Meso Numerical Study Of Slip Damage In Plastic Zone Of Crack Tip Under Overload

Polycrystalline pure copper is widely used in oil and steam supply pipelines,filters,valves,pumps and other devices because of its good ductility and corrosion resistance.It is often subjected to various variable amplitude fatigue loads in actual working conditions.To analyze its damage tolerance,improve the residual strength,and predict the crack life,it is necessary to describe the fatigue damage,crack initiation and propagation under variable amplitude loads.Adding single peak tension /compression overload to constant amplitude cyclic load is the most common form of variable amplitude load.Because polycrystalline pure copper materials contain many grains with different orientations and sizes,the slip deformation is also different under load.The non-uniformity of deformation will lead to the stress concentration and slip damage concentration in the local area of the material,while overload will make the stress-strain field at the crack tip more complex.The further expansion of fatigue crack is usually closely related to the stress concentration and slip damage of the material at the crack tip.In this paper,the macro chaboche cycle model is combined with the meso crystal plastic finite element method.Taking polycrystalline pure copper as an example,the simulation analysis from the macro standard compact tensile specimen overall model to the meso representative element(RVE)sub model and then to the sub crystal scale is realized,and its single peak tensile /compression overload cycle is simulated respectively The evolution and distribution of meso deformation inhomogeneity and slip damage in the plastic zone at the crack tip before crack initiation under a single tension compression cross overload cycle.The main research work and related results are as follows:1.In order to accurately evaluate the meso heterogeneity of the force deformation trend of the grain surface at the front edge of the crack tip under overload,the rod parameter is introduced.It is found that compared with the constant amplitude cycle,the grain surface mainly bears shear stress at the moment of tensile overload and tensile stress at the moment of compressive overload;With the increase of overload multiple,the compression trend of single peak tensile overload and single tension compression cross overload increases at the tensile peak time of subsequent cycles,while the difference between the stress deformation trend of single peak compression overload and constant amplitude cycle at the tensile peak time of subsequent cycles can be almost ignored.2.In order to study the damage evolution and non-uniformity of RVE model before crack initiation under overload,taking the modified Fatemi society criterion as the fatigue index parameter,it is found that compared with constant amplitude cycle,overload reduces the damage nonuniformity of RVE model.Under overload cycles,the damage increases instantaneously and then the damage growth slows.The hysteresis effect of single tension compression cross overload cycle is stronger than that of single peak tension overload cycle.The hysteresis of single peak tensile overload cycle is caused by compressive residual stress.The hysteresis of single peak compressive overload cycle is caused by the fact that the competitiveness of plastic passivation of crack tip material is stronger than that of tensile residual stress.3.In order to solve the problem that there may be errors in the simulation calculation accuracy on the grain scale under weak plastic slip,locate the grain with the largest damage.Starting from the sub grain scale,the normal distribution statistics of the total slip deformation of the lattice slip system in the grain are carried out.The causes of its distribution are analyzed by means of the longitudinal stress distribution in the grain and the generalized Schmidt factor(GSF)distribution.The results show that compared with the constant amplitude cycle,With the increase of overload multiple,the increase of longitudinal stress and the maximum and mean value of GSF at the moment of overload leads to the instantaneous increase of total slip deformation and the increase of slip non-uniformity;The decrease of longitudinal stress at the peak time of subsequent cyclic tensile and the expansion of GSF interval will inhibit the original slip growth rate and improve the non-uniformity of the distribution of total slip deformation in the crystal.