Analysis Of The Relationship Between Inhomogeneity Of Metallic Material Deformation And Fatigue Life In Low Cycle

The fatigue of the material under the reciprocating load is an important factor that affects the service life of the engineering structure.People have studied it for more than 160 years.The classical method of fatigue life analysis is based on experience.Both theoretical and engineering applications need improvement.This paper discusses a new fatigue life analysis method,focusing on the polycrystalline structure of metal materials and the inhomogeneous mesomorphic deformation caused by the crystal deformation mechanism,and studies the evolution of inhomogeneous deformation with the number of cycles under low-cycle fatigue deformation,leading to localized mesoscopic strain increase and its relationship with material fatigue failure.Considering the polycrystalline structure of the material,the Voronoi polycrystal aggregates are used as representative units to describe the structure and mechanical behavior of the material,and the crystal plasticity constitutive theory is used to numerically simulate the cyclic plastic deformation process of the material on the grain scale.On this basis,the inhomogeneity evolution of the meso-deformation of the material was studied.The statistical analysis of the evolution of the inhomogeneity of the material's reciprocating meso-deformation was used to summarize the law of low-cycle fatigue failure,and a new method for the prediction of the low-cycle life has been developed.The research in this paper mainly makes the following progress:1.The cyclic plastic behaviors of the 310S stainless steel such as the Bauschinger effect and the non-Masing property at different temperatures were studied through a test system,and a lot of test data with important significance was obtained,and the parametric calibration of the material's macro-cycle plasticity model and crystal plastic constitutive model was performed.2.For 310s stainless steel,the multi-sample method was used to study the follow-up yield surface after monotonic and cyclic preloading under different paths.The influence of plastic deformation on the anisotropic hardening behavior of the induced metal materials was discussed.Experiments have shown that the shape of the subsequent yield surface is strongly related to the preload path,and the shape curvature of the subsequent yield surface changes greatly in the preload direction,and the cyclic preload causes the dimensional expansion of the subsequent yield surface in the vertical preload direction.These phenomena indicate that the successor surface described by the classical plasticity theory does not match the actual situation and needs improvement.3.A representative unit of the Voronoi polycrystal aggregate that can reflect the meso-structure of the metal material was used to test the subsequent yield surface after tension and pressure preloading by applying periodic boundary conditions and incorporating a plastic constitutive model.The results show that the method can describe the cycle characteristics of polycrystalline metal materials very well,and can reflect the heterogeneity of the meso-scale deformation of the material during cyclic loading.It can also reflect the evolution of material yield characteristics and plastic deformation induced anisotropic hardening characteristics consistent with macroscopic measurements.4.The fatigue test of 310S stainless steel was simulated using representative elements of Voronoi polycrystals combined with crystal plastic constitutive model with periodic boundary conditions.The statistical standard deviation of loading axial strain in the representative unit,the statistical standard deviation of the first principal strain,and the statistical mean value of the first principal strain,etc.,which characterize the inhomogeneous deformation of the material,are analyzed with the evolution of cycle times.Compared with the range of cycle life of 102 to 104,there is a critical value for these parameters.Critical values can be used to determine the occurrence of low-cycle metal fatigue.The inspection confirmed that if the material is known to be recycled,this method only needs to obtain the critical value of the fatigue parameter of the material through a test under the strain amplitude,and the fatigue life of the material under other strain amplitudes can be predicted by simulation calculation.5.The crystal plasticity simulation of the fatigue test of the GH4169 alloy was performed using a representative element of a Voronoi polycrystal with a free surface.The non-uniform deformation of the surface was characterized by statistical standard deviations of the loading axial strain of the free surface elements,the statistical mean value of the first principal strain,and the statistical standard deviation of the free surface node displacements,confirming that they can all be used to determine the occurrence of metal low-cycle fatigue failure.The predicted results are consistent with the measured results.