Daeyong Kim

Measurements of anisotropic yielding, bauschinger and transient behavior of automotive dual-phase steel sheets

In order to present better prediction capability in computational analysis, mechanical properties of the dualphase high strength steel have been characterized especially for anisotropy as well as the Bauschinger and transient behavior. As for the anisotropy, the non-quadratic anisotropic yield function Yld2000-2d has been utilized and its material parameters have been obtained using the uni-axial tension tests as well as the hydraulic bulge test. To measure the hardening behavior including the Bauschinger and transient behavior, a newly developed test method has been applied for the uni-axial tension/compression and compression/tension tests, in which solid blocks along the both sides of the sheet specimen prevent buckling. From the tension/compression curves, the equations to describe isotropic and kinematic hardening behavior have been obtained. The modified Chaboche model has been confirmed to well represent the hardening behavior including the Bauschinger and transient behavior.

Effect of hardening laws and yield function types on spring-back simulations of dual-phase steel automotive sheets

In order to simulate the spring-back of DP-steel automotive sheets, the effect of hardening laws and yield function types has been studied based on finite element simulations performed for 2-D draw bending and S-rail tests. Specifically, the performance of the combined isotropic-kinematic hardening based on the modified Chaboche model was compared with those of the pure isotropic hardening and kinematic hardening laws, along with the non-quadratic anisotropic yield (Yld2000-2d) and Mises yield functions. As for the 2-D draw bending test, numerical results were compared with experimental results for verification purposes.

Semi-Analytic Hybrid Method to Predict Springback in the 2D Draw Bend Test

A simplified numerical procedure to predict springback in a 2D draw bend test was developed based on the hybrid method which superposes bending effects onto membrane solutions. In particular, the procedure was applied for springback analysis of a specially designed draw bend test with directly controllable restraining forces. As a semi-analytical method, the new approach was especially useful to analyze the effects of various process and material parameters on springback. The model can accommodate general anisotropic yield functions along with nonlinear isotropic-kinematic hardening under the plane strain condition. For sensitivity analysis, process effects such as the amount of bending curvature, normalized back force and friction, as well as material property effects such as hardening behavior including the Bauschinger effect and yield surface shapes were studied. Also, for validation purposes, the new procedure was applied for the springback analysis of the dual-phase high strength steel and results were compared with experiments.

Decoupling Thermal and Electrical Effect in an Electrically Assisted Uniaxial Tensile Test Using Finite Element Analysis

Application of intermittent electric pulses during uniaxial tensile test changes the mechanical behavior owing to electroplastic effect. The electric current increases the temperature of the specimen due to Joule heating. It is, therefore, necessary to decouple the thermal effect from the overall behavior to understand the contribution of electric current in the mechanical behavior. In the present work, an electro-thermo-mechanical finite element study of an electrically assisted uniaxial tensile test of Al5052 alloy is performed to isolate the thermal effect. The simulated results yielded the thermal effect due to the electric current. By comparing the experimental and simulated results, the contribution of electric current is decoupled from that of thermal effect. It is found that the thermal component contributes significantly to the instantaneous stress drop and long-range permanent softening observed in experiment. The electric current, in addition to the instantaneous stress drop and permanent softening, affects the reloading behavior. The present work can be utilized to develop simpler constitutive models for the mechanical behavior of metals subjected to pulsed electric current.

Fracture criterion for AZ31 Mg alloy plate at elevated temperature

The fracture criterion was characterized for an AZ31 Mg alloy plate with the 3.0 mm thickness at the elevated temperature of 250 °C in this work. In order to properly characterize the fracture criterion, its mechanical properties were also characterized. As for mechanical properties, simple tension tests were performed to calibrate the Hill1948 yield function. Also, in order to account for the hardening deterioration (softening) behavior beyond the uniform deformation limit, the flow curves of the Mg alloy plate were numerically obtained based on the inverse calibration method, in which strain rate sensitivity was also considered. As for the fracture criterion, effective fracture strains, which are dependent on stress triaxiality and deformation paths, were numerically characterized utilizing experimental data based on specimens with four different shapes newly developed. For comparison purposes, empirical fracture criteria such as the Cockcroft-Latham, Brozzo, Ayada and Clift models were also calibrated. For validation purposes, the five fracture criteria were applied for a real part (an EL-cover) drawing case and the result confirmed that the fracture criterion developed in this work performed best among the five models tried out.

Experimental study on forming behavior of high-strength steel sheets under electromagnetic pressure

To experimentally investigate the forming behavior of automotive high-strength steel sheets under electromagnetic pressure, three high-strength steel sheets (DP780, DP980, and CP1180) were freely formed into hemi-elliptical protrusions. The experiments were performed using a spiral flat coil and open-cavity die. Aluminum driver plates were utilized for improving the forming efficiency. The forming behavior was investigated by evaluating characteristics such as peak height, limit height, and springback angle for various charge voltages. The effect of the aluminum driver plate on the forming behavior of the high-strength steel sheet was also observed.

Influence of constraint condition on rolling behavior of magnesium alloy at low temperatures

Influence of constraint condition on rolling behavior of a magnesium alloy sheet at low temperature was investigated by embedding the magnesium alloy sheet within a block of an aluminum alloy having a higher strength. Magnesium alloy sheets rolled with the aluminum alloy block exhibited better rolling capability at a low temperature of 373 K than the sheets rolled without the block. The constraint placed on the magnesium alloy sheet by the aluminum alloy block impedes the initiation of edge cracks in the sheet during rolling by reducing the tensile stress generated along the rolling direction near the edge region of the magnesium alloy sheet. In addition, the constraint develop compressive stress along the transverse direction in the magnesium alloy sheet, which causes tensile twin and rotates the basal texture of the sheet to an orientation that is more capable of accommodating deformations along the normal direction. It is for these reasons that the embedded magnesium alloy sheet exhibits an improved rolling capability.

Experimental Study on Electromagnetic Forming of High Strength Steel Sheets with Different Dimensions of Aluminum Driver Plate

Recently, the potential of the electromagnetic forming process has been introduced to form the shallow longitudinal reinforcement ribs in the lateral walls of roll formed parts, made of high strength steel sheets of 340MPa tensile stress grade [1]. However, it seems that the application may not be easy for high strength steel sheet because of its high tensile strength and low electric conductivity. In order to overcome this difficulty, aluminum driver plate could be considered to enhance the formability of high strength steel sheets in the electromagnetic forming process. In this paper, in order to investigate the effect of aluminum driver plate on forming height of high strength steel sheet in electromagnetic forming process, DP780 workpiece sheets were formed into a hemi elliptical protrusion shape with Al1050 driver plate of various thicknesses and sizes. Experiments were performed with a flat spiral coil actuator connected to an electromagnetic forming system. The results, the aluminum driver plate helps to increase the forming height of high strength steel sheets. In addition, the forming height of high strength steel sheet increases as the thickness and size of a driver plate increases.

Experimental Formability Investigation for FSW Sheets with Respect to Base Material`s Directional Combination

In order to investigate the formability of friction stir welded(FSW) tailor welded blanks(TWB) with respect to the base material`s directional combination, aluminum alloy AA6111-T4 sheets were welded with three different conjoining types: RD-RD, TD-RD and TD-TD. Here, RD and TD represent rolling and transverse directions, respectively. For experimental formability study, three tests with gradual complexity were performed: the simple tension test with various weld line directions for uni-axial elongation, the hemisphere dome stretching test for biaxial stretching and the cylindrical cup deep drawing test. As a result, all three forming tests showed that RD-RD type samples exhibited the best formability, while TD-TD type sheets showed the least formability performance.

Determination of forming limits of high strength sound-deadening laminated sheet

High strength sound-deadening laminated sheet consists of high strength steel sheets and a viscoelastic polymeric adhesive with high vibrational damping characteristics. In this study, to evaluate formability of sound-deadening laminated sheet, forming limit curves (FLC) were determined. The high strength sound-deadening laminate sheet was fabricated with dual phase (DP) 590 steel sheets as outer skins and polymeric adhesive as a core by roll bonding process. The punched dome tests (Nakajima tests) were performed with varying specimen width. On the specimen prior to deformation, a stochastic pattern was applied to the surface using a white and black spray. Until the fracture occurs, the strain distribution was measured in the stereo image using digital image correlation (DIC) system. FLC of the laminated was successfully determined by capturing limit strains near fracture as well as calculating them according to the ISO standards. FLC of the laminated sheet is slightly higher than that of DP590 sheet but is not significantly different.