• Proceedings of the Korean Society of Precision Engineering Conference
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• 1996.11a
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• pp.933-937
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• 1996

In this paper, the behavior of plastic zone in the notch tip was studied under Loye's Micro Vickers Hardness Measurement Method. The direction forming maximum plastic zone was estimated by finite element analysis. In the experiments, cold rolling sheet SGCD3, SK5 and hot rolling sheet SS41, S4SC was used to study the influence of carbon contents on plastic zone. The standard hardness test specimen and the notch hardness test specimen was made and loaded cyclically. The specimen was aged to stabilize the hardness. After aging treatment, the notch specimen was made and simple tension load of 50% yield strength was applied. The hardness test at the notch tip until the hardness data of standard hardness specimen was checked was performed.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2006.05a
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• pp.254-257
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• 2006

In the last few years, ductile fracture criteria based on various hypotheses have been developed and utilized with FEM to predict forming failure. The accurate deformation analysis by the FEM and the decision of damage parameters are the most important factors in these approaches. In this paper, several conventional integral forms of fracture criteria were introduced and the test method to determine damage parameters by using notched specimen was suggested. Based on the results, damage parameters obtained under the different stress system (tensile and compression) are compared and analyzed.

• Transactions of Materials Processing
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• v.7 no.3
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• pp.215-224
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• 1998

A semi-solid forming has a lot of advantages compared to the die casting. squeeze casting and conventional forging. therefore, semi-solid forming process is now becoming industrial interest for the production of metal components and metal matrix composites. However the material behaviour in the semi-solid temperature range is not sufficiently known although it controls the whole process through forces and geometry evolutions because the behaviour of metal slurries is complex. The semi-solid materials(SSMs) fabricated under electric-magnetic stirring condition is necessary to be applicated in forming process. A reheating conditions were studied with the reheating time holding time and reheating temperatures. The microstructure of SSM(specimen size : d39${\times}$h85) at the condition of heating time 10min and heating temperature 590${\circ}C$ is the most globular and finest one. The microstructure of SSM(specimen size : d76${\times}$h60) reheated under the three step reheating conditions is most globular and finest.

• Journal of the Korean Ceramic Society
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• v.33 no.4
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• pp.448-454
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• 1996

The microstructural evolution of BaTiO3 ceramics during cubic-hexagonal phase transformation was investiga-ted. In the case of phase transformation from cubic to hexagonal BaTiO3 the hexagonal phase nucleated at the surface region of specimen. On the other hand in the case of that from hexgonal phase to cubic, cubic phase was initiated at the center region of specimen. And fast grain growth and irregular grain boundary shape could be also observed during these transformation processes. Besides low densified hexagonal BaTiO3 specimen was made with low forming pressure. The phase transformation of these specimens toward cubic phase was relatively retarded comparing with dense hexagonal BaTiO3 specimens. was made low forming pressure.. The phase transformation of these specimens toward cubic phase was relatively retarded comparing with dense hexagonal BaTiO3 specimens. These results were explained that hexagonal BaTiO3 had lowder surface energy than cubic phase.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.10
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• pp.1463-1470
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• 2010

Microforming, which exploits the advantages of metal forming technology, appears very promising in manufacturing microparts since it enables the production of parts using various materials at a high production rate, it has high material utilization efficiency, and it facilitates the production of parts with excellent mechanical properties. However, the conventional macroscale forming process cannot be simply scaled down to the micro-scale process on the basis of the extensive results and know-how on the macroscale process. This is because a so-called "size effect" occurs as the part size decreases to the microscale. In this paper, we attempt to develop an effective analytical and experimental modeling technique for explaining the effects of the grain size and the specimen size on the behavior of metals in microscale deformation processes. Copper sheet specimens of different thicknesses were prepared and heat-treated to obtain various grain sizes for the experiments. Tensile tests were conducted to investigate the influence of specimen thickness and grain size on the flow stress of the material. In addition, an analytical model was developed on the basis of phenomenological experimental findings to quantify the effects of the grain size and the specimen size on the flow stress of the material in microscale and macroscale forming.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.37 no.9
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• pp.939-945
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• 2009

The dome stretching tests and tension tests have been performed to obtain a forming limit curve(FLC) for the copper alloy which is used for manufacturing the regenerative cooling thrust chamber. For experimental investigation of the forming limit curve, we have used in-plane tension specimen to obtain tension-compression strain state as well as out-of-plane specimen to obtain tension-tension strain state through dome stretching test. All specimens were divided into longitudinal and transverse directions according to the orientation of test specimen. The test results showed that in the tension-tension region, copper alloy revealed a maximum major strain of 62.3% and a maximum minor strain of 58.6%. In the tension-compression region, the maximum major strain and the maximum minor strain were measured to be 60.5% and 25.8%, respectively.

• Journal of the Korean Ceramic Society
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• v.46 no.1
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• pp.94-101
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• 2009

The physical properties of artificial aggregates made from clay and inorganic wastes with poor plasticity depends largely on forming method. The artificial aggregates composing of coal fly ash, stone sludge and clay were fabricated using 4 different forming methods and those physical properties were comparatively analyzed. The surface of aggregates made through the extrusion forming process was dense and smooth but was rough for the aggregates obtained by crushing a tile-shaped green body. The aggregates made by pelletizing process had a weak green strength and bumpy surface. The shell generated at surface during a high temperature sintering process induced the most aggregates to be bloated due to a dense shell. But the aggregates made through pelletizing process with dense surface layer showed no significant change in bulk density with sintering temperatures. The water absorption of aggregates decreased with sintering temperature, and that of pelletized specimen was standing $1.8{\sim}2.2$ times higher than that of made by other forming methods. It is concluded that the aggregates having various properties could be fabricated from one batch by using different forming methods.

• Transactions of Materials Processing
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• v.27 no.2
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• pp.115-122
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• 2018

Most domestic and international standards on the forming limit diagram (FLD) including ISO 12004-2, use a 'position-dependent method,' which determines the forming limit from a strain distribution measured on the specimen after necking or fracture. However, the position-dependent method has inherent problems such as the incidence of asymmetry of a strain distribution, the estimation of missing data near fracture, the termination time of test, and the deformation due to the new stress equilibrium after a fracture, which is blamed for causing sometimes a significant lab-to-lab variation. The 'time-dependent method,' which is anticipated to be a new international standard for evaluating the forming limit, is expected to greatly improve these intrinsic disadvantages of the position-dependent method. It is because the time-dependent method makes it possible to identify and accurately determine the forming limit, just before the necking point from the strain data as continuously measured in a short time interval. In this study, we propose a new time-dependent method based on a Gaussian fitting of strain acceleration with the introduction of 'normalized correlation coefficient.' It has been shown in this study that this method can determine the forming limit very stably and gives a higher value, which is in comparison with the results of the previously studied position-dependent and time-dependent methods.

• Journal of Korea Foundry Society
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• v.22 no.5
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• pp.231-237
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• 2002

The structure and properties of HSLA steel obtained by Osprey forming process were investigated. The porosities were generated at the bottom of specimen due to the quenching effect of cold substrate during forming. The steel had a bainitic structure. The grain size were different among positions due to the cooling rates. The elements such as Mn, S, Si, Cr, Mo and etc. were contained in inclusions. They were formed at grain boundries. The size of them was $1{\sim}2{\mu}m$ and has nothing to do with the chemical composition. MnS and NbC were precipitated during rolling and aging.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2003.10a
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• pp.21-24
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• 2003

In hot forming process of the backward end-bulkhead of a pressure hull, the blank diameter and the tool clearance are the critical factors which influence wrinkling defect, forming load and shape completeness of the product. Two F.E.A softwares with the elasto-plastic material model and rigid plastic model were utilized to predict the occurrence of wrinkling defect. Tool clearance was determined by considering the increase of blank thickness, die strength and the stretching effect. Heat treatment condition after the hot forming to recover the original properties of the material was estabilished by specimen-based heat treating experiment.