Issue 17

V. Crupi et alii, Frattura ed Integrità Strutturale, 17 (2011) 32-41; DOI: 10.3221/IGF-ESIS.17.04 34 The polymeric sandwiches consist of a PVC foam core and two GFRP (glass-fibre reinforced plastic) laminated skins. The measured total density is about 0.68 g/cm 3 . The PVC foam core has a density of 0.075 g/cm 3 and a thickness of about 15 mm, while the average thickness is 3 mm for the upper skin and 7 mm for the lower one. One of the two investigated laminates is made of alternated fibre-glass MAT and GFRP [0°/90°] biaxial layers and has a thickness of about 10 mm, the other one has an additional [0°/90°] Kevlar reinforced layer and two GFRP [± 45°] biaxial layers and a thickness of about 9 mm. All of these typologies of composite materials (sandwiches and laminates) are commonly used for yacht structures. Two different commercial aluminium alloy foam sandwiches have been investigated. The first one ( Schunk GmbH, Heuchelheim, Germany ), consists of an AlSi7 foam core and two AlMn1 faces obtained by extrusion (integral skins), the second one ( Alulight GmbH, Ranshofen, Austria ) is made of an AlSi10 core and two Al (99.5 %) faces, bonded to the core by an epoxy adhesive. The measured total density is 0.87 g/cm 3 for AFS Schunk and 0.95 g/cm 3 for AFS Alulight , while the core density, calculated by assuming a density of 2.73 g/cm 3 for the aluminium skins and a face thickness of 1 mm, is 0.45 g/cm 3 for AFS Schunk and 0.53 g/cm 3 for AFS Alulight . The following aluminium honeycomb sandwiches typologies were, also, investigated: 1/8-5052-0.0020 and 1/4-5052- 0.0025; the designation corresponds to cell size (inch) – alloy - foil thickness (inch). The first typology consists of hexagonal cells with diameter of 3 mm and thickness of 0.05 mm and has a honeycomb density equal to about 130 kg/m 3 , the second one has hexagonal cells with diameter of 6 mm and thickness of 0.06 mm and has a honeycomb density equal to about 80 kg/m 3 . The honeycomb core is made of AA5052 aluminium alloy and the two skins are realized by AA5754 H32 aluminium alloy. The skin thickness is about 1 mm. Methods Low-velocity impact tests were performed using a CEAST Fractovis Plus drop-weight impact test machine (Fig. 2). The instrumented impactor, having a hemispherical tip with diameter of 20 mm, hit the centre of the specimens. The tests were conducted on the different typologies of composites with an impactor mass of about 7 kg. The drop height was adjusted to generate different level of initial impact energies (from 56 to 284 J), each with a corresponding impact velocity (from 4 to 9 m/s). The specimens were clamped by a rigid metallic plate with a diameter of 40 mm, as shown in Fig. 2, without crushing the sample. (a) (b) Figure 2 : a) Drop-weight impact test machine; b) Clamping system. The impact damage of the composites, undetectable by a visual inspection, was analyzed by the Y.CT Vario focus X-ray computed tomography system (Fig. 3). This unit is equipped with an X-ray source having maximum voltage and current of 225 kV and 7.1 mA, respectively, depending on the focal spot size that can be chosen among these values: 250 µm, 300 µm, 500 µm and 800 µm. The detector system is a flat panel with a resolution of 1920 x 1536 pixels. The scans, reported in this paper, were conducted with 250 µm focus and X-rays were set at a voltage of 210 kV and at a current of about 1.1 mA,. Different filters (Al, Cu and Sn filters) were used depending on the sample. It is worth noting that the source- detector and the source-sample distances are critical in determining the magnification of the sample and, thus, the resolution of the scan: the closer the sample was to the source and the further away from the detector, the higher the magnification. A conical X-ray beam scanned the sample, which was rotated at increments of 0.5°/s for each rotation