Crystal Plasticity Theory And Numerical Study Of Axial Compression Of Micro-scale Metal Columns

In recent years,a large number of micro-compression experiments have shown that when the material size is reduced to the micron/submicron scale,many new features will be shown,for example,when the external characteristic size of the material decreases,the yield strength will be higher and the strain localization of plastic deformation will become more obvious.However,based on the conventional continuum theory,it is impossible to explain the scale effect phenomenon and the local slip deformation behavior due to the small size,so the computational analysis method,the experimental test technology and the structural safety evaluation theory system which are established on the basis of this dissertation will face new challenges.At present,the research on the mechanical properties of micro-scale materials is one of the most important problems in the field of mechanics and materials science.Based on the conventional crystal plasticity theory,this dissertation mainly adopts the method of combining theory analysis with numerical simulation.By introducing the hardening equation containing the evolution information of dislocation and considering the theory of strain gradient effect,the conventional crystal plasticity theory is modified and developed respectively,and the theory model is implemented by finite element method with the help of the Abaqus user subroutine(UMAT)function.The axial compressive plastic response of micro-scale metal single crystal was analyzed.The specific research contents include:(1)In order to develop scientific theoretical model and calculation method,the deformation behavior of micro-scale material was predicted more accurately and reliably,and the physical nature of plastic deformation was revealed.Within a conventional crystalline plastic frame,in this dissertation,a hardening equation with dislocation density evolution information was introduced,a crystal plasticity model based on dislocation density was established,and the one-dimensional numerical and three-dimensional finite element method was used to simulate the axial compression process of micro-scale Ni single crystal column.The rationality and reliability of thenew model are verified by comparing the simulation results with the experimental data.(2)The actual micro-compression experiments will inevitably introduce many errors,so the data obtained under different environmental conditions tend to be dispersive and unfavorable to each other.The micro-compression experiment of single slip Oriented Ni single crystal column was studied,the mechanical response of Ni micro-cylinder was studied by using the crystal plasticity theory model based on dislocation density and the method of finite element calculation,and the effects of the common experimental error factors such as crystal orientation,transverse confinement,contact mismatch and geometrical taper were investigated.By comparing the results,the effects of these common experimental environmental errors on the axial mechanical test results of the micro-cylinder were evaluated.(3)In the process of axial compression,the micro-scale metal single crystal column has a significant size effect.In order to investigate the effect of strain gradient on the axial mechanical behavior of the micro-column,a low order strain gradient plasticity model for the tangent modulus algorithm was constructed under the conventional crystal plasticity theory.The linear relationship between strain gradient increment and strain gradient was proposed in this model,and a hardening modulus expression containing plastic strain and strain gradient was established.By the finite element method,the axial compression process of single crystal Ni was simulated.the rationality and reliability of the new model are verified by comparing the simulation results with t with the predicted results of strain gradient theory based on the forward Euler algorithm.