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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stressed productions, the ductility of DX51D grade material changes significantly. In order to prevent failures such as cracks, ruptures and fractures in the production process of components, the ductility of material must be known and determined before the production phase with the help of numerical calculations. Being used as a raw material of DX51D and having ductile fracture parameters available in the literature, hot-rolled S235JR and hot-rolled S275JR grade materials are chosen for initial hemming simulations to observe fracture event. As mentioned before, the fracture curve of S235JR grade material is taken from Kõrgesaar et al. (2018) . For the S275JR grade material, Pantousa and Karavasilis (2021) did temperature-dependent experimental fracture investigation work to evaluate fracture initiation. According to their experimental program, S275JR grade’s Ho sford-Coulomb ductile fracture model parameters in room temperature condition are used in this study for initial numerical calculations. For projection of these fracture behaviour of steels to DX51D grades, experimental tensile test results are used for calibration of the Hosford- Coulomb ductile fracture model’s b parameter. With the help of modified fracture models, hemming process simulations are carried out accurately and ductile fracture of material’s outer bending surface can be predicted as similar to a real-world scenario. 2. Problem definition The cracking and rupturing problems that occurred in the bending area of the material after the hemming process can be seen in Fig. 1. This hemming operation was performed for a long time with DX51D material grade without having problems. The operation does not seem very difficult in terms of application but the high plastic deformation that comes out during the bending process makes the choice of material very important. Since DX51D material does not have any restriction on yield stress according to EN 10346:2015, sometimes low-strength and sometimes high strength productions are made. For this reason, determining the yield limit will be useful in preventing problems such as tearing and cracking in such operations in the future.

Fig. 1. Fracture images of parts and EN 10346:2015 standard details

3. Ductile fracture modelling Ductile fracture modelling was carried out in order to detect the cracks that occur in the hemming operation with finite element analysis. Details of the finite element model which was created in Altair HyperMesh software are given in Fig. 2. The die and punch components were modelled to present rigid behaviour. The part was formed from cylindrical geometry with a punch radius of 1 mm. The die component was fixed for the process and the imposed displacement boundary condition was applied for the punch. The sheet component is 2.5 mm thick and was modelled