Issue 17

V. Crupi et alii, Frattura ed Integrità Strutturale, 17 (2011) 32-41; DOI: 10.3221/IGF-ESIS.17.04 39 (a) (b) (c) Figure 14 : Tomograms of GFRP-PVC foam core panel after the impact (v = 7 m/s); the ply in (c) is on the lower skin of the sample. For that concerns the two AFS typologies ( Schunk and Alulight ), the complete failure of the specimens (foam core thickness 9 mm and skin thickness 1 mm) occurred after the impact tests at v=7 and 8 m/s. Their impact behaviour is different respect to the polymeric sandwiches; the AFS experienced some foam cell crushing and extensive out-of-plane displacement even at low impact energy. The capacity of energy absorption is strongly influenced by the foam quality, that resulted to be better for the AFS Alulight panels, that require an energy amount of 142 J for the complete failure with the rupture of the lower skin, whereas for the AFS Schunk panels it is sufficient an energy value of 122 J. The data scattering, observed in the tests, is due to the different porosity distributions of the aluminium foams, so it is important to check the foam quality by means of non-destructive techniques, such as the CT. Figs. 15 and 16 show the tomographic images just for AFS Alulight and Schunk panels after the impact at v = 2 and 4 m/s. These tomographic images allow a better understanding of their failure mode, that is characterized by the progressive crushing of the foam cells with a more uniform distribution of the impacted load. The post-impact investigation of the specimens confirms the results of the tests conducted by Compston et al. [7]: the AFS specimens experienced extensive ductile fracture with large out-of-plane displacement compared to the impact behaviour of the polymeric sandwiches and laminates. Moreover, the AFS structures are relatively intact compared to the more catastrophic and localized fracture of the polymeric sandwiches, so they exhibit a better post-impact damage tolerance and mechanical properties. Low velocity impact tests were performed, also, on honeycomb panels (core thickness 9 mm and skin thickness 1 mm). The experimental results confirm, as expected, that the honeycomb cells of smaller size ( d = 3 mm) are able to absorb greater amounts of energy at a given impact velocity and require an energy value of 128 J for the complete failure respect to the 116 J necessary for the failure of panels with larger cells ( d = 6 mm). The collapse of the honeycomb sandwich occurs for the initial deformation of the upper skin and for the buckling of the core cells as confirmed by the CT investigations (Fig. 17). The specimens experienced ductile fracture with out-of-plane displacement. The analysis of the tomographic images (Fig. 17) of the panels with smaller cells ( d = 3 mm) reveals that the collapse of the cells is located in the area concerned by the impact, while the rest of the structure remains almost intact. Instead, with the same initial energy of impact, the collapse of the panels with larger cells ( d = 6 mm) affects almost all cells. Figure 15 : Tomographic images of the damaged AFS panels (left Alulight ; right Schunk ) after the impact (v = 2 m/s). Figure 16 : Tomographic images of the damaged AFS panels (left Alulight ; right Schunk ) after the impact (v = 4 m/s).