| Two-or three-dimensional chiral structures made by periodic arrangements of unit cells with chirality show a significant rotational response under axial forces due to deformation asymmetry,which provides an opportunity to develop multifunctional metamaterials with unique mechanical properties.By designing the structural geometric dimension,a better compression-twist effect can be obtained.It is valuable for engineering applications.Cylindrical shells have a wide range of applications due to their hollow structural form,but existing compression-twist cylindrical shells suffer from low twist rates or low structural stiffness with low bearing capacity.In order to design a chiral metamaterial cylindrical shell with both a high twist rate and high stiffness at the same time,this dissertation uses a combination of theoretical analysis,experimental study,and finite element simulation to discuss the compression-twist performance of the improved tetra-chiral cylindrical shell with different geometrical parameters and carries out a multi-objective optimization design of the structure by using NSGA-II genetic algorithm.Firstly,the twist rate of the structure can be improved by changing the shape of the chiral unicellular central ring which increases the ligament equivalent length.For the modified tetra-chiral cylindrical shell,the connection between ligaments and center rings is regarded as an elastic support with a certain rotational stiffness,and a theoretical calculation method of the structural twist rate in this case was introduced.For a group of structures with different ligament angles,the compression-twist deformation patterns were studied by quasi-static compression experiments.The specimens were made by 3D printing.Twist angle and compression displacement of the structure during the process were obtained according to the DIC(Digital Image Correlation)method.Then compared experimental results with simulation results to verify the reliability of simulation method used in this work.Secondly,Abaqus was used to simulate the deformation process of tetra-chiral cylindrical shells with different geometric parameters under large compression displacement as well as tension loading,and the twist rate and effective elastic modulus obtained from the simulation were compared with the theoretical derivation results to study the corresponding changes of structural compression-twist characteristics systematically when changing its geometric dimension and number of chiral cells.Finally,in order to comprehensively improve the compression-twist performance of the structure,a multi-objective structural optimization design of the tetra-chiral compression-twist cylindrical shell was carried out by using the Isight optimization platform.Choosed structural twist rate and compressive stiffness as evaluation indexes.The optimal Latin hypercube experimental design method was used to select the sample points among the design domain randomly and uniformly.Then performed simulation calculations.The degree of contribution of each design variable to the output response was analyzed from the results,and the results were fitted with a quadratic polynomial response surface approximation model and then iterative optimization was performed based on NSGA-II genetic algorithm.Finally obtained the set of Pareto non-inferiority solutions and Pareto fronts that satisfied the conditions.Some of the design variables corresponding to the optimal solution were modeled and then simulated and calibrated.Due to the contradiction between the two objectives,it is necessary to select the appropriate design point from the Pareto front according to the engineering practical needs.After comparison and verification,it was found that the optimized design method in this work can effectively improve the compression-twist performance of the structure,which provides a reference for the design and application of subsequent compression-twist metamaterials. |
PDF Full Text Request