As the carrier of simulation calculation, grid plays a decisive role in the quality of simulation results. The selection of mesh generation should mainly consider the following aspects: cell density, cell type, cell shape, mesh generation method and mesh generation algorithm.
There is a problem in the application of direct method in solving factor intensity factor, that is, according to the theory of fracture mechanics, when the radius approaches 0, the stress at the crack tip will be infinite. In other words, the crack tip has the characteristics of singularity. At present, there are many problems when FEM method is used to simulate tensile test, which makes it difficult to realize. Among many difficulties, the most important one is that the fields at the tip in the middle of crack propagation have singularity. To solve this problem, many scholars have carried out extensive research and proposed the most effective isoparametric singular element method. The main content is the application of singular elements that can avoid special cracks. Only by moving all the nodes in the edge around the crack to the 1 / 4 point of the crack tip in the CT sample. In this way, the stress singularity at the crack tip can be realized. Therefore, when setting the simulation parameters, the singularity value is set to 0.25.
In the finite element simulation research for the test, the mesh refinement is usually carried out for the main research content to achieve the high precision and detailed information of the research part. Due to the singularity of the crack tip, it is necessary to divide different grids when meshing the sample. A specific area is designated at the crack tip by the method of subdivision to divide the corresponding type of grid. Therefore, in this experimental simulation, the generation and propagation area of the crack is refined.
In ABAQUS, there are many types of meshing for simulated parts. There are three main types of meshes. One is quadrilateral element (Quad) (in this type, all meshes are composed of this type of elements). The second is quadrilateral element (at the main body of the part, the mesh can mostly be this regular element) – triangular element (Quad dominated) (triangular element is used for optimization at similar edge and angle transition positions). H is a triangular element (TRI) (in this type, its mesh consists entirely of triangular element types). Considering the calculation accuracy, for the grid near the crack tip, the fineness decreases outward layer by layer, and the modified 6-node quadratic plane stress triangular element (cps6m) is adopted. For other regions, 8-node quadrilateral quadratic plane stress complete integration element (cps8) can be used in the simulation.
In ABAQUS, there are three types of grid division: structured grid. The second is to sweep the M-grid (sweep). Third, its own grid (free). According to the content of this research and the characteristics of grid generation, two combined grid generation types, free grid and sweeping grid, can be used. In the transition area between the crack tip and the specimen, the swept grid is used, and the other areas of the specimen are free grid.
The finite element analysis model of the structure is shown in the figure. Under the above working conditions, the fatigue crack propagation m of 42CrMo steel compact tensile specimen is numerically simulated.
