• Transactions of the Korean Society of Mechanical Engineers A
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• v.29 no.11 s.242
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• pp.1480-1487
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• 2005

Metal matrix composites(MMCs) have been rapidly becoming one of the strongest candidates for structural materials fur many high temperature application. However, among the various high temperature environments in which metal matrix composites was applied, thermal shock is known to cause significant degradation in most MMC system. Due to the appreciable difference in coefficient of thermal expansion(CTE) between reinforcement and metal matrix, internal stresses are generated following temperature changes. Infernal stresses affect degradation of mechanical properties of MMC by causing microscopic damage in interface and matrix during thermal cycling. Therefore, the nondestructive evaluation on thermal shock damage behavior of SiC/A16061 composite has been carried out using ultrasonics. For this study, SiC fiber reinforced metal matrix composite specimens fabricated by a squeeze casting technique were thermally cycled in the temperature range 298$\~$673 K up to 1000cyc1es. Three point bending test was conducted to investigate the efffct of thermal shock damage on mechanical properties. The relationship between thermal shock damage behavior and the propagation characteristics of surface wave and SH-ultrasonic wave was discussed by considering the result of SEM observation of fracture surface.

• Advanced Composite Materials
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• v.18 no.1
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• pp.61-75
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• 2009

The effect of porous matrix on thermal fatigue behavior of 3D-orthogonally woven SiC/SiC composite was evaluated in comparison with that having relatively dense matrix. The porous matrix yields open air passages through its thickness which can be utilized for transpiration cooling. On the other hand, the latter matrix is so dense that the air passages are sealed. A quantity of the matrix was varied by changing the number of repetition cycles of the polymer impregnation pyrolysis (PIP). Strength degradation of composites under thermal cycling conditions was evaluated by the $1200^{\circ}C$/RT thermal cycles with a combination of burner heating and air cooling for 200 cycles. It was found that the SiC/SiC composite with the porous matrix revealed little degradation in strength during the thermal cycles, while the other sample showed a 25% decrease in strength. Finally it was demonstrated that the porous structure in 3D-SiC/SiC composite improved the thermal fatigue durability.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.11
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• pp.2913-2921
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• 1994

In SMA(shape memory alloy), the degradation by fatigue is one of the most important problems to be overcome, when SMA is used for robot-actuator material. The actuator is operated repeatitively for long time and its repeating operation develops the fatigue degradation of SMA. The fatigue degradation changes the transformation temperature and deformation behavior and results in inaccurate operation control of robot. Accordingly, the changing behavior of transformation temperature and deformation which results from repeating operation is to be investigated in advance and the scheme to resolve those problems have to be made for the design of actuator. In this study, the fatigue tests were carried out on SMA specimens prepared to have different condition of aging time and pre-strain with the direct-current heating-cooling method, which was a general method of operation in robot actuators. The behavior of transformation temperature and deformation were examined and analyzed in each specimen and the study was performed to establish the optimistic manufacturing condition of SMA against the fatigue degradation.

• Proceedings of the Korean Fiber Society Conference
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• 1996.04a
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• pp.210-212
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• 1996

• Elastomers and Composites
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• v.53 no.4
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• pp.213-219
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• 2018

The degradation mechanism and physical properties of an FKM O-ring were observed with thermal aging in this experiment. From X-ray photoelectron spectroscopy (XPS) analysis, we could observe carbon (285 eV), fluoro (688 eV), and oxygen (531 eV) peaks. Before thermal aging, the concentration of fluoro atoms was 51.23%, which decreased to 8.29% after thermal aging. The concentration of oxygen atoms increased from 3.16% to 20.39%. Under thermal aging, the FKM O-ring exhibited debonding of the fluoro-bond by oxidation. Analysis of the C1s, O1s, and F1s peaks revealed that the degradation reaction usually occurred at the C-F, C-F2, and C-F3 bonds, and generated a carboxyl group (-COOH) by oxidation. Due to the debonding reaction and decreasing mobility, the glass transition temperature of the FKM O-ring increased from $-15.91^{\circ}C$ to $-13.79^{\circ}C$. From the intermittent CSR test, the initial sealing force was 2,149.6 N, which decreased to 1,156.2 N after thermal aging. Thus, under thermal aging, the sealing force decreased to 46.2%, compared with its initial state. This phenomenon was caused by the debonding reaction and decreasing mobility of the FKM O-ring. The S-S curve exhibited a 50% increase in modulus, with break at a low strain and stress state. This was also attributed to the decreasing mobility due to thermal aging degradation.

• Nuclear Engineering and Technology
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• v.56 no.2
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• pp.728-744
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• 2024

The presence of hydrogen absorbed by zirconium-based cladding materials during reactor operation can trigger degradation mechanisms and endanger the rod integrity. Ensuring the durability of the rods in extended time-frames like dry storage requires anticipating hydrogen behavior using numerical modeling. In this context, the present paper describes a hydrogen post-processing tool for Falcon - HYPE, a PSI's in-house tool able to calculate hydrogen uptake, transport, thermochemistry, reorientation of hydrides and hydrogen-related failure criteria. The tool extracts all necessary data from a Falcon output file; therefore, it can be considered loosely coupled to Falcon. HYPE has been successfully validated against experimental data and applied to reactor operation and interim storage scenarios to present its capabilities.

• Korean Journal of Metals and Materials
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• v.49 no.8
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• pp.596-600
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• 2011

Thermal degradation behavior of a $WO_3-TiO_2$ monolithic catalyst was investigated in terms of structural, morphological, and physico-chemical analyses. The catalyst with 4 wt.% $WO_3$ contents were prepared by a wet-impregnation method, and a durability test of the catalysts were performed in a temperature range between $400^{\circ}C$ and $800^{\circ}C$ for 3 h. An increase of thermal stress decreased the specific surface area, which was caused by grain growth and agglomeration of the catalyst particles. The phase transition from anatase to rutile occurred at around $800^{\circ}C$ and a decrease in the Brønsted acid sites was confirmed by structural analysis and physico-chemical analysis. A change in Brønsted acidity can affect to the catalytic efficiency; therefore, the thermal degradation behavior of the $WO_3-TiO_2$ catalyst could be explained by the transition to a stable rutile phase of $TiO_2$ and the decrease of specific surface area in the SCR catalyst.

• Transactions of the Korean Society of Machine Tool Engineers
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• v.14 no.1
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• pp.44-51
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• 2005

Material degradation is a multibillion-dollar problem which affects all the industries amongst others. The last decades have seen the development of newer and more effective techniques such as Focused-ion beam(FIB), Transmission electron microscopy(TEM), Secondary-ion mass spectroscopy(SIMS), auger electron spectroscopy(AES), X-ray Photoelectron spectroscopy(XPS) , Electrochemical impedance spectroscopy(EIS), Photo- stimulated luminescence spectroscopy(PSLS), etc. to study various forms of material degradation. These techniques are now used routinely to obtain information on the chemical state, depth profiling, composition, stress state, etc. to understand the degradation behavior. This paper describes the use of these techniques specifically applied to materials degradation and failure analysis.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2000.07a
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• pp.857-860
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• 2000

The objective of this paper is to demonstrate degradation phenomena through DC degradation tests and predicts degradation phenomena as a function of time from the tests. The ZnO varistor used in this investigation were fabricated by standard ceramic techniques. Especial, these were sintered in nitrogen atmosphere, at 2 h, for $1300^{\circ}C$. The conditions of DC degradation test were 115$\pm$$2^{\circ}C$for 0, 2, 4, and 8 h, respectively. To demonstrate the degradation phenomena of ZnO varistors, Voltagecurrent analyses were performed before and after the degradation test, and frequency analyses were used with the time of the degradation tests. It was found that the degradation occurred in not grain but grain boundary and the degradation behavior of varistors was unsymmetrically degraded with the direction of tests.

• The Korean Journal of Ceramics
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• v.3 no.3
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• pp.195-198
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• 1997

Post-firing process of electronic ceramic, such as electroding and encapsultion with resin, often causes damage by thermal shock. The thermal shock behavior of $BaTiO_3$ ceramics was investigated by the down-quench test, where the relative strength retained is determined after the sample is quenched from an elevated temperature into a fixed temperature bath. The critical temperature drop, $\DeltaTc$, was evaluated for three kinds of sintered $BaTiO_3$ ceramics, which were formed by extrustioin, uniaxial pressing using granules, and uniaxial pressing using powders. A drastic loss in strength caused by microcracking was observed for the specimens quenched with $\DeltaT\geq150^{\circ}C$. This concentp can be adopted as a method of the quality control by monitoring the sudden drop of the strength of capacitor products after each exposure to heat.