Experimental investigation of the effects of adhesive thickness on the fracture behavior of structural acrylic adhesive joints under various loading rates

Designing joints requires a clear understanding of the appropriate thickness of the adhesive used. Structural acrylic adhesives have rarely been studied in terms of their thickness effect on the joint performance. To this end, the fracture resistance of a second-generation acrylic (SGA) adhesive was experimentally investigated by conducting a double cantilever beam (DCB) test. Because the SGA adhesive whitened when plastically deformed, the change of the plastic region with the crack growth was visualized. Therefore, the relationship between the fracture energy and the adhesive thickness was explained in terms of the plastically deformed area. With a thinner adhesive layer, the entire layer was whitened, and a linear relationship was obtained. In this region, the fracture energy increased from approximately 2 kJ/m(2) with an adhesive thickness of 0.2 mm to approximately 4 kJ/m(2) with 0.6 mm thickness. With more increase in the thickness, the fracture energy increased over 8 kJ/m(2). Increasing the thickness, however, resulted in partial whitening, yielding a non-linear relationship. Moreover, increasing the loading rate changed fracture behavior. At the opening speed of 2.5 m/s, stick-slip crack propagation was observed in any adhesive thickness, and the critical fracture energy dramatically decreased. In contrast, the arrest fracture energy under unstable crack propagation was independent of the loading rate and the adhesive thickness as approximately 1 kJ/m(2).