An amplitude curve allows arbitrary time variations of loads, boundary conditions, and temperature fields to be given throughout a step (using step time) or throughout the analysis (using total time). By default, the values of loads, boundary conditions, and temperature fields either change linearly with time throughout the step (ramp function) or are applied immediately and remain constant throughout the step (step function), depending on the step type; see Table 11–1.
Table 11–1 Default amplitude variations.
| Step | Default Amplitude Variation |
|---|---|
| General Static | Ramp |
| Heat Transfer (transient) | Step |
| Heat Transfer (steady-state) | Ramp |
However, many problems require a more elaborate definition of time variation for prescribed variables according to their type and the type of step in which they are used. For example, different amplitude curves can be used to specify time variations for different loadings. One common example is the combination of thermal and mechanical load transients: usually the temperatures and mechanical loads have different time variations during the step. Different amplitude curves can be used to specify each of these time variations. Amplitude curves can be referred to by any number of boundary conditions, loads, and predefined fields.
If a boundary condition, load, or predefined field refers to an amplitude curve and is not redefined in subsequent steps, the following rules apply:
If the associated amplitude was given in terms of total time, the prescribed condition continues to follow the amplitude definition.
If the amplitude was given in terms of step time, the prescribed condition remains constant at the magnitude associated with the end of the previous step.
You can define tabular amplitudes in the Nonlinear Structural Analysis and Thermal Analysis workbenches. Tabular amplitudes use a table of values to define the amplitude for use with boundary conditions, loads, or temperature fields.