Adhesive bonding of CFRP with a 3D-printed short-fiber composite: An experimental study on the effects of geometry and adhesive system on joint performance

The introduction of fiber-reinforced thermoplastic polymers in additive manufacturing has marked a significant turning point for this technology, offering the opportunity to extend its inherent flexibility to industrial sectors traditionally reliant on thermosetting composites. This innovation opens new applicative scenarios, such as the creation of more complex assemblies or the repair of damaged composite structures, where intricate-shape, functional 3D-printed parts can be implemented into pre-existing composite ones. Building on this premise, this experimental study explores design solutions for the fabrication of adhesive joints between dissimilar composite materials, specifically a polyamide-matrix composite reinforced with short carbon fibers (Onyx by Markforged), produced via Fused Deposition Modeling (FDM), and a thermoset Carbon Fiber Reinforced Polymer (CFRP) laminate, fabricated through vacuum-assisted resin infusion technique. Two sets of joints – each comprising six joint series configured according to both single overlap and more complex (''multi-layer'') geometries – are evaluated in parallel, employing two commercial adhesives (acrylic and epoxy), and specific surface-preparation procedures (solvent cleaning, abrasion, or low-pressure plasma), tailored to the substrate-adhesive combination. A comparison between the cases is proposed based on the results of tensile shear tests, focusing on the mechanical and failure behavior of the joints as a function of the geometry and the adhesive system used, as well as on process-related aspects and application implications.