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The low stiffness resulting from substantial core porosity within the re-entrant honeycomb sandwich panel can be addressed by introducing rib-reinforcements. To investigate its static and vibration characteristics, an energy-based 2D equivalent-homogenization model (2D-EHM) was derived through asymptotic analysis of the leading terms in the governing equations considering the energy-related characteristics. The accuracy of the constructed model for predicting the elastic bending performance of the sandwich panel was verified by comparing the results of a three-point bending experiment with 3D-printed specimens. Comparative analysis with a 3D FE model demonstrates that the 2D-EHM exhibits maximum errors of only 4.65% and 6.46%, respectively, in analyzing static deformation and natural frequency while improving computational efficiencies by up to 59- and 173-folds. Parameter analysis reveals that the performance of SP-RRH is minimally affected by the reinforcement-to-honeycomb wall thickness ratio. However, its performance is compromised when the honeycomb walls align in an equilateral triangle with the reinforcing walls. Comparing specific stiffness as well as static and vibration characteristics of sandwich panels with different core configurations (including traditional and diamond reinforced re-entrant honeycombs) shows that inclusion of rib reinforcements enhances deformation resistance and natural frequencies through increased specific stiffness. These findings provide valuable insights for optimizing design of re-entrant honeycomb sandwich panels. [Display omitted] • VAM-based 2D model of SP-RRH accurately validated against 3D-FEM results. • Equivalent stiffness obtained via asymptotic analysis of unit cell's energy functional. • The tailorability of the unit cell facilitates the customization of its properties. • Rib-reinforcing walls enhance deformation resistance of sandwich panels. • The performance of SP-RRH is influenced by equilateral triangular cellular structure. [ABSTRACT FROM AUTHOR]

Cellular structures such as honeycombs are used to manage impact loads in many engineering applications, where often the impact load is oblique (non-axial). Although these structures have been extensively studied under axial impacts, a key question remains largely unaddressed: How may their axial and shear responses be independently tailored for oblique impacts? Here we test whether incorporating curvature in the form of axisymmetric shape changes in the cell walls of viscoelastic circular cell honeycombs enables independent control over their shear and compressive responses during oblique impacts. We developed detailed finite element models of the circular cell honeycombs made of the 3D printable viscoelastic material Agilus. Using a new rig, we validated the predictions of the finite element models under oblique impacts. We performed a full-factorial computational design-of-experiments, varying the cell shape from concave to convex with different amplitudes and impact velocities from 2.5 to 7.5 m/s in order to determine their effects on the shear and compressive responses of the honeycombs under oblique impacts. The finite element models demonstrate remarkable agreement with experimental oblique impacts for both concave and convex cells. Our full-factorial computational search shows that both convex and concave cells have similar compressive stresses. However, the post-buckling shear stress of the concave cells are up to 300% larger than the convex cells. In addition, increasing the impact velocity increases both compressive and shear stresses by 211% due to the viscoelastic nature of the material. Our results show that these new recoverable honeycombs provide a compelling control over their shear response under oblique impacts, which is nearly independent from their compressive response. These new honeycombs can be used to manage oblique impacts in a wide range of products, such as helmets, shoes and seats, where enabling an independent control over shear and compressive responses will open new design avenues. In addition, the proposed simple control mechanism, i.e. the curvature change, can be exploited in future to actively morph the honeycomb cells during use for real-time performance adjustment. [Display omitted] [ABSTRACT FROM AUTHOR]