Study On Dynamic And Static Mechanical Properties Of Macro/micro Negative Poisson's Ratio Structures

With the development of high and new technology,special performance requirements for metal materials are put forward in engineering field.With the development of high technology,special performance requirements for metal materials are put forward in engineering field.The special deformation characteristics and excellent mechanical properties of negative Poisson's ratio structures have attracted wide attention in academic field.In the previous studies,the research mainly focus on two-dimensional structures,and lack of systematic research on threedimensional negative Poisson's ratio structures.There are few applications of negative Poisson ratio structures.Therefore,exploring the mechanical properties of the negative Poisson's ratio structures will provide a theoretical basis for the application of the negative Poisson's ratio structures.In this paper,three-dimensional re-entrant structure is selected as the research object.The mechanical properties of material are investigated by quasi-static compression experiment and dynamic separation Hopkinson pressure bar experiment.Preparation re-entrant structure with different support thickness by SLM and micro-assembly techniques.And explore the influence of forming process and size parameter on structure performance.This paper provides a theoretical basis for the engineering application of negative Poisson's ratio bone implants.SLS process is used to prepare the nylon re-entrant structure,and the auxetic effect is verified by axial compression experiment.Before studying the mechanical properties of the structure,we need to know the mechanical properties of the material in advance.In this paper,dynamic and static mechanical experiments are carried out with tin-phosphorus bronze.The influence of size effect on mechanical properties of materials is analyzed.The result shows that both in dynamic and static experiments,the sample shows obvious characteristic size effect and grain size effect.The relationship between initial grain size and flow stress obeys the Hall-Petch rule.Based on the stress-strain relation of quasi-static experiment and dynamic Hopkinson pressure bar experiment,the parameters of Johnson-Cook constitutive model are calibrated.The isothermal stress-strain relationship is obtained via the split Hopkinson pressure bar experiment and quasi-static preload experiment.Based on the Johnson-Cook constitutive model,the damage model of microscale high strain rate is established.Combined with the Freudenthal fracture criterion,a multi-strain rate micro-scale damage model is builded.The damage and fracture of bronze material can be predicted by this model.Three-dimensional re-entrant structures were prepared by SLM and micro-assembly techniques and mechanical properties test.The results indicate that both SLM and micro-assembly structure could achieve auxetic effect.The absolute value of Poisson's ratio decreases with the increase of support thickness.The negative Poisson's ratio effect of micro-assembly structure is more obvious but Young's modulus is relatively small.During elastic stage,two structures are instability,and local fracture occurs on SLM structure.Study on microscale negative Poisson's ratio find that the top element of structure is instability.The load-displacement curve indicate that the structure appears stress cycling phenomenon.This is due to the failure of the structure layer by layer and verified by numerical simulation.