Multiscale Designs Of The Chitinous Nanocomposite Of Beetle Horn And Fabrication Of Bioinspired Structural Materials

After a long period of natural selection,they can achieve excellent comprehensive mechanical and functional properties by self-assembly into ingenious structural design,although organisms employ components with simple composition and poor performance.Among them,beetle horn is not only a natural biological material evolved by organisms,but also a natural weapon.It is endowed with extraordinary characteristics of fracture resistance and energy-absorbing efficiency through multiscale designs,and it can balance the offence and persistence nature of each other in the same material.In this work,many structural characterization and mechanical experiments were designed to investigate the structure-mechanical property relations of horn of the male beetle Allomyrina dichotoma.The purpose of this work is to provide a theoretical basis for the design of new bioinspired structural materials.The various microscopic techniques were used to systematically characterize and analyze the horn from the macroscopic shape,geometry,and connection with the body to the mesoand microscopic architecture,moisture content,and chemical and structural characteristics.The horn consists of a dense cuticle and an internally filled cellular core.At the macroscale,the horn is gradually bent upwards towards the apex and forms a flat fork.The equivalent diameter of the cross-sections of horn gradually increases from the middle part towards the bottom.The moisture content of the horn decreases from the tip,reaches a minimum at the slender middle part,and then increases towards the bottom.In the exocuticle indicates a gradual enrichment of the elements P and S.At the microscopic,the fraction of circumferentially-oriented nanofibrils progressively increases,while that of radially-oriented nanofibrils decreases,towards the inner of the exocuticle.The endocuticle is typical plywood structure composed of nanofibrils bundle,and the orientation of each layer presents varying tilted degrees.The basement membrane is a dense layer with no preferential structural orientation.The basic components of the horn are chitin and protein.It was found that as the structural orientation of the exocuticle gradually changed from radial to circular,and the intensity of the components shows a gradual decrease by Raman spectroscopy.Due to the typical plywood structure,the intensity of the components shows changes periodically in the endocuticle.The mechanical properties of the horn were tested at multiscale and different hydration states.Macroscopic compression experiments reveal that the horn has higher compressive strength in air-dried state,and the horn has better plasticity in rehydrated state.The results of the Micro-indentation test show the hardness of the cuticle is higher in the air-dried state than in the rehydration state.Under the same hydration state,the cucitle indicates a gradient decrease of hardness from exocuticle to endocuticle and there is no clear varying trend of hardness at different cross-sectional positions.The nano-indentation technique was used to analyze the cucitle at the tip and middle position.The results show that the nano-hardness and Young's modulus increases from the exocuticle,and then decreases towards the endocuticle,but this change is quite the opposite in the cuticle of the middle position.Since the structural orientation of the layers in the endocuticle exhibits varying tilted degrees,the measured nano-hardness and Young's modulus have large deviations.The effect of hydration on the mechanical properties of horn was further explained by dynamic mechanical analysis.With the increase of moisture content,the storage modulus and size shrinkage decrease gradually,while the loss modulus and loss factor increase gradually.Finite element simulation is used to analyze the macroscopic shape and geometry of horn,which can alleviate the stress concentration,retard the formation of crack and show better load-carrying capacity.When the plywood structure of the endocuticle is subjected to a force,its structure can be reoriented to better adapt to external stress-strain conditions.By revealing the unknown mysteries in nature,the key principles of materials design in natural biomaterials are extracted,which can provide useful enlightenment for the preparation of high-performance bioinspired structural materials.At the end of this work,we employed freeze casting technology to explore the preparation of bioinspired structural materials with graphene as reinforcing phase.The resulting materials are mainly including the metallic foam with high strength,light weight and high elasticity molded on the directed porous structure of wood,and the metal-based bioinspired composites with high strength,high hardness and wear resistance molded on the laminated structure of abalone shell nacre.