Papers by Author: Fuh Kuo Chen

Abstract: With the ongoing development of product process, there is a growing demand on micro products. Though the macro-drawing process has been well-developed, the design concepts may not be directly applicable to the micro-drawing due to the size effect occurred in the micro-forming processes. In the present study, experiments were conducted first to establish the stress-strain curves, r-values and work hardening exponents of 304 stainless steel sheets with different grain sizes. The experiment results reveal that the stress-strain and r-value become smaller and the work hardening exponent increases for larger grain sizes. The difference between stress-strain curves in various directions of 0°, 45° and 90°, respectively, is significant when the grain size increases. The stamping of a vibration motor shell of cell phone, which bears a circular cylindrical shape, was also examined in the present study. The finite element simulations were performed to evaluate the formability of the multi-stage drawing process with initial die design. The forming characteristics were identified and an optimum die design was then developed with the use of the finite element analysis. The stamping process with multi-stage tooling design based on the finite element analysis was implemented and the actual stamping experiments were conducted to verify finite element analysis. The experimental results confirm the validity of the modified tooling design and the efficiency of the finite element analysis.

Abstract: In the present study, the Bauschinger effect exhibited in the advanced high strength steel under cyclic bending and reversed bending deformation was examined by both the experimental approach and the finite element analysis. The cyclic tension-compression tests were first conducted for the DP590 steel sheet to determine the material constants required in the Yoshida-Uemori model used in the finite element simulations. Since the deformation mode occurred in the reversed bending tests is similar to that presented in the sheet metal passing across the draw bead or die corner, a three-point reversed bending test apparatus was also developed and the experiments were conducted in the present study. The reversed bending test results clearly demonstrate that the Bauschinger effect presents in the reversed bending process. It confirms that the cyclic reversed bending tests can be applied to examine the Bauschinger effect exhibited in the sheet metal forming process. The finite element analysis was also performed to simulate both the U-hat bending and cyclic reversed bending processes. The comparison of the simulation results with the experimental data reveals that the finite element predictions in both springback and reversed bending load are more accurate if the Yoshida-Uemori model is adopted. It implies that consideration of the Bauschinger effect is necessary in the sheet metal forming if a reversed loading path is present during the forming process.

Abstract: Due to the complexity of hot stamping mechanism, including the coupling of material formability, thermal interaction and metallurgical microstructure, it makes the process design more difficult even with the aid of the finite element analysis. In the present study, the experimental platforms were developed to measure and derive the friction and heat transfer coefficients, respectively. The experiments at various elevated temperatures and contact pressures were conducted and the friction coefficients and heat transfer coefficients were obtained. A finite element model was also established with the experimental data and the material properties of the boron steel calculated from the JMatPro software. The finite element simulations for the hot stamping forming of an automotive door beam, including transportation analysis, hot forming analysis and die quenching analysis were then performed to examine the forming properties of the door beam. The validation of the finite element results by the production part confirms the efficiency and accuracy of the developed experimental platforms and the finite element analysis for the process design of hot stamping.

Abstract: The Bauschinger effect exhibited in the advanced high strength steel sheets was examined by conducting cyclic tension-compression tests. The experimental device for performing the cyclic tension and compression tests with a single sheet specimen was developed in the present study. The experimental apparatus consists of a novel constraint jig that could prevent the sheet specimen from buckling during the compression test. The efficiency of the developed clamping device was validated by both the finite element analysis and the cyclic tension and compression tests conducted in the present study. The test results reveal that the advanced high strength steel exhibits significant Bauschinger effect. It is also confirmed that the finite element prediction of springback present in the stamping of advanced high strength steel sheets is more consistent with the experimental results if the Bauschinger effect is considered.

Abstract: Due to the requirement of lightweight in the automotive body structure design, the application of advanced high strength steel (AHSS) has been widely adopted in the automotive industry. However, the technical difficulties are also experienced in the forming process of stamping the advanced high strength steel. One of the major defects is springback. In this study, both the experimental approach and the finite element analysis were adopted to examine the springback phenomenon occurred in the stamping of a front bumper inner made of 590Y advanced high strength steel. The die compensation approach was employed to adjust the amount of springback to make the dimension of the automotive part conforming to the design specification. The accurate dimension of the production part validates the finite element analysis and the die compensation approach adopted in the present study provides a useful guideline for improving the springback defect in the stamping of advanced high strength steel sheets.

Abstract: In the present study, a set of novel clamping apparatus that could deliver biaxial stretching motions with the use of a uniaxial tensile testing machine was designed and manufactured. The conversion of uniaxial motion into biaxial stretching motions is achieved by a sliding mechanism that consists of two blocks sliding in two mutually perpendicular grooves, respectively. During the biaxial tension test, a cross-shaped specimen sitting in the grooves are stretched by the two blocks driven by a pulling rod. The different stress ratios could be obtained by adjusting the groove surface shape and the lengths of specimen wings. In the clamping apparatus design stage, the finite element simulations were performed to examine the validity of the sliding mechanism and the frictional force generated between the sliding blocks and the grooves. The coefficient of friction was determined afterwards from the comparison of the pulling forces obtained in the experiments with those calculated by the finite element simulations. In addition, the optimum geometry and dimension of the cross-shaped specimen used in the biaxial tension tests were investigated by the finite element analysis as well. The slotted specimen proposed by Kuwabara et al. was taken as the basic design. A sufficiently large area in the central region of specimen where the principal stress directions aligned with the groove direction was obtained for gluing the strain gauges to the specimen for the biaxial stretching tests. The number of slots and associated slot widths were also examined to optimize the shape of the specimens. The proposed clamping apparatus was manufactured and the biaxial tension tests were conducted with cross-shaped specimens made of advanced high strength steel sheets. The validity of the designed clamping apparatus used for biaxial tension tests was confirmed and the congruence of various yield criteria applied to the advanced high strength steel sheets subjected to biaxial stress states was discussed.

Abstract: The dynamic behavior of a radiator support frame impacted by a free-falling engine hood was studied with the use of the 3-D finite element simulations. The engine room including an engine hood, a radiator support frame, and a hood ledge was modeled first. Taking computing efficiency and accuracy into account, approaches for selecting element size, cleaning up complicated features of CAD files were then studied. In order to make the simulations more efficient, the contact spot weld model was adopted. To further reduce the computing time, a theoretical model was also proposed in the present study to calculate the instantaneous angular velocity of the engine hood at the incipient impact to the radiator support frame. The engine hood free-falling tests were also conducted to validate the finite element simulations. The consistency between the experimental data and the simulation results confirms the validity of the finite element model constructed in the present study.

Abstract: The press forging of AZ31 magnesium-alloy sheets for producing thin-walled structural components used in the electronics industry was examined by both the finite element analysis and the experimental approach in the present study. The experimental results obtained from the compression tests and ring compression tests were employed in the finite element simulations to investigate the effects of process parameters, such as forming temperature, friction condition, embossment location, and sheet thickness on the formation of embossments in a press forging process. The finite element simulation results reveal that a cost-effective press forging process of AZ31 magnesium-alloy requires an optimum combination of the above parameters. The detailed examination of the effects of the process parameters on the formation of embossments made in the present study could provide a design guideline for a press forging process of AZ31 magnesium-alloy sheets.

Abstract: The stamping process for manufacturing cell phone cases with magnesium alloy AZ31 sheets was studied using both the experimental approach and the finite element analysis. The formability of AZ31 magnesium-alloy sheet at elevated temperatures was studied first. The experimental results reveal that the forming of AZ31 sheets becomes possible as long as the sheet is heated to an elevated temperature and 200oC is an optimum forming temperature to start with. An optimum stamping process, including die geometry, forming temperature, and blank dimension, for manufacturing the cell phone cases was examined by the finite element analysis. The finite element analyses performed for the cell phone were validated by the good agreement between the simulation results and the experimental data. It also confirms that the cell phone cases can be produced with AZ31 magnesium-alloy sheets at elevated temperature by the stamping process. It provides an alternative to the electronics industry in the application of magnesium alloys.

Abstract: The springback behavior of an invar sheet and its perforated form were examined in the present study. The mechanical properties for invar sheet and perforated invar-sheet at elevated temperatures were first obtained from tensile tests. The test results suggest that both invar sheet and perforated invar-sheet have favorable formability at temperature higher than 200oC. An analytical model was also established to predict the springback of the invar sheet and its perforated form under bending conditions at various elevated temperatures. In order to verify the predicted results, the V-bending tests were conducted for the invar sheet at various temperatures ranging from room temperature to 300
. The experimental data indicate that the springback decreases with the rise in temperature for both invar sheet and perforated invar-sheet. The good agreement between the experimental data and the predicted values confirms the validity of the proposed theoretical model as well.