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An investigation of modelling approaches for material instability of aluminum sheet metal using the GISSMO-model

The most generally accepted tool for the assessment of formability of sheet material up to the present date is the forming limit diagram (FLD). It allows the identification of critical areas in a sheet-forming simulation where critical thinning or the appearance of cracks is to be expected. The FLD is determined experimentally in Nakajima tests and is widely used in the design of sheet forming processes. However it is known that the assessment based on the FLD is incomplete. Cracks are observed also in areas, which should be safe according to the FLD and vice versa. There is a strong need for numerical methods, which allow exact quantitative predictions about critical areas in the sheet forming process. Literature investigations have shown that FE-Analysis in combination with advanced material models constitutes a reliable supplement to the FLD. The General Incremental Stress State dependent Material Model (GISSMO) in LS-Dyna offers the description of necking, damage and failure for sheet materials under complex loading conditions. Within a joint research project between AMAG rolling GmbH and LKR an advanced concept (GISSMO-Modelling) for the assessment of sheet forming processes was compared to the standard method (FLD). The relevant mechanical properties were determined experimentally und a material card for GISSMO was calibrated. The Validation of the material model was carried out and the predictive quality of the two concepts was compared systematically. The results of the present paper show, that the use of the GISSMO model leads to a good prediction of the maximum drawing depth of the cross-tool test. The location of fracture was also predicted correctly. The assessment based on the FLD is too conservative. The location of fracture was determined, but the maximum drawing depth according to the FLD is 30% below the measured one. Simulations with different mesh sizes were performed and validated.