• Nuclear Engineering and Technology
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• 제49권4호
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• pp.744-751
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• 2017

The fatigue crack propagation behaviors of Z3CN20.09M duplex stainless steel (DSS) were investigated by studying oxide films of specimens tested in $290^{\circ}C$ water and air. The results indicate that a full oxide film that consisted of oxides and hydroxides was formed in $290^{\circ}C$ water. By contrast, only a half-baked oxide film consisting of oxides was formed in $290^{\circ}C$ air. Both environments are able to deteriorate the elastic modulus and hardness of the oxide films, especially the $290^{\circ}C$ water. The fatigue lives of the specimens tested in $290^{\circ}C$ air were about twice of those tested in $290^{\circ}C$ water at all strain amplitudes. Moreover, the crack propagation rates of the specimen tested in $290^{\circ}C$ water were confirmed to be faster than those tested in $290^{\circ}C$ air, which was thought to be due to the deteriorative strength of the oxide films induced by the mutual promotion of oxidation and crack propagation at the crack tip. It is noteworthy that the crack propagation can be postponed by the ferrite phase in the DSS, especially when the specimens were tested in $290^{\circ}C$ water.

• 한국세라믹학회지
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• 제28권10호
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• pp.785-792
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• 1991

An investigation of the crack propagation behavior of Al2O3-33Vol.% SiCw at 140$0^{\circ}C$ was conducted with various loading frequencies. Higher crack propagation was observed in lower frequency and higher load ratios. Interface sliding fracture due to glassy phase from the oxidation of SiCw and cavitation along grain boundary of diffusional creep appeared to be the main mechanism of fatigue fracture in slower crack propagation while interface sliding and whisker pull out aided by glassy phase formation played main role of fatigue fracture for higher crack growth condition. The frequency effect on deformation behavior was discussed with a Maxwell model.

• 대한기계학회논문집
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• 제19권8호
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• pp.1989-1998
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• 1995

Characteristics of the flame propagation for normal and abnormal combustion in hydrogen premixture in a cylindrical constant-volume combustion chamber are studied numerically. A detailed hydrogen oxidation kinetic mechanism, mixture transport properties and a model describing spark ignition process are used. The calculated pressure-time history of the stable deflagration wave propagation agrees well with the experiment. The ignition of the premixture in the unburned gas, initiated by the hot spot, causes a transition from deflagration to detonation under some initial temperature and pressure. Under the initial conditions with high temperature and pressure, excessive ignition energy initiates a strong blast wave and a detonation wave that follows. The chemical reaction in the detonation wave is much more vigorous than that in the deflagration wave and the peak pressure in the detonation wave is much higher than the equilibrium value.

• 대한금속재료학회지
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• 제48권12호
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• pp.1078-1083
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• 2010

In this study, high temperature oxidation behavior of Alloy 617 was investigated to evaluate the effect of grain size for Alloy 617. The grain size of grain-refined Alloy 617 (GR617) was reduced to $5{\mu}m$ from $71{\mu}m$ for as-received Alloy 617 (AR617) by recrystallization after cold rolling. After high temperature aging, the oxide layers of AR617 and GR617 consisted of $Cr_2O_3$ external oxide scale and $Al_2O_3$ internal oxide. The external oxide scale resulted in a Cr-depleted zone and a carbide free zone below the scale. The depth of the carbide free zone was deeply formed in GR617. On the other hand, the depth of the internal oxide layer in GR617 was shorter than that in AR617. After a 3-point bending test, crack propagation of GR617 was more restricted than that of AR617 because of the different microstructure of the internal oxide.

• 한국결정성장학회지
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• 제24권4호
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• pp.151-157
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• 2014

The degradation mechanisms of thermal barrier coatings (TBCs) were investigated in different thermal fatigue condition in terms of microstructural analyses. The isothermal and cyclic oxidation tests were conducted to atmospheric plasma sprayed-TBCs on NIMONIC 263 substrates. The delamination occurred by the oxide layer formation at the interface, the Ni/Cr-based oxide was formed after Al-based oxide layer grew up to ${\sim}10{\mu}m$ in the isothermal condition. In the cyclic oxidation with dwell time, the failure occurred earlier (500 hr) than in the isothermal oxidation (900 hr) at same temperature. The thickness of Al-based oxide layer of the delaminated specimen in the cyclic condition was ${\sim}4{\mu}m$ and the interfacial cracks were observed. The acoustic emission method revealed that the cracks generated during the cooling step. It was considered that the specimens were prevented from the formation of the Al-based oxide by cooling treatment, and the degradation mode in the cyclic test was dominantly interfacial cracking by the difference of thermal expansion coefficients of the coating layers.

• Nuclear Engineering and Technology
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• 제27권6호
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• pp.825-832
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• 1995

SA508 Cl.3 원자로 압력용기강에서 하중변수와 온도가 공기 중에서 피로 균열성장률에 미치는 영향에 대하여 조사하였다. 피로 균열성장률 시험은 12.7mm 두께의 CT(compact tension) 시편을 이용하였으며, 균열길이 추정은 컴플라이언스 방법을 사용하였다. 시험은 0.1, 0.5 하중비와 1, 10Hz의 하중주파수로 상온에서 40$0^{\circ}C$ 까지 온도를 변화시키면서 수행하였다. 12$0^{\circ}C$ 이하의 비교적 낮은 온도에서는 피로균열전파속도는 하중주파수와 온도에 영향을 받지 않았지만, 12$0^{\circ}C$이상의 경우 피로 균열성장률은 온도가 높을수록, 하중주파수가 낮을수록 증가하였다. 이러한 피로균열진전속도의 빨라짐은 균열선단에서의 산화속도의 증가로 인한것으로 생각된다. 또한 하중비의 영향으로 균열닫힘과 산화의 상호작용으로 피로 균열성장률은 상온에서 두드러졌다.

• 전기학회논문지
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• 제56권9호
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• pp.1614-1618
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• 2007

We have demonstrated the experimental results to obtain the immunity of FN (Fowler Nordheim) stress for S-RCAT (Spherical-Recess Cell Array Transistor) which has been employed to meet the requirements of data retention time and propagation delay time for sub-100-nm mobile DRAM (Dynamic Random Access Memory). Despite of the same S-RCAT structure, the immunity of FN stress of S-RCAT depends on the process condition of gate oxidation. The S-RCAT using DPN (decoupled plasma nitridation) process showed the different degradation of device properties after FN stress. This paper gives the mechanism of FN-stress degradation of S-RCAT and introduces the improved process to suppress the FN-stress degradation of mobile DRAM.

• Corrosion Science and Technology
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• 제4권4호
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• pp.136-139
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• 2005

The effect of oxidation and hot corrosion on the solid particle erosion was investigated for hot-pressed silicon nitride using as-polished, pre-oxidized and pre-corroded specimens by molten sodium sulfates. Erosion tests were performed at 22, 500 and $900^{\circ}C$ using angular silicon carbide particles of mean diameter $100{\mu}m$. Experimental results show that solid particle erosion rate of silicon nitride increases with increasing temperature for as-polished or pre-oxidized specimens in consistent with the prediction of a theoretical model. Erosion rate of pre-oxidized specimens is lower than that of as-polished specimens at $22^{\circ}C$, but it is higher at $900^{\circ}C$. Lower erosion rate at $22^{\circ}C$ in the pre-oxidized specimens is attributed due to the blunting of surface flaws, and the higher erosion rate at $900^{\circ}C$ is due to brittle lateral cracking. Erosion rate of pre-corroded specimens decreases with increasing temperature. Less erosion at $900^{\circ}C$ than at $22^{\circ}C$ is associated with the liquid corrosion products sealing off pores at $900^{\circ}C$ and the absence of inter-granular crack propagation observed at $22^{\circ}C$.

• 한국정밀공학회지
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• 제19권10호
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• pp.84-91
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• 2002

The shaft with the circular cross section have symmetric structural combination parts to keep the rotating balance. Hence the crack usually emanate from symmetric combination parts due to the stress concentration of these parts. In this study to estimate the fatigue behavior of symmetric cracks, the fatigue test was performed by using rotary bending tester and the specimen with symmetric defects in circular cross section. From the facts the characteristics of crack initiation and propagation on the symmetric surface cracks in circular cross section was examined. Also we observed the internal crack using oxidation coloring and investigated the fatigue behavior using the relationship between surface crack and internal crack. As a result of fatigue lift of symmetric cracks was reduced to 35% compared to single crack’s. We examined the characteristics of fatigue behavior in element with symmetric cracks using internal crack propagation rate and maximum stress intensity factor range obtained from approximation method.

• International Journal of Precision Engineering and Manufacturing
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• 제5권4호
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• pp.36-42
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• 2004

The shaft with the circular cross section has symmetric structural combination parts to keep the rotating balance. Hence the crack usually initiates from symmetric combination parts due to the stress concentration of these parts. In this study to estimate the fatigue behavior of symmetric cracks, the fatigue test was performed by using a rotary bending tester and the specimen with symmetric defects in circular cross section. The characteristics of crack initiation and propagation on the symmetric surface cracks in circular cross section were examined. We also observed the internal crack using the oxidation coloring method and investigated the fatigue behavior using the relationship between the surface crack and the internal crack. As a result, the fatigue life of symmetric cracks was reduced by 35% compared to that of a single crack. We examined the characteristics of fatigue behavior of elements with symmetric cracks using internal crack propagation rate and maximum stress intensity factor range that were obtained from an approximation method.