Impact response and residual performance of hybrid graded sandwich panels for UAV structural applications

Composite sandwich structures are widely used in unmanned aerial vehicle (UAV) components due to their high stiffness-to-weight ratio; however, their vulnerability to impact-induced damage remains a major limitation for structural reliability. Conventional sandwich panels with uniform foam or honeycomb cores often experience either excessive indentation or brittle collapse under low-velocity impact, leading to significant degradation of post-impact stiffness. Although impact damage mechanisms in sandwich composites are well established, experimental validation of hybrid graded core architectures at full structural scales remains limited. In this study, a combined experimental and numerical investigation is conducted to evaluate the impact response and residual bending performance of hybrid graded sandwich panels for UAV applications. Sandwich panels incorporating carbon-fiber-reinforced polymer (CFRP) face sheets and three core configurations, namely uniform foam, uniform honeycomb, and hybrid graded foam-honeycomb, were subjected to a 25 J low-velocity impact followed by residual three-point bending tests. The results demonstrate that the hybrid graded configuration significantly improves impact tolerance by promoting progressive damage evolution. The graded panels exhibited a sustained force plateau, higher energy absorption (48.6 J), and effective damage confinement (∼4 cm 2 ), compared with lower energy absorption (28.5 J for foam and 35.4 J for honeycomb) and larger delamination areas (8 cm 2 and 12 cm 2 ) in conventional cores. Residual bending tests revealed stiffness retention approximately 90%, substantially higher than that of uniform core configurations. These improvements arise from both core architecture and the controlled interaction of face-sheet damage, core crushing, and limited delamination governing post-impact load transfer. Finite element simulations developed in Abaqus/Explicit accurately reproduced the impact response and damage evolution, with deviations within ∼11%. This study provides a validated full-scale demonstration of hybrid graded sandwich structures for UAV applications and establishes a quantitative correlation between damage morphology and residual structural performance, offering practical design guidance for impact-tolerant lightweight structures.