• Journal of the Korean Institute of Electrical and Electronic Material Engineers
• /
• v.15 no.3
• /
• pp.244-252
• /
• 2002

The electrical and dielectric behavior fort DC accelerated aging stress of P $r_{6}$ $O_{11}$-based Zno varistors cnsisting of ZnO-P $r_{6}$ $O_{11}$-CoO-C $r_2$ $O_3$-E $r_2$ $O_3$ were investigated with sintering temperature in the range of 1325~1345$^{\circ}C$. The varistor ceramics with increasing sintering temperature were more densified. A more densified varistors leaded to high stability for DC accelerated aging stress. Furthermore, the stability for DC accelerated aging stress was increased with the leakage current and dtan $\delta$/dV decreasing in order of 1325longrightarrow1335longrightarrow1345longrightarrow134$0^{\circ}C$ in sintering temperature. It was found that the stability for DC stress is affected more greatly by the leakage current and dtan $\delta$/dV than the densification. It is considered that the stability of varistors for DC stress can be estimated by considering the factors, such as the densification, leakage current, and dtan $\delta$/dV. As a result, the varistor sintered at 134$0^{\circ}C$ exhibited the highest stability, with %$\Delta$ $V_{lmA}$=-1.54%, %$\Delta$$\alpha$=-2.49%, %$\Delta$ $I_{\ell}$=+240.68%, 5%$\Delta$tan$\delta$=+29.96%.96%.96%.%.

• Korean Journal of Materials Research
• /
• v.26 no.5
• /
• pp.281-286
• /
• 2016

In the present study, the tensile properties and dynamic strain aging of an Fe-24.5Mn-4Cr-0.45C alloy were investigated in terms of strain rate. During tensile testing at room temperature, all the stress-strain curves exhibited serrated plastic flows related to dynamic strain aging, regardless of the strain rate. Serration appeared right after yield stress at lower strain rates, while it was hardly observed at high strain rates. On the other hand, strain-rate sensitivity, indicating a general relationship between flow stress and strain rate at constant strain and temperature, changed from positive to negative as the strain increased. The negative strain-rate sensitivity can be explained by the Portevin Le Chatelier effect, which is associated with dynamic strain aging and is dependent on the strain rate because it is very likely that the dynamic strain aging phenomenon in high-manganese steels is involved in the interaction between moving dislocations and point-defect complexes.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
• /
• 2000.07a
• /
• pp.119-122
• /
• 2000

Recently polymer insulators are being used for outdoor high voltage applications. Polymer insulators for transmission line have significant advantages over porcelain and glass insulators, especially for ultra-high voltage transmission lines. Their advantages are light weight, vandalism resistance and hydrophobicity. Polymer insulators are a relatively new technology, but their expected life is still unknown. Therefore these estimating technique are very important. Their life time is related to weathering and operating condition. Multi-aging test is requested because aging factor is occurred by multi-aging than unique aging. The aging test about polymer insulators have mainly carried out by IEC 61109. This paper presents multi-stress chamber experiments and tracking wheel test to examine the tracking and erosion performance of polymer insulator for transmission. Multi-stress testing is able to demonstrate deficiencies of polymer insulator materials and designs, including the nature of interfaces in insulation design. We have investigated IEC 61109 Annex C (5000h aging test) and CEA tracking wheel test as test methods of artificial accelerated aging. The aging degree of polymer insulator is estimated by leakage current, measurement of hydrophobicity degree, damage conditions of insulator surface, withstand voltage test etc.

• International Journal of Safety
• /
• v.1 no.1
• /
• pp.36-42
• /
• 2002

This paper is aimed at predicting the life of rubber insulating gloves under normal operating stresses from relatively rapid test performed at higher stresses. Specimens of rubber insulating gloves are subject to multiple stress conditions, i.e. combined electrical and thermal stresses. Two modes of electrical stress, step voltage stress and constant voltage stress are used in specimen aging. There are two types of test for electrical stress in this experiment: the one is Breakdown Voltage (BDV) test under step voltage stress and thermal stress and the other is lifetime test under constant voltage stress and temperature stress. The ac breakdown voltage defined as the break-down point of insulation that leakage current excesses a limit value, l0mA in this experiment, is determined. Because the very high variability of aging data requires the application of statistical model, Weibull distribution is used to represent the failure times as the straight line on Weibull probability paper. Weibull parameters are deter-mined by three statistical methods i.e. maximum likelihood method, graphical method and least squares method, which employ SAS package, Weibull probability paper and FORTRAN, respectively. Two chosen models for predicting the life under simultaneous electrical and thermal stresses are inverse power model and exponential model. And the constants of life equation for multistress aging are calculated using numerical method, such as Gauss Jordan method etc.. The completion of life equation enables to estimate the life at normal stress based on the data collected from accelerated aging test. Also the comparison of the calculated lifetimes between the inverse power model and the exponential model is carried out. And the lifetimes calculated by three statistical methods with lower voltage than test voltage are compared. The results obtained from the suggested experimental method are presented and discussed.

• Proceedings of the KIEE Conference
• /
• 2000.07c
• /
• pp.1948-1950
• /
• 2000

Polymeric composite insulators have been in use for outdoor insulation. However, our knowledge about their long-term performance in an outdoor environment is still very limited. Especially, these insulators are subjected to the environmental stress such as ultraviolet radiation, electrical stress, mechanical load, etc.. Hence, in this paper, we simulated the factors having influence on aging of polymer insulators using the combined aging test chamber. In order to evaluate an aging characteristics with time, we measured leakage current on these insulators to make out the degree of aging.

• Korean Journal of Materials Research
• /
• v.23 no.1
• /
• pp.18-23
• /
• 2013

In this study, the effects of cryogenic treatment cycles on the residual stress and mechanical properties of 7075 aluminum alloy (Al7075) samples, in the form of a tube-shaped product with a diameter of 500 nm, were investigated. Samples were first subjected to solution treatment at $470^{\circ}C$, followed by cryogenic treatment and aging treatment. The residual stress and mechanical properties of the samples were systematically characterized. Residual stress was measured with a cutting method using strain gauges attached on the surface of the samples; in addition, tensile strength and Vickers hardness tests were performed. The detailed microstructure of the samples was investigated by transmission electron microscopy. Results showed that samples with 85 % relief in residual stress and 8% increase in tensile strength were achieved after undergoing three cycles of cryogenic treatments; this is in contrast to the samples processed by conventional solution treatment and natural aging (T4). The major reasons for the smaller residual stress and relatively high tensile strength for the samples fabricated by cryogenic treatment are the formation of very small-sized precipitates and the relaxation of residual stress during the low temperature process in uphill quenching. In addition, samples subjected to three cycles of cryogenic treatment demonstrated much lower residual stress than, and similar tensile strength compared to, those samples subjected to one cycle of cryogenic treatment or artificial aging treatment.

• Journal of Life Science
• /
• v.34 no.1
• /
• pp.68-77
• /
• 2024

Airborne particulate matter (PM) is an environmentally hazardous pollutant that originates from various sources. PM is comprised of solid particles and liquid droplets of diverse composition and size. Hazardous chemical compositions of PM include elemental and organic carbon, organic compounds, biological compounds and metals. Upon acute and chronic PM exposure, toxic contaminants enter and accumulate within physiological systems and prompt cell structure changes accompanied with intracellular endoplasmic reticulum stress, mitochondrial dysfunction, oxidative stress, inflammation, lipid accumulation, and cell cycle arrest. Ultimately, these cellular response leads to the development of key characteristics of aging. In addition, PM internalization enhances autophagy reflux and lysosomal dysfunction, which is involved in cell aging. Previous studies have emphasized a positive association between PM and increased mortality or decreased lifespan, although these are evidenced mostly by observational studies. Direct evidence of the link between PM and aging is still limited. This review evaluates the evidence from not only observational studies but also in vitro and in vivo evidence of PM on aging progression and age-related diseases development. This evidence is based on age-associated cellular changes including endoplasmic reticulum stress, mitochondrial dysfunction, oxidative stress, inflammation, adipose accumulation, autophagy, which strengthen the association between PM exposure and aging. Understanding the underlying cellular responses under PM may allow for the development of new therapeutic targets for PM-induced aging.

• Korean Journal of Occupational Health Nursing
• /
• v.30 no.4
• /
• pp.175-185
• /
• 2021

Purpose: The purpose of this study was to explore the influence of care stress for older patients, self-perceptions of aging, and anxiety about aging on preparation for retirement in clinical nurses. Methods: This was a cross-sectional descriptive study in which participants were 222 nurses who had at least six months of work experience and were involved in caring for older patients in the last six months at the work site. Data were collected from August to September, 2020 using an online survey questionnaire. Data were analyzed using the IBM SPSS/WIN 25.0 program. Results: The mean level of preparation for retirement in the participants was moderate (3.21 out of 5 points). Among the sub-domains of preparation for retirement, financial preparation had the lowest score. Self-perceptions of aging and aging anxiety were significant predictors for retirement preparation in nurses, accounting for 16% of the variable's total variance. Conclusion: A greater level of preparation for retirement was associated with a positive perception of aging and a decreased level of aging anxiety in the clinical nurses. Further research should focus on exploration of specific determinants of financial preparation for retirement and development of intervention strategies for improving preparation for retirement in the nursing workforce.

• Journal of the Korean Society for Heat Treatment
• /
• v.12 no.1
• /
• pp.21-30
• /
• 1999

The present study were investigated changes of precipitation behaviour of laves phase in ferrite single phase and ferrite-martensite dual phase and precipitation of laves phase under stress. Hardness changes in ferrite phase appeared two hardness peaks by precipitation of initial fine precipitator and laves phase in 3Mo-0.3Si and 3Mo-0.3Si-C specimens, respectively. Hardness changes in martensite phase of 3Mo-0.3Si-C specimen was lower in the initial stage of aging by carbide precipitation and after this, increased by re-hardening due to precipitation of laves phase. In the ferrite phase, laves phase was mainly precipitated, whereas in the martensite phase, carbide was preferentially formed during the initial stage of aging and with increasing aging time, laves phase and carbide were simultaneously precipitated by precipitation of laves phase at around carbide. In the ferrite-martensite interface, laves phase was mainly precipitated and carbide was mainly formed at boundary of lath martensite than grain boundary. Adding the stress in aging, fine precipitator of inital precipitation of laves phase precipitated in (100) of perpendicular to tensile direction and has grown to only followed<010>direction and also, volume fraction of laves phase increased. Consequently, the stress added was accelerated initial precipitation of laves phase.

• Journal of Physiology & Pathology in Korean Medicine
• /
• v.27 no.3
• /
• pp.331-335
• /
• 2013

This study investigated the relationship between drinking and health factors including vaso-aging degree, stress index, stress resistance, fatigue and activity of autonomic nerve system etc. The subjects were 20,925 persons who had received health examination by Korean Medicine and submitted questionnaires about drinking history, for 8 months from March to November. The vaso-aging degree were measured by APG (Accelerated Photoplethysmograph). The stress index and resistance, fatigue and activity of autonomic nerve system were measured by HRV (Heart Rate Variability). We analyzed the relationship between drinking and various variables by chi-square test with SPSS ver. 19.0. Regarding the relationship between drinking and vaso-aging degree, there was significant differences(p<0.05). But regarding the relationship between drinking and the other factors, there were no statistically differences. This study suggests that the vaso-aging degree is affected by drinking alcohol.