PSI - Issue 61

Frank Schweinshaupt et al. / Procedia Structural Integrity 61 (2024) 214–223 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Analogous to the setup used for fine blanking experiments, a finite element (FE) model was designed for numerical analysis in Forge NxT 3.2 software, which uses an implicit solver for calculation. Taking advantage of the symmetry, a quarter of the blanked part geometry was modeled (Fig. 2a). All modeled tool elements have been connected to rigid plates for force or movement initiation as well as fixed clamping. The process forces are applied to the blank holder and counter punch with the parameters used in the experiments. The blanking velocities investigated are initiated by moving the die based on measured velocity profiles, whereas the blanking punch is fixed in space. Die, blanking punch, blank holder, counter punch and blank were meshed as 3D bodies with tetrahedral elements. To evaluate the meshing quality, a mesh convergence analysis was carried out based on averaged temperature and stress values in the defined measuring fields of the shear zone. At a minimum edge length of 0.35 mm, the temperature and stress values converged with sufficient accuracy. In the area of the punch surfaces and blanking edges on the die, the elements were therefore meshed with a minimum edge length of = 0.3 mm both in the sheet metal blank as well as in the contact area of the corresponding tool elements. With increasing distance from the contact surfaces as well as blanked part area, the minimum edge length was increased in two steps to ≈ 1.2 mm, so that in each case there is a size difference of factor two to neighboring volume elements. All tool elements were modeled elastically for realizing thermally coupled material behavior. Table 2 shows the physical and thermophysical parameters used for die and blanking punch (Böhler S390) as well as blank holder and counter punch (cold work tool steel X155CrVMo12-1 or 1.2379). Table 2: Parameters used for Young ’s modulus , Poisson's ratio , Density , Thermal conductivity , Specific heat capacity , Coefficient of thermal expansion at 20 °C regarding the tool elements (die, blanking punch, blank holder, counter punch) Material / GPa / - / kg/m 3 / W/(m ⋅ K) / J/(kg ⋅ K) / 10 -6 /K Böhler S390 231 0.285 8100 17 420 10 1.2379 218 0.285 7680 29.2 465 10.1 The material behavior of the sheet metal material 42CrMo4+AC was modeled thermoviscoplastically using a remeshing algorithm. The flow behavior was modeled temperature and strain rate dependent according to Hensel and Spittel (1978) using model parameters of Landolt-Börnstein database (Spittel and Spittel (2009)). Equation 1 gives the parameters used for modeling the true stress f as a function of temperature , true strain = pl and strain rate ̇ . f ( , , ̇) = 929.822 ⋅ −0.00064⋅ ⋅ 0.08896 ⋅ ̇ 0.0 4 ⋅ −0.0080 MPa (1) Except for the used Poisson's ratio of = 0.285, the physical as well as thermophysical parameters of 42CrMo4+AC were modeled temperature dependent in the range of 20 to 400 °C (Table 3). Table 3: Temperature dependent parameters used for Young’s modulus , Density , Thermal conductivity , Specific heat capacity , Coefficient of thermal expansion regarding the sheet metal material 42CrMo4+AC according to Spittel and Spittel (2009) as well as Richter (2011) Temperature / °C / GPa / kg/m 3 / W/(m ⋅ K) / J/(kg ⋅ K) / 10 -6 /K 20 212 7818.7 40.38 475.18 10.79 100 207 7792.3 42.36 488.62 11.23 200 199 7762.5 42.33 502.02 11.98 300 192 7730.4 40.50 522.15 12.54 400 184 7696.1 38.00 546.61 13.06 In order to consider deletion of elements during the shearing process and an associated thermal influence on the shear zone, the Cockcroft-Latham normalized ductile fracture criterion L modified by Oh et al. (1979) was used. The criterion (Equation 2) is based on the maximum principal stress max , the equivalent stress and the equivalent true or plastic strain at fracture ̅ f . Elements are deleted when the computed fracture criterion reaches the defined critical fracture threshold ri . L = ∫ max ̅ ̅ f 0 d ̅ with element deletion at L ≥ ri (2)