• Transactions of the Korean Society of Mechanical Engineers A
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• v.24 no.10 s.181
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• pp.2597-2602
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• 2000

The most effective means of evaluating remaining life is through the creep testing of samples removed from the component. But sampling of large specimen from in-service component is actually impossible. So, sampling device and small-specimen creep tester have been applied. Sampling device has been devised to extract mechanically small samples by hemispherical, diamond -coated cutter from the surface of turbine rotor bores and thick-walled pipes without subsequent weld repairs requiring post weld heat treatment. A method of manufacturing small creep specimen, 2min gage diameter and 10min gage length, using electron beam welding to attach grip section, has been proven. Small-specimen creep tester has been designed to control atmosphere to prevent stress increment by oxidation during experiment. To determine whether the small specimens successfully reproduce the behavior of large specimens, creep rupture tests for small and large specimens have been performed at identical conditions. Creep rupture times based on small specimens have closely agreed within 5% error compared with that of large specimen. The errors in rupture time have decreased at longer test period. This comparison validates the procedure for fabricating and testing on small specimen. This technique offers potential as an efficient method for remaining life assessment by direct sampling from in -service high temperature components.

• Journal of the Korea Concrete Institute
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• v.20 no.6
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• pp.681-687
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• 2008

This study presents a technique to quantitatively measure the corrosion level of a reinforcing bar using infrared thermography system. We found out electric heating method having an important effect on thermal data in previous study. This study purposed an efficient way for grip standardization to reduce constriction resistance problem, and providing reliable thermal data using infrared thermographic method. Using vise type earth clamp, the surface resistance of rebar was smaller than that of traditional plier type earth clamp through temperature-distribution relationship. Also this study contains experiments with toque wrench to improve heating contact problem.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.11a
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• pp.629-632
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• 2008

As concrete is a material which is subject to wide quality fluctuations by temperature, grip of seasonal feature and maintenance of ambient temperature and humidity to secure the quality required after casting concrete are able to keep away from harmful effects. In case of summer, a high temperature has caused rapid hydration reaction of cement in early age, which has caused to decrease strength by autogenous shrinkage. Therefore we need to consider a countermeasure for decrease in the hydration heat of hot-weather concrete, according to minimize water and cement content and use mineral admixtures In this experimental research, the compressive strength development for replacement ratio of mineral admixtures, curing temperature and methods of concrete was investigated to confirm the effects of mixture design and curing condition on compressive strength of concrete.

• International Journal of Automotive Technology
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• v.6 no.1
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• pp.45-52
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• 2005

Interrupted creep tests for investigating the structural degradation during creep were conducted for a Mod.9Cr-1Mo steel in the range of stress from 71 to 167 MPa and temperature from 873 to 923 K. The change of hardness and tempered martensitic lath width was measured in grip and gauge parts of interrupted creep specimens. The lath structure was thermally stable in static conditions. However, it was not stable during creep, and the structural change was enhanced by creep strain. The relation between the change in lath width and creep strain was described quantitatively. The change in Vickers hardness was expressed by a single valued function of creep LCR(life consumption ratio). Based on the empirical relation between strain and lath width, a model was proposed to describe the relation between change in hardness and creep LCR. The comparison of the model with the empirical relation suggests that about 65% of hardness loss is due to the decrease of dislocation density accompanied by the movement of lath boundaries. The role of precipitates on subboundaries was discussed in connection with the abnormal subgrain growth appearing in low stress regime.

• Journal of the Korean Society for Precision Engineering
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• v.21 no.2
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• pp.168-176
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• 2004

Interrupted creep tests were carried out on the Mod.9Cr-1Mo steel in order to investigate the structural degradation during creep. The ranges of creep stress and temperature were from 71 to 167MPa and 873 to 923k, respectively. The change of hardness and tempered martensitic lath width were measured in the grip and gauge parts of interrupted specimens. The lath structure was thermally stable in static conditions, but was not stable during creep, and the structural evolution was enhanced by creep strain. The relation between the change in lath width and strain was described in the from, $\delta$W= a ($W_s-W_o$)$cdot;varepsilon$, where $\varepsilon$ is the strain, $W_o$is the initial lath width, $W_s$ is the final lath width depending solely on stress, and a is the constant of the magnitude of 0.67 $\mu$m /strain. The change in Victors hardness was expressed by a one-valued function of creep life consumption ratio. Based on the empirical relation between strain and lath width, a model was proposed to explain the relation between change in hardness and creep life consumption ratio. The model revealed that about 65$%$ of dislocations in lath structures were eliminated by the migration of subboundaries.

• Elastomers and Composites
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• v.56 no.3
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• pp.152-163
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• 2021

Liquid butadiene rubber (LqBR) is used as a processing aid and plays a vital role in the manufacture of high-performance tire tread compounds. In this study, center-functionalized LqBR (C-LqBR) was polymerized with different vinyl content via anionic polymerization. The effects of the vinyl content on the properties of the compounds were investigated by partially replacing the treated distillate aromatic extract (TDAE) oil with C-LqBR in silica-filled rubber compounds. C-LqBR compounds showed a low Payne effect and Mooney viscosity regardless of the vinyl content, because of improved silica dispersion due to the ethoxysilyl group. As the vinyl content of C-LqBR increased, the optimum cure time (t₉₀) increased owing to a decrease in the number of allylic hydrogen. Moreover, the glass transition temperature (T_g) of the compound increased, and snow traction and abrasion resistance performance decreased, whereas wet grip improved. The energy loss characteristics revealed that the hysteresis attributed to the free chain ends of C-LqBR was dominant.

• Applied Microscopy
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• v.43 no.4
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• pp.164-172
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• 2013

In this work, microstructural and hardness evolution of the X20 steel upon short-term creep test ($550^{\circ}C$ to $650^{\circ}C$, $180^{\circ}C$ to 60 MPa) was studied by using scanning electron microscope, electron backscattered diffraction, and transmission electron microscope, microhardness tester. After creep rupture, gauge and grip part of the specimens were microstructurally analyzed. Creep at the $650^{\circ}C$/60 MPa resulted in a rupture at 1,460 hours with growth of lath width from 1.31 to $2.87{\mu}m$ and a grain growth with a more equiaxed feature. There is a close relationship between Microhardness and lath width. The formation and coarsening of Laves phase, which was observed up to $600^{\circ}C$ of creep temperature, was accelerated by the applied stress. Slight coarsening of the $M_{23}C_6$ was observed in the $550^{\circ}C$ and $600^{\circ}C$ crept or aged specimens. The coarsening of $M_{23}C_6$ depended on the temperature, where specimens crept at $650^{\circ}C$ showed higher growth rate. The microstructural evolution of X20 after short-term creep test was extensively discussed in relation to the long-term creep/aging test reported in literatures.

• Korean Journal of Materials Research
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• v.21 no.11
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• pp.596-603
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• 2011

The very high temperature gas reactor (VHTR) is one of the next generation nuclear reactors for its safety, long-term stability, and proliferation-resistance. The high operating temperature of over 800$^{\circ}C$ enables various applications with high energy efficiency. Heat is transferred from the primary helium loop to the secondary helium loop through the intermediate heat exchanger (IHX). The IHX material requires creep resistance, oxidation resistance, and corrosion resistance in a helium environment at high operating temperatures. A Ni-based superalloy such as Alloy 617 is considered as a primary candidate material for the intermediate heat exchanger. In this study, the microstructures of Alloy 617 crept in pure helium and air environments at 950$^{\circ}C$ were observed. The rupture time in helium was shorter than that in air under small applied stresses. As the exposure time increased, the thickness of outer oxide layer of the specimens clearly increased but delaminated after a long creep time. The depth of the carbide-depleted zone was rather high in the specimens under high applied stress. The reason was elucidated by the comparison between the ruptured region and grip region of the samples. It is considered that decarburization caused by minor gas impurities in a helium environment caused the reduction in creep rupture time.

• Korean Journal of Materials Research
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• v.33 no.10
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• pp.406-413
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• 2023

This study used optical and scanning electron microscopy to analyze the surface oxidation phenomenon that accompanies a γ'-precipitate free zone in a directional solidified CM247LC high temperature creep specimen. Surface oxidation occurs on nickel-based superalloy gas turbine blades due to high temperature during use. Among the superalloy components, Al and Cr are greatly affected by diffusion and movement, and Al is a major component of the surface oxidation products. This out-diffusion of Al was accompanied by γ' (Ni₃Al) deficiency in the matrix, and formed a γ'-precipitate free zone at the boundary of the surface oxide layer. Among the components of CM247LC, Cr and Al related to surface oxidation consist of 8 % and 5.6 %, respectively. When Al, the main component of the γ' precipitation phase, diffused out to the surface, a high content of Cr was observed in these PFZs. This is because the PFZ is made of a high Cr γ phase. Surface oxidation of DS CM247LC was observed in high temperature creep specimens, and γ'-rafting occurred due to stress applied to the creep specimens. However, the stress states applied to the grip and gauge length of the creep specimen were different, and accordingly, different γ'-rafting patterns were observed. Such surface oxidation and PFZ and γ'-rafting are shown to affect CM247LC creep lifetime. Mapping the microstructure and composition of major components such as Al and Cr and their role in surface oxidation, revealed in this study, will be utilized in the development of alloys to improve creep life.

• The Korean Journal of Community Living Science
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• v.23 no.2
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• pp.137-144
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• 2012

This study was undertaken to develop protective gloves for rose farmers who work on thorny plant in Korea. Prototype of protective gloves was designed and evaluated in terms of thermal comfort and mobility. Gloves were made with arm protectors attached to them, so that they could protect the lower part of arms, and rubber bands were inserted into the arm protectors for them not to slip down. The bending part of each finger was punched in order to give ventilation. Also, the bending parts of the fingers in the upper and lower part of gloves were inserted with sponges and were stitched together in order to enhance gripping movement. According to compared evaluation of the developed gloves and the existing gloves, temperature inside the gloves didn't show any significant differences, but humidity inside the gloves showed significant differences. There were significant differences in terms of comparison of objective mobility, that is, pegboard run-time and grip power, from statistical aspects. In addition, a comparison of subjective discomfort showed significant differences and so the suitability of developed gloves was proved.