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Auxetic honeycomb sandwich structures (AHS) composed of a single material generally exhibit comparatively lower energy absorption (EA) and platform stress, as compared to traditional non-auxetic sandwich structures (TNS). To address this limitation, the present study examines the use of aluminum foam (AF) as a filling material in the re-entrant honeycomb sandwich structure (RS). Filling the AHS with AF greatly enhances both the EA and platform stress in comparison to filling the TNS with AF, while the auxetic composite honeycomb sandwich structure effectively addresses interface delamination observed in traditional non-auxetic composite sandwich structures. Subsequently, the positive–negative Poisson's ratio coupling designs are proposed to strengthen the mechanical features of a single honeycomb sandwich structure. The analysis results show that the coupling structure optimizes the mechanical properties by leveraging the high bearing capacity of the hexagonal honeycomb and the great interaction between the re-entrant honeycomb and the filling material. In contrast with traditional non-auxetic sandwich structures, the proposed auxetic composite honeycomb sandwich structures demonstrate superior EA and platform stress performance, suggesting their immense potential for utilization in protective engineering. [ABSTRACT FROM AUTHOR]

• The proposed hybrid AuxHex structure consists of auxetic and honeycomb cells. • The equations for modulus and strength along x - and y -directions are derived. • Theoretical results are in agreement with simulation and experimental results. • Mechanical properties and specific energy absorption are enhanced 16% and 38%. The architected mechanical metamaterials have garnered significant research attention for a variety of engineering application due to their remarkable mechanical properties and unique deformation behavior. Herein, the in-plane uniaxial compressive response and energy absorption capacity of a novel hybrid configuration, AuxHex structure, which consists of auxetic and hexagonal honeycomb cells, are systematically investigated through theoretical, finite element simulation and experimental methods. A series of AuxHex sandwich core panels have been fabricated with nylon material by using additive manufacturing route. The relationships for Young's modulus and plastic collapse stress along different loading directions are derived and validated through the comparative analysis. In addition, the deformation mechanism and failure modes of the AuxHex structure have also been discussed in detail. The AuxHex structures have exhibited superior Young's modulus, collapse strength and energy absorption than traditional honeycomb structures. In the x -direction, the energy absorption capacity was improved by 38%, which can be attributed to the uniform and stable deformation mode of the unit cell. The theoretical prediction results of Young's modulus and plastic collapse stress are consistent with finite element simulation and experimental results. The AuxHex structures demonstrate a novel design strategy for the architected metamaterials through the combination of various cellular cells. The hybrid structures will play an important role in both the load-bearing and energy-absorbing applications, and it demonstrates a novel design strategy for the architected metamaterials through the combination of various cellular cells. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Sandwich panels/sacrificial cladding systems composed of an auxetic core sandwiched between two faceplates can be considered an effective protective measure for reinforced concrete structures from blast loads. In this study, the effectiveness of re-entrant honeycomb core sandwich (RHS) protective panel systems, which exhibit a negative Poisson's ratio, to protect one-way simply supported reinforced concrete (RC) slabs subjected to a close-range detonation from high explosive charges was experimentally evaluated. Three large-scale field blast trials were carried out, namely an RC slab without protection as the reference test and two RC slabs with RHS protective systems under similar blast loading conditions. The RC slabs with RHS protective systems experienced catastrophic failure, unlike the unprotected reference slab that experienced only localised concrete damage. LS-DYNA numerical models were validated and agreed well with the experimental results. It was demonstrated that the transferred blast impulse to the main structure can be magnified by a factor of 1.6 by using an RHS protective system compared to an unprotected structure. Furthermore, the effect of the mass of the protective system was numerically studied. The numerical results showed that the blast protection level of the main structure is governed by the inertial resistance of the protective system rather than through energy absorption of the auxetic honeycomb core structure. • Response of RC panels protected by auxetic sandwich panels to close-in explosion is investigated experimentally and numerically. • The damage magnification effect of the auxetic sandwich panels is captured experimentally which has not been reported so far. • The impulse transferred to the main structure could be magnified by a factor of 1.6 compared to the unprotected structure. • The duality of energy absorption and inertial resistance of auxetic protective systems need to be considered at close standoff distances. [ABSTRACT FROM AUTHOR]

• A novel composite is proposed by filling layered re-entrant honeycombs with concrete. • The mechanical properties and failure modes of two types of specimens under uniaxial compression are investigated. • The proposed auxetic composite exhibits an enhanced stiffness and shear resistance. • The effects of cell angle and cell wall thickness on the mechanical properties are investigated. Auxetic materials exhibit desirable mechanical properties, e.g., fracture resistance, shear resistance and energy dissipation due to their unique deformation characteristics. Honeycomb sandwich structures also have superior properties, e.g., energy absorption performance and low density. These desirable characteristics enable them as promising alternatives for building construction to meet the modern construction requirement which a higher safety standard is needed. In this study, a novel concrete composite with auxetic layered honeycomb was designed, manufactured and investigated for exploring the load-bearing performance of auxetic metamaterials under quasi-static compression. The layered honeycomb sandwich structure was analyzed by quasi-static compression test and its energy absorption performance was evaluated. Numerical models, validated by experimental results, were employed in parametric studies to further examine their performance. The results show that the layered structure has higher initial peak stress and stable platform stress. Moreover, it could improve the shear resistance and overall stability of the structure. These findings are beneficial to the applications of auxetic metamaterials in building construction. [ABSTRACT FROM AUTHOR]