Strain-rate dependent anisotropic yield criteria for auto-body steel sheets

Abstract

A proper description of anisotropy is one of the most important aspects in the simulation of the sheet metal forming process since sheet metals generally exhibit a significant anisotropy of the mechanical properties due to their crystallographic structure and the characteristics of the rolling process. Many researchers, hence, have been proposed diverse anisotropic yield criteria to model the anisotropic yield behavior of sheet metals. The deformation of steel sheets generally involves strain rate effects during the forming processe. 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 previous research works, it was demonstrated that the stress-strain relationship of metals are highly influenced by the strain rate. However, there have been few studies on the effect of strain-rate on the anisotropic yield behavior of materials. This study deals with the modeling of strain-rate dependent anisotropy yield criteria of auto-body steel sheets including two advanced high strength steel (AHSS) sheets of TRIP590 and DP780 and a conventional cold-rolled steel sheet of SPCC. Uniaxial tensile tests were performed at different strain rates varying the loading angle from the rolling direction to investigate the effect of the strain rate on the anisotropy of material properties by obtaining yield stresses and r-values. A new method was proposed to measure the r-value at intermediate strain rates with the aid of digital image correlation (DIC). Anisotropic yield behavior of auto-body steel sheets was modeled with the Hill48, Yld89 and Yld2000-2d yield criteria at strain rates ranging from 0.001/s to 100/s. The performance of the three yield criteria was evaluated by comparisons of theoretically calculated planar distributions of the uniaxial yield stress and the r-value with ex...