• Bulletin of the Korean Chemical Society
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• v.31 no.12
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• pp.3791-3793
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• 2010

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.5
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• pp.1192-1202
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• 1994

Fatigue behavior and life prediction were presented for thermal-mechanical and isothermal low cycle fatigue of 12Cr forged steel used for high temperature applications. In-phase and out-of-phase thermal-mechanical fatigue test at 350 to 600.deg. C and isothermal low cycle fatigue test at 600.deg. C were conducted using smooth cylindrical hollow specimen under strain-control with total strain ranges from 0.006 to 0.015. Cyclic softening behavior was observed regardless of thermal-mechanical and isothermal fatigue tests. The phase difference between temperature and strain in thermal-mechanical fatigue resulted in significantly shorter fatigue life for out-of-phase than for in-phase. The difference in fatigue lives was dependent upon the magnitudes of inelastic strain ranges and mean stresses. Increase in inelastic strain range showed a tendency of intergranular cracking and decrease in fatigue life, especially for out-of-phase thermal-mechanical fatigue. Thermal-mechanical fatigue life prediction was made by partitioning the strain ranges of the hysteresis loops and the results of isothermal low cycle fatigue tests which were performed under the combination of slow and fast strain rates. Predicted fatigue lives for out-of-phase using the strain range partitioning method showed an excellent agreement with the actual out-of-phase thermal-mechanical fatigue lives within a factor of 1.5. Conventional strain range partitioning method exhibited a poor accuracy in the prediction of in-phase thermal-mechanical fatigue lives, which was quite improved conservatively by a proposed strain range partitioning method.

• Journal of the Korean Society of Clothing and Textiles
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• v.40 no.2
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• pp.305-314
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• 2016

This paper investigated the applicable possibility of PTT and wool staple fibers to the air vortex system as high quality yarns for a high emotional and comfort garment. It was found that the tactile hand of vortex yarn knitted fabrics was harsher than ring and compact yarns knitted fabrics. It was observed that formability and sewability of air vortex yarn knitted fabrics seemed worse than ring and compact yarns due to low tensile and compressional resilience and high bending and shear hysteresis of air vortex yarn knitted fabrics. It revealed that wicking and drying rates of air vortex yarn knitted fabric were better than ring and compact yarns; in addition, the heat keepability of vortex yarn knitted fabric was higher than ring and compact yarns due to low thermal conductivity and max heat flow rate ($Q_{max}$). Any difference of thermal shrinkage between air vortex and ring yarn knitted fabrics was not shown, but pilling characteristic of air vortex yarn knitted fabric was superior. However, it was shown that wicking, drying, thermal property and pilling characteristics of air vortex yarn knitted fabric were superior due to air vortex yarn structure with parallel fibers in the core part and periodical and fasciated twists in the sheath part of the yarns.

• Journal of the Korea Institute of Building Construction
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• v.4 no.2
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• pp.113-118
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• 2004

This study is a basic experiment on quality management of the compression strength of high strength concrete, aiming. at quality management of high strength mass concrete by giving the temperature hysteresis of the mass test pieces to managerial test pieces. Different from ordinary concrete, high strength concrete generally shows the temperature high rising caused by hydration heat inside the concrete. It is known that, in mass concrete, thermal stress occurs due to the difference in temperature between the inside and the outside, which causes a significant difference in compression strength between structure beams and managerial test pieces. It is also reported that there is a large difference between the compression strength of cylindrical managerial test pieces of standard underwater curing and the strength of structure beam concrete. Thus, this study made concrete test pieces in an optimal mix ratio for each strength level, and also created thermal insulation curing box and managerial test pieces. Then it carried out comparative analysis in relation to core strength and suggested equipment and a technique that can control the strength of high strength concrete mass more conveniently and accurately.

• Current Applied Physics
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• v.18 no.12
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• pp.1486-1491
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• 2018

In recent years, mica has been successfully used as a substrate for the growth of flexible epitaxial ferroelectric oxide thin films. Here, we systematically investigated the flexibility of mica in terms of its thickness, repeated bending/unbending, extremely hot/cold conditions, and successive thermal cycling. A $20-{\mu}m-thick$ sheet of mica is flexible even up to the bending radius of 5 mm, and it is durable for 20,000 cycles of up- and down-bending. In addition, the mica shows flexibility at 10 and 773 K, and thermal cycling stability for the temperature variation of ca. 400 K. Compared with the widely used flexible polyimide, mica has a significantly higher Young's modulus (ca. 5.4 GPa) and negligible hysteresis in the force-displacement curve. These results show that mica should be a suitable substrate for piezoelectric energy-harvesting applications of ferroelectric oxide thin films at extremely low and high temperatures.

• Journal of the Korean Ceramic Society
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• v.24 no.6
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• pp.586-592
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• 1987

The role of the strain energy and phase stability in the diffuse phase transition have been investigated in the highly disordered solid solution, (Pb1-xBax)(Zr0.4Ti0.6)O3 (0.2 x 0.4). X-ray diffraction analysis indicates that tetragonality (c/a) decreases with the increasing Ba content. Also as the Ba content increases, phase transition becomes more diffuse and at the same time dielectric relaxation as a function of measured frequencies in the 1KHz-10MHz range occurs very pronouncedly. In the Ba content range, 0.2 x 0.35, hysteresis loops are routinely observed and the loop is observed to narrow shape as the Ba content increases but becomes very slim at 40mol% Ba content. Moreover thermal analysis shows that there is no abrupt change in the thermal expansion coefficient below the apparent transition temperature at which dielectric constant becomes maximum. From the above results, it has been concluded that creation of the strain energy due to the distorthion that occurred during the phase transition suppresses diffuse phase transition.

• Journal of the Korean Ceramic Society
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• v.27 no.3
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• pp.345-354
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• 1990

The densification behaviro and resulting properties of cordierite-aluminium titanate composites containing 5 to 40wt.% aluminium titanate were investigated. Compared with cordierite monolithics a substantial increase of sintering temperature range for composites was observed, which was due to the formation of cordierite and glass phase at relatively low temperatures. The bending strength of composites showed its maximum at 30wt.% aluminium titanate content, which was about 50% increase relative to the cordierite monolithics, then decreased by a small amount at 40wt.% aluminium titanate content. The decrease was explained by the increase of microcracks whose presence was confirmed by the hysteresis of thermal expansion curve of composites. However, the microcracks formed was not severe enough to produce a significant decrease in strength, which was also evidenced by the continuous increase of thermal expansion coefficient up to 40wt.% aluminium titanate content.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2003.06a
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• pp.733-736
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• 2003

This paper presents two systems of two-stage electrohydraulic servovalve with a nozzle-flapper pilot stage, which is controlled by stack-type piezoelectric elements. Two flapper moving mechanisms proposed in this research can compensate for the hysteresis problem and thermal expansion of the piezoelectric elements. The experimental results show that the first flapper moving mechanism has the frequency response of over 500 Hz and the second one has the response of over 600 Hz. And the first simplified servovalve system rising the first flapper moving mechanism has the frequency response of about 150 Hz, and the second system has the response of about 300 Hz at the supply pressure of 210 bar

• Journal of the Korean Society for Precision Engineering
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• v.20 no.6
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• pp.71-80
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• 2003

This paper presents a two-stage electrohydraulic servovalve with a nozzle-flapper pilot stage, which is controlled by stack-type piezoelectric elements. The flapper moving mechanism developed in this research can compensate for the hysteresis problem and thermal expansion of the piezoelectric elements. The experimental result shows that this flapper moving mechanism has the frequency response of about 600 Hz. And a simplified servovalve system using this flapper moving mechanism has the frequency response of about 300 Hz at the supply pressure of 210 bar.

• International Journal of Precision Engineering and Manufacturing
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• v.4 no.6
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• pp.29-37
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• 2003

This paper presents a two-stage electrohydraulic servovalve with a nozzle-flapper pilot stage, which is controlled by stack-type piezoelectric elements. The flapper moving mechanism developed in this research can compensate for the hysteresis problem and thermal expansion of the piezoelectric elements. The experimental result shows that this flapper moving mechanism has the frequency response of about 600 Hz. And a simplified servovalve system using this flapper moving mechanism has the frequency response of about 300 Hz at the supply pressure of 210 bar.