• 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 KSME Conference
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• 2001.06a
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• pp.243-249
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• 2001

In this study, the progressive inelastic deformation, so called, thermal ratchet phenomenon which can occur in high temperature liquid metal reactor was simulated with thermal ratchet structural test facility and 316L stainless steel test cylinder. The inelastic deformation of the reactor baffle cylinder can occur due to the moving temperature distribution along the axial direction as the hot free surface moves up and down under the cyclic heat-up and cool-down of reactor operations. The ratchet deformations were measured with the laser displacement sensor and LVDTs after cooling the structural specimen which experiences thermal load up to $550^{\circ}$ and the temperature differences of about $500^{\circ}C$. During structural thermal ratchet test, the temperature distribution of the test cylinder along the axial direction was measured from 28 channels of thermocouples and the temperatures were used for the ratchet analysis. The thermal ratchet deformation analysis was performed with the NONSTA code whose constitutive model is nonlinear combined kinematic and isotropic hardening model and the test results were compared with those of the analysis. Thermal ratchet test was carried out with respect to 9 cycles of thermal loading and the maximum residual displacements were measured to be 1.8mm. It was shown that thermal ratchet load can cause a progressive deformation to the reactor structure. The analysis results with the combined hardening model were in reasonable agreement with those of the tests.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.26 no.2
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• pp.193-198
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• 2017

Rubber materials are widely used for anti-vibration in various industries such as railways, automobile, and aviation. However, various factors hinder the accurate prediction of mechanical properties and lifetime of these materials. Particularly, a stress softening phenomenon Mullins effect greatly affects the accuracy of test results by reducing the initial peak stress. Although the Mullins effect has been studied previously, research on its temperature dependence is lacking. In this study, we performed experiments to estimate the temperature dependence of the Mullins effect. Dumbbell specimens made of natural rubber (NR65) was mounted on a stress softening tester and placed in a heat chamber, where they were tested at temperature of 25, 50, and $80^{\circ}C$. Further, five test sets, each consisting of 10 loading/unloading cycles were sequentially performed at predetermined time intervals. Based on the test results, we assessed the effect of temperature and time interval on stress softening and recovery.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.14 no.10
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• pp.3941-3954
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• 2020

In this paper, the climate change in Haiyan County in recent decades was analyzed in detail with the methods of moving average, Mann-Kendall non-parametric mutation test and wavelet analysis. According to the variation trend of meteorological factors such as temperature, relative humidity, wind speed, pan evaporation and precipitation in recent decades, the climate of Haiyan County has a tendency of drought, which is becoming more and more serious. From the results of the analysis, the sunshine hours and the air temperature in Haiyan County have an obvious upward trend. The average surface temperature has increased by 2.75 ℃ from 1976, and its largest increase occurred in the late 1970s and 1980s. At this stage, the average surface temperature increased by 1.37 ℃. The relative humidity has a decreasing trend that has decreased by 2.75％. From 1976 to the present, there are two quasi-3a cycles and one quasi-6a cycle. The precipitation and evaporation showed the opposite change trend, in which the trend of precipitation fluctuated upward, while the trend of evaporation showed a fluctuating downward tendency, which led to the serious loss of water in the feeding area. The wind direction in Haiyan County are mainly from west to east, and its wind speed has a trend of slight increase.

• Journal of the Korean Society for Heat Treatment
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• v.11 no.4
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• pp.295-304
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• 1998

The effects of retained austenite and carbon content in the retained austenite on the tensile strength-elongation balance and spot weldability of TRIP high strength steel sheet have been investigated. The retained austenite of granular type increased with increasing intercritical annealing and austempering temperature, and film type was increased with the increase of austempering time. The volume fraction of retained austenite increased with decreasing intereritical annealing temperature, and the maximum value was obtained at austempering temperature of $400^{\circ}C$. The values of tensile strength-elongation balance increased with decreasing intercritical annealing temperature and maximum value was obtained at austempering temperature of $400^{\circ}C$. The maximum value of tensile strength-elongation balance was obtained at a retained austenite content of about 12%. Tensile shear strength of the specimens with retained austenite was higher than that of the normalizing specimens. With increasing welding current and time, the tensile shear strengh and nugget diameter increased, while nugget thickness showed the peak value and then decreased. The optimum range of welding condition at the given welding pressure of 350kgf was 7~11kA and 10~15 cycles.

• Restorative Dentistry and Endodontics
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• v.45 no.3
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• pp.37.1-37.8
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• 2020

Objectives: This study aims to compare the cyclic fatigue resistance of VDW.ROTATE, TruNatomy, 2Shape, and HyFlex CM nickel-titanium (NiTi) rotary files at body temperature. Materials and Methods: In total, 80 VDW.ROTATE (25/0.04), TruNatomy (26/0.04), 2Shape (25/0.04), and HyFlex CM (25/0.04) NiTi rotary files (n = 20 in each group) were subjected to static cyclic fatigue testing at body temperature (37℃) in stainless-steel artificial canals prepared according to the size and taper of the instruments until fracture occurred. The number of cycles to fracture (NCF) was calculated, and the lengths of the fractured fragments were measured. The data were statistically analyzed using a 1-way analysis of variance and post hoc Tamhane tests at the 5% significance level (p < 0.05). Results: There were significant differences in the cyclic fatigue resistance among the groups (p < 0.05), with the highest to lowest NCF values of the files as follows: VDW.ROTATE, HyFlex CM, 2Shape, and TruNatomy. There was no significant difference in the lengths of the fractured fragments among the groups. The scanning electron microscope images of the files revealed typical characteristics of fracture due to cyclic fatigue. Conclusions: The VDW.ROTATE files had the highest cyclic fatigue resistance, and the TruNatomy and 2Shape files had the lowest cyclic fatigue resistance in artificial canals at body temperature.

• The Journal of Korean Academy of Prosthodontics
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• v.28 no.1
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• pp.165-191
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• 1990

Although a number of studies have been performed to assure that residual stress caused by a mismatch of alloy porcelain thermal expansion can contribute to clinical failure of a ceramometal restoration, the interactive influence of cooling rate on the magnitude of thermal expansion difference and on bond strength between them have not been extensively analyzed. The objective of this study was to determine the influence of cooling rate and the number of firing cycles on the expansion mismatch and the flexural failure resistance of metal porcelain strip. Tested alloys included one Pd-Ag alloy, one Ni-Cr-Be alloy with two kinds of porcelain, Vita and Ceramco. Metal specimens were cast into rods with a height of 13mm and a diameter of 5mm. Subsequently, the castings were subjected to scheduled firing cycles without porcelain. And the porcelain specimens after being fired were trimmed into a bar with a final dimension of $5{\times}5{\times}25mm$. Thermal expansions of the alloys and porcelains were measured by using a push rod or a differential dialometer respecitvely. Porcelain glass transition temperatures and expansion values were derived alloy-porcelain pairs were assessed by comparing expansion values of the components at a porcelain glass transition temperature. Calculations were made using combinations of a Ni-Cr alloy or Pd-Ag alloy with each of two porcelain products. Metal-porcelain strip specimens were subjected to four point loading in an Instron testing machine until crack occured at the metal-cramic interface at the time of sharp decrease of load on recorder. On the basis of this study, the following conclusions may be stated: 1. Regardless of the kinds of ceramometal combinations, both of calculated and experimental data revealed that the double fired specimens exhibited a significantly lower flexural strength. 2. By the rise of the amount of mismatch, bond strength were decreased. 3. Thermal expansion value of Pd-Ag alloys were higher than that of Ni-Cr alloys. 4. Expansion curves of metal were proportional to the increase of temperature and were not affected by the experimental conditions, however porcelains did not show the same magnitude of metal, and a shift of the glass transition temperature to higher temperatures was observed when cooled rapidly 5. Alloy-porcelain thermal compatibility appeared more dependent on the porcelain than the alloy.

• Journal of the Korean Electrochemical Society
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• v.23 no.3
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• pp.57-65
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• 2020

It is crucial to investigate the thermal degradation of lithium-ion batteries (LIBs) to understand the possible malfunction at high temperature. Herein, we investigated the effects of surface film formation on the thermal degradation of lithium cobalt oxide (LiCoO₂, LCO) cathode that is one of representative cathode materials. Cycling test at 60℃ exhibited poorer cycleability compared with the cycling at 25℃. Cathodes after the initial 5 cycles at 60℃ (60-LCO) exhibited higher impedance compared to the cathode after initial 5 cycles at 25℃ (25-LCO), resulting in the lower rate capability upon subsequent cycling at 25℃, although the capacity values were similar at the lowest C-rate of 0.1C. In order to understand degradation of the LCO cathode at the high temperature, we analyzed the cathodes surface using X-ray photoelectron spectroscopy (XPS). Among various peaks, intensity of lithium hydroxide (LiOH) increased substantially after the operation at 60℃, and the C-C signal that represents the conductive agent was distinctly lower on 60-LCO compared to 25-LCO. These results pointed to an excessive formation of cathode-electrolyte interphase including LiOH at 60℃, leading to the increase in the resistance and the resultant degradation in the electrochemical performances.

• The Journal of Korean Academy of Prosthodontics
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• v.29 no.1
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• pp.111-137
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• 1991

Although a number of studies have been performed to assure that residual stress caused by a mismatch of alloy porcelain thermal expansion can contribute to clinical failure of a ceramometal restoratoin, the interactive influence of cooling rate on the magnitude of thermal expansion difference and on bond strength between them have not been extensively analyzed. The objective of this study was to determine the influence of cooling rate and the number of firing cycles on the expansion mismatch and the flexural failure resistance of metal porcelain strip. Tested alloys included one Pd-Ag alloy, one Ni-Cr-Be alloy with two kinds of porcelain, Vita and Ceramco. Metal specimens were cast into rods with a height of 13mm and a diameter of 5mm. Subsequently, the castings were subjected to scheduled firing cycles without porcelain. And the porcelain specimens after being fired were trimmed into a bar with a final dimension of 5 x 5 x 25mm. Thermal expansions of the alloys and porcelains were measured by using a push rod or a differential dialometer respectively. Porcelain glass transition temperatures and expansion values were derived alloy- porcelain pairs were assessed by comparing expansion values of the components at a porcelain glass transition temperature. Calculations were made using combinations of a Ni-Cr alloy or Pd-Ag alloy with each of two porcelain products. Metal- porcelain strip specimens were subjected to four point loading in an Instron testing machine until crack occured at the metal-cramic interface at the time of sharp decrease of load on recorder. On the basis of this study, the following conclusions may be stated : 1. Regardless of the kinds of ceramometal combinations, both of calculated and experimental data revealed that the double fired specimens exhibited a significantly lower flexural strength. 2. By the rise of the amount of mismatch, bond strength were decreased. 3. Thermal expansion value of Pd-Ag alloys were higher than of Ni-Cr alloys. 4. Expansion curves of metal were proportional to the increase of temperature and were not affected by the experimental conditions, however porcelains did not show the same magnitude of metal, and a shift of the glass transition temperature to higher temperatures was observed when cooled rapidly. 5. Alloy- porcelain thermal compatibility appeared more dependent on the porcelain than the alloy.

• 한국연소학회:학술대회논문집
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• 2004.11a
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• pp.231-237
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• 2004

Vacuum carburizing continues to gain acceptance as an alternative to atmosphere carburizing particularly in the car industry. The advantages of low-pressure carburization over atmospheric gas carburization is not only the creation of a surface entirely free of oxide and the environmentally friendly nature of these methods but also an improvement in deformation behaviour achieved by combining carburization with gas quenching, a reduction in batch times by increasing the carburization temperature, low gas and energy consumption and the prevention of soot to a large extent. In present study, an improved vacuum carburizing method is provided which is effective to deposit carbon in the surface of materials and to reduce cycle time. Also LPC process simulator was made to optimize to process controls parameters such as pulse/pause cycles of pressure pattern, temperature, carburizing time, diffusion time. The carburizing process was simulated by a diffusion calculation program, where as the model parameters are proposed with help the experimental results and allows the control of the carburizing process with good accordance to the practical results. Thus it can be concluded that LPC process control method based on the theoretical simulation and experimental datas appears to provide a reasonable tool for prototype LPC system.