PSI - Issue 28

Vinicius Carrillo Beber et al. / Procedia Structural Integrity 28 (2020) 1950–1962 V.C. Beber and M. Brede / Structural Integrity Procedia 00 (2019) 000–000

1955

6

 thick adherend shear test (TAST) joints with predominantly shear stresses.

Butt joints, when loaded under tension, present a state of nearly homogeneous tensile stress within the adhesive layer (Schneider et al., 2015). In scarf joints the angle between load and bonding surface controls the ratio of tensile and shear stresses (Beber et al., 2015). Thick adherend shear test (TAST) joints are commonly used to assess the shear strength of adhesives. Differences in the state of stress among different joint configurations are known to alter their fatigue behaviour under cyclic loading (Schneider et al., 2015). Butt joints are expected to show the highest stress multiaxiality levels due to the high constraint of adherends, TAST joints usually present multiaxiality values close to zero, and the stress multiaxiality of scarf joints lie between the butt and TAST joints (Beber et al., 2017). The geometry of the joints is shown in Figure 3.

(a)

(b)

Figure 3 - Geometry of butt, scarf and TAST joints used in the investigation: (a) elastic adhesive, (b) structural adhesive

3.4. Manufacturing of adhesively bonded samples The adhesive bonding process for both adhesive is schematically presented in Figure 4 . The surface preparation process for bonding was different for the structural and elastic adhesives. For the elastic adhesive, the aluminium substrate had the following treatment: (i) cleaning with solvent, (ii) application of primer, (iii) waiting for a period of 30 min to let volatiles of the primer to evaporate. For the structural adhesive the surface preparation consisted of: (i) cleaning with the solvent methyl ethyl ketone (MEK), (ii) corundum FEPA 220 blasting with 6 bar of pressure, and (iii) final cleaning with MEK solvent. Alignment devices (Figure 5a) were employed to ensure the correct positioning of adhesives during cure and reproducible adhesive layer thickness. The adhesives were applied to substrates aiming to obtain adhesive layer thicknesses of 0.35 mm for the structural adhesives and 5 mm for the elastic adhesives. Finally, the cure of the adhesives was different, i.e. 24h/RT+ 2h/70°C for the structural adhesive and 2 weeks at RT for the elastic adhesive. The size of the joints between the adhesives is very different, since their application is also very distinct (one for thin adhesive layer and other for thick adhesive layer). This difference in size can be seen in (Figure 5b) in which joints are placed next to each other during their curing at room temperature.