Issue 17

V. Crupi et alii, Frattura ed Integrità Strutturale, 17 (2011) 32-41; DOI: 10.3221/IGF-ESIS.17.04 35 step. This procedure was then repeated until a full rotation of 360° was achieved, and a total of 1440 projections were then obtained to be used in the 3D profile generation. The sizes of the voxels and images were 0.033÷0.050 mm and 2048 x 2048 pixels respectively. The integration time was chosen equal to 500 ms. It is important to underline that this NDT technique doesn’t require to cut and polish the samples for carrying out the X-ray measurements. This allows a significant savings of time and the investigation of the internal damage without perturbing the impacted specimen. The system, based on a variable focal-spot size technology, creates the cross-sectional images of three-dimensional objects using X-rays. A volumetric representation of the item to be inspected is obtained as a result of the CT . Both the material inner and outer structures and the geometric dimensions of the item to be inspected are recognizable. Figure 3 : Y.CT Vario cone-beam X-ray tomography system. R ESULTS AND D ISCUSSION Analysis of investigated composites using CT he 3D reconstruction of an aluminium honeycomb panel with 3 mm cell size was obtained by means of the CT as shown in Fig. 4. The cell sizes can be measured by CT system; as example the analysis in terms of grey levels intensity, along the middle line, drawn on the sample middle section, is reported in Fig. 4. The investigation of the aluminium foams by means of CT is very useful to check their quality in terms of porosity distribution, that influence the mechanical properties of the foams. Fig. 5 shows the CT reconstruction of an AFS Schunk panel; the tomogram allows the investigation of the foam porosity and the localization and quantification of the pores by checking the grey levels intensity. Because of the non homogeneity of the AFS panels, it is strongly necessary to check the sample before carrying out the impact test and to discard defective samples, as the one shown in Fig. 5. CT analyses allowed, also, the investigation of a polymeric sandwich with glass-fibre reinforced skins and foam core for quality control of flaws and for the evaluation of thickness and geometric properties of core and skin layers (Fig. 6). The thickness and the typology of each laminate layer can be evaluated by CT, the dimensions of a biaxial ply of a GFRP laminated composite, obtained by checking the greyscale levels, are shown in Fig. 7. Energy absorption and collapse modes of the composites under impact loading The low-velocity impact tests produced the complete failure of the GFRP laminated composite at an initial impact velocity of 8 m/s with an energy absorption of about 217 J, whereas a test at v = 9 m/s was necessary for the failure of the hybrid Kevlar/fiber-glass laminate, confirming the higher impact strength of Kevlar fiber composites than that of GFRP.