Issue 48

T. Febra et alii, Frattura ed Integrità Strutturale, 48 (2019) 242-248; DOI: 10.3221/IGF-ESIS.48.25 246 Some samples were inspected by C-Scan technique, supplied by Physical Acoustics Cooperation , in order to analyze the damaged zone. Figure 5 shows the typical and representative images of the BGFn mat and Km mat samples, impacted by 5J. It is possible to observe that, in spite of the higher damaged area of the Km mat sample, the C-Scan image for BGFn mat insert suggests a much more intense damage. These observations are in accordance with the lower dynamic stiffness obtained in Fig. 3b) for the BGFn mat inserts. Similarly to what happens in sandwich composites, three types of failure can be observed: rupture of the superficial polymer layers, delamination of the interface and in extreme conditions, rupture of the fibers [20]. Multi-impact tests The results of the multi-impact tests (in which the same specimen was subjected to successive impacts with impact energy increasing successively 1J for each test) are summarized in Figures 6 and 7, in terms of the recovered energy (in percentage of the impact energy) and the dynamic stiffness, respectively. Fig. 6 shows the same tendencies of the single impact tests presented in Fig. 3a), indicating that biaxial glass fibre netting inserts promote the highest values of recovered energy. However, the multiaxial glass fibre mesh insert shows the lower recovered energy and perforation energy. Recovered energy decreases parabolic until failure, according with F. Cucinotta et al. [20]. Also, as in the single impact tests presented in Fig. 3b) Kevlar inserts exhibit the highest dynamic stiffness, in contrast to glass fibre inserts (Fig. 7). In any case, a significant difference was observed on the dynamic stiffness values for the multi-impacted specimens in comparison with the single impacted tests, decreasing significantly after the peaking for an intermediate impact energy. This fact is caused by the accumulated damage occurred at the intermediate impact energy. Figure 6 : Recovered energy after multi-impact tests. Figure 7 : Dynamic stiffness after multi-impact tests. Figure 8 : Comparison of the recovered energy for multi-impact and single impact tests.