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Study on the Anisotropy of Auto-body Steel Sheets considering the Strain Rate(변형률속도를 고려한 차체강판의 이방성에 관한 연구)
The sheet metal forming is an effective process which is widely used in many industries. In sheet metal forming processes, the quality of deformed parts are influenced by many process parameters, such as the shape of the die, the shape and thickness of the initial blank, the material properties, the blank holding force, the friction, and so on. The main concern of many industries is to satisfy good formability at the high speed. The sheet metal forming process becomes simple and fast for the improvement of productivity. The deformation of steel sheets generally involves strain rate effects during the forming process. When the deformation of steel sheets is accelerated, the strain rate effect becomes significant. Therefore, it is essential to calculate the final shape and deformation history of a product considering the strain rate effect in the forming process. In the previous researches, it was demonstrated that material properties can be changed by the strain rate effect. However, the change of material properties has not been considered in the forming analysis since it is difficult to measure the change of the material properties with an experimental method. Among a number of material properties, especially, the r-value and the yield stress have a significant effect on the initial anisotropic state of auto-body steel sheets. The initial anisotropic state has an effect on the amount of spring-back since it calculates the different residual stress and strain distribution after the forming process. Hill48 quadratic yield criterion has been widely used for simulations of forming process of auto-body steel sheets due to the simplicity of its numerical formula and the low computing cost. It shows good performance to approximate anisotropy of auto-body steel sheets. This paper deals with the anisotropic material properties and the initial yield state considering the strain rate. Specimens of SPCC and DP590 were extracted at interval of 15o from 0o(RD; rolling direction) to 90o(TD; transverse direction). Uniaxial tensile tests were performed according to the strain rate and the loading angle from RD to TD in order to obtain flow stress curves and tensile properties. The r-values were measured with high speed camera by analyzing the deformation history during the tensile test. To obtain reliable r-values, the measuring range is limited to the necking instability strain of steel sheets. Anisotropy of auto-body steel sheets is described by using Hill48 and Yld89(Barlat89) yield functions according to the strain rates ranged from 0.001/sec to 100/sec. Hill48 and Yld89 yield loci of auto-body steel sheets with respect to the strain rate are constructed in order to visualize initial yield state. The performance of two yield criteria is evaluated by comparing yield loci constructed in the principal stress plane. It shows that the initial yield states are different when the strain rate is considered to describe anisotropy of steel sheets. Anisotropic yield function is modified to describe anisotropic yield state of steel sheets according to the strain rate effectively by approximating r-value and yield stress with respect to the strain rate.