Study On Design And Mechanical Properties Of Bionic Composites Based On Plant Fiber Microstructure

Fiber reinforced composite,due to their characteristics of light weight,high strength,high modulus and flexible designability,has become the main bearing structure material for aircraft,vehicles,ships and other high-end equipments.With the rapid development of these high-end equipments and technologies,higher requirements are put forward for the strength and toughness of fiber reinforced composite.However,the strength and toughness of materials are often contradictory.Strong materials are always brittle,while tough materials are usually soft.In the study of this problem,the rich biomaterials provide many new ideas for finding the answer to the problem.After a long-term arms race in nature,a variety of effective structures have been evolved,which endows biomaterials with excelent comprehensive mechanical properties,and provide a lot of inspiring design strategy of composites.Therefore,exploring the microstructure and toughening mechanism of biomaterials,and developing new structural forms of fiber-reinforced composites through biomimetic strategies are expected to obtain toughened composites from the perspective of innovative structural design.In this paper,the structure-mechanical properties relationship of plant fiber is studied from the perspective of material science and mechanics,and the microscopic characteristic structure is extracted to establish the corresponding biomimetic structure model.The 3D printing technology is utilized to fabricate the bionic composites with enhanced energy absorption and bearing capacity.The main research contents of this paper are as follows:（1）Scanning electron microscopy（SEM）and X-ray diffraction（XRD）measurements were used to analyze the microstructure characteristics of plant fibers.The influence of structural parameters on the tensile properties of plant fibers was studied and the tensile deformation mechanism was revealed.It was found that the higher the crystal inity of cellulose and the smaller MFA were,the higher the tensile strength and modulus of plant fibers were.On the contrary,the higher the fracture toughness of plant fiber,the higher the energy absorption performance.At the same time,the gradient porous structure was also conducive to the improvement of energy absorption performance.The failure mode of hemp fiber and sisal fiber also changed with the varation of cellulose crystal inity and MFA.Hemp fiber and sisal fiber showed brittle fracture behavior,whereas coir fiber exhibited ductile fracture behavior（2）Taking the heterogeneous structure of cell-wall layers at the micro-scale as a biomimetic template,a bionic heterogeneous laminated composite was designed and fabricated by 3D printing technology.The effect of SCF/CCF configuration on mechanical properties and failure modes of bionic heterogeneous laminated composites was systematically studied.The results showed the SCF/CCF heterostructure deflected the crack,elongating the crack propagation path and increasing energy dissipation,which improved the energy-absorption and damage-resistant of composites.Therefore,the impact resistance of bionic heterogeneous laminated composite was significantly improved.（3）Taking the helical structure of the S2 layer at the micro-scale as a biomimetic template,a bionic helical laminated composite was designed and fabricated by 3D printing technology.The effect of the helical angleαon the mechanical properties and failure mode of the bionic helical laminated composite was systematically studied.The results showed that the isotropic helical structure increased the crack propagation path and energy dissipation,which improved the energy-absorption and damage-resistant of composites.（4）Taking the gradient porous structure of coir fiber at the meso-scale as a biomimetic template.a bionic gradient honeycomb composite was designed and fabricated by 3D printing technology.Firstly,the mechanical properties and failure modes of traditional honeycomb（UH）and bionic gradient honeycomb（FGH）were compared.The results showed that FGH possessed better bending and impact properties.Then the effect of the maximum honeycomb wall thickness T_max and the honeycomb gradient index N on the mechanical properties and failure modes of FGH was systematically studied.（5）A bionic honeycomb sandwich composite（BSH）was designed and prepared by combining the fiber panel with helical structure and the honeycomb core with gradient structure.The effect of the maximum honeycomb wall thickness T_max,honeycomb gradient index N and helical angleαon the mechanical properties and failure modes of BSH was systematically studied.The results showed that the helical panel increased the crack propagation path and energy dissipation,while the stress distribution of BSH was optimized and the material utilization of BSH was increased.When the BSH was partially damaged,it was still able to resist deformation and absorb energy,which accounted for the simultaneous improvement of bending and impact performance.（6）In order to further improve the ability of BSH to absorb energy and resist damage,the structural parameters of BSH（the maximum honeycomb wall thickness Tmax and the honeycomb gradient index N）were optimized for improvement of energy absorption and resistance to damage,by combining experimental design method,numerical analysis method,surrogate model method and multi-objective optimization theory.Compared with the initial design,the optimized peak force(F_max)and specific energy absorption（SEA）of BSH were significantly improved.