Experimental and numerical investigation of the impact behavior of additively manufactured auxetic structures

One of the challenges encountered in the selection of auxetic structures is represented by the specific loading conditions. This study investigates the mechanical performance of five auxetic structure types: reentrant, arrowhead, tetra-chiral, hybrid honeycomb (HB structure) and an x shaped cell (x-cell) structure, additively manufactured from polylactic acid (PLA+) and printed on Creality K2 Plus 3D printer by fused filament fabrication (FFF) technology. The mechanical performance was evaluated by Charpy impact tests on 25 specimens, five for each structure. The specimens were designed according to ASTM D6110-04 and had nominal dimensions of 125 × 12.7 × 12.7 mm. The technological parameters used for printing are deposited layer height 0.20 mm, infill percentage 100%, printing temperature 220°C and bed temperature 50°C. To ensure comparability between the different types of structures, the technological parameters were the same for each specimen. The experimental results show the influence that the internal structure has on the impact resistance. In the case of the hybrid structure, this was approximately 30% higher than the tetra-chiral structure which is the next ranked, while the x-cell, arrowhead and reentrant structures recorded very close results, with differences of up to 3%. Thus, the difference in behavior highlights the need to adapt geometry to the type of stress. The finite element analysis (FEA) showed similar results to the experimental ones and confirms the internal geometry as an important factor for impact resistance and for choosing the type of auxetic structure.