There are two parameters that define the event in a mechanical event simulation. These are specified in a table within the Event tab of the Analysis Parameters dialog box. The length of the event is specified in the Duration column (first column). The Number of time steps for which you want the results to be calculated is 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 appropriate. The second column heading of the event table updates accordingly.
Regardless of whether you choose Capture rate or Time step size, the input values are adjusted so the number of time steps, or Total steps, is an integer. For example, if you enter a Duration of 0.4 seconds and a Capture rate of 23 steps per second, the dialog changes the Capture rate to 22.5 steps per second so that the Total steps is 9.
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 is an impact or drop test. Use a coarse capture rate while the object is traveling towards the impact. Then, use a fine capture rate to capture the rapidly occurring conditions during the impact event.
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:
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.) It is recommended that you create a backup copy of all files before restarting so that you may easily return to this same restart point.
It is always safer to use a time step size that is 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!). Such time step reductions are typical for high-speed impact problems involving stiff materials. 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 mechanical event simulation are described on the following pages:
| Page | Description |
|---|---|
| Defining Load Curves | Define how the applied loads change over time. |
| Controlling the Output Results | Indicate to output additional results for the Results environment. |
| Advanced Settings | The advanced analysis options can be accessed by pressing the Advanced button on the Analysis Parameters dialog box, from which advanced controls for iterating, convergence, time-step management, and so on. are found. |