Improve the convergence rate and robustness of your finite element simulation.
It is a widely accepted notion that good convergence (or any convergence at all) is difficult to achieve in a progressive failure simulation of a composite structure. In fact, many progressive failure simulations terminate early, not due to global structural failure, but rather due to the inability of the finite element code to obtain a converged solution at a particular load increment. Helius PFA significantly improves the overall convergence rate and robustness of finite element simulations of progressive failure of composite structures. When Helius PFA is used in conjunction with Nastran to perform a progressive failure analysis, the increased robustness of the solution greatly diminishes the need for time incrementation reductions (or cut-backs). As a result, the analysis can be completed much faster than without the use of Helius PFA. In order to take full advantage of the superior convergence characteristics, you must change some of the default settings that govern the nonlinear solution process used by Nastran. These changes can be enacted with the NLPARM entry.
In Nastran, the default settings for the nonlinear solution process are based on the fundamental assumption of the Newton-Raphson algorithm that the nonlinear response of the composite structure is sufficiently smooth at both the local and global levels. However, in a progressive failure simulation of a composite structure, the nonlinear response of the composite structure is not smooth, especially at the local level. This situation is primarily responsible for the difficulty in obtaining convergence. Helius PFA is specifically designed to efficiently handle this localized 'jagged' material response; however, the default settings of Nastran must be changed to allow Helius PFA to improve the convergence characteristics of the finite element simulation. These default settings can be changed via the NLPARM entry. In this case, the NLPARM entry is used to significantly increase the number of equilibrium iterations performed before the code evaluates a need for a reduction in step size. It is also used to specify the number of increments in the step and the force convergence tolerance.
Specific options used with the NLPARM entry are discussed later (Nonlinear Solution Control Parameters). For now, be aware the NLPARM entry is used to provide Helius PFA with the freedom to drastically improve the speed and robustness of convergence in progressive failure simulations.