• 폴리머
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• 제36권1호
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• pp.88-92
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• 2012

일정 하중에 지속적으로 노출되는 고분자의 장기간 사용을 위해서는 재료의 수명을 평가할 수 있는 가속화된 시험 방법이 필요하다. 반복 피로하중 시험법은 이러한 방법들 중 하나로 많은 관심을 받고 있다. 본 연구에서는 X-선 회절법을 이용하여 고밀도 폴리에틸렌의 반복 피로하중에 의한 구조적 변화와 크리프 변형을 비교하고자 하였다. 이를 위하여 별도의 인장시험기를 제조, X-선 회절기에 부착하여 장시간 변형 과정을 성공적으로 관찰하였다. 그 결과 크리프와 반복 피로하중 사이의 거시적이고 뚜렷한 차이에도 불구하고 결정화도, 결정크기 및 면간거리와 같은 결정의 미세구조는 두 방법에서 거의 동일하게 관찰되었다. 그러나 항복점 전(BYP), 항복점(YP) 그리고 항복점 후(AYP)로 각각 변형시킨 후 시험한 시료의 경우 AYP와 다른 두 시료간 뚜렷한 구조적 차이를 확인할 수 있었다.

• 한국지반신소재학회논문집
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• 제4권3호
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• pp.11-19
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• 2005

2가지 형태의 지오그리드의 장기안정성을 평가하였다. 멤브레인 연신형 지오그리드는 지수함수 형 인장특성을 나타내었으며 섬유형 지오그리드는 파단점에 근접할수록 빠른 인장특성의 증가를 나타내었다. 단기 가속 크리프 시험은 섬유형 지오그리드에는 실시되었지만 멤브레인 연신형 지오그리드의 경우 열적특성 때문에 실온에서만 실시 하였다. 멤브레인 연신형 지오그리드의 크리프 변형률은 인장시험에 의한 극한 인장변형률보다 크게 나타났다. 섬유형 지오그리드의 크리프 변형에 의한 감소인자는 멤브레인 연신형 지오그리드보다 작게 나타났다. 이 결과로부터 섬유형 지오그리드가 멤브레인 연신형 지오그리드보다 크리프 변형에 안정성이 있음을 알 수 있었다.

• 한국신뢰성학회:학술대회논문집
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• 한국신뢰성학회 2005년도 학술발표대회 논문집
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• pp.69-73
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• 2005

The failure of geogrids used for soil reinforcement application can be defined as an excessive creep strain which causes the collapse of slopes and embankments. Accordingly, the lifetime is evaluated as a time to reach the excessive creep strain using two accelerated creep testing methods, time-temperature superposition(TTS) and stepped isothermal methods(SIM). TTS is a well-accepted acceleration method to evaluate creep behavior of polymeric materials, while SIM was developed in the last ten years mainly to shorten testing time and minimize the uncertainty associated with inherent variability of multi-specimen tests. The SIM test is usually performed using single rib of geogrids for temperature steps of $14^{\circ}C$ and a dwell time of 10,000 seconds. However, for multi-ribs of geogrids, the applicability of the SIM has not been well established. In this study, the creep behaviors are evaluated using multi-ribs of polyester geogrids using SIM and TTS creep procedures and the newly designed test equipment. Then the lifetime of geogrids are predicted by analyzing the failure times to reach the excessive creep strains through reliability analysis.

• 방사성폐기물학회지
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• 제18권2_spc호
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• pp.237-245
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• 2020

사용후핵연료를 저장하는 볼트체결 저장용기의 격납경계를 형성하는 주된 구성요소는 금속 밀봉재이다. 이러한 금속 밀봉재는 열과 방사선에 의해 그 성질이 저하된다. 또한, 금속 밀봉재가 강한 열에 장기간 노출되면 크리프 현상이 발생한다. 이러한 크리프는 밀봉시스템에 응력 이완을 가져와서, 결국 밀봉재의 건전성을 해치게 된다. 이러한 응력 이완은 금속 밀봉재의 밀봉성능 저하로 이어지며, 저하의 정도에 따라 저장용기의 누설을 야기할 수 있다. 또한, 볼트 체결력의 감소도 밀봉성능 저하에 영향을 미친다. 본 논문에서는 금속 밀봉재의 격납건전성과 볼트체결력 감소를 평가하기 위해 수행한 가속화 시험의 결과에 대하여 기술한다. 전 시험기간 동안 각 시편에서의 누설률, 볼트 변형률, 금속 밀봉재 주변 온도를 계측하여 분석하였고, 금속 밀봉재는 저장기간 50년 동안 격납건전성을 유지함을 입증하였다. 또한, 가속화 시험의 타당성에 대해서 기술하였다.

• Applied Microscopy
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• 제43권4호
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• pp.164-172
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• 2013

In this work, microstructural and hardness evolution of the X20 steel upon short-term creep test ($550^{\circ}C$ to $650^{\circ}C$, $180^{\circ}C$ to 60 MPa) was studied by using scanning electron microscope, electron backscattered diffraction, and transmission electron microscope, microhardness tester. After creep rupture, gauge and grip part of the specimens were microstructurally analyzed. Creep at the $650^{\circ}C$/60 MPa resulted in a rupture at 1,460 hours with growth of lath width from 1.31 to $2.87{\mu}m$ and a grain growth with a more equiaxed feature. There is a close relationship between Microhardness and lath width. The formation and coarsening of Laves phase, which was observed up to $600^{\circ}C$ of creep temperature, was accelerated by the applied stress. Slight coarsening of the $M_{23}C_6$ was observed in the $550^{\circ}C$ and $600^{\circ}C$ crept or aged specimens. The coarsening of $M_{23}C_6$ depended on the temperature, where specimens crept at $650^{\circ}C$ showed higher growth rate. The microstructural evolution of X20 after short-term creep test was extensively discussed in relation to the long-term creep/aging test reported in literatures.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2005년도 추계학술대회 논문집 Vol.18
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• pp.315-316
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• 2005

This paper describes the AC accelerated ageing characteristics of zinc oxide varistors for the station class lightning surge arresters. ZnO varistors whose rated voltage were 3.27kV, 4.00kV, and 6.00kV were manufactured with general ceramic production methods. The power loss of sample A varistors rapidly increased and eventually showing the thermal run-away, but sample B and sample C varistors exhibited negative creep phenomena in power losses during the test.

• Journal of Welding and Joining
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• 제15권6호
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• pp.32-40
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• 1997

Solid state diffusion bonding is the joining process performed by creep and diffusion, which is accelerated by heating below melting temperature and proper pressing, in vacuum or shielding gas atmosphere. By this process we can obtain sufficient joint which can't be expected from the fusion welding. For Ti-6Al-4V alloy, the optimum solid state diffusion bonding condition and mechanical properties of the joint were found, and micro void morphology at bond interface was observed by SEM. The results of tensile test showed sufficient joint, whose mechanical properties are similar to that of base metal. 850$^{\circ}$C, 3MPa is considered as the optimum bonding condition. Void morphology at interface is long and flat at the initial stage. As the percentage of bonded area increases, however, small and round voids are found. Variation of void shape can be explained as follows. As for the void shrinkage mechanism, at the initial stage, power law creep is the dominant, but diffusion mechanism is dominant when the percentage of bonded area is increased.

• 전기학회논문지P
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• 제59권2호
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• pp.202-205
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• 2010

In this paper, mechanical reliability(Life-time) estimation method for 25.8kV SIS(Solid Insulated Switchgear) has been studied. Recently enacted KEPCO's standard includes clause that have to submit a warrantable reliability data for life-time(over B10 25 years) of an epoxy-solid insulating material. Accordingly, this research was carried out on the ALT(Accelerated Life Test) and Life-Estimation method for SIS's insulating material. Mechanical life-time estimation for SIS's insulating material is to verify reliability for tensile creep & fatigue stress, which is the major mechanical stress of SIS. This study proved that SIS's reliability for mechanical stress and established that confidence for estimation results in further verification test.

• 대한기계학회논문집
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• 제13권2호
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• pp.236-242
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• 1989

본 연구에서는 주로 보일러의 과열기 및 재열기등의 관재료로 널리 사용되고 있는 304 스테인레스강에 대하여 실제 고온부식현상을 재현한 부식환경하인 630.deg. C, 690.deg. C 및 750.deg. C 3가지 온도범위에서 크리프시험을 실시하고 고온부식이 크리프파단 특성에 미치는 영향을 부식조건이 없는 고온대기중의 실험결과와 비교 검토하였다.

• Geomechanics and Engineering
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• 제8권2호
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• pp.187-220
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• 2015

Soil disturbance induced by installation of mandrel driven vertical drains decreases the in situ horizontal hydraulic conductivity of the soil in the vicinity of the drains, decelerating the consolidation rate. According to available literature, several different profiles for the hydraulic conductivity variation with the radial distance from the vertical drain, influencing the excess pore water pressure dissipation rate, have been identified. In addition, it is well known that the visco-plastic properties of the soil also influence the excess pore water pressure dissipation rate and consequently the settlement rate. In this study, a numerical solution adopting an elastic visco-plastic model with nonlinear creep function incorporated in the consolidation equations has been developed to investigate the effects of disturbed zone properties on the time dependent behaviour of soft soil deposits improved with vertical drains and preloading. The employed elastic visco-plastic model is based on the framework of the modified Cam-Clay model capturing soil creep during excess pore water pressure dissipation. Besides, nonlinear variations of creep coefficient with stress and time and permeability variations during the consolidation process are considered. The predicted results have been compared with V$\ddot{a}$sby test fill measurements. According to the results, different variations of the hydraulic conductivity profile in the disturbed zone result in varying excess pore water pressure dissipation rate and consequently varying the effective vertical stresses in the soil profile. Thus, the creep coefficient and the creep strain limit are notably influenced resulting in significant changes in the predicted settlement rate.