In order to solve the difficult problem of the finite element meshing of complex assembly, this paper proposes the solution to the problem of the connection relationship between the parts in a complex assembly and the mixture of several typical models (cavity, thin shell, screws and bolts). Slicing, sweeping) meshing method, using the Ansys software for the example model using free meshing and hybrid meshing respectively. The results show that the hybrid meshing method has fewer units and higher quality than the free meshing method. The hybrid meshing method can be applied to all models with similar geometric features. 1 Introduction 2 complex assembly finite element mesh generation method 2.1 How to handle the connection relationship between the parts of a complex assembly 2.2 Meshing method for cavity and thin shell 2.2.1 cavity division method 2.2.2 Thin Shell Partitioning Method 2.3 Meshing method for screws and bolts 3 instance verification 3.1 Using a free meshing method 3.2 Using Hybrid Mesh Division Using the free mesh and hybrid meshing to get the relevant data of the unit and node, the list comparison can be seen from the data in the above table: the number of free mesh units is nearly 1 million more than the mixed mesh unit. And the unit is a tetrahedron; the order of the unit is also lower than that of the hybrid network, and the cell quality is worse than that of the hybrid grid (hexahedron). 4 Summary Wood Design PVC Decorative Film
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Meshing is a key step in the finite element analysis and calculation. The accuracy of the meshing will directly affect the accuracy and speed of the calculation, and even the calculation will not converge due to the unreasonable meshing. Meshing can be divided into three steps: defining cell attributes (including real constants), defining grid attributes on the geometric model, and dividing the grid. Due to the complexity of the assembly model and the high requirements for model simplification and the accuracy of the calculation results, the following issues need to be resolved in terms of meshing:
(l) Connections between various parts of a complex assembly
The simulation of the connection relationship between the components of a complex assembly directly relates to the accuracy of the simulation calculation. The connection relations include solid connection, welding, threaded connection, and contact. What kind of relationship simulation is used depends on the specific conditions and the accuracy requirements of the calculation results.
(2) Meshing of complex cavities and thin shells
Complex cavities and thin shells are extracted by using 2D shell elements to simulate the middle plane or by using 3D solid elements. If 3D solid elements are used, tetrahedron elements are used (the number of elements is too large, the quality is poor, calculation is time-consuming and the accuracy is better than that of hexahedrons. For the difference, it is also divided into hexahedral units by using methods such as mapping and sweeping. These are problems that need to be solved.
(3) Meshing of screws and bolts
For screws and bolts, whether to use 2D rod units or 3D solid elements depends on the specific problem to be solved; if the latter is adopted, because the number of units and nodes of complex assemblies is very large, it is necessary to consider how to The large number and poor quality of tetrahedral elements translate into fewer, higher quality hexahedral elements.
In this paper, the meshing method of complex assembly models with threaded joints is studied. At the same time, the finite element models generated by free meshing and hybrid meshing are compared.
Complex assembly finite element mesh generation method can be summarized as follows three aspects:
Because it involves static contact nonlinearity and modal and random vibration analysis, and the screws and bolts are an important focus, the simulation of the screw connection uses three-dimensional bolts, simplified models of screws (omitting factory threads) and bolt pre-tensioning units. ; And the contact simulation, using gas-dimensional contact unit.
Because 3D screws and bolts are to be calculated, 3D solid elements are also used for the meshing of the cavity and thin shell. At the same time, in order to reduce the number of the overall single file and improve the quality of the grid, dividing the parts into regular bodies and then sweeping the grid can divide the grid into hexahedrons.
For a 3D complex entity that has been built, if its topological form in one direction is always the same, the grid can be divided by using the (manual or fully automatic) sweep meshing function; almost all of the units formed in this way are Hexahedral unit. In general, it is a very good method to use a sweep mesh to form a mesh. For complex geometric entities, a regular hexahedral mesh can be automatically formed after some necessary segmentation and bonding processes. The grid division method has greater advantages and flexibility.
Here is an example. As shown in Fig. 1(a), when the cavity is divided into grids, it can be first constructed into a closed annulus, divided into two layers (as shown in Fig. 1(b)); and then the outer layer is divided into two parts: Divide two layers (as shown in Figure 1 (c)); afterwards, sweep grids can be used separately. 
The thin shell model with threaded holes is cut into the screw holes and other parts. After the slicing, the parts are bonded together so that they are continuous on the cell grid; afterwards, the scan meshes can be used separately. . The example is as follows: Figure 2 (a) shows the front view of the thin shell, in the vicinity of the threaded hole to construct a part of a cylindrical or cylindrical surface, the other part is then cut into small pieces, can be divided into Figure 2 (b) The adhesive bodies of a plurality of shells are shown so that each part can be swept and divided so as to ensure effective control of the number of units and the quality of the unit. 
The screw (bolt) is cut into two layers: the screw cylinder surface is extended to the head surface of the screw (bolt), and then the entire surface of the screw (bolt) is cut. The two sub-entities are then glued. Connected together can be divided using the sweep grid. Here is an example: Figure 3 (a) shows a solid model of a screw, Figure 3 (b) shows an exploded view after cutting, and Figure 3 (c) shows the result after meshing. 
Take the XX model component as an example to verify the mesh division method described in the article. The grids are divided by the free meshing method and the hybrid meshing method described in this paper.
Free meshing is one of the most automated meshing techniques. It can automatically generate triangles or quadrilateral meshes on surfaces (planes, surfaces) and automatically generate a four-body mesh on the body. Under normal circumstances, you can use ANSYS's smart size control technology (SMARTSIZE command) to automatically control the size and density of the grid, you can also manually set the size of the grid (AESIZE, LESIZE, KESIZE, ESIZE series of commands) Controls the distribution of the density and selects the subnetwork algorithm (MOPT command). For a complex geometric model, this kind of network separation method saves time and effort, but the disadvantage is that the number of units is usually very large, the computational efficiency is reduced, and this method can only generate tetrahedral units for a three-dimensional complex model. If a hexahedral element is selected, this method automatically degenerates the hexahedral element into a tetrahedron element with the same order. Therefore, it is better not to use a linear hexahedron element because the element is degenerate into a linear tetrahedron element and has a rigid surface. The stiffness of the calculation accuracy is poor; if the secondary hexahedron element is used, because it is a degenerate form, the number of nodes is the same as the hexahedron prototype element, but there are only a few nodes in the same position. Therefore, the TCHG command can be used to model the The degenerate form of the tetrahedral element changes to a non-degenerate tetrahedral element, reducing the number of nodes per element and increasing the efficiency of the solution. After the free meshing of the assembly model, the finite element model shown in Figure 4 is obtained (Figure 4). (a) is the overall model, and Fig. 4(b) is the screw model). 
Hybrid meshing means that in the geometric model, according to the characteristics of each part, various meshing methods such as free, map, sweep, etc. are respectively adopted to form a finite element model with a comprehensive effect as good as possible. Using a hybrid mesh (body was divided and swept as described above and the surface was divided freely), the finite element model shown in Figure 5 was obtained. 
In order to improve the calculation accuracy and reduce the calculation time, we should first consider the division of the hexahedral mesh into the areas suitable for sweeping and mapping meshes. This mesh can be linear (no middle node) or quadratic. (There is a node in the middle.) If there is no suitable area, it should try to create a suitable area (especially for the area or part of interest) by using a variety of Boolean operations such as cutting the servant. Secondly, it must be impossible to cut it. In the area divided by a tetrahedral free mesh, a hexahedral element with a middle node is used to perform free meshing (automatically degenerate into a unitary pattern suitable for a free division form). At this time, the fx: domain has been scanned or mapped. At the interface of the meshed area, the pyramid transition unit will be formed automatically (the hexahedral element without the middle node has no pyramid degradation form).
In short, it is extremely important to use a variety of means to establish a high-quality, high-efficiency finite element model. There are only a few parts introduced here. If necessary, special meshing software (Hypermesh, etc.) can be used. The constant exploration, summarization and verification of many engineering problems are the most effective means of ensuring the efficient handling of complex models.