Application of a “concave-side rule” approach for assessing formability of stretch-bent steel sheets

In sheet forming, inhomogeneous through-thickness deformation, (e.g. due to a combined deformation of simultaneous stretching and bending when sheet material is drawn over a defined tool radius), is known to have influence on materials formability (i.e. onset of necking). Thus, it is preferable to take this influence into account when assessing the formability of sheet metal in a forming process. For that reason, in 2003 Tharett and Stoughton introduced the so-called “Concave-Side Rule” (CSR) approach to assess the formability of stretch-bent steel sheets. In this study the predictive quality of the CSR approach is analyzed. Therefore a series of Angular Stretch Bend Tests (ASBT) is performed. H340LAD (micro-alloyed steel) sheet specimens, with a thickness of 1.5 mm are stretch-bent over punches of various radii from 1 mm to 20 mm to produce different severity of bending in the test specimens. Results of optical on-line surface strain measurements of the test specimens are used to calibrate Finite-Element (FE) simulations of the ASBTs. From these FE-simulations, numerical results are used to assess the predictive quality of the CSR approach for H340LAD. The rule is found to be valid for H340LAD sheet material stretch-bent with punch radii R ≥ 10 mm. Whereas predictive quality decreases for more pronounced inhomogeneous through-thickness deformation (i.e. stretch-bending deformation using punch radii R < 10 mm).