PSI - Issue 33

Sergey Smirnov et al. / Procedia Structural Integrity 33 (2021) 259–264 Author name / Structural Integrity Procedia 00 (2019) 000–000

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from those when this material is used as a monolithic part. Besides, the issue of the adhesive properties of the material and the substrate is of importance. Various procedures for evaluating the adhesive strength of the coating relative to the substrate are discussed in the literature Campbell (2004), Duncan et al. (2001), Smirnov (2019) particularly, modified breakaway and shear methods. ASTM's adhesives standards are helpful in the evaluation and testing of adhesives. The use of Arcan devices is promising for studying adhesive failure under the tension+shear loading condition. The aim of this study is to test a modified Arcan device for investigating the effect of the stress state on the adhesive strength of the epoxy/aluminum-alloy junction. 2. Materials and methods The paper studies a polymer coating/substrate system in the form of modified Arcan specimens, such as Oskui et al. (2016), Pucillo et al. (2011), Prakash et al. (2015), Breda et al. (2018) (Fig. 1, a). Polymer interlayers were made from glue based on ED-20 epoxy, i.e. 4,4’-isopropylidenediphenol resin (Sverdlov Plant, Dzerzhinsk, Russia), with an epoxy number of 21.1%, cured by polyethylenepolyamine. The substrate system was produced from a hot-rolled plate made of the 1570 aluminum-magnesium-lithium alloy used in aerospace industry. The polymer coating was applied onto specimen halves, which were then joined together and left for 24 hours for polymerization at room temperature. The thickness of the glue layer was 0.2±0.02 mm. The area of adhesion was 150 mm 2 .

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b

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Fig. 1. A general view of the test specimen (a) and modified Arcan specimens (the angle α between the tension axis and the junction plane: b – 0°; c – 45°)

The testing was performed on a Zwick/Roell Z2.5 universal testing machine. The prepared specimens were mounted in the grips of the machine, and the angle α between the tensile direction and the normal to the junction plane was varied. The angles of 0, 22.5, 45, 67.5, and 90° were considered, an example being given in Fig. 1, b-c. As the angle was varied, the stress state patterns were changed (α = 0 – breakaway under the action of normal tensile stresses; α = 90° – shear under the action of tangential stresses; at the intermediate values of α there is a complex tension+shear stress state). The testing was performed at +23 °С. The specimens were tested at a speed of 1 mm/min to fracture. The change of the load Р was recorded during the testing. The obtained experimental data on P max were statistically analyzed using the coefficient of variation by Box et al. (2005). 3. Results Table 1 shows the experimental values of the failure load Р max , averaged from the results of testing 3 to 5 specimens for each angle α, and the coefficients of variation. It is assumed in statistics that the degree of data dispersion is considered to be insignificant if the coefficient of variation is below 10%, moderate if the latter ranges between 10% and 20%, and significant if the coefficient of variation exceeds 20% or is at most 33%. It is obvious that, for most of the angles, the data dispersion is insignificant, this being indicative of experimental data repeatability. The exception is the value for the angle 67.5, which is most likely to be attributable to the technical matters of the experiment.