Effect Of Residual Stress,Grain Size And Texture Coefficient On Mechanical Behavior Of Nanoindentation

Size effect and residual stress are two key factors that affect the mechanical behavior of material indentation.The traditional elastoplastic mechanics has not considered the effects of the two.In view of their importance,the study of the two in the theoretical framework of traditional mechanics has become one of the hot topics in solid mechanics in recent years.Therefore,this article has carried out a series of researches on the mechanical behavior of material indentation around these two basic points.The main work is as follows:First,the theoretical research work on nanoindentation experiments is carried out.From a theoretical perspective,the study of size effects is relatively complete,while the study of residual stresses is currently mostly limited to experimental and numerical solutions.Theoretical and computational considerations of the effects of both on the behavior of nanoindentation have not yet been studied.In view of this,on the basis of the strain gradient theory based on physical mechanism,the residual stress is described by the geometric dislocation density,and the theory is extended so that the effects of size effect and residual stress on the nonlinear deformation behavior of materials can be considered simultaneously.In addition,a corresponding stress update algorithm is constructed,and the algorithm is added to the commercial finite element software ABAQUS through the user subroutine UMAT to build a computing platform.Secondly,combined with the results of indentation experiments on metallic glass,the effects of size effect and initial residual stress on the mechanical behavior of nanoindentation were numerically analyzed.The experimental results show that the residual stress has a significant effect on the material hardness and load-displacement curve.The theoretical prediction results in this article fully reproduce this phenomenon,and give a mechanism analysis based on the settlement.Residual compressive stress hardly affects the size of the plastic impact zone,but the compressive stress field generated by the indentation and the residual compressive stress field are superimposed,which makes the stress value after indentation larger,and the residual tensile stress has a greater impact on the plasticity affected zone.They are all affected by the dislocations necessary for geometry.The existence of residual stress will cause rapid dislocation proliferation in the sample,and the dislocation density is almost 10 times that without residual stress.In addition,the model successfully predicts the indentation size effect(ISE)of MgO materials,which increases hardness as the indentation depth decreases.The agreement between experimental results and theoretical predictions further illustrates the effectiveness of the model developed in this paper.Finally,the effect of grain boundaries on the mechanical behavior of nanoindentation of bicrystal materials was studied.The effects of different indentation positions and grain boundary distances on the mechanical response of nanoindentations were numerically studied.The results show that when the distance between the tip of the indenter and the grain boundary is small,the plastic strain gradient between the tip of the indenter and the grain boundary is high,which results in the accumulation of geometrically necessary dislocations,which results in the hardening effect of the grain boundary.If the indenter tip is too far away from the grain boundary,the hardening effect will disappear.On this basis,a Voronoi method was used to establish a polycrystalline model,and the effects of crystal plane texture coefficient and grain size on the mechanical behavior of polycrystalline nickel nanoindentations were studied.The calculation results show that the higher the degree of preferred orientation of the(111)crystal plane of the polycrystalline nickel material,the greater the resistance to plastic deformation.