A Study on the Formability of Sheet Steel

Abstract

The formability of two sheet steels, SSPDX (0.8mm thickness) and SPMY (0.9 mm thickness), are examined by means of a plane-strain tensile test and an in-plane stretching test. The parameter m of Hill's nonpolynomial yield function is calculated by applying Wagoner's method. The assumption m=2 is recognized for both steels because the difference between effective stress-strain curves of the uniaxial and plane-strain test can almost be neglected. The experimental data of the in-plane stretching test are in good agreement with those predicted by modified M-K theory. Because of less roughness, i.e., a slower growth rate, the limit strain for SSPDX is greater than that for SPMY. The fracture mechanism of the in-plane stretching specimen observed is almost identical to those for isotropic fcc metals reported. However, that of the plane-strain tensile specimen is quite different. The crack is initiated at the center of the specimen by shear deformation and then propagated in the manner of a mode I-type crack. This difference is well explained in terms of anisotropy and strain distribution.