Quantitative Analysis Of Plastic Localization Of Metal In Tensile Deformation

Generally,metal plastic forming is a kind of non-uniform deformation.The nonuniform flow of metal is caused by uneven strain which is the result of the stress distributed unevenly in the preforms.The uneven stress is caused by many factors such as boundary conditions,friction and shapes of preforms.This kind of uneven plastic deformation can be explained by continuum mechanics which is assumed to be uniform within the material.However,the uniform and continuous material does not exist,and there are some defects such as uneven distribution of grains,voids,structure segregation and metallurgical defects.Therefore,the mechanical properties of the material are not homogeneous and the geometric dimensions of machined surfaces are uneven inevitably.During the unstable flow stage,due to the inevitable existence of nonuniformity of mechanical properties and geometrical dimension,the deformation prefer to develop in the weakest place.Thus,the real flow inside preform body and the actual development of free boundary are different from non-uniform flow of metal plastic predicted by traditional continuum mechanics.With the development of modern plastic precision forming technology aimed at lightweight,green manufacturing and near-net forming,it is necessary to describe the true plastic flow behavior of metal accurately and clearly.However,non-uniformity of mechanical properties and geometrical dimensions are difficult to characterize accurately using the existing methods,and are constantly evolving during the plastic deformation,whose effects on the uneven plastic flow are not able to be explained reasonably by the existing theory tools based on continuum mechanics.Based on the discussion above,in this paper,we attempted to propose a quantitative analysis method of metal true plastic non-uniform flow behavior considering non-uniformity of mechanical properties and geometrical dimension.That is to say,try to use the load-elongation curve to determine the non-uniformity of metal plastic deformation.In order to facilitate the analysis,the simplest force condition--the uniaxial tension of the metal rod specimen was considered in this paper.Firstly,the mechanics relationships between the degree of metal uneven deformation and the test variable(load,elongation)are established based on the mechanical equation of state,which lays the theoretical foundation of this study.Secondly,finite element inverse method and mechanics analysis method are proposed respectively,the true tensile loadelongation curve of metal rods are considered as judge criterion,and then obtain quantitative results of the true uniaxial plastic deformation.Finally,the quantitative analysis results are verified by dynamic optical measurement of uniaxial tension of metal rods.The main research results are as follows:(1)In order to quantitatively describe the non-uniformity of plastic deformation after necking of the specimen,two kinds of characterization methods of non-uniformity are defined based on the overall deformation and the local minimum radius of specimen.(2)Under the consistent premise of the material properties,the specimen size,the test conditions and the boundary conditions,non-uniformity of plastic deformation after uniaxial tensile necking has a one-to-one relationship with the test variable(load and elongation).In other words,any point on the load-elongation curve can correspond to the unique geometry shape of uneven plastic deformation of metal material,thus,the non-uniformity of plastic deformation can be determine by the load-elongation curve during the metal tensile deformation process.(3)The metal hardening curve over a large range of strains obtained by the torsion test was used as a material model input for subsequent finite element simulation and mechanical analysis.(4)A quantitative analysis method to determine the uneven plastic deformation based on finite element inverse analysis was proposed.By setting various sizes of initial imperfection,the uniaxial tension of the metal rods was simulated in finite element software to obtain the simulation results of different load-elongation curves and corresponding plastic deformation unevenness,which were compared with the real sample load-elongation curve to determine the corresponding non-uniformity of plastic deformation.(5)A quantitative analysis method to determine non-uniform plastic deformation based on mechanics iterative was presented.The ratio of the length to be deformed to the length of the minimum cross section was always regarded as an unknown variable,which was solved based on the hardening equation,the force balance and the geometric displacement relations,and the process is approximated to the true tensile load curve by iterative calculation repeatedly,and finally the non-uniformity of plastic deformation during the stretching process is determined.(6)The plastic non-uniformity of metal specimen in the tensile test was calculated based on the dynamic optical measurement,which was different in numerical from that obtained by the methods of finite element and the iterative analysis,but similar in variation tendency.Thus,the correctness of the quantitative analysis method of nonuniform plastic deformation proposed in this paper was verified.