Design And Analysys Of Programming Shape-memory Negative-poission-ratio Structure Based On 3D Printing Technology

Two-dimensional lattice structures with specific geometric features have been reported to have a negative Poisson’s ratio,termed as auxetic metamaterials,i.e.,stretching induces expansion in the transversal direction.In this paper,we designed a novel auxetic metamaterial,which by utilizing the shape memory effect of the constituent materials,the in-plane moduli and Poisson’s ratios can be continuously tailored.During deformation,the curved meshes ensure the rotation of the mesh joints to achieve auxetics.The rotations of these mesh joints are governed by the mesh curvature,which continuously changes during deformation.Due to the shape memory effect,the mesh curvature after printing can be programmed,which can be used to tune the rotation of the mesh joints and the mechanical properties of auxetic metamaterial structures,including the Poisson’s ratios,moduli,and fracture strains.Using the finite element method,the deformation of these auxetic meshes was analyzed.Finally,we designed and fabricated gradient/digital patterns and cylindrical shells,and used the auxetics and shape memory effects to reshape the printed structures.In the third chapter of this paper,the geometric model design and finite element simulation method of two-dimensional grid structure are mainly introduced.The tension and expansion deformation of grid and element are verified by finite element simulation.In this paper,a superstructure element is designed,which is induced by the rotation of nodes.The viscoelastic model was used to simulate the thermomechanical behavior of PLA.The model parameters of PLA were checked by the results of thermo-mechanical tests.The same thermo-mechanical loading conditions were adopted in the finite element simulation.The results of finite element simulation reveal the deformation characteristics of the grid structure under tension and compression,and prove that the grid structure has remarkable expansion characteristics.In Chapter 4,the tensile deformation of uniform grids,pre-programmed grids and non-uniform digital grids with different curvatures are studied by uniaxial tensile test and finite element simulation.The shape memory effect of PLA can be used to control in-plane modulus and Poisson’s ratio from both experimental and simulation aspects.For unprogrammed specimens,the influence of curvature on the elastic modulus and Poisson’s ratio of grids was analyzed by uniaxial tensile test and simulation.The effect of programming strain on in-plane modulus and Poisson’s ratio was studied for the sample after programming.Based on the above results,we design a non-uniform digital grid.The experimental and simulation results show that the non-uniform grid can realize complex non-uniform in-plane deformation field and can be used to control in-plane expansion.In the fifth chapter of this paper,we have prepared cylindrical shells using this kind of expansion supermaterial.The rugby/saddle-shaped deformation mode of cylindrical shells after tension/compression deformation is analyzed experimentally and simulated.Based on the results of in-plane deformation research,two kinds of non-uniform digital cylindrical shells are further designed.The influence of non-uniform element arrangement on out-of-plane deformation is compared.It is proved that non-uniform digital cylindrical shells can realize out-of-plane buckling through non-coordinated expansion of expansion element.The research results provide an effective way for the low-cost and fast manufacturing of complex three-dimensional shells.