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. stretchbending deformation using punch radii R < 10 mm).
The state of deformation in deep drawing operations is characterized by superimposed stretching and bending (i.e. stretch-bending). Bending effects, especially for Advanced High Strength Steels (AHSS) are known to influence the material formability. Traditional formability measures such as the Forming Limit Curve (FLC) fail to reliably predict stretch-bending formability. Consequently, to ensure an efficient and economical use of AHSS in the industrial application, current research work is focusing on the reliable numerical prediction of stretch-bending formability of AHSS sheets.Within this work, a phenomenological concept to predict the forming limit (e.g. the onset of necking) in deep drawing processes taking bending effects into account is presented. The proposed concept is based on curvature-dependent (i.e. regarding the principle curvatures κ1 and κ2 of the stretch-bend (convex) sheet surface) forming limit surfaces representing the probability of failure and is calibrated with experimental results from stretch-bending tests and conventional forming test such as a Nakazima test. The results of the phenomenological forming limit criterion are promising and show a more accurate prediction of the drawing depth at failure than the conventional FLC approach. The method contributes also to a probabilistic view on the forming limit of deep drawing parts.
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