Extension of a Failure Criterion for Hemming Applications in the FEM Simulation

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In the FEM calculation of sheet metal forming processes to determine the failure case, the forming limit diagram is basically used. To determine the failure case at bending condition, the forming limit diagram can not be used. This behaviour was shown by many authors. Bending tests with an aluminium material (AC170PX) have shown that a high deformation ratio can be achieved without failure. Based on the loading conditions and the previous strain path through the deep-drawing process, a resulting bendability at a certain point can be obtained. Depending on the pre-damage and the mentioned loading conditions of the material failure will be occurring during bending at different times. Current developments of failure criteria consider the failure as in ductile fracture or shear fracture, which must be considered separately in the simulation. To rule out a separate analysis of the mode of failure in the post-processing, an existing failure criterion is extended and will be presented in this work. For the applications flanging and hemming the following extension of a stress-based failure criterion is proposed. Based on the triaxiality and the equivalent plastic strain a monitoring of the stress ratio is implemented in the FEM simulation. During the forming simulation the monitoring system observe the stress ratio based on the principal stresses resulted from the integration (Gauss) point of the shell element. According to the evaluation of the stress ratio evolution, a relevant definition will take into account how the damage will be accumulated. If the critical value of damage in the integration point of the shell element is reached, failure will be occur based on the position of the sheet thickness.