Papers by Author: Beong Bok Hwang

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: This paper is concerned with the analysis of plastic deformation of bimetal co-extrusion process. Extrusion is related to large deformation of material and leads to non-homogeneous deformation within work-piece material. The mechanism of plastic deformation during the composite rod extrusion is much more complicated than that in single metal extrusion. Deformation patterns of co-extrusion of two different materials are characterized by several process parameters. In this paper, the analysis is focused to investigate the effect of contact conditions along the interface between two different materials. The rigid-plastic finite element method was applied to the analysis of co-extrusion process. The selected materials are AA 1100 aluminum alloy as hard material and CDA 110 as soft one. This type of material selection was to examine the effect of hard core and soft sleeve and vice versa in terms of deformation pattern. The initial composite billets were prepared by inserting the core material in tight (0.023mm) and weak (0.012mm) interference bonding, respectively. Four different cases of co-extrusion process in terms of material combination and interference bonding were simulated to investigate the effect of material arrangement between core and sleeve, and of bonding on the plastic zones. It is concluded from the simulation results that the plastic zones in this co-extrusion process are not influenced much by the selection of material arrangements or bonding condition between construction materials. However, it was seen from the simulation results that the extrusion ratio of each construction material, i.e. homogeneity of co-extrusion, depends much on the material arrangement and the bonding condition.

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: This paper is concerned with the long-term performance of geo-textile (GT) composites in terms of creep deformation and frictional properties. Composites of PVA GT and HDPE GM were made to investigate the advanced properties of long-term performance related to waste landfill applications. The same experiments were also performed for typical polypropylene and polyester GT and compared to PVA GT/HDPE GM composites. The main purpose of this study is to develop high performance GT composites with GM by using PVA GT which is capable of improving frictional property and thus enhances long-term performance of GT composites. In the present experiments, GT composites of PVA GT/HDPE GM, PVA GT of 600, 1000, 1500, 2000g/m2 and HDPE GM were prepared in thermal bonding process. Polyester and polypropylene GT were also made in needle punching process. The creep deformation of GT composites was measured and evaluated in accordance with ASTM D5262. Frictional characteristics of GT composites tested in this study were conducted with compact direct shear apparatus in accordance with ASTM D5321. It was concluded from the present experimental study that friction coefficient of GT composites is relatively large compared with those of polyester and polypropylene non-woven GT as long as the friction media has similar size to the particles of domestic standard earth. In the event that 20% of the maximum tensile strength was added to polypropylene and polyester non-woven geo-textiles, creep deformation reached to 10% or higher, making it even impossible to find reduction factor.

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: This paper is concerned with the performance of geo-textile (GT) against chemical condition. GT is generally adopted for the upper part of geo-membrane (GM) for waste landfills and thus it is very important to consider the performance of GT against certain chemical environments until landfill is completed. In this study, PVA geo-textile/HDPE geo-membrane was prepared to investigate the waste landfill related properties in terms of long-term performance against chemical conditions imposed. GT composites of PVA GT/HDPE GM, PVA GT and HDPE GM were produced in thermal bonding process. Polyester and polypropylene GT were also manufactured in needle punching process. The experiments have been conducted under a modified version of EPA 9090 test method which is very similar to the method of evaluating chemical resistance of flexible membrane liner by the US Environmental Protection Agency (EPA). In this testing method, samples immersed in chemical of different solutions up to 150 days at 30 day interval were obtained to find tensile strength holding rate and chemical resistance. The analysis in this paper is focused to evaluate the effect of different pH conditions and temperature environments on geo-synthetics weights strength retention. It was concluded from the experiments that tensile strength of GT composites against leachate were reduced by 10 to 20% in both polypropylene and polyester non-woven GT. The reduction was more significant at temperatures of 50 °C than that at 25 °C. The experiments conducted in this study demonstrated that PVA GT is excellent in terms of chemical resistance.

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: Carbon, aramid and glass fibers are inherently superior to conventional textile fibers in terms of mechanical properties as well as other chemical characteristics. Because of inherent advantages and disadvantages associated with each material, it is generally better to hybridize them to fully benefit of their high performance in many practical applications. In this paper, the possibility of hybridizing Carbon/Aramid-, Carbon/Glass- and Aramid/Glass- matrices has been investigated through the commingling process. In the experiment, several process parameters were selected and they include pressure, yarn oversupply-rate and different nozzle types. As a result of experiments, it was concluded that the hybridized materials has shown better performance than individual reinforced filament yarns in terms of mechanical properties. For small tensile forces, the Carbon/Glass/matrix combination turned out to be good enough for general purpose applications. However, for high tensile applications, Carbon/Aramid or Aramid/Glass with matrix combinations was better than the other material combinations. The hybridization process was also investigated under an air pressure of 5 bar, a yarn oversupply-rate of 1.5% for reinforced filaments, and 3.5% to 6% for matrix materials, respectively. It was also shown from the experimental results that Carbon/Glass/matrix combination may be desirable for small tensile force applications and Carbon/Aramid/matrix and Glass/Aramid/matrix combinations most suitable for heavy tensile force applications, respectively. As a matrix material, polypropylene and polyester have shown better performance than polyether-ether-keeton in terms of tensile property.

Abstract: This paper is concerned the breaking elongation properties of Carbon/Aramid-, Carbon/Glass- and Aramid/Glass- matrix hybridized commingling yarns. The hybrid yarns produced by commingling process were investigated in terms of breaking elongation property. In experiments, carbon (CF), aramid (AF), and glass (GF) filament yarns were combined. In this study, selected matrix materials include Polyether-ether-Keeton (PEEK), and polyester (PES), or polypropylene (PP). The volume content of filament in hybrid yarn cross section was maintained at 50% for both reinforced and matrix, ant hybrid yarns count at 600 tex, respectively. The reinforcement to matrix filament combination was selected as 1:1 proportion. The effect of different air pressures and material combinations was investigated in terms of breaking elongation. In experiments, each type of hybrid yarn sample has been tested 20 times at the testing speed of 10mm/min. under 3 bar of yarn clamping pressure. Since breaking elongation is one of most important properties in textile fiber, it was examined closely with reference to the first breaking point of commingling-hybrid yarns. It was concluded from experiments that hybrid yarns with higher breaking elongation and higher tensile strength tend to show better force-elongation relationship. It was also known from experiments that the combination of two reinforcement filament yarns gives always much better results than a single reinforcement filament yarns in terms of elongation property. GF/AF/matrix is shown very much better elongation properties. PP and PES gives higher elongation than PEEK as a matrix material.