Specific Features of FE-Meshes used in Lagrange Formulation for the Analysis of Metal Forming Processes

2nd Symposium on Trends in Unstructured Mesh Generation, University of Colorado, Boulder, August 1999

MESHING
RESEARCH
CORNER

Institute of Metal Foming and Metal Forming Machine Tools (IFUM), University Hannover
meinen@office.ifum.uni-hannover.de

Abstract
Due to steadily increasing demands on die forging industry, the process simulation is of continuously importance. The Finite Element Method (FEM) is frequently used to simulate forming processes for the purpose of predicting the quality of the final product and the loading on the tool. Most forging processes reduce the original height of the workpiece by 60-70 percent. Therefore, considerable distortions of the finite element mesh occur and some elements can even degenerate. Consequently, the results obtained are unreliable and it may even be impossible to pursue the calculation any further. In order to continue the analysis for such large deformations, it becomes necessary to define a new mesh during the FE Simulation (remeshing) and the values of the field variables (stress, velocities, etc.) must be transferred to the new mesh elements (rezoning).

The numerical results are depending on the element shape, so it is important to have a good quality mesh to become reliable results. If in a meshing-algorithm, the element length depends on geometrical and/or numerical criteria, it is called an adaptive meshing. So, one needs a small element length where the gradients of the field variables are big and vice versa. In the numerical simulation of bulk metal forming processes the gradients are normally big at the workpiece surface and at part details with small dimensions. A priori one should use a mesh with fine elements on the surface and bigger elements in the inner site of the workpiece. Also, the meshing-module has to be robust, conform, work automatically and should be able to mesh any kind of geometry, because of the different shapes of the workpiece during the simulation. It is very important that there is nearly no change of volume during the remeshing. A general automatic, adaptive meshing procedure for 2D problems, which takes into account the issues described above, has been developed at the Institute of Metal Forming and Metal Forming Machine Tools (IFUM). It can be used to remesh a deformed mesh as well as to generate the initial one. The meshing-module is based on triangle- elements. Some metal forming examples should be shown and discussed.

Next, we present a meshing-module for 3D geometry based on tetrahedron- elements. This kind of element is particularly well suited to describe a complex geometry. The 3D meshing-module employs an optimised algorithm to create automatically, adaptive mesh with good quality elements. The input requirement is a discrete description by a grid of triangular facets of the surface of the part to be meshed. Based on this surface grid, the meshing-algorithm creates automatically a new volume mesh, which has optimised features for the analysis of metal forming processes.