PSI - Issue 61

T. Stoel et al. / Procedia Structural Integrity 61 (2024) 206–213 T. Stoel et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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laminate length of 10 mm is sufficient to inhibit artificial effects of the clamped laminate length on the stress state in the shear zone. Therefore, a squared laminate with a length of 25 by 25 mm was used. The laminate was meshed with 8 node continuum hexahedron elements with reduced integration points (C3D8R).

(a) FE-model of near-net-shape blanking

(b) Meshing of the CFRP laminate (top view)

el = 1 mm

Symmetry plane

Blank holder

2 mm

Punch

Shear zone

el = 0.1 mm

Die

CFRP laminate

z

el = 1 mm

Counter punch

Symmetry plane

y

x

Figure 2: Near-net-shape blanking process model (a) and meshing of the laminate (b); Legend: el – Element size Furthermore, an enhanced hourglass control algorithm was used. In order to achieve a high modeling accuracy, the area of the laminate near the shear zone was significantly finer meshed than the rest of the laminate as shown in Figure 2b. The smallest element size was set to el = 0.1 mm. No significant effect of further mesh refinement on the blanking force was detected. 2.4. Contact and boundary conditions To define interactions between the laminate and the tool parts a general contact algorithm with a uniform Coulomb friction coefficient of = 0.3 was used. The blanking process was performed by setting the die immovable and translating the punch in negative z-direction with a constant blanking velocity of b = 45 mm/s. Blank holder and counter force were set in a way to achieve a surface pressure on the laminate of 10 % and 45 % of the empirically determined transverse compressive strength ⊥ of the respective laminate (see Table 3). Table 3: Values for blank holder and counter force in dependence of the transverse compressive strength ⊥ Magnitude of the force Unit BH ( f = 60 %) CP ( f = 60 %) BH ( f = 70 %) CP ( f = 70 %) kN 24 12 29 15 By applying the needed process forces due to a concentrated force to the blank holder and counter punch, a strong oscillation of the corresponding reaction forces was observed. To eliminate those oscillations, a closed loop controlled indirect force application for the blank holder force by Shirobokov et al. (2018) was used, modified, and made applicable for the counter force. Therefore, the motion of blank holder and counter punch was implemented by means of incremental displacements. After each time increment, the difference between the set force set and the applied force ( BH and CP ), which is determined by the resulting reaction force R , is checked. If the applied force is lower than the set force, the counter punch or blank holder is displaced in the direction of the laminate by ± = 1 ∙ 10 −5 mm. If the difference is negative, the counter punch or blank holder is displaced in the opposite direction of the laminate by − = 1 ∙ 10 −5 mm. Otherwise the respective tool element is not moved. Furthermore, a damping factor is used to − (10 % of ⊥ ) + (45 % of ⊥ ) kN 108 55 132 68