• Proceedings of the Korean Geotechical Society Conference
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• 2000.03b
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• pp.317-324
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• 2000

In this study, ten drilled shafts were constructed for evaluating the application of NDT(Non-Destructive Testing) techniques. The drilled shafts, 0.4 m in diameter and 7.0 m in length, were constructed at Namyangju site in Namyangju City. One of the shafts was constructed with no defect, and the other shafts were constructed with the defects of soft bottom, necking, bulging, cave-in and/or weak concrete. Then, these techniques were applied to the bridge foundations for studying unknown bridge foundation characteristics.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.11
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• pp.2253-2261
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• 2002

Recently the hydroforming technology has drawn a lot of attention because of its capability to produce high quality and light weight parts. In the present study, the tube expansion - one of the simplest hydroforming processes, has been investigated in order to understand fundamental phenomena such as deformation characteristics and effect of process parameters. As a result, the most important process parameters, which determine the state of stress at the expanded zone, were found to be pressure and die displacement. If the stress becomes equi-axial tension at the zone, necking occurs at some distance from the weld line and develops into a crack along the axial direction. Some aspects of mechanical property measurements as well as distributions of hardness and microstructure are also discussed in this paper.

• Proceedings of the KSME Conference
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• 2004.11a
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• pp.1075-1079
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• 2004

Steam was vertically discharged into water through mini nozzles of various diameters (d=0.115, 0.520, 1.55mm). The condensation was observed and categorized into several types of condensation regimes for each of the nozzles. Compared with the regimes in the previous researches, the regimes of 'internal necking with attached bubble' and 'internal chug with detached bubble' were newly observed. Depending on a nozzle, some regimes expanded, shrank, or moved in the regime map. For the nozzle of 1.55mm, the regime map was similar to Chan and Lee (1982) except that the regime of 'internal chug' was not observed. For the nozzle of 0.115mm, the regime of 'internal chug' appeared even at high pool temperature.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 2002.04a
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• pp.579-584
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• 2002

In press forming of sheet metals, the material sheet is usually subjected to very large plastic strain under in-plane stressing. Moreover, the sheet also very often is subjected to out-of-plane compressive force between tools such as the upper and lower dies, the blank holder and the die, and so forth. In this paper, it is clearly demonstrated theoretically that out-of-plane stress may notably raise the forming limit strain and thus it cm be effectively utilized to avoid earlier fracture of the sheet in press forming.

• Journal of Welding and Joining
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• v.21 no.1
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• pp.42-47
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• 2003

Stainless rolling stocks are usually fabricated by spot welding process without painting. Indentation on the surface of the car body after spot welding injures the beauty of the stainless rolling stocks. In this study, insulated electrode tips to reduce the indentation are developed and applied to the actual spot welding works. The developed tips are composed of head, neck, hole for cooling water, body and resistance material. They provide large surface contact area with the base materials during spot welding and enhance the current density by necking. Experimental results using the developed tips show that small indentation and sufficient tensile shear strength is produced due to large contact area and enhanced current density.

• Journal of the Korean Society for Precision Engineering
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• v.17 no.2
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• pp.169-175
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• 2000

The object of this study was to investigate the feasibility of solid-phase forming of the composites and to characterize the material behavior in the biaxial stretch forming. The materials tested contained 20%, 30%, and 40% glass fibers by weight in a polypropylene matrix. Biaxial stretch forming tests were performed at three forming speeds of 10mm/sec, 1mm/sec, and 0.1mm/sec and at four forming temperatures of $75^{\circ}C, 100^{\circ}C, 125^{\circ}C, and 150^{\circ}C$ to investigate effects of forming speed and forming temperature. The microscopic observation of a formed part was conducted at various strain levels to characterize the material behavior. The strain distribution on a formed part was measured and displayed on the farmed geometry with a contour display The material behavior of the composite in the biaxial stretch forming was strongly influenced by the forming conditions.

• Proceedings of the KSME Conference
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• 2007.05a
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• pp.642-647
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• 2007

Determination of loading conditions for tube hydroforming(THF) process that implies an amount of the increment in axial feeding and internal pressure for each step is one of the most important constituents at the process design level. On account of the fact that those design factors mentioned above are imposed simultaneously during the process, suitable loading conditions are required to obtain robust products without any failure such as buckling, necking, bursting and so on. In which, especially, bursting is well known as the most frequently occurred failure in general THF process. In this study, therefore, determination of the loading condition based on the adaptive method was carried out to obtain safe loading paths. In addition, forming limit curves are applied to evaluate the derived loading conditions by using the simulation results. Consequently, it is found that described method in this study for THF process design is useful and has a feasibility.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2000.10a
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• pp.49-52
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• 2000

The hole flanging experiments are performed on a flat circular plates with a hole in the center and the flangeability and fracture behavior of TRIP steel and ferrite-bainite duplex steel were examined. Over the ranges of conditions investigated, the flangeability of ferrite-bainite duplex steel is better than TRIP steel and the term 'hole flanging capacity' introduced in this study. Fracture behavior of TRIP steel and ferrite-bainite duplex steel was characterized by petal formation at the edge of flange. In case of crack propagation, crack tip diversion that is supposed to be responsible for flangeability occurs more severely on HSLA Steel.

• Transactions of Materials Processing
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• v.10 no.6
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• pp.465-470
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• 2001

Hole flanging experiments are performed on flat circular plates with a hole in the center and the flangeability and fracture behaviors of TRIP steels and ferrite-Bainite duplex steels were examined. In the hole flanging, deformation by lip and petalling occurs when plates are struck by punches of various shapes and high circumferential strains induced in the target material cause radial cracking and the subsequent rotation of the affected plate material in a number of symmetric petals. In all cases, failure of the plate was due to lip fracture that results from multiple localized neckings that take place around the hole periphery where straining is most severe and a somewhat regular pattern was observed in a fracture shape. The neck characteristics in flange formation and the transition from the lip to petal mode at which fracture occurs were compared with two materials.

• Transactions of Materials Processing
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• v.24 no.5
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• pp.348-353
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• 2015

The PEPS(Polar-coordinated Effective Plastic Strain) FLD(Forming Limit Diagram), a new type of FLD based on a polar representation of the EPS(Effective Plastic Strain), appears to be an effective solution to the problem of non-linear strain path effects. This method has the advantages of the familiar strain-based diagram for linear loading, but without the strain-hardening limitations of the stress-based diagram, or non-intuitive aspects of alternate Cartesian diagrams based on effective plastic strain. In the current study, the PEPS FLD was applied to the development process of an aluminum automobile-body panel, including the necking or crack prediction, die design, and die modification. As a result, the PEPS FLD provided improved formability of aluminum sheet as compared to deriving the potential formability with non-linearity.