Stress Analysis Of 18CrNiMo7-6 Alloy Steel Considering Microstructure

Fatigue failure of mechanical components brings great safety hazards to the safety service of key equipment.The main cause of fatigue failure is that the surface integrity of components during manufacturing or use is damaged and stress concentration occurs.With the deepening of research,the degradation of material properties and the fatigue failure of some key components are not only related to the destruction of surface integrity,but also depend to a large extent on the microstructure of the material.The non-uniformity of the microstructure is the main reason for the uneven stress generated by the microstructure under the action of external force.The non-uniformity of the microstructure is the main reason for the uneven stress generated by the microstructure under external force.The localized stress concentration is another important cause of fatigue failure of engineering components.Therefore,studying the relationship between microstructure and stress concentration on the fatigue life of materials has become a hot topic in the study of metal materials.In this thesis,18CrNiMo7-6 alloy steel was used as the research object.The constitutive parameters of the crystal were calibrated by establishing the microstructure geometry model and using ABAQUS finite element simulation software,and considering the combination of crystal plasticity theory and material macroscopic response.Finally,the stress concentration simulation analysis of the microstructure model is carried out,and the stress singularity is studied.The main work is as follows:?a?According to the principle of Voronoi diagram,a two-dimensional polycrystalline microstructure finite element model with random crystal orientation,different grain size and shape,and capable of reflecting real microstructure is established by combining MATLAB and ABAQUS.The elastic anisotropy constantsC₁₁,C_{1 2}andC₄₄ of the single crystal of 18CrNiMo7-6 alloy steel were determined.The crystal plastic finite element method is used to simulate the tensile test of the polycrystalline model and the stress-strain curve matching of the18CrNiMo7-6 obtained by the experiment.A set of optimal crystal plasticity parameters,i.e.the initial yield stress?₀?28?200MPa,stage I stress?_s?28?270MPaand initial hardening modulush₀?28?500MPa,are determined by an error function.?b?Based on the single crystal elastic constitutive model and the crystal plastic constitutive model,the finite element simulation of the uniaxial tension of the microstructure finite element model is carried out.The effects of different mesh densities,loading loads,crystal size and crystal orientation on microstructure stress concentrations were investigated.The results of the two models are compared and analyzed.The results show that the loading load has little effect on the stress concentration;The stress concentration factor increases with the increase of the mesh density,and the elastic model increases more than the plastic model.The difference between the two is 27%to 56%;Different crystal orientations have a greater influence on the stress distribution state of microstructures,and the range of stress concentration factors is 23%to 33%higher than the latter;Both models have a phenomenon in which the maximum Mises stress increases as the crystal size decreases,and the former is about 20%higher than the latter.?c?By establishing a finite element model,the stress singularity at the junction of the three grains is analyzed by the equal biaxial loading method,and the singularity index of each grain is given.The effects of different loading loads,three crystal plastic parameters and crystal orientation on grain stress singularity were analyzed.It is found that the loading load,?_s andh₀ have no effect on the stress singularity of the grains;Under certain conditions?₀ has a strong influence on the stress singularity of the grain when it changes between 20 MPa and 60 MPa,and has no effect between 70 MPa and 270 MPa;The change in crystal orientation has a greater effect on the stress singularity of the grains,and the results of the two different crystal orientations differ by an order of magnitude.