PSI - Issue 36

Valeriy Kharchenko et al. / Procedia Structural Integrity 36 (2022) 137–144 Valeriy Kharchenko, Eugene Kondryakov, Oleg Katok et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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The following characteristics of the material for steel 65G were specified for the striker and support: ( σ Т = 785 MPa, σ В = 980 MPa, ρ = 7800 kg / m 3 , Е = 2.15 × 10 5 MPa, μ = 0.3). The choice of contact conditions can significantly affect the results of calculations. The subsequent calculations define rigid contact conditions between the plate and the support, as well as the plate and the striker without friction. Fig. 1 illustrates the FE models for two types of punches: hemispherical and flat. They are used to simulate perforation processes in dynamic and static formulations. ¼ part of the model is simulated under the corresponding symmetry conditions. A series of test calculations with different FE dimensions have been carried out to verify the accuracy and convergence of the calculation results. From the obtained results the minimum FE dimensions of 50 µm within the contact zone are chosen.

Fig. 1. FE models with hemispherical (a) and flat (b) punches for the calculation of static and dynamic perforation processes.

3. Results and Discussions Based on the calculation results, the analysis of the stress-strain state and the character of material fracture was performed. Fig. 2 illustrates the equivalent strain at different time points under dynamic perforation by flat (Fig.2(a)) and hemispherical (Fig.2(b)) punch. It can be seen that there is a “plugging” type of fracture under flat punch perforation . After the moment of circular crack initiation (t = 760 μs), it propagates at a high velocity and the total time of fracture is 20 μ s. This type of fracture under dynamic flat punch perforation is common even for ductile materials. Under hemispherical punch perforation (Fig.2(b)) a “petaling” type of fracture is observed. In this case, microcracks nucleate on the lower plate surface before fracture. The time between the onset of crack initiation and complete plate fracture is ̴ 240 μ s. Figure 3 shows the time dependences of force (a), punch velocity (b), and strain rate within the fracture zone (c) for dynamic flat and hemispherical punch perforation. It is obvious that the forces in the flat punch are significantly higher than in the hemispherical punch (Fig.3(a)), as well as their rate of fall after the start of fracture. The speed of the hemispherical punch decreases more slowly during loading than the flat punch (Fig.3(b)). The strain rate during loading with a flat punch reaches values of 1200 s -1 , for a flat punch, the strain rate does not exceed 900 s -1 . The duration curves of stress and logarithmic strain components in the plate fracture zone are shown in Fig.4 and Fig.5 for dynamic perforation by flat and hemispherical punches, respectively. It is apparent that throughout the entire loading process in the plate fracture zone, stress components σ 11 and σ 33 dominate, while strains ε 12 and ε 22 dominate among the strain components.