Analysis of the adhesive spread and the thickness of the adhesive bonded joint depending on the compressive force applied to bonded materials with different surface structure

The thickness of the adhesive layer in a bonded joint is one of the important factors determining its strength and its recommended value varies depending on the type of the adhesive used and the type of load on the joint. Specimens for testing adhesively bonded joints with a specified thickness of the layer are usually produced under laboratory conditions using spacers of a specified thickness, and the remaining excess adhesive is removed from the specimen. In industrial conditions, this method of obtaining an appropriate thickness of the adhesive layer is not always possible and desirable. The method of adhesive spreading between the joined plates under the influence of pressure without spacers was analysed. For this purpose, 3 pairs of aluminium plates were used, differing in surface texture and roughness for the Ra parameter at 0,25 μm, above 3 μm, and above 5 μm. 0.5 ml of Bonapur RZ45 polyurethane adhesive was compressed between the plate pairs without additional force (except for the own weight of the plates) and with load of 50 N, 100 N, 250 N, and 500 N. The surface where the adhesive would be distributed was checked to estimate the resulting thickness of its layer. The size of the layer ranged from 0.09 (for the force 500 N) to 0.53 mm (without additional force, only plate). The values obtained of the layer thickness in the case of using additional force are lower than recommended in the literature (0.5–2 mm) for polyurethane adhesives. The measurements showed that the roughness of the surface does not significantly affect the differences in the resulting thickness of the adhesive layer, whereas the visible surface machining footprint causes changes in the way the adhesive spreads and impacts the shape of the adhesive on the plate surface.