Experimental Investigation of the Mechanical Behavior of Aluminum Adhesive Joints under Mixed-Mode Loading Conditions

In recent years, structural adhesives have rapidly become the preferred alternative to resistance spot welding in fabricating stronger, lighter aluminum connections. Connections inevitably undergo and must withstand complex quasi-static and/or dynamic loads during their service life. Therefore, understanding how loading conditions affect the mechanical behavior of adhesive joints is vital to their design and the advancement of structural safety. Quasi-static and dynamic tests are performed to analyze both the strength and failure modes of aluminum 6062 substrates bonded by an adhesive (Darbond EP-1506) for an array of loading directions. An Arcan test device, which enables application of mixed-mode loads ranging from pure peel (mode I) to pure shear (mode II) to the adhesive layer, is employed in quasi-static testing. A self-designed medium-speed test machine is utilized to perform dynamic testing. To avoid severe system oscillations exhibited in force measurements during dynamic testing, the Arcan test device is replaced by a set of integrated fixtures to improve stiffness and reduce fixture weight. To induce manifold mechanical responses in the adherend and adhesive materials, the specimens are subjected to a mixture of transverse and shear forces. Experimental results demonstrate that the fracture displacement increases with an increase in testing angle of the applied force, indicating that ductile fracture predominates brittle fracture and governs failure when shear loading is significant in mixed loading scenarios, especially for dynamic tests. Furthermore, dynamic strengthening is observed in samples subjected to peel loading, but is undiscovered in samples tested under mixed loading conditions, where interfacial failure is more likely to occur compared to cohesive failure. This observation reveals that the cohesive zone is reinforced by the strain rate effect, whereas the interface between the adhesive and adherend does not experience this phenomenon.