Papers by Author: K.H. Min

Abstract: This paper is concerned with the pressure distribution along the die-powder interface in long parts. The pressure exerted on the interface at various points on the moving and stationary punch, and also on the sidewall of container was investigated by the finite element method. A plasticity theory describing asymmetric behavior of powdered metals in tension and compression was briefly summarized. The yield criterion applied to the sintered powdered metals had been modified for describing this asymmetric behavior. The material properties of copper powders under compaction were also briefly described for the completeness of the paper. The copper powders were selected as a model material in the present study. The main purpose of this study is to investigate the pressure distribution along the interface of tooling quantitatively by the finite element method so that the results could be applied usefully to the design of tooling, especially container design for powdered metal compaction. Geometrical condition for analysis was confined to the Class II components which is very long parts without steps. It was concluded from the simulation results that the pressure exerted on the moving punch increases sharply near the outer circumference of punch and the pressure on the sidewall decreases at a distance from moving punch to fixed punch. It was also seen from the simulation that the pressure on the stationary punch is not significantly built up and decreases toward outer periphery. These trends were seen amplified with severe frictional conditions imposed on the tooling and powder interface.

Abstract: Combined extrusion processes generally have advantages of forming in terms of the minimum deformation power since the material is pressed through two or more orifices simultaneously. This paper is concerned with the analysis of forming load characteristics of a forward-backward can extrusion process using thick-walled pipe as an initial billet. The combined tube extrusion process was analyzed by using a commercial finite element code. A thick-walled pipe was selected as an initial billet and the punch geometry has been chosen on the basis of ICFG recommendation. Several tool and process parameters were employed in this analysis and they are punch nose radius, backward tube thickness, punch face angle, and frictional conditions, respectively. The main purpose of this study is to investigate the effect of process parameters on the force requirements in combined extrusion process. The possible extrusion process to form a forward-backward tube parts in different process sequences were also simulated to investigate the force requirements in sequential operations, i.e. separate operations. It was easily concluded from the simulation results that lower forming load was predicted for the combined extrusion, compared to those for separate sequential operations. It was also revealed that the punch nose radius and the punch face angle have little effect on the force requirements and the forming load increases significantly as the frictional condition along tool-workpiece interface becomes severe. The simulation results in this study suggest that the combined extrusion process has strong advantage in terms of force requirements as long as the simultaneous material flow into multiple orifices could be closely controlled.

Abstract: In this paper, the forming limit of flange in radial extrusion process was analyzed by the rigid-plastic finite element method. The selected model material for simulation and experiments was AA 3105 aluminum alloy. The predictions from simulation were made in terms of axial and circumferential strains. Experiments also have been conducted to compare with the simulation results with regards to deformation pattern. Furthermore, the deformation pattern in forming of flange section was closely investigated and categorized in three cases such as sticking, separating and cracking. The analysis in this paper is focused on the transient extrusion process of material flow into the gap in radial direction for different gap heights and die corner radii. The results of present study were summarized in terms of evolution of surface strains in axial and circumferential directions measured from the finite element meshes located in the region where surface cracking occurred in experiments. The forming limit line was drawn in the relationship of circumferential and axial strain. It was concluded from this study that the forming limit line is influenced mainly by circumferential strain on free surface of flange. It was also predicted that ductile fracture on flange surface is likely to occur in the middle of flange gap under the condition of sticking deformation and near bottom of flange gap under the condition of separating deformation, respectively. The forming limit of flange in terms of flange diameter was expected about 2.5do, which is 2.5 times the diameter of original billet.

Abstract: Fibers made of elements such as carbon, aramid and glass have higher mechanical properties than other conventional textile fibers and they enable the production of light weight composites as end products. Furthermore, commingling hybrid yarns generally have a characteristic feature so that their components are distributed homogeneously enough over the yarn cross section. A normal air texturerising machine was modified to produce commingling hybrid yarns for test samples. Different process parameters were applied to produce the hybridized yarn samples. However, these process parameters turned out to have little effect on the filament distribution over the hybrid yarn cross section in terms of homogeneity. The analysis in this paper is focused on the pattern of mixing of filaments over a cross section of hybrid yarns according to different combinations of reinforcement and matrix filament yarns through microscopic view. The volume content of filament in hybrid yarn cross section was maintained at 50% for both reinforced and matrix, and the hybrid yarns count at 600 tex throughout experiments. It was concluded from the experiments that the diameters of reinforcement and matrix filaments have strong effects on the pattern of mixing of filaments over a cross section of hybrid yarns such that the hybrid yarns with more or less equal diameters of reinforcement and matrix filaments showed considerably even distributions over the hybrid yarn cross section.

Abstract: Geo-composites are generally made by hybridizing of some components among geo-textile, geo-grid, geo-membrane, geo-net, and other materials. Due to practical applicability of geotechnical structures, the demand of geo-composites, especially for drainage application, has gradually increased. In the present study, the geo-composites bonded with geo-grid in chemical process were investigated experimentally in terms of strain evaluation and creep response values. Geo-grid plays an important role as a reinforcing material. Three kinds of geo-grid were prepared as strong yarn polyester and they were woven type, non-woven type, and wrap knitted type. The sample geo-grids were then coated with PVC. The rib tensile strength tests were conducted to evaluate geo-grid products in terms of tensile strength with regard to single rib. The test was performed according to GRI-GGI. The test results revealed that the tensile strains at the maximum tensile strength showed very good tensile deformation characteristics in the range of 10.0-13.0% in terms of mono-rib performance. Any significant trends have not found between warp knitted and woven type geo-grid in terms of the tensile strength ratios. Further experimental analysis has been conducted to investigate the wide-width strip tensile strength, contact point strength and creep features of the geo-grid samples used in this study. It was concluded again from the experiments that the tensile and creep strains of the geo-grid showed so stable values that the geo-grid prepared in this study could protect geo-textile partially in practical structure.

Abstract: This paper is concerned with the analysis of the forming load characteristics of a forward-backward can extrusion in both combined and sequence operation. A commercially available finite element program, which is coded in the rigid-plastic finite element method, has been employed to investigate the forming load characteristics. AA 2024 aluminum alloy is selected as a model material. The analysis in the present study is extended to the selection of press frame capacity for producing efficiently final product at low cost. The possible extrusion processes to shape a forward-backward can component with different outer diameters are categorized to estimate quantitatively the force requirement for forming forward-backward can part, forming energy, and maximum pressure exerted on the die-material interfaces, respectively. The categorized processes are composed of combined and/or some basic extrusion processes such as sequence operation. Based on the simulation results about forming load characteristics, the frame capacity of a mechanical press of crank-drive type suitable for a selected process could be determined along with securing the load capacity and with considering productivity. In addition, it is suggested that different load capacities be selected for different dimensions of a part such as wall thickness in forward direction and etc. It is concluded quantitatively from the simulation results that the combined operation is superior to sequence operation in terms of relatively low forming load and thus it leads to low cost for forming equipments. However, it is also known from the simulation results that the precise control of dimensional accuracy is not so easy in combined operation. The results in this paper could be a good reference for analysis of forming process for complex parts and selection of proper frame capacity of a mechanical press to achieve low production cost and thus high productivity.

Abstract: This paper is concerned with forward rod extrusion combined simultaneously with backward tube extrusion process in both steady and transient states. The analysis has been conducted in numerical manner by employing a rigid-plastic finite element method. AA 2024 aluminum alloy was selected as a model material for analysis. Among many process parameters, major design factors chosen for analysis include frictional condition, thickness of tube in backward direction, punch corner radius, and die corner radius. The main goal of this study is to investigate the material flow characteristics in combined extrusion process, i.e. forward rod extrusion combined simultaneously with backward tube extrusion process. Simulation results have been summarized in term of relationships between process parameters and extruded length and volume ratios, and between process parameters and force requirements, respectively. The extruded length ratio is defined as the ratio of tube length extruded in backward direction to rod length extruded in forward direction, and the volume ratio as that of extruded volume in backward direction to that in forward direction, respectively. It has been revealed from the simulation results that material flow into both backward and forward directions are mostly influenced by the backward tube thickness, and other process parameters such as die corner radius etc. have little influence on the volume ratio particularly in steady state of combined extrusion process. The pressure distributions along the tool-workpiece interface have been also analyzed such that the pressure exerted on die is not so significant in this particular process such as combined operation process. Comparisons between multi-stage forming process in sequence operation and one stage combined operation have been also made in terms of forming load and pressure exerted on die. The simulation results shows that the combined extrusion process has the greatest advantage of lower forming load comparing to that in sequence operation.