Abstract
A numerical technique is presented in order to assess formability conditions for tearing type sheet metal failures in automotive stamping applications. The proposed technique is based on the plastic instability and uses Swift’s diffuse necking and Hill’s localized necking concepts. Both necking models are transformed into a set of differential equations that may be applied both for proportional and non-proportional loadings and expressed an incremental form suitable for finite element (FE) analysis. Next, the numerical models are implemented into Ls-Dyna FE program and applied to predict the forming limit curve (FLC) of a high-strength dual-phase steel using in-plane proportional loadings of a single shell element. Then cup drawing processes of the same steel grade are simulated and failure heights for three square blanks are predicted. Allowable maximum punch strokes predicted with the necking models are compared with the results from square cup drawing tests under the same blankholder force. Model predictions were in accord with the experimental data and determined to be conservative for this steel grade. An investigation of cup failure heights determined with conventional technique revealed that the FE post-processing using the experimental FLC resulted in erroneous failure predictions and not conservative.
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