• pp.142-145
• /
• 2005

The life expectancy of a transformer largely depends on the temperature-rise it experiences. If the temperature-rise exceeds limits specified in the design standards, the aging of insulating materials is accelerated and the capability of the cooling medium is deteriorated. Consequently, applicable limits for the temperature-rise are essential in designing the transformer and the coolers, demanding the estimation of the transformer's thermal behavior. In order to analyze the temperature characteristics of the transformer, numerical analysis by way of the commercial CFD code has been carried out, and temperature-rise testing to verify computed results was performed. The results obtained in this study show that there is a good agreement between computed outcomes and experimental outcomes.

• Proceedings of the Korean Institute of Interior Design Conference
• /
• 2003.05a
• /
• pp.174-177
• /
• 2003

The progress of urbanization has been accelerated by the development of machinery after the Industrial Revolution and the speed of change has been increased after the invention of computers. However, the problem of residence, the space for human life in the city, is still not complete. Ever since apartments rose as a solution for the residential problem caused by urbanization after the 1970s and now after 30 years, remodelling is preferred among the 2 choices, reconstruction and remodelling of apartments, in respect of economy and efficiency. Especially, in remodelling collective residence, construction, equipment environment, sight, etc. are considered and only after testing these various aspects can remodelling succeed. Aiming at studying the need of visual identity among many considerations in remodelling, this study investigates and analyzes the remodelling cases of domestic and overseas common residence and suggests the need of visual identity. In Korea where construction design for common residence is monotonous, expressing visual identity can be even more important. The common residence area that symbolizes its unique characteristic will elevate not only its economic value but also the psychological value of its residents.

• Proceedings of the Korean Society for Agricultural Machinery Conference
• /
• 1993.10a
• /
• pp.291-296
• /
• 1993

Taking a bledes axle of rotary tiller as a example, this paper discusses influences of four loading essential factors, which are strengthened amplitude, cycle times, loading sequence and loading frequency. in fatigue life. Determination principles of above four factors and monitoring methods of fatigue damage by local strain are dealt with. The actual field testing check of farm machinery is rapidly simulated by laboratory program fatigue test can shorten the period of development and improvement of a product. In the time of in-door simulation test, damage monitoring and four loading essential factors, which are strengthened amplitude , cycle times, loading sequence and loading frequency, have to be dealt with . If these problems are solved successfully, it is possible to accelerated test speed, reduce costs and manhours, and raise accuracy of test result. However strengthening method, loading pattern and influence of loading frequency on test result have not so far been discu sed systematically, damage monitoring is even more a difficult problem. Authors have studied above problems with the object of blades axle of rotary tiller.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
• /
• 2002.07b
• /
• pp.1022-1025
• /
• 2002

This paper presents the reliability of planar InP/InGaAs avalanche photodiodes (APD's) with recess etching, which is very crucial for the commercial 10-Gb/s optical receiver application. A versatile design for the planar InP/InGaAs APD's and bias-temperature tests to evaluate long-term reliability at temperature from 200 to $250^{\circ}C$. The reliability is examined by accelerated life tests by monitoring dark current and breakdown voltage. The lifetime of the APD's is estimated by a degradation activation energy. Based on the test results, it is concluded that the planar InP/InGaAs APD's with recess etching shows the sufficient reliability for practical 10-Gb/s optical receivers.

• Proceedings of the KIEE Conference
• /
• summer
• /
• pp.551-553
• /
• 2004

SF_6 gas has been increasingly used as the insulating and arc-suppressing medium in switchgears which are used as the protection devices of power system. Nowadays, with these trends, SF_6 load break switches have rapidly substituted for the air or oil-type load break switches, in order to get higher reliability. And, the national standard was made for reliability assessment of SF_6 gas load break switch and testing facilities was also set in KERI(Korea Electrotechnology Research Institute ). This paper introduces the synopsis for the reliability assessment requirements specified in the standard and the assessment results.