Bending and Impact Performance of Hybrid Sandwich Composites with Different Cellular Core Geometries

Sandwich composite structures with cellular cores offer significant potential for lightweight and high-performance engineering applications; however, the influence of core geometry on the combined stiffness–strength–energy absorption–weight balance remains insufficiently understood. In particular, systematic and mass-normalized comparisons of conventional, auxetic, and chiral core topologies manufactured under identical conditions are still limited in the literature. In this study, hybrid sandwich composites consisting of carbon fiber/epoxy skins and additively manufactured thermoplastic polyurethane cores with diverse cellular architectures—including honeycomb, grid, column, star-shaped, re-entrant, arc re-entrant, and hexachiral honeycomb geometries—were experimentally investigated. All cores were fabricated using fused deposition modeling with identical printing parameters to ensure fair comparison. The mechanical performance of the sandwich structures was evaluated through three-point bending and Charpy impact tests, focusing on bending strength, bending modulus, maximum displacement, impact energy, and impact strength. The results were analyzed using both absolute and mass-normalized metrics to assess true structural efficiency. The findings reveal that core geometry plays a dominant role in governing load-transfer mechanisms, deformation behavior, and energy absorption capacity. Among the investigated configurations, the hexachiral honeycomb core exhibited superior performance in terms of stiffness, strength, and impact energy absorption, while lightweight geometries such as honeycomb and grid cores demonstrated competitive specific performance after normalization. Auxetic geometries showed enhanced deformation capability but comparatively lower mass-normalized efficiency. The results provide clear experimental evidence that optimal sandwich core design requires simultaneous consideration of geometry and material efficiency, offering valuable guidelines for the development of lightweight, impact-resistant composite structures.