PSI - Issue 35

İbrahim Yelek et al. / Procedia Structural Integrity 35 (2022) 51 – 58 Yelek et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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4. Numerical results According to the results of the numerical analysis, the ductile fracture behaviour of the material could be observed as seen in Fig. 5(c). Two different models were established according to the normal and high-level DX51D grade material mechanics as mentioned before. The black-dotted curve represents high-level mechanics of DX51D as seen in Fig. 5(a). The ductile fracture initiation was predicted early phases of the process because the material did not have the ductility to withstand the amount of bending during the process. Besides, according to the black dashed line which represents normal-level DX51D mechanics as seen in Fig. 5(a), the hemming process was successfully performed without any damage during the process since this material has higher ductility with relatively lower strength as seen in Fig. 5(d). In addition, simulation results show that the stress state in the regions where maximum plastic strain occurs in operations such as bending and hemming generally corresponds to stress triaxiality values equivalent to plane strain valley as shown in Fig. 5(a). For the presentation of plastic strain and stress triaxiality contours for both models after the bending process, simulation results are given in Fig. 5(b).

Fig. 5. a) Fracture locus of materials and equivalent strain path of an initially failed element, b) Equivalent strain and stress triaxiality results, c) Modified fracture locus of D.Pantousa et al. for failed hemming process with high-level mechanics of DX51, d) Modified fracture locus of M. Kõrgesaar et al. for successful hemming process with normal-level mechanics of DX51 5. Conclusions Ductile fracture initiation models for the hemming process of DX51D grade material with different tensile mechanics are proposed to accurately simulate and predict fracture event in numerical calculations. Due to the similarity of tensile mechanics fracture parameters of structural steel grades are chosen from literature to initially perform calculations. With the help of these fracture parameters, the post-necking stress-strain curves of normal and high-level mechanics of DX51D grade materials were extracted by using FEM iteratively. Some simulations were done to successfully match results with experimental tensile data. Correlation of experimental data with simulations was done by modifying the fracture curves of structural steel grades S235JR and S275JR. For modification, the b