Issue 46

L. Sorrentino et alii, Frattura ed Integrità Strutturale, 46 (2018) 285-294; DOI: 10.3221/IGF-ESIS.46.26 287 called Fibrelam ® Grade 5. The first core material chosen for the core was the DIAB Divinycell P60 panel, that is made of a recyclable closed cell thermoplastic foam. It is addressed to public transport, industrial applications and wind applications, since it offers excellent FST properties (fire and toxicity of fumes), resistance to high temperatures, good thermal insulation and low water absorption. It also offers good mechanical properties, such as excellent resistance to chemical agents and a very low density of 60 kg/m 3 . This core is compatible with any type of resin, polyester, vinylester, epoxy and it can be used with most prepregs because it resists up to the temperature of 150 °C, so there are no problems for the prepreg oven polymerization. It was used a panel of 8 mm thickness. The composite skins were constituted by glass fiber/epoxy matrix prepreg fabric. The composite fabric consisted of fibers woven at [0/90]; thirteen fabrics were overlapped and cured at a temperature of 125°C for 90 minutes by means of vacuum bagging in autoclave to obtain a board thickness of about 2 mm, so the total thickness of the sandwich structure was 12 mm. Composite material density is 1850 kg/m 3 ; moreover, this material has a compressive strength of 650 MPa and a tensile strength of 1750 MPa. The other core material was the Fibrelam ® , that is made of unidirectional glass fiber skins with a cross pattern of thickness 0.38 mm glued to an aramid alveolar panel to have a total sandwich thickness of about 10 mm. It has excellent resistance to compression (5.5 MPa) and to impact (4.5 Nm), that are compatible with stone application. The Fibrelam ® has a remarkable stiffness, testified by a maximum deformation of 0.8 mm/m, which allows the sandwich structure to be used without any support and a flames extinguish time of 4 seconds after a 60-second exposure; its density is 2.5 kg/m 2 . Perlato Royal of Coreno and Absolute Black granite were the natural stones chosen for the tiles. Perlato Royal of Coreno is characterized by a high strength to wear and to impact that together to its resistance to weather agents carried out to be used for buildings. It has a density of 2650 kg/m 3 ; it has a compressive strength of 166 MPa and a tensile strength of 10 MPa. The Absolute Black granite is very hard and since it is available in small volumes, it has a great commercial value. It has a density of 3030 kg/m 3 ; it has a compressive strength of 295 MPa and a tensile strength of 27 MPa. Two adhesives were used to bond the self-produced composite sandwich to stone tiles: the Scotch-Weld TM EC-2216 B/A Gray of 3M TM and the Scotch-Weld TM AF163-2k of 3M TM . The first is an epoxy resin Scotch-Weld TM EC-2216 B/A Gray of 3M TM , whose polymerization time is 7 days at 24°C, 2 hours at 66°C and 30 minutes at 93 °C. It is commonly used for composite material and it is suitable for aerospace applications. The second adhesive is a thermosetting epoxy resin Scotch-Weld TM AF163-2k of 3M TM , which occurs as a veil. It polymerizes in 90 minutes at 107.2 °C. The commercial sandwich Hexcel Fibrelam ® was glued to stone only by means of the resin Scotch-Weld TM AF163-2k of 3M TM , since it allows to reach the best geometrical precision in gluing, so taking full advantage of the high precision of the commercial sandwich. All the samples manufactured are shown in Tab. 1, five repetitions were carried out for each parameter combination. After bonding to sandwich panels, each hybrid tile was cut with a diamond saw to produce samples measuring 40 mm x 200 mm for 3-point bending tests. The 3-point bend test was considered for evaluating the structural properties of the obtained samples, according to the UNI EN 12372 standard for stone products. In detail, the width of the sample should be between 25 mm and 100 mm and it has to be twice the size of the largest grain in the stone. The sample length should be six times its thickness, while the distance between the two pins that sustain the sample should be five times its thickness. Factor # levels Level value Stone type 2 Coreno Perlato Royal, Absolute Black Granite Stone thickness [mm] 3 3,4,6 Core type 2 DIAB P60, Fibrelam Grade 5 Adhesive type 2 EC-2216, AF163-2k Table 1 : Experimental plan. Process to prepare the sample with core DIAB P60 and EC-2216 adhesive Three marble tiles and three granite tiles, whose dimensions were 200 mm x 200 mm x 10 mm, were brushed on the raw side by means of an abrasive paper and, then, they were cleaned by a cloth soaked in ethylic alcohol. The same operations were repeated on the composite laminate of 2.2 mm thickness, once cut through a band-saw at 205 mm x 205 mm. A