Lightweight cellular sandwich panels with CNT-reinforced resin fillers for superior impact resistance

Sandwich panels with architected lattice cores offer high stiffness-to-weight efficiency, yet their low-velocity impact performance remains strongly governed by core material behavior rather than geometry alone. While carbon nanotube reinforcement has been widely studied in bulk polymer composites, its role as an injected filler within additively manufactured lattice sandwich cores remains insufficiently understood. The novelty of this work lies in the architecture-level experimental investigation of 3D-printed polylactic acid (PLA) X-frame sandwich panels filled with epoxy reinforced by controlled multi-walled carbon nanotube (MWCNT) concentrations (0–1.25 wt%), and in the systematic evaluation of how CNT loading governs crack propagation, through-thickness damage transmission, and failure morphology under low-velocity impact. Low-velocity drop-weight tests were conducted at impact energies of 22.34 J and 37.23 J (impact velocities of 1.71 m/s and 2.21 m/s). Damage evolution was quantified using front- and back-face macro-crack length measurements and indentation diameter analysis. The results reveal a pronounced non-linear dependence on CNT content. Intermediate CNT concentrations (0.5–0.75 wt%) produce the most damage-tolerant response, characterized by highly localized indentation and complete suppression of back-face cracking at both impact energies. In contrast, unreinforced panels exhibit extensive through-thickness cracking, while excessive CNT loading (1.25 wt%) restores brittle splitting behavior. Furthermore, increasing impact energy induces a shift from global splitting to more localized deformation in certain compositions, highlighting the energy-dependent nature of failure mechanisms. Overall, the study identifies an effective CNT concentration window specific to resin-filled X-frame sandwich cores and provides architecture-level design guidance for lightweight impact-resistant sandwich structures where minimizing damage footprint and suppressing rear-face damage are critical.