PSI - Issue 35

Toros Arda Akşen et al. / Procedia Structural Integrity 35 (2022) 82 – 90 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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6.2. Hole expansion test simulations Hole expansion process of TRIP590 and TWIP940 steel sheets were simulated using Hypela2 user subroutine in implicit Marc software. This subroutine is employed in order to define the material behavior along with the hypoelastic model description (Marc vol. D (2018)). Blank was meshed with fully integrated hexahedral solid elements known as Hex7 in Marc. Quarter of the blank was considered due to the orthotropic symmetry conditions. Die, punch and blank holder were modeled as rigid surfaces. Friction coefficient between the blank and the other tools were assumed as 0.1. Two elements were used in thickness direction. RD of the sheet was determined as x direction. Segment to segment contact algorithm was adopted with Coulomb bilinear friction model. Punch stroke values were specified as 15.8 mm and 12.8 mm for TRIP590 and TWIP940 steels, respectively. Generated mesh layout and the finite element model of the hole expansion process can be seen in Fig. 5.

Fig. 5. FE Model of hole expansion process

7. Results Initially, in order to assess the performance of the yield criterion, HET simulations were performed up to the fracture strokes of TRIP590 and TWIP940 steels. Numerical and experimental results were compared in Table 6.

Table 6. Numeric and experimental results for the specified punch stroke values TRIP590 h = 15.8 mm TWIP940 h = 12.8 mm Experiment HER 68.4% 39.1% True Strain 0.521 0.33 FEM HER 62.10% 43.17% True Strain 0.483 0.359

Predicted HER values show good agreement with the experimental results. For TRIP590 steel, the difference between the predicted and experimental HER is less than %9.21. As for the TWIP940, higher HER value was predicted in comparison to the experiment and %11.8 difference was observed. In the second stage of the present study, punch stroke values which the failure occurs were estimated using plastic work criterion. Damage indicators were assigned as the threshold values of failure in Hypela2 user subroutine. Punch stroke predictions and the experimental values were compared in the Table 7.

Table7. Punch stroke prediction obtained from the plastic work criterion TRIP590

TWIP940

Experiment

Stroke [mm] Stroke [mm]

15.8 14.2

12.8 12.9

Prediction (Plastic Work Criterion)