Multi-scale Modeling Of Royal Palm Tissue And The Bionic Applications Based On Its Tissue Structure

As the results of the natural evolution from the mechanical interaction between the plant individual and its habitat, the adaptive structure of the plant has always holden the sources for bionic innovations and answers for tough pratical problems in various fields including the mechanical engineering. The knowledge of the structure-mechanical relationship in plant materials is of importance in the direct application of these mateirals or in the practice with purpose of bionic design. Based on this idea, this paper firstly describes the multi-scale modelling of royal palm tissue. Then its structural-mechanical relationship is investigated with the verified multi-scale models. According to some of the critical conclusions from the study of the structural-mechanical relationship, two types of bionic designs for honeycomb structures are proposed. Their mechanical properties and functions in adjusting the shear stress distribution as core materials in the composite main spar of wind turbine blade are studied.At the microscale, the mechanical modeling of cell wall material begins with the adjustment of cell wall simplified model. By considering the gradual transition area between the S1 and S2 layers, the interlayer model introduced in this thesis is able to deliever 80 and 11 percent more accurate predictions of cell wall modulus in the longitudinal and transverse directions, respectively, than the conventional models. The softening/hardening effect and the hygroexpansion behavior of the cell wall under the variation of moisture conditions can also be more accrutely predicted with the interlayer model.At the mesoscale, with the calculated data from the interlayer model and some literature datas, the cell wall mechanical properties for the modelling of the cellular structure of the sheath tissue in the royal palm are established. Then, based on the Delaunay triangulation, the area-weighted tessellation method is developed and introduced to rebuild the natural cellular structure of the palm tissue from SEM images. With this reconstruction method, the RVE of the sheath tissue and the model of the single vascular bundle are created and analysed with finite element methods. The FE mdoels are verified with experimental measurements of the sheath and single vascular bundle samples. Then, this verified FE model is employed to uncover the structure-mechanical relationship of the sheath tissue under static and dynamic loading scenarios. The mechanical functions of the different regions inside the vascular tissue in maintaining the resistance of sheath tissue to various mechanical risks are discussed.At the macroscale, the FE model established at the mesoscale is used to obtain the tissue mechanical properties from sheath to petiole. Together with the measured data of tissue densities, the entire branching structure is able to be analysed through numerical approaches. The modeled results under different loading conditions reveals that the mechanical flexibility and enhanced frictional energy dissipation are attributed to the graded distribution of stiffness and the Poisson’s ratio from sheath to petiole in the branching structure, respectively.Inspried by the method of adjusting the Poisson’s ratio in the palm tissues through the variation of volume fractions of stiff regions, the idea of releasing coupled strain into blank regions is proposed. With that idea, the RSRE network structure showing zero Poisson’s ratio is created. This abilty is then numerical and experimentally proved and expaned into 3D space. The 3D RSRE structure exihibits zero Poisson’s ratio under axial loading along all three directions in 3D space. And the low Poisson’s effect of the RSRE honeycomb make it a fine choice in reducing the shear stress on the interface between the skin and core in a honeycomb sandwich.The angular-graded honeycomb is design in order to mimic the stiffness distribution in the royal palm branching system. By setting the structural parameters, the elastic modulus increases from the inner to the outer layers, while the variation of the shear modulus reverses. This bionic stiffness distribution in the angular-graded honeycomb significantly reduced the maxium shear stress inside the structure. The shear stress distribution is also much more moderate compared to a regular hexagonal honeycomb. Then the angular-graded honeycomb and the RSRE honeycomb are designed as the core structures for the spar and the shear web, respectively, in the composite main spar of wind turbine blade. The in-vitro and in-situ mechanical behaviors of the bionic main spar model and the regular model with hexagonal honeycombs are compared. The results show that the shear stresses on the skin-core interfaces of the spar and the shear web are both reduced with the bionic angular-graded honeycomb and the RSRE honeycomb.