PSI - Issue 33

M.F.M.O. Rosas et al. / Procedia Structural Integrity 33 (2021) 115–125 Rosas et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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2. Materials and methods 2.1. Materials

The tubular adherends were made of the high-strength AW6082 T651 aluminum alloy, which was characterized in Moreira and Campilho (2015) in bulk tension as defined in the standard ASTM-E8M-04. Analysis to the test data resulted in the following properties: Young’s modulus ( E ) of 70.07  0.83 GPa, tensile yield stress (  y ) of 261.67  7.65 MPa, tensile failure strength (  f ) of 324.00  0.16 MPa and tensile failure strain (  f ) of 21.70  4.24%. The ductile epoxy Araldite ® 2015 was selected for this work (Araldite ® from Huntsman, Basel, Switzerland). The ductility in two of these adhesives is highly relevant because bonded joints typically develop peak stresses, and ductile adhesives provide an improved behavior in these cases due to plasticization allowance before failure (Nunes et al. 2016) . Actually, the adhesives’ ductility avoids the occurrence of brittle failures and enables that a larger portion of the bond participates in the transfer of loads between adherends (Davis and Bond 1999). This adhesive was duly characterized in references (Campilho et al. 2011, Campilho et al. 2013). Bulk adhesive testing was considered to estimate the relevant mechanical properties, using the NF T 76-142 standard, resulting in E ,  y ,  f and  f . The shear mechanical properties were derived from Thick Adherend Shear Tests (TAST), in bonded specimens with C45E steel adherends. All this procedure was done in accordance with the 11003-2:2001 ISO standard. Fracture characterization (i.e., G IC and G IIC ) was accomplished by Double-Cantilever Beam (DCB) and End Notched Flexure (ENF) tests, respectively (Campilho et al. 2011, Campilho et al. 2013), with specimens showing the same t A of the bonded joints in this work. All properties are summarized in Table 1.

Table 1. Mechanical and fracture properties of the adhesive Araldite ® 2015 (Campilho et al. 2011, Campilho et al. 2013). Property Araldite ® 2015 Young’s modulus, E [GPa] 1.85±0.21 Poisson’s ratio,  0.33 a Tensile yield stress,  y [MPa] 12.63±0.61 Tensile strength,  f [MPa] 21.63±1.61 Tensile failure strain,  f [%] 4.77±0.15 Shear modulus, G [GPa] 0.70 b Shear yield stress,  y [MPa] 14.6±1.3 Shear strength,  f [MPa] 17.9±1.8 Shear failure strain,  f [%] 43.9±3.4 Toughness in tension, G IC [N/mm] 0.43±0.02 Toughness in shear, G IIC [N/mm] 4.70±0.34 a manufacturer’s data b estimated from the Hooke’s law using E and 

2.2. Geometry, fabrication and testing

The tubular joint geometry and dimensions are shown in Fig. 1 and Table 2. The fabrication process initiated by milling the aluminum tubes in a manual lathe, using circular bars as raw material, up to reaching dimensions near to the final adherend diameters. A carbide insert end mill was used to trim the outer diameters, and a carbide drill attached to the lathe’s tailstock cut the butt holes. In order to avoid air entrapment in the joints during bonding, it was necessary to drill a hole with Ø1 mm in a manual vertical drill and transversely in the un-bonded region of the outer adherends, because the hole drilled to reach the internal diameter of the tubes is not through-all (Sekercioglu 2007). After the milling process, grit blasting with corundum sand and cleaning with a degreaser were applied to make the surfaces rough for bonding. At the beginning of joint fabrication, three nylon wires equally spaced with Ø0.2 mm were used along the inner perimeter of the outer tube, guaranteeing the concentricity of the two tubes, in which t A =0.2 mm was reached. Then the adhesive was spread in both bonding surfaces so that the tubes were positioned with a digital caliper to ensure the correct L O and then carefully assembled together, to avoid any rotation movements between tubes, because this would lead to adhesive expulsion and lacks of adhesion. After bonding, it was necessary to left the specimens to cure, at room temperature and during one week, in a jig ensuring the concentricity of the tubes. Finally, the milling process was used to remove the excess of adhesive. A Shimadzu-