• Computers and Concrete
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• v.8 no.5
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• pp.491-510
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• 2011

Before deciding if structures exposed to high temperature are to be repaired or demolished, their final state should be carefully examined. Destructive and non-destructive testing methods are generally applied for this purpose. Compressive strength and color change in mortars are observed as a result of the effects of high temperature. In this study, ordinary and pozzolan-added mortar samples were produced using different aggregates, and exposed to 100, 200, 300, 600, 900 and $1200^{\circ}C$. The samples were divided into two groups and cooled to room temperature in water and air separately. Compression tests were carried out on these samples, and the color change was evaluated by the Munsell Color System. The relationships between the change in compressive strength and color of mortars were determined by using a multi-layered feed-forward Neural Network model trained with the back-propagation algorithm. The results showed that providing accurate estimates of compressive strength by using the color components and ultrasonic pulse velocity design parameters were possible using the approach adopted in this study.

• Tunnel and Underground Space
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• v.30 no.6
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• pp.559-572
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• 2020

This paper performs laboratory experiments for borehole stability considering temperature and true triaxial stress condition, and observes a thermo-mechanical behavior of the rock under stress and temperature conditions of deep underground. China yellow sandstone and Hwangdeung granite specimens were used to perform a true triaxial compression test. Mechanical tests were carried out under nine confining pressure conditions, and thermo-mechanical tests using granite samples were carried out under six confining pressure conditions at 60-100℃. In the mechanical tests, maximum principal stress at borehole breakout was proportional to intermediate principal stress. In the thermo-mechanical tests, it was confirmed that thermal stress is added to the stress field of the borehole with the increase in temperature, resulting in additional breakout progress. To analyze the results of the laboratory experiment, Mogi-Coulomb failure criterion was used. The results of traditional triaxial compression test on cylindrical specimens and borehole breakout under true triaxial compressions matched well with Mogi-Coulomb failure criterion.

• Korean Journal of Metals and Materials
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• v.46 no.7
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• pp.449-457
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• 2008

Compression deformation behaviors at high temperature as a function of temperature and strain rate were investigated in the 6061 aluminum alloy, which is used for automobile wheel. Compression tests were carried out in the range of temperatures $300{\sim}475^{\circ}C$ and strain rate $10^{-3}{\sim}10^{-1}sec^{-1}$. By analyzing these results, strain rate sensitivity, deformation temperature sensitivity, the efficiency of power dissipation, Ziegler's instability criterion, etc were calculated, which were plastic deformation instability parameters as suggested by Ziegler, Malas, etc. Furthermore, deformation processing map was drawn by introducing dynamic materials model (DMM) and Ziegler's Continuum Criteria. This processing map was evaluated by relating the deformation instability conditions and the real microstructures. As a result, the optimum forging condition for the automobile wheel with the 6061 aluminum alloy was designed at temperature $450^{\circ}C$, strain rate $1.0{\times}10^{-1}sec^{-1}$. It was also confirmed by DEFORM finite element analysis tool with simulation process.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2007.05a
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• pp.123-126
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• 2007

Deformation behavior of $Zr_{55}Cu_{30}Al_{10}Ni_5$ (at. %) bulk metallic glass (BMG) fabricated by suction casting method has been investigated at elevated temperatures in this study. The BMG was first verified to have an amorphous structure thru X-ray diffraction (XRD) and differential scanning calorimetry (DSC). A series of compression tests has consequently been performed in supercooled liquid temperature region to investigate the high temperature deformation behavior. A transition from Newtonian to non-Newtonian flow appeared to take place depending upon both the strain rate and test temperature. A processing map based on a dynamic materials model has been constructed to estimate a feasible forming condition for this BMG alloy.

• Journal of Powder Materials
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• v.1 no.1
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• pp.35-41
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• 1994

Mechanical alloying process of Al-8wt.% Ni powder was investigated for the various milling time in order to get the steady state powder. High temperature deformation behaviors of the sintered specimens were investigated by activation energy calculated after high temperature compression tests at the strain rates of 2.5$\times$10-3 s-1, 2.5$\times$10-2 s-1 and 2.5$\times$10-1 s-1 at the temperature range between $350^{\circ}C$ and $450^{\circ}C$. The steady state was obtained after 1000 minutes of milling with the PCA of 1.5 wt.% stearic acid under the condition of grinding media to powder weight ratio of 50 : 1 and impeller rotating speed of 300 rpm. True activation energy of Al-8wt.% Fe alloy was estimated to be 181 kJ/mole at the temperature range of 350~ $400^{\circ}C$ and 265 kJ/mole at the range of 400~$450^{\circ}C$.

• Transactions of Materials Processing
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• v.16 no.4 s.94
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• pp.309-312
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• 2007

Deformation behavior of $Zr_{55}Cu_{30}Al_{10}Ni_5$(at. %) bulk metallic glass(BMG) fabricated by suction casting method has been investigated at elevated temperatures in this study. The BMG was first verified to have an amorphous structure with the analysis of X-ray diffraction(XRD) and differential scanning calorimetry(DSC) data. A series of compression tests has consequently been performed in the region of supercooled liquid temperature to investigate the behavior of high temperature deformation. A transition from Newtonian to non-Newtonian flow appeared to take place depending upon both the strain rate and test temperature. A processing map based on a dynamic materials model has been constructed to estimate a feasible forming condition for this BMG alloy.

• Journal of Powder Materials
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• v.11 no.2
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• pp.149-157
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• 2004

High temperature deformation behavior of activated sintered W powder compacts was investigated. The compression tests were carried out in the temperature range between 900 and 110$0^{\circ}C$ at the strain rate of $10^{-3}s^{-1}$. The sintered specimens of Ni-doped submicron W powder compacts showed decrease in W grain size with increasing the Ni content. As the result, the flow stress was significantly increased with increasing the Ni content. We obtained Ni-activated sintered W compacts with the relative density of 94 $\pm$ l％and the average grain size of less than 5${\mu}{\textrm}{m}$. A moderate true strain up to 0.60 was obtained without fracture even at 110$0^{\circ}C$ with the strain rate of $10^{-3}s^{-1}$ for the activated W compact despite adding the 1.0 wt％Ni to submicron W powder.

• Journal of Powder Materials
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• v.28 no.5
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• pp.389-395
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• 2021

Oxide dispersion-strengthened (ODS) steel has excellent high-temperature properties, corrosion resistance, and oxidation resistance, and is expected to be applicable in various fields. Recently, various studies on mechanical alloying (MA) have been conducted for the dispersion of oxide particles in ODS steel with a high number density. In this study, ODS steel is manufactured by introducing a complex milling process in which planetary ball milling, cryogenic ball milling, and drum ball milling are sequentially performed, and the microstructure and high-temperature mechanical properties of the ODS steel are investigated. The microstructure observation revealed that the structure is stretched in the extrusion direction, even after the heat treatment. In addition, transmission electron microscopy (TEM) analysis confirmed the presence of oxide particles in the range of 5 to 10 nm. As a result of the room-temperature and high-temperature compression tests, the yield strengths were measured as 1430, 1388, 418, and 163 MPa at 25, 500, 700, and 900℃, respectively. Based on these results, the correlation between the microstructure and mechanical properties of ODS steel manufactured using the composite milling process is also discussed.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2004.05a
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• pp.80-83
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• 2004

High temperature deformation behavior of AZ31 Mg alloy was investigated in this study on the basis of a processing map $(\varepsilon\approx0.6)$. To construct a processing map, compression tests were carried out at wide range of temperatures and strain rates $(T=250\~500^{\circ}C,\;\varepsilon=10^{-4}\~100/s)$. Two regions of high deformation efficiency $(\eta)$ were identified as: (1) a dynamic recrystalization (DRX) domain at $250^{\circ}C$ and 1/s and (2) a superplasticity domain at $450^{\circ}C$ and $10^{-4}/s$. Possible deformation mechanisms operating at high temperature were discussed in relation to the activation energy. A two-stage deformation method was found to be effective in enhancing the superplasticity of AZ31 Mg alloy. From the two-stage deformation method, tensile elongation of $1200\%$ was obtained at the superplastic domain.

• Transactions of Materials Processing
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• v.14 no.4 s.76
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• pp.384-391
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• 2005

The nickel-based alloy Nimonic 80A possesses the excellent strength, and the resistance against corrosion, creep and oxidation at high temperature. Its products are used in aerospace engineering, marine engineering and power generation, etc. Control of forging parameters such as strain, strain rate, temperature and holding time is important because change of the microstructure in hot working affects the mechanical properties. Change of the microstructure evolves by recovery, recrystallization and grain growth phenomena. The dynamic recrystallization evolution has been studied in the temperature range of $950\~1250^{\circ}C$ and strain rate range of $0.05\~5s^{-1}$ using hot compression tests. The metadynamic recrystallization and grain growth evolution has been studied in the temperature range of $950\~1250^{\circ}C$ and strain rate range $0.05,\;5s^{-1}$, holding time range of 5, 10, 100, 600 sec using hot compression tests. Modeling equations are proposed to represent the flow curve, recrystallized grain size, recrystallized fraction and grain growth phenomena by various tests. Parameters in modeling equations are expressed as a function of the Zener-Hollomon parameter. The modeling equation for grain growth is expressed as a function of the initial grain size and holding time. The modeling equations developed were combined with thermo-viscoplastic finite element modeling to predict the microstructure change evolution during hot forging process. The grain size predicted from FE simulation results is compared with results obtained in field product.