| In order to adapt to the natural environment,natural materials have formed nearly perfect structures through continuous evolution,and show good mechanical characteristics such as compressive resistance,impact resistance and energy absorption.In order to solve the conflict between compression resistance,impact resistance and light weight of protective structures,new inspiration and solutions have been sought from nature,and many bionic structures and mechanical characteristics of natural biomaterials have been studied.In this paper,the hierarchical cellular structure and its mechanical characteristics of thousands-of-eyes-Bodhi（TEB）are discussed,and the bionic design inspired by hierarchical cellular structure features of TEB is carried out,and the effects of structure level,structural parameters,relative density and strain rate on the mechanical properties of the novel bionic hierarchical cellular structure are studied.The main research work and contents are as follows:（1）The hierarchical cellular structure and static mechanical characteristics of the TEB are studied.Firstly,scanning electron microscopy（SEM）studies reveal that the TEB hierarchically exhibit three distinct cellular structures,the filled-cells cellular structure(>10-3m)at macroscopic scale,and closed-cells cellular structure(10-4-10-5m)and pen-cells cellular structure(10-6m)at different microscopic scales,and the structure is anisotropic.Secondly,the Vickers hardness test,static compression test and punching shear test in the dry state show that the superior mechanical properties of the sample are closely related to the hierarchical cellular structure.The sample dissipates a large amount of energy through the deformation mechanisms of three hierarchical cellular structures.Microscopically,the multiple micro-cracks are firstly generated from the open-cell cellular structures,and the closed-cell cellular structures are deformed and crushed in which the multiple micro shear bands and cell walls interlocking phenomenon can be found in the tests.Macroscopically,the filled-cell cellular structures are stretched and damaged with the extrusion of filler.The novel hierarchical cellular structure of TEB possesses excellent mechanical properties,which hinder the catastrophic failure and increase the toughness and strength.Finally,the static compression tests show that the increase of water content increases the ductility and energy absorption of the sample,reduces the elastic modulus and compressive strength,and delays the appearance of macroscopic cracks.The hardness,compressive strength,elastic modulus and energy absorption of the sample in the longitudinal direction are better than those in the transverse direction,and the deformation behavior in the longitudinal and transverse directions has significant anisotropy.（2）The dynamic mechanical characteristics of hierarchical cellular structure of TEB are studied.Firstly,based on the SHPB system,the dynamic compressive tests at different strain rates are carried out in the longitudinal and transverse directions.And the three-wave method is used to verify the reliability of the test results.The results show that the dynamic impact mechanical properties of the samples have obvious strain rate dependence and anisotropy.Secondly,the macroscopic deformation processes of the samples under different strain rates are studied based on the high-speed camera.The results show that the number of macroscopic cracks increases with the increase of strain rate.Finally,the failure mechanisms of the samples are analyzed by the statistical method of fragment size and SEM.The tests results show that the dynamic failure mechanisms of each order cellular structure of the samples exist a significant strain rate dependence.The samples dissipate the impact energy through the dynamic compression mechanisms of each order cellular structure which improves the energy absorption and impact resistance.（3）Based on the structure characteristics of the TEB,a novel bionic hierarchical cellular structure is designed,and its static mechanical characteristics are studied.Firstly,inspired by the hierarchical structure of the TEB,a novel bionic hierarchical cellular structure is designed and prepared by 3D printing technology,a novel bionic hierarchical cellular structure is designed and prepared by 3D printing technology.Secondly,the crushing resistance capability and deformation modes of the samples are investigated by test and simulation methods.The compression deformation processes of the samples are recorded by a CCD camera,and the strain field distribution is analyzed by DIC method.Finally,comparing the mechanical behaviors of pure square tube,second-order and bionic hierarchical cellular structures,it is found that the structure hierarchy affects the deformation mode,improves the strain field distribution,delays the strain concentration,and enhances the specific energy absorption and specific strength.It is also noted that the novel bionic hierarchical cellular structure exhibits different deformation modes and strain field distributions in the transverse and longitudinal directions,and shows better crushing resistance in the longitudinal direction.In addition,compared with other typical artificial protective structures,the novel bionic hierarchical cellular structure possesses better specific energy absorption and specific strength.（4）The static mechanical characteristics of the novel bionic hierarchical cellular structures with different structural parameters are studied.First of all,the novel bionic hierarchical cellular structures with different structural parameters are designed and fabricated using 3D printing technology.Secondly,the static compression test method combined with CCD and DIC technology is adopted.The results show that the mechanical properties of the novel bionic hierarchical cellular structure improve with the increase of the cell wall thickness,but there is a non-monotonic relationship with the tube-wall thickness and the diameter of the circular hole.There are"level",shear and"V"shear deformation modes of the novel bionic hierarchical cellular structures with different cell-wall thickness,and it is found that the the novel bionic hierarchical cellular structures with different tube-wall thickness and circular hole diameter have better Mechanical properties when the"level"deformation mode occurs.Meanwhile,based on the results of numerical simulation and DIC analyses,it can be find that the strain field distribution of the novel bionic hierarchical cellular structures with different wall thickness is dominated by the deformation mode of the unit cell,and it is also find that the strain field distribution of the novel bionic hierarchical cellular structures with the tube-wall thickness of 0.7 mm and the diameter of the circular hole of 0.6 mm is relatively uniform.Finally,according to Gibson-Ashby theory,a semi-empirical theoretical model of the relationship between the static mechanical properties and the relative density of the novel bionic hierarchical cellular structure is established.（5）The dynamic mechanical characteristics of the novel bionic hierarchical cellular structures with different relative densities are studied.First of all,the novel bionic hierarchical cellular structures with different relative densities are designed and manufactured by 3D printing technology.Secondly,the dynamic impact compression tests under different strain rates are carried out based on the SHPB system.The results show that the energy absorption of the novel bionic hierarchical cellular structure improves with the increase of strain rate,but the sensitivity of the peak stress of the the novel bionic hierarchical cellular structures with different relative densities to strain rate is different.The dynamic impact mechanical properties of the novel bionic hierarchical cellular structure enhance with the increase of relative density.Finally,the dynamic deformation responses of the novel bionic hierarchical cellular structures under different strain rates are analyzed based on the high-speed camera,and it is find that the that the strain rate and relative density have obvious effects on the dynamic deformation responses of the novel bionic hierarchical cellular structure. |
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