Papers by Author: Beom Soo Kang

Abstract: Multi-stage deep drawing process for rectangular cups with extreme aspect ratio using finite element analysis is performed. The process is mainly consists of four forming stages including blanking, drawing, ironing and trimming. However, main deformation of the rectangular cup is completed during the drawing-ironing procedure. Tool design and blank modification for the multi-stage deep drawing process are presented. To consider the deep drawing and the ironing operations, the multi-stage deep drawing process is applied to obtain the rectangular cup by using each numerical simulation models from first to fifth drawing. Based on the design results of the initial blank, the multi-stage deep drawing process is performed, but it is shown that severe earing phenomenon is occurred at the upper flange part. To solve the severe deformation at the upper flange due to normal anisotropy of the used sheet material, initial blank modification is carried out. The simulation results for the rectangular cup are compared with the final configuration before and after the modification of the blank shape. The predicted result is confirmed that the modified blank shape not only improve the quality of a deep-drawn product but also reduce the cost of production.

Abstract: In order to reduce the elastic recovery of a sheet material and eliminate a great number of solid dies used in the forming process of various shapes, a flexible stretch forming process (FSFP) is considered in this study. Especially, the relationship among design variables, such as the punch size, objective radius of curvature, and elastic pad thickness is quantitatively evaluated to find out their respective influences on the shape errors of a formed sheet plate using the statistical method based on the FE simulation result planned by the three-way factorial design. The shape errors are divided into two types based on the material behavior according to the widthwise- and tensile- directions. The correlations of the shape errors and the design variables are estimated through the Pearson correlation analyses. The punch size has a strong positive linear correlation with the widthwise- and tensile- shape errors, and the correlation between the objective curvature radius and tensile-direction shape error is weak and negative. Although the effect of the elastic pad thickness is less than those of the other variables, it prevents effectively surface defects. Subsequently, the mathematical model is assumed to clarify their relationship. Two regression equations are estimated in terms of the design variables regarding the widthwise- and tensile- shape errors. The shape errors could be inferred by the assumed model in the particular combination of the design variables; then, the acceptable punch size and elastic pad thickness can be determined according to the objective curvature radius.

Abstract: The ability to predict the forming severity with respect to crack and failure is essential to analysis of sheet metal forming process. The forming limit diagram (FLD) is commonly used to gauge the formability of sheet metal. In this article, forming limit diagrams of cold rolled carbon steel (JIS-SPCC), which widely used to produce the parts of automobile, are obtained by performing experiment and FE simulation with the Nakajima-test. By using the GTN (Gurson-Tvergaard -Needleman) damage mechanical model, a failure criterion based on void evolution was examined in this FE simulation. The parameters of GTN model are determined through comparison of experimental and numerical result with Nakajima-test. These parameters acceptably can be used in GTN model using given material. In application case, the reliability of the GTN model for failure criterion in simulation with automotive part was confirmed.

Abstract: Metallic sandwich panels based on lattice cell structures have been developed for a wide range of potential applications with their lightweight and multi-functionality. Structural performance of sandwich panels can be predicted from the studies on the mechanical properties of a unit cell. Numerical investigations on the unit cell can provide efficient guidelines for the design of overall core structures for a specific application. When any types of external forces are applied on the sandwich panel, each truss member of the unit cell undergoes severe plastic deformation without any restrictions so that the deformation behavior is strongly dependent on mesh density and element type. Therefore, in order to improve simulation accuracy and minimize calculation time, it is necessary to investigate the influence of element type and mesh density on that. In this work, as the preparatory stage to predict the mechanical behavior of a pyramidal unit cell, a series of finite element simulations for various element sizes and types were carried out. The influence of mesh density and element type on the simulation accuracy was investigated in diverse aspects; calculation time, resultant load, deformed geometry, effective modulus and peak stress.

Abstract: In this work, in order to predict the forming failure of AZ31 magnesium alloy sheet in drawing process at elevated temperatures, a series of square cup tests at various temperatures and FE analyses were carried out. The critical damage values and the mechanical properties dependent on strain rates and temperatures were evaluated from uniaxial tensile tests and those were utilized to the forming failure prediction using FE analysis. Based on the plastic deformation history obtained from FE analysis and Cockcroft and Latham’s ductile fracture criterion, the fracture initiation time and location were predicted and verified with the experimental results.

Abstract: This study is dedicated to three-dimensional finite element analysis of seaming process, which consists
of bending, curling and caulking process, of a large tubular mechanical bonded structure. The seaming process is often used to improve a high bonding strength as avoiding any kind of defect. Finite element simulations of the seaming process were preformed for two different initial conditions with pre-analyzed results and without those from bending process. The mechanical bonding strength of the seamed area in the large tubular structure was estimated and compared through finite element analysis among several different analysis conditions of the bending and the caulking. Tensile test for the specimen extracted from the large tubular mechanical bonded structure was also executed and compared with the results of finite element analysis, in order to verify which initial condition in finite element analysis was suitable for this kind of multi stage seaming process. As a result, the effect on an accuracy of finite element analysis for the multi stage seaming process was evaluated in this study. Finally, it is noted that the pre-analyzed results from bending process should be considered in order to obtain the accurate results from finite element analysis.

Abstract: Mechanical damping systems have been widely used to various industrial structures and are mainly hydraulic and pneumatic devices nowadays. This article presents an experimental investigation of a nano colloidal damper. Particularly for colloidal damper, the hydraulic oil is replaced by a colloidal suspension, which is consisted from a nano-porous matrix with controlled architecture and a lyophobic fluid. Nano colloidal damper test rig and the measuring technique of the hysteresis were described in this study. Influence of the water volume and particle diameters upon the nano colloidal damper hysteresis was investigated. As a result, the proposed nano colloidal damper (NCD) is proved as an effective one, which can be replaced for the conventional hydraulic damper.

Abstract: The main goal of this study was to develop the technique of process design and manufacturing for a rectangular deep drawn cup with very narrow width by using finite element analysis scheme and a series of experiments. The manufacturing process of this rectangular cup required several intermediate steps to generate the final shape. The multi-stage deep drawing process was applied to finite element analysis, and a continuous progressive press was employed in a series of experiments. Final specifications of the rectangular deep drawn cup were length of 33.70mm, height of 48.30mm and width of 3.46mm, respectively. In this study, finite element analysis for this drawing process was carried out from the first to the seventh stage, and a series of practical experiment was performed. These simulated results of the rectangular cup were compared with the prototypes of the experiments in view of the deformed shape in each mid-part. The results of finite element analysis showed good agreement with those from the experiments.

Abstract: Preform design in tube hydroforming implies the design of an intermediate shape between initial tube and the final product enabling to be fabricated without defects and excessive loss of material. A carefully selected preform can contribute significantly to reduce production cost and improve formability, since thinned sections may not be able to endure internal pressure during expansion whereas excessive thickening may lead to wrinkles. Generally, preform design in hydroforming was mainly carried out through the trial-and-error approach. Even though a series of numerical simulations for several predetermined preformed shapes were conducted, optimum configuration could not be obtained and could not be suggested the general procedure for preform design as well. In this work, a simple numerical approach to the preform design for formability enhancement was introduced based on the deformation history during forward hydroforming simulation. The proposed approach was implemented to a hydroforming process of an automobile subframe component in order to be satisfied the required specification after hydroforming, and the conceptual application has been proved to be successful on its effectiveness and feasibility. Therefore, it is shown that preform design approach proposed in this study will provide one of feasible methods to satisfy the increasing practical demands for improvement of the formability in hydroforming processes.

Abstract: Loading path is one of the most influential parameter in tube hydroforming(THF) process. Load history has a major effect on failures such as buckling, necking, bursting, and so on. Because loading conditions that consist of axial feeding and internal pressure are imposed simultaneously. Therefore suitable loading path should be determined to prevent onset of failures i.e. bursting on final products. This paper deals with the procedure on determination of the loading path in order to ensure the robustness of the final products after the THF. In order to verify the availability and feasibility of the proposed methodology a subframe model of engine cradle module in automotive is implemented. In this study, thinning ratio and forming limit stress diagram is used to demonstrate the improvement of the finished product. The result shows that the developed algorithm has successfully promoted the effectiveness and feasibility in the THF. Consequently, it is shown that the automatic approach on the determination of loading condition which is proposed in this paper will provide a valuable method to satisfy the increasing practical demands for designing process condition in THF.