• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1994.03a
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• pp.55-62
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• 1994

For theoretical estimate of closed-die forging pressure, upper-bound method is applied to the involute tooth profile. In the analysis, the deformation regions have been divided into several zones. A constant frictional stress has been assumed on the contacting surfaces. Utilizing the formulated velocity fields, numerical calculations have been carried out to investigate the effects of various parameters, such as module, number of teeth and friction factor, on the forging of spur gears. Very close agreement was found between the predicted values of forging load and those obtained from experimental results.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1996.04a
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• pp.761-765
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• 1996

This paper describes the forging of circular tooth profiled gears as a series of development of simulator for non-axisymmetric parts that being used at the pump pulley, timing belt pulley etc. in automobiles. The half pitch of gear is divided into 6 deformation regious and kinematically admissible velocity fields for those regions are proposed. The neutral surface is introduced torepresent inner flow of material during forging operation with flat punch and, for each step, it is assumed as a circle. The upper bound solutions obtained from the suggested kinematically admissible velocity fields are in good agreement with experimental results and they are useful to predict the capacity of forging press for forging of circular gears.

• International Journal of Precision Engineering and Manufacturing
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• v.1 no.2
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• pp.94-104
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• 2000

A kinematically admissible velocity field for near-net shape forging of a crown gear form is proposed. This takes into account the profiled shape of the teeth chosen by approximating these kinematically as radially straight taper teeth, (rectangular and trapezoidal teeth). The upper bound to the forging load, the relative forging pressure and the deformed configurations, with both the initially solid circular cylindrical and hollow billets, are predicted using the velocity field at varying incremental punch movements considering differing frictional factors. These and other results are given and commented upon.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1996.11a
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• pp.134-138
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• 1996

A simple kinematically admissible velocity field for closed-die forging of clutch teeth is analysed which takes account of the profiled teeth chosen kinematically by approximating these as straight taper teeth. The upper bound load and the deformed configurations are predicted by the velocity field at varying punch movements considering differing frictional factors. Experiments were carried out using a model material of plasticine at room temperature where talcum powder was used as a lubricant. The theoretical predictions of the forging load and the relative pressures are found to be in reasonably good agreement with the experimental results.

• Journal of the Korean Society for Precision Engineering
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• v.16 no.9
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• pp.89-95
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• 1999

The compression behavior of semi-solid materials in studied from a viewpoint of yield criteria and analysis methods. To describe the behavior of materials in semi-solid state, several theories have been proposed by extending the concept of plasticity of porous compressible materials. in the present work, the upper-bound method and the finite element method are used to model the simple compression process using yield criteria of Kuhn and Doraivelu. Segregation between solid and liquid which cause defect of product is analysed for Sn-15%Pb alloy is compared with the experimental result of Charreyron et al..

• Journal of the Korean Society for Precision Engineering
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• v.12 no.11
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• pp.91-97
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• 1995

In this study, a deformation model for the regular polygonal-shaped tubes from hollow-cylindrical billets is proposed and a kinematically admissible velocity field is obtained from this deformation model. The final stage upper-bound extrusion load and the average extruded length are determined by minimizing the total power consumption with respect to chosen parameters. Experiments have been carried out with hard solder billets at room temperature. The theoretical predictions of the extrusion load are in good agreements with the experimental results and there is generally reasonable agreement in average extruded height between theory and experiment.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.21 no.9
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• pp.1364-1372
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• 1997

In this paper, the forging of helical gears has been investigated. Punch is tooth-shaped as is the die insert. The punch compresses a cylindrical billet placed in a die insert. As a consequence the material of billet flows into the tooth region. The forging has been analysed by using the upper-bound method. A kinematically admissible velocity field has been developed, wherein, an involute curve has been introduced to represent tooth profile of the gear. Numerical calculations have been carried out to investigate the effects of various parameters, such as module, number of teeth, helix angle and friction factor on the forging of helical gears. Some forging experimentswere carried out with aluminum alloy to show the validity of the analysis. Good agreement was found between the predicted values of the forging load and obtained from the experimental results.

• Transactions of Materials Processing
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• v.8 no.4
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• pp.374-383
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• 1999

To determine the flow stress of semi-solid materials, a new combined method has been studied by experimental and analytic technique in the current approach. Using backward extrusion experiment and its numerical analysis, the characterization scheme of semi-solid materials according to the change of initial solid volume fraction has been proposed. Because that solid volume fraction is sensitive to temperature change, it is required to precisely control the temperature setting. Model materials can guarantee the establishment of material characterization technique from the noise due to temperature change. Thus, clay mixed with bonded abrasives was used for experiment and the change of initial solid fraction was copied out through the variation of mixing ratio. Upper bound method was adapted to increase in efficiency of the calculation in numerical analysis and new kinematically admissible velocity field was employed to improve the accuracy of numerical solution. It is thought that the material characterization scheme proposed in this study can be applied to not only semi-solid materials, but also other materials that is difficult to obtain the simple stress state.

• Proceedings of the Korean Geotechical Society Conference
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• 2001.03a
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• pp.41-48
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• 2001

In this study, two factors are simultaneously considered for assessing tunnel face stability: one is the effective stress acting on the tunnel face calculated by upper bound solution; and the other is the seepage force calculated by numerical analysis under the condition of steady-state groundwater flow. The seepage forces calculated by numerical analysis are compared with the results of a model test. From the results of derivations of the upper bound solution with the consideration of seepage forces acting on the tunnel face, it could be found that the minimum support pressure for the face stability is equal to the sum of effective support pressure and seepage pressure acting on the tunnel face. Also it could be found that the average seepage pressure acting on the tunnel face is proportional to the hydrostatic pressure at the same elevation and the magnitude is about 22% of the hydrostatic pressure for the drainage type tunnel and about 28% for the water-proof type tunnel. The model tests performed with a tunnel model had a similar trend with the seepage pressure calculated by numerical analysis. From the model tests it could be also found that the collapse at the tunnel face occurs suddenly and leads to unlimited displacement.

• Proceedings of the Korea Water Resources Association Conference
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• 2016.05a
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• pp.201-201
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• 2016

The asymptotic extreme value distributions of maxima are a natural choice when designing against future extreme events like flood peaks or wave heights, given a stationary time series. The generalized extreme value distribution (GEV) is often utilised in this context because it is seen as a convenient single expression for extreme event analysis. However, the GEV has a drawback because the location of the distribution bound relative to the data is a discontinuous function of the GEV shape parameter. That is, for annual maxima approximated by the Gumbel distribution, the data is also consistent with a GEV distribution with an upper bound (no lower bound) or a GEV distribution with a lower bound (no upper bound). A more consistent single extreme value expression for design purposes is proposed as the Weibull distribution of smallest extremes, as applied to transformed annual maxima. The Weibull distribution limit holds here for sufficiently large sample sizes, irrespective of the extreme value domain of attraction applicable to the untransformed maxima. The Gumbel, Type 2, and Type 3 extreme value distributions thus become redundant, together with the GEV, because in reality there is only a single asymptotic extreme value distribution required for design purposes - the Weibull distribution of minima as applied to transformed maxima. An illustrative synthetic example is given showing transformed maxima from the normal distribution approaching the Weibull limit much faster than the untransformed sample maxima approach the normal distribution Gumbel limit. Some New Zealand examples are given with the Weibull distribution being applied to reciprocal transformations of annual flood maxima, where the untransformed maxima follow apparently different extreme value distributions.