| Negative Poisson's Ratio(NPR)structure is a mechanical superstructure with negative Poisson's ratio effect.This structure shrinks horizontally in compression and expands horizontally in tension.Because of this unique mechanical property,the negative Poisson's ratio structures exhibit higher stiffness,higher shear strength and satisfactory energy absorption capacity,and have been widely used in aerospace,automotive,defense equipment,biomedical and other fields.Metal elliptical hole negative Poisson's ratio structure is a new mechanical super structure developed from elastic buckling induced negative Poisson's ratio structure.It has received a great deal of attention from researchers due to its structural simplicity,adjustable mechanical properties and large effective strain range of negative Poisson's ratio.However,due to the late start of the study and the limited preparation process,the research on the energy absorption characteristics of metal elliptical hole negative Poisson's ratio structure is relatively scarce,and the relationship between structural parameters and energy absorption characteristics has not been discussed in detail.Studies have shown that the introduction of the spatial gradient in many porous structures can effectively improve the mechanical properties and energy absorption characteristics of materials.Therefore,it is of great research value to explore whether the introduction of spatial gradient in elliptical hole negative Poisson's ratio structure can improve its mechanical properties and energy absorption characteristics.In this study,316 L elliptical hole negative Poisson's ratio structures with different structural parameters were designed and prepared by Selective Laser Melting(SLM).Through experiments and finite element simulation,the effects of different structural parameters on the compression properties and energy absorption characteristics of 316 L elliptical hole negative Poisson's ratio structure were studied.The influence of structural parameters on Poisson's ratio and elastic modulus of materials was analyzed.The macroscopic deformation behavior and the mechanism of negative Poisson's ratio effect of the elliptical hole were revealed.Different gradient elliptical hole negative Poisson's ratio structures were designed and prepared.The compression properties and energy absorption characteristics of the non-gradient structure and gradient structure were studied.The deformation behavior and failure mechanism of gradient negative Poisson's ratio structure were discussed.The main research results of this paper are as follows:1.Sandblasting treatment improves the surface quality of SLM-forming materials.Layered micro-pool morphology is formed during the forming process of the sample,and there are both cellular and columnar crystals in the pool.The phases of 316 L stainless steel powder and SLM formed samples are composed of austenite and a small amount of ferrite.The elastic modulus of the SLM-formed tensile part is 187.5±1.8 GPa and the tensile strength is 625 MPa.2.The length to short axis ratio has a significant influence on the compression properties of the elliptical hole negative Poisson's ratio structures.The compression strength,energy absorption capacity,and average energy absorption efficiency of the elliptical hole negative Poisson's ratio structure decrease with the increase of the length to short axis ratio.With the increase of the length to short axis ratio,the peak energy absorption efficiency is improved.When the length to short axis ratio is 1:1,namely,a circular hole,the sample does not show a negative Poisson's ratio effect.The elastic modulus and Poisson's ratio decrease with the increase of the length to short axis ratio.3.Porosity has a significant effect on the compression properties of the elliptical hole negative Poisson's ratio structures.The compressive strength,energy absorption capacity,and average energy absorption efficiency of the elliptical hole negative Poisson's ratio structures decrease with the increase of porosity,while the peak energy absorption efficiency increases with the increase of porosity.The elastic modulus and Poisson's ratio of the negative Poisson's ratio structure of the elliptical hole decrease with the increase of porosity.4.The introduction of axial porosity gradient fails to improve the compression properties of the elliptical hole negative Poisson's ratio structure.The compressive strength and energy absorption capacity of the gradient elliptical hole negative Poisson's ratio structures are lower than those of the non-gradient elliptical hole negative Poisson's ratio structure.The introduction of porosity gradient has little effect on energy absorption efficiency and elastic modulus.5.The introduction of axial porosity gradient leads to the change of deformation behavior of elliptical hole negative Poisson's ratio structure,and the gradient structure has a more obvious local deformation area.The different deformation characteristics of the non-gradient structure and gradient structure lead to different variation trends of Poisson's ratio. |
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