NLCTRL

Format

Example

Definitions

Comments

1. The NLCTRL Bulk Data Entry is selected by the Subcase Information Entry NLCTRL=ID.
2. A nonlinear implicit subcase uses one of the following control entries: NLPARM, NLCTRL or TSETPNL Bulk Data Entries. TSTEPNL is used for a nonlinear transient subcase. NLCTRL Bulk Data Entry is an alternative for the NLPARM Bulk Data Entry to control the convergence, based on considerations illustrated in the following comments, and can improve convergence for the nonlinear solver. Many field options that are supported in NLPARM and NLADAPT can also be used directly in NLCTRL through keywords.
3. TTERM is the duration of time until the subcase is terminated. In the case of subcase continuation, it is the local subcase time that is different between the start time and the final time of the subcase. In a nonlinear transient subcase, the specified value of TTERM is computed depending on the Ni and DTi fields of the TSTEP Bulk Data Entry, if they are specified.
4. DT is the user-specified initial time. If NINC and DT are specified at the same time, then the NINC file is ignored.
5. DIRECT=NO is used for activating the adaptive time increment scheme in both Nonlinear Static Analysis and Nonlinear Transient Analysis. The time step in the next increment is adjusted based on the number of iterations. The time step is changed if there are highly nonlinear difficulties in geometry and material calculations. In Nonlinear Transient Analysis, the automatic time stepping is checked to control the displacement error related to velocity and acceleration. At the end, the smaller value from both will be chosen. For more details, refer to the TSTEP Bulk Data Entry.
6. For those subcases using NLCTRL, the special time adjustment of expert system PARAM,EXPERTNL will be ignored. However, if PARAM,EXPERTNL,CNTSTB is specified, contact stabilization is performed in those subcases with NLCTRL.
7. TOLF is the tolerance of force criterion. The relative error of force is defined as:
   $$E_F=\frac{R_{\max }}{F_{ref}}\le TOLF$$
   Where,

   $R_{\max }$

   Maximum residual force of any degree of freedom in translational direction

   $F_{ref}$

   Reference force which can be specified directly or calculated by OptiStruct for the model based on the loading history

   For the adaptive time increment scheme, $F_{ref}$ is the time average force, $F_{tim,avg}$ , over a number of increments, which can be expressed as:

   $$F_{tim,avg}=\frac{1}{N_{avg}}{\displaystyle \sum _{i=1}^{N_{avg}}(F_{avg,i})}$$

   Where,

   $F_{avg,i}$

   Spatial average force of the model in the $i$ increment

   $N_{avg}$

   Number of active increments

   The spatial average force, $F_{avg}$ , is the average value of the degrees-of-freedom with significant non-zero force when you check all grid forces and element forces, including applied force, reaction force and element internal force.

   $$F_{avg}=\frac{1}{N_{act}}{\displaystyle \sum _{i=1}^{N_{act}}{f}_{act}}$$
   Where, $N_{act}$ is the number of active degrees-of-freedom.

   Moment convergence criterion in the rotational direction is handled separately and in the same way as force convergence criterion.

8. TOLU is the tolerance of displacement criterion. The relative error of displacement is calculated only based on the current increment:
   $$E_U=\frac{\delta U_{\max }}{\text{Δ}{U}_{\max }}\le TOLU$$
   Where,

   $\delta U_{\max }$

   Maximum displacement correction in one iteration.

   $\text{Δ}{U}_{\max }$

   Maximum displacement change in the current increment for one single degree of freedom in translational direction.

   The rotation convergence criterion is handled separately in the same way as the displacement criterion.
9. Alternative tolerances TOLFLI and TOLMLI are used in iteration numbers equal to or larger than ITER.
10. Alternative tolerances TOLUZF and TOLRZM are used in iterations when the time average value of force and moment are insignificant which means there is no active loading.
11. If the reference force REFF or the reference moment REFM is specified, the reference value is replaced with a specified value and remains constant throughout the analysis:
    $$F_{ref}=REFF$$
    or
    $$M_{ref}=REFM$$
12. Line search method using MAXLS and LSTOL is only supported in SMDISP NLSTAT. For more information, refer to the NLPARM Subcase Information Entry.
13. When EXTRA=YES the linear extrapolation of displacement is performed at the beginning of an increment, based on the displacement change in the last increment. This option is only supported in LGDISP NLSTAT.
14. For the STABLIZ option, refer to the NLADAPT Bulk Data Entry.
15. Both NLPARM and NLCTRL Bulk Data Entries can not be specified together in the same nonlinear implicit subcase.
16. Detailed convergence check information for implicit nonlinear subcases, increments, and Newton-Raphson iterations are printed in the _nl.out file. Additional options for checking Nonlinear Analysis run information and monitoring progress of Nonlinear Implicit Analysis jobs are listed in Runtime Monitoring in the User Guide.
17. For more information on the augmented Lagrange multiplier method, refer to Augmented Lagrange Multiplier (ALM) Method (Nonlinear Analysis) in the User Guide.
18. When the augmented Lagrange multiplier method for nonlinear contact is activated, MAXITER is respected within each augmentation of a time increment.
19. If cutback occurs, the time increment is immediately reduced to 25% of the current time increment. Subsequently, if an increment converges without issue, the scaling factor for the time increment is successively increased from 1.0 to 1.5 in the next few increments, aiming to achieve a stable solution path.