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• The effect of thickness and reinforced strut gradients on the crushing response of RRH is explored. • Two plateau stresses of RRH specimens are derivated theoretically with a relative error of less than 7 %. • Both thickness and catenary gradients inspire a multi-step deformation mode and multi-plateau stress. • Some suitable thickness and catenary gradients are discovered under different impact velocities. Re-entrant honeycombs have demonstrated promising applications in engineering fields attributed to better manufacturability and adaptability. Herein, the reinforced re-entrant honeycombs (RRH) with thickness and reinforced strut gradients are proposed, respectively, and their in-plane energy absorption is investigated through experiment, theoretical analysis, and simulation. Metallic specimens of uniform and graded RRH are manufactured through a step-by-step method. The deformation mode and plateau stress (PS) of these specimens are significantly influenced by both thickness and catenary gradients. Two PSs of RRH specimens are predicted theoretically with a relative error of less than 7.0 %. Subsequently, the deformation mode and energy absorption of uniform and graded RRH, subjected to varying load rates, are investigated thoroughly with a numerical method verified against the experimental data. The results show that both thickness and catenary gradients can inspire multi-step deformation mode and prevent instability deformation. Compared to uniform RRH (U-RRH), the specific energy absorptions (SEA) of thickness gradient (TG)-123 (123 represents that the thicknesses from impact end to fixed end are t 1 , t 2 , t 3) and TG-213 are enhanced by 52.2 % and 49.8 %, respectively, under quasi-static compression, the SEA of TG-132 and TG-123 are raised by 41.6 % and 20.2 %, respectively, under low-velocity impact, and the SEA of strut gradient (SG)-123 is 66.7 % higher than that of U-RRH under high-velocity impact. More significant auxetic deformation does not always mean better energy absorption because there is also an increased tendency of global instability and earlier densification points. This work provides a novel approach to designing high-energy absorption honeycombs with low structural complexity. [ABSTRACT FROM AUTHOR]

• Thermal-force external coupling enhances polyimide molecular chain orientation. • Orientation modulation increases the honeycomb shear strength 10 times to 1.07 MPa. • Excellent thermal performance of polyimide aerogel honeycomb (T d5% >650 °C). • Fabricating integrated lightweight honeycomb structures using the formwork method. Lightweight honeycomb structures are widely used in applications that require high strength-to-weight ratios and low densities, including aircraft, automobiles, and various engine components. However, due to the immaturity of microstructure conditioning techniques and the fact that highly porous structures usually fracture during stretching, it is challenging to obtain both high-strength and lightweight porous structures. Herein, a newly developed multilayer interconnected polyimide aerogel-based paper honeycomb is successfully prepared based on the ice crystal reverse template method and multi-level structure regulation via chemical and physical interactions. The synergistic effect of the hot extrusion-stress strategy and the heat imidization process provides the submicron layer of the paper honeycomb wall with more orderly molecular chain stacking, excellent orientation, and an enhanced folding degree between layers. The prepared polyimide aerogel paper honeycombs have low density (approx. 8–9 kg/m3), good shear strength of 1.07 MPa (W direction) and 0.86 MPa (L direction), excellent specific compressive strength (0.38 MPa), and excellent thermal stability. These integrally molded polyimide paper honeycombs, as sandwich resin-based composite structures, provide a new versatile platform for aerospace and marine applications. [ABSTRACT FROM AUTHOR]

• The novel helical layered square honeycombs exhibit higher strength (∼176 %) and specific energy absorption (∼502 %) than regular square honeycomb. • The quasi-static compressive behaviors of helical-layered and regular square honeycombs are compared. • The deformation modes of helical-layered square honeycombs are dominated by the relative density and the number of helical layer. • The mechanical properties and energy absorption capacity improved by increasing the number of helical layer. • The failure mechanisms of helical-layered square honeycombs are dominated by layer cracking. Bionics shape design of cellular structures has the potential advantage of improving their both mechanical properties and energy absorption capabilities to the conventional honeycomb structures. In this article, inspired by square porous wood and DNA double helix, we demonstrate the creation of the novel helical layered square honeycombs (HLSHs) that simultaneously exhibit high strength and excellent energy absorption capacity. In-plane compression experiments and finite element simulations revealed that helical structure can effectively strengthen every unit cell to resist local stress concentration which always leads to rapid strain localization and finally followed by catastrophic collapse. Furthermore, the stiffness, strength and energy absorption capability can be continuously improved by increasing the number of helical layers. More specifically, the HLSH-4L (4 layers) exhibits higher collapse stress (∼176 %) and specific absorption energy (∼502 %) compared with that of regular square honeycomb (RESH). The investigation of deformation and failure behavior show that the helical structure can supply high cell-buckling resistance and facilitate the "X" shape deformation band. Besides, the helical structure also caused the transition of failure mechanisms from printing fibers tearing in RESH to layer cracking in HLSHs. This study can present insight into the design of lighter and stronger honeycomb. [ABSTRACT FROM AUTHOR]

This study presents a novel three-dimensional orthogonal accordion structure (3D-OAS) with multi-directional zero Poisson's ratios (ZPR) achieved by combining two-dimensional accordion honeycomb structures (2D-AHS) orthogonally. The analysis of its mechanical performance involved deriving equivalent elastic properties using beam theory. Subsequently, a three-dimensional Cauchy equivalent model (3D-CEM) was established using the variational asymptotic method to convert structural orthotropy into material orthotropy. Uniaxial compression tests were conducted on unit-cell and multi-layer 3D-printed specimens to confirm the multi-directional ZPR effects. The in-plane and out-of-plane static behaviors of 3D-OAS panels were evaluated, affirming the accuracy and effectiveness of the equivalent model. Numerical simulations showed that the 3D-CEM required fewer elements and nodes, representing 0.85% and 0.83% of the computing time required by 3D FE model for in-plane and out-of-plane deformation analyses, respectively. The 3D-OAS panel experienced no significant vertical deformation during in-plane tension but exhibited cylindrical deformation with ZPR characteristics under out-of-plane bending. Specifically, normal stresses exceeded shear stresses at the intersection of the two perpendicular horizontal struts. The inclined angle of strut minimally influenced the Poisson's ratio of 3D-OAS but notably impacted the elastic moduli. These findings provide important theoretical support for the design and application of three-dimensional accordion honeycomb structures. • 3D-OAS achieves multi-directional ZPR effects through orthogonal combination. • Mechanical analysis involves deriving equivalent elastic properties. • 3D-CEM transforms structural orthotropy into material orthotropy. • Uniaxial compression tests on unit-cell and multi-layer 3D-printed specimens. • Efficient 3D-CEM with reduced elements and nodes for numerical simulations. [ABSTRACT FROM AUTHOR]

• Through hypervelocity impact experiments, the impact process was recorded by high-speed camera, and the damage effects of reactive projectile and Al2024 projectile on the aluminum alloy honeycomb sandwich panel double-layer structure were compared. • The damage enhancement mechanism of reactive projectile to double-layer structure was revealed by numerical simulation and theoretical analysis. • The influence of impact-induced detonation reaction of reactive projectile on the "channel effect" of honeycomb sandwich panel was obtained. In this paper, the experiments of Al2024 and reactive projectiles with hypervelocity impact on aluminum alloy honeycomb sandwich panel double-layer structure respectively were carried out by using two-stage light-gas gun, and the impact processes were recorded through high speed camera. According to the analysis of the experimental results, the debris clouds motion process and the damage effects on double-layer structure were compared. The damage enhancement mechanism of reactive projectile was revealed through numerical simulation and theoretical methods. The impact-induced detonation reaction of the reactive projectile can significantly reduce the "channel effect" of the honeycomb sandwich panel by destroying the honeycomb core cell wall, increase the perforation on the back facesheet of the honeycomb sandwich panel, and generate the debris cloud with higher temperature and faster expansion velocity. The debris cloud induced by the reactive projectile has a larger load distribution area on the rear plate, avoiding the concentration of loads in the center of the rear plate, and cause large area impact and thermal combined damage effects on the rear plate and internal space of the double-layer structure, while ensuring that the target structure cannot be penetrated. The reactive projectile efficiently applies the kinetic energy and chemical energy released by the impact-induced reaction to the interior of the target structure, the waste of kinetic energy caused by whole structure penetration can be avoided, resulting in significantly higher damage effects than the inert Al2024 projectile. [ABSTRACT FROM AUTHOR]

• The vibration suppression performance of composite honeycomb hemispherical shells with functional gradient protection coating is studied. • A mathematical model is proposed for predicting the dynamic behaviors of such shells with advanced coating. • The solution principle of the natural characteristics and vibration responses of such coated shells is clarified. • Some important design suggestions are summarized to improve the vibration resistance of such coated shells. The vibration reduction performance of composite honeycomb hemispherical shells (CHHSs) coated with functional gradient protection coating (FGPC) are investigated in this work. Using the first-order shear deformation theory and the power-law distribution rule, the virtual spring technique, the regional decomposition method, and the Newmark-Beta approach, etc., a dynamic model of the FGPC CHHSs under base excitation is formulated to solve the inherent characteristics and displacement responses in time and frequency domains. After a set of convergence analyses are completed to ascertain an appropriate segment number and the stiffness values of virtual springs employed in the predictive model, the forecasted vibration parameters are verified using the literature and experimental results that are performed on uncoated and coated shells. The maximal natural frequency errors of the current model compared to the experimental results are 3.8% and 4.8%, and the displacement response errors under different excitation amplitudes are less than 10.3% and 12.7%, respectively, which demonstrate the correctness of such a model. Finally, the impact of key structural and material parameters on the vibration behaviors of the FPGC CHHSs is evaluated. To improve their vibration suppression capability, it is recommended to choose a high gradient index of coating material and a large thickness ratio of the FGPC to the overall shell with a reasonable moduli ratio of Material A to Material B of the FGPC to improve vibration reduction capability. This study offers a practical model tool and several important design recommendations for vibration prediction and dynamic attenuation of honeycomb sandwich hemispherical shell structures in aerospace engineering. [ABSTRACT FROM AUTHOR]

• The CFRCs honeycomb sandwich structures are fabricated integrally by 3D printing technique. • The damage types at each stage of the flatwise compression process of 3D printed CFRCs honeycomb sandwich structures are successfully characterized. • A 3D printing method for CFRCs honeycomb sandwich structures with multilayer sandwich and core/panel variable printing thickness is proposed. • The compressive strength and energy absorption capacity of multilayer CFRCs honeycomb sandwich structures have obvious advantages. Continuous fiber reinforced composites (CFRCs) 3D printing technology has the advantage of integrated fabricating for honeycomb sandwich structures, the load-bearing capacity of the fabricated honeycomb sandwich structures still needs to be investigated and improved. This study aims to reveal compression failure modes of 3D printed CFRCs honeycomb sandwich structures with various internal configurations by conducting flatwise compression tests. The deviations of single-layer CFRCs honeycomb cores with different core sizes and core heights and their impacts on the compression failure process were analyzed. A 3D printing method with multilayer cores and core/panel variable printing thickness was proposed, and the effects of multilayer core structure and different core printing parameters on the compressive properties of honeycomb sandwich structures were investigated. The results showed that interlayer cracking caused by core buckling deformation was the leading cause of failure of 3D printed CFRCs honeycomb sandwich structures when subjected to compression. The core buckling deformation could be suppressed by printing multilayer cores and using low core printing thickness, and the compression strength and specific energy absorption of the sandwich structures reached 38.1 MPa and 25.4 kJ/kg, respectively. This study provides guidance for the customization of the 3D printing process of CFRCs honeycomb sandwich structures. [Display omitted] [ABSTRACT FROM AUTHOR]

• Hybrid re-entrant/hexagonal CF/PA12 honeycombs were 3D printed via FFF. • Temperature-dependent mechanical and piezoresistive characteristics were evaluated. • Hybrid honeycombs excel in enhancing in-plane strength (+64 %) and energy absorption (+125 %). • Hybrid honeycombs exhibit remarkable strain-sensing abilities, boasting gauge factors of ∼146. • The hybrid honeycombs retain 40–60 % of their mechanical properties at 125 °C. This study investigates a novel self-sensing honeycomb composite structure composed of two distinct cellular layers with differing unit cell architectures, specifically hexagonal and re-entrant designs. Short carbon fiber (CF)/polyamide 12 (PA12) composite filaments with 0, 5 or 15 wt.% CF content were utilized to additively manufacture the honeycomb structures via Fused Filament Fabrication (FFF), and their mechanical and piezoresistive self-sensing characteristics were experimentally investigated under quasi-static in-plane and out-of-plane compression at both room temperature and elevated temperatures. The results reveal that the hybrid hexagonal/re-entrant (HR) honeycombs mechanically outperform their non-hybrid double-layer and single-layer counterparts under in-plane loading, reporting an increase in collapse strength and energy absorption by factors of 1.64 and 2.25, respectively. These improvements are attributed to the mechanical interactions occurring at the interface between the auxetic and non-auxetic layers within the hybrid structure, effectively enhancing its structural attributes. Furthermore, the double-layer honeycombs display excellent strain-sensing capabilities within the elastic regime, with gauge factors reaching values as high as 146. Mechanical tests conducted at elevated temperatures reveal that the CF/PA12 honeycombs retain a significant portion of their elastic modulus, strength and energy absorption even at 125 °C, while maintaining high gauge factors of up to 72.4. These honeycombs also exhibit pronounced thermoresistive behavior, evidenced by a decrease in electrical resistance of up to 41.3 % with increasing temperatures from 25 to 125 °C. Considering their exceptional combination of thermo-mechanical, thermoresistive and piezoresistive characteristics, these hybrid double-layered CF/PA12 honeycombs hold promise for potential applications in multifunctional lightweight structures, offering integrated temperature and strain-sensing capabilities. [Display omitted] [ABSTRACT FROM AUTHOR]

• A novel bi-material multistable auxetic honeycomb (BMAH) with reusable and enhanced energy-absorbing phases was designed. • The crushing stress–strain curve shows three stress plateaus, and BMAH maintains a stable Poisson's ratio even at high strain levels, transitioning from near-zero to negative and then to positive. • Under the selected material combination, the second stress plateau is 8 times greater than the first, and the third stress plateau is 17 times greater than the first, significantly improving energy absorption efficiency. • A theoretical model was established to reveal the deformation mechanism of BMAH and predict the plateau stresses and strain range for each deformation stage. • Effects of geometrical parameters and crushing velocity on BMAH stress plateaus and Poisson's ratio were investigated, and a programmable guide for designing stress plateaus and specific energy absorption is presented. The ideal energy absorption structures, in addition to possessing reusability and high energy absorption efficiency, should also possess multiple desired properties to fulfill various functional applications. In this study, a bi-material multistable auxetic honeycomb (BMAH) is fabricated by using bi-material 3D printing technology. Under in-plane crushing, multiple stress plateaus can be achieved and the Poisson's ratio can be stably tuned, allowing for a transition from near-zero to negative and then to positive. The above characteristics are attributed to the specimen's multi-path deformation. The first stress plateau is only associated with recoverable elastic deformation. The second and third stress plateaus, which are 8 times and 17 times higher respectively than the first, are associated with plastic deformation, significantly enhancing energy absorption efficiency. The deformation mechanism is theoretically analyzed, and the effects of geometric parameters on the performance of BMAH are investigated. In addition, the effect of crushing velocity on the crushing behavior of the BMAH is also discussed. As the crushing velocity increases, the total number of stress plateaus transitions from three initially to two and then to one. The developed BMAH exhibits significant potential applications in multi-stage energy absorbers and smart sensors. [ABSTRACT FROM AUTHOR]

• Symmetry-corrugate hierarchical design strategy is proposed to develop novel honeycombs. • Symmetry-corrugate hierarchical honeycombs exhibit stable and efficient mechanical properties. • Period coefficients and amplitude coefficients have significant effects on crashworthiness. • A crashworthiness prediction model is developed by machine learning algorithms. Honeycombs are extensively utilized as energy absorber due to their stabilized mechanical properties. Therefore, we propose a novel symmetry-corrugate hierarchical design strategy to construct symmetry-corrugate hierarchical honeycombs (SCHH). Crashworthiness and mechanical properties of SCHH are investigated by experiments and numerical simulation. SCHH exhibits a stable and efficient mechanical property in the compression test. Furthermore, numerical simulations have been conducted to investigate the parametric design of SCHH. The results show the effects of different period coefficients φ T and amplitude coefficients φ A on deformation modes and crashworthiness. Specifically, as φ T increases and φ A decreases, the deformation changes from progressive deformation mode (P-mode) to mixed deformation mode (M-mode) to global bending deformation mode (G-mode). P-mode and M-mode have better crashworthiness. The effect of φ T on the crashworthiness of SCHH is more significant than φ A. In addition, SCHH provides the highest crashworthiness compared with other honeycombs. Specifically, the energy absorption capacity and crushing stability are improved by 53.8 % and 68.5 %, respectively, compared with conventional honeycomb. Eventually, a crashworthiness prediction model of SCHH is developed by machine learning algorithms, and the model is verified to have high accuracy. In this study, the proposed strategy and methods are expected to further improve the crashworthiness of honeycomb. [Display omitted] [ABSTRACT FROM AUTHOR]

• Failure behavior of 3D printed spherical honeycomb sandwich structures. • The low-velocity impact response of spherical honeycomb sandwich structures is studied. • Impact response and failure behavior are influenced by structural parameters. • Curvature radius has significant influence on the impact resistance. • Honeycomb orientation (concentric and coaxial) has significant influence on the impact resistance. The impact resistance of spherical honeycomb sandwich structures is affected by the orientation and curvature radius of the honeycomb, which are different from those of planar honeycomb sandwich structures. In this study, concentric and coaxial spherical honeycomb sandwich structures with different curvature radii are designed, and the specimens are prepared using 3D printing technology. The impact resistance of concentric and coaxial spherical honeycomb sandwich structures with different curvature radii are investigated through low-velocity impact experiments. It is shown that the impact resistance of the concentric structure decreases with the increase of curvature radius, which is better than that of the coaxial structure with the same curvature radius. This study provides a reference for the design and application of the spherical honeycomb sandwich structure. [ABSTRACT FROM AUTHOR]

• Auxetic origami honeycomb structure 3D-printed using stainless steel. • Impact tests on cushioning performance of auxetic origami structures conducted. • Auxetic origami structures exhibit higher plateau stress and specific energy absorption. • A smaller width-to-thickness ratio leads to higher energy absorption. • The folding angle significantly affects the stress and energy absorption. The honeycomb structure with a negative Poisson's ratio, inspired by the Miura origami unit, exhibits three-dimensional negative Poisson's ratio characteristics and effective energy dissipation performance. This unique property provides significant potential in the field of protection applications. This study aims to comprehensively understand the impact of auxetic and origami structures on the cushioning performance of honeycomb structures. For this purpose, 316 L stainless steel specimens were fabricated using 3D printing technology and subjected to drop hammer impact tests, and the results were verified by finite element simulation. This paper focuses on assessing the deformation mode and energy dissipation performance of origami auxetic honeycomb structures with varying width-to-thickness ratios and folding angles under different impact energy levels. The results indicate that the negative Poisson's ratio origami specimens exhibit higher plateau stress and specific energy absorption compared to their positive and zero Poisson's ratio origami counterparts with the same geometry. In addition, a smaller width-to-thickness ratio and higher input impact energy enhance the cushioning performance of honeycomb structures. [ABSTRACT FROM AUTHOR]

The presence of serrated ligaments in the re-entrant auxetic honeycomb (abbreviated as serrated RAH) effectively enhances the stiffness while maintaining auxeticity in the re-entrant direction. To investigate its auxetic characteristics, a 3D equivalent Cauchy model (3D-ECM) was established using the variational asymptotic method. The unique aspect is that the engineering constants are obtained by homogenizing the representative unit-cell and then utilized in the 3D-ECM for global analysis. Subsequently, the resulting global responses are fed into the recovery relationships for localized field analysis. The accuracy of the 3D-ECM and the recovered local field distributions were confirmed by comparing them with the results obtained from the three-dimensional FE model (3D-FEM) and the experimental data collected from the 3D-printed specimen. The effects of the geometric parameters on the negative Poisson's ratio and engineering constants were in-depth discussed. Specifically, an optimal value of 0.5 for the re-entrant ratio and d-value (corresponding to the re-entrant angle of 90 degree) yield maximum auxeticity and comparatively higher elastic moduli. In addition, the 3D-ECM greatly enhanced the computational efficiency without compromising the accuracy in predicting global behaviors and local field distributions. The unit-cell tailorability in the proposed model offers valuable guidance for optimizing the design of serrated RAHs. • The presence of serrated ligaments enhance stiffness while maintaining auxeticity. • Engineering constants are obtained by homogenizing the unit-cell within 3D-ECM. • Auxeticity of the serrated RAH is investigated using a VAM-based 3D-ECM. • The accuracy of the 3D-ECM is validated against 3D-FEM and experimental data. • A re-entrant angle of 90-degree yield maximum auxeticity and higher elastic moduli. [ABSTRACT FROM AUTHOR]

• Energy absorption of MRHC on two in-plane principal directions is comparable. • The MRHC are theoretically, experimentally and numerically studied. • More pronounced NPR effect can be attained by implementing misplaced design. • MRHC possesses in-plane bidirectional excellent two plateau stresses. Reinforced honeycomb exhibits exceptional energy dissipation capability. A misplaced design was introduced into reinforced honeycomb (RHC) in this paper, and a novel misplaced reinforced honeycomb structure was proposed (MRHC). The in-plane mechanical properties of MRHC was investigated experimentally, numerically and theoretically. MRHC possesses two plateau stresses in the two in-plane principle directions during low-velocity impact, while the RHC structure demonstrates this characteristic solely in one in-plane direction. The collapse modes and energy absorption performance of the MRHC structure under various impact velocities were investigated. The plateau stress behavior of the MRHC structure was investigated under various unit rotation angles. The two plateau stresses of the MRHC structure exhibit higher values when compared to the previously proposed structure with two plateau stresses. The structures exhibiting two-stage deformation characteristics can determine the extent of collision damage by analyzing the transformation of their structural configuration, establishing a foundation for the design of intelligent structures. [ABSTRACT FROM AUTHOR]

• A corner-enhanced biomimetic spider web hierarchical structure was designed. • Bio-inspired spiderweb honeycomb structures can effectively enhance energy absorption efficiency and achieve rapid energy dissipation. • Bio-inspired spiderweb honeycomb structures corner enhancement and graded enhancement mechanisms are revealed. • The effects of helix radius ratio and radial thickness on the specific energy absorption of honeycomb were investigated. To improve the energy absorption properties of honeycomb structures based on the underlying mechanism of uniform stress dispersion and to effectively increase the strength of a spiderweb, a corner-enhanced spiderweb hierarchical honeycomb structure was proposed herein. This study involved a quasistatic compression test and simulation of a corner-enhanced spiderweb hierarchical structure complemented by 3D printing. The deformation pattern of the common spiderweb honeycomb (CSH) was //-shaped. However, the deformation pattern of the honeycomb after corner enhancement tended to be monolithic, forming an X shape. The deformation characteristics of both the CSH and the first-order local cellular elements were analyzed. The corner-enhanced honeycomb cellular elements dispersed the load onto the inner wall to achieve a common load-bearing effect. Further numerical simulation analysis was carried out in conjunction with the experiments, and local destruction analysis was performed for single-cell elements. Corner enhancement could increase the plastic hinge energy absorption of the structure, with a first-order corner-enhanced spiderweb honeycomb (CESH-F) exhibiting a 46.3 % improvement in specific energy absorption compared to that of a conventional CSH. The layers were reinforced to compress the load uniformly, and a third-order corner-enhanced spiderweb honeycomb (CESH-T) had a 25.7 % improvement in specific energy absorption compared to that of the CESH-F. In addition, optimization was performed for corner-enhanced spiderweb honeycomb (CESH) according to different helix radii. The effect of the radial line thickness on honeycomb-specific energy absorption was explored, and the radial line thickness was positively proportional to honeycomb-specific energy absorption. These findings offered valuable insights into the design of lightweight and high-energy-absorbing biomimetic structures. [ABSTRACT FROM AUTHOR]

• The proposed auxetic honeycomb possesses more excellent auxetic behavior and specific energy absorption than the conventional honeycomb. • The interaction between the structural parameters is analyzed. • The multi-objective optimization of the enhanced auxetic honeycomb (EAH) was investigated. • The theoretical model and Box-Behnken Design model were used to predict the mechanical properties of EAH. In response to the challenge of low stiffness and energy absorption capacity, various designs of negative Poisson's ratio (NPR) structures have emerged. However, these designs often lack a comprehensive analysis of other mechanical properties, leading to subpar mechanical performance. Previous studies have explored the mechanical properties response of enhanced re-entrant honeycombs (ERH) under impact conditions, revealing limitations in optimizing the structure's performance with a single objective. Therefore, this study aims to enhance ERH structural parameters to achieve superior mechanical performance through theoretical derivations and geometric optimizations. The results demonstrate that the proposed theoretical model is consistent with finite element analysis and response surface (RS) predictions. Expressions for Young's modulus, specific energy absorption, and compressive strength are proposed, facilitating the identification of structural parameters that meet specific requirements during reverse design. Furthermore, a multi-objective optimization approach optimizes the geometric parameters based on maximum energy absorption and compressive strength. The mechanical behavior of the optimized ERH is investigated using the finite element method, revealing the energy absorption capacity of 13.78 J/g while maintaining Poisson's ratio at -1.06. Additionally, the deformation mode of the optimized structure showcases enhanced stability compared to traditional honeycomb structures. The theoretical model and RS method were used to guide the design of ERH and promote the application of NPR structures. [ABSTRACT FROM AUTHOR]

• A novel light-weight and high load-bearing C/C honeycomb structure was fabricated. • The accurate model of honeycomb structure containing void defects is established. • A multi-scale damage model for C/C honeycomb structure is developed. • Effects of weave modes and defects on mechanical and failure features are given. The combination of honeycomb structure and carbon/carbon (C/C) composites offers unique high load-bearing and light-weight advantages in satellite-bearing platforms. However, the defects generated by the imperfect preparation process are inevitable. This paper employs the chemical vapor infiltration (CVI) technique to fabricate novel C/C honeycomb structures and considers the presence of internal void defects. Based on micro-computerized tomography (μ-CT) images, the void distribution and porosity as well as the micro-structure characteristics are acquired. Meso-scale representative volume element (RVE) models is established to predict the performance of honeycomb wall material. A secondary development of ABAQUS is performed to establish a model of honeycomb structure containing void defects. Multi-scale models are developed for investigating the mechanical performance of the C/C honeycomb structure. Experimental methods and finite element simulations are utilized to examine the effects of porosity and different weave structures on the compression and shear properties, as well as damage modes of C/C honeycomb structures in detail. The results demonstrate that the presence of low porosity ρ = 1 % severely diminishes the load-bearing capacity of the honeycomb and affects its damage pattern. Twill-2 weave C/C honeycomb inherits the advanced performance of the composites and exhibits superior comprehensive properties compared to plain weave and twill-1 weave honeycomb structures. [ABSTRACT FROM AUTHOR]

• Cuttlebone-inspired honeycomb with asymmetric sinusoidal walls is first proposed. • Effect of the wall amplitude and gradience on the crushing performances are revealed. • The mechanism of honeycomb is fully studied through FEM and the elastic theory. Inspired by cuttlebone microstructure, this study proposes a novel bionic square-tube cuttlebone-inspired honeycomb (S-CIH) structure prepared by digital light processing (DLP) method. Fifteen configurations of honeycombs of same mass including S-CIH, regular honeycomb (S-RH) and sinusoidal honeycomb (S-SH) structures are evaluated and compared in terms of the deformation behavior and compressive performances under out-of-plane compression, especially energy-absorbing capacity. The results indicate that the S-CIH structure achieves the improved energy absorption 8.859 J/g, over 40 % and 60 % than that of S-RH and S-SH structure with same relative density and mass, respectively. The mechanism analysis of the asymmetric corrugated walls disposed in cross and parallel in S-CIH, reveals that the magnitude of amplitude determines the buckling mode thus folding mode of walls, and the magnitude of amplitude gradient determines whether the whole honeycomb structure can achieve a progressive failure. Optimal structural ratio can enable S-CIH delay the complete failure and improve the long-term failure-resistance. This indicates that S-CIH design has the potential ability to strengthen and toughen other brittle matrix materials, and the cuttlebone-like architecture provides a new biological design of mechanically efficient meta-honeycomb beyond conventional honeycomb structure. [Display omitted] [ABSTRACT FROM AUTHOR]

• Introduced an optimized and expanded three dimensional steady-state transition structure. • Proposed a novel impact-resistant structure based on the steady-state transition design. • Demonstrated significant energy absorption features and energy oscillations in the steady-state transition structure under impact. • Highlighted the development potential of steady-state transition structures in the field of impact resistance. Advancements in materials science and mechanical system have spurred widespread investigation into non-traditional structures. In particular, engineers have shown interest in steady-state transition structures with phase transition properties. In this paper, a three-dimensional steady-state transition structure was established using the genetic algorithm and three-dimensional expansion, on which the low-velocity impact tests were carried out. Force response and energy absorption of the steady-state transition structures under impact were studied. From the force response and energy absorption curves, it can be seen that the steady-state transition structures absorb and store portion of impact energy temporarily, gradually releasing it after impact peak. This unique energy behavior results in a steady state transition structure with a long impact response time and a low peak impact force under impact. In addition, investigation into energy transfer within these structures reveals that energy oscillations occur among multilayered bistable surfaces during steady-state transitions, further improving the energy dissipation. Further, the dynamic response and damage modes of the honeycomb structure and the steady state transition structure were compared, showing that the damage mode of the steady state transition structure under the impact is an overall damage, in contrast to the penetration damage of the honeycomb structure. Notably, the steady-state transition structures showcase a higher proportion of elastic strain energy during impacts, effectively delaying structural damage onset, and dispersing impact energy, thus enhancing impact resistance. In light of these findings, the demonstrated behavior of steady-state transition structures in absorbing and dissipating impact energy showcases their potential as promising avenues for the development of more effective impact protection strategies. [ABSTRACT FROM AUTHOR]

• Size-dependent behaviors of auxetic honeycomb sandwich nanoplates is developed. • The proposed approach utilizes a nonlocal strain-gradient isogeometric analysis which incorporates the effects of both nonlocality and strain gradient. • The sandwich nanoplate consists of a core layer featuring an auxetic honeycomb and two outer skin layers reinforced with graphene nanoplatelets. • The length scale parameters can high-efficiently predict size effects. • Some novel benchmark numerical results are illustrated and introduced. Honeycomb structures, which are known for being lightweight and stiff, are still being researched and developed. They have been used in a wide range of industries, but their full potential has not yet been realized. In this study, a novel computational approach for exploring the size-dependent behaviors of auxetic honeycomb sandwich nanoplates is developed. The proposed approach employs a nonlocal strain-gradient isogeometric analysis integrating the influences of nonlocality and strain gradient into the nanoplate structures. The sandwich nanoplate consists of a core layer featuring an auxetic honeycomb with a negative Poisson's ratio, complemented by two outer skin layers reinforced with graphene nanoplatelets (GNPs). This configuration not only achieves exceptional lightweight characteristics through the utilization of auxetic honeycomb cells but also enhances structural stiffness by incorporating GNPs into the skin layers. The material properties of the core layer are determined using cellular cell formulas, while the reinforcement of the two outer skin layers with GNPs is calculated using the modified Halpin-Tsai model. Numerous numerical examples are conducted to investigate the influence of various parameters on the frequencies of the auxetic honeycomb sandwich nanoplates. Notably, the geometrical dimensions of the auxetic honeycomb cells and the nonlocal and length scale parameters emerge as significant influencers on the results. As the first analysis of honeycomb structures at small dimensions, our findings stand as valuable benchmarks for future analyses. [ABSTRACT FROM AUTHOR]

• Hybrid hierarchical square honeycombs (HHSHs) with different configurations were fabricated and tested for evaluating their in-plane elastic modulus. • 11 machine learning (ML) models are trained using a database with 234 results from experimentally validated FE models. • A modified theory for the in-plane elastic modulus of HHSH is developed based on the XGBoost model and existing theory. • The upper and lower bounds of the in-plane elastic modulus of HHSH are determined by the modified theory. • Based on the SHAP method, a quantitative comparison of the importance of hierarchical parameters is conducted. In this study, experimental, finite element (FE) simulation, machine learning (ML), and theoretical techniques are employed to investigate the in-plane elastic modulus (E HHSH) of hybrid hierarchical square honeycombs (HHSHs). First, HHSHs with different configurations were fabricated using a 3D printer, and in-plane quasi-static compression tests were conducted on them. Then, 234 FE models are simulated to determine the E HHSH of HHSHs with various configurations, and the results are used to train 11 ML models. Comparative analysis demonstrates that the Extreme Gradient Boosting (XGBoost) model has the best predictive capability. Moreover, a modified theory for E HHSH is established based on the XGBoost model and existing theory, and its exceptional predictive capability is verified by comparing with experimental, FE, and existing theoretical results. Finally, the upper and lower bounds of E HHSH are determined by the modified theory, and the Shapley Additive Explanation (SHAP) method is used to identify the importance of different geometric parameters on tailoring E HHSH. The combination of theoretical and ML techniques provides a promising approach for developing a robust prediction model of material properties. [Display omitted] [ABSTRACT FROM AUTHOR]

• A new non-uniform rectangular honeycomb sandwich panel is designed based on Bezier curves to enhance material utilization. • The effective elastic constants of non-periodic honeycombs are obtained through the geometric equivalent method and the strain energy-based homogenization method. • The analytical solutions of bending and buckling responses of the non-uniform rectangular honeycomb sandwich panel are derived by using parabolic shear deformation theory. • The weight/performance multi-objective optimization design of the non-uniform rectangular honeycomb sandwich panel is conducted. Honeycomb sandwich panels have a wide range of applications in aerospace, marine and energy industry due to their high stiffness-to-weight ratio. In this study, a new non-uniform rectangular honeycomb core is presented to promote its material utilization ratio. The non-uniform wall of honeycomb core is described using Bezier curves, and the effective elastic constants are obtained by both the geometric equivalent method and the strain energy-based homogenization method. To analyze the bending and buckling behaviors of the sandwich panel, the parabolic shear deformation theory is adopted, and the governing equations are derived by employing Hamilton's principle. Then, the closed-form solutions of bending and buckling responses of the non-uniform rectangular honeycomb sandwich panel are derived. Furthermore, multi-objective design formulations are established, with the objectives of minimizing the weight and deflection, and maximizing the critical buckling load. Illustrative examples demonstrate that the non-uniform design of the honeycomb core can remarkably increase the material utilization rate and improve the mechanical performance of sandwich panels. [ABSTRACT FROM AUTHOR]

• A bistable structure was proposed by introducing cosine-shaped beams. • Configuration and geometric parameter can modulate the ν of the structure. • The structural tunable vibration isolation property was verified by experiments and simulations. With the increasing demand for vibration-isolation structures with tunable and multifunctional properties driven by the rapid development of complex engineering applications, metamaterials have emerged as promising candidates for this purpose owing to their extraordinary properties derived from their unique structural configurations. Owing to their unique deformation mechanisms, negative Poisson's ratio structures have been extensively studied in terms of vibration isolation and energy absorption; however, research on their performance tunability remains lacking. This paper describes a bistable honeycomb mechanical metamaterial (BHMM) consisting of cosine-shaped beams, which exhibits transformable Poisson's ratio and tunable vibration isolation properties. The proposed structure exhibited bistable properties, i.e., undergoing a configuration transition under external forces and retaining its existing configuration after the loading was removed. Furthermore, the two stable configurations exhibited distinctive elastic vibration characteristics, allowing the manipulation of elastic wave propagation on a subwavelength scale via local resonant mechanisms. Overall, this study presents a design methodology for multi-performance tunable metamaterials that satisfy the demands of the external environment. For example, the tunable vibration-isolation property of metamaterial makes it intriguing safety protection structures during crash events. [ABSTRACT FROM AUTHOR]

• 2.5D woven carbon-fiber honeycomb sandwich structures was manufactured and investigated both experimentally and numerically. • Effects of varied parameters of the sandwich structure on the mechanical, especially the energy absorption characteristic were investigated through quasi-static indentation experiments. • The panel strength and honeycomb core strength were found synergistically determine the failure mode of the sandwich structure. Here, a new type of sandwich structure made of fiber metal laminates (FMLs) as facesheet and 2.5D woven carbon-fiber (CFRP) honeycomb as core was developed. The FMLs/CFRP honeycomb sandwich structures were subjected to quasi-static indentation tests to investigate the mechanical response with the variation of panel structure, honeycomb cell element size, honeycomb core wall thickness, and honeycomb core height. In addition to experiments, a finite element method (FEM) model was also created in ABAQUS/Explicit to analyze the damage mechanisms and failure modes of honeycomb sandwich structures under quasi-static indentation. Both the experimental and FEM simulation results revealed that the honeycomb sandwich structure underwent two different failure modes under quasi-static loading: localized damage dominated by indentation and global damage due to overall bending deformation. The combination of panel strength and honeycomb core strength determined the failure mode of the sandwich structure. Damage analysis of the honeycomb core showed that it mainly underwent fiber compression damage and matrix compression damage under quasi-static indentation. This study provides guidance to optimize the design of honeycomb sandwich structures. [ABSTRACT FROM AUTHOR]

In recent years, origami honeycomb metamaterials have garnered extensive attention from researchers. Re-entrant origami honeycomb metamaterials exhibit excellent mechanical performances by virtue of lightweight, high-energy-absorbing characteristics and diverse configurations. In this study, the gradient re-entrant origami honeycomb metamaterials are designed by changing significant geometric parameters. To investigate the effects of geometric parameters variations on compressive behaviors, the theoretical analysis model of the re-entrant origami honeycomb is built to predict the compression stress under low-velocity and high-velocity impact. Subsequently, the energy absorption of the uniform re-entrant origami honeycomb (UROH), the gradient-thickness re-entrant origami honeycombs (GTROHs) and the gradient-angle re-entrant origami honeycomb (GAROHs)are investigated under different compression velocities. The impact resistances of UROH, GTROHs and GAROHs are discussed systematically. By adjusting the gradient distribution of thickness, the deformation modes and Poisson's ratio curves of the structure could be altered. Furthermore, under the high-velocity impact, the specific energy absorption (SEA) of the unidirectionally negative GTROH is increased by 36% than that of UROH at the strain of 0.29. The SEA of the unidirectionally positive GTROH is higher by 14.4% than that of UROH at the strain of 0.8. In addition, the unidirectionally positive GTROH exhibits better impact resistance performance than UROH by comparing the peak stress and SEA value under different compression velocities. This study is expected to provide the new route for designing advanced origami-inspired honeycomb structures and improving buffer protection devices. • Gradient re-entrant origami honeycombs are proposed by varying significant parameter of cells. • An analytical model is established to estimate the plateau stress of the present structure. • Deformation modes and energy absorption of gradient origami honeycombs are investigated. • The crashworthiness of uniform and gradient re-entrant origami honeycombs are discussed. [ABSTRACT FROM AUTHOR]

• The mechanical behavior of carbon fiber reinforced polymer honeycomb cores under in-plane and out-of-plane quasi-compression loads was explored. • Digital image correlation was used to obtain full-field strain evolution and distribution of plastic hinges during compressive loading. • In out-of-plane compression, the cores underwent progressive failure, forming a plastic hinge line. • In transverse direction compression, the cores produced continuous diagonal shear bands. • In ribbon direction compression, diagonal shear band first appeared in the cores, followed by layer-by-layer crushing. In order to meet the urgent demand for the application of composite honeycomb sandwich structures, Carbon fiber reinforced polymer (CFRP) honeycomb cores were made by the hot press molding method. The mechanical behavior of the honeycomb cores under in-plane and out-of-plane compression loadings was explored, including deformation process, energy absorption mechanism, and damage modes. The research focused on the correlation between macroscopic response and local deformation, as well as the influence of loading direction on the deformation process. Digital image correlation (DIC) was used to obtain full-field strain evolution and distribution of plastic hinges during compressive loading. Especially, 2D-DIC was established to obtain the full-field strain distribution of the front and side of the cores during in-plane compression. The experimental results showed that progressive failure of the cores occurred under out-of-plane compression. The deformation was dominated by the bending deformation and the compressive deformation of cell walls when compressed in the W and L directions, corresponding to diagonal shear and layer-by-layer failure, respectively. In addition, the failure mechanism was identified by optical microscope and scanning electron microscope (SEM). This study provided a better understanding of the compressive response of the CFRP honeycomb structures. [ABSTRACT FROM AUTHOR]

• Honeycomb sandwich structures with continuous density gradients are designed. • Flexural behavior is sensitive to horizontal but insensitive to vertical gradients. • Achieved favorable flexural resistance with horizontal arched density distribution. • Detrimental effects from thin cover plates and honeycomb layers are indicated. Sandwich structures with graded cellular cores exhibit promising flexural performances, but their reliance on gradient continuity and distribution warrants further exploration. In this study, seven honeycomb sandwich structures with diverse continuous density gradients, encompassing horizontal, uniform and vertical variations, were designed based on the Voronoi technique and fabricated via 3D printing. Quasi-static three-point bending tests and numerical simulations were conducted to comprehensively analyze the flexural response of density-graded honeycomb sandwich structures. The results reveal a significant influence of the honeycomb core's density distribution on flexural performance, with pronounced sensitivity to horizontal distribution (perpendicular to the loading direction) and less sensitivity to vertical distribution. A favorable flexural resistance is achieved by strategically assigning higher relative density cells to the mid-span of a honeycomb core layer, gradually diminishing towards the ends, which was further optimized into an arched density distribution along the horizontal direction. The influences of cover plate and honeycomb thicknesses on the deformation mechanisms and energy absorption were also investigated, and indicate that thin cover plates and honeycomb layer have a detrimental effect on the bending load-bearing and energy absorption of the sandwich structure. [ABSTRACT FROM AUTHOR]

• A novel origami honeycomb acoustic metastructure is proposed as an effective impact energy absorber. • The metastructure features superior low-frequency sound and impact energy absorption capacities. • The origami foldcore and the top panel are the key components in absorbing impact energy. • Energy absorption efficiency of the metastructure can be enhanced by tuning its geometric parameters. The ancient art of origami has inspired the design and fabrication of numerous acoustic and energy-absorption mechanical metamaterials and metastructures. This study introduces an origami-inspired honeycomb acoustic metastructure designed for engineering applications that demand both high impact resistance and effective sound absorption. This acoustic metastructure is formed by bonding a panel containing periodically distributed perforated holes and a panel without perforations to a periodical array of trapezoidal origami-bellow units. Acoustic wave propagation and drop-weight impact simulations are conducted to investigate the acoustic performance and low-velocity impact behavior of the metastructure. The results show that the origami-bellow honeycomb structure exhibits superior sound absorption capacity over two frequency ranges, outperforming the conventional honeycomb-core sandwich structure. Furthermore, it is shown that, in comparison with both the conventional hexagonal honeycomb-core and the Miura-ori foldcore sandwich structures, the proposed acoustic metastructure demonstrates higher levels of impact resistance and energy absorption capacity. We also show that the mechanical performance of the introduced metastructure under low-velocity impact loads can be further enhanced by increasing the story height of the origami-bellow units. Besides, the matrix material of the origami foldcore can be tailored to suit various scenarios, providing noise control and collision robustness for a diverse range of engineering applications. [Display omitted] [ABSTRACT FROM AUTHOR]

• The deformation behaviors of aluminum honeycomb sandwich panels with artificial defects of different sizes under in-panel compression were investigated using mirror-assisted MV-DIC. • Dual-surface in-plane deformation fields and through-thickness strain fields were measured. • The honeycomb sandwich panel exhibits local undulations and strain concentrations in the surface with defects. • Only defects with sufficiently large sizes leads to decreases in the buckling load. • The bending direction of the buckled specimen is influenced by the local undulations caused by the defect. Honeycomb sandwich panels have been extensively utilized in the aerospace and automotive industries for their prominent advantages of energy absorption, high strength and tailorable anisotropic mechanical properties. Nevertheless, the common presence of local core-to-skin disbonding poses an urgent need to understand the mechanical behaviors of defective honeycomb sandwich panels. The present work investigates the deformation behaviors and evolutions of aluminum honeycomb sandwich panels with varying sizes of circular artificial core-to-skin disbonding defects using mirror-assisted multi-view digital image correlation (MV-DIC). Based on the 3D profiles and dual-surface deformation fields obtained with mirror-assisted MV-DIC, it is revealed that disbonding defects induced local surface undulations with severe local strain concentrations and determined the mode of gradual bending and buckling. Defects with a diameter below 20 mm induced minor local surface undulations and slight decreases in buckling load. When the diameters of defects reach 30 mm, extensive surface convex deformation occurs and the buckling load decreases by 23 %. The results validate mirror-assisted MV-DIC as a cost-effective and practical technique for characterizing dual-surface 3D deformation of sheet composite materials and structures. [ABSTRACT FROM AUTHOR]

• A core stagger layered strategy is proposed for the honeycomb sandwich panels. • Experiments indicate availability of the strategy and accuracy of the numerical simulation. • For a given mass and geometrical size, the increase of layer number and cell size consolidate this effect. • The strategy provides a well choice to improve protective capacity of sandwich panels under air blast loading. Metallic honeycomb sandwich panels have been widely used in energy dissipation due to large plastic deformation under impact/blast loading. In this paper, the dynamic response of a honeycomb sandwich panel with stagger-layered core under intensive near-field air blast loading is investigated. Comparative experiments are conducted for stagger-layered and typical honeycomb sandwich panels with the same geometrical size and mass. Numerical simulation has been undertaken based on the experiments. Numerical results coincide well with experimental results on the deformation/failure modes and center-point deflection of back face sheet. The influence of layer number as well as the geometrical parameters on the energy absorption ability of sandwich panels is clarified. Results show that stagger-layer arrangement of honeycomb core greatly improves energy dissipation capability of sandwich panels compared to typical one with the same mass. The increase of layer number and cell size benefits this effect. These findings indicates that core stagger-layer arrangement is a well choice to improve energy dissipation ability of sandwich panels under air blast loading, especially for highly intensive near-field blast loading. [ABSTRACT FROM AUTHOR]

• Out-of-plane stress-strain response of a sandwich panel differs from that of cellular core. • Adhesive height determines densification initiation strain. • Method correctly characterizes densification region, accounts for adhesive effect. • Significant energy absorbed in the densification region for panels with adhesive. • Larger fillets reduced the overall absorbed energy due to lower densification initiation strains. Lightweight aluminum honeycomb sandwich panels are widely used in industrial engineering, particularly in aircraft applications. Due to their thin-walled nature, the honeycomb cell walls are especially susceptible to buckling under out-of-plane compressive loading, while the adhesive fillet that secures the honeycomb to the face-sheet remains intact. This paper investigates the effect of the adhesive geometry and bonding constraints on the out-of-plane compressive response through experimental, Finite Element (FE), and mathematical methods. A hybrid method accounting for adhesive bonding between the face-sheet and the cell walls, and different adhesive fillet geometries was proposed to predict strain onset and stress-strain relationships during densification. The resulting stress-strain curves predicted the energy absorbed by the panels to within 3.5 % of the experimental results. The widely used material model for honeycomb core does not account for densification and underpredicted the absorbed energy by 15.6 %. These findings emphasize the need for detailed adhesive analysis in the design of lightweight, thin-walled sandwich structures. [ABSTRACT FROM AUTHOR]

• Effects of heating and cooling thermal shock on thermally induced vibration (TIV) are explored. • Influence of geometric nonlinearity under heating/cooling thermal shock forms is revealed. • The TIV characteristics of sandwich beams with auxetic honeycomb cores are discussed and compared with traditional honeycomb. • The instability conditions of the clamped beam with immovable ends under heating thermal shock are given. The nonlinear thermally induced vibration (TIV) characteristics of sandwich beams with auxetic honeycomb cores under general boundary conditions are studied. Based on first-order shear deformation theory, the dynamic governing equations are obtained through Hamilton principle, in which geometric nonlinearity and temperature dependence are considered. The temperature field along the thickness is calculated by finite element method and nonlinear TIV responses are solved via Newton-Raphson-Newmark method. The effects of temperature dependency, thermal shock forms, geometric nonlinearity, thicknesses of beam and face-sheet, boundaries and honeycomb geometrical parameters are studied. Results show that the reduction of the absolute inclined angle of the auxetic honeycomb can weaken the TIV responses. The influence of geometric nonlinearity on quasi-static and dynamic responses depends on the thermal shock form. Thermal buckling is the necessary condition for the occurrence of the TIV in clamped beams with immovable ends. Moreover, the research provides some design references to suppress the TIV responses of the sandwich beam with auxetic honeycomb core. [ABSTRACT FROM AUTHOR]

• A novel composite auxetic reentrant honeycomb structure is designed and fabricated. • The underwater impact properties of composite reentrant honeycomb structure were explored and analyzed by experiment and finite element analysis. • The effect of gradient form, peak pressure and decaying time on the fluid-structure interaction was analyzed and revealed. • The effect of cavitation of composite reentrant honeycomb structure was analyzed and investigated. In this paper, the effects of the gradient form, peak pressure and decaying time on the fluid-structure interaction of composite auxetic re-entrant honeycomb structures was experimentally and numerically investigated. The specimens for this experiment were manufactured by hot-press molding method. The acrylic transparent tube was utilized to accommodate water medium, pistons and specimen during experiment. The exponential decaying shock wave was simulated by accelerating the fly plate to impact the piston based on the one-stage light gas gun system. The incident velocity of the fly plate and process of the fluid-structure interaction response were monitored by high frame rate camera. In addition, the velocity of the shock wave in water was measured by transducers installed at different measuring points. The voltage signal of the dynamic overpressure collected by transducers was amplified by the charge amplifier and recorded by the oscilloscope. On the other hand, finite element software Abaqus/Explicit was applied to simulate the cavitation process of water domain and dynamic response of specimens. The experimental results and numerical results were in good agreement. The results indicated that the increase of the thickness of the fly plate had a certain effect on the propagation velocity of the shock wave, and the increase of the peak pressure would make the cavitation generation position close to the fluid-structure interface and make the cavitation duration longer. And the average structure which has a minimum transformation of 8.12 mm shows better underwater impact resistance than the gradient structure. [ABSTRACT FROM AUTHOR]

• A novel re-entrant double-arrow auxetic honeycomb was proposed. • Three typical deformation modes under different impact velocities were revealed. • Theoretical models of single-plateau and double-plateau stresses were established. • The effects of impact velocity and geometric parameters were explored. A novel re-entrant double-arrow honeycomb (RDAH) has been proposed and investigated by combining re-entrant hexagonal honeycomb and double-arrow honeycomb. Quasi-static compression experiments of 3D printed samples in different loading directions were carried out. Deformation modes and stress–strain response have been explored and identified. The corresponding finite element (FE) models were established and verified, and the dynamic crushing behaviors of RDAH were further investigated by FE simulations. The results show that RDAH has three typical deformation modes under different impact velocities. When subjected to quasi-static loading and low-velocity impact along the horizontal wall, two stress plateau regions of the stress–strain curves can be observed, while there is only one stress plateau when loaded perpendicular to the horizontal wall. Based on deformation modes, theoretical models of plateau stresses of RDAH under low-velocity and high-velocity impact were further established. The theoretical results of the plateau stresses agree well with the FE simulation results. The effects of impact velocity and geometric parameters on densification strain, Poisson's ratio and energy absorption were also discussed. The results show that the RDAH exhibits obvious negative Poisson's ratio effect in both directions. The energy absorption of RDAH is higher when loaded along the horizontal wall and can be 29.45 % higher than the traditional RH structure. The energy absorption of RDAH increases with the decrease of inclined wall l 1 , the decrease of inclined angle θ (perpendicular the horizontal wall), and the increase of inclined angle θ (along the horizontal wall). [ABSTRACT FROM AUTHOR]

• A design strategy of embedding origami into honeycomb is proposed. • Two types of origami-embedded honeycombs are constructed. • The design strategy is able to improve SEA and weaken anisotropy. • The high energy-absorbing mechanism of origami design is clarified. Origami has been exploited to various marvelous materials to manifest mechanical properties. In this study, inspired by the distinctive deformation modes of kresling-origami with rotation limitation, a design strategy of embedding kresling-origami into conventional honeycomb for improving energy absorption capacity is proposed. According to different embedding approaches, i.e., replacing or adding cell walls, two types of origami-embedded honeycombs are constructed. To investigate the energy absorption properties of proposed origami-embedded honeycombs, 3D-printed specimens are manufactured and tested under quasi-static compression. The experiment results show that proposed origami-embedded honeycomb have higher specific energy absorption and three-axial comparable energy absorption performance relative to conventional ones. It indicates the design strategy of embedding kresling-origami is practical for improving energy absorption capacity of conventional honeycomb and weakening its anisotropy. In addition, the energy absorption performance of origami-embedded honeycombs with different geometrical configurations is investigated through an established and experimentally validated numerical simulation model. Furthermore, the mechanism (i.e., high energy-absorbing deformation modes of kresling-origami under boundary constraints) of embedding design strategy is clarified via comparison analysis between experiments and simulations. This strategy carves out a novel way to optimize the mechanical properties of honeycomb and may inspire new innovations of metamaterials. [Display omitted] [ABSTRACT FROM AUTHOR]

The triangular chiral (Tri-Chi) honeycomb is a novel negative Poisson's ratio structure, featuring fully triangular cells inspired by the anti-tetra chiral design. This work aims to reveal the deformation mechanisms of the Tri-Chi honeycomb and employs the variational asymptotic method to determine the effective plate properties of sandwich panels with Tri-Chi honeycombs (i.e., SP-TCH). Based on this, a 2D dimension reduction model (2D-DRM) is developed, and the warping functions are solved through asymptotic analysis of the leading terms in the energy functional. Comparative analysis with the 3D FE model demonstrates that the 2D-DRM exhibited maximum errors of only 6.83% and 2.93% respectively in analyzing tension-bending coupling behavior and natural frequency, with the computational efficiencies being improved by approximate 55-fold and 32-fold, respectively. Furthermore, the parameter analysis indicate that the structural stiffness and natural frequencies of SP-TCH were higher than those of stiffened triangle honeycomb sandwich panel with the same dimensions and materials. The facesheet layup design had a significant impact on the equivalent stiffness and vibration characteristics, whereas the included angle had a significant influence on the Poisson's ratio of the Tri-Chi core layer, leading to a reversal from a negative to a positive ratio as the included angle increased. • The deformation mechanisms of the novel Tri-Chi honeycomb is revealed. • 2D dimension reduction model is developed using variational asymptotic method. • Effects of geometric parameters and layup configurations are investigated. • Structural stiffness and natural frequencies of SP-TCH are higher than SP-STH. • Included angle has impact on the Poisson's ratio of the Tri-Chi core. [ABSTRACT FROM AUTHOR]

• 3D curved-walled same-phase chiral honeycombs are proposed. • Out-of-plane experiments and FEM simulations are carried out. • Efficient folding mode with compression-torsion coupling effect is discovered. • Plastic hinge model is developed to estimate out-of-plane mean stress. • Thickness gradient design is implemented. Honeycombs are widely-used protective structures with exceptional properties. However, the performance improvement relying on regular configuration optimization and parametric study are relatively limited, because main plastic deformation only concentrates in folding line regions under progressive folding mode. To address this issue, 3D curved-walled same-phase chiral strategy is designed for honeycombs, and their crash performance is studied by experiments, numerical simulations, and theoretical analysis. The proposed honeycombs exhibit approximate progressive folding mode with controllable local compression-torsion coupling effect under out-of-plane loads. Therefore, the shearing energy is obviously increased, and the plastic hinge lines have been prolonged to increase bending energy. According to experimental results, the proposed honeycombs can achieve specific energy absorption SEA and energy absorption efficiency η respectively of 30.92J/g and 73.7 %, which are 14.6 % and 24.9 % larger than traditional honeycomb with same thickness. Moreover, their mean stress is effectively estimated based on plastic hinge analysis, and the contribution of coupling effect on crashworthiness is demonstrated. Furthermore, out-of-plane thickness gradient design can be implemented to further promote the crashworthiness. The proposed honeycomb with thickness gradient of Δ t / t a v e = 1.5 displays SEA and η respectively 10.9 % and 23.1 % larger than the non-gradient model under same equivalent density. This study for the first time introduces compression-torsion coupling mechanism into honeycombs, and provides a reference for developing high-performance honeycombs and expanding their potentials. [Display omitted] [ABSTRACT FROM AUTHOR]

• A novel model for auxetic honeycomb thin plates, utilizing modified Gibson functions, is developed. • The tensile and compressive deformation are considered in the equivalent elastic formulas. • Nonlinear primary/super-/sub- harmonic resonances of auxetic honeycomb plates are analyzed. • The geometric parameters of auxetic honeycomb unit cells exhibit a significant influence on nonlinear dynamic behaviors. Auxetic honeycombs possess many excellent properties, making them ideal cores for sandwich structures that can absorb energy effectively. This paper investigates the nonlinear vibrations of auxetic honeycomb composite plates, focusing on the primary resonance, super-/sub-harmonic resonances of the composite plates. Two pure panels sandwich an auxetic honeycomb core to form the honeycomb composite plate. The proposed modified Gibson function enables the derivation of effective material properties. By considering geometric nonlinearity, the nonlinear motion equations are derived through the implementation of the Hamilton principle, and then nonlinear ordinary differential equations are given by introducing stress functions and the Galerkin method. The nonlinear response curves are then determined by the multiple scale method. The impact of important parameters on the nonlinear vibration of auxetic honeycomb composite plates is evaluated in this study. [ABSTRACT FROM AUTHOR]

• A novel methodology for fabrication of aluminium honeycomb core by FSW. • Compressive performance is studied with different core height and number of cells. • Good specific compressive properties without crack or debonding in the weld. • Deformation behaviour shows symmetric folding and progression of folds. Aluminium honeycomb structure has been recognized as an exceptional lightweight energy absorber in transportation, construction and aerospace industries. An innovative fabrication method based on friction stir welding is proposed to enhance the crashworthiness property of aluminium honeycomb and address the challenges with conventional fabrication methods. Moreover, a lab-scale fixture is designed to demonstrate the feasibility of the suggested method for industrial use. Various fabricated honeycombs with different core heights and cell counts are subjected to quasi-static flatwise compression tests to investigate their crushing characteristics. The crashworthiness parameters like yield stress, average plateau stress, specific energy absorption are analysed and compared with theoretically predicted crushing strength. The failure mode of the structure is also discussed, showing the influence of non-optimal welding parameters. Experimental results indicate that average crush force and specific energy absorption do not differ significantly with cell count and core height. The specimens under flatwise compression fold with plastic hinges symmetrically positioned without any rupture of the weld. Specific compression property of the fabricated honeycomb has been compared with other aluminium honeycombs fabricated using other methods, which favours the friction stir welded honeycomb over the other conventional counterparts. [ABSTRACT FROM AUTHOR]

• HCSP showed enhanced damage and perforation resistance under combined loading. • The enhanced damage of HCSP was caused by fracture of perforation hole. • The enhanced perforation resistance of HCSP was due to face sheets deforming. The performance of sandwich structures against individual shock loading and fragment impact loading has been extensively examined, but little is known about their performance in the presence of simultaneous shock and impact loading. Based on a recently developed composite projectile to simulate combined shock and single fragment impact loading, the performance of aluminum honeycomb core sandwich panels (HCSPs) against combined loading is methodically examined. The achieved results reveal that the combined loading exhibits a synergistic effect compared to the single loading, i.e., increased damage of the sandwich panel and enhanced perforation resistance of the sandwich panel. After that, a three-dimensional finite element simulation is performed to explore the underlying mechanism of the synergistic effect. Numerical analysis indicates that the increased damage is a result of perforation-induced reduction of load-carrying capacity of sandwich panel, while the enhanced perforation resistance is the result of deflecting-induced perforation time delay of the perforation process. Finally, the parametric investigation of sandwich geometries such as asymmetric face sheets, core density, and core height is comprehensively conducted. The gained results reveal that both performance and synergistic effect due to combined loading are sensitive to these parameters. [ABSTRACT FROM AUTHOR]

• Field tests and a well-verified numerical model reveal the failure mechanism of HSBs subjected to blast loads. • The auxetic HSBs showed less global plastic displacement and greater core compression than conventional HSBs. • The blast performance of HSBs without debonding of the face sheets was significantly improved. • Decreasing the thickness of the cell side walls and top face sheet while maintaining the total mass exhibited less deformation. Little research has been conducted on comparing the blasting characteristics between the regular hexagonal and auxetic honeycomb sandwich beams (RHSBs and AHSBs). To address this gap in knowledge, the study utilised both experimental and numerical analyses. The HSBs consisted of steel top and rear face sheets bonded to a stainless- steel honeycomb core. The parameters considered in this test included the core configuration (regular vs. auxetic hexagonal) and face-core bonding method (adhesive vs. integrated). Results revealed that at scaled distances of 0.8617 m/kg1/3 and 0.7971 m/kg1/3, the AHSB core exhibited a reduction in mid-span displacement of only 3.3 % and 3.7 %, respectively, compared to the RHSB core. However, the compression value of the AHSB core significantly exceeded that of the RHSB core by 171.5 % and 161.5 %, respectively. This finding indicates that auxetic honeycomb cores possess enhanced energy absorption capacity to withstand blast loading. In addition, using unbonded face sheets increases the range of local cell deformation and decreases the global flexural bending by 22.66 % and 24.14 % at scaled distances of 0.8617 m/kg1/3 and 0.7971 m/kg1/3, respectively, compared to the AHSBs with face sheet debonding. To further investigate the damage mechanism of HSBs subjected to blast loading, a well-validated finite element model was employed. Notably, parametric simulations demonstrated that the dynamic behaviour of AHSBs under blast loading is significantly influenced by cell wall thickness, face sheet thickness, and cell angle. [ABSTRACT FROM AUTHOR]

• A bolted reinforced honeycomb was experimentally and numerically studied. • Both Poisson's ratio and in-plane mechanical properties were researched. • The study characterizes the effects of the relative thickness of the corrugated plate and reinforced plate. • Three design strategies - layered, internal and external, and space filling - were adopted to further regulate Poisson's ratio. The use of large honeycomb structures is becoming increasingly widespread, with many specialized honeycomb structures being produced through additive manufacturing and laser cutting technologies. However, the high manufacturing costs and low efficiency associated with these methods limit their applicability. In this paper, reinforced honeycombs with a total length of nearly 1 m were assembled using bolts. The effects of relative thickness and corrugated plate angle on Poisson's ratio and mechanical properties were investigated both experimentally and numerically. Additionally, three design strategies - layered, internal and external, and space filling - were adopted to further regulate Poisson's ratio. The results show that relative thickness, angle, and the aforementioned design strategies can all be used to regulate Poisson's ratio. As the thickness of the reinforced plate increases, energy absorption (EA) increases, but specific energy absorption (SEA) decreases. The reinforced honeycomb structure (RHC) exhibits the best energy absorption and SEA at a 60° angle. Through hierarchical, internal, and external designs, this paper shows that the reinforced honeycomb structure with one layer of reinforcement plus one layer of non-reinforcement (1+1-RHC) has the largest negative Poisson's ratio effect. Furthermore, the transition structure between RHC and 1+1-RHC is studied through unreinforced space filling design. The fabrication method presented here promises low-cost and large-scale fabrication with the ability to program Poisson's ratio, making it suitable for applications in protective gear and smart energy absorbing devices. [ABSTRACT FROM AUTHOR]

Metamaterials and lightweight structures are widely used in various engineering applications and additive manufacturing technologies have enhanced their potential. Curved elements have been increasingly used in the design of planar mechanical metamaterials or honeycombs. A honeycomb cellular structure topology with cell walls parametrized using cubic Bézier curves is proposed and analyzed here. The effective Young's modulus of the Bézier-based honeycomb is characterized semi-analytically using Castigliano's second theorem. Semi-analytical estimations of the effective Young's modulus were contrasted to those obtained with numerical finite element models and measured on additively manufactured samples. By considering the relative influence of bending, axial, and shear loads, the deformation mechanism of the honeycomb can be predicted and explained in terms of the load orientation, which depends on the definition of the Bézier curve. A large region achievable by tailoring the design parameters of the curve is presented in the so-called Ashby maps and compared to other existing topologies. The methodology presented here enables designers to propose topologies with a customized effective stiffness, expanding the design space for honeycombs in various engineering applications. • The effective Young's modulus of Bézier-based honeycombs is obtained semi-analytically using Castigliano's second theorem. • The contribution of bending, axial and shear loads in the deformation of Bézier-based honeycombs is quantified. • The role of curvature of the constituent elements on stiffness is quantified by means of energy approaches. • Analytical estimation of the effective Young's modulus agrees with finite element simulations and compressive mechanical tests. [ABSTRACT FROM AUTHOR]

• A novel honeycomb structure with zero Poisson's ratio is proposed. • Relationships between three elastic constants and geometric parameters are studied. • The zero poisson's ratio property is maintained within a relatively small fluctuation range. The superior performance of honeycomb structure has facilitated its extensive application in various fields. In this paper, a novel honeycomb structure is proposed, which exhibits zero Poisson's ratio behavior. Firstly, the in-plane mechanical properties are analyzed based on the classical Castigliano's theorem to obtain expressions connecting the three elastic constants of the structure and geometric parameters. Then, the theoretical expressions are verified by homogenized finite element methods and 3D-printed experimental models, which help to determine the relationship between the in-plane mechanical properties and the geometric parameters. The results show that the mechanical properties of the honeycomb structure can be tailored by changing the geometric parameters, while ensuring that the zero Poisson's ratio property is maintained within a relatively small fluctuation range. This study reveals that the mechanical properties of the novel honeycomb structure are tunable, which has outperformed some honeycomb structures on adjusting range. [ABSTRACT FROM AUTHOR]

In response to the demand for the ultra-high stability and light-weight optical structure in high-resolution spacecraft, the present study proposes a novel carbon/carbon (C/C) honeycomb structure which integrates the superiorities of C/C composite and honeycomb structure. The C/C honeycomb was fabricated by chemical vapor infiltration (CVI) processing with continuous carbon fiber preform, and then the L - and W -direction shear experiments were conducted. A multi-scale damage model is established to describe the mechanical and damage behavior of the C/C honeycomb, which includes the damage model and constitutive model in both meso-scale and macro-scale. The effects of yarn orientation, side length, wall thickness and height of C/C honeycomb on L - and W -direction shear characteristics as well as damage modes of the novel C/C honeycomb are comprehensively researched. The results show that the C/C honeycomb has excellent shear properties when the yarn orientation is ± 45° as well as the side length and wall thickness are about l = 6 mm and t = 0. 3 mm , respectively. With the yarn orientation shifts from 0°/90° to ± 45°, the damage region transforms from top and bottom surfaces of the C/C honeycomb structure to honeycomb walls. As the side length increases and the wall thickness decreases, the damage region is distributed obliquely along the honeycomb wall. This research contributes to the design and optimization of optical-mechanical structures in high-resolution spacecraft. • A novel light-weight and high load-bearing C/C honeycomb structure was fabricated. • A multi-scale damage model is proposed to evaluate the mechanical behavior of C/C honeycomb structure. • Optimal parameters for improving light-weight and high load-bearing integration performance of C/C honeycomb structure are suggested. • Main damage mode and its variation of C/C honeycomb with different yarn orientation, side length, wall thickness and height of core are investigated. [ABSTRACT FROM AUTHOR]

This paper introduces a novel hierarchical core structure to sandwich panel for bearing the blast loading, in which each vertex of a regular hexagonal cell was replaced with a smaller hexagonal unit. The finite element (FE) models of such hierarchical honeycomb sandwich panels were established and validated with the experiments under different impulse loads. The hierarchical honeycomb cores were compared with the regular honeycomb counterpart in terms of the peak deflection on the back facesheet, compression and specific energy absorption (SEA) of the core. The results showed that the maximum deflection at the back facesheet of the hierarchical honeycomb sandwich panels were smaller than the regular honeycomb counterpart for a higher level of blast load (specifically, the dimensionless impulse higher than 0.06). It was found that the structural hierarchical parameter γ (i.e. the ratio of the newly-introduced smaller hexagonal edge length (L 1) to the regular hexagon edge length (L 0)), had limited influence on the maximum deflection of back facesheet of the sandwich panel, but had a significant effect on the SEA of the cores. Image 1 • A novel sandwich panel with hierarchical honeycomb core for improving blast resistance was first developed. • Parametric study on blasting resistance of the novel sandwich panel was conducted. • The novel sandwich panel has excellent blasting resistance compared with traditional sandwich panel. [ABSTRACT FROM AUTHOR]

Re-entrant anti-trichiral honeycomb is a typical auxetic material. The collapse process can be divided into three stages while its negative Poisson's ratio (NPR) effect under large deformation is still unclear. In the present paper, the NPR effects of re-entrant anti-trichiral honeycombs under large deformation are studied by numerical and theoretical method. Numerical results are verified by the experiments with respect to their deformation mechanism and stress-strain behavior. Theoretical formulas are derived to predict the NPR effect of the honeycomb under large deformation. The analytical predictions are in good agreement with the FEM results. It is revealed that the NPR effect of the honeycomb is magnified with the global strain in the second stage while is weaken in the third stage of the deformation process. The decrease of the ligament-ratio and the mean radius will result in more obvious NPR effect. Variation of cell-wall thickness doesn't have much impact on the Poisson's ratio in the second stage, while smaller cell-wall thickness will induce larger NPR in the third stage. The present work is supposed to shed light on the design and fabrication of re-entrant anti-trichiral honeycombs with desired Poisson's ratio. Image 1 • Analytical predictions of the NPR effect under large deformation exhibits a good agreement with the numerical results. • A special deformation mechanism and stress-strain behavior are illustrated by the experiments and numerical simulations. • The negative Poisson's ratio (NPR) of the honeycombs varies differently with strain at different stages. • Variation of cell-geometry parameters greatly influence the NPR effect of the honeycombs. • More evident NPR effect is exhibited when the honeycomb gradually develops into the conventional re-entrant honeycomb. [ABSTRACT FROM AUTHOR]

Abstract Modal performance of micro-perforated sandwich beams with honeycomb-corrugation hybrid cores was investigated. Finite element methods and modal analysis techniques have been used to predict their vibration characteristics (i.e. their natural frequencies and mode shapes). It is shown that the natural frequencies of sandwich beam with micro-perforation are slightly lower than those of corresponding order of sandwich beam without micro-perforation based on the experimental and three-dimensional finite element calculated results. To reveal the effect of micro-perforation diameters on the natural frequencies of sandwich beams, a dimensionless frequency parameter ϖ was proposed. The results demonstrate that the frequency parameter of sandwich panels decrease near-linearly with the increase of micro-perforation ratio. In addition, the frequency parameter ϖ is sensitive not to such parameters as the face sheet thickness ratio, the slenderness ratio of corrugated member and the relative density of filling honeycomb, but to the configuration of micro-perforation. Highlights • Novel lightweight sandwich constructions with micro-perforated square honeycomb-corrugation hybrid cores are proposed. • Natural frequencies increase with the facesheet thickness and filling honeycomb, decrease with the corrugation thickness. • Frequency parameter of sandwich panels decrease near-linearly with the increase of micro-perforation ratio. • Frequency parameter is not sensitive to geometry parameters, however, to the configuration of micro-perforation. [ABSTRACT FROM AUTHOR]

Abstract This paper systematically investigates the low-velocity impact response and resulting damage behavior of aluminum honeycomb sandwich structures with carbon fiber reinforced plastic (CFRP) face sheets by combining the experimental and numerical methods. Low-velocity impact tests are conducted to determine and quantify the effects of structural parameters, such as face sheet thickness, cell wall thickness, honeycomb core height and hexagon side length, on the impact load, energy absorption, and failure mode. To further elucidate the impact behavior, a user VUMAT subroutine is developed to predict the progressive failure behavior of composite face sheets. Numerical simulation is performed to characterize the impact response and explore the deformation/failure mechanisms. The predicted impact load, energy absorption, contact time and failure mode agree well with measured counterparts. These studies reveal the face sheet thickness has particularly significant influence on the impact resistance performance of honeycomb structures. Cell wall thickness and side length of honeycomb core have notable effect on the impact load and structural stiffness of such structures, while which do not play a great role in energy absorption. Increase in core height has comparatively little effect on initial stiffness and energy absorption but makes the second peak loads decreasing for the perforation cases. Highlights • Low-velocity impact response of honeycomb sandwich structures is studied. • VUMAT is developed to explore damage state and failure mechanism of composite. • Impact response and damage behavior are seriously affected by structural parameters. • Face sheet thickness has significant influence on the impact resistance performance. • Cell wall thickness and core side length have notable influence on the impact load. [ABSTRACT FROM AUTHOR]

Abstract Furniture users as a rule consider low prices, lower product weight and easy transport of furniture items to the intended location as obvious advantages. However, practically no studies in available literature are found to assess the effect of variable climate conditions on the stiffness of shelves and furniture made from honeycomb panels. The aim of this study was to determine the effect of these changes on the value of Young's modulus and on the stiffness of whole furniture units subjected to torsional loads. Tests were performed on furniture units, which elements were manufactured in five structural design variants. It was shown that the applied materials retain high quality of the furniture units only at the temperature T = 26 °C and humidity H = 40%. An increase in the values of these climate parameters to T = 28 °C and H = 85% results in a drastic deterioration of furniture quality. Graphical abstract fx1 Highlights • The effect of humidity and air temperature on the quality of furniture manufactured from honeycomb panels was studied. • Changes in stiffness of horizontal furniture elements were determined. • Changes in stiffness of furniture bodies were determined. • Results of calculations based on the analytical and numerical models were compared. [ABSTRACT FROM AUTHOR]