• Journal of the Korean Society for Nondestructive Testing
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• v.12 no.3
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• pp.7-11
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• 1992

Accurate measurement of green density of compacted part in the powder metallurgy industry is rather fundamental but extremely important process that decide the quality of the sintered part. In case of green sheet P/M product, the green density as well as the distribution of the density must be examined for the same reasons. Currently in most cases, density measuring process is being performed applying conventional Archimedes principles. However this method is not only time-consuming but also often inaccurate because of the inherent nature of the process, such as part sectioning, closing of surface porosity with wax and weighing in air and in water. Therefore, it is necessary to develop a faster and more accurate method to measure the density of green sheet P/M product. In this work, a nondestructive density measurement device using gamma-ray absorption principles was constructed and the optimum condition for measuring green density of P/M sheet and its distribution was sought. The results showed that this method was very effective in terms of measuring time and accuracy.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.05a
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• pp.477-479
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• 2008

Drawability and other mechanical properties of sheet metals are strongly dependent on their crystallographic orientations. In this paper the formability of the AA 5052 Al alloy sheets was investigated after asymmetric rolling and subsequent heat treatment. In most cases, after asymmetric rolling specimens showed a fine grain size and subsequent heat treated specimens showed that the ND//<111> texture component were observed. The anisotropy of formability is often described by the plastic strain ratios (r-value) as a function of the angle to the rolling direction in sheet metal. For a good formability, a high r-value is required in sheet metals. In the asymmetry rolled and subsequent heat treated Al alloy sheet, the variation of the plastic strain ratios have been investigated in this study. The plastic strain ratios of the asymmetry rolled and subsequent heat treated AA 5052 Al alloy sheets were higher than those of the original Al sheets. These could be related to the formation of ND//<111> texture components through asymmetric rolling in Al sheet.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.21 no.12
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• pp.2209-2219
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• 1997

A split hopkinson bar could be used for obtaining the high strain rate material properties of sheet metals for an autobody. In high speed tensile tests of sheet matals, a new design of a tension split Hopkinson bar apparatus is needed. The design of grips and an anvil length are numerically analyzed with ABAQUS/Explicit for the new apparatus of split Hopkinson bars. From the experiments with the new apparatus, the material properties of SPCEN in the high strain rate state have been acquired and compared with quasi-static experimental results. The material properties of SPCEN as well as other sheet metals in an autobody are indispensible for the analysis of crashworthness. Nevertheless the experiment of sheet metal in the high strain rate state has not been done or reported.

• 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.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 2004.10a
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• pp.162-169
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• 2004

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 can be effectively utilized to avoid earlier fracture of the sheet in press forming.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 2003.04a
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• pp.340-345
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• 2003

By the use of a similar numerical method as that in the previous paper, the forming limit strain by coating method of clad sheet metals is investigated, in which the FEM is applied and J2G(J2-Gotoh's corner theory) is utilized as the plasticity constitutive equation. Clad two-layer sheets and sheets bonded with dissimilar sheets on both surface planes are stretched in a plane-strain state, with various work-hardening exponent n-values and thicknesses of each layer. Processes of shear-band formation in such composite sheets are clearly illustrated. It is concluded that, in the clad state, the higher limiting strain of one layer is reduced due to the lower limiting strain of the other layer and vice versa, and does not necessarily obey the rule of linear combination of the limiting strain of each layer weighted according thickness.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1996.10a
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• pp.38-47
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• 1996

Sectional analysis program for plane strain or axisymmetric geometry of aluminum alloy sheet metals was developed. For modeling the anomalous behavior of aluminum alloy, Barlat's strain rate potential and Hill's 1990 non-quadratic yield theory arranged under the plane stress assumption were employed. 2-D rigid-viscoplastic FEM formulation based on the bending-augmented membrane theory was derived, solving simultaneously force equilibrium as well as non-penetration condition. Isotropic hardening law was also assumed for yielding behavior. To verify the validity and availability of the developed program, 2-D stretch/draw forming process for plane strain geometry and cylindrical cup deep drawing process for axisymmetric geometry were simulated.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1998.06b
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• pp.8-18
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• 1998

The effect of press warm forming in cylindrical deep drawing of stainless-aluminum clad sheet metals are examined . The temperature of die and blank holder is varied from room temperature to 20$0^{\circ}C$, while the punch is kept cooled during test to increase the fracture strength of workpiece on the punch corner area. Test materials chosen for experiments are STS304-Al050-STS304, STS304-A1050-STS430-, STS304 and Al050 metals and teflon film as a lubricant is used on both sides of a workpiece. The limit drawing ration as well as quality of drawn cups (distribution of thickness and hardness)are investigated and discussed.

• Transactions of Materials Processing
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• v.5 no.4
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• pp.320-326
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• 1996

Based upon experimental observations the authors have shown in the previous studies that the orientations of orthotropy axes of anisotropic sheet metals are subjected to change during tensile loading at angles to the rolling direction. To predict the rotations of orthotropy axes under general plane stress conditions, a simple phenomenological model is proposed which accounts for the effect of work hardening. Predictions from the model are compared against the experiments for 0%, 3%, and 6% of 1st tensile prestrains in the rolling direction and 2nd tensile prestrains at 30$^{\circ}$, 45$^{\circ}$ and 60$^{\circ}$ to the 1st prestrains axis. The model showed good agreements with the experimental observations. A new interpretation of the experimental data is suggested regarding the rotations of orthotropy axes.

• Transactions of Materials Processing
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• v.7 no.1
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• pp.81-93
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• 1998

Identification of forming limits of sheet metals is an important task to be done before the sheet metal forming processes. The information of the forming limit is indispensable for design of deformed shapes and related forming processes. This procedure becomes more important than ever as the auto-body becomes complicated and the number of auto-body parts is reduced for lower production cost. To identify the forming limit of sheet metals stretching with a hemispherical punch has gained popularity because of the convenient experimental procedure. The stretching experiment however has localized deformation or the shear band is originated from the non-unifrom deformation in the critical circum-stance instead of the absolute criterion. More accurate information of the forming limit therefore could be obtained by a more appropriate experiment to the real process. In this papaer an experiment program is devised to practivally identify the forming limits of sheet metals for auto-body parts. The experiment program contains not only stretching but deep-drawing Both forming experiments use the same hemispherical punch while they use different specimens. Deep-drawing experiments use speci-mens cut out in circular arc on both sides of circular blank to make it torn during the deep-drawing They also use speciments cut out straight in one side of a circular blank to make it deformed unevenly which causes local deformation during the deep-drawing. The experimental result demonstrates that the forming limit diagrams in the two cases show difference in their effective magnitude. The forming limit curve from deep-drawing is located lower than that from stretching. It is noted from the result that the deep-drawing process causes acceleration of localized deformation in comparison with the stretching process. From the experimental result the maximum value of forming limit could be pre-dicted for safe design.