The Study On Axial Compression Properties Of Three-dimensional Negative Poisson’s Ratio Cellular Structure

The lightweight design in engineering is paid more attention by people, since the energy conservation and emission reduction are becoming the most important problem around the world. To solve this problem, cellular structure with advantages of lightweight and high performance is developing ripidly. As a new type of cellular structure, negative Poisson’s ratio cellular structure can present transversal compressive strain under axial compression, which shows deformation of compression-contraction. Benefiting from this behavior of deformation, the negative Poisson’s ratio cellular structure not only has the properties of traditional cellular structure, but also has some special properties such as effect of enhanced properties, which has an important significance for improving the performance of engineering equipments.The main method of obtaining negative Poisson’s ratio cellular structure is on the bisis of building negative Poisson’s ratio foam. The cellular structure obtained by this way can not present stable performance due to unsymmetrical cells. But the technology of 3D printer has changed this situation, the building of complex structure comes true, which means it is necessary to find a new three-dimensional negative Poisson’s ratio cell. On the basis of existed two-dimensional negative Poisson’s ratio cell, a new three-dimensional negative Poisson’s ratio cell is presented in this paper, which can present compressive strain in the two directions of cross section and has more obvious effect of enhanced properties. Through the theoretical equations of relative density, the relative density is defined by cell angle, thickness ratio, length ratio and cell wall scale ratio. Through studying on the characteristers of this cellular structure, the relationships between relative density and geometric parameters are studied, and the ranges of geometric parameters are defined.In the research of mechanical properties in cellular structure, the most common way is assuming it as the continuous medium and defining its properties through effective Young’s modulus and effective Poisson’s ratio. So the theoretical equations of effective Young’s modulus and effective Poisson’s ratio are presented through axial compression mechanical model of unit cell, which is proved by results of FEA. On the other hand, the main yield reasons of cellular structure are elastic buckling and plastic collapse under axial compression. So the theoretical equations of yield stress are presented, which is also proved by results of FEA. The requirements of this two yield modes are studied, which show that the plastic collapse is the only reason of yield. Beside that, the relationships between effective properties and geometric parameters are also studied through FEA method.The most important characterister of negative Poisson’s ratio cellular structure is the effect of enhanced properties, which has increscent effective properties under axial compression. The change of cell shape under loading is the main reason of this effect, subject to deformation of compression-constriction. So it is limited to define effective properties defined by relative density, and necessary to consider more geometric parameters. The FEA model considering of geometric nonlinear is presented to study on the mechanism of the effect of enhanced propertites, and the relationships between effective properties and geometric parameters under larger deformation are also studied. The results show that this negative Poisson’s ratio cellular structure has high strength-to-weight ratio and obvious effect of enhanced stiffness.The most common usage of cellular structure is energy absorption device, negative Poisson’s ratio cellular structure is the same as that. Benefiting from the effect of enhanced stiffness, it has lower relative density and higher plateau stress at the same time, which has important significance for energy absorption. The effects of geometric parameters and impact velocity on performance of energy absorption, the deformation in the process of impact and the mechanism of energy absorption are studied through axial impact FEA model. Unlike traditional cellular structure, the stress in plateau stage is nolonger stable, which is presenting the effect of enhanced plateau stress. This makes the compressive deformation changed, and the process of it can be divided into four stages:elastic stage, plateau stage, enhanced plateau stress stage and densification stage. The mechanism of enhanced plateau stress and its effect on energy absorption are studied through the theoretical model of enhanced plateau stress stage. At the same time, the modeling energy absorption diagrams are presented to study on the effect of geometric parameters on energy absorption, which makes it much easier to use.It is necessary to use design and optimization methods to meet the requirements of this negative Poisson’s ratio cellular structure in the engineering. So the optimization models of maximum absorbed energy, minimum structure mass and minimum peak stress are presented, and the best shape of cell is presented through multi-objective genetic algorithm. To verify the effectiveness of this cellular structure, it is applied in the vehicle body. And the lightweight negative Poisson’s ratio cellular structure crush box is designed, which meets the requirements of performances and law.