• Journal of Applied Reliability
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• v.12 no.3
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• pp.177-186
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• 2012

This paper presents a comparison of the failure mechanism for high power converted white LEDs(3W) with the commercially available YAG:Ce and silicate phosphor. We carry out the normal aging life test for 10,000 hours, the high temperature aging test for 8,000 hours, the high temperature and humidity aging test for 8,000 hours and the current aging testing for 5,000 hours. The optical and electrical parameters of LEDs were monitored, such as lumen, correlated color temperature (CCT), chromaticity coordinates(x, y), thermal resistance, I -V curve and spectrum intensity. The stress induced a luminous flux decay on LED in all experiments and causes a failure. So we try to find out what's a main failure mechanism for a high power LED.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 1998.06a
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• pp.37-40
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• 1998

In this paper, Pb(Z $r_{0.53}$ $Ti_{0.47}$) $O_3$+x[wt%]C $r_2$ $O_3$ ceramics were fabricated, and thermal aging effects on the specimen were investigated. After thermal aging, the temperature coefficients of their resonant frequence became stable and only the specimen with C $r_2$ $O_3$ additives in them could have lower temperature coefficientient

• Proceedings of the Korean Institute of IIIuminating and Electrical Installation Engineers Conference
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• 2006.05a
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• pp.49-53
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• 2006

In this paper the thermal aging characteristics of transformer fluid were considered. First, the temperature rise characteristic in transformer winding was discussed. The temperatures were rapidly increased immediately with applying load and then the temperature was followed the exponential temperature rise model. Next, Capacitance and leakage current characteristics of the transformer fluid were measured during the over-loading operation. The capacitance of vegetable fluid and leakage current were evidently increased wit aging time. So the simple methods could be applied to diagnosis of the transformer aging

• Transactions of the Korean Society of Mechanical Engineers A
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• v.27 no.8
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• pp.1398-1408
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• 2003

Tensile and LCF(low cycle fatigue) tests were carried out in air at wide temperature range 20$^{\circ}C$-750$^{\circ}C$ and strain rates of 1${\times}$10$\^$-4//s-1${\times}$10$\^$-2/ to ascertain the influence of strain rate on tensile and LCF properties of prior cold worked 316L stainless steel, especially focused on the DSA(dynamic strain aging) regime. Dynamic strain aging induced the change of tensile properties such as strength and ductility in the temperature region 250$^{\circ}C$-600$^{\circ}C$ and this temperature region well coincided with the negative strain rate sensitivity regime. Cyclic stress response at all test conditions was characterized by the initial hardening during a few cycles, followed by gradual softening until final failure. Temperature and strain rate dependence on cyclic softening behavior appears to result from the change of the cyclic plastic deformation mechanism and DSA effect. The DSA regimes between tensile and LCF loading conditions in terms of the negative strain rate sensitivity were well consistent with each other. The drastic reduction in fatigue resistance at elevated temperature was observed, and it was attributed to the effects of oxidation, creep and dynamic strain aging or interactions among them. Especially, in the DSA regime, dynamic strain aging accelerated the reduction of fatigue resistance by enhancing crack initiation and propagation.

• Proceedings of the Korea Concrete Institute Conference
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• 2001.11a
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• pp.445-450
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• 2001

The paper investigates the relationships between dynamic elastic modulus and static elastic modulus or compressive strength according to curing temperature, aging, and cement type. Based on this investigation, the new model equations are proposed. Impact echo method estimates the resonant frequency of specimens and uniaxial compression test measures the static elastic modulus and compressive strength. Type I and V cement concretes, which have the water-cement ratios of 0.40 and 0.50, are cured under the isothermal curing temperature of 10, 23, and 50 $^{\circ}C$. Cement type and aging have no large influence on the relationship between dynamic and static elastic modulus, but the ratio of dynamic and static elastic modulus comes close to 1 as temperature increases. Initial chord elastic modulus, which is calculated at lower strain level of stress-strain curve, has the similar value to dynamic elastic modulus. The relationship between dynamic elastic modulus and compressive strength has the same tendency as the relationship between dynamic and static elastic modulus. The proposed relationship equations between dynamic elastic modulus and static elastic modulus or compressive strength properly estimates the variation of relationships according to cement type, temperature, and aging.

• Nuclear Engineering and Technology
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• v.54 no.2
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• pp.709-731
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• 2022

Loss of fracture resistance due to thermal aging degradation is a potential limiting factor affecting the long-term (80+ year) viability of nuclear reactors. To evaluate the effects of decades of aging in a practical time frame, accelerated aging must be employed prior to mechanical characterization. In this study, a variety of chemically and microstructurally diverse austenitic stainless steels were aged between 0 and 30,000 h at 290-400 ℃ to simulate 0-80+ years of operation. Over 600 static fracture tests were carried out between room temperature and 400 ℃. The results presented include selected J-R curves of each material as well as K_0.2mm fracture toughness values mapped against aging condition and ferrite content in order to display any trends related to those variables. Results regarding differences in processing, optimal ferrite content under light aging, and the relationship between test temperature and Mo content were observed. Overall, it was found that both the ferrite volume fraction and molybdenum content had significant effects on thermal degradation susceptibility. It was determined that materials with >25 vol% ferrite are unlikely to be viable for 80 years, particularly if they have high Mo contents (>2 wt%), while materials less than 15 vol% ferrite are viable regardless of Mo content.

• Proceedings of the KSME Conference
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• 2008.11a
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• pp.1391-1393
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• 2008

In this paper, the long-term reliability for 1.25G transceiver in use of high speed optical access network is investigated. High temperature storage tests and accelerated life tests are used to long-term reliability. Accelerated aging test have been during 3,000 hour of the three accelerated aging conditions by caused high temperature stress. Mean life is assumed to follow the Arrhenius relationship and analysis from the failure data obtained in the accelerated aging conditions.

• Journal of Korea Foundry Society
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• v.24 no.4
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• pp.231-237
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• 2004

Bake hardenable steel utilizes the phenomenon of strain aging to provide an increase in the yield strength of formed components. An increase of the carbon content will improve the bake hardening response: more solutes are available to pin mobile dislocations and to form the clusters more rapidly. But aging resistance decrease as increasing solute carbon. In order to under-stand the compatibility between bake hardenability and aging resistance. The optimum solute carbon control methods during manufacture should be determined. In this paper, the effect of continuous heat cycle conditions such as soaking temperature, rapid cooling start temperature, cooling rate on BH(Bake Hardenability), AI(Aging Index), YP-El(Yield Point Elongation) and other mechanical properties have been investigated. and following results were obtained. In the case of soaking temperature, BH increases with higher soaking temperature because of NbC $dissolution(830^{\circ}C)$, Therefore the solute carbon and BH at $850^{\circ}C$ and $870^{\circ}C$ are higher than these at $810^{\circ}C$. But BH at $870^{\circ}C$ is a little lower than that at $850^{\circ}C$ owing to the ferrite grain size. The measurement of amount of dissolution C using IFT(Internal Friction Test) can explain the relation of solute carbon and BH.

• KIEAE Journal
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• v.15 no.6
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• pp.11-18
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• 2015

Purpose: The current study analyzed trends in thermal insulation performance with aging, and condensation characteristics caused by the former. Method: Thermal insulation and condensation prevention performance of an architecture were assessed using Temperature Difference Ration Inside, or TDRi. Subjects of this quantitative analysis in thermal insulation performance change due to aging included recently constructed apartments and aged apartments older than 40 years. Time series comparison and analysis were conducted to observed changes in the thermal insulation performance and condensation characteristics. Result: Analysis showed that wall insulation performance degraded with aging regardless of fortified insulating material usage or insulating material type, which caused increased danger of condensation. In addition, when fortified insulating material was installed on the connection between the walls, insulation performance degradation was lower compared to cases in which fortified materials were not used. In all cases from 1 to 10, the rate of thermal insulation performance degradation increased after 20 years of aging.

• Nuclear Engineering and Technology
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• v.29 no.5
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• pp.361-367
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• 1997

The phase separation in ferrite phase of duplex stainless steel is the primary cause of thermal aging embrittlement of the LWR primary pressure boundary components. In this study the phase separation of simulated duplex stainless steel was detected by Mossbauer spectroscopy and magnetic property analysis by VSM(Vibrating Specimen Magnetometer). The simulated duplex stainless steels, Fe-Cr binary, Fe-Cr-Ni ternary, and Fe-Cr-Ni-Si quarternary allots, were aged at 370 and 40$0^{\circ}C$ up to 5,340 hours. It was observed from Mossbauer spectra analysis that internal magnetic field increases with aging time and from VSM that the specific saturation magnetization and Curie temperature increase with aging time. These result are indicative that phase separation into Fe-rich region and Cr-rich region is caused by thermal aging in the temperature range of 370~40$0^{\circ}C$ In cases of specimens containing Ni, the increase of specific saturation magnetization is much higher. This implies that Ni seems to promote Fe-Cr interdiffusion, which accelerates the phase separation into Fe-rich $\alpha$ phase and Cr-rich $\alpha$' phase.