In lightweight constructions fibre-reinforced polymers are often combined with light metals (e.g. aluminium) to hybrid composites, making their joint quality a key factor for component properties. Adhesive bonding is advantageous here, as it saves weight and eliminates the need for drilled or punched holes in the bulk material. At various points in the life cycle of a hybrid composite, it may be necessary to systematically dismantle these bonds for repair, reuse or recycling. However, structural adhesives (i.e. epoxy-amine) irreversibly bond after the curing process. New strategies must therefore be developed in order to meet the recycling demands of the European Climate Pact.

This study links aspects of component design with its end-of-life phase by developing a switchable bond that enables debonding on demand of the hybrid structure. To do so, Thermally Expandable Microspheres (TEMs) with concentrations of 0-50 wt.-% are mixed into an epoxy-based adhesive, which is further used to adhesively bond single-lap joints (SLJ) of carbon fibre reinforced polymers (CFRP) and aluminium. To initiate the debonding, the rear side of the aluminium adherend is heated with infrared (IR)-radiation. As a result of heat conduction, the temperature at the adhesive interface increases, which leads to the expansion of the TEMs in this region. This method is therefore material-friendly against the thermally more sensitive CFRP on the backside of the bond line. At the same time, it is assumed that the interface design, including the concentration of TEMs in this region as well as the aluminium surface structure, has significant impact on the debonding mechanisms and resulting failure behaviour (adhesive or cohesive). For this reason, different pre-treatment methods of the aluminium adherend (i.e. acetone cleaning and laser surface structuring) as well as the concentration of TEMs in the adhesive were compared regarding their microstructural and mechanical properties in SLJs. Following the debonding stage, the residual shear strength of the SLJ was correlated to the joint properties and the debonding parameters (temperature and time). After mechanical testing, the fracture surfaces were microscopically analysed to classify their predominant debonding mechanisms in comparison to reference specimens. One of the most important findings of these experiments is that debonding can be achieved by a one-sided material-friendly energy input through the thickness of the aluminium adherend. For example, even short-term IR-heating at 200 °C for 1 min can reduce the bond strength of SLJs with 10 wt.-% TEMs by > 50 %.