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• Inspired by pomelo peel and bamboo structure, corrugated and layered honeycombs are designed and fabrication by metal 3D printing. • The deformation modes and energy absorption capacity of the bionic inspired honeycombs are experimentally investigated with quasi-static and dynamic drop-weight tests. • The influence of impact speed on the crashworthiness performance of the honeycombs is investigated. To improve the energy absorption of traditional structures, impact resistant natural materials are usually mimicked to develop novel structures with desirable energy absorption. In this work, inspired by the unique 3D corrugated microstructure of vascular bundles in pomelo peel, a new design of honeycomb structure with corrugated characteristics is proposed. Further, motivated by special nodal bulkhead architecture of bamboo, layered honeycombs with diaphragms are designed and fabrication by metal 3D printing technology. The quasi-static and dynamic crushing responses and energy absorption characteristics of six types of bio-inspirational honeycombs (ISWH, LSWH, ISCH, LSCH, IPCH and LPCH) are studied using quasi-static testing machine and dynamic drop-weight impact experimental systems. To investigate the effect of impact speed on the crashworthiness performance of the honeycombs, the tests of the honeycombs impacted by the drop-weight with two different mass (106.07 and 59.60 Kg) and three impact velocity (V 1 =6.23, V 2 =8.82 and V 3 =11.77 m/s), respectively, are performed. The experimental results indicate that the crush failure mode of the layered honeycombs is more regular than that of the integrated honeycombs. Failure of all layered honeycombs occurs first at the junction of the honeycomb and diaphragms. Under impact loading, the layered corrugated wall honeycombs exhibit a greater final crushing distance than their corresponding integrated honeycombs. The design of corrugated wall can effectively reduce the initial peak force of the integrated honeycombs, while the layered honeycomb design also facilitates the reduction of the initial peak force, thus avoiding damage to personnel or equipment from excessive peak collision forces. The layered design will reduce the specific energy absorption of the integrated honeycombs in most cases, but a reasonable combination of layered and corrugated wall design can also effectively increase the specific energy absorption of the honeycombs under a specific loading speed. And, the layered design can also effectively improve the crush force efficiency of the corresponding integrated honeycombs. [ABSTRACT FROM AUTHOR]

Highlights • A novel hierarchical honeycomb was constructed inspired by pomelo peel. • Crushing resistant and energy absorption capability of the structure was studied. • An analytical-numerical approach was developed to unravel the underlying mechanism. • Deformation modes were governed by the geometric parameter-equivalent thickness. • The crushing resistance and energy absorption can be enhanced greatly. Abstract Hierarchical materials born of variable natural cellular and intricate architecture are demonstrated to have the potential to achieve outstanding mechanical properties, thus make them excellent constituents for impact protection. Inspired by the unique microstructure of pomelo peel, this study constructed a novel hierarchical honeycomb and investigated the crushing resistance along with energy absorption capabilities of such structural materials. An integrated analytical-numerical approach was developed to fully elucidate the underlying quantitative structure-property relations by parametric studies on the evaluation of different hierarchical orders and equivalent thickness. It is revealed that the deformation modes of pomelo peel inspired honeycomb are governed by the geometric parameter-equivalent thickness, where three deformation modes (hexagonal mode, transitional mode and coin mode) and two localized band ("V" mode and "I" mode) can be observed under out-of-plane and in-plane crushing, respectively. Additionally, in conjunction with theoretical and numerical studies, improved crushing resistance and energy absorption properties of the pomelo peel inspired honeycomb can be obtained via increase of structural hierarchy and variation of geometric dimensions. The crushing resistance criteria, SEA (specific energy absorption) and equivalent plateau stress of hierarchical honeycomb, can be enhanced up to 1.5 and 2.5 times than its counterpart for traditional honeycomb under out-of-plane and in-plane crushing, respectively. The promising results of pomelo peel inspired honeycomb may exhibit a novel perspective on providing the superior mechanical properties of natural cellular materials and offer insights for applications of bio-inspired engineering materials. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

Highlights • Blast response of gradient honeycomb sandwich panels were examined. • Influence of core arrangement on the blast response were investigated. • The energy absorption of gradient cores was calculated. • The use of basalt FMLs can enhance the blast resistance of sandwich panels. Abstract In this paper, the blast responses of fiber metal laminates (FMLs) and gradient aluminum honeycomb sandwich panels with FML as skins were experimentally investigated. The woven basalt fabric was chosen as the composite reinforcement in FMLs due to its excellent mechanical and eco-friendly properties. Five different core layer arrangements were considered for sandwich panels in the experiments by arranging honeycomb core layers with different cell geometric dimensions. The deformation/failure modes of sandwich panels were obtained in the experiments, in terms of FML face-sheets and gradient honeycomb cores. The energy absorption of gradient honeycomb cores was quantitatively analyzed by digitizing the deformation/failure region of cores. The results showed that the use of basalt FMLs as face sheets can greatly enhance the blast resistance of sandwich panels compared with that with aluminum sheets as skins. The blast resistance of gradient sandwich panels not only depends on the geometric dimensions and arrangements of core layers, but also related to the intensity of target load which will cause different deformation/failure mechanism of panels. Thus, in order to obtain the best blast resistance, a well-design gradient sandwich panel should have suitable core layer arrangements and geometry to satisfy the intensity of target blast load. The results obtained from current study can give valuable reference to the using of sandwich panels in engineering protection field. [ABSTRACT FROM AUTHOR]

Analytical and numerical analyses are carried out in order to reveal the importance of the cellular materials topology for their dynamic compaction. The aim is to distinguish between the deformation and energy absorption mechanisms of materials which exhibit local structural softening, such as out-of-plane loaded honeycomb, and materials with local structural hardening (foam). It is shown that the dynamic out-of-plane compaction of honeycombs does not obey the law of shock wave propagation and a new phenomenological model of the velocity attenuation is proposed. It is revealed that the absorbed energy by the honeycomb is proportional to the area under the dynamic stress-strain curve, which is defined by the foil material properties, in contrast to the shock-wave propagation model where the absorbed energy is proportional to the area under the shock chord. Comparisons with the predictions of the Rigid Perfectly-Plastic Locking (RPPL) model, which approximates well the average quasi-static stress-strain characteristic of a honeycomb, are discussed. Special attention is given to the effects of neglecting the honeycomb topology on their response to impact loading. Finite element simulations are carried out to verify the proposed theoretical model revealing the major factors which influence the dynamic response of out-of-plane loaded honeycombs to high velocity impact. [ABSTRACT FROM AUTHOR]

• In this paper, 6 kinds of Bi-directional corrugated honeycomb aluminum (BDCHA) is used to carry out compression-shear loading results with different oblique angles and installation directions by simulation (experiments are used to verify accuracy of the simulation). • The deformation mode and mechanical properties are observed under different oblique angles and installation directions. • Comparing with wood, the energy absorption shows the feasibility of BDCHA as a shock-absorbing filling material using for nuclear spent fuel transportation casks. When filling materials used for nuclear spent fuel transportation casks, they will inevitably be subjected to oblique impact. The study of material performance during oblique impact is very important for their safety. In this paper, Bi-directional corrugated honeycomb aluminum (BDCHA) is used to carry out compression-shear loading results with different oblique angles (20°, 30°, 45°) and installation directions (X-FACE, Y-HOLE) by simulation (experiments are used to verify accuracy of the simulation). Quasi-static tests perform 6 kinds of materials dates with different cell thickness/cell size ratios, and observe the deformation mode and mechanical properties. The results show that the deformation of the material is mainly the torsional buckling deformation. In addition, BDCHA mainly bears the shear and compress stress. When the oblique angle and installation directions are different, the stress is different. The material of the cell thickness/cell size ratio affects the stress of the material. Finally, comparing with wood, the energy absorption shows the feasibility of BDCHA as a shock-absorbing filling material using for nuclear spent fuel transportation casks. [ABSTRACT FROM AUTHOR]

• Comparative impact tests of novel FHCB were performed. • Foam filling enhances impact resistance. • Gradient design can optimize foam energy absorption. Auxetic structures have attracted a lot of attention as they can meet the requirements of impact resistance and lightweight performance of protective structures in aerospace and defense industries. To enhance the impact resistance and improve the capacity of energy absorption of auxetic structures, foam-filled hybrid-chiral honeycomb beams (FHCB) were designed and fabricated through SLM (Selective laser melting) printing method and foaming method. The localized impulse loading was generated through the impact from foam projectile, accelerated by light gas gun. The dynamic responses of EHCB (empty hybrid-chiral honeycomb beam) and FHCB were compared. In the experimental results, FHCB showed enhanced impact resistance and higher SEA (specific energy absorption) value than EHCB. A validate finite element model was used to study the influence of cell-wall thickness on the impact resistance of EHCB and FHCB. Results indicated that by changing the cell-wall thickness, the potential energy absorption capacity of the filled foam can be fully utilized. Furthermore, the impact resistance of FHCB can be effectively improved by using a gradient cell-wall thickness arrangement. This study can provide reference for the optimal design of blast mitigation and impact resistant structures. [ABSTRACT FROM AUTHOR]

Double arrowhead honeycombs (DAHs) are auxetic cellular materials with negative Poisson's ratio (NPR). The quasi-static and impact behaviors of both uniform and functionally graded DAHs are explored. Analytical expressions for the in-plane Young's modulus, Poisson's ratio and quasi-static yield stress of uniform DAHs are derived and are found to be consistent with finite element (FE) predictions, showing that the quasi-static yield stress of DAHs is dependent upon the geometrical parameters and NPR. Systematical simulations of uniform DAHs subject to different impact velocities reveal that there exists a critical impact velocity beyond which the dynamic plateau stress is insensitive to the effect of NPR. A semi-empirical expression of the plateau stress is obtained. For functionally graded DAHs, they are shown to only have improved energy absorption capacity under high velocity impacts. Moreover, the results of uniform DAHs are shown to be able to be adopted to interpret the simulated plateau stress of functionally graded DAHs under impact loading. The deformation mechanisms associated with the impact resistance of DAHs are also discussed. [ABSTRACT FROM AUTHOR]

• Dynamic response of sandwich plates with GLARE face-sheets under metal foam projectile impact is studied. • Effects of impulse and thickness on dynamic response are studied by single impact experiments. • The ability of resisting multiple impacts and the ultimate failure mode are measured. • The numerical results are in good agreement with experimental ones. Aluminum honeycomb sandwich plate with fiber metal laminate (FML) as panel can combine the advantages of FML face-sheet and honeycomb core, and has advantages of high specific strength / specific stiffness, lightweight and good energy absorption performance. In this paper, the dynamic response of aluminum honeycomb sandwich plates with Glass Laminate Aluminum Reinforced Epoxy (GLARE) face-sheets under metal foam projectile impact is studied. The single foam projectile impact test is conducted to study the dynamic response of honeycomb sandwich plates with GLARE face-sheets. The debonding between metal and composite layers of the front face-sheet, the fiber fracture, buckling, shear and fracture of core are observed. Also, the multiple foam projectile impact test is carried out to study the impact resistance of the sandwich plate. The numerical calculation is conducted, numerical results are compared with experimental ones. Influences of the impulse, face-sheet thickness, fiber laying angle, honeycomb height, wall thickness of core and cell length of honeycomb on impact resistance and energy absorption of sandwich plates are discussed. It is found that the improvement range of the maximum deflection of back face-sheet gradually decreases with the increase of face-sheet thickness. In addition, by increasing the impulse, decreasing the face-sheet thickness and decreasing the core stiffness, the energy absorption of sandwich plates under metal foam projectile impact can be enhanced. These findings can be used to guide the optimal design of FML sandwich structures under metal foam projectile impact. [ABSTRACT FROM AUTHOR]

The dynamic crushing of circular-celled honeycombs under out-of-plane impact is studied within a wide range of impact velocity by using both theoretical and numerical methods, and verified by the experiments. Two kinds of honeycombs with different cell package, i.e., the SP and HP honeycombs, are considered. The results of the honeycombs are discussed and compared with those of the cylindrical tubes under axial compression. An analytical expression is deduced to predict the honeycombs' crushing stress, showing good agreements with the numerical results. It is shown that the energy absorption per unit mass of the HP honeycomb and the SP honeycomb is at least 13.3% and 6.4%, respectively, greater than that of the cylindrical tubes. And the HP honeycomb can absorb at least more 23% energy per unit volume than the SP ones mainly due to the denser package mode of the cells. The different energy absorption ability among the two kinds of honeycombs and the cylindrical tubes can be attributed to the different constraints around the cylindrical cells and tubes, which is verified by the experiments. A non-dimensional “dynamic sensitivity index” is used to quantitatively evaluate the sensitivity of the structure's load capacity to the impact velocity, showing the most sensitivity of the tube and the least sensitivity of the HP honeycomb, which are confirmed by the numerical results. [ABSTRACT FROM AUTHOR]

• The dynamic responses and energy absorption characteristics of aluminium honeycomb sandwich panels under ice wedge impact are investigated. • A three-dimensional nonlinear finite element model of AHSP under ice impact is established. • The ice wedge impact tests of AHSPs were conducted. • The plastic deformation energy absorption and ice facture energy dissipation mechanism is revealed. In this paper, the dynamic responses and energy absorption characteristics of aluminium honeycomb sandwich panels (AHSPs) under ice wedge impact are numerically and experimentally investigated. A three-dimensional nonlinear finite element model of AHSP under ice impact is established based on a concrete constitutive ice model in the commercial package ANSYS/LS-DYNA. The impact force-displacement curves and energy absorption properties as well as the crushing process of ice wedge are numerically achieved. In addition, the ice wedge impact tests of AHSPs were conducted by using a horizontal impact experimental apparatus. Furthermore, the effects of impact parameters and structural parameters on the dynamic responses and energy absorption are performed. Results show that the numerical results of ice wedge-AHSP impact dynamic responses are consistent well with the experimental results. The permanent deformation profiles of face sheets are approximately axisymmetric about the vertical and horizontal centerline. The ice impact energy is mainly converted into the plastic deformation energy of AHSP and rebound kinetic energy of ice wedge as well as the fracture energy of ice wedge. With the increase of the impact energy and thickness of face sheets, the energy dissipation ratio of ice broken increases dramatically and the energy dissipation by ice wedge makes an increasingly contribution to the impact energy absorption. [ABSTRACT FROM AUTHOR]

Abstract: Commercial aluminum honeycombs with various cell configurations are experimentally tested to study the influence of cell number and central angle on the out-of-plane crush resistance of the structures. The boundary effect is found to have significant impact on the crush strength of the structure when the number of cells is small and the central angle is observed to get a difference less than 10% in the strength of the honeycombs. Numerical analyses based on whole honeycomb model and Y-shaped element model are carried out to simulate the crush and deformation process of the specimens. The adhesive bonding of the double thickness foil is considered in the simulation and the numerical results show good agreement with the experimental data and theoretical predictions. Finally, the reason for the small influence of central angle on the out-of-plane strength of honeycombs is investigated and the interaction effect between wall thickness and central angle is believed to account for it. [Copyright &y& Elsevier]

• The ballistic response of cylindrical sandwich shell was explored • Hemispherical and ogive nosed impactors were hit normally at the outer periphery. • Ballistic resistance and failure modes was dependent on the impactor shape • Target shows the more resistance against hemispherical impactor than ogive impactor • Thicker face sheet and cell wall, small side length caused higher resistance The energy absorption (EA) and ballistic behaviour of the cylindrical sandwich shells, which comprised of honeycomb core (AL-3003 H18) and cylindrical face sheets (AL-1100 H12), were investigated by carrying out experiments and numerical simulations. Two distinct nosed shapes, ogive and hemispherical shape impactor of mass 52.5 gram, length 50.8 mm and diameter 19 mm, were impacted normally at the outer curved surface of the sandwich structure. A compressed air gun was used for experiments and the incidence velocities of both the impactor were kept within the range of 50 -146 m/s. The results in the form of ballistic limit, local and global deformation and the failure modes of the sandwich structure were reproduced through numerical simulations which were carried out through commercially available finite element code Abaqus/Explicit. Johnson cook material model was used for the face sheet whereas for the honeycomb core and the adhesive layers, ductile damage material model was employed. The validated numerical model was further extended to discover the influence of facesheet thickness, side length, curvature and cell wall thickness on the ballistic resistance, deformation, failure mode and EA of the sandwich shell structure. The influence of the face sheet thickness on the performance of the cylindrical sandwich structure was more significant compared to the honeycomb core geometric parameters. However, decreasing the side length, and increasing cell wall thickness and curvature enhanced the EA and ballistic resistance marginally. [ABSTRACT FROM AUTHOR]

Abstract: Honeycomb cellular structures and multi-cell prismatic columns are highly efficient and effective energy dissipating components and are widely used in the crashworthiness design of vehicles. Due to the complex features during large plastic deformation, only few special sections have been theoretical modeled for their energy absorption capacity under axial compression. In this paper, based on a simplified FE model, the energy absorption characteristics of angle elements with three panels are investigated by using the non-linear finite element code LS-DYNA. Theoretical models are proposed to predict the crush resistance of three-panel angle elements with different angles. Numerical results show that the proposed theoretical model can predict the energy absorption of these angle elements with good accuracy. [Copyright &y& Elsevier]

Abstract: Sandwich structures have been widely employed to build lightweight components having good mechanical properties and energy absorbing capacity, especially in the transport industry. The aim of this paper was the analysis of static and low-velocity impact response of two typologies of aluminium honeycomb sandwich structures with different cell size. The static bending tests produced various collapse modes for panels with the same nominal size, depending on the support span distance and on the honeycomb cell size. Simplified collapse models were applied to explain the experimental observations and a good agreement between predicted and experimental limit loads was achieved. Low-velocity impact tests were, also, performed on the two typologies of aluminium honeycomb sandwiches and a theoretical approach, based on the energy balance model, has been applied to investigate their impact behaviour. The failure mode and damage of the honeycomb panels have been investigated using the 3D Computed Tomography. [Copyright &y& Elsevier]

Abstract: Energy absorption characteristics of regular polygonal columns and angle elements under dynamic axial compression are investigated by using non-linear explicit finite element code LS-DYNA. The influence of central angle on deformation mode and mean crushing force of angle elements is studied. Based on two types of deformation mechanisms known by experiments, two types of initial indentation triggers are introduced. By assuming appropriate boundary condition on the edges, a simplified finite element model is adopted in the analysis of angle elements and validated by comparing with full polygonal column model. Numerical investigations are also carried out to study the influence of angle on angle elements with three and four panels by using the simplified model. Several useful conclusions are drawn about the axial crushing of polygonal columns and angle elements and can be used to guide the design of crashworthiness structures. In addition, a comparison is conducted between the numerical results and theoretical predictions of the mean forces of some special angle elements. Good agreement is obtained. [Copyright &y& Elsevier]

Abstract: Dynamic cushioning tests were conducted by free drop and shock absorption principle. The effect of paper honeycomb structure factors on the impact behavior was analyzed. Results of many experiments show that the dynamic impact curve of paper honeycomb sandwich panel is concave and upward; the thickness and length of honeycomb cell-wall have a great effect on its cushioning properties; increasing the relative density of paper honeycomb can improve the energy absorption ability of the sandwich panels; the thickness of paper honeycomb core has an up and down fluctuant effect on the cushioning properties; with the increase of the thickness of paper honeycomb core, the effect dies down; flexible corrugated paperboard as liners can improve the compression resistance and cushioning properties of paper honeycombs. The research results can be used to optimize the structure design of paper honeycomb sandwich panel and material selection for packaging design. [Copyright &y& Elsevier]

• A novel graded origami honeycomb was proposed. • Dynamic experiments on a 2 × 1 core validate the numerical approach. • The performance of ungraded and graded 6 × 6 cores was compared numerically. • The gradient was shown to influence the energy absorption capacity; and, • the force transmitted to the supporting structure. Efforts to improve the effectiveness of cellular structures in impact scenarios has led to the introduction of gradients in their mechanical properties. A plethora of studies have shown that these gradients can be implemented to increase the energy absorption or limit the force transfer to the structure they are protecting. Whilst these findings are promising, the cost and difficulty associated with manufacturing these graded cellular structure limits their application. Origami techniques provide a convenient way to transform a 2D sheet to a 3D structure. Additionally, the underlying crease pattern can be varied to grade the topology of the structure. In this study, the ability to imbue honeycombs with a graded mechanical behaviour using origami techniques is investigated under dynamic loading. Grading is applied to the crease pattern through the variation of a simple geometric parameter. The typical honeycomb expansion manufacturing technique is preserved, as the crease pattern has one degree of freedom and is developable. 2 × 1 representative origami honeycomb cores were manufactured using a sequential stamping technique from aluminium sheets and experimentally tested using the Direct Impact Hopkinson bar and Taylor type test setup. These results validated a numerical approach that was then extrapolated to a numerical model of a 6 × 6 ungraded and graded core. Similar advantages, as those found for typical grading methods, were observed for the graded origami honeycomb when subjected to dynamic loading. These results show that origami techniques can successfully be applied to introduce a gradient in the mechanical behaviour, providing an attractive alternative to standard grading methods. [ABSTRACT FROM AUTHOR]

• Taylor impact and direct impact tests are performed on density graded honeycombs. • The double shock deformation mode proposed previously is confirmed by the experimental observations. • Analytical modeling for Taylor type impact is implemented based on a one-dimensional shock theory. • The effect of gradient is discussed in terms of the stress at impact section, energy absorption and transmitted stress of the graded honeycombs. Aluminum honeycomb samples with density gradients were fabricated and impact tests were conducted in order to possibly observe the double shock deformation mode proposed previously. Two different arrangements were employed for impact tests. In scenario 1, a graded honeycomb sample together with a backup mass block impinged onto an elastic Hopkinson bar, at a given velocity (Taylor type impact test). In scenario 2, a stationary sample supported at the back by a Hopkinson bar was impacted upon by a block at a velocity (direct impact test). Deformations of the graded honeycombs and force histories were recorded in both the loading scenarios. The deformation mode was related to the impact velocity. Observation of the deformation process, in particular, the distributions of the local engineering strains, confirms the double shock mode. Forward and backward densification fronts were observed. Furthermore, analytical modeling for Taylor type impact is implemented based on a one – dimensional shock theory. Compared with the experiments and numerical simulations, the analytical model can predict the stresses at both the proximal and distal ends. The effect of gradient is discussed in terms of the stress at impact section, energy absorption and transmitted stress of the graded honeycombs. [ABSTRACT FROM AUTHOR]