Issue 47

F. Moroni et alii, Frattura ed Integrità Strutturale, 47 (2019) 294-302; DOI: 10.3221/IGF-ESIS.47.22 295 different manufacturing routes can be used for bonding composite parts: i) secondary bonding; ii) co-bonding; iii) co-curing (with or without adhesive). The last two methods are generally preferred over the first one for bonding small- to medium- size components because of the smaller number of manufacturing steps [8]. However, the majority of papers in the literature test fracture behavior of secondary bonded joints. A comparison of the fracture toughness of co-bonded and joints co- cured without adhesive is generally missing, too. Most of the times adherents are made by a cross- and/or angle ply stacking of unidirectional (UD) plies, while in several applications wowen, satin or braided textiles are used for manufacturing purposes. This paper is therefore aimed at testing and comparing co-cured (without adhesive) and co-bonded joints of textile CFRP adherents. The experiments concern the tensile strength and the mode I and mode II fracture toughness, to assess whether co-bonding can guarantee the same performance of a co-cured composite joint. E XPERIMENTS Materials and specimen manufacturing satin-weave (5H) carbon fiber C280 T1100 12K, pre-impregnated with 38% 2573 epoxy resin, was supplied by Toray with a ply nominal thickness of 0.3mm. The structural adhesive used in this work is AF 163-2U Scotch-Weld thermosetting modified epoxy, supplied by 3M in the form of unsupported film with a 0.15 kg/m 2 mass and 0.14 mm nominal thickness. Mechanical properties from the suppliers technical datasheets are reported in Tab. 1. Material Modulus of elasticity (GPa) Tensile Strength (MPa) T1100 CF + 2573 epoxy resin adherent 0° tensile 89 1900 90° tensile 87 1740 0° compressive 76 800 90° compressive 80 740 0° Flexural 75 1060 0° ILSS - 74 90° ILSS - 73 AF 163 adhesive film 1.1 48 Table 1 : Properties at environmental temperature of the materials used in this work. The composite joint was co-cured a 130°C for 120 min in a vacuum bag with an applied external pressure of 6 bars while, in the case of co-bonded joint, the adhesive film was placed on a cured, sandpapered and cleaned with acetone CFRP adherent, then pre-preg plies were laid over the adhesive and the resulting layup was consolidated at 130°C for 120 min in a vacuum bag with applied external pressure of 6 bars. The experimental plan includes the following tests: - TRAZ-BJ: tensile test on butt-bonded cylindrical joints, in order to extract the average tensile strength of the adhesive used in a joint; - TRAZ-CI: tensile test on cylinders of CFRP, stressed in direction 3 (perpendicular to the lamination plane) in order to extract the ILTS (InterLaminar Tensile Strength); - TRAZ-CE: tensile test on cylinders of composite material, stressed in direction 3 (perpendicular to the lamination plane) in order to extract the elastic module in direction 3 of the laminate; - DCB: mode I delamination/debonding test - ENF: mode II delamination/debonding test ILTS and co-bonded joint average tensile strength The nominal geometry of TRAZ-CI and TRAZ-BJ specimens is shown in Fig. 1(a) and (b), respectively, that comply with the ASTM D 7291 standard. The threaded aluminum ends are bonded to the composite using 3M 9323 A/B two-part, structural epoxy adhesive, which is commonly used by Dallara in cold bonding of composites and it is generally stronger than the composite itself. The specimens are attached to the testing machine (MTS 810 with Trio Sistemi e Misure RT3 controller) using two universal joints for self-alignment. In the case of TRAZ-CI tests, beside the extensometer three strain gages are placed at angles of 120° (Fig. 2) in order to have the value of the out-of-plane Young's modulus measured on the composite, but also to monitor that the strain is uniform across the gage section. The tests have been conducted under displacement control at a crosshead displacement rate of 0.002mm/sec. A