Influence of core height gradient on bending performance and energy absorption of multi-layer glass fiber reinforced polypropylene honeycomb sandwich structures

To enhance the load-carrying capacity and flexural resistance of thermoplastic composite sandwich structures, this paper systematically investigates the flexural behavior of multi-layer glass fiber reinforced polypropylene (GF/PP) honeycomb sandwich structures via three-point bending tests and sandwich beam theory, and analyzes the correlations of layer count and gradient configurations with failure modes, flexural stiffness, and energy absorption. Results indicate that incorporating intermediate facesheets transforms the failure mode from catastrophic brittle fracture to controllable progressive failure, significantly improving ultimate load capacity and failure displacement. Notably, the positive gradient configuration, through the soft contact and the stiffness regulation method in the later stage, as well as the core material failure resistance of the intermediate facesheets, demonstrates superior ultimate bearing capacity and bending stiffness compared to the negative gradient and uniform configurations, and achieves an increase in energy absorption. Furthermore, a finite element (FE) model was developed to elucidate the underlying load-bearing and strengthening mechanisms under different gradient designs. Combined with experimental data, this study demonstrates that a positive core height gradient achieves the best overall balance among bending stiffness, strength, and energy absorption, providing a theoretical foundation and optimization strategy for designing lightweight, high-performance composite structures.