Dae-Cheol Ko

Application of artificial neural network and Taguchi method to preform design in metal forming considering workability

This study describes a new method of perform design in muti-stage metal forming processes considering workability limited by ductile fracture. The finite element simulation combined with ductile fracture criterion has been performed in order to predict ductile fracture. The artificial neural network using the Taguchi method has been implemented for minimizing objective functions relevant to the forming process. The combinations of design parameters used in finite element simulation are selected by orthogonal array in statistical design of experiments. The orthogonal array and the result of simulation are used as train data for artificial neural networks. The cold heading process is taken as an example of designing preforms which do not form any fracture in the finished product. The results of analysis to validate the proposed design method are presented.

The prediction of central burst defects in extrusion and wire drawing

Abstract Central burst defects cause very serious problem in the quality control of products, because it is impossible to detect the defects by means of a simple surface inspection of the workpiece. Since the occurrence of these kinds of defects is, therefore, troublesome in industrial practice, it is important to predict the conditions that may lead to the defects, and by using this prediction it may be possible to choose appropriate process conditions and to modify the forming processes to produce sound and reliable products. This paper suggests an approach to simultaneously accomplish both the prediction of central burst defects and the analysis of deformation in extrusion and wire drawing. The proper ductile fracture criterion is employed in this investigation to estimate the occurrence of central burst defects. Based on the results of analyses, it has been possible to obtain the numerical boundaries that divide the safe and danger zones in reduction in area vs. semi-die angle plane for central burst defects. To verify the effectiveness of the proposed approach, numerical predictions and experimental results are compared, the latter showing a good agreement with the former.

Prediction of surface-fracture initiation in the axisymmetric extrusion and simple upsetting of an aluminum alloy

Abstract Most bulk metal forming processes may be limited by ductile fracture, such as an internal and surface fracture developing in the workpiece. It is important to predict the conditions within the deforming workpiece which may lead to fracture, as by using this prediction it may be possible to choose appropriate process conditions and to modify the forming processes to produce sound products. This paper suggests a scheme to simultaneously accomplish both the prediction of surface-fracture initiation and the analysis of deformation in the axisymmetric extusion and simple upsetting of an aluminum alloy. The Cockcroft-Latham criterion, which has been applied successfully to a variety of loading situations, is used in the present investigation to estimate if and where surface fracture will occur during the deformation process. The numerical predictions and experimental results for the two types of metal-forming process under consideration are compared. The proposed scheme successfully predicts the site of surface-fracture initiation and the corresponding level of deformation observed experimentally.

Effect from Cyclic Fatigue of Nickel-Titanium Rotary Files on Torsional Resistance

INTRODUCTION The aim of this study was to evaluate the effect of cyclic fatigue on the torsional resistance of nickel-titanium (NiTi) rotary instruments. METHODS ProFile (#25/0.06; Dentsply Maillefer, Ballaigues, Switzerland) and ProTaper (F1, Dentsply Maillefer), both of which have the same external diameter at D5, were tested using a fatigue testing machine for the mean number of cycles of failure (mNCF). Then, new files were cyclic precycled to 4 conditions (ie, 0%, 25%, 50%, and 75% of the mNCF) before the torsional resistance test was performed on these cyclic preloaded files. A uniform clockwise rotation was applied to the file in a straight state in a torsion tester. The torsional load and distortion angle were recorded during rotation until the file succumbed to the torque. The toughness was computed. The results were analyzed statistically using one-way analysis of variance and the Duncan post hoc comparison to find any differences between groups at a significance level of 95%. RESULTS In both ProFile and ProTaper groups, the 75% preloading groups had significantly lower torsional strength than other preloaded files. In the ProFile group, the 50% and 75% preloading groups had a smaller distortion angle until fracture than the 25% and no preloading groups. The 75% preloading group showed a lower toughness value than the 25% and no preloading groups. In the ProTaper group, all preloading groups had less distortion and toughness than the no preloading group. Fractographic examinations revealed the 75% preloaded files showed less amount of reverse-wound flute than other preloading groups. CONCLUSIONS Within the limitations of the methodology, it could be concluded that approximate 75% cyclic fatigue may reduce the torsional resistance of NiTi rotary instruments significantly.

Finite-element simulation of the shear process using the element-kill method

Abstract The major objective of the present paper is to establish an analytical technique in order to closely understand and analyze the actual shearing process. First of all, isothermal and non-isothermal finite element (FE)-simulation of the shearing process are carried out using the finite-element software DEFORM. Based on preliminary simulation using DEFORM, the FE program to analyze the two-dimensional shearing process is developed. The ductile-fracture criterion and the element-kill method are used also to estimate if and where a fracture will occur and to investigate the features of the sheared surface in the shearing process. It can be seen that the developed program combined with the ductile fracture criterion and the element-kill method has enabled the achievement of FE-simulation from the initial stage to the final stage of the shearing process. The effects of the punch–die clearance on the shearing process are investigated also. In order to verify the effectiveness of the proposed technique, the simulation results are compared with known experimental data. The results of the present work are in close agreement with published experimental results.

Finite element analysis for the semi-solid state forming of aluminium alloy considering induction heating

Abstract The major objective of this study is to establish an analytical technique in order to investigate the behavior of semi-solid material considering induction heating of the workpiece. The induction heating process is analyzed using the commercial finite element software, ansys . The finite element program, sfac2d , for the simulation of deformation in the semi-solid state is developed in the present study. The behavior of semi-solid material is described by a viscoplastic model for the solid phase and by Darcy’s law for the liquid flow. Simple compression and closed-die compression processes considering induction heating are analyzed. To validate the effectiveness of the proposed analytical technique, the results of simulation are compared with those of experiment.

Finite element analysis for the wear of Ti–N coated punch in the piercing process

Abstract This paper describes a computer modeling scheme using the finite element method in order to investigate the wear mechanism and to predict the wear profile of Ti–N coated tool in the piercing process according to the volume of production. Archard’s wear model is reformulated in an incremental form to predict tool wear and then the wear depth on the tool is calculated at each step in the deformation using the results of finite element analysis, taking in consideration the sliding velocity and normal pressure over the contact area. To analyze the quantitative wear of the tool, wear coefficients of the coated and the inner layer of tool are determined by the result using a wear test, such as pin-on-disk wear test. To verify the effectiveness of the suggested scheme, the scheme is applied to wear analysis on punch during piercing process in multi-stage forming process of piston pin and simulation results are compared with experimental ones. It is known that the simulation results are in good agreement with those obtained experimentally.

Influence of Tool Shape on Hole Clinching for Carbon Fiber-Reinforced Plastic and SPRC440

Carbon fiber-reinforced plastic (CFRP) is a lightweight material that can potentially replace structural steel components in automobiles. The hole-clinching process is a mechanical clinching technique for joining brittle or low-ductility materials, such as CFRP, with ductile materials. In this study, the influence of tool shape on the hole-clinching process for CFRP and SPRC440 was investigated using FE-analysis and experiments. The parameters of the tool shape investigated were the punch corner radius and the punch diameter. The geometrical interlocking shapes of hole-clinched joints were characterized by neck thickness and undercut. Based on the desired joint strength of 2.5 kN, hole-clinching tools were designed on the basis of the relationship between joint strength and geometrical interlocking. FE-analysis and hole-clinching experiments were performed with the designed hole-clinching tools to investigate the geometrical interlocking shape as well as joinability, including neck fracture, undercut, and hole expansion, resulting from changes in tool parameters. Joint strength was evaluated to verify the effectiveness of hole clinching by a single lap shear test.

Development of an analytical scheme to predict the need for tool regrinding in shearing processes

This paper describes an analytical scheme for the prediction of the tool wear and the need for tool regrinding in the sheet metal shearing process. The finite element program developed is used for the analysis of the shearing process in this study. In order to predict tool wear, Archards wear model is reformulated in an incremental form and then the wear depth on the tool is calculated at each step in the deformation using the result of finite element analysis, taking consideration of the sliding velocity and normal pressure over the contact area. In general, the need for regrinding of the shearing tool is determined on the basis of allowable burr height on the final product. Therefore, based on this criterion, the analysis of the shearing process is iteratively performed using the worn profile on the tool in order to predict the need for tool regrinding. To analyze the quantitative wear of the tool, the parameters included in the wear model are determined by the result using a wear test, such as a pin-on-disk wear test for the material of the shearing tool and sheet metal. The effectiveness of the proposed scheme is verified by comparison with experimental results. It is shown that the simulation results are in good agreement with those obtained experimentally and the need for regrinding of the shearing tool can be determined for an allowable burr height.

Rigid-thermoviscoplastic finite element simulation of non-steady-state orthogonal cutting

Abstract The objective of this study is to develop a rigid-thermoviscoplastic finite element program for the analysis of orthogonal cutting process. Deformation of the workpiece is considered as rigid-viscoplastic and the numerical solution is obtained from the coupled analysis between plastic deformation and temperature fields. The chip and burr formation are simulated for the non-steady-state orthogonal cutting using the developed program. To validate the program the predicted results at the chip and burr formation stage are compared with the published ones. The results of this study are in close agreement with the published ones. The case of isothermal cutting process is also considered to study the thermal effect on the machining process.