Flexural behavior of hybrid sandwich composites incorporating 3D-printed glass fiber-reinforced nylon cores and carbon fiber/epoxy face sheets

Composite sandwich structures are widely used in weight-critical applications such as aerospace and automotive engineering due to their high strength-to-weight ratio. This study investigates the flexural performance of sandwich structures fabricated with 3D-printed glass fiber-reinforced nylon (GF-PA6) cores, comparing fully printed composite sandwich structures (PCSS) to hybrid composite sandwich structures (HCSS) reinforced with woven carbon fiber/epoxy face sheets. Two core geometries, triangular and tri-hexagonal, were fabricated at infill densities of 20% and 40% using fused filament fabrication (FFF). HCSS configurations were produced by bonding carbon fiber/epoxy face sheets to the printed cores via vacuum-assisted wet layup. All specimens were tested under three-point bending per ASTM C393, with stress characterization performed using sandwich beam theory applied consistently to both PCSS and HCSS configurations. HCSS specimens achieved facing stress values of 184.40–349.70 MPa, compared to 86.96–103.35 MPa for PCSS, with the failure mode transitioning from brittle fracture in PCSS to progressive classical sandwich failure in HCSS, involving local face-sheet indentation, core shear failure, face-core debonding, and fiber pull-out. Increasing infill density from 20% to 40% produced significantly larger improvements in facing stress in HCSS (+57.1% and +89.6% for triangular and tri-hexagonal geometries, respectively) than in PCSS (+8.3% and +18.8%), confirming that a denser core more effectively transfers load to the carbon fiber face sheets. A density-dependent trade-off between strength and energy absorption was observed, with 20% infill HCSS exhibiting greater displacement capacity and energy absorption despite lower facing stress. These findings highlight the effectiveness of combining 3D-printed fiber-reinforced thermoplastic cores with high-stiffness composite face sheets, and provide insight into how core density and architecture govern flexural performance and failure behavior in additively manufactured sandwich composites.