13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- energy is calculated as the difference between the initial kinetic energy and the final one and given in Table 2 for each test. The ballistic limit velocity Vbl can be estimated using the average absorbed energy Ea = 161 J as: √ (1) It can be concluded that the ballistic limit velocity is equal to 5.0 m/s with an error of +/- 3%. 2.3.3. Force during impact The force induced by the sheet on the striker can be plot during impact with the help of the force sensor (Fig. 6). Firstly, when the striker is reaching the sheet, a force peak can be observed. Secondly, cracks appear and so the force is continuously growing to be maximal when the striker cylindrical part is reached (test 7 to 12) or when the striker start to bounce back (test 1 to 6). After, the force is decreasing. If the conical nose goes through the sheet, the force sensor will hit the sheet involving a new increase of force. 2.4. Conclusion on experimental results It can be concluded that the ballistic limit velocity is about 5 m/s +/- 3% using a 13 kg striker. Four or more than four petals always appear in the sheet during perforation. The perforation absorbed energy tends to be constant and equal to 161 J for impact velocity between 5 m/s and 7 m/s. 3. Modeling of perforation Experimental tests are compared with two models: an analytical model based on energetic consideration and a shell finite elements model. 3.1 Perforation analytical model Figure 3. Analytical model. An energetic approach for perforation is proposed by Nazeer [13]. A conical nose striker impacting thin plates involves bending deformation, stretching deformation and N cracks’ propagation (fig 3).
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