• 한국세라믹학회지
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• 제36권8호
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• pp.871-875
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• 1999

Theoretical equation to calculate thermal fatigue life was derived in which slow crack growth theory was adopted. The equation is function of crack growth exponent n. Cyclic thermal fatigue tests were performed at temperature difference of 175, 187 and 200$^{\circ}C$ respectively. At each temperature difference critical thermal fatigue life cycles of the alumina ceramics were 180,37 and 7 cycles. And theoretical thermal fatigue life cycles were calculated as 172, 35 and 7 cycles at the same temperature difference conditions. Therefore thermal fatigue behavior of alumina ceramics can be represented by derived equation. Also theoretical single cycle critical thermal shock temperature difference can be calculated by this equation and the result was consistent with the experimental result well.

• 한국초전도저온공학회:학술대회논문집
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• 한국초전도저온공학회 2003년도 추계학술대회 논문집
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• pp.274-277
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• 2003

Critical current(Ic) degradation of HTS tapes after bending is one of the hottest issues in HTS development and application studies. Many people are measuring Ic degradations for effect of bending radius. However even if the bending radius is larger than the breaking radius a HTS tapes can be damaged by repetitive bending, which is unavoidable in the winding processes. Therefore, We studied the Ic degradation after repetitive bending. at 77K with self-field, of Bi-2223 tapes processed by "Powder-in-Tube" technique, which was made by America Superconductor Corporation(AMSC) and superconductiing tapes that strain is imposed measured critical current by temperature difference of L$N_2$ and normal temperature. Like this, critical current could measure that degradation about 1~3％ by temperature difference. These results will amount the most important basis data in power electric machine of superconductivity cable, magnet, etc that winding work is require.

• 대한기계학회논문집A
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• 제34권2호
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• pp.129-138
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• 2010

공업용 세라믹은 자체의 특이한 물리적 특성으로 인하여 극한의 열 및 화학적 환경에서도 적용할 수 있는 우수한 고온 재료이다. 세라믹은 고온에서 저온으로 빠르게 이동되면 열충격을 받는다. 본 연구에서는 열충격에 대한 매개변수로 임계온도차이를 제안한다. 세라믹 부품에 대한 임계온도차이는 부품 크기와 대류열전달계수 등에 의해 영향을 받는다. 부품의 열충격 특성은 비정상 열응력에 의해 평가된다. 비정상 열응력이 파단계수를 초과한다면 열충격 균열이 표면에서 시작된다고 가정할 수 있다. 물에 대한 임계온도차이는 공기에 대한 임계온도차이보다 적다. 본 연구에서 사용된 국내 승용차용 삼원 촉매 담체는 반경 및 축방향 온도차이가 임계온도차이 아래에 존재하므로 충분한 열충격 성능을 가지고 있었다.

• 한국세라믹학회지
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• 제35권10호
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• pp.1094-1100
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• 1998

The thermal fatigue behavior of alumina ceramics was investigated by water quenching method. Single-quench thermal shock tests were performed to decide the critical thermal shock temperature difference ($\Delta$Tc) which was found to be 225$^{\circ}C$ Cyclic thermal shock fatigue tests were performed at temperature diff-erences of 175$^{\circ}C$, 187$^{\circ}C$ and 200$^{\circ}C$ respectively. After cyclic thermal shock fatigue test the distributions of retained strength and crack were observed. Retained strength was measured by four point bending method and crack observation method bydye penetration. In terms of the retained strength distribution the critical number of thermal shock cycles(Nc) were 7 for $\Delta$T=200$^{\circ}C$, 35 for $\Delta$T=187$^{\circ}C$ and 180for $\Delta$T=175$^{\circ}C$ respec-tively. In terms of the crack observation the critical number of thermal shock cycles were 5 for $\Delta$T==200$^{\circ}C$ 20 for $\Delta$T==187$^{\circ}C$ and 150 for $\Delta$T=175$^{\circ}C$ respectively. The difference of Nc investigated by two different methods is due to the formation of the longitudinal cracks which had no effect on the four point bending strength. Therefore the thermal fatigue behavior of alumina ceramics could be more accurately described by the crack observation method than the retained strength measurement method.

• 한국세라믹학회지
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• 제34권7호
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• pp.767-773
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• 1997

Thermal shock behavior of alumina ceramics were studied by quenching the heated alumina specimen into the water of various temperatures over 0~10$0^{\circ}C$. The critical thermal shock temperature difference ( Tc) of the specimen decreased almost linearly from 275$^{\circ}C$ to 20$0^{\circ}C$ with increase in the cooling water temperature over 0~6$0^{\circ}C$. It is probably due to the increase of the maximum cooling rate which is dependent of the convection heat transfer coefficient. The convection heat transfer coefficient is a function of the temperature of the cooling water. However, the critical thermal shock temperature difference( Tc) of the specimen increased at 25$0^{\circ}C$ over 80~10$0^{\circ}C$ due to the film boiling of the cooling water. The maximum cooling rate, which brings about the maximum thermal stress of the specimen in the cooling process, was observed to increase linearly with the increase in the quenching temperature difference of the specimen due to the linear relationship of the convection heat transfer coefficient with the water temperature over 0~6$0^{\circ}C$. The critical maximum cooling rate for thermal shock fracture was observed almost constant to be about 260$\pm$1$0^{\circ}C$/s for all water temperatures over 0~6$0^{\circ}C$. Therefore, thermal shock behavior of alumina ceramics is greatly influenced by the convection heat transfer coefficient of the cooling water.

• International Journal of Automotive Technology
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• 제7권6호
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• pp.697-702
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• 2006

The change in the critical speed due to surface temperature of automotive disk brakes may be analyzed both theoretically as well as experimentally. Juddering of disk brakes is closely related to its critical speed. In analyzing the critical speed, if $\sigma$ is positive, Disk develops TEI(Thermo-Elastic Instability) resulting in juddering in disk brakes. And $\sigma$ is affected not only by the critical speed but also by the initial temperature of disk surface. As the initial temperature of the disk surface rises, the critical speed decreases and juddering is developed more easily. Also, when hot spots are developed by TEI, they show large temperature difference in small local range.

• 한국세라믹학회지
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• 제30권3호
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• pp.229-235
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• 1993

Thermal history and thermal stress of alumina specimen, which occured from thermal shock process, were calculated by finite difference method. Stress intensity factor and crack growth in cyclic thermal fatigue were calculated from single thermal shock temperature history and thermal stress. Cyclic thermal life were estimated by bending strength after cyclic thermal shock under critical thermal shock temperature. Calculated stress intensity factor was compared with real experimental thermal fatigue life of specimen. Fatigue life until critical stress intensity factor and real experimental result were comparable.

• 열처리공학회지
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• 제12권3호
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• pp.206-214
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• 1999

The effect of cooling rate on carbide precipitation during quenching of austenitic manganese steel was investigated by optical microscopy, image analyzer and numerical analysis. A computer program based on the finite difference method for analyzing heat treatment processes was developed in order to evaluate cooling rates and the possibility of carbide precipitation during quenching. The area ratio of carbide precipitated in the austenite matrix was measured by the image analyzer, and used to determine the critical point of carbide precipitation. Temperature-dependent critical cooling rates at the critical points were calculated using the present simulation program, The calculated results showed a good agreement with the experimental ones.

• 한국표면공학회지
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• 제39권1호
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• pp.18-24
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• 2006

Although stainless steels have the excellent corrosion resistance by passive film, they are susceptible to pitting corrosion in the environment containing halogen elements such as chloride ions. The resistance to pitting corrosion can be evaluated by measuring the critical pitting temperature (CPT). CPT values can be obtained using immersion, potentiodynamic and potentiostatic polarization test methods. Results on duplex 2205 stainless steels showed that CPT values were measured as $50^{\circ}C,\;55^{\circ}C\;and\;61^{\circ}C$, respectively for immersion, potentiodynamic and potentiostatic polarization test methods, depending upon the different test methods, even though the difference between CPT values are not much.

• 한국재료학회지
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• 제21권2호
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• pp.73-77
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• 2011

Specific heat of a crystal is the sum of electronic specific heat, which is the specific heat of conduction electrons, and lattice specific heat, which is the specific heat of the lattice. Since properties such as crystal structure and Debye temperature do not change even in the superconducting state, the lattice specific heat may remain unchanged between the normal and the superconducting state. The difference of specific heat between the normal and superconducting state may be caused only by the electronic specific heat difference between the normal and superconducting states. Critical temperature, at which transition occurs, becomes lower than $T_{c0}$ under the influence of a magnetic field. It is well known that specific heat also changes abruptly at this critical temperature, but magnetic field dependence of jump of specific heat has not yet been developed theoretically. In this paper, specific heat jump of superconducting crystals at low temperature is derived as an explicit function of applied magnetic field H by using the thermodynamic relations of A. C. Rose-Innes and E. H. Rhoderick. The derived specific heat jump is compared with experimental data for superconducting crystals of $MgCNi_3$, $LiTi_2O_4$ and $Nd_{0.5}Ca_{0.5}MnO_3$. Our specific heat jump function well explains the jump up or down phenomena of superconducting crystals.