Low-Speed Impact Behavior of 3D-Printed Polylactic Acid-Based Auxetic Core Sandwich Structures Filled with Polyurethane Foams

Auxetic sandwich structures have attracted considerable attention in recent years due to their unique deformation mechanisms, enhanced impact resistance, and superior energy absorption capabilities. However, studies investigating the combined effects of auxetic core geometry and polyurethane foam filling on the low-velocity impact behavior of sandwich structures remain limited. Therefore, this study systematically investigates the low-velocity impact behavior of sandwich structures with four different auxetic core geometries, such as re-entrant core (RESS), tetra-chiral core (TCSS), double-arrowhead core (DASS), and star-shaped core sandwich structures (SSSS). Each core sandwich structure is fabricated using additive manufacturing and is prepared in 3 different forms as foam-unfilled (FUF), 40 density polyurethane foam-filled (40DFF), and 60 density foam-filled (60DFF). The low-velocity impact tests of each sandwich structure are performed at the different impact energy levels of 6.04 and 10.74 J. The contact force history and contact force–displacement variation, crashworthiness indicators, damage analysis, and deformation fields obtained by means of the digital image correlation (DIC) technique are evaluated in detail to determine the unit cell core geometry and foam density on the low-velocity impact response. The existence of foam material provides a more uniform distribution of impact loads and controlled damage progression. Moreover, the crashworthiness indicators show an overall improvement with increasing foam density. In particular, the 60DFF structures exhibit higher stiffness, whereas the FUF structures show more localized and abrupt failure behavior. The impact performance of sandwich structures is significantly influenced by the core geometry, foam-filling condition, foam density, and the applied impact energy.