| Implicit finite element (FE) codes offer better accuracy for sheet metal forming simulations when compared with explicit FE codes, but suffer from numerical disadvantages, including difficulty in parallelizing, rapid increase of CPU time with the number of elements, and, most particularly, no assured solution (non-convergence). It is the last of these that has been addressed as the most serious obstacle to widespread adoption of implicit FE codes. Among the implicit FE codes, the SHEET-3 program exhibits remarkable stability and strong convergence by use of its two special features: the mesh-normal direction and the N-CFS algorithm. But these two features alone do not always guarantee accuracy, convergence, and robustness of SHEET-3.; To enhance the reliability of implicit FE sheet forming simulations for use with the N-CFS algorithm and the mesh-normal formulation, the following improvements have been implemented and tested. (1) A contact search method, consisting of a global search procedure and a newly developed local search procedure, was formulated to enforce the contact search along the mesh normal direction for mesh-patch tool description with irregular triangular mesh patches. (2) An improved contact capability within the N-CFS algorithm was formulated to take into account shell thickness in contact analysis within the framework of shell elements by defining a new contact distance error. (3) An improvement of numerical tolerances in the SHEET-3 program was provided to enhance the efficiency and application to a wide range of problem dimensions.; Numerical tests are presented to evaluate these aforementioned improvements. The accuracy, stability and robustness are greatly improved compared with previous versions of the SHEET-3 program. |
