Hello!
The best method I use is to take a look to the NX Nasran Analysis Monitor when launching the nastran job from inside FEMAP, here I see how the solution progress, not need to take a look to *.F06 file:
The most crucial data for successful nonlinear static solutions are contained in the
NLPARM bulk data entry. NLPARM defines strategies for the incremental and iterative solution processes. It is difficult to choose the optimal combination of all the options for a specific problem. However, based on a considerable number of numerical experiments, the default option was intended to provide the best workable method for a general class of problems. Therefore, if you have little insight or experience in a specific application, you should start with the default option. However, if you have some experience and insight in a specific problem, you can change the default values, if you keep in mind the following guidelines:
•Computing cost for each line search is comparable to that of an iteration.
•The SEMI method usually provides better convergence than the AUTO method at the expense of higher computing cost.
•Default tolerances for the convergence criteria may be somewhat conservative. However, loose tolerances may cause difficulties in the subsequent steps.
•The quasi-Newton method is effective in most problems. However, it seems to have adverse effects in some problems, e.g., creep analysis.
•The line search method is effective to cope with difficulties in convergence in some problems. More extensive line searches may be exercised by a large value of MAXLS and/or a smaller value of LSTOL. On the other hand, line searches may have adverse effects in some problems, e.g., plane stress plasticity.
•The arc-length method should be used if the problem involves snap-through or postbuckling deformation. Then the Bulk Data entry NLPCI must be attached.
At each iteration, the computing time for convergence without the stiffness matrix update is estimated and compared with the computing time for the matrix update in order to determine whether the update is more efficient. This decision is deferred in the first two iterations after a new stiffness is obtained. If the solution tends to diverge, however, the update decision will be made effective immediately. The stiffness matrix will be updated upon convergence if the number of iterations required for convergence is greater than KSTEP. The SEMI option is identical to the AUTO option except for one additional stiffness update after the first iteration which always occurs unless the solution converges in a single iteration. With the ITER option, the stiffness matrix is updated at every KSTEP iterations. Thus, the full Newton-Raphson iteration is exercised if KSTEP is 1. If KSTEP > MAXITER, the stiffness will never be updated.
The MAXITER field is an integer representing the number of iterations allowed for each load increment. If the number of iterations reaches MAXITER without convergence, the load increment is bisected and the analysis is repeated. If the load increment cannot be bisected (i.e., MAXBIS is reached or MAXBIS = 0) and MAXDIV is positive, the best attainable solution is computed and the analysis is continued to the next load increment. If MAXDIV is negative, the analysis is terminated.
NX Nastran performs the convergence test at every iteration with the criteria you specify in the CONV field. You can specify any combination of U (for displacement), P (for load), and W (for work). All the specified criteria must be satisfied to achieve convergence, except for an absolute convergence condition, under which the solution is converged regardless of criteria.
I hope the above helps you to understabd better how nonlinear solutions runs in NX NASTRAN.
Best regards,
Blas.
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Blas Molero Hidalgo
Ingeniero Industrial
Director
IBERISA
48011 BILBAO (SPAIN)
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