Study On Deformation Behavior Of Crack Tip Plastic Zone Based On Mesoscopic Inhomogeneity

Metal material deformation generally exhibits heterogeneous deformation,which is not limited to the material’s inherent heterogeneous state.In the fatigue process,the distribution of plastic deformation in different loading steps also reflects inhomogeneity.The cumulative damage in plastic deformation will lead to crack tip initiation and propagation,and different trends of change will be reflected in the early stages.Previously,most crack tip analyses focused on overload behavior.The reason is that the overload behavior at the crack tip results in different residual stress changes during the fatigue process,which can inhibit or promote crack driving at the crack tip.This application is widely used in rail,drilling,oil collection and other fields.However,from a microscopic point of view,the crystallographic orientation of the crack tip position produces different crack initiation tendencies,resulting in different mechanical properties,and this mechanism is complex,which requires further research.In this paper,the sub-model method combining macro and micro is used to establish a standard compact tensile specimen with polycrystalline pure copper material as an example.The representative volume unit is established by selecting the local area of the crack tip,and the simulation analysis of the material grain level is realized.The main work is as follows :(1)In order to simplify the data distribution of RVE in the loop process and reduce calculation errors,this study used entropy calculation to analyze the inhomogeneity changes of the material during the cyclic process.This method divides all grids in RVE into strain intervals,and uses the volume fraction of grids in different intervals as the basic data for analyzing inhomogeneity.Compared with conventional methods,the entropy method is conducive to simplifying data distribution and reflecting non-uniformity changes.(2)To solve the problem of possible errors in calculating the inhomogeneity of crack tip position during the cycling process,a local analysis method was used to calculate the local entropy inhomogeneity of crack tip grains under different overload forms,in order to improve their trend changes.The results show that compared with the constant amplitude loading,using the local analysis method to calculate inhomogeneity in small volume regions of RVE improves the inhomogeneity numerical changes of the material.At the same time,the regions with larger inhomogeneity are concentrated in the crack initiation area,providing a basis for selecting representative grains for orientation selection and slip system analysis in subsequent studies.(3)In order to further optimize the trend of crack initiation under crack tip overload,this study focused on areas with larger inhomogeneity,and analyzed the influence of microcrystal orientation characteristics from the perspective of crystal plasticity.Based on the shear strain of different slip systems within grains,the study investigated the accumulation of slip and inhomogeneity.The results show that compared with constant amplitude loading,when tensile overload is added in the overload form,as the overload multiple increases,the subsequent accumulation rate of intra-granular slip decreases.When the grain orientation tends toward "soft" grains,this will further optimize the plastic deformation of the crack tip position.