Auxetic structures are a type of metamaterial characterized by a negative Poisson’s ratio an‎d special geometrical configurations. The negative Poisson’s ratio in these structures leads to unique mechanical properties such as low weight, high shear strength, an‎d high energy absorption, which are investigated in the present study. In this thesis, the deformation behavior an‎d energy absorption capacity of an auxetic structure based on a composite unit cell were compared with those of honeycomb an‎d re-entrant structures. The samples were fabricated using 3D printing with TPU filament an‎d filled with polyurethane foam. Their mechanical performance under quasi-static loading was examined through compression tests an‎d finite element simulations. The main objective was to eva‎luate the role of auxetic geometry an‎d the effect of foam core filling in enhancing stiffness, compressive strength, an‎d specific energy absorption, an‎d to provide design guidelines for energy-absorbing an‎d lightweight structural applications. The methodology included the geometric design of the unit cells, fabrication of experimental specimens by FDM printing, preparation an‎d injection of polyurethane foam into the cells, quasi-static compression testing, an‎d 3D modeling with mesh sensitivity analysis an‎d energy validation. Numerical an‎d experimental results showed that filling the core with foam had a significant effect on the structural properties. The compressive strength of the foam-filled composite sample was reported as 2.327 MPa, while the unfilled composite, re-entrant, an‎d honeycomb structures showed 1.521 MPa, 0.198 MPa, an‎d 0.089 MPa, respectively. Thus, the foam-filled sample exhibited approximately 2 to 25 times higher compressive strength than the other structures. In terms of stiffness, the foam-filled composite sample under a load of 300 N an‎d a displacement of 2 mm demonstrated 13- an‎d 11-times greater stiffness than the honeycomb an‎d re-entrant structures, respectively, an‎d 4 times higher stiffness than the unfilled composite sample. This increase in stiffness highlights the critical role of the foam in stress distribution an‎d in preventing local wall buckling. One of the most important findings was the significant increase in energy absorption in the foam-filled condition. The energy absorption of the foam-filled composite structure was, on average, twice that of the unfilled one, showing good repeatability in both numerical an‎d experimental results. This indicates that the combination of auxetic geometry with polyurethane foam can greatly enhance energy absorption efficiency an‎d serve as an effective design strategy for energy absorbers. In the numerical part, Abaqus software based on the finite element method was used. To verify the quasi-static nature of the analysis, the ratio of kinetic energy to internal energy remained below conventional thresholds (less than 5–10%), an‎d the kinetic energy trend did not show significant fluctuations, confirming the adequacy of the analysis setup an‎d the reliability of the quasi-static results. Mesh sensitivity analysis was also performed. The results of the present study are consistent with previous research, demonstrating that filling the core with polyurethane foam increases specific energy absorption by 101% compared to the unfilled case.

عليرضا شهيدي ريزي

مهدي جوان بخت , حسين جعفرزاده