Analysis Method Of Meso-inhomogeneous Deformation And Its Application In Fatigue Crack Initiation Prediction

The orientation,shape and size of the crystals among different grains are different,and the starting conditions of each slip system in different grains are different when metals are subjected to the same load,so the deformation of polycrystalline metals shows a non-uniform state in the meso-scale.The inhomogeneity of deformation in metals will lead to the concentration of stress in some areas of the material,and the initiation of fatigue cracks are related to stress concentration closely.Therefore,the inhomogeneity of meso deformation has an important impact on the manufacturing process and fatigue performance of mechanical products.In order to improve the safety of service performance of mechanical engineering materials and products,reveal the law of mesoinhomogeneous deformation in metal materials and its influence on fatigue crack initiation process,it is necessary and urgent to investigate the deformation inhomogeneity of polycrystalline metals in the meso-scale under cyclic loading.In this paper,taking ultra-fine grained copper as an example,the representative volume element(RVE)model is developed to simulate the mechanical response of material under different strain amplitudes with the finite element simulation method combined with the crystal plasticity constitutive theory.A method of describing strain inhomogeneity in the meso-scale is proposed.The relationship between the local inhomogeneous deformation of the material and the location of crack initiation is explored.The fatigue crack initiation position is predicted by using the parameter of meso-inhomogeneous deformation,and the accumulation of plastic strain and cumulative dissipated strain energy are used as the classical fatigue indicators to verify the prediction results.The main innovative research work in this paper is as follows:1.According to the morphology of equiaxed grains of ultra-fine grained copper,the Voronoi diagram representing equiaxed grains is constructed by secondary screening of grains,and the orientation data of Voronoi diagram is given by ultra-fine grained copper.Based on the constitutive theory of crystal plastic,the mechanical response of ultra-fine grained copper during cycling is simulated.2.In order to evaluate accurately the meso-inhomogeneous deformation of ultrafine crystal copper under cycling,an analysis method based on deviation normalization method is proposed.This method preprocess the strains of all meshes in the RVE with deviation normalization first,then the standard deviation of the preprocessed data is calculated,and the standard deviation is considered as the evaluation parameter of the deformation inhomogeneity in the meso-scale.To verify the accuracy of the deviation normalization method,this method is compared with different methods to describe the deformation inhomogeneity in the meso-scale under the same cycling conditions,and the deviation normalization method is found to be the most accurate method for evaluating inhomogeneous deformation.3.In order to study the relationship between the material strain inhomogeneity and the fatigue crack initiation location,the fatigue crack initiation location is predicted based on the local strain inhomogeneity in the material RVE model.It is assumed that the local area with the largest strain inhomogeneity is the most prone to generate fatigue crack,and the mesh with the largest longitudinal strain in this area is the specific crack initiation location.Classical cumulative plastic strain method and cumulative dissipation of strain energy method are used to verify the prediction results,and it is found that the accuracy of prediction results is relatively high.