Perform Restart Analyses

The information on this page applies to the following analysis types, unless otherwise indicated:

Mechanical Event Simulation (MES)
Static Stress with Nonlinear Material Models
MES Riks Analysis

A convenient feature of nonlinear stress is the ability to restart an analysis from the last converged time step or previously saved restart step. This is useful for many situations. For example, if a nonlinear stress analysis is running and you need to use the computer for another task, you can stop the analysis and restart it when the other task is complete. Also if a power outage occurs in the middle of the analysis, it can be restarted from the last converged time step. Another situation that is made possible by the restart feature is the ability to use different time-step sizes during an analysis. An example of this would be if you are performing an analysis that begins with the tightening of a chain that then pulls a load. To properly simulate the tightening of the chain, a small time step will be required. However, once the chain is taut, the simulation becomes simplified. Using the same time-step size will result in unnecessarily large result files and analysis time.

Note: If you want to stop a nonlinear stress analysis, and resume/restart it later, you can cancel the running job from the Mechanical Simulation Job Manager. However, if the solver is in the process of writing temporary or restart files when the solution is canceled, you may not be able to restart the analysis. If a restart fails, you will have to rerun the solution from the beginning.

Note: If you anticipate the need to change the time step size during an event (before running the simulation), you can define multiple time intervals within the event tab of the Analysis Parameters dialog box. This feature is available for MES and nonlinear static analyses. See one of the following two pages, whichever is applicable:

Saving Multiple Restart Times

In some situations, it is necessary to set the restart parameters before running the original model. By default, the data that enables a restart analysis is saved only for the last time step. To restart at a time-step earlier than the last step, then set the Interval to save restart data drop-down to one of the following values:

Last step only Only the last converged time step is saved (regardless of whether the step is at the capture rate or in between a capture rate).
Last N steps The restart data for the last N steps is saved, where N is input in the N = field. Restart data older than N steps are deleted as the analysis proceeds. (See the paragraph below for clarification on the meaning of N steps.)
Every N steps The restart data for every Nth step is saved, starting with step N. (See the paragraph below for clarification on the meaning of N steps.) The last time step is also saved (regardless of whether the step is at the capture rate or in between a capture rate).

The definition of N steps depends on the capture rate and the other output options (specified on the Setup Model Setup Parameters Output tab) as follows:

If additional output is not being saved with the Output results of all time steps then only time-steps corresponding to the capture rate are considered for saving, as set above. Thus, restart data corresponding to the capture rate is saved. Converged steps at intermediate time steps, which occur when the time-step has been automatically reduced, are not saved.
If Output results of all time steps is activated, then every converged solution is considered for saving, as set above.
Time steps which correspond to the capture rate are an N step.

Perform Restart Analyses

There are numerous methods that can be used to restart a nonlinear stress analysis, depending on whether you are using the same model or a different model. The following items cannot be changed when using any of the restart methods:

Mesh.
Element loads, such as pressure.
Boundary conditions.
Cross sectional data in the Element Definition that changes the stiffness of the part.
Material properties that change the stiffness of the part.
Other items as indicated below.

The following items can be changed when using any of the restart methods:

Mass density.
Load curves.
Other items as indicated below.

The different restart methods are as follows:

Resume or Extend an Analysis using the Solver Manager: You can use this method to extend a completed analysis duration to determine what will happen if the event continues. You can also resume a partially completed solution with or without extending the duration. The time-step size (capture rate) cannot be changed. Changes to the original Duration are ignored. (The total duration is changed by using the Extend Total By input, as described below):
1. When extending the event, make sure that the load curves extend out for the additional time that will be added to the analysis, if needed.
2. Click Analysis Analysis Solver Manager.
3. Click the MES Restart button in the appropriate design scenario row.
  - Activate the Resume checkbox.
  - To extend the duration, specify how much longer you want to extend the analysis in the Extend Total By field. If you want to resume a partially completed solution without extending the original event duration, leave this value set to zero.
  - Click OK. The analysis will resume from the last completed time step.
4. Click Analyze. If you extended the event, the results will go from time 0 to the original duration + the time specified in the Extend Total By input field.

Resuming the Current Analysis from a Specified Time Step: This method is used when want to resume the analysis from a time step prior to the last completed one. The results of the additional run will be appended to the original run beginning at the specified restart time step, overwriting any later results. For example, if the original analysis reached step 35 of 50, and if you resume from step 20, the original steps 21 through 35 will be overwritten. All of the files from the previous solution must be present. The time-step size (capture rate) cannot be changed. The steps for this type of restart are as follows:
1. Set the Restart pull-down on the Setup Model Setup Parameters Advanced Time-Step tab to Resume current analysis.
2. Use the Resume from step pull-down menu to select which time-step to resume from. Step 0 has a special meaning. Specifically, it represents the last converged time step.
3. Click OK to accept the advanced analysis parameters and click OK to exit the Analysis Parameters dialog box.
4. Start the analysis using either Analysis Analysis Run Simulation or Solver Manager.
5. The results will go from time 0 to the originally specified duration.

Restarting From Another Analysis: This method provides more flexibility but places the new results in a new model. The steps to perform this type of restart are as follows:
1. Copy the other analysis to a new design scenario or a new model.
2. Nodal loads (magnitude and/or direction) can be changed.
3. The time step (capture rate) can be changed.
4. In the new model, set the Duration as the amount of time to add to the other analysis. Thus, the total analysis time will equal the time in the original analysis plus the duration in the new model.
5. In the new model, set the load curves based on the cumulative time, starting with the time from the original analysis. The load curves between time 0 to the time in the original analysis is ignored in the new model.
6. In the new model, set the Restart pull-down on the Setup Model Setup Parameters Advanced Time-Step tab to Restart from another analysis.
7. Click the Browse button to choose the Name of model to restart from.
8. Use the Resume from step pull-down to select which time-step to resume from. Step 0 has a special meaning. Specifically, it represents the last converged time step.
9. Start the analysis using either Analysis Analysis Run Simulation or Solver Manager.
10. The results will be calculated as follows:
  1. The Time in the Results environment starts at the time of the restart step (from the previous analysis), not at zero. The end time is the start time plus the event duration of the new model.
  2. The Time Step number in the Results environment goes from 0 to number of time steps in the new model.
  Note: The Model Units of the restart analysis must be identical to the Model Units of the original analysis.
  
  When using the method Restart From Another Analysis, the results for the entire analysis will be in two or more models. Animations from each run can be appended to create one animation for the entire analysis. Use the following procedure:
  1. If the Time Step number is shown in the annotations, consider hiding the annotation or removing the text. Since each analysis starts at time step number 0, this number would not proceed smoothly when the animations are appended.
  2. Create the first animation as normal. (Results Contours Captures Animate Save as AVI) Note the Target Resolution while setting it up. This will be needed for the additional animations.
  3. Save the presentation for the first analysis. (Right-click the presentation name in the browser and choose Save with Model.) This will save the result type, view of the model, legend settings, and so on.
  4. Open the next model.
  5. Use the Results Options Others Tools Import Presentations menu and select the first model. This will load the saved presentations into the current model.
  6. Right-click the saved presentation in the Saved Presentations branch of the tree view and choose Activate. This will open a new results window using the previous settings.
  7. Access the animation dialog (Results Contours Captures Animate Save as AVI). Select the previous animation file name and be sure to choose the option to append the animation.
  8. Set the appropriate step numbers to be animated from the current analysis. Normally, the last step of the previous analysis and step 0 of the current analysis are the same, so set the Start Step to 1.
  9. Set the Target Resolution to the same value as the first animation, if necessary.
  10. Click the Save button. This appends the animation for the current model into the previous animation.
  11. Repeat steps d through j for each restarted analysis.