Issue 17

V. Crupi et alii, Frattura ed Integrità Strutturale, 17 (2011) 32-41; DOI: 10.3221/IGF-ESIS.17.04 37 laminated composite (lower images of Fig. 8). The cross-sectional images of the impact-damaged area were taken in the frontal through-thickness direction of the specimen, considering a distance between two slices of about 0.4 mm. Only the most representative slices of the two damaged specimens are shown in Fig. 8. In the hybrid Kevlar/fiber-glass laminate the impact damage mechanism is dominated by failure of mat and biaxial fiberglass layers, whereas [0°/90°] Kevlar layer, positioned near the bottom face, don’t fail completely, but show out-of- plain deformation. That is confirmed by the tomogram of the middle section of an impacted laminate after a test at v = 8 m/s, shown in Fig. 9, where shear cracks are also observed. Figure 8 : Tomograms of laminated specimens impacted at energy of 126 J. Figure 9 : Tomogram of an hybrid Kevlar/fiber-glass laminate after the impact (v = 8 m/s). Fig. 10 shows that damage mechanism of a GFRP laminate after the impact at 6 m/s is characterized by delamination and fiber/matrix debonding. Three major damages can be found through the thickness: broken fibers, delaminations and transverse matrix cracks (Figs. 8 and 11). As described by Abrate [4], the damage process is initiated by matrix cracks which then induce delaminations, that is the debonding between adjacent laminas. Shear cracks, positioned at an angle from the midsurface, were observed in the tomogram of Fig. 11a relative to the middle section of impacted panel, confirming the important contribute of the shear stresses. In the low-velocity impact tests, the matrix cracks appeared in the first layer impacted because of the high, localized contact stresses and damage propagated through the thickness towards the bottom face, resulting in a pine tree pattern [4, 12] as shown in Fig. 11a. Large delamination damage occurs at the back face and progressively becomes smaller toward the impact face (Fig. 11b). The matrix cracking and fiber breakage were easily detected by a visual inspection as the near surface matrix cracking manifested itself in a stress- whitened or discolored patch, as reported in literature [14]. The longitudinal matrix interlaminar cracks below the impacted surface and the presence of delaminations at the bottom of the specimen can be noted in the tomograms of Fig. 11 relative to different transverse sections of the impacted specimens. In the low-velocity impact tests performed on the polymeric sandwiches, the first impacted skin is the 3 mm thick one. The tests, carried out at impact velocity values lower than 9 m/s, did not produce the complete failure of the specimens; the impactor penetrated only the upper skin and the core. The complete failure with the perforation of the lower skin, occurred only after the impact test at v=9 m/s and the energy amount, required to produce the complete failure, was evaluated to be 263 J. The post-impact specimens showed a localized damage and very little out of plane displacement. The CT investigations of the impacted polymeric sandwiches allow a better understanding of their failure mode, which is characterized by the rapid crushing of the PVC core (Figs. 12 - 14). Careful observation of the failure sequences in these specimens at various impact energies showed that the failure was produced by the formation of matrix cracks in the central region of the skin under the impactor (Fig. 12) and, for tests at higher impact energy, the cracks propagated along