• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2007.10a
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• pp.190-194
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• 2007

The forging process design and microstructure evolution for gas turbine disk of a Waspaloy is investigated in this study. Parameters related to deformation are die and preform geometry, and forging temperature of die and workpiece. Die and preform design are considered to reduce the forging load, and to avoid the forging defects. Blocker and finisher dies for multistage forging are designed and the initial billet geometry is determined. The control of hot forging parameters such as strain, strain rate and temperature also is important because the microstructure change in hot working affects the mechanical properties. The dynamic recrystallization evolution has been studied in the temperature range 900-$1200^{\circ}C$ and strain rate range 0.01-1.0s-1 using hot compression tests. Modeling equations are required represent the flow curve, recrystallized grain size, recrystallized volume fraction by various tests. In this study, we used to thermo-viscoplastic finite element modeling equation of DEFORM-2D to predict the microstructure change evolution during thermo-mechanical processing. The microstructure is updated during the entire thermal and deformation processes in forging.

• Structural Engineering and Mechanics
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• v.73 no.4
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• pp.407-426
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• 2020

In this present paper, a semi-analytical mesh-free method is employed for the three-dimensional free vibration analysis of a bi-directional functionally graded piezoelectric circular structure. The dependent variables have been expanded by Fourier series with respect to the circumferential direction and have been discretized through radial and axial directions based on the mesh-free shape function. The current approach has a distinct advantage. The nonlinear Green-Lagrange strain is employed as the relationship between strain and displacement fields to observe thermal impacts in stiffness matrices. Nevertheless, high order terms have been neglected at the final steps of equations driving. The material properties are assumed to vary continuously in both radial and axial directions simultaneously in accordance with a power law distribution. The convergence and validation studies are conducted by comparing our proposed solution with available published results to investigate the accuracy and efficiency of our approach. After the validation study, a parametric study is undertaken to investigate the temperature effects, different types of polarization, mechanical and electric boundary conditions and geometry parameters of structures on the natural frequencies of functionally graded piezoelectric circular structures.

• Structural Engineering and Mechanics
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• v.48 no.4
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• pp.547-567
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• 2013

Nonlinear behavior of functionally graded material (FGM) plates under thermal loads is investigated here using an efficient sinusoidal shear deformation theory. The displacement field is chosen based on assumptions that the in-plane and transverse displacements consist of bending and shear components, and the shear components of in-plane displacements give rise to the sinusoidal distribution of transverse shear stress through the thickness in such a way that shear stresses vanish on the plate surfaces. Therefore, there is no need to use shear correction factor. Unlike the conventional sinusoidal shear deformation theory, the proposed efficient sinusoidal shear deformation theory contains only four unknowns. The material is graded in the thickness direction and a simple power law based on the rule of mixture is used to estimate the effective material properties. The neutral surface position for such FGM plates is determined and the sinusoidal shear deformation theory based on exact neutral surface position is employed here. There is no stretching-bending coupling effect in the neutral surface-based formulation, and consequently, the governing equations and boundary conditions of functionally graded plates based on neutral surface have the simple forms as those of isotropic plates. The non-linear strain-displacement relations are also taken into consideration. The thermal loads are assumed as uniform, linear and non-linear temperature rises across the thickness direction. Closed-form solutions are presented to calculate the critical buckling temperature, which are useful for engineers in design. Numerical results are presented for the present efficient sinusoidal shear deformation theory, demonstrating its importance and accuracy in comparison to other theories.

• Steel and Composite Structures
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• v.42 no.6
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• pp.779-789
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• 2022

This work presents a non-linear cylindrical bending analysis of functionally graded plate reinforced by single-walled carbon nanotubes (SWCNTs) in thermal environment using a simple integral higher-order shear deformation theory (HSDT). This theory does not require shear correction factors and the transverse shear stresses vary parabolically through the thickness. The material properties of SWCNTs are assumed to be temperature-dependent and are obtained from molecular dynamics simulations. The material properties of functionally graded carbon nanotube-reinforced composites (FG-CNTCRs) are considered to be graded in the thickness direction, and are estimated through a micromechanical model. The non-linear strain-displacement relations in the Von Karman sense are used to study the effect of geometric non-linearity and the solution is obtained by minimization of the total potential energy. The numerical illustrations concern the nonlinear bending response of FG-CNTRC plates under different sets of thermal environmental conditions, from which results for uniformly distributed CNTRC plates are obtained as benchmarks.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2002.10a
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• pp.212-215
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• 2002

Interfacial evaluation and damage sensing of the carbon fiber/epoxy-amine terminated (AT)-polyetherimide (PEI) composites were performed using micromechanical test and electrical resistance measurement. As AT-PEI content increased, the fracture toughness of epoxy-AT-PEI matrix increased, and thus their interfacial shear strength (IFSS) was improved due to the improved toughness. After curing process, the changes in electrical resistance (ΔR) with increasing AT-PEI contents increased gradually because of the changes in thermal expansion coefficient (TEC) and thermal shrinkage of matrix. Matrix fracture toughness was correlated to the IFSS, residual stress and electrical resistance. The results obtained from the electrical resistance measurement during curing process, reversible stress/strain, and durability test were consistent with modified matrix toughness properties.

• Elastomers and Composites
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• v.53 no.4
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• pp.213-219
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• 2018

The degradation mechanism and physical properties of an FKM O-ring were observed with thermal aging in this experiment. From X-ray photoelectron spectroscopy (XPS) analysis, we could observe carbon (285 eV), fluoro (688 eV), and oxygen (531 eV) peaks. Before thermal aging, the concentration of fluoro atoms was 51.23%, which decreased to 8.29% after thermal aging. The concentration of oxygen atoms increased from 3.16% to 20.39%. Under thermal aging, the FKM O-ring exhibited debonding of the fluoro-bond by oxidation. Analysis of the C1s, O1s, and F1s peaks revealed that the degradation reaction usually occurred at the C-F, C-F2, and C-F3 bonds, and generated a carboxyl group (-COOH) by oxidation. Due to the debonding reaction and decreasing mobility, the glass transition temperature of the FKM O-ring increased from $-15.91^{\circ}C$ to $-13.79^{\circ}C$. From the intermittent CSR test, the initial sealing force was 2,149.6 N, which decreased to 1,156.2 N after thermal aging. Thus, under thermal aging, the sealing force decreased to 46.2%, compared with its initial state. This phenomenon was caused by the debonding reaction and decreasing mobility of the FKM O-ring. The S-S curve exhibited a 50% increase in modulus, with break at a low strain and stress state. This was also attributed to the decreasing mobility due to thermal aging degradation.

• Korean Journal of Microbiology
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• v.20 no.3
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• pp.125-133
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• 1982

Mutational experiments were performed to imporve the cellulase productivity of Aspergillus phoenicis KU175, isolated from the southern part of Korea, as a high cellulase producer. By treatment ultra-violet light nad 4-NQO(4-Nitroquinoline-N-Oxide), mutation waas induced, and treatment ultra-violet light and 4-NQO (4-Nitroquinoline-N-Oxide), mutation was induced, and A.phoenicis KU175-115 was finally selected for its highest avicelase production. Avicelase production of the mutant was increased about 2 times compared with those of the wild strain. However, activities of other hydrolytic enzymes, such as amylase, protease and nuclease, of the mutant strain didn't show a marked difference compared with those of the nuclease, of the mutant strain didn't show a marked difference compared with the wild strain, except slight increase in ribonuclease activity and slight decrease in glucoamylase activity. Avicelases from the mutant strain selected were purified from wheat bran culture by successive salting out, followed by dialysis and column chromatography, and their charcteristics were compared with thosw of the wild strain. Avicelase was separated into three peaks in the mutant strain as well as in the case of wild strain. Avicelase II activity of the mutant strain was prominently higher than that of the wild strain, while avicelase I and III activities of those were equivalent. The optimal pH ranges and stability of avicelase II from the mutant strain were pH4-5 and pH3.5-6.0, respectively, as well as in the case of the wild strain. The optimal temperature and thermal stability of avicelase II from the mutant strain were $40{\sim}50^{\circ}C\;and\;20{\sim}55^{\circ}C$, respectively. These results were same as those of the wild strain. By the using of Eadie-Hofastee plot, $K_m\;and\;V_{max}$ of avicelase II from the mutant and the wild strain were calculated to be 2.29mg/ml and $4.84{\mu}g$ reducing sugar as glucose per min equally, from the line fitted to the data by the least square method. Activity of avicelase II from the mutant strain was slightly activated by $Mg^{++}\;but\;inhibited\;by\;Cu^{++}, \;Mn^{++}\;and\;Zn^{++}$, as well as in the case of the wild strain. Therefore, it was concluded that the mutant didn't induce the formation of another avicelase isozyme, or the changes in the properties of avicelase, but induce the changes in the productively of the same avicelase II by the action of regulatory gane.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.10a
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• pp.199-202
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• 2008

In this study, we investigated the thermal properties of $Zr_{66.4}Nb_{6.4}Cu_{10.5}Ni_{8.7}Al_{8.0}$ by using a differential scanning calorimeter (DSC), and then analyzed the composition of dendrite phase by using X-ray diffraction (XRD). A series of uniaxial compression tests has been performed under the strain rates between $10^{-5}/s$ and $10^{-2}/s$ at room temperature and near SLR. This BMGC has higher high temperature strength than other Zr-based monolithic BMGs because in-situ formed crystalline phases hinder a feasible viscous flow of amorphous matrix. Warm formability is also estimated by laboratory-scale extrusion test within supercooled liquid region. It was found that BMGC has poor formability compared with nother Zr-based bulk metallic glass composite presumably due to large volume fraction of 'brittle' crystalline phases distributed within amorphous matrix.

• KIEE International Transactions on Electrophysics and Applications
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• v.4C no.2
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• pp.51-54
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• 2004

In this paper, the thermal and mechanical properties of elastic epoxy for the application of high voltage products were investigated. Glass transition temperature (Tg) of elastic epoxies cannot be found from room temperature to 20$0^{\circ}C$ by DSC (Differential Scanning Calorimetry). Weight reduction occurred at 285$^{\circ}C$ and 451$^{\circ}C$ according to a thermogravimeter. The first temperature was affected by addictives and the second by epoxies characteristic. Maximum tensile strain showed 28.3kgf/$\textrm{cm}^2$/$\textrm{cm}^2$ at 20% of mechanical stress in addictives 35 (phr). The SEM (Scanning electron microscope) micrograph of the fracture surface observed void and tearing of elastic epoxy at addictives 35 (phr). On the other side, the SEM micrograph of the rigid epoxy showed a broken trace.

• Proceedings of the Materials Research Society of Korea Conference
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• 2003.11a
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• pp.170-170
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• 2003

We have implanted on sapphire substrate with various ions and investigated the properties of GaN epilayers grown on implanted sapphire substrate by metal organic chemical vapor deposition (MOCVD). Sapphire is typical substrate for GaN epilayers. However, there are many problems such as lattice mismatch and thermal coefficient difference between sapphire substrate and GaN. The ion implanted substrate's surface had decreased internal tree energies during the growth of the GaN epilayer, md the misfit strain was relieved through the formation of an AlN phase on the ions implanted sapphire(0001) substrates. [1] The crystal and optical properties of GaN epilayer grown in ions implanted sapphire(0001) substrate were improved.