Fabrication of a novel continuous fiber 3D printed thermoset all-composite honeycomb sandwich structure with polymethacrylimide foam reinforcement

Continuous fiber-reinforced composite (CFRC) 3D printing provides attractive solutions for integrated fabrication of all-composite honeycomb sandwich structures. However, weak matrix strength and insufficient interlayer bonding degrade the performance of 3D-printed thermoplastic honeycomb sandwich structures. The curing-mold-dependent thermoset honeycomb sandwich structures are challenging to fabricate integrally by 3D printing. This study addressed these issues by filling the honeycomb cells with lightweight polymethacrylimide (PMI) foam during 3D printing, achieving co-curing support and enhanced performance of the integrally 3D-printed honeycomb sandwich structures. The novel foam-reinforced structure was studied using three-point bending tests and compared to the foam-free bonding-based structure. With a minor weight gain (9.48 %), the PMI foam reinforcement increased the peak load capacity of the honeycomb sandwich structure by 198.31 % (4.32 kN). The specific bending stiffness and the specific core shear strength were increased by 49.32 % and 165.69 %, respectively. Finally, the failure flow maps and stress state were analyzed to reveal structural failure modes and PMI foam reinforcing mechanisms.