• Journal of the Korean Wood Science and Technology
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• v.24 no.3
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• pp.101-109
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• 1996

Effects of process variables were evaluated in physical properties of the wood fiber-thermoplastic fiber composites using nonwoven web method. Turbulent air mixer using compressed air was employed to mix wood fiber with two types of thermoplastic polypropylene and nylon 6 fibers. The optimal hot press temperature and time were found to be $190^{\circ}C$ and 9 minutes in wood fiber-polypropylene fiber composite and to be $220^{\circ}C$ and 9 minutes in wood fiber-nylon 6 fiber composite. As the density of wood fiber-polypropylene fiber composite and wood fiber-nylon 6 fiber composite increased, the physical properties were improved The density appeared to be the most significant factor on physical properties in the statistical analysis. The composition ratio of polypropylene or nylon 6 fiber to wood fiber was considered not to be statistically significant factor. The thickness swelling decreased somewhat in wood fiber-polypropylene fiber composite and wood fiber-nylon 6 fiber composite as the content of synthetic fiber increased. As the increase of mat moisture content, dimensional stability was improved in wood fiber-polypropylene fiber composite but not in wood fiber-nylon 6 fiber composite.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.23 no.6
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• pp.547-554
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• 2014

CVD SiC is a perfect material used for molds/dies in hot press molding of glass lens. In its fabrication process, nano-precision polishing is essential finally. For this purpose, a novel polishing method using MCF (Magnetic Compound Fluid) slurry is proposed. In this method, MCF slurry is supplied into a given gap between the workpiece and a MCF slurry carrier, and constrained within the polishing zone by magnetic forces from permanent magnet. In this paper, after an experimental rig used to actually realize the proposed method has been constructed, the fundamental polishing characteristics of CVD SiC such as the effects of process parameters including MCF slurry composition on work-surface roughness were experimentally investigated. As a result, nano-precision surface finish of CVD SiC was successfully attained with MCF slurry and the optimum process parameters for obtaining the smoothest work-surface were determined.

• Journal of the Korean Society for Heat Treatment
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• v.6 no.1
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• pp.9-16
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• 1993

To investigate the properties of diffusion bonding of 7050 Al alloy, the diffusion bonding joints have been produced in self-made diffusion bonding hot-press which admits a defined application of the bonding pressure during the heating phase and also rapid cooling after the bonding process with various bonding condition. The strength of the bond increases with increasing the bonding time and temperature. Shear test at toom temperature showed that high strength up to 70% that of parent metal (320 MPa), 220 MPa for the specimen bonded 14 hr at $560^{\circ}C$, with 3 MPa. In this case, however, there is large deformation more than 20% reduction in thickness. The results were correlated with joint characteristics found by optical microstructure and by fractography by SEM. When the strengths of the bonds are more than 50% that of parent metal, a great deal of dimples stretched along the direction of shear stress are observed over the fractured surface of the bond. On the microstructure of the bond line, initial mophology of the bond line disapeared for the grain boundary migration with increasing the bonding time.

• Journal of the Korean Society for Heat Treatment
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• v.26 no.6
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• pp.279-287
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• 2013

Oxide Dispersion Strengthened (ODS) Fe with $Al_2O_3$ dispersoid was successfully produced by reactive milling with a mixture of Fe, $Fe_3O_4$ (Magnetite), $Fe_2O_3$ (Hematite) and Al reactants at cryogenic temperature. The milled powders were consolidated by Vacuum Hot Press (HP) at 1323 K, and the consolidated materials were characterized by Transmission Electron Microscopy (TEM), Scanning Transmission Electron Microscopy (STEM), and Energy Dispersive Spectroscopy (EDS); the yield strength and the hardness of the consolidated materials were determined by compressive test and Vickers hardness test at room temperature. The grain size of the materials was estimated by X-ray Diffraction technique using the scherrer's formula. The TEM observations showed that the microstructure was comprised with a mixture of nanocrystalline Fe matrix and $Al_2O_3$ nano-dispersoids with a bimodal size distribution; the 0.2% off-set yield strength of the materials was as high as $758{\pm}29$ MPa and the Vickers hardness was $358{\pm}2$. The effect of the cryogenic milling and addition of extra Fe powder was discussed on the suppression of MSR (Mechanically induced Self-sustaining Reaction) for the desired microstructural evolution of ODS alloys.

• Journal of the Korean Ceramic Society
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• v.55 no.4
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• pp.400-405
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• 2018

FCM nuclear fuel, a concept proposed as an accident tolerant fuel in light water reactors, consists of TRISO fuel particles embedded in a SiC matrix. The uniform dispersion of internal TRISO fuel particles in the FCM fuel is very important for improving the fuel efficiency. In this study, FCM sintered pellets with various volume ratios of TRISO-coated particles were prepared by hot press sintering. The distribution of TRISO-coated particles was quantitatively analyzed using X-ray ${\mu}CT$ and expressed as a dispersion uniformity index. TRISO-coated particles were most uniformly dispersed in the FCM pellets prepared using only overcoated TRISO particles without mixing of additional SiC matrix powder. FCM pellets with uniformly dispersed TRISO particle volume fraction of up to 50% were prepared using overcoated TRISO particles with varying thickness.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2009.05a
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• pp.334-336
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• 2009

An elastic-plasticity model during the austenitic decomposition was derived and implemented to incorporate the two important deformation behaviors observed during the phase transformations: the volumetric strain and transformation induced plasticity due to the temperature change and phase transformation. To obtain transformed phase volume fractions during cooling, the fourth order Runge-Kutta method was used to solve the Kirkaldy's phase kinetics model which is function of temperature, austenitic grain size and chemical composition. The volumetric strain was calculated by considering the densities of constituent phases, while the transformation induced plasticity was based on the micro-plasticity due to the volume mismatch between soft austenitic phase and other harder phases. The constitutive equations were implemented into the implicit finite element software and a simple boundary value problem was chosen as a model problem to validate the effect of transformation plasticity on the deformation behavior of steel under cooling from high temperature. It was preliminary concluded that the transformation plasticity plays a critical role in relaxing the developed stress during forming and thus reducing the magnitude of springback.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.29 no.10 s.241
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• pp.1315-1320
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• 2005

TiNi alloy fiber was used to recover the original shape of materials using its shape memory effect. The shape memory alloy plays an important role within the metal matrix composite. The shape memory alloy can control the crack propagation in metal matrix composite, and improve the tensile strength of the composite. In this study, TiNi/A16061 shape memory alloy(SMA) composite was fabricated by hot press method, and pressed by a roller for its strength improvement. The four kinds of specimens were fabricated with $0\%,\;3.2\%,\;5.2\%\;and\;7\%$ and volume fraction of TiNi alloy fiber, respectively. A fatigue test has performed to evaluate the crack initiation and propagation for the TiNi/A16061 SMA composite fabricated by かis method. In order to study the shape memory effect of the TiNi alloy fiber, the test has also done under both conditions of the room temperature and high temperature. The relationship between the crack growth rate and the stress intensity factor was clarified for the composite, and the cold rolling effect was also studied.

• Advances in Energy Research
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• v.5 no.3
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• pp.255-275
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• 2017

The advantages of using natural circulation (NC) as a cooling system, has prompted the worldwide development to investigate this phenomenon more than before. The interesting application of the NC in low power experimental facilities and research reactors, highlights the obligation of study in these laminar flows. The inherent oscillations of NC between hot source and cold sink in low Grashof numbers necessitates stability analysis of cooling flow with experimental or numerical schemes. For this type of analysis, numerical methods could be implemented to desired mass, momentum and energy equations as an efficient instrument for predicting the behavior of the flow field. In this work, using the explicit, implicit and Crank-Nicolson methods, the fluid flow parameters in a natural circulation experimental test loop are obtained and the sensitivity of solving approaches are discussed. In this way, at first, the steady state and transient results from explicit are obtained and compared with experimental data. The implicit and crank-Nicolson scheme is investigated in next steps and in subsequent this research is focused on the numerical aspects of instability prediction for these schemes. In the following, the assessment of the flow behavior with coarse and fine mesh sizes and time-steps has been reported and the numerical schemes convergence are compared. For more detail research, the natural circulation of fluid was modeled by ANSYS-CFX software and results for the experimental loop are shown. Finally, the stability map for rectangular closed loop was obtained with employing the Nyquist criterion.

• Earthquakes and Structures
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• v.8 no.4
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• pp.935-956
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• 2015

Structural Health Monitoring (SHM) of steel towers has become a hot research topic. From the literature, it is impractical and impossible to develop a "general" method that can detect all kinds of damages for all types of structures. A practical method should make use of the characteristics of the type of structures and the kind of damages. This paper reports a feasibility study on the use of measured modal parameters for the detection of damaged braces of tower structures following the Bayesian probabilistic approach. A substructure-based structural model-updating scheme, which groups different parts of the target structure systematically and is specially designed for tower structures, is developed to identify the stiffness distributions of the target structure under the undamaged and possibly damaged conditions. By comparing the identified stiffness distributions, the damage locations and the corresponding damage extents can be detected. By following the Bayesian theory, the probability model of the uncertain parameters is derived. The most probable model of the steel tower can be obtained by maximizing the probability density function (PDF) of the model parameters. Experimental case studies were employed to verify the proposed method. The contributions of this paper are not only on the proposal of the substructure-based Bayesian model updating method but also on the verification of the proposed methodology through measured data from a scale model of transmission tower under laboratory conditions.

• Steel and Composite Structures
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• v.12 no.3
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• pp.183-205
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• 2012

This paper presents an investigation of the effect of inclined stiffeners on the load-carrying capacity of simply-supported hot-rolled steel I-beams under various load conditions. The study is carried out using finite element analysis. A series of beams modeled using 3-D solid finite elements with consideration of initial geometric imperfections, residual stresses, and material nonlinearity are analyzed with and without inclined stiffeners to show how the application of inclined stiffeners can offer a noticeable increase in their lateral-torsional buckling (LTB) capacity. The analysis results have shown that the amount of increase in LTB capacity is primarily dependent on the location of the inclined stiffeners and the lateral unsupported length of the beam. The width, thickness and inclination angle of the stiffeners do not have as much an effect on the beam's lateral-torsional buckling capacity when compared to the stiffeners' location and beam length. Once the optimal location for the stiffeners is determined, parametric studies are performed for different beam lengths and load cases and a design equation is developed for the design of such stiffeners. A design example is given to demonstrate how the proposed equation can be used for the design of inclined stiffeners not only to enhance the beam's bearing capacity but its lateral-torsional buckling strength.