Forming limit diagrams with an FE-based approach for sheets under non-proportional loading

In this study, LS-DYNA was used to predict the experimental forming limit diagrams (FLDs) for pre- strained sheets in aluminum alloy Al2008-T4 found by Graf and Hosford [1]. The original data of Graf and Hosford [1] includes numerous pre-straining situations, but it was here chosen only to investigate pre-straining by biaxial and uniaxial tension. In order to generate the FLDs, several analyses of a square patch were run systematically to construct the different points. Pre-straining was applied by first stretching a somewhat larger patch to a given pre-strain, and then trimming this patch to the standard square patch. The material model used in the analyses includes two instability criteria; a non- local criterion to detect incipient localized necking and a through-thickness shear instability criterion [2-5]. The objective was to study whether the effects of pre-straining on the FLD could be predicted by the chosen modelling approach, and good results were obtained.