Adhesive bonding of 3D-printed short- and continuous-carbon-fiber composites: An experimental analysis of design methods to improve joint strength

This experimental work focuses on the effect of various joint-design factors on the adhesive bonding of mixed short-(Onyx) and continuous-carbon-fiber reinforced Nylon-6 parts, 3D-printed via Fused Filament Fabrication technology. A low-pressure plasma treatment has been preliminarily performed to maximize the adhesion at the composite's surface-adhesive interface. This approach has allowed the definition of a performance benchmark beyond which the intrinsic non-homogeneity of the composite determines joint failures due to delamination of substrates. Starting from this condition, adhesive-bonding parameters such as adherend overlap length (12.5, 25, or 50 mm) and geometry, adhesive type (epoxy or polyurethane), and substrate configuration are considered, investigating their influence on both the mechanical behavior of single-lap joints and modality of crack propagation across the bonded laminates. Supported by failure analysis, the experimental findings highlight that higher load levels (up to +251.4%) can be achieved by increasing the joint stiffness using higher values of overlap length thanks to lower peel stresses at the adherend ends where material discontinuities are mainly concentrated. Alternatively, joining of continuous-fiber-only adherends is considered to provide a further high-performing approach that involves the exclusion of the Onyx shell from the areas designed to be bonded. Moreover, heterogeneous joining between additively-manufactured composites and conventional carbon-fiber-reinforced-polymer materials is also investigated, providing pivotal insights about the applicability of 3D-printed composites also in pre-existing composite structures.