Structural Design And Mechanical Performance Characterization Of Several Kinds Of Lattice Materials

In recent years,lattice materials have attracted extensive attention due to their excellent mechanical properties.Joints or nodes with complex geometrical configuration at the connection between rods and rods are commonly found in lattice materials.The existence of nodes will definitely have impact on the mechanical properties of the lattice materials.The prediction of mechanical properties of high relative density solid lattice materials by traditional linear model is too low,and low relative density hollow lattice materials are highly sensitive to defects.It is necessary to establish an analysis model of high-density lattice materials to comprehensively evaluate the influence of nodal overlapping effect on their mechanical properties.Based on the nodal effect,thus,series of lightweight functional lattice materials are designed.Firstly,by considering nodal material overlapping effect,bending and shear force in strut and introducing a new concept of equivalent length,a theoretical prediction model for the high density octet lattice is established to predict their relative stiffness and relative strength.This has been validated by both FE simulations and experiments.These results demonstrate that the relative compressive stiffness and strength not only depend on the relative density,but also relate to the effect of strut joint,bend and shear.The nodal material overlapping effect in strut joint and the coupling effect between the shearing force and the bending moment will raise the relative compressive stiffness and strength.Then,by introducing spherical nodes and smooth connection around nodes,we design a stretching-dominated lattice material that can absorb very large energies while at the same time retaining a low density.A few examples of lattice materials are considered and we show that a new class of body centered cubic(BCC)shellular lattice material has the best mechanical properties for shock absorption: they are ultrastiff,ultrastrong,and they possess high specific energy absorption at a low relative density.After that,based on a design concept of enhancing the resistance of struts to bending and shearing deformation and making full use of nodal effect,by replacing solid struts with hollow struts,a class of simple-cubic closed tubular lattice material with limited loading direction dependence along with high mechanical properties and irregular stable nonlinear response is presented.The fabrication of its complex structure was made possible by direct laser writing at the microscale.Experiments and simulations demonstrate that both the elastic modulus and the yield strength of the simple-cubic closed tubular lattice are significantly larger than those of the simple-cubic truss lattice,regardless of the loading direction.At a relative density of 0.1 and compared to the truss lattice,the closed tubular lattice can absorb respectively 4.45 times and 6.14 times as much energy along directions [100] and [110].Its average normalized Young’s modulus and yield strength are respectively 28% and 53% larger than those of the most outstanding openform shellular metamaterial with the same mass.Such excellent mechanical properties make it a promising candidate for applications to load-bearing and energy absorption.Next,we propose a new class of light-weight elastic isotropic bending-dominated truss lattice by replacing the inner node of the BCC lattice with a SC lattice.We use analytical and numerical predictions for the design of their elastic moduli and collapse strength.Additional numerical simulations reveal that the proposed lattices not only exhibit elastic isotropy but also nearly isotropic nonlinear response.In particular,our material with relative density below 1% almost attains the upper bound of Poisson’s ratio for isotropic materials.Uniaxial compression tests are performed to confirm the design.Results show that the fabricated materials have a relative modulus 2 times larger and a relative collapse strength and specific energy absorption about 1.6 times larger in contrast to BCC truss lattices.Thus,our material can be consider as a noteworthy alternative to absorb energy and to transformation elastodynamics.Finally,a new class of isotropic and reusable cork-like lattice material that is designed from an hybrid truss-lattice material with complex node connections to show an isotropic Poisson’s ratio close to zero is proposed.Optimization is conducted using a multiobjective genetic algorithm,assisted by an elliptical basis function neural network,and coupled with finite element simulations.The optimal micro-structured lattice material,fabricated by direct laser writing technique with a lattice constant of 300 m,has an almost isotropic Poisson’s ratio smaller than 0.08 in all directions.It can recover 96.6%of its original shape after a compressional test exceeding 20% strain.