13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- 2.3. Experimental results 2.3.1. Description of the perforation test A 2 mm 2024 AA sheet is impacted by the striker. Thus, the striker nose induces plastic strain in the center of the sheet. More than four cracks appear, two in the laminated direction, two in the transversal direction, and sometimes in the 45° direction (Table 2). If the conical nose does not go through the plate, the striker bounces. If the conical nose goes through the sheet, the force sensor hits the sheet involving a bounce. All are analyzed before this bounce. Table 2. Testing plan and main results. Test number 1 2 3 4 5 6 7 8 9 10 11 12 Total mass (kg) 13 13 13 13 13 13 13 13 13 13 13 13 Drop test height (m) 0.4 0.4 1 1 1.5 1.5 1.8 1.8 2 2 2.2 2.5 Impact velocity (m/s) 2.79 2.71 4.24 4.20 5.00 4.81 5.52 5.58 5.72 5.82 6.30 6.90 Petals number 4 4 5 5 6 4 6 5 4 4 6 5 Residual velocity (m/s) -1.4 -1.3 -1.3 -1.5 -1.3 -1.1 2.6 3.2 2.6 2.6 4.0 4.6 Absorbed Energy (J) 51 48 117 115 163 150 155 160 168 152 157 170 Peak Force (kN) 8.7 8.4 9.1 8.8 10.0 9.2 9.3 9.5 9.5 9.2 9.2 Unknown Maximal Force (kN) 6.6 7.1 8.2 8.6 9.4 9.4 9.6 9.4 9.9 9.6 9.6 Unknown Figure 2. Residual velocity versus initial velocity. 2.3.2. Velocity results It is usual to plot the residual velocity of the striker as a function of the initial velocity. In Fig. 2, if the striker does not perforate the sheet completely, the residual velocity is taken equal to 0. Classical results show that starting from the ballistic limit the velocity Vbl is a continuous growing curve [8]. Here this hypothesis is confirmed. In Fig. 2, pictures are taken at the end of the perforation (before the bounce). Thus it can be concluded, that using a 13 kg conical striker, the ballistic limit velocity to perforate a 2 mm sheet of 2024 AA is between 5 and 5.52 m/s. To obtain a better approximation of the ballistic limit velocity, the definition of the absorbed energy during perforation is used. This
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