Multi-step simulations need not be defined in a single job. Indeed, it is usually desirable to run a complex simulation in stages. This allows you to examine the results and confirm that the analysis is performing as expected before continuing with the next stage. The ABAQUS restart analysis capability allows a simulation to be restarted and the model's response to additional load history to be calculated.
The restart analysis capability is discussed in detail in “Restarting an analysis,” Section 7.1.1 of the ABAQUS Analysis User's Manual.
The ABAQUS/Standard restart (.res) file and the ABAQUS/Explicit state (.abq) file contain the information necessary to continue a previous analysis. In ABAQUS/Explicit the package (.pac) file and the selected results (.sel) file are also used for restarting an analysis and must be saved upon completion of the first job. In addition, both products require the output database (.odb) file. Restart files can become very large for large models; when restart data are requested, they are written for every increment or interval by default. Thus, it is important to control the frequency at which restart data are written. Sometimes it is useful to allow data to be overwritten on the restart file during a step. This means that at the end of the analysis there is only one set of restart data for each step, corresponding to the state of the model at the end of each step. However, if the analysis is interrupted for some reason, such as a computer malfunction, the analysis can be continued from the point where restart data were last written.
When restarting a simulation using the results of a previous analysis, you specify the particular point in the simulation's load history from which to restart the analysis. The model used in the restart analysis, however, must be the same as the model used in the original analysis up to the restart location. Specifically,
the restart analysis model must not modify or add any geometry, mesh, materials, sections, beam section profiles, material orientations, beam section orientations, interaction properties, or constraints that are already defined in the original analysis model; and
similarly, it must not modify any step, load, boundary condition, field, or interaction at or before the restart location.
Continuing an interrupted run
The restart analysis continues directly from the specified step and increment of the previous analysis. If the given step and increment do not correspond to the end of the previous analysis (for example, if the analysis was interrupted by a computer malfunction), ABAQUS will try to finish the original step before trying to simulate any new steps.
In ABAQUS/Explicit in cases where restart is being performed simply to continue a long step (which might have been terminated because the time limit for the job was exceeded, for example), you can restart the run by using the Recover job type as shown in Figure 11–9.
Continuing with additional steps
If the previous analysis completed successfully and, having viewed the results, you want to add additional steps to the load history, the specified step and increment should be the last step and last increment of the previous analysis.
Changing an analysis
Sometimes, having viewed the results of the previous analysis, you may want to restart the analysis from an intermediate point and change the remaining load history in some manner—for example, to add more output requests, to change the loading, or to adjust the analysis controls. This can be necessary, for example, when a step has exceeded its maximum number of increments. If an analysis is restarted because the maximum number of increments has been exceeded, ABAQUS/Standard thinks that the analysis is partway through a step, tries to complete the step, and promptly exceeds the maximum number of increments again.
In such situations you should indicate that the current step should be terminated at the specified step and increment. The simulation may then continue with the new steps. For example, if a step allowed only a maximum of 20 increments, which was less than the number necessary to complete the step, a new step should be defined in which the entire step definition, including applied loads and boundary conditions, is identical to that specified in the original run with the following exceptions:
The number of increments should be increased.
The total time of the new step should be the total time of the original step less the time completed in the first run. For example, if the time of the step as originally specified was 100 seconds and the analysis ran out of increments at a step time of 20 seconds, the duration of the step in the restart analysis should be 80 seconds.
Any amplitude definitions specified in terms of step time need to be respecified to reflect the new time scale of the step. Amplitude definitions specified in terms of total time do not need to be changed, provided the modifications given above are used.
The magnitudes of any loads or prescribed boundary conditions remain unchanged since they are always total values in general analysis steps.
