Issue 17

V. Crupi et alii, Frattura ed Integrità Strutturale, 17 (2011) 32-41; DOI: 10.3221/IGF-ESIS.17.04 33 An extensive series of experimental tests were performed in order to obtain the mechanical characterization of aluminium foams under static and dynamic loading conditions [5]. Hazizan et al. [6] carried out low velocity impact tests in order to investigate the response of sandwiches, made of aluminium honeycomb core and glass fibre reinforced/epoxy skins. Compston et al. [7] compared the low velocity impact behaviour of aluminium foam and polymeric foam sandwiches and different damage modes were observed: the polymeric foam sandwiches exhibited localized damage (skin fracture and core crushing) with negligible permanent out-of plane strain, whereas the aluminium foam sandwiches experienced little fracture with extensive permanent out-of plane strain. Moreover the post impact characterization tests showed that the aluminium foam specimens exhibited lower strain, suggesting a better damage tolerance respect to the polymeric foam sandwiches. In a previous research paper of the authors [8], the structural response of aluminium foam sandwiches under static and impact loading was compared with that of the PVC foam sandwiches. Aim of the present research was the investigation and the comparison of the structural response of laminated and sandwich (with polymeric and aluminium core) panels under impact loading. The failure mode and the damaged structure of the impacted composites have been investigated by an X-ray computed tomography (CT) system, that allows a three-dimensional reconstruction of the analyzed object. X-ray CT improves the use of X-ray radiography by imaging detailed cross-sectional views of the specimens, thereby resolving through-thickness delamination and matrix cracks. Cone-beam CT is a three dimensional imaging technique which is used non-destructively to inspect the inner structure of an object by transmission measurements using X-rays. A large number of projection images are obtained by rotating the sample. After a reconstruction process, the volume rendering of the external and internal geometries of the part is created. CT is a very useful tool to identify structural inhomogeneities, voids, fractures, microcracks and porous structures in both metallic and polymeric composites where there is a significant difference in density. This non-invasive technique has been used to characterize quantitatively the microstructure and the internal architecture of different typologies of closed-cell aluminium alloy foam [9] and to obtain the data for finite element models of open-cell aluminium foam specimens [10]. Schilling et al. [11] demonstrated that the CT system can detect the damage and internal flaws, including delamination and microcraking, in fibre-reinforced polymeric matrix composites. Laminated composites, stitched with varying stitch densities and stitch thread thickness, were subjected to low-velocity impact tests at different energy values and the impact damages were investigated using an X-ray micro-computed tomography, that was able to show detailed through-thickness matrix cracks distribution and 3D delamination damage pattern [12]. It was noted that a CT image alone provided much better resolution of the outline of the damage area than the planar view of X-radiography [13]. M ATERIALS AND M ETHODS Materials n this paper, the following different typologies of composites have been investigated (Fig 1): laminated composites, PVC foam core sandwiches, aluminium foam sandwiches ( AFS ) and aluminium honeycomb sandwiches. Figure 1 : Investigated composite materials. I