There are two parameters that define the event in a static stress with nonlinear materials simulation. These are specified in a table within the Event tab of the Analysis Parameters dialog box. The length of the event must be specified in the Duration column (first column). The Number of time steps for which you want the results to be calculated must be specified in the second column.
Alternatively, the Capture rate (that is, the number of calculated steps per second) or the Time step size can be specified to define the event. Use the drop-down list to change the Step input method from the Number of time steps to either of the two alternative methods, as desired. The second column heading of the event table will update accordingly.
Regardless of the input method you choose, the number or time steps must be an integer. Therefore, if you enter a combination of parameters that results in a non-whole number of time steps, the parameters will be automatically adjusted. For example, a duration of 0.3 seconds and a capture rate of 25 per second would represent 7.5 time steps (0.3 * 25 = 7.5). The program will adjust the inputs to result in a whole number of time steps. Changing either set of input will update the other set.
You can specify multiple time increments within the event table. Use the Insert Row, Add Row, or Delete Row buttons to add or delete time intervals. An example of when you may wish to use multiple intervals where plastic deformation of a material causes accelerated changes in the deformation of a structure. Use a coarse capture rate while the stresses are linear and the model relatively stable. Then, use a fine capture rate to capture the more rapidly occurring conditions during plastic deformation.
Two additional columns show the Total time and the Total steps. These quantities are cumulative, adding the time and number of steps from the preceding rows to those of the current row. The four event parameters are related to each other as follows:
The Output results of all time steps option in the Output tab of the Analysis Parameters dialog box is supported for nonlinear solutions run using the Autodesk Nastran solver. This option is mapped to an equivalent Nastran option.
In any nonlinear analysis, the most critical parameter is the size of the time step. Note the following:
The above findings should be used as guidelines when choosing a time step size.
During a given analysis, you might reach a point in the event where convergence difficulty is encountered. The restart feature allows the user to change the time step size and continue a run from the last converged step, even if the original event was not set up with multiple capture intervals. (See the page Advanced Settings: Performing a Restart Analysis below.) It is recommended that you save all files before restarting so that you may easily return to this same restart point.
It is always safer to use a time step smaller than required. However, doing so can significantly add to the processing time and the size of the output files.
Finally, it may sometimes be necessary to reduce the time step by orders of magnitude during an analysis (not just by a factor of two!). These reductions can be achieved either by setting up multiple time intervals in the event table or by using the restart feature.
The other steps in the Analysis Parameters required to set up the nonlinear static analysis are described on the following pages:
| Page | Description |
|---|---|
| Defining Load Curves | Define how the applied loads change over the event. |
| Controlling the Output Results | Indicate whether or not to output additional binary results for viewing in the Results environment. |
| Advanced Settings | The advanced analysis options can be accessed by pressing the Advanced button on the Analysis Parameters dialog, from which advanced controls for iterating, convergence, time-step management, and so on. are found. |