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    Bending performance of the AuxOcta multi-cellular beam structure
    (Elsevier Science Ltd, 2023) Kahraman, Mehmet Fatih; Genel, Kenan
    Auxetic cellular tubes are potential candidates for several engineering applications requiring resistance to bending due to the enormous demand for lightweight and high mechanical performance. Interestingly, we found that the preferential buckling mode can change the direction of deformation in the beam cross-section by applying negative Poisson's ratio cells in the lateral direction and is an effective modifying region. The bending response and energy absorption of multi-cellular tube with auxetic cells (Aux) and novel design tubes with auxetic and octagonal cells (AuxOcta) have been investigated with numerical and experimental systematically. These unique auxetic structures are produced by direct metal laser sintering (DMLS) with 316L stainless steel. The bending results show that the Aux tube indicates gradual local deformation in the loading region, while the cells of the AuxOcta tube beam are homogeneously deformed in the loading region. Moreover, AuxOcta structure is optimized to improve load-carrying and energy-absorbing capacities. This optimized AuxOcta structure (AuxOcta-G) offers superior bending performance from the test results. Compared to the Aux structure, the improvement in the specific load carrying (SLC) capacity of the AuxOcta-G structure above is 15% for 5 mm displacement, while the improvement in the specific energy absorption (SEA) approximately reaches 16%. The Finite Element Method (FEM) results showed that AuxOcta and AuxOcta-G structures offer a preferred behavior with a wide displacement range. This study provides a baseline for future investigation of hybrid beam design with a negative Poisson cross-section.
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    Comparative failure behavior of metal honeycomb structures under bending: A finite element-based study
    (Pergamon-Elsevier Science Ltd, 2024) Kahraman, Mehmet Fatih; İriç, Sedat; Genel, Kenan
    In recent years, metallic honeycomb structures have been popularly researched due to their significant influence on structural strength and rigidity. In this study, the bending response of multi-cellular beam structures with four (hollow tube (HT), square (Squ), hexagonal (Hex) and octagonal (Octa) and Re-entrant honeycomb (ReH)) cross-sections are numerically investigated. Furthermore, using the nonlinear finite element codes LS-DYNA, a comparative study of the energy absorption characteristics between structures with auxetic and non-auxetic beam crosssections was carried out. The ReH specimen used in the finite element (FE) validation study was manufactured using the Direct metal laser sintering (DMLS) additive manufacturing method to accommodate the complex geometries. FE method analysis are carried out to systematically investigate the influence of the geometrical configuration and identify the failure mechanism on the bending performance. The results show that in the HT structure, an upper corner fracture occurred because of folding in the contact area of the indenter. For Squ, Hex and Octa beam structures, failure occurs due to localized stress caused by buckling in the cell walls. On the other hand, the cross-sectional area of ReH structure tends to shrink under the bending load. Thus, the influence of the local buckling effect could be minimized despite the high displacements. This situation has ensured that the reduction in the moment of inertia of the crosssection remains limited. Furthermore, the specific energy absorption (SEA) capacity of the ReH beam significantly performed 11.3, 3.76 and 1.77 times better than the multi-cellular beam with Hex, Squ and Octa honeycomb beams, respectively. Accordingly, it was understood that the failure of the re-entrant cross-section under severe deformation was more limited than the others. This study is expected to contribute to evaluating the load-bearing capacity of metallic honeycomb structures, including understanding the failure process.
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    Effect of angle and thickness of cell wall on bending behavior of auxetic beam
    (Elsevier, 2024) Kahraman, Mehmet Fatih; Genel, Kenan
    Auxetic (Aux) structures, which exhibit unique mechanical properties due to the tunability of negative Poisson's ratio (NPR), have become great attention in recent years. In this work, the bending response and energy absorption behavior of beam structure in which the auxetic cells are oriented along the beam axis under bending are systematically investigated for various thicknesses and angles (55-85 degree) of walls experimentally and numerically. The beam structure was manufactured from AISI 316 L stainless steel by Powder Bed Fusion (PBF) additive manufacturing method, and three-point bending test was employed to determine the mechanical properties. The results revealed that the auxetic cross-section subjected to loading shrinks relating to NPR, orienting the outer wall inward thus reducing local buckling effect causing the main damage mechanism. It was also found that increasing the cell wall angle with the same relative density (rho = 0.485) owing to the change in the negative Poisson's ratio of the cross-section was beneficial in improving the load carrying and energy absorption capacity of Aux beam. However, increasing the cell wall angle up to a certain value (75 degrees) provides a significant benefit in the bending performance of the beam. The cell wall angle takes larger values resulting in a negligible increase in performance, whereas damage occurs at lower displacements. Moreover, the performance of the Aux beam can be improved by functionally graded thickness of the auxetic cell wall, increasing the specific load carrying (SLC) and specific energy absorption (SEA) capacity. By grading the cell wall thickness of the structure with the best mechanical performance according to the cell angle 75 degrees, it is understood that SLC and SEA values can be increased by 19.4% and 25.4%, respectively. This research is estimated to ensure a valuable reference for improving the bending response of the auxetic cross-section beams.