The program uses time as a tracking parameter in all static and transient analyses, regardless of whether they are actually time-dependent. You can therefore use one consistent counter or tracker in all cases, eliminating the need for analysis-dependent terminology. Time always increases monotonically, and most occurrences in nature happen over a period of time, however brief the period may be.

In a transient analysis or rate-dependent static analysis (creep or viscoplasticity), time represents actual, chronological time in seconds, minutes, or hours. You assign the time (TIME) at the end of each load step while specifying the load history curve.

In a rate-independent analysis, however, time becomes a counter that identifies load steps and substeps. By default, the program automatically assigns time = 1.0 at the end of load step 1, time = 2.0 at the end of load step 2, and so on. Any substeps within a load step are assigned the appropriate, linearly interpolated time value. By assigning your own time values in such analyses, you can establish your own tracking parameter. For example, if a load of 100 units is to be applied incrementally over one load step, you can specify time at the end of that load step to be 100, so that the load and time values are synchronous.

If you obtain a deflection-vs.-time graph in the postprocessor, it means the same as deflection vs. load. An example of the usefulness of this technique is in a large-deflection buckling analysis, where the objective may be to track the deflection of the structure as it is loaded incrementally.

Time adopts yet another meaning when using the arc-length method in the solution. In this case, time equals the value of time at the beginning of a load step, plus the value of the arc-length load factor (the multiplier on the currently applied loads). ALLF does not have to be monotonically increasing (that is, it can increase, decrease, or even become negative), and it is reset to zero at the beginning of each load step. As a result, time is not considered a "counter" in arc-length solutions.

The arc-length method is an advanced solution technique. For more information about using it, see Nonlinear Structural Analysis in the Structural Analysis Guide.

A load step is a set of loads applied over a given time span. Substeps are time points within a load step at which intermediate solutions are calculated. The difference in time between two successive substeps can be called a time step or time increment. Equilibrium iterations are iterative solutions calculated at a given time point purely for convergence purposes.