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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]

The integration of two vertically crossed re-entrant honeycombs gives rise to a 3D re-entrant honeycomb (3D-RH), which displays intricate three-dimensional auxeticity. To simplify the modeling complexity, this study employs asymptotic analysis of the energy functional stored in the unit cell of the 3D-RH to develop unified constitutive models for 3D structures and panels with multiple length scales. Based on this, an equivalent 3D Cauchy continuum model is established for multi-layer 3D-RH, while a 2D equivalent plate model is developed for sandwich panel with single-layer 3D-RH (SP-3D-RH). The accuracy and effectiveness of these equivalent models are subsequently confirmed through investigations of the X-shaped compressive deformation of multi-layer 3D-RH, as well as the in-plane auxetic deformation and out-of-plane dome-shaped deformation of single-layer 3D-RH. Compared to traditional hexagonal and re-entrant honeycomb sandwich panels, the SP-3D-RH exhibited enhanced equivalent tensile stiffness while effectively mitigating maximum deformation and strain energy under bending and tension conditions. However, due to its relatively low equivalent shear stiffness, the SP-3D-RH exhibited significant deformation and strain energy when subjected to torsional loads, resulting in local stress concentration in the connect region between adjacent facesheets. Parameter analysis shows that the facesheet-to-strut thickness ratio had a substantial impact on the equivalent stiffness and in-plane behavior of the SP-3D-RH, while adjusting the horizontal strut length, strut depth, and re-entrant angle of the 3D-RH can improve the out-of-plane behavior. [Display omitted] • Equivalent models improve efficiency and accuracy in static analysis of 3D-RH. • 3D-EHM captures the X-shaped compressive deformation of multi-layer 3D-RH. • SP-3D-RH shows significant deformation and strain energy under torsional loads. • The facesheet-to-strut thickness ratio affects stiffness and behavior of SP-3D-RH. • 3D-RH in sandwich panel mitigates deformation under tension and bending. [ABSTRACT FROM AUTHOR]

• Classification of the deformation modes of sandwich panels with Miura-ori cores is proposed. • An analytical meso‑scale model of panels' deformation with strong origami cores is developed. • The applicability of the homogenisation approach to the core material properties is analysed. • Experiments were conducted to validate the finite element studies and analytical models. • Comparison of the bending strengths of Miura-ori, PVC and honeycomb core panels is presented. Collapse mechanisms of sandwich panels with Miura-ori cores are analysed and classification of the deformation regimes under three-point bending is proposed. It is revealed that the homogenisation approach to the core material properties is not applicable to the deformation of panels with relatively strong origami cores due to the immediate simultaneous local deformations of the core and face sheets. To describe this deformation regime, an analytical model based on the development of new stationary hinge lines in Miura-ori cells and rigid motion rotation of the adjacent planes is proposed. The model pertains to a single degree-of-freedom model governed by the global bending angle of the panel. On the other hand, the homogenisation approach to the core material properties proved to be applicable to bending analysis of panels with relatively soft Miura-ori cores. This approach is used to analytically obtain the collapse load related to several deformation modes. Parametric FE analysis is conducted to further explore the deformation regimes of panels with different face sheet thicknesses. Experiments were conducted to validate the finite element studies and analytical models. A good agreement between the analytically predicted collapse forces and those obtained by the FE simulations is shown. Deformation mode maps are constructed based on the analytical and FE analyses in terms of the face sheet thicknesses and core strength. A brief comparison between the bending strength of sandwich panels with Miura-ori cores and panels with PVC and honeycomb cores is presented, demonstrating the superiority of the honeycomb cores loaded in the out-of-plane direction. On the other hand, the possibility of the development of high shear strains in metallic Miura-ori cores without damage gives them advantages compared to three-point bending of panels with low density PVC foam cores. [Display omitted] [ABSTRACT FROM AUTHOR]

The core layer of accordion honeycomb sandwich panel (accordion HSP) exhibits exceptional affinity for one-dimensional curvature deformations due to its special zero Poisson's ratio effect. This study thoroughly examines its static and dynamic characteristics by decomposing the analysis into unit-cell constitutive modeling and a two-dimensional equivalent-oriented model (2D-EOM) through the variational asymptotic method. The unit-cell constitutive modeling enables determining equivalent stiffness properties for the 2D-EOM. To validate the accuracy and efficiency of the proposed 2D-EOM, its results are compared against those obtained from a detailed 3D finite element model, including in-plane and out-of-plane behaviors of the core layer and sandwich panel, as well as free and random vibration under diverse excitation conditions. The comparison results demonstrate a significant three-fold increase in the equivalent stiffness of the accordion honeycomb in the direction of the added ligaments, as compared to the re-entrant honeycomb. Simultaneously, the acceleration amplitude of the accordion HSP is effectively reduced by 4% due to the involvement of lower high-order frequencies, rendering it exceptionally suited for shockproof meta-structural applications. This equivalent model not only fulfills accuracy requirements but also improves computational efficiency by 15 times, providing an effective approach for preliminary design of honeycomb sandwich panels. • Exceptional affinity for 1D curvature deformations due to ZRP effect of accordion core. • Comprehensive analysis of static and random dynamic characteristics of accordion HSPs. • Enhanced computational efficiency of 2D-EOM while maintaining sufficient accuracy. • Increased stiffness achieved by incorporating ligaments into accordion honeycombs. • Reduced acceleration amplitude due to involvement of lower higher-order frequencies. [Display omitted] [ABSTRACT FROM AUTHOR]

• We present an analytical model able to predict the effective elastic constants of SSHCs. • The model accounts for the effect of thick faces and assumes that the core is weak. • SSHCs are significantly stiffer than conventional honeycombs under in-plane loading. • SSHCs show reductions in the out-of-plane shear moduli of at least 10%. • For in-plane loading, the Gibson–Ashby exponents of SSHCs range from 1.6 – 1.8. Sandwich-structured honeycombs (SSHCs) are hierarchical structures comprising sandwiched cell walls and are known to exhibit enhanced mass-specific properties. Here, we present an analytical model capable of predicting the effective elastic properties of hexagonal SSHCs, employing a sandwich beam theory that accounts for the effect of thick faces and regards the core as structurally weak. The analytical solutions of the nine elastic constants are compared with the numerical predictions obtained from a finite element-based homogenization technique, and an excellent agreement is reported for a wide range of architectural parameters such as the beam size, core thickness, and angle. Overall, it is found that SSHCs outperform conventional (monolithic) honeycombs in terms of their in-plane elastic and shear moduli, reporting values up to 20 times the monolithic counterparts of identical mass. In contrast, the out-of-plane shear moduli of the SSHCs showed reductions of at least 10% as compared to the traditional monolithic honeycomb structures. [Display omitted] [ABSTRACT FROM AUTHOR]

This study proposes a novel sandwich panel with an improved star-shaped honeycomb (SP-ISH) by replacing sharp corners with cubic struts and lengthening inclined ribs of traditional honeycomb. To more effectively analyze its special dynamic characteristics, the bending Poisson's ratio was derived and effective engineering constants were obtained through asymptotic analysis of the energy functional stored in periodic unit cells. Based on this analysis, a two-dimensional equivalent downscaling model (2D-EDM) was developed to investigate the free vibration of SP-ISH under various boundary conditions, and the influence of initial deformation on free vibration was also discussed. The frequency-domain and time-domain analyses were employed to identify the resonance characteristics of the SP-ISH under harmonic loading. Our findings demonstrate that 2D-EDM can accurately predict the free and forced vibration of SP-ISH and improve computational efficiency by reducing DOFs. The results indicate that the maximum resonance response was obtained by applying excitation at the center of the sandwich panel. Additionally, under cyclic loading, a sharp change in local stress amplitude was observed in the facesheet-core interface region. Compared with traditional star-shaped honeycomb, the incorporation of extended ribs and added struts could significantly reduce the maximum resonance amplitude by 3.8%. Notably, the natural frequencies of SP-ISH were affected by initial compression (up to a 40.6% reduction) and tensile deformation (up to a 26.4% increase). [Display omitted] • Improved SSH design with added cubic struts and elongated inclined ribs. • VAM-based 2D-EDM is developed for free and forced vibration of SP-ISH. • SP-ISH has superior dynamic characteristics other honeycomb sandwich panels. • Initial deformation from support movement impacts free vibration. • 2D-EDM accurately predicts resonance amplitude for various excitations. [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]

Well-designed honeycomb sandwich panels are known to have superior blast performance compared to their corresponding solid panel of the same mass. However, the residual structural capacity of honeycomb sandwich panels and their blast resilience has not been systematically studied. Here, we investigate the structural behavior of all-metal honeycomb sandwich panels after shock loading using detailed numerical simulations. The initial shock is varied from relatively small intensities to moderate intensities sufficient to create material failure and significant plastic deformation in the panel. The structural response of the shock-loaded panels is investigated under quasi-static punch indentation and in-plane compression. The maximum load carrying and energy absorption capacities of shock-loaded panels are quantified for a wide range of initial shock intensities and different panel core densities. Failure maps for the honeycomb panels were constructed for each quasi-static loading condition by considering three failure modes: core failure, face sheet failure, and total panel detachment from its support. This study provides new insights into the behavior and structural resilience of the shock-loaded sandwich panels, while further highlighting their potential in the development of resilient structural systems. [ABSTRACT FROM AUTHOR]

• Elasticity and failure behaviors of the resin-coated aramid paper are analysed. • A novel segmented equivalent mechanical model of the material is proposed. • Equivalent mechanical parameters of multi-layer structure are derived theoretically. • Proposed model performs well in ultrasonic cutting simulation of Nomex honeycomb. • Difficulty of modelling and CPU time of simulation are reduced significantly. Nomex honeycomb, as the core part of sandwich structure, has been widely used in the aviation and aerospace industries. Ultrasonic cutting, as a technology that can reduce machining defects, has been highlighted in recent years, but some problems in the cutting process still need to be investigated significantly. Finite element method (FEM) offers an alternative way of in-depth understanding of the problems. However, the multi-layer structure of the Nomex honeycomb cell wall, which is the resin-coated aramid paper, makes it difficult to establish a detailed multi-layered finite element (FE) model at a macro-scale level. Therefore, a novel segmented material mechanical model, which is applicable to the equivalent single-layer model, is proposed based on the analysis of the mechanical behaviour of the multi-layer structure of the resin-coated aramid paper. Moreover, the calculating method of equivalent mechanical parameters of the multi-layer structure are proposed theoretically. The proposed method is first verified by FE models at a micro-scale level in both single-layer and multi-layer approaches. Additionally, simulations and experiments are performed to verify that the proposed method performs well in the simulation of ultrasonic cutting for Nomex honeycomb, and the difficulty of modelling and CPU time of simulation are reduced significantly. [Display omitted] [ABSTRACT FROM AUTHOR]

• A series of potential variable thickness honeycombs with enhanced biomimetic cells (VTHEBs) are proposed. • The crashworthiness mechanism of VTHEBs is revealed through experiments and simulations. • Theoretical models of VTHEBs under out-of-plane loading are developed. Honeycomb materials are often used as the inner core of composite sandwich structures and are widely applied in impact resistance equipment design. Inspired by biomimetic materials, a series of potential variable thickness honeycombs with enhanced biomimetic cells (VTHEBs) are proposed and analyzed in this study, including the variable thickness honeycomb with bionic circular cells (VTHBC), the variable thickness honeycomb with bionic triangular cells (VTHBT), and the variable thickness honeycomb with bionic hexagonal cells (VTHBH). The crashworthiness properties of VTHEBs are studied by experimental, numerical, and theoretical analysis. The experimental results reveal the mechanical responses of VTHEBs under out-of-plane loading and validate the numerical models established by LS-DYNA. It is also observed that VTHBC shows better crashworthiness than VTHBT and VTHBH. Compared with the corresponding regular variable thickness honeycomb (RVTH), the variable thickness honeycomb with enhanced bionic cells has superior material distribution, which helps to improve the crushing force level and energy absorption potential. Besides, the numerical models are used to explore the effects of different design parameters (thickness deviation coefficient, cell size coefficient) on the mechanical performances of VTHEBs. Finally, the theoretical models of crushing forces are derived to predict the crushing performances of VTHEBs. The results show that the theoretical models can approximate the overall trend of experimental and numerical results with acceptable accuracy. This study provides an interesting design concept for superior crashworthy structures, which can be expected to be applied to impact resistance equipment in the future. [Display omitted] [ABSTRACT FROM AUTHOR]

• Two crashworthiness indexes of honeycomb sandwich structures are proposed. • The proposed indexes are validated by various honeycomb structures. • The mechanism of crashworthiness indexes is clarified. It is expected to quantitatively determine the crashworthiness of honeycomb sandwich structures in engineering. However, it is challenging due to diverse cell features, hybrid component materials, as well as varied types of impact load. In this study, two dimensionless indexes (i.e., horizontal dent offset displacement per unit depth ϕ and the ratio of horizontal to vertical energy absorption η) are proposed for sandwich structures based on the mechanical response characteristics (deformation and energy absorption) of sandwich structures under low-speed oblique impact. Unlike conventional crashworthiness indexes such as dent depth, specific energy absorption, damage area, etc., the proposed indexes take into account the influence of impact angle (θ) on the mechanical behavior of sandwich structures. Multiple sets of simulation of honeycomb sandwich structures, including conventional hexagonal honeycombs, honeycomb-filled structures and negative Poisson's ratio structures, obliquely impacted (θ varies from 15° ~ 75°) by a rigid body are performed by using the finite element method, whereby the crashworthiness index values of all the sandwich structures are obtained. The index values (ϕ and η) become smaller as the crashworthiness is enhanced (where the wall thickness of the sandwich structure is thickened from 0.02 ~ 0.1 mm). It manifests that the proposed indexes can capture the crashworthiness of sandwich structures under low-speed oblique impact reasonably well. In addition, the proposed indexes can more clearly distinguish the crashworthiness of sandwich structures of different configurations than conventional indexes. The index values (ϕ and η) show that the oblique crashworthiness of the negative Poisson's ratio structure, hexagonal honeycomb, and honeycomb-filled structure becomes stronger in order. [Display omitted] [ABSTRACT FROM AUTHOR]

• Dynamic responses of auxetic DAHSPs under paper tube-guided air blasts is experimentally conducted. • Large-scale metallic DAHSPs with diverse core configurations are designed and fabricated via folding and vacuum brazing. • Fair comparisons on the blast resistance among different design strategies to vary the core relative density is performed. • Optimal thickness allocation within each layer/among various layers and foam filling for performance promotion are pursued. • Numerical simulations reveal the shock focusing effect and indicate reasonable agreement with the experimental data. Auxetic double arrowhead honeycombs (DAHs) featured by negative Poisson's ratio is attracting increasing attention owing to their tailorable mechanical properties and outstanding energy absorption capability. In this paper, the two-dimensional metallic DAHs were produced and used as the sandwich cores to construct the large-size double arrowhead honeycomb sandwich panels (DAHSPs). A comprehensive experimental investigation on their dynamic responses under the paper tube-guided air blasts was conducted. It majorly focused on the influence of core configurations induced by the variation of core wall thickness, horizontal distance, inclined angles, thickness allocations and foam filling on the deformation/failure modes, associated mechanisms and blast resistance of the target panels. Fair comparisons among different design strategies were performed to evaluate their relative performance. Experimental results show that relative to increasing the core wall thickness, the decrease of horizontal distance is more remarkable to reduce the back face deflection upon the similar weight expense, whereas the effect of inclined angles is limited due to the competition between enhanced bending stiffness and deteriorated transmitted load. A more efficient approach to decrease the back face deformation is to enlarge the inclined angles when the core relative density is relatively low but to decrease the horizontal distance at a high level of relative density. A further improvement of blast resistance can be achieved by allocating more mass to the stuffer wall rather than the tendon one, or by introducing a gradient DAH core instead of the homogeneous one. Compared to adjusting the metallic DAH core itself, the lightweight polyurethane (PU) foam filling is a more competitive manner to promote the blast performance. The preliminary numerical simulations confirmed the shock focusing effect of the paper tube support and indicated reasonable agreement with the experimental results. The present study is supposed to shed light on the design, manufacturing and optimization of auxetic sandwich structures for protection applications. [Display omitted] [ABSTRACT FROM AUTHOR]

• Low-velocity impact and residual flexural behaviors of honeycomb panels with different structural configurations are investigated. • Core height has little effect on low-velocity behavior, but affects residual flexural strength significantly. • Residual flexural strength almost reduces 20%−30% even though superficial impact damage remains barely visible. • Bending failure pattern is related to core stiffness, but independent of the extent of impact-induced damage. • Predicted impact load, absorbed energy, residual flexural strength and failure modes match well with experimental ones. This paper aims to evaluate the influences of impact-induced damage on the residual flexural strength of honeycomb core sandwich panels with different structural configurations by combining the experimental, numerical and theoretical methods. Low-velocity impact tests and three-point bending tests after impact are carried out to determine and quantify the effects of structural configuration and impact energy on the impact damage and residual flexural strength of such structures. Subsequently, an integrated FE model with the VUMAT subroutine is developed to further investigate the damage states and failure mechanisms for the impact and bending simulation. The numerical results match well with the experimental ones in terms of impact load, absorbed energy, residual flexural strength and failure mechanisms. Results indicate that increasing cell wall thickness or decreasing side length of honeycomb core has significant effects on peak load, while increasing core height has little effect. Specimens with lower core stiffness fail through core buckling and crushing under the bending load, while specimen with higher core stiffness fails by top face sheet fracture. The residual flexural strength reduces markedly even through the impact damage is barely visible, indicating that it has a strong correlation with impact energy and structural configuration of cores. Image, graphical abstract [ABSTRACT FROM AUTHOR]