PSI - Issue 81

Nazar Loboda et al. / Procedia Structural Integrity 81 (2026) 221–227

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interface. Moisture can enter the adhesive joint in several ways: by penetrating the adhesive volume, seeping at the phase interface, capillary penetration through micropores and cracks, and spreading in the permeable substrate before reaching the adhesive layer.

Nomenclature CZM cohesive zone model  normal stiffness , s t K K shear stiffnesses C G fracture energy G shear modulus  Poisson’s ratio P load force F bonding area t adhesive thickness l  displacement n K

maximum tangential stress

2. Experimental Study An experimental technique was developed and test equipment was modernized for the study of adhesive joints under shear loading. Experimental tests were carried out on a BISS 202 servohydraulic testing machine with a maximum load of 25 kN. For reliable fixation of specimens, special mechanical grips were developed to ensure the location of the load line of the power circuit along the central axis of the adhesive layer, which minimizes bending stresses (Fig. 1.a). The test specimens consisted of two anodized aluminum plates bonded together with a multicomponent polyurethane adhesive (Fig. 2). The adhesive layer thickness was approximately 0.2 mm. The type of adhesive used is typically employed in the aviation industry, where it must provide the design strength of structures at altitudes of up to 12,000 m and at low temperatures down to -60 °C. To carry out tests at low temperatures, a specimen cooling system was developed, which included a climatic chamber, a liquid nitrogen supply system, a control unit with a microprocessor regulator and a full-scale control system for thermal and strain gauges. The climatic chamber was designed as an isothermal container made of 50 mm thick polystyrene foam, covered with a shielded thermal insulation layer. During the tests, the thermo-strain gauge system performed online recording of the temperature-time dependence, which was recorded by a thermocouple fixed directly to the surface of the specimen. The test procedure involved placing the specimen in a climatic chamber, cooling it to a specified temperature (down to -60 ° C) with a holding time of 30 minutes, and then loading it with an axial force P until failure at a loading rate of 10 mm/min.

Fig. 1. (a) Testing machine; (b) fractured specimens.