Biomimetic Design Of Bend-twist Coupling Thin-walled Beams Based On The Structure Of Rice Stem

Plant stem supports the whole plant weight,ensuring the plant can complete normal physiological activities,thus a certain rigidity is required to prevent plant buckling,and also a certain degree of flexibility is required to prevent brittle fracture under extreme external loads.Structure with both the merits of rigidity and flexility provides an important source of inspiration for the design of flexible beam in engineering.In this paper,taking rice,a common herbaceous plant,as an example,the mechanical simulation of rice stem structure is carried out via multi-scale simulation to analyze the effects of geometric and structural parameters at different scales on the performance of rice stems,and the static and dynamic properties of the stem structure are studied comprehensively.A bionic design for the bending-torsional coupling beam structure is carried out by applying the structure-function principle of rice stalks.The following concludsions can be made:(1)Multiscale structure-functional relationship of rice stem and its experiamental verificationThe mechanical properties of rice stem in three directions were studied based on multi-scale simulation.The rule of mixtures,Halpin-Tsai equation and the laminate model were used to calculate the cell wall material properties at the microscale level,the cellular structure of rice stem at mesoscale scales was reconstructed by using the image-based skeleton method.The resulting cellular structure was used to establishe a multi-scale mechanical model.Structural parameters at different length scales,including the volume fractions of mechanical tissue layer and vascular bundle,the cell wall thicknesses of fiber and parenchyma cells,the volume fraction of S2 layer in the cell wall,microfibril angle in S2 layer,the volume fraction and the mechanical properties of cell wall compositions,were studied by parametric analysis and sensitivity analysis.The effect of different variables at multiscales on the anisotropic material properties of the stem was obtained.(2)Effects of gradient structure parameters and material parameters on the lodging resistance of rice stemThrough the Rayleigh-Ritz method,the two-parameter shape function is used to derive the axial critical buckling load for the three-step beam.The axial stiffness gradient parameters and length gradient parameters were selected as independent variables,and the material utilization ratio of stems under different parameters was studied.Three-point bending test was used to measure the actual gradient parameter values of rice stems in the late filling stage,and experimental data were compared with theoretical data.Moreover,a simplified fluid-solid coupling finite element model of rice stem was established based on the actual measured parameters.The dynamic response behavior of rice stem with approximate rigid body,isotropic flexible body and anisotropic flexible body at different wind speeds was studied.The contribution of bending and torsion behavior of rice stem to unloading of wind load under different wind speeds was studied.(3)Torsional behaviors of stiffened plate and rectangular thin-walled beamsAccording to the distribution characteristics of the material of the plant stem section,the torsional stiffness weakening effect of the longitudinal ribs on the plate structure and the rectangular thin-walled beam was studied by simulation and experiment.The simulated and experimental results show that the existence of the ribs changes the stress transmission path of the thin-walled structure during torsion,causing the local stress concentration of the structure and expanding the invalid material area of the structure.Moreover,the weak connection between the reinforcing ribs and the outer wall or the flat plate makes it difficult to transmit the torsional stress to the inner material of the reinforcement materials.Therefore,the presence of the reinforcement ribs weakens the torsional rigidity of the thin-walled structure with the same material,confirming that the weak connected structure between the vascular bundles of the plant stems is an important reason for the plants to have low torsional rigidity.(4)Bionic design of bend-twist coupling thin-walled beamRice stem need to reduce their torsional stiffness while maintaining a certain bending stiffness in order to reduce the wind load by the torsional deflection effect,which is similar to the working state of the bending and torsion coupling beam.According to the weakening effect of vascular bundles on the torsional stiffness of plant stems and the gradient distribution pattern of vascular bundle,two different reinforced bend-twist coupling thin-walled beams were propsoed.Combining with the gradient distribution of axial material and structure of rice stems,a bend-twist coupling bionic beam with gradient distribution characteristics is designed,and the multi-objective optimization design with the maximum critical buckling force and the minimum maximum stress under 1000 N lateral load is carried out.By comparing the difference between the mechanical properties of the optimized model and the original model,the superiority of the structure of rice and other herbaceous plant stems and its potential used in the field of bionic design of the bending-torsional coupling thin-walled beam structure are reflected.