Experimental investigation and COPRAS-based selection of FFF-printed double-walled polygonal conical structures for crashworthiness applications

This study presents a comprehensive experimental investigation into the crashworthiness performance of thin-walled, double-walled polygonal conical structures fabricated via Fused Filament Fabrication (FFF) using enhanced polylactic acid (PLA+) material. The influence of the cross-sectional geometry (square, hexagonal, and octagonal) and of the taper strategies applied to the internal and external walls is systematically evaluated through 30 quasi-static axial compression tests on 15 configurations. Specific Energy Absorption (SEA), Crushing Force Efficiency (CFE), Mean Crushing Force (MCF), and Initial Peak Crushing Force (IPCF) are examined as crashworthiness indicators, and the most efficient design is determined through the Complex Proportional Assessment (COPRAS) method. The experimental results showed that increasing the vertex number from square to octagonal cross-sections increased SEA by up to 122%, while the outer-cell conicity reduced the IPCF of the octagonal tubes by 10.9% and increased their CFE by 13.1% compared with the non-tapered octagonal design. According to the COPRAS analysis, the octagonal tube with outer-cell conicity (T8_OCAG) was identified as the optimal configuration with a performance index of 100%, an average SEA of 18.57 J/g, and a CFE of 0.62–0.63. The selection was verified through four additional multi-criteria decision-making methods (TOPSIS, VIKOR, EDAS, and MOORA) and through weight sensitivity analyses, which confirmed the robustness of the identified optimum. The results demonstrate that the synergy between the polygonal cross-section, the strategically applied wall conicity, and additive manufacturing offers significant potential for developing lightweight, high-performance passive safety components.