PSI - Issue 41

A.E.S. Pinheiro et al. / Procedia Structural Integrity 41 (2022) 60–71 Pinheiro et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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tensile yield stress of 261.67±7.65 MPa, tensile strength of approximately 360 MPa, tensile strain of 21.70±4.24% and Poisson’s ratio of 0.33. The respective stress -strain curves ( σ - ε ) of the aluminum adherends were experimentally obtained according to the ASTM-E8M-04 standard (ASTM-E8M-04 2004). To promote bonding between the adherends, three types of adhesives were selected for the tubular adhesive joints: Araldite ® AV138 (brittle epoxy adhesive), Araldite ® 2015 (moderately ductile epoxy adhesive), and SikaForce ® 7752 (structural polyurethane adhesive, which combines high ductility with moderate strength). These adhesives were characterized in previous works and the respective mechanical and fracture properties were obtained (Campilho et al. 2011, Campilho et al. 2013, Faneco et al. 2017). Experimental tests on bulk specimens were performed, aiming to determine the tensile mechanical properties ( E , σ y , σ f and ε f ) of all the adhesives analyzed in this work. For the assessment of the equivalent shear properties, thick-adherend shear tests (TAST) were employed using steel adherends. The fracture properties were determined by means of the double-cantilever beam (DCB) tests for G IC , and end-notched flexure (ENF) for G IIC . The obtained mechanical and fracture properties are listed in Table 2. Table 2. Mechanical and fracture properties of the Araldite ® AV138, Araldite ® 2015 and SikaForce ® 7752 (Campilho et al. 2011, Campilho et al. 2013, Faneco et al. 2017). Property Araldite ® AV138 Araldite ® 2015 SikaForce ® 7752 Young’s Modulus, E (GPa) 4.89 ± 0.81 1.85 ± 0.21 493.81 ± 89.6 Poisson’s ratio, ν 0.35 b 0.33 b 0.33 b Tensile yield stress, σ y (MPa) 36.49 ± 2.47 12.63 ± 0.61 3.24 ± 0.5 Tensile strength, σ f (MPa) 39.45 ± 3.18 21.63 ± 1.61 11.49 ± 0.3 Tensile failure strain, ε f (%) 1.21 ± 0.10 4.77 ± 0.15 19.18 ± 1.4 Shear modulus, G (GPa) 1.56 ± 0.01 0.56 ± 0.21 187.75 ± 16.4 Shear yield stress, τ y (MPa) 25.1 ± 0.33 14.60 ± 1.30 5.16 ± 1.1 Shear strength, τ f (MPa) 30.2 ± 0.40 17.90 ± 1.80 10.17 ± 0.6 Shear failure strain, γ f (%) 7.8 ± 0.7 43.90 ± 3.40 54.82 ± 6.4 Toughness in tension, G IC (N/mm) 0.20 a 0.43 ± 0.02 2.36 ± 0.2 Toughness in shear, G IIC (N/mm) 0.38 a 4.70 ± 0.34 5.41 ± 0.5 a estimated values from Neto et al. (2012) b manufacturer’s data 2.3. Joint fabrication and testing Joint fabrication initiated by preparing the tubular adherends. The manufacturer provided the adherends as 120 mm-long solid round bars with diameters higher than d SI and d SE . The bars were milled to their final dimensions in a lathe. The outside dimensions were milled with an end mill with carbide insert. On the other hand, the holes were opened using a carbide drill. To enable the bonding process to occur smoothly, i.e., without inside pressure created due to the air entrapment in the joint (Sekercioglu 2007), a transverse hole (Ø1 mm) was drilled in the un-bonded region of the outside adherend, using a manual dri ll. Subsequently to the adherends’ milling, the bonding surfaces were roughened by grit blasting with corundum abrasive particles, and after cleaned with acetone. The joint assembly followed, in which Ø0.2 mm diameter calibrated nylon wire was used to ensu re the tubes’ concentricity during assembly and curing. This was accomplished by placing three of these wires equally spaced along the inner part of the outside adherend, which is exactly the available space between the two adherends. After assembly, this assures the correct placement between the adherends and guarantees the design value of t A =0.2 mm. Adhesive pouring was done in both adherends, and assembly took place exclusively with a longitudinal movement between the tubes, i.e., without rotation, to prevent adhesive expulsion from the bonding sites. The correct L O was achieved by measuring the relative position between both adherends with a digital caliper. The tubular joints were then placed in a jig to prevent misalignments between the tubes and left to cure for one week at room temperature (RT). As the final fabrication stage, the adhesive excess resulting from the assembly process was trimmed in a vertical mill with care to avoid damaging the joints. After this, the specimens were ready for testing, which was undertaken in an Autograph AG-X machine (Shimadzu), using a 100 kN load cell. The joints were tested at Room-Temperature (RT) with a loading rate of 1 mm/min. The load-displacement ( P -  ) data was the test output, which was then analyzed to produce the results that will be presented in this work. Five tests were considered for each joint condition (set L O -adhesive type) and, for each joint condition, at least four valid tests were taken and averaged for comparison with the different tested methods.