Research On Automatic Hexahedral Mesh Generation Algorithm And FEM Numerical Simulation Technologies In Three-Dimensional Metal Forming Processes

In the course of industry modernization, 'progress in technology and innovation in science' is the theme of this course. The advanced manufacturing technique, represented by CAD/CAM/CAE (Computer Aided Design/Manufacture/Engineering), is most active part in fields of application and academic research. With the development of computer technology and the metal forming theory, the 3-D FEM (Finite Element Method) numerical simulation for metal forming process can be realized on computer. The detailed information in the metal forming process can be obtained through the numerical simulation; it gives a powerful tool and means for the optimization of process design and die.In this dissertation, automatic hexahedral mesh generation and remeshing technique, automation and computation strategy of three-dimensional rigid or rigid-visco plastic finite element method based metal forming processes are studied.In the part of rigid or rigid-visco plastic FEM, a method that the finite element surface mesh is used to represent to the whole die is suggested. The STL format file widely used in most CAD software is introduced to represent the die surface in FEM simulation process and the format is kept unchanged in all steps. A new approach about construction of local coordinate system is established, it guarantees the boundary conditions and calculate results transform form the global to the local coordinate system correctly. The formula about the frictional stress between the die and the workpiece in three-dimensional case and its contribution to the stiffness matrix and node load matrix is derivated. A new strategy to adjust the touch-die nodes in three-dimensional FEM for element volume control is presented. "Original Length Method (OLM)" can reduce the touch-die element volume change and improve the reliability of simulation effectively. An especial research on volume loss in numerical simulation using rigid-plastic FEM is carried out. The reasons for volume loss arediscussed, and the conclusion is that volume loss is inevitable but it can be controlled. Finally, some methods about the relationship between calculate efficiency and simulation accuracy are given.Although mesh generation is a "bottleneck" problem to star the numerical simulation by FEM, automatic mesh regeneration (remeshing) is an unavoidable necessity to smoothly continue the finite element analysis of large plastic deformation where severe mesh distortion will occur and the geometry will undergo constant change so that the shape of FE model before remeshing is no longer as regular as its initial shape. Element type choice in the numerical simulation of metal forming is discussed and the result shows that hexahedral element is more perfectly than the tetrahedral element because it may increase the accuracy of the solution and decrease the overall element count. Since the automation generation of hex meshing has significant payoff, a number of approaches have been explored. The existing algorithms can be classified into three types: Mapped Meshing, Paving/Plastering Algorithm and Grid-based Method. After comparing the advantages and disadvantages in all of these three types, the modified grid-based method is selected to generate the hex mesh in simulation of metal forming processes.In this dissertation, a new method for automatic hex mesh generation combined with the modified grid-based method and "Surface Element Layer (SEL)" method is studied in detail. It first recognizes the given model generated in most CAD software or distorted hexahedral finite elements. This course includes surface, characteristic lines and characteristic vertex construction and surface curvature and the whole mesh density function establishment. Secondly it generates a regular mesh inside the bounding box that circumscribes the given domain to be meshed. In-or-out status of regular mesh modes and elements are checked using even and odd parity check rule; the core mesh is generated by removal process of out status elements. Then, the geometric features of the core mesh (i.e. sharp edges and co...