Through-thickness 3D-printed bio-inspired fibers to enhance mode I fracture resistance of composite bonded joints

Composite bonded joints are susceptible to sudden and brittle failure, especially under mode I opening loading conditions. To improve crack resistance under mode I, conventional solutions use through-thickness reinforcements, such as rivets, which is effective in delaying the crack growth but damage the performance of the composite. This study explores an alternative through-thickness reinforcement consisting of 3D-printed overlapping curl (OC) fibers with sacrificial bonds and hidden lengths (SBHL) to enhance fracture resistance under mode I of composite bonded joints. The OC fibers are embedded through the thickness of a structural epoxy, oriented perpendicular to the crack propagation direction, enabling effective activation of hidden length unfolding under mode I opening conditions. This through-thickness reinforcement fully exploits the SBHL toughening mechanism without introducing detrimental effects to the CFRP substrate, in contrast to conventional techniques such as Z-pinning or riveting. Double cantilever beam experiments demonstrate that the OC topology is essential for effective toughening, as it anchors the OCs within the adhesive and prevents pull-out, allowing hidden length unfolding that leads to extended crack bridging distance and increases the mode I energy release rate by up to {119}{%}. Furthermore, numerical simulations are conducted to systematically explore the influence of OC mechanical parameters, providing design guidelines for improving the fracture performance of CFRP adhesively bonded joints.