• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 2000.04a
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• pp.635-640
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• 2000

Thermal deformation of machine tools can be evaluated from the analysis of the whole temperature field. However, it is extremely inefficient and impossible to acquire the whole temperature field by measuring temperatures of every point. So, a temperature estimator, which can estimate the whole temperature field from the temperatures of just a few points, is required. In this paper, 1-dimensional heat transfer problem is modeled with modal analysis and state space equations. and then state observer is designed to estimate the intensity of heat source and the whole temperature field in real-time. The reliability of this estimator is verified by making a comparison between solutions by the proposed method and the exact solutions of examples. The proposed method is applied to the estimation of temperature distribution in a ball screw system.

• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.5
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• pp.1166-1180
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• 1990

The study is concerned with the two-dimensional thermo-viscoplastic finite element analysis for radial forging as an incremental forging process. The deformation and temperature distribution of the workpiece during radial forging are studied. The analysis of deformation and the analysis of heat transfer are carried out for simple upsetting of cylinder by decoupling the above two analyses. A method of treatment for heat transfer through the contact region between the die and the workpiece is suggested, in which remeshing of the die elements is not necessary. Radial forging of a mild steel cylinder at the elevated temperature is subjected to the decoupled finite element analysis as well as to the experiment. The computed results in deformation, load and temperature distribution are found to be in good agreement with the experimental observations. As an example of viscoplastic decoupled analysis of hot radial forging, forging of a square section into a circular section is treated. The stresses, strains, strain rates and temperature distribution are computed by superposing material properties as the workpiece is rotated and forged incrementally. It was been thus shown that proposed method of analysis can be effectively applied to the hot radial forging processes.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.05a
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• pp.273-276
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• 2008

Thermoplastic resin takes place stress relaxation and creep according to temperature and time. In room temperature, time dependent deformation (TDD) of polymer was carried out at previous study. In this study, it evaluates time dependent deformation to relate temperature. Nanoscale indents can be used as cells for molecular electronics and drug delivery, slots for integration into nanodevices, and defects for tailoring the structure and properties. Therefore, it is important to control pattern depth for change of indent depth by creep when using Nanoindenter. For evaluating TDD at high temperature, it is occurred thermal-nanoindentation test by changing hold time at maximum load. Temperature is putted at $90^{\circ}C$, hold time at maximum loads are putted at 1, 10, 50, 100, 200, 300 and 500s.

• Transactions of Materials Processing
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• v.7 no.2
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• pp.168-176
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• 1998

Hot workability of the AA5083 alloy ws investigated by torsion test at temperature ranges of $350{\sim}520^{\circ}C$ and strain rates of 0.5, 1.0, and 3.0/sec. The flow stress and hot ductility of the AA5083 alloy as a function of deformation variables such as temperature and train rate were studied. The microstructural evolution of the AA5083 alloy was studied in relation to Zener-Hollomon parameter (Z=exp( /RT) Also the hot restoration mechanism of the AA5083 alloy was small when Z val-ues were higher than $1.73{\times}1016/sec(370^{\circ}C,\;0.5/sec)$ In addition the difference microstructures during hot deformation. It was found that the increase of flow curves and deformed microstructures during hot deformation. It was found that the increase of flow stress of the AA5083 alloy was small when Z val-ues were higher than $1.73{\times}1016/sec(370^{\circ}C.\;0.5/sec)$. However under the low Z values less than $1.73{\times}1016/sec(370^{\circ}C,\;0.5/sec)$ the flow stress increase with increasing the Z values. The large dispersoid particles in the matrix grain decreased the flow strain of the AA5083 alloy because it caused the stress concentration during hot deformation.

• Journal of Korean Society of Steel Construction
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• v.10 no.2 s.35
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• pp.201-209
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• 1998

The presence of elevated temperature can alter significantly the structural response of fibre-reinforced laminated composites. A thermal environment causes degradation in both strength and constitutive properties, particularly in the case of fibre-reinforced polymeric composites. Furthermore, associated thermal expansion, either alone or in combination with mechanically induced deformation, can result in buckling, large deflections, and excessively high stress levels. Consequently, it is often imperative to consider environmental effects in the analysis and design of laminated systems. Exact analytical solutions of higher-order shear deformation theory is developed to study the thermal buckling of cross-ply and antisymmetric angle-ply rectangular plates. The buckling behavior of moderately thick cross-ply and antisymmetric angle-ply laminates that are simply supported and subject to a uniform temperature rise is analyzed. Numerical results are presented for fiber-reinforced laminates and show the effects of ply orientation, number of layers, plate thickness, and aspects ratio on the critical buckling temperature and compared with those obtained using the classical and first-order shear deformation theory.

• Transactions of Materials Processing
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• v.18 no.7
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• pp.544-549
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• 2009

This study aimed to elucidate the deformation mechanism during low-temperature superplasticity of fine-grained Ti-6Al-2Sn-4Zr-2Mo-0.1Si alloy in the context of constitutive equation. For this purpose, initial coarse equiaxed microstructure was refined to $2.2{\mu}m$ via dynamic globularization. Globularized microstructure exhibited large superplastic elongations(434-826%) at temperatures of $650-750^{\circ}C$ and strain rate of $10^{-4}s^{-1}$. It was found that the main deformation mechanism of fine-grained material was grain boundary sliding accommodated by dislocation motion with both stress exponent (n) and grain size exponent (p) values of 2. When the alpha grain size, not sub-grain size, was considered to be an effective grain size, the apparent activation energy for low-temperature superplasticity of the present alloy(169kJ/mol) was closed to that of Ti-6Al-4V alloy(160kJ/mol).

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.11
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• pp.2913-2921
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• 1994

In SMA(shape memory alloy), the degradation by fatigue is one of the most important problems to be overcome, when SMA is used for robot-actuator material. The actuator is operated repeatitively for long time and its repeating operation develops the fatigue degradation of SMA. The fatigue degradation changes the transformation temperature and deformation behavior and results in inaccurate operation control of robot. Accordingly, the changing behavior of transformation temperature and deformation which results from repeating operation is to be investigated in advance and the scheme to resolve those problems have to be made for the design of actuator. In this study, the fatigue tests were carried out on SMA specimens prepared to have different condition of aging time and pre-strain with the direct-current heating-cooling method, which was a general method of operation in robot actuators. The behavior of transformation temperature and deformation were examined and analyzed in each specimen and the study was performed to establish the optimistic manufacturing condition of SMA against the fatigue degradation.

• Journal of the Korean Society for Precision Engineering
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• v.21 no.10
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• pp.162-169
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• 2004

High temperature uniaxial compression tests in the alpha single phase region were carried out on the Ti -43mo1%Al intermetallic compound, in order to obtain oriented lamellar microstructure. The compression deformation temperatures and strain rates are from 1573k to 1623k and 1.0x10$^{-4}$ s to 5.0x10$^{-3}$ s, respectively. Fully lamellar microstructure was observed after the uniaxial compression deformation in a single phase region followed by cooling to room temperature. Lamellar colony diameter depended on strain rates and test temperatures. The diameter varied between 8601m and 300fm. Stress-strain curve showed a work softening and the size of lamellar colony diameter varied depending on peak stresses. This shows the occurrence of dynamic recrystallization. Texture measurements after the uniaxial compression deformation, showed the development of fiber during dynamic recrystallization. It is seen that the area for the maximum pole density existed in 35 degrees away from the compression plane. The texture sharpens with a decrease in strain rate

• Journal of the Korean Society for Heat Treatment
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• v.16 no.2
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• pp.83-89
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• 2003

As a research for developing fine-grained high strength low carbon steels, the effects of austenitization temperature and hot deformation on microstructure was investigated in 0.15 wt.% carbon steels with microalloying elements such as Nb and Ti. When the steels were reheated at $1250^{\circ}C$, Nb containing steel showed very coarse austenite grain size of $200{\mu}m$ whereas Nb-Ti steel did fine one of $70{\mu}m$ because Ti carbonitrides could suppress the austenite grain growth. In case of 50% reduction at $850^{\circ}C$, the austenite grains in the Nb steel partially recrystallized while those in the Nb-Ti steel fully recrystallized probably due to finer prior austenite grains.For the Nb-Ti steel, ferrite grain size was not sensitively changed with austenitization temperature and compression strain and, severe deformation of 80% reduction was not essentially necessary to refine ferrite grains to about $3{\mu}m$ which could be obtained through lighter deformation of 40% reduction.

• Korean Journal of Materials Research
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• v.30 no.2
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• pp.57-60
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• 2020

Titanium aluminides have attracted special interest as light-weight/high-temperature materials for structural applications. The major problem limiting practical use of these compounds is their poor ductility and formability. The powder metallurgy processing route has been an attractive alternative for such materials. A mixture of Ti and Al elemental powders was fabricated to a mechanical alloying process. The processed powder was hot pressed in a vacuum, and a fully densified compact with ultra-fine grain structure consisting of Ti₃Al intermetallic compound was obtained. During the compressive deformation of the compact at 1173 K, typical dynamic recrystallization (DR), which introduces a certain extent of grain refinement, was observed. The compact had high density and consisted of an ultra-fine equiaxial grain structure. Average grain diameter was 1.5 ㎛. Typical TEM micrographs depicting the internal structure of the specimen deformed to 0.09 true strain are provided, in which it can be seen that many small recrystallized grains having no apparent dislocation structure are generated at grain boundaries where well-developed dislocations with high density are observed in the neighboring grains. The compact showed a large m-value such as 0.44 at 1173 K. Moreover, the grain structure remained equiaxed during deformation at this temperature. Therefore, the compressive deformation of the compact was presumed to progress by superplastic flow, primarily controlled by DR.